Glutamic acid decarboxylase immunoreactivity of olivocochlear neurons in the organ of Corti of guinea pig and rat.

The distribution of glutamic acid decarboxylase (GAD)-like immunoreactivity in the organ of Corti of guinea pig and rat was studied under the light microscope. Indirect immunohistochemical techniques were used. Cochleae were first incubated with a specific antiserum to rat brain GAD and then stained through an avidin-biotin-horseradish peroxidase (HRP) procedure. GAD-like immunoreactivity was visualized as staining with HRP reaction product. Surface preparations were prepared from the immunoreacted cochleae. GAD-like immunoreactivity was found in the inner spiral bundle, tunnel spiral bundle, tunnel crossing fibers, outer hair cell synaptic regions and outer spiral bundles. Little staining was seen in the basal turn. Most of the immunoreactivity was seen in the third and lower fourth turn of the guinea pig cochlea, but even there many efferent fibers and endings were unstained. It is concluded that GAD-like immunoreactivity is present in a subpopulation of cochlear efferents that contains elements from both the medial and the lateral olivocochlear system. Future studies are needed to determine whether this subpopulation is GABA-ergic (i.e. uses gamma-aminobutyric acid as a neurotransmitter) and/or cholinergic.