Distribution of ionotropic glutamate receptor subunit immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet of the hamster

Abstract

Glutamate is thought to mediate the effects of light on the circadian pacemaker contained in the suprachiasmatic nucleus. Glutamate can reset this pacemaker both in vivo and in vitro while glutamate antagonists can reduce photically induced phase shifts in activity rhythms and c-fos expression in the suprachiasmatic nucleus. Most behavioural and gene expression experiments investigating circadian rhythms use hamsters, but the majority of the anatomical data on the presence and distribution of selected glutamate receptor subunits in the suprachiasmatic nucleus has been collected from rat. In the present study, we examined the distribution of ionotropic glutamate receptor subunits in the hamster suprachiasmatic nucleus using mono- and polyclonal antibodies directed against these subunits. In addition, we examined the distribution of immunostaining for these subunits in a second structure of the mammalian circadian system, the intergeniculate leaflet of the thalamus since it also is thought to receive glutamatergic input from the retina and is important in the entrainment of circadian rhythms. The results indicated that all of the subunits investigated (GluR1, GluR2/3, GluR4, GluR5/6/7, and NMDAR1) were present in the suprachiasmatic nucleus and that all but GluR4 were present in the intergeniculate leaflet. Each of the subunits investigated had a unique pattern of distribution and intensity of staining. The distribution of immunoreactivity for these subunits in the hamster suprachiasmatic nucleus and intergeniculate leaflet differed from that reported in the rat. The presence of these subunits in the suprachiasmatic nucleus and intergeniculate leaflet implies the presence of functional NMDA and non-NMDA receptors in these structures that may have a role in photic entrainment of the circadian pacemaker.