| Summary: | Prostaglandins are a group of important cell-signaling molecules involved in the regulation of ovarian maturation, oocyte development, egg laying and associated behaviors in invertebrates. However, the presence of prostaglandin E<sub>2</sub> (PGE<sub>2</sub>), the key enzymes for PGE<sub>2</sub> biosynthesis and its interference by drugs were not investigated previously in the ovary of ticks. The present study was undertaken to assess the modulation of the PGE<sub>2</sub>-mediated pathway in the eclosion blocking effect of flumethrin and terpenoid subfraction isolated from <i>Artemisia nilagirica</i> in <i>Rhipicephalus annulatus</i> ticks. The acaricidal activities and chemical profiling of the terpenoid subfraction were performed. The localization of the cyclooxygenase1 (COX1) and prostaglandin E synthase (PGES) enzymes and the quantification of PGE<sub>2</sub> in the ovaries of the ticks treated with methanol (control), flumethrin and terpenoid subfraction were also undertaken. In addition, the vitellogenin concentration in hemolymph was also assayed. Both flumethrin and the terpenoid subfraction of <i>A. nilagirica</i> elicited a concentration-dependent inhibition of fecundity and blocking of hatching of the eggs. The COX1 could not be detected in the ovaries of treated and control ticks, while there was no significant difference observed in the concentration of vitellogenin (Vg) in them. The presence of PGES in the oocytes of control ticks was confirmed while the immunoreactivities against PGES were absent in the vitellogenic oocytes of ticks treated with flumethrin and terpenoid subfraction. The levels of PGE<sub>2</sub> were below the detection limit in the ovaries of the flumethrin-treated ticks, while it was significantly lower in the ovaries of the terpenoid subfraction-treated ticks. Hence, the prostaglandin E synthase and PGE<sub>2</sub> were identified as very important mediators for the signaling pathway for ovarian maturation and oviposition in ticks. In addition, the key enzyme for prostaglandin biosynthesis, PGES and the receptors for PGE<sub>2</sub> can be exploited as potential drug targets for tick control. The detection of PGES by immunohistochemistry and quantification of PGE<sub>2</sub> by LC-MSMS can be employed as valuable tools for screening newer compounds for their eclosion blocking acaricidal effects.
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