Excision-activated chloride channels in cultured postnatal rat hippocampal neurons

Halothane and propofol modulation of gamma-aminobutyric acidA receptor single-channel currents

Halothane and propofol enhance the activity of the gamma-aminobutyric acid (GABA) system, which is one of the most important systems in the mechanism of anesthesia. To determine whether halothane and propofol enhance GABAergic responses by the same mechanism, we performed single-channel patch-clamp experiments with rat cortical neurons in primary culture. Each of the open-time and closed-time distributions of GABA(A) receptor single channels was expressed by a sum of fast and slow time constants. Neither halothane nor propofol changed the single-channel conductance. Halothane increased the probability of the channel being open via a prolongation of the slow phase of open time, whereas propofol increased the channel open probability via a shortening of the slow phase of closed time. Thus, although both halothane and propofol augmented the channel open probability, thereby causing an increase in charge transfer during inhibitory transmitter action, they acted by different mechanisms.

GABA-Induced Cl- current in cultured embryonic human dorsal root ganglion neurons

gamma-Aminobutyric acid (GABA)-activated channels in embryonic (5-8 wk old) human dorsal root ganglion (DRG) neurons in dissociated culture were characterized by whole cell and single-channel techniques. All DRG neurons when held at negative holding membrane potentials displayed inward current to micromolar concentrations of GABA applied by pressure pulses from closely positioned micropipettes. The current was directly proportional to the concentration of GABA (EC50, 111 microM; Hill coefficient, 1.7). DRG neurons also responded to micromolar concentrations of pentobarbital and alphaxalone but not to cis-4-aminocrotonic acid (CACA), glycine, or taurine. Baclofen (100 microM) affected neither the holding currents nor K+ conductance (when patch pipettes were filled with 130 mM KCl) caused by depolarizing pulses. Whole cell GABA-currents were blocked by bicuculline, picrotoxin, and t-butylbicyclophosphorothionate (TBPS; all at 100 microM). The reversal potential of whole cell GABA-currents was close to the theoretical Cl- equilibrium potential, shifting with changes in intracellular Cl- concentration in a manner expected for Cl--selective channels. The whole cell I-V curve for GABA-induced currents demonstrated slight outward rectification with nearly symmetrical outside and inside Cl- concentrations. Spectral analysis of GABA-induced membrane current fluctuations showed that the kinetic components were best fitted by a triple Lorentzian function. The apparent elementary conductance for GABA-activated Cl- channels determined from the power spectra was 22.6 pS. Single-channel recordings from cell-attached patches with pipettes containing 10 microM GABA indicated that GABA-activated channels have a main and a subconductance level with values of 30 and 19 pS, respectively. Mean open and closed times of the channel were characterized by two or three exponential decay functions, suggesting two or three open channel states and two closed states. Single channels showed a lack of rectification. The actions of GABA on cultured human embryonic DRG neurons are mediated through the activation of GABAA receptors with properties corresponding to those found in the CNS of human and other mammalian species but differing from those of cultured human adult DRG neurons.