SYNAPTIC-INTERACTIONS OF SUBSTANCE-P IMMUNOREACTIVE NERVE-TERMINALS IN THE BARORECEPTOR AND CHEMORECEPTOR REFLEXES OF THE CAT

The neurochemical anatomy and synaptic interactions of morphologically identified chemoreceptor or baroreceptor afferents in the nucleus of the solitary tract (NTS) are poorly understood. A substantial body of physiological and light microscopic evidence suggests that substance P (SP) may be a neurotransmitter contained in first order sensory chemo - or baroreceptor afferents, however ultrastructural support for this hypothesis is lacking. In the present report we have traced the centra l projections of the carotid sinus nerve (CSN) in the cat by utilizing the transganglionic transport of horseradish peroxidase. Medullary ti ssues including the commissural NTS (cNTS) were processed for the hist ochemical visualization of transganglionically labeled CSN afferents a nd for the immunocytochemical detection of SP by dual labeling light a nd electron microscopic methods. At the light microscopic level, dense bilateral labeling with TMB was found in the tractus solitarius (TS) and cNTS, caudal to the obex. Rostral to the obex, significant ipsilat eral TMB labeling was detected in the dorsal, dorso-lateral, and media l subnuclei of the NTS, as well as in the TS. Significant staining of SP immunoreactive processes was detected in most subnuclei of the NTS. The cNTS was examined by electron microscopy. Either HRP or SP were r eadily identified in single labeled unmyelinated axons, myelinated axo ns, and nerve terminals in the cNTS. SP immunoreactivity was also iden tified in unmyelinated axons, myelinated axons, and nerve terminals in the cNTS which were simultaneously identified as CSN primary afferent s. These ultrastructural data support the hypothesis that SP immunorea ctive first order neurons are involved in the origination of the chemo - and baroreceptor reflexes. Axo-axonic synapses were observed between CSN primary afferent terminals and: (a) unlabeled nerve terminals; (b ) other CSN primary afferent terminals; and (c) terminals containing S P. Axo-axonic synapses were also observed between CSN primary afferent s which contained SP, and other SP terminals. These observations may m ediate the morphological bases for multiple forms of presynaptic inhib ition in the cNTS, including those involved in cardiorespiratory integ ration. In conclusion, our results indicate that SP immunoreactive ner ve terminals may be important in both the origination and the modulati on of the chemo- and/or baroreceptor reflexes.