Effects of the cannabinoid receptor agonist CP 55,940 and the cannabinoid receptor antagonist SR 141716 on intracranial self-stimulation in Lewis rats

Abstract

Lewis rats were trained to self-stimulate the medial forebrain bundle (MFB) using a rate-frequency paradigm. They were then tested for the effects of the cannabinoid receptor agonist CP 55,940, the selective cannabinoid receptor antagonist SR 141716 and the dopamine D 1 receptor antagonist SCH 23390. CP 55,940 (0, 10, 25 and 50 μg/kg i.p.) had no effect on MFB self-stimulation behaviour as assessed by the M 50, the stimulation frequency at which half-maximal response rates were obtained. With SR 141716, only a very high dose (20 mg/kg i.p.) caused a significant inhibition of the rewarding efficacy of the stimulation. This was seen as an increase in the M 50. All other doses of SR 141716 (0, 1, 3, 10 mg/kg i.p.) were ineffective in modulating the M 50. By comparison, a relatively low dose (0.06 mg/kg i.p.) of SCH 23390 caused a large increase in M 50. These results indicate a relatively modest influence, if any at all, of exogenous or endogenous cannabinoids on reward-relevant neurotransmission.