Biomedical subjects

B Malpaux

Publications and source records attributed to B Malpaux.

At least 73 records · Page 4Linked to original sources

PubMed1994

Active immunization against melatonin in Ile-de-France ewes and photoperiodic control of prolactin secretion and ovulatory activity.

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Short-day effects of melatonin on luteinizing hormone secretion in the ewe: evidence for central sites of action in the mediobasal hypothalamus.

Experiments were designed to localize the central sites of action of melatonin involved in the control of seasonal reproduction. Ewes were exposed to long days and received microimplants of melatonin in the preoptic area (n = 9), anterior hypothalamus (n = 4), dorsolateral hypothalamus (n = 4), or mediobasal hypothalamus (n = 12). The effects of implants were determined by comparison with control ewes (untreated or sham-operated, n = 10) and with ewes treated with an s.c. implant of melatonin (n = 8) or ewes subjected to short days (n = 8). All ewes were ovariectomized and treated s.c. with a silastic capsule of estradiol. Melatonin released in the preoptic area as well as in the anterior and lateral hypothalamus did not cause any difference as compared with the controls (no stimulation of LH secretion and no inhibition of prolactin secretion). In contrast, melatonin implanted in the mediobasal hypothalamus caused an increase in LH secretion in 7 of the 12 ewes on Day 53.0 +/- 4.2 after implantation (mean +/- SEM). Their response was not different compared with that of ewes treated s.c. with melatonin or exposed to short days either in terms of timing (Day 56.3 +/- 6.2 and 59.5 +/- 3.1, respectively, for controls) or of amplitude of the LH response. Similarly, melatonin caused only a reduction of prolactin secretion in the mediobasal, s.c., and short-day groups. It is concluded that the mediobasal hypothalamus or the surrounding tissues could be the sites of action of melatonin involved in the control of seasonal reproduction.

Animals↗

PubMed1993

[Can melatonin be used in out-of-season reproduction in domestic mammals?].

Melatonin, synthetized by the pineal gland, is the chemical messenger which allows seasonal animals to perceive day length changes. The nervous message, transformed into a hormonal one, triggers pulsatile activity of the LHRH neurons. Its sites and mode of actions are not completely elucidated. The most frequent mode of distribution is the sub-cutaneous implant, which induces an advance of the cyclical ovulatory activity of ewes and goats. The date of fertilization is advanced and fecundity of females is improved. It can be used alone, or in association with other hormonal treatments, or after an artificial photoperiodic treatment. Under these conditions, it allows a quantitative and qualitative increase in out-of-season sperm production in rams and he-goats.

Animals↗

PubMed1992

Oestradiol increases the extracellular levels of amine metabolites in the ewe hypothalamus during anoestrus: a microdialysis study.

Giving a subcutaneous oestradiol implant during anoestrus to ovariectomized ewes inhibits pulsatile LH secretion. This effect results from an increased negative feedback of oestradiol and depends on the synthesis of biogenic amines, mainly from the mediobasal hypothalamus. In the present study, we examined the effect of oestradiol on the extracellular levels of amines and their metabolites. Eight ewes were sampled by microdialysis from the lateral retrochiasmatic area, including the dopaminergic A15 nucleus, during inhibition of LH secretion by long days. Two dialysis sessions were carried out on each ewe; one after a 10-day oestradiol treatment and the other one after 10 days without oestradiol treatment. Half of the ewes were first oestradiol-treated then untreated, the other half received the treatment in the reverse order. Oestradiol caused a decline in pulsatile LH secretion without affecting the secretion of prolactin. This steroid also led to a significant increase in the levels of amine metabolites: 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid in the extracellular medium. These results demonstrate the effect of oestradiol on aminergic activity as related to changes in hormonal secretions during long days (16 h of light per 24 h). Thus our data support the hypothesis that amines inhibit gonadotrophic secretion during anoestrus in the ewe and suggest that there is an activation of the aminergic neurones from the retrochiasmatic area in this regulatory mechanism.

Anestrus↗

PubMed1991

Synchronization of the circannual reproductive rhythm of the ewe by discrete photoperiodic signals.

