Serotonin Induces a Cyclic AMP-Mediated Outwardly Rectifying Slow K+ Current in a Single Identified Snail Neuron.

In the F2 neuron of the parietal ganglion of the snail Helix aspersa either bath or iontophoretic application of serotonin (5-HT) induces an outward current. This current has a long latency (10 - 60 s) and a slow time course, a 500 ms iontophoretic application of 5-HT evoking a response lasting 3 - 5 min. This slow outward current reverses at -80 mV, a value equal to EK. After doubling the extracellular K+ concentration the reversal potential of the 5-HT response is shifted by 19 mV, as predicted by the Nernst equation. The I-V curves reveal that the 5-HT-induced slow outward current is outwardly rectifying. This 5-HT response is blocked by intracellular Cs+ and tetraethylammonium (TEA+) and by extracellular TEA+ and Ba2+, but is not affected by the removal of extracellular Ca2+ or the intracellular injection of ethyleneglycol-bis-(beta-amino-ethylether)-N,N,N',N'-tetra-acetic acid (EGTA). These results indicate that the outwardly rectifying slow outward current induced by 5-HT in the F2 neuron is carried by K+ and is Ca2+-independent. In the single isolated F2 neuron, 5-HT induces a 2.5-fold stimulation of the adenylate cyclase activity. In addition, both the intracellular injection of 3',5'-adenosine monophosphate (cAMP) and the application of forskolin mimic the effect of 5-HT on the F2 neuron. The phosphodiesterase inhibitor isobutylmethylxanthine also induces a slow outward current and potentiates the 5-HT response. The intracellular injection of a synthetic 20-residue peptide inhibitor of the cAMP-dependent protein kinase blocks the slow outward K+ current induced by 5-HT. These results show that in the F2 neuron, 5-HT elicits a slow K+ current via the stimulation of adenylate cyclase, an increase in intracellular cAMP and the activation of the cAMP-dependent kinase which probably phosphorylates a population of outwardly rectifying K+ channels or some protein/s associated with these channels.