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

Physiological role for GABAA receptor desensitization in the induction of long-term potentiation at inhibitory synapses

GABAA receptors (GABAARs) are pentameric ligand-gated ion channels distributed throughout the brain where they mediate synaptic and tonic inhibition. Following activation, these receptors undergo desensitization which involves entry into long-lived agonist-bound closed states. Although the kinetic effects of this state are recognised and its structural basis has been uncovered, the physiological impact of desensitization on inhibitory neurotransmission remains unknown. Here we describe an enduring form of long-term potentiation at inhibitory synapses that elevates synaptic current amplitude for 24 h following desensitization of GABAARs in response to agonist exposure or allosteric modulation. Using receptor mutants and allosteric modulators we demonstrate that desensitization of GABAARs facilitates their phosphorylation by PKC, which increases the number of receptors at inhibitory synapses. These observations provide a physiological relevance to the desensitized state of GABAARs, acting as a signal to regulate the efficacy of inhibitory synapses during prolonged periods of inhibitory neurotransmission.

Mutation of a glutamate receptor motif reveals its role in gating and delta2 receptor channel properties

Despite its importance in the cerebellum, the functions of the orphan glutamate receptor delta2 are unknown. We examined a mutant delta2 receptor channel in lurcher mice that was constitutively active in the absence of ligand. Because this mutation was within a highly conserved motif (YTANLAAF), we tested its effect on several glutamate receptors. Mutant delta2 receptors showed distinct channel properties, including double rectification of the current-voltage relationship, sensitivity to a polyamine antagonist and moderate Ca 2+ permeability, whereas other constitutively active mutant glutamate channels resembled wild-type channels in these respects. Moreover, the kinetics of ligand-activated currents were strikingly altered. We conclude that the delta2 receptor has a functional ion channel pore similar to that of glutamate receptors. The motif may have a role in the channel gating of glutamate receptors.

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.

GABAA receptor subunit composition and functional properties of Cl- channels with differential sensitivity to zolpidem in embryonic rat hippocampal cells

Using flow cytometry in conjunction with a voltage-sensitive fluorescent indicator dye (oxonol), we have identified and separated embryonic hippocampal cells according to the sensitivity of their functionally expressed GABAA receptors to zolpidem. Immunocytochemical and RT-PCR analysis of sorted zolpidem-sensitive (ZS) and zolpidem-insensitive (ZI) subpopulations identified ZS cells as postmitotic, differentiating neurons expressing alpha2, alpha4, alpha5, beta1, beta2, beta3, gamma1, gamma2, and gamma3 GABAA receptor subunits, whereas the ZI cells were neuroepithelial cells or newly postmitotic neurons, expressing predominantly alpha4, alpha5, beta1, and gamma2 subunits. Fluctuation analyses of macroscopic Cl- currents evoked by GABA revealed three kinetic components of GABAA receptor/Cl- channel activity in both subpopulations. We focused our study on ZI cells, which exhibited a limited number of subunits and functional channels, to directly correlate subunit composition with channel properties. Biophysical analyses of GABA-activated Cl- currents in ZI cells revealed two types of receptor-coupled channel properties: one comprising short-lasting openings, high affinity for GABA, and low sensitivity to diazepam, and the other with long-lasting openings, low affinity for GABA, and high sensitivity to diazepam. Both types of channel activity were found in the same cell. Channel kinetics were well modeled by fitting dwell time distributions to biliganded activation and included two open and five closed states. We propose that short- and long-lasting openings correspond to GABAA receptor/Cl- channels containing alpha4beta1gamma2 and alpha5beta1gamma2 subunits, respectively.

Noninvasive measurements of the membrane potential and GABAergic action in hippocampal interneurons

Neurotransmitters affect the membrane potential (Vm) of target cells by modulating the activity of receptor-linked ion channels. The direction and amplitude of the resulting transmembrane current depend on the resting level of Vm and the gradient across the membrane of permeant ion species. Vm, in addition, governs the activation state of voltage-gated channels. Knowledge of the exact level of Vm is therefore crucial to evaluate the nature of the neurotransmitter effect. However, the traditional methods to measure Vm, with microelectrodes or the whole-cell current-clamp technique, have the drawback that the recording pipette is in contact with the cytoplasm, and dialysis with the pipette solution alters the ionic composition of the interior of the cell. Here we describe a novel technique to determine the Vm of an intact cell from the reversal potential of K+ currents through a cell-attached patch. Applying the method to interneurons in hippocampal brain slices yielded more negative values for Vm than subsequent whole-cell current-clamp measurements from the same cell, presumably reflecting the development of a Donnan potential between cytoplasm and pipette solution in the whole-cell mode. Cell-attached Vm measurements were used to study GABAergic actions in intact CA1 interneurons. In 1- to 3-week-old rats, bath-applied GABA inhibited these cells by stabilizing Vm at a level depending on contributions from both GABAA and GABAB components. In contrast, in 1- to 4-d-old animals, only GABAA receptors were activated resulting in a depolarizing GABA response.

