Properties of slow early potassium current in neurons of snail Helix pomatia

Effects of weak environmental magnetic fields on the spontaneous bioelectrical activity of snail neurons

We examined the effects of 50-Hz magnetic fields in the range of flux densities relevant to our current environmental exposures on action potential (AP), after-hyperpolarization potential (AHP) and neuronal excitability in neurons of land snails, Helix aspersa. It was shown that when the neurons were exposed to magnetic field at the various flux densities, marked changes in neuronal excitability, AP firing frequency and AHP amplitude were seen. These effects seemed to be related to the intensity, type (single and continuous or repeated and cumulative) and length of exposure (18 or 20 min). The extremely low-frequency (ELF) magnetic field exposures affect the excitability of F1 neuronal cells in a nonmonotonic manner, disrupting their normal characteristic and synchronized firing patterns by interfering with the cell membrane electrophysiological properties. Our results could explain one of the mechanisms and sites of action of ELF magnetic fields. A possible explanation of the inhibitory effects of magnetic fields could be a decrease in Ca(2+) influx through inhibition of voltage-gated Ca(2+) channels. The detailed mechanism of effect, however, needs to be further studied under voltage-clamp conditions.

The fruit essential oil of Pimpinella anisum L. (Umblliferae) induces neuronal hyperexcitability in snail partly through attenuation of after-hyperpolarization

AIM OF THE STUDY

Many biological actions of Pimpinella anisum L. (Ainse), including antiepileptic activity have been demonstrated; however, there is no data concerning its precise cellular mechanisms of action. We determined whether the fruit essential oil of anise affect the bioelectrical activity of snail neurons in control condition or after pentylenetetrazol (PTZ) induced epileptic activity.

MATERIALS AND METHODS

Intracellular recordings were made under the current clamp condition and the effects of anise oil (0.01% or 0.05%) alone or in combination with PTZ were assessed on the firing pattern, action potential configuration and postspike potentials.

RESULTS

Anise oil changed the firing pattern from regular tonic discharge to irregular and then to bursting in intact cells or resulted in the robustness of the burst firing and the steepness of the paroxysmal shift induced by PTZ treatment. It also significantly increased the firing frequency and decreased both the after-hyperpolarization potential (AHP) following single action potential and the post-pulse AHP.

CONCLUSIONS

Likely candidate cellular mechanisms underlying the hyperexcitability produced by anise oil include enhancement of Ca(2+) channels activity or inhibition of voltage and/or Ca(2+) dependent K(+) channels activity underlying AHPs. These finding indicates that a certain caution is needed when Pimpinella anisum is used for treating patients suffering from epilepsy.

The functional consequences of paraoxon exposure in central neurones of land snail, Caucasotachea atrolabiata, are partly mediated through modulation of Ca2+ and Ca2+-activated K+-channels

Toxicity of paraoxon has been attributed to inhibition of cholinesterase, but little is known about its direct action on ionic channels. The effects of paraoxon (0.3 microM-0.6 microM) were studied on the firing behaviour of snail neurones. Paraoxon significantly increased the frequency of spontaneously generated action potentials, shortened the afterhyperpolarization (AHP) and decreased the precision of firing. Short periods of high frequency-evoked trains of action potentials led to an accumulation in the depth and duration of post-train AHPs that was evidenced as an increase in time to resumption of autonomous activity. The delay time in autonomous activity initiation was linearly related to the frequency of spikes in the preceding train and the slope of the curve significantly decreased by paraoxon. The paraoxon induced hyperexcitability and its depressant effect on the AHP and the post-train AHP were not blocked by atropine and hexamethonium. Calcium spikes were elicited in a Na+ free Ringer containing voltage dependent potassium channel blockers. Paraoxon significantly decreased the duration of calcium spikes and following AHP and increased the frequency of spikes. These findings suggest that a reduction in calcium influx during action potential may decrease the activation of calcium dependent potassium channels that participate in AHP generation and act as a mechanism of paraoxon induced hyperexcitability.

Mechanism of 4-aminopyridine block of the transient outward K-current in identified Helix neuron

The block of the transient outward K-current, I(K(A)) by 4-aminopyridine (4-AP) and blood-depressing substances (BDS) was investigated in identified Helix pomatia neurons (LPa3) using the two microelectrode voltage-clamp technique. The present study shows that 4-AP inhibits I(K(A)) in snail neurons in a voltage- and concentration-dependent manner. The 4-AP block of I(K(A)) involves the block of both open and closed states of the channel, however binding to open channels is preferred. It is suggested that 4-AP have two binding sites on the identified Helix neuron. One site causes an open channel block, which affects the N-type inactivation, and binding to the second site induces closed channel block, which affects C-type inactivation. In control solution the inactivating phase of the current is biexponential, suggesting simultaneous presence of two types of inactivation. The counterplay of these mechanisms results in the crossover of the current traces recorded from control and 4-AP blocked channels. It is assumed that use-dependence does not occur through blocker 'trapping', but rather by a different mechanism. BDS had no effect on Helix I(K(A)), suggesting that transient potassium channels in LPa3 neuron are not Kv3.4 type channels.