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BIROIESCEPTOR REFLEX By William James Marquis The purpose of my research was an attempt to elucidate the neural pathways in the medulla involved in the baroreceptor reflex concentrating on the sympatho-inhibitory component and to ascertain the relative contribution of proposed medullary sites mediating this reflex. Past research in this area has yielded much conflicting data and inter-. pretations as to the importance of various medullary sites involved in the reflex. Stimulation techniques were employed in an attempt to characterize the cardiovascular responses elicited from proposed medullair sites in the baroreceptor reflex pathway. Stimulation of HTS produced a large depressor response as well as a pronounced bradycardia suggesting that both components of the baroreceptor reflex (sympatho-inhibition nd vagal activation) were present at this site. Stimulation of PEN m resulted in a large depressor response. The heart rate changes (small ‘ t bracvcardia) were shown to be mediated by sympatho-inhibitory fibers (1' o the heart and thus, no vagal component of the reflex was present at this site. Siniliar responses were obtained by electrical activation of the inferior olivary nucleus. ihe second phase of the experiments involved ablation of various . medullar' sites in vaqotonised cats in order to ascertain the importance and contl ibution of the lcsioned areas to the sympatho—inhibitory component of the baroreceptor reflex. Baroreccptor reflex activity was modified by the following techniques: BISO, i.v. injection of NE, and stimulation of nuclear sites proposed to be in the pathway. Ablation of NTS prevented the compensatory reflex adjustments to barorcccptor activation. This was shown by the following: The pressor response to BLCO was significantly reduced; the aeores sor response to IYHJstimulation was substantially potentiated; and mean blood pressure tincre Ha ed followinga Ml ties. The data obtained from EFT ablation experiments ind:1 ated that the lesioned area did not contribute signifi- cantly to the sympatho inhibitory pathway of the ‘aroreceptor reflex: The pres sor reaponse to BIC Jfiwas GdUCQLl onlyl 104 following bi latera.l ablation of this nucleus. Midline ablation data revealed that the classic "depressor area“ was a crucial substrate for the sympatho- inhibitory corp :ncnt of ihe barorecentor reflex: The pressor response to BLCO was sienificaztly reduced: following ablation; the presso- re- Sponse to i.v. HE was 5fi_fiiPlLunt1’ enla ced; and snail pressor responses elicited oy HTS slimulad ;3en were pstcntiat cd following midline ablatiei. Evidence was presented indicu,ing that pressor neurons may reside in classic "depressor areas" of the midline mec lullary reticule r for— maticn and may constitute the site of baroreceptor induced sympathe- inhibition to the blood vessels: Fressor responses were recorded J low s imulus parameters to nit-l1rers; and blood pressure decreased after midline ablation poss|h.v implying the destruction of pressor 11011330113 0 THE HEDULLAR‘ 931°“"'WlfiTIOd OF THE BAN RECEPTOR RCFI BK By W'illiam James Larc uir rnvrr "IS 4.114).] Submitted to Hichigan State University in partial fulfillment of the \ requirements for the d3 rec of 113517113 oa- M” :rcr \)\I.1" -\ .J Department of Pharmacology 1rw .LCI -L I ACKNOWLEDGHEETS The author respectfully acknowledges the support of Dr. G.L. Gebber. He wishes to thank Dr. T.N. Brody, Dr. J. look and Dr. C. Chou for serving on his committee. TABLE OF CONTENTS Page ACKIIO‘ZITij‘GFLIEITI‘SOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOoI. ii TABLTQ OF COIETEE‘“? 0.0.....0IOOOOOOOOOOOOOOOOOOOO0.... iii HST OITAKBLESO000......O00.000.00.00...00.0.0.0...O. iv LIST 0? FIGIIRESOOOOOOOOOCOOOOOOOOOOOO00.00.00.000... v INTRODUCTIOI‘I....................................... l NETIIODS............................................ 19 RESULTS................................o........... 24 DISCUSSION......................................... 65 SUI~§~IARY.........o.................................. 74 REFEEIJCESOOOOOOOOOOOOOOOOOOOOOOOO0.000000000000000 75 Table 1+ LIST OF TABIES The effect of vagotomy on cardiovascular resyonses elicited by inferior olive stimulation. The effect of hTS ablation on baroreceptor . reflex activity. The effect of FIR ablation on baroreceptor reflex activity. The effect of dorsal lateral reticular formation ablation on baroreceptor reflex activity. The effect of midline "depressor area" ablation on baroroceptor reflex activity. The effect of midline ablation on cardio- vascular responses clicitcd by HTS stimulation. Page an 1:6 50 52 51+ 64 Figure l \‘1 \O 10 LIST OF IIJJR YLIS Page~ The effect of vaqotory on the cardiovascular 30 responses elicited by KTS stimulation. Volta~e response curve shoving the cf fect of 32 vavoto: y on heart rate rcs ponses el.Lcited by “TS Stil.d1dvi()ho Voltage-res ponse curve showi ng t} c effect of 34 vagotomy on depressor responses evoked by NTS stimllation. Voltage-rears nse Clere showing tlze ”1f ct of 36 vagotony on denressor resy:onses evolced by P”IJ stimulation. Voltage-response curve showing the effect of 38 vagotomy on the heart rate responses evoked by YEN stimulation. The effect of va rying volta.:ge intrns ities on the no card: ova.scular responses evoked by stimulation of \_’\~ . . . Lu‘ao Vo 3_t a :e-rcsponse curve shom n5 tin effect of “2 vzby otomy on the depress or res tronses elicited by inferior olive stixil: tion. The effect of 2T3 ablation on the pressor #8 response and heart rate evoked by bilateral occlusion of the carotid arteries in vagoti- mized cats. Correlation gra}>h relating the c}ange of mean I .56 blood pressure levels and tie chan7c in the pressor response to 1 L00 following midline ablation. In The effect of midline “depressor area" ablation 5 on baroreccptor re..lcx activity. Figure 11 12 Page The effect of blood pressure level on the 60 pressor response to LLCO following midline ablation. ' The effect of midline ablation on the cardio- 62 vascular responses elicited by his stimulation. INTRODUCTION The baroreoeptor reflex is probably the most important visceral reflex in the body, yet little is known about its central anatomical organization. In as much as abnormal cardiovascular functions in- cluding hypertension and cardiac arrhythmias have been attributed to baroreceptor reflex impairment, it is important to delineate the the medullary neural pathways of the reflor. Also, a clearer under- standing of the medullary baroroceptor reflex pathways is important in order to interpret supramedullary modifications of the reflex which are integrated at the medullary level. The purpose of my research was an attempt to elucidate the neural pathways in the medulla involved in the baroreceptor reflex, concentrating on the sympatho-inhibitory component and to ascertain the relative contribution of proposed medullary sites mediating this reflex. The following will be a description of some of the pertinent past research in baroreceptor physiology and anatomy. It may be stated at the onset that the literature is beset with controversies regarding the neuroanatomical pathways of the reflex as well as the contribution and importance of supramedullary structures on cardiovascular function. Traditionally, the medulla has been regarded as the neural center integrating central cardiovascular control. Dittmar and Owsjannikow in the 1870's were the first to enter into an investigation of the medullary vasomotor center (Bard, 1960). They found that transections of the brain stem from above downward had no effect on blood pressure until they reached a pontine level near the rostral border of the trapezoid body. As sections were made more caudally they produced a greater fall in blood pressure. The maximal effect was observed 3-4 mm above the calamus scriptorius. At this point the degree of decrease in blood pressure was the same as that obtained by section of the spinal cord at 0-1. These authors also showed that cardio- vascular reflexcs elicited by stimulation of the sciatic nerve were integrated in this same general ponto;medullary area necessary for maintainance of tonic blood pressure. Further information regarding the position and nature of the bulbar cardiovascular center was presented by Ranson and Billingsley (1916). They stimulated the floor of the nth