Multiple mechanisms of antagonism of gamma-aminobutyric acid (GABA) responses

Synaptic Zinc Enhances Inhibition Mediated by Somatostatin, but not Parvalbumin, Cells in Mouse Auditory Cortex

Cortical inhibition is essential for brain activity and behavior. Yet, the mechanisms that modulate cortical inhibition and their impact on sensory processing remain less understood. Synaptically released zinc, a neuromodulator released by cortical glutamatergic synaptic vesicles, has emerged as a powerful modulator of sensory processing and behavior. Despite the puzzling finding that the vesicular zinc transporter (ZnT3) mRNA is expressed in cortical inhibitory interneurons, the actions of synaptic zinc in cortical inhibitory neurotransmission remain unknown. Using in vitro electrophysiology and optogenetics in mouse brain slices containing the layer 2/3 (L2/3) of auditory cortex, we discovered that synaptic zinc increases the quantal size of inhibitory GABAergic neurotransmission mediated by somatostatin (SOM)- but not parvalbumin (PV)-expressing neurons. Using two-photon imaging in awake mice, we showed that synaptic zinc is required for the effects of SOM- but not PV-mediated inhibition on frequency tuning of principal neurons. Thus, cell-specific zinc modulation of cortical inhibition regulates frequency tuning.

Methods for recording and measuring tonic GABAA receptor-mediated inhibition

Tonic inhibitory conductances mediated by GABA_A receptors have now been identified and characterized in many different brain regions. Most experimental studies of tonic GABAergic inhibition have been carried out using acute brain slice preparations but tonic currents have been recorded under a variety of different conditions. This diversity of recording conditions is likely to impact upon many of the factors responsible for controlling tonic inhibition and can make comparison between different studies difficult. In this review, we will firstly consider how various experimental conditions, including age of animal, recording temperature and solution composition, are likely to influence tonic GABA_A conductances. We will then consider some technical considerations related to how the tonic conductance is measured and subsequently analyzed, including how the use of current noise may provide a complementary and reliable method for quantifying changes in tonic current.

A novel, rapid, inhibitory effect of insulin on alpha1beta2gamma2s gamma-aminobutyric acid type A receptors

In the CNS, GABA and insulin seem to contribute to similar processes, including neuronal survival; learning and reward; and energy balance and food intake. It is likely then that insulin and GABA may interact, perhaps at the GABA(A) receptor. One such interaction has already been described [Q. Wan, Z.G. Xiong, H.Y. Man, C.A. Ackerley, J. Braunton, W.Y. Lu, L.E. Becker, J.F. MacDonald, Y.T. Wang, Recruitment of functional GABA(A) receptors to postsynaptic domains by insulin, Nature 388 (1997) 686-690]; in it a micromolar concentration of insulin causes the insertion of GABA(A) receptors into the cell membrane, increasing GABA current. I have discovered another effect of insulin on GABA(A) currents. Using a receptor isoform, alpha(1)beta(2)gamma(2s) that is the likely main neuronal GABA(A) isoform expressed recombinantly in Xenopus oocytes, insulin inhibits GABA-induced current when applied simultaneously with low concentrations of GABA. Insulin will significantly inhibit currents induced by EC(30-50) concentrations of GABA by about 38%. Insulin is potent in this effect; IC(50) of insulin was found to be about 4.3 x 10(-10) M. The insulin effect on the GABA dose responses looked like that of an antagonist similar to bicuculline or beta-carbolines. However, an effect of phosphorylation on the GABA(A) receptor from the insulin receptor signal transduction pathway cannot yet be dismissed.

Mapping convulsants' binding to the GABA-A receptor chloride ionophore: a proposed model for channel binding sites

Gamma-aminobutyric acid (GABA) type A receptors play a key role in brain inhibitory neurotransmission, and are ligand-activated chloride channels blocked by numerous convulsant ligands. Here we summarize data on binding of picrotoxin, tetrazoles, beta-lactams, bicyclophosphates, butyrolactones and neurotoxic pesticides to GABA-A ionophore, and discuss functional and structural overlapping of their binding sites. The paper reviews data on convulsants' binding sensitivity to different point mutations in ionophore-lining second trans-membrane domains of GABA-A subunits, and maps possible location of convulsants' sites within the chloride ionophore. We also discuss data on inhibition of glycine, glutamate, serotonin (5-HT3) and N-acetylcholine receptors by GABA-A channel blockers, and examine the applicability of this model to other homologous ionotropic receptors. Positioning various convulsant-binding sites within ionophore of GABA-A receptors, this model enables a better understanding of complex architectonics of ionotropic receptors, and may be used for developing new channel-modulating drugs.

