THE INDUCTION OF FOS-LIKE PROTEINS IN THE SUPRACHIASMATIC NUCLEI AND INTERGENICULATE LEAFLET BY LIGHT-PULSES IN DEGUS (OCTODON-DEGUS) AND RATS

In nocturnal rodents, exposure to light results in an increase in Fos expression in two regions that receive direct retinal input: the supra chiasmatic nuclei (SCN) of the hypothalamus and the intergeniculate le aflet (IGL) of the thalamus. The induction of Fos within the SCN of no cturnal, rodents is phase dependent, with Light presented during the s ubjective night increasing Fos expression and light presented during t he subjective day having little effect. By contrast, Fos expression in creases in the IGL when light is presented during the subjective day o r night. It is unclear whether Fos is part of the pathway mediating li ght-induced phase shifts in diurnal rodents. In the present study, the ability of light to induce immunostaining for Fos in the SCN and IGL was compared in diurnal rodents, Octodon degus (degus), and nocturnal rats. Degus and rats were either maintained in constant darkness or ex posed to a I-h light pulse at circadian time (CT) 4 or 16. Degus exhib it robust phase shifts at each of those circadian hours, whereas rats demonstrate phase shifts only at CT 16. In degus, exposure to a l-h li ght pulse at CT 16 resulted in an increase in the number of Fos-immuno positive (Fos(+)) cells in the ventrolateral SCN. By contrast, a l-h l ight pulse at CT 4 resulted in a decrease in the number of Fos' cells in the dorsomedial portion of the SCN. In rats, a light pulse presente d at CT 16 resulted in an increase in Fos(+) cells throughout the SCN, and a pulse at CT 4 had no effect on Fos staining. Both degus and rat s showed increases in Fos expression in the IGL after Light exposure a t CTs 4 and 16. The authors conclude that light pulses presented at ti mes that produce phase shifts in activity rhythms also alter Fos expre ssion in the SCN and IGL of degus. Although these effects of light exp osure on Fos expression are not identical in diurnal and nocturnal rod ents, it is likely that Fos and other immediate early genes are part o f the pathway mediating the effects of light in both diurnal and noctu rnal rodents.