Dopamine acts through a D₂-like receptor on a jellyfish motor neuron

Dopamine, which is present in nerve-rich tissues of the hydromedusa Polyorchis penicillatus, produces membrane hyperpolarization in identified motor neurons from this jellyfish. In this study we demonstrate that the inhibitory action of dopamine is mediated by conventional drug-receptor interactions which are reversible, saturable and specific. When 10 μM dopamine was applied by micro-spritzing onto voltage-clamped (holding potential, −20 mV), cultured “swimming” motor neurons, an outward current of about 1 nA was evoked. Using this technique, we established a potency order for several amines: dopamine≫norepinephrine>tyramine >octopamine>β-phenylethylamine. Dopamine is effective at concentrations betweeen 1 × 10^-8 and 1 × 10^-3M. Several dopamine receptor blockers such as fluphenazine, haloperidol and spiperone reduced the dopamine-induced current in a concentration-dependent manner. Although propranolol, a β-adrenergic blocker, reduced the dopamine response and SKF 83566, a D₁ blocker, increased the response, it appears that the dopamine receptors in these jellyfish neurons share pharmacological properties with mammalian D₂ dopamine receptors.