The soluble N-ethylmaleimide-sensitive factor attached protein receptor complex in growth cones: molecular aspects of the axon terminal development

Soluble N-ethylmaleimide-sensitive factor attached protein (SNAP) receptor (SNARE) mechanisms are thought to be involved in two important processes in axonal growth cones: (1) membrane expansion for axonal growth and (2) vesicular membrane fusion for mature synaptic transmission. We investigated the localization and interactions among the proteins involved in SNARE complex formation in isolated growth cone particles (GCP) from forebrain. We demonstrated that the SNARE complex is present in GCPs morphologically without synaptic vesicles (SVs) and associated with growth cone vesicles. However, the apparently SV-free GCP was lacking in the regulatory mechanisms inhibiting SNARE complex formation proposed in SV fusion, i.e., the association of synaptotagmin with the SNARE complex, and vesicle-associated membrane protein (VAMP)-synaptophysin complex formation. The core components of the SNARE complex (syntaxin, SNAP-25, and VAMP) accumulated for several days before postnatal day 7, when SVs first appeared, and preceded the accumulation of marker proteins such as synaptophysin, SV2, and V-ATPase. Our present results suggest that the SNARE mechanism for vesicular transmitter release is not fully functional in growth cones before the appearance of SVs, but the SNARE mechanism is working for membrane expansion in growth cones, which supports our recent report. We concluded that the regulation of the SNARE complex in growth cones is different from that in mature presynaptic terminals and that this switching may be one of the key steps in development from the growth cone to the presynaptic terminal.

Upregulation of GABAA current by astrocytes in cultured embryonic rat hippocampal neurons

Embryonic rat hippocampal neurons were cultured on poly-D-lysine (PDL) or a monolayer of postnatal cortical astrocytes to reveal putative changes in neuronal physiology that involve astrocyte-derived signals during the first 4 d of culture, GABA-induced Cl- current (IGABA) was quantified using outside-out and whole-cell patch-clamp recordings beginning at 30 min, when cells had become adherent. The amplitude and density (current normalized to membrane capacitance) of IGABA in neurons grown on astrocytes became statistically greater than that recorded in neurons grown on PDL after 2 hr in culture (HIC). Although the current density remained unchanged in neurons on astrocytes, that in neurons on PDL decreased and became statistically lower beginning after 2 HIC. The differences in amplitude and density of IGABA in the two groups of neurons were maintained during the 4 d experiment. The upregulation effect of astrocytes on neuronal IGABA required intimate contact between the neuronal cell body and underlying astrocytes. Suppression of spontaneous Cac2+ elevations in astrocytes by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid that was loaded intracellularly decreased their modulatory effects on IGABA. IGABA in all cells was blocked completely by bicuculline and exhibited virtually identical affinity constants, Hill coefficients, and potentiation by diazepam in the two groups. Outside-out patch recordings revealed identical unitary properties of IGABA in the two groups. More channels per unit of membrane area could explain the astrocyte enhancement of IGABA. The results reveal that cortical astrocytes potentiate IGABA in hippocampal neurons in a contact-dependent manner via a mechanism involving astrocyte Cac2+ elevation.

