Distribution of GABAergic neurons and terminals in the auditory system of the barn owl

Antisera to GAD (glutamic acid decarboxylase) and GABA were used to determine the distribution of GABAergic cells and terminals in the brainstem and midbrain auditory nuclei of the barn owl. The owl processes time and intensity components of the auditory signal in separate pathways, and each pathway has a distinctive pattern of GAD- and GABA-like immunoreactivity. In the time pathway, all the cells of the cochlear nucleus magnocellularis and nucleus laminaris receive perisomatic GABAergic terminals, and small numbers of GABAergic neurons surround both nuclei. The ventral nucleus of the lateral lemniscus (anterior division) contains both immunoreactive terminals and some GABAergic neurons. In the intensity pathway, dense immunoreactive terminals are distributed throughout the cochlear nucleus angularis, which also contains a small number of GABAergic neurons. The superior olive contains two GABAergic cell types and immunoreactive terminals distributed throughout the neuropil. All the neurons of the nucleus of the lateral lemniscus (ventral part) appear to be GABAergic, and this nucleus also contains a moderate number of immunoreactive terminals. Immunoreactive terminals are distributed throughout the neuropil of the ventral nucleus of the lateral lemniscus (posterior division), whereas multipolar and small fusiform GABAergic neurons predominate in the dorsal regions of the nucleus. The time and intensity pathways combine in the inferior colliculus. The central nucleus of the inferior colliculus contains a larger number of fusiform and stellate GABAergic neurons and a dense plexus of immunoreactive terminals, whereas the external nucleus contains slightly fewer immunoreactive cells and terminals. The superficial nucleus contains dense, fine immunoreactive terminals and a small number of GABAergic neurons.