Permeability changes induced by L-glutamate at the crayfish neuromuscular junction

Characterization of postsynaptic Ca2+ signals at the Drosophila larval NMJ

Postsynaptic intracellular Ca(2+) concentration ([Ca(2+)](i)) has been proposed to play an important role in both synaptic plasticity and synaptic homeostasis. In particular, postsynaptic Ca(2+) signals can alter synaptic efficacy by influencing transmitter release, receptor sensitivity, and protein synthesis. We examined the postsynaptic Ca(2+) transients at the Drosophila larval neuromuscular junction (NMJ) by injecting the muscle fibers with Ca(2+) indicators rhod-2 and Oregon Green BAPTA-1 (OGB-1) and then monitoring their increased fluorescence during synaptic activity. We observed discrete postsynaptic Ca(2+) transients along the NMJ during single action potentials (APs) and quantal Ca(2+) transients produced by spontaneous transmitter release. Most of the evoked Ca(2+) transients resulted from the release of one or two quanta of transmitter and occurred largely at synaptic boutons. The magnitude of the Ca(2+) signals was correlated with synaptic efficacy; the Is terminals, which produce larger excitatory postsynaptic potentials (EPSPs) and have a greater quantal size than Ib terminals, produced a larger Ca(2+) signal per terminal length and larger quantal Ca(2+) signals than the Ib terminals. During a train of APs, the postsynaptic Ca(2+) signal increased but remained localized to the postsynaptic membrane. In addition, we showed that the plasma membrane Ca(2+)-ATPase (PMCA) played a role in extruding Ca(2+) from the postsynaptic region of the muscle. Drosophila melanogaster has a single PMCA gene, predicted to give rise to various isoforms by alternative splicing. Using RT-PCR, we detected the expression of multiple transcripts in muscle and nervous tissues; the physiological significance of the same is yet to be determined.

Ca2+-independent, but voltage- and activity-dependent regulation of the NMDA receptor outward K+ current in mouse cortical neurons

To test the novel hypothesis that the K+ efflux mediated by NMDA receptors might be regulated differently than the influx of Ca2+ and Na+ through the same receptor channels, NMDA receptor whole-cell currents carried concurrently or individually by Ca2+, Na+ and K+ were analysed in cultured mouse cortical neurons. In contrast to the NMDA inward current carried by Ca2+ and Na+, the NMDA receptor outward K+ current or NMDA-K current, recorded either in the presence or absence of extracellular Ca2+ and Na+, and at different or the same membrane potentials, showed much less sensitivity to alterations in intracellular Ca2+ concentration and underwent little rundown. In line with a selective regulation of the NMDA receptor K+ permeability, the ratio of the NMDA inward/outward currents decreased, and the reversal potential of composite NMDA currents recorded in physiological solutions shifted by -8.5 mV after repeated activation of NMDA receptors. Moreover, a depolarizing pre-pulse of a few seconds or a burst of brief depolarizing pulses selectively augmented the subsequent NMDA-K current, but not the NMDA inward current. On the other hand, a hyperpolarizing pre-pulse showed the opposite effect of reducing the NMDA-K current. The voltage- and activity-dependent regulation of the NMDA-K current did not require the existence of extracellular Ca2+ or Ca2+ influx; it was, however, affected by the duration of the pre-pulse and was subject to a time-dependent decay. The burst of excitatory activity revealed a lasting upregulation of the NMDA-K current even 5 s after termination of the pre-pulses. Our data reveal a selective regulation of the NMDA receptor K+ permeability and represent a novel model of voltage- and excitatory activity-dependent plasticity at the receptor level.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Neuromuscular glutamatergic and GABAergic channels

Our laboratory has worked extensively on glutamatergic and GABA-ergic channels, predominantly in crayfish, but also in locust, Drosophila and recently Ascaris. Channel currents were recorded in the different modes of the patch-clamp technique (Hamill et al., 1981). The opening kinetics of the channels were derived from open and closed time histograms obtained from single channel recordings. From these, channel conductances could also be evaluated. The most relevant data were obtained by very rapidly rising and falling pulses (time of change about 0.1 ms) of agonists applied to outside-out patches containing the respective channels (Franke et al., 1987). From such recordings we constructed dose-response curves for peak and steady-state currents, for the rise times of the currents and for the time constants of desensitization. In double-pulse experiments we measured recovery from desensitization and predesensitization due to low agonist concentrations. For most of the channel types, we succeeded in constructing a reaction scheme which in computer simulations mimicked channel behaviour to a good approximation.