Depolarizing GABA-activated Cl- channels in embryonic rat spinal and olfactory bulb cells

1. We have compared the electrical properties of the Cl- channels activated by GABA in cells acutely dissociated from embryonic (E) spinal cord (SC) and olfactory bulb (OB) regions at E15 using different configurations of the patch-recording technique. By in situ analysis these cells express GABAA receptor mRNAs encoding a common set of subunits (alpha 2, beta 2, and beta 3). SC cells also express alpha 3, alpha 5 and gamma 2s transcripts. 2. Whole-cell recordings revealed current responses to GABA (0.5 microM to 1 mM) in 242 out of 294 cells. In both SC and OB cells, currents evoked by 2 microM GABA could be potentiated by diazepam (DZP) in a dose-dependent manner with an EC50 of approximately 50 nM in both SC and OB. The maximal effect was approximately 300%. Both SC and OB cells exhibited GABA-activated currents that were only partially sensitive to zinc even at high micromolar concentrations. The effect of DZP and the relatively modest sensitivity to zinc suggest the presence of gamma subunits in both preparations. 3. Spectral analysis of current responses in twenty-six cells showed that power spectra could be fitted by three exponential components (tau 1-3) in the cells of both areas. The tau of the longest-lasting component (tau 3) was significantly different in the cells of the two areas: approximately 50 ms in OB and 80-100 ms in SC. No statistically significant differences in the average inferred unitary conductance between the two cell types could be resolved. 4. Single-channel properties were examined directly using the cell-attached configuration. GABA-activated channels could be recorded in only 89 out of well-sealed 984 patches and most of them exhibited multiple channel activity. The mean open time in the response to 10 microM GABA was significantly shorter in OB cells (12 ms) compared to SC cells (25 ms) while the average conductance values were not significantly different between the two cell types. 5. On average, Cl- channels reversed polarity when the on-cell patch pipette potential was approximately -30 mV. Thus, in these embryonic neurons, micromolar GABA activates Cl- channels, which, when open, effectively depolarize cells by approximately 30 mV. 6. Cl- channels activated by GABA are open longer in embryonic SC cells than in OB cells. This statistically significant difference in native GABAA receptor Cl- channel properties correlates with, and may be related to differences in subunit mRNA expression.

Characteristics of GABAA channels in rat dentate gyrus

Single channel currents were activated by GABA (0.5 to 5 microM) in cell-attached and inside-out patches from cells in the dentate gyrus of rat hippocampal slices. The currents reversed at the chloride equilibrium potential and were blocked by bicuculline (100 microM). Several different kinds of channel were seen: high conductance and low conductance, rectifying and "nonrectifying." Channels had multiple conductance states. The open probability (Po) of channels was greater at depolarized than at hyperpolarized potentials and the relationship between Po and potential could be fitted with a Boltzmann equation with equivalent valency (z) of 1. The combination of outward rectification and potential-dependent open probability gave very little chloride current at hyperpolarized potentials but steeply increasing current with depolarization, useful properties for a tonic inhibitory mechanism.

Functional heterogeneity of hippocampal GABAA receptors

gamma-Aminobutyric acid type A (GABAA) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose-response curves of rapidly and slowly desensitizing cells revealed EC50 values of 8.5 and 37.3 microM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABAA receptor channels was 28-31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn2+. These data suggest the presence of physiologically and pharmacologically distinct GABAA receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.

Single channel properties at the synaptic site

The properties of single channels activated during spontaneous postsynaptic currents in small cultured rat hippocampal neurons were investigated in low-noise whole cell recordings. The technique of nonstationary fluctuation analysis, which has previously been applied to sodium currents, was modified so that fluctuations were measured around the least-squares scaled fit of the ensemble average to individual synaptic currents. This had the effect of separating channel gating fluctuations from the quantal fluctuations of scale from event to event. Single channel amplitude was estimated from the variance--current distribution, and the kinetics of channel gating fluctuations were studied. Channels involved in the non-N-methyl-D-aspartate (non-NMDA) phase of the excitatory glutamatergic postsynaptic current showed a single channel amplitude of 1.5 pS, while those in the NMDA phase had a conductance of 42 pS. The single channel conductance estimated for the inhibitory chloride synaptic current was 14 pS. In addition, NMDA phase channel openings could be resolved directly in the whole cell current against the low noise level afforded by the small cells. Single channel lifetime and amplitude distributions of the channels activated during the postsynaptic current were measured, and confirmed the accuracy of the fluctuation method.