Heavy metals modulate the activity of the purinergic P2X4 receptor

To further characterize the nature of the regulatory metal-binding sites of the rat P2X(4) receptor, several transition heavy metals were tested to examine their ability to mimic the facilitator action of zinc or the inhibitory action of copper. cDNA coding for the rat P2X(4) receptor was injected into Xenopus laevis oocytes; the two-electrode voltage-clamp technique was used to measure and quantify the ATP-evoked currents in the absence or presence of the metals. Cadmium facilitated the ATP-gated currents in a reversible and voltage-independent manner; maximal potentiation occurred within less than 1 min. Cadmium displaced leftward, in a concentration-dependent manner, the ATP concentration-response curve. In contrast, mercury reduced the ATP-gated currents in a reversible, time, and concentration manner. Maximal inhibition occurred after about 5 min of metal application. Cobalt also augmented the ATP-evoked currents, but its action was long lasting and did not reverse even after 45 min of metal washout. Other metals such as lead, nickel, manganese, silver, or gallium did not significantly alter the ATP-gated currents. The co-application of cadmium plus zinc or mercury plus copper caused additive effects. Mutation of H140 by alanine (H140A) augmented both the cadmium-induced facilitation and the mercury-induced inhibition. In contrast, the H241A mutant showed characteristics indistinguishable from the wild type. The H286A mutant showed a normal cadmium-induced potentiation, but an increased mercury inhibition. Out of the metals examined, only cadmium mimicked closely the action of zinc, evidencing commonalities. While mercury mimicked the action of copper, both metals apparently interact at distinct metal-binding sites. The present findings allow us to infer that heavy metals modulate the P2X(4) receptor by acting in at least three separate metal-binding sites.

Modulation by bicuculline and penicillin of the block by t-butyl-bicyclo-phosphorothionate (TBPS) of GABA(A)-receptor mediated Cl(-)-current responses in rat striatal neurones

1. T-butyl-bicyclo-phosphorothionate (TBPS) is a prototypical representative of the cage-convulsants which act through a use-dependent block of the GABA(A)-receptor-ionophore complex. Using current recordings from cultured neurones of rat striatum the manner was investigated in which two antagonists, bicuculline and penicillin, presumably acting at the agonist binding site and in the ionic channel, respectively, modify the rate of block by TBPS. 2. Penicillin (5 or 10 mM) did not slow the rate of block by TBPS, but produced a significant enhancement of block rate, which, however, was inversely related to the degree of antagonism by penicillin of the GABA-induced current. 3. Bicuculline (10 microM) reduced the rate of block by TBPS. However, this effect was 3 fold weaker than its GABA-antagonistic action. The slowing of block rate and the current antagonism exhibited a biphasic, positive-negative relationship. Co-application of bicuculline (100 microM) in a concentration that produced nearly complete antagonism and TBPS (10 microM) resulted in a marked ( approximately 40%) reduction of subsequent GABA response amplitudes compatible with a direct, bicuculline-induced conformational change in the receptor required for the binding of and block by TBPS. 4. The lack of protection afforded by the channel blocker penicillin as well as the lack of correlation between bicuculline antagonism of the Cl(-)-current and its efficiency in protecting against TBPS block is evidence against an open channel blocking mechanism for TBPS. TBPS does, therefore, not appear to gain access to its binding site via the open pore but through alternative routes regulated from the agonist binding site.

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.

The alpha9 nicotinic acetylcholine receptor shares pharmacological properties with type A gamma-aminobutyric acid, glycine, and type 3 serotonin receptors

In the present study, we provide evidence that the alpha9 nicotinic acetylcholine receptor (nAChR) shares pharmacological properties with members of the Cys-loop family of receptors. Thus, the type A gamma-aminobutyric acid receptor antagonist bicuculline, the glycinergic antagonist strychnine, and the type 3 serotonin receptor antagonist ICS-205,930 block ACh-evoked currents in alpha9-injected Xenopus laevis oocytes with the following rank order of potency: strychnine > ICS-205,930 > bicuculline. Block by antagonists was reflected in an increase in the acetylcholine (ACh) EC50 value, with no changes in agonist maximal response or Hill coefficient, which suggests a competitive type of block. Moreover, whereas neither gamma-aminobutyric acid nor glycine modified ACh-evoked currents, serotonin blocked responses to ACh in a concentration-dependent manner. The present results suggest that the alpha9 nAChR must conserve in its primary structure some residues responsible for ligand binding common to other Cys-loop receptors. In addition, it adds further evidence that the alpha9 nAChR and the cholinergic receptor present at the base of cochlear outer hair cells have similar pharmacological properties.