ventricle with a needle electrode and found two discrete reaction points which responded with changes in blood pressure: A pressor center ( #0 mm Hg. increase in blood presszre) about 1 mm in diameter in the region of the ala cinera to the fovea inferior, and a depressor center ( #0 mm Hg. decrease in blood pressure) in the area postrcma just lateral to the obex. The authors concluded that these points could represent; a) afferent endings of vasomotor neurons, b) receptive nuclei of afferent vasomotor neurons, or c) true vasodilator and vasoconstrictor centers giving origin directly to autonomic efferents. The nex significant research in this area was done by Hanson and Wang (1939)c sing the newly desivned Horsley-Clark stereotaxic instrument, they were able to explore the depths of the medulla in an attempt to further delineate cardiovascular centers. In general, pressor points were 5 read difquely in the dorsolatoral reticular .. i A formation, and depressor points in the ventromodial portions of the reticular formation. Thus, these authors showed that the surface areas previously reported by Hanson and Billingsley were just the dorsal extensions of these diffuse areas in the reticular formation. In 1946, Alexander amplified the concept of cardiovascular centers by presenting a detailed localization of pressor and depressor areas in the pons and medulla. As can be seen from the following figure, the pressor center was found to occupy an extensive region of the lateral reticular formation in the caudal pons and rostral two-thirds of the medulla, while the depressor center includes much of the medial reticular formation in the caudal one-half of the medulla. Relative distribution of the bulbar pressor and depressor regions: pressor area indicated by crosshatching; depressor area by horizontal ruling. I, II, and III - levels of transection Reproduced from Alexander, l9k6. In attempting to assess the origin of tonic sympathetic nerve activity controlling blood pressure, transections were made as indicated on the diagram and recordings were taken from the inferior cardiac nerve and cervical sympathetic trunk to indicate changes in cardiac and vaso- motor function. Transection as far caudally as the lower one—third 'of the pens had no significant effect on blood pressure or tonic activity along the sympathetics. Section at I (the level of the auditory tubercle) resulted in a considerable fall in mean blood pressure corre- lated with a significant reduction in tonic sympathetic activity. This transection removed a significant portion of the rostral pressor region. Section at II (slightly rostral to the obex) produced a maximal fall in blood pressure together with a complete disappearance of activity in the sympathetics. This section, as can be seen from the diagram, removed a large part of the pressor region while leaving a major portion of the depressor region intact.’ Further section at III (C-l cord section) raised blood pressure levels from previous low levels and restored some activity in the sympathetic outputs thus implying that the depressor region exhibited tonic activity. That is, the C-1 cord section eliminated a tonic depressor activity descending to the spinal cardiovascular centers from the depressor center in the’ medulla. This finding, according to Alexander, established the depressor center as a functional entity rather than merely a region through which inhibitory afferents travel to reach the depressor center. The question remains, however, whether this effect depends on the intrinsic activity of a group of midline neurons or is the result of an inflow of impulses from baroreceptor afferents or other sources of a depressor reflex. Alexander (lQhS) also studied somatic pressor reflexes produced by stimulation of the sciatic nerve. Fell wing removal of portions of reduction of its discharge rate following proceedurcs which altered blood pressure levels including i.v. injection of vasoconstrictor and vasodilator agents and bilateral carotid occlusion. These neurons, so defined, could be divided into two groups: Population I- neurons that exhibited an increase in their discharge when arterial pressure rose and a decrease when pressure fell. These neurons ceased firing during bilateral carotid occlusion; Population 11- those neurons that behaved in the opposite way, i.e. decreased their firing rate with an increase in blood pressure and increased their firing rate with a decrease in blood pressure. The ratio of .ype II to type I was about 5 to 1. It was suggested by the author that circulatory control may be achieved by some form of reciprocal innervation between these two systems of neurons similiar to the reciprocity seen in the respiratory centers. On the other hand, it is equally possible that the two systems may Operate independently and integrate their activity (inhibitory in the case of type-I neurons and excitatory in the case of type-II) at the spinal cord level. The distribution of type I and type II units did not reveal separate anatomical areas for these units. Type I and type II cardiovascular neurons were found scattered throughout the medulla, intermingled with each other, with the majority of the detect- .able units within medial structures. In a study done by Wang and Przybyla (1967) employing similiar techniques as Salmoiraghi, cardiovas ular ncur ns (type II) were located almost exclusively in the periventricular grey and adjacent dorsal reticular formation. These neurons decreased their firing rates ,4 . . . .. n . from 30~lOOp following a rise in blooo pressure. rrom this study, and stu dies eznploying les ioning techniques, hese authors concluded that the areas responsible for ca iovascular integration are localized in the dorsal lc-tera 1 portions of the medulla. This group has also demonstrated that unraro.ull structures are not essential for cardiovascular integration. This was pointed out by showing that midcollieular dccerebration did not abolish the increase in arterial pressure s on aft er 8(CtiOP :ing of the hr ffer nerves or inhibit the carotid occlusion presser response. Also, in another study, Chai and Wang (1962) shlo;ed hat c; rdioactcleration elicited by dorsal medullary stimulation, bilateral carotid occlusion, or sciatic nerve stimulation was not reduced after midcollicular section. 6“ ) disputes this View that medullary centers \0 However, Maxining (1 located in the dorsal reticular formation maintain vascular tone and form the central synaptic link cont roLJ.ing cardiovascular reflex adjustments. He showed that extensive lesions in the medullary vaso— motor area did not significantly alter the cardiovascular reflex ad" justments to bilateral carotid occlusion, to stimulation of the sciatic nerve, r to hypothalamic stimulation. In addition the preparations :ere critically dependent upon supramedullary connections to maintain ascular tone and c: rculatory reflex adjustments for midcollicular section in the lesioned animal brought about a reduction in blood pressure and a loss of vascul.' r reflex responses. Thus, according to Kenning, unlovedlllarv centers exert tonic as well as phasic influences on vascular ini cardiac activity independent of the medullary vaso- motor area. Evidence from Feiss' laboratory supports this view of ~ fr y ‘ m anning(Pe ciss, 190p). he ShOICd that cardioaeo elation produced by CO stimulation in the dorsal medulla was eliminated by lesions caudal to the hypothalamus and sugeest d thit die cardioacce leration elicited by stimulation in this area was due to activation of afferent pathways to higher CNS structures. Also, Domino (1968) sho*,.