Biphasic behavioural stimulation induced by a monoamine oxidase-inhibiting antidepressant

1. Male Wistar rats were injected i.p. with phenelzine at doses of 12.5-150 mg/kg and their behaviour monitored for 12 hr. 2. Phenelzine at doses of 50 mg/kg and above induced biphasic behavioural stimulation. During the first hr after injection the rats exhibited stereotyped rearing, sniffing and headbobbing and were hyperactive and hyperreactive. In contrast, 2-12 hr post-injection, the drug treated rats exhibited WDS, forepaw padding, grooming, splayed hindlimbs and backward walking. 3. It seems likely that the initial phase of stimulation is due to DA release whereas the subsequent response may be the result of an accumulation of the indoleamines 5-HT and T due to MAO inhibition.