Identification of a GABA-activated chloride-mediated synaptic potential in rat pars intermedia.

Intracellular recordings from melanotrophs in acutely isolated, intact pituitaries revealed inhibitory postsynaptic potentials (IPSPs) resulting from both pituitary stalk stimulation and exogenous gamma-aminobutyric acid (GABA) application. The stalk-stimulated and GABA-evoked IPSPs displayed identical conductance increases to chloride ions. Both responses reversed in polarity at the same membrane potential and were blocked with bicuculline, a GABAA antagonist. We conclude that activation of this synapse releases GABA which acts on a classical GABAA receptor to increase conductance to chloride in the melanotroph.