SUBSTANCE-P IMMUNOREACTIVE NERVE-TERMINALS IN THE DORSOLATERAL NUCLEUS OF THE TRACTUS-SOLITARIUS - ROLES IN THE BARORECEPTOR REFLEX

Physiological and light microscopic evidence suggest that substance P (SP) may be a neurotransmitter contained in first-order sensory barore ceptor afferents; however, ultrastructural support for this hypothesis is lacking. We have traced the central projections of the carotid sin us nerve (CSN) in the cat by utilizing the transganglionic transport o f horseradish peroxidase (HRP). The dorsolateral subnucleus of the nuc leus tractus solitarius (dlNTS) was processed for the histochemical vi sualization of transganglionically labeled CSN afferents and for the i mmunocytochemical visualization of SP by dual labeling light and elect ron microscopic methods. Either HRP or SP was readily identified in si ngle-labeled unmyelinated axons, myelinated axons, and nerve terminals in the dlNTS. SP immunoreactivity was also identified in unmyelinated axons, myelinated axons, and nerve terminals in the dlNTS, which were simultaneously identified as CSN primary afferents. However, only 15% of CSN terminals in the dlNTS were immunoreactive for SP. Therefore, while the ultrastructural data support the hypothesis that SP immunore active first-order neurons are involved in the origination of the baro receptor reflex, they suggest that only a modest part of the total sen sory input conveyed from the carotid sinus baroreceptors to the dlNTS is mediated by SP immunoreactive CSN terminals. Five types of axo-axon ic synapses were observed in the dlNTS. SP immunoreactive CSN afferent s were very rarely involved in these synapses. Furthermore, SP termina ls were never observed to form the presynaptic element in an axo-axoni c synapse with a CSN afferent. Therefore, SP does not appear to be inv olved in the modulation of the baroreceptor reflex in the dlNTS. (C) 1 998 Elsevier Science B.V.