A relationship between substance P receptor and retinal fibers in the rat suprachiasmatic nucleus.

The suprachiasmatic nucleus (SCN) in the hypothalamus controls many of the circadian rhythms in mammalian species. In the present study, we investigated the location of substance P receptor (SPR)-containing neurons in the rat SCN, using a specific antibody against SPR, which corresponds to the NK-1 subtype of tachykinin receptors, and also examined the synaptic relationship between SPR-containing neurons and retinal fibers at the ultrastructural level. An SPR-immunoreactive meshwork of labeled somata and dendrites was identified in the SCN. The strongest SPR-immunoreactivity was observed in the dorsal and lateral parts of the SCN. Many labeled somata were identified there and their dendrites protruded ventrally from their somata. A few SPR-immunoreactive somata were observed also in the ventral part of the SCN and within the optic tract. In the SCN of eye-enucleated animals, degenerating retinal fibers were shown to terminate on SPR-immunoreactive dendrites forming asymmetrical axo-dendritic contacts.