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

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.

Chloride ions in the pore of glycine and GABA channels shape the time course and voltage dependence of agonist currents

In the vertebrate CNS, fast synaptic inhibition is mediated by GABA and glycine receptors. We recently reported that the time course of these synaptic currents is slower when intracellular chloride is high. Here we extend these findings to measure the effects of both extracellular and intracellular chloride on the deactivation of glycine and GABA currents at both negative and positive holding potentials. Currents were elicited by fast agonist application to outside-out patches from HEK-293 cells expressing rat glycine or GABA receptors. The slowing effect of high extracellular chloride on current decay was detectable only in low intracellular chloride (4 mm). Our main finding is that glycine and GABA receptors "sense" chloride concentrations because of interactions between the M2 pore-lining domain and the permeating ions. This hypothesis is supported by the observation that the sensitivity of channel gating to intracellular chloride is abolished if the channel is engineered to become cation selective or if positive charges in the external pore vestibule are eliminated by mutagenesis. The appropriate interaction between permeating ions and channel pore is also necessary to maintain the channel voltage sensitivity of gating, which prolongs current decay at depolarized potentials. Voltage dependence is abolished by the same mutations that suppress the effect of intracellular chloride and also by replacing chloride with another permeant ion, thiocyanate. These observations suggest that permeant chloride affects gating by a foot-in-the-door effect, binding to a channel site with asymmetrical access from the intracellular and extracellular sides of the membrane.

Acetylcholine receptors in spider peripheral mechanosensilla

Peripherally located parts of spider mechanosensory neurons are modulated by several neurotransmitters released from apposed efferent fibers. Activities of acetylcholine (ACh) synthesizing enzyme choline acetyltransferase (ChAT) and ACh degrading enzyme acetylcholine esterase (AChE) were previously found in some efferent fibers. ChAT activity was also present in all the mechanosensory neurons, while AChE activity was only found in some. We show that spider mechanosensory neurons and probably some efferent neurons are immunoreactive to a monoclonal antibody against muscarinic ACh receptors (mAChRs). However, application of muscarinic agonists did not change the physiological responses or membrane potentials of neurons in the lyriform organ VS-3. Similarly, the sensitivities of the neurons of trichobothria (filiform hairs) remained unchanged after application of these agonists. Therefore, activation of mAChRs may only modulate the function of spider mechanosensory neurons indirectly, for example, by affecting the release of other transmitter(s). However, a subgroup of VS-3 neurons was inhibited by ACh, which also depolarized the membrane similar to these neurons' responses to GABA, suggesting that ACh activates anion channels in these neurons. Interestingly, all of the neurons responding to ACh were the rapidly adapting Type A neurons that were previously shown to express AChE activity.

Use of 82Br- radiotracer to study transmembrane halide flux: the effect of a tranquilizing drug, chlordiazepoxide on channel opening of a GABAA receptor

We used the short-lived radionuclide, 82Br- to follow gamma-aminobutyrate (GABA) receptor-mediated halide exchange into membrane vesicles from rat cerebral cortex in millisecond and second time regions using quench-flow technique. The radioisotope was prepared by neutron capture [81Br-(n,gamma)82Br-] on irradiation of a natural isotope of bromine, 81Br- in a neutron flux. 82Br- decays by beta-emission with secondary gamma-emission. Possible advantages of 82Br- over 36Cl- in anion tracer measurements include, (a) a short lifetime (t1/2 = 35.3 hr), which alleviates contamination and disposal problems, (b) high counting efficiency (1.54) due to the secondary radiation, (c) measurement with a gamma-counter as well as a beta-counter, (d) a simple preparation not requiring subsequent purification steps giving a specific activity depending on the irradiation time. With 6 hr irradiation time the specific activity was sufficient to make measurements with < 1 mM Br-, which is less than the bromide concentration known to affect the properties of GABAA receptor. The radiotracers, 82Br- and 36Cl- could be compared with the same solution composition. In conditions where a direct effect of binding of halide to receptor does not contribute to a difference in measured ion-flux, 82Br- was translocated only marginally faster than 36Cl-. The effect of chlordiazepoxide (CDPX) (2-250 microM) on the progress of GABA (10 microM)-mediated 82Br- uptake was measured in a time range of 200 msec to 20 sec using quench-flow technique. The two phases of anion exchange previously reported in this experimental model with GABA alone were observed. The rate of 82Br- exchange was increased 2.3-fold at 30-60 microM CDPX and was not further increased with increasing [CDPX]. The rate of halide exchange is a measure of open channel concentration. The isotope exchange rate constant, J, in a membrane vesicle preparation, is a measure of the membrane permeability per internal volume/surface area, J = PmA/V. Receptor desensitization rate was also increased by CDPX, but unlike the isotope exchange rate, it continued to increase up to at least 250 microM CDPX.

