Calcium dependent gating of the L-glutamate activated, excitatory synaptic channel on crayfish muscle

A receptor for presynaptic glutamatergic autoinhibition is a glutamate transporter

Monoquantal excitatory postsynaptic currents were recorded by means of a perfused macropatch electrode from 9 to 15 micro m stretches of crayfish neuromuscular junctions. The excitatory transmitter l-glutamate superfused to a terminal inhibits quantal release by activating autoreceptors [Parnas et al. (1996) Eur. J. Neurosci., 8, 116-126]. Substances related to glutamate that do not activate glutamatergic postsynaptic channels, but are substrates of glutamate transporters, elicited analogous inhibitions, e.g. l- and d-aspartate and some other glutamate transport blockers. As expected, all transport blockers prolonged synaptic currents. Blockers that bind to the transporter receptors but are not transported did not inhibit release, but prevented inhibition by the transport substrates. It appears that autoinhibition is elicited by transport of glutamate or its analogues. Transport into cells is powered by symport of three Na+. To block the transport step electrochemically, extracellular Na+ concentration was lowered to one-quarter, but this surprisingly left the inhibition of release by glutamate unaffected, showing inhibition to be associated to a step between binding and transport. After binding a substrate, glutamate transporters open a parallel Cl- channel. Replacement of extracellular Cl- prevented Cl- current, and release inhibition by glutamate or aspartate was blocked. It is suggested that the flow of Cl- across the cell membrane, after binding a transport substrate, mediates autoinhibition. We measured a related reduction of presynaptic action potentials.

Calcium currents, transmitter release and facilitation of release at voltage-clamped crayfish nerve terminals

1. The presynaptic terminals at crayfish (Procambarus spp.) opener neuromuscular junctions were voltage clamped. Calcium currents were measured during (ICa) and following (tail ICa) presynaptic depolarizations; EPSPs or IPSPs were simultaneously recorded from the (postsynaptic) muscle fibre directly beneath the presynaptic impalement. 2. For short (< or = 6 ms) presynaptic depolarizations, most of the transmitter release occurred during the tail ICa. EPSP or IPSP amplitudes at the end of the 6 ms pulse (end EPSP or end IPSP) increased monotonically with the integral of the ICa ([symbol: see text]ICa). The suppression potential for transmitter release was near the apparent reversal potential for ICa. 3. When the end EPSP or end IPSP amplitude was plotted against the peak ICa elicited during a presynaptic pulse (peak ICa), large and small depolarizations which evoked the same peak ICa evoked different amounts of transmitter release. The differences in transmitter release were eliminated when end EPSP amplitude was plotted against [symbol: see text] ICa, suggesting that transmitter release during a depolarization depends only upon calcium current and not upon a subsequent voltage-dependent step. 4. The synaptic transfer function of various measurements of EPSP or IPSP amplitude vs. [symbol: see text]ICa evoked during a presynaptic depolarization was a power function having an exponent of about 3. Similar measurements of EPSP amplitude vs. [symbol: see text]tail ICa evoked following a presynaptic depolarization had an exponent of about 2. 5. Facilitation of an EPSP or IPSP was not due to increases in calcium current at the test depolarization. 6. When the conditioning depolarization was increased and the test depolarization remained constant, EPSP amplitude at the test depolarization and facilitation increased . When the conditioning depolarization remained constant and the test depolarization was increased, EPSP amplitude at the test depolarization increased, while facilitation decreased. 7. Our data suggested that transmitter release at crayfish neuromuscular junctions is a non-linear function of calcium influx, and that facilitated release utilizes intracellular calcium differently from non-facilitated release. These data contradict simple models of facilitation which combine the residual calcium hypothesis with the calcium co-operativity hypothesis of non-facilitated release.

