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Comparison of the effects of phorbol dibutyrate and low-frequency stimulation of synaptic inputs on the excitability of myenteric AH neurons

  • Cellular Neurophysiology
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Abstract

Low-frequency stimulation of synaptic inputs to after-hyperpolarising (AH) neurons in the guinea-pig small intestine causes sustained increases in excitability that far outlast the stimulus period. This excitation has been called sustained, slow, post-synaptic excitation (SSPE). Intracellular microelectrodes were used to record the effects of the protein kinase C (PKC) stimulant, phorbol dibutyrate (PDBu), and compare these with changes seen during the SSPE, in AH neurons of the small intestine of the guinea-pig. PDBu (1 nM–1 µM) increased excitability, depolarised the membrane and increased input resistance concentration dependently, mimicking the effects of low-frequency stimulation of pre-synaptic inputs. These changes developed slowly after the start of infusion and were only slowly reversible after wash out. PDBu suppressed a late after-hyperpolarising potential (AHP) that depends on Ca2+ entry via voltage-gated Ca2+ channels during the action potential. The effects of PDBu (10 nM) on the late AHP were indistinguishable from those observed during the SSPE. PDBu, at a concentration that inhibited the AHP, had no effect on the action potential half-width or the slope of its first repolarisation phase (the early phase of repolarisation is slowed by the Ca2+ influx of the action potential). Thus PDBu inhibited K+ channel opening underlying the late AHP, but did not suppress Ca2+ entry during the action potential. The hyperpolarisation-activated cation current (I h) in intrinsic primary afferent neurons (IPANs) was not affected by PDBu. We conclude that PDBu mimics the sustained excitation caused by low-frequency stimulation of synaptic inputs to IPANs by closing IK channels responsible for the AHP or restricting their opening by Ca2+ and by reducing the current carried by K+ channels that are active at rest. IK channels, the opening of which results in the AHP, have consensus sites for PKC and are likely targets for phosphorylation during the SSPE.

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Acknowledgements

This work was supported by Pfizer Pharmaceuticals and by a grant from the National Health and Medical Council of Australia.

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Authors and Affiliations

  1. Dept Anatomy and Cell Biology and Centre for Neuroscience, University of Melbourne, VIC 3010, Parkville, Australia

    Mitsuhisa Kawai, Trung V. Nguyen, Martin J. Stebbing & John B. Furness

  2. Discovery Biology Research Group, Pfizer Global Research and Development, Nagoya Laboratories, Pfizer Inc., Aichi, Japan

    Mitsuhisa Kawai

  3. Lab ITIS, UMR CNRS-Univ Mediterranée, Marseille, France

    Nadine Clerc

  4. United Graduate School of Veterinary Science, Gifu University, Gifu, Japan

    Mitsuhisa Kawai & Seiichi Komori

Authors
  1. Mitsuhisa Kawai
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  2. Trung V. Nguyen
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  3. Martin J. Stebbing
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  4. Nadine Clerc
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  5. Seiichi Komori
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  6. John B. Furness
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Correspondence to John B. Furness.

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Kawai, M., Nguyen, T.V., Stebbing, M.J. et al. Comparison of the effects of phorbol dibutyrate and low-frequency stimulation of synaptic inputs on the excitability of myenteric AH neurons. Pflugers Arch - Eur J Physiol 447, 298–304 (2003). https://doi.org/10.1007/s00424-003-1179-8

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  • DOI: https://doi.org/10.1007/s00424-003-1179-8

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