The present study was conducted to study the effect of immunoneutralization against endogenous inhibin on FSH, LH, oestradiol-17 beta and progesterone secretion and to investigate the effect of removal of endogenous inhibin on subsequent follicular development in the hamster. After treatment with anti-inhibin serum (inhibin-AS) at 1100 h on day 2 of the oestrous cycle (day 1 = day of ovulation), a marked increase in plasma levels of FSH and a slight increase in plasma levels of LH were noted and pituitary contents of FSH, but not LH, were also increased. In the group treated with inhibin-AS, superovulation occurred on day 1 of the following cycle. Plasma levels of oestradiol-17 beta markedly increased with the increase in the number of ovulations induced by human chorionic gonadotrophin (hCG) as compared with those in control animals. In the second cycle, plasma concentrations and pituitary contents of FSH in the animals given 200 microliters inhibin-AS still showed high values as compared with those in the animals treated with control serum, although superovulation did not occur on day 1 of the third cycle. Plasma concentrations and pituitary contents of LH in the hamster given 200 microliters inhibin-AS tended to decrease as compared with those in control animals during the second cycle. Plasma concentrations of oestradiol-17 beta in the animals treated with 200 microliters inhibin-AS changed in a similar way to controls. A marked increase in plasma concentrations of progesterone was noted on days 1 and 2 of the second cycle in the group receiving inhibin-AS. The twice daily injection of 1 IU hCG during the second cycle to the animals given 200 microliters inhibin-AS induced superovulation on day 1 of the third cycle. These results indicate that circulating inhibin may be an important indicator of the number of developing follicles and may maintain the species-specific number of developing follicles through suppression of FSH secretion in the cyclic hamster. They also suggest that high levels of inhibin slightly suppress plasma levels of LH, indicating that plasma LH may also regulate follicular development in the cyclic hamster.

The time course for loss of ability of Graafian follicles to secrete inhibin and estradiol was investigated during induced follicular atresia. Cyclic hamsters were hypophysectomized on Day 1 (estrus) and injected s.c. with 30 IU eCG. Thereafter, these animals were given a single i.p. injection of antiserum to eCG on the morning of Day 4 to induce follicular atresia in a rapid and predictable manner. A drastic fall in plasma levels of estradiol and testosterone was noted within 1 h, whereas relatively high levels of plasma inhibin were maintained until 12 h, followed by an abrupt decline by 24 h. The first histological signs of pyknosis in granulosa cells appeared by 4 h, and breakdown of the mural granulosa layer was observed in most follicles by 8-12 h after immunoneutralization of circulating eCG. According to immunohistochemical analysis, inhibin activity was unchanged in granulosa cells at 12 h followed by a slight decline by 24 h, whereas positive reaction for aromatase in these cells rapidly declined by 8 h. Immunoreactivity of 17alpha-hydroxylase/C17,20-lyase (CYP 17) was also reduced in theca cells by 8 h. These results indicate that granulosa cells continue to secrete inhibin during the process of follicular atresia, although these cells quickly shut off the secretion of estradiol, and theca cells shut off the secretion of testosterone. The present results indicate as well that a rapid decline of estradiol and testosterone in plasma is an early sign of atresia in antral follicles. These results, therefore, suggest that the loss of enzymatic activity of aromatase in granulosa cells and CYP 17 in theca cells is a part of the process of follicular atresia.

To investigate the physiological importance of oestradiol-17beta and inhibin in the regulation of gonadotrophin secretion in the cyclic golden hamster, females were passively immunized against two hormones. When 200 microL antiserum against oestradiol-17beta (oestradiol-AS) was given on Day 3 (Day 1 = day of ovulation), the preovulatory gonadotrophin surge was completely blocked for 24 h and the length of the oestrous cycle was also prolonged for one day. In the group given 200 microL oestradiol-AS on Day 3, basal levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) increased slightly and superovulation (19.6+/-0.8, mean+/-s.e.m.) occurred. When 200 microL antiserum against inhibin (inhibin-AS) was given at 1100 hours on Day 3, a dramatic increase in plasma FSH and a slight increase in LH were noted, resulting in superovulation (38.2+/-2.6) on the expected Day 1. The present study indicates clearly that inhibin plays a major role in regulating the specific ovulation rate in the hamster through the control of FSH secretion. Present results also indicate that oestradiol-17beta suppresses basal LH secretion. Oestradiol-17beta may act as an indicator of the follicular maturation, and the high plasma concentration of oestradiol-17beta noted from Day 3 to Day 4 may play an important role in determining the timing of initiation of the preovulatory gonadotrophin surge.

Mechanisms responsible for a daily increase in plasma gonadotropins during lactation of golden hamsters (Mesocricetus auratus) were investigated. A daily afternoon (1700 h) increase in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone was observed until at least day 15 of lactation (day 0 of lactation = day of parturition). On the other hand, plasma estradiol did not show any significant change at 1100, 1700 and 2300 h. Such increases occur in hamsters nursing 8 pups. In hamsters nursing 2 pups, however, only a daily increase in LH was noted on day 5 of lactation and the diurnal rhythm of LH was difficult to see because of the increase in basal levels of LH on day 10 and 15 of lactation. The diurnal increases in plasma LH and FSH disappeared within 2 days after removal of the litter on day 10 of lactation and by day 14 of lactation (4 days after litter removal) most animals had resumed ovulating. Ovariectomy on day 2 of lactation did not affect the profile of plasma LH on subsequent days. In contrast, the basal levels of plasma FSH increased, suggesting the absence of an inhibitory ovarian signal (probably inhibin). A negative relationship existed between plasma estradiol and the daily surges of LH and FSH at 1700 h; the daily rise in gonadotropins occurred when plasma levels of estradiol were low. The afternoon increases in plasma LH and FSH were prevented by treatment with physiological levels of estradiol, comparable to those observed in the cyclic hamster. Antiserum against LHRH injected at 1100 h on day 10 of lactation completely suppressed the daily rise of plasma LH and FSH. These results indicate that during lactation low levels of estradiol are required for the expression of diurnal increase in plasma LH and FSH in the hamster. Moreover, the low levels of estradiol results in the daily release of LHRH.

In the present study, we investigated changes in plasma concentrations of inhibin, estradiol-17β, progesterone and FSH in the golden hamster (Mesocricetus auratus) superovulated by equine chorionic gonadotropin (eCG) treatment. In addition, the hamsters were injected at various times with human chorionic gonadotropin (hCG) to determine the follicular development after treatment with eCG. After treatment with 30 IU eCG at 1100 h on day 1 of the estrous cycle (day 1=day of ovulation), superovulation (36.5 ± 1.1 v.s. 13.5 ± 0.4 in controls treated with saline) occurred on the expected next day 1. Plasma concentrations of inhibin and estradiol-17β increased and plasma concentrations of FSH decreased in the animals given 30 IU eCG. Plasma concentrations of progesterone also increased after eCG treatment; however, they decreased to control levels within 48 h after eCG injection. The number of follicles capable of ovulating after hCG administration was dramatically increased within 36 h after eCG injection (43.4 ± 3.4). Thereafter, the number of follicles capable of ovulating decreased to about a half of the peak value during the next 6 h period, then increased gradually to the numbers of ovulations observed on the next day 1. Plasma inhibin and estradiol-17β were augmented and plasma concentrations of FSH were suppressed in the animals in which follicular development was stimulated, though there is a difference in the changing pattern between plasma inhibin and estradiol-17β. The present results also suggest that plasma concentrations of inhibin may be an indicator for the number of developing follicles and estradiol-17β may be an indicator for follicular maturation.