The influence of genotype on the rewarding and locomotor activating effects of morphine and ethanol was examined in the place conditioning paradigm. Two inbred mouse strains (C57BL/6J and DBA/2J) were exposed to a differential conditioning procedure in which each mouse received four pairings of a distinctive floor stimulus with IP injection of morphine (0, 2.5, 5 or 10 mg/kg) or ethanol (0, 1, 2, 3 or 4 g/kg). A different floor stimulus was paired with saline. Conditioning trials lasted 30 min and each experiment concluded with a floor preference test in the absence of drug. In accord with previous studies, morphine evoked a dose-dependent increase in activity during conditioning that was greater in C57BL/6J mice than in DBA/2J mice. In contrast, ethanol produced a dose-dependent increase in activity that was greater in DBA/2J than in C57BL/6J mice. Both strains showed conditioned place preference with morphine, but only the DBA/2J strain showed conditioned place preference with ethanol. No conditioned place aversion was seen. With both drugs, stronger place preference conditioning was obtained in DBA/2J mice, supporting the general conclusion that sensitivity to drug reward is influenced by genotype. The fact that the same genotype is more sensitive to the rewarding effects of two different drugs supports theories postulating commonality in the biological mechanisms of drug reward. Although the outcome of the ethanol study supports predictions of the psychomotor stimulant theory of addiction concerning the relationship between drug-induced activation and reward, the outcome of the morphine study does not.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol-induced locomotor stimulation has been proposed to be positively correlated with the rewarding effects of ethanol (Wise and Bozarth 1987). The present experiments provided a test of this hypothesis using a genetic model. Three behavioral indices of the motivational effects of ethanol (drinking, taste conditioning, place conditioning) were examined in mice from two independent FAST lines, selectively bred for sensitivity to ethanol-induced locomotor stimulation, and mice from two independent SLOW lines, selectively bred for insensitivity to ethanol-induced locomotor stimulation. In a single-bottle procedure, mice were allowed access to drinking tubes containing ethanol in a concentration (1-12% v/v) that increased over 24 consecutive days. FAST mice consumed greater amounts of ethanol solution. In a two-bottle procedure, mice were allowed access to tubes containing water or various concentrations of ethanol (2-8% v/v) over 6 days. FAST mice generally showed greater preference for ethanol solutions than SLOW mice. In a conditioned taste aversion procedure, mice received access to saccharin solution followed by injection of 2.5 g/kg ethanol (IP). SLOW mice developed aversion to the saccharin flavor more readily than FAST mice. In a series of place conditioning experiments, tactile stimuli were paired with various doses of ethanol (0.8-2.0 g/kg). During conditioning, FAST mice showed locomotor stimulation after 1.0, 1.2 and 2.0 g/kg ethanol while SLOW mice did not. During testing, mice conditioned with 1.2 g/kg and 2.0 g/kg ethanol showed conditioned place preference, but there were no line differences in magnitude of preference. These results indicate that genetic selection for sensitivity to ethanol-stimulated activity has resulted in genetic differences in ethanol drinking and ethanol-induced conditioned taste aversion but not ethanol-induced conditioned place preference. Overall, these data provide mixed support for the psychomotor stimulant theory of addiction.

Two experiments examined the effect of ambient temperature during ethanol exposure on development of conditioned taste aversion to saccharin. In both studies, male albino rats receiving saccharin-ethanol (1.5 g/kg, IP) pairings followed by 6-h exposure to a 32 degrees C environment developed a weaker saccharin aversion than did rats experiencing ethanol at room temperature. Exposure to the warm environment reduced ethanol-induced hypothermia, but enhanced ethanol's motor-impairing effect. The influence of ambient temperature on ethanol-induced taste aversion may be due to changes in body temperature, neural sensitivity, or elimination rate. Although alternative accounts cannot be entirely dismissed, this outcome suggests that ethanol-induced hypothermia plays a role in determining strength of conditioned taste aversion and thus may be involved in the regulation of oral ethanol intake in rats.

Mice selectively bred for sensitivity (COLD) or insensitivity (HOT) to the hypothermic effect of ethanol were tested in three tasks purported to assess ethanol's hedonic properties: place conditioning, taste conditioning, and ethanol drinking. In the place conditioning task, distinctive tactile (floor) stimuli were differentially paired with injection of ethanol (2.25 g/kg) or saline, and preference for the tactile stimuli was assessed during a choice test without ethanol. In the taste conditioning task, fluid-deprived mice were given repeated access to saccharin followed by injection of ethanol (2.25 g/kg). In the drinking task, mice were given access on alternate days to a single drinking tube containing water or ethanol in a concentration that gradually increased from 1 to 12% (v/v) over days. HOT mice showed greater conditioned preference for ethanol-paired flavor cues, and greater aversion for ethanol-paired flavor cues, and drank less ethanol at concentrations above 5% than COLD mice. HOT mice also showed higher levels of ethanol-stimulated activity than COLD mice. Control experiments indicated that the lines did not differ in initial preference for the tactile and flavor stimuli used in the conditioning tasks. Because the same line differences were seen in mice selected from two genetically independent populations, these studies offer strong evidence of genetic correlations between ethanol's thermal effect and its effect on activity, place conditioning and taste conditioning. Evidence of a genetic correlation between ethanol's thermal effect and ethanol drinking, however, is weaker since it is based on a line difference observed in only one of the genetic replicates. (ABSTRACT TRUNCATED AT 250 WORDS)

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