In order to elucidate the mechanism of the resumption of follicular activity and ovulation in rats, levels of FSH, LH and prolactin in plasma and pituitary gland and ovarian follicular development were quantified after removal of the litter on day 3 of lactation (day of parturition = day 0 of lactation). Such removal resulted in ovulation of 13 oocytes 4 days later, a number comparable with that found in normal cyclic rats. Plasma levels of prolactin were high during lactation but markedly decreased after removal of the litter. Although plasma concentrations of FSH and LH did not change during days 3–7 of lactation, there was an FSH surge between 24 and 30 h after removal of the litter. Plasma concentrations of LH also increased slightly but significantly by 24 h after removal of the litter and this value persisted during the following 2 days. Surges of FSH, LH and prolactin occurred at 17.00 h 3 days after pups were removed. Removal of the litter did not increase pituitary contents of FSH, LH and prolactin and a marked reduction in pituitary levels of FSH and LH, but not of prolactin, occurred at 17 00 h 3 days after removal of the litter.

A quantitative study of follicular development indicated that follicles larger than 401 μm in diameter were absent during days 3–7 of lactation. However, the number and size of antral follicles increased by 30 h after removal of the litter, probably due to the increases in plasma levels of FSH and LH, and follicles larger than 601 μm in diameter appeared 3 days after the young were removed. Although ovulation could not be induced by human chorionic gonadotrophin from days 3 to 5 of lactation, its administration 30 h after removal of the litter produced ovulation in all rats by the following morning.

These results indicated that a moderate increase in FSH, although below the amounts released at the preovulatory surge, together with basal levels of LH which were within the range observed on the day of dioestrus during the normal cycle were responsible for the initiation of follicular maturation after removal of the litter.

A selective surge of FSH with a small concomitant rise in LH occurred invariably in rats when ovulation was induced by injecting human chorionic gonadotrophin (HCG) at various reproductive stages such as day 15 of lactation and in 29-day-old immature rats as well as in dioestrous animals. No FSH surge occurred on day 3 of lactation or in 26-day-old immature rats in which ovulation could not be induced by HCG. The FSH surge occurred 6–18 h after HCG treatment regardless of the time of day of injection of HCG. Ovulation began by 12 h and was completed by 18 h after injection of HCG.

Pituitary responsiveness to luteinizing hormone releasing hormone (LH-RH) with respect to FSH release strikingly increased at 01.00 h on day 1 after HCG injection at 17.00 h of dioestrus (day 0) to levels similar to those of the group at 01.00 h of oestrus, when the greatest response was noted during the normal cycle. With regard to LH release pituitary responsiveness to LH-RH at 01·00 h on day 1 markedly increased but the response was only about half of the response at 01·00 h of oestrus and one third of the response at 17.00 h of pro-oestrus when the greatest response was noted during the normal oestrous cycle.

These results indicate that during ovulation the pituitary gland of the rat is highly responsive to LH-RH with respect to the release of FSH, for which secretory changes in the ovary after an ovulating dose of HCG may be responsible.

Maximal levels of progesterone in the plasma after premature ovulation induced by either the administration of human chorionic gonadotrophin (HCG) or LH-releasing hormone (LH-RH) to dioestrous (day 0) rats were observed from 33 to 45 h but decreased 3 h earlier than after spontaneous ovulation. This suggested an earlier decline in the secretory activity of corpora lutea formed from premature ovulations than that of corpora lutea formed during a normal oestrous cycle.

The next spontaneous ovulation occurred 4 days (day 5) after premature ovulation induced by LH-RH on day 0. A single s.c. injection of 2·5 μg oestradiol-17β (OE₂) at 10.00 h on day 2 to these animals advanced the next spontaneous ovulation by 1 day. A normal number of oocytes was shed, indicating that earlier secretion of oestrogen on day 2 had advanced the next spontaneous ovulation. A single injection of 2·5 μg OE₂ to normal 4-day cyclic rats at metoestrus failed to advance the next ovulation. An earlier decline of progesterone levels in the plasma of rats after premature ovulation as compared with spontaneous ovulation may explain the greater effectiveness of oestrogen in the former group.

The progesterone surge was observed during the period of premature ovulation in both HCG- and LH-RH-treated groups. This progesterone release in the periovulatory period may be responsible for the inhibition of gonadotrophin surges on the expected day of prooestrus (day 1).

When 1·0 μg luteinizing hormone releasing hormone (LH-RH) was given i.v. three times at 1 h intervals from 17.00 to 19.00 h on the day of dioestrus (day 0) to regular 4 day cyclic rats, premature ovulation was induced the next morning (day 1) with the number of ova present comparable to normal spontaneous ovulation. The next spontaneous ovulation occurred on the morning of day 5, 4 days after premature ovulation induced by LH-RH.

Plasma concentrations of FSH and LH showed transient rises and falls within 1 h of administration of LH-RH; concentrations of FSH in the plasma decreased from 20.00 h on day 0 but markedly increased again from 23.00 h on day 0 to 02.00 h on day 1 and these high levels persisted until 14.00 h on day 1, with only a small increase of plasma LH during this period. The duration of increased FSH release during premature ovulation induced by LH-RH treatment was 6 h longer than the FSH surge occurring after administration of HCG on day 0. Surges of gonadotrophin were absent on the afternoon of day 1 (the expected day of pro-oestrus) and the surges characteristic of pro-oestrus occurred on the afternoon of day 4 and ovulation followed the next morning. The pituitary content of FSH did not decrease despite persisting high plasma levels of FSH during premature ovulation induced by either LH-RH or HCG on day 0.