*cd statistically significant decreas es in blood pressure followin'r midpontm ebrainstem transection and suggested that portions of the CNS above the section may be exerting tonic influences on blood pressure. Thus, the question of sunroreeillarv influence 01 tonic blood pres sure and integration of cardiovascular reflexe31 is still not resolved. Having discussed the concept of cardiovascular centers and the integration of cardiovascular ref exes in the medulla, I would now like to specifically discuss the baroreceptor reflex including its physio- logical functions and anatomical representation in the medulla as well as the influence of supremed.ullary structures on the reflex. The baroreceptor reflex is essentierlly a re*'t1ve control sys tem. Baro- receptor nerve endings are found predominately in the carotid sinus region and the arch of the aorta as well as in the right atrium and along the pulmonary arteries. An increase in blood pressure 0 uses mechanical. distortion of stretch receptors in these regions and increases tonic nerve activi y along the carotid sinus nerve, the aortic depressor nerve in the cat, and the vague. These fibers terminate in the medulla where they eventually cause inhibition of sympathetic tone to the V"; .4 blood vessels at a nedullary site or in th spinal cord to buffer the rise in pressure, nd vafal activation to slow the heart. Also, de- creases in blood pressure result in a decrease in activity along baroreceptor afFCI ents wi h a concomitant increase in blood pressure and heart rate due to release of tonic inhibit ion controlling these cardiovascular responses. There is also a sympathetic innervation controlling heart rate and cardiac contractility which is modified by baroreceptor afferent input. In the total pcpulation of baroreceptors, there is considerable variation in threshold. At a low but effective pressure, only a: minority consisting of the most excitable units contribute to the discharge of afferent nerve impulses. As the arterial pressure rises, more and more units are recruited to augment the impulse bombardment of the baroreceptor afferents. At the same time,those units already active discharge more frequently. The relationship between mean blood pressure and impulse activity has been determined by Bronk and Stella (193U) and is shown in the following graph: 120 ‘ ”p. Impulses / sec of a single 60 _‘ fiber 1. 1 60 120 mean blood pressure Hence, the rate of impulse discharge is closely related to the level of pressure within the sinus. It was also shown that a pulsatile pressure is a more effective stimulus in exciting baroreccptor disc.arge } than a steady pressure (Heal et al, 1952). 10 It appears that other physiological functions are mediated or influenced by fibers carried in the baroreceptor afferents. Chemo- receptors are located in the carotid and aortic bodies supplied by afferent fibers of the carotid sinus nerve (CSH) and the vagus. Impulse activity in these fibers is only slight in the anesthesized, spontaneously breathing animal, but is greatly increased by anoxic anoxia, hypercapnia and acidosis. Chemoreceptor stimulation causes both reflex vasoconstirction and hyperpnea. Baust and Heinemann (1967) have suggested that the baroreceptors play a role in the regu- lation of sleep and wakefullness. They concluded from their studies that the monotonous synchronous inflow from the baroreceptors is in part responsible for the onset and maintainance of synchronized sleep.‘ A related study by Bonvallet (1954) showed that EEG activity is in- fluenced by stimulation of carotid sinus afferents; increased pressure resulted in EEG synchronization via a reflex inhibition of ascending reticular neurons. Bonvallet also showed in this study that inter- ruption of the baroreceptor pathway at the nucleus of the tractus solitarius (HTS) intensifies the arousal pattern in the EEG induced by somatic or auditory stimulation of the reticular activating system. Zanchetti et a1 (1960) demonstrated that diencephalic centers respon- sible for sham rage are under a tonic inhibitory influence from the carotid sinus and aortic presscceptors. They showed that de—affcr- entation of the pressoceptors results ih an increase of somatic and autonomic outbursts, while increasing intrasinusal pressure blocked Spontaneously occuring outbursts of sham raje. Also in acute decor- ticate cats, bilateral carotid occlusion induced outbursts of sham ll rage which we‘e abolished by carotid sinus nerve section. In addition to the afore mentioned activities, baroreceptor afferent impulses also modify the spinal somatic reflexes as well as influence ADH secretion (Share, 1965, Rothballer, 1963). From these studies, then, it is evident that blood pressure changes mediated by baroreceptcr afferents result in widespread and diverse physiological modifications which might conceivably alter the direct cardiovascular compensatory mechanisms mediated by the baroreceptor reflex. Turning now to the neural representation of the baroreceptor reflex in the medulla, I would like to discuss papers which attem. to elucidate the medullary neural pathways as well as mention some studies demonstrating supramodullary modifications of the reflex. As will be shown, there are.hufih.gaps in our knowledge of the inter— actions betwecn the various inputs and the integratiVe centers of the medulla concerned with cardiovascular function. It has been proposed from evidence derived from degeneration studies that the principal site of termination of baroreceptor afferents is in the middle third of the nucleus of the tractus solitarius (HTS) near the obex (Cottle, 1964, Kerr, 1962). Dcscrepencies exist as to where second order neurons project mediating s‘mpatho-inhibition and vagal activation, and if, and to what extent, classic "depressor centers" of the midline re- ticular formation participate in the baroreceptor reflex. Morest (1967) failed to show connections from HTS to the midline based on axonal degeneration studies following lesions in the posterior one-half of HTS. The most significant projections of HTS were traced to the dorsal lateral reticular forration of the medulla. This area ‘12 has been implicated as the primary integrating center for cardiovascular reflexes as well as the most promizent locus of tonic cardiovascular neurons (Chai and Wang, 1968). Projections were also traced by Morest to nucleus ambiguous after HTS lesions. This finding supports the contention of Gunn et a1 (1968) who postulated that nucleus ambiguous is the site of origin of vagal cardiomotor efferents. Advances in electrophysiologieal recording techniques have re- sulted in a more precise exploration of medullary pathways involved in cardiovascular reflex mechinisms. Sampson and Biscoe (1968) re- corded electrical potentials cvoked in the brain stem by stimulation of the carotid sinus nerve. Based on onset latencies, they showed monosynaptic connections to HTS which relayed monosynaptically to the ventromedial reticular formation of the medulla. Recording extra— cellularly, they detected positive potentials (indicating a decrease in excitability) in the following medial areas: 0.5-1.8 mm from the midline; 2 to 5 mm from the ventral surface; and from ~10 to - 13 mm (Horsley-Clark coordinants in the A-P plane. In several trials, when the recording electrode penetrated one of the cells in this area, a hyperpolarizing IPSP was detected with the same time course as the extracellular positivity. Histolo:ica1 sections revealed involvement of the following medial nuclear groups: Subnucleus ventralis reticularis oblongata; nucleus reticularis gigantocellularis; and possibly the. medial nuclei of the raphd. They also concluded from their study that secondary fibers may be crossing the midline at many levels in the reticular formation based on potentials recorded contralaterally from A-8 to A-16. 13 Crill and Rois (1968) observed a similiar distribution of baro— receptor afferents. They explored the medulla with stimulating electrodes and recorded antidromic potentials in the carotid sinus nerve and the aortic depressor nerve in the cat. Potentials were recorded following stimulation in 3T3 as well as in the medial reticular formation inclu- ding paramedian reticular nucleus (PEN), nucleus gigantocellularis, and nucleus medullae oblongatae centralis. They also recorded potentials from stimulation of the cuneate complex suggesting that tonic baro— receptor input may be responsible for the spontaneous activity recorded at this site by many investigators. Further evidence that the midline is involved in the baroreceptor pathway is presented by Humphrey (1967). He applied single shock stimulation to the ipsilateral carotid sinus nerve and recorded poten- tials within two relatively distinct zones of the medial reticular fromation. One region consisted of a ventral midlinc strip about 1 mm in dorsal—ventral length bounded ventrally by the internal arcuate fibers. The second zone consisted of an area approximately 1 mm from the midline which extended ventrally from 2 to h mm beneath the floor of the fourth ventricle. In addition to the medial regions, potentials