Desensitization of GABA-induced currents in cultured rat hippocampal neurons

The basic characteristics of desensitization of the GABAA receptor were investigated in cultured rat hippocampal neurons (three days to four weeks in vitro) using whole cell patch clamp techniques. GABA at 10-500 microM was perfused on to neurons for 30 or 60 s, with 60 s intervals of wash with control bath solution between perfusions. Desensitization, evaluated by peak-to-plateau ratio and time constants of current decay (tau), was dose-dependent and culture age-dependent. Desensitization was observed as early as three days in culture, the earliest time tested. At all ages, higher concentrations of GABA induced both larger and faster desensitization. Desensitization was markedly voltage-dependent and decreased with depolarization; peak-to-plateau ratio went from 6.3 to 1.4 and tau went from 4.6 to 26.8 s when holding potentials were changed from -80 mV to +30 mV. Low concentrations of GABA (1-2 microM) perfused for 2-60 s, which did not induce any current, had no effect on the maximal response nor desensitization produced by a subsequent application of 100 microM GABA. This finding suggests that GABA receptors were not desensitized without first being activated.

Non-uniform distribution of GABA activated chloride channels in cultured cortical neurons

Several different neurotransmitter receptors and ion channels exist in non-random distributions in the membrane of nerve and muscle cells, a pattern which appears to have functional implications. The data presented here, based on the distribution of single channels in outside-out patches, directly demonstrate that receptors for gamma-aminobutyric acid (GABA) and their associated chloride channels (GABAA receptors) exist almost exclusively in clusters in the membranes of cultured rat cortical neurons. This non-uniform distribution is present both in innervated and uninnervated neurons.

Nonstationary fluctuation analysis and direct resolution of single channel currents at postsynaptic sites

In order to measure unitary properties of receptor channels at the postsynaptic site, the noise within the decay phases of inhibitory postsynaptic currents (IPSCs) and of N-methyl-D-aspartate (NMDA)-dependent excitatory postsynaptic currents (EPSCs) in rat hippocampal neurons was studied by nonstationary fluctuation analysis. Least squares scaling of the mean current was used to circumvent the wide variation in amplitude of postsynaptic currents. The variance of fluctuations around the expected current was analyzed to calculate single channel conductance, and fluctuation kinetics were studied with power spectra. The single channel conductance underlying the IPSC was measured as 14 pS, whereas that underlying the EPSC was 42 pS. Openings of the EPSC channel could also be resolved directly in low-noise whole-cell recordings, allowing verification of the accuracy of the fluctuation analysis. The results are the first measurements of the properties of single postsynaptic channels activated during synaptic currents, and suggest that the technique can be widely applicable in investigations of synaptic mechanism and plasticity.

Variation in the pattern of [Ca2+]i change induced by acetylcholine in cultured hippocampal neurons

Acetylcholine (ACh) caused various patterns of change in the intracellular Ca2+ concentration ([Ca2+]i) in cultured rat hippocampal neurons. We studied the underlying mechanisms of the [Ca2+]i changes with simultaneous recording of [Ca2+]i and membrane potential/current. In most cases, [Ca2+]i rise was accompanied by a membrane depolarization. The [Ca2+]i change was significantly reduced when the membrane was voltage clamped, which implies that most of the [Ca2+]i rise results from the Ca2+ influx through the voltage-gated Ca2+ channel activated by the membrane depolarization. The membrane depolarizations were classified into two types, one associated with membrane conductance decrease and the other associated with membrane conductance increase. The former results from potassium conductance ((gK+) decrease, and the latter may result from the activation of a Na(+)-permeable channel. However, [Ca2+]i elevation was also observed in some neurons showing membrane hyperpolarization in response to ACh. This seems to show that ACh liberates Ca2+ from the intracellular Ca2+ store, resulting in the activation of a calcium-dependent K+ channel (KCa). The variations of ACh response in the hippocampal neurons seem to result from a variety of muscarinic acetylcholine receptors and various species of ion channels governed by those receptors.

Change in calcium permeability caused by quinolinic acid in cultured rat hippocampal neurons

The calcium permeability of receptor channels activated by quinolinic acid (QUIN) in cultured rat hippocampal neurons was investigated using the whole-cell voltage-clamp method. In Na+-free, 10 mM Ca2+ solution with the internal solution containing 165 mM Cs+, QUIN elicited prominent inward currents at -60 mV, and the reversal potential of the QUIN-induced current was -5.8 +/- 1.2 mV, indicating that QUIN-activated channels are highly permeable to Ca2+ (permeability ratio PCa2+/PCs+ = 5.9). This result was substantiated by microfluorometry using fura-2, which revealed that QUIN caused a marked increase in the intracellular Ca2+ concentration even after the voltage-dependent Ca2+ channels had been suppressed by La3+.

