Evidence of a supraspinal opioid analgesic mechanism engaged by lateral hypothalamic electrical stimulation.

Brief trains of electrical stimulation were applied to the tails of rats to evaluate pain thresholds in the presence and absence of concurrent brain stimulation. Lateral hypothalamic (LH) stimulation, particularly in the dorsolateral medial forebrain bundle, elevated threshold for eliciting a post-stimulus vocalization response. Thresholds for eliciting a simple vocalization and motor response--both of which are organized at lower levels of the CNS than post-stimulus vocalization--were not significantly affected. This restricted form of analgesia was reduced by the opioid antagonist, naltrexone. Rewarding effects of stimulation in these LH sites, as evaluated in tests of self-stimulation threshold, were not affected by naltrexone. These results suggest that LH stimulation activates an opioid analgesic mechanism that is selectively active at a supraspinal level and diminishes the affective consequences of otherwise noxious stimuli.