Naloxone attenuation of the effect of cocaine on rewarding brain stimulation.

Antagonism of the threshold lowering effect of cocaine for brain stimulation reward by naloxone was investigated. Rats with bipolar electrodes implanted in either the median forebrain bundle (MFB) or the ventral tegmental area (VTA) were trained on a rate-independent threshold procedure. Effective threshold lowering doses of cocaine (10-15 mg/kg i.p.) were determined for each subject. A moderate dose of naloxone (4 mg/kg i.p.) effectively blocked the threshold lowering action of the cocaine. Lower (2 mg/kg) and higher (8 mg/kg) doses of naloxone attenuated but did not completely block the cocaine effect. These results provide further evidence for a catecholamine/endogenous opioid interaction in central reward.