Acute dystonia as an idiosyncratic response to neuroleptics in baboons

Among 25 baboons, Papio papio, 2 consistently showed acute dystonic reactions, with mouthing, compulsive gnawing and limb and trunk dystonia, following the intravenous administration of neuroleptics and related drugs (haloperidol, 0-6-1-2 mg/kg; pimozide 0-5-2-5 mg/kg; chlorpromazine 5-25 mg/kg; metoclopramide 1-5-1-7 mg/kg; oxyperomide 0-25-1-0 mg/kg). The syndrome was not seen after thioridazine (3-7 mg/kg). The dystonic responses occurred within 1-2 h of drug injection and lasted for 2-24 h. They were abolished for 1-3 h within 1-2 min of the intravenous injection of acetylcholine antagonists (benztropine 0-2 mg/kg; hyoscine 0-02 mg/kg). Pre-treatment with a combination of reserpine (2 mg/kg) and alpha-methylparatyrosine (2 X 200 mg/kg) substantially reduced the dystonic response to haloperidol. A second larger dose of haloperidol (5 mg/kg), given 60-90 min after 0-5 mg/kg) initially reduced the intensity of the dystonic response, but after 29 min induced vomiting and generalized seizures in the idiosyncratic baboons. The hypothesis is advanced that the dystonic responses result from release of dopamine on to a sub-population of receptors in the striatum that are relatively insensitive to blockade by neuroleptics.