Single-trial conditioned place preference using intravenous morphine.

Experiments were performed to investigate single-trial conditioned place preference (CPP) using intravenous morphine in rats. Single-trial CPP was obtained when morphine (8 mg/kg) was paired for either 15 or 30 min with a distinct white compartment. When morphine administration was delayed for either 15 or 25 min after the beginning of a 30-min exposure to the white compartment, single-trial CPP was not obtained. Intravenous naloxone (2 mg/kg) also blocked single-trial CPP when administered 15 min after the beginning of the 30-min exposure to the white compartment with morphine, but naloxone by itself did not alter place preference. The results from these experiments indicate that single-trial CPP using intravenous morphine may offer a useful animal model to assess the reinforcing efficacy of the initial drug experience.