Cholinergic, Gaba-Ergic, and Noradrenergic Input to Cochlear Granule Cells in the Guinea Pig and Monkey

Abstract

In most mammals, the granule cell zone of the cochlear nuclei forms a peripheral capsule surrounding both the dorsal and ventral nuclei. Part of the capsule is a cellular layer composed of granule cells and “star” stellate cells (Moore, 1986). The heaviest accumulation of these small neurons occurs on the lateral free surface of the nuclei, but thinner layers of cells extend between the nuclei and around the medial edge of the cochlear complex (Fig. 1, guinea pig). The granule cell area also includes the dorsal nucleus molecular layer, a lamina formed by axons of granule cells in all parts of the capsular zone (Fig. 1, Mugnaini et al., 1980). Much of the afferent input to the granule cell area appears to arise either from intrinsic cochlear axons or from descending projection systems originating more centrally in the nervous system. Some of these projections have been demonstrated by histological methods for localizing neurotransmitters and transmitter-related enzymes. In particular, a concentration of cholinergic, GABA-ergic, and noradrenergic terminals has been demonstrated in the peripheral granule cell area of mammals such as the cat, rat, chinchilla, and guinea pig. In the case of the cholinergic and GABA-ergic projections systems, the input to the cochlear granule cell zone may arise from side branches of systems of axons projecting to the cochlea.