Opioid influence on some aspects of stereotyped behavior induced by repeated amphetamine treatment

Rats were administered repeated IP injections of dl-amphetamine (AMPH) according to a chronic escalating dose schedule (three doses per 24 hr, for four days, two days or one day). Animals treated for four days exhibited a diminished oral stereotypy in response to a challenge of 12 mg/kg AMPH or 2 mg/kg SC apomorphine (APO), 72 hr after withdrawal. Pretreatment with 2 mg/kg IP naloxone (NAL) during the period of chronic AMPH administration prevented the reduction in oral stereotypy induced by AMPH or APO. No differences were detected among the mean of stereotypy scores from the different treatments in response to a challenge dose of 6 mg/kg AMPH. Neurochemical data showed that NAL pretreatment reversed the depletion of striatal dopamine content induced by chronic AMPH. When repeated injections of AMPH were given only one day, the diminished stereotypy response to AMPH or APO was not observed. Animals treated simultaneously with 1 mg/kg IP morphine or 5 micrograms/kg IP beta-endorphin and repeated AMPH injections for one day, showed a reduced stereotyped response to AMPH or APO. These results suggest that opioid peptides are involved in the mechanisms underlying the decrease in oral behaviors following AMPH treatment.