INHIBITORY EFFECTS OF ELECTRICAL-STIMULATION OF THALAMIC NUCLEUS SUBMEDIUS ON THE NOCICEPTIVE RESPONSES OF SPINAL DORSAL HORN NEURONS IN THE RAT

The aim of the present study is to examine whether stimulation of the thalamic nucleus submedius (Sm) exerts an inhibitory influence on the long latency responses (C-responses) of the spinal cord dorsal horn ne urons evoked by noxious cutaneous electrical stimulation, in an attemp t to provide electrophysiological evidence for involvement of the Sm i n modulation of nociception. Single unit extracellular recordings from the dorsal horn neurons were obtained with glass micropipettes in pen tobarbital-anesthetized rats. A total of 71 nociceptive neurons, inclu ding 61 wide dynamic range (WDR) and 10 nociceptive specific (NS) neur ons, were studied in 29 rats. Electrical stimulation of either ipsilat eral or contralateral Sm markedly suppressed the C-responses in most ( 75%, 53/71) of these neurons, and facilitated the responses in only a few neurons. In general, the inhibitory effect was dependent on both t he stimulus intensity and the length of stimulus train, and the stimul us threshold for the inhibition to be elicited was about 50 mu A when a 300-ms train of 0.2-ms pulses at 200 Hz was used. The inhibitory eff ect outlasted the Sm stimulation about 500 ms. Inhibition of C-respons es could also be produced by stimulation of the dorsal hypothalamic ar ea (DA). On the other hand, stimulation of the structures in the media l thalamus surrounding Sm had no obvious influences on the C-responses of the dorsal horn neurons. The findings of this study provided furth er support for the hypothesis that Sm may be implicated in the descend ing modulation of nociception.