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Previous studies have shown that while depletion of brain serotonin by the administration of p-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase, blocks the daily surge of LH in oestrogen-treated ovariectomized rats, restoration of serotonin synthesis by treatment with its immediate precursor, 5-hydroxytryptophan (5-HTP), at a critical time of day, reinstates the surge. The present study indicates that the experimental procedure involving serotonin depletion and its subsequent replenishment may also be used to control the preovulatory LH surge and ovulation in intact cyclic rats provided that (1) the PCPA is administered subcutaneously rather than intraperitoneally and (2) the 5-HTP is given in conjunction with carbidopa, a peripheral decarboxylase inhibitor: the latter observation providing further evidence for a central role for serotonin in the control of ovulation. These precautions were unnecessary when oestrogen was administered at the same time as the PCPA. It appears that PCPA administered intraperitoneally results in a suppression of the preovulatory rise in oestrogen secretion (and may have additional deleterious effects at the level of the ovaries) and that 5-HTP, in the absence of supplementary oestrogen, may block ovulation by peripheral action after conversion to serotonin. This study indicates the need for caution when using pharmacological 'cocktails' to investigate neuroendocrine events underlying ovulation when the experiments are carried out in the presence of the ovaries.
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SUMMARY
In order to determine whether the timing of ovulation in rats was controlled by an endogenous circadian rhythm, the hour of ovulation was determined by observing tubal ova during laparotomy in adult rats exposed to full animal room illumination (150 lux) during daily photoperiods of 14 h (full LD), continuous 150 lux illumination (full LL), daily dim (0·2 lux) photoperiods of 14 h (dim LD), continuous 0·2 lux illumination (dim LL) or continuous darkness (DD). Rats in all groups except those exposed to full LL continued normal cyclic ovulation. By the second oestrous cycle, most rats in the full LL group failed to ovulate, even though they showed characteristic cyclic changes in the vaginal smear pattern. The hour at which ovulation occurred was similar in rats exposed to full LD, dim LD or DD but was delayed in rats exposed to full LL or dim LL; the longer the period of exposure, the greater was the delay. For a given length of exposure, ovulation was delayed more in full LL than in dim LL. The full LL used in this study produced persistent vaginal oestrus within 40 days, whereas the dim LL did not. The delayed ovulation in rats exposed to dim LL was associated with a delayed preovulatory surge of LH. These results are consistent with the hypothesis that the timing of the preovulatory surge of LH and ovulation are controlled by an endogenous circadian rhythm, which in most rats has a periodicity in continuous light of slightly longer than 24 h.
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ABSTRACT
Two experiments were conducted on White Leghorn hens to investigate the effect of different concentrations of LH on the frequency of induction of ovulation in vitro of ovarian follicles collected 18–20 h before the expected time of ovulation from birds of three different ages (7, 18 and 30 months) or of the same age but laying short (three to four) or long (more than seven) sequences of eggs. The ability of LH to induce ovulation was directly related to the age of the donors of the follicles. Follicles derived from hens laying short sequences of eggs were less responsive to the ovulation-inducing effects of LH than were follicles from birds laying long sequences. These observations suggest that the sensitivity of ovarian follicles to the ovulation-inducing effects of LH declines with age and is greater in hens laying long sequences of eggs than those laying short sequences.
J. Endocr. (1985) 106, 67–69
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Search for other papers by HILDA M. BRUCE in
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SUMMARY
Stilboestrol was administered in the drinking water at several different concentrations to groups of normal adult rats and mice.
Continuous vaginal cornification was obtained with the higher dose levels and both species showed a capacity for repeated coitus at intervals corresponding approximately to those separating oestrous periods in untreated animals. The response was much more uniform in the mice.
Inhibition of ovulation required dose levels well above those that clearly influenced the nature of the vaginal smear. The great majority of ovulated eggs were found to undergo spermatozoon penetration, and supplementary spermatozoa were commonly seen. Results suggest that the frequency of spermatozoon penetration was increased by moderate oestrogen dosage. Fertilization appeared to be quite normal.
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Department of Pharmacology, Chelsea College, Manresa Road, London, SW3 6LX, and *Department of Physiology, St George's Hospital Medical School, Tooting, London, SW17 OQT
(Received 12 May 1977)
There is evidence accumulating to suggest that prostaglandins (PG) have a hypothalamic action in mediating the preovulatory surge of luteinizing hormone from the pituitary gland. Intraventricular injection of inhibitors of PG synthesis has been shown to block ovulation (Behrman, Orczyk & Greep, 1972; Orczyk & Behrman, 1972) and this effect can be overcome by subsequent administration of PG or luteinizing hormone releasing hormone. Similarly, PGE2, administered intraventricularly, can overcome the ovulatory blockade induced by the α-adrenoceptor antagonist phentolamine (Linton, Perkins & Whitehead, 1977). In this study, we have investigated the action of the prostaglandin antagonist N-0164 (Eakins, Rajadhyaksha & Schroer, 1976) on ovulation in the rat.