were observed in the dorsal reticular formation, ventral to the nuclei of the 9th, 10th, and 12th cranial nerves (area of HTS). No evoked potentials were recorded from the "pressor regions" of the lateral reticular formation. Since the onset latencies of these potentials indicated polysyiaptic pathways, Humphrey stimulated points in NTS that had responded to carotid sinus nerve stimulation and recorded potentials in the same areas as shown previously. Thus, the polysy aptic 14 nature of the pathway was confirmed, with HTS a relay point in these pathways between sinus nerve afferents and the depressor regions of the medullary reticular formation. As mentioned previously, stimulation of baroreceptors is known to produce such diverse effects as respiratory changes, alteration of somatic reflexes, EWG changes, etc, so these pathways delineated by Hinphrey may be mediating such effects rather than hypotensive effects. Recent electrophysiological evidence of Niura and AGiS (1969) indicates that baroreceptor afferents terminate in IVE as well as HTS. This dual termination in the medulla was suggested by the short latency, monosynaptic reSponses recorded at these sites by stimulation of the carotid sinus nerve. This indicated a direct pathway whereby carotid sinus nerve activity could reach the medial reticular formation without synapsing in HTS. It is interesting to note that FYN may be involved in the baroreceptor pathway. Studies have shown that its integrity appears necessary for sustaining the depressor responSes elicited by electrical stimulation of forebrain (Lofving, 1961), and muscle nerves (Heymens and Neil,l958) as well as some cardiovascular responses elicited from the cerebellum Kiura and Reis, 1968). Thus, supranedullary modulating effects on baroreceptor activity may be integrated at this site. Additional evidence implicating the midline depressor area in the baroreceptor reflex was presented by Chai and Wang (1968). Followiig ablation of discrete areas in the midline, they showed that the pressor response to bilateral carotid occlusion ($100) was significantly reduced. They also showed that ablation of the periventricular grey .15 or the "pressor area" in the dorsal later 1 reticular formation re- duced the BLCO response as well as the pressor response to stimulation of the sciatic nerve. From these results they postulated that afferent pathwa eye of the barorecept or reflex pas sthrough the midline depressor region and on to the lateral press or areas. From the foreioing, it appears that NTS is the first relay station in the burorccoo or re-1lex pathway. As stated prcviously, the des— crepensies exist as to the projections from this area to the site of synpatho—inhibition of tonic pressor neurons which may be in the medulla or the spinal cord (lim et al, 1938, Seller nd Illcrt, 1968, AlExander, 19h6). Termination of projections from XTS in the depressor areas has been proposed by several authors previously mentioned. This is com— patable with the anatomical description of reticular neurons shown by Brodal (1957) and the Scheibels' (1957). Their studies demonstrate t‘at reticular neurons located in the medial regions of the brain stem characteristically have long axomi WfiiCh ascend or descend or both thus serving as effector neurons whereas neurons in the laseral reticular fromation send their axons medially and are, by nature, association or sensory neurons. This important anatomical description of reticular neurons seems to have been neglected by many physiologists wor kin ng in this area. Since most of the "pressor area" is situated outside those regions w ich give off reticulospinal fibers, one must question the proposals th show the fine 1 synaptic link in the medulla at the lateral reticular fronation. How do the effector axons of these neurons reach the intentediola teral columns of the spina.l cord which contm the cells of the final common pathway to the vessels? One sH1011d 16 also question the efficiency of a meandering pathway that goes from lateral areas to the midline and back to the pressor regions in the lateral reticular formation such as proposed by Chai and Wang (1968). Teleolotically, one would not expect such an arrangement. The most efficient pathway compatable with anatomical and physioloqical evidence would be from HTS to the midline and down to the spinal cord placing the site of sympatho~inhibition in the cord or perhaps in the medullary "depressor region”— a possibility which to my knowledqo has never been preposed. I will be presenting some evidence that is compatable with thiS'idea. Before concluding a discussion of the neural pathways, I would like to briefly describe 5 me studies demonstrating supranedullary modifications of the baroreceptor reflex. Knowledge of these mechanisms may aid in elucidating the pathways and neural structures involved in the medullary representation of the reflex. Since there have been many studies recently demonstrating supramedullary nodifications of the baroreceptor reflex, I will limit my discussion to those which have implicated a specific medullary termination site along the baro- receptor pathway. lockman (1969) has shown that stimulation of the fastigial nucleus in the cerebellum in spinal cats inhibits the brady- cardia produced by stimulation of the carotid sinus nerve. The degree of inhibition depended on the amount of impulse traffic in the baro- receptor afferents. Since there is a prominant efferent fastigio- bulbar pathway to PEN and since carotid sinus nerve afferents have been shown to terminate around this nucleus, it was postulated that cerebellar modification (inhibition) of the baroreceptor reflex occurs 17 at this site. Carlaresu and Henry (1970) have implicated nucleus intercalatus (NIC) as a site of integration of cardiovascular reflexes in the medulla. Stimulation of this area (parahypoglossal) produces an increase in blood pressure and heart rate. NIc receives projections from the hypothalamus as well as from HTS (Forest, 1967). Thus, this site may be part of a descending hypothalamic pathway.that acts to inhibit compensatory baroreceptor reflexes in order to maintain an adequate blood flow to critical tissues in times of stress and exertion. [Smith and Kathen (1966) have shown that stimulation of medial portions of the inferior olivary nucleus can inhibit the bradycardia and the depressor effect of carotid sinus stretch and suggested that this area may serve to integrate supramedullary influences from the midbrain and hypothalamus on barorecoptor compensatory reflexes. From these studies, it ap;erars clear that suprabulbar influences on cardiovascular functions are not exerted by modulating the excita- bility of the " primary cardiovascular neurons " located in the medullary reticular formation but rather act on discrete loci presumably in the baroreceptor reflex pathway which are distinct from the tonic vaso- motor neurons. Also, it is evident that the reflex is quite labile and appears to be overidden by neural centers above the medulla during environmental stress including emotions, exercise and perhaps certain pathological conditions such as hypertension. In an attempt to characterize the cardiovascular responses elicited from proposed medullary sites in the baroreceptor reflex, stimulation techniques were employed. he following nuclear sites were investigated: NTS; TEN; and the inferior olivary nucleus. The participation and contribution of sympathetic and parasympathetic components of the reflex were evaluated by obtaining cardiovascular responses before and after vagotomy. The second phase of the experiments involved ablation of various medullary sites in order to ascertain the importance and relative contribution of the lesioned areas to the baroreceptor reflex. 