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B. P. Fitzgerald and L. B. Mellbye

Summary. Two experiments were performed to test the hypothesis that the seasonal suppression of gonadotrophin pulse frequency in anoestrous horse mares reflects inhibitory neural mechanisms. In a preliminary experiment (Exp. 1) conducted in February, 4 anoestrous mares were sedated by repeated intravenous injections of xylazine, an α2-adrenergic receptor agonist. On the day of treatment, 1–2 LH pulses were observed in xylazine-treated mares. In contrast, during a 12-h period only 1/8 untreated control mares exhibited a LH pulse. In Exp. 2, the effect of xylazine-induced sedation on pulsatile gonadotrophin release was examined in 4 anoestrous mares on two occasions before (18 November and 9 December) and after (23 December and 6 January) an abrupt, artificial increase in day length. Treatment with xylazine was associated with an overall increased FSH (P < 0·01) and LH (P < 0·05) pulse frequency, compared with that observed during 12-h pretreatment periods. To evaluate an effect of treatment at the various time during the experimental period, the change in FSH pulse frequency was analysed, since occasionally FSH pulses were unaccompanied by a change in serum LH values indicative of a LH pulse. On two occasions before increased daylength only 1/4 and 3/4 mares exhibited an increase in FSH pulses; in contrast, 14 days after increased daylength (23 December), 4/4 mares exhibited increased FSH pulse frequency associated with treatment. After 27 days of increased daylength (6 January), endogenous FSH pulse frequency was greater than before increased daylength and treatment with xylazine was unaccompanied by a further increase.

The results support the hypothesis that an inhibitory catecholaminergic neural system may play a role in the seasonal suppression of pulsatile gonadotrophin secretion in the anoestrous mare. The activity of this inhibitory system appears to decrease towards the approach of the breeding season.

Keywords: gonadotrophins; pulse frequency; seasonal inhibition; xylazine; anoestrous mares

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B. P. Fitzgerald and F. J. Cunningham

Summary. Daily determinations of FSH and prolactin in plasma were made for 10–12 weeks after parturition in ewes which lambed either in the middle of (December) or late in (February) the breeding season. Fluctuations in the plasma concentrations of FSH could not be related to the time after parturition or to the occurrence or otherwise of oestrus and ovulation. However, there was evidence of an increased secretion of prolactin post partum but only in those ewes which lambed in February. The lack of an elevated level of prolactin during the post-partum period in the December-lambing ewes was associated with an earlier return to oestrus by these animals. The suppression of prolactin concentrations by treatment with bromocriptine to undetectable values in the December-lambing ewes was not associated with an earlier return to oestrus. The removal of lambs at various times post partum from those ewes which lambed in February was not associated with any marked changes in prolactin secretion. The results suggest that when the plasma concentrations of prolactin are low post partum there is a greater likelihood of an earlier resumption of breeding activity in the ewe.

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P. J. Silvia, L. Johnson and B. P. Fitzgerald

Summary. In mares, the amount of gonadotrophin-releasing hormone (GnRH) is low in the hypothalamus during seasonal anoestrus, but by early spring, concentrations of GnRH are high. The timing of this response was characterized more precisely by determining concentrations of GnRH in hypothalamic tissue collected immediately before and at various times after the winter solstice (22 December 1986). Ovaries, pituitary gland, hypothalamus and a blood sample were collected from six groups of mares (6–12 mares per group) at death, 1 week before day of the winter solstice and 1, 2, 3 and 12 weeks afterwards. No significant changes in weight of the anterior pituitary gland or concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were observed in the anterior pituitary gland (P > 0·1). Mean diameter of the largest follicle, number of follicles ≥ 20 mm in diameter and concentrations of LH and FSH in serum remained unchanged for weeks −1 to +3 (P > 0·05), then increased significantly by week 12 (P < 0·001). Content and concentration of GnRH in the median eminence was low at −1 week, increased gradually (P < 0·05) to a maximum by + 1 week, then decreased gradually (P < 0·05) to low values at 12 weeks. Means (±sem) for −1, +1 and +12 weeks were 33·5 ± 5·5, 117·7 ± 18·6 and 29·8 ± 3·7 ng GnRH, respectively. Mean content of GnRH in the preoptic area of the hypothalamus showed a reciprocal pattern. Since content of GnRH in the median eminence was increasing on the day of the winter solstice, the impetus for resurgence of GnRH synthesis may not be lengthening photoperiod, but refractoriness to the previous short photoperiod. Increased gonadotrophin content observed at 12 weeks suggests that the coincidental reduction in content of GnRH may be due to an increase in secretion. One role of lengthening photoperiod may therefore be to boost production of GnRH during a period of increasing release.

Keywords: mare; LH; GnRH; hypothalamus; pituitary; photoperiod

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B. P. Fitzgerald, Janet D. Evins and F. J. Cunningham

Summary. The concentrations of prolactin in plasma were measured before and after the injection of TRH in pregnant ewes and in non-pregnant ewes during the breeding season and seasonal anoestrus. During the first 80 days of pregnancy basal concentrations of prolactin were low and stable but thereafter increased progressively to reach maximum values at Day 140. During a comparable time of year (December-April) basal concentrations of prolactin in non-pregnant ewes were not significantly different from those found between Days 20 and 120 of pregnancy. At other times of the year basal concentrations of prolactin in non-pregnant ewes were elevated during anoestrus and declined markedly at the onset of the breeding season. In pregnant and non-pregnant ewes the responsiveness of the pituitary gland to stimulation with TRH was significantly correlated with the basal concentration of prolactin in plasma: the raised concentration of prolactin at Day 140 of pregnancy and in July in non-pregnant ewes was associated with the greatest release of prolactin from the pituitary gland. A change in the responsiveness of the pituitary gland may play an important role in the overall control of prolactin secretion in the ewe.

