Anion interaction at the inhibitory post-synaptic membrane of the crayfish neuromuscular junction

Electrophysiology of ionotropic GABA receptors

GABA_A receptors are ligand-gated chloride channels and ionotropic receptors of GABA, the main inhibitory neurotransmitter in vertebrates. In this review, we discuss the major and diverse roles GABA_A receptors play in the regulation of neuronal communication and the functioning of the brain. GABA_A receptors have complex electrophysiological properties that enable them to mediate different types of currents such as phasic and tonic inhibitory currents. Their activity is finely regulated by membrane voltage, phosphorylation and several ions. GABA_A receptors are pentameric and are assembled from a diverse set of subunits. They are subdivided into numerous subtypes, which differ widely in expression patterns, distribution and electrical activity. Substantial variations in macroscopic neural behavior can emerge from minor differences in structure and molecular activity between subtypes. Therefore, the diversity of GABA_A receptors widens the neuronal repertoire of responses to external signals and contributes to shaping the electrical activity of neurons and other cell types.

The mixture effect on ionic selectivity and permeability of nanotubes

Ion-selective nanotubes have great potential in applications such as ion separation, desalination, and power generation. However, their performance is often limited by the deteriorated selectivity in mixed salt solutions. To reveal the underlying mechanism of the mixture effect on ion transport through nanotubes, we perform molecular dynamics (MD) simulations on ion transport through carbon nanotubes (CNTs) and polymer nanopores with a pore diameter of ∼1 nm and a charge density of -1 e nm^-2. Based on the simulation results, when a single salt solution is replaced by a mixed salt solution, the ionic selectivity drops as the permeability of higher permeable ions decreases much greater than that of lower permeable ions. This is because the adsorption of lower permeable ions on the inner surface of nanotubes blocks the ion flux and increases the entrance barrier to the nanotube, and the adsorption is significantly reduced in the mixed salt solution. Such a reduction results from the occupancy of higher permeable ions on the adsorption sites as they have a higher adsorption tendency albeit weaker adsorption compared with lower permeable ions. These studies will help design the next generation of nanostructures to circumvent the mixture effect and show high permeability and selectivity in real applications.

Venus Flytrap HKT1-Type Channel Provides for Prey Sodium Uptake into Carnivorous Plant Without Conflicting with Electrical Excitability

The animal diet of the carnivorous Venus flytrap, Dionaea muscipula, contains a sodium load that enters the capture organ via an HKT1-type sodium channel, expressed in special epithelia cells on the inner trap lobe surface. DmHKT1 expression and sodium uptake activity is induced upon prey contact. Here, we analyzed the HKT1 properties required for prey sodium osmolyte management of carnivorous Dionaea. Analyses were based on homology modeling, generation of model-derived point mutants, and their functional testing in Xenopus oocytes. We showed that the wild-type HKT1 and its Na(+)- and K(+)-permeable mutants function as ion channels rather than K(+) transporters driven by proton or sodium gradients. These structural and biophysical features of a high-capacity, Na(+)-selective ion channel enable Dionaea glands to manage prey-derived sodium loads without confounding the action potential-based information management of the flytrap.

Studies on the mechanism of action of picrotoxinin and other convulsants at the crustacean muscle GABA receptor

The actions of picrotoxinin, bicuculline and penicillin-G were investigated on the GABA-receptor system of lobster muscle by using intracellular recording. The highly potent antagonist, picrotoxinin, produced a lateral shift and depression in the maximum of the GABA dose--conductance curve (designated as mixed antagonism); bicuculline, a weak antagonist, caused only a depression in the maximum with little or no lateral shift, whereas penicillin-G, an even weaker antagonist, produced a greater depression at the top of the dose--response curve. The possible sites of antagonist action were examined, with a critical re-evaluation of a drug-receptor model previously proposed to account for the antagonistic behaviour of picrotoxinin (the mixed antagonistic model); this model was extended to include the actions of bicuculline and penicillin-G. Antagonism was examined (i) towards different GABA receptor agonists; (ii) in various external anion media; (iii) at varying external pH; and (iv) when two different antagonists were combined. The GABA agonists were differentially antagonized by picrotoxinin and bicuculline, but external pH and substituent anions caused only minor perturbations to the inhibition. Combination experiments suggested at least three sites for GABA antagonists binding on crustacean muscle: (i) the GABA recognition site or sites; (ii) the ionic selectivity site in the ionophore; and (iii) a highly lipophilic site which may be part of the GABA receptor or ionophore. The mixed antagonism model accounted for the pH and external anion data but required modification to a cyclic scheme to explain the antagonism of a partial agonist. A model based on two-state receptor theory could only account for the antagonism of GABA if picrotoxinin was assumed not only to perturb L (the R rightleftharpoons T conformation constant) but also to affect the agonist binding affinity. It is suggested that picrotoxinin and bicuculline may antagonize GABA responses by stabilizing the closed form of the activated channel, whereas penicillin-G may block the channel in the open state.