19 1"”de Cats weighing 2.0 to 3.5 kg. were used for t} ese experiments. They :cre anesthes ized with Dial-”rethane (O. 7 ec/lcg.) ). A heating pad was =mployed to maintain rectal temperature at 37<.Cm Tracheal cannulation was performed on all ca ts so that artificial ventilation could be administered when necessary. Blood pressure was recorded from a femoral artery with a Statham P.23 series pressure transducer connected to a Grass polygraph. Bee t to beat chanses in heart rate were recorded using a Grass eardiotachometer } ich triggers to each femoral pulse wave. Drugs were injected into the left femoral vein. . The carotid arteries were disected free in the neck rejion and separated by a loose lieaturc to insure eas* V C.) (3 r) j k" (’3 P. O" !.J. }—1 H. d ‘4 H) O ’- Sm.ll oulld a claxps were used for bilateral carotid occlusion. The Q duration of the occlusi n was limited to 30 seconds in order to minimize cerebral anoxia which may distort the cardiovascular responses measured. In several experiments, the vagal trunk was ligated so that vagotomy could be performed to ascertain the relative contributions of the parasympathetic co omponents of cardiova Heul r fu netion. Brain St mulation Stimuli ne-re applied before ard after vagotomy to selected areas of the medulla to characterize the cardiovascular responses of proposed sites in the bare: wrxez tcr r-:fl‘}'z :nd to evaluate the sympathetic and para sympathetic centriouil 115 of these responses. Stimuli were delivered by means of a square wave stimulator the output of which was passed through a stimulus isolation unit to insulated stainless ste. e1 bi polar or eonccnt rie electrodes. The cats were placed in a stereotaxic frame, A occlusion proceedurcs. 20 Pbrtions of the occipital and parietal bones were removed to permit electrode iltcemrnt. Partial cerebellectomy was performs :d by suction to allow a clear visualization of the dorsal medulla. 111‘s stimulation: with the vagi intact, the middle third of 1sz just rostral to the obex was explored until a maximal bradycardia was elicited. This point was used throughout the experiment and marked by a leisoning instrument (1.5 ma) for subsequent histolosical exm mi- nation. Vo .re.retion,e curves were plotted varying the voltage from 2.5 to 10 V ana keeping the duration (0.1 ms) and frequency (20 cps) constant. FMS s imulation: The electrodes were carefully positioned according to the coord1: nt 0 Barman (1968) and a locus in FER was selected 01 :hich yielded a large dcprrssor response (approximately P—13 mm, 0.5 mm from the midline, and 2 L 2.5 mm from the dorsal surface). Voltage- response curves were otta d(2. 5 o 15 V ), keeping the duration (0.5 ms) and frequency (50 cps) constant. Inferior oliva.ry nucleus stimulation: The electrode placement was determined by exploring the inferior olive and locating large depressor responses. The approximate coordinants were as follows: P-ll mm, 0.5 mm from the midline, and 3.5 to u.5 mm from the dorsal surface. Voltages varied from 2.5 to 10 V. :0.5 ms and 50 cps were kept constant. Ablat ions Ablat ions were ma.e bilaterally in HTS, PEN, the dorsxal lateral reticular formation (DLRF) of the medulla, and the extent of the midline depressor area with a Steelting Lesion Producing Device. Nan 1o ablation: Cardiovascular responses were recorded following uni Wla ral and bilateral NTS ablation. A 15 minute stabilisation 21 period ensued between lesioninj and recording responses. FUN ablation: The lesioning electrode was positioned so that the ablations would include the site of monosynaptic termination of CSN afferents described by Huira and Reis. lesions were made bilaterally 0.75 mm rostral and 0.75 mm caudal to the obex. The lateral coor- dinant was 0.75 mm from the midline; the dorsal-ventral coordinant was 2.5 nm from the dorsal surface. Responses were recorded 15 minutes after lesion. Extended lesions were made in most experiments in 1 mm steps in the rostral plane to include the anterior portions of PEN. The Optimal lesioning current was 3 ma/l5 sec. DLRF ablation: esions were made bilaterally in the DLRF including an area from the obex to 6 mm above the obex. The electrodes were placed 3 mm lateral to the midline and 2.5 mm from the dorsal surface. Responses were recorded after the total extent of the lesion. Lesioning parameters were 5 to 10 ma/ZO sec. Midline depressor area ablation: The lesions were performed in the following manner in an attempt to destroy most of the midline depressor area: Lesion #1; 0.5 and 1.5 mm above the obex, bilaterally, 2.5 and #00 mm below the dorsal surface of the medulla, and 0.75 mm from the midline. Therefore a total of 8 lesion points. Iesion #2; 2.5 and 3.5 mm above the obex with the same dorsal-ventral and medio— 1ateral coordinants. Lesion £3; b.5 mm above the obex with the same dorsal-ventral and medic-lateral coordinants. Cardiovascular responses were recorded 15 minutes after each set of 8 lesions. The lesioning parameters were 3 ma/15 sec. 22 Barorecrptor Activation Activity in baroreceptor reflex pathways was modified in the follo ring manner: Bilateral carotid occlusion; i.v. injection of NE; and sti. ilzmt on of nuclear sites proposed to be in the pathway. When the carotids are clamped bilaterally distal to the stretch receptors in the car tid sinus region, a pronounced reflex rise in blood pressure would crsue lx>couse there would be few impulses along baroreceptor afferents to inhibit sympathetic vascular tone..That is, the tonic sympatao-inhibitory input would be reduced. Injection of NE raises blood pressure and increases baroreceptor input. If the ablations interrupt ad the reflex path:.avs, one would expect the HE pres sor response to be enhanced since the reflex mechanisms could not compensate for the rise in pressure. The third method employed to Hlt r ba roreccptor reflex activity involved electrical activation of nuclear sites pro- posed in the pathwar. By testing the cardiovascular responses elicited by stimulation of these sites before and after ablation, one can as cer— tain the relative importance of the ablated site to the baroreceptor pat.1way. Histolozg The extent of the lesions as well as the placement of stimulating electrodes ':ere eluc Ldated by gross section after fixation in 10% buffered formalin. Histological verification wa attempted on selected brains from each set of experiments employin" the following proceedures: l) fixation in formalin for several days. 2) dehydration in various concentrations of alcohol and xylene. 3) embedded in paraffin. h) sectioned in 20 micron thickness and 5) stained with cresyl violet 23 for cells and luxol fast blue for fiber tracts. Analysis of Data All values are eXpressed as the mean 1 SE. Statistical analysis was performed with the Student t test for paired and grouped data. A "P" value of (0.05 was considered to indicate statistical significance. Also, a coefficient of correlation was obtained in one group of ex- periments employing the method proposed by Bravais and Pearson (lewis, 1966). A: .f RESUL ea U) In the first set of execriments, NTS, the proposed site of termi- nation of baroreeeptor afferents, was stimulated before and after vagotomy in order to characterize the cardiovascular responses and to ascertain the relative contributions of parasympathetic and sym- pathetic components mediating these responses. Figure 1 shows typical responses to NTS stimulation. It is evident that HTS mediates both sympatho-inhibition to the vessels as well as vagal activation. The responses seen following vagotomy indicate that the bradycardia resulting from NTS stimulation is predominantly mediated by parasym- pathetic fibers, although there appears to be a small sympatho-inhibitory contribution as indicated by the residual bradycardia following vagotcm . The fact that the depressor response is essentially unchanged following varotomy suggests that this is a sympatho-inhibitory response independent of vagal influence. Figure 2 su marizes the results of NTS stimulation on heart rate responses in 7 cats. Here again it is clear that vagotomy $1?