Inhibitory synaptic input to identified rubrospinal neurons in Macaca fascicularis: an electron microscopic study using a combined immuno-GABA-gold technique and the retrograde transport of WGA-HRP.

Rubrospinal neurons of the magnocellular division of the red nucleus of Macaca fascicularis were retrogradely labeled following spinal cord microinjections of wheat germ agglutinin-horseradish peroxidase, as demonstrated by the chromagen tetramethylbenzidine, identifying the mesencephalic cells of origin of this descending motor pathway. The tissue was processed for electron microscopy and subsequently tested on the electron microscope grid for immunoreactivity of gamma aminobutyric acid (GABA) in presumed local circuit neuronal somata, in dendrites, and in axonal terminals. Results demonstrate the presence of retrogradely labeled rubrospinal neurons of medium and large diameters (30-90 microns) and immunoreactive neurons of small size (less than 20 microns in diameter) within the nucleus. In addition, there are substantial numbers of GABAergic, presumably inhibitory, synaptic structures contacting somata and primary, medium, and small sized dendrites, as well as spineheads of rubrospinal neurons. The immunoreactive presynaptic profiles exhibit two different morphological appearances: one axonal and the other dendritic. Axonal terminals contain densely packed pleomorphic to flattened vesicles and form primarily symmetrical synapses with somata and all regions of the dendritic arbor. GABAergic profiles resembling presynaptic dendrites (PSDs) are also present. These profiles possess scattered flattened to pleomorphic synaptic vesicles in a translucent cytoplasm and are often postsynaptic to axonal terminals of unknown origin, or to GABAergic profiles. GABAergic local circuit neurons (LCNs), the neurites of which remain within the confines of the nucleus, appear to be contacted primarily by cortical and cerebellar afferents. These LCNs may or may not possess axons and thus may represent both the source of the GABAergic axonal terminals as well as that of the PSDs. Inhibitory afferents from other sources, such as the mesencephalic reticular formation, may also account for GABAergic terminals involved in this inhibition. We propose that the level of excitability of rubrospinal neurons and their subsequent activation of spinal motor neurons and interneurons is significantly regulated by the local circuit GABAergic inhibitory interneuronal population of the nucleus proper and probably by axons entering the nucleus from an extranuclear source.