INHIBITORY EFFECTS OF ELECTRICAL-STIMULATION OF THALAMIC NUCLEUS SUBMEDIUS AREA ON THE RAT TAIL-FLICK REFLEX

This study in lightly anesthetized rats found that unilateral electric al stimulation delivered to the ventral part of the thalamic nucleus s ubmedius (Sm), the thalamic reuniens nucleus (Re) and the hypothalamic dorsal area (DA) markedly depressed the TF reflex, and this inhibitor y effect increased following increasing stimulation intensity. Stimula tion in the dorsal part of Sm did not produce any or only slight depre ssion of the TF reflex. Furthermore, an ipsilateral electrolytic lesio n of the ventrolateral orbital cortex (VLO) eliminated the unilateral Sm-evoked inhibition, but not the inhibition elicited by Re and DA and contralateral Sm stimulation. Finally, after bilateral electrolytic l esions of the ventrolateral periaqueductal gray (FAG) the DA and Re an d contralateral Sm-evoked inhibitions were also eliminated. The result s suggest that the Sm plays an important role in modulation of nocicep tive inputs, and this role of Sm is mediated by the VLO and leads to a ctivation of the RAG descending inhibitory system and depression of th e nociceptive inputs at the spinal cord level.