Ultrastructural examination of the corticocollicular pathway in the guinea pig: a study using electron microscopy, neural tracers, and GABA immunocytochemistry

Projections from auditory cortex (AC) can alter the responses of cells in the inferior colliculus (IC) to sounds. Most IC cells show excitation and inhibition after stimulation of the AC. AC axons release glutamate and excite their targets, so inhibition is presumed to result from cortical activation of GABA ergic IC cells that inhibit other IC cells vialocal projections. However, it is not known whether cortical axons contact GABA ergic IC cells directly. We labeled cortico collicular axons by injecting fluorescent dextrans into the AC inguine a pigs. We visualized the racer with diaminobenzidine and processed the tissue for electron microscopy. We identified presumptive GABA ergicpro files with post- embedding anti GABA immunogold his to chemistry on ultrathin sections. We identified dextran-labeled cortical b out on sin the IC and identified their post synaptic targets according to morphology(e. g., spine, dendrite) and GABA-reactivity. Cortical synapses were observedinall IC subdivisions, but were comparatively rare in the central nucleus. Cortical boutons contain round vesicles and few mitochondria. They form asymmetric synapses with spines (most frequently), dendritic shafts and, leastoften, with cell bodies. Excitatory boutons in the IC can be classified as large, medium or small; most cortical boutons belong to the small excitatory class, while aminority(similar to 14%) belong to the medium excitatory class. Approximately 4% of the cortical targets were GABA- positive; these included dendritic shafts, spines, and cell bodies. We conclude that the majority of cortical boutons contact non-GABA ergic(i. e., excitatory) IC cells and as mall proportion (4%) contact GABA ergic cells. Given that most IC cells show inhibition (as well as excitation) after cortical stimulation, it is likely that the majority of cortically-driven inhibition in the IC results from cortical activation of a relatively small number of ICGABA ergic cells that have extensive local axons.