Lack of self-administration of cocaine in dopamine D1 receptor knock-out mice

Abstract

Evidence suggests a critical role for dopamine in the reinforcing effects of cocaine in rats and primates. However, self-administration has been less often studied in the mouse species, and, to date, "knock-out" of individual dopamine-related genes in mice has not been reported to reduce the reinforcing effects of cocaine. We studied the dopamine D1 receptor and cocaine self-administration in mice using a combination of gene-targeted mutation and pharmacological tools. Two cohorts with varied breeding and experimental histories were tested, and, in both cohorts, there was a significant decrease in the number of D1 receptor knock-out mice that met criteria for acquisition of cocaine self-administration (2 of 23) relative to wild-type mice (27 of 32). After extinction of responding with saline self-administration, dose-response studies showed that cocaine reliably and dose dependently maintained responding greater than saline in all wild-type mice but in none of the D1 receptor knock-out mice. The D1-like agonist SKF 82958 (2,3,4,5,-tetrahydro-6-chloro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrobromide) and the D2-like agonist quinelorane both functioned as positive reinforcers in wild-type mice but not in D1 receptor mutant mice, whereas food and intravenous injections of the opioid agonist remifentanil functioned as positive reinforcers in both genotypes. Finally, pretreatment with the D1-like antagonist SCH 23390 [R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-01] produced surmountable antagonism of the reinforcing effects of cocaine in the commonly used strain C57BL/6J. We conclude that D1 receptor knock-out mice do not reliably self-administer cocaine and that the D1 receptor is critical for the reinforcing effects of cocaine and other dopamine agonists, but not food or opioids, in mice.

Figure 1.

Summary of data for acquisition of cocaine self-administration (0.5 or 1.0 mg/kg per injection). Abscissa, Experimental history including operant naive mice (left pair of bars) and mice that had previous operant training with food reinforcement (right pair of bars). Ordinate, Percentage of mice that met criteria for acquisition of cocaine self-administration under the FR 1 schedule. Criteria for acquisition were stable levels of responding (within 20% across 2 consecutive sessions), a minimum of 20 active nose pokes, and at least 70% of nose pokes in the active hole. Numbers at top of each bar indicate number of mice that met criteria/group size. Asterisks indicate statistical significance (*p < 0.05, ***p < 0.001; all data analyzed by χ² test for genotype). The cocaine dose for all data shown was 1.0 mg/kg per injection.

Figure 2.

Cocaine self-administration in mice previously trained with food reinforcement: acquisition and effect of cocaine dose. Abscissas, Session number (top row) or cocaine dose in milligrams per kilogram per injection (bottom row); points above zero depict data from saline self-administration. Ordinates, Nose pokes (top row) and cocaine injections self-administered (bottom row) in 3 h sessions under the FR 1 schedule. Cocaine dose was 1.0 mg/kg per injection during acquisition (top row). Filled symbols show data for wild-type (+/+) mice, and open symbols show data for mutant (−/−) mice. Squares show data for reinforced nose pokes (active hole), and diamonds show data for nonreinforced nose pokes (inactive hole). In top left, group size decreases with increasing session number as acquisition tests ceased for each mouse as soon as they met criteria for self-administration. After acquisition tests, 2 of the 20 wild-type mice did not complete dose–effect determinations because of catheter failure or illness (bottom left). After acquisition tests, half of the mutant mice were tested with various cocaine doses in consecutive sessions (bottom right), and the other half were tested with food and cocaine reinforcement alternately (see Fig. 4). All symbols depict group means and SEM. Asterisks indicate significantly different from saline self-administration by pairwise comparison (*p < 0.05, **p < 0.01) following significant main effect of cocaine dose by ANOVA.

Figure 3.

Nose-poke behavior reinforced with liquid food: acquisition and effect of food concentration. Abscissas, Session number (top row) or liquid food concentration (bottom row). Ordinates, Nose pokes in 2 h sessions under the FR 1 schedule. Liquid food concentration was 100% during acquisition (top row). Filled symbols show data for wild-type (+/+) mice, and open symbols show data for mutant (−/−) mice. Squares show data for reinforced nose pokes (active hole), and diamonds show data for nonreinforced nose pokes (inactive hole). Circles and triangles show data for a second determination of the concentration–effect curve for reinforced and nonreinforced nose pokes, respectively. All symbols depict group means and SEM. Significant main effects included genotype, session number, hole selection, food concentration, and multiple main interactions (for details, see Results).

