[GABA-A receptors: molecular substrate for the development of new anxiolytic agents]

Inhibitory effects of amiloride on the current mediated by native GABA(A) receptors in cultured neurons of rat inferior colliculus

1. The diuretic amiloride is known to modulate the activity of several types of ion channels and membrane receptors in addition to its inhibitory effects on many ion transport systems. However, the effects of amiloride on some important ion channels and receptors, such as GABA(A) receptors, in the central nervous system have not been characterized. 2. In the present study, we investigated the functional action of amiloride on native GABA(A) receptors in cultured neurons of rat inferior colliculus using whole-cell patch-clamp recordings. 3. Amiloride reversibly inhibited the amplitude of the GABA-induced current (I(GABA)) in a concentration-dependent manner (IC(50) 454 +/- 24 micromol/L) under conditions of voltage-clamp with a holding potential at -60 mV. The inhibition depended on drug application mode and was independent of membrane potential. Amiloride did not change the reversal potential of I(GABA). Moreover, amiloride induced a parallel right-ward shift in the concentration-response curve for I(GABA) without altering the maximal value and Hill coefficient. 4. The present study shows that amiloride competitively inhibits the current mediated by native GABA(A) receptors in the brain region, probably via a direct action on GABA-binding sites on the receptor. The findings suggest that the functional actions of amiloride on GABA(A) receptors may result in possible side-effects on the central nervous system in the case of direct application of this drug into the cerebrospinal fluid for treatment of diseases such as brain tumours.

Pharmacodynamic and pharmacokinetic effects of flumazenil and theophylline application in rats acutely intoxicated by diazepam

Background/Aim. The majority of symptoms and signs of acute diazepam poisoning are the consequence of its sedative effect on the CNS affecting selectively polisynaptic routes by stimulating inhibitory action of GABA. The aim of the present study was to examine the effects of combined application of theophylline and flumazenil on sedation and impaired motor function activity in acute diazepam poisoning in rats. Methods. Male Wistar rats were divided in four main groups and treated as follows: group I - with increasing doses of diazepam in order to produce the highest level of sedation and motor activity impairment; group II - diazepam + different doses of flumazenil; group III - diazepam + different doses of theophylline; group IV - diazepam + combined application of theophylline and flumazenil. Concentrations of diazepam and its metabolites were measured with LC-MS. The experiment was performed on a commercial apparatus for spontaneous motor-activity registration (LKBFarad, Sweden). Assessment of diazepam- induced neurotoxic effects and effects after theophylline and flumazenil application was performed with rotarod test on a commercial apparatus (Automatic treadmill for rats, Ugo Basile, Italy). Results. Diazepam in doses of 10 mg/kg and 15 mg/kg produced long-time and reproducible pharmacodynamic effects. Single application of flumazenil or theophylline antagonized effects of diazepam, but not completely. Combined application of flumazenile and theophylline resulted in best effects on diazepaminduced impairment of motoric activity and sedation. As a result of theopylline application there was better elimination of diazepam and its metabolites. Conclusion. Combined application of flumazenil and theophylline resulted in the best antidotal effects in the treatment of diazepam poisoned rats. These effects are a result of different mechanisms of their action, longer half-life of theophylline in relation to that of flumezenil and presumably the diuretic effect of theophylline.