Cyclic AMP-stimulated sodium current in identified feeding neurons of Lymnaea stagnalis.

Iontophoretic injection of cAMP elicits a slow, transient inward current in identified buccal feeding motoneurons and in the giant cerebral interneuron of the snail, Lymnaea stagnalis. The current is voltage independent, and is abolished in the absence of extracellular Na+. Application of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) causes a marked increase in both amplitude and duration of cAMP-stimulated inward current. The amplitude of the current is reduced following prolonged application of depolarizing pulses to the cell. However, generation of high-frequency bursts of action potentials lasting up to 20 s has no significant effect on the amplitude of the cAMP-induced current measured subsequently. Bath application of the cAMP analogue 8-chlorophenylthio-cAMP or of IBMX leads to enhanced bursting activity in buccal motoneurons. It is suggested that cAMP sensitivity in feeding motoneurons provides a mechanism for adjusting the cells' responsiveness to rhythmic synaptic inputs during the generation of feeding motor output.