Low doses of morphine reduce voluntary alcohol consumption in rhesus monkeys

Endogenous opioids and alcohol dependence: opioid alkaloids and the propensity to drink alcoholic beverages

Rats consume alcoholic beverages in a wide variety of circumstances. Opioid antagonists, naloxone and naltrexone, decrease intake of many ingesta, including alcoholic beverages. Small doses of morphine increase intake of alcoholic beverages. Further, the effects of small doses of morphine are persistent and there is no sign that tolerance to morphine's ability to increase alcohol intake develops as seen with morphine's ability to produce analgesia. Morphine's effects can combine with other variables that enhance intake of alcoholic beverages to produce very large daily intakes of ethanol. These generalizations, from a large number of separate experiments, support the conclusion that alcoholism is a special case of an ingestive disorder involving opioidergic systems.

Endorphins and experimental addiction

Animal studies suggest that the endogenous opioid systems in the brain play an important role in the initiation and maintenance of drug dependence. Opioids in the ventral tegmental area (VTA) may be involved in rewarded behaviors and, consequently, in the initiation of drug self-administration that may be associated with addiction proneness. Opioids in the limbic forebrain are particularly implicated in subsequent drug self-administration, which may be associated with craving, maintenance, and relapse. Alcohol intake in monkeys is reduced after treatment with naltrexone in a graded, dose-dependent manner. Naltrexone also is associated with a greater decrease in alcohol consumption after imposed abstinence. These findings support the idea that endorphins play a role in alcohol-drinking behavior, particularly after a period of abstinence during the so-called catch-up phenomenon. Recent studies of recovering alcoholic patients provide evidence that opiate antagonists attenuate the craving for alcohol and decrease and/or postpone relapse into addictive behavior.

Neuropeptides and alcohol addiction in monkeys

Neuropeptides have been implicated in experimental drug addiction. Desglycinamide (Arg8) vasopressin (DGAVP) attenuates heroin and cocaine intake during initiation of drug self-administration in rats. beta-Endorphin is self-administered in rats and a role of endogenous opioids in cocaine reward has been proposed. The present studies deal with voluntary alcohol consumption in monkeys under free choice conditions. Monkeys initiated alcohol drinking within a few days and after a stable drinking pattern was acquired increased their ethanol consumption during a short period following interruption of the alcohol supply (relapse). The alcohol drinking behavior seems under the control of reinforcement principles. DGAVP reduced the acquisition of alcohol drinking in the majority of treated monkeys. Initiation of alcohol drinking induced modifications in neuroendocrine homeostasis e.g. an increased plasma beta-endorphin. Both the opioid antagonist naltrexone and the opioid agonist morphine dose-dependently decreased alcohol intake during continuous supply and after imposed abstinence. The monkeys were more sensitive to both drugs after imposed abstinence. The effects are interpreted in the context of the endorphin compensation hypothesis of addictive behavior. It is suggested that endorphins may be particularly implicated in craving for addictive drugs and in relapse of addictive behavior.