Ion effects on gating of the Ca(2+)-activated K+ channel correlate with occupancy of the pore

We studied the effects of permeant ions on the gating of the large conductance Ca(2+)-activated K+ channel from rat skeletal muscle. Rb+ blockade of inward K+ current caused an increase in the open probability as though Rb+ occupancy of the pore interferes with channel closing. In support of this hypothesis, we directly measured the occupancy of the pore by the impermeant ion Cs+ and found that it strongly correlates with its effect on gating. This is consistent with the "foot-in-the-door" model of gating, which states that channels cannot close with an ion in the pore. However, because Rb+ and Cs+ not only slow the closing rate (as predicted by the model), but also speed the opening rate, our results are more consistent with a modified version of the model in which the channel can indeed close while occupied, but the occupancy destabilizes the closed state. Increasing the occupancy of the pore by the addition of other permeant (K+ and Tl+) and impermeant (tetraethylammonium) ions did not affect the open probability. To account for this disparity, we used a two-site permeation model in which only one of the sites influenced gating. Occupancy of this "gating site" interferes with channel closing and hastens opening. Ions that directly or indirectly increase the occupancy of this site will increase the open probability.

Receptor-mediated presynaptic facilitation of quantal release of acetylcholine induced by pralidoxime in Aplysia

1. Possible interactions of contrathion (pralidoxime sulfomethylate), a reactivator of phosphorylated acetylcholinesterase (AChE), with the regulation of cholinergic transmission were investigated on an identified synapse in the buccal ganglion of Aplysia californica. 2. Transmitter release was evoked either by a presynaptic action potential or, under voltage clamp, by a long depolarization of the presynaptic cell. At concentrations higher than 10(-5) M, bath-applied contrathion decreased the amplitude of miniature postsynaptic currents and increased their decay time. At the same time, the quantal release of ACh was transiently facilitated. The facilitatory effect of contrathion was prevented by tubocurarine but not by atropine. Because in this preparation, these drugs block, respectively, the presynaptic nicotinic-like and muscarinic-like receptors involved in positive and negative feedback of ACh release, we proposed that contrathion activates presynaptic nicotinic-like receptors. 3. Differential desensitization of the presynaptic receptors is proposed to explain the transience of the facilitatory action of contrathion on ACh release. 4. The complexity of the synaptic action of contrathion raises the possibility that its therapeutic effects in AChE poisonings are not limited to AChE reactivation.

Actions of anaesthetics and avermectin on GABAA chloride channels in mammalian dorsal root ganglion neurones

1. The gamma-aminobutyric acid (GABA)-mimetic actions of some anaesthetics and the antehelminthic avermectin B1a were examined on freshly isolated mammalian dorsal root ganglion (DRG) neurones by use of suction electrodes and a single electrode voltage clamp. 2. Pentobarbitone (60 microM-3 mM), chloralose (600 microM-1 mM), etomidate (10-100 microM), alphaxalone (10-60 microM) and avermectin (10-60 microM) directly activated chloride channels in GABA-sensitive DRG neurones. The agonist action was sensitive to block by bicuculline and picrotoxinin. 3. Steady-state current-voltage (I-V) curves for the anaesthetics were either linear, or rectified in the opposite direction to steady-state I-V curves obtained with GABA. Current relaxations in response to voltage jumps were also of the opposite direction. An extra surge of current ('bounce') was commonly observed on washout of some of these agonists. 4. Pentobarbitone was ineffective as an agonist at alkali pH (10.4 and 9.4), but was approximately twice as effective at acid (5.4) than at normal (7.4) pH values. 5. These results suggest that some anaesthetics and avermectin are capable of 'blocking' GABA channels in addition to activating them.

Characteristics of GABA-activated chloride channels in mammalian dorsal root ganglion neurones

