The potent and specific GABA agonists muscimol and THIP have been injected into the caudal and rostral parts of the ventral tegmental area (VTA) containing the A 10 dopaminergic cell bodies. Muscimol (10 and 25 ng), THIP (100-500 ng) and GABA (100 μg) all induced a compulsive hypermotility with low exploratory activity following bilateral injection into the caudal VTA. In addition, the rats showed fighting, attack behavior and increased food intake. The hypermotility was attenuated by picrotoxin (3 mg/kg), haloperidol (0.1-0.5 mg/kg) and completely prevented by reserpine (7.5 mg/kg) plus α-methyltyrosine (200 mg/kg). Picrotoxin (100 ng) injected into the caudal VTA induced mild sedation whereas bicuculline methiodide (BMI) (120 ng) induced convulsions. Morphine (5 μg) injected bilaterally into the caudal VTA induced a different behavioral stimulation shown as rearing, motility, sniffing and licking. Bilateral apomorphine (5 μg) and carbachol (2.5 μg) injections into caudal VTA produced sedation and catalepsy. In the rostral VTA muscimol and THIP decreased spontaneous activity, whereas BMI and picrotoxin induced strong hypermotility. The results indicate that different kinds of hyperactivity can be elicited from the VTA and it is suggested that this reflects the regional and topographical organization of the VTA dopamine projections to cortical and limbic areas. The effects of GABA agonists injected into caudal VTA seem to mimic the effects described after electrolytic or 6-hydroxydopamine lesions of VTA, whereas the morphine response resembles the behavioral stimulation produced by systemic administration of dopaminergic stimulants such as amphetamine or cocaine. © 1979.
