Recent studies have shown that nerve-rich tissues in the margin of Polyorchis penicillatus (Eschscholtz), one of the hydromedusae, contain dopamine. The present experiments were conducted to determine the physiological action of dopamine at the cellular level. In the current-clamp mode, dopamine, ranging from 10~ to 10~moll~, applied to cultured swimming motor neurons of this jellyfish produced hyperpolarizations accompanied by a decrease of firing rate or complete inhibition of spiking produced by anodal break excitation. Dopamine in the voltage-clamp mode elicited outward currents at more positive levels than -55 mV, which is the reversal potential of the response. The results of a series of ionic experiments suggest that the inhibitory effect of dopamine is caused by an increased permeability to potassium ions.
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