Zn2+ inhibition of recombinant GABAA receptors: an allosteric, state-dependent mechanism determined by the gamma-subunit

1. The gamma-subunit in recombinant gamma-aminobutyric acid (GABAA) receptors reduces the sensitivity of GABA-triggered Cl- currents to inhibition by Zn2+ and transforms the apparent mechanism of antagonism from non-competitive to competitive. To investigate underlying receptor function we studied Zn2- effects on macroscopic and single-channel currents of recombinant alpha 1 beta 2 and alpha 1 beta 2 gamma 2 receptors expressed heterologously in HEK-293 cells using the patch-clamp technique and rapid solution changes. 2. Zn2+ present for > 60 s (constant) inhibited peak, GABA (5 microM)-triggered currents of alpha 1 beta 2 receptors in a concentration-dependent manner (inhibition equation parameters: concentration at half-amplitude (IC50) = 0.94 microM; slope related to Hill coefficient, S = 0.7) that was unaffected by GABA concentration. The gamma 2 subunit (alpha 1 beta 2 gamma 2 receptor) reduced Zn2+ sensitivity more than fiftyfold (IC50 = 51 microM, S = 0.86); increased GABA concentration (100 microM) antagonized inhibition by reducing apparent affinity (IC50 = 322 microM, S = 0.79). Zn2+ slowed macroscopic gating of alpha 1 beta 2 receptors by inducing a novel slow exponential component in the activation time course and suppressing a fast component of control desensitization. For alpha 1 beta 2 gamma 2 receptors, Zn2+ accelerated a fast component of apparent desensitization. 3. Zn2+ preincubations lasting up to 10 s markedly increased current depression and activation slowing of alpha 1 beta 2 receptors, but had little effect on currents from alpha 1 beta 2 gamma 2 receptors. 4. Steady-state fluctuation analysis of macroscopic alpha 1 beta 2 gamma 2 currents (n = 5) resulted in control (2 microM GABA) power density spectra that were fitted by a sum of two Lorentzian functions (relaxation times: 37 +/- 5.6 and 1.41 +/- 0.15 ms, means +/- S.E.M.). Zn2+ (200 microM) reduced the total power almost sixfold and accelerated the slow (23 +/- 2.8 ms, P < 0.05) without altering the fast (1.40 +/- 0.16 ms) relaxation time. The ratio (fast/slow) of Lorentzian areas was increased by Zn2+ (control, 3.39 +/- 0.55; Zn2+, 4.9 +/- 0.37, P < 0.05). 5. Zn2+ (500 microM) depression of previously activated current amplitudes (% control) for alpha 1 beta 2 gamma 2 receptors was independent of GABA concentration (5 microM, 13.2 +/- 0.72%; 100 microM, 12.2 +/- 2.9%, P < 0.8, n = 5). Both onset and offset inhibition time courses were biexponential. Onset rates were enhanced by Zn2+ concentration. Inhibition onset was also biexponential for preactivated alpha 1 beta 2 receptors with current depression more than fourfold less sensitive (5 microM GABA, IC50 = 3.8 microM, S = 0.84) relative to that in constant Zn2+. 6. The results lead us to propose a general model of Zn2+ inhibition of GABAA receptors in which Zn2+ binds to a single extracellular site, induces allosteric receptor inhibition involving two non-conducting states, site affinity is state-dependent, and the features of state dependence are determined by the gamma-subunit.

Bicuculline and gabazine are allosteric inhibitors of channel opening of the GABAA receptor

Anesthetic drugs are known to interact with GABAA receptors, both to potentiate the effects of low concentrations of GABA and to directly gate open the ion channel in the absence of GABA; however, the site(s) involved in direct gating by these drugs is not known. We have studied the ability of alphaxalone (an anesthetic steroid) and pentobarbital (an anesthetic barbiturate) to directly activate recombinant GABAA receptors containing the alpha 1, beta 2, and gamma 2L subunits. Steroid gating was not affected when either of two mutated beta 2 subunits [beta 2 (Y157S) and beta 2 (Y205S)] are incorporated into the receptors, although these subunits greatly reduce the affinity of GABA binding. These observations indicate that steroid binding and subsequent channel gating do not require these particular residues, as already shown for barbiturates. Bicuculline or gabazine (two competitive antagonists of GABA binding) reduced the currents elicited by alphaxalone and pentobarbital from wild-type GABAA receptors; however, gabazine produced only a partial block of response pentobarbital or alphaxalone, and bicuculline only partially blocked responses to pentobarbital. These observations indicate that the blockers do not compete with alphaxalone or pentobarbital for a single class of sites on the GABAA receptor. Finally, at receptors containing alpha 1 beta 2 (Y157S) gamma 2L subunits, both bicuculline and gabazine showed weak agonist activity and actually potentiated responses to alphaxalone. These observations indicate that the blocking drugs can produce allosteric changes in GABAA receptors, at least those containing this mutated beta 2 subunit. We conclude that the sites for binding steroids and barbiturates do not overlap with the GABA-binding site. Furthermore, neither gabazine nor bicuculline competes for binding at the steroid or barbiturate sites. The data are consistent with a model in which both gabazine and bicuculline act as allosteric inhibitors of channel opening for the GABAA receptor after binding to the GABA-binding site.

