The ionic mechanism of the excitatory action of glutamate upon the membranes of motoneurones of the frog

Impact of calcium-activated potassium channels on NMDA spikes in cortical layer 5 pyramidal neurons

Active electrical events play an important role in shaping signal processing in dendrites. As these events are usually associated with an increase in intracellular calcium, they are likely to be under the control of calcium-activated potassium channels. Here, we investigate the impact of calcium-activated potassium channels onN-methyl-d-aspartate (NMDA) receptor-dependent spikes, or NMDA spikes, evoked by glutamate iontophoresis onto basal dendrites of cortical layer 5 pyramidal neurons. We found that small-conductance calcium-activated potassium channels (SK channels) act to reduce NMDA spike amplitude but at the same time, also decrease the iontophoretic current required for their generation. This SK-mediated decrease in NMDA spike threshold was dependent on R-type voltage-gated calcium channels and indicates a counterintuitive, excitatory effect of SK channels on NMDA spike generation, whereas the capacity of SK channels to suppress NMDA spike amplitude is in line with the expected inhibitory action of potassium channels on dendritic excitability. Large-conductance calcium-activated potassium channels had no significant impact on NMDA spikes, indicating that these channels are either absent from basal dendrites or not activated by NMDA spikes. These experiments reveal complex and opposing interactions among NMDA receptors, SK channels, and voltage-gated calcium channels in basal dendrites of cortical layer 5 pyramidal neurons during NMDA spike generation, which are likely to play an important role in regulating the way these neurons integrate the thousands of synaptic inputs they receive.

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.

Glycine-activated currents are changed by coincident membrane depolarization in developing rat auditory brainstem neurones

1. During early ontogeny, glycine receptors (GlyRs) exert depolarizing responses which may be of developmental relevance. We have used the gramicidin-perforated patch technique to elucidate the mechanism of glycine-activated currents in developing neurones of the rat lateral superior olive (LSO). 2. When the holding potential was set to -60 mV, perforated-patch recordings revealed glycine-induced inward currents in 59 %, outward currents in 5 % and biphasic currents in 34 % of the LSO neurones tested (n = 44). The biphasic currents were characterized by a transient outward phase which was followed by an inward phase. 3. Ion substitution experiments showed that both Cl- and HCO3- contributed to the glycine- induced biphasic current responses. 4. In the biphasic responses, the reversal potential of the glycine-induced current (Egly) depended on the response phase. A strong shift of Egly from a mean of -72 mV during the outward phase of the glycine response to a mean of -51 mV during the inward phase was observed, suggesting a shift of an ion gradient. 5. When the membrane potential was depolarized, 'tail' currents were induced in the presence of glycine. An increased duration or amplitude of the evoked depolarizations resulted in a proportional enlargement of these tail currents, indicating that they were produced by a shift of an ion gradient. Since changes of the HCO3- gradient are negligible, because of the carbonic anhydrase activity, we suggest that these tail currents were caused by a shift of the Cl- gradient. 6. We conclude that Cl- accumulates intracellularly during the activation of GlyRs and, consequently, Egly moves towards more positive values. 7. Coincident depolarizing stimuli enhanced intracellular Cl- accumulation and the shift of Egly, thereby switching hyperpolarizing to depolarizing action. This change could assist in an activity-dependent strengthening and refinement of glycinergic synapses during the maturation of inhibitory connectivity.

The role of calcium in apoptosis

One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid redistribution of ionised calcium throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Various physiological stimuli increase [Ca2+]i transiently and, thereby, induce cellular responses. However, under pathological conditions, changes of [Ca2+]i are generally more pronounced and sustained. Marked elevations of [Ca2+]i activate hydrolytic enzymes, lead to exaggerated energy expenditure, impair energy production, initiate cytoskeletal degradation, and ultimately result in cell death. Such Ca(2+)-induced cytotoxicity may play a major role in several diseases, including neuropathological conditions such as chronic neurodegenerative diseases and acute neuronal losses (e.g. in stroke).

Cobalt accumulation in neurons expressing ionotropic excitatory amino acid receptors in young rat spinal cord: morphology and distribution

Excitatory amino acids (EAA) acting on N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate receptors play an important role in synaptic transmission in the spinal cord. Quantitative autoradiography and physiological experiments suggest that NMDA receptors are localized mainly in lamina II while kainate and AMPA receptors are found on both dorsal and ventral horn neurons. However the cell types expressing EAA receptors and their laminar distribution is not known. We have used a cobalt uptake method to study the morphology and distribution of spinal cord neurons expressing AMPA, kainate, or NMDA excitatory amino acid receptors in the lumbar enlargement of the rat spinal cord. The technique involved superfusion of hemisected spinal cords of 14 day-old rat pups in vitro with excitatory amino acid receptor ligands in the presence of CoCl2. Cobalt has been shown to enter cells through ligand-gated ion channels in place of Ca2+. Cells which accumulated cobalt ions following activation by ionotropic excitatory amino acid receptors were visualized histochemically. The cobalt uptake generated receptor-specific labeling of cells, as the NMDA receptor antagonist D-(-)-2-amino-(5)-phosphonovaleric acid (D-AP-5) (20 microM) blocked the NMDA, but not kainate-induced cobalt uptake. The kainate-induced cobalt labeling was reduced by the non-selective excitatory amino acid receptor antagonist kynurenic acid (4 mM). Passive opening of the voltage-gated Ca(2+)-channels by KCl (50 mM) did not result in cobalt uptake, indicating that cobalt enters the cells through ligand-gated Ca(2+)-channels. AMPA (500 microM), kainate (500 microM), or NMDA (500 microM) each induced cobalt uptake with characteristic patterns and distributions of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal perikarya while NMDA-induced uptake was the lowest. AMPA and kainate, but not NMDA superfusion, resulted in cobalt labeling of glial cells. Our results show that the cobalt uptake technique is a useful way to study the morphology and distribution of cells expressing receptors with ligand-gated Ca2+ channels.

