Synaptic structures of the nociceptive primary afferent central terminals in the substantia gelatinosa of spinal trigeminal nucleus candalis

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The present study was designed to elucidate the relationship of CGRP-, SP-immunoreactive (CGRP-, SP-IR) primary afferent central terminals with GABA-immunoreactive (GABA-IR) interneurons in the mouse dorsal horn. Pre-embedding immunostaining for GABA was done on free-floating sections with ABC method. Ultrathin sections were collected on Formvar-coated nickel grids. The grids were immunostained for CGRP or SP with postembedding immunogold technique. The GABA-IR cell bodies were distributed throughout the superficial dorsal horn. The CGRP- or SP-IR terminals were presynaptic to the CABA-IR dendrite of interneurons in the substantia gelatinosa. These findings suggest that the information transmitted by primary afferent fibers can be relayd directly to GABAergic neurons, and inhibitory circuits are activated by nociceptive information.In order to directly demonstrate the synapses between CI-terminals and GABA- and Met-Enkephalinergic interneurons, capsaicin treatment and immunolabeling methods were utilized. Superficial layr of the lumbar spinal cord dorsal horn and spinal trigeminal nucleus candalis of the mouse were examined by electron microscopy after post-embedding immunogold labeling procedures. Double-labeling with different size of IgG gold was used for GABA (15nm) and Met-Enk (5nm). Glomerular CI-terminals made presynaptic contacts with surrounding dendrites exhibiting GABA and Met-Enk immunoreactivities (IR). Nonglomerular CI-terminals also made presynaptic contacts with GABA- and Met-Enk-IR soma. Frequently a single dendrite and somata showed double labeling, i.e.GABA and Met-Enk might coexist in some of the interneurons. The present findings strongly suggest that nociceptive primary afferent neurons modulate the pain transmission by themselves via inhibitory GABAergic and Met-Enkephalinergic interneurons in the superficial layr of dorsal horn and spinal trigeminal nucleus caudalis.