Inhibition of 5-HT Neurotransmission Increases Clonidine Protective Effects on Naloxone-induced Conditioned Place Aversion in Morphine-dependent Rats

Previous pharmacological studies have implicated serotonergic brain systems in opiate-withdrawal-precipitated conditioned place aversion. To assess this hypothesis, we tested the effects of either (i) a near-total 5,7-dihydroxytryptamine-induced lesion (90% depletion) or (ii) an acute serotonin (5-HT) inhibition induced by the specific stimulation of 5-HT1A autoreceptors (8-OHDPAT 5–100 μg/kg), on naloxone-induced conditioned place aversion in morphine-dependent rats. Morphine dependence was induced by the implantation of morphine slow-release pellets. The protective properties of clonidine (an alpha-2 adrenergic agonist classically given for opiate detoxification) were also tested after inhibition of 5-HT transmission. Serotonergic lesions in morphine-dependent rats failed to alter naloxone-induced conditioned place aversion but increased the sensitivity to the protective effects of clonidine. Acute neuropharmacological blockade of serotonin transmission also potentiated the clonidine effects on naloxone-induced conditioned place aversion. When combined with the 5-HT1A agonist 8-OHDPAT, clonidine was also found to be more potent. Further understanding of this serotonin/noradrenaline interaction might help devise new therapeutic treatments for the acute opiate withdrawal syndrome.