Effect of Dopamine Agonists and Antagonists on DOPA Formation in the Substantia Nigra

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Abstract
The effect of different psychotropic drugs on the rate of dopa accumulation after administration of a decarboxylase inhibitor (NSD 1015) was compared in the substantia nigra (SN) and caudate nucleus (CN) by a new radioenzymatic method. Inhibition of monoamine oxidase with pargyline or stimulation of dopamine (DA) receptors with apomorphine, N-n-propyl-norapomorphine or D-amphetamine reduced dopa formation in the CN and SN to the same extent. Inhibition of DA receptors with haloperidol or (-)sulpiride and depletion of DA concentration with reserpine enhanced dopa formation to a greater extent in the CN than in the SN. Apomorphine antagonized not only the effect of haloperidol and (-)sulpiride, but also, even more effectively, that of reserpine. DA synthesis in the SN is apparently controlled by both end-product inhibition and DA receptor-mediated mechanisms.