Comparison of the effects of phorbol dibutyrate and low-frequency stimulation of synaptic inputs on the excitability of myenteric AH neurons.
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 microM) 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 (Ih) 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.
Kawai
Mitsuhisa
M
Dept Anatomy and Cell Biology and Centre for Neuroscience, University of Melbourne, VIC 3010, Parkville, Australia.
Nguyen
Trung V
TV
Stebbing
Martin J
MJ
Clerc
Nadine
N
Komori
Seiichi
S
Furness
John B
JB
D000200:Action Potentials / Q000187:drug effects* / Q000502:physiology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation / Q000379:methods; D006168:Guinea Pigs; D009197:Myenteric Plexus / Q000187:drug effects* / Q000502:physiology; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology*; D013569:Synapses / Q000502:physiology*
D015240:Phorbol 12,13-Dibutyrate