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M. Caillol, M. Mondain-Monval and B. Rossano

Summary. In brown hares, which are induced ovulators, sexual behaviour occurs episodically at the beginning of pregnancy. From Day 34 (length of pregnancy is 41 days), the frequency of sexual chases followed by mating, ovulation and fertilization increased and 59% of pregnant females presented a natural superfoetation. The pattern of circulating luteinizing hormone (LH), follicle-stimulating hormone (FSH), oestradiol and progesterone was studied in 13 pregnant females left permanently with a male, and in 10 females isolated from males around Day 20 of pregnancy. In the 2 groups, FSH concentrations were high at the beginning and end of pregnancy. All females presented a peak value of FSH in the last 4 days of pregnancy, regardless of mating stimuli. This peak value was higher for females left permanently with a male than for isolated ones. Oestradiol concentrations fluctuated between 20 and 100 pg/ml, without any clear correlation with sexual behaviour, stage of pregnancy or profiles of other hormones. Prepartum matings occurred when progesterone values were still > 50 ng/ml; they were followed by a transient rise in LH and by a periovulatory progesterone secretion, with values above 100 ng/ml in the morning after mating. Such modifications of LH and progesterone were not detected before Day 34, suggesting that mating stimuli are not able to induce an LH surge at the beginning of pregnancy. After Day 34, mating can induce an LH surge, ovulation and superfoetation.

Keywords: brown hare; sexual behaviour; pregnancy; superfoetation; steroids; gonadotrophins

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M. Bonnin, M. Mondain-Monval and B. Dutourné

Summary. Oestrogen levels were low during most of gestation, but there was a significant increase (P <0·05) in oestradiol concentrations at implantation. Early pregnancy was characterized by high levels of progesterone which decreased significantly (P < 0·001) thereafter, but there was no decline in progesterone or rise in oestrogen levels at parturition. There was no difference in the length of progesterone secretion between pregnant and non-pregnant females.

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L. Martinet, M. Mondain-Monval and R. Monnerie

Summary. Mink are seasonal photosensitive breeders; testis activity is triggered when days have less than 10 h light. Increasing and decreasing plasma concentrations of prolactin induce the spring and autumn moults. In a 5 year experiment, males were maintained under short days (8 h light:16 h dark) at 13°C or long days (16 h light:8 h dark) at 21°C, winter and summer conditions, respectively. Under winter and summer conditions, circannual cycles of prolactin secretion and moulting were observed at intervals of about 11 months. Recurrence of testis cycles was not evident. In a second experiment, males were maintained under an 8 h light:16 h dark cycle from the winter solstice or under 10 h light:14 h dark, 12 h light:12 h dark or 14 h light:10 h dark cycles from 10 February. Under 8 h light:16 h dark cycle, testis regression was slightly later than under natural conditions, indicating photorefractoriness. However, mink remained sensitive to light: the longer the photoperiod, the faster the testis regression. In a third experiment, males were transferred under 8 h light:16 h dark or 16 h light:8 h dark from 15 May (group 1), 12 June (group 2) or 4 July (group 3); males submitted to long days received melatonin capsules on the day of transfer. Increasing concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and testis volume were shown by half the males in group 2 and nearly all the males in group 3; the constant release of melatonin from implants was more efficient than short days; but in the three groups, prolactin concentrations decreased in the few days after short-day or melatonin treatment. Overall, the results demonstrate endogenous circannual rhythms of prolactin secretion, body weight and moulting. Although a refractory period to short days was observed, the annual cycle of testis activity totally relies on the annual changes in daylength.

Keywords: circannual rhythm; photorefractoriness; melatonin; testis; prolactin; moult; mink

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M. Caillol, M. Mondain-Monval, M. Meunier and B. Rossano

Summary. Female hares were given an i.v. injection of 5 μg luteinizing-hormone-releasing hormone (LHRH) between Days 7 and 19 (n = 21), 20 and 33 (n = 17) and 34 and 41 (n = 17) of pregnancy, and in the 3 days after parturition (n = 16). Whatever the stage of pregnancy, the LHRH injection induced a release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and an acute secretion of progesterone; these hormonal responses increased significantly during pregnancy, to reach values similar to those observed in nonpregnant, nonpseudopregnant females during the breeding season in the 3 days after parturition. However, the release of LH remained monophasic in pregnant and post-partum females, in contrast to the unmated females during the reproductive season, in which there was a biphasic profile. The proportion of ovulating females after LHRH treatment was ∼60% at the beginning and end of pregnancy; and, after parturition, fell to 23% between Days 20 and 33. After Day 33, the pituitary response to LHRH was significantly higher in ovulating than in nonovulating females. At the beginning of pregnancy, 67% of females aborted after LHRH injection; after Day 20, the incidence of abortion decreased significantly and was 0% from Day 34. The amplitude and duration of progesterone secretion by the new corpora lutea resulting from ovulation after LHRH injection were similar to those of corpora lutea induced in nonpregnant females during the breeding season. These results show that, throughout pregnancy, despite high circulating progesterone concentrations, the pituitary and the ovary can respond to an LHRH injection by a release of LH and FSH, and by an ovulation. Before Day 34, the neural pathways between mating stimuli and induced ovulation are therefore blocked mainly at the hypothalamic level; from Day 34, the pituitary content of LH and FSH increases, and mating can be followed by an endogenous release of LHRH sufficient to induce a gonadotrophin surge and an ovulation. The mechanisms overriding the hypothalamic blockade of induced ovulation and allowing superfoetation remain to be determined.

