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 muM) 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.