| Summary: | Imidacloprid is a neonicotinoid pesticide used in large-scale agricultural systems, home gardens, and veterinary pharmaceuticals. Imidacloprid is a small molecule that is more water-soluble than other insecticides, increasing the likelihood of large-scale environmental accumulation and chronic exposure of non-targeted species. Imidacloprid can be converted to the bioactive metabolite desnitro-imidacloprid in the environment and body. Little is known about the mechanisms by which imidacloprid and desnitro-imidacloprid induce ovarian toxicity. Thus, we tested the hypothesis that imidacloprid and desnitro-imidacloprid differentially affect antral follicle growth and steroidogenesis in vitro. Antral follicles were dissected from the ovaries of CD-1 mice and cultured in media containing vehicle control or 0.2 µg/mL–200 µg/mL of imidacloprid or desnitro-imidacloprid for 96 h. Follicle morphology was monitored, and follicle size was measured every 24 h. At the end of the culture periods, media were used to quantify follicular hormone levels, and follicles were used for gene expression analysis of steroidogenic regulators, hormone receptors, and apoptotic factors. Imidacloprid did not affect follicle growth or morphology compared to the control. Desnitro-imidacloprid inhibited follicle growth and caused follicles to rupture in culture compared to the control. Imidacloprid increased progesterone, whereas desnitro-imidacloprid decreased testosterone and progesterone compared to the control. Desnitro-imidacloprid also changed estradiol compared to the control. At 48 h, IMI decreased the expression of <i>Star</i>, <i>Cyp17a1</i>, <i>Hsd17b1</i>, <i>Cyp19a1</i>, and <i>Esr2</i> and increased the expression of <i>Cyp11a1</i>, <i>Cyp19a1</i>, <i>Bax</i>, and <i>Bcl2</i> compared to the control. IMI also changed the expression of <i>Esr1</i> compared to the control. At 48 h, DNI decreased the expression of <i>Cyp11a1</i>, <i>Cyp17a1</i>, <i>Hsd3b1</i>, <i>Cyp19a1</i>, and <i>Esr1</i> and increased the expression of <i>Cyp11a1</i>, <i>Hsd3b1</i>, and <i>Bax</i> compared to the control. At 72 h of culture, IMI significantly decreased the expression of <i>Cyp19a1</i> and increased the expression of <i>Star</i> and <i>Hsd17b1</i> compared to the control. At 72 h, DNI significantly decreased the expression of <i>Cyp11a1</i>, <i>Cyp17a1</i>, <i>Hsd3b1</i>, and <i>Bax</i> and increased the expression of <i>Esr1</i> and <i>Esr2</i>. At 96 h, IMI decreased the expression of <i>Hsd3b1</i>, <i>Cyp19a1</i>, <i>Esr1</i>, <i>Bax</i>, and <i>Bcl2</i> compared to the control. At 96 h, DNI decreased the expression of <i>Cyp17a1</i>, <i>Bax</i>, and <i>Bcl2</i> and increased the expression of <i>Cyp11a1</i>, <i>Hsd3b1</i>, and <i>Bax</i> compared to the control. Together, these data suggest mouse antral follicles are targets of neonicotinoid toxicity, and the mechanisms of toxicity differ between parent compounds and metabolites.
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