GABA receptors of bipolar cells from the skate retina: actions of zinc on GABA-mediated membrane currents

GABA receptors of bipolar cells from the skate retina: actions of zinc on GABA-mediated membrane currents. J. Neurophysiol. 78: 2402-2412, 1997. gamma-Aminobutyric acid (GABA)-induced currents were recorded from isolated bipolar cells of the skate retina using perforated patch-clamp methodology. Pharmacological analysis of the responses, using selective agonists and antagonists of the major classes of GABA receptor, revealed the presence of both GABAA and GABAC receptors at both the dendrites and axon terminals of the bipolar cells. The two receptor types showed very different reactions to zinc, a divalent metallic cation that was detected in the synaptic terminal region of skate photoreceptors. Currents mediated by the activation of GABAC receptors were down-regulated by zinc, a feature that is typical of the action of zinc on GABAC receptors. On the other hand, the effects of zinc on GABAA receptor-mediated activity was highly dependent on zinc concentration. Unlike the GABAA receptors on other neurons, responses mediated by activation of the GABAA receptor of skate bipolar cells were significantly enhanced by zinc concentrations in the range of 0. 1-100 mu M; at higher concentrations of zinc (>100 mu M), response amplitudes were suppressed below control levels. The enhancement of GABAA receptor activity on skate bipolar cells showed little voltage dependence, suggesting that zinc is acting on the extracellular domain of the GABAA receptor. In the presence of 10 mu M zinc, the dose-response curve for 4,5,6, 7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; a GABAA agonist that suppresses GABAC-activated currents) was shifted to the left of the curve obtained in the absence of zinc, but without a significant change in the response maximum. This finding indicates that the enhancing effect of zinc is due primarily to its ability to increase the sensitivity of the GABAA receptor. The novel enhancement of neuronal GABAA receptor activity by zinc, observed previously in the GABAA-mediated responses of skate Müller (glial) cells, may reflect the presence of a unique subtype of GABAA receptor on the bipolar and Müller cells of the skate retina.

Allosteric modulation of GABAA receptors in acutely dissociated neurons of the suprachiasmatic nucleus

The gamma-aminobutyric acid (GABA)-induced response was investigated in acutely dissociated suprachiasmatic nucleus (SCN) neurons of 11- to 14-day-old rats, under the voltage-clamp condition of nystatin-perforated patch recording. At a holding potential of -40 mV, application of GABA induced inward currents in a concentration-dependent manner. Pentobarbital and 5 beta-pregnan-3 alpha-ol-20-one (pregnanolone) similarly induced inward currents. GABA-induced inward currents were suppressed in a concentration-dependent manner by pretreating neurons with a GABAA receptor antagonist, bicuculline. Bicuculline (3 x 10(-6) M) shifted the concentration-response curve of GABA to the left in a competitive manner. Reversal potential of the GABA response (EGABA) was -3.4 +/- 0.7 mV, close to the theoretical Cl- equilibrium potential of -4.1 mV. Pretreating SCN neurons with diazepam, pentobarbital, and pregnanolone enhanced the 3 x 10(-6) M GABA response. Diazepam (3 x 10(-8) M), pentobarbital (3 x 10(-5) M), and pregnanolone (10(-7) M) shifted the concentration-response curve of GABA to the left without changing the maximal amplitude of GABA responses. EGABA in the presence of diazepam, pentobarbital, or pregnanolone was the same as that in their absence. These results show that the GABA response in acutely dissociated SCN neurons is mediated by the GABAA receptor. Because the GABAA receptor of SCN neurons is allosterically augmented by diazepam, pentobarbital, and pregnanolone, similarly as in other regions of the central nervous system, the present study opens up ways to functionally modulate the GABAA receptors in SCN.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Mutations affecting the glycine receptor agonist transduction mechanism convert the competitive antagonist, picrotoxin, into an allosteric potentiator

Contrary to its effects on the gamma-aminobutyric acid type A receptor, picrotoxin antagonism of the alpha 1 subunit of the human glycine receptor is shown to be competitive, not use-dependent, and nonselective between the picrotoxin components, picrotin, and picrotoxinin. Competitive antagonism and non-use dependence are consistent with picrotoxin binding to a site in the extracellular domain. The mutations Arg-->Leu or Arg-->Gln at residue 271 of the glycine receptor alpha 1 subunit, which are both associated with human startle disease, have previously been demonstrated to disrupt the transduction process between agonist binding and channel activation. We show here that these mutations also transform picrotoxin from an allosterically acting competitive antagonist to an allosteric potentiator at low (0.01-3 microM) concentrations and to a noncompetitive antagonist at higher (> or = 3 microM) concentrations. This demonstrates that arginine 271 is involved in the transduction process between picrotoxin binding and its mechanism of action. Thus, the allosteric transduction pathways of both agonists and antagonists converge at a common residue prior to the activation gate of the channel, suggesting that this residue may act as an integration point for information from various extracellular ligand binding sites.

