DISTRIBUTION OF GLUTAMATE-IMMUNOREACTIVE, GLYCINE-IMMUNOREACTIVE AND GABA-IMMUNOREACTIVE NERVE-TERMINALS ON DENDRITES IN THE CAT SPINAL MOTOR NUCLEUS

The dendritic tree constitutes more than 93% of the receptive membrane area of a spinal motoneuron, yet little is known about its synaptic i nputs. In this study we examined the distribution of glutamate-, GABA- and glycine-like immunoreactivity in boutons apposing dendrites in the L7 spinal cord motor nucleus, by use of postembedding immunohistochem istry on serial sections. We examined 799 boutons apposing 401 cross-s ectioned dendrites of different calibre (range 0.2-15 mu m), and 14 fi rst-order (stem) dendrites. Thirty-five percent (35%) of the boutons w ere immunopositive for glutamate and 59% for GABA and/or glycine. Amon g the latter, 30% showed glycine immunoreactivity only and 24% were im munoreactive for both GABA and glycine. Very few were immunoreactive o nly for GABA (5%). As few as 6% of the boutons were judged as not enri ched for any amino acid analysed. The fine structural characteristics of the boutons were in accordance with previous descriptions. The samp le of dendrites was arranged in calibre bins in order to facilitate di stribution analysis. Stem dendrites differed from the other bins, with a high total bouton covering (61%) and a high bouton density. Sixty-n ine percent of the membrane covering was by glycine and/or GABA-immuno reactive boutons, whereas 18% was covered by boutons enriched in gluta mate. For non-stem dendrites, bouton covering fell from 33% to 12% wit h decreasing calibre. However, bouton apposition length decreased in p arallel, yielding a fairly uniform bouton density among dendrites of d ifferent calibre. The lack of correlation between packing density and dendrite calibre was also evident when the sample of dendrites was bro ken down into subsamples based on content of amino acid immunoreactivi ty. The latter analysis also revealed that both the relative covering and density of boutons containing inhibitory amino acids (57%; glycine and/or GABA) and glutamate (38%), respectively, did not vary systemat ically with dendrite calibre. Combined, the data indicate that in non- stem dendrites the proportion of excitatory and inhibition inputs does not change systematically throughout the dendritic arborizations of s pinal alpha-motoneurons. Thus, spinal motoneurons can, with respect to the general synaptic architecture, be divided into two main compartme nts, i.e. the proximal soma-juxtasomatic compartment (including stem d endrites) and the distal dendritc compartment. The proximal domain is under a powerful glycine and/or GABA influence. Finally, based on the data presented here and previously published data, it was calculated t hat spinal alpha-motoneurons receive in the range of 50-140x10(3) syna ptic boutons.