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SUMMARY
Serotonin (50 and 100 mg/kg), given subcutaneously, inhibited induced ovulation in immature rats treated with pregnant mare serum gonadotrophin (PMSG). A single injection was effective if given 52–55 h after the PMSG, i.e. in the 3 h period after the critical period before ovulation. The injection of serotonin inhibited the release of luteinizing hormone (LH) from the pituitary since the pituitary levels were higher than in the control animals and there was complete inhibition of the plasma LH surge. The antiovulatory effect was reversed by administration of progesterone and endogenous plasma progesterone levels were reduced in the late evening after serotonin treatment. The site of action of serotonin appeared to be peripheral since it inhibited induced ovulation in hypophysectomized rats but was without effect in intact rats treated intraventricularly. It is suggested that progesterone is essential for the occurrence of induced ovulation and serotonin inhibits either its secretion at the ovarian level or its passage away from the ovary.
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Department of Gynecology and Obstetrics, and Human Development Center, University of Kansas Medical Center, Kansas City, Kansas 66103, U.S.A.
(Received 18 December 1974)
Knowing the exact age of the corpus luteum (CL) may be of importance in studies designed to assess the factors that influence CL survival and luteolysis. Control of the CL in vitro, from the moment of ovulation until luteolysis, is an objective of a sequence of experiments utilizing organ cultures (Fainstat & Emanuele, 1972; Baranczuk, Cheatum & Fainstat, 1975) and organ incubations in this laboratory. The accurate determination of CL age, based upon the time of ovulation in vitro, is desirable for correct interpretations of data from our in-vitro experiments. The purpose of this present study was to determine the influence of time on the likelihood of ovulation in vitro. Identifying the time of the peak incidence of ovulations would aid the selection of the optimal time
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It is now well established that progesterone can both facilitate and inhibit ovulation in the rat. In the adult female rat these responses are dependent on the stage of the oestrous cycle as was shown by Everett (1948) and recently confirmed by Zeilmaker (1966). A hypothalamic site of action for progesterone in the facilitation of ovulation was demonstrated by Barraclough, Yrarrazaval & Hatton (1964). On the other hand, ever since Hohlweg (1934) observed the induction of corpus luteum formation by oestrogen in prepubertal female rats, a close connexion between the preovulatory rise of oestrogen and ovulation-inducing gonadotrophin release was suggested by many investigators, and Döcke & Dörner (1965) concluded that oestrogen may increase the sensitivity of the hypophysis to the stimulating effect of the hypothalamic gonadotrophin-releasing factor(s). The present study was made to see whether a synergism between oestrogen and progesterone in the induction of ovulation can be shown in
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SUMMARY
Ovulation in the sheep could be blocked by chlorpromazine used as a neural blocking agent. The neural stimulus leading to the release of sufficient gonadotrophin to cause ovulation was shown to occur within 2 hr. after the onset of oestrus.
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SUMMARY
The effect of age and dose of a single injection of pregnant mare serum gonadotrophin (PMS) on spontaneous ovulation in immature Wistar rats is described. Ovulation could be induced by human chorionic gonadotrophin (HCG) at least 6 days before it occurred when pregnant mare serum gonadotrophin alone was given. Chlorpromazine was shown to block pregnant mare serum gonadotrophin-induced ovulation at a dose level (0·25 mg. in a 60 g. rat) which has no effect on the ovulatory response to human chorionic gonadotrophin. Higher doses interfered with the action of injected human chorionic gonadotrophin. Ovulation could be induced in the chlorpromazine-blocked animals by the systemic injection of an extract of bovine median eminence, but the sensitivity was too low to use this response for an assay method.
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It has been estimated that the ovulatory release of luteinizing hormone (LH) in rats lasts for approx. 2 hr. (Everett, 1961), but no estimate of the amount of gonadotrophin secreted is available. In the present study we have blocked ovulation by a single oral dose of a potent oestrogen antagonist—ICI 46,474 [Trans-1-(P-β-dimethylaminoethoxyphenyl)-1,2-diphenylbut-l-ene] (Walpole, 1968), and then attempted to determine the amount of LH required to restore ovulation fully in such rats. Evidence presented earlier supports the view that this anti-oestrogen blocks ovulation by interfering with the positive oestrogen feedback, since ovulation in the treated rats can be fully restored either by oestrogen or by LH releasing factor (Labhsetwar, 1970).
