Distribution of neurophysin II immunoreactive nerve fibers within the subnuclei of the nucleus of the tractus solitarius of the rat.

The location of neurophysin II immunoreactive nerve fibers and preterminal processes has been examined in various functionally distinct subnuclei of the nucleus of the tractus solitarius (nTS) using the indirect immunofluorescence method for immunocytochemistry combined with cytoarchitectonic identification. The nTS is responsible for integrating respiratory and autonomic reflex activity: the vlnTS, vnTS, ni and nI are associated with respiratory activity; the dlnTS and dnTS are important sites for the integration of baroreceptor and chemoreceptor activity; the ncom, dnTS and dlnTS integrate cardiac afferent activity and the mnTS mediates both cardiovascular and gastrointestinal effects. At levels caudal to the obex, the ncom contained the largest number of neurophysin II immunoreactive nerve fibers and the mnTS and dmnX contained moderate neurophysin II immunoreactivity. At levels rostral to the obex the region of the dorsal medulla adjacent to the mnTS and dnTS (PVR and dPSR) showed the densest immunoreactivity and the mnTS, dmnX and vPSR showed moderate immunoreactivity. At the rostral pole of the nTS, neurophysin II immunoreactive nerve terminals were seen in the dendritic regions of cells in dmnX and mnTS. This selective distribution of neurophysin II immunoreactive nerve terminals in the cardiovascular and gastrointestinal subnuclei of the nTS implicates a direct, descending, hypothalamic, oxytocin-neurophysin II containing pathway interacting with these nTS functions. These results confirm the hypothesis (Sawchenko and Swanson) that descending neurophysin II immunoreactive pathways represent an important neuronal system for the hypothalamic regulation of cardiovascular (vasomotor) and gastrointestinal nuclei in the brainstem.