Distribution pattern of inhibitory and excitatory synapses in the dendritic tree of single masseter alpha-motoneurons in the cat

Little is known about the differences in the distributions of inhibitory an d excitatory synapses in the dendritic tree of single motoneurons in the br ainstem and spinal cord. In this study, the distribution of gamma-aminobuty ric acid (GABA)-, glycine-, and glutamate-like immunoreactivity in axon ter minals on dendrites of cat masseter alpha-motoneurons, stained intracellula rly with horseradish peroxidase, was examined by using postembedding immuno gold histochemistry in serial ultrathin sections. The dendritic tree was di vided into three segments: primary (Pd) and distal (Dd) dendrites and inter mediate (Id) dendrites between the two segments. Quantitative analysis of 1 75, 279, and 105 boutons synapsing on 13 Pd, 54 Id, and 81 Dd, respectively , was performed. Fifty percent of the total number of studied boutons were immunopositive for GABA and/or glycine and 48% for glutamate. Among the for mer, 27% showed glycine immunoreactivity only and 14% were immunoreactive t o both glycine and GABA. The remainder (9%) showed immunoreactivity for GAB A only. As few as 346 of the boutons were immunonegative for the three amin o acids. Most boutons immunoreactive to inhibitory amino acid(s) contained a mixture of spherical, oval, and flattened synaptic vesicles. Most boutons immunoreactive to excitatory amino acid contained clear, spherical, synapt ic vesicles with a few dense-cored vesicles. When comparisons of the inhibi tory and excitatory boutons were made between the three dendritic segments, the proportion of the inhibitory to the excitatory boutons: was high in th e Pd (60% vs. 37%) but somewhat low in the Id (46% vs. 52%) and Dd (44% vs. 53%. The percentage of synaptic covering and packing density of the inhibi tory synaptic boutons decreased in the order Pd, Id, and Dd, but this trend was not applicable to the excitatory boutons. The present study provides p ossible evidence that the spatial distribution patterns of inhibitory and e xcitatory synapses are different in the dendritic tree of jaw-closing alpha -motoneurons. (C) 1999 Wiley-Liss, Inc.