Concanavalin A reduces habituation in the tectum of the frog

In the tectum of Rana pipiens, responses to repeated flashes of light to the ipsilateral eye display considerable habituation. We have employed the plant lectin concanavalin A (Con A), which can diminish desensitization of glutamate receptors in vitro, in order to examine whether desensitization of glutamate receptors contributes to this habituation. The ipsilateral eye's projection reaches each tectal lobe indirectly, being relayed from the opposite tectal lobe via the tecto-isthmo-tectal projection. One of the sites along this pathway at which habituation may take place is the retinotectal synapse, where glutamate is a putative transmitter. Pretreatment of one lobe of the tectum with Con A significantly diminished the habituation of responses recorded in the other tectal lobe to light offset, with less of an effect on responses to light onset. These data suggest that OFF habituation may reflect glutamate receptor desensitization at the retinotectal synapse. In contrast, recordings from retinotectal terminals indicate that a primary site of habituation of ON responses is within the retina.

Dual regulation of cation-selective channels by muscarinic and alpha 1-adrenergic receptors in the rabbit portal vein

1. The excitatory actions of phenylephrine (Phe) and acetylcholine (ACh) on the smooth muscle of rabbit portal vein were investigated and compared by using whole-cell and single channel configurations of the patch clamp technique, in combination with a modified concentration jump method. 2. At negative holding potentials with KCl (0.1 mM EGTA) electrodes, rapid applications of Phe (> 1 microM) and ACh (> 10 microM) both resulted in biphasic responses consisting of fast outward or inward currents and a long-lasting inward current with an increased noise level. 3. The slow inward current was still recorded when the cell was dialysed with caesium aspartate solution complemented with 10 mM EGTA, in order to eliminate contributions of calcium-dependent conductances (K+ and Cl- currents). Phe was more potent at activating the current than ACh. Ion replacement experiments revealed that both the Phe- and ACh-induced slow inward currents are cation-selective conductances (CS currents). 4. The I-V relationships of the Phe- or ACh-induced CS currents were similar both for the instantaneous peak and the steady state. The tail current analysis over a wide range of membrane potentials (-150 to +100 mV) showed that depolarizations to very positive potentials (> +50 mV) from near the resting membrane potential (-40 mV) can produce a several-fold increase in the steady-state activation of the CS currents, but the maximal activations were in most cases not observed even at +100 mV. 5. Externally applied Cd2+ produced a quick and reversible inhibition of both the Phe- and ACh-induced CS currents. This inhibition seemed almost voltage independent and the concentrations of half-inhibition were 100 and 129 microM for Phe and ACh, respectively. 6. Single channel activities were recorded in the presence of Phe or ACh using the outside-out membrane patches. The unitary conductances and reversal potentials of the Phe- and ACh-activated channels were 23 pS, +5.2 mV and 25 pS, +4.1 mV, respectively, and the open lifetimes evaluated for 50 microM Phe and 500 microM ACh were of similar order (the longer open times at -60 mV for Phe and ACh were 4.3 and 4.4 ms, respectively). In addition, the relative open probability (Po,rel) was no more than 0.2 in the voltage range of -100 to -30 mV for both 50 microM Phe and 500 microM ACh, suggesting low open probabilities of the Phe- and ACh-activated CS channels near the resting membrane potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic transmission at high pressure: Effects of [Ca2+]o

1. The effects of pressure on synaptic currents were examined in crayfish abdominal muscles. 2. Helium pressure (10.1 MPa) considerably decreased extracellularly-recorded excitatory junctional potentials associated with increased short-term facilitation. 3. These effects could be mimicked by a reduction of [Ca2+]o, and partially compensated by an increase in [Ca2+]o. 4. Pressure also reduced the amplitude of the extracellular nerve terminal potentials (ENTP) by up to 25%, and significantly increased synaptic delay in a [Ca2+]o-dependent manner. 5. The interaction between compression and various [Ca2+]o were analysed in terms of an existing model of transmitter release. The results were consistent with the hypothesis that high pressure decreases the maximal Ca2+ influx into nerve terminals. 6. The decreased ENTP and increased synaptic delay suggest that additional processes may be involved in pressure effects on synaptic transmission.

