Alphaxalone activates a Cl- conductance independent of GABAA receptors in cultured embryonic human dorsal root ganglion neurons.

Whole cell and cell-attached patch-clamp techniques characterized the neurosteroid anesthetic alphaxalone's (5alpha-pregnane-3alpha-ol-11,20-dione) effects on GABAA receptors and on Cl- currents in cultured embryonic (5- to 8-wk old) human dorsal root ganglion neurons. Alphaxalone applied by pressure pulses from closely positioned micropipettes failed to potentiate the inward Cl- currents produced by application of GABA. In the absence of GABA, alphaxalone (0.1-5.0 microM) directly evoked inward currents in all dorsal root ganglion neurons voltage-clamped at negative membrane potentials. The amplitude of the current was directly proportional to the concentration of alphaxalone (Hill coefficient 1.3 +/- 0.15). The alphaxalone-induced whole cell current was carried largely by Cl- ions. Its reversal potential was close to the theoretical Cl- equilibrium potential, changing with a shift in the external Cl- concentration as predicted by the Nernst equation for Cl- ions. And because the alphaxalone-current was not suppressed by the competitive GABAA receptor antagonist bicuculline or by the channel blockers picrotoxin and t-butylbicyclophosphorothionate (TBPS; all at 100 microM), it did not appear to result from activation of GABAA receptors. In contrast to GABA-currents in the same neurons, the whole cell current-voltage curves produced in the presence of alphaxalone demonstrated strong inward rectification with nearly symmetrical bath and pipette Cl- concentrations. Fluctuation analysis of the membrane current variance produced by 1.0 microM alphaxalone showed that the power density spectra were best fitted to double Lorentzian functions. The elementary conductance for alphaxalone-activated Cl- channels determined by the relationship between mean amplitude of whole cell current and variance was 30 pS. Single-channel currents in cell-attached patches when the pipette solution contained 10 microM alphaxalone revealed a single conductance state with a chord conductance of approximately 29 pS. No subconductance states were seen. The current-voltage determinations for the single-channels activated by alphaxalone demonstrated a linear relationship. Mean open and shut times of single alphaxalone-activated channels were described by two exponential decay functions. Taken together, the results indicate that in embryonic human DRG neurons, micromolar concentrations of alphaxalone directly activate Cl- channels whose electrophysiological and pharmacological properties are distinct from those of Cl- channels associated with GABAA receptors.     

Patch-clamp studies of chloride channels activated by gamma-aminobutyric acid in cultured hippocampal neurones of the rat

The properties of the GABA-activated ion channel in hippocampal neurones prepared from 17- to 19-day-old fetal rats in dispersed cell cultures were studied with the patch-clamp techniques. These neurones had chemosensitivity to gamma-aminobutyric acid (GABA) in the absence of synaptic inputs. GABA activated chloride ion channels selectively in these neurones. The GABA-induced Cl- current was detectable with greater than 1 microM GABA. The amplitude of the current increased with higher concentrations of GABA and apparently saturated at 100 microM. The effective single-channel conductance (gamma) was estimated to be 19 pS from the mean and variance of the steady-state GABA-induced current fluctuation in the whole-cell recordings. The power density spectra for GABA-induced current fluctuations in the whole-cell as well as cell-free outside-out membrane patch recordings had more than a single Lorentzian component. The application of GABA induced discrete pulse-like current flows through the cell-free outside-out membrane patch, after the number of channels activated by GABA had decreased due to the rundown. The single-channel conductance estimated from the amplitude of the current pulse was 29 pS when the intra- and extracellular Cl- concentrations were 150 and 120 mM, respectively. In addition to the above conducting state, the GABA channel had several open states with lower conductances. The apparent discrepancy between the effective single-channel conductance estimated from noise analysis and the single-channel conductances directly measured with individual channels may be due, at least in part, to the presence of multiple conducting states.     