Reinterpreting the anomalous mole fraction effect: the ryanodine receptor case study

The origin of the anomalous mole fraction effect (AMFE) in calcium channels is explored with a model of the ryanodine receptor. This model predicted and experiments verified new AMFEs in the cardiac isoform. In mole fraction experiments, conductance is measured in mixtures of ion species X and Y as their relative amounts (mole fractions) vary. This curve can have a minimum (an AMFE). The traditional interpretation of the AMFE is that multiple interacting ions move through the pore in a single file. Mole fraction curves without minima (no AMFEs) are generally interpreted as X displacing Y from the pore in a proportion larger than its bath mole fraction (preferential selectivity). We find that the AMFE is also caused by preferential selectivity of X over Y, if X and Y have similar conductances. This is a prediction applicable to any channel and provides a fundamentally different explanation of the AMFE that does not require single filing or multiple occupancy: preferential selectivity causes the resistances to current flow in the baths, channel vestibules, and selectivity filter to change differently with mole fraction, and produce the AMFE.

The anomalous mole fraction effect in calcium channels: a measure of preferential selectivity

The cause of the anomalous mole fraction effect (AMFE) in calcium-selective ion channels is studied. An AMFE occurs when the conductance through a channel is lower in a mixture of salts than in the pure salts at the same concentration. The textbook interpretation of the AMFE is that multiple ions move through the pore in coordinated, single-file motion. Instead of this, we find that at its most basic level an AMFE reflects a channel's preferential binding selectivity for one ion species over another. The AMFE is explained by considering the charged and uncharged regions of the pore as electrical resistors in series: the AMFE is produced by these regions of high and low ion concentration changing differently with mole fraction due to the preferential ion selectivity. This is demonstrated with simulations of a model L-type calcium channel and a mathematical analysis of a simplistic point-charge model. The particle simulations reproduce the experimental data of two L-type channel AMFEs. Conditions under which an AMFE may be found experimentally are discussed. The resistors-in-series model provides a fundamentally different explanation of the AMFE than the traditional theory and does not require single filing, multiple occupancy, or momentum-correlated ion motion.

Thiocyanate as a probe of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Immediately following exposure to thiocyanate (SCN–)-containing solutions, the cystic fibrosis conductance regulator Cl– channel exhibits high unitary SCN– conductance and anomalous mole fraction behaviour, suggesting the presence of multiple anion binding sites within the channel pore. However, under steady-state conditions SCN– conductance is very low. Here I show, using patch clamp recording from CFTR-transfected mammalian cell lines, that under steady-state conditions neither SCN– conductance nor SCN– permeability show anomalous mole fraction behaviour. Instead, SCN– conductance, permeability, and block of Cl– permeation can all be reproduced by a rate theory model that assumes only a single intrapore anion binding site. These results suggest that under steady-state conditions the interaction between SCN– and the CFTR channel pore can be understood by a simple model whereby SCN– ions enter the pore more easily than Cl–, and bind within the pore more tightly than Cl–. The implications of these findings for investigating and understanding the mechanism of anion permeation are discussed.Key words: chloride channel, permeation, anion binding, multi-ion pore behaviour, rate theory model.

CFTR: mechanism of anion conduction

CFTR: Mechanism of Anion Conduction. Physiol. Rev. 79, Suppl.: S47-S75, 1999. - The purpose of this review is to collect together the results of recent investigations of anion conductance by the cystic fibrosis transmembrane conductance regulator along with some of the basic background that is a prerequisite for developing some physical picture of the conduction process. The review begins with an introduction to the concepts of permeability and conductance and the Nernst-Planck and rate theory models that are used to interpret these parameters. Some of the physical forces that impinge on anion conductance are considered in the context of permeability selectivity and anion binding to proteins. Probes of the conduction process are considered, particularly permeant anions that bind tightly within the pore and block anion flow. Finally, structure-function studies are reviewed in the context of some predictions for the origin of pore properties.

Effects of replacing extracellular chloride with formate on the inhibitory neuromuscular junction of the crayfish opener muscle

The effect of partially replacing extracellular chloride by formate on the inhibitory junction of the crayfish opener muscle is investigated. Inhibitory postsynaptic potential (IPSP) amplitude, recorded in muscle fibers and presynaptic axons, increases significantly in the formate saline whereas input resistance and resting membrane potential of muscle fibers are not affected. The increase in IPSP amplitude is mainly due to an increase in IPSP driving force while the GABA mediated conductance change underlying IPSP is not altered. The waveform of presynaptic action potential is slightly altered by formate substitution where an after-depolarizing potential is decreased. This change does not seem to affect the probability of transmitter release because the magnitude of synaptic facilitation is unchanged. In conclusion, formate substitution significantly increases IPSP amplitudes by increasing its driving force without affecting presynaptic release mechanisms.

Cystic fibrosis transmembrane conductance regulator (CFTR) anion binding as a probe of the pore

We compared the effects of mutations in transmembrane segments (TMs) TM1, TM5, and TM6 on the conduction and activation properties of the cystic fibrosis transmembrane conductance regulator (CFTR) to determine which functional property was most sensitive to mutations and, thereby, to develop a criterion for measuring the importance of a particular residue or TM for anion conduction or activation. Anion substitution studies provided strong evidence for the binding of permeant anions in the pore. Anion binding was highly sensitive to point mutations in TM5 and TM6. Permeability ratios, in contrast, were relatively unaffected by the same mutations, so that anion binding emerged as the conduction property most sensitive to structural changes in CFTR. The relative insensitivity of permeability ratios to CFTR mutations was in accord with the notion that anion-water interactions are important determinants of permeability selectivity. By the criterion of anion binding, TM5 and TM6 were judged to be likely to contribute to the structure of the anion-selective pore, whereas TM1 was judged to be less important. Mutations in TM5 and TM6 also dramatically reduced the sensitivity of CFTR to activation by 3-isobutyl 1-methyl xanthine (IBMX), as expected if these TMs are intimately involved in the physical process that opens and closes the channel.

