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There is increased evidence that serotonin (5-HT) plays an inhibitory role in the control of ovulation. Increased hypothalamic 5-HT and intraventricular injection of the drug block ovulation (Kordon, 1969; Schneider & McCann, 1970; Domanski, Przekop & Skubiszewski, 1972; Labhsetwar, 1972; Mess, Treintini & Tima, 1972). Serotonin injected in vivo blocks spontaneous ovulation (Labhsetwar, 1971) and reduces hypothalamic but not cortical oxygen consumption in vitro (Campos & Ladosky, 1972; Ladosky & Campos, 1972).
Baumgarten, Björklund, Lachenmayer, Nobin & Stenevi (1971) introduced 5,6-dihydroxytryptamine (5,6-DHT) for long-lasting selective depletion of brain 5-HT. Due to these properties 5,6-DHT is a better tool than p-chlorophenylalanine previously used (Labhsetwar, 1972). In these experiments 50 μg 5,6-DHT were infused into the third ventricle of intact male and female rats and gonadectomized animals. All were bled 10 days later and luteinizing hormone (LH) was assayed by radioimmunoassay. Results were compared with those from saline-perfused intact and gonadectomized
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Search for other papers by D. P. GILMORE in
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SUMMARY
In the majority of normal cycling female rats, a single injection of norethisterone (0·5 mg.) delayed ovulation by 24 hr. The number of animals showing delayed ovulation was dependent upon the stage of the cycle and the time of day at which the steroid was given. Injection on day 1 (oestrus) and day 3 (dioestrus 2) was most effective (80% delay) followed by day 2 (dioestrus 1, 50% delay) and day 4 (pro-oestrus, 0% delay). Oestradiol benzoate (5 μg.) given to norethisterone-treated rats on day 3 largely overcame the delaying action of the steroid. Similarly the injection of progesterone (1 mg.) on day 4 advanced ovulation by 24 hr. in most of the norethisterone-treated rats. It is suggested that norethisterone exerts its effects at both pituitary and hypothalamic levels.
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Search for other papers by W. F. STRAUSS in
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SUMMARY
In immature female rats injected with PMSG at 30 days of age (day 30), ovulation occurs between the hours of 02.00 and 03.00 on day 33. If progesterone is injected at 10.00 h on day 32, the onset of ovulation is advanced by 1–2 h. In rats that were not given progesterone, ovulation was blocked by phenobarbitone sodium administered on day 32 before 13.50 h. However, pretreatment with progesterone at 10.00 h caused ovulation to occur in spite of phenobarbitone treatment at 13.50 h. An early release of ovulatory gonadotrophin from the anterior pituitary gland cannot completely account for progesterone's capacity to reverse the blockade of ovulation by phenobarbitone, because when phenobarbitone treatment was advanced by 2–4 h, ovulation still occurred in most progesterone-treated rats.
Search for other papers by P. VAN DER SCHOOT in
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Injection of sodium pentobarbitone (Nembutal) into rats between 12.00 and 13.00 h on the day of pro-oestrus was fully effective in blocking the expected ovulation. In 75% of the rats, ovulation of the present generation of large follicles occurred 24 h later (delayed ovulation). Injection of Nembutal between 12.00 and 13.00 h on the day of pro-oestrus and at the same time on the subsequent day was fully effective in blocking the ovulation twice: in only two rats out of 13 did ovulation of the present generation of follicles still occur. When unilateral ovariectomy was performed immediately after injection of a single dose of Nembutal into pro-oestrous rats, delayed ovulation was significantly inhibited. However, after inhibition of ovulation by either two injections of Nembutal or one injection and unilateral ovariectomy, delayed ovulation could be induced by treatment with a small dose of oestradiol benzoate during the Nembutal-induced anaesthesia. It thus seemed that delayed ovulation failed because of disruption of oestrogen production after administration of Nembutal. The concentration of oestradiol-17β in the plasma of Nembutal-treated pro-oestrous rats decreased rapidly during the 24 h after treatment. It is concluded that this decrease in the concentration of oestradiol is due to the inherent ageing of preovulatory follicles manifesting itself when exposure to the ovulatory surge of LH is inhibited.
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Ovarian follicular development was studied in the rat during a 15-day period preceding first ovulation. Ovaries were obtained by unilateral ovariectomy performed at various ages and the rats were allowed to live until the day after first ovulation. The timing of this ovulation was compared with that in unoperated, paired control rats of the same age. For estimation of gonadotrophin levels, blood was taken from the paired control rats at the time when experimental rats were unilaterally ovariectomized. There was no evidence that unilateral ovariectomy had any influence on the timing of first ovulation. Therefore the ovaries obtained could be dated in relation to first ovulation, and follicular growth during the final prepubertal period could thus be studied in a genuine developmental sequence. Results revealed that follicular growth leading to first ovulation starts at ±8 days before this ovulation; follicular processes taking place are comparable to those found during the adult 5-day cycle but proceed more slowly.
Gonadotrophin concentrations accompanying the follicular dynamics and measured at 11·00 h, showed a clear tendency for FSH concentrations to decrease with increasing age, i.e. approaching first ovulation. Concentrations of LH did not show a definite pattern and were generally low, although in some individual rats relatively high LH values (> 100 μg/l) were found in the period of 5–3 days before first ovulation.
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The present study examined the ovarian progesterone levels in rats during the normal 5-day oestrous cycle and in rats with an experimentally induced delayed or advanced ovulation. Ovarian androgen and oestrogen concentrations were also studied in rats with an advanced ovulation.
