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Charybdotoxin and iberiotoxin but not apamin abolish the slow after-hyperpolarization in myenteric plexus neurons
W A Kunze1, J C Bornstein, J B Furness
1Department of Physiology, University of Melbourne, Parkville, Victoria, Australia.
Abstract:
Myenteric neurons of guinea-pig ileum were studied with intracellular microelectrodes. The specific toxins charybdotoxin, iberiotoxin and apamin were used to characterize the prolonged after-hyperpolarizations of AH neurons in this preparation. Charybdotoxin and iberiotoxin blocked prolonged after-hyperpolarizations in 23 of 24 AH neurons, but apamin had no effect on 5 of 5 AH neurons. Abolition of the after-hyperpolarizations was accompanied by depolarization and increases in input resistances of those AH neurons affected, but the shapes of action potentials were unchanged. The excitability of the AH neurons was enhanced as shown by an increase in the number of action potentials evoked by a 500-ms depolarizing current pulse or by a train of 15-ms depolarizing current pulses (10Hz). The other class of myenteric neurons, S neurons, was also investigated. The 19 S neurons studied fired action potentials only at the start of a 500 ms depolarization, but the toxins had no effect on this behaviour or on their other properties. Intracellular injection of Neurobiotin into the neurons studied and subsequent immunohistochemical staining to localise the calcium-binding protein, calretinin, indicated that all major classes of S neurons were included in the sample. Thus, the prolonged after-hyperpolarizations in AH neurons may be due to opening of a large-conductance (BK) calcium-dependent potassium channel, but similar channels play little or no role in regulation of the excitability of S neurons.
Insights
Charybdotoxin and iberiotoxin block prolonged after-hyperpolarizations in guinea-pig ileum AH neurons, suggesting a role for large-conductance calcium-dependent potassium channels. S neurons were unaffected, indicating distinct channel roles.
Area of Science:
- Neuroscience
- Gastroenterology
- Ion Channel Physiology
Background:
- Myenteric neurons control gastrointestinal function.
- Two main classes, AH and S neurons, exhibit distinct electrophysiological properties.
Purpose of the Study:
- To investigate the ion channels responsible for prolonged after-hyperpolarizations in AH neurons.
- To determine the role of these channels in AH and S neuron excitability.
Main Methods:
- Intracellular microelectrode recordings from guinea-pig ileum myenteric neurons.
- Application of specific potassium channel blockers: charybdotoxin, iberiotoxin, and apamin.
- Neurobiotin injection and immunohistochemistry for calretinin to identify neuron types.
Main Results:
- Charybdotoxin and iberiotoxin abolished prolonged after-hyperpolarizations in 23/24 AH neurons, increasing excitability.
- Apamin had no effect on AH neurons.
- S neurons showed no changes in response to toxins, despite inclusion of all major classes.
Conclusions:
- Prolonged after-hyperpolarizations in AH neurons are likely mediated by large-conductance (BK) calcium-dependent potassium channels.
- BK channels play a minimal role in regulating S neuron excitability.
- Distinct roles for ion channels in different enteric neuron subtypes.