Comparative distribution of NK1, NK2, and NK3 receptors in the rat brainstem auditory nuclei.

While the distribution of substance P in the auditory system is well illustrated, the localization of its receptors has not yet been documented. The goal of our study was to characterize the distribution of the tachykinin receptors NK1-R, NK2-R and NK3-R in the brainstem auditory nuclei of the adult rat using immunohistochemical techniques. The immunoreactivity of the neurokinin receptors was found to be widely distributed in most neurons of the cochlear nucleus (CN), the lateral superior olive (LSO), the medial nucleus of the trapezoid body (MNTB) and in the inferior colliculus (IC). Immunoreactivity was generally confined to post-synaptic targets (neuronal cell body and proximal or primary dendrites) in all auditory nuclei. However, unlike brainstem nuclei, the IC showed, in addition to neuronal cell body staining, a positive axonal immunolabeling (axons and pre-synaptic terminals) with the anti-NK1-R antibody. This axonal staining, revealing a pre-synaptic expression of NK1-R, is in good agreement with the known presence of substance P in the IC neurons. The absence of axonal staining in the superior olivary complex nuclei which projects afferent to the IC indicated that the NK1-R labeled axons are rather intrinsic IC fibers or descending thalamic projections to the IC. Overall, the wide distribution of the three types of tachykinin receptors observed in the present study argues for an important role of tachykinin neuropeptides in the central auditory system.