Anions modulate cation-induced responses of single units of the frog glossopharyngeal nerve

Fibers of the frog glossopharyngeal nerve that are sensitive to water stimulation also respond to Ca, Mg and Na salts. During stimulation with a salt, the cation and the anion are applied together and the anion could influence the response to the cation. We examined this interaction using single unit recordings at the level of fungiform papilla. Nerve impulses of large amplitude were recorded in response to the stimulation of adjacent papillae with increasing concentrations of various Ca, Mg and Na salts. For a given cation, the elicited spike frequency depended on the anion. For example, the responses of single fibers to Ca2+ concentrations above 0.1 mM were maximal with CaSO4 and minimal with Ca(SCN)2. The rank order of efficiency was CaSO4 > CaCl2 = CaBr2 = Ca(NO3)2 > Ca(SCN)2 for Ca2+ ions at 5 mM. The effects of these anions were reversed for Mg and Na salts, the rank orders being Mg(SCN)2 > Mg(NO3)2 > MgBr2 = MgCl2 > MgSO4, for Mg2+ ions at 200 mM, and NaSCN > NaI > NaNO3 > NaBr > NaCl >> NaF = Na2SO4, for Na+ ions at 500 mM. All these sequences correspond to the lyotropic rank order of the anions. In stimulation by a mixture of Ca and Na salts, which have different rank orders with respect to anions, either the response to Ca2+ ions or the response to Na+ ions could be eliminated as a result of mutual antagonism between Ca2+ and Na+ ions. In this case, the rank order of anions was dependent only on the cation that was able to exert a stimulatory effect in the mixture. Threshold concentrations for Ca, Mg and Na salts are influenced by cationic properties, but not by anionic properties. We hypothesize that anions can modulate the efficacy of cation transduction by binding to a membrane element that interacts with each of the three distinct receptors for Ca2+, Mg2+ and Na+ ions without altering the affinities of these receptors for the respective cations. The present results cannot be interpreted in terms of permeability of the apical membrane to anions and changes in surface potential on the apical membrane. The possibility is discussed that an anion-selective paracellular pathway between taste cells is responsible for the effect of anions on the cation-induced response.

GABA-gated anion channels in intact crayfish opener muscle fibres and stretch-receptor neurons are neither activated nor desensitized by glutamate

The influence of glutamate on the GABA-activated Cl- conductance was studied in the slowly adapting stretch-receptor neuron and dactylopodite opener muscle fibre of the crayfish (Astacus astacus) using a two-microelectrode and a three-microelectrode voltage clamp, respectively. Glutamate (0.5-1.0 mM) had no effect on the GABA-activated conductance in either preparation. This indicates that the availability of the inhibitory channels for activation of GABA is not influenced by glutamate. The present results are in sharp contrast to those obtained by Franke et al. (J Comp Physiol A 159:591-609, 1986) in experiments on excised membrane patches, which suggested that glutamate is capable of both activating and desensitizing inhibitory postsynaptic channels in the crayfish opener muscle fibre.

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

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

The inhibitory chloride channel of the lobster Panulirus penicillatus neuromuscular junction

1. Single channel activity was recorded from muscle membranes of the lobster Panulirus penicillatus using the patch-clamp technique. 2. Cell-attached, outside-out and inside-out patches were prepared from the deep abdominal extensor muscle. 3. Low amplitude single channel currents were observed in most patches, and were identified as being chloride-currents. 4. The chloride channel was active spontaneously, and tended to desensitize when outside-out patches were exposed to a small jet of glutamate. 5. Amplitude histograms of single channel currents presented a well defined peak of 8 pA at a membrane potential of -160 mV, while open and closed time histograms were fit to single exponential functions with tau open of 3.27 msec and tau closed of 31.58 msec.

Anion channels with multiple conductance levels in a mouse B lymphocyte cell line

1. Multiple conductance level ion channels were recorded in excised and cell-attached patches from cells of a mouse B lymphocyte hybridoma line. The reversal potential for the single-channel current was unaffected by the species of cation on the cytoplasmic face of the patch, but changed as the Cl- concentration was altered, indicating that the channel is anion selective. 2. The permeability sequence determined from reversal potentials was F- greater than I- greater than SCN- greater than Br- greater than Cl- greater than glucuronate greater than NO3- greater than aspartate. This was different from the conductance sequence (Cl- greater than SCN- = F- greater than Br- greater than NO3- greater than I- greater than glucuronate greater than aspartate), indicating interaction of ions within the pore of the channel. Consistent with this was the observation of anomalous mole fraction dependence with a mixed solution of thiocyanate and chloride. 3. In addition to the main open level (about 400 pS; excised patch, symmetrical 165 mM-Cl-), three subconductance levels and one supraconductance level were observed. These were concluded to be integral components of the same channel based on coincidence of appearance and identical permeabilities. 4. The channel is voltage dependent, with open probability in excised patches increasing with more positive potentials. The channel was reversibly blocked in a voltage-dependent manner by SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid), a stilbene derivative, on the cytoplasmic face. 5. Several differences were noted between cell-attached and excised-patch recordings. The multiple conductance level channel was less frequently seen in cell-attached patches but could often be induced to appear by prolonged application of positive voltages. This induced channel in attached patches showed an altered voltage dependence which could be partially mimicked in excised patches by including cyclic AMP and ATP in the solution on the cytoplasmic side of the membrane.

