Abstract
The aim of this ultrastructural study was to analyse quantitatively the distribution of γ-aminobutyric acid (GABA)-like immunoreactivity in axon terminals apposed to somatic and proximal dendritic membranes of cat motoneurons in lumbar column 2. Preembedding immunocytochemistry was used to count the GABAergic terminals contacting profiles of eighteen α- and six γ-motoneurons. Of the 1293 terminals counted on the somatic and proximal dendritic compartments of α-motoneurons, 197 were GABAergic. In contrast, a total number of only 62 terminals were counted on γ-motoneurons, of which 8 were GABAergic. These populations of GABAergic terminals were less numerous than the population of glycinergic terminals observed in a previous study. The morphometric characteristics of GABAergic synapses were analyzed using postembedding immunocytochemistry. Most of the GABAergic terminals contained pleomorphic vesicles (F-type boutons, flattened or pleomorphic vesicles). All terminals presynaptic (P boutons) to large terminals containing sphericle vesicles (M-type boutons, characteristic of α-motoneurons), were GABA-immunopositive. These results suggest that there are different distributions of the GABAergic control of excitability on γ- and α-motoneurons. GABA appears to be strongly involved in post-synaptic inhibition of α-motoneurons, whereas γ-motoneurons receive very few GABAergic inhibitory inputs. Morphological correlates of GABAergic presynaptic inhibition were seen on α- but not on γ-motoneurons.
Published Version
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