METHIOTHEPIN MALEATE

Serotonin (5-hydroxytryptamine) is an essential neurotransmitter involved in regulating various behaviors in plant-parasitic nematodes, including locomotion, egg laying, feeding, and mating. However, the functional role of serotonin in root-knot nematode invasion of host plants and the molecular mechanisms underlying feeding behavior remain poorly understood. In this study, we tested the effects of exogenous serotonin and the pharmacological compounds fluoxetine and methiothepin on the feeding behaviors of Meloidogyne graminicola. Our results suggested that M. graminicola possesses an endogenous serotonin signaling pathway and that serotonin plays a crucial role in modulating feeding behaviors in M. graminicola second-stage juveniles. We also identified and cloned the serotonin synthesis enzyme tryptophan hydroxylase ( Mg-tph-1) in M. graminicola and investigated the role of endogenous serotonin by generating RNA interference nematodes in Mg-tph-1. Silencing Mg-tph-1 substantially reduced nematode invasion, development, and reproduction. According to the immunostaining results, we speculated that these serotonin immunoreactive cells near the nerve ring in M. graminicola are likely homologous to Caenorhabditis elegans ADFs, NSMs, and RIH serotonergic neurons. Furthermore, we investigated the impact of phytoserotonin on nematode invasion and development in rice by overexpressing OsTDC-3 or supplementing rice plants with tryptamine and found that an increase in phytoserotonin increases nematode pathogenicity. Overall, our study provides insights into the essential role of serotonin in M. graminicola host plant parasitism and proposes that the serotonergic signaling pathway could be a potential target for controlling plant-parasitic nematodes.

In the present study, we found that methiothepin (a nonselective 5‐hydroxytryptamine [5‐HT] receptor antagonist) inhibited antigen‐induced degranulation in rat basophilic leukemia cells and mouse bone marrow‐derived mast cells. Although antigen stimulation induces release of histamine and serotonin (5‐HT) by exocytosis and mast cells express several types of 5‐HT receptor, the detailed role of these receptors remains unclear. Here, pretreatment of cells with methiothepin attenuated increased intracellular Ca2+ concentration, phosphorylated critical upstream signaling components (Src family tyrosine kinases, Syk, and PLCγ1), and suppressed TNF‐α secretion via inhibition of Akt (a Ser/Thr kinase activated by PI3K)and ERK phosphorylation. Furthermore, it inhibited PMA/ionomycin‐induced degranulation; this finding suggested that methiothepin affected downstream signaling. IκB kinase β phosphorylates synaptosomal associated protein 23, which regulates the fusion events of the secretory granule/plasma membrane after mast cell activation, resulting in degranulation. We showed that methiothepin blocked PMA/ionomycin‐induced phosphorylation of synaptosomal associated protein 23 by inhibiting its interaction with IκB kinase β. Together with the results of selective 5‐HT antagonists, it is suggested that methiothepin inhibits mast cell degranulation by downregulating upstream signaling pathways and exocytotic fusion machinery through mainly 5‐HT1A receptor. Our findings provide that 5‐HT antagonists may be used to relieve allergic reactions.