Effects of pentazocine and concomitant clonidine on opioid receptors in the rat brain.

The changes in opioid receptors (Op-R) caused by repeated administration of pentazocine and the effect of concomitant clonidine were investigated. Binding of [3H] naloxone was markedly decreased in the absence of Na+, but was increased in the presence of Na+ in the diencephalon-mesencephalon of chronic pentazocine-treated rats. No significant changes were observed in the cerebral cortex of pentazocine-treated rats. The pentazocine-induced changes in Op-R were abolished by the concurrent use of clonidine, an alpha-adrenergic agonist, which has been shown to relieve the withdrawal symptoms of morphine. This result indicated that the behavioral action of clonidine can also be observed at the Op-R level.