Motivational effects of ethanol in DARPP-32 knock-out mice

Abstract

DARPP-32 (dopamine and adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa) is an important component of dopaminergic function in brain areas thought to be important for drug and alcohol addiction. The present experiments characterized the acquisition of ethanol-induced conditioned taste aversion, ethanol-induced conditioned place preference, and ethanol self-administration in DARPP-32 knock-out (KO) mice compared to wild-type (WT) controls. For taste conditioning, KO and WT mice received access to 0.2 m NaCl solution followed immediately by intraperitoneal injection of 0-4 gm/kg ethanol. Ethanol produced dose-dependent conditioned taste aversion that was the same in both genotypes. For place conditioning, KO and WT mice received eight pairings of a tactile stimulus with ethanol (2 gm/kg, i.p.), and a different stimulus with saline. Ethanol produced increases in locomotor activity during conditioning, with KO mice showing higher activity levels after ethanol compared to WT mice. WT mice, but not KO mice, acquired conditioned preference for the ethanol-paired stimulus. In the self-administration procedure, KO and WT mice were trained to lever press for access to 10% v/v ethanol. Subsequently, the mice had 23 hr/d access to food, ethanol, and water. Response patterns were determined using 0-30% v/v ethanol concentrations. WT mice displayed concentration-dependent responding for ethanol. Responding on the ethanol lever by KO mice did not change as a function of ethanol concentration. Saccharin (0.2% w/v) was subsequently added to the ethanol mixture, and responding was examined at 0, 5, 10, and 20% ethanol concentrations. Ethanol responding increased in both genotypes, although WT mice showed higher rates at all concentrations.

Fig. 1.

Mean (±SEM) NaCl intakes (in milliliters) for DARPP-32 KO mice and WT mice during the taste-conditioning study. On trials 1–4 (conditioning), after 60 min access to 0.2 mNaCl, groups received either saline or ethanol (2 gm/kg or 4 gm/kg, i.p.). On trial 5, groups received a final 60 min access period to NaCl. Trials were conducted every 48 hr, and subjects received 2 hr access to water between trials.

Fig. 2.

Mean (±SEM) activity counts per minute for DARPP-32 KO mice and WT mice during place-conditioning trials. On CS+ trials groups received ethanol (2 gm/kg, i.p.). On CS+ trials the same groups received saline. Immediately after injections, subjects were placed in the conditioning chambers for a 5 min trial. CS+/CS− trials were counterbalanced for order of presentation.

Fig. 3.

Mean (±SEM) seconds per minute spent on the grid floor during floor choice testing for DARPP-32 KO mice and WT mice. Test 1 occurred after four conditioning trials, test 2 after two additional conditioning trials, and test 3 after two more additional conditioning trials. Grid+ groups had previously received pairings of the grid floor with drug treatment (and hole floor with saline), whereas Grid− groups had previously received pairings of the grid floor with saline (and hole floor with drug treatment). Conditioned place preference (*p < 0.05) is shown when time spent on grid floor by the Grid+ group exceeds time spent on the grid floor by the Grid− group.

Fig. 4.

Mean (±SEM) number of ethanol lever responses per 23 hr session (left panel), food lever responses (middle panel), and milliliters of water intake (right panel) during phase 1 for DARPP-32 KO mice (n = 7) and WT mice (n = 8). Ethanol was presented on an FR4 schedule of reinforcement. Food was presented on an FR1 schedule of reinforcement. Each subject's data was averaged into four session blocks.

Fig. 5.

Mean (±SEM) number of ethanol lever responses per session (left panel), food lever responses (middle panel), and milliliters of water intake (right panel) during phase 2 for DARPP-32 KO mice (n = 7) and WT mice (n = 8). Ethanol was presented on an FR4 schedule of reinforcement. Food was presented on an FR1 schedule of reinforcement. Subjects were presented with each ethanol concentration (% v/v) for four consecutive 23 hr sessions.

Fig. 6.

Mean (±SEM) number of ethanol lever responses per session (left panel), food lever responses (middle panel), and milliliters of water intake (right panel) during phase 3 for DARPP-32 KO mice (n = 7) and WT mice (n = 8). Ethanol was presented on an FR4 schedule of reinforcement. Food was presented on an FR1 schedule of reinforcement. Subjects were presented with each ethanol concentration (% v/v) for four consecutive 23 hr sessions.