Extracellular potassium, glial and neuronal potentials in the solitary complex of rat brainstem slices

Extracellular K+ activities (aKe) and neuronal and glial membrane potentials were recorded in the nucleus tractus solitarius (NTS) and in the dorsal vagal motor nucleus (DVMN) of rat brainstem slices after orthodromic stimulation of the tractus solitarius (TS). In glial cells, repetitive stimulation of the TS induced depolarizations of up to 30 mV followed by repolarizations which were fitted by exponential curves with a time constant of 1.6-5 s. Similar stimulations induced elevations of aKe of up to 8 mM, the recovery of which was fitted by single exponential curves with a time constant ranging between 1.6 and 4 s. These elevations in aKe were reduced by 75% during blockage of synaptic transmission in low Ca2+, high Mg2+ solution, and by 24% with application of 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX, 50 microM). Perfusion with a low Mg2+ solution increased the aKe response to stimulation of the TS, an effect that was reduced by the addition of 2-amino-5-phosphono-valeric acid (AP7, 50 microM) to the bath. No significant change in aKe and glial potential was seen when superfusing high concentrations of the C-terminal octapeptide of cholecystokinin (CCK8, 1-5 microM) and C-terminal tetrapeptide (CCK4, 50-100 microM). The effect of TS stimulations on solitary complex neurons suggests that extracellular K+ concentration is increased during synaptic activation of non-NMDA or NMDA ionotropic receptors. Conversely, slow depolarizations elicited by repetitive afferent activity or excitation by CCK agonists develop in neurons in the absence of measurable extracellular K+ fluctuations or glial depolarization.

Intracellular Na+ activity in cultured mouse oligodendrocytes

Na(+)-selective double-barrelled microelectrodes were used to measure the intracellular Na+ activity (aiNa) and membrane potential (Em) in oligodendrocytes from cultures of embryonic mouse spinal cord. In Na(+)-free solutions aiNa rapidly fell from its baseline of about 15 mM to values below 1 mM. Elevation of the K+ concentration in the bath ([K+]o) from 5.4 to 15 or 50 mM elicited an aiNa decrease of 4.7 or 9.0 mM, respectively. Ouabain blocked the aiNa decrease in response to 50 mM K+ by 37%. Bath application of 1 mM glutamate resulted in a membrane depolarization of 4.5 mV and a concomitant rise of aiNa by 8.6 mM. aiNa increased by approximately 11 mM after washout of a solution containing 20 mM NH4+. This aiNa increase was not blocked by amiloride, excluding a major contribution of a Na+/H+ antiporter. We conclude that, in cultured oligodendrocytes, transmembraneous Na+ movements are involved in pH regulation, glutamate causes an influx of Na+, and that the Na+/K+ pump and passive KCl uptake contribute to K+ accumulation.

Glutaminergic responses of neuropile glial cells and Retzius neurones in the leech central nervous system

The effects of glutaminergic agonists on neuropile glial cells and on Retzius neurones in the central nervous system (CNS) of the leech, Hirudo medicinalis were investigated using double-barrelled ion-sensitive microelectrodes. In both types of cells, bath-application of L-glutamate (Glu), kainate (Ka) and quisqualate (Qui) elicited substantial membrane depolarizations which were accompanied by increases of the intracellular Na+ activity aiNa and by concomitant decrease of the intracellular K+ activity aiK. In the glial cells, these alterations of aiNa and aiK were preceded by a transient decrease of aiNa and an increase of aiK upon administration of Ka and Qui. In both glial cells and neurones, N-methyl-D-aspartate (NMDA) did not affect Em, aiK and aiNa. As found for Ka, the neuronal as well as the glial responses to glutaminergic agonists persisted during inhibition of synaptic transmission in high Mg2+, low Ca2+ solutions. The results indicate that leech neuropile glial cells have a Ka/Qui-preferring non-NMDA glutamate receptor similar to that in the Retzius neurones.

Intracellular and extracellular changes of [Ca2+] in hypoxia and ischemia in rat brain in vivo

