Related Experiment Videos
Potassium channel blocking actions of beta-bungarotoxin and related toxins on mouse and frog motor nerve terminals
1Department of Physiology and Pharmacology, University of Strathclyde, Glasgow.
Abstract:
1. beta-Bungarotoxin and other snake toxins with phospholipase activity augment acetylcholine release evoked from mouse motor nerve terminals before they produce blockade. This action of the toxins is independent of their phospholipase A2 activity, but the underlying mechanism for the facilitation of release is unclear. To determine whether the toxins affect ionic currents at motor nerve terminals, extracellular recordings were made from perineural sheaths of motor nerves innervating mouse triangularis sterni muscles. 2. Perineural waveforms had a characteristic shape, with two major negative deflections, the first being associated with nodal Na+ currents and the second with terminal K+ currents. Block of the K+ currents revealed a Ca2+-dependent component. 3. During the facilitatory phase of its action, beta-bungarotoxin (150 nM) reduced the second negative component of the perineural waveform by 30-50%. 4. The reduction could be a consequence of a decreased K+ ion contribution or of an increase in the current carried by Ca2+. As beta-bungarotoxin had similar effects in solutions which contained no added Ca2+, it is unlikely to be acting on the Ca2+ current. Also, it is unlikely to be blocking the Ca2+-activated K+ current, which is suppressed in zero Ca2+ conditions. 5. Other prejunctionally active snake toxins (taipoxin, notexin and crotoxin) had similar effects to those of beta-bungarotoxin, but a similar basic phospholipase of low toxicity from cobra venom had no effect. 6. Thus, beta-bungarotoxin and related toxins block a fraction of the K+ current in the motor nerve terminals of mouse preparations. Such an effect could explain the facilitation of acetylcholine release caused by these toxins before the onset of presynaptic blockade. 7. In frog cutaneous pectoris preparations, f-bungarotoxin reduced endplate potential amplitude but had little effect on perineural waveforms. Therefore, the consequences of toxin binding must be different in frog terminals.
Insights
Snake toxins like beta-bungarotoxin enhance acetylcholine release by blocking potassium (K+) currents at motor nerve terminals, independent of their phospholipase A2 activity. This action precedes presynaptic blockade, offering a mechanism for initial neurotransmitter augmentation.
Area of Science:
- Neuroscience
- Toxicology
- Molecular Biology
Background:
- Snake toxins, including beta-bungarotoxin, are known to affect neurotransmission at motor nerve terminals.
- These toxins initially augment acetylcholine release before causing blockade, but the mechanism for this facilitation is not fully understood.
- The role of phospholipase A2 activity in this facilitatory effect has been questioned.
Purpose of the Study:
- To investigate the effect of beta-bungarotoxin and related snake toxins on ionic currents at motor nerve terminals.
- To determine the mechanism by which these toxins facilitate acetylcholine release.
- To explore whether the observed effects are dependent on phospholipase activity.
Main Methods:
- Extracellular recordings from perineural sheaths of motor nerves in mouse triangularis sterni muscles.
- Analysis of perineural waveforms to identify contributions of Na+ and K+ currents.
- Testing the effects of beta-bungarotoxin in varying calcium (Ca2+) concentrations and comparing with other snake toxins and a non-toxic phospholipase.
Main Results:
- Beta-bungarotoxin (150 nM) reduced the second negative deflection in perineural waveforms, indicative of terminal K+ currents, by 30-50%.
- This reduction was observed even in low Ca2+ solutions, suggesting it was not due to altered Ca2+ currents or Ca2+-activated K+ currents.
- Other potent snake toxins (taipoxin, notexin, crotoxin) showed similar effects, while a basic phospholipase from cobra venom did not.
- F-bungarotoxin had different effects in frog preparations, reducing endplate potential amplitude but not significantly altering perineural waveforms.
Conclusions:
- Beta-bungarotoxin and related snake toxins inhibit a fraction of K+ currents in mouse motor nerve terminals.
- This blockade of K+ currents likely explains the initial facilitation of acetylcholine release observed before presynaptic blockade.
- The mechanism of action differs between mouse and frog nerve terminals, indicating species-specific effects of these toxins.