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

Abstract

The cAMP-dependent Na+ current (lNa,cAMP) modulates excitability in many molluscan neurons. Rapid activation of lNa,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 lNa,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 lNa,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 lNa,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 lNa,cAMP channel activity were observed, where opening probability and open times differed, but conductance was similar, 36.7 pS. These observations suggest that lNa,cAMP activation occurs by direct binding of cAMP to a regulatory site at the channel, rather than by phosphorylation. Copyright © 1993 Society for Neuroscience.