Effect of external Cd2+ and other divalent cations on carbachol-activated non-selective cation channels in guinea-pig ileum

1. Inhibitory actions of the external divalent cations, Cd2+, Ni2+, Mn2+ and Co2+ on carbachol (CCh)-induced inward current (Ins,ACh) were investigated in caesium aspartate-loaded single longitudinal smooth muscle cells of guinea-pig ileum, using a rapid solution-switching device, under voltage-clamped conditions. 2. Cd2+ (20-1000 microM), when added to the external solution, reduced Ins,ACh rapidly and reversibly. This effect occurred dose dependently, the relationship being adequately described by a Michaelis-Menten equation with a Hill coefficient (n) of 1.0 and a dissociation constant (Kd) of 98 microM. 3. The inhibitory action of Cd2+ was associated neither with agonist concentration (CCh) nor with changes in reversal potential, and was voltage independent. 4. The appearance and removal of the effect of Cd2+ were both rapid (a few hundred milliseconds), in sharp contrast with a relatively slow time course of atropine or CCh action (of the order of seconds). 5. Other divalent cations (Ni2+, Mn2+, Co2+, Mg2+) applied externally also suppressed Ins,ACh, but less potently. The sequence of apparent potency was Cd2+ (98 microM) greater than or equal to Ni2+ (131 microM) much greater than Co2+ (700 microM) greater than or equal to Mn2+ (1000 microM) much greater than Mg2+ (approximately 10 mM). 6. External Ca2+ increased Ins,ACh dose dependently and antagonized the inhibitory effect of Cd2+. However, this effect may not be a simple competition with Cd2+. 7. These results show that external divalent cations strongly modulate Ins,ACh channels, possibly through direct interaction with the channel protein.

Evoked phasic release in frog nerve terminals obtained after block of Ca2+ entry by Cd2+

Cutaneous pectoris muscles of frogs were isolated, mounted in a chamber and superfused with Ringer's solution. With a macro-patch-clamp electrode placed on a section of a motor nerve terminal, quantal synaptic currents were elicited by depolarizing pulses and recorded. The electrode tip and the section of the terminal recorded from were perfused rapidly by Ringer's solution alone or containing 20-500 microM Cd2+ to block Ca2+ inflow. Separate superfusion of the muscle and the rest of the terminal with normal or elevated Ca2+ Ringer's solution provided a sufficiently high resting Ca2+ concentration in the terminal even when Ca2+ was blocked by Cd2+. The depolarization level of maximal Ca2+ inflow into the terminal was found by measuring maximal test pulse facilitation, Fc. In control solution as well as in the case of Cd2+ block, the rate of phasic release after depolarizing pulses rose further when depolarization was increased past the level of Fc, and reached a saturation level which was maintained at estimated depolarizations up to +200 mV. Block of Ca2+ inflow by Cd2+ decreased release substantially, but did not suppress it. The depression of release was greater in the range of large Ca2+ inflow (around Fc) than for very large depolarizations. The time course of phasic release was unaltered by blockage of Ca2+ inflow. It is concluded that Ca2+ inflow contributes to the promotion of evoked release only in the depolarization range in which Ca2+ inward current is large.(ABSTRACT TRUNCATED AT 250 WORDS)

Concanavalin A: a tool to investigate neuronal plasticity

Neuronal plasticity is the ability of neurons to alter their cellular properties in response to changes in their environment. These changes are typically triggered by the binding of specific ligands, such as neurotransmitters, growth factors or other neuromodulators, to receptors on the neuronal membrane surface. Since the extracellular domains of many of these receptors are glycosylated, they can also be bound by lectins--proteins with high affinity binding sites for polysaccharides. Different lectins have different affinities for various sugar residues. This feature has made lectins useful in the investigation of the regional localization and relative mobility of different classes of glycosylated membrane receptors, and in the subsequent purification of the receptors. This article reviews some of the different kinds of neuronal plasticity produced by the plant lectin concanavalin A (Con A), such as enhancement of neurite outgrowth, modulation of neurotransmitter responses, and alteration in the specificity and strength of synaptic connections.

