Diffuse noxious inhibitory controls (DNIC): evidence for post-synaptic inhibition of trigeminal nucleus caudalis convergent neurones.

Activity produced by direct microelectrophoretic application of glutamate onto 19 convergent neurones in trigeminal nucleus caudalis, was strongly depressed during and after the application of heterotopic noxious conditioning stimuli: noxious heat (52 degrees C) applied to the tail, noxious pinches applied to the tail or hindpaws and intraperitoneal injections of bradykinin produced mean reductions in activity of 80-90%. The same noxious conditioning stimuli had no effect on the activities of any of 5 noxious-only or 5-non-noxious-only neurones. These effects were similar to those previously reported to influence peripherally evoked activities of nucleus caudalis convergent neurones and which have been termed diffuse noxious inhibitory controls (DNIC). It is therefore proposed that DNIC act on nucleus caudalis convergent neurones by a final post-synaptic inhibitory mechanism involving hyperpolarization of the neuronal membrane. Consistent with this hypothesis, it was also found that the noxious conditioning stimuli could restore firing of convergent neurones which had been excessively depolarised by large doses of glutamate.