Responses of rat substantia nigra compacta neurones to L‐DOPA

1 The electrophysiological actions of l‐DOPA were studied on substantia nigra dopamine‐containing neurones by use of intracellular recordings, current and voltage clamp, in vitro. 2 l‐DOPA (3–300 μm), applied by superfusion, decreased the spontaneous firing activity of the neurones, hyperpolarized the membrane potential and produced an outward current. 3 These effects of l‐DOPA were graded and were antagonized by sulpiride, a D2‐receptor antagonist. 4 The l‐DOPA activated membrane hyperpolarization persisted in slices exposed to tetrodotoxin while it was markedly reduced or abolished in solutions with no calcium plus cobalt (2 mm). 5 In slices treated with carbidopa, an inhibitor of the aromatic amino acid decarboxylase enzyme, the actions of l‐DOPA were greatly reduced. 6 We propose that l‐DOPA inhibits the firing of substantia nigra zona compacta neurones by being converted to dopamine which is then released by these cells.