Low dose of morphine and the consumption of a sweetened ethanol solution: differential effects on acquisition and maintenance.

The influence of a low dose of morphine was investigated on the acquisition and maintenance of consumption of a sweetened ethanol solution with water as the alternative in a two-bottle choice procedure. During acquisition in Experiment 1, morphine failed to significantly increase the consumption of a sweetened ethanol solution compared to either a postinjection period in the same animals or a no-treatment control group. Although morphine significantly increased sweetened ethanol consumption when compared to a saline control group, this appears to be due to a stress response to the injections, which suppressed ethanol consumption in the saline animals. During maintenance in Experiment 2, morphine significantly increased consumption of sweetened ethanol in all groups compared to consumption following saline control injections. There was no difference in this effect among the three groups, suggesting that prior history with morphine was not a factor. In addition, rats that were exposed to morphine during both experiments drank significantly more sweetened ethanol following injections in Experiment 2 than in Experiment 1. This suggests that morphine's potentiation of ethanol consumption is due to its interaction with endogenous opioid receptors that modulate the reward value of ethanol rather than more general mechanisms affecting satiety or taste. The results of these experiments provide support for both the Deficit and Surfeit Hypotheses of ethanol consumption, both of which suggest that endogenous opioid receptors are responsible, in part, for ethanol's reinforcing properties.

Morphine enhances selection of both sucrose and ethanol in a two-bottle test.

The influence of a low dose of morphine on the self-selection of alcohol and sucrose solutions is investigated. When given a choice between sucrose sweetened ethanol and plain water, rats show a significant preference for the sweetened ethanol. However, when given a choice between sweetened ethanol and sweetened water, rats increase consumption of sweetened water. These results suggest that the low-dose morphine enhancement of sweetened alcohol solutions is mediated by the reinforcing properties of sucrose not ethanol. However, when rats receive small doses of morphine and a choice between unsweetened ethanol and water, the rats increase consumption of ethanol. Therefore, a low dose of morphine enhances the self-selection of both sucrose and ethanol solutions. This provides additional confirmation that opioids may enhance the rewarding properties of a variety of appetite reinforcers.

Opiates and alcohol self-administration in animals.

During the past 25 years, numerous animal studies have demonstrated a relationship between alcohol consumption and opiates. Converging lines of evidence suggest that (1) alcohol consumption enhances opioid receptor activity and (2) conditions associated with relative deficiencies in opioid receptor activity stimulate increases in alcohol preference. This evidence leads to the hypothesis that alcohol drinking is reinforced, in part, by enhanced opioid receptor activity; thus, these effects should be blocked by opiate antagonists. In fact, the animal data are consistent with this prediction. Opiate antagonists reduce excessive alcohol intake without reducing the ingestion of other biologically important reinforcers.

Alcohol drinking in rats during and following morphine injections.

Adult male Sprague-Dawley rats given daily SC injections of 2.5 mg/kg or 10 mg/kg morphine sulfate (for eight days) decreased preference for a 5% (v/v) ethanol solution, and then increased ethanol preference during the postinjection period, relative to a saline control group. These bidirectional effects were directly related to morphine dosage. The results were interpreted as supporting the theory that alcohol drinking is reinforced through its interaction with the endogenous opioid system and can compensate for deficiencies in opioid receptor activity.

The bidirectional effects of shock on alcohol preference in rats.

Rats given a choice between a 5% alcohol solution and water will dramatically increase alcohol preference on the days following experience with inescapable electric footshocks, compared with unshocked animals. Although, total alcohol preference did not differ during shock days, an interaction occurred between shock stress and alcohol preference. Rats that initially preferred alcohol decreased alcohol preference during shock days, whereas, rats that initially avoided alcohol increased alcohol preference during shock days. Therefore, the stress of inescapable electric footshock has bidirectional effects on alcohol preference. These bidirectional effects depend on the temporal dynamics of alcohol consumption in relation to the shock experience and the initial alcohol preference.

The influence of control over appetitive and aversive events on alcohol preference in rats.

The effect of control of food reinforcement or shock termination on alcohol drinking was examined in two experiments. In the first experiment, rats unable to control the delivery of food pellets preferred alcohol more than rats which had control over food. Similarly, in the second experiment, rats unable to control electric footshock termination preferred alcohol more than rats which could escape shock. These results showed that the psychological dimension of control over environmental events influences alcohol preference in rats.

Weight gain in postweaned rats as a function of predictable and unpredictable feeding schedules.

In two experiments rats given unpredictable feeding schedules gained less weight over a 24 or 30 day training period than rats which consumed an equivalent amount of food on a predictable schedule. These results suggest that irregular feeding schedules disrupt food utilization.