Anion conductances of the giant axon of squid Sepioteuthis

Anion conductances of giant axons of squid, Sepioteuthis, were measured. The axons were internally perfused with a 100-mM tetraethylammonium-phosphate solution and immersed in a 100-mM Ca-salt solution (or Mg-salt solution) containing 0.3 microns tetrodotoxin. The external anion composition was changed. The membrane currents had a large amount of outward rectification due to anion influx across Cl- channels of the membrane (Inoue, 1985). The amount of outward rectification depended on the species of anion used and was strongly influenced by temperature and internal pH. In contrast to the anion conductances themselves, the conductance relative to Cl- (gA/gCl) was found to be quite stable against changes in the membrane potential, temperature, and pH. It is therefore suggested that each gA/gCl is an intrinsic quantity of the Cl- channel of the squid axon membrane. The sequence and values of gA/gCl obtained in this study were NO3- (1.80) greater than I- (1.40) greater than Br- (1.07) greater than Cl- (1.00) greater than MeSO3- (0.46) greater than H2PO2- (0.33) greater than CH3COO- (0.29) greater than SO4(2-) (0.06).

The anion selectivity of the gamma-aminobutyric acid controlled chloride channel in the perfused spinal ganglion cell of frog

The selectivity of the GABA-controlled Cl- channel in the membrane of the dorsal root ganglion cell of the frog has been measured in internally perfused cells by means of current and voltage clamp. When Cl- was replaced by various anions, the 10(-5) or 10(-4) M GABA-induced reversal potentials (EGABA) for Br-, I-, NO3-, ClO4-, SCN-, BF4- and ClO3- were more negative than that for Cl-, despite the fact that, in solution, the test anions are either larger than or similar in size to Cl-.

Non-selective conductance in calcium channels of frog muscle: calcium selectivity in a single-file pore

Voltage-clamp studies were carried out to compare currents through Ca2+ channels (ICa) with Na+ currents (Ins) through a non-selective cation conductance blocked by micromolar concentrations of external Ca2+. The gating of both currents was found to have similar time and voltage dependence. The amplitudes of ICa and Ins varied widely, but Ins was always large in fibres with large ICa, and small in fibres with small ICa. Both ICa and Ins were blocked by the specific Ca2+ channel blocker nifedipine, with half-blockage concentrations that were virtually identical (KD = 0.9 microM for ICa and 0.7 microM for Ins). ICa and Ins were also equally sensitive to block by diltiazem (KD = 80 microM). These parallels between Ins and ICa are most easily explained if Ins flows through Ca2+ channels. Apparently, Ca2+ channels bear high-affinity Ca2+-binding sites, and are highly permeable to monovalent cations when Ca2+ is absent. Ba2+ currents (IBa) and ICa were measured in external solutions containing mixtures of Ba2+ and Ca2+. IBa is blocked by Ca2+, as is Ins. Adding Ba2+ to Ca2+ produces only small or no increases in current, as if Ba2+ is only sparingly permeant when Ca2+ is present. Membrane currents in Ba2+/Ca2+ mixtures show anomalous mole-fraction behaviour, suggesting that Ca2+ channels are single-file, multi-ion pores. Complex current transients are observed under maintained depolarizations in Na+/Ca2+ and Ba2+/Ca2+ mixtures. They suggest that in ion mixtures, Ca2+ channels transport Ca2+ in preference to Na+ and Ba2+. Hence Ca2+ channels are selective for Ca2+, even though current amplitudes suggest that the Na+ or Ba2+ permeabilities in the absence of Ca2+ are as high as, or higher than, the Ca2+ permeability. We conclude that the selective permeability of Ca2+ channels depends on the presence of Ca2+. In model calculations, our observations are explained as a consequence of Ca2+ channels being single-file pores. It is proposed that Ca2+ channels derive much of their ion selectivity from high-affinity Ca2+ binding sites located in an otherwise unselective aqueous pore.

Mechanism of ion permeation through calcium channels

Calcium channels carry out vital functions in a wide variety of excitable cells but they also face special challenges. In the medium outside the channel, Ca2+ ions are vastly outnumbered by other ions. Thus, the calcium channel must be extremely selective if it is to allow Ca2+ influx rather than a general cation influx. In fact, calcium channels show a much greater selectivity for Ca2+ than sodium channels do for Na+ despite the high flux that open Ca channels can support. Relatively little is known about the mechanism of ion permeation through Ca channels. Earlier models assumed ion independence or single-ion occupancy. Here we present evidence for a novel hypothesis of ion movement through Ca channels, based on measurements of Ca channel activity at the level of single cells or single channels. Our results indicate that under physiological conditions, the channel is occupied almost continually by one or more Ca2+ ions which, by electrostatic repulsion, guard the channel against permeation by other ions. On the other hand, repulsion between Ca2+ ions allows high throughput rates and tends to prevent saturation with calcium.