Pharmacological characteristics of GABAA responses in postnatal suprachiasmatic neurons in culture

The suprachiasmatic nucleus (SCN) is considered to be an endogenous circadian pacemaker. Previous studies have suggested functional roles of gamma-aminobutyric acid (GABA) in the control of circadian rhythms. In this study, the responses to applied GABA in cultured SCN neurons dissociated from postnatal rat hypothalamus were investigated using whole-cell voltage-clamp techniques. GABA and muscimol induced a large current response (EC50 values 5.3 and 1.6 microM, respectively), which was blocked by the GABAA antagonist bicuculline. This current response was also blocked by Zn2+ (0.5-50 microM) in a concentration-dependent manner, but was not potentiated by diazepam (10 microM) or ethanol (21 mM). These characteristics seem to correspond to those of GABAA receptors that lack gamma-type subunits.

Differential modulation of GABAA receptor-channel complex by polyvalent cations in rat dorsal root ganglion neurons

The effects of divalent and trivalent cations on the gamma-aminobutyric acid (GABA)-induced chloride current were studied with rat dorsal root ganglion neurons in primary culture using the whole-cell configuration of the patch-clamp technique. Lanthanum (La3+) reversibly potentiated the GABA-induced current with a half-maximal effective concentration (EC50) of 231 microM and the maximal potentiation to about 300% of control. La3+ did not seem to compete with chlordiazepoxide, pentobarbital or picrotoxin for binding sites, which indicated that the La3+ binding site was distinct from any of the benzodiazepine, barbiturate and picrotoxin binding sites on the GABA receptor-channel complex. Copper (Cu2+) reversibly suppressed the current induced by GABA with an EC50 of 19 microM in a non-competitive manner. Zinc (Zn2+) and Cu2+ had a very similar action on GABA response in terms of potency and efficacy. The degree of suppression of GABA-induced current by Cu2+ and Zn2+ was not affected by La3+, whereas Cu2+ antagonized the blocking action of Zn2+ in a concentration-dependent manner. Therefore, La3+ does not interfere with the binding site(s) for Cu2+ and Zn2+, whereas Cu2+ and Zn2+ may share a common site. These results are consistent with the presence of at least two distinct binding sites for polyvalent cations on the GABA-receptor channel complex: one for positive regulation by La3+ and the other for negative regulation by Cu2+ and Zn2+. These two sites are likely to be located at or near the external orifice of the chloride channel.

Selective potentiation of GABA-mediated Cl- current by lanthanum ion in subtypes of cloned GABAA receptors

The effect of lanthanum ion (La3+) on gamma-aminobutyric acid (GABA)-mediated Cl- currents was examined in the alpha 1 beta 2 or alpha 1 beta 2 gamma 2 subtype of GABAA receptors expressed in a human kidney cell line (A293), using a whole-cell configuration of patch-clamp techniques. La3+ dose-dependently stimulated the Cl- currents in the alpha 1 beta 2 gamma 2 subtype with an EC50 of 21.3 +/- 3.5 microM with a maximal potentiation of 240 +/- 16% as normalized to the GABA response at 5 microM. In the alpha 1 beta 2 subtype, however, the ion marginally potentiated GABA response, a maximal stimulation being less than 70% with an EC50 for La3+ near 200 microM. The stimulation of GABA response by La3+ in the alpha 1 beta 2 gamma 2 subtype was due to a decrease in the half maximal concentration for GABA and was more pronounced at the negative membrane potentials. This selectivity of La3+ toward the subtypes of GABAA receptors contrasts to that of Zn2+ which inhibits the currents in the alpha 1 beta 2, but not in the alpha 1 beta 2 gamma 2 subtype (Neuron, 5: (1990) 781-788). It appears that these polyvalent cations are useful in understanding the molecular basis for the functional diversity and in characterizing the molecular organization of native GABAA receptors.

Non-competitive inhibition of GABAA responses by a new class of quinolones and non-steroidal anti-inflammatories in dissociated frog sensory neurones

1. The interaction of a new class of quinolone antimicrobials (new quinolones) and non-steroidal anti-inflammatory agents (NSAIDs) with the GABAA receptor-Cl- channel complex was investigated in frog sensory neurones by use of the internal perfusion and 'concentration clamp' techniques. 2. The new quinolones and the NSAIDs (both 10(-6)-10(-5) M) had little effect on the GABA-induced chloride current (ICI) when applied separately. At a concentration of 10(-4) M the new quinolones, and to a lesser degree the NSAIDs, produced some suppression of the GABA response. 3. The co-administration of new quinolones and some NSAIDs (10(-6)-10(-14) M) resulted in a marked suppression of the GABA response. The size of this inhibition was dependent on the concentration of either the new quinolone or the NSAID tested. The inhibitory potency of new quinolones in combination with 4-biphenylacetic acid (BPAA) was in rank order norfloxacin (NFLX) much greater than enoxacin (ENX) greater than ciprofloxancin (CPFX) much greater than ofloxacin (OFLX), and that of NSAIDs in combination with ENX was BPAA much greater than indomethacin = ketoprofen greater than naproxen greater than ibuprofen greater than pranoprofen. Diclofenac, piroxicam and acetaminophen did not affect GABA responses in the presence of ENX. 4. In the presence of ENX or BPAA, there was a small shift to the right of the concentration-response curve for GABA without any effect on the maximum response. However, the co-administration of these drugs suppressed the maximum of the GABA concentration-response curve, indicating a non-competitive inhibition, for which no voltage-dependency was observed.5. Simultaneous administration of ENX and BPAA also suppressed pentobarbitone (PB)-gated Icl. On the other hand, both PB and phenobarbitone reversed the inhibition of GABA-induced Ic, by coadministration of ENX and BPAA.6. The effect on GABAA responses of co-administration of new quinolones and NSAIDs was not via an interaction with benzodiazepine receptors coupled to the GABAA receptor, since this effect was not reversed by Rol5-1788 or diazepam.7. It is concluded that the co-administration of new quinolones and some of the NSAIDs inhibit GABAergic transmission, and could result in convulsions.