Changes in intra- and extracellular free calcium concentration were evaluated with ion-selective microelectrodes during periods of anoxia and ischemia in three different regions of intact rat brain. Recordings stable for at least 2 min and in most cases for 4-6 min were chosen for analysis. Under normoxic conditions neuronal [Ca2+]i varied between less than 10(-8) and 10(-7) M from cell to cell but no systematic regional differences were observed. Elimination of O2 or interruption in blood flow caused, within 30-60 s, slight intracellular alkalinization followed by a small rise in [Ca2+]i, a mild degree of hyperpolarization, and disappearance of electrical activity in the cortex, in that order. It is postulated that a decline in cellular energy levels, as manifested by H+ uptake associated with creatine phosphate hydrolysis, leads to an increase in [Ca2+]i, which activates Ca2(+)-dependent K+ channels and consequently enhances gK. 2-4 min later there was a sudden, large rise in [K+]e, a fall in [Ca2+]e and a rapid elevation of [Ca2+]i. The magnitude of the latter was greatest in a high proportion of hippocampal neurons in area CA1 and some cortical cells, while it was smallest and relatively delayed in thalamic neurons. In the hippocampus area CA1 increases in [Ca2+]i to as much as 6-8 x 10(-4) were observed; some of these could be reversed when O2 or blood flow were restored to normal. Pretreatment of animals with ketamine and MK-801, antagonists of excitatory amino acid transmitters, markedly slowed and decreased the rises in [Ca2+]i. The effects of the two agents were most pronounced in the hippocampus. It is concluded that the receptor-operated channels are largely responsible for Ca2+ entry into certain cells during hypoxia/ischemia. This pathway may be of primary importance in parts of the hippocampus and cortex, regions of the brain that are particularly vulnerable to O2 deprivation and which receive high glutamatergic input and have an abundance of excitatory amino acid receptors.

Relationships between the neuronal sodium/potassium pump and energy metabolism. Effects of K+, Na+, and adenosine triphosphate in isolated brain synaptosomes

The relationships between Na/K pump activity and adenosine triphosphate (ATP) production were determined in isolated rat brain synaptosomes. The activity of the enzyme was modulated by altering [K+]e, [Na+]i, and [ATP]i while synaptosomal oxygen uptake and lactate production were measured simultaneously. KCl increased respiration and glycolysis with an apparent Km of about 1 mM which suggests that, at the [K+]e normally present in brain, 3.3-4 mM, the pump is near saturation with this cation. Depolarization with 6-40 mM KCl had negligible effect on ouabain-sensitive O2 uptake indicating that at the voltages involved the activity of the Na/K ATPase is largely independent of membrane potential. Increases in [Na+]i by addition of veratridine markedly enhanced glycoside-inhibitable respiration and lactate production. Calculations of the rates of ATP synthesis necessary to support the operation of the pump showed that greater than 90% of the energy was derived from oxidative phosphorylation. Consistent with this: (a) the ouabain-sensitive Rb/O2 ratio was close to 12 (i.e., Rb/ATP ratio of 2); (b) inhibition of mitochondrial ATP synthesis by Amytal resulted in a decrease in the glycoside-dependent rate of 86Rb uptake. Analyses of the mechanisms responsible for activation of the energy-producing pathways during enhanced Na and K movements indicate that glycolysis is predominantly stimulated by increase in activity of phosphofructokinase mediated via a rise in the concentrations of adenosine monophosphate [AMP] and inorganic phosphate [Pi] and a fall in the concentration of phosphocreatine [PCr]; the main moving force for the elevation in mitochondrial ATP generation is the decline in [ATP]/[ADP] [Pi] (or equivalent) and consequent readjustments in the ratio of the intramitochondrial pyridine nucleotides [( NAD]m/[NADH]m). Direct stimulation of pyruvate dehydrogenase by calcium appears to be of secondary importance. It is concluded that synaptosomal Na/K pump is fueled primarily by oxidative phosphorylation and that a fall in [ATP]/[ADP][Pi] is the chief factor responsible for increased energy production.

Glutamate stimulation of [3H]dopamine release from dissociated cell cultures of rat ventral mesencephalon

In dissociated cell cultures of fetal rat ventral mesencephalon preloaded with [3H]dopamine, glutamate (10(-5)-10(-3) M) stimulated the release of [3H]dopamine. Glutamate stimulation of [3H]dopamine release was Ca2+ dependent and was blocked by the glutamate antagonist, cis-2,3-piperidine dicarboxylic acid. Glutamate stimulation of [3H]dopamine release was not due to glutamate neurotoxicity because (1) glutamate did not cause release of a cytosolic marker, lactate dehydrogenase, and (2) preincubation of cultures with glutamate did not impair subsequent ability of the cells to take up or release [3H]dopamine. Thus, these dissociated cell cultures appear to provide a good model system to characterize glutamate stimulation of dopamine release. Release of [3H]dopamine from these cultures was stimulated by veratridine, an activator of voltage-sensitive Na+ channels, and this stimulation was blocked by tetrodotoxin. However, glutamate-stimulated [3H]dopamine release was not blocked by tetrodotoxin or Zn2+. Substitution of NaCl in the extracellular medium by sucrose, LiCl, or Na2SO4 had no effect on glutamate stimulation of [3H]dopamine release; however, release was inhibited when NaCl was replaced by choline chloride or N-methyl-D-glucamine HCl. Glutamate-stimulated [3H]-dopamine release was well maintained (60-82% of control) in the presence of Co2+, which blocks Ca2+ action potentials, and was unaffected by the local anesthetic, lidocaine. These results are discussed in terms of the receptor and ionic mechanisms involved in the stimulation of dopamine release by excitatory amino acids.