Inhibitory effects of L-glutamate on central processes of crustacean leg motoneurons

In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by beta-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections--a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.

Ion permeation properties of the glutamate receptor channel in cultured embryonic Drosophila myotubes

Ion permeation properties of the glutamate receptor channel in cultured myotubes of Drosophila embryos were studied using the inside-out configuration of the patch-clamp technique. Lowering the NaCl concentration in the bath (intracellular solution), while maintaining that of the external solution constant, caused a shift of the reversal potential in the positive direction, thus indicating a higher permeability of the channel to Na+ than to Cl- (PCl/PNa < 0.04), and suggesting that the channel is cation selective. With 145 mM Na+ on both sides of the membrane, the single-channel current-voltage relation was almost linear in the voltage range between -80 and +80 mV, the conductance showing some variability in the range between 140 and 170 pS. All monovalent alkali cations tested, as well as NH4+, permeated the channel effectively. Using the Goldman-Hodgkin-Katz equation for the reversal potential, the permeability ratios with respect to Na+ were estimated to be: 1.32 for K+, 1.18 for NH4+, 1.15 for Rb+, 1.09 for Cs+, and 0.57 for Li+. Divalent cations, i.e. Mg2+ and Ca2+, in the external solution depressed not only the inward but also the outward Na+ currents, although reversal potential measurements indicated that both ions have considerably higher permeabilities than Na+ (PMg/PNa = 2.31; PCa/PNa = 9.55). The conductance-activity relation for Na+ was described by a hyperbolic curve. The maximal conductance was about 195 pS and the half-saturating activity 45 mM. This result suggests that Na+ ions bind to sites in the channel. All data were fitted by a model based on the Eyring's reaction rate theory, in which the receptor channel is a one-ion pore with three energy barriers and two internal sites.

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.

Permeation of calcium through excitatory amino acid receptor channels in cultured rat hippocampal neurones

1. N-methyl-D-aspartate (NMDA)-, quisqualate- and kainate-induced currents were recorded in cultured rat hippocampal neurones using the whole-cell voltage-clamp technique. To isolate the inward currents carried by Ca2+ and other divalent cations (Sr2+, Ba2+, Mn2+ and Mg2+), both Na+ and K+ in the control external solution were replaced with the impermeant cation N-methylglucamine (NMG). 2. Replacement of Na+, K+ and Ca2+ with NMG abolished NMDA-, quisqualate- and kinate-induced inward currents. In Na(+)-, K(+)-free (abbreviated simply as Na(+)-free) solution containing 10 mM-Ca2+ NMDA caused prominent inward currents at -60 mV. In this solution with the internal solution containing 165 mM-Cs+, the reversal potential of the NMDA-induced current was -5.0 +/- 0.7 mV (n = 36), indicating a value of PCa/PCs = 6.2 for the ratio of the permeability coefficients of Ca2+ and Cs+ according to the constant-field equation. 3. NMDA elicited inward current responses at -60 mV in Na(+)-, Ca2(+)-free solution containing 10 mM-Sr2+, Ba2+, or Mn2+, but not in Na(+)-free, 10 mM-Mg2+ solution. On the basis of reversal potential measurements, the permeability sequence of NMDA receptor channels among the divalent cations was determined to be Ba2+ (1.2) greater than Ca2+ (1.0) greater than Sr2+ (0.8) greater than Mn2+ (0.3) much greater than Mg2+ (less than 0.02). 4. The reversal potential of the quisqualate-induced current was more negative than -80 mV in Na(+)-free, 10 mM-Ca2+ solution, indicating a value of PCa/PCs less than 0.18. 5. Kainate-induced current responses were classified into two types. In the type I response the reversal potential of the kainate-induced current was more negative than -80 mV in Na(+)-free, 10 mM-Ca2+ solution, indicating that the Ca2+ permeability of this type of kainate channel is as low as that of the quisqualate channel. In the neurones which showed a type I response, there was a tendency of outward rectification in the current-voltage plots of the kainate response in control solution. 6. In the type II response kainate caused prominent inward currents at -60 mV in Na(+)-free, 10 mM-Ca2+ solution. The reversal potential was -23.3 +/- 5.6 mV (n = 17), indicating a permeability ratio PCa/PCs = 2.3. In the neurones which showed a type II response, a remarkable inward rectification was observed in the current-voltage plots of the kainate response in control solution. 7. Type II kainate channels showed relatively poor selectivity among divalent cations.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Anomalous voltage dependence of channel blockade at a crustacean glutamate-mediated synapse

