Glutamate shifts the phase of the circadian neuronal firing rhythm in the rat suprachiasmatic nucleus in vitro.

The effect of glutamate (GLU) on the phase of the circadian neuronal firing rhythm of the rat suprachiasmatic nucleus (SCN) was examined in vitro. GLU application in late subjective night produces phase-advances whereas GLU applied in early subjective night produces either phase-delays or a bimodal peak in the firing rhythm that appears to be a combination of phase-advances and phase-delays. Optic chiasm (OC) stimulation, or exposure of animals to light just prior to sacrifice, produces phase-delays in early subjective night, and phase-advances in late subjective night. None of GLU, light and OC stimulation produces significant phase-changes during subjective day. The results indicate that GLU is the neurotransmitter of the retinohypothalamic tract.