NEUROBIOLOGY OF WITHDRAWAL MOTIVATION - EVIDENCE FOR 2 SEPARATE AVERSIVE EFFECTS PRODUCED IN MORPHINE-NAIVE VERSUS MORPHINE-DEPENDENT RATS BY BOTH NALOXONE AND SPONTANEOUS WITHDRAWAL

In drug-naive rats, the rewarding effects of morphine are blocked by l esions of the tegmental pedunculopontine nucleus (TPP), but not by neu roleptics. In dependent rats (chronically treated with morphine), morp hine reward is blocked by neuroleptics, but not by TPP lesions. Just a s this activation of opiate receptors in naive versus dependent rats p roduces different mechanisms of reward, this study concludes that redu ced opioid activity on these opiate receptors produces different mecha nisms of aversion. Neuroleptics blocked the conditioned place aversion s produced by naloxone and spontaneous withdrawal in morphine dependen t, but not naive, rats, without attenuating the somatic withdrawal syn drome induced by naloxone in dependent rats. The researchers suggest t hat the aversive effects of endogenous opioid withdrawal in naive rats are mediated by different neural substrates than the aversive effects of exogenous opioid withdrawal in dependent rats.