1. The voltage dependence and concentration dependence of blockade of glutamate-activated currents by the diquaternary amine, chlorisondamine, were examined in a marine crustacean muscle. 2. Chlorisondamine results in the splitting of focally recorded synaptic current decays into two exponential components. The fast component becomes faster with increases in drug concentration and with hyperpolarization. The slow decay rate is unchanged or faster with hyperpolarization and the relative amplitude of the slow component is increased with hyperpolarization. 3. The alteration of synaptic current decay rates by chlorisondamine over the range of 5 to 100 microM and -80 to -140 mV is quantitatively consistent with a simple channel blockade model with a zero-voltage blocking rate of 6 x 10(5) M-1 s-1 at 12 degrees C with a voltage dependence of about 40 mV per e-fold change. The unblocking rate is about 5 s-1 at 0 mV and increases with hyperpolarization with a voltage dependence of about 30 mV per e-fold change. 4. The dose dependence and voltage dependence of blockade of ionophoretically activated glutamate currents by chlorisondamine are qualitatively consistent with the kinetic estimates. 5. The anomalous voltage dependence of the unblocking process is considered in terms of the possibility that the relief from blockade by chlorisondamine occurs by transit of chlorisondamine through the ion channel opened by glutamate.

Changes in binomial parameters of quantal release at crustacean motor axon terminals during presynaptic inhibition

1. The effects of presynaptic inhibition on quantal release of transmitter were investigated at neuromuscular junctions of the motor axon supplying one of the limb muscles of a crab (Pachygrapsus crassipes). 2. Binomial analysis of transmitter release recorded at selected neuromuscular junctions with an extracellular 'macro-patch' electrode indicated high probability of release (p) from a limited number of available sites (n). During presynaptic inhibition, both n and p were reduced. 3. The binomial model provided a good description of results from non-inhibited junctions. During presynaptic inhibition, results from some junctions could be described by the binomial model, while those from other junctions could not. An interpretation of this finding is that presynaptic inhibition differentially affects the probability of release at various release sites of the neuromuscular junctional complex. 4. A morphological study of the region of transmitter release under the macropatch electrode was made. Release-dependent uptake of horseradish peroxidase (HRP) into presynaptic terminals was restricted to the region under the recording electrode, by perfusing the preparation with calcium-free solution containing HRP. Transmitter release, and HRP uptake, occurred only at the site of the electrode, which was filled with a calcium-containing solution. Subsequently, serial sections were prepared for electron microscopy and the region of transmitter release was reconstructed. 5. Numerous axo-axonal synapses were found in the HRP-labelled region. Thus, the morphological prerequisite for presynaptic inhibition exists at the site of transmitter release, and not exclusively at a more remote region. 6. The number of morphologically identified excitatory neuromuscular synapses exceeded the 'release sites' estimated from the binomial model (n) by a wide margin. Morphological differences among synapses were observed. It is proposed that not all morphologically identified synapses participated in transmitter release under the experimental conditions employed. Thus, morphologically defined synapses are likely to be non-uniform in their response properties, including probability of transmitter release (p).

