Mechanism of stimulation of cyclic‐GMP level in a neuronal cell line mediated by serotonin (5‐HT 3 ) receptors

The mechanism by which serotonin (5‐HT 3 ) receptors mediate a rise in cyclic‐GMP level was investigated in a neuronal cell line. Inhibitors of phospholipase A 2 (mepacrine) and of lipoxygenase (eicosatetraynoic acid or nordihydroguaiaretic acid) suppressed the action of serotonin. On the other hand, inhibition by hemoglobin indicates a role for nitric oxide which could be in part responsible for the cyclic‐GMP effect as an intercellular stimulant. The suppression of the serotonin effect by the arginine analogues N ω ‐methyl‐L‐arginine and canavanine is consistent with the notion that nitric oxide could be released from arginine. The serotonin‐induced rise of cyclic‐GMP level depends on the presence of extracellular Ca 2+ with half‐maximal stimulation at 0.3 mM Ca 2+ . The serotonin‐stimulated rise of cyclic GMP was inhibited by (a) addition of inorganic blockers of Ca 2+ ‐permeable channels (La 3+ , half‐maximal inhibitory concentration (IC 50 ) 0.04 mM; Mn 2+ , IC 50 , 0.4 mM; Co 2+ , IC 50 , 0.9 mM; Ni 2+ , IC 50 , 1.2 mM) and (b) of organic blockers (diltiazem: IC 50 , 6 μM, methoxyverapamil: IC 50 , 3 μM and (c) intracellular application of the Ca 2+ chelator bis‐( O ‐aminophenoxy)‐ethane‐ N,N,N′,N′ ‐tetraacetic acid (IC 50 , 2 μM). Thus, two pathways for the activation of soluble guanylate cyclase by serotonin are possible: (a) via lipoxygenase products of arachidonic acid and/or (b) via nitric oxide or a related nitroso compound. Serotonin mediates a rise of cytosolic Ca 2+ activity due to entry of extracellular Ca 2+ . It still has to be investigated which step depends on a rise of cytosolic Ca 2+ activity that appears to be a prerequisite for activation of guanylate cyclase.