Site-specific acylation of GABA-gated Cl- channels: effects on 36Cl- uptake

Radioligand binding studies indicate that p-isothiocyanato-t-butylbicycloorthobenzoate (p-NCS-TBOB) specifically acylates GABA-gated chloride channels. Preincubation of synaptoneurosomes with p-NCS-TBOB followed by washing resulted in a concentration dependent (63-500 nM) inhibition of both muscimol-stimulated chloride uptake and [355]t-butylbicyclophosphorothionate (TBPS) binding. The extent of acylation (assessed by inhibition of [35S]TBPS binding) was highly correlated (r = 0.89; p less than 0.001) with the inhibition of muscimol-stimulated Cl- uptake. Neither basal Cl- uptake nor [3H]muscimol binding to GABAA receptors were affected by p-NCS-TBOB. Preincubation with the nonacylating 'cage' convulsant t-butylbicycloorthobenzoate (500 nM) followed by washing had no effect on either muscimol-stimulated Cl- uptake or [35S]TBPS binding. These findings indicate that p-NCS-TBOB interferes with the efficacy of muscimol promoted channel openings, but does not affect the recognition qualities of GABAA receptors. p-NCS-TBOB should prove useful in electrophysiological and biochemical studies examining the properties of GABA-gated Cl- channels.

Transmitter-activated ion channels as the target of chemical agents

The transmitter-activated ion channels are known to be important target sites of a variety of therapeutic and toxic agents. The GABA-activated chloride channel has been shown to be modulated by general anesthetics, alcohols, and the pyrethroid, cyclodiene and lindane insecticides. The general anesthetics halothane, enflurane and isoflurane greatly augmented the GABA-activated current before desensitization took place, and suppressed it after desensitization at clinically relevant concentrations equivalent to 1-2 minimum alveolar concentrations. The stimulating effect appears to be a mechanism of general anesthesia. It seems that general anesthetics have a specific affinity for the GABA receptor-channel complex. Ethanol also augmented the GABA-activated peak chloride current with little or no effect on the desensitized sustained current. Longer chain alcohols n-butanol, n-hexanol, n-octanol, and n-decanol also exerted the same type of effect, with the potency and efficacy increasing with lengthening of the carbon chain. The GABA receptor-channel complex has also been shown to be an important target site of certain insecticides. The type II pyrethroids deltamethrin and fenvalerate augmented the GABA-activated peak chloride current when applied concurrently with GABA, but the effect was diminished as the pyrethroids were applied for long periods of time prior to GABA application. The latter effect might explain the controversy in the literature regarding the pyrethroid action on the GABA system. The type I pyrethroid allethrin suppressed the GABA-activated peak chloride current when co-applied with GABA. Both types of pyrethroids suppressed the N-methyl-d-aspartate-induced current. Lindane and the cyclodienes dieldrin, endrin, heptachlor-epoxide, and isobenzan suppressed the GABA-activated chloride current. These effects can account for the convulsant action of lindane and the cyclodienes.

Functional and molecular distinction between recombinant rat GABAA receptor subtypes by Zn2+

gamma-Aminobutyric acid receptor (GABAAR) channels in different neurons display heterogeneous functional properties. Molecular cloning revealed a large number of GABAAR subunits that assemble into GABAAR subtypes with different functional properties, suggesting that the subunit combination determines the functional properties of the receptor. In this study, the subunit composition of GABAARs is related to a functional distinction between Zn2(+)-sensitive and Zn2(+)-insensitive receptor subtypes. GABAARs reconstituted in transiently transfected fibroblasts from combinations of cDNAs encoding alpha and beta subunits are potently blocked by Zn2+. The presence of a gamma subunit in any combination with the other subunits leads to the formation of GABAARs that are almost insensitive to Zn2+. These data provide a structural correlate to the functional heterogeneity of the action of Zn2+ on GABAARs in native membranes and show that Zn2+ insensitivity of GABA-activated currents indicates the presence of a gamma-subunit in the assembled GABAAR channel.

