Equilibrium Potential For Chloride Ions Research Articles

The role of chloride ions in the maintenance of resting membrane potential in rat fast and slow muscles during hypogravity modeling.

Antiorthostatic hindlimb suspension reduces resting membrane potential of rat fast and slow muscles within 7 days. Changes in Na+/K+ pump activity and shifts of the equilibrium potential for chloride ions are the main mechanism of the changes in the resting potential of muscle fibers. The latter is presumably associated with increased intracellular ion current due to activation of the second active Na+, K+, 2Cl- symport. Reduction of the membrane potential is related to muscle denervation. However, membrane depolarization of muscle fibers during antiorthostatic suspension cannot be explained solely by changes in the mechanisms of neurotrophic control from motor neurons.

ANO1‐ther brick in the wall – role of Ca2+‐activated Cl− channels of interstitial cells of Cajal in cholinergic motor control of gastrointestinal smooth muscle

ANO1‐ther brick in the wall – role of Ca²⁺‐activated Cl⁻ channels of interstitial cells of Cajal in cholinergic motor control of gastrointestinal smooth muscle

The possible additional role of the cystic fibrosis transmembrane regulator to motoneuron inhibition produced by glycine effects

In the present work we study the contribution of the chloride channel of the Cystic Fibrosis Transmembrane Regulator (CFTR) in the postsynaptic inhibition of somatic motoneurons during rapid-eye-movement (REM) sleep atonia. Postsynaptic inhibition of motoneurons is partially responsible for the atonia that occurs during REM sleep. Disfacilitation is an additional mechanism that lowers motoneuron excitability in this state. Postsynaptic inhibition is mediated by the release of glycine from synaptic terminals on motoneurons, and by GABA that plays a complementary role to that of glycine. In this work we look in brain stem motoneurons of neonatal rats at a mechanism unrelated to the actions of glycine, GABA or to disfacilitation which depends on the chloride channel of the CFTR. We studied the presence of CFTR by immunocytochemistry. In electrophysiological experiments utilizing whole cell recordings in in vitro slices we examined the consequences of blocking this chloride channel. The effects on motoneurons of the application of glycine, of the application of glibenclamide (a CFTR blocker) and again of glycine during the effects of glibenclamide were studied. Glycine produced an hyperpolarization, a decrease in motoneuron excitability and a decrease in input resistance, all characteristic changes of the postsynaptic inhibition produced by this neurotransmitter. Glibenclamide produced an increase in input resistance and in motoneurons' repetitive discharge as well as a shift in the equilibrium potential for chloride ions as indicated by the displacement of the reversal potential for glycinergic actions. In motoneurons treated with glibenclamide, glycine produced postsynaptic inhibition but this effect was smaller when compared to that elicited by glycine in control conditions. The fact that blocking of the CFTR-chloride channel in brain stem motoneurons influences glycinergic inhibition suggests that this channel may play a complementary role in the glycinergic inhibition that occurs during REM sleep.

Molecular cloning of a GABA receptor subunit from Laodelphax striatella (Fallén) and patch clamp analysis of the homo-oligomeric receptors expressed in a Drosophila cell line

A cDNA encoding a gamma-aminobutyric acid (GABA) receptor subunit was cloned from the small brown planthopper Laodelphax striatella. The L. striatella GABA receptor subunit was found to have high amino acid sequence similarity to the bd-type splice variant of the Drosophila GABA receptor Rdl subunit and several other GABA receptor subunits, with identities of over 70%. The cDNA was inserted into the expression vector pAc5.1-lac-Hygro. Clonal cell lines stably expressing homo-oligomeric L. striatella GABA receptors were generated by transfecting the vector into D.mel-2 cells. Expression of functional GABA receptors in the cell lines was demonstrated by whole-cell patch clamp recordings. GABA induced inward currents with an EC(50) value of 29 microM and a Hill coefficient of 1.7. The GABA-evoked responses reversed close to the Nernst equilibrium potential for chloride ions. The amplitudes of agonist-induced currents were found to be in the order muscimol (100 microM) >/= GABA (100 microM) > isoguvacine (100 microM) > cis-4-aminocrotonic acid (CACA) (100 microM) > 5-(4-piperidyl)-3-isoxazolol (4-PIOL) (1 mM). Antagonists such as fipronil (100 nM), 4'-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) (100 nM), dieldrin (100 nM) and SR95531 (gabazine) (1 microM) suppressed GABA-induced currents. The functional expression of a GABA receptor from an agricultural pest presents a unique opportunity to discover new molecules active at this important target site.

