Neurons that express the AMPA receptor GluR2/3 subunits in suprachiasmatic nuclei of Syrian hamsters colocalize either vasoactive intestinal peptide, peptide histidine isoleucine or gastrin-releasing peptide.

Abstract

In mammals, many circadian rhythms are driven by a clock located inside the suprachiasmatic nucleus of the hypothalamus. They are synchronized to environmental light-dark cycles by information coming directly from the retina via glutamatergic afferents. In rodents, retinal fibres make direct synaptic contacts with neurons synthesizing vasoactive intestinal peptide and gastrin-releasing peptide. These two neuropeptides, administered alone or combined with the peptide histidine isoleucine, phase-shift the clock in the same way that light does. Using ICC and light and electron microscopy, our study demonstrates that subunits 2 and 3 of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamatergic receptors are colocalized in neurons expressing one or other of these three neuropeptides. Double-labelled neurons were located in the ventral and lateral ventral parts and near the symmetrical plane of the intermediate and caudal thirds of the nucleus. In light microscopy, brown and granular blue stainings of chromogens revealing both antigens were easily identifiable and spatially separated in perikarya. In electron microscopy, almost all the cells observed in these zones expressed the receptor subunits. A few labelled dendritic profiles, some of them post-synaptic, were observed; axon terminals were always unlabelled. Colocalization with vasoactive intestinal peptide and gastrin-releasing peptide was confirmed by the immunogold technique in perikarya and some dendrites. The present study suggests that peptidergic neurons expressing the AMPA receptors are involved in photic entrainment of the clock by the retina without excluding some glutamatergic information coming from other hypothalamic nuclei.