LONG-TERM GABA-A RECEPTOR ACTIVATION INCREASES [CA2+](I) IN SINGLE LACTOTROPHS

Prolactin (PRL) release by pituitary lactotrophs is inhibited by gamma -aminobutyric acid (GABA). We have investigated the effect of long-las ting activation of GABA(A) receptors bn membrane potential and cytosol ic free calcium concentration ([Ca2+](i)) in single identified lactotr ophs. Membrane potential was recorded using the perforated patch-clamp technique and [Ca2+](i) using indo 1 as a fluorescent Ca2+ probe. Whe n cells were bathed in muscimol (10 mu M) for 30 min, [Ca2+](i) unexpe ctedly increased in 53% of the lactotrophs. This was due to a Ca2+ inf lux, since it was inhibited by Ca2+-free extracellular medium or by Ca 2+ channel blockers such as the dihydropyridine PN 200-110. In cells i ncubated in muscimol, wash of muscimol from the cell membrane was full y reversible. Wash hyperpolarized the cell membrane potential and redu ced [Ca2+](i) to the levels found in control cells. This effect was mi micked by picrotoxin, a GABA-operated Cl- channel blocker, thus suppor ting the involvement of a muscimol-induced Cl- flux. Conversely, under similar experimental conditions, static assays of PRL release reveale d an inhibition of release by muscimol, unaffected by the dihydropyrid ine PN 200-110. Our observations suggest that GABA(A) receptors may no t regulate the process of exocytosis within the Ca2+-regulated steps.