Functional significance of GABA ergic and glycinergic neurons making contact with the masticatory matoneurons-intracellular recording and HRP injection technique-

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Little is known about boutonal synaptic ultrastructure of interneurons contacting trigeminal motoneurons. To address this issue, physiologically identified premotor neurons in the rostrodorsomedial part of cat oral nucleus (Vo.r) were labeled by intracellular injections of horseradish peroxidase (HRP), and ultrastructure of their axon terminals was analyzed qualitatively and quantitatively. In addition, the effects of glycine antagonist strychnine, GABAA antagonist bicuculline, NMDA antagonist 2-amino-5-phosphovalerate (APV) and non-NMDA antagonist 6-cyno-7-nitroquinoxaline-2, 3-dione (CNQX) on Vo.r-induced postsynaptic potentials in trigeminal motoneurons were examined to evaluate potential signal substances of the premotor neurons. Labeled boutons made synaptic contacts with either jaw-closing or-opening motoneurons. All the boutons contained pleomorphic vesicles and most of them formed one synapse on the soma or the primary dendrite with symmetric specializations. A morphometric analysis indicated that the means of bouton volume and surface area, apposed surface area, total active zone area, and mitochondrial volume were not different between boutons on jaw-closing and-opening motoneurons, with exception of vesicle number and density, which were higher for boutons on jaw-closing motoneurons. The five morphological parameters were positively correlated with bouton volume, with exception of vesicle density, which tended to be negatively correlated. Application of strychnine and bicuculline suppressed Vo.r-induced inhibitory postsynaptic potentials (IPSPs) in jaw-closing motoneurons. Abolition of Vo.r-induced excitatory postsynaptic potentials in jaw-opening motoneurons with APV and CNQX unmasked IPSPs.The present results suggest that premotor neurons in the Vo.r are inhibitory, and that positive correlations between the ultrastructural parameters associated with synaptic release and bouton size are applicable to the interneurons as they do in primary afferents.