Cuneothalamic relay neurons are postsynaptic to glycine-immunoreactive terminals in the rat cuneate nucleus

This study was aimed to clarify whether the cuneothalamic relay neurons (CT Ns) in the rat cuneate nucleus contained glycine or whether the neurons wer e modulated directly by presynaptic glycine-IR terminals. For this purpose, retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and immunoperoxidase labelling for glycine have been u sed to ascertain if the CTNs in the rat are glycine-immunoreactive (glycine -IR). Our results have shown that the WGA-HRP-labelled CTNs (mean area = 31 8 +/- 6.5 mu m(2)) were not reactive for glycine. Glycine immunoreactivity, however, was localized in some small-sized neurons (mean area 210 +/- 6.2 mu m(2)) and axon terminals associated with the CTNs. The synaptic organiza tion between the glycine-IR terminals and CTNs was further analyzed using a nti-glycine postembedding immunogold labelling. By electron microscopy, the immunogold-labelled glycine-LR terminals containing pleomorphic synaptic v esicles formed symmetrical synaptic contacts with the dendrites, dendritic spines, and somata of CTNs. Quantitative estimation showed that the mean ra tios of glycine-IR terminals to total terminals associated with the soma, p roximal dendrites and distal dendrites of the CTN were 49.5, 45.2, and 45.8 %, respectively. The higher incidence of glycine-IR terminals on the soma, however, was not significantly different from that of the proximal and dist al dendrites. Notwithstanding the above, this study has shown a large numbe r of glycine-LR terminals making direct synaptic contacts with CTNs, sugges ting that glycine is one of the important neurotransmitters involved in pos tsynaptic inhibition on the cuneothalamic relay neurons to modulate incomin g somatosensory information from forelimb areas in the rat. Synapse 37:222- 231, 2000. (C) 2000 Wiley-Liss, Inc.