Interaction of Ca-channel blockers and high pressure at the crustacean neuromuscular junction

Exposure to high pressure causes a significant depression of synaptic transmission. We examined the effects of various Ca-channel blockers and their interaction with high pressure on excitatory neuromuscular junction currents (EJCs) of lobster abdominal muscles. Reduced [Ca2+]o to half of normal concentration or exposure to 40-60 microM CdCl2, 10-20 microM NiCl2 and 1 microM omega-conotoxin decreased EJCs by 50%. Nifedipine, Nitrendipine and Bay K-8644 were ineffective. Either Ca-blockers or reduced [Ca2+]o, enhanced EJC suppression exerted by high pressure. The data suggest that high pressure primarily affects Ca2+ inflow at the presynaptic terminals through N-type voltage-gated Ca-channel.

Effect of membrane potential on acetylcholine-induced inward current in guinea-pig ileum

1. The whole-cell patch clamp technique with caesium aspartate internal solution was used with single isolated cells from the longitudinal muscle layer of guinea-pig ileum, to investigate the voltage-dependent gating of ACh-induced inward current. 2. In voltage clamp experiments, at holding potentials ranging from -80 to -30 mV, ACh (300 microM) produced a slow sustained inward current in physiological salt bath solution (PSS). The measurements of the reversal potentials on substituting Na+ by other monovalent and divalent cations showed that this current is through non-selective cation channels (Ins, ACh). 3. During hyperpolarizations, Ins, ACh instantaneously increased in amplitude and then relaxed to a new steady-state level. The I-V relationship of the instantaneous peak was linear with a reversal potential of 0 mV, while that of the steady state was bell-shaped. The time course of relaxation appeared to be monoexponential and its time constants were reduced by stronger hyperpolarizations. 4. These results were not affected by the organic Ca2+ antagonists D600 or nitrendipine (10 microM). Under this condition, maximal chord conductance of Ins, Ach which was observed at 0 mV was about 1.5 nS. The steady-state activation relationship was well fitted by Boltzmann's equation with a half-maximal activation (Vh) of -50 mV and a slope factor (k) of -15 mV at membrane potentials negative to 0 mV, but over 0 mV the degree of activation was again decreased. The time constants for relaxation also appeared to follow a sigmoid curve. 5. In current clamp experiments, superfusion of ACh (300 microM) depolarized the membrane up to -10 to 0 mV. Inward current injection resulting in the moderate hyperpolarization of the membrane (-70 to -80 mV) attenuated ACh-induced depolarization and stronger hyperpolarization (less than -80 mV) abolished it. 6. These results show that ACh-induced depolarization is controlled by the membrane potential, which is explained by the voltage-dependent gating of Ins, Ach.

Rapid activation, desensitization, and resensitization of synaptic channels of crayfish muscle after glutamate pulses

Completely desensitizing excitatory channels were activated in outside-out patches of crayfish muscle membrane by applying glutamate pulses with switching times of approximately 0.2 ms for concentration changes. Channels were almost completely activated with 10 mM glutamate. Maximum activation was reached within 0.4 ms with greater than or equal to 1 mM glutamate. Channel open probability decayed with a time constant of desensitization of 2 ms with 10 mM glutamate and more rapidly at lower glutamate concentrations. The rate of beginnings of bursts (average number of beginnings of bursts per time bin) decayed even faster but approximately in proportion to the glutamate concentration. The dose-response curve for the channel open probability and for the rate of bursts had a maximum double-logarithmic slope of 5.1 and 4.2, respectively. Channels desensitized completely without opening at very low or slowly rising glutamate concentrations. Desensitization thus originates from a closed channel state. Resensitization was tested by pairs of completely desensitizing glutamate pulses. Sensitivity to the second pulse returned rapidly at pulse intervals between 1 and 2 ms and was almost complete with an interval of 3 ms. Schemes of channel activation by up to five glutamate binding steps, with desensitization by glutamate binding from closed states, are discussed. At high agonist concentrations bursts are predominantly terminated by desensitization. Quantal currents are generated by pulses of greater than 1 mM glutamate, and their decay is determined by the duration of presence of glutamate and possibly by desensitization.

