We review recent studies in our laboratory that have investigated the neural mechanisms underlying photic entrainment of the mammalian circadian system. The results from studies of extracellular single-unit recordings and of photic induction of Fos-like immunoreactivity (Fos-lir) indicate that excitatory amino acid (EAA) transmission, and particularly activation of the N-methyl-D-aspartate (NMDA) receptor subtype, is important for conveying photic information to suprachiasmatic nucleus (SCN) cells. We have also found that a subregion of the SCN still shows Fos-lir after blockade of EAA receptors, and we have evidence suggesting that these cells are innervated by a distinct subdivision of the retinal projection to the SCN. In addition, we have found that photic responses of cells in the intergeniculate leaflet (which projects to the SCN) and of SCN cells are modulated by serotonin (5-HT) via a receptor that resembles the 5-HT1A subtype.
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