Characterization of GABA current in rat septal cholinergic neurons in culture and its modulation by metal cations.

1. With the aim of characterizing the functional and pharmacological properties of gamma-aminobutyric acid-receptor (type A; GABAAR) channels expressed in a given type of brain neurons, we obtained whole cell voltage-clamp recordings from rat septal cholinergic neurons in primary culture. 2. GABA-induced currents (IGABAS) were accompanied by an increase in conductance and reversed the direction at the Cl- equilibrium potential. The permeability sequence of GABA-activated Cl- channel for various anions (ratio to Cl- permeability) was found to be SCN- (1.72) > or = I- (1.60) > or = Br- (1.55) > Cl- (1) > HCO2- (0.42) >> F- (0). 3. GABA applied by local perfusion produced an IGABA that enhanced in amplitude with increasing concentrations (EC50 = 21.4 microM; nH = 1.70) and desensitized at higher concentrations. Bicuculline inhibited IGABA (20 microM) competitively (IC50 = 0.963 microM) with a dissociation constant of approximately 0.5 microM. Picrotoxin depressed IGABA (20 microM) noncompetitively in a dose- and use-dependent manner (IC50: approximately 100 microM for peak IGABA). 4. IGABA was potentiated by diazepam at 1 and 10 microM (by 20% at 1 microM for IGABA evoked at 10 microM) but not at 0.1 and 100 microM. IGABA was facilitated by pentobarbital sodium (5-200 microM) in a dose-dependent fashion with an increase in an apparent affinity for GABA (by 251% at 50 microM). 5. Zn2+ (2-100 microM) inhibited IGABA (limited to approximately 20% decrease) in a noncompetitive manner. IGABA was also depressed by Cu2+ and Cd2+ (by 50 and 15%, respectively, at 100 microM); its recovery process from their depressions was slow and fast, respectively. The Cu2+ but not Cd2+ action persisted in the presence of Zn2+. La3+ (50-500 microM) potentiated IGABA by enhancing an apparent affinity for GABA (by 158% by 100 microM La3+); this action disappeared in the presence of pentobarbital but not Zn2+. La3+ applied alone induced an inward current at negative holding potentials, but this was not due to the activation of GABAAR channels. 6. Septal cholinergic neurons were endowed with GABAAR channels that were similar to those on other types of neurons in the affinities for GABA, bicuculline, picrotoxin, and pentobarbital, and in an anion-permeability sequence, but were greatly different in the actions of metal cations. The septal GABAA response was depressed strikingly by Cu2+ and to a little extent by Zn2+, whose action was shared by Cd2+ but not by Cu2+, while being potentiated by La3+, possibly through the action on a pentobarbital site. It is suggested that a distinction in properties among native GABAAR channels may be largely reflected in the actions of not only Zn2+ but also Cu2+ and La3+.