1. The properties of gamma-aminobutyric acid (GABA)-activated chloride channels in dorsal root ganglion (DRG) neurones obtained from rats and cats were examined using the single-electrode voltage clamp in conjunction with suction-electrode techniques. 2. GABA-evoked currents showed voltage-sensitive kinetics. Time constants (tau D) were measured from voltage-jump relaxations and tau D became briefer with membrane hyperpolarization. tau D was 33 ms at -120 mV with 60 microM-GABA and changed e-fold for 188 mV. tau D decreased as GABA concentration was increased - the extrapolated tau D at 'zero' GABA concentration was approximately equal to 50 ms at -120 mV. 3. The steady-state current in GABA was curvilinear, rectifying at negative potentials. The instantaneous current was linear with symmetrical chloride concentrations (140 mM) on both sides of the cell membrane. 4. Muscimol was a more effective agonist than GABA, while piperidine-4-sulphonic acid and ethylenediamine monocarbamate were only weakly effective agonists. Taurine and glycine had no detectable agonist activity. 5. Ion substitution experiments revealed the permeability sequence I- greater than Br- greater than Cl- greater than F- greater than propionate (1.88 greater than 1.21 greater than 1.0 approximately equal to 0.1 approximately equal to 0.1). 6. The presence of iodide and bromide ions externally caused an increase in chloride efflux at membrane potentials more negative than -40 mV, and caused a prolongation of voltage-jump relaxations. Relaxations in fluoride and propionate solutions were faster than those seen in chloride.

Outward rectification of inhibitory postsynaptic currents in cultured rat hippocampal neurones

1. Inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) were recorded from cultured hippocampal neurones of the embryonic rat at 22 degrees C, using the whole-cell patch-clamp technique with a low-Cl-, 145 mM-potassium gluconate solution in the patch pipette. Individual synaptic events were elicited at low frequency (0.05-0.1 Hz) by stimulating a presynaptic neurone either by direct intracellular current injection, or by applying a brief pulse of L-glutamate. 2. In target neurones voltage clamped at -40 mV, outwardly directed IPSCs of mean amplitude 0.23 nA were recorded. The IPSCs were depressed by the GABA antagonist bicuculline, and reversed polarity between -50 and -80 mV (mean -64 mV), as did current responses to gamma-aminobutyric acid. The IPSPs and IPSCs reversed as a single phase; no bicuculline-resistant 'late' synaptic event was observed. 3. The IPSCs had variable kinetics, with rise times between 1 and 5 ms (mean 2.9 ms) at -40 mV, and slower, monoexponential, decay phases (decay time constant, tau IPSC, 10-40 ms at -40 mV). In some cells, tau IPSC clearly increased with depolarization. 4. The IPSC reversal potential was -64 +/- 9 mV (n = 23) under the experimental conditions used; this suggests that the synaptically activated channels are approximately 25 times more permeable to Cl- than to the gluconate anion. 5. The peak conductance associated with the IPSC showed outward rectification. The synaptic conductance measured at -40 mV was 1.7 times greater than that measured at -100 mV; at -20 mV, synaptic conductance was 2.5 times greater than at -100 mV. This outward rectification can be explained by a constant field model under these experimental conditions of asymmetric Cl- concentrations.

Ion channels and postsynaptic potentials

Neurotransmitters open transmembrane ion channels in target membranes. At the motor endplate, the open time of channels activated by acetylcholine determines the time course of the endplate current. Endplate channels are selective for cations and their conductance and open times are influenced by the nature of the permeating cation. Similar effects have been seen at anion-selective inhibitory synapses. Some 'blocking' drugs that depress postsynaptic responses to neurotransmitters produce effects not consistent with a simple model in which they block open channels. Channels activated by neurotransmitters can show subconductance states that may reflect the fundamental mode of operation of a variety of ion channels.

Action of enflurane on cholinergic transmission in identified Aplysia neurones

Effects of enflurane on the cholinergic transmission in Aplysia neurones were studied by current and voltage clamp methods. Acetylcholine (ACh) evoked three types of postsynaptic responses on different identified neurones: (1) a depolarizing response due to an increase in Na and K conductances (D-response), (2) a fast hyperpolarizing response due to an increase in C1 conductance (C1-response), and (3) a slow hyperpolarizing response due to an increase in K conductance (K-response). Enflurane altered neither the action potential nor the membrane resistance of the neurones but depressed the three ACh-induced responses, non-competitively, in a dose-dependent manner. The K-response was less suppressed than the other two. Blockade of the closed state of ion channel was suggested by a reduction in the first ACh response evoked 1 min after administration of enflurane. The anaesthetic facilitated the decay of the neurally evoked e.p.s.c. and i.p.s.c. in suggesting a reduction in the mean open time of the postsynaptic ion channel. It is concluded that enflurane depresses excitatory and inhibitory cholinergic transmission by reducing the postsynaptic currents.

