The rewarding action of acute cocaine is reduced in ?-endorphin deficient but not in ? opioid receptor knockout mice.
Nguyen AT, Marquez P, Hamid A, Kieffer B, Friedman TC, Lutfy K. The rewarding action of acute cocaine is reduced in ?-endorphin deficient but not in ? opioid receptor knockout mice. Eur J Pharmacol. 2012 Jul 05; 686(1-3):50-4.