Direct relation between etonitazene dose and response rate: responding under a single FI per session.

Response-contingent injections of etonitazene (ETZ) have been shown to reinforce rats' lever pressing behavior. The objective of the present study was to determine the relation between response rate and ETZ dose when ETZ was administered subcutaneously once per session by the experimenter contingent upon completion of a 10-min fixed-interval (FI) schedule. When injections of the saline vehicle replaced drug injections, response rates dropped to low levels; rates subsequently increased above saline levels when drug injections were reintroduced, demonstrating that ETZ was serving as a reinforcer. A range of ETZ doses (0.01, 0.1, 1, 5.7, and 10 microg/kg) was administered subcutaneously to six rats, resulting in response rates that were directly related to drug dose. These findings are consistent with other studies that have found an increase in reinforcing effects with increases in drug dose. Thus, studies in which drug is administered once per session may be used to measure the reinforcing effects of drugs directly from rate measures, as the response rate in these studies is unaffected by satiation or direct drug effects.