Armando G. Amador

Seasonal differences in testicular receptors and steroidogenesis

Gonadal function in most animal species exhibits considerable annual fluctuations, with gametogenesis and fertility often being confined to a short and rigidly controlled breeding season. In males, production of androgenic steroids by the testis is usually maximal immediately before and during the breeding season. In the golden hamster, seasonal regression of the testes is associated with decrease in the total content of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) receptors, and similar findings have been reported for other mammalian species. However, the concentration of LH and FSH receptors per unit of testis weight is typically elevated rather than suppressed during testicular regression. Reduction in the number of testicular LH and PRL receptors in adult golden hamsters exposed to short photoperiod is due primarily to suppression of pituitary PRL release under these circumstances. Regulation of seasonal changes in testicular FSH binding, as well as regulation of the levels of LH, PRL and FSH receptors in other seasonally breeding species remain to be elucidated. Reduction in the content of LH receptors in the testes is accompanied by reduced capacity to produce androgens in response to LH stimulation. Although these events are likely to be causally related, other mechanisms are also involved. In particular, seasonal regression is accompanied by reduced capacity of the testes to convert C21 steroid precursors into biologically active androgens. Seasonal loss of FSH receptors was reported to be accompanied by increased rather than reduced responsiveness of the Sertoli cells to FSH, thus resembling the situation in immature animals. It can be concluded that alterations in the ability of the testes to bind pituitary gonadotropins and to respond to gonadotropic stimulation are among the mechanisms responsible for seasonal shifts between gonadal activity and quiescence.

Seasonal variations in circulating levels of progesterone and estradiol in unmated adult female woodchucks (Marmotamonax) in captivity

The annual profile of serum levels of progesterone (P) and estradiol-17 beta (E2) was characterized in a seasonally breeding rodent, the woodchuck (Marmota monax). Hormonal levels were determined in serum samples taken at weekly or biweekly intervals from unmated female woodchucks maintained all year indoors under controlled conditions of photoperiod and temperature. Annual fluctuations included a rise of E2 levels during late January through February, followed by a modest increase in plasma P concentrations by late March, the latter attaining peak values during April and May. A temporal dissociation of peak values of circulating levels of P and E2 during the annual reproductive cycle was also detected. The timing of changes in serum levels of P and E2 in these captive woodchucks corresponded to reproductive events during the normal breeding season of the woodchuck in the southern part of its range.

Genetics and Luteinizing Hormone Receptors

Testicular luteinizing hormone receptors (LH-Rs) that bind either LH or human chorionic gonadotropin (hCG) are required for the differentiation of the Leydig cell (1). This is indicated by the correlation between the onset of Leydig cell differentiation and the appearance of hCG during the development of the human fetus. The number of Leydig cells also correlates with the levels of hCG during pregnancy. Leydig cells, and therefore LH-Rs, are present by week 8 of pregnancy in the human (2–4), and by day 15 in rodents (5).