Immunfluorescence study of neuropeptides in identified neurons of the rat auditory superior olivary complex

The present study was conducted to investigate the distribution and immunoh istochemical characteristics of ascending and descending projection neurons of the rat superior olivary complex (SOC), a group of interrelated brainst em nuclei. Ascending neurons were identified by injection of cholera toxin B subunit (CTB) into the central nucleus of the inferior colliculus (IC), d escending neurons were labeled by application of Fluoro-Gold (FG) into the scala tympani of the cochlea, ipsilaterally to the IC injection. In accorda nce with the literature, we observed neurons innervating the IC located in the lateral superior olivary nucleus (LSO) and dorsal periolivary groups (D PO) on both sides, in the superior paraolivary nucleus (SPO) predominantly ipsilateral, as well as in the ipsilateral medial superior olivary nucleus (MSO) and the medial nucleus of the trapezoid body (MNTB). Cochlear project ion neurons were found predominantly in the ipsilateral LSO as well as in t he bilateral SPO, DPO, MSO and MNTB. In addition, a considerable population of neurons in the ipsilateral LSO and SPO were identified as being both as cending and descending. To further characterize these double-projecting neu rons, brainstem sections were incubated in antisera directed against differ ent neuroactive substances. The majority of ascending/descending cells in t he LSO contained calcitonin gene-related peptide, but not substance P (SP), met-enkephalin (ENK) or tyrosine hydroxylase (TH). Some of these neurons a pparently were contacted by ENK- or SP-immunoreactive fibers and terminals. In addition, we found TH-immunoreactive neurons in the lateral MNTB region . These neurons, which were labeled upon tracer injection into the cochlea (but not upon IC injection), probably belong to the CI catecholaminergic ce ll group and may represent a division of the uncrossed olivocochlear bundle . The present results reveal the existence of a previously unknown subpopul ation of SOC neurons that project to both the cochlea and the inferior coll iculus. Their CGRP immunoreactivity and their uncrossed projection pattern provide evidence that they may belong to the cholinergic, putatively excita tory cell group.