Electrophysiological evidence for the role of substance P in retinohypothalamic transmission in the rat

The retinohypothalamic tract (RHT) is a neural pathway through which photic time cues are delivered directly to the mammalian circadian pacemaker in t he suprachiasmatic nucleus (SCN). Although the excitatory amino acid glutam ate is the primary neurotransmitter in the RHT, other Substances such as su bstance P (SP) also have been suggested to play a role. The present study t ested the hypothesis that SP participates in retinohypothatamic transmissio n and selectively modulates either N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated neurotransmission, The SP antagonist L-703,606 depressed the excitatory postsynaptic current (EPSC) evoked by optic nerve stimulatio n in SCN neurons in rat hypothalamic slices. The SP antagonist also had a s imilar depressive effect on the NMDA and non-NMDA receptor-mediated compone nts of the EPSC. These results suggest that SP is an excitatory neuromodula tor contributing to the expression of both the NMDA and non-NMDA receptor-m ediated components of retinohypothalamic transmission. (C) 1999 Elsevier Sc ience Ireland Ltd. All rights reserved.