Medial thalamic injection of opioid agonists: mu-agonist increases while kappa-agonist decreases stimulus thresholds for pain and reward.

Selective agonists for mu- and kappa-opioid receptor types were infused, bilaterally, into the intralaminar central lateral nucleus of the rat. Subcataleptic doses of the mu-agonist, DAGO (0.25 and 1.0 microgram), elevated tailshock threshold for eliciting pain vocalization and motor responses. The hyperalgesic effect of U50,488 is not likely to be the result of antagonist action at a mu 2-isoreceptor; the general mu-antagonist, naloxone, and its less lipophilic quaternary analogue, both failed to produce a significant reduction in pain thresholds. Paralleling their effects on pain, DAGO and U50,488 elevated and reduced, respectively, lateral hypothalamic electrical stimulation threshold for positive reinforcement. These results suggest that medial thalamic opioid mechanisms are not exclusively involved in pain modulation but may generally regulate the responsiveness of the organism to motivating stimuli. Moreover, mu- and kappa-receptors may mediate opposite behavioral effects of opioid peptides.