Release of glutamate and of free fatty acids in vasogenic brain edema

The pathophysiological potential of mediator substances in manifestations of secondary brain damage is attracting increased attention. This is particularly true of the excitatory transmitters glutamate and arachidonic acid. Noxious properties of these compounds in central nervous tissue have been demonstrated. The current study was performed to determine whether glutamate and arachidonate are released in brain tissue secondary to focal trauma. For this purpose, a cold injury of exposed cerebral cortex was induced in cats. Marked accumulation of glutamate was observed in interstitially drained edema fluid, reaching 10 to 15 times the level that was assessed in normal cerebrospinal fluid (CSF) prior to trauma. The extracellular release of glutamate was further dramatically enhanced by a critical decrease of the cerebral perfusion pressure due to a malignant increase of intracranial pressure. Under these conditions, glutamate concentrations 1000 to 1500 times normal levels accumulated in vasogenic edema fluid, demonstrating a relationship between the extent of the release of glutamate in damaged brain and the severity of the insult. Although under normal conditions glutamate concentrations in plasma were considerably higher than in the interstitial fluid, the pronounced increase of glutamate in this compartment due to trauma cannot be explained by transport of the compound together with the plasma-like edema from the intravascular space. Corresponding findings were obtained for free fatty acid concentrations in edema fluid. Almost all fatty acids that were studied had a significantly higher concentration in edema fluid than in normal CSF obtained as a control prior to trauma. However, contrary to the findings for glutamate, fatty acid concentrations in edema fluid were lower than in plasma. Accumulation of fatty acids in vasogenic edema fluid might, therefore, have resulted from uptake of the material together with edema fluid through the breached blood-brain barrier. Arachidonic acid was an exception. Its concentrations were significantly higher in edema fluid than in plasma, suggesting that it was released from cerebral parenchyma as the underlying mechanism of its extracellular accumulation. The current observations provide further support for a mediator function of glutamate and arachidonic acid in acute traumatic lesions of the brain. Quantitative assessment of the release of highly active mediator substances in brain tissue may facilitate analysis of the therapeutic efficiency of specific treatment aimed at interfering with the release or pathological function of mediators of secondary brain damage.

Electrophysiological characterization of a nicotinic acetylcholine receptor on leech neuropile glial cells

Ion-selective double-barrelled microelectrodes were used to measure the activities of intracellular K+, Na+, Cl-, and H+ (aiK, aiNa, aiCl, pHi) and membrane potential (Em) in neuropile glial cells as well as extracellular K+ activity (aeK) in the neuropile of the leech, Hirudo medicinalis, during bath application of carbachol. As measured with conventional single-barrelled microelectrodes, acetylcholine (ACh), nicotine, carbachol, tetramethylammonium (TMA), and choline elicited concentration-dependent (10(-6)-5 X 10(-3) M) transient membrane depolarizations of up to 60 mV amplitude whereas muscarine (10(-6)-10(-3) M) did not affect Em. alpha-Bungarotoxin (10(-7) M), decamethonium (10(-5) M), d-tubocurarine (5 X 10(-5) M), and strychnine (5 X 10(-5) M) blocked the carbachol depolarization by about 90%. Atropine (5 X 10(-5) M) blocked the response by about 75%, whereas hexamethonium was only effective at millimolar concentrations. Average baseline levels of aeK in the neuropile and of aiK, aiNa, and aiCl in the neuropile glial cells were about 3, 70, 10, and 7 mM, respectively. During the carbachol depolarization aeK and aiNa transiently increased, whereas aiK decreased. In contrast, a rise of aiK and a fall of aiNa were observed during glial depolarizations in solutions with elevated K+ concentration. aiCl increased during both the carbachol- and the K+-induced depolarization. During carbachol, pHi transiently fell by about 0.2 units from its average baseline level of 6.9, whereas an alkalinization of small amplitude was observed in high-K+ solutions. Bath-applied choline, TMA, and decamethonium rapidly accumulated in the neuropile glial cells as intracellularly monitored with double-barrelled microelectrodes filled with Corning K+ exchanger resin, which is highly selective for these agents. The results suggest that leech neuropile glial cells have a nicotinic ACh receptor coupled to a cation channel. It is hypothesized that this channel might also be permeable to choline, TMA, and decamethonium.

The role of excitatory amino acids in synaptic transmission in the hippocampus

We have highlighted some aspects of the action of excitatory amino acid transmission in the hippocampus. Fast epsps can be blocked by CNQX to reveal a component of synaptic transmission which is mediated by NMDA receptors. Extracellular recordings of ionic activities show that NMDA and non-NMDA ionophores are permeable to the major monovalent cations, while NMDA ionophores also appear to be permeable to Ca2+. Interactions of agonists applied by iontophoresis may be correlates of phenomena such as LTP, which can be evoked by appropriate synaptic stimulation.

The dependence of motoneurone membrane potential on extracellular ion concentrations studied in isolated rat spinal cord

