ABSTRACTVOLTAGE-GATED Ca2+ CHANNEL PLASTICITY AND CONTROL OF NEUROMUSCULAR TRANSMISSION BY THE ENTERIC NERVOUS SYSTEMByEileen Rodriguez-TapiaFunctional gastrointestinal disorders (GI) disorders are characterized by alterations in the function of the GI tract that occur without clear evidence for structural or biochemical abnormalities. These disorders comprise about 41% of the total GI complications in the United States and altered motility of the GI muscles is a hallmark characteristic. The enteric nervous system (ENS) controls motility of GI muscles through an intrinsic neural circuit. Voltage-gated Ca2+ channels (VGCC) regulate neurotransmitter release and as such they could play a critical role in modulation of intestinal motor patterns. However, the contribution of different VGCC subtypes within the motility circuit is not very well understood. Therefore, the overall goal of these studies was to provide detail insight into the physiological role and significance of P/Q- and R-type VGCC in enteric neuromuscular transmission and investigate potential homeostatic synaptic changes in response to deficits in these VGCC subtypes. Three animal models were used to study the role of VGCC subtypes in controlling intestinal motility. Chapter 3 uses the longitudinal muscle of the guinea pig small intestine to investigate the contribution of R- and N-type VGCC in nerve-evoked contractions and relaxations. Chapter 4 uses the α1E knockout (KO) mouse, which has a genetic deletion of the gene encoding for the α1 subunit forming the pore of R-type VGCC. Here the physiological relevance of R-type channels is investigated in the colon of α1E KO mice. Chapter 5 uses the mouse called Tottering (TG), which has a spontaneous missense loss-of-function mutation in the α1A subunit of P/Q-type VGCC. Mechanical responses of the muscle and intracellular recordings of junction potentials were conducted to investigate the contribution of R- and P/Q-type VGCC to neuromuscular transmission. The studies discussed in this dissertation revealed that R- and P/Q-type VGCC contribute to neuromuscular transmission within the ENS. (1) R-type VGCC contribute to inhibitory neuromuscular transmission of the longitudinal muscle in the guinea pig small intestine. Specifically, activation of these channels is coupled to the prominent nitrergic component of longitudinal muscle relaxation. (2) Absence of the α1E subunit of the R-type VGCC did not alter colonic motility patterns. A homeostatic plastic change was identified that explained maintenance of colonic motility in face of total absence of the α1E subunit. The nature of this change was an up-regulation in the contribution of L-type VGCC during generation of purinergic component of the inhibitory junction potentials (IJPs). The nitrergic component of the IJP was still decreased in α1E KO mice and the resting membrane potential of circular muscle cells, which is regulated by ongoing release of nitric oxide, was significantly depolarized. (3) Loss-of-function of P/Q-type VGCC only produced subtle alterations in colonic propulsive motility in vivo and in vitro. Intracellular recordings of IJPs revealed that these electrical events were either enhanced or at normal levels in TG mice. Here, an up-regulation in the contribution of L-type channels during generation of the IJPs was also identified in TG mice. Contribution of L-type channels to the IJPs could serve to produce the observed normal nerve-evoked relaxation of circular muscle rings and colonic propulsive motility. Taking together these studies provide evidence for the contribution of both R- and P/Q-type VGCCs to enteric neuromuscular transmission and for the ability of the ENS to adapt to challenges that disrupt the function of VGCC subtypes. L-type channels serve as a “back-up plan” to sustain neuromuscular transmission in-face of alterations to other VGCC subtypes. This plasticity of L-type channels maintains physiologically appropriate colonic function and overcome alterations to components of the enteric neuromuscular junction.
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