The influence of the permeant ions thallous and potassium on inward rectification in frog skeletal muscle

A three-electrode voltage-clamp method was used to investigate the inactivation of Tl currents through the inward rectifier of frog sartorius muscle fibres, and the interaction between the permeant ions Tl+ and K+. In 80 mM-Tl Ringer inward currents inactivated on hyperpolarization along an exponential time course, with time constants that initially increased and then fell with increasing hyperpolarization. Because of the inactivation process steady-state conductances were smaller than instantaneous conductances at all potentials in Tl Ringer. The steady-state conductance increased to a maximum value at around - 100 mV in 80 mM-Tl Ringer, and then fell with increasing hyperpolarization. In K Ringer the steady-state conductance was greater at all potentials than the instantaneous conductance because K currents activate (rather than inactivate) on hyperpolarization. Time constants of Tl inactivation were the same when measured from the decay of current during a single pulse, or from the rate of recovery from inactivation using either a two- or a three-pulse method, indicating that inactivation obeys first-order kinetics. In 80 mM-Tl Ringer steady-state inactivation increased with increasing hyperpolarization, e-fold every 48 mV. This would be consistent with the site at which inactivation occurs experiencing 0.5 of the membrane voltage field. Tl+ was more permeant than K+ through the inward rectifier, the permeability ratio PTl+/PK+ being 1.66. In solutions containing both Tl+ and K+ the membrane showed an anomalous mole-fraction dependence of conductance, the resting potential being more negative, and both instantaneous and steady-state conductances smaller than those recorded in solutions containing only Tl+ or only K+. The reduction in the amplitude of the instantaneous conductance in Tl-K mixtures was voltage-dependent, the block being initially increased and then falling with increasing hyperpolarization. Inward currents also inactivated on hyperpolarization in Tl-K mixtures. The time constants of inactivation, and the extent of inactivation which occurred, became less dependent on membrane potential in these solutions. When K+ is the major permeant ion in solution, Tl+ has a blocking effect on the currents carried by K+, and the degree of block is voltage-dependent. Increasing [Tl]o increased the block at all potentials. The results of our experiments in solutions containing both Tl+ and K+ are discussed in terms of an interaction between these ions within the channel.

Some properties of solubilized GABA receptor

gamma-Aminobutyric acid (GABA) receptor was solubilized from synaptic membrane of the rat brain by various detergents. Nonidet P-40, a non-ionic detergent, was found to be an effective solubilizing agent, since it caused no interference on the receptor binding assay, yielded a [3H]muscimol binding protein with a high specific activity and no aggregation, and preserved good stability of the solubilized fraction. Ammonium sulfate precipitation of the solubilized supernatant significantly increased the binding of [3H]muscimol to GABA receptor, possibly by removing heat-stable and small molecular inhibitory substances. The specific [3H]muscimol binding to the soluble fraction obtained by Nonidet P-40 treatment and subsequent ammonium sulfate precipitation, was saturable with KD 13 and 64 nM, and Bmax 3.4 and 1.8 pmol/mg protein, respectively. The enhancement of the [3H]muscimol binding by diazepam as found in synaptic membrane was also detected in the soluble fraction. Molecular weight of the [3H]muscimol binding site was determined by gel filtration on Sephadex G-200 and was calculated to be 270,000 daltons. This value was identical with that of the [3H]flunitrazepam binding site which appeared in the same solubilized fraction. These results indicate that the properties of solubilized GABA receptor are identical to those of membrane-bound GABA receptor. Furthermore, the present results suggest that both GABA and benzodiazepine receptors may reside on the same macromolecule in synaptic membrane.

Inhibitory postsynaptic currents at Aplysia cholinergic synapses: effects of permeant anions and depressant drugs

Inhibitory postsynaptic potentials (i.p.s.ps) and, under voltage-clamp conditions, inhibitory postsynaptic currents (i.p.s.cs) were recorded in neurons in buccal ganglia of Aplysia juliana. The decay of i.p.s.cs was exponential with a single time constant, tau, which decreased with membrane depolarization. In external solutions containing iodide or bromide ions instead of chloride ions, tau varied according to the sequence tau (I) greater than tau (Br) greater than tau (Cl), and the voltage sensitivity of tau was altered. In iodide solution, the voltage sensitivity of tau was reversed. Furthermore, the foreign halides depressed the peak current amplitude and shifted the reversal (zero-current) potential to more positive membrane potentials. In low concentrations of sodium pentobarbitone (100-200 microns), the decay of i.p.s.cs became biphasic. Increasing drug concentration and membrane hyperpolarization had differential effects on the rates and relative amplitudes of the two components of i.p.s.c. decay. Octanol (0.5-1 mM) reduced the amplitude of i.p.s.ps and increased the rate of decay of i.p.s.cs without changing the voltage sensitivity of tau. The effect of foreign halides and barbiturates on i.p.s.c. decay were interpreted in terms of a reaction between the anion and an ion-binding site(s) associated with the anion-selective channel, which affects the probability of anions entering the channel and normal channel closure.

Effectiveness of some anions in sustaining the efferent inhibition in the frog labyrinth

Efferent inhibition in the frog labyrinth is sustained by the release of acetylcholine (ACh) which opens a Cl(-)-channel in the hair cell membrane. To investigate more closely the nature of the permeability change underlying the ACh reaction, the external Cl(-) was replaced by anions of increasing hydrated size, and to test the possible role of a Cl(-)-pump in the sensory cells, drugs were applied which are known to block active cl(-) pumping in other systems. Experiments indicate that the ACh-operated inhibitory channel of the hair cell is larger than at other inhibitory synapses (or approximately 0.7 nm), while pharmacological treatments (DNP, NaN3, acetazolamide, ammonium acetate, DIDS) fail to demonstrate any active distribution of Cl(-) across the hair cell membrane.