1. Intracellular recordings from ninety-nine motoneurones have been made in an in vitro hemisected spinal cord preparation. Their mean resting membrane potential in normal artificial cerebrospinal fluid (CSF) was -71 +/- 0.5 mV (+/- S.E.M.). The mean amplitude of the action potential was 84.0 +/- 1.4 mV (n = 50), and the mean input conductance was 101 +/- 7 nS (n = 49). 2. Both membrane potential and input conductance were sensitive to changes in [K+]o, [Na+]o, [Cl-]o and [Ca2+]o. 3. Replacement of extracellular Ca2+ by Mn2+ resulted in less than 1 mV hyperpolarization and a decrease in input conductance from 102 +/- 7 to 93 +/- 6 nS (n = 15). 4. At high [K+]o (greater than 10 mM) the membrane potential followed the potential predicted by the Nernst equation for K+ ions with a slope of 58 mV per 10-fold change in [K+]o. At low [K+]o (less than 10 mM) there was significant deviation from K+ equilibrium potential (EK). 5. [K+]i was found to be 106 mM when estimated from the reversal potential of the after-hyperpolarization of the antidromic action potential. 6. The reversal potential of the recurrent inhibitory postsynaptic potential (IPSP) in normal CSF was used to calculate [Cl-]i. This was 6.6 mM, which is less than would be expected if Cl- was passively distributed, indicating the presence of an outwardly directed Cl- pump. 7. Decreasing [Cl-]o from control (134 mM) to 4 mM resulted in a depolarization of 6.9 +/- 0.9 mV and a decrease in input conductance from 102 +/- 5 to 90 +/- 5 nS (n = 14) in 3 mM [K+]o. 8. Decreasing [Na+]o from 156 to 26 mM by substitution with choline resulted in a 6.2 +/- 0.5 mV hyperpolarization and a decrease in input conductance from from 102 +/- 4 to 76 +/- 4 nS (n = 5) in 3 mM [K+]o. 9. The input conductances for Na+, Cl- and K+ at the resting potential were calculated. After allowing for a microelectrode leak conductance, the relative input conductances were gNa/gK = 0.13 and gCl/gK = 0.25.

The role of divalent cations in the N-methyl-D-aspartate responses of mouse central neurones in culture

1. Single-channel currents activated by N-methyl-D-aspartate (NMDA) agonists were analysed in the presence of various extracellular concentrations of divalent cations in outside-out patches from mouse neurones in primary culture. 2. In nominally Mg2+-free solutions the opening and closing of the channels leads to rectangular current pulses, the mean duration of which varies little with membrane potential. After addition of Mg2+, the single-channel currents recorded at negative potentials appear in bursts of short openings separated by brief closures. 3. The duration of the short openings decreases with increasing Mg2+ concentration, while the duration of the short closures is independent of the Mg2+ concentration. Depolarization increases the duration of the short openings and decreases the duration of the short closures. 4. The dependence of the burst structure on the Mg2+ concentration and on membrane potential is compatible to a first approximation with a model in which Mg2+ ions enter the open channel and block it by binding at a deep site. A better approximation requires, however, additional assumptions such as Mg2+ permeation and/or interactions between Ca2+ and Mg2+. 5. Increasing the extracellular Ca2+ concentration from 1 to 100 mM produces three effects on the currents flowing through NMDA channels. It shifts the reversal potential towards a positive value (+30 mV); it reduces the outward current flowing through the NMDA channels at very positive potentials; it reduces the inward current flowing at negative potentials. 6. The interpretation of the effects of Ca2+ appears to require three hypotheses: that Ca2+ permeates the NMDA channel, that there exists a significant surface potential at the entrance of the NMDA channel in physiological solutions and that both Ca2+ and monovalent cations bind to the channel, binding being stronger in the case of Ca2+ ions. 7. While Co2+ and, to a lesser extent, Mn2+ mimic the effects of Mg2+ on the NMDA channel, Ca2+, Ba2+ and Cd2+ do not. The distinction between Mg2+-like and Ca2+-like divalent cations corresponds to a difference in the speed of exchange of the water molecules surrounding the cations in solutions. Thus, it is possible that permeation occurs for all the divalent cations, but is slower for those which are slowly dehydrated.

Continuous electrophysiological measurements of changes in cell volume of motoneurons in the isolated frog spinal cord

One type of ion-sensitive micro-electrode (K+ ligand Corning 477317) is sensitive to large quaternary ammonium ions such as choline or tetramethylammonium (TMA+). We have now used such electrodes for continuous electrophysiological measurements of changes in cell volume of motoneurons in the isolated frog spinal cord. The electrodes were double-barrelled with tip diameters of 1 micron. The reference barrel was filled with 100 mM choline or 100 mM TMA+ in 1 M Mg2+-acetate, the sensitive barrel contained the Corning K+ ligand. After the impalement of a motoneuron, choline or TMA+ diffused into the cell and about 1 h later, a steady-state concentration of these ions in the range of 10-20 mM was reached. Following this period, the motoneurons were activated by repetitive electrical stimulation or by application of amino acids via the bathing solution. All these stimuli led to a transient rise of the intracellular concentrations of choline or TMA+ (indicating a cell shrinkage of 3-10% difference to control volume).

Excitatory postsynaptic currents in response to different synaptic inputs of frog spinal motoneurons

Excitatory postsynaptic currents (EPSCs) evoked by the primary afferents (dorsal root; DR) and the descending lateral column (LC) fibers were studied in frog spinal motoneurons under voltage clamp with two separate electrodes. The average rise time and half-width of the EPSCs were shorter for LC-EPSCs than for DR-EPSCs, though the values of the parameters for LC- and DR-EPSCs were distributed within a similar range. The relation between the amplitudes of the EPSP and EPSC was almost linear. The amount of current required to generate a 1 mV increment in the EPSP was 5.0 +/- 2.3 nA for the DR-EPSC and 3.8 +/- 1.2 nA for the LC-EPSC. The decay time was shortened by hyperpolarization and prolonged by depolarization in DR- and LC-EPSCs and spontaneous EPSCs. The reversal potential ranged from -30 to -5 mV and was almost identical for DR- and LC-EPSCs and spontaneous EPSCs in individual motoneurons. The current-voltage relation was linear from -100 to +50 mV for these EPSCs. Spontaneous EPSCs became more prominent and frequent during a large hyperpolarization or a large depolarization. These results suggest that the ionic mechanisms underlying EPSC are similar for the functionally different excitatory synapses located on motoneurons.

