Enhanced locomotor, reinforcing, and neurochemical effects of cocaine in serotonin 5-hydroxytryptamine 2C receptor mutant mice

Abstract

Brain serotonin [5-hydroxytryptamine (5-HT)] systems substantially influence the effects of cocaine; however, the contributions of individual 5-HT receptor subtypes to the regulation of cocaine responses are unclear. A line of mutant mice devoid of 5-HT2C receptors was used to examine the contribution of this receptor subtype to the serotonergic modulation of cocaine responses. Mutants display enhanced exploration of a novel environment and increased sensitivity to the locomotor stimulant effects of cocaine. In an operant intravenous self-administration model under a progressive ratio schedule of reinforcement, mutants display elevated levels of lever pressing for cocaine injections, indicating that the drug is more reinforcing in these mice. Moreover, mutants exhibit enhanced cocaine-induced elevations of dopamine (DA) levels in the nucleus accumbens, a brain region implicated in the stimulant and rewarding properties of cocaine. In contrast, phenotypic differences in dorsal striatal DA levels were not produced by cocaine treatment. These findings strongly implicate 5-HT2C receptors in the serotonergic suppression of DA-mediated behavioral responses to cocaine and as a potential therapeutic target for cocaine abuse.

Fig. 1.

Novelty-induced locomotion and habituation.a, Distance traveled during the first 2 hr in the locomotor monitoring apparatus on days 1–4 for mutant (n = 62) and WT (n = 63) mice. A 2 × 4 ANOVA with day as a repeated measure demonstrated a significant effect of day (F_(3,121) = 130.2; p < 0.0001) and genotype (F_(1,123) = 50.9; p< 0.0001) and an interaction of day and genotype (F_(3,121) = 5.9; p= 0.001) for distance traveled in the 2 hr before injection. Also shown is distance traveled during 5 min bins for the first 2 hr of exposure to the activity monitoring apparatus for days 1–4 (b–e, respectively). In all figures, values represent mean ± SEM, with WT indicated by filled symbolsand mutant indicated by open symbols.

Fig. 2.

Effect of cocaine administration on locomotion.a, Distance traveled during the first hour after cocaine injection. A 2 × 4 ANOVA for dose and genotype revealed a significant effect of dose (F_(3,117) = 67.3; p < 0.0001) and genotype (F_(1,117) = 14.9; p< 0.001) but no interaction. Also shown is distance traveled during 5 min bins on day 4 with 0 (b), 7.5 (c), 15 (d), and 30 (e) mg/kg cocaine given by intraperitoneal injections at 120 min. The peak locomotion values for the 5-HT2C receptor mutants were significantly different from those of the WT mice (F_(1,117) = 8.2; p= 0.005) and increased significantly with dose (F_(3,117) = 85.7; p< 0.0001) without an interaction of dose and genotype.

Fig. 3.

Effect of cocaine administration on DA levels. DA dialysate concentrations were analyzed by mixed factorial repeated-measures ANOVA. Significant differences among individual means were confirmed by Fisher's post hoc tests. Baseline dialysate DA levels from the NAcc and the contralateral DStr of WT (n = 10; NAcc, 2.9 ± 0.5 nm; DStr, 4.2 ± 0.4 nm) and mutant (n= 9; NAcc, 3.8 ± 1.3 nm; DStr, 5.6 ± 0.8 nm) mice were not significantly different. The effect of cocaine (15 mg/kg; arrow) on dialysate DA levels from each region is thus expressed as the percentage change from baseline.a, Cocaine induced a significant increase in NAcc DA levels (F_(9,53) = 35.8;p < 0.0001) with a significant effect of genotype (F_(1,7) = 4.7; p < 0.05) as well as a significant genotype-by-time interaction (F_(9,153) = 3.6; p< 0.0005). Subsequent simple effects analyses revealed that NAcc DA levels were significantly higher in 5-HT2C mutant mice relative to WT controls at both the 10 and 20 min postcocaine time points (p < 0.05 at each time point) as denoted byasterisks. b, Cocaine also significantly elevated DStr DA levels over time (F_(9,153) = 35.8; p< 0.0001) but with no significant effect of genotype.

Fig. 4.

Cocaine self-administration, extinction, and food reinstatement. Values indicate reinforcers obtained by mutant and WT mice under a PR schedule during three consecutive sessions of stable responding (x-axis). Lefty-axes refer to the total number of reinforcers obtained, and righty-axes refer to the corresponding number of active lever presses (ratio completed) for delivery of each reinforcer. a, Cocaine self-administration: Mutants completed a ratio of ∼20 lever presses, compared with 10 by WT mice, and thus obtained significantly more cocaine injections. Repeated-measures ANOVA confirmed a significant effect of genotype for the number of cocaine injections (F_(1,19) = 7.5; p< 0.05) but no effect of session or session-by-genotype interaction.Asterisks indicate significant (p < 0.05) phenotypic differences.b, Extinction: When saline was substituted for cocaine, mutant and WT mice extinguished lever pressing at a comparable rate. Repeated-measures ANOVA confirmed a significant effect of session on the number of saline injections within subjects (F_(3,51) = 21.6; p< 0.0001) but no effect of genotype or session-by-genotype interaction. c, Reinstatement with nondrug reinforcer. After saline substitution, condensed milk was made available, and both groups reinstated lever-pressing behavior at a comparable rate. Repeated-measures ANOVA confirmed a significant effect of session within subjects on the number of milk reinforcers obtained (F_(3,72) = 25.3; p< 0.0005) but no effect of genotype or session-by-genotype interaction.