“ J substantially reduces the bradycardia evoked by hrs seinulation. Also, ‘ I ‘3 P.) ’1 U) C" hat the vagal pathway eminating from the stimulated area in NTS is uncrossed as ipsilateral vagotomy reduces heart rate responses to about the same level seen after bilateral vagotomy. The next figure (figure 3) represents the depressor responSes resulting from NTS stimulation at various voltages. It appears that the vague does not contribute to the depressor responses as the responses seen after vagotomy are similar at most voltage parameters. In the next series of experiments, paramedian reticular nucleus (PEN) was stimulated. The purpose of this was to ascertain the 25 cardiovascular reSponses as this area was proposed by Miura and Reis (1969) to be a monosynaptic termination site of CSN afferents. Also, the degree of vagal participation in the responses could be evaluated following bilateral vagotomy. Figure h summarizes the results of 5 experiments depicting the depressor responses to FfiU stimulation at varying voltages. It is evident that there is a large depressor component (60 mm Hg.) which is still present after vagotony indicating a pure sympatho-inhibitory response. .Figure 5 shows the concomitant fall in heart rate during ‘EH stimulation. Tne small bradyeardia evoked by electrical activation of FF? is evident after vagotomy implying that the decrease in heart rate is mediated by sympathe— inhibitory fibers to the heart. Figure 6 represents typical cardio- vascular responses to PEN stimulation. The important point to be noted is that the large depressor responses are converted to small pressor responses at low voltage intensities. This unexpected ob; servation was seen in several experiments and may sugfiest that pressor neurons reside in the classic "depressor regions" of the medulla. Further explanations will be offered in the discussion section. The next series of eXperiments involved electrical activation of the inferior olivary nucleus. This nuclear mass is located in the classic "depressor refiions" of the medial reticular formation and has been implicated as a possible integrative center for supramedullary influences (midbrain, cerebellum and hypothalamus) on the baroreceptor reflex. Figure 7 represents graphically the results of the effect of inferior olivary stimulation on blood pressure in 3 cats. Depressor responses are similar before and after vagotomy. Table 1 summarizes 26 the cardiovascular responses evoked by stimulation of this area. It is noted that heart rate is hardly influenced by stimulation of the inferior olive. The next group of experiments employed ablation techniques to interrupt preposed baroreceptor reflex pathway sites. These experiments were done on vagotomized cats in order to evaluate the sympathoinhibitory component of the baroreccptor reflex. Activity in barorzceptor reflex pathways was modified in the followin: manner: Bilateral carotid occlusion; i.v. injection of NE; and stimulation of nuclear sites proposed to be in the pathway. By employing these techniques, one is able to directly alter barereceptor reflex activity and evaluate the changes in activity which would occur if ablations interrupted part of the neural substrate of the reflex. The first series of ablations was done on HTS. After control responses were obtained, NTS was lesioned on the right side. Responses were retested after a 15 rinute stabilization interval. NTS was then lesioned on the left side and the responses were again tested. Table 2 summarizes the data obtained from 6 cats. Following ablation mean blood pressure was increased, however not significantly. The bilateral carotid occlusion response (BLCO), the most sensitive indicator of baroreceptor reflex activity, was significantly reduced after uni- lateral NTS ablation. Further reduction of the response occurred after bilateral ablation: It was reduced to 45% of the control value. Presser responses to 1E increased slightly after bilateral NTS ablation. The interaction studies done with PEN stimulation revealed that the depressor response seen in control was augmented following bilateral 2? ablation of ITS. Figure 8 shows the typical reductions of the pressor response to BLCO following unilateral and bilateral HTS lesion. This reduction was seen in every animal tested. In order to evaluate the importance of I33 in the baroreceptor pathway, ablations were performed at this site in 6 cats. The lesion- ing electrode was positioned to include the area of ‘K? where Niura and Reis detected monosynaptic terminations of CSN afferents. Table 3 summarizes the results. Of interest is the observation that the BLCO response was net significantly reduced; reduction was only to 90¢ of control value. Also, blood pressure decreased following ab- if PEN was involved in the reflex 0.. lation which would not be echete pathway. The next group of ablations involved the dorsal lateral reticular formation of the medulla. This area, as mentioned previously, was implicated by Chai and Wang to represent the site of tonic pressor output to the vessels as well as the central integrating locus for cardiovascular reflexes. Table b lists the results from 3 cats. It is evident from this data that central cardiovascular function is markedly depressed. However, gross inepection of the lesioned site revealed massive destruction probably extending to interrupt afferent fibers of the reflex. Also, it was noted that extensive bleeding occurred. Therefore, it is doubtful that these results represent an accurate assessment of the role of the dorsal lateral reticular for- mation in baroreceptor function. Recvaluation of this area is planned in the future with the purpose of producing more discrete lesions. Since PEN ablation did not significantly alter baroreceptor reflex '28 function and because midline depressor areas have been implicated in th. reflex pathway, it was decided to extend nidline ablations to include an area from the obex to about 5 nm above the obex. The lesioning sites were described in the methods section. Table 5 lists the data from 10 cats. A significant reduction of the BLCO response was noted after the second set of lesions implying that the midline \ area encumpassed by the lesion was part of the barorcceptor pathway. J. \ However, examination of the data revealed that when mean blood pressure increased after lesion (cat $7), the BLCO response also was increased from control; and when resting blood pressure decreased, the BLCO .s correlation. The alue ob- i,J. response decreased. Figure 9 shows th 22 which was significant \) tained for a coefficient of correlation was 0. 1“! If. , r - 1- ~-‘ ‘ "‘A . . at the 5p level. Table 5 also shows that the TCSPONbr to i.v. he 18 J significantly potentiated after two sons of lesions. It is also noted that heart rate increased progressively following lesions. Figure 10 depicts the typical reduction of the pressor response to BLCO following the first and second set of lesions in which resting blood pressure had decreased. The bottom panel represents the potentiation of the JE pressor response in the same animal after lesioninq. The following figure (fig. ll) shows an example of potentiation of the BLCO response after ablation in which blood pressure has increased from control values. Another finding which may implicate the midline in the bare— receptor pathway is illustrated by figure 12. It is evident that a small pressor response elicited by NTS stimulation becomes prejressivcly potentiated as more of the nidline is ablated. Table 6 summarizes the data comparizg blood pressure responses to 2T3 stimulation before and after lesion. F.) \3 Figure l The effect of vajotony on .he cardiovascular responses elicited by ‘1‘" n19 stimulation. Panel B shows the Llood pressure and heart rate responses to TS stimulation with the vagi intact. Panel A shows the 0‘ cardiovascular responses after bilateral vagotofiy. The downward ) deflections of the time has. indicate the duration of HTS stimulation. Parameters of stimulation are: 10 V., 0.1 m.s., and 20 c.p.s. Bitblood pressure in mm H*. H3=heart rate in beats/min- t \J » EL .WN . .. ~.Ja«m.\1\..u~...1f.u9\é .3}; 4,321)...‘ 31.45 12.7.3?..:..mmimrn_...{E...w:1:.m.Wp,~w..w.m...mw.w.722 ... v ; .11}... . fl. .. . . . ._ .. . L. .3» o. L ... ... > up? n t . . fl .4 .... a. ...; ..:... - ...- .... ... . p ...... mm:iEm..:?...T??;~_.n_.:.42:131:;Fizfiré: 3.1.3.231. 11A. 41.374 {6.13.153 *m 5.4.. . 3.143.... .131); mliveDJua If}? P... H.” x. ..f . .J . 5!. at; . ., ......2 ..., ..om Nut/3., . ...). ,ofll ...! 3. «1.9.73? a! 9.3:). ...- 3.).§.3 ...!n..?..0 ,, - /r$f-mrP.-7.I...P.fi.,’.3z.fi.?j . .. . .. . ., .. r. t L ... ... n 4w. .. .. .. . ,. .w. h .C‘ I ix 0 0‘ 1‘. .L ax C." 1‘ ...l./.... w . x .. f I. iii? 5/ lam ) 3.13 a. 6.3.... e: Figure 2 Voltage-Response Curve showing the effect of vazotomy on heart rate responses elicited by HTS stimulation at varying voltage intensities. Each point represents the mean and the standard error of 7 experiments. The stars represent heart rate responses with both vagi intact; open circles represent heart rate responses following ipsilateral vagotomy; triangles represent heart rate responses after bilateral vagotomy. The voltage intensities were varied from 2.5 to 10 V. The duration. (0.1 ms) and frequency (20 cps) were kept constant. 