Glycine receptors in a population of adult mammalian cones

Glycinergic interplexiform cells provide a feedback signal from the inner retina to the outer retina. To determine if cones receive such a signal, glycine was applied on cultured porcine cone photoreceptors recorded with the patch clamp technique. A minor population of cone photoreceptors was found to generate large currents in response to puff application of glycine. These currents reversed close to the calculated equilibrium potential for chloride ions. These glycine-elicited currents were sensitive to strychnine but not to picrotoxin consistent with the expression of alpha-beta-heteromeric glycine receptors. Glycine receptors were also activated by taurine and beta-alanine. The glycine receptor antibody mAb4a labelled a minority of the cone photoreceptors identified by an antibody specific for cone arrestin. Finally, expression of the beta subunit of the glycine receptor was demonstrated by single cell RT-PCR in a similar proportion (approximately 13%) of cone photoreceptors freshly isolated by lectin-panning. The identity of cone photoreceptors was assessed by their specific expression of the cone arrestin mRNA. The population of cone photoreceptors expressing the glycine receptor was not correlated to a specific colour-sensitive subtype as demonstrated by single cell RT-PCR experiments using primers for S opsin, cone arrestin and glycine receptor beta subunit. This glycine receptor expression in a minority of cones defines a new cone population suggesting an unexpected role for glycine in the visual information processing in the outer retina.

Actions of glutamate and ivermectin on the pharyngeal muscle of Ascaridia galli: A comparative study with Caenorhabditis elegans

The actions of glutamate and ivermectin were examined in the pharynx of Ascaridia galli and the results compared with those on the pharynx of Caenorhabditis elegans. In both preparations glutamate elicits a depolarization and inhibition of pharyngeal pumping, but the response of the pharynx of A. galli was much less than for C. elegans. This may be either because the pharyngeal membrane potential of the former is closely linked to the equilibrium potential for chloride ions (E Cl) while that of C. elegans is independent of E Cl, or that there is a lower density of glutamate receptors on the pharyngeal muscle of A. galli compared with C. elegans. The maximum depolarization to glutamate of the pharyngeal muscle was 4.5±0.8 mV in A. galli while it was >25 mV in C. elegans. Picrotoxin was a weak antagonist of the glutamate response in both species. Flufenamic acid, pentobarbitone and flurazepam had no significant effect on either preparation at concentrations up to 100 μM. Three glutamate receptor agonists, ibotenate, kainate and quisqualate were all more potent than glutamate on the A. galli pharyngeal muscle. In contrast, only ibotenate was more potent than glutamate in C. elegans pharynx, the other two agonists being approximately 20 times less potent. The potency of ivermectin differed markedly between the two species, being approximately three orders of magnitude less potent on the pharynx of A. galli compared with C. elegans. This study demonstrates clear differences between the properties of the pharyngeal muscle of the two species and shows that care must be taken when extrapolating data from free-living to parasitic species of nematode.

Nigral GABAergic inhibition upon cholinergic neurons in the rat pedunculopontine tegmental nucleus.

