Several distinct receptor binding enkephalins in olivocochlear fibers and terminals in the organ of Corti.

Biochemical studies centering on the use of reverse-phase high-performance liquid chromatography (HPLC) and radioimmunoassays (RIA) demonstrate the presence in the guinea pig organ of Corti of at least 3 enkephalin-related peptides, two of which are identified as Met- and Leu-enkephalin, respectively. Enkephalins were identified and quantitated by HPLC-RIA in the isolated second turn of the organ of Corti, but were not found in stria vascularis or auditory nerve dissected from the cochlea. Three enkephalin-immunoreactive HPLC fractions inhibited the binding of labeled naloxone to rat brain membranes. All enkephalins identified by the combined HPLC-RIA procedure had an apparent molecular weight similar to that of Met- and leu-enkephalin peptide standards. Immunocytochemistry, performed with the best-characterized Met-enkephalin antiserum used in the RIAs, localized the enkephalin-like immunoreactivity to lateral efferent fibers and terminals under inner hair cells of the organ of Corti. Other antisera raised against Met-enkephalin, not used for RIA, visualized enkephalin-like immunoreactivity in medial efferent fibers under outer hair cells as well. This enkephalin-like immunoreactivity may reflect the presence in the medial efferent system of other structurally similar peptides in addition to those detected biochemically. Efferent fiber lesion, by evulsion of the vestibular nerve close to the vestibulocochlear anastomosis in which the olivocochlear fibers run, eliminated enkephalin-like immunoreactivity and the enkephalin-related peptides identified by HPLC-RIA.