The effect of veratridine on Helix pomatia neurones

Effects of veratridine on sodium currents and fluxes

Veratridine causes Na+ channels to stay open during a sustained membrane depolarization by abolishing inactivation. The consequential Na+ influx, either by itself or by causing a maintained depolarization, leads to many secondary effects such as increasing pump activity, Ca2+ influx, and in turn exocytosis. If the membrane is voltage clamped in the presence of the alkaloid, a lasting depolarizing impulse induces, following the "normal" transient current, another much more slowly developing Na+ current that reaches a constant level after a few seconds. Repolarization then is followed by an inward tail current that slowly subsides. Development of these slow currents is enhanced by additional treatment with agents that inhibit inactivation. Most of these phenomena can be satisfactorily explained by assuming that Na+ channels must open before veratridine binds to them, and that the slow current changes reflect the kinetics of binding and unbinding. It is unclear, however, where the alkaloid stays when it is not bound. Although the effect sets in promptly, once this pool is filled, access to it from outside must be impeded since in most preparations veratridine can only partially be washed out. Cooling acts as if the available concentration is reduced, but this reversible "reduction" takes much longer to develop than the cold-induced changes in kinetics. Several authors assume that the binding site, site 2, is accessed from the lipid phase of the membrane. Considerations of this kind are often based on experiments with batrachotoxin, the widely used site-2 ligand which has a much higher affinity and acts as a full agonist in contrast to the partial agonist veratridine. Batrachotoxin thus lends itself to binding studies using radiolabeled derivatives. Such experiments may eventually lead to the characterization of neurotoxin site 2; the first promising steps have been taken. Modern techniques of molecular biology will almost certainly be successful, and one hopes for point-mutated channels with distinctly different reactions also to veratridine. A considerable amount of research is still required to clarify the structural basis for the numerous allosteric interactions with other sites, the mechanism of the very large potential shift of activation, the reduced single-channel conductance and selectivity, and the chemical nature of the different affinities of the site-2 toxins. Note Added in Proof. A report on point mutations with effects on neurotoxin site 2 (see Sect. 8) has just appeared: Wang S-Y, Wang GK (1988) Point mutations in segment I-S6 render voltage-gated Na+ channels resistant to batrachotoxin. Proc Natl Acad USA 95:2653-2658. In microliter muscle Na+ channels expressed in mammalian cells, mutation Asn434Lys leads to complete, Asn434Ala to partial insensitivity to 5 mM batrachotoxin. (Asn434 corresponds to Asn419 of Trainer et al. 1996). The mutant channel displays almost normal current kinetics and in the presence of veratridine little, if any, slow tail current. However, veratridine inhibits peak Na+ currents in the mutant which may point to a complex structure of site 2.

Rapid and slow gating of veratridine-modified sodium channels in frog myelinated nerve

The properties of voltage-dependent Na channels modified by veratridine (VTD) were studied in voltage-clamped nodes of Ranvier of the frog Rana pipiens. Two modes of gating of VTD-modified channels are described. The first, occurring on a time scale of milliseconds, is shown to be the transition of channels between a modified resting state and a modified open state. There are important qualitative and quantitative differences of this gating process in nerve compared with that in muscle (Leibowitz et al., 1986). A second gating process occurring on a time scale of seconds, was originally described as a modified activation process (Ulbricht, 1969). This process is further analyzed here, and a model is presented in which the slow process represents the gating of VTD-modified channels between open and inactivated states. An expanded model is a step in the direction of unifying the known rapid and slow physiologic processes of Na channels modified by VTD and related alkaloid neurotoxins.

Kinetics of veratridine action on Na channels of skeletal muscle

Veratridine bath-applied to frog muscle makes inactivation of INa incomplete during a depolarizing voltage-clamp pulse and leads to a persistent veratridine-induced Na tail current. During repetitive depolarizations, the size of successive tail currents grows to a plateau and then gradually decreases. When pulsing is stopped, the tail current declines to zero with a time constant of approximately 3 s. Higher rates of stimulation result in a faster build-up of the tail current and a larger maximum value. I propose that veratridine binds only to open channels and, when bound, prevents normal fast inactivation and rapid shutting of the channel on return to rest. Veratridine-modified channels are also subject to a "slow" inactivation during long depolarizations or extended pulse trains. At rest, veratridine unbinds with a time constant of approximately 3 s. Three tests confirm these hypotheses: (a) the time course of the development of veratridine-induced tail currents parallels a running time integral of gNa during the pulse; (b) inactivating prepulses reduce the ability to evoke tails, and the voltage dependence of this reduction parallels the voltage dependence of h infinity; (c) chloramine-T, N-bromoacetamide, and scorpion toxin, agents that decrease inactivation in Na channels, each greatly enhance the tail currents and alter the time course of the appearance of the tails as predicted by the hypothesis. Veratridine-modified channels shut during hyperpolarizations from -90 mV and reopen on repolarization to -90 mV, a process that resembles normal activation gating. Veratridine appears to bind more rapidly during larger depolarizations.

The effect of pentylenetetrazol on inward currents of non-bursting neurons and its role in plateau formation

The epileptogenic drug, pentylenetetrazol (PTZ) produces paroxysmal depolarization shifts in molluscan neurons that are similar to PDSs seen at a mammalian epileptic focus. Most research on molluscan neurons indicates that PTZ acts by altering ionic somatic conductances. This study was carried out to investigate the effect of PTZ on inward currents in isolated neurons of the pond snail, Lymnaea stagnalis, and to investigate how these altered currents might lead to the production of PDSs. In concentrations from 10 to 60 mM, PTZ decreased maximum inward current conductance and shifted the inactivation and activation curves to the left with the former shift being consistently greater. There was no change in reversal potential or time constants for activation and inactivation of inward currents. The effects of the PTZ-induced alterations in the inward currents were studied by incorporating them along with alterations of outward currents seen in this and other studies in a computer model for molluscan neuronal firing. The composite model reproduced in large part the intermediate changes in electrical activity seen before the development of the PDS as well as the PDS.

Annotation. Action of pyrethroid insecticides on the vertebrate nervous system

The neurotoxic action of the synthetic pyrethroid insecticides has received much interest in recent years, as the number of available pyrethroids and their practical applications have greatly increased. Although the majority of pyrethroids have a low oral toxicity to mammals, they may cause severe neurotoxic symptoms whenever they reach the nervous system in sufficient amount. The principal effect of pyrethroids in the vertebrate nervous system is to induce repetitive activity, particularly in the sensory nervous system. This repetitive activity originates from a prolongation of the transient increase in sodium permeability of the nerve membrane associated with excitation. Available evidence indicates that pyrethroids primarily interfere with the sodium channels in the nerve membrane. All active pyrethroids affect sodium channel gating in a similar manner, but marked differences in neurotoxic activity exist between the various pyrethroids, especially between alpha-cyano and non-cyano compounds.