Cation permeability change caused by L-glutamate in cultured rat hippocampal neurons

The ionic mechanism of the membrane permeability changes caused by L-glutamate in hippocampal neurons prepared from 17- to 19-day-old fetal rat in dispersed cell cultures was studied with the whole-cell variation of the patch electrode voltage-clamp technique. The cultured hippocampal neurons became sensitive to glutamate 7 days after plating, and thereafter the sensitivity gradually increased. The conductance increase caused by glutamate was voltage-sensitive, decreasing with membrane hyperpolarization at potentials more negative than -40 mV. The relative permeability of glutamate-activated channels to alkali metal and alkaline earth cations was estimated by reversal potential measurements. The alkali metal cations, Li+, Na+, K+, Rb+ and Cs+ were permeant to the glutamate channels, and the selectivity among them was weak. The alkaline earth cations, Ca2+, Sr2+ and Ba2+ were more permeant than the alkali metals. The permeability ratios of these divalent cations relative to Na+ were 2.4 (Ca2+), 2.4 (Sr2+) and 2.8 (Ba2+), respectively. Mg2+ was much less permeant and the permeability ratio (PMg/PNa) was only 0.1. Anion conductance made no contribution to the glutamate-induced current. Functional implications of the glutamate-induced increased in Ca2+-influx were discussed.

Properties of the kainate channel in rat brain mRNA injected Xenopus oocytes: ionic selectivity and blockage

The properties of kainate receptor/channels were studied in Xenopus oocytes injected with mRNA that was isolated from adult rat striatum and cerebellum and partially purified by sucrose gradient fractionation. Kainate (3-1000 microM) induced a smooth inward current that was competitively inhibited by gamma-D-glutamyl-aminomethanesulfonate (GAMS, 300 microM). In striatal mRNA-injected oocytes, the kainate current displayed nearly linear voltage-dependence and mean reversal potential (Er) of -6.1 +/- 0.5 mV. In cerebellar mRNA-injected oocytes; Er was nearly identical (-5.1 +/- 1.2 mV) but there was marked inward rectification of the kainate current. Ion replacement studies reveal that the kainate channel is selective for cations over anions, but relatively non-selective among small monovalent cations. Large monovalent cations such as tetrabutylammonium are impermeant and induce a non-competitive block of kainate current that is strongly voltage-dependent. Divalent cations are relatively impermeant in the kainate channel and Cd++ and other polyvalent metals were shown to block kainate current by a mechanism that is only weakly voltage-dependent. A model of the kainate channel is proposed based upon these observations.

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)

Permeation and block of N-methyl-D-aspartic acid receptor channels by divalent cations in mouse cultured central neurones

