Opioid receptors and the discriminative stimulus effects of ethanol in squirrel monkeys Mu and delta opioid receptor mechanisms

Mu and delta plod receptors modulate the reinforcing effects of ethanol however their role in the subjective effects of ethanol is not well understood This study evaluated the contribution of mu and delta opioid receptors to the subjective effects of ethanol using drug discrimination procedures Monkeys were trained to discriminate ethanol from saline under a schedule ol food delivery In tests ethanol engendered increases in drug-lever responding reaching a maximum of >80% The mu opioid receptor agonists fentanyl and buprenorphine and the delta opioid receptor agonists SNC 80 and SNC 162 did not substitute for the discriminative stimulus effects of ethanol As pretreatments the full agonists fentanyl and SNC 80 enhanced the effects of low doses of ethanol and fentanyl attenuated the effects of the ethanol training dose Although the possibility of pharmacological antagonism of the effects of ethanol cannot be ruled out, a more likely alternative is that the diminished effects of ethanol were due to perceptual masking of the ethanol stimulus In contrast the partial agonists buprenorphine and SNC 162 did not alter ethanol s effects Finally the discriminative stimulus effects of ethanol were attenuated following administration of presumably mu-selective doses of the antagonist naltrexone but not after administration of the delta opioid receptor antagonist naltrindole The ability of naltrexone to block the discriminative stimulus effects of ethanol likely reflects its capacity to attenuate ethanol-induced increases in endogenous opioids in particular beta-endorphin because attenuation of the ethanol stimulus was not accompanied by significant suppression of response rate (C) 2010 Elsevier B V All rights reserved