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)

Excitatory amino acids and intracellular pH in motoneurons of the isolated frog spinal cord

Double-barrelled pH-sensitive micro-electrodes were used to measure changes of intracellular and extracellular pH in and around motoneurons of the isolated frog spinal cord during application of excitatory amino acids. It was found that N-methyl-D-aspartate, quisqualate and kainate produced a concentration-dependent intracellular acidification. Extracellularly, triphasic pH changes (acid-alkaline-acid going pH transients) were observed during the action of these amino acids. The possible significance of such pH changes for the physiological and pathophysiological effects of excitatory amino acids are discussed.

Ionic requirements for neurotoxic effects of excitatory amino acid analogues in rat cerebellar slices

The ionic requirements for the neurotoxic effects of N-methyl-D-aspartate and kainate in incubated slices of developing rat cerebellum were studied using light and electron microscopy. Under normal conditions, 30 min exposure to 100 microM N-methyl-D-aspartate followed by a 90 min recovery period in agonist-free medium resulted in the necrosis of differentiating granule cells and deep nuclear neurons, while the corresponding effect of 100 microM kainate was the death of Golgi cells. Substitution of 96% of the Cl- in the medium with isethionate did not prevent the toxicity of either agonist. However, all the ordinarily vulnerable cells survived and exhibited normal ultrastructure if the slices were exposed to the excitants in a Ca2+-free medium and were subsequently allowed to recover in a Ca2+-containing solution. Prior to this recovery period, granule, Golgi and deep nuclear neurons exposed to N-methyl-D-aspartate were markedly swollen but their mitochondria were hypercontracted and there was no clumping of chromatin or obvious swelling of the rough endoplasmic reticulum or Golgi apparatus, in contrast to observations made on slices exposed to this agonist in normal medium. Substitution of all the Na+ in the medium with a mixture of choline (118 mM) and Tris (25 mM) itself caused necrosis of granule cells and deep nuclear neurons and an intense microvacuolation of Purkinje cells, due, in large part, to high amplitude mitochondrial swelling. A low (25 mM) Na+ medium was well tolerated under control conditions. This medium protected granule cells but not deep nuclear neurons from the toxicity of N-methyl-D-aspartate and failed to prevent kainate-induced death of Golgi cells. It is concluded that the acute neurotoxic effects of the two excitatory amino acid receptor agonists in the slices are dependent on extracellular Ca2+ and are independent of extracellular Cl-. Where apparent, the protective effect of reducing extracellular Na+ on the toxicity of N-methyl-D-aspartate is likely to reflect the involvement of this ion in the primary depolarizing mechanism.

NMDA-receptor activation increases cytoplasmic calcium concentration in cultured spinal cord neurones

Excitatory amino acids act via receptor subtypes in the mammalian central nervous system (CNS). The receptor selectively activated by N-methyl-D-aspartic acid (NMDA) has been best characterized using voltage-clamp and single-channel recording; the results suggest that NMDA receptors gate channels that are permeable to Na+, K+ and other monovalent cations. Various experiments suggest that Ca2+ flux is also associated with the activation of excitatory amino-acid receptors on vertebrate neurones. Whether Ca2+ enters through voltage-dependent Ca2+ channels or through excitatory amino-acid-activated channels of one or more subtype is unclear. Mg2+ can be used to distinguish NMDA-receptor-activated channels from voltage-dependent Ca2+ channels, because at micromolar concentrations Mg2+ has little effect on voltage-dependent Ca2+ channels while it enters and blocks NMDA receptor channels. Marked differences in the potency of other divalent cations acting as Ca2+ channel blockers compared with their action as NMDA antagonists also distinguish the NMDA channel from voltage-sensitive Ca2+ channels. However, we now directly demonstrate that excitatory amino acids acting at NMDA receptors on spinal cord neurones increase the intracellular Ca2+ activity, measured using the indicator dye arsenazo III, and that this is the result of Ca2+ influx through NMDA receptor channels. Kainic acid (KA), which acts at another subtype of excitatory amino-acid receptor, was much less effective in triggering increases in intracellular free Ca2+.

In vitro neurotoxicity of excitatory acid analogues during cerebellar development

