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Ruffing DM, Domino EF. 
“Interaction of Synthetic Opioid Metenkephalin Peptide Analogs, Lilly 127623 and FK 33-824 with Indole Hallucinogens: Antagonism of N,N-Dimethyltryptamine- and LSD-Induced Disruption of Food Rewarded Bar Pressing Behavior in the Rat”. 
Psychopharmacology. 1983;80:315 -318.
The selected opioid metenkephalin synthetic peptide analogs Lilly (LY) 127623 and FK 33-824 were tested for behavioral dose effects and potential interaction with N,Ndimethyltryptamine (DMT) and lysergic acid diethylamide25 (LSD) in adult male Holtzman rats trained on a positive reinforcement fixed-ratio 4 (FR-4) behavioral bar pressing schedule, i.e., a reward of 0.01 ml sugar-sweetened evaporated milk was earned on every fourth bar press. DMT (3.2 mg/kg) and LSD (0.1 mg/kg), administered IP followina a 0.9% NaC1 15 - 20-min control pretreatment, disrupted established food-rewarded FR-4 bar pressing in a consistent and reproducible manner. Animals pretreated IP with predetermined behaviorally noneffective doses of LY 127623 (0.01 - 0.32 mg/kg) and FK 33-824 (0.001 - 0.01 mg/kg) 15-20 min prior to receiving DMT demonstrated significant antagonism to DMT-induced disruption of FR-4 bar pressing, while doses of 0.10 - 0.32 mg/kg LY 127623 and 0.00032 - 0.0032 mg/kg FK 33-824 significantly antagonized LSD-induced behavioral effects.
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