Injection of a GABA antagonist into the mesopontine reticular formation abolishes haloperidol-induced catalepsy in rats.

THE pedunculopontine nucleus and its adjacent structure of the mesopontine reticular formation are known as a mesencephalic locomotor region, since either electrical or chemical stimulation of these regions induces locomotion in decerebrate animals. In parkinsonism, it is presumed that the pedunculopontine nucleus is under GABAergic overinhibition from the basal ganglia. To reveal the behavioural effects of GABAergic disinhibition of the mesopontine reticular formation in parkinsonism, picrotoxin, a GABAA antagonist, (5 or 10 ng/ 0.25 microliter) or vehicle was injected unilaterally into the mesopontine reticular formation of rats via implanted cannulae after induction of catalepsy using haloperidol (1.5 mg kg-1, i.p.). Injection of the larger dose of picrotoxin, but not the smaller dose nor the vehicle, abolished the catalepsy with or without spontaneous locomotor activity. The present result suggests that the disinhibition of the brainstem output structures contributes to the recovery of mobility in the cataleptic state induced by blocking the dopaminergic transmission of the basal ganglia.