A repeated 28-day oral dose toxicity study of methoxychlor in rats, based on the 'Enhanced OECD Test Guideline 407' for screening endocrine-disrupting chemicals

Abstract

In association with the international validation project to establish an OECD Enhanced Test Guideline 407, we performed a 28-day repeated-dose toxicity study of methoxychlor, a chlorinated hydrocarbon pesticide with pro-estrogenic and anti-androgenic activities. Attention was paid to the sensitivity of certain additional parameters for detecting endocrine related effects of endocrine disrupting chemicals based on the existing TG 407. Seven-week-old Crj:CD(SD)IGS rats were allocated to one of four groups, each consisting often males and ten females, and methoxychlor was administered once daily by gavage at doses of 0 (control), 20, 100 or 500 mg/kg body weight per day. Male rats were killed on the day after the 28th administration. Female rats were killed on the day of the diestrus stage during 4 days after the 28th administration. Male rats receiving methoxychlor showed mainly atrophy of mammary acinus in the 20 mg/ kg and higher groups, together with decreases in prostate and seminal vesicle weights, and atrophy of epididymis, prostate, seminal vesicle and coagulating gland in the 100 and 500 mg/kg groups. In addition, decrease in serum testosterone level, increase in follicle-stimulating hormone level, decrease in testis and epididymis weights, atrophy of semiferous tubules and Leydig cells, decrease in the number of sperm in the caudal epididymis and their motility were observed in the 500 mg/kg group. Female rats receiving methoxychlor showed mainly abnormal estrous cycles, decrease in serum luteinizing hormone level, decrease in ovary weight, proliferation of mammary acinus, atrophy of ovary due to decrease in follicles and corpus luteum in histopathology, hypertrophy of endometrial epithelium of uterus and vagina epithelium in the 100 and 500 mg/kg groups. Among the parameters tested in the present experimental system, effects of methoxychlor on endocrine-related organs were detected with regard to serum hormone, organ weights, histopathological examination in both sexes, estrus cycle in females and sperm examination in males. Based on these results, a no-observed-adverse-effect level (NOAEL) in the present study was estimated to be below 20 mg/kg per day. In particular, the adverse effects were effectively detected in organ weights of accessory sex organs and histopathological examination.

Specific transgenerational imprinting effects of the endocrine disruptor methoxychlor on male gametes

“…This might explain why no effect of MXC on sperm counts was observed. Moreover, the discrepancy between our results and those of Okazaki et al (2001) might be explained by the higher doses of MXC used in the latter study. Our results, however, show for the first time the direct effects of MXC on the methylation pattern of four out of the five paternally imprinted and maternally imprinted genes tested in the sperm.…”

“…MXC administration in adult male rats during the 28 days was found to induce a decrease in the number of sperm cells in the caudal epididymis (Okazaki et al 2001). In our study, the short time that elapsed between the end of the MXC treatment and the sperm collection would not allow the spermatogonial cells to become sperm.…”

“…Next morning, females with a copulation plug were separated into two groups. The dose of a developmental exposure to MXC, with which no adverse effect was observed, has been shown to vary between 20 mg/kg per day (Okazaki et al 2001) and !20 mg/kg per day (Armenti et al 2008) in rats. It has been reported to be about 40 mg/kg per day in mice (Swartz & Eroschenko 1998).…”

“…MXC administration in adult male rats has been found to induce abnormalities in male reproductive tract, affecting testes, prostate, and seminal vesicle development as well as spermatogenesis (Okazaki et al 2001).…”

Interactions of Endosulfan and Methoxychlor Involving CYP3A4 and CYP2B6 in Human HepaRG Cells

“…Methoxychlor induces follicular atresia, reduces ovulation rate, and decreases embryo implantation in rats and mice (Tiemann, 2008). It also reduces the weight of testes, prostate, and seminal vesicles and causes disorders of spermatogenesis in male rats (Okazaki et al, 2001). However, mechanisms of action of endosulfan and methoxychlor in humans remain poorly understood.…”