Although many species display endogenous circannual rhythms of biological activity that are synchronized by day length, the specific photoperiodic requirements for synchronizing such rhythms are not established for any species. We tested the hypothesis that the circannual reproductive rhythm of sheep can be synchronized by exposure to just one or two discrete blocks of photoperiodic information each year. Ewes were pinealectomized to prevent their ability to transduce photoperiodic information into altered reproductive neuroendocrine activity. During the 53/4 yr following pinealectomy, specific photoperiodic signals were restored for discrete periods of time via replacement of 24-h patterns of melatonin, the pineal hormone that transmits photic information to the reproductive neuroendocrine axis. The ewes were kept in a 12-mo photoycycle that alternated between short (8L:16D) and long (16L:8D) days every 6 mo and that was 6 mo out of phase with the geophysical year. Pineal-intact control ewes exhibited synchronous annual reproductive cycles. Noninfused pinealectomized control ewes did not exhibit synchronous cycles. Pinealectomized ewes infused with alternating 70-day blocks of short- and long-day patterns of melatonin every 6 mo for the first 21/2 yr of the experiment exhibited synchronous annual reproductive cycles that were 6 mo out of phase with those of ewes maintained outdoors. This synchrony persisted when the frequency of the melatonin treatment was reduced to just one 70-day block of a long-day pattern of melatonin each 365 days. Cycle period was 368 +/- 3 days; standard deviation of the date of onset of reproductive induction averaged only 3 days. Our study provides the first direct evidence that a single block of photoperiodic information a year can synchronize a circannual rhythm.

Animals↗

PubMed1990

Photoperiodic requirements for timing onset and duration of the breeding season of the ewe: synchronization of an endogenous rhythm of reproduction.

A study was conducted to test the hypothesis that different portions of the annual photoperiodic cycle play different roles in timing the breeding season of the ewe, Ovis aries, an animal in which an endogenous rhythm generates the seasonal reproductive transitions. Adult female sheep were pinealectomized to disrupt transduction of photoperiodic cues at 4 times of the year (summer and winter solstices, vernal and autumnal equinoxes), and the effects on seasonal reproductive neuroendocrine activity were evaluated. Time of pinealectomy greatly influenced the subsequent seasonal reproductive cycle such that the following inferences are possible. Lengthening days between the winter and summer solstices synchronize reproductive onset to the appropriate time of year. The relatively long days around the summer solstice act to suppress reproductive activity and forestall the start of the breeding season until late summer/early autumn. The shortening days between the summer solstice and autumnal equinox maintain a normal intensity and duration of reproductive neuroendocrine induction during the impending breeding season. However, the shortening days between the autumnal equinox and winter solstice (i.e., after breeding season onset) do not appear to play a critical role in maintaining the breeding season of that year, but may provide important cues for timing the breeding season of the following year.

Animals↗

PubMed1990

A role for short days in sustaining seasonal reproductive activity in the ewe.

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 less than 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.

Animals↗

PubMed1989

Social cues can play a role in timing onset of the breeding season of the ewe.

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 2 1/2-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.

Animals↗

PubMed1989

Regulation of the onset of the breeding season of the ewe: importance of long days and of an endogenous reproductive rhythm.

Three experiments were conducted to evaluate the role that the increasing day lengths of late winter and spring play in timing the breeding season of the Suffolk ewe. In the first experiment, ewes were denied their normal complement of increasing day length by maintaining them on the photoperiod experienced at the winter solstice. This prevented the breeding season from occurring the subsequent autumn. In the second experiment, ewes were exposed to increases in day length at different time-intervals after the winter solstice: the normal time, later than normal or earlier than normal. Once the summer solstice photoperiod was reached, it was maintained until the end of the study. When increasing photoperiod was provided early, the breeding season was advanced; when it was provided late, reproduction was delayed. In the third experiment, ewes were exposed to a continuously increasing photoperiod matching the maximal rate of rise in natural conditions; this treatment was begun on the spring equinox and continued until mid-autumn. The steadily increasing photoperiod did not alter the time of reproductive onset in the autumn. These findings support the following conclusions for timing of the breeding season of the Suffolk ewe. (1) The lengthening photoperiod between the winter and summer solstices is required for the occurrence of the breeding season in the autumn. (2) The time of initial exposure to this lengthening photoperiod provides an important cue for determining when the reproductive period occurs. (3) The time of onset of the breeding season does not depend upon the decreasing photoperiod after the summer solstice, nor does it require the photoperiod to stop increasing as the summer solstice approaches. These findings have been incorporated into a conceptual model for temporal regulation of the annual reproductive cycle of the ewe. An important component of this model is a critical role for increasing photoperiod to initiate a process in the late winter-spring which ultimately leads to an obligatory reproductive onset in the autumn.

