Novel synaptic responses mediated by dopamine and gamma-aminobutyric acid in neuroendocrine cells of the intermediate pituitary.

Neuroendocrine cells of the pituitary pars intermedia contain pro-opiomelanocortin and secrete mainly alpha-melanocyte-stimulating hormone and, in lesser amounts, corticotropin and endorphin. We have recorded intracellularly from rat pars intermedia cells in vitro and analysed the synaptic inputs to these cells. Electrical stimulation of the pituitary stalk evoked a biphasic postsynaptic response in pars intermedia cells. An initial inhibitory postsynaptic potential was most likely mediated by gamma-aminobutyric acid (GABA) because there was a conductance increase, and it was blocked by bicuculline, a GABA antagonist. A more prolonged hyperpolarization was also observed which could last 10-30 s and was associated with a decreased whole cell conductance. This late hyperpolarization was blocked by the dopamine antagonists, chlorpromazine and domperidone, and was therefore most likely dopamine-mediated.