Distributions of GABAergic and glutamatergic neurons in the brains of a diurnal and nocturnal rodent.

Light influences the daily patterning of activity by both synchronizing internal clocks to environmental light-dark cycles and acutely modulating arousal states, a process known as masking. Masking responses are completely reversed in diurnal and nocturnal species. In nocturnal rodents, masking is mediated through a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) whose projections are similar in diurnal and nocturnal rodents. This raises the possibility that differences in responsivity to signals that these cells release might underlie chronotype differences in masking. We explored one aspect of this hypothesis by examining the distribution of excitatory and inhibitory neuronal populations in many ipRGC target areas of a diurnal species (Nile grass rat) and a nocturnal one (Norway rat). We discovered that while many of these regions were very similar in these two species, there were striking differences in the ventral lateral geniculate nucleus (vLGN; higher density of glutamate cells in Norway rats) and in the lateral habenula (LHb; GABAeric cells present in grass rats, but not Norway rats). These patterns raise the possibility that the vLGN and LHb contribute to differences in masking and/or circadian regulation of diurnal and nocturnal species.