Differential effect of zinc on the vertebrate GABAA-receptor complex

1. gamma-Aminobutyric acid (GABA) responses were recorded from rat superior cervical ganglia (SCG) in culture using the whole cell recording technique. 2. Zinc (50-300 microM) reversibly antagonized the GABA response in embryonic and young post-natal neurones, while neurones cultured from adult animals were far less sensitive and occasionally resistant to zinc blockade. Cadmium (100-300 microM) also antagonised the GABA response, while barium (100 microM-2 mM) was ineffective. 3. The differential blocking effect of zinc on cultured neurones of different ages also occurred in intact SCG tissue. 4. The GABA log dose-response curve constructed with foetal or adult cultured neurones was reduced in a non-competitive manner by zinc. This inhibition was minimally affected by the membrane potential. 5. The GABA response recorded intracellularly from guinea-pig pyriform cortical slices was enhanced by zinc (300-500 microM), which occurred concurrently with a decrease in the input conductance of the cell. The enhancement was unaffected by prior blockade of the GABA uptake carrier by 1 mM nipecotic acid. This phenomenon could be reproduced by barium (300 microM) and cadmium (300 microM). 6. We conclude that the vertebrate neuronal GABAA-receptor becomes less sensitive to zinc with neural (GABAA-receptor?) development, and the enhanced GABA response recorded in the CNS is a consequence of the reduction in the input conductance and not due to a direct effect on the receptor complex.

Differential effects of iodide and chloride on allosteric interactions of the GABAA receptor

t-[35S]Butylbicyclophosphorothionate [( 35S]TBPS) has been shown to bind to the GABAA receptor complex. The binding is modulated allosterically by drugs that interact at components of the receptor complex. The present studies were designed to evaluate the influence of ionic environment and state of equilibrium on the allosteric modification of [35S]TBPS binding. In both I- and Cl- under nonequilibrium conditions, diazepam, gamma-aminobutyric acid (GABA), and pentobarbital (PB) stimulate and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) inhibits [35S]TBPS binding. In addition, there is an inhibitory component to the effect of GABA and PB at higher drug concentrations. These effects are blocked by the appropriate antagonists for each drug. In Cl-, the stimulation of [35S]TBPS binding by drugs disappears at equilibrium, whereas the inhibition by GABA and PB persists. The inhibitory effect of DMCM in Cl- also disappears at equilibrium. When assayed in I- at equilibrium, however, DMCM stimulates [35S]TBPS binding. In addition, bicuculline, which is without effect under nonequilibrium conditions in either Cl- or I-, stimulates [35S]TBPS binding in I- at equilibrium. The persistent effects of DMCM, bicuculline, and GABA in I- are accompanied by alterations in the affinity of [35S]TBPS for its receptor. In addition, the stimulation of [35S]TBPS binding by GABA is associated with a decreased number of [35S]TBPS binding sites. These data demonstrate that receptor complex interactions with anions influence the responsiveness to drug binding.

GABA-induced chloride current in rat isolated Purkinje cells

Electrical and pharmacological properties of the gamma-aminobutyric acid (GABA)-induced current in the rat isolated cerebellar Purkinje cell bodies were studied using a concentration-jump method, which is termed as a "concentration-clamp" technique. This technique enables the rapid exchange of external solution around the neurons to be perfused internally under a voltage-clamp condition. The peak amplitude of GABA response increased sigmoidally with the increase of the concentration of GABA. The value of the GABA concentration that evokes a half-maximal response (Ka) was 5 X 10(-5) M, and the Hill coefficient was 1.8. The current-voltage relationship for the GABA response showed nonlinearity at membrane potentials more negative than -40 mV. The reversal potential of GABA-evoked current was close to the equilibrium potential of Cl- (ECl), indicating that the current elicited by GABA is carried by Cl-. Both the activation and inactivation phases of GABA-induced Cl- current (ICl) consisted of fast and slow components. These time constants in both phases decreased as the concentration of GABA increased. Strychnine and bicuculline inhibited the GABA-induced ICl in a dose-dependent manner, and the inhibition of the GABA response by bicuculline was competitive. Pentobarbital sodium augmented the GABA response and modified the inactivation phase. The augmentation of the GABA response by pentobarbital was more profound at lower concentrations of GABA and was accompanied by a change in the Hill coefficient from 2 to 1. The properties of the GABA response in cerebellar Purkinje cells were thought to be basically similar to those previously reported in other preparations.

