Ionic conductances related to GABA action on secretory and biosynthetic activity of pars intermedia cells

We have examined the action of GABA on the electrical, secretory and synthetic activities of rat and porcine intermediate lobe (IL) cells in primary culture. Chloride and calcium currents were investigated using patch-clamp techniques. A chloride current activated by 1-100 microM isoguvacine, a specific GABA-A agonist, and antagonised by bicuculline and SR 95103 was recorded at the whole cell and single channel level current. Whole cell calcium currents were investigated and shown to be reduced by 40 microM cadmium, zero external calcium and 10 microM baclofen, a specific GABA-B receptor agonist. Both GABA-B receptor activation and use of calcium deficient medium inhibited peptide release from IL cells. Finally, pro-opiomelanocortin (POMC) mRNA levels were measured using a hybridization technique. Removal of calcium from the culture medium or long-term (48 hr) incubation with 10 microM GABA or muscimol (a mixed GABA-A and GABA-B agonist) significantly reduced POMC mRNA levels.