The neurotoxic effects of the selective excitatory amino acid receptor agonists, quisqualate, kainate and N-methyl-D-aspartate, were studied in slice preparations of cerebellum from rats at different stages of postnatal development. With increasing age, (i) Purkinje cells became more vulnerable to kainate and quisqualate but remained insensitive to N-methyl-D-aspartate (up to 300 microM); (ii) Golgi cells became more sensitive to kainate, quisqualate and N-methyl-D-aspartate; (iii) granule cells became more sensitive to kainate, less sensitive to N-methyl-D-aspartate and remained unaffected by quisqualate (up to 100 microM), and (iv) basket and stellate cells and, up to 14 days of age, neurones of the deep cerebellar nuclei, became more vulnerable to kainate and quisqualate, but their sensitivity to N-methyl-D-aspartate stayed the same. The neurotoxicity of N-methyl-D-aspartate, but not that of kainate in 8-day-old cerebellar slices was prevented by 2-amino-5-phosphonovaleric acid; tetrodotoxin did not affect the toxicity of the agonists in 8-day-old or adult slices. The results with kainate are consistent with other studies indicating an insensitivity of the immature brain to its neurotoxic effects, but suggest that this property is not a peculiarity of kainate. Alterations in excitatory potency can explain some of the observed developmental changes. However, other observations cannot readily be accounted for on the basis of either changes in excitatory potency, the functional maturation of cerebellar circuits, changes in synaptic density, or the developmental appearance of Ca2+ channels in susceptible cells, suggesting that additional factors play an important role in the neurotoxic effects of the excitants.

The action of N-methyl-D-aspartic acid on mouse spinal neurones in culture

Neurones from the ventral half of mouse embryo spinal cord were grown in dissociated culture and voltage clamped. The current-voltage relation of responses evoked by N-methyl-D-aspartic acid (NMDA), L-glutamic acid and kainic acid was recorded in media of different ionic composition. On removal of Mg2+ from the extracellular solution, responses to NMDA and L-glutamate became less voltage sensitive, such that NMDA responses were no longer associated with a region of negative slope conductance. The antagonism of NMDA responses produced by application of Mg2+ to neurones bathed in nominally Mg2+-free solutions shows voltage dependence and uncompetitive kinetics. Voltage-jump experiments showed that the voltage-dependent action of Mg2+ occurred rapidly, and with complex kinetics. Ni2+ and Cd2+, two potent blockers of calcium currents in spinal cord neurones, had significantly different potencies as NMDA antagonists, Ni2+ being of greater potency than Mg2+, and Cd2+ considerably weaker. The voltage-dependent block of NMDA responses produced by physiological concentrations of Mg2+ is sufficient to explain the apparent increase in membrane resistance produced by NMDA in current-clamp experiments, and the ability of NMDA to support repetitive firing. Substitution of choline for Na+ produced a hyperpolarizing shift in the reversal potential for responses evoked by kainic acid consistent with an increase in permeability to Na+ and K+. In choline-substituted solutions, the reversal potential of NMDA responses was more positive than that recorded for kainic acid, and in addition NMDA responses showed enhanced desensitization.

Excitatory amino acid-induced responses of frog motoneurones bathed in low Na+ media: an intracellular study

Motoneurones of the frog spinal cord slice preparation were impaled with microelectrodes and superfused at 7 degrees C with the excitatory amino acids glutamate, quisqualate or N-methyl-D-aspartate. The role of Na+ in the action of these amino acids was studied by comparing amplitude matched depolarizations obtained in control Ringer solution with the responses recorded from the same cells after replacing (86-100%) Na+ by choline or glucosamine. Effective replacement of extracellular Na+ proved to be a rather slow process requiring 30-60 min. In glucosamine solution depolarizations evoked by glutamate, N-methyl-D-aspartate or quisqualate were abolished or strongly reduced with recovery following return to control Ringer. In choline solution, glutamate and N-methyl-D-aspartate effects were blocked whereas the quisqualate response was surprisingly unaffected. Mn2+ (2 mM) added to choline solution strongly diminished the action of quisqualate. These results suggest that Na+ was important in mediating amino acid responses and that quisqualate activated an additional conductance mechanism (perhaps to Ca2+) unmasked only in choline-containing solution.

The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

The ionic mechanism of postsynaptic inhibition in frog spinal motoneurones was studied with conventional and with ion-sensitive microelectrodes. In these neurones the inhibitory postsynaptic potential was depolarizing, its reversal potential being 15 mV less negative than the resting membrane potential. During the inhibitory postsynaptic potential the input resistance of the motoneurones was reduced to 20% of the resting value, indicating a strong increase of membrane conductance. The Cl- equilibrium potential calculated from intra- and extracellular Cl- activity measurements coincided with the reversal potential of the inhibitory postsynaptic potential to within a few millivolts. During repetitive inhibitory postsynaptic activity the intracellular Cl- activity decreased markedly, while the extracellular Cl- activity increased slightly. These changes of intra- and extracellular Cl- activities were no longer found after suppression of the inhibitory postsynaptic potential by strychnine. Blockade of an active, inward-going Cl- transport system in motoneurones by NH+4 led to a shift of the Cl- equilibrium potential and the reversal potential of the inhibitory postsynaptic potential towards the resting membrane potential. After prolonged action of NH+4, the Cl- equilibrium potential approached the membrane potential to within 5 mV, while the reversal potential of the inhibitory postsynaptic potential and resting membrane potential coincided. The difference between Cl- equilibrium potential and membrane potential after blockade of the Cl- pump is traced back to interfering intracellular ions, such as HCO-3 or SO42-, leading to an overestimation of intracellular Cl- activity and to the calculation of an erroneous Cl- equilibrium potential. Inhibitory amino acids like gamma-aminobutyrate or beta-alanine evoked depolarizations with reversal potentials similar to that of the inhibitory postsynaptic potential. These depolarizations were associated with a marked decrease of neuronal input resistance during inhibition. During the actions of these compounds a decrease of intracellular and a small increase of extracellular Cl- activity were found. The activities of other ions (K+, Ca2+ and Na+) did not change significantly, with the exception of extracellular K+ activity, which was slightly increased. Evidence is presented that the inhibitory postsynaptic potential, as well as the depolarizing action of inhibitory amino acids in motoneurones, is the result of an increase in membrane Cl- permeability and an efflux of Cl- from these cells, while other ions do not seem to be involved.