We investigated, in a midbrain parasagittal slice preparation of Wistar rats (postnatal day 9-17), the synaptic inhibition of neurons in the pedunculopontine tegmental nucleus (PPN), which was mediated by gamma (gamma)-amino-butyric acid (GABA). Whole-cell patch-clamp recording was used, in combination with a single-cell reverse transcription-polymerase chain reaction amplification technique, to record synaptic potentials and to identify the phenotype of the recorded PPN neuron. In the presence of the ionotropic glutamate receptor antagonists, 6-cyano-2, 3-dihydroxy-7-nitro-quinoxaline-2, 3, dione, and dl-2-amino-5-phosphonovaleric acid, single electrical stimuli were applied to the substantia nigra pars reticulata (SNr), one of the basal ganglia output nuclei. Stimulation of the SNr evoked inhibitory postsynaptic potentials (IPSPs) in 73 of the 104 neurons in the PPN. The IPSPs were abolished with a GABAA receptor antagonist, bicuculline. Inhibitory postsynaptic currents of the neurons were reversed in polarity at approximately -93.5 mV, which was close to the value of the equilibrium potential for chloride ions of -88.4 mV. Single-cell reverse transcription-polymerase chain reactions revealed that approximately 30% (9/32) of the PPN neurons that received inhibition from the SNr expressed detectable levels of choline acetyltransferase mRNA. These findings show that output from the SNr regulates the activity of cholinergic PPN neurons through GABAA receptors.

Postsynaptic Currents in Dorsal Root Ganglion Neurons Co-cultured with Spinal Cord Neurons

In co-cultured dorsal root ganglion (DRG) neurons and spinal cord neurons from newborn rats, using a voltage-clamp technique in the whole-cell configuration enabled us to observe in DRG neurons the effects evoked by extracellular local electrical stimulation of cells corresponding to spinal cord neurons in their morphological characteristics. Such stimulation caused the appearance of postsynaptic currents (PSC) in DRG neurons in 9% of the cases. The mean delay of these currents (measured from the stimulus leading edge) was 4.7 ± 0.29 msec, the mean time to peak was 2.6 ± 0.77 msec, and the decay time constant τ = 14.5 ± 1.04 msec. The reversal potential of evoked PSC (ePSC) was close to the equilibrium potential for chloride ions estimated by the Nernst equation. Application of 20 μM bicuculline induced practically complete and reversible ePSC block. The conclusion was drawn that these currents arise due to activation of the chloride channels operated by GABA receptors and, hence, represent an inhibitory PSC. Thus, one may deem it proved that spinal cord neurons can establish functional inhibitory synapses with DRG neurons.

Co-existence in DUM neurones of two GluCl channels that differ in their picrotoxin sensitivity.

Whole cell patch-clamp electrophysiology was used to study the effects of L-glutamate on dissociated cockroach (Periplaneta americana) dorsal unpaired median (DUM) neurones. Application of L-glutamate via pressure-ejection pipette resulted in a two-component hyperpolarization, consisting of an initial transient and a second, prolonged phase. Under voltage-clamp, using isotonic chloride in the saline and intrapipette solutions, two L-glutamate-gated inward currents were characterized. Their reversal potentials were close to the equilibrium potential for chloride ions. One component was selectively activated by ibotenate and was sensitive to picrotoxin (100 microM), BIDN (10 microM) and the phenylpyrazole insecticide fipronil (10 microM), known to be potent blockers of insect GABA-gated chloride channels. The second component was insensitive to picrotoxin (100 microM) and BIDN (10 microM). These findings demonstrated, for the first time, the co-existence of two pharmacologically distinct native insect neuronal L-glutamate-gated chloride channels.

Hyperpolarization-activated inward potassium and calcium-sensitive chloride currents in beating pacemaker insect neurosecretory cells (dorsal unpaired median neurons)

Hyperpolarization-activated inward currents were studied in single adult cockroach Periplaneta americana pacemaker neurosecretory cells, identified as dorsal unpaired median neurons using the whole-cell patch-clamp technique. Under current clamp, injection of negative current produced a hyperpolarization of the cell membrane with a sag in the membrane potential toward the resting value. Under voltage clamp, the whole-cell current–voltage relationship exhibited an unexpected biphasic aspect. The global hyperpolarization-activated inward current could be dissociated by means of 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid and tetraethylammonium chloride sensitivity, ionic selectivity, voltage dependence and activation threshold as inward potassium and calcium-sensitive chloride currents. The inward potassium current was activated around −80 mV. The reversal potential followed the potassium equilibrium potential when the extracellular potassium concentration was raised. This current was not dependent on the external sodium concentration and was sensitive to 10 mM tetraethylammonium chloride or 5 mM barium chloride. The hyperpolarization-activated inward calcium-sensitive chloride current was activated in a range of potential 20 mV more positive than the potassium current. The estimated reversal potential (−71 mV) was very close to the equilibrium potential for chloride ions (−73 mV). Intracellularly applied 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid, 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid and external application of 1 mM zinc chloride, calcium-free saline or high concentrations of intracellular 1,2-bis-(2-aminophenoxy)ethane- N, N, N′, N′-tetra-acetate blocked the inward chloride current. Current-clamp experiments indicated that the inward potassium current accounted for inward rectification of dorsal unpaired median neurons. Our findings report, for the first time in pacemaker neurosecretory cells, the co-existence of two distinct hyperpolarization-activated inward currents which have specialized function in pacemaker activity.