Membrane properties underlying patterns of GABA-dependent action potentials in developing mouse hypothalamic neurons

Spikes may play an important role in modulating a number of aspects of brain development. In early hypothalamic development, GABA can either evoke action potentials, or it can shunt other excitatory activity. In both slices and cultures of the mouse hypothalamus, we observed a heterogeneity of spike patterns and frequency in response to GABA. To examine the mechanisms underlying patterns and frequency of GABA-evoked spikes, we used conventional whole cell and gramicidin perforation recordings of neurons (n = 282) in slices and cultures of developing mouse hypothalamus. Recorded with gramicidin pipettes, GABA application evoked action potentials in hypothalamic neurons in brain slices of postnatal day 2-9 (P2-9) mice. With conventional patch pipettes (containing 29 mM Cl-), action potentials were also elicited by GABA from neurons of 2-13 days in vitro (2-13 DIV) embryonic hypothalamic cultures. Depolarizing responses to GABA could be generally classified into three types: depolarization with no spike, a single spike, or complex patterns of multiple spikes. In parallel experiments in slices, electrical stimulation of GABAergic mediobasal hypothalamic neurons in the presence of glutamate receptor antagonists [10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 microM 2-amino-5-phosphonopentanoic acid (AP5)] resulted in the occurrence of spikes that were blocked by bicuculline (20 microM). Blocking ionotropic glutamate receptors with AP5 and CNQX did not block GABA-mediated multiple spikes. Similarly, when synaptic transmission was blocked with Cd(2+) (200 microM) and Ni(2+) (300 microM), GABA still induced multiple spikes, suggesting that the multiple spikes can be an intrinsic membrane property of GABA excitation and were not based on local interneurons. When the pipette [Cl-] was 29 or 45 mM, GABA evoked multiple spikes. In contrast, spikes were not detected with 2 or 10 mM intracellular [Cl-]. With gramicidin pipettes, we found that the mean reversal potential of GABA-evoked current (E(GABA)) was positive to the resting membrane potential, suggesting a high intracellular [Cl-] in developing mouse neurons. Varying the holding potential from -80 to 0 mV revealed an inverted U-shaped effect on spike probability. Blocking voltage-dependent Na+ channels with tetrodotoxin eliminated GABA-evoked spikes, but not the GABA-evoked depolarization. Removing Ca(2+) from the extracellular solution did not block spikes, indicating GABA-evoked Na+ -based spikes. Although E(GABA) was more positive within 2-5 days in culture, the probability of GABA-evoked spikes was greater in 6- to 9-day cells. Mechanistically, this appears to be due to a greater Na+ current found in the older cells during a period when the E(GABA) is still positive to the resting membrane potential. GABA evoked similar spike patterns in HEPES and bicarbonate buffers, suggesting that Cl-, not bicarbonate, was primarily responsible for generating multiple spikes. GABA evoked either single or multiple spikes; neurons with multiple spikes had a greater Na+ current, a lower conductance, a more negative spike threshold, and a greater difference between the peak of depolarization and the spike threshold. Taken together, the present results indicate that the patterns of multiple action potentials evoked by GABA are an inherent property of the developing hypothalamic neuron.     

Membrane channels activated by taurine in cultured mouse spinal cord neurons

Patch-clamp methods were used to compare biophysical properties of anion channels activated by taurine and gamma-aminobutyric acid in the membrane of cultured mouse spinal neurons. Outside-out patches were voltage clamped at -80 mV at a temperature of 21-23 degrees C. Bath application of GABA (1.5-2 microM) or taurine (5-40 microM) induced chloride-dependent single-channel currents in 14/20 patches tested. Amplitude distributions of these currents showed peaks corresponding to conductance levels of 8, 16, 27 and 46 pS. Only a few percent of GABA-induced events reached the 46 pS level, while 30% of taurine-induced currents were of this size. The average lifetime of taurine-activated channels in the open state was 1.0 +/- 0.07 ms, significantly shorter than the corresponding value for GABA (1.6 +/- 0.08 ms). Taurine-induced currents were abolished by 10 microM strychnine, but persisted in the presence of 50 microM bicuculline.     

Gamma-aminobutyric acid modulates chloride channel activity in cultured primary bovine lactotrophs

