Evaluation of the male pubertal assay's ability to detect thyroid inhibitors and dopaminergic agents

The male pubertal onset assay is under consideration as an alternate Tier I screening assay to detect potential endocrine active chemicals (EACs) acting through a variety of steroid hormone and thyroid hormone receptor-mediated and non-receptor-mediated mechanisms. This study focused on the assay's ability to detect several non-receptor-mediated EACs. Weanling male CD rats (21 days old) were dosed for 30 d by gavage with vehicle (0.5% METHOCEL) or the following EAC classes (mg/kg/d): a potent thyroid agent (6-propylthiouracil, PTU, 240), a weak thyroid agent (phenobarbital, PB, 50 or 100), a dopamine antagonist (haloperidol, HALO, 2 or 4), or a dopamine agonist (bromocryptine, BRC, 10 or 50). In vehicle-treated males, preputial separation (PPS) occurred at 44.4 +/- 2.0 days of age. Age at PPS was delayed with PTU and 50 BRC, treatments that also delayed growth. Absolute testes and/or epididymal weights were decreased by PTU and 100 PB. BRC (50) and PB (100) decreased absolute prostate and seminal vesicle weights. Relative thyroid weights were altered by HALO, PTU, and PB, agents that significantly decreased serum T(4) levels. PTU increased serum thyroid-stimulating hormone (TSH) by 8.5 times and markedly altered thyroid histology, whereas HALO and PB did not significantly increase TSH and had marginal effects on thyroid histology. Thus, this assay detected both strong (PTU) and weak (PB) thyroid agents as well as the dopamine agonist BRC; however, its ability to detect dopamine antagonists remains unproven. These results confirm that thyroid weight measurements, although not required in the current male pubertal assay protocol, may add valuable information for interpretation of thyroid effects. Due to the apical nature of the male pubertal assay end points, additional work will be required to establish definitive criteria for a positive result in this assay.