Changes in intracellular free Ca ion concentration evoked by electrical activity in cat spinal neurons in situ

In cats under allobarbitone anaesthesia, Ca2+-sensitive microelectrodes were inserted into the lumbosacral spinal neurons to measure intracellular free Ca2+ concentration [Ca]i. In 72 resting motoneurons, the global mean [Ca]i was 7.9 microM (SD +/- 25.9). In the 36 "best" cells (with resting and action potentials better than 60 mV), mean [Ca]i was 1.6 microM (SD +/- 1.64). Activation of motoneurons by antidromic or direct stimulation evoked mean increases in [Ca]i of about 90 nM when stimulating for 30 s at 10 Hz, and 170 nM at 20 Hz. The mean time to half-recovery was 23 s (SD +/- 14.5). Orthodromic stimulation consistently produced smaller increases in [Ca]i. Measurements in motor axons showed a comparable resting level of [Ca]i, but only minimal changes during stimulation, even at 100 Hz. Sensory axons (also recorded within the spinal cord) similarly failed to show any increase in [Ca]i during high frequency stimulation. In some interneurons, however, particularly large and rapid increases in [Ca]i could be evoked by dorsal root stimulation at 1-5 Hz. Unresponsive cells (presumably neuroglia), with a typically high and stable resting potential, had a variable [Ca]i giving a mean of 32 microM (SD +/- 63.0). A tentative theoretical analysis of the magnitude and time course of delta [Ca]i evoked in motoneurons by tetanic stimulation is consistent with remarkably slow apparent diffusion of intracellular Ca2+ (1/250 of rate of diffusion in water), such as might be expected in the presence of very efficient mechanisms of Ca2+ sequestration.

The effects of potassium channel blocking agents on the responses of in vitro frog motoneurones to glutamate and other excitatory amino acids: an intracellular study

Intracellular recordings were obtained from motoneurones of the slice preparation of the in vitro frog spinal cord to assess the participation of K+ channels in the generation of amino acid-induced excitation. The amino acids were applied by fast superfusion and at low temperature to reduce their uptake processes. Intracellular Cs+, bath-applied tetraethylammonium or 4-aminopyridine prolonged the spike and blocked its afterhyperpolarizations. Tetraethylammonium and 4-aminopyridine enhanced the synaptic activation of motoneurones which could be abolished by including Mn2+ in the superfusion fluid. Mn2+ reduced the duration of action potentials already prolonged by 4-aminopyridine but appeared to cause plateaus to develop on tetraethylammonium-prolonged spikes. The hypothesis that amino acids depolarize motoneurones by closing K+ channels was tested by observing the effects of K+ channel blocking agents on amino acid responses. Neither tetraethylammonium nor 4-aminopyridine reduced the excitatory depolarizations evoked by glutamate, N-methyl-D-aspartate, quisqualate or DL-homocysteate. Intracellular Cs+ blocked responses to amino acids irrespective of their receptor preference; an action which has been interpreted as non-specific.

Depolarization of feline primary afferent fibres by acidic amino acids

When administered micro-electrophoretically into the spinal grey matter of cats anaesthetized with pentobarbitone, acidic amino acids known to be neuronal excitants lower the threshold of electrically stimulated muscle and cutaneous primary afferent fibres and terminations. This depolarizing effect was not observed with fibres stimulated in the white matter. Depolarization by micro-electrophoretic potassium and excitant amino acids appeared not be be associated with an alteration in terminal membrane conductance since there was no change in synaptically evoked primary afferent depolarization. Excitant amino acid depolarization was not blocked by the gamma-aminobutyric acid antagonist bicuculline methochloride, but was reduced by selective excitant amino acid antagonists. The results are discussed in relation to the probable absence of specific excitant amino acid receptors on afferent terminals, the depolarizing effect of the amino acids on myelinated fibres and non-myelinated terminals being more likely a consequence of changes in the extracellular medium associated with the depolarization and firing of neurones.

Intracellular free sodium and potassium, post-carbachol hyperpolarization, and extracellular potassium-undershoot in rat sympathetic neurones

Double-barrelled ion-sensitive microelectrodes were used to record the free intracellular Na+- and K+-concentrations [( Na+]i, [K+]i) and to determine their relation to changes in membrane potential and extracellular K+ [( K+]e) in rat sympathetic ganglia. The application of 50 mumol/l carbachol resulted in an elevation of [K+]e followed by a post-carbachol [K+]e-undershoot. The membrane depolarization of the sympathetic neurones was associated with an increase in [Na+]i and a decrease in [K+]i. A membrane hyperpolarization and a recovery of [K+]i and [Na+]i to their baseline levels were observed during the [K+]e-undershoot. The time course of the [K+]e-undershoot correlated exactly with the duration of the rise in [Na+]i and decrease of [K+]i. No K+-reuptake occurred in the presence of ouabain. These data confirm, by direct measurements of intracellular ion concentration changes, the contribution of the Na+,K+-pump to the post-carbachol membrane hyperpolarization and [K+]e-undershoot.