1. Spinal cord and hippocampal neurones in cell culture were voltage clamped using the tight-seal, whole-cell recording technique. The concentration of sodium and a series of divalent cations in the extracellular media was varied to study permeation through excitatory amino acid receptor channels activated by the selective agonists N-methyl-D-aspartic acid (NMDA), kainic acid and quisqualic acid. 2. On raising the extracellular calcium concentration, with [Na+]o held constant at 105 mM, the reversal potential of responses to NMDA shifted in the depolarizing direction. This shift was adequately described by the extended constant-field equation over the range 0.3-50 mM-calcium. Using ionic activity coefficients we calculate a value of PCa/PNa = 10.6. Under the same experimental conditions the reversal potential of responses to kainic and quisqualic acids was much less affected by raising the calcium concentration, such that PCa/PNa = 0.15. A depolarizing shift of the NMDA reversal potential was also recorded during application of 20 mM-barium, strontium or manganese, suggesting permeation of these ions. The permeability sequence was Ca2+ greater than Ba2+ greater than Sr2+ much greater than Mn2+. No depolarizing shift of the NMDA reversal potential occurred during application of 20 mM-cobalt, magnesium or nickel. 3. In experiments in which the extracellular Na+ concentration was varied the extended constant-field equation was adequate in predicting shifts of the NMDA reversal potential recorded on varying [Na+]o over the range 50-150 mM, but failed to accurately predict the reversal potential of responses to NMDA with 10 mM-[Ca2+]o and only 10 or 20 mM-[Na+]o. These results imply an apparent increase in PCa/PNa on lowering [Na+]o and may result from interaction of permeant ions within the channel. 4. Barium and to a lesser extent calcium, but not strontium (all 20 mM), reduced the slope conductance of responses to NMDA recorded within +/- 15 mV of the reversal potential; over this limited range of membrane potential the current-voltage relationship remained linear in the presence of each of these ions. In contrast manganese produced a strong, voltage-dependent block of responses to NMDA, similar to that produced by magnesium, such that even close to the reversal potential the NMDA current-voltage relationship was highly non-linear. Thus manganese both permeates and blocks the NMDA receptor channel. 5. Raising the extracellular calcium concentration, from 0.1 to 5 mM, had two effects on the conductance mechanism activated by NMDA.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of CDP-choline on phospholipase A2 and cholinephosphotransferase activities following a cryogenic brain injury in the rabbit

Within the tissue surrounding the necrotic lesion, following a cryogenic injury of the brain, there is a definite activation of phospholipase A2 (at 2 and 4 hr post lesion) that accounts, at least in part, for the phospholipid breakdown. There is also an activation of cholinephosphotransferase (at 2 hr post lesion) that may correspond to an early process of phospholipid resynthesis. Oral CDP-choline in this model is able to completely inhibit the activation of phospholipase A2, but has no detectable effect on cholinephosphotransferase activity. The beneficial effect of CDP-choline might be explained by a prevention of destruction rather than by an enhancement of reconstruction of phospholipids.

Delayed neurotoxicity of excitatory amino acids in vitro

The acute neurotoxicity produced by glutamate and related excitatory amino acids is probably caused by depolarization leading to excessive anionic and cationic fluxes and osmotic lysis. Recently, a more delayed form of glutamate neurotoxicity, which is critically dependent upon calcium influx, has been described in cultured neocortex. We investigated this phenomenon in cultures of dispersed rat hippocampal neurons. When these cultures were briefly incubated with various excitatory amino acids in low extracellular chloride, there was no acute toxicity, but a gradual drop-out of neurons occurred over the next day. When calcium was removed from the extracellular medium during amino acid incubation, this late neuronal loss was not seen. Interestingly, blocking excitatory amino acid receptors in cultures after the amino acid exposure also prevented this delayed neuronal death. In addition, these treated cultures contained neurons with normal physiological properties, and had concentrations of adenosine triphosphate that were close to control values. The findings suggest an amino acid-induced calcium influx may elevate the release of endogenous excitatory transmitter, likely glutamate, and/or increase the sensitivity of these neurons to glutamate. These in vitro observations may partially explain the delayed neuronal loss seen in some pathological conditions affecting man.

Properties of ion channels closed by light and opened by guanosine 3',5'-cyclic monophosphate in toad retinal rods

