Intracellular recordings from intact pituitary melanotrophs show that, in the same cell, inhibitory postsynaptic potentials resulting from either pituitary stalk stimulation or exogenous dopamine are abolished by D 2 receptor antagonists, display identical conductance changes, are reversed in polarity at the same membrane potential and are sensitive to pertussis toxin pretreatment. The reversal potential of the inhibitory postsynaptic potential shows a 65 mV shift with a 10-fold change in external potassium concentration, which is close to that predicted by the Nernst equation. We conclude that activation of this synapse releases dopamine which acts on a D 2 receptor to increase potassium conductance via a G-protein-mediated mechanism. This is the first characterization of an inhibitory dopaminergic synapse in the mammalian nervous system.
