Search Results

You are looking at 1 - 5 of 5 items for

  • Author: B. Malpaux x
Clear All Modify Search
Free access

B. Malpaux and F. J. Karsch

Summary. This study tested the hypothesis that short days can prolong the breeding season of the ewe when reproductive activity is initiated by an endogenous process, as opposed to when it is driven by exposure to short days. Suffolk ewes were ovariectomized and treated with Silastic capsules containing oestradiol; reproductive activity was monitored from serum concentrations of LH. In this model, a rise in LH is indicative of onset of the breeding season and the duration of the elevation in LH is an indicator of length of reproductive activity. The ewes were subjected to 6-month alternations between long and short photoperiods such that the LH rise began during exposure to the inhibitory long photoperiod (i.e. it resulted from an endogenous process). When short days were provided soon after such a spontaneous onset of the LH rise, the duration of reproductive activity was greater than that observed when ewes were maintained in long days throughout the LH elevation (194 ± 10 vs 155 ± 15 days; P < 0·02). Since the transition from anoestrus to the breeding season in Suffolk ewes maintained outdoors does not require a decrease in daylength and appears to be generated by an endogenous process, our results support the hypothesis that shortening photoperiod sustains the natural breeding season which begins in early autumn.

Keywords: seasonal reproduction; photoperiod; photorefractoriness; LH; breeding season; sheep

Free access

N. L. Wayne, B. Malpaux and F. J. Karsch

Summary. Female Suffolk sheep were pinealectomized around the vernal equinox to eliminate the major environmental input to the reproductive system (photoperiod) and then either isolated from, or maintained with, pineal-intact gonad-intact sheep. The ewes were ovariectomized and treated with constant-release oestradiol implants and reproductive state was monitored by measuring serum LH concentrations. Pinealectomized ewes that were isolated from the normal flock showed a 2½-month delay in onset of the seasonal rise in LH values compared with that of pineal-intact controls (18 November vs 5 September). On the other hand, pinealectomized ewes that were maintained with the flock showed an onset of the seasonal rise in LH that was not delayed. These results suggest a timekeeping role for social cues for timing onset of the breeding season in an animal that normally relies on photoperiodic signals for temporal regulation of the seasonal reproductive cycle.

Keywords: photoperiod; pineal gland; seasonal reproduction; sheep; social cues

Free access

B. Malpaux, J. E. Robinson, M. B. Brown and F. J. Karsch

Summary. Three groups of ovariectomized Suffolk ewes bearing s.c. Silastic implants of oestradiol were subjected to a 90-day priming treatment of an inhibitory long photoperiod (16 h light/day; 16L:8D). On Day 0 of the experiment, they were moved to stimulatory photoperiods. One control group was transferred to 12L:12D and a second control group was transferred to 8L:16D; both groups remained in those photoperiods to determine the timing of reproductive induction and refractoriness. The experimental group was transferred to 12L:12D on Day 0 and then to 8L:16D on Day 55 to determine whether the further reduction in daylength could delay the development of refractoriness. Reproductive neuroendocrine condition was monitored by serum concentrations of LH and FSH. Both gonadotrophins remained elevated for a longer period of time in the experimental group receiving the second reduction in daylength than in either control group, indicating that the second photoperiodic drop delayed the onset of photorefractoriness. Measurement of 24-h patterns of circulating melatonin suggests that the prolonged stimulation of reproductive neuroendocrine activity in the experimental group resulted from a lengthening of the nocturnal melatonin rise. These findings indicate that refractoriness to an inductive photoperiod can be temporarily overcome by exposure to a shorter daylength, and that the change in duration of the nocturnal increase in melatonin secretion is important in photoperiodic signalling. Thus, in natural conditions, the decreasing autumnal daylength, and the resulting expansion of the nocturnal elevation in melatonin secretion, may be utilized to produce a breeding season of normal duration.

Keywords: seasonal reproduction; photorefractoriness; melatonin; oestradiol negative feedback; photoperiodic history; sheep

Free access

A. Daveau, B. Malpaux, Y. Tillet, G. Roblot, R. Wylde and P. Chemineau

An experiment was conducted to determine whether active immunization against melatonin could modify the perception of abrupt photoperiodic changes in ewes. Two groups each containing six intact Ile-de-France ewes were submitted to alternate periods of short days for 2.5 months and long days for 2.5 days for about 70 weeks. Three series of active immunizations against a melatonin conjugate were carried out during the first of the three long-day periods. Control ewes were actively immunized at the same time against human serum albumin. Blood samples were taken once a week throughout the experiment to measure antibody titre and affinity, and prolactin and progesterone concentrations. Sera of all treated ewes demonstrated higher antibody titres than those of control ewes. Antisera were highly specific, as evidenced by the absence of displacement of iodinated melatonin in the presence of ten melatonin metabolites. Significant effects of photoperiod and of the interaction between treatment and photoperiod on prolactin concentration were detected. Prolactin concentrations in plasma of the control ewes were high during long days and low during short days. However, in the treated ewes, with the exception of the first period of long days, prolactin concentrations were not influenced by photoperiodic changes. Ovulatory activity of control ewes, as demonstrated by progesterone measurements, was stimulated by short days and inhibited by long days. In contrast, ovulatory activity of treated ewes, after a response identical to that of control ewes after the first photoperiodic shift from long to short days, showed a complete desynchronization of ovulatory activity relative to photoperiodic changes. Immunization against melatonin can therefore modify the influence of artificial photoperiodic changes on prolactin secretion and ovulatory activity in ewes.

Free access

B. Malpaux, A. Daveau, F. Maurice, A. Locatelli and J-C. Thiéry

An experiment was designed to determine whether the pars tuberalis is the site of action of melatonin involved in the photoperiodic control of LH and prolactin secretion in sheep. In an attempt to produce a 'short-day' effect on these hormones (i.e. stimulation of LH secretion and inhibition of prolactin release), microimplants of melatonin were placed either around the pituitary stalk (n = 6) or in the third ventricle (n = 5) as a control for the efficacy of the microimplant. Two sham-operated groups were treated with empty microimplants around the pituitary stalk (n = 4) or in the third ventricle (n = 3). A further two control groups were given either no melatonin (n = 5) or a melatonin implant s.c. (n = 6). Administration of a melatonin implant s.c. is known to stimulate LH secretion and inhibit prolactin release in photoperiodically inhibited ewes. During the experiment (over 75 days), there was no significant increase in LH concentrations for the ewes receiving melatonin around the pituitary stalk. A similar lack of response was noted in the untreated or sham-treated ewes. In contrast, LH concentrations increased in ewes treated with the melatonin microimplant in the third ventricle on day 37 (± 4) and remained high until day 62 (± 4). Similarly, in the ewes given an implant s.c., LH concentrations rose on day 39 (± 4) and remained high until the end of the study. Furthermore, melatonin caused an inhibition of prolactin secretion relative to controls only when delivered to these sites (i.e. s.c. and into the third ventricle, but not around the pituitary stalk). These results do not support the hypothesis that the pars tuberalis is a site of action of melatonin involved in the photoperiodic control of gonadotrophin or prolactin secretion in ewes.