The effects of GABA, the analogues muscimol and baclofen, and the antagonist bicuculline were investigated on cultured primary bovine lactotrophs using the patch clamp technique. Under voltage clamp in the whole cell mode using solutions containing chloride as the only permeable ion, GABA application increased the amplitude of mean membrane current and fluctuations of current about this mean. Statistical analysis of current fluctuations induced by GABA showed that the power density spectra in 8 of 12 cells were best fitted to double Lorentzian functions and the variance was smallest around the estimated equilibrium potential of chloride ions. The underlying channel open time estimated from noise analysis was only weakly voltage-dependent. The variance of current noise increased with GABA concentration within the range of 1-30 microM, although a slight decrease of variance in one cell could be observed at 30 microM, suggesting that desensitization to GABA might occur. Muscimol mimicked the effect of GABA but baclofen was without effect under these conditions. Bicuculline reduced the GABA-activated membrane current fluctuations. GABA- or muscimol-activated channels recorded in isolated outside-out patches had a slope conductance of about 20 pS. Mean open times of the channel were characterized by two exponential decay functions. We conclude that bovine lactotrophs have GABA-activated chloride channels, which appear to be voltage-independent. In addition, the action of GABA appears to be mediated through the GABAA receptor subtype.     

Preliminary analysis of the GABA-induced current in cultured CNS neurons of the cutworm moth, Spodoptera litura

Properties of GABA-induced current in cultured CNS (ganglion) neurons of cutworm moths (Spodoptera litura) were studied using a whole-cell patch-clamp technique. CNS neurons ranging from 10 to 20 microm in diameter were cultured for 4-7 days in MGM-464 medium. GABA-induced a current response in these neurons in a sigmoidally dose dependent manner where the Hill coefficient and EC50 were 2.2 and 33.0 microM, respectively. The reversal potential of GABA-induced current was -2.5 mV, which is close to the Cl- equilibrium potential that was calculated from chloride ion concentrations in the present experimental environment. Furthermore, the GABA-induced current response depended on the extracellular chloride ion concentration, indicating that the receptor regulates the Cl- permeability of cells. The GABA-induced current was completely inhibited by the GABA(A) antagonist, SR95531, and activated by the GABA(A) agonist, muscimol but not by the GABA(B) agonist, baclofen. On the other hand, the GABA(C) agonist, CACA, also induced a little smaller current than the GABA-induced response. The pharmacological behaviors of the GABA-induced currents suggest that these cells contain GABA receptors that belong to the GABA receptor family including the Rdl GABA receptors identified in Drosophila neurons.     

GABA-activated whole-cell currents in isolated retinal ganglion cells

1. We have begun to analyze neurotransmitter-activated conductances in retinal ganglion cells by measuring the response of single voltage-clamped adult goldfish ganglion cells to gamma-aminobutyric acid (GABA). Here we describe 1) our method of identifying ganglion cells in vitro after their dissociation from papain-treated retinas, and 2) the response of these cells to GABA in the tight-seal whole cell configuration of the patch-clamp method (cf. 41) after 1-4 days of primary cell culture. 2. Ganglion cell somata were backfilled in situ by injections of horseradish peroxidase (HRP) into the optic nerve. After dissociation of the retinas containing these cells, HRP reaction product was localized to cells that retained the size, shape, and an intracellular organelle characteristic of ganglion cells in situ. These features enabled us thereafter to identify ganglion cells in vitro without retrograde marker transport. 3. GABA (3-10 microM) elicited inward currents and substantial noise increases in almost all ganglion cells at negative holding potentials. Reversal potential measurements in salines containing different chloride concentrations indicated that GABA produces a chloride-selective conductance increase in ganglion cells. Bicuculline (10 microM) reversibly inhibited ganglion cell GABA responses. Baclofen (10 microM) alone elicited no responses in ganglion cells. 4. Noise analysis of GABA-activated whole cell currents yielded elementary conductance estimates of 16 pS, with a slow time constant of 30 ms plus a faster component of 1-2 ms. No significant voltage dependence of these values was observed between -20 and -80 mV. 5. We have thus devised a means of identifying ganglion cells dissociated from adult retinas, identified GABAA receptors (cf. 16) on these cells, and found that the responses mediated by these receptors resemble those found in other regions of central nervous system (CNS). These results are consistent with the notion that GABA may function as an inhibitory transmitter at synapses on ganglion cells.     

