A study of the crustacean axon repetitive response. I. The effect of membrane potential and resistance

A new configuration for voltage clamp of axons used to demonstrate nerve conduction blockade by a 2,5-disubstituted pyrrolidine

An original voltage-clamp technique on axons from crayfish Procambarus clarkii is described in this paper. Its advantages are: a fast dissection leading to the availability of several fibers of different diameters (10-500 microns) that may contain different ion channels; and use of a double-electrode voltage clamp on a chosen fiber with good clamping characteristics (short time clamp and good space clamp, small leak conductance). Because of the absence of exogenous lipidic phase in the superfusion chamber, this technique appears particularly suited to studying how liposoluble neurotoxins affect nerve conduction. This method has been successfully applied to test the effect of a synthetic derivative (2-(1non-8enyl)-5(1non-8enyl)pyrrolidine (Pyr 9)) of ant venom alkaloids from Monomorium species on nerve conduction. We present here evidence of a strong blocking effect on inward current involved in spike conduction. The resting potential of the treated axons did not change and it appears that only the inward current was affected.

Neural repetitive firing: modifications of the Hodgkin-Huxley axon suggested by experimental results from crustacean axons

The Hodgkin-Huxley equations for space-clamped squid axon (18 degrees C) have been modified to approximate voltage clamp data from repetitive-firing crustacean walking leg axons and activity in response to constant current stimulation has been computed. The m infinity and h infinity parameters of the sodium conductance system were shifted along the voltage axis in opposite directions so that their relative overlap was increased approximately 7 mV. Time constants tau m and tau h, were moved in a similar manner. Voltage-dependent parameters of delayed potassium conductance, n infinity and tau n, were shifted 4.3 mV in the positive direction and tau n was uniformly increased by a factor of 2. Leakage conductance and capacitance were unchanged. Repetitive activity of this modified circuit was qualitatively similar to that of the standard model. A fifth branch was added to the circuit representing a transient potassium conductance system present in the repetitive walking leg axons and in other repetitive neurons. This model, with various parameter choices, fired repetitively down to approximately 2 spikes/s and up to 350/s. The frequency vs. stimulus current plot could be fit well by a straight line over a decade of the low frequency range and the general appearance of the spike trains was similar to that of other repetitive neurons. Stimulus intensities were of the same order as those which produce repetitive activity in the standard Hodgkin-Huxley axon. The repetitive firing rate and first spike latency (utilization time) were found to be most strongly influenced by the inactivation time constant of the transient potassium conductance (tau b), the delayed potassium conductance (tau n), and the value of leakage conductance (gL). The model presents a mechanism by which stable low frequency discharge can be generated by millisecond-order membrane conductance changes.

Effects of thiamine antagonists on nerve conduction. I. Actions of antimetabolites and fern extract on propagated action potentials

To assess the hypothesis that thiamine is directly involved in the permeability changes at the sodium channel during nerve conduction, the effects of thiamine antagonists on lobster giant axon resting and action potentials were determined. Thiamine antimetabolites, in millimolar concentrations, reversibly decreased the maximum rate of rise and amplitude of the action potential while increasing its duration. In particular, thiamine tert-butyl disulfide (TTBD) elicited the formation of pronounced shoulders during repolarization, lengthening the action potential by 2-50 times, depending on dose. Antimetabolites also depolarized the resting membrane, but this change was poorly reversible and may indicate a dual mechanism for antimetabolite action. An extract of the fern, Pteris aquilina, reversibly decreased the maximum rate of rise of the action potential and depolarized the resting potential. It also elevated and prolonged the action potential after-depolarization, sometimes causing repetitive activity. The strength of these actions was correlated with the antithiamine potency of the extract, and was diminished by addition of thiamine to the extract.

Membrane properties of the stretch receptor neurones of crayfish with particular reference to mechanisms of sensory adaptation

1. Membrane properties of neurones of the two morphologically different types of stretch receptor of crayfish, the slowly adapting (RM(1)) and the rapidly adapting (RM(2)) receptors, were investigated with two microelectrodes inserted into the same neurone.2. The action potential was usually larger in the slowly adapting than in the rapidly adapting neurone. But the distributions of the height were not sharply delimited, and there was an overlap from the two groups of neurone.3. There were no marked differences in the current-voltage relationship between the two types.4. Under voltage clamp, depolarizations evoked a large delayed outward current, which slowly diminished during maintained depolarization (K-inactivation). Under a moderate depolarization, development of the K-permeability increase was very slow.5. When stimulated by intracellularly applied constant currents, the slowly adapting neurone always adapted slowly, and gave rise to long-lasting trains of spikes, whereas the rapidly adapting neurone never produced maintained repetitive discharges.6. The same marked differences in the adaptation behaviour of spike discharge between the two types were also observed when the neurones were stimulated by constant currents applied through external electrodes.7. When the stimulating point was shifted along the axon of the slowly adapting neurones, the ability to produce long-lasting repetitive discharges was found to be confined to the axonal region near the soma, where the diameter was very small, and where impulses were first initiated.8. Possible ionic mechanisms of the adaptation of the spike generating membrane were discussed. The importance of slowly occurring changes in the Na- and K-permeabilities and changes in the electromotive force of the membrane due to electrogenic pump was emphasized.

Light-induced changes in dye-treated lobster giant axons

Single giant axons from the lobster circumesophageal connective were studied using the sucrose gap voltage-clamp technique. The axon area in the gap was bathed in acridine orange for several minutes and then rinsed for several minutes. Subsequent illumination resulted in progressive prolongation of electrically stimulated action potentials to durations of 150 msec. The prolongation was accompanied by an increase in threshold. Currents in voltage clamp were altered such that transient current inactivation was greatly slowed. The turn on of transient current was somewhat slowed, the voltage at which peak transient current could be obtained was shifted to more positive internal potentials, and transient current at all potentials was decreased. Steady-state current was similarly affected. Low calcium following illumination partially counteracted some of the changes, but not the slowing of inactivation. Low calcium increased the duration of prolonged action potentials. Selective alteration of parameters in the Hodgkin-Huxley equations brought about a qualitative match between computations and data.

Bioelectric phenomena related to protein-fixed charge in a crab nerve fiber

The bioelectrical characteristics of a crab nerve fiber subjected to solutions containing the impermeant anions ferrocyanide, glutamate, or ethanolsulfate substituted for chloride include prolonged slow depolarizations that elicit prolonged trains of impulses sulperimposed on the plateau portion. Propionate and nitrate depress repetitive firing while thiocyancite has only a slighit effect. Nerves treated with ferrocyanide lose potassium and gain sodium, which fact may account for their depolarization. On the other hand, studies with interference microscopy reveal shift by ferrocyanide in the interference fringe pattern of the sheath material; this suggests a reorientation of fixed charges in the protein layers by direct action of the anion on these layers. This may also account for the electrical manifestations observed.