Female Wistar rats weighing between 230 and 280 g were maintained under a controlled lighting schedule (12
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* Department of Physiology & Biochemistry, The University, Whiteknights, Reading, RG6 2AJ, and † Imperial Chemical Industries Ltd, Pharmaceuticals Division, Mereside, Alderley Park, Macclesfield, Cheshire, SK104TG
(Received 4 April 1975)
Fraps (1961) proposed that in the hen an 'excitation' hormone secreted by the developing follicle initiated the release of the luteinizing hormone (LH) required for ovulation. Since progesterone induced premature ovulation in the hen (Fraps & Dury, 1943) it was considered the most likely candidate for such a role. Evidence showing that plasma progesterone rises either immediately before, or coincidentally with, the plasma LH surge is consistent with this hypothesis (Furr, Bonney, England & Cunningham, 1973). It has been demonstrated recently, however, that oestradiol (Senior & Cunningham, 1974) and testosterone (Etches, 1974) are also increased before the preovulatory rise in LH, which means that they cannot be precluded from consideration. Certainly there is ample evidence that oestradiol induces the LH
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Recently a single hypothalamic hormone, termed luteinizing hormone releasing hormone (LH-RH), capable of releasing luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary has been chemically characterized and synthesized (Matsuo, Arimura, Nair & Schally, 1971; Matsuo, Baba, Nair, Arimura & Schally, 1971) and has been shown to be biologically active in several species (Schally, Arimura & Kastin, 1973). Initial attempts to demonstrate LH-RH action in 4-day cyclic mice treated with sodium barbitone in pro-oestrus were unsuccessful because barbitone did not consistently block ovulation (R. G. Gosden, unpublished results). In this report we demonstrate the action of LH-RH in adult female mice as shown by the induction of ovulation and the increase of serum LH.
Young virgin (8–14 weeks) CFLP mice (Carworth Europe) were injected with 3 i.u. pregnant mare serum gonadotrophin (Intervet) i.p. at 16.00–17.00 h without reference to the stage of the oestrous cycle. Approximately 44 h later
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SUMMARY
The response to gonadotrophins in an assay based on the induction of ovulation in immature mice was reduced by treatment of the experimental animals with barbitone. The responses to follicle-stimulating hormone and luteinizing hormone (LH) were reduced to a similar degree. Lack of specificity of the assay for LH is probably not due simply to the endogenous secretion of this hormone.
Progesterone and oestradiol-17β had no definite effect on the assay but norethynodrel significantly reduced the slope of the dose-response line. Feeding the mice with methylthiouracil increased the sensitivity to a small but significant extent.
Assays depending on the induction of ovulation in immature, pregnant and mature dioestrous mice were compared. Mice of the first two types gave more sensitive and precise assays of human urinary gonadotrophin.
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It has been demonstrated in the rat that on the day of prooestrus there is a critical period for the initiation of the release of the 'ovulating gonadotrophic complex' from the pituitary gland (Everett, 1952). Thus, in animals kept under controlled lighting conditions with illumination from 5 a.m. to 7 p.m. the critical period is between 2 and 4 p.m., ovulation occurring between 1 and 2.30 a.m. the next day.
Bourdel (1961) showed that rabbit anti-ovine-LH serum could neutralize endogenous LH in immature female rats, and more recently Bourdel & Li (1963) found that this antiserum, when injected into adult female rats daily for 12 days beginning 36 hr. in advance of presumed oestrus, could suppress this oestrus and subsequent ones. If the first injection were given only 12—16 hr. in advance, the first expected oestrus occurred normally, although subsequent oestrous periods were suppressed. Similarly, Young, Nasser & Hayashida (1963)
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The activity of the thyroid gland is highest during oestrus in regularly cyclic adult female rats, and it has been suggested (Brown-Grant, 1962) that this may be due to a period of hypothalamic stimulation of pituitary TSH secretion during prooestrus. The release of luteinizing hormone that leads to ovulation can be blocked by a suitably timed injection of 'Nembutal' (sodium pentobarbitone) during the day of prooestrus and if the increase in thyroid activity were also inhibited this would provide additional evidence in favour of this concept.
Rats were kept under the same conditions and the vaginal cycles followed in the same way as in earlier studies (Brown-Grant, 1962). Thyroid activity was measured by determining the 2·5 hr. uptake of 131I by the gland on the morning of the predicted day of oestrus. The occurrence or failure of ovulation was determined by the examination of serial sections of the ovaries.
