Central action of narcotic analgesics. II. Locomotor activity and narcotic analgesics

The effect of morphine, codeine, fentanyl and pentazocine on locomotor activity of rats and mice and open-field performance of rats were tested. All the analgesics tested produced a depressive action in the rat. In mice a depressive action was produced by pentazocine and codeine. Fentanyl increased the exploratory and basal locomotor activity of mice. Morphine increased the exploratory activity, but, given at doses of 2.5 and 10 mg/kg decreased the basal locomotor activity. The increase of locomotor activity in mice by morphine and fentanyl is caused by an indirect stimulation of catecholamine receptors.