CAMP-actiyated Na⁺ current of molluscan neurons is resistant to kinase inhibitors and is gated by cAMP in the isolated patch

The cAMP-dependent Na⁺ current (l_Na,cAMP) modulates excitability in many molluscan neurons. Rapid activation of l_Na,cAMP by cyclic nucleotide, its ion dependence, and its blockade by divalent cations resemble cyclic nucleotide-activated cation currents in vertebrate photoreceptors and olfactory receptors, where activation has been found to be independent of kinase activity. We tested the phosphorylation dependence of l_Na,cAMP in neurons of the feeding and locomotory networks of the predatory marine snail Pleurobranchaea. Identified neurons of pedal and buccal ganglia were axotomized for recording the l_Na,cAMP response to iontophoretic injection of cAMP under two-electrode voltage clamp. Intracellular injections of specific peptide inhibitor of protein kinase A had no blocking effects on activation of l_Na,cAMP by iontophoretic injection of cAMP. Inward single-channel currents were activated in excised inside-out patches during exposure to cAMP in salines without added ATP. Sodium was the major current carrying ion. Two distinct types of l_Na,cAMP channel activity were observed, where opening probability and open times differed, but conductance was similar, 36.7 pS. These observations suggest that l_Na,cAMP activation occurs by direct binding of cAMP to a regulatory site at the channel, rather than by phosphorylation.