The Role of N-Methyl-D-Aspartate-Type Glutamatergic Neurotransmission in the Photic Induction of Immediate-Early Gene Expression in the Suprachiasmatic Nuclei of the Syrian Hamster

This study investigated the role of N‐methyl‐D‐aspartate (NMDA)‐type glutamatergic neurotransmission in mediating the photic induction of immediate‐early gene expression in the Suprachiasmatic nucleus (SCN) of the Syrian hamster. Activation of c‐fos, c‐jun and egr‐1 was assessed by immunocytochemical detection of their protein products. To characterize the circadian basis to the inductive effects of light, hamsters were allowed to free‐run in constant dim red light and received a 1 h light pulse at different circadian phases relative to activity onset (defined as CT 12). In control animals which did not receive light pulses, c‐fos and egr‐1 expression was absent or restricted to a small area of the dorsolateral region of the SCN, and expression of c‐jun could not be detected in the SCN. In hamsters killed after presentation of a light pulse at either CT 14 or CT 20, there was a marked increase in c‐fos and egr‐1 immunoreactivities throughout the ventrolateral division of the SCN. In contrast, light pulses given at CT4 or CT 8 failed to activate immediate‐early gene expression. Light pulses did not induce c‐jun immunoreactivity at any circadian phase tested. Staining for c‐fos was maximal 1 h after the start of the light pulse and had started to decline by 2 h. At this later time, c‐jun expression was still undetectable. To compare the distribution of retinal afferents with that of c‐fos induction, hamsters held on a light schedule of 16 h light: 8 h dark received an intraocular injection of cholera toxin‐horseradish peroxidase conjugate 3 days before exposure to a 1 h light pulse given 2 h after lights off. Comparison of adjacent sections processed for c‐fos immunoreactivity or for cholera toxin‐horseradish peroxidase revealed that light‐induced c‐fos expression was precisely restricted to retinal terminal fields in the SCN. Light pulses also induced c‐fos expression in the retinoreceptive ventral lateral geniculate nucleus and intergeniculate leaflet but not in the retinal fields of the dorsal lateral geniculate nucleus, indicating that the expression of cfos in response to light is spatially specific. The aim of the subsequent experiments was to investigate the role of NMDA‐type glutamatergic neurotransmission in mediating the effects of light on c‐fos expression in the SCN. To determine whether NMDA had the potential to activate c‐fos expression in the SCN, hamsters were infused with 2.5 nmol NMDA or vehicle via an intracerebroventricular (icv) cannula