Synaptic current at the squid giant synapse

Optimization of the leak conductance in the squid giant axon

We report on a theoretical study showing that the leak conductance density, G{L} , in the squid giant axon appears to be optimal for the action potential firing frequency. More precisely, the standard assumption that the leak current is composed of chloride ions leads to the result that the experimental value for G{L} is very close to the optimal value in the Hodgkin-Huxley model, which minimizes the absolute refractory period of the action potential, thereby maximizing the maximum firing frequency under stimulation by sharp, brief input current spikes to one end of the axon. The measured value of G{L} also appears to be close to optimal for the frequency of repetitive firing caused by a constant current input to one end of the axon, especially when temperature variations are taken into account. If, by contrast, the leak current is assumed to be composed of separate voltage-independent sodium and potassium currents, then these optimizations are not observed.

Compartmentalization of the submembrane calcium activity during calcium influx and its significance in transmitter release

Quantitative modeling indicates that, in presynaptic terminals, the intracellular calcium concentration profile during inward calcium current is characterized by discrete peaks of calcium immediately adjacent to the calcium channels. This restriction of intracellular calcium concentration suggests a remarkably well specified intracellular architecture such that calcium, as a second messenger, may regulate particular intracellular domains with a great degree of specificity.

Evidence for L-glutamate as the neurotransmitter of the squid giant synapse

The effect of a spider toxin (JSTX), a specific blocker of glutamate receptors, was studied in the squid stellate ganglion. JSTX irreversibly blocked the excitatory postsynaptic potential in a dose-dependent manner. The toxin neither affected the spike in the postsynaptic nor the presynaptic fibers. Spontaneous miniature potentials recorded from thin stellate nerves were suppressed by the toxin. Iontophoretically applied L-glutamate depolarized the postsynaptic membrane of the giant axon and this potential was also blocked by JSTX. Kainic acid also depolarized the postsynaptic membrane but this was partially blocked by JSTX, indicating that JSTX differentiated kainate receptor from glutamate one. The results strongly suggest that L-glutamate is the neurotransmitter in the giant synapse of squid.

Blockade of synaptic transmission in the squid giant synapse by a spider toxin (JSTX)

We studied the effect of spider toxin (JSTX)--specific blocker of glutamate receptor--on giant synapse of the squid stellate ganglion. JSTX irreversibly blocked the excitatory postsynaptic potential (EPSP) without affecting the presynaptic action potential or the antidromic action potential. L-glutamate depolarized the postsynaptic membrane and suppressed EPSP which may be due to desensitization. The action of glutamate was completely blocked in the presence of JSTX. The results suggest that glutamate is the transmitter at the giant synapse of squid.

Rate-limiting step of inhibitory post-synaptic current decay in Aplysia buccal ganglia

1. In neurones BL and BR 3, 6, 8, 9, 10 and 11 of Aplysia buccal ganglia, cholinergic inhibitory post-synaptic potentials are produced by activity in either of two presynaptic cells. In order to analyse the synaptic conductance change, neurones were voltage-clamped inhibitory post-synaptic currents (i.p.s.c.) recorded. 2. The synaptic conductance change rises to an average peak value of 0.65 micromho and decays exponentially with single time constant tau of 19 msec. 3. We have attempted to identify the rate-limiting step responsible for i.p.s.c. decay from among the following possibilities: (1) acetylcholine (ACh) supply, (2) ACh removal by diffusion, (3) ACh removal by hydrolysis or (4) a slow unbinding or conformational change closing open synaptic current channels. 4. Cooling prolongs tau, with Q10 of 5.2. Cooling and eserine treatment together produce greatly prolonged, exponentially decaying i.p.s.c.s with tau > 150 msec. These results suggest that ACh removal, either by diffusion or hydrolysis, is not the rate-limiting step. 5. Prolonging synaptic action potential time course with intracellular injection of tetraethylammonium broadens the i.p.s.c. peak but does not affect the decay tail, suggesting that the rate-limiting step is not ACh release. 6. The spectrum of ACh-induced current fluctuations is fitted by a double Lorentzian with cut-off frequencies of 7.8 and 47 Hz. The frequency of the slower component corresponds to the macroscopic i.p.s.c. decay tau. 7. We conclude that a slow conformational change closing open synaptic current channels is likely to determine i.p.s.c. decay. We cannot, however, exclude either delayed diffusion or a late tail of slow ACh release as possibilities.

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.

An attempt at an integral interpretation of nerve excitability

A qualitatively consistent integral interpretation of biochemical, electrophysiological, and biophysical data on nerve activity is given in terms of a basic excitation unit. This operational term models a dynamically coupled assembly of membrane components accounting for graded and all-or-none responses upon stimulation. The analysis contains a series of suggestions linking controversial interpretations and is aimed at stimulation of experimental studies providing the basis for a quantitative integral theory of nerve excitation.

Chemical events in conducting and synaptic membranes during electrical activity

Evidence has accumulated in recent years for the central role of proteins and enzymes in the function of cell membranes. In the chemical theory proposed for the generation of bioelectricity, i.e., for the control of the ion permeability changes of excitable membranes, the protein assembly associated with the action of acetylcholine plays an essential role. Support of the theory by recent protein studies in which the excitable membranes of the highly specialized electric tissue were used will be discussed. A scheme is presented indicating the possible sequence of chemical reactions that change ion permeability after excitation. A sequence of chemical events within the excitable membranes of the synaptic junctions, i.e., within the pre- and postsynaptic membranes, similar to that proposed for the conducting membranes, is presented in a second scheme as an alternative to the hypothesis of the role of acetylcholine as a transmitter between two cells.

The propagation of transient potentials in some linear cable structures

1. Analytical solutions have been given for the time course of voltage transients occurring in one-dimensional cable structures, with linear uniform membrane properties.2. These solutions show how the time course of the voltage transients generated at different distances are affected by variations in the time course of current injection (at one point on the cable) and by alterations in the length of the cable, with either sealed or open end terminations. These effects are illustrated with computed results.3. Simple measurements from the time course of a membrane potential displacement, occurring in a one-dimensional cable structure following a brief current injection across the membrane, are used to evaluate the parameters which describe the cable model. These parameters are the membrane time constant, the electrotonic length of the cable, the cable end termination, and in the case of a post-synaptic potential, the distance between the electrode and the active synapse.

Synaptic electrogenesis in eel electroplaques

Whether evoked by neural or by chemical stimulation, the synaptic membrane of eel electroplaques contributes a depolarizing electrogenesis that is due to an increased conductance for Na and K. The reversal potential (E(S)) is the same for the two modes of synaptic activation. It is inside-positive by about 30-60 mv, or about midway between the emf's of the ionic batteries for Na (E(Na)) and K(E(K)). The total conductance contributed by synaptic activity (G(S)) varied over a fivefold range, but the individual ionic branches, G(SSNa), and G(SSK), change nearly equally so that the ratio G(SSNa):G(SSK) is near unity. G(SSK) increases independently of the presence or absence of Na in the bathing medium, and independently of the presence or absence of the electrically excitable G(K) channels. When activated, the synaptic membrane appears to be slightly permeable to Ca and Mg. When the membrane is depolarized into inside positivity the conductance of the synaptic components decreases and approaches zero for large inside-positive values. Thus, the synaptic components become electrically excitable when the potential across the membrane becomes inside-positive, responding as do the nonsynaptic components, with depolarizing inactivation.