Naltrindole, a delta-opioid antagonist, blocks MDMA's ability to enhance pressing for rewarding brain stimulation

Twelve rats were each fixed with a chronically indwelling bipolar electrode for stimulation of the medial forebrain bundle as it courses through the hypothalamus. These rats were trained to press a bar for intracranial stimulation of 0.3-s trains of 60 Hz sine waves for 10 min daily at three intensities. One intensity was just above threshold for maintaining pressing, one intensity was a high intensity that sustained considerable pressing, but not maximum pressing, and the other was intermediate to the others. After stable rates of pressing were obtained, rats received MDMA daily. MDMA significantly increased rates of pressing. Prior to a day when rats received MDMA, they also received an injection of naltrindole, a selective delta-opioid receptor antagonist. Naltrindole blocked MDMA's enhancement of pressing for reinforcing brain stimulation.