Light can directly and acutely alter arousal states, a process known as “masking”. Masking effects of light are quite different in diurnal and nocturnal animals with light increasing arousal and activity in the former and suppressing in the latter. Few studies have examined chronotype differences in masking or the neural substrates contributing to this process. However, in nocturnal mice, masking responses are mediated through a subset of retinal ganglion cells that are intrinsically photosensitive (termed ipRGCs) due to their expression of the melanopsin protein. The goal of the studies in this dissertation was to first characterize masking responses in day- and night-active animals and then to evaluate the possibility that differences in ipRGC projections or the circuitry within their targets might contribute to species differences in masking.First, I compared behavioral and brain responses to light across individuals within a species (the Nile grass rat, Arvicanthis niloticus). In this diurnal species some individuals become night-active when given access to a running wheel, while others do not. I found that masking responses to light and darkness in these animals were dependent upon the chronotype of the individual. Additionally, the responsiveness of neurons within two brain regions, the intergeniculate leaflet (IGL) and olivary pretectal area (OPT), was associated with the behavioral response of the animal to light.Next, I compared behavioral responses to light and darkness across species, the diurnal grass rat and the nocturnal Norway rat (Rattus norvegicus: Long Evans (LE) strain). Overall, light suppressed general activity in LE rats, while darkness increased it, a pattern very different from that seen previously in grass rats, in which light stimulates activity, but darkness has no effect (Shuboni et al., 2012). I also found that light induced sleep and resting behavior in LE rats and suppressed it in grass rats and that these effects lasted for at least a full hour.To determine whether differences in the projections of ipRGCs may account for species differences in masking, I characterized the melanopsin system of the grass rat and compared it to that previously described in nocturnal rodents. I found that the grass rat retina contained the same basic subtypes of melanopsin cells and that the majority of these cells (87.7%) contained the neuropeptide, pituitary adenylate cyclase-activating polypeptide (PACAP), while 97.4% of PACAP cells contained melanopsin. Since, within the retina, PACAP is found almost exclusively in ipRGCs, I then examined the distribution of PACAP-labeled fibers originating in the retina to characterize ipRGCs projections to the brain. I found that although these were similar to those of nocturnal species, some differences existed in their density in the dorsal and ventral lateral geniculate nucleus (dLGN and vLGN) and in the rostrocaudal extent of the OPT.Finally, to determine whether differences exist in some features of the internal circuitry of ipRGC target areas, I first examined whether there were differences in retinal input to light responsive neurons within ipRGC target areas in a diurnal and nocturnal brain. Within the IGL, the majority of light responsive neurons had close contacts with retinal fibers in both grass rats and LE rats. I then determined whether differences exist in excitatory (glutamate) and inhibitory (GABA) neuronal populations in multiple ipRGC target areas. In many areas the distributions of glutamate and GABA cells were similar in the two species, but there were differences in the vLGN (more glutamate in LE rats than grass rats) and in the lateral habenula (GABA present in grass rats but not LE rats). Overall, these studies provide insight into chronotype differences in behavioral responses to light, as well as the brain regions that may mediate those differences.
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