Previous studies suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by GABA-mediated presynaptic mechanisms in the trigeminal principal sensory nucleus (Vp) than other sensory nuclei. However, it is unknown if such a mechanism is common to functionally different classes of primary afferent in the same nucleus or across the nuclei. To address these issues, the present study focused on synaptic microcircuits associated with slowly adapting (SA) mechanosensory afferents innervating the periodontal ligaments in the cat Vp and attempted to examine GABA, glycine, and glutamate immunoreactivity in axon terminals involved in the circuits. Afferents were physiologically characterized before injection of horseradish peroxidase (HRP) and preparation for electron microscopy. HRP-labeled afferent boutons were serially sectioned and immunostained with antibodies against GABA, glycine, and glutamate using a postembedding immunogold method. All the afferent boutons examined contacted non-primary dendrites and they were frequently postsynaptic to unlabeled axons (p-endings). Axodendritic and axoaxonic contacts per afferent bouton were 1.3 (46/35) and 2.0 (70/35), respectively. Most p-endings were immunoreactive for GABA (63/70) and also glycine was co-stained in the majority of the p-endings (49/63). Thirty percent of p-endings with the colocalization of GABA and glycine participated in synaptic triads where a p-ending formed a synapse with the same dendrite as the afferent bouton. None of the p-endings was immunoreactive for glutamate. Most afferent boutons were enriched with glutamate but were immunonegative for GABA and glycine. This study provides evidence suggesting that transmission from SA afferents is strongly controlled presynaptically by GABAergic interneurons with colocalized glycine, and that a proportion of these interneurons, involved in synaptic triads, may also have postsynaptic inhibitory actions on target neurons of the SA afferents.