GABA innervation in adult rat oculomotor nucleus: a radioautographic and immunocytochemical study

GABA innervation in the adult rat oculomotor nucleus (n.III) was investigated using two complementary approaches: radioautography after incubation of brain slices with tritiated GABA ([3H]GABA) along with local in vivo microinjections of the tracer, and GABA immunocytochemical procedures involving antibodies directed against a GABA-glutaraldehyde-protein conjugate. As determined by radioautography after in vitro or in vivo labelling, the [3H]GABA uptake sites in the n.III mainly involved axon terminals. These were distributed throughout the neuropil and were often closely apposed to unlabelled motoneuron somata. A small number of glial cells also showed preferential accumulation of the tracer. The GABA-immunostaining likewise involved axon terminals throughout the nucleus, but no glial cells were immunopositive. In the dorsal region of the structure, occasional GABA-immunostained internuclear neurons were observed among unstained motoneuron cell bodies. Electron microscopic examination of [3H]GABA-labelled or GABA-immunostained profiles in n.III revealed axon terminals of around 1 micron in diameter, always filled with small, round synaptic vesicles homogeneously distributed throughout the axoplasm. These boutons frequently contained mitochondria and one or more large granular vesicles. In single thin sections, 35% of [3H]GABA-labelled, and 19% of GABA-immunostained varicose profiles exhibited a synaptic differentiation, suggesting the existence of a predominantly if not entirely junctional innervation. These synapses mostly involved dendritic trunks or dendritic branches and were usually of the symmetrical type. A few, which were always symmetrical, were also observed on large somata of motoneurons. Some of the dendrites synaptically contacted by GABA-immunostained axon terminals were themselves GABA immunoreactive. These data substantiate the idea that GABA is involved in the control of motoneuron activity in n.III, and provide a structural basis for the inhibitory role of this transmitter in oculomotor function.