Figure 5

In mammals, photic regulation of the circadian system is mediated by unknown photoreceptors which are located in the retina. These photoreceptors project to the SCN directly through a mono-synaptic projection from retinal ganglion cells known as the retinohypothalamic tract or RHT. The RHT appear to be both necessary and sufficient for synchronization of the circadian system by light. Evidence suggests that the amino acid glutamate (Glu) and the peptides substance P (SP) and NAAG are signaling molecules at this synaptic connection.

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published 2000