Immunohistochemical localization of substance P and substance P receptor (NK1) in the olivary pretectal nucleus of the rat

The olivary pretectal nucleus (OPN) is the first central nucleus in the pup illary light reflex are (PLR). Substance P (SP) is a neuropeptide present i n the OPN, The present immunohistochemical study, performed at the ultrastr uctural level, aimed to determine the synaptic localization of SP and SP re ceptor in the OPN. Three types of SP-positive terminals were found. The mos t abundant type was of retinal origin, characterized by electron-lucent mit ochondria and round vesicles, organized in glomerular structures, making as ymmetric synaptic contacts with dendrites, and profiles containing pleomorp hic vesicles, also making synaptic contacts with dendrites. The second type of SP-immunoreactive terminal contained electron-dense mitochondria and pl eomorphic vesicles. This type made symmetric synaptic contacts and may orig inate from the ventral part of the lateral geniculate nucleus. The third ty pe of SP-irnmunoreactive terminals contained electron-dense mitochondria, c lear round vesicles, and made an asymmetric synaptic contact. This type ori ginates from the contralateral OPN. SP receptors of the NK1 subtype were re vealed to be on dendrites and were part of the glomerular-like arrangement. On account of the present observations, it can be concluded that retinal p rojections to the OPN use SP as a neurmodulator and synapse on NK1 receptor -containing dendrites of large neurons projecting to the Edinger-Westphal n ucleus. Since SP also modulates the parasympathetic component of the PLR, w e postulate that SP plays a modulating role in all components of the PLR.