Rectification of single GABA-gated chloride channels in adult hippocampal neurons

The properties of single chloride channels activated by gamma-aminobutyric acid (GABA) were investigated with hippocampal slices from adult guinea pigs. After the slices were treated with proteolytic enzymes, gigaseal recordings were made from excised patches of pyramidal or granule cell membranes. This newly developed preparation permits the application of patch-clamp techniques to the adult mammalian central nervous system. Guinea pig hippocampal slices were prepared in a conventional manner. Once prepared, the slices were treated with two different enzymes for brief periods and gently agitated. Slices generally split apart along the boundaries of the cell body regions, exposing neuronal somata. Standard patch-clamp techniques were used for the gigaseal recordings from excised patches. Solutions for both sides of the patches consisted of symmetrical concentrations of chloride, with all cation channels blocked. GABA at concentrations of 0.5-1.0 microM was added to the solution for the extracellular side of the patches. At transmembrane potentials negative to the chloride reversal potential (0 mV), the conductance through the GABA-gated chloride channels was approximately 20 pS. When the transmembrane potential was changed to positive values, the chloride conductance increased dramatically. For example, at +40 mV the conductance through the GABA-gated channels was almost 40 pS. Ramp-clamp commands were used to measure the current-voltage (I-V) relationship through single open channels. The open-channel I-V curves displayed outward rectification. The relationship between open-channel conductance and voltage could be fitted reasonably well by a single energy-barrier model for the channel, with the higher energy barrier placed near the cytoplasmic side of the membrane (at a fractional distance through the membrane of 0.73).(ABSTRACT TRUNCATED AT 250 WORDS)     

Whole-cell voltage-clamp study of the fading of GABA-activated currents in acutely dissociated hippocampal neurons

The lability of the responses of mammalian central neurons to gamma-aminobutyric acid (GABA) was studied using neurons acutely dissociated from the CA1 region of the adult guinea pig hippocampus as a model system. GABA was applied to the neuronal somata by pressure ejection and the resulting current (IGABA) recorded under whole-cell voltage clamp. In initial experiments we examined several basic properties of cells in this preparation. Our data confirm that passive and active membrane properties are similar to those which characterize cells in other preparations. In addition, GABA-dependent conductance (gGABA), reversal potential (EGABA), and the interaction of GABA with pentobarbital and bicuculline all appeared to be normal. Dendritic GABA application could cause depolarizing GABA responses, and somatic GABA application caused hyperpolarizations due to chloride (Cl-) movements. Repetitive brief applications (5-15 ms) of GABA (10(-5) to 10(-3) M) at a frequency of 0.5 Hz led to fading of successive peaks of IGABA until, at a given holding potential, a steady state was reached in which IGABA no longer changed. Imposing voltage steps lasting seconds during a train of steady-state GABA responses led initially to increased IGABA that then diminished with maintenance of the step voltage. The rate of decrease of IGABA at each new holding potential was independent of the polarity of the step in holding potential but was highly dependent on the rate of GABA application. Application rates as low as 0.05 Hz led to fading of IGABA, even with activation of relatively small conductances (5-15 nS). Since IGABA evoked by somatic GABA application in these cells is carried by Cl-, the Cl- equilibrium potential (ECl) is equal to the reversal potential for IGABA, i.e., to EGABA. The fading of IGABA with changes in holding potential can be almost entirely accounted for by a shift in ECl resulting from transmembrane flux of Cl- through the GABA-activated conductance. Maneuvers that prevent changes in the intracellular concentration of Cl-ions, [Cl-]i, including holding the membrane potential at EGABA during repetitive GABA application or buffering [Cl-]i with high pipette [Cl-], prevent changes in EGABA. Desensitization of the GABA response (an actual decrease in gGABA) occurs in these neurons during prolonged application of GABA (greater than 1 s) but with a slower time course than changes in EGABA. Whole-cell voltage-clamp techniques applied to tissue-cultured spinal cord neurons indicated that rapid shifts in EGABA result from repetitive GABA application in these cells as well.(ABSTRACT TRUNCATED AT 250 WORDS)     

Histamine activates chloride conductance in motor neurons of the lobster cardiac ganglion.

