Prior to any investigation of toxicant effects on sexual development it is necessary to have a complete understanding of the relevant physiology of reproductive development. Beginning at conception, development of males and females diverge to form the respective reproductive systems. From the prenatal period to the interval following puberty, radical changes take place in the hypothalamo-pituitary-gonadal axis of males and females. The complexity of each of these systems and their development is mirrored in the many possibilities for the means by which chemicals may produce adverse effects. For example, a chemical that affects hormone synthesis may, if administered at the proper time, affect hypothalamic development. As a consequence, pubertal development may not occur normally. In this chapter, we have outlined the basics of reproductive development and provided examples of adverse effects by endocrine disrupting chemicals (EDCs) on such development.
- To develop β2-adrenergic receptor (AR) agonists with higher selectivity, it is essential to evaluate the cardiac side effects which are the most serious side effects of this class of drugs. We studied receptor occupancy of β1-ARs in rats as a possible cause for the side effect of β2-AR agonists, namely myocardial fibrosis. Myocardial fibrosis in rats was observed on Day 7 after the administration of salbutamol and terbutaline, both of which are selective β2-AR agonists, at higher dose levels. To evaluate receptor occupancy, plasma concentrations of (R)-salbutamol and (R)-terbutaline, plasma protein binding and the EC50 for chronotropic effects in rats were determined. Based on the plasma concentrations, the plasma protein binding and EC50, receptor occupancy-time profiles were constructed. The relationship between the receptor occupancy-time profile under the curve, the AUCΦ, and the degree of myocardial fibrosis was evaluated with a multiple correlation analysis. Myocardial fibrosis was significantly correlated (r2 > 0.78) to the AUCΦ with the threshold above approximately 50%, but not to plasma concentrations. These results indicate that the receptor occupancy theory is also useful for the evaluation of the chronotropic side effects of β2-AR agonists.
Testosterone propionate (TP) was supplemented to male rats for assessment of its ameliorating effect on testicular toxicity with thiamphenicol (TAP). A total of 20 male Sprague-Dawley rats were treated orally with TAP at 200 mg/kg/day for up to 4 weeks. In addition, 5 male rats were allotted to the control group receiving vehicle only. Ten of the 20 treated rats had a Silastic capsule® (containing about 80 mg of TP) implanted in the dorsal skin at Week 2 and assigned to the TAP-TP group, while the other 10 treated rats were in the TAP group. After Weeks 3 and 4, five of both treated groups were examined for weight and histology of the testis and accessory genital glands, and for staging analysis of the seminiferous tubules. The same parameters were also assessed in the control group after Week 4. Weights and morphology of the seminal vesicle and prostate recovered remarkably from the TAP toxicity after TP supplement. However, no ameliorating effects of TP were obtained for the testis in either weight, morphology, or staging analysis of the seminiferous tubules.
- A relationship has been reported between trace elements and diabetes mellitus. This study examined the concentration of several trace elements (Zn, Mg, Ca, Fe, Cu, Mn, P) in each organ from spontaneous type 2 diabetic model rat (GK rat) and compared with control. Results showed that diabetic rat excreted 1.25-, 1.5-fold more zinc and magnesium, respectively, than control in the urine. Contents of zinc and magnesium in kidney and testis and fatty tissue were lowered significantly in GK rat. Zinc in liver and magnesium in lung were also lowered. Concentrations of other trace elements also changed in some organs from GK rats compared with those from control. These results demonstrate that trace element imbalances come out significantly in mild diabetes and the need to instruct a nutritionally balanced diet that takes account of trace elements in diabetes from the early stage.
We surveyed interpretation of the ICH guidelines concerning reproductive toxicology. Valid responses were obtained from Japan (JPN), Europe (EUR) and the US. The results obtained were compared to those at the time of a previous survey targeted at JPN facilities in 1995-1996 as well as compared among all three regions. Compared to the previous survey in Japan, the number of facilities performing toxicokinetics (TK) in rats has slightly increased. This result was considered to represent changes of attitude toward TK in reproductive toxicity studies. Differences in interpretation of the guidelines between JPN, EUR and the US were widely seen. Clear differences were noted in sperm examinations, postnatal tests, fetal examinations, some examinations for F1 animals after culling and TK. Researchers in the West seemed to be interpreting the ICH guidelines more flexibly from the scientific point of view. JPN researchers appeared to interpret the guidelines, including notes, as rigid requirements. Most of the parts which produced different interpretations were the notes in the guidelines. The force of mention in the notes should be defined in the future. In addition, there were doubts about some parts, including notes, which had been found to have become unsuitable for the implementation of studies because of scientific progress or from long experience in using the guidelines. Therefore, updates of the guidelines may be needed in the future as well as the remedy of interpretation by JPN researchers. In JPN, the number of reproductive toxicity studies has decreased. The scanty experience in JPN therefore raises apprehension of appropriate selection and stagnating development of methodology, and might hinder the maintenance of the guidelines. In the future, the cooperation of CROs as well as global collaboration will be essential not only to scientific developments of reproductive toxicology but also updates of the guidelines.
This study was undertaken to detect key parameters of rat sperm motion in relation to male fertility by comparing the differences in sperm motion induced by treatment with α-chlorohydrin (ACH), known to produce spermatotoxicity, and nitrobenzene (NTB), known to produce testicular toxicity. Male rats received ACH (5 or 20 mg/kg/day) or NTB (60 mg/kg/day) for either 3 days or 18 days. Epididymal sperm was assessed for motility using a Hamilton-Thorne Sperm Analyzer (HTM-IVOS). Numerical data for statistical analysis and graphical renditions of sperm motion using parameters in radar charts and reconstructed sperm tracks were analyzed to evaluate sperm motion. Males were allowed to copulate with untreated females and cesarean sections were conducted in order to examine the effects of drug administration on male fertility. Linearity of sperm track (linearity (LIN) and/or straightness (STR)) decreased and/or beat cross frequency (BCF) increased only in ACH groups (5 or 20 mg/kg/day), although the percentage of motile sperm, sperm velocities (average path velocity (VAP), curvilinear (VCL), and straight line velocity (VSL)) and amplitude of lateral head displacement (ALH) decreased on Day 18 in both ACH and NTB (60 mg/kg/day) groups. Furthermore, from the individual reconstructed sperm tracks, it was clear that ACH-treated spermatozoa were characterized by abnormal motion ("jerking") with low vigor (low velocities) and little or no forward progression. Finally, only ACH treatment led to a reduction in pregnancy rate or infertility. Therefore, our results suggest that linearity (especially VSL, STR and LIN) in sperm motion is a key parameter for assessing a chemical's potential to induce male infertility.