NALTRINDOLE, A DELTA-OPIOID ANTAGONIST, BLOCKS MDMAS ABILITY TO ENHANCE PRESSING FOR REWARDING BRAIN-STIMULATION

Twelve rats were each fixed with a chronically indwelling bipolar elec trode for stimulation of the medial forebrain bundle as it courses thr ough the hypothalamus. These rats were trained to press a bar for intr acranial stimulation of 0.3-s trains of 60 Hz sine waves for 10 min da ily at three intensities. One intensity was just above threshold for m aintaining pressing, one intensity was a high intensity that sustained considerable pressing, but not maximum pressing, and the other was in termediate to the others. After stable rates of pressing were obtained , rats received MDMA daily. MDMA significantly increased rates of pres sing. Prior to a day when rats received MDMA, they also received an in jection of naltrindole, a selective delta-opioid receptor antagonist: Naltrindole blocked MDMA's enhancement of pressing for reinforcing bra in stimulation.