Figure 4.

Alternating daily test sessions with either cocaine or food as the reinforcer in groups of D₁ receptor mutant (−/−) mice. Abscissas, Session number (top row) or reinforcers available (bottom row). Ordinates, Reinforced nose-poke responses per session under the FR 1 schedule (top row) or percentage of mutant mice that met criteria for acquisition of responding (bottom row). Food concentration was 100% in all panels and is depicted in dark gray bars, and cocaine was 0.32, 1.0, or 3.2 mg/kg per injection and is depicted in lighter gray bars. Numbers at top of each bar indicate number of mice that met criteria/group size. Asterisks indicate statistical significance (*p < 0.05, **p < 0.01; all data analyzed by χ² test for reinforcer type, i.e., food or cocaine).

Figure 5.

Operant behavior in representative individual mice. Abscissas, Time within a session (top and middle) or cocaine dose in milligrams per kilogram per injection (bottom). Ordinates, Cumulative number of liquid food deliveries (top) or cocaine injections (middle) within a session, or total cocaine injections earned in a 3 h session under the FR 1 schedule (bottom). Each slanted tick mark indicates a reinforced nose-poke response (top and middle). Bold cumulative records and filled squares show data from a wild-type mouse, and thin cumulative records and open squares show data from a mutant mouse. Top and middle show data from single sessions, and bottom shows data averaged from two determinations for each dose of cocaine for each mouse. Mouse #30 was representative of the group of wild-type mice, whereas mouse #40 was representative of those two mutant mice that initially met criteria for cocaine self-administration but did not self-administer any dose of cocaine at rates profoundly higher than saline. Nonreinforced (inactive) nose pokes were extremely low throughout the tests (e.g., 3 inactive nose pokes for mouse #30 and 1 inactive nose poke for mouse #40 during the tests shown in the middle).

Figure 6.

Self-administration of the D₁-selective agonist SKF 82958 and the D₂-selective agonist quinelorane in wild-type (+/+) or D₁ receptor mutant (−/−) mice. Abscissas, Training history (left column) or unit dose in micrograms per kilogram per injection of SKF 82958 (top right) or in milligrams per kilogram per injection of quinelorane (bottom right); points above zero depict data from saline self-administration. Ordinates, Percentage of mice that met criteria for self-administration of direct dopamine agonists (left top, 10 μg/kg per injection of SKF 82958; bottom left, 0.32 mg/kg per injection of quinelorane) or number of injections per 3 h session under the FR 1 schedule (right column). Numbers at top of each bar in left column indicate number of mice that met criteria/group size. All symbols in right column depict group means and SEM. Asterisks in left column indicate statistical significance (*p < 0.05, **p < 0.01) analyzed by Mann–Whitney U test for genotype. Asterisks in right column indicate significantly different from saline self-administration by pairwise comparison (*p < 0.05, **p < 0.01) following significant main effect of unit dose by ANOVA. Some D₁ receptor mutant (−/−) mice were also given access to several doses of each dopamine agonist but none of them met criteria for self-administration (data not shown).

Figure 7.

Self-administration of the opioid agonist remifentanil in wild-type (+/+) or D₁ receptor mutant (−/−) mice. Abscissas, Dose of remifentanil self-administered (0.01 mg/kg per injection or zero, i.e., saline). Ordinates, Injections per 3 h session under the FR 1 schedule. All bars indicate group means and SEM. Asterisks indicate significant main effect of dose by ANOVA (**p < 0.01).

Figure 8.

Self-administration of cocaine after pretreatment with vehicle or the D₁-selective antagonist SCH 23390 in C57BL/6J mice. Abscissas, Dose of SCH 23390 administered before self-administration of 1.0 mg/kg per injection of cocaine (left) or dose of cocaine (milligram per kilogram per injection) self-administered after pretreatment with vehicle or 0.18 mg/kg of the D₁-selective antagonist SCH 23390 (right). Symbols above zero depict data from vehicle pretreatment (left) or saline self-administration (right). Ordinates, Cocaine injections per 2 h session. All symbols depict group means and SEM. Asterisks indicate significantly different from vehicle pretreatment by pairwise comparison (**p < 0.01) following significant main effect of SCH 23390 dose (left) or a SCH 23390 × cocaine dose interaction (right) by ANOVA.