Role of dopamine D2-like receptors in cocaine self-administration: studies with D2 receptor mutant mice and novel D2 receptor antagonists

Abstract

Dopamine receptor subtypes have been classified generally as D1-like (e.g., D1, D5) or D2-like (D2, D3, D4), and converging evidence suggests that D2-like receptors may be especially important in mediating the abuse-related effects of cocaine. However, it has been difficult to differentiate the roles of the D2-like receptor subtypes in the behavioral effects of cocaine because of the relatively low selectivity of drugs for D2, D3, and D4 receptors in vivo. The goal of the present series of studies was to investigate the contributions of D2-like receptor subtypes in the reinforcing effects of cocaine using new genetic and pharmacological tools. First, we evaluated cocaine self-administration behavior, and related effects of nonselective D2-like drugs, in mutant mice that lack the D2 receptor but express D3 and D4 receptors. When high doses of cocaine on the descending limb of the cocaine dose-effect function were available, D2 mutant mice self-administered at higher rates than their heterozygous or wild-type littermates, but the ascending limb of the cocaine dose-effect function did not differ between genotypes. Elevated rates of drug intake were not attributable to nonspecific increases in response rate, because response rates maintained by presentation of a range of food concentrations were significantly lower in D2 mutant mice than in wild-type mice. In wild-type mice, pretreatment with the D2-like antagonist eticlopride increased rates of self-administration of high doses of cocaine, and the D2-like agonist quinelorane served as a positive reinforcer when substituted for cocaine. However, these effects of eticlopride and quinelorane were not observed in mice that lacked the D2 receptor. Next, we compared the effects of novel antagonists selective for different D2 receptor subtypes on cocaine self-administration behavior in outbred rats. In rats, a D2 selective antagonist increased rates of self-administration of high doses of cocaine and also combinations of cocaine and the D2-like agonist quinelorane, whereas D3/D4 antagonists were ineffective. Collectively, these findings suggest that the D2 receptor is not necessary for cocaine self-administration, but this receptor subtype is involved in mechanisms that limit rates of high-dose cocaine self-administration. Our results also suggest that D3 and D4 receptors do not play major roles in the modulation of cocaine self-administration by D2-like drugs.

Fig. 1.

Behavior maintained by liquid food in wild-type mice (left panels, open symbols), heterozygous mutant mice (middle panels, hatched symbols), and homozygous mutant mice that lack the dopamine D₂ receptor (right panels, filled symbols). The top panels show the number of food reinforcers delivered when 100% liquid food was available. The bar on the right side of each top panel shows the mean number of reinforcers per hour in each group of mice. The left side of each top panel shows a representative cumulative record for the first hour of a session from one mouse in each group. For cumulative records, the tracing moved to the right with the passage of time and incremented upward each time a response was emitted. Angled tick marks on the cumulative record indicate the delivery of a reinforcer (under an FR 1 schedule). Thebottom panels show the mean number of food reinforcers per hour as a function of the concentration of liquid food in water. Bars over symbols depict SEM. Asterisks indicate that the number of food reinforcers earned was significantly lower in mutant mice than in wild-type mice by pairwise comparisons following overall main effect by ANOVA (n = 14–17; *p < 0.05; **p < 0.01).

Fig. 2.

Behavior maintained by intravenous cocaine injections in wild-type mice (left panels, open symbols), heterozygous mutant mice (middle panels, hatched symbols), and homozygous mutant mice that lack the dopamine D₂ receptor (right panels, filled symbols). The top panels show the number of injections delivered when 1.0 mg/kg per injection of cocaine was available. The bar on the right side of each top panel shows the mean number of injections per hour in each group of mice. The left side of each top panel shows a representative cumulative response record for the first hour of a session from one mouse in each group. Bars over symbols depict SEM. In top left, center andright panels, respectively, mean data are shown from groups of 16, 17, and 14 mice after baseline criteria for cocaine self-administration were met. In bottom left, center,and right panels, respectively, mean data are shown from up to 13, 15, and 11 mice that were tested with various cocaine doses.Asterisks indicate that the number of cocaine injections earned was significantly higher in mutant mice than in wild-type mice by pairwise comparisons following overall main effect by ANOVA (analyzed within-subjects in groups of 6, 12, and 7 mice that were tested with every dose of cocaine; **p < 0.01).

