Peripheral Opioid Receptors May Mediate a Portion of the Aversive and Depressant Effect of EtOH: CPP and Locomotor Activity

Abstract

Previous investigators have reported that peripheral opioid receptors (located in the gut) may produce aversive effects when activated. Opioid receptors can be activated by endogenous opioids or by-products of ethanol (EtOH) metabolism [e.g., tetrahydroisoquinolines (TIQs)]; both are stimulated following EtOH consumption. Therefore, we tested the hypothesis that a portion of the aversive or depressant effects of EtOH may be mediated through, or modulated by, peripheral opioid receptors. Conditioned place preference (CPP) and locomotor activity were the dependent variables. Prior to EtOH gavage, we antagonized the peripheral opioid receptors with methylnaltrexone (MNTX), an opioid antagonist that does not easily pass through the blood–brain barrier. The effects of EtOH were found to be dose dependent: 1.5 g/kg was hedonically neutral but depressed locomotor activity; 2.25 g/kg EtOH was aversive and also depressed locomotor activity. MNTX (32 mg/kg) treatment was rewarding and stimulated motor activity (especially during the first conditioning session). When combined, 1.5 g/kg EtOH tended to enhance the rewarding effects of MNTX whereas MNTX blocked the aversive effects of 2.25 g/kg EtOH. During the first conditioning session EtOH attenuated the motor stimulant effects of MNTX whereas MNTX antagonized the motor depressant effects of EtOH; there was little effect of MNTX on EtOH-induced motor depression during subsequent conditioning sessions. Pretreatment with various doses of MNTX (0.1, 1.0, 10.0, 32.0 mg/kg) of rats receiving 1.5 g/kg EtOH indicated the effects of MNTX were dose dependent. Drug-induced locomotor activity and time spent in the conditioned compartment were positively correlated, suggesting that both behaviors were homologous. The data suggest that peripheral opioid receptors participate in mediating or modulating a portion of the behavioral effects of EtOH.