Rotational behavior as a classically conditioned response to pentobarbital administration

Pentobarbital stimulus control of rotational behavior was investigated in rats with unilateral 6-hydroxydopamine lesions of substantia nigra. In conditioning trials, lesioned rats were injected simultaneously with 10 mg/kg pentobarbital and 0.05 mg/kg apomorphine and their rotational (circling) behavior observed and counted. Subsequent to three consecutive daily conditioning sessions, animals were re-introduced to the rotation environment and tested with saline or pentobarbital. Pentobarbital, but not saline, administration was followed by a brief epoch of rapid contralateral rotation. After additional conditioning trials in which pentobarbital and apomorphine administration were paired, test sessions with 1 g/kg ethanol and with 10 mg/kg chlordiazepoxide were conducted. Most animals did not rotate in response to ethanol administration and most did rotate in response to chlordiazepoxide. Finally, in order to determine the persistence of the conditioned effect, animals were tested with pentobarbital 15 weeks after their last conditioning session and were found to rotate actively in response.