Rats immobilized for 2 h daily for 7 days showed an increased behavioral response (forepaw treading and hind-limb abduction) to 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) 24 h after the last stress session. An injection of naloxone before each stress session fully antagonized the increased behavioral reactivity to 5-MeODMT. Treatment with morphine or beta-endorphin associated with each...