Depressor action of l-threo-dihydroxyphenylserine in the rat nucleus tractus solitarii

Microinjections of l - threo -dihydroxyphenylserine ( l - threo -DOPS, 0.1–3 ng), a synthetic precursor amino acid of noradrenaline, into the medial area of the nucleus tractus solitarii produced dose-dependent depressor and bradycardic responses in anesthetized rats treated with or without i.p. 3-hydroxybenzylhydrazine, a central inhibitor of l -aromatic amino acid decarboxylase. d - threo -DOPS (3 ng) produced no effect. l -Dihydroxyphenylalanine ( l -DOPA) methyl ester (1 μg), a competitive antagonist of l -DOPA, microinjected into the nucleus tractus solitarii, blocked the depressor and bradycardic responses to l - threo -DOPS. l - threo -DOPS itself produces vasodepressor actions without its conversion to noradrenaline, probably via a recognition site for l -DOPA in the rat nucleus tractus solitarii.