Keywords: hare; pregnancy; superfoetation; LHRH; gonadotrophins; progesterone; ovulation

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M. Caillol, M. Meunier, M. Mondain-Monval and P. Simon

Summary. In the brown hare, fertile mating takes place from the beginning of December to September. Pituitary and ovarian response to a monthly i.v. injection of 5 μg LHRH was studied from September 1983 to October 1984 in 2 groups of 6 hares. The basal concentrations of LH remained undetectable until the end of January, rose from 0·23 ± 0·14 ng/ml from February to a maximum of 1·44 ± 0·57 ng/ml in July. LHRH injection was always followed by a release of LH. Between September and December, the LH value peaked 15 min after injection and returned to basal concentrations 2 h later. From January, this pattern altered and a second peak of LH appeared 2 h after injection. Peak levels 15 min after LHRH were around 10 ng/ml between September and December, increased from 47·0 ± 8·0 ng/ml in January to 106 ± 33 ng/ml in July and decreased in August (69·4 ± 10·6 ng/ml). The values of the second peak rose from 11·0 ± 2·2 ng/ml in January to 90·6 ± 12·4 ng/ml between March and July and decreased in August (24·5 ± 5·1 ng/ml). The LH surge induced by LHRH was always followed by a transient rise in progesterone. During the breeding season, this progesterone secretion increased considerably. Ovulation was possible between January and August and the number of ovulating females was maximum between March and July. The amount and duration of progesterone secretion during the resulting pseudopregnancies increased during the breeding season.

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M. Caillol, M. Mondain-Monval, M. Meunier and A. S. McNeilly

Summary. In the brown hare, fertile mating takes place from the beginning of December to September. Seasonal variations of basal concentrations of LH and FSH, and pituitary response to a monthly i.v. injection of LHRH were studied in intact control females and in females ovariectomized during the seasonal anoestrus (OVX1) or during the breeding season (OVX2). In intact females, both basal and LHRH-stimulated LH levels showed an annual variation, with minimal values during anoestrus. During the breeding season, the LH response to LHRH exhibited a biphasic pattern. In contrast, there was no clear seasonal variation in basal and LHRH-stimulated FSH concentrations. After ovariectomy during anoestrus, basal LH remained low for 2 months and began to increase in December. After ovariectomy during the breeding season, LH basal concentrations increased within a few days after the operation. Thereafter, LH values remained high in both groups of females until September, and decreased significantly as in intact females. The pattern of LH release after LHRH remained monophasic in the two groups of ovariectomized females. In OVX1 females, the LH response increased as early as October, was maximum from December to April and decreased progressively until October. In OVX2 females, the LH response decreased regularly after ovariectomy to a minimum in October. In the 2 groups of ovariectomized females, basal FSH concentrations and pituitary response to LHRH rose rapidly after ovariectomy and did not vary significantly thereafter. These results showed a direct central effect of season on the regulation of basal concentrations of LH, modulated by a negative feed-back of ovarian secretions during the breeding season. In intact hares, the enhanced LH response after LHRH during the breeding season was related to an acute positive effect of ovarian secretions. The regulation of FSH was less dependent on season and remained under a negative control of the ovary throughout the year.

Keywords: brown hare; LH; FSH; LHRH; ovariectomy; season

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M. Caillol, M. Mondain-Monval, M. Meunier and B. Rossano

Summary. The pituitary and ovarian responses to a monthly i.v. injection of 5 pg luteinizing-hormone-releasing hormone (LHRH) were studied in three groups of young doe hares, born in January–February (group I), in April (group II) or at the end of the breeding season (August–September, group III). The LHRH injection was always followed by a release of LH and progesterone, which did not differ among the three groups at 3 months of age. The pituitary and ovarian responses to LHRH increased gradually from the age of 3 months in groups I and III and from the age of 9 months in group II. One female of the ten born in January–February ovulated and reached puberty in June, at the age of 4 months, but with a weak pituitary response. The females born in April displayed a seasonally delayed puberty, at 9 months of age (two of five females ovulated in the next January). Four of the five females born at the end of the breeding season ovulated after LHRH when 5 months old (in February), with a full pituitary–ovarian response. The low pituitary response of group I in June–August, even if 10–20% of females ovulated after LHRH, suggests a need for a period of short days. Then, the most favourable conditions for the hare to reach puberty would be a period of short decreasing daylengths during the fall, followed by increasing daylengths after the winter solstice.

