Substance P neurones in medullary baroreflex areas and baroreflex function of capsaicin-treated rats. Comparison with other primary afferent systems

Adult rats (age 13–17 weeks) treated 20 h after birth with capsaicin (50 mg/kg s.c.) showed a 50% loss of substance P-immunoreactive nerve terminals in the thoracic substantia gelatinosa as revealed by quantitative immunohistochemistry and radioimmunoassay. Other spinal substance P systems were unchanged. A similar decrease of immunoreactive substance P axons and nerve terminals was observed in medullary primary afferent systems, namely in IX and X rootlets, tractus spinalis nervi V, tractus solitarius and substantia gelatinosa nervi V. Other medullary substance P fibres, e.g. in the reticular formation, nucleus motorius nervi X and XII, inferior olive and area postrema, were not obviously changed. The nucleus tractus solitarii receiving, among others, IX and X primary afferents, e.g. from baroreceptors and chemoreceptors, showed a particular pattern of depletion: although the substance P axons penetrating from the tractus solitarius mainly into the intermediate and caudal nucleus tractus solitarii were markedly reduced in capsaicin-treated rats, the total number of substance P nerve terminals in the intermediate and caudal nucleus tractus solitarii was only moderately decreased and appeared normal in the cranial portion of the nucleus. The number of substance P-immunoreactive cells in the nucleus tractus solitarii was unchanged. Radioimmunoassay of substance P levels showed a 60% decrease in the microdissected substantia gelatinosa nervi V, but only insignificant decreases (by 20%) in microdissected divisions of the nucleus tractus solitarii and reticular formation. Thus, capsaicin destroyed a considerable part of primary spinal and medullary substance P afferents. In the entire nucleus tractus solitarii most of substance P is contained in a capsaicin-insensitive, probably intrinsic system. However, the intermediate and caudal divisions contain, in addition, primary afferent IX and X substance P terminals which were partly destroyed by capsaicin. In view of a recent proposal that substance P is the transmitter released from baroreceptor afferents, blood pressure and baroreflex function were measured in capsaicin-treated (50 and up to 600 mg/kg) rats; neither differed from controls. While, at first sight, this finding militates against a transmitter role of substance P in the baroreceptor reflex pathway, incomplete destruction of primary substance P afferents in the nucleus tractus solitarii by capsaicin and the possible development of compensatory mechani