Effect of baclofen on cocaine self-administration in rats reinforced under fixed-ratio 1 and progressive-ratio schedules.

RATIONALE

Recent reports have indicated that the gamma-aminobutyric acid (GABA)B agonist baclofen attenuates the reinforcing effects of cocaine.

OBJECTIVES

To further evaluate the effect of baclofen on cocaine self-administration under a fixed ratio (FR) and progressive ratio (PR) schedule of reinforcement.

METHODS

In the first series of experiments, three dose-response curves were generated that examined the effect of three doses of baclofen (1.8, 3.2, or 5.6 mg/kg, i.p.) against four unit-injection doses of cocaine (0.19, 0.38, 0.75, and 1.5 mg/kg per injection) reinforced under a FRI schedule. For comparison, an additional group of rats was pretreated with haloperidol (32, 56, or 100 microg/kg, i.p.). A separate experiment examined the effect of baclofen (1.8, 3.2, or 5.6 mg/kg, i.p.) on responding for concurrently available cocaine or food reinforcement.

RESULTS

Under the FR1 schedule, baclofen suppressed intake of low but not high unit injection doses of cocaine. In contrast to haloperidol, baclofen had no effect on the distribution of inter-injection intervals and, instead, produced long pauses in cocaine self-administration. Baclofen dose dependently reduced cocaine-reinforced responding on a PR schedule; concurrent access to a food-reinforced lever demonstrated that the animals retained the capacity to respond at high rates.

CONCLUSION

The effect of baclofen pretreatment on cocaine self-administration is dependent on the unit injection dose of cocaine and on the response requirements of the schedule.