Evidence for protein kinase involvement in long-term postsynaptic excitation of intrinsic primary afferent neurons in the intestine

We have investigated the effects of protein kinase inhibitors on the sustained slow postsynaptic excitation (SSPE) that is evoked by prolonged stimulation of synaptic inputs to intrinsic primary afferent neurons (IPANs) in the small intestines of guinea pigs. Stimulation of synaptic inputs to the IPANs caused depolarisation, increased input resistance, and increased excitation that continued after the cessation of stimulation. The excitation was substantially reduced by the broad-spectrum kinase inhibitor staurosporine (1 μM), PKC inhibitors Ro 31-8220 (3.3 μM) and calphostin C (1 μM), but not by the PKA inhibitor H89 (1 μM). At a higher concentration, 10 μM Ro 31-8220 reduced the excitability of axons to electrical stimulation. Phorbol dibutyrate (1 μM) caused excitability increases, membrane depolarisation, and increased input resistance that mimicked the SSPE. We conclude that the generation of the SSPE requires a phosphorylation step that is mediated by protein kinase C.