GABA-like immunoreactive innervation and dendro-dendritic contacts in the ventrolateral dendritic bundle in the cat S1 spinal cord segment: an electron microscopic study.

The motoneurons (MNs) in the ventrolateral nucleus (VLN) of the upper sacral spinal cord segments in the cat supply the external sphincters and the ischiocavernosii muscles. The dendrites of the MNs in the VLN are arranged into rostro-caudally oriented bundles (ventrolateral dendritic bundle, VLB). In this study we describe the distribution and synaptic arrangement of gamma-aminobutyric acid-immunoreactive (GABA-IR) axonal bouton profiles innervating the VLB. This was accomplished using the peroxidase-antiperoxidase technique and a polyclonal antibody raised against glutaraldehyde-conjugated GABA. The VLN receives an extensive innervation of GABA-IR axonal bouton profiles that surround both cell bodies and dendrites. Twenty-five per cent of the total number of vesicle-containing axonal profiles in the VLN neuropil were estimated to be GABA-IR. On cell bodies in the alpha-motoneuron size-range, the membrane covering of GABA-IR bouton profiles was about 18% and they constituted about 29% of the total membrane covering of axonal bouton profiles. Quantitative analysis of GABA-IR bouton profiles on dendrites revealed membrane covering figures rather similar to those on the cell bodies. They were not randomly distributed within the dendritic arborisations. Instead, they were very infrequent (2.5% of the covering) on small calibre dendrites (< 1 microns) as compared to larger dendrites (> 1 microns, 14-18.5% of the covering), although the total membrane covering of axonal bouton profiles was rather similar for all dendrites (42-52%). The data on membrane covering by GABA-IR boutons presented here may be low estimates due to technical limitations, indicating that the GABAergic input to this region might be even more extensive. A frequent finding was that one and the same GABA-IR bouton made synaptic contact with two to three adjacent dendrites. This type of synaptic arrangement among the VLN MNs indicates a divergence of the GABAergic input at the terminal level. In addition, the postsynaptic dendrites involved in such arrangements often disclosed dendro-dendritic contacts. In total, 44% of the bundled dendrites in the VLN disclosed direct dendro-dendritic contact regions. These contacts were most often of the puncta adherentia type, while desmosome-type contacts were less frequent. None of the dendro-dendritic contacts studies had the characteristics of a gap junction. Taken together, the present results indicate that GABA may be a transmitter substance in a large fraction of the synaptic input to the VLN MNs.(ABSTRACT TRUNCATED AT 400 WORDS)