Distribution of neuropeptide immunoreactive nerve terminals within the subnuclei of the nucleus of the tractus solitarius of the rat

Abstract

The location of substance P, enkephalin and somatostatin (SRIF), and neurophysin II immunoreactive nerve terminals and preterminal processes in the caudal part of the nucleus of the tractus solitarius (nTS) was examined by the indirect immunofluorescence method for immunocytochemistry combined with cytoarchitectural identification of nuclear subgroups in the same tissue. In 22 Sprague‐Dawley rats we examined 14‐μm‐thick serial sections of the dorsal medulla at levels from 1 mm caudal to 2 mm rostral to the obex. These sections were incubated with substance P, enkephalin, somatostatin, and neurophysin II antisera. All four peptides were examined in each case and five typical levels (two caudal and three rostral to the obex) were selected for comparison of terminal distribution between peptides. All sections were photographed under the fluorescence microscope and then counter‐stained with cresyl violet. This method of analysis revealed distinct patterns of neuropeptide immunoreactivity in the subnuclei of the nTS that varied according to the level of the section. The nTS is responsible for integrating respiratory, cardiovascular (baroreceptor and cardiac), and gastrointestinal functions. The ventrolateral subnucleus (Vl)nTS, ventral subnucleus (v)nTS, interstitial subnucleus (ni)nTS, and intermediate subnucleus (nI)nTS are the major respiratory subnuclei with vlnTS and vnTS prominently associated with pulmonary afferents, ni associated with laryngeal afferents, and nI with tracheal afferents. The vlnTS, vnTS, and ni showed a moderate density of somatostatin‐positive nerve terminals, scattered substance P and enkephalin immunoreactivity, and no neurophysin II‐positive terminals. The nI showed moderate density of substance P immunoreactive nerve terminals. The subnuclei of the nTS receiving baroreceptor and chemoreceptor afferents—dorsolateral and dorsal (dl and d) subnuclei of nTS—showed scattered substance P immunoreactive nerve terminals. The commissural nucleus of nTS (ncom), which receives most of the cardiac afferents, showed a moderate density of enkephalin‐positive immunoreactive nerve terminals. The medial subnucleus (m)nTS at levels rostral to the obex, the primary site for the termination of gastrointestinal afferents, showed substance P immunoreactivity in moderate amounts and week immunoreactivity for all the other neuropeptides. An important result of these experiments was the observation that regions of the medulla adjacent to the nTS, i.e., the ventral parasolitarius region (vPSR), dorsal (d)PSR, and the periventricular region (PVR) showed the densest amounts of immunoreactive nerve terminals. These regions represent the sites where dendrites of nTS neurons project (Cajal, '09). The vPSR containing dense collections of substance P‐ and enkephalin‐positive immunoreactive nerve terminals is adjacent to respiratory neurons in the vnTS and vlnTS; the dPSR containing dense enkephalin‐positive nerve terminals is adjacent to the baroreceptor and chemoreceptor region in the dlnTS; and the PVR containing intense substance P‐ and enkephalin‐positive immunoreactivity is adjacent to the gastrointestinal subnucleus—the mnTS. These results indicate that distinct patterns of neuropeptide immunoreactivity exist within the various subnuclei of the nTS and adjoining regions of the dorsal medulla. In addition, for a given subnucleus of the nTS the peptide located in the perikaryal region was different from the peptide found in the dendritic zone; e.g., the vlnTS contains somatostatin immunoreactivity in the perikaryal region and substance P and enkephalin in the adjacent dendritic region—the vPSR. Thus different neuropeptides might be used to serve independent functional effects on soma and dendrites of a given class of nTS neurons.