1. Individual motor neurons of the lobster cardiac ganglion were voltage clamped with two microelectrodes. Superfusion of histamine evoked a concentration-dependent membrane current. The mean effective concentration (EC50) for the concentration-effect relationship was 28 microM. 2. The amplitude and polarity of the histamine-activated current depended on intracellular and extracellular Cl- concentration. The membrane potential at which the current polarity reversed was a function of the Cl- equilibrium potential. 3. The histamine-activated Cl- conductance was voltage dependent, increasing with depolarization. As a consequence, the histamine-evoked current showed outward rectification. 4. We conclude that histamine activates a Cl- conductance with biophysical properties similar to the crustacean Cl- conductance activated by gamma-aminobutyric acid (GABA) and to the histamine responses described in lobster olfactory and stomatogastric neurons. 5. The response to histamine was competitively inhibited (IC50 = 7 microM) by cimetidine, an H2 subtype inhibitor in mammals. Ranitidine, pyrilamine, chlorpheniramine, diphenhydramine, and cyproheptadine were 50-100 times less potent than cimetidine. Tubocurarine, a Cl- channel blocker, blocked with an IC50 of 20 microM, but picrotoxin did not begin to inhibit the histamine response until concentrations exceeded 0.1 mM. 6. These results suggest that the response cannot easily be classified with the use of the pharmacological categories developed in mammals. Like the Cl(-)-dependent responses to various neurotransmitters in a number of invertebrates, the histamine response in the lobster cardiac ganglion was inhibited by tubocurarine. 7. Both GABA and histamine had similar effects on the motor neurons, but only GABA inhibited pacemaker bursts. In this respect, GABA more resembles the endogenous inhibitory postsynaptic potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nicotine modulates evoked GABAergic transmission in the brain

The effects of nicotine on evoked GABAergic synaptic transmission were examined using whole cell recordings from neurons of the lateral spiriform nucleus in embryonic chick brain slices. All synaptic activities were abolished by the GABA(A) receptor antagonist, bicuculline (20 microM). Under voltage-clamp with KCl-filled pipettes (holding potential -70 mV), nicotine (0.1-1.0 microM) increased the frequency of spontaneous GABAergic currents in a dose-dependent manner. Nicotine enhanced electrically evoked GABAergic transmission only at relatively low concentrations of 50-100 nM (but not 25 nM), which approximate the concentrations of nicotine in the blood produced by cigarette smoking. At higher concentrations nicotine had either no effect (0.25 microM) or diminished (0.5-1.0 microM) evoked GABAergic neurotransmission. Nicotine had no significant effect on the postsynaptic current induced by exogenous GABA (30-50 microM). These data imply that nicotine levels attained in smokers are sufficient to enhance evoked GABAergic transmission in the brain, and that this effect is most likely mediated through activation of presynaptic nicotinic receptors.     

Persistent activation of GABA(A) receptor/Cl(-) channels by astrocyte-derived GABA in cultured embryonic rat hippocampal neurons

Whole cell patch-clamp recordings using Cl(-)-filled pipettes revealed more negative levels of baseline current and associated current variance in embryonic rat hippocampal neurons co-cultured on a monolayer of astrocytes than those cultured on poly-D-lysine. These effects were mimicked by culturing neurons on poly-D-lysine in astrocyte-conditioned medium (ACM). The baseline current and variance decreased immediately in all cells after either local perfusion with saline or exposure to bicuculline, an antagonist of GABA at GABA(A) receptor/Cl(-) channels. Baseline current and variance in all cells reached a nadir at approximately 0 mV, the calculated equilibrium potential for Cl(-). Perfusion of ACM rapidly induced a sustained current in neurons, which also reversed polarity at approximately 0 mV. Bicuculline attenuated or eliminated the ACM-induced current at a concentration that completely blocked micromolar GABA-induced current. Quantitative analyses of spontaneously occurring fluctuations superimposed on the ACM-induced current revealed estimated unitary properties of the underlying channel activity similar to those calculated for GABA's activation of GABA(A) receptor/Cl(-) channels. Bicuculline-sensitive synaptic-like transients, which reversed at approximately 0 mV, were also detected in neurons cultured in ACM, and these were immediately eliminated along with the negative baseline current and superimposed current fluctuations by perfusion. Furthermore bicuculline-sensitive synaptic-like transients were rapidly and reversibly triggered when ACM was acutely applied. ACM induced an increase in cytoplasmic Ca(2+) in cultured embryonic hippocampal neurons that was completely blocked by bicuculline and strychnine. We conclude that astrocytes release diffusible substances, most likely GABA, that persistently activate GABA(A) receptor/Cl(-) channels in co-cultured neurons.     

