Effects of serotonin and cAMP on calcium currents in different neurones of Helix pomatia.

Effects of application of serotonin (5-HT) and intracellular administration of cyclic adenosine monophosphate (cAMP) on voltage-gated calcium current (ICa) were studied in isolated, intracellularly perfused Helix pomatia neurones. Two types of the effects of 5-HT (1-10 microM) were observed in different neurones: reversible inhibition (by about 20%) or reversible potentiation (up to 50%) of the current amplitude. Some cells did not respond to 5-HT application. In cells with the potentiating effect of 5-HT, ICa could also be increased by intracellular introduction of cAMP (100 microM), but not the guanosine analogue, cGMP (50-100 microM). These effects were not additive and could be potentiated by theophylline (5 mM) and 3-isobutyl-1-methylxanthine (IBMX, 100-500 microM); they could be mimicked by forskolin (10-50 microM) and abolished by tolbutamide (1-5 mM) or protein kinase inhibitor (500 micrograms/ml), indicating that cAMP-dependent phosphorylation mediates the potentiating action of 5-HT on ICa. In neurones showing inhibitory effect of 5-HT, neither cAMP nor forskolin increased ICa. Methiothepin (10-50 microM), a 5-HT1,2 receptor antagonist, irreversibly inhibited the potentiating effect of 5-HT, while antagonists of 5-HT2 receptors cyproheptadine (10-50 microM) or ketanserine (10-60 microM) and of 5-HT3 receptors ISC 205-930 (10-50 microM) or cocaine (5-25 microM) had no effect on ICa and its enhancement by 5-HT. It is suggested that in certain snail neurones the possibility of cAMP-dependent up-regulation of ICa correlates with the presence of 5-HT1-like receptors in the neuronal membrane.