Abstract
The cAMP-dependent Na+ current (INa,cAMP) modulates excitability in many molluscan neurons. Rapid activation of INa,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 INa,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 INa,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 INa,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 INa,cAMP channel activity were observed, where opening probability and open times differed, but conductance was similar, 36.7 pS. These observations suggest that INa,cAMP activation occurs by direct binding of cAMP to a regulatory site at the channel, rather than by phosphorylation.