Keywords: brown hare; season of birth; puberty; LHRH; LH; progesterone; ovulation

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I. Stoufflet, M. Mondain-Monval, P. Simon and L. Martinet

Summary. Peripheral plasma progesterone concentrations exhibited an increase 10 days before implantation, coinciding with the resumption of blastocyst growth and with a decrease in plasma androgen values (DHA, androstenedione, testosterone). No definite pattern of oestrone was observed and oestradiol concentrations remained undetectable. The production of steroids by dispersed luteal cells showed that the growth of the corpora lutea paralleled that of blastocysts and resulted in hypertrophy followed by hyperplasia of the luteal cell. The production of progesterone in the medium increased with blastocyst size up to implantation; it was enhanced by mink charcoal-treated serum, but prolactin, LH, FSH or a combination of these hormones did not affect the progesterone production, whatever the stage of diapause. DHA and androstenedione secretion increased in the two last stages of blastocyst growth and was enhanced by LH. The conversion of androstenedione and testosterone into oestrone and oestradiol was observed at all stages of embryonic diapause, indicating that corpora lutea contain aromatase activity even at an early stage. The secretion of oestrone was higher than that of oestradiol. The non-luteal tissue contributed up to 50% of the steroid production; while progesterone and androgen production remained constant, that of oestradiol decreased at the end of the delay period. These results indicated a change in the size and the secretory capacity of the luteal cell related to blastocyst development and implantation. Although progesterone was the main product of the corpora lutea, androgens and oestrogens were also secreted.

Keywords: delayed implantation; mink; corpus luteum; steroid hormones; in vivo; in vitro

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O. M. Møller, M. Mondain-Monval, A. Smith, E. Metzger and R. Scholler

Summary. During pro-oestrus, baseline LH concentrations for 9 vixens (pooled data) ranged from 0·8 to 5·3 ng/ml. In each vixen, baseline levels were interrupted by elevations of LH ranging from 3·1 to 10·4 ng/ml. A major preovulatory LH surge was detected in all the vixens. The LH peak ranged from 13·5 to 73·0 ng/ml with an average of 27·8 ± 18·8 (s.d.) ng/ml. Plasma LH concentrations declined to a basal level of 1·3 ± 1·0 ng/ml within 48 h of the peak value. The duration of the LH surge was 1–3 days. The LH peak occurred 1 or 2 days before any sexual receptivity was observed. All the vixens were mated twice 2–5 days after the LH peak; 8 conceived. Plasma concentrations of oestradiol-17β increased gradually during the last 6–7 days before oestrus and reached maximum values (124–373 pg/ml) at the time of the preovulatory LH peak. The first significant increase in plasma progesterone concentration occurred simultaneously with the LH peak. During oestrus (normally 3–5 days), progesterone levels rose steeply, attaining a mean concentration of 57·0 ± 17·5 ng/ml when the vixens went out of heat. Androstenedione and testosterone values changed similarly, both increasing at the beginning of pro-oestrus and reaching maximum values (805–1879 pg/ml and 328–501 pg/ml respectively) 1 day before to 1 day after the oestradiol-17β peak.

The electrical resistance of the vaginal tract increased rapidly during the last 2–3 days of pro-oestrus, reaching a maximum value (300–640 Ω) ~2 days after the oestradiol-17β peak that corresponded with the onset of sexual receptivity. Towards the end of oestrus, the values fell to 100–200 Ω.

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M. Mondain-Monval, A. J. Smith, P. Simon, O. M. Møller, R. Scholler and A. S. McNeilly

Summary. A heterologous radioimmunoassay system developed for the sheep was shown to measure FSH in the plasma of the blue fox. FSH concentrations throughout the year showed a circannual rhythm with the highest values (61 ·6 ± 14·8 ng/ml) occurring shortly before or at the onset of the mating season, a pattern similar to that of LH. The concentration of FSH then declined when androgen concentrations and testicular development were maximal at the time of the mating season (March to May). Thereafter, concentrations remained low (25·2 ± 4·1 ng/ml) in contrast to those of LH. Implantation of melatonin in August and in February maintained high plasma values of FSH after the mating season (142·3 ± 16·5 ng/ml) in association with a maintenance of testicular development and of the winter coat. The spring rise of prolactin was suppressed by melatonin treatment. The release of FSH after LHRH injection was also increased during this post-mating period in melatonin-treated animals, in contrast to the response of the control animals which remained low or undetectable.

These results suggest that changes both in the secretions of FSH and prolactin may be involved in the prolongation of testicular activity and in the suppression of the spring moult after melatonin administration.

Keywords: blue fox; FSH; melatonin; LHRH; seasonal cycle