Antagonistic responses of the radular protractor and retractor to the same putative neurotransmitters

1. Mechanisms of antagonistic responses of the radular protractor and the retractor to the same neurotransmitters, octopamine and serotonin, were investigated in a mollusc, Rapana thomasiana. 2. Hyperpolarization induced by octopamine or serotonin was found to depend mainly on the increase in Cl- conductance of the membrane in both the radular protractor and the retractor. 3. In the protractor, the enhancement of acetylcholine depolarization during the application of octopamine or serotonin was exhibited even when the membrane potential was varied by current application. Spontaneous firing was blocked by these amines, but larger spikes with higher frequency were elicited on giving a depolarizing current above threshold. 4. In the retractor, the inhibition of glutamate depolarization during the application of octopamine or serotonin was observed irrespective of membrane potential. Spontaneous spikes were stopped by these amines but reappeared on depolarization above threshold. Reappeared spikes during serotonin application were smaller than control spikes, while those during octopamine application were about the same as or larger than control ones. 5. The modes of actions of octopamine and serotonin on both muscles were considered.

An analysis of the inhibitory post-synaptic current in the voltage-clamped crayfish muscle

1. Inhibitory post-synaptic currents (i.p.c.s) were recorded from the feed-back current through a wire electrode inserted longitudinally into the opener muscle fibre of the claw in the crayfish (Cambarus clarkii). 2. I.p.s.c. rose to its peak in about 3-4 msec and decayed approximately exponentially. The decay time constant at -100 mV was 9.4 msec. 3. The decay time constant decreased as the membrane was hyperpolarized and increased during depolarization. The time constant (tau) depends on voltage (V) according to the relation tau = a exp (AV), with a = 18.6 msec and A = 0.0065 mV-1. Voltage dependence was opposite in direction to that seen at frog end-plates, but in the same direction as that of e.p.s.c. in crayfish muscle. 4. At lower temperatures, the rise and fall times of i.p.s.c.s were prolonged. Q10 for the decay time constant was 2.4 between 22.6 and 12.5 degrees C. 5. When pH was decreased from 7.2 to 5.5, the decay time constant increased by about 50%, with little change in the voltage dependence of the time course. 6. When chloride in the solution was changed to iodide, the decay time constant was increased by a factor of 3, while voltage dependence of the time course was not changed. In bromide solution the decay time constant increased by about 50%. 7. Peak amplitudes of i.p.s.c.s were approximately linear as the membrane was depolarized, but they levelled off as the membrane was hyperpolarized beyond reversal potential. The non-linear I-V relation did not result from inadequate voltage clamping, nor from a change in the inside concentration of chloride. After equilibration with iodide solution the I-V relation was approximately linear. 8. The decay time constant was increased after repetitive nerve stimulation. This prolongation became more pronounced at lower temperatures. 9. The kinetic process of the transmitter action is discussed. It is suggested that the rate limiting process for i.p.s.c. is binding and unbinding of the transmitter to the receptor.

Effects of internal free calcium upon the sodium and calcium channels in the tunicate egg analysed by the internal perfusion technique

1. The unfertilized egg of the tunicate, Halocynthia roretzi, was intracellularly perfused with various solutions. 2. The perfusion apparatus consisted of lower and upper compartments which were connected by a small glass funnel. A denuded egg cell without chorion was dropped into the funnel and brought into close contact with the glass wall of the funnel. The membrane of the egg faced to the lower compartment was ruptured by a slight difference of hydrostatic pressure and the inside of the egg was perfused with the internal solution flowing through the lower compartment. The current across the upper membrane was analysed by voltage-clamp technique. 3. The egg cell in contact with 400 mM-Na external solution and perfused intracellularly with 400 mM-Na for 30 min showed a relatively low Na reversal potential, +6 mV, in comparison with +60 mV in the intact egg in standard artificial sea water. The exchange efficiency was monitored by observing the shift of Na reversal potential during perfusion with high Na internal perfusate. 4. The internal perfusate containing F- ions stabilized the egg membrane and kept the excitability for 1--2 hr during the intracellular perfusion. With the internal F- perfusate the intracellular cationic content was changed to 400 mM-Na, K, Rb or Cs (external solution of 400 mM-Na) and permeability ratios of the egg Na channel were estimated as PNa:PK:PRb:PCs=1.0:0.14:0.05:0.04. The internal F- perfusate abolished Ca current which was consistently observed in the intact egg, while the internal Cl- perfusate kept both Na and Ca current as in the intact egg. However with the internal Cl- perfusate the egg cell could not be kept in good condition more than 20-30 min. 6. The effects of intracellular free Ca ions upon the egg Na and Ca channels were analysed by using Ca ion-buffered internal Cl- and high Na perfusate. The results showed that internal Ca ions above 10(-6) reduced the Ca current and enhanced the Na current at the same time. In the range between 10(-5) and 10(-4) M the Ca current became half of the control obtained with zero free Ca perfusate while the Na conductance at the zero current level doubled. The internal Ca ions above one mM seemed to abolish the Ca current and to reduce the Na current as well. The reciprocal effect of intracellular Ca ions upon the egg Na and Ca channels was demonstrated in the concentration range from 10(-6) to 10(-3) M.