1. In patch-clamp recordings from outer segments of dark-adapted rod photoreceptors, single-channel recordings were obtained from the light-sensitive conductance when divalent cations were omitted from the pipette solution bathing the extracellular face of the recorded patch of membrane. 2. Activity of the light-sensitive channel was suppressed by light within the normal response range of the dark-adapted rod. During dim, steady illumination, the rate of opening of the channel fluctuated dramatically, as expected qualitatively from statistical fluctuations in the number of photoisomerizations occurring within the effective collecting area of the recorded patch. 3. The light-sensitive channel flickered rapidly in the open state, so that individual events appeared as a burst of openings and closings. The average duration of a burst was 0.78 +/- 0.03 ms (mean +/- S.E.). The average duration of an individual opening was 0.18 +/- 0.008 ms. The average closed duration within a burst was 0.37 +/- 0.02 ms. 4. Hyperpolarization of the recorded patch had no effect on average burst or open duration, although opening frequency increased slightly (+18.6 +/- 4.9%; n = 13; mean +/- S.E.). Average single-channel current increased linearly with hyperpolarization, giving an estimated single-channel conductance of 20.5 +/- 1.1 pS. By extrapolation of the relation between channel current and hyperpolarization, the dark driving force was estimated to be about 48 mV. 5. In addition to reducing the rate of channel events, dim non-saturating light also reduced the average duration of a burst of openings and the average duration of openings within a burst. 6. About 50% of cell-attached patches showed no channel activity in darkness. Light-suppressable channel activity could be induced in these silent patches by perfusing the outer segment with low-Ca2+ Ringer solution. Similarly, activity could be increased dramatically by low-Ca2+ Ringer solution in patches that did show channel activity in the dark. From the maximal channel activity observed during low-Ca2+ perfusion, the lower limit for the number of channels per patch was 20-70, corresponding to an estimated channel density of 100-350 channels micron-2. 7. After recording light-sensitive channel activity in the intact rod, the patch of membrane was excised, exposing the intracellular membrane face. Application of guanosine 3',5'-cyclic monophosphate (cyclic GMP) to the intracellular face activated channels (Haynes, Kay & Yau, 1986; Zimmerman & Baylor, 1986; Matthews, 1986d, 1987) whose properties could then be compared directly with the light-sensitive channels recorded earlier in the same patch of membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: II. Mechanism of action

The specificity and mechanism of the neurotoxic action of kainic acid (KA) was investigated by histological methods in the isolated retina of toads and goldfish. Particular attention was paid to the earliest and most sensitive response to KA in the outer plexiform layer (OPL). Of 21 compounds tested as potential mimics of KA neurotoxicity in the OPL, only the enantiomers of glutamate and aspartate mimicked KA, inducing a low-level neurotoxic effect at concentrations 5,000-10,000-fold higher than concentrations of KA giving comparable effects. Further, of 22 compounds tested as potential blockers of KA neurotoxicity in the OPL, only D-gamma-glutamylglycine, D,L-alpha-amino pimelic acid, sodium pentobarbital, D,L-alpha-amino adipic acid, L-glutamate, and L-aspartate blocked KA neurotoxicity (IC50 values of 0.1, 0.3, 0.3, 2, 5, and 15 mM, respectively). In ionic substitution experiments, KA-induced vacuolization was found to require sodium and chloride ions but not calcium ions in the extracellular medium. These findings support the hypothesis that KA combines with specific receptors in the membrane of susceptible neurons in the retinal OPL, leading to prolonged opening of membrane channels permeable to sodium and potassium ions. An accompanying equilibrating chloride influx may result in intracellular ion excess, leading to osmotic swelling and vacuolization. The membrane receptors involved in mediating the action of KA in the OPL are likely to be a class of postsynaptic or extrasynaptic glutamate receptor.

Glutamate and the pathophysiology of hypoxic--ischemic brain damage

Information obtained over the past 25 years indicates that the amino acid glutamate functions as a fast excitatory transmitter in the mammalian brain. Studies completed during the last 15 years have also demonstrated that glutamate is a powerful neurotoxin, capable of killing neurons in the central nervous system when its extracellular concentration is sufficiently high. Recent experiments in a variety of preparations have shown that either blockade of synaptic transmission or the specific antagonism of postsynaptic glutamate receptors greatly diminishes the sensitivity of central neurons to hypoxia and ischemia. These experiments suggest that glutamate plays a key role in ischemic brain damage, and that drugs which decrease the accumulation of glutamate or block its postsynaptic effects may be a rational therapy for stroke.

Glutamate neurotoxicity in cortical cell culture is calcium dependent

Brief exposure to glutamate produced widespread neuronal death in mature, but not young, cortical cell cultures. Extracellular sodium replacement or addition of tetrodotoxin produced only minor reduction in this toxic neuronal loss. However, removal of extracellular calcium markedly reduced neuronal loss, and elevation of extracellular calcium accentuated neuronal loss. These observations suggest that the toxicity of glutamate on cortical neurons may depend primarily on the presence of extracellular Ca, probably through a mechanism which is distinct from simple 'excitotoxicity'.