The changes in uterine weight indicated that the pattern of oestrogen secretion from the day of premature ovulation induced by LH-RH to the day of the next spontaneous ovulation was similar to that of the normal 4 day oestrous cycle. When 10 i.u. HCG were given on day 0, an increase in oestrogen secretion occurred on day 2, 1 day earlier than in the group given LH-RH on day 0. This advancement of oestrogen secretion was assumed to be responsible for the gonadotrophin surges on day 3.

Similar numbers of fully developed follicles were found by 17.00 h on day 2 after premature ovulation induced by either LH-RH or HCG, suggesting that the shorter surge of FSH during premature ovulation induced by HCG had no serious consequences on the initiation of follicular maturation for the succeeding oestrous cycle in these rats.

Administration of LH-RH on day 0 had no direct effect on the FSH surge during premature ovulation. Secretory changes in the ovary during ovulation may be responsible for this prolonged selective release of FSH.

Laboratory of Veterinary Physiology, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183, Japan

(Received 2 May 1977)

When an amount of human chorionic gonadotrophin (HCG) sufficient to cause ovulation is given to 4-day cyclic rats on the day of dioestrus, premature ovulation is induced the next morning (Eto & Imamichi, 1955). The pattern of release of follicle-stimulating hormone (FSH) responsible for the initiation of follicular maturation of the next set of follicles (Schwartz, 1969; Welschen & Dullaart, 1976) after HCG-induced ovulation has not been previously evaluated. The present communication is concerned with this problem and indicates that a large amount of FSH is released within 12 h of administration of HCG, with only a small concomitant rise in the concentration of luteinizing hormone (LH).

Adult female Wistar rats were maintained under a 14 h light : 10 h darkness schedule (lights on 05.00 h), and those showing three or

When bovine follicular fluid (BFF) was given i.p. three times at intervals of 3 h from 17.00 to 23.00 h to dioestrous rats pretreated with 10 i.u. human chorionic gonadotrophin (HCG) at 17.00 h on the day of dioestrus (day 0), the selective surge of FSH at 02.00 h on day 1 was suppressed in a dose-dependent manner. Three i.p. injections of 0·5 ml BFF completely suppressed the FSH rise in plasma at 02.00 h on day 1, but the time of premature ovulation induced by HCG was not altered. In these animals treated with HCG and BFF, however, the selective surge of FSH occurred as a delayed surge from 05.00 to 23.00 h on day 1. After seven i.p. injections of 0·5 ml BFF (from 17.00 h on day 0 to 11.00 h on day 1) the delayed surge of FSH took place from 17.00 h on day 1 to 11.00 h on day 2, indicating that waning of BFF with a decrease in inhibin secretion by the ovaries may be responsible for the delay of the FSH surge.

The next spontaneous ovulation in rats treated with HCG and BFF occurred on day 5, a delay of ovulation of 1 day compared with animals given HCG on day 0 with no BFF. Initiation of follicular maturation or selection of growing follicles for the succeeding oestrous cycle appeared to be retarded by the delay of the FSH surge in HCG- and BFF-treated animals.

The pituitary content of FSH in animals given HCG and three i.p. injections of 0·5 ml BFF increased strikingly until 11.00 h on day 1, when the delayed FSH surge was already in progress. These results suggest that the ability of the pituitary gland to synthesize FSH is high during the period of ovulation.

The present investigation was performed to elucidate the mechanism of the initiation of follicular maturation after inhibition of ovulation in rats treated with pentobarbitone sodium at 13.30 h and progesterone at 14.00 h on the day of pro-oestrus (day 0 denotes the day of these treatments). Ovulation was completely inhibited and the next spontaneous ovulation occurred on day 5, the expected day of the next oestrus. Follicular responsiveness to injection of human chorionic gonadotrophin (hCG) indicated that preovulatory follicles at the time of treatment with pentobarbitone and progesterone regressed by 05.00 h on day 2. Maturation of a new set of follicles began from 17.00 h on day 2 and all rats were induced to ovulate by hCG injection by 17.00 h on day 3, the number of oocytes ovulated being comparable to normal ovulation.

In the animals receiving pentobarbitone sodium and progesterone treatment, two selective rises in plasma FSH, which had peak levels at 05.00 h on day 1 and 11.00 h on day 2, were observed without a rise in LH. Preovulatory surges of FSH and LH occurred on the afternoon of day 4.

These results suggest that the second rise in FSH was induced by regression of Graafian follicles present at the time of treatment with pentobarbitone sodium and progesterone and that this surge of FSH was responsible for initiation of maturation of a new set of follicles destined to ovulate in the subsequent cycle. The mechanism of induction and the role of the first rise of FSH from the night of day 0 to the morning of day 1 cannot be explained at present.

To investigate the mechanism of the selective surge of FSH during the period of ovulation induced by human chorionic gonadotrophin (hCG) in dioestrous rats, inhibin activity in ovarian vein plasma was determined at varying time-intervals after treatment with hCG using the primary monolayer culture system of anterior pituitary cells.

Inhibin activity in ovarian vein plasma had already decreased 6 h after injection of hCG, when concentrations of FSH in the plasma were still low in three of four animals. Inhibin activity further decreased 12–18 h after hCG, when a selective surge of FSH occurred. Inhibin activity increased to the level before hCG treatment 24 h after the treatment, when ovulation was completed and the FSH surge terminated.

These results suggest that the selective surge of FSH occurs as a consequence of the decrease in inhibin secretion from the ovary, which is perhaps due to the ovulation dose of hCG altering the functional activity of the granulosa cells in the large Graafian follicles.