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

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 recep tor antagonist SR 141716 and the dopamine D-1 receptor antagonist SCH 23390 . CP 55,940 (0, 10, 25 and 50 mug/kg i.p.) had no effect on MFB self-stimul ation behaviour as assessed by the M-50, the stimulation frequency at which half-maximal response rates were obtained. With SR 141716, only a very hig h dose (20 mg/kg i.p.) caused a significant inhibition of the rewarding eff icacy of the stimulation. This was seen as an increase in the M-50. All oth er doses of SR 141716 (0, 1, 3, 10 mg/kg i.p.) were ineffective in modulati ng 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. (C) 2001 Elsevier Science Inc. All righ ts reserved.