Modulation of sympathetic transmission by neuronally-released dopamine.

1 When rabbits were pretreated with Fla-63, there was a marked inhibition of dopamine-beta-hydroxylase such that, after incubation of the ear arteries with [3H]-dopamine 47.2% of the tritium in the tissue was retained as unchanged dopamine. 2 [3H]-dopamine was released by stimulation of the sympathetic nerves in ear arteries taken from rabbits pretreated with Fla-63 and incubated with [3H]-dopamine. 3 The dopamine antagonists metoclopramide (1.0 microM) and ergometrine (1.0 microM) enhanced the stimulation-induced efflux of tritium in ear arteries taken from rabbits pretreated with Fla-63 and incubated with [3H]-dopamine, but not when the arteries were incubated with [3H]-noradrenaline. 4. These results suggest that if dopamine is present in the transmitter stores, it can be released by stimulation of the sympathetic nerves, and if the amount is adequate, it can activate an inhibitory feedback loop where prejunctional dopamine receptors are present.