Masking of behavior by external stimuli works with the circadian system to ensure that animals are active during the correct time-of-day. Light for diurnal and nocturnal species produces different masking responses, enhancing activity for diurnal species and suppressing activity for nocturnal species. Few studies have examined the neural mechanisms of masking; none these experiments use animals active during the day. The first experiment of this dissertation uses the protein of the immediate-early gene cFOS to compare activation of brain regions to light between nocturnal mice and diurnal grass rats during a time-point where they showed a distinct behavioral dichotomy in response. Grass rats showed a consistent increase in activation in areas that receive retinal innervation or were related to sleep/arousal, while mice showed either no difference or a decrease in activation with the exception of the SCN. This study demonstrates the differences in behavioral and neurological responses to masking pulse of light between a nocturnal and diurnal rodent species.We next examined the functional role of the ventral subparaventricular zone (vSPZ) on masking to light in the grass rat. The vSPZ uniquely responded to light in diurnal grass rats, receives direct retinal innervation from the eye and after ablation showed a possible alteration in masking. Schwartz et al. (2009)showed that grass rats with damage to the vSPZ had bouts of arrhythmia in LD conditions, indicating that the masking system may be altered. In Chapter 3, we tested the functional for of the region by bilateral lesioning the vSPZ and directly testing the effects of light on masking. Animals with complete ablations still increased activity in response to light in two different masking protocols. In Chapter 4, we examined another brain structure that could have played a role in masking, the pineal gland. Melatonin is a hormone produced by the pineal gland that plays a role in circadian rhythms and seasonality. The expression of melatonin is sensitive to light exposure, additionally, the presence of the hormone also feedbacks and influences the effects of light on the brain. Additionally, pinealectomy in rats alters the behavior profile of activity in LD conditions. Removal of the pineal gland in the grass rats did not influence the ability of animals to mask to light or the animal's circadian rhythm of activity.In summary, the work presented in this dissertation demonstrates the immediately impact of light on behavior and the possible brain regions that may play a functional role in the masking response. We established that two neural structures when ablated or removed do not impact the grass rat's ability to respond to light with an increase in activity. Here we broaden the scope of research into the neural mechanisms of masking to include a diurnal species, beginning the journey toward understanding the neural changes needed for a species to transition between temporal niches.
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