Animals↗

PubMed1988

Characteristics of the melatonin signal that provide the photoperiodic code for timing seasonal reproduction in the ewe.

The following is a progress report of our studies to identify important features of the circadian pattern of melatonin secretion which provide the photoperiodic code for daylength in regulating seasonal breeding in the Suffolk ewe. The first series of experiments evaluated two conceptual models of how melatonin codes for daylength: the circadian timing of the melatonin elevation as opposed to the length of the time melatonin is elevated during each 24-hr period (phase vs duration). Strong support has been gathered for the duration hypothesis. No evidence was obtained to support a role for phase; nevertheless, this hypothesis could not be discounted definitively. A second series of studies evaluated the importance of the previous melatonin pattern to the interpretation of a given melatonin signal. Evidence is presented that a fixed melatonin pattern can maintain a given reproductive response only for a limited length of time and that this response can be prolonged by appropriate changes in the melatonin pattern. Thus, change is an important feature of the melatonin signal. Further, the nature of the melatonin change appears to be crucial, specifically whether the nocturnal elevation increases or decreases in duration. Thus, transfer to a common photoperiod can promote either reproductive induction or arrest, depending upon whether the transfer leads to a decrease or increase in daylength. This has important ramifications to the photoperiodic timekeeping process in those species of mammals which utilize daylength to time their seasonal reproductive cycle.

Animals↗

PubMed1988

How does melatonin code for day length in the ewe: duration of nocturnal melatonin release or coincidence of melatonin with a light-entrained sensitive period?

The main objective of the study was to test the hypothesis that the phase of melatonin release with respect to the light-dark cycle mediates the effects of photoperiod on the reproductive response of the ewe. To test the phase hypothesis, we eliminated endogenous melatonin secretion by pinealectomy and then restored physiological levels of serum melatonin with rises of the same duration but at different phases of the light-dark cycle (either at night or in the middle of the day). Serum melatonin patterns were determined by radioimmunoassay in samples taken hourly for 24 h. The reproductive state was monitored by measuring serum luteinizing hormone (LH) in ovariectomized ewes treated with constant-release estradiol implants. Infusion of a long-day pattern of melatonin was equally effective in maintaining reproductive suppression when given during the night or the middle of the day. LH remained low for approximately 150 days and then rose as ewes became refractory to the inhibitory melatonin signal. These results do not support the phase hypothesis. Rather, they are consistent with the hypothesis that the duration of the nocturnal secretion of melatonin codes for day length.

Animals↗

PubMed1988

Termination of the breeding season in the Suffolk ewe: involvement of an endogenous rhythm of reproduction.

Two studies were performed to determine if the transition into anestrus in the Suffolk ewe results from the lack of a decrease in photoperiod, a signal suggested to be important in maintaining the breeding season, or from an obligatory turn-off that reflects the expression of an endogenous rhythm of reproduction. Shortly after the autumnal equinox, three groups of ovariectomized ewes bearing s.c. Silastic implants of estradiol were placed in different lighting environments. A control group was exposed to normal variations in day length until the winter solstice and held on that day length thereafter. The two experimental groups experienced either a continuously decreasing day length or a continuously increasing duration of elevated melatonin levels during each 24-h period until mid-March. Reproductive activity, assessed by circulating levels of luteinizing hormone (LH), was marginally extended (2-3 wk) in the experimental groups. In a second experiment, two groups of ovary-intact ewes were exposed to photoperiodic treatments shortly after the winter solstice. A control group was held on the winter solstice day length (10L:14D) until the end of the study in mid-March. The experimental group experienced a 3-h photoperiodic reduction to 7L:17D. It remained to determine if that abrupt decrease could maintain reproductive induction. Again, the cessation of reproductive induction was marginally delayed in the experimental group (2-3 wk). This marginal delay in the arrest of reproductive activity seen in both experiments indicates that the lack of decrease in day length around and after the winter solstice may play some role in timing the end of the breeding season. However, our inability to prevent or markedly delay the termination of reproductive activity leads to the conclusion that the primary reason for the transition into anestrus is an obligatory turn-off. This obligatory process may be the expression of an underlying endogenous rhythm of reproduction.