36 60 O v 53. d9 NI QSVBHDBG VOLTS ' Figure 5 Voltage—Response Curve showing the effect of vacotony on the heart rate responses evoked by PEN stimulation. The points represent the mean and standard error of 5 CXperiments. The open circles represent the heart rate responses evoked by YEN stimulation with the vagi intact; stars represent the heart responses after bilateral vazotomy. The voltage intensities were varied from 2.5 to 15 V. The duration (0.5ms) and frequency of stimulation (50 cps) were kept constant. m (.13 60 O ‘1 EH Ni 39V M ’5 «5 O N 3" ...i VOLTS . AJ/ ".YL L-L C “ $ .. a 2.. ‘AH A3- -....v‘p. #- +‘w ...: vi. a . 'LS TQPTCSL tr ~“.h’ "..fi 7" ‘5'- v :..1. p“; . . ‘C‘v’v'. ' u L o- - rf. .'. 7' v...’ Iv“- .3 L L.‘ f t 1025 O L dvacJ . "4- afi- 'Ah.’ I“ .1 .L 1" ”F *5—vi‘c ’~~ ... e o - I“. - I . ties..The 351 .,.,..L avg}... ‘ a ..u u-..u. CD 5’ ‘1‘ 1+0 . 3/ x3. on m???) 2.74.33 #7511?!“ Ws.§/. .W.??a. .... W13?HH\.\ f/JJJ 7L1»..\.. /. 4.3.3.3.... WN _. Q m n , g I n .. ... . 5.. ((c (4.7. c c c 6(Q we... c (c.( ((12; ml(..<£\ «It .... (.6. 0 ON Gog (all?! ..l)\/l...ll..l00N Ml ..L oom >3 .>w >9 >2 Figure 7 ‘4 Voltage-Response odrve shov‘n the effect of vazotomy on the depressor responses elicited by inferior olive stimulation. Each point represents the mean and standard error of 3 eXperiments. The stars represent the depressor responses elicited by stimulation of the inferior olive with the vagi intact. The circles represent the responses elicited by stimulation of the same point in the inferior alive after bilateral vagotory. The voltages were varied from 2.5 to 10 V. The duration (0.5ms) and frequency of stimulation (50 cps) were kept constant. #2, o— On ”50> nN 0v 330 .n .3 V'fi‘ "In... "bV d "'1 CU. t 1+3 Table'l The effect of vagotomy on cardiovascular responses elicited by inferior olive stimulation. The numbers represent the mean and standard errors of 3 experiments. The voltage intensities were varied from 2.5 to 10 V. The duration and frequency of stimulation were kept constant: 005 ms and 50 CPS. 00.0 00.0«00.0- . . 00.0 s0.mnxe.m- s m.~ 03.2.8.0... 00.2%..NH- $.0«$.0+ 0N.se$.flm.. s 0.0 3.080.? issued? . 00.0 _ Hosesmém- s we. 00.2%.? mméflofim- $.flmmé... 3.1%.00- s 0H .m... e Kim use . Ami e. gem zseafi 0.0220900; 485200 S ablation on baroreceptor reflex activity. Each number represents the near and standard error obtained from 6 cats. Al3.P.= change in mean blood pressure (- denotes a decrease and + denotes an increase in BP). A H.R. = change in heart rate. B.L.C.O. = bilateral carotid occlusion. N.E. = Sorepinepharine ’f-"T— 4 5 ' .. .— v. 3 : ,,., - .-. .... nn— Paramedian netiCular nucleus. * denotes sihnifiCance at P<0.05 f i 'Q' n ‘ ‘rfi-rv j'~r-' . f, ‘ '. -\-.. -- -. . :- 0“ ‘ ‘ nysf‘.- - A A as Leasured 05 p;;TCd t test. Les_3ninQ Sinhkifli ra-an/t»rs were 2—3 ma/lO sec. mm. B” ...... oJNH ..HQ mm...” . HdAHm £3” « mm.mm + £4. ... 8.3... fl :3“ a £5: and: $4M: :mfih Kama fl mmJHn mmdmu omom q unmomm .1 mm.m flofima . Hmd max. .53ng :mé h mwomw + Hmm max 86 « QWONW‘r oOoUo‘Hom 3.0.... N96 .+. 9.8“- $.wm... Sm omé .w mm.mH mméan 59%.. .>OH .Em A... ......m. .... m. a...» R; 23$ .....m new. .Hommkco 1;? - f‘ P gure C "" 4‘1" ; r v " ,‘- ,L; a‘ $- .ne e-.:co 0. “To colat-on on tne pressor response and heart rate «I. evoked by bilateral occlusion of th, carotid arteries in vagOtomized (,7 cats. Panel A refrcsents the control pres or and heart ate responses to 3.33. Panel B retresents the carciovascular responses to 3130 S abl?tlc”. 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O ‘. Table h ,Shonv 0 re.l Y‘ {3 I.‘ s “+5 th( -..v n. v.2 mm: «vs (A 5 v A \— O- . 1 . L .A-. . gro~ -_ xx 7 "o 1 ‘ ‘ Ton n the The 193 .\ ’1‘: hr » —\ g C 52 $63 Rim 03 mm mm RAH $4. 0 2 m 93$ “.92 OJNH 0.03 0.9: $4: m.:m 0.9 0.3. 9% oMom MI om 0N 0H zomeqqmd Hmom :0.man mowm mH.m “no.3 3.9... m6? H:.OHH 0.0NH ow . on Hmm moz . .033 Q. N... .0635 0.3mfi o.mma .mom 0&3 0.02 gm 24% ON , 0H . WoENM Momwzoo Table 5 The effect of midline "depressor area" ablation on barorcccntor reflex activity. The individual responses are shown for each of 10 eXperiments. The mean and standard error are represented at the bottom of each group. An explanation f the lesioned sites is presented in Kethods. The heading lesion 0.5 & 1.5 represents responses obtained 15 minutes following lesioning from the obex to 1.5 mm above the obex; values under Lesion 2.5 § 3.5 represent responses following lesions which ' attem ted to destroy the midline depressor area from the obex to 3.5 mm above the obex; values under lesion h.5 represent the responses following lesions from he obex to 4.5 mm above the obex. In some cases following the extended lesions, blood pressure dropped too low to record responses. # = experimental animal number; ESP = mean blood pressure; HR = heart rat:; BLCO = bilateral carotid occlusion; and RE = om.mH ms.H am.m N~.m om.0H . ow.OH mfi.fl 0H.o me.OH n:.m .m.m mm.sm ow.o ow.m: «.mmm m.mHH . n.~m 0.“ ~.mm n.mH~ o.m0H x on w om mam moH 0H mm m we mam OOH mm 0 mm mom mm m om n ma mom “NH no a an Now mud w no a 03 0mm . NmH Hm w mm Nam 05H m 03H 3 mm mmfi med .0 ONH Ha ow mNN mmH “Ha sfi mm mmm mm m msfl m an mum no a m: m m +4? mm m we a mu mmm no me u an ONN moa m mm 0 NH New mm a Emflmz ewe a? mm mm; . Emfimz E? .54. mm “mi 8.8 . 00.8 m.: m Hmmu m.n q m.~ ZOHmma -, mH.e 5H.H “@.m on.“ mm.w ow.“ o:.H ow.m no.n ow.m m.m a; m.wm N.oH m.Hm :.ao~ m.NHH m.oa m.HH o.mm m.umfi :.HHH x mm m mm wmm mHH Nu m Ha CNN mm OH on m an wmfi N ow HH we NmH 00H a mu HH Nu mmH NHfi mm o ms mma Baa w mm m em sow emfl on m mm Nmfi omfl m m a om oafi mom a an ow we sea 03H e um ea on mam .m mm HH me new mm m mOH OH ms «Hm “OH mm mH mm awn 00H s cm Na we mafi moa am 0H we mmfl mm n no m mm swfi mm mm m mm nmfi mad N n: ma No mmm um om ma mm mHN uma H Emémz mic mmq mm mm: Emémz m: mm... mm mm," m 08m 08m m.H e m.o ZOHmug nomezoo Figure 9 1‘ Correlation graoh relating the change Ol mean blood pressure levels and the change in the pressor response to ULCO following midline ablation. The ordinant represents the change in mean blood pressure 63MB?) following midline ablation extending from the obex to 3.5 mm above the obex. Increases in ER? are represented in the top half of the ordinant; decreases 'Jo n RB? are shown on the bottom half. The abcissa represents changes in the pressor response (AIR?) to BLCO following the lesion ( positive changes to the right-decreases in pressor responses to the left). Each point represents the difference between control values and values obtained following lesion (n=lO). The coefficient of correlation was 0.722 which was significant at the 5; level. oo 0v U D ON a? _. 08m 00 on ON ON CV 00 u n Om emfiq 0v oo Figure 16 The effect of midline "depressor area" ablation on baroreceptor reflex activity. The top two panels represent the reduction of the pressor response to 5130 following ablation along with heart rate responses. The bottom two panels show the potentiation of the pressor response to NE following ablations along with the heart rate changes. Panel A in both cases represents the control values; Panel B shows the responses observed after midline ablation of 2 mm of the midline from the obex to 2 mm above the obex; Panel C shows the responses following extended midline ablation in the same animal to include an area from the obex to about u mm above the obex. The downward deflections from the time base in the top panels represent the clamping and unclamping of the carotids on both sides. The downward deflection in the bottom panels represents the i.v. injection of NE (1 ug kg). loo— loom L oom ,_b}aobfl ”My .-Lw. bum-w»- . $5.35; h€¢.<<(¢¢ W.»».»» m A ..l .xerepbs . fl. ..» m «.¢c¢mn «ac _ .J .; a.‘ o.. .iw tron 3‘. V. was .x_m.?§ ‘3..ch ....f?.o§o&...«5.> .w Ta .. .H x w ... 1% s! cas¢\ e .e 1¢cuxcme U m nvm 122:3... .xr.p,»p.p» ecccuq.ut .55». \p. . ,‘q. , u;s a» . s;o.¢...«.. c, .2. n2 «tau... . u; «gazeu‘ com 4 J 09 I CON .... oom .. ._ .wif)‘; v)... .. .5, on M. riv.f..,o.\u).u\\))}./a 31% .hoabbmvh u‘tb)’ ‘ .. . . .. t ... (cg-vea‘ViWi.‘ac¢......n¢.n.\tc.‘r.(.f fumbinfih..u.o.hb (in mafihpbs‘bb 5‘9h? ‘ u . . .4 ‘0.- » J. ¢¢que.q. “veg“. wmq ; J .Mbsb» .aq» mWNF .mgm...