PKA-mediated inhibition of a novel K+ channel underlies the slow after-hyperpolarization in enteric AH neurons.

Postspike after-hyperpolarizations (AHPs) control the excitability of neurons and are important in shaping firing patterns. The duration of some of these events extends to tens of seconds and they can render neurons inexcitable for much of their time course. While consensus is strong that the medium duration (< 1 s AHPs are mediated by the opening of small conductance Ca2+-activated K+ channels, the K+ channels mediating slow AHPs (> 5 s in a subset of enteric (AH) neurons) have an intermediate unit conductance (IKCa). Using whole-cell and excised-patch recording, we have demonstrated that the cAMP-protein kinase A (PKA) pathway regulates the activity of these channels. In whole-cell mode, forskolin (0.003-1 microM) inhibited the current underlying the slow AHP (IsAHP) by 90 %, and this was partially sensitive to inhibition of PKA with internal Rp-cAMPS (500 microM). Rp-cAMPS alone increased the current following break-in and caused a 20 mV hyperpolarization, suggesting that PKA maintains slow AHP channels in the closed state. Internal perfusion of the inhibitory peptide PKI5-24 slightly increased the IsAHP and opposed the inhibitory action of forskolin. Internal perfusion of the catalytic subunit of PKA (PKAcat) suppressed the IsAHP by 50 % without affecting membrane potential or action potential configuration. In inside-out patches containing IKCa-like channels, PKAcat decreased the open probability of IKCa-like channels while alkaline phosphatase activated them. These results suggest that the IKCa-like channels that underlie the slow AHP in myenteric AH neurons are subject to inhibition by PKA-dependent phosphorylation and that PKA plays an integral role in their gating.