Fig. 3.

Behavior maintained by intravenous cocaine injections (1.0 mg/kg per injection) in wild-type mice (+/+), heterozygous (+/−) mice, and homozygous (−/−) mutant mice that lack the dopamine D₂ receptor. Abscissa, Genotype. Ordinate, Mean number of cocaine injections earned per hour. Bars over symbols depict SEM. There were no statistically significant gender differences in cocaine self-administration baseline values.Asterisks indicate that the number of cocaine injections was significantly higher in mutant mice than in wild-type mice by pairwise comparisons following overall main effect by ANOVA (n = 5, 6, 5 for wild-type, heterozygous, and homozygous female mice, respectively, and n = 11, 11, 9 for wild-type, heterozygous, and homozygous male mice, respectively; **p < 0.01).

Fig. 4.

Behavior maintained by intravenous cocaine injections in C57BL/6 mice after pretreatment with the D2-like antagonist eticlopride (filled symbols) or vehicle (open symbols). Abscissa, Pretreatment dose of eticlopride (left panel) or unit dose of cocaine per injection (right panel). In the left panel, the unit dose of cocaine was 1.0 mg/kg per injection. In the right panel, the pretreatment dose of eticlopride was 0.18 mg/kg, given intraperitoneally. Ordinate, Mean number of cocaine injections earned per hour. Bars over symbols depict SEM.Asterisks indicate that the number of cocaine injections earned was significantly higher after pretreatment with eticlopride by pairwise comparisons with vehicle pretreatment following overall main effect by ANOVA (n = 5-6; ∗∗p < 0.01).

Fig. 5.

Effects of the D2-like antagonist eticlopride on cocaine self-administration (top panels) and reinforcing effects of the D2-like agonist quinelorane (bottom panels), in wild-type mice (left panels), heterozygous mutant mice (middle panels), and homozygous mutant mice that lack the dopamine D₂ receptor (right panels). Top panels, The bars on the right side of each top panel show the mean number of cocaine injections per hour after vehicle pretreatment or after pretreatment with 0.18 mg/kg eticlopride. The left sideof each top panel shows representative cumulative records for the first hour after vehicle (solid lines) or eticlopride pretreatment (hatched lines) from one mouse in each group. The pretreatment dose of eticlopride was usually 0.18 mg/kg, as shown in cumulative records for wild-type and heterozygous mice. In several D2 mutant mice, a tenfold higher dose of eticlopride was also tested (1.8 mg/kg, shown in cumulative record attop right, mouse #66). Bars over symbols depict SEM. Asterisks indicate that the number of cocaine injections earned was significantly higher after pretreatment with eticlopride by pairwise comparisons with vehicle pretreatment following overall main effect by ANOVA (n = 10, 13, and 8, for wild-type, heterozygous, and homozygous mice, respectively; **p < 0.01). Bottom panels,Abscissa, Unit dose of quinelorane. S indicates saline injections. Ordinate, Mean number of quinelorane injections earned per hour. Asterisks indicate that the number of quinelorane injections earned was significantly higher than the number of saline injections earned by pairwise comparisons following overall main effect by ANOVA; (n = 4, 4, 8, for wild-type, heterozygous, and homozygous mice, respectively; *p< 0.05, **p < 0.01).

Fig. 6.

Effects of novel D2-like antagonists on the self-administration of cocaine or cocaine combined with the D2-like agonist quinelorane in male Sprague Dawley rats. Abscissa, Pretreatment dose of novel D2-like antagonists (top and bottom rows) or unit dose of cocaine per injection (center row). In the top and bottom rows, the unit dose of cocaine was 1.0 mg/kg per injection, and in the bottom row this dose of cocaine was combined with 1.0 μg/kg per injection of quinelorane (hatched and filled bars). In the center row, the pretreatment dose of novel D2-like antagonists was 5.6 mg/kg, given intraperitoneally. Ordinate, Mean total number of drug injections earned in 2 hr (top andbottom rows) or 20 min (center row). Bars over symbols depict SEM. Asterisks indicate that the number of drug injections earned was significantly different after pretreatment with the novel D2-like antagonist by pairwise comparisons with vehicle pretreatment following overall main effect by ANOVA (n = 8, 6, and 5, for top, middle,and bottom rows, respectively; *p < 0.05, **p < 0.01).