Cocaine self-administration behaviors in ClockΔ19 mice

Abstract

Rationale: A key role has been identified for the circadian locomotor output cycles kaput (Clock) gene in the regulation of drug reward. Mice bearing a dominant negative mutation in the Clock gene (ClockΔ19 mice) exhibit increased cocaine-induced conditioned place preference, reduced anxiety- and depression-like behavior, increased sensitivity to intracranial self-stimulation, and increased dopaminergic cell activity in the ventral tegmental area.

Objectives: We sought to determine if this hyperhedonic phenotype extends to cocaine self-administration and measures of motivation.

Methods: Two separate serial testing procedures were carried out (n = 7-10/genotype/schedule). Testing began with acquisition of sucrose pellet self-administration, implantation of intravenous catheter, acquisition of cocaine self-administration, and dose-response testing (fixed ratio or progressive ratio). To evaluate diurnal variations in acquisition behavior, these sessions occurred at Zeitgeber 2 (ZT2) or ZT14.

Results: WT and ClockΔ19 mice exhibited similar learning and readily acquired food self-administration at both ZT2 and ZT14. However, only ClockΔ19 mice acquired cocaine self-administration at ZT2. A greater percentage of ClockΔ19 mice reached acquisition criteria at ZT2 and ZT14. ClockΔ19 mice self-administered more cocaine than WT mice. Using fixed ratio and progressive ratio schedules of reinforcement dose-response paradigms, we found that cocaine is a more efficacious reinforcer in ClockΔ19 mice than in WT mice.

Conclusion: Our results demonstrate that the Clock gene plays an important role in cocaine reinforcement and that decreased CLOCK function increases vulnerability for cocaine use.

Fig. 1

Normal CLOCK function is not required for acquisition of responding for sucrose pellet reinforcers during the early phase of the light or dark cycle. a The percent of mice that reached acquisition criteria for each session (presented as a cumulative percentage). Open (ZT14 group) and closed (ZT2 group) circles denote ClockΔ19 mice, and open (ZT14 group) and closed (ZT2 group) squares denote WT mice. b Latency to acquire sucrose pellet self-administration at ZT2 and ZT14. Open bars denote ClockΔ19 mice and gray bars denote WT mice. c Lever discrimination (mean number of active and inactive lever presses) for the three consecutive sessions that enabled each subject to meet acquisition criteria. Open bars denote ClockΔ19 mice active lever presses, bars with vertical lines denote ClockΔ19 mice inactive lever presses, gray bars denote WT mice active lever presses, and checkered bars denote WT mice inactive lever presses. Open (inactive lever) and closed (active lever) circles denote ClockΔ19 mice, and open (inactive lever) and closed (active lever) squares denote WT mice. N=9–10/genotype/ZT

Fig. 2

ClockΔ19 mice exhibit a greater propensity to initiate cocaine use, reduced latency to acquire cocaine self-administration, and increased cocaine intake. a Percent of mice reaching acquisition criteria. Open (ZT2⇒14 group) and closed (ZT14 group) circles denote ClockΔ19 mice, and open (ZT2⇒14 group) and closed (ZT14 group) squares denote WT mice. Dashed vertical line between sessions 10 and 11 denotes the transition ZT2⇒14 (described in the “Materials and methods” section). b Cocaine infusions earned for ZT2 and ZT14 sessions. Open bars denote ClockΔ19 mice and gray bars denote WT mice. c Lever discrimination (mean number of active and inactive lever presses). Open bars denote ClockΔ19 mice active lever presses, bars with vertical lines denote ClockΔ19 mice inactive lever presses, gray bars denote WT mice active lever presses, and checkered bars denote WT mice inactive lever presses. ***p<0.001; ****p<0.0001. N=7–10/genotype/ZT

Fig. 3

Cocaine is a more efficacious reinforcer in ClockΔ19 mice than in WT mice. a Cocaine infusions earned in an FR1 descending dose–response schedule. b Dose–response lever discrimination (mean number of active and inactive lever presses). Open bars denote ClockΔ19 mice active lever presses, bars with vertical lines denote ClockΔ19 mice inactive lever presses, gray bars denote WT mice active lever presses, and checkered bars denote WT mice inactive lever presses. Open circles denote ClockΔ19 mice and gray squares denote WT mice. N=7/genotype

Fig. 4

ClockΔ19 mice displayed greater motivation to obtain cocaine infusions in a progressive ratio schedule of reinforcement. a Number of lever presses at which the breakpoint occurred. b Dose–response lever discrimination (mean number of active and inactive lever presses). Open bars denote ClockΔ19 mice and gray bars denote WT mice. *p<0.05; **p<0.01; ****p<0.0001. N=4–5/genotype