SHORT APPLICATIONS OF GAMMA-AMINOBUTYRIC - ACID INCREASE INTRACELLULAR CALCIUM CONCENTRATIONS IN SINGLE IDENTIFIED RAT LACTOTROPHS

We have investigated the direct effect of GABA receptor agonists on th e cytosolic free calcium concentration ([Ca2+](i)) and the membrane po tential of rat lactotrophs in primary culture. [Ca2+](i) was recorded in single identified lactotrophs by dual emission microspectrofluorime try using indo-1 as intracellular fluorescent calcium probe. Whole cel l and perforated patch-clamp were performed. A short application of GA BA (10(-5) M, 10 s) induced a marked transient [Ca2+](i) increase in 6 6% of lactotrophs, which could be readily mimicked by muscimol (10(-5) M). By contrast, neither L-homocarnosine (10(-3) M) nor baclofen (10( -5) M), a GABA(B) agonist, had any effect on [Ca2+](i). The GABA-induc ed [Ca2+](i) increase was antagonized by picrotoxin (10(-5) M), bicucu lline methiodide(10(-5) M) and strychnine (10(-4) M), demonstrating GA BA(A) receptor specificity. Furthermore, clonazepan (1.5 x 10(-4) M) c ould potentiate the GABA effect on [Ca2+](i). The [Ca2+](i) increase d isappeared in the absence of Ca2+ in the extracellular medium or in th e presence of Ca2+ channel blockers (cadmium, PN 200-110). GABA and mu scimol depolarized the membrane potential with a concomitant fall in c ell input resistance, thus suggesting, as in other cell types, the ope ning of receptor-operated chloride channels. When Ca2+ entry was preve nted by the use of cadmium (500 x 10(-6) M), GABA still elicited membr ane depolarization but did not raise [Ca2+](i). Our results suggest th at a short application of GABA leads to Ca2+ entry through voltage-gat ed Ca2+ channels in single lactotrophs. This Ca2+ influx is due to dep olarization of the prolactin cell.