Frequency-dependent depression of inhibition in guinea-pig neocortex in vitro by GABAB receptor feed-back on GABA release

1. The mechanisms involved in the lability of inhibition at higher frequencies of stimulation were investigated in the guinea-pig in vitro neocortical slice preparation by intracellular recording techniques. We attempted to test the possibility of a feedback depression of GABA on subsequent release. 2. At resting membrane potential (Em, -75.8 +/- 5.2 mV) stimulation of either the pial surface or subcortical white matter evoked a sequence of depolarizing and hyperpolarizing synaptic components in most neurones. An early hyperpolarizing component (IPSPA) was usually only obvious as a pronounced termination of the EPSP, followed by a later hyperpolarizing event (IPSPB). Current-voltage relationships revealed two different conductances of about 200 and 20 nS and reversal potentials of -73.0 +/- 4.4 and -88.6 +/- 6.1 mV for the early and late component, respectively. 3. The conductances of IPSPA and IPSPB were fairly stable at a stimulus frequency of 0.1 Hz. At frequencies between 0.5 and 2 Hz both IPSPs were attenuated with the second stimulus and after about five stimuli a steady state was reached. Concomitantly IPSPs were shortened. The average decrease in synaptic conductance between 0.1 and 1 Hz was 80% for the IPSPA and 60% for the IPSPB. At these frequencies the reversal potentials decreased by 5 and 2 mV, respectively; Em and input resistance (Rin) were not consistently affected. 4. The amplitudes of field potentials, action potentials and EPSPs of pyramidal cells were attenuated less than 10% at stimulus frequencies up to 1 Hz, suggesting that alterations in local circuits between the stimulation site and excitatory input onto inhibitory interneurones may play only a minor role in the frequency-dependent decay of IPSPs. 5. Localized application of GABA produced multiphasic responses. With low concentrations and application near the soma an early hyperpolarization prevailed followed by a depolarizing late component. Brief application of GABA at low frequencies induced constant responses; at higher frequencies, the responses sometimes declined. The current-voltage relationships of the two GABA responses were similar to each other and to the early IPSP. An apparently fivefold higher conductance was estimated at lower Ems, suggesting that the GABA response had a voltage sensitivity. The slope conductance of IPSPs was decreased by up to 50% for tens of seconds after postsynaptically detectable effects of GABA had dissipated. 6. Application of the GABA uptake inhibitor nipecotic acid (50-500 microM) reduced the conductance of both components of orthodromically evoked inhibition and shortened the IPSP at low frequencies, but had no additional effects at higher stimulation rates.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of GABA on neurotransmission in frog tectal slices

The regional distribution of gamma-aminobutyric acid (GABA) was determined in the central nervous system of the frog. The highest amount of GABA was found in the tectum (35.7 +/- 2.3 mmol/kg protein). A thin tectal slice (400-600 microns thick) preparation was developed in which the postsynaptic potential (PSP) could be elicited. The addition of GABA to the perfusion medium depressed the PSP in a dose-dependent manner in the concentration range 1-12.5 mM at 28 degrees C. Nipecotic acid, a GABA uptake inhibitor, at a concentration of 0.5 mM shifted the dose-response curve for GABA to the left. These results from the frog, a cold-blooded animal, were in good agreement with those for hippocampal slices from the guinea-pig.

Ion channels activated by quinolinic acid in cultured rat hippocampal neurons

The membrane responses to quinolinic acid, an excitotoxic brain metabolite, were studied in cultured rat hippocampal neurons with the patch-clamp technique. In the whole-cell recording mode, pressure applications of quinolinic acid elicited inwardly directed membrane currents over a membrane potential range of -60 to -5 mV. The current response reversed at about 0 mV. The current-voltage (I-V) relation of the response had a negative slope conductance at membrane potentials more negative than -40 mV. On removal of Mg2+ from the extracellular solution, the current response showed no region of negative slope conductance at potentials more positive than -60 mV. In Mg2+-free solution applications of quinolinic acid elicited discrete pulse-like current flows through the outside-out membrane patch. The single channel conductance was 40-46 pS over a membrane potential range of -40 to -80 mV, and 50-55 pS at membrane potentials more positive than +30 mV, showing an outward rectification. These values of the single channel conductance were similar to those of the main conducting state of the channels activated by N-methyl-D-aspartate (NMDA). The responses to quinolinic acid were completely suppressed by the NMDA receptor antagonist (+/-)-2-amino-5-phosphonovaleric acid. The results indicate that quinolinic acid selectively activates NMDA receptors in the cultured rat hippocampal neurons.