Electrophysiological properties of mouse horizontal cell GABAA receptors

GABA-induced currents have been characterized in isolated horizontal cells from lower vertebrates but not in mammalian horizontal cells. Therefore horizontal cells were isolated after enzymatical and mechanical dissociation of the adult mouse retina and visually identified. We recorded from horizontal cell bodies using the whole cell and outside-out configuration of the patch-clamp technique. Extracellular application of GABA induced inward currents carried by chloride ions. GABA-evoked currents were completely and reversibly blocked by the competitive GABAA receptor antagonist bicuculline (IC50 = 1.7 microM), indicating expression of GABAA but not GABAC receptors. Their affinity for GABA was moderate (EC50 = 30 microM), and the Hill coefficient was 1.3, corresponding to two GABA binding sites. GABA responses were partially reduced by picrotoxin with differential effects on peak and steady-state current values. Zinc blocked the GABA response with an IC50 value of 7.3 microM in a noncompetitive manner. Furthermore, GABA receptors of horizontal cells were modulated by extracellular application of diazepam, zolpidem, methyl 6,7-dimethoxy-4-ethyl-beta-carboxylate, pentobarbital, and alphaxalone, thus showing typical pharmacological properties of CNS GABAA receptors. GABA-evoked single-channel currents were characterized by a main conductance state of 29.8 pS and two subconductance states (20.2 and 10.8 pS, respectively). Kinetic analysis of single-channel events within bursts revealed similar mean open and closed times for the main conductance and the 20.2-pS subconductance state, resulting in open probabilities of 44.6 and 42.7%, respectively. The ratio of open to closed times, however, was significantly different for the 10.8-pS subconductance state with an open probability of 57.2%.     

Patch-clamp study of ion channels activated by GABA and glycine in cultured cerebellar neurons of the mouse

gamma-Aminobutyric acid (GABA) and glycine receptor channels have been investigated using the patch-clamp technique. Six out of 12 membrane patches exposed to GABA as well as to glycine responded to both substances, 3 to only GABA, two to only glycine and one was insensitive to both transmitters which were applied in micromolar concentrations. Channel currents reversed at the potential predicted by the constant field equation (using Cl- activities), indicating that the charge was carried by Cl-. Although multiple-conductance states were induced by GABA as well as by glycine, the phenomenon was much more pronounced after glycine applications. GABA channel permeabilities ranged from 3.7 X 10(-14) to 6.0 X 10(-14) cm3 s-1. With Cl- distributed symmetrically at 145 mM, these values would correspond to conductances of 19 and 31 pS, respectively. Glycine channel permeabilities ranged from 3.3 X 10(-14) to 14.7 X 10(-14) cm3 s-1 (corresponding conductances 17 and 76 pS).     

Effect of extracellular pH on GABA-activated current in rat recombinant receptors and thin hypothalamic slices.

We studied the effects of extracellular pH (pHo) on gamma-aminobutyric acid (GABA)-mediated Cl- current in rat hypothalamic neurons and recombinant type-A GABA (GABA(A)) receptors stably expressed in human embryonic kidney cells (HEK 293), using whole cell and outside-out patch-clamp recordings. In alpha3beta2gamma2s receptors, acidic pH decreased, whereas alkaline pH increased the response to GABA in a reversible and concentration-dependent manner. Acidification shifted the GABA concentration-response curve to the right, significantly increasing the EC50 for GABA without appreciably changing the slope or maximal current induced by GABA. We obtained similar effects of pH in alpha1beta2gamma2 receptors and in GABA-activated currents recorded from thin hypothalamic brain slices. In outside-out patches recorded from alpha3beta2gamma2 recombinant receptors, membrane patches were exposed to 5 microM GABA at control (7.3), acidic (6.4), or alkaline (8.4) pH. GABA activated main and subconductance states of 24 and 16 pS, respectively, in alpha3beta2gamma2 receptors. Alkaline pH(o) increased channel opening frequency and decreased the duration of the long closed state, resulting in an increase in open probability (from 0.0801 +/- 0.015 in pH 7.3 to 0.138 +/- 0.02 in pH 8.4). Exposure of the channels to acidic pH(o) had the opposite effects on open probability (decreased to 0.006 +/- 0.0001). Taken together, our results indicate that the function of GABA(A) receptors is modulated by extracellular pH. The proton effect is similar in recombinant and native receptors and is dependent on GABA concentration. In addition, the effect appears to be independent of the alpha-subunit isoform, and is due to the ability of H+ to alter the frequency of channel opening. Our findings indicate that GABAergic signaling in the CNS may be significantly altered during conditions that increase or decrease pH.     

