Indications of an opioid mechanism in TIQ-induced alcohol drinking : studies in the food-restricted and food-satiated rat

The purpose of this series of experiments was to examine the preference for alcohol in the food-restricted and food-satiated rat using two different methods to induce alcohol drinking. One group of rats was trained to drink alcohol using the procedure of food-induced drinking (FID); the other rats were infused intracerebrally either with tetrahydropapaveroline (THP) in a dose of 1.0 µg, 100 ng or 10 ng or with artificial cerebrospinal fluid. In each group of experimental animals the self-selection of alcohol was tested over a series of ascending concentrations, ranging from 3 to 30%.

When ad lib. food was restored to the food-restricted rat, the intake of alcohol declined substantially in the FID rat, an effect which was less pronounced in the THP-treated rat. A diurnal variation in the 24-hour pattern of alcohol intake was observed in the FID but not the THP-treated rat.

The intracerebroventricular infusion of 100 ng or 10 ng of THP enhanced alcohol drinking, whereas the 1.0 µg dose did not alter alcohol preference. The same dose-dependent effect of THP on alcohol self-selection occurred when this alkaloid was infused into the hippocampus. It was also shown that THP somewhat selectively augmented preference for alcohol in the middle range (i.e., 9-15%) of concentrations offered to the rat. These concentrations were consistently rejected by the animal prior to THP treatment.

The results of the present experiments indicate that the FID rat drank alcohol for the calories provided by this fluid. There was no evidence that the food-satiated, THP-infused rat consumed alcohol for its caloric value. Since the hippocampus, a structure which is known to contain opiate receptors, was shown to be sensitive to the effect of THP, these findings suggest that an opiate mechanism may mediate the effect of THP to enhance alcohol self-selection.