Crayfish muscle fiber: ionic requirements for depolarizing synaptic electrogenesis

Functional characteristics of L-glutamate, N-methyl-D-aspartate and kainate receptors in isolated brain synaptic membranes

L-Glutamic acid (L-Glu) and other excitatory amino acids and amino acid analogs enhanced [35S]thiocyanate (SCN-) uptake in isolated-resealed synaptic membrane vesicles. The SCN- uptake was used as a measure of membrane depolarization to evaluate the characteristics of functional excitatory amino acid receptors in the synaptic membranes. N-Methyl-D-aspartate (NMDA) and L-Glu produced additive effects on SCN- accumulation indicating the presence of distinct L-Glu and NMDA receptors. On the other hand, kainic acid (KA) and L-Glu shared either common receptor sites or ion channels. The effects of antagonists on NMDA, L-Glu, and KA stimulation of SCN- influx were consistent with previously reported electrophysiologic observations in intact neurons.

L-Glutamate effects on electrical potentials of synaptic plasma membrane vesicles

The electrogenic nature of the L-glutamate-stimulated Na+ flux was examined by measuring the distribution of the lipophilic anion [35S]thiocyanate (SCN-) into synaptic membrane vesicles that were incubated in a NaCl medium. Concentrations of L-glutamate from 10(-7) to 10(-4) M added to the incubation medium caused an enhanced intravesicular accumulation of SCN-. Based on the SCN- distribution in synaptic membrane vesicles it was calculated that 10 microM L-glutamate induced an average change in the membrane potential of + 13 mV. L-Glutamate enhanced both the Na+ and K+ conductance of these membranes as determined by increases in SCN- influx. Other neuroexcitatory amino acids and amino acid analogs (D-glutamate, L-aspartate, L-cysteine sulfinate, kainate, ibotenate, quisqualate, N-methyl-D-aspartate, and DL-homocysteate) also increased SCN- accumulation in synaptic membrane vesicles. These observations are indicative of the activation by L-glutamate and some of its analogs of excitatory amino acid receptor ion channel complexes in synaptic membranes.

Lithium and rubidium: effects on the rhythmic swimming movement of jellyfish (Aurelia aurita)

The effects of adding LiCl, RbCl, KCl or NaCl to sea water at concentrations up to 30 mmoles/1 on the frequency of contraction of jellyfish (Aurelia aurita) suggest that studies on phylogenetically low animals with relatively simple nervous systems may be of use to determine mechanisms of action of lithium and rudidium on movements.

Ionic mechanisms associated with the depolarization by glutamate and aspartate on human and rat spinal neurones in tissue culture

The action of glutamate and aspartate was studied on the membrane potential of human and rat spinal neurones in tissue culture. Both amino acids caused a depolarization of the cell membrane, the size of which was dependent on the concentration of the amino acids in the bathing fluid. In order to study ionic mechanisms associated with the amino acid depolarization, the ionic composition of the extracellular fluid was altered. Removal of sodium ions from the bathing solution reversibly reduced or abolished the depolarization produced by glutamate and aspartate suggesting that the action of these amino acids is associated with an increased sodium permeability. Substituting lithium for sodium ions also reversibly abolished the depolarization by glutamate indicating that in contrast to the effect of lithium on the action potential, this ion cannot replace sodium for the glutamate depolarization. These experiments show that the method of tissue culture is a suitable model to study ionic mechanisms underlying the action of neurotransmitters in the mammalian and especially in the human CNS.