Conductance increases produced by glycine and gamma-aminobutyric acid in lamprey interneurones

1. Conductances of individual neurones in the isolated lamprey spinal cord were measured with separate intracellular electrodes for recording potentials and for passing current pulses during application of glycine or GABA (0.1-1.0 MM) in Ca-free bathing fluid. Large, reversible increases in conductance were produced in giant interneurones by both amino acids, but Müller axons and sensory dorsal cells were unaffected. 2. Conductance increases produced by glycine and by GABA were selective for Cl. Both conductance increases were linearly related to external Cl concentrations and repeated exposure to the amino acids in Cl-free fluid progressively reduced the conductance increases to less than 1% of their values in normal Cl. 3. Strychnine was a competitive antagonist of glycine, while GABA was antagonized competitively by bicuculline and non-competitively by picrotoxin. 4. The sensitivity of giant interneurones to glycine and GABA increased at low temperatures, in Na-free fluid, and after repeated exposure to the amino acids. Sensitization may have been produced by inhibition of uptake mechanisms for glycine and GABA in the spinal cord. 5. Discharges of interneurones recorded extracellularly were inhibited by bath-applied glycine and GABA, but directly elicited action potentials of axons were unaffected. Strychnine and Cl-free fluid in the presence of Ca produced seizures in lamprey spinal cord. 6. The conclusions of these experiments are that different receptors for glycine and for GABA are present on giant interneurones, that glycine is the better candidate for an inhibitory transmitter in the lamprey spinal cord, and that GABA produces effects similar to those which have been well studied in arthropod muscle.

Anomalous permeabilities of the egg cell membrane of a starfish in K+-Tl+ mixtures

The electrical properties of "inward" rectifying egg cell membranes of the starfish mediastera aequalis have been studied in the presence of K(+)-Tl(+) mixtures. When the ratio of the external concentrations of these ions is changed while their sum is kept constant, both the conductance and the zero-current membrane potential go through a minimum, showing clear discrepancies from theoretical results based on conventional electrodiffusion models (E.g., Goldman's equation). By contrast, when the ration of the two concentrations is fixed and their sum varied, the potential follows an ideal Nernst slope, consistent with Goldman's equation. The membrane conductance which, according to previous studies on similar membranes, is to be viewed as a function of the displacement of the membrane potential from its resting value deltaV, shows marked differences between the cases in which K(+) or Tl(+) are the predominant ions: when K(+) is the predominant permeant ion in solution, the addition of small amounts of Tl(+) inhibits the current, while corresponding blocking effects of K(+) on the current are not observed when Tl(+) is the predominant permeant ion. Also, the time course of the conductance during voltage clamp is different in the two cases, being much faster in Tl(+) than in K(+) solution for comparable values of deltaV. Most of the above features are accounted for by a model in which it is assumed that the ionic channels have external binding sites for cations and that their permeability properties depend on the species of the cation bound (K(+)or Tl(+) in the present experiments).

Electrophysiological study of the antiluminal membrane in the proximal tubule of Necturus: effect of inorganic anions and SCN-

1. A study has been made of the effects of anionic substitutions on the electrical potential difference (p.d.) and conductance characteristics of the antiluminal (peritubular) membrane of the proximal tubule of Necturus kidney. The tubular lumina were filled with oil in order to minimize potential and conductance contributions from luminal membrane and from paracellular shunt pathway.2. Isosmotic substitutions, [A](o) for [Cl](o), produced the following average changes in membrane p.d. (mV): F(-) +1.7, BrO(3) (-) +0.1, Br(-) -4.5, ClO(3) (-) -5.2, I(-) -7.9, NO(3) (-) -12.1, ClO(4) (-) -17.8, SCN(-) -25.3.3. The amplitude of the depolarization caused by increase in K concentration (K-depolarization) in the peritubular perfusate was found to increase during perfusion of the tissue with ClO(4) (-) (by 78%), SCN(-) (45%), I(-) (23%), NO(3) (-) (20%), Br(-) (16%); it decreased with F(-) (by 17%).4. Comparison of membrane p.d. at peak K-depolarization in the control state (during KCl perfusion) with that obtained in the experimental state (during KA perfusion) was found to be more reliable than determination of bi-ionic potentials as a qualitative estimate of the permeabilities of the various anions (P(A)) relative to that of chloride (P(Cl)).5. Study of both peak K-depolarization p.d. and bi-ionic potentials yielded the following sequence for halide anion permeabilities: P(F) > P(Cl) > P(Br) > P(I). The peritubular membrane was found to be substantially more permeable to NO(3) (-), ClO(4) (-) and SCN(-) than to Cl(-).6. The sequence of membrane conductances during anionic substitutions was Cl(-) approximately BrO(3) (-) < Br(-) </= ClO(3) (-) < I(-) approximately F(-) < NO(3) (-) < ClO(4) (-) < SCN(-).7. From the changes in p.d. induced by K-depolarization, the absolute values of p.d. at peak K-depolarization and from the changes in membrane conductance induced by anionic substitutions, it may be inferred that Br(-), I(-), NO(3) (-), ClO(4) (-) and SCN(-) all increase P(K); and that F(-) increases P(Na) (though a smaller increase in P(K) cannot be excluded).

