Naloxone enhancement of DMT and LSD-25 induced suppression of food-rewarded bar pressing behavior in the rat.

The narcotic antagonist naloxone was tested to determine its possible interaction with N,N-dimethyltryptamine (DMT) and lysergic acid diethylamide-25 (LSD) in adult male Holtzman rats trained to press a bar on a fixed-ratio four schedule (FR4), i.e., every fourth press earned a reward of 0.01 ml sugar sweetened milk. LSD (0.1 mg/kg) or increasing doses of DMT (1.0, 3.2, and 10.0 mg/kg) were administered i.p. to disrupt food-rewarded fixed ratio bar pressing in a dose related fashion. Pretreatment (5--10 min) with behaviorally ineffective doses of naloxone (1.0--5.6 mg/kg) dramatically enhanced the effects of DMT and LSD. The content of DMT in the brain and liver of rats injected with DMT alone (10 mg/kg) and with a 5 min pretreatment of naloxone (3.2 mg/kg) was determined by radiochemical analysis at 30 and 90 min after 14C-DMT injection. There was no significant difference for either brain or liver 14C-DMT levels when control DMT rats were compared with the naloxone pretreated rats. These results seem to rule out interference by naloxone with the metabolism of DMT as a mechanism of the observed behavioral potentiation.