Permeability changes produced by L-glutamate at the excitatory post-synaptic membrane of the crayfish muscle

1. Permeability changes produced by L-glutamate at the neuromuscular junction of the crayfish (Cambarus clarkii) were investigated by application of the drug iontophoretically to the voltage-clamped junction and measuring the resulting 'glutamate current'. 2. Reversal potentials were determined by measuring the glutamate current at different membrane potentials. They were +39-1 +/- 3-6 mV (mean +/- S.E. of mean) in normal solution and +16-5 +/- 2-0 mV in solutions made twice as hypertonic by the addition of sucrose. 3. Decreasing external Na+ concentration shifted the reversal potential in the negative direction; increased Na+ in the positive direction. 4. The relation between the amplitude of the glutamate current and extracellular Na+ concentration was approximately linear. 5. Alteration of the external K+ or Cl- concentration did not affect the amplitude or reversal potential of glutamate current. 6. In Na+-free solution the application of L-glutamate produced a small inward current at the resting potential and its amplitude was augmented by increasing the external Ca2+ concentration. 7. Increasing the Ca2+ concentration in the normal Na+ media produced no appreciable effect on the reversal potential but decreased the amplitude of glutamate current. 8. The results indicate that L-glutamate increases the membrane permeability mainly to Na+ and slightly to Ca2+. 9. The time course of glutamate current was shorter than that of the concentration calculated from the diffusion equation and it was simulated more closely by the square of the concentration.

Ionic mechanism of the excitatory synaptic membrane of the crayfish neuromuscular junction

1. The reversal potential for the excitatory neuromuscular junction of the crayfish (Cambarus clarkii) was measured using the voltage clamp method. The potential change was recorded with an intracellular microcapillary and the negative phase of the output of the feed-back amplifier was connected to the stainless-steel wire which was inserted longitudinally into the muscle fibre. 2. When the excitatory nerve was stimulated, a transient feed-back current flowed inwardly through the membrane. This current was called the excitatory junctional current (e.j.c.). 3. Reversal potentials were determined by extrapolating the e.j.c.s measured at different membrane potentials. They were about 10-20 mV positive with respect to the bath solution (11-5 +/- 1-2 mV, mean +/- S.E.). 4. The reversal potential for the iontophoretically applied glutamate was identical with that for the e.j.c. 5. In hypertonic solutions, the reversal potentials for e.j.c. and glutamate became more negative. 6. When the sodium concentration of the bath solution was decreased, the reversal potential became more negative. 7. When the chloride and potassium concentration were altered, little, if any, change was observed in the reversal potential. 8. It was concluded that the e.j.c. was carried mainly by sodium ions. Contribution of other ions, possibly calcium ions, was discussed.

Cooperative interaction of glutamate and aspartate with receptors in the neuromuscular excitatory membrane in walking limbs of the lobster

When applied to lobster muscle fibers, L-glutamate, L-aspartate, and combinations of the two amino acids can induce membrane depolarization. Under normal conditions, a quantitative analysis of the depolarization response or change in membrane conductance was precluded by nonlinearities in the voltage-current relationship of the membrane. By including gamma-aminobutyrate (GABA) in the bathing medium, the voltage-current relationship was made linear in the depolarizing direction over a range of 15-20 mV from the resting potential. However, a meaningful examination of the increase in membrane conductance caused by glutamate and aspartate was still not possible. Therefore, the depolarization responses caused by the excitatory amino acids were taken as a quantitative reflection of receptor activation in the excitatory postsynaptic membrane. In the presence of GABA, aspartate by itself, at concentrations up to 10 mM, had little excitatory activity, whereas glutamate effected an appreciable membrane depolarization at concentrations of 0.1 to 0.2 mM. Aspartate, at concentrations which exhibited no activity alone, markedly enhanced the excitatory action of glutamate. Aspartate shifted the glutamate dose-response curve to the left, but did not appear to affect the maximum depolarization response elicited by glutamate. These observations are consistent with the concept that aspartate increases the affinity between glutamate and the glutamate binding sites. Limiting slopes of log-dose versus log-response curves for the excitatory action of glutamate suggest that the interaction of glutamate with excitatory receptors is a cooperative process. The possibility exists that individual receptors contain multiple and distinct glutamate and aspartate binding sites. These results support the view that neuromuscular excitation in the lobster is mediated by glutamate and aspartate functioning synergistically.

