MODULATION OF GAMMA-AMINOBUTYRIC-ACID (GABA) RECEPTORS AND THE FEEDING RESPONSE BY NEUROSTEROIDS IN HYDRA-VULGARIS

gamma-Aminobutyric acid (GABA) receptors are present in membrane prepa rations from Hydra vulgaris, one of the most primitive organisms with a nervous system. These receptors are sensitive to muscimol and benzod iazepines and appear to be important in the regulation of the feeding response. The effects of neurosteroids. general anaesthetics, and GABA antagonists on GABA(A) receptors in membranes prepared from Hydra and on the feeding response have now been investigated. The neurosteroids tetrahydroprogesterone and tetrahydrodeoxycorticosterone increased [H -3]GABA binding to hydra membranes with nanomolar potency (EC50, 141 /- 11 and 623 +/- 36 nM, respectively) and high efficacy (maximal incr ease 79+/-6.5 and 62+/-4%, respectively), whereas the 3 beta-hydroxy e pimer of tetrahydroprogesterone was ineffective. The benzodiazepine re ceptor ligands diazepam (100 mu M), clonazepam (100 mu M) and abecarni l (30 mu M) enhanced [H-3]GABA binding to Hydra membranes by 22; 20 an d 24%, respectively; effects abolished by the specific benzodiazepine antagonist flumazenil (100 mu M). On the contrary, the peripheral benz odiazepine receptor ligand 4'chlorodiazepam failed to affect [H-3]GABA binding to Hydra membranes. The general anaesthetics propofol and alp haxalone similarly increased (+38% and +30%, respectively) [H-3]GABA b inding. Moreover, [H-3]GABA binding to Hydra membranes was completely inhibited by the GABA(A) receptor antagonist SR 95531, whereas bicucul line was without effect. The modulation of GABA(A) receptors in vitro by these various drugs correlated with their effects on the glutathion e-induced feeding response in the living animals. Tetrahydroprogestero ne and tetrahydrodeoxycorticosterone (I to 10 mu M) prolonged, in a do se-dependent manner, the duration of mouth opening induced by 10 mu M glutathione, with maximal effects of +33 and +29%, respectively, appar ent at 10 mu M neurosteroid. Alphaxalone (10 mu M) similarly increased (+33%) the effect of glutathione. The effects of steroids on the feed ing response were inhibited by SR 95531 in a dose-dependent manner; t- butylbyclophosphorothyonate (1 mu M), a specific Cl- channel blocker, which pei se, like picrotoxin but not bicuculline, shortened the durat ion of the response, also counteracted the steroids effects at 1 mu M. These results suggest that the modulation of GABA(A) receptors by ste roids is an ancient characteristic of the animal kingdom and that the pharmacological properties of these receptors have been highly conserv ed through evolution. (C) 1998 IBRO. Published by Elsevier Science Ltd .