Inhibition of Ca2+ inflow at nerve terminals of frog muscle blocks facilitation while phasic transmitter release is still considerable

Action potentials were triggered in the motor nerve by a suction electrode and calcium currents (iCa) in the nerve terminals were measured by means of a perfused macro-patch-clamp electrode on the distal portion of the end-plates. Postsynaptic currents were blocked by adding d-tubocurarine, whereas presynaptic Na+ (iNa) and K+ (iK) currents were blocked by adding tetrodotoxin (TTX), tetraethylammonium and 3,4-diaminopyridine, respectively, to the perfusate of the electrode. The current components which could be suppressed by addition of Cd2+ to the perfusate were taken as presynaptic iCa. The observed effects on the presynaptic current components were very similar to those reported previously. If the electrode was perfused with Ringer's solution containing the blockers for iNa and iK, the same, obviously complete block of iCa was obtained by 50 and 100 microM Cd2+, an average of 96% block by 20 microM Cd2+ and 50% block by about 5 microM Cd2+. Using the same type of electrode and similar locations on motor nerve terminals, postsynaptic quantal currents and twin-pulse facilitation (Fd) were elicited by variable-duration (0.5-3 ms) depolarizing pulses. When the electrode was perfused with Ringer's solution containing TTX, 20 microM Cd2+ added to the perfusate reduced the rate of phasic release of quanta insignificantly for short depolarizing pulses and by a factor of about 10 for longer pulses. Fd was blocked almost completely. Addition of 50 microM Cd2+ to the perfusate had a greater depressive effect on release after short depolarizing pulses and reduced release after longer pulses by a factor of about 100.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium dependence of quantal release triggered by graded depolarization pulses to nerve terminals on crayfish and frog muscle

Quantal transmitter release was measured in small portions of neuromuscular junctions by means of a perfused macro-patch-clamp electrode. Release was elicited by graded current pulses through the recording electrode (excitation blocked by TTX). On increasing the stimulation current from a threshold amplitude, release rose steeply for several orders of magnitude and finally approached a saturation level of about 10 quanta/pulse. Reduction of the Ca concentration in the perfusate of the electrode, Cae, depressed the saturation level of release relatively little and had practically no effect on the threshold current amplitude, as long as the Ca concentration in the superfusion of the bath, Cab, remained high. When Cab was reduced too, the depression of release was more severe. The dependence of release on Cae was determined for a large range of Cae for saturating depolarization pulses. In crayfish, at 0 Cab, in double-logarithmic release-Cae plots the maximum slope was on average 3.9, and this slope dropped to on average 2.1 in 13.5 mM Cab. In frog, at 0 Cab, the respective double-logarithmic slope was 3.5, while in 1.8 mM Cab this slope declined dramatically, the rate of release decreasing on average only by a factor of 3.8 from 10 mM to 0.02 mM Cae. These results are interpreted by the assumption that the resting Ca concentration in the terminal, Cair, has strong influence on the rate of release due to depolarization pulses in low Cae, and that Cab has control on Cair in the terminal.