Dorsal root potentials and dorsal root reflexes: a double-edged sword.

The nature of dorsal root reflexes (DRRs) and their possible role in peripheral inflammation and the consequent hyperalgesia are reviewed. The history of DRRs and the relationship of DRRs to primary afferent depolarization and presynaptic inhibition in pathways formed by both large and fine afferents are discussed. Emphasis is placed on the mechanisms underlying primary afferent depolarization, including the anatomical arrangement of the synapses involved, how depolarization can result in inhibition by decreasing transmitter release, the role of excitatory amino acids and GABA, the manner in which the equilibrium potential for chloride ions is determined in primary afferent fibers, and forms of presynaptic inhibition that do not utilize GABA(A) receptors. There is then a discussion of neurogenic inflammation, including the role of the release of neuropeptides such as substance P and calcitonin gene-related peptide from sensory nerve endings. Evidence is reviewed that links DRRs to a substantial part of the swelling of the knee joint in acute experimental arthritis and to the flare reaction in the skin following intradermal injection of capsaicin. Possible mechanisms by which the level of DRR activity might be enhanced following inflammation are suggested. The consequences of this increase in DRRs may include exacerbation of hyperalgesia as well as of peripheral inflammation. The conversion of an inhibitory process, presynaptic inhibition, to an excitatory one by DRRs can thus lead to pathological consequences.

Extracellular divalent cations block a cation non-selective conductance unrelated to calcium channels in rat cardiac muscle.

1. The effect of removing extracellular divalent cations on resting potential (Vrest) and background conductance of rat cardiac muscle was studied. Vrest was measured with 3 M KCl-filled microelectrodes in papillary muscles, or with a patch electrode in ventricular myocytes. Whole-cell membrane currents were measured in myocytes using step or ramp voltage commands. 2. In both muscles and single cells, decrease or removal of Ca2+o and Mg2+o caused a nifedipine-resistant depolarization, which was reversed upon readmission of Ca2+o or Mg2+o (half-maximal effect at 0.8 mM Ca2+o or 3 mM Mg2+o in muscles). 3. In single myocytes, removal of Ca2+o and Mg2+o had no effect on the seal resistance in nonruptured cell-attached recordings, but reversibly induced a current with a reversal potential (Vrev) of -8 +/- 3.4 mV (with internal Cs+; mean +/- S.E.M., n = 23) during whole-cell recordings. The current was insensitive to nifedipine (3-100 microM) or amiloride (1 mM). Vrev was insensitive to changes in the equilibrium potential for chloride ions (ECl). 4. The current induced in the absence of extracellular divalent cations was blocked in a concentration-dependent manner by Ca2+o. (At -80 mV, the affinity constant KCa was 60 microM with a Hill coefficient of 0.9) KCa was voltage dependent at positive but not negative potentials. Mg2+o, Ni2+o, Sr2+o, Ba2+o, Cd2+o and Gd3+o also blocked the current. 5. In 0 mM Na+ (145 mM NMDG+), the inward component of the divalent cation-sensitive current was decreased and Vrev shifted to more negative potentials. 6. These results suggest that a novel conductance pathway, permeable to monovalent cations but not to Cl- and blocked by divalent cations, exists in ventricular myocytes.

Stimulation within the rostral ventrolateral medulla can evoke monosynaptic GABAergic IPSPs in sympathetic preganglionic neurons in vitro.