Postsynaptic fall in intracellular pH and increase in surface pH caused by efflux of formate and acetate anions through GABA-gated channels in crayfish muscle fibres.

H(+)-selective microelectrodes and a two- or three-microelectrode voltage clamp were used to examine the influence of weak-acid, carboxylate anions on the actions of GABA on postsynaptic intracellular pH, surface pH and on membrane potential in fibres of the crayfish leg opener muscle. Substitution of 30 mM Cl- by formate or acetate promoted a GABA-induced decrease in intracellular pH, which was coupled to an increase in surface pH and to a depolarization. Such effects were not seen in the presence of an equivalent amount of lactate, methanesulphonate or glucuronate. Both the GABA-induced depolarization and the fall in internal pH promoted by formate and acetate were blocked by picrotoxin, and the fall in pH was reversibly inhibited by a K(+)-induced depolarization. The rate of the fall in intracellular pH produced by GABA (0.2 mM) was about 0.02 pH units/min in the presence of formate and 0.03 pH units/min in the presence of acetate. Under steady-state conditions, both 30 mM formate and acetate (but not lactate) induced a positive shift in the reversal potential of GABA-activated current, which was accounted for by a relative permeability vs Cl- of formate and acetate of 0.5 and 0.15, respectively. The conductance sequence of the anions was identical to the permeability sequence, i.e. Cl- greater than formate greater than acetate greater than lactate approximately equal to 0. This sequence is strictly correlated to the Stokes diameter of the anions. The relative permeabilities of the anions indicate that the effective diameter of the GABA-gated channel is about 0.5 nm. The fact that the GABA-induced acidosis was slower in the presence of formate than in the presence of acetate suggests that, in the former case, the rate-limiting step in the fall in internal pH is the entry of non-dissociated formic acid. All the above results are consistent with a scheme where GABA induces a channel-mediated efflux of permeant weak-acid anions, which gives rise to an inward (depolarizing) current and to an intracellular acidosis. A comparison of the permeability properties of crayfish and vertebrate GABA-gated channels suggests that effects similar to those seen in this work are likely to occur in mammalian and other vertebrate neurons in the presence of permeant weak-acid anions.     

Augmentation of GABA-induced chloride current in frog sensory neurons by diazepam

The effect of diazepam (DZP) on the GABA-induced macroscopic and microscopic Cl- current was investigated in isolated frog sensory neurons using both 'concentration-clamp' and patch-clamp techniques. At concentration range between 10(-9) and 10(-4) M, DZP itself evoked no response but potentiated time- and dose-dependently the subthreshold GABA responses, though at high DZP concentrations beyond 10(-5) M the potentiation ratio decreased. The potentiation effect was long-lasting and desensitized slowly over the course of several 10 minutes after washing-out of DZP. DZP potentiated GABA response without shifting the GABA reversal potential. The entire GABA dose-response curve was shifted in a parallel manner to the left by adding DZP without changing cooperatively: the Hill slope was 2.0. The potentiation of GABA response by DZP did not depend on either inward or outward direction of the Cl- current but slightly on the membrane potential. The time constants of activation of desensitization of GABA-gated Cl- current consisted of fast and slow components, respectively. The slow components were concentration-dependent, and significantly changed in the presence of DZP, while DZP had little effects on fast components. In the 'inside-out' configuration, the addition of DZP activated GABA-receptor ionophore complexes under subthreshold without changing the single Cl- channel conductance. It is concluded that DZP may act at a site to modulate GABA binding, in which DZP increases GABA binding affinity and also affects the kinetics of GABA-gated Cl- channels, indicating that DZP has dual action on the GABA-induced responses.     

Unitary, quantal and miniature GABA-activated synaptic chloride currents in cultured neurons from the rat superior colliculus.