Twin pulse facilitation in dependence on pulse duration and calcium concentration at motor nerve terminals of crayfish and frogs

Phasic release from motor-nerve terminals of crayfish and frogs was elicited and recorded by means of a macro-patch-clamp electrode through which the terminal was depolarized in graded pulses. The tip of the electrode was perfused and the Ca concentration around the terminal, Cae, was controlled independent from that in the superfusion of the muscle, Cab. Release increased with pulse duration with a double-logarithmic slope of 5 to 9 in crayfish and frogs, which represents a form of "early facilitation" (Katz and Miledi 1968). In crayfish, this relation was shifted to longer pulse durations on lowering Cae, while in frogs, in addition, the saturation level of release was suppressed at low Cae. Responses to twin pulses with intervals of 7-10 ms showed facilitation, Fd. When pulse duration of the twin pulses was increased, starting from about 0.5 ms, Fd increased to a maximum, but declined for longer pulses which elicited release approaching the saturation range. On lowering Cae, the maximum of Fd, Fd, increased in amplitude and was shifted to larger pulse durations. Also reduction of Cab increased Fd. The effects of pulse duration and of Cae and Cab on Fd are predicted by the residual Ca theory of facilitation, if it is assumed that changes of Cae produce corresponding changes in Ca inflow during depolarization, and if the resting intracellular Ca concentration is influenced by the extracellular Ca concentration. The large values of early facilitation can not be explained by the residual Ca theory of facilitation and may indicate the action of another depolarization dependent factor which joins in the control of release.

Calcium and depolarization dependence of twin-pulse facilitation of synaptic release at nerve terminals of crayfish and frog muscle

Transmitter quanta were elicited from nerve terminals of crayfish and frog muscle by depolarization pulses through a macro-patch-clamp electrode. The rates of quantal release for twin pulses and their ratio, twin pulse facilitation, Fd, were determined. When the electrode was perfused with normal Cae (13.5 mM for crayfish, 1.8 mM for frog), Fd was low for threshold depolarizations, increased to a maximum at medium depolarizations, decreased when the rate of release due to the first pulse approached saturation, and increased again for larger depolarizations. If under these conditions the superfusion of the muscle outside the electrode was changed from normal to 0 Cab and high Mgb solution, Fd increased. When the Ca concentration around the terminal, Cae, was reduced to levels at which release did not reach the saturation level for large depolarizations, Fd in dependence on depolarization did not show the minimum at higher depolarizations. The amplitude of Fd measured for large, constant depolarization pulses showed a maximum at a Cae below that of the normal solution. The maximum of Fd was much higher if the superfusion of the bath contained 0 Cab and high Mgb than when normal bathing solution was superfused. The maxima of Fd at a low value of Ca inflow are predicted by the "residual Ca" theory of facilitation, if release is influenced by a resting low internal Ca concentration, Cair, and reaches a saturation level for large Ca-inflow. It is also predicted that decreasing Cair (as in low Cab) will increase Fd.

Membrane depolarization evokes neurotransmitter release in the absence of calcium entry

The discovery that Ca2+ is necessary for the release of neurotransmitter, the primary means by which nerve cells communicate, led to the calcium hypothesis of neutransmitter release, in which release is initiated after an action potential only by an increase in intracellular Ca2+ concentration near the release sites and is terminated (1-2 ms) by the rapid removal of Ca2+. Since then, the calcium-voltage hypothesis has been proposed, in which the depolarization of the presynaptic terminals has two functions. First, in common with the calcium hypothesis, the Ca2+ conductance is increased, thereby permitting Ca2+ entry. Second, a conformational change is induced in a membrane molecule that renders it sensitive to Ca2+, and then binding of Ca2+ to this active form triggers release of neurotransmitter. When the membrane is repolarized, the molecule is inactivated and release is terminated, regardless of the local Ca2+ concentration at that moment. This hypothesis, in contrast to the calcium hypothesis, accounts for the insensitivity of the time course of release to experimental manipulations of intracellular Ca2+ concentration. Furthermore, it explains rapid termination of release after depolarization, even though Ca2+ concentration may still be high. Here we describe experiments that distinguish between these two hypotheses and find that our results support the calcium voltage hypothesis.

