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Summary1. The drug HA-966 (1-hydroxy-3-amino-pyrrolidone-2), which chemically resembles the cyclic form of GABA, has been studied for neuro-pharmacological properties and for effects on the catecholamine content of the corpus striatum. 2. The acute effects on spontaneous behaviour of rodents included flaccid catalepsy and reversible tranquillization in doses which were 5% or less of the lethal dose. Long lasting depression of the CNS, followed by complete recovery, was produced in the cat and the dog. In the monkey HA-966 caused periodical sleeping episodes. 3. The exploratory behaviour and the amphetamine-induced motor activity in mice were blocked by HA-966. The toxicity of amphetamine in aggregated mice was only moderately reduced, suggesting that HA-966 differs from neuroleptics. 4. Tremors induced by chemical agents (nicotine, zinc and tremorine) were markedly inhibited by HA-966. The muscarinic effects of tremorine were not reduced by HA-966, indicating a selective central antitremor effect. 5. HA-966 elevated the threshold to strychnine convulsions and abolished the ipsilateral flexor reflex, while not having motor endplate blocking properties. It is suggested that HA-966 depresses central internuncial neurones. 6. In rats and rabbits HA-966 produced synchronous EEG and inhibited the sensory arousal in doses not causing sedation. In the monkey the drug caused a periodical dissociation between 'sleep-EEG' and behaviour. 7. In rat brain, HA-966 selectively elevated the dopamine content in the corpus striatum, while no changes in noradrenaline and 5-hydroxytryptamine contents could be demonstrated. The effect was still present when dopa synthesis was inhibited with a-methyl-p-tyrosine. 8. Several effects of intravenously administered HA-966 became manifest after an appreciable delay and in hepatectomized mice the effects were much reduced. It is postulated that HA-966 is converted to a pharmacologically active metabolite.HA -966, drug for extrapyramidal diseases 9. The results are discussed in the light of current views on drug therapy in extrapyramidal conditions and a GABA-related hypothesis as to the mode of action of HA-966 is presented.

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