Immunohistochemical demonstration of neuroactive substances in the inner ear of rat and guinea pig.

The presence and localization of different neuropeptides and other putative neurotransmitters or -modulators were examined by immunohistochemistry in the cochleovestibular end organs and in neurons innervating them in rats and guinea pigs. In the organ of Corti neural elements beneath inner hair cells showed immunoreactivity for enkephalin (ENK), calcitonin gene-related peptide (CGRP), L-glutamate decarboxylase (GAD), substance P (SP) and tyrosine hydroxylase (TH). Nerve chalices of type I vestibular hair cells contained SP and GAD, but not consistently. SP was only occasionally observed in neuronal cell bodies of the 8th cranial nerve but fine fibers with different neuroactive substances were seen in the nerve trunk in the following relative numbers: TH greater than SP greater than CGRP greater than ENK. The present data demonstrate the presence of several different neuroactive substances in the rat and guinea pig inner ear suggesting a multiplicity of neurotransmitters or -modulators in this system.