Characterization of GABA- and glycine-induced currents of solitary rodent retinal ganglion cells in culture

Ganglion cells were fluorescently labeled, dissociated from 7- to 11-day-old rodent retinas, and placed in tissue culture. Whole-cell recordings with patch electrodes were obtained from solitary cells lacking processes, which permitted a high-quality space clamp. Both GABA (1-200 microM) and glycine (10-300 microM) produced large increases in membrane conductance in virtually every ganglion cell tested, including ganglion cells from different size classes in both rats and mice. Taurine evoked responses similar to those of glycine, but considerably greater concentrations of taurine (150-300 microM) were necessary to observe any effect. Since 20 microM GABA produced approximately the same response as 100 microM glycine, the effects of these two concentrations were compared under various conditions. When recording with chloride distributed equally across the membrane, the reversal potential of the agonist-induced currents was approximately 0 mV. When the internal chloride was reduced by substitution with aspartate, the reversal potential shifted in a negative direction by about 42 mV, indicating that the current was carried mainly by chloride ions. Strychnine (1-5 microM) completely and reversibly blocked the actions of glycine (100 microM) but not those of GABA (20 microM); however, higher concentrations of strychnine (20 microM) nearly totally inhibited the current elicited by GABA (20 microM). The responses to glycine (100 microM) were not affected by bicuculline methiodide (20 microM) or picrotoxinin (20 microM). In contrast, bicuculline methiodide (10 microM) and picrotoxinin (10 microM) reversibly blocked the current evoked by GABA (20 microM); d-tubocurarine (100 microM) only slightly decreased the response to GABA (20 microM). The antagonists were effective over a wide range of holding potentials (-90 mV to +30 mV). The responses to a steady application of both GABA and glycine decayed in a few seconds when recorded under conditions of both symmetric and asymmetric chloride across the membrane. During this decay the current and conductance decreased simultaneously, reflecting receptor desensitization rather than a change in the driving force for chloride caused by agonist-induced ionic fluxes. The time-course of desensitization was usually described by a single exponential with time constants for GABA (20 microM) and glycine (100 microM) of 4.0 +/- 1.6 s and 4.4 +/- 1.9 s (mean +/- S.D.), respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Cation permeability change caused by L-glutamate in cultured rat hippocampal neurons

The ionic mechanism of the membrane permeability changes caused by L-glutamate in hippocampal neurons prepared from 17- to 19-day-old fetal rat in dispersed cell cultures was studied with the whole-cell variation of the patch electrode voltage-clamp technique. The cultured hippocampal neurons became sensitive to glutamate 7 days after plating, and thereafter the sensitivity gradually increased. The conductance increase caused by glutamate was voltage-sensitive, decreasing with membrane hyperpolarization at potentials more negative than -40 mV. The relative permeability of glutamate-activated channels to alkali metal and alkaline earth cations was estimated by reversal potential measurements. The alkali metal cations, Li+, Na+, K+, Rb+ and Cs+ were permeant to the glutamate channels, and the selectivity among them was weak. The alkaline earth cations, Ca2+, Sr2+ and Ba2+ were more permeant than the alkali metals. The permeability ratios of these divalent cations relative to Na+ were 2.4 (Ca2+), 2.4 (Sr2+) and 2.8 (Ba2+), respectively. Mg2+ was much less permeant and the permeability ratio (PMg/PNa) was only 0.1. Anion conductance made no contribution to the glutamate-induced current. Functional implications of the glutamate-induced increased in Ca2+-influx were discussed.

The effects of L-glutamate on cultured insect neurones

The effects of L-glutamate on insect cultured neurones were studied under current and voltage-clamp conditions using conventional and whole-cell patch-clamp techniques. Brief pressure or iontophoretic application of L-glutamate produced either a depolarisation or hyperpolarisation. The current underlying the depolarisation was inwardly directed and reversed at around 0 mV while the hyperpolarisation was caused by an outward current that reversed between -60 and -80 mV. Single channel currents underlying the depolarisation were readily recorded from cell attached patches and showed multiple conductance states. Channel activity corresponding to the hyperpolarising response has not yet been observed.

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).