The ovarian progesterone concentrations during the normal cycle showed two peaks: one during metoestrus and dioestrus 1 and the second during the afternoon of pro-oestrus.
Rats treated with pentobarbitone sodium (35 mg/kg body weight) at 13.00 h on the day of pro-oestrus showed no increase in ovarian progesterone levels on that day, but did show an increase during the afternoon of the next day.
Rats injected intraperitoneally with an ovulation-inducing bolus of human chorionic gonadotrophin (hCG; 20 i.u.) at 15.00 h on the day before pro-oestrus (dioestrus 2) showed increased ovarian progesterone levels as early as 0·5 h after hCG injection. Lowered levels of ovarian androgen and oestrogen values compared with saline-injected rats were first found 2 and 6 h respectively after hCG injection.
In conjunction with data obtained previously these findings show that normal, delayed and advanced ovulations are preceded by an increase in ovarian progesterone concentrations and a subsequent decline in ovarian androgen and oestrogen levels.
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Search for other papers by C. M. CHRISTENSON in
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Search for other papers by M. X. ZARROW in
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SUMMARY
Pheromonal facilitation of pregnant mare serum gonadotrophin (PMSG)-induced ovulation was compared in four strains of immature mice after exposure to adult intact males, castrated males, castrated males after treatment with testosterone propionate, or exposure to adult females. Psychobehavioural modification of their response was studied after transferring PMSG-injected females between cages, or after their exposure to either electric foot shock or adverse auditory stimuli. Significant facilitation of the average number of ova released and percentage of mice ovulating was found in females of the strains C3HeB/FeJ and BALB/cJ, but not in strains DBA/2J and C57BL/6J after exposure to adult males. Exposure to castrated males treated with testosterone propionate resulted in a response essentially the same as that to intact males, whereas exposure to adult females inhibited ovulation uniformly in all the strains studied except BALB/cJ. Auditory stimuli prevented this phenomenon in strains C3HeB/FeJ and C57BL/6J, but an obvious facilitation was observed in strains BALB/cJ and DBA/2J. Electric shock facilitated ovulation in strain DBA/2J but not in the other three strains.
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Search for other papers by K. BROWN-GRANT in
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SUMMARY
The effects of different doses of pregnant mare serum gonadotrophin (PMS) were examined in 29-day-old immature rats; a dose great enough to produce follicles capable of ovulating in response to the additional injection of human chorionic gonadotrophin (HCG) 56 hr. later in the majority of animals also caused significant oestrogen secretion as judged by uterine weight. Maximal uterine growth resulted from doses of PMS that led to a significant incidence of 'spontaneous' ovulation. In contrast, administration of ovine follicle-stimulating hormone (FSH) resulted in the ripening of large numbers of follicles with much less evidence of oestrogen secretion; 'spontaneous' ovulation did not occur in FSH-treated animals but ovulation could be induced by additional treatment with oestrogen and progesterone in combination but not by either steroid alone. Previous exposure to oestrogen appears to be necessary for the facilitatory action of progesterone on luteinizing hormone release to be demonstrable and the FSH-primed immature rat is a convenient preparation to study this interaction.
Search for other papers by R. SRIDARAN in
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Search for other papers by C. E. McCORMACK in
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SUMMARY
Continuous monitoring of wheel-running activity and determination of the time of ovulation in rats by serial laparotomies revealed that ovulation followed the onset of running at prooestrus by approximately 9 h (range 7–11 h). This temporal relationship held in rats in which the period of the circadian rhythm had been modified (entrained) by daily exposure to 14 h photoperiods, and in rats in dim continuous light whose rhythms were non-entrained (freerunning). Knowledge of this temporal relationship between the two rhythms made it possible to give bright light signals at known points in the circadian cycle of the rat and to observe the effects on the timing of running and ovulation in subsequent cycles. Giving daily light signals near the onset of running (i.e. at subjective dusk) delayed, whereas giving signals near the end of running (i.e. at subjective dawn) advanced, the time of running and ovulation in subsequent cycles. These results indicate that in rats exposed to the usual laboratory photoperiod the delaying effect of dusk light and the advancing effect of dawn light balance one another; thus the preovulatory surge of LH occurs at a relatively consistent time at prooestrus.
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Search for other papers by K. B. RUF in
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The 'positive feedback' effect of exogenous oestradiol-17β in advancing ovulation induced by pregnant mare serum gonadotrophin (PMSG) has been used in the present study as a model in which to test the possible oestrogenic or antioestrogenic effects of the catechol oestrogens, 2-hydroxyoestradiol (2-OHE2) and 4-OHE2. Sprague–Dawley rats of 26 days of age were injected with 20 i.u. PMSG together with either vehicle alone or test steroids. The animals were killed 72 h later and the Fallopian tubes were examined for the presence of ova. Advancement of induced ovulation by treatment with oestradiol was confirmed; 2-OHE2, in doses of up to 100 pg, influenced neither the time of ovulation nor the number of ova present but 4-OHE2 was equipotent with oestradiol in doses varying from 0·5 pg (the minimum effective dose for both steroids) to 10 μg. The possible antioestrogenic effect of 2-OHE2 was tested by giving a 100 pg dose either at the same time or 2 h before PMSG plus 2 pg oestradiol or 4-OHE2. The effects of oestradiol and 4-OHE2 were not altered by this treatment. These data show that, in this model of'positive feedback', 2-OHE2 has neither an oestrogenic nor an antioestrogenic action but that 4-OHE2 has a potent oestrogenic action, thus raising the question of a physiological role for 4-OHE2 in the regulation of ovulation.