‘ «..ea ... “a a 9% v c h «%‘< AXUN .a.. x: mm x: mm \n u) ‘n v The effect of alood pressure level on the pressor response to ELCO following nidline ablation. Panel 5 represents the control pressor response to BIC? when blord pressure is quite low. Panel B shows the enharced pressor response in the sane cat after lesion in which blood pressure levels have increased. Between panels A & B, midline ablation was pirJOTfiZQ extending 3.5 Ln in the L-P plane. The heart rate changes an H m m F.) r 0 O m T 5‘ :3 1 ’d If (D n. O 3 L; Q. Q. (‘3 H H o L . :CLlOH U} of the time base represent clamp;.g and unclenping of the carotids (about 30 sec. occlusion). _ —-——..——.._‘-——__. -_ _—-,.. '— _ - 60 -.-- "9,. , n“- Lisa- 77’ ‘i —‘..‘“,. ... .4 L; 0-5. '84?" rw “i F b» h 6mm a m 61 Figure 12 The effect of midline ablation on the cardiovascular responses elicited by NTS stimulation. Blood pressure and heart rate responses to NTS stimulation are represented: Panel A depicts the control cardiovascular responses to HTS stimulation; Panel B shows the responses to HTS stimulation after bilateral vagotomy; Panel C shows the responses in the same cat after midline ablation extending from the obex to about 2 mm above the obex; Panel D represents the responses to NTS stimulation after extended midline ablation to include an area from the obex to about a mm above the obex in the A~P plane. The downward deflections $ Y5“ of the time base represent the duration of His stimulation ( about 10 sec.). .p aech.5.¢.)...>>.¥J suox . a a Yttrx‘K‘w... §«.T‘\(¢ £¢§§< m om.maa.a Hm.m»oa.a .m.m mm.wum.wfl mm.msoa.m .m.m .> OH seameq wwom Hoppcoo es m.a e m.o mowmca 6‘5 DISCUSSION The results oi the stimulation experiments characterize the cardiovascular responses elicited from proposed medullary sites in the baroreceptor reflex pathway as well as indicate the relative contributions of sympathetic and parasympathetic components. The NTS stimulation data reveals that the middle hird of this nucleus mediates both depressor and bradycardia responses. Since vagotomy substantially reduced the bradycardia but not the depressor response, it appears that there is a sympatho—inhibitory component mediating blood pressure chanfies as well as a paiasympathetic vagal component mediating heart rate responses. These findings therefore suggest that NTS is a likely site in the baroreeeptor reflex pathway. Characterization of the cardiovascular responses from TEN stim- ulation revealed a large depressor response which persisted following vagotomy implying a pure sympatho-inhibitory response. Since the small bradycardia elicited by PEN stinulation was -resent after vago— tomy, it is likely that the vagal component of the baroreeeptor reflex does not extend to this nucleus. The fact that large depressor respon- ses were elicited at this site sugsests that TNN’may be involved in the sympatho-inhibitory pathway of the reflex. An uneXpected response occurred, however,when PEN was stimulated at low voltage intensities as shown in figure 6. The large depressor responses evoked at high stimulus parameters were converted to small pressor responses at low intensities. Possible CXplanations are as follows: 1) There may be pressor neurons located in the classic "depressor area" with low thresholds which are masked at high stimulus intensities bit manifest at low voltages which are subthreshol_d to surrounding depressor neurons or 2)T he large depressor responses may be due to current spread to inhibitory afferents coming in toward PEN and other midline nuclei. At low voltages current spread would be minima_ and pressor neurons endogenous to PKH would predomim te. This idea that pressor neurons exist near the midline would be corpatable with Brodal's anatomical arraneement of the reticular formation which I mentioned earlier. That is, the site of sympatho-inhibition should be located near the midline since only midline reticular formation neurons send axons in a rostral—c auda 1 plane. A descending pathway with long axons to the intermediolateral cell column in the spinal cord is, of course, the effector component of the reflex that regulates vascular tone. The results obtained from electrical activation of the inferior olivary nucleus indicate that this site may be involved in the symm patho-inhibitory component of the baroreceptor reflex. The fact that d pressor respons were observed before and after vagotomy and the obs rvation that heart rate was unaltered by inferior olive stimulation suggest that the cardiovascular responses elicited from this site were uncontaminated by vagal influences. Since the stimulation experiments revealed cardiovas ular responses similar to the :e cE‘araeteristic cf the bar oreceptor reflex, it was decided to ablate these medullary sites and asce rJ;ain the effect of ablation on be roreeeptor reflex activity induced by bile teral carotid occlusion, i.v. injection of‘ lE, and stimulation techniques as descritied in Kethcds, The data presented in Trble 2 reveal that KTS is an essential 67 nuclear substrate in the baroreceptor reflex pathway as ablation of this site prevents the compen mt ry refit ox adjustments to baroreceptor activation. This is shown by the significant reduction of the BLCO response following ablation. then the carotids are clamped, tonic sympath0~inhibitor y impulse traffic is essentially abolished with a concomitant large pressor reSponse. If the baroreceptor reflex pathway- is interrupted one would see less of a release of inhibition and, consequently, a reduction of the pressor respom Of course, the possibility exist sthzt the ablation ray be interrupting the SN af- ferents in their course toward ITS in the tra ctus solatarius. In this case a maximum reduction of th CO response would be expected. Another finding suggestingi‘ TS involvement in the pathway is that the depress or response to PRU stimu latien is substantially potentiated after ablation. This potentiation indicates that HTS ablation prevents reflex adjustments to the decrease in blood pressure and, consequently, enhancement of the response is seen. Another indication of HTS in- VOIVement in the baroreceptor rel lex pathway is that the pressor response to i.v. injection of NE '8 slightly auvmented following bilateral NTS ablation. Further evidence implicating HTS in the pathway is revealed by the mean blood pressure levels: Mean blood pressure is increased after ablation, however not significantly. One might expect a greater increase in blood pressure levels after \iS ablation if this site was involved in the barorcceptor pathway since sites mediating tonic inhibitory input to pressor regions would be eliminated. Three factors may be mas :509, 1952. Peiss, C.U. Central control ofs *nnqo etic cardio_acceleration in the cat. J. Physiol., London 15-:225—237, 1960. Ranson, 8.3., and P.R. Billingsley. Vasomotor reactions from stimulation of the floor of the fourth ventricle. Am J._Physiol. ul:85, 1916. Ransom, W.R., and Wang, S.C. Autonomic responses to electrical stim— ulation of the lower brain stem. J. Comp. Jeurol. 71:437. 1939. Rothballer, A.B. Pathways of secretion and regulation of posterior pituitary factors. Res. Publ. Assoc. Res. Nervous mental Disease. 93:86-131, 1963. Sa 1moiraghi, G. C. "Cardiova: cular" no areas in brainstem of cat. Jo Heuro ophysiel. 49.11a—l97 1962. Sampson, S.R., and? .J. Biscoe. Electrical potentials evoked in the brainstem by stimulation of the sinus nerve. Brain Res. 9.398-901, 1968. Scheibel, M.E., and A.B. Scheibel.tructura1 substrates for inte- grative patterns in the brainstem reticular core. In: Reticulai Formation of the Brain. Boston: little, Brown, 1957, p 31-55. Seller, H., and E. Illert. A descending svwnathownnibitory t1 act in the ventrolateral column 01 the cat. Pfu,ers Arch. ges. Physiol., 313:393—360, 1969. Share, L. Effects of carotid occlusion and left atrial distention on plasma vasopressin titer. Am J. Physiol. 208:219—223, 1965. Smith, O.A., Jr., and M.A. Nathan. Inhibition of the carotid sinus reflex by stimulation of tho infe~ior olive, Science 154;674, 1966. Wang, 8.0., and A.C. Przybyla. Neurophys iological characteristics of cardiovascular neurons in the medulla oblongata of the cat. J. of NeurOphysiol. 30:695~660,1967. Zanchetti. A., C.Bartore11', E. Dizzi, and A. 'bretti. Inhibitory control of sinocarotid pres oceptive aifc-rents on hypothalami c autonomic activity and sham r.ge beh havior. Arch. ital. Biol., 98:308- 326, 1960. H N l ”'Ttuil/fiwig’lujr’fllfl Ali/1111171111111 A“