Dihydropyridine-sensitive low-threshold calcium channels in isolated rat hypothalamic neurones

1. Low-voltage-activated Ca2+ channels which produce a transient inward current were studied in neurones freshly isolated from the ventromedial hypothalamic region of the rat. Membrane currents were recorded using a suction-pipette technique which allows for internal perfusion under a single-electrode voltage clamp. A concentration-jump technique was also used for rapid drug application. 2. In most cells superfused with 10 mM-Ca2+, a transient inward Ca2+ current was evoked by a step depolarization to potentials more positive than -65 mV from a holding potential of -100 mV. Such a low-threshold Ca2+ current could easily be separated from a high-threshold, steady type of Ca2+ current by selecting the holding and test potential levels, as well as by resistance to the wash-out during cell dialysis. 3. Activation and inactivation processes of the low-threshold Ca2+ current were highly potential dependent at 20-22 degrees C. For a test potential change from -60 to +20 mV, the time to peak of the current decreased from 45 to 9 ms, and the time constant of the current decay decreased from 90 to 40 ms. The steady-state inactivation occurred at very negative potentials, reaching a 50% level at -93 mV. Recovery from inactivation showed a time constant between 2.63 and 0.94 s for a potential change from -80 to -120 mV. 4. The amplitude of the low-threshold Ca2+ current depended on the external Ca2+ concentration [( Ca2+]o), approaching saturation at 100 mM [Ca2+]o. Ba2+ substituted for Ca2+ reduced the current amplitude by 30-50% while Sr2+ produced no definite changes in the current amplitude. 5. The low-threshold Ca2+ current was blocked by various di- or trivalent cations in the sequence of La3+ greater than Zn2+ greater than Cd2+ greater than Ni2+ greater than Co2+. The corresponding apparent dissociation constants (KD) were 7 x 10(-7), 1 x 10(-4), 3 x 10(-4), 6 x 10(-4) and 3 x 10(-3) M. 6. Various organic Ca2+ antagonists were effective in blocking the low-threshold Ca2+ current in the following sequence: flunarizine greater than nicardipine greater than nifedipine greater than nimodipine greater than D600 (methoxyverapamil) greater than diltiazem. The corresponding KDs were 7 x 10(-7), 3.5 x 10(-6), 5 x 10(-6), 7 x 10(-6), 5 x 10(-5) and 7 x 10(-5) M. These Ca2+ antagonists induced a use-dependent decrease in the current amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Electrophysiological detection of acetylcholinesterase activity using concentration clamp on physically isolated Aplysia neurons

In this study, we have used electrophysiological techniques to evaluate the acetylcholinesterase (AChE) activity on single neurons physically isolated from the pedal ganglia of Aplysia californica and kurodai. Acetylcholine (ACh) was applied to cells having Na-dependent response using a concentration clamp technique which allows a step-like complete change of external medium around the cell (diameter more than 120 micron) within 30 msec. When the neuron was exposed to ACh (50 microM) initially by rapid and brief (400-800 msec) flow and continuously after stopping the perfusion, the induced current rose to a peak and then decayed in the presence of ACh. However, the current increased again when the rapid flow of the same ACh solution around the cell was resumed. The increase in the current by the resumption of the perfusion was not seen when carbachol was substituted for ACh or when extraordinarily high concentration (10 mM) of ACh was used. Furthermore, this increase in the current was blocked by the antiAChE agent, edrophonium, in a dose-dependent manner. These results suggest that acetylcholinesterase (AChE) causes a local depletion of ACh at the membrane surface where the rate of hydrolysis of ACh exceeds the rate of ACh diffusion from bulk solution and the increase in the current by the resumption of the perfusion resulted from the restoration of ACh concentration at the membrane surface. This electrophysiological indication of AChE activity may be a useful tool for the study of AChE by other than biochemical means.

Intracellular picrotoxin blocks pentobarbital-gated Cl- conductance

Using the 'inside-out' configuration of frog sensory neurons, we studied the effect of intracellular picrotoxin on the pentobarbital-gated single channel response of Cl- -current (iCl). The pentobarbital-induced iCl showed no voltage-dependency and the single channel conductance (gamma Cl) was 16 +/- 3.1 pS (n = 6). Picrotoxin caused the pentobarbital-gated Cl- channels to react in a flickering pattern and then finally caused them to cease their opening altogether. This inhibitory action of picrotoxin was reversible.

Non-competitive inhibition of GABA currents by phenothiazines in cultured chick spinal cord and rat hippocampal neurons

The gamma-aminobutyric acid (GABA) inhibiting properties of several classes of antipsychotic medications were studied using gigaseal whole-cell voltage-clamp techniques in cultured chick spinal cord and rat hippocampal neurons. At doses above 1 microM trifluoperazine, chlorpromazine and thioridazine blocked GABA currents in a non-competitive fashion decreasing the maximal transmitter response without altering the half-maximal effective concentration. In contrast, haloperidol was ineffective against GABA at concentrations up to 100 microM. Among the agents studied trifluoperazine was the most potent GABA inhibitor with half maximal effect at 12 microM. Trifluoperazine (100 microM) also inhibited glycine-gated chloride currents in spinal cord neurons to an extent comparable to GABA (85 +/- 6% inhibition) but reduced glutamate currents by less than 35% in either spinal cord or hippocampal neurons.