Dose-dependent conditioned place preference produced by etonitazene and morphine

The conditioned place preference (CPP) paradigm was used to study the reinforcing properties of etonitazene in comparison with those of morphine. Increasing doses of etonitazene (2.5-15-mu-g/kg i.p.) and morphine (1-80 mg/kg i.p.) induced a dose-dependent CPP. High doses of etonitazene (25-40-mu-g/kg) did not elicit CPP. In addition, these reinforcing properties were related to behavioral modifications such as analgesia, assessed with the tail-flick method, and increased catalepsy, evaluated by a scoring system. It is concluded that neither the strong behavioral effects induced by etonitazene nor tolerance to such effects account for the results. These findings are discussed with regard to the possibility that etonitazene could interfere with associative learning motivated by reward.