Intergeniculate leaflet lesions result in differential activation of brain regions following the presentation of photic stimuli in Nile grass rats.

The intergeniculate leaflet (IGL) plays an important role in the entrainment of circadian rhythms and the mediation of acute behavioral responses to light (i.e., masking). Recently, we reported that IGL lesions in diurnal grass rats result in a reversal in masking responses to light as compared to controls. Here, we used Fos as a marker of neural activation to examine the mechanisms by which the IGL may influence this masking effect of light in grass rats. Specifically, we examined the patterns of Fos activation in retinorecipient areas and in brain regions that receive IGL inputs following 1-h light pulses given during the early night in IGL-lesioned and sham-operated grass rats. Three patterns emerged: (1) IGL lesions had no effect on the Fos response to light, (2) IGL lesions resulted in a reversal in Fos responses to light, and (3) IGL lesions resulted in a lack of a Fos response to light. Of specific interest were the suprachiasmatic nucleus (SCN) and the olivary pretectal nucleus (OPT), both of which are retinorecipient and connect reciprocally with the IGL. Light-induced Fos expression in the SCN was unaffected by IGL lesions, whereas the OPT exhibited a significant reduction in Fos expression following a light pulse in animals with IGL lesions. Altogether, our results suggest that the OPT, but not the SCN, exhibits a reversal in Fos responses to light following IGL lesions that reverse masking responses in diurnal grass rats. Our results suggest that interconnections between the IGL and downstream brain areas (e.g., OPT) may play a role in determining the direction of the behavioral response to light.