The effect of calcium ions on the glutamate response and its desensitization in crayfish muscle fibres

The responses of crayfish muscle fibres to bath application or long ionophoresis of L-glutamate were studied in normal and low Ca2+ solutions. The smaller responses recorded in low Ca2+ solutions have characteristics suggesting a faster desensitization. Desensitization and recovery have complex kinetics. Desensitization is faster and recovery slower when external Ca2+ concentration is reduced. Both components of the recovery phase, which can be fitted by the sum of two exponentials, are affected by the external Ca2+ concentration. Recovery can be accelerated by external Ca2+ ionophoresis onto desensitized glutamate receptors. Responses to brief glutamate pulses of low intensity are not affected by Ca2+ reduction. For higher intensities, signs of desensitization are detectable early in the rising phase of the response. Concanavalin A (Con A) blocks both desensitization and Ca2+ dependence with similar time courses. Whether or not the preparation has been treated with Con A, the slowly rising responses recorded in isotonic Ca2+ do not show signs of desensitization. Con A causes a partial blockade of the glutamate response. The Ca2+ dependence of the glutamate response can be explained by the Ca2+ dependence of the desensitization process, the cation acting at ectocellular sites of the muscle membrane.

Modulation of neuronal responses to L-glutamate in Aplysia

Potentiation of the excitatory response to L-glutamate (Glu) by L-aspartate (Asp), similar to that which has been described at the crustacean neuromuscular junction, is observed in Aplysia neurons which are glutamate sensitive. Potentiation of the inhibitory responses to ionophoretically applied Glu in neurons preconditioned with Asp permits experiments which serve to differentiate among four hypotheses previously proposed to explain the underlying mechanism of the phenomenon. The potentiation is inhibited by cooling (Q10 = 1.3 +/- 0.2) and is blocked in Na+-free seawater, where the response to Glu applied alone is increased in both amplitude and duration. These results are most consistent with the view that Glu is normally removed from the extracellular medium through an active reuptake process which is Na+ dependent, is slightly temperature sensitive, and may be blocked by Asp. Potentiation of the excitatory response to L-glutamate (Glu) by L-aspartate (Asp) has been previously described at the crustacean neuromuscular junction (Kravitz et al., 1970; Nistri and Constanti, 1979). This potentiation has been attributed to an Asp-induced change in conformation of the Glu receptor, thereby increasing its affinity for Glu (Shank and Freeman, 1975); suppression of the rate of desensitization of the Glu receptor induced by Asp (Dudel, 1977); blockade by Asp of a Glu reuptake process (Crawford and McBurney, 1977); and release, triggered by Asp, of a bound store of Glu (Constanti and Nistri, 1978).(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent drug blockade of L-glutamate activated channels of the crayfish

The actions of d-tubocurarine (d-TC) and local anaesthetics on the L-glutamate activated channel at the voltage-clamped crayfish neuromuscular junction were studied. The effect of d-TC and local anaesthetics on the dose-response relationship between ionophoretically applied L-glutamate and synaptic current suggested that both acted as non-competitive inhibitors. The amount of inhibition was voltage dependent, and increased as the membrane potential was hyperpolarized. This voltage-dependent block was also manifest in a flattening of the I-V relationship between L-glutamate induced current and membrane potential in the presence of d-TC. However, the reversal potential for the L-glutamate activated channel was not affected; it was about +7 mV in both the presence and absence of d-TC. The neurally evoked excitatory post-synaptic current (e.p.s.c.) was depressed in the presence of these drugs and this effect was also voltage dependent. The time course of the e.p.s.c. was affected, but less so than expected if the L-glutamate activated channel were identical to the channel opened by acetylcholine at the vertebrate neuromuscular junction. Possible reasons for this discrepancy are discussed.