Spontaneous and evoked quantal neurotransmitter release at the neuromuscular junction of the larval housefly, Musca domestica

The release of neurotransmitter was monitored at the neuromuscular junctions of larval housefly ventrolateral muscles 6a and 7a, using intracellular recording, and a loose patch clamp technique to isolate discrete release sites. Transmitter release occurred spontaneously and could also be evoked by neural stimuli. Spontaneous discharges consisted of events which were randomly distributed in time and of bursts of temporally ordered events. Evoked and spontaneous release occurred in a quantal manner. The quantal content of evoked excitatory postsynaptic currents (EPSCs) was dependent upon the extracellular calcium concentration, increasing with a 3.8 power dependency. The relationship between the quantal content of a response and extracellular calcium concentration was offset by the presence of magnesium in the bathing saline. The rates of decay of miniature EPSCs (mEPSCs) and EPSCs were also found to increase with extracellular calcium concentration, consistent with a non-diffusion limited block of the glutamate receptor-channel complex by calcium ions (KB 2.5 x 10(4) s-1 M-1, P less than 0.01). The frequency of random mEPSCs (0.26 +/- 0.32 Hz, n = 24 cells) was independent of the extracellular calcium concentration. Random mEPSCs were not inhibited by 1 microM tetrodotoxin which blocked mEPSC bursts and neurally evoked responses. EPSCs evoked during mEPSC bursts had a significantly lower quantal content than those EPSCs recorded from the same nerve terminal between bursting, indicating that both of these forms of release recruited quanta from a common pool of transmitter. Following a neurally evoked EPSC the mEPSC frequency was potentiated severalfold, this delayed release was influenced by EPSCs with large quantal contents evoked in saline containing elevated calcium concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid activation and desensitization by glutamate of excitatory, cation-selective channels in locust muscle

Outside-out patches of membrane were excised from extensor tibiae muscles of locusts. L-Glutamate or its agonists were applied to such patches in short pulses by means of a lipid filament switch. Cationselective, excitatory channels were activated by quisqualate, L-glutamate and aspartate (in decreasing order of effectivity), but not by ibotenate, kainate, N-methyl-D-aspartate and glycine. At high agonist concentrations, channel activation reached a peak within 1 ms. Two kinetic types of channels have been identified: L-channels with on average relatively long and S-channels with short openings. Both types of channel openings showed surprisingly high rates of desensitization, channel activity declining after the initial surge to zero with time constants of about 25 and 3 ms, respectively. The L-channels exhibit open times close to those of channels recorded in M omega-seal studies. The S-channel has not been reported previously.

Ionic permeabilities of L-glutamate activated, excitatory synaptic channel in crayfish muscle

Excitatory single channel currents triggered by L-glutamate were measured in outside-out excised patches of crayfish muscle membrane. If an 'intracellular' solution was present in the pipette and normal extracellular solution with added glutamate (10(-3) M) passed the outside of the patch, the single channel currents, i1, had amplitudes of -8 pA at a patch potential of -70 mV. If in the extracellular solution Na+ was replaced by Li+ or Ca2+, the amplitudes of single channel currents were reduced by about 30%. Only about 20% of the channel current amplitude remained on replacement of Na+ by choline. Replacement of Na+ reduced the variance of channel amplitude distributions to the level of the baseline. Presence of Na+ thus induces an additional variance of open channel current. When the proportions of Na+/choline were varied, the resulting channel currents could be separated in Na+, Ca2+ and choline components. The amplitude of the Na+ component, i1,Na, could be described by a constant channel permeability pi Na = 110 10(-15) cm3 s-1 according to the constant field equation. Ba2+ could replace Ca2+ without change in single channel current, while replacement of Ca2+ by Mg2+ reduced the channel currents by 20%. The following permeabilities of the single channel were estimated (in 10(-15) cm3 s-1): pi Na = 110, pi K = 86, pi Ca = 30, pi Mg = 24, pi Ba = 30, pi Li = 84 and pi choline = 11. These permeabilities were obtained inserting ionic concentrations. The respective permeabilities are listed also as calculated on the basis of ionic activities.(ABSTRACT TRUNCATED AT 250 WORDS)