ONTOGENY OF GAMMA-AMINOBUTYRIC-ACID IN EFFERENT FIBERS TO THE RAT COCHLEA

Cochlear efferent innervation originates in two different groups of ne urons located in the superior olivary complex. A first group of olivoc ochlear neurons (lateral efferent neurons) lies in the lateral superio r olive. They send axons to the organ of Corti, where they synapse wit h radial afferent dendrites of primary auditory neurons, postsynaptic to the inner hair cells. The second group of neurons (medial efferent neurons) is found in medial subnuclei of the superior olivary complex and sends axons to synapse with outer hair cells. Subpopulations of bo th medial and lateral olivocochlear neurons probably use gamma-aminobu tyric acid (GABA) as a neurotransmitter. We have used an immunoperoxid ase technique to detect GABA-like immunoreactivity (GABA-LI) in postna tal maturing rat cochleas. The GABA-LI appeared in the inner hair cell region by P3 (P1 = birth) and reached a mature appearance by P15-P16. In the outer hair cell region, GABA-like immunoreactive fibers and te rminals could not be identified until P9 and they were only found in t he apical end of the cochlea. There was a dual gradient of maturation of GABA-LI in the cochlea. The GABA-LI appeared first at the cochlear base and then extended towards the apex. It also appeared earlier (abo ut a week) in the inner hair cell region than in the outer hair cell r egion. This dual gradient of maturation is in close agreement with pre vious data concerning the maturation of the cochlea.