MULTIPLE MODULATORY EFFECTS OF THE NEUROACTIVE STEROID PREGNANOLONE ON GABA(A) RECEPTOR IN FROG PITUITARY MELANOTROPHS

1. The effects of the neuroactive steroid pregnanolone (5 beta-pregnan -3 alpha-ol-20-one) on the electrical response to GABA were investigat ed in cultured frog pituitary melanotrophs using the patch-clamp techn ique. 2. Low concentrations of pregnanolone (0.01-1 mu M) in the extra cellular solution enhanced the current evoked by submaximal concentrat ions of GABA(A) receptor agonists and prolonged the GABA-induced inhib ition of the spontaneous action potentials in a dose-dependent manner. 3. Pregnanolone augmented the opening probability of the single GABA- activated channels but did not modify the conductance levels. 4. Pregn anolone (1 mu M) shifted the GABA dose-response curve towards the low GABA concentrations, reducing the EC50 from 4.2 to 1.8 mu M. 5. Intern al cell dialysis with pregnanolone (1 or 1.0 mu M) did not alter the G ABA-evoked current. 6. Pregnanolone accelerated the desensitization of both the current and conductance increases caused by GABA. 7. High co ncentrations of pregnanolone (30 mu M) markedly and reversibly diminis hed the current evoked by 10 mu M GABA. 8. At high concentrations (10- 30 mu M), pregnanolone induced an outward current which reversed at th e chloride equilibrium potential. 9. It is concluded that, in frog pit uitary melanotrophs, pregnanolone exerts a dual inverse modulation and a direct activation of the GABA(A) receptor-channel depending on the concentrations of both GABA and steroid. Pregnanolone acts on an extra cellular site on the GABA(A) receptor inducing conformational changes of the receptor-channel complex, resulting in a desensitized less-cond ucting state.