The GABA/GAD innervation within the inner spiral bundle in the mouse cochlea.

Stains with antibodies to glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in the cochlea of postnatal and adult mice reveal within the inner spiral bundle a distinctive neuronal plexus intimately associated with the inner hair cells. This innervation provides endings that cradle the receptor poles of the sensory cells and lateral end collaterals that wind between the cells, distributing endings alongside and around them. Some GAD-positive fibers enter the inner pillar bundle, from where they distribute tunnel fibers to the outer hair cells and recurrent collaterals to the inner hair cells. The GABAergic innervation within the inner spiral bundle is present along the entire cochlear axis, with the highest density in its basal half. Stainings against calcitonin gene-related peptide (CGRP), which localizes in the cholinergic counterpart of the inner spiral bundle, reveal that some of these fibers parallel the GABAergic circuit. The present data, together with our previous demonstration of compound (serial, converging, triadic) efferent synapses within this pathway (Sobkowicz et al. (1995) Abst. ARO 18, 171) evidences the presence of a distinctive innervation to the inner hair cells, hitherto unrecognized. The expression of growth-associated protein-43 (GAP-43) within the inner hair cell innervation in the adult cochlea provides evidence for a continuous synaptic turnover and plasticity, thus emphasizing its functional importance.