The inhibitory responses of identified sympathetic preganglionic neurons (SPNs) to stimulation within the rostral ventrolateral medulla (RVLM) were studied to determine their nature and pharmacology. Whole cell patch-clamp recordings were made from 36 SPNs in the upper thoracic segments of the spinal cord in a neonatal rat brain stem-spinal cord preparation. Neurons were identified as SPNs on the basis of their antidromic activation after stimulation of the ipsilateral segmental ventral root and their morphology and location in the intermediolateral cell column and intercalated nucleus. In all SPNs, electrical stimulation of the RVLM evoked fast excitatory postsynaptic potentials (EPSPs) that were mediated by non-N-methyl-D-aspartate (NMDA) and NMDA receptors. These excitatory responses were the most prominent response in control artificial cerebrospinal fluid and have been studied previously. In 22 of the SPNs, RVLM stimulation also elicited fast inhibitory postsynaptic potentials (IPSPs), which increased in amplitude as the membrane was depolarized. Five of these neurons were not studied further as they responded occasionally with IPSPs that had highly variable onset latencies indicating the involvement of a polysynaptic pathway. In the remaining SPNs (n = 17), the evoked IPSPs persisted in the presence of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3,-dione and D,L-2-amino-5-phosphonopentanoic acid. In eight of these SPNs, it was necessary to block the EPSPs to reveal the IPSPs. In the 7 SPNs tested, the onset latencies of the IPSPs were not significantly different from the onset latencies of the fast EPSPs. The low sweep-to-sweep fluctuations in onset latency of individual IPSPs (absolute average deviation: 0.4 ms) indicated that the IPSPs were elicited by activation of a monosynaptic pathway. The amplitudes of the IPSPs decreased in amplitude as the membrane was hyperpolarized and reversed in polarity at -70.3 +/- 1.7 mV (mean +/- SD), which was close to the equilibrium potential for chloride ions. In addition, in seven SPNs, bath applications of 5 microM bicuculline, a gamma-aminobuturic acid-A (GABAA) antagonist, abolished or reduced the evoked IPSPs. Five SPNs also were studied that displayed ongoing IPSPs. The amplitudes of these IPSPs increased with membrane depolarization and were blocked by bath applications of 5 microM bicuculline, suggesting that they also were mediated by activation of GABAA receptors. These results demonstrate the existence of a bulbospinal GABAergic pathway impinging directly onto SPNs. This pathway may be tonically active in the neonatal rat brain stem-spinal cord preparation.

Human astrocytoma cells express a unique chloride current

Human astrocytoma cells were studied using whole-cell patch-clamp recording. Voltage-dependent outwardly-rectifying anion currents were identified in primary cultures of six freshly resected human brain tumors and in seven established anaplastic astrocytoma/glioblastoma cell lines (U251MG, CH235MG, U373MG, U105MG, D54MG, SK-MG-1, and STTG1). Anion currents were not observed in normal, non-neoplastic glial cells, nor in human tumor-derived cells of non-glial origin (melanoma, breast cancer, neuroblastoma, rhabdomyosarcoma). Currents activated at potentials > 50 mV and showed large transients upon termination of voltage steps. Currents reversed at the predicted equilibrium potential for chloride ions and could also be recorded when Cl- was replaced by F-, Br- or I-. Currents were inhibited by the Cl- channel blockers chlorotoxin, DIDS, and DNDS. These Cl- currents may play a role in the growth control of astrocytoma cells.

Chloride current activated by hypotonicity in cultured human astrocytoma cells

Cultured astrocytoma cells were voltage clamped with pipettes where [Ca2+] in the pipette was buffered to 10(-7) M with 10 mM 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and membrane currents recorded. In isotonic solution predominantly K+ currents were evoked by depolarizing or hyperpolarizing commands and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 1 mM) had little effect on inward and outward currents evoked by voltage steps to negative and positive potentials. If the osmolarity of the bathing solution was reduced from 280 to 200 mosmol l-1, a large current developed, which rectified outwardly and reversed close to the equilibrium potential for chloride ions, ECl. Upon stepping to potentials positive to about +30 mV in hypotonic solution, this outward current inactivated over 2-3 s; this decline was greater with N-methyl-D-glucamine chloride (NMDG-Cl) in the pipette than when KCl or sodium glutamate was present. Holding at various positive potentials inactivated the current, with half-inactivation occurring around +50 mV. The current reactivated over 2-3 s at potentials negative to about +30 mV. The current evoked by hypotonic solution was inhibited by various putative chloride channel blockers with the order of potency DIDS > SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) approximately NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid) > niflumic acid > 9-AC (anthracene 9-carboxylic acid). The currents inhibited by these compounds reversed close to the calculated ECl. It is concluded that hypotonic solution evokes a large anionic current in these cultured astrocytoma cells largely carried by chloride ions. This current is absent in isotonic solution, when currents are carried mainly by potassium ions. It is likely that the current elicited by hypotonic solution is part of the mechanisms regulating astrocyte volume in vivo.

