Toshio Narahashi

Interactions of aminopyridines with potassium channels of squid axon membranes

The effects of aminopyridines on ionic conductances of the squid giant axon membrane were examined using voltage clamp and internal perfusion techniques. 4-Aminopyridine (4-AP) reduced potassium currents, but had no effect upon transient sodium currents. The block of potassium channels by 4-AP was substantially less with (a) strong depolarization to positive membrane potentials, (b) increasing the duration of a given depolarizing step, and (c) increasing the frequency of step depolarizations. Experiments with high external potassium concentrations revealed that the effect of 4-AP was independent of the direction of potassium ion movement. Both 3- and 2-aminopyridine were indistinguishable from 4-AP except in potency. It is concluded that aminopyrimidines may be used as tools to block the potassium conductance in excitable membranes, but only within certain specific voltage and frequency limits.

Mechanism of excitation block by the insecticide allethrin applied externally and internally to squid giant axons

Abstract The mechanisms of excitability block by allethrin have been studied by means of internal electrode, internal perfusion, and sucrose-gap voltage-clamp techniques in the giant axons of the squid. Allethrin at a concentration of 10–100 μ M blocked the action potential with slight depolarization either from inside or from outside the nerve membrane. The effect was partially reversible. The membrane currents associated with step depolarizations were inhibited by allethrin. With external application of allethrin, the early transient component of the membrane conductance was more strongly inhibited than the late steady-state component, whereas both components were inhibited to the same extent by internal application of allethrin. The time to peak early transient membrane current was slightly delayed either by external or by internal application of allethrin. There was an indication that with internal application of allethrin the sodium conductance increase was not as quickly inactivated as normal. The excitability block by allethrin can primarily be ascribed to the inhibition of the early transient membrane conductance. Further mechanisms of allethrin action are discussed in comparison with other blocking agents.

Effects of Insecticides on Excitable Tissues

Publisher Summary This chapter describes the mechanism of the nerve excitation by insecticides and discusses the changes in the nervous function caused by insecticides. The action on the nerve membrane is discussed in the chapter. In the chapter, the electrophysiological techniques are applied to various problems of the mode of action of insecticides. This includes the mechanism involved in the temperature effect on insecticidal activity, the resistance of insects to insecticides, and the structure-activity relationship. Mode of action of insecticides has been studied extensively for the past two decades since the development of a variety of synthetic insecticides. One of the most remarkable achievements in this field is the study of the metabolism of insecticides, which includes activation and degradation. Another contribution worthy of note is the study of the inhibition of cholinesterases (ChEs) by a number of insecticides, most of which are either organophosphates or carbamates. The action of insecticides on the nervous system may be classed into three categories: (1) functional changes in the nervous system as a result of insecticide intoxication; (2) biochemical mechanisms, which are responsible for the functional changes; (3) biophysical or physico-chemical mechanisms, which are responsible for the functional changes.

Site of action and active form of local anesthetics.

Publisher Summary Local anesthetics penetrate the nerve membranes in the uncharged molecular forms, are dissociated into cations inside the axon, and exert the blocking action in the cationic forms from inside the nerve membrane. This chapter presents the interpretation of the existing data in the literature using the present model. The experimental data is described in a summarized form in the chapter and, the data is in support of the concept that local anesthetics block the action potential by combining with a site on the inside surface of the nerve membrane in their charged form. The effects of pH changes on the blocking potency of tertiary amine local anesthetics can be accounted for by this model. Moreover, quaternary amines are able to block the action potential much more strongly from inside than from outside the nerve membrane. The chapter discusses several factors that may affect the present analyses. These factors are the concentrations in the unstirred membrane layer, the possible dilution of local anesthetics, the possibility that both charged and uncharged forms of local anesthetics are active in blocking the action potential, and the possibility that the active sites on receptors in the nerve membrane are changed in their sensitivity to the local anesthetics when pH is altered because of titration of active groups. However, within the limit of pH change used in the study described in the chapter, the fourth factor that may affect the present analyses can be excluded for some reasons. The chapter highlights these reasons. The last factor that may affect these analyses is the penetration of certain quaternary compounds to the membrane.

Tetrodotoxin Derivatives: Chemical Structure and Blockage of Nerve Membrane Conductance

The nerve-impuilse-blocking actions of derivatives of tetrodotoxin have been tested on lobster and squid axons. The block produced by deoxytetrodotoxin was similar to that produced by tetrodotoxin and was probably caused by tetrodotoxin contamination. Tetrodaminotoxin and anhydrotetrodotoxin also produced a similar block but at such high concentrations that tetrodotoxin contamination cannot be ruled out. The hydroxyl group of C4 and the hemilactal oxygen links play an important role for the nerve-blocking action.

DDT: Interaction with Nerve Membrane Conductance Changes

The falling phase of action potentials of lobster giant axons is prolonged by DDT; finally a plateau phase is produced like cardiac action potentials. In axons poisoned with DDT, peak transient (sodium) currents associated with step depolarizations are turned off very slowly, and steady-state (potassium) currents are markedly suppressed. These two changes would cause the prolongation of action potentials and are considered the major ionic mechanisms of DDT action.

Effects of Insecticides on Nervous Conduction and Synaptic Transmission

The intoxication of an organism with insecticide involves a variety of steps and reactions (Narahashi, 1971a). The uptake of insecticide is the first step to occur, and a number of factors such as lipid solubility and vapor pressure of insecticide are related to this process (Chapter 1). The insecticide that has entered the body is then transported to various organs and may undergo a variety of biotransformations in which it is either converted into a more potent compound or degraded to one which is relatively nontoxic (Chapters 2, 4, and 5). The active form of the insecticide eventually reaches its target site and exerts effects characteristic of the insecticide and the tissue concerned.

Tetrodotoxin Does Not Block Excitation from Inside the Nerve Membrane

Tetrodotoxin does not block the action potential or membrane sodium current when internally perfused through the giant axon of a squid at much higher concentrations than those required for blocking by external application. It is suggested that the gate for the sodium channel is located on the exterior surface of the axon, because tetrodotoxin is not lipid soluble.

Saxitoxin and Tetrodotoxin: Comparison of Nerve Blocking Mechanism

Saxitoxin at concentrations of 3 x 10-8 to 3 x 10-7 mole per liter blocks the conduction of lobster giant axon with no change in resting potential. Recovery of washed axons is faster in those that had been treated with saxitoxin than it is in those that were treated with tetrodotoxin. Peak transient increase in nerve membrane conductance is selectively blocked by saxitoxin with no change in late steady-state increase in conductance. The major mechanism of saxitoxin blockage is the same that of tetrodotoxin blockage.

Condylactis Toxin: Interaction with Nerve Membrane Ionic Conductances

A toxin from the Bermuda anemone Condylactis gigantea causes the early transient conductance change of crayfish giant axon membranes to persist without affecting the shape of its turning-on. The increase in late steadystate conductance is either not affected or slightly suppressed. The effect on the conductance components can adequately account for the prolonged action potential observed in the treated axon.