Dopamine (D2) or gamma-aminobutyric acid (GABAB) receptor activation hyperpolarizes rat melanotrophs and pertussis toxin blocks these responses and the accompanying fall in [Ca2+]i.

The effects, on membrane potential, of dopamine (DA) and gamma-aminobutyric acid (GABA), transmitters present in the secreto-inhibitor innervation to the melanotrophs, were monitored in primary cultures of rat melanotrophs with bis-oxonol. DA and GABA, acting through D2 and GABAB receptors, hyperpolarized the melanotrophs. Hyperpolarization was not suppressed by tetrodotoxin but was prevented by pertussis toxin and may thus be due to a G protein mediated mechanism. Pertussis toxin also blocked the effects mediated by the two receptors to reduce intracellular free Ca2+ ([Ca2+]i).