Spontaneous and GABA-evoked chloride channels on pituitary intermediate lobe cells and their internal Ca requirements

On porcine intermediate lobe (IL) endocrine cells, spontaneously opening chloride channels have been studied and compared to GABA-A activated chloride channels. Elementary currents were recorded mainly from outside-out patches excised from IL cells maintained in culture for 1-4 weeks. Spontaneous inward currents were observed in Cs-loaded cells after replacing Na in the extracellular medium by the impermeant ion choline. This activity, at an internal calcium concentration of 10(-8) M corresponded to a channel for chloride ions with a main conductance level of 26 pS, and substates around 11 pS. The sequence of permeabilities to halides was I greater than Br greater than Cl. These conductance characteristics were common to the GABA-operated channels which also showed a main conductance substate of 23-31 pS. The open time of the 26 pS level mostly encountered in spontaneous activity, was distributed along two modes: one, the most frequent, around 1 ms, and the other around 4 ms. This latter mode was the predominant one observed during GABA and isoguvacine applications but in addition a bursting activity of 19 ms duration was also seen. Specific GABA-A receptor antagonists (bicuculline and SR42641, 1 microM) blocked activity evoked by GABA (1-10 microM), but did not affect spontaneous events. These spontaneous Cl events were only observed in a restricted range of internal Ca concentrations, i.e. between 1 nM and 0.1 microM, and were practically abolished at Cai 1 microM. The GABA-induced activity of Cl channels was also Ca-sensitive, being reduced when Cai reached 1 microM.

gamma-Aminobutyric acid (GABA) mediated transmembrane chloride flux with membrane vesicles from rat brain measured by quench flow technique: kinetic homogeneity of ion flux and receptor desensitization

Transmembrane chloride flux mediated by the GABAA receptor and the desensitization of the receptor were followed using quench flow technique with 36Cl- and a membrane preparation from rat cerebral cortex. Measurements in short times allowed these two processes to be resolved. In general the ion-flux activity was desensitized in two phases. A fast phase took place in circa 200 ms (100 microM GABA) followed by a slower phase in several seconds. A minority (10%) of the membrane preparations did not display the fast phase. It is desirable to be able to separate these two phases of desensitization to facilitate analysis of the responses of the receptor. A short preincubation with GABA removed the fast phase from a subsequent measurement. In the absence of the fast phase the whole ion-flux equilibration was seen as a single phase. The measurements presented covering a time range of 0.01 seconds to 10 seconds show a single phase of ion flux which can be described by a first order ion influx process and a single first order desensitization process with a half time of circa 1 s (100 microM GABA). The results imply a single kinetically homogeneous population of vesicles containing a single population of GABA receptor (remaining active) with a single phase of desensitization. An understanding of this homogeneity, and how to ensure it, gives a basis for quantitatively testing the effects of drugs on these responses. Ion flux measurements with quench flow technique are a suitable tool for investigation of the mechanism of action of neurotransmitter receptors from brain.

Two desensitization processes of GABA receptor from rat brain. Rapid measurements of chloride ion flux using quench-flow techniques

Two rapid phases of GABA receptor desensitization, which proceeded with a 10-fold difference in rates, were detected in two types of experiment with membrane vesicle preparations from rat cerebral cortex. The time course of GABA-mediated 36Cl- influx progressed in two phases. The 36Cl- influx was decreased, by preincubation with GABA, in two phases. Measurements were made in the time range 10-1000 ms. The major loss of channel opening activity occurred in the faster phase, which was complete in 100 ms with saturating GABA concentrations. The remaining activity decreased in a slower phase in a few seconds with a 10-fold slower rate. The faster phase of desensitization was more than 10-fold faster than previously observed and the slower phase was slightly faster than previously reported measurements with GABA receptor. Both desensitization processes had a similar dependence on GABA concentration with a half response at approximately 100 microM GABA.

Potentiation of gamma-aminobutyric-acid-activated chloride conductance by a steroid anaesthetic in cultured rat spinal neurones

