Mechanisms of biphasic surges of testosterone in response to human chorionic gonadotrophin (hCG) in stallions

Abstract

Human chorionic gonadotrophin (hCG) is included in stimulation tests for diagnosis of cryptorchidism and endocrine function of testes in stallions. Although the exact mechanisms are unknown, circulating testosterone (T) concentration shows a clear biphasic increase in response to a single administration of hCG (Tsunoda. et. al. Animal Reproduction Science. 2010; 121S: S165-S167). In the first experiment, concentrations of circulating T and estradiol-17β (E) in response to intravenous administration of 10,000IU hCG were measured. Concentrations of circulating testosterone were also measured in adult Thoroughbred stallions (n=4). In the second experiment, the same experiment was done using adult male golden hamsters (n=45) to figure out mechanisms of biphasic response of T. Adult male golden hamsters were used as a model of stallions, because golden hamsters are typical long day seasonal breeders same as stallions. In the first experiment, blood samples were taken hourly for 6 hours, and daily for 7 days after administration of hCG. Circulating T concentrations in stallions showed a clear biphasic increase within 6 hours and between 2 and 4 days after administration of hCG. Circulating T concentrations at the second peak were significantly higher than those at the first peak. Circulating E concentrations after hCG administration was similar to that of T at the first peak, though the second increase was not detected. Circulating hCG concentrations sharplyincreased just after administration, then followed by an abrupt decline, and low levels were maintained until day 7. It indicated that concentrations of hCG decreased completely when the second peak of T concentrations appeared in circulation. In the second experiment, 25 male golden hamsters were administered 100IU of hCG, and 20 hamsters were administered saline. Blood samples and testes were taken from 5 golden hamsters 5 hours, 24 hours, 3, 5, and 7 days after administration, respectively. Testicular tissues were investigated by real-time PCR for cytochrome P450scc (P450scc), 3β-hydroxy-steroid dehydrogenase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), and receptors of luteinizing hormone (LHR). Testes were also histologically evaluated by hematoxylin-eosin staining. The results of changes in circulating T and hCG concentrations in male golden hamsters were similar to those in stallions. Expression of P450cc, 3β-HSD, 17β-HSD and number of LHR in testes 3 days after administrations of hCG were significantly higher as compared with those at 0 hour. The number of Leydig cells in the histological section 3 days after administration of hCG were also higher compared to 0 hour. The present study strongly suggests that the increase in numbers of LHR and Leydig cells, together with upregulation of steroid enzymes in testes causes the second increase of T plasma concentration after hCG in stallions.