A comparative study of the effects of glutamate and kainate on the lobster muscle fibre and the frog spinal cord

1 The depolarizing actions of glutamate and its conformationally restricted analogue kainate were investigated on the lobster muscle fibre and the frog spinal cord using intracellular and extracellular recordings, respectively. 2 Bath-applied kainate was less potent than glutamate on the lobster fibre but more potent on the frog cord. From the log-log transformation of dose-response curves it was proposed that more than one glutamate molecule was necessary to activate both the lobster and the frog receptor sites. In the frog, at least three kainate molecules were thought to be required for receptor activation. 3 The ionic dependence of glutamate and kainate responses appeared different for the two tissues. 4 Some possible explanations of the differential tissue sensitivity to kainate are discussed.

Effect of penicillin on an excitatory synapse

When penicillin, an epileptogenic agent, was applied to the neuromuscular junctions of the superficial flexor muscles of crayfish, the excitatory junctional potential (EJP) amplitudes were increased by 50-200%. This effect of the drug was not due to changes in the passive electrical properties of the muscle cell membrane or to an increase in its chemical sensitivity to acetylcholine (ACh), the presumed transmitter at the junction studied. Inactivating the penicillin with the enzyme penicillinase, or substituting acetate for penicillin in the test solutions eliminated the effect on EJPs, showing that the penicillin ion was the active agent. Penicillin ions did decrease the frequency of spontaneous miniature EJPs and increase the amplitude or presynaptic spikes recorded extracellularly, suggesting that augmentation of EJPs may have been due to alterations at the presynaptic nerve terminals.

Anion permeation in the proximal tubule of Necturus kidney: the shunt pathway

The effect of foreign anions on transepithelial potential difference and transepithelial input conductance was studied in the isolated perfused Necturus kidney. Two microelectrodes (recording and current-injecting) were inserted into the lumen of single proximal tubules and the peritubular perfusate was shifted reversibly for 30-60 sec from a physiologic Ringer's solution to a test solution in which chloride was replaced isosmotically by a foreign anion. The permeability sequence, obtained by potential measurements, was: lactate less than glutamate less than gluconate less than pyruvate less than benzene sulfonate less than or equal to acetate less than or equal to F less than propionate less than BrO3 less than formate less than ClO3 less than Cl than ClO4 less than I less than or equal to Br less than NO3 less than SCN. Transepithelial conductance decreased when the tissue was perfused with anions less permeable than chloride but the conductance sequence was different from the permeability sequence. Such discrepancies were more pronounced during perfusion with hyperpolarizing anions; ClO4 and I- (both more permeable than chloride) produced an important decrease in transepithelial conductance, followed by incomplete reversibility when the perfusion was shifted again to chloride Ringer's. The results are best explained by the presence of weak positive fixed charges, governing anion permeation, at the shunt pathway of the proximal tubule. An analysis of the data allows tentative estimates of shape and size of the sites.

Mechanism of anion permeation through the muscle fibre membrane of an elasmobranch fish, Taeniura lymma

1. Properties of anion permeation through the membrane of skeletal muscle fibres of the stingray, Taeniura lymma, were studied with intracellular recording and polarization techniques.2. The Cl conductance of the resting membrane in the normal stingray saline at pH 7.7 is 8-10 times greater than the K conductance.3. The Cl conductance decreases with decreasing external pH, with an apparent pK of 5.3, whereas the K conductance is independent of pH between 4 and 9.4. The Q(10) of the Cl conductance is about 2.0, compared with a value of 1.2-1.4 for the K conductance.5. The Cl conductance is proportional to the external Cl concentration when observed after the fibre is equilibrated in the test solution.6. The permeability sequence obtained by potential measurement is SCN > NO(3) > Cl = Br > I > ClO(3) and the permeability ratio is independent of the mole fraction of anions.7. The conductance sequence determined by total replacement of the external Cl with other anion species differs from the permeability sequence and the conductance observed for partial replacement deviates significantly from that expected from the independence principle.8. Possible mechanisms of anion permeation are discussed.

Variations in the permeability properties of the inhibitory post-synaptic membrane of the crayfish neuromuscular junction when activated by different concentrations of GABA

1. The membrane conductance of the crayfish muscle was measured with intracellular micro-electrodes. Increase in the membrane conductance induced by various concentrations of gamma-amino butyric acid (GABA) was measured in the Br(-), NO(3) (-), I(-) and CNS(-) solutions and compared with that in Cl(-) solution. When the membrane was activated by lower concentrations of GABA, the resulting membrane conductance in NO(3) (-) and I(-) solutions was larger than in Cl(-) solution, while the membrane conductance activated by higher concentrations in NO(3) (-) and I(-) solutions were smaller than in Cl(-) solution. The membrane conductance activated by GABA was larger in Br(-) solution and smaller in CNS(-) solution than the membrane conductance activated in Cl(-) solution, throughout the concentration range of GABA examined.2. The reversal potential of inhibitory junctional potentials (i.j.p.s) was measured shortly after replacing Cl(-) by foreign anions: Br(-), NO(3) (-), I(-) and CNS(-). In I(-) solution the i.j.p.s produced by lower stimulation frequencies showed a reversal potential at a more hyperpolarized level than those produced by higher stimulation rates.3. Shortly after replacing Cl(-) by I(-), the GABA potential induced by iontophoretic application showed a triphasic potential change at the ;reversal' potential. The reversal level of the GABA potential produced by smaller doses was at a more hyperpolarized level than that produced by larger doses.4. Possibilities of a concentration effect of GABA are discussed. It is suggested that the relative permeability of the crayfish inhibitory membrane to anions changes depending on the concentration of GABA.