Opioid-mediated inhibition from the subnucleus caudalis of spinal trigeminal nucleus to the neurons in the subnucleus oralis

Abstract The influence of enkephalin-containing neurons in the subnucleus caudalis of the spinal trigeminal nucleus (STN caudalis) on the subnucleus oralis of the STN (STN oralis) was examined using chloral hydrate-anesthetized rats. In the STN oralis, conditioning stimuli applied to the STN caudalis 10–50 ms preceding the test stimulus inhibited spikes produced by tooth pulp stimulation in type B interneuron, which was activated by orthodromic stimulation but not by thalamic stimulation, without affecting those of the relay neuron. When the type B intereurons were further classified into type B 1 and type B 2 neurons, which were characterized by the occurrence of the STN caudalis-induced inhibition with long and short latencies, respectively, microiontophoretically applied naloxone reduced the STN caudalis-induced inhibition of the orthodromic spikes of type B 1 interneurons with little effects on type B 2 interneuron. Furthermore, naloxone-reversible inhibition of tooth pulp-induced spikes of the type B 1 inteneurons were also observed during iontophoretic application of enkephalin. These results suggest that the type B 1 interneurons in the STN oralis are inhibited by opioid peptides-containing neurons in the STN caudalis.