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.
R. T. Duggan, M. J. Bryant and F. J. Cunningham
Summary. Plasma concentrations of LH, FSH, progesterone and total oestrogens were determined (by radioimmunoassay) throughout late pregnancy and lactation in samples taken by cardiac catheter from 10 Camborough hybrid sows. At 2–3 weeks post partum, 5 were housed individually with their litters (Group I) and 5 grouped with their litters, a boar being introduced 1 day later (Group II).
Plasma progesterone fell 1–2 days pre partum and oestrogens at parturition, each remaining low in early lactation. Plasma LH was undetectable in most samples in this period, but FSH fell slightly during parturition, showing signs of resurgence from Day 10 post partum.
Group I sows remained in lactational anoestrus until weaning. In Group II there was increased plasma LH activity, and all Group II sows displayed oestrus during lactation. Only one farrowed to service at the lactational oestrus; of the remainder, oestrus in one was clearly anovulatory, and in two others was incomplete or atypical judged by plasma progesterone concentration. The data suggest that whilst oestrus can be induced by appropriate lactation management, poor conception rates to service at this oestrus may be due to the absence or impairment of ovulation.
P. G. Knight, R. T. Gladwell and F. J. Cunningham
Summary. An in-vitro superfusion technique was used to study basal and depolarization-induced (32 mmol K+/l) release of LHRH from the mediobasal hypothalamus (MBH) of pullets at 8–25 weeks of age. Plasma LH concentrations and the incremental change (ALH) after an i.v. injection of 1 or 15 μg synthetic ovine LHRH/kg body weight were also determined. Between 8 and 25 weeks of age, significant (P < 0·01) increases in basal and depolarization-induced release of LHRH (93 and 330%, respectively) were accompanied by a significant (P < 0·01) rise in the residual LHRH content of MBH tissue (152%), observations which suggest that the ability of the hypothalamus to synthesize and secrete LHRH increases as sexual maturation proceeds. However, plasma LH, which reached a maximum concentration of 2·05 ± 0·43 μg/l at 15 weeks, fell significantly (P < 0·05) to 1·14 ± 0·05 μg/l at 25 weeks. Since ΔLH in response to exogenous LHRH showed a marked and progressive decline between 12 and 20 weeks of age, the low plasma concentration of LH typical of the mature hen is probably attributable to a direct negative-feedback action of ovarian steroids on the anterior pituitary gland rather than to an impaired secretion of LHRH from the median eminence. It is suggested that a dramatic increase in the responsiveness of LHRH nerve terminals in the MBH to depolarization by 32 mmol K+/l between 20 and 25 weeks of age (mean age at onset of lay 21 ·9 weeks; range 19–25 weeks) may reflect the development of hypothalamic responsiveness to the positive feedback action of progesterone.
M. WELLS, P. S. BROWN and F. J. CUNNINGHAM
T. R. MORRIS, O. MELEK and F. J. CUNNINGHAM
The time of peak concentration of LH in the peripheral plasma of ten laying hens kept on a 14 hr light/13 hr dark cycle was determined by radioimmunoassay. A single peak of LH occurred in each hen 3½ hr before oviposition or 4 hr before ovulation. These are similar intervals to those reported for laying hens kept on a 14 hr light/10 hr dark cycle. The mean time of the observed peak in plasma LH was 2 hr 9 min after the beginning of the dark period. This is about 3 hr earlier than corresponding LH peaks reported for hens under normal lighting.
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.
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.
N. F. Cunningham, N. Saba, C. D. H. Boarer and J. J. P. Hattersley
Summary. Plasma progesterone and gonadotrophin levels were studied in anoestrous ewes treated during June or July with a subcutaneous progesterone implant and/or an injection of oestradiol or PMSG. Of 32 ewes treated with progesterone during July, 9 showed a gonadotrophin surge after removal of the implant, and 10 ewes showed oestrous behaviour during the following 4 days. Six ewes conceived at this induced oestrus. Progesterone treatment during June was much less effective, with only 2 of 19 treated ewes showing a gonadotrophin surge and oestrous behaviour. Administration of PMSG at the time of implant removal in the June experiment was followed by a gonadotrophin surge and oestrous behaviour in 18 of 19 ewes, and 15 ewes conceived at the induced oestrus. An injection of PMSG, without progesterone pretreatment, stimulated a gonadotrophin surge and ovulation, but did not result in oestrous behaviour. The treatments employed appeared to initiate cyclic ovarian activity in the July experiment, but not in the June experiment.
P. G. Knight, Susan C. Wilson, R. T. Gladwell and F. J. Cunningham
Summary. Concentrations of LHRH, dopamine, noradrenaline and adrenaline in the anterior hypothalamus—preoptic region (AH—POR) and posterior hypothalamus-median eminence (PH—me) were determined in hens killed at different times in relation to the first ovulation of a sequence. The occurrence of a preovulatory rise in plasma LH concentration 4–6 h before the expected time of ovulation was confirmed. This rise in plasma LH was accompanied by a significant (P < 0·01) 50% reduction in the LHRH content of the AH—POR and PH—me while the subsequent fall in plasma LH was accompanied by a restoration of the LHRH content of both regions to their former levels. Although no significant fluctuations in the hypothalamic content of either dopamine, noradrenaline or adrenaline were detected during the ovulatory cycle, significant correlations between LHRH content and catecholamine content were observed in the AH—POR (P < 0·05) and PH—me (P < 0·01). Thus mean levels of each amine followed the same temporal pattern as LHRH content with minimum values being observed shortly before the peak of the preovulatory surge of LH. These findings support the conclusion that an enhanced secretion of LHRH from the median eminence, possibly associated with an increased activity of catecholaminergic neurones, is a prerequisite for the preovulatory release of LH in the hen.