Serotonin-induced nitric oxide production in the ventral nerve cord of the earthworm, Eisenia fetida

Effect of serotonin on nitric oxide (NO) production in the ventral nerve cord (VNC) of the earthworm Eisenia fetida was investigated by a bio-imaging and an electrochemical technique. In the bio-imaging, the spatial pattern of NO production in VNC was visualized using an NO-specific fluorescent dye, diaminofluorescein-2 diacethyl (DAF-2 DA). Application of serotonin (100 μM) increased NO production in VNC by about 65% (P<0.05), compared with basal NO production. The increase was mainly from the nitergic neurons in the ventral side of VNC. In the electrochemical technique, real-time basal and serotonin-induced NO production was estimated with an NO-specific electrode. On the ventral surface of VNC, the estimated basal NO production was stable at 200±52 nM, and was transiently augmented to 840±193 nM by the addition of 10 μM serotonin. In conclusion, the estimated basal NO production in the earthworm VNC is relatively high compared with other nervous systems earlier reported, and transiently augmented by serotonin. Our results suggest that NO signaling in VNC is involved in neuromodulation by serotonin.