Amiridin and tacrine modulation of the activity and plasticity of the cholinoreceptors of neurons of the common snail: phenomenology and mechanisms.

The influence of amiridin and tacrine on the membrane potential, activity, and plasticity of cholinoreceptors was investigated using the methods of recording of intracellular potentials and transmembrane currents in identified RPa3 and LPa3 neurons of the common snail. Amiridin and tacrine (1-100 mumole/liter) do not exert appreciable influences on the membrane potential of the cells. Both compounds modulate the activity of cholinoreceptors, judging from their influence on the inward current induced by local application of the acetylcholine: the duration of the ACh current increases and its amplitude varies biphasically (a short-latency increase is succeeded by a decrease). Amiridin and tacrine intensify the extinction of the ACh current induced by repeated applications of ACh to the soma. The acetylcholinesterase inhibitor, physostigmine, exerts a similar modulatory influence on the ACh current and on its extinction, and also prevents the manifestation of the effects of amiridin and tacrine. Amiridin and physostigmine directly influence the cholinoreceptors and the ionic membrane channels controlled by them by similarly altering the voltage-current characteristics (VAC) of the ACh current, and by shifting the reversal potential of the ACh current in the negative direction, bringing it closer to the equilibrium potential for chloride ions; this may be interpreted as a result of the nonidentical action of these compounds on the various ionic currents controlled by cholinoreceptors. The hypothesis is advanced that the modulatory influence of amiridin, tacrine, and physostigmine on the activity and plasticity of the cholinoreceptors is governed by their direct membrane-cytoplasmic action.

Properties of amino acid neurotransmitter receptors of embryonic cortical neurons when activated by exogenous and endogenous agonists.

1. The properties of receptors for amino acid neurotransmitters expressed by developing cortical neurons were studied with the use of whole-cell recording in the intact cerebral cortex of embryonic turtles in vitro. The inhibitory agonist gamma-aminobutyric acid (GABA) and the excitatory agonist glutamate were focally applied to single cells under voltage clamp, and the ionic dependence, voltage dependence, and pharmacological sensitivity of the responses were characterized. The responses mediated by a glutamate receptor subtype, the N-methyl-D-aspartate (NMDA) receptor, produced by glutamate and by evoked release of an endogenous excitatory agonist, were compared further. Fluctuation analysis was used to characterize the properties of the NMDA channels and the mechanism of action of receptor antagonists. 2. When postmitotic neurons first appeared at stage 15, all neurons tested responded to GABA with a current that reversed at the equilibrium potential for chloride ions and that was sensitive to the GABAA receptor antagonist bicuculline methiodide (BMI). As development proceeded, an increasing proportion of neurons also responded with a BMI-insensitive current that reversed near the equilibrium potential for potassium ions. This current was blocked by the GABAB receptor antagonist 3-amino-2-propyl phosponic acid (phaclofen). The GABAB agonist baclofen, however, failed to produce a detectable postsynaptic current. 3. Neurons at stage 15 showed a biphasic response to glutamate that reversed at the equilibrium potential for cations. All neurons tested showed a slow, sustained response associated with an increase in current variance compared with background, and, as development proceeded, an increasing proportion also exhibited a fast, transient response. Both fast and slow responses varied linearly with voltage in the absence of Mg2+ ions, but the addition of Mg2+ ions to the bathing medium attenuated the slow response at hyperpolarized potentials. As a result, the current-voltage relation of the slow response in the presence of Mg2+ ions exhibited a region of negative slope conductance, like that of currents mediated by NMDA receptors. 4. The fast and slow responses to glutamate differed in their pharmacological sensitivity. The fast responses were sensitive to the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas the slow responses were sensitive to the NMDA receptor antagonist D(-)-2-amino-5-phosphonovalerate (D-APV). 5. When cells were held at -70 mV, glutamate evoked a fluctuating current consisting of channel currents with a mean open time, tau, of 4.42 +/- 0.47 (SE) ms in early postmitotic neurons at stage 15 and 4.99 +/- 0.38 ms at stages 17-20.(ABSTRACT TRUNCATED AT 400 WORDS)

