We review recent studies in our laboratory that have investigated the
neural mechanisms underlying photic entrainment of the mammalian circa
dian system. The results from studies of extracellular single-unit rec
ordings and of photic induction of Fos-like immunoreactivity (Fos-lir)
indicate that excitatory amino acid (EAA) transmission, and particula
rly 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 sho
ws Fos-lir after blockade of EAA receptors, and we have evidence sugge
sting 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 rece
ptor that resembles the 5-HT1A subtype.