Endogenous opioids and smoking: a review of progress and problems.

The present report examines efforts to elucidate the role of opioid mechanisms in the reinforcement of smoking. A number of approaches have been used to evaluate nicotine-opioid interactions. Opiate agonists such as heroin or methadone have been found to increase cigarette smoking reliably in humans, and morphine has been shown to increase the potency and efficacy of nicotine in rats. There is also an extensive literature documenting the nicotine-stimulated release of endogenous opioids in various brain regions involved in the mediation of opiate reinforcement. Blockade studies using opioid antagonists have not been as conclusive. Although animal models have demonstrated commonalities between nicotine withdrawal and the opiate abstinence syndrome, including reversibility by morphine, and although the impact of nicotine on certain response systems such as respiratory reflexes has clearly been shown to involve opioid mediation, attempts to demonstrate opioid modulation for the key indicators of smoking reinforcement--cigarette consumption and nicotine self-administration--have been fraught with difficulty. Resolution of the apparent contradictions will require taking into account: (a) the biphasic properties of nicotine-opioid effects at higher doses and anti-opioid effects at lower doses; (b) the contributions of the opioid receptor populations--mu, kappa, sigma--stimulated at various dose levels; (c) the possibility that endogenous-opioid activity is entrained primarily during the acquisition or re-acquisition of nicotine self-administration; (d) the possibility that the endogenous opioid response does contribute to nicotine reinforcement but only as a delimited component of the neuroregulatory cascade of nicotine; and (e) the possibility that opioids contribute primarily to nicotine reinforcement under special conditions such as stress. Taking these considerations into account should allow studies on endogenous opioid effects to begin to do justice to the complexity of both smoking behavior and the actions of nicotine.