1. Intracellular recordings from cultured rat spinal cord neurones demonstrated that Cl(-)-dependent responses to GABA (gamma-aminobutyric acid) (but not glycine) were increased in amplitude and duration by the steroid anaesthetic alphaxalone (3 alpha-hydroxy-5 alpha-pregnane-11,20-dione) at submicromolar concentrations that produced little or no effect on passive electrical properties. The non-anaesthetic 3 beta-hydroxy analogue was without effect on GABA-evoked responses. 2. Under voltage clamp, membrane currents evoked by GABA were potentiated by alphaxalone without change in the reversal potential for the GABA-evoked response. Fluctuation analysis of GABA-evoked currents suggested that the mean open-time of GABA-activated channels was prolonged from 30 to 74 ms in the presence of the anaesthetic. 3. Higher concentrations of alphaxalone, similar to those reported during surgical anaesthesia, increased membrane conductance in the absence of exogenously applied GABA. Under voltage clamp, current responses to alphaxalone reversed at the same potential as did responses to GABA, suggesting that they result from increased Cl- conductance. 4. Alphaxalone responses were reduced by the GABA antagonist bicuculline. Fluctuation analysis of current responses to the anaesthetic suggest that they result from the activation of ion channels of long (100 ms) open-time and elementary conductance indistinguishable from that of channels activated by GABA (20 pS). Taken together, these findings indicate that the steroid anaesthetic is able to directly activate Cl- conductance normally activated by GABA in spinal neurones. 5. The actions of the steroid at GABA-receptor-Cl(-)-channel complexes are similar to those produced by the anaesthetic barbiturates (e.g. pentobarbitone), although obtained at 50-100-fold lower concentrations. These effects on the inhibitory Cl(-)-conductance mechanism may be partly responsible for the depressant actions of alphaxalone on the mammalian central nervous system.

Blocking effects of cobalt and related ions on the gamma-aminobutyric acid-induced current in turtle retinal cones

Red-sensitive cone photoreceptors were isolated from the turtle retina, and GABA-induced currents were recorded under voltage clamp. The effect of Co2+, widely used as a blocker of chemical synapses, on the GABA-induced current was studied. Co2+ blocked the GABA-induced current evoked by local application either at the synaptic region (cone pedicle) or at the extra-synaptic region (cell body). 5 microM-Co2+ suppressed the GABA-induced current by 50%, and a few hundred microM-Co2+ blocked it almost completely. Co2+ suppressed the GABA-induced current non-competitively: the saturating response amplitude decreased without a change in the threshold or saturating dose of GABA. The blocking was not voltage dependent in the physiological range of the membrane potential. Ni2+ and Cd2+ also blocked the GABA-induced current non-competitively, and were as effective as Co2+. Tetraethylammonium (25 mM) showed a similar but weaker blocking effect. On the other hand, Mg2+ (20 mM), Mn2+, Sr2+, Ba2+ (10-100 microM each), D-600 (10 microM) or Cs+ (10 mM) did not affect the GABA-induced current. The Ca current in the turtle cones was blocked almost completely by 20 mM-Mg2+ or 4 mM-Co2+, or strongly suppressed by 10 microM-D-600. However, Cd2+ and Ni2+ (10 microM each) blocked the Ca current by ca. 50%, and Co2+ and Mn2+ (10 microM each) suppressed it only partially. The blocking of the GABA-induced current by these agents was, therefore, not directly related to the blocking of the Ca current and/or Ca-mediated currents. These observations present a warning on the use of some divalent cations, such as Co2+, Ni2+ or Cd2+, as a presynaptic blocker at the GABAergic synapse. High concentrations of Mg2+ are recommended as a more appropriate blocker.

GABA activates different types of chloride-conducting receptor-ionophore complexes in a dose-dependent manner

We report here evidence for 3 new subtypes (alpha 1, alpha 2 and beta) of type-A GABA receptor-channel complexes that conduct chloride ions. The chloride current, ICl, was isolated in the frog sensory neuron by a combination of voltage clamp and internal perfusion. Analysis of the variance of GABA-induced ICl fluctuations shows that the channel population N decreases exponentially with single-channel conductance gamma in such a way that alpha 2 less than alpha 1 less than beta for gamma and alpha 2 much greater than alpha 1 greater than beta for N, and that the population-rank plot fits Zipf's law. Various aspects of the GABA-induced ICl are understood from dose-dependent activation and inactivation of these functionally distinct receptor-channel types. The steady-state ICl is mediated by alpha 1 at low but by beta units at high GABA concentrations, and the pronounced ICl peak at intermediate and high doses reflects the desensitization of alpha 1 and alpha 2 receptors, respectively. Picrotoxin blocks alpha 1 and alpha 2 and has no effect on beta channels. Patch-clamp recordings indicate two distinct classes of GABA-gated chloride conductances that appear to correspond to the alpha 1 and beta types. The presence of these different ICl components explains why the dose-response relationship cannot be fitted well by a single Hill equation; the fitting requires a synthesis of 3 suitable Hill equations.