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D. K. Vanderwall, W. J. Silvia and B. P. Fitzgerald

The reproductive tracts of nine thoroughbred mares were examined by ultrasound to determine the day of ovulation (day 0). Mares were fitted with intercavernous sinus cannulae on the day before the start of sample collection of pituitary venous effluent rich in oxytocin. Intercavernous sinus blood samples were collected for at least 36 h at 5 min intervals beginning at noon on day 13 (n = 2), day 15 (n = 5) or day 16 (n = 2) after ovulation. Concentrations of oxytocin and 13,14-dihydro-15-keto prostaglandin F (PGFM) in plasma were determined by radioimmunoassay. Three high-magnitude surges of PGFM (> 1 ng ml−1) were found in these samples. Three high magnitude pulses of oxytocin (> 200 pg ml−1) were also observed, one associated with each of the PGFM surges. In each of these cases, the oxytocin pulse appeared to follow or coincide with the onset of the PGFM surge. Lower magnitude pulses of both hormones were detected throughout the bleeding period in every mare. The average interval between these pulses was 122.3 min for oxytocin and 121.0 min for PGFM. The interval between pulses for individual mares varied from 90 to 199 min for oxytocin, and from 87 to 213 min for PGFM. However, there was no correlation between PGFM and oxytocin pulse intervals among mares. Within each mare, there was no discernable association between low magnitude pulses of oxytocin and PGFM. From these data, it was concluded that high-magnitude surges of PGF are associated with similar surges of oxytocin from the posterior pituitary gland, and that PGF may induce their secretion. The posterior pituitary gland also appears to secrete oxytocin in a pulsatile manner at a frequency of approximately 1 pulse every 2 h but these pulses do not appear to be associated with the low magnitude pulses of PGF secreted from the uterus.

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B. P. Fitzgerald, H. I'Anson, R. G. Loy and S. J. Legan

Summary. Administration of a GnRH agonist (5 μg) every 12 h to long-term ovariectomized ewes for 5 or 10 days during the breeding season suppressed mean LH levels from around 6 to 1 ng/ml on Days 1 and 4 after treatment; on Day 1 after treatment LH pulse frequency and amplitude were lower than pretreatment values. On Day 4 after treatment LH pulse frequency was restored to pretreatment levels (1 per h) whereas LH pulse amplitude had only slightly increased from 0·5 to 1 ng/ml, a value 25% of that before treatment. This increase in amplitude was greater the shorter the duration of treatment. Ovariectomized ewes treated with the agonist for 5 days exhibited both negative and positive feedback actions after implantation of a capsule containing oestradiol; however, compared to control ewes treated with oestradiol only, the positive and negative feedback actions of oestradiol were blunted.

These results suggest that the recovery of tonic LH concentrations after GnRH agonist-induced suppression is limited primarily by changes in LH pulse amplitude. The results also demonstrate that the feedback actions of oestradiol are attenuated, but not blocked, by GnRH agonist treatment.

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J. S. Walton, Janet D. Evins, B. P. Fitzgerald and F. J. Cunningham

Summary. The plasma concentrations of FSH, LH and prolactin in ewes were measured at frequent intervals during 24-h periods in anoestrus at different times from the onset of the breeding season. Ewes kept under natural daylength conditions were compared with ewes in which the onset of the breeding season was advanced by exposure to constant short days (8L:16D). The concentrations of FSH during mid-anoestrus did not vary during the day and there was no effect of short days or any changes which could be associated with the onset of ovulation. In all of the ewes pulsatile releases of LH were observed on each sampling occasion. During mid-anoestrus the occurrence of LH pulses varied between 1 and 3/day and one of the pulses appeared to be synchronized with dawn. Exposure to short days did not affect the frequency of LH release. In both groups of ewes an increased frequency of LH pulses was observed in the period 12–14 days before the first ovulation but this was associated with a decrease in the magnitude of each pulse. Prolactin concentrations were raised during anoestrus and tended to be higher during the hours of darkness and in the early morning. Exposure to short days for 3 weeks abolished these diurnal changes and reduced the concentrations to non-detectable amounts.

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B. P. Fitzgerald, Helen I'Anson, R. G. Loy and Sandra J. Legan

Summary. To determine whether tonic LH secretion in ovariectomized mares is characterized by pulsatile release, frequent blood samples (every 15 min) were collected from 5 ovariectomized horse mares in nine 8-h periods between February and May. Mean serum LH concentrations increased 9-fold and were associated with a 4-fold increase in mean LH pulse frequency. These results provide the first evidence of pulsatile LH release in ovariectomized mares and suggest that the increasing daylengths of the spring months may increase serum LH by increasing LH pulse frequency.

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B. P. Fitzgerald, K. J. Affleck, S. P. Barrows, W. L. Murdoch, K. B. Barker and R. G. Loy

Summary. Two groups of mares were exposed to an abrupt, artificial increase or a natural increase in daylength. In both groups, mean LH pulse frequency increased with time of year and was accompanied by a reciprocal decrease in LH pulse amplitude. A non-pulsatile pattern of LH secretion was observed in some mares sampled close to the day of ovulation. Maximum mean LH pulse frequency and the onset of the breeding season occurred earlier in those mares exposed to an abrupt artificial increase in daylength. In blood samples collected frequently, mean serum LH concentrations increased in relation to time of year. However, during 60 days before ovulation, when LH pulse frequency increased, mean daily serum LH values only increased on Day – 3 before ovulation. The magnitude of the periovulatory LH rise was greater before the second than the first ovulation of the breeding season. These results support the hypothesis that, in the mare, a photoperiod-induced seasonal alteration in LH pulse frequency and/or amplitude may play a role in the onset of the breeding season.