Expression of substance P, CGRP, and GABA in the vestibular periphery, with special reference to species differences

The present study was conducted to elucidate species differences in the distribution of neuroactive substances, including substance P (SP), calcitonin gene-related peptide (CGRP) and gamma-aminobutyric acid (GABA), in the vestibular periphery of various animals (chicken, pigeon, rat, guinea pig and squirrel monkey). SP-like immunoreactivity was found in a subset of the primary vestibular afferents with no marked species differences in the staining pattern. In contrast, the localization of CGRP- and GABA-like immunoreactivities in the efferent nerve fibers varied according to species. This difference in the distribution pattern of neuroactive substances, found in the efferent system, may indicate that each species has a chemically (and probably functionally) distinct efferent system which is related to its specific environment and/or evolution.