The effect of apomorphine on the open-field behavior of rats: alone and in pairs

Male rats were observed in the open-field while alone and while in pairs in an alternating series of trials. The trials extended over a 78 min session following injections of either saline (0.9%) or apomorphine (5.0 mg/kg, IP) into the observed member of each pair. Contrary to the literature on apomorphine stereotypy, apomorphine did not induce continuous sniffing of the environment and continuous gnawing in most rats. Sniffing of the environment remained at normal levels but there was an increase in nodding the head in the vertical plane while keeping the snout close to the floor. Apomorphine-induced hyperactivity was attributed to two factors: a sustained increase in the duration of bouts of locomotion and a failure of the frequency of bouts of locomotion to habituate to novelty. Apomorphine eliminated all social behavior directed toward the other rat, however apomorphine rats showed they were sensible to the presence of the other by increasing their locomotion and rearing when the partner was introduced.