Miniature and evoked inhibitory junctional currents and gamma-aminobutyric acid-activated current noise in locust muscle fibres

gamma-Aminobutyric acid (GABA) current noise and inhibitory junctional currents (i.j.c.s) have been examined to give properties of the GABA receptor and its associated synaptic channel. Various procedures were used to identify muscle bundles receiving inhibitory innervation. In normal bathing medium the decay time constant of the i.j.c. was tau i.j.c. = 7.6 +/- 0.7 ms (clamp potential, Vm = -80 mV; temperature, T = 21 degrees C). Most muscle fibres were sensitive to ionophoretically applied GABA, irrespective of the presence of inhibitory innervation. GABA current noise obtained at junctional sites gave spectra which were fitted usually with a single Lorentzian component, or occasionally with the sum of two Lorentzians. The conductance of the single inhibitory channel was, gamma (GABA) = 21.6 +/- 0.9 pS (Vm = -80 mV; T = 21 degrees C). The mean 'burst length' of the openings produced by a single receptor activation was tau noise = 4.0 +/- 0.8 ms, at Vm = -80 mV. This decreased exponentially with hyperpolarization. On average tau i.j.c. exceeded tau noise although good agreement was found in some fibres. I.j.c.s were examined in greater detail after excitatory synaptic receptors had been desensitized with 10(-3) M-L-glutamate to abolish all excitatory synaptic activity. Their decay time constant was tau i.j.c. = 7.2 +/- 0.4 ms, and their rise time was 3.3 +/- 0.12 ms, at Vm = -80 mV. An e-fold decrease in tau i.j.c. resulted from a 103 +/- 7.9 mV hyperpolarization; time to peak showed a smaller dependence on Vm. The mean size of the inhibitory quantal event (i.e. response to a single transmitter packet) was estimated from fluctuations in i.j.c. amplitude. Mean quantal content of the i.j.c. was about 30 at normal levels of release. Mean amplitude of the directly measured miniature i.j.c. = 0.65 +/- 0.08 nA at Vm = -80 mV (V eq approximately equal to -40 mV). The amplitude of the quantal event showed a non-linear dependence on Vm. The burst length of the inhibitory channel, produced by a single receptor activation, is longer in duration (at -80 mV) and exhibits greater voltage dependence than the burst length of the excitatory glutamate-activated channel in these fibres. It is estimated that a single quantum of GABA opens about 600-1000 post-synaptic chloride channels.

Some effects of procaine at the toad end-plate are not consistent with a simple channel-blocking model

Miniature end-plate currents (m.e.p.c.s) were recorded extracellularly from toad sartorius muscle fibres exposed to solutions containing procaine at pH 5.4, 7.4 and 9.9. The decay phase of m.e.p.c.s was analysed to determine whether the effects of procaine were consistent with a sequential channel-blocking model. Averaged m.e.p.c.s measured in the presence of procaine were biphasic, decaying as the sum of two exponential components. However, about 10-15% of m.e.p.c.s decayed as single exponentials and were not biphasic. At pH 9.9 the relative amplitudes of the fast and slow phases were generally consistent with the decay time constants, according to the predictions of the blocking model. Such a correlation was not found at pH 5.4 or 7.4. In addition, the rate of decay of m.e.p.c.s at pH 5.4 did not increase as predicted with procaine concentration. These results demonstrate that the sequential blocking model is unable to account for all of the effects of procaine on m.e.p.c. decay. In addition, the finding that some m.e.p.c.s are single exponentials, while most are biphasic, suggests a heterogeneity of receptor-channel complexes.

Analysis of glycine-activated inhibitory post-synaptic channels in brain-stem neurones of the lamprey

Voltage-clamp techniques were used to measure fluctuations in membrane current produced by the application of glycine to Müller cells in the brain stem of the lamprey. The power density spectrum of the glycine-induced current 'noise' was consistent with the hypothesis that glycine activated a single population of conductance channels with open times determined by first-order kinetics. In normal bathing solution the channel conductance was 73 +/- 12 pS (mean +/- S.D.) and the channel open time 34 +/- 6 msec at 5 degrees C. The reversal potential for the response was 66 +/- 5 mV. Neither channel conductance nor mean open time was voltage-dependent. Replacement of Cl- in the bathing solution by isethionate and sulphate reversibly abolished the response to glycine. Increasing intracellular Cl-, either by using Cl- -filled micropipettes or by raising extracellular K+, decreased channel conductance. This unexpected decrease was a direct effect of intracellular Cl- and was not related to coincident changes in reversal potential. Channel open time was unaffected by intracellular Cl- concentration. Reducing extracellular Cl- concentration from 126.5 to 31 mM reduced channel conductance at all levels of intracellular Cl- without affecting open time. Increasing the temperature of the preparation resulted in increase in channel conductance and a decrease in mean open time. Q10S for the effects were of the order of 1.3 and -2.3 respectively in the range 4-14 degrees C.