Lever-press responding maintained by contingent intraperitoneal administration of etonitazene in Long Evans hooded rats

Lever pressing maintained by intraperitoneal (IP) injections of etonitazene was established in five Long Evans hooded rats. Each training session consisted of an 8-min fixed interval (FI) during which lever pressing was maintained by food pellets delivered at the end of the interval. Food delivery was accompanied by illumination of stimulus lights in the chamber. Every 20th response during the 8 min interval also produced a brief illumination of the stimulus lights (FI 8 min (FR 20:S)). Administration of etonitazene was then introduced. Immediately following food delivery, the rat received an IP drug injection and was returned to the operant chamber for 30 min. During this confinement, the stimulus lights remained illuminated. This procedure resembles conditioned place preference in that an environment is paired with the effects of an investigator-delivered drug. When food pellet delivery subsequently was discontinued, responding persisted when followed by drug, but not saline, administration. Alternating blocks of sessions with administration of etonitazene (6.0-9.0 mu g/kg) or saline produced corresponding increases or decreases in responding. These results indicate that etonitazene can function as a reinforcer when administered to rats by the IP route, and thus extend the range of conditions under which drug reinforcement can be investigated. (C) 1998 Elsevier Science B.V. All rights reserved.