Possible involvement of a chloride conductance in the transient outward current of whole-cell voltage-clamped ferret ventricular myocytes

The transient outward current was studied, using the whole-cell patch-clamp technique, in isolated ventricular cells from the ferret heart. In the presence of 4-aminopyridine and cadmium chloride which respectively blocked the Ca-insensitive and the Ca-dependent outward currents, a residual transient outward current was observed in about 30% of the cells tested. This current was suppressed in external hypochloride solution, completely inhibited by SITS (3 mM) and reversed at the equilibrium potential for chloride ions. This suggests the presence of a chloride permeability which could contribute to the repolarization phase of the cardiac action potential.

Studies on the ionic selectivity of the GABA-operated chloride channel on the somatic muscle bag cells of the parasitic nematode Ascaris suum

The reversal potential for the GABA-elicited hyperpolarization (EGABA) of the muscle bag cells of the parasite nematode Ascaris suum is near to the equilibrium potential for chloride ions (ECl) indicating that this is a chloride-mediated event. The value of EGABA in the presence of different anions indicates the selectivity sequence of the GABA-operated chloride channel as SCN- greater than Br-, I- greater than ClO3-, NO3-, Cl- greater than C2H5COO- greater than CH3COO-, HCOO-, OHCH2CH2SO3H-, CH3SO4-, BrO3-, SO4(2-). The cut-off point in terms of relative hydrated size for permeation is between ClO3- and BrO3- suggesting that the diameter of the channel is between 0.29 and 0.33 nm. The selectivity sequence of the resting membrane chloride channels was indicated by the degree of muscle cell hyperpolarization upon addition of the anion to the bathing medium and was SCN- greater than NO3- greater than I-, Br-, greater than ClO3- greater than Cl-. These sequences are consistent with the anions interacting with a cationic site of low field strength at the selectivity determining portion of the channel for both the GABA-operated and the resting chloride channel.

Voltage clamp analysis of the kinetics of piperidine-induced chloride current in isolated Aplysia neurons

The effect of piperidine (Pip) on isolated Aplysia neurons was investigated using the voltage clamp and concentration clamp techniques in which neurons were perfused internally and externally with Na+,K+ free solution. Pip induced a Cl-current (ICl) in a dose-dependent manner for doses ranging from 10(-4) M to 10(-2) M. The dose-response curve gave an apparent dissociation constant of 8.4 x 10(-4) M and a Hill coefficient of 1.7. The current-voltage relationship was linear in the voltage range examined (-70 to +30 mV). The equilibrium potential for Pip induced current was close to the calculated equilibrium potential for chloride ions (ECl), (-10.7 mV). The activation phase of the ICl was characterized by a single exponential at all concentrations. The time constant of this phase decreased with increasing concentrations of Pip but did not depend on the membrane potential. The deactivation phase of the ICl proceeded on a single exponential curve at concentrations of Pip less than 5 x 10(-4) M, but on a double exponential at concentrations of 5 x 10(-4) M and higher. The deactivation time constant also decreased with increasing concentrations of Pip, but showed no potential dependence. Pip- and ACh-induced IClS were not blocked by 10(-4) M atropine. However, Pip-induced ICl was abolished with 10(-4) M d-tubocurarine (dTC), and the ACh-induced ICl was depressed by the same dose of dTC. These results suggest that Pip acts on at least two components of the nicotinic receptor-Cl channel complex in Aplysia neurons to elicit the ICl.