Pentobarbital: selective depression of excitatory postsynaptic potentials

The effects of pentobarbital (Nembutal) on synaptic transmission and postsynaptic potentials were studied by the use of several invertebrate preparations. Pentobarbital selectively and reversibly depressed both excitatory postsynaptic potentials and sodium-dependent postsynaptic responses to putative excitatory transmitters without affecting either inhibitory postsynaptic potentials or chloride- and potassium-dependent postsynaptic responses to putative transmitters. A selective depression of postsynaptic excitatory events was also observed with other central nervous system depressants (ethanol, chloroform, chloralose, diphenylhydantoin, and urethane). The results suggest that central and peripheral depression observed during general anesthesia is due to a selective depression of excitatory synaptic events.

Salivary excretion of lithium. II. Functional analysis

Salivary Li+ was actively secreted by the cat submaxillary gland, inversely to the rate of flow, at concentrations above serum level. Stop-flow studies indicated ductal secretion of Li+ by a HgCl2 and ouabain-sensitive mechanism. Salivary secretion of Li+ did not resemble Na+ but was similiar to K+ secretion.

Conductance changes associated with receptor potentials in Limulus photoreceptors

The receptor potential in Limulus photoreceptors appears to be a consequence not of permeability changes in the cell membrane but of alterations in a light-sensitive constant-current generator.

Effects of some organic cations on generator potential of crayfish stretch receptor

The generator potential of both slowly and rapidly adapting crayfish stretch receptor cells can still be elicited by mechanical stimuli when all the Na of the bathing medium is replaced by various organic cations. In the presence of tris(hydroxymethyl)aminomethane (Tris), the generator potential is particularly large, about 30-50 % of that in the control saline, while spike electrogenesis of the cell is abolished. Persistence of the generator response is not due to retention of Na by a diffusion barrier, and ionic contributions to the electrogenesis by Ca and Cl can also be excluded. Thus, whereas the electrogenesis of the generator membrane must be due to an increased permeability to monovalent cations, the active receptor membrane appears to be less selective for different monovalent cations than is the receptor component of some other cells, or the conductile component of the stretch receptor neuron.

Effects of lithium on different membrane components of crayfish stretch receptor neurons

Unlike several other varieties of input membrane, that of the crayfish stretch receptor develops a generator potential in response to stretch when all the Na of the medium is replaced with Li. However, Li depolarizes the receptor neuron, the soma membrane becoming more depolarized than that of the axon. During exposure to Li the cell usually fires spontaneously for a period, and when it becomes quiescent spike electrogenesis fails in the soma but persists in the axon. These effects are seen in the rapidly adapting as well as the slowly adapting cells. The block of spike electrogenesis of the soma membrane is only partly due to the Li-induced depolarization and a significant role must be ascribed to a specific effect of Li.

Permeability of alkali metal cations in lobster muscle. A comparison of electrophysiological and osmometric analyses

Single muscle fibers from lobster walking legs are effectively impermeable to Na, but are permeable to K. They shrink in hyperosmotic NaCl; they swell in low NaCl media which are hyposmotic or which are made isosmotic with the addition of KCl. In conformity, the membrane potential is relatively insensitive to changes in external Na, while it responds according to the Nernst relation for changes in external K. When the medium is made isosmotic or hyperosmotic with RbCl the volume and membrane potential changes are of essentially the same magnitudes as those in media enriched with KCl. The time courses for attaining equilibrium are slower, indicating that Rb is less permeant than K. Substitution of CsCl for NaCl (isosmotic condition) produces no change in volume of the muscle fiber. Addition of CsCl (hyperosmotic condition) causes a shrinkage which attains a steady state, as is the case in hyperosmotic NaCl. Osmotically, therefore, Cs appears to be no more permeant than is Na. However, the membrane depolarizes slowly in Cs-enriched media and eventually comes to behave as an ideal Cs electrode. Thus, the electrode properties of the lobster muscle fiber membrane may not depend upon the diffusional relations of the membrane and ions, and the osmotic permeability of the membrane for a given cation may not correspond with the electrophysiologically deduced permeability. Comparative data on the effects of NH(4) and Li are also included and indicate several other degrees of complexity in the cell membrane.