The aim of this study was to identify the conductance change induced by one quantum of gamma-aminobutyric acid from axonal release sites on cultured superior colliculus neurons. Unitary (single cell-activated) inhibitory postsynaptic currents and spontaneous synaptic activity were recorded with patch clamp techniques in the whole cell configuration while superfusing the entire neuron with normal saline. Miniature inhibitory postsynaptic currents were recorded in the presence of tetrodotoxin and in reduced [Ca2+]o/[Mg2+]o. In addition, the membrane area contributing to synaptic activity was limited to a narrow window of 50 microns. Smaller neurons were chosen for recording to render a standard deviation of the "instrumental" noise of less than 1.5 pA at a holding voltage of -80 mV. After two weeks in vitro, the percentage of synaptically connected tectal neurons exceeded 50%. At holding voltages of -80 mV (Cl- equilibrium potential -12 mV) minimal amplitudes of unitary inhibitory postsynaptic currents were as low as 7-10 pA, while maximal amplitudes exceeded 500 pA. The mean time to peak and time constant of decay were 3.0 and 34.4 ms, respectively (n = 31). Fluctuating unitary inhibitory postsynaptic currents were deemed to be compound postsynaptic responses. Multiple Gaussian equations could be fitted to the amplitude histograms of unitary postsynaptic currents. This procedure rendered a quantal size between 5.0 and 10.9 pA (mean 7.1 pA; S.D. 1.78 pA) in five neurons from mature cultures. The amplitudes of statistically determined quantal inhibitory postsynaptic currents were slightly smaller than the independent estimate from somatic miniature inhibitory postsynaptic currents. The latter had a mean amplitude of 9.1 pA (S.D. 3.3 pA, n = 23), a mean time to peak of 1.65 ms (n = 9), and a mean time constant of decay of 16.2 ms (n = 9). Single channel recording from outside-out patches showed three to four main conductance states ranging from 9 to 22 pS. Single channel closures at the 21-24 pS level were occasionally observed during relaxation of miniature currents. The small size of whole cell quantal inhibitory postsynaptic currents and somatic miniature currents indicates that one GABA quantum opened only 5-15 single Cl- channels.     

GABAA-receptor-activated current in dorsal root ganglion neurons freshly isolated from adult rats

1. gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter affecting dorsal root ganglion (DRG) neurons. This study compares properties of current activated by the GABAA receptor in two populations of DRG neurons. DRG neurons were isolated from adult rat with the use of enzymatic and mechanical means. Within hours of being isolated, neurons were recorded from with the use of the whole-cell variant of the patch-clamp technique. 2. One population of neurons exhibited an afterdepolarizing potential (ADP), a low threshold for action-potential generation (-45 to -50 mV), a short-duration action potential (less than 2 ms) that was abolished in the presence of 1-2 microM tetrodotoxin (TTX), and an insensitivity to 50 nM capsaicin. The second population of neurons exhibited a high threshold for action-potential generation (less than -40 mV), a shoulder on the falling phase of the action potential, insensitivity of action-potential generation to TTX (1-2 microM), and a depolarizing response to application of 50 nM capsaicin. 3. Sensitivity to GABA (over the range of 1-1,000 microM) was comparable for the two populations of neurons. 4. GABA-activated current was greater in ADP neurons than in non-ADP-type neurons of a comparable diameter (30-50 microns). The mean +/- SE amplitude of current activated by 10 microM GABA in ADP neurons was 0.310 +/- 0.050 nA (range = 0.110-0.460 nA, n = 8), and 0.037 +/- 0.016 nA (range = 0.010-0.130 pA, n = 7) in non-ADP neurons. Ten microM GABA elicited cell firing in ADP neurons but not in non-ADP neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

GABA triggers a Cl- efflux from cultured mouse oligodendrocytes

gamma-Aminobutyric acid (GABA) has been shown to depolarize the membranes of astrocytes and oligodendrocytes taken from different tissues and species. The mechanism mediating this depolarization was identified, in cultured rat brain astrocytes, as an activation of GABA receptor-linked Cl- channels. A subpopulation of cultured oligodendrocytes from mouse spinal cord also responded to GABA with a membrane depolarization. In the present study we demonstrate that, in oligodendrocytes, the depolarization was accompanied by a decrease in intracellular Cl- activity [( Cl-]i) as measured with ion-selective microelectrodes. At rest, [Cl-]i was elevated above the passive distribution, and upon application of GABA, [Cl-]i decreased towards the level of passive distribution. Furosemide blocked the Cl- inward carrier which led to a passive Cl- distribution; in the presence of furosemide, GABA no longer elicited a membrane depolarization or a change in [Cl-]i. We conclude that oligodendrocytes, which were depolarized by GABA, expressed GABA-activated Cl- channels. Oligodendrocytes which were unresponsive to GABA demonstrated a non-passive Cl- distribution indicating that they did express inward-directed Cl- carriers, but no GABA-activated Cl- channels.