The female rats used were of Alderley Park Strain I, originally derived from Wistar rats and randomly bred for several generations under specific pathogen-free conditions. They were exposed to 14 hr. of artificial illumination (06.00–20.00 hr.) daily. Only
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It is presumed that steroid hormones inhibit ovulation by blocking the release of appropriate hormones from the hypothalamic-pituitary system. Recent studies have suggested the possibility that the block is at the level of the ovary itself. Purshottam, Mason & Pincus (1961) have shown that a number of steroids, as well as certain other pharmacologically active agents, will partially block the release of ova from ovaries of immature mice treated with pregnant mares' serum and human chorionic gonadotrophin (HCG). Wright (1960) reported inhibition by oestrogen of ovulation induced in isolated ovaries of frogs with pituitary extracts, whereas these same oestrogens actually augmented steroid-induced ovulation. Our preliminary studies also showed oestrone and oestradiol to be inactive in blocking progesterone-induced ovulation from frog ovarian fragments, although oestriol, deoxycorticosterone acetate, cortisone (but not cortisol) and 19-nortestosterone were effective (unpublished), as was HCG (Edgren & Carter, 1962). We shall report here the unsuccessful attempt to
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SUMMARY
Pubertal female rats received sodium pentobarbitone (PB; 45 mg/kg body wt) at various hours on the day of first pro-oestrus. Maximal blockade of ovulation, in about 60% of the rats, occurred after PB treatment at 12.00, 13.00 and 14.00 h. The number of small antral follicles (100–499 × 105 μm3) was reduced 1 day after PB treatment in both blocked and ovulating rats. In the ovaries of non-ovulating rats signs of stimulation by LH such as dispersion of cumulus cells, oocyte maturation and early luteinization were sometimes present; in ovulating rats cystic corpora lutea with entrapped ova were found in addition to normal corpora lutea. Gonadotrophin measurements after PB treatment (14.00 h) in pubertal and adult rats showed (at 17.00 h) reduced levels of both LH and FSH, these levels being lower in the adults. Gonadotrophin levels of blocked and ovulating pubertal rats overlapped.
In PB-treated, pubertal rats in which ovulation was postponed by 1 day, vaginal oestrus was prolonged by 1 day and the subsequent dioestrus by 2 days.
The pubertal rat is thus less sensitive to PB treatment than the adult. PB treatment of the younger animal influences not only the ovulatory process but also follicular growth and, presumably, the length of the approaching cycle.
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SUMMARY
To study the mechanism of the ovulation-inhibiting effect of chlormadinone acetate, different quantities of this progestogen were implanted into one or both ovaries of dioestrous female rats. Introduction of the subcutaneously effective dose into one ovary suppressed ovulation in both, but implantation of half of this quantity did not influence spontaneous ovulation. On the other hand, 1/100 of the subcutaneously effective dose, implanted into the medio-basal hypothalamus or the anterior pituitary of adult dioestrous female rats, and of prepubertal females simultaneously injected with oestradiol benzoate, inhibited spontaneous and oestrogen-induced ovulations respectively. In juvenile rats the main site of action was the median eminence—anterior pituitary region. Chlormadinone was then implanted into the anterior hypothalamus or the anterior pituitary of adult dioestrous rats, and the median eminence was electrically stimulated during the 'critical period' in pro-oestrus. Since implants in the adenohypophysis prevented ovulation whereas implants in the anterior hypothalamus did not, a hypophysial site of action is suggested. On the basis of these results and of former findings on the action of oestrogens in ovulation, a hypothesis involving a competitive antagonism between oestrogen and progestogen at the hypophysial level is advanced to explain the acute ovulatory effects of these steroids.
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Rats with a 4-day oestrous cycle were given a single dose of atropine (100, 300, 500 or 700 mg/kg body wt) at 13.00 h on the days of oestrus, dioestrus 1, dioestrus 2 or pro-oestrus and were autopsied on the next expected day of oestrus. The doses of atropine (in mg/kg body wt) necessary to block ovulation during the cycle were 300 at oestrus, 100 at dioestrus 1 or 2 and 700 at pro-oestrus. A single dose of atropine (100 mg/kg) at oestrus, dioestrus 1 or dioestrus 2 was given at 09.00, 13.00, 17.00 or 21.00 h, autopsy again being performed on the next expected day of oestrus. The ability of atropine to block ovulation appeared to have a circadian rhythm, with a maximum blockade at 13.00 h on dioestrus 1 and dioestrus 2 and a minimum at 21.00 h on the same days. Hormone replacement (human chorionic gonadotrophin at oestrus, dioestrus 1 or 2, oestradiol benzoate at dioestrus 2 or progesterone at pro-oestrus) re-established normal ovulation in rats whose ovulation was blocked with atropine (100 mg/kg) on dioestrus 1 at 13.00 h. When ovulation was blocked with atropine but no hormone replacement had been given, rats ovulated 24 h after the next expected day of oestrus.
Results obtained in these experiments suggest the existence of a circadian rhythm of gonadotrophin secretion thoughout the oestrous cycle and a close relationship between that rhythm and the cholinergic system.