Animals↗

PubMed1988

Reproductive refractoriness of the ewe to inhibitory photoperiod is not caused by alteration of the circadian secretion of melatonin.

The 24-hour pattern of melatonin secretion was determined in 5 Suffolk ewes during extended exposure to a long day length to assess whether the eventual loss of response (photorefractoriness) to inhibitory long days resulted from an alteration in the circadian secretion of melatonin. Determinations of the secretory profile of melatonin were made once in short days (8 h light/day) before the switch to long days, and 11 times throughout the 250-day period of exposure to long days (16 h light/day). Samples were obtained hourly for 24 or 48 h. Reproductive state was assessed by response to estradiol-negative feedback, monitored as serum LH in ovariectomized ewes bearing estradiol implants. The characteristic secretory pattern of melatonin (low during the day, high at night), the duration of the melatonin elevation, and its phase relative to the light/dark cycle did not change as ewes became refractory to the inhibitory effects of long days. These results are consonant with the hypothesis that refractoriness of the ewe to inhibitory day length does not result from an alteration of the circadian rhythm of melatonin secretion.

Circadian Rhythm↗

PubMed1988

Importance of pituitary and neural actions of estradiol in induction of the luteinizing hormone surge in the ewe.

Two experiments were performed to test the importance of both pituitary and neural sites of action of estradiol in inducing the surge of luteinizing hormone (LH) in the ewe. Both experiments were conducted using an animal model in which pulsatile secretion of gonadotropin-releasing hormone (GnRH) and endogenous secretion of ovarian steroids were eliminated by ovariectomy during seasonal anestrus and treatment with Silastic implants which maintained a luteal-phase level of serum progesterone. The hormonal requirements for the surge were then evaluated by systematic application of GnRH and estradiol signals using pulsatile infusion pumps (for GnRH) and Silastic implants (for estradiol). In experiment 1, the circulating level of estradiol and frequency of GnRH pulses were increased either abruptly or progressively (i.e. mimicking the changes in the estrous cycle between luteolysis and just before the LH surge). Abrupt increments led to an LH surge in all ewes; progressive rises to the same absolute levels did not. However, sudden application of a further large increase in GnRH upon the progressive rise elicited an LH surge in every instance. In experiment 2, a GnRH pulse pattern known to be effective in inducing the LH surge was applied under conditions of differing estradiol concentration: no estradiol, basal estradiol, basal rising to peak estradiol. The GnRH signal elicited high-amplitude surges of LH only in the presence of a peak estradiol concentration. Our findings are consistent with the conclusion that two actions are required for a rise in estradiol to elicit a full-amplitude surge of LH in the ewe: an action on the brain to evoke a sudden increase in GnRH release and an action on the pituitary to maximize its response to GnRH.

Animals↗

PubMed1988

Importance of changing photoperiod and melatonin secretory pattern in determining the length of the breeding season in the Suffolk ewe.

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.

Animals↗

PubMed1987

Reproductive refractoriness of the ewe to inductive photoperiod is not caused by inappropriate secretion of melatonin.

The 24-h pattern of melatonin secretion was evaluated in Suffolk ewes during prolonged exposure to an inductive photoperiod to assess whether altered secretion of melatonin could account for the eventual loss of response to stimulatory photoperiod (photorefractoriness). Secretory patterns of melatonin were determined approximately every two weeks in samples obtained hourly for 24-48 h. Sampling was begun one week before the switch from inhibitory (long) to inductive (short) photoperiod and continued for 150 days, by which time all but one of the ewes were unresponsive to that stimulatory day length. Melatonin was measured in two different radioimmunoassay systems. Reproductive state was monitored by luteinizing hormone secretion in ovariectomized ewes bearing constant-release estradiol implants. No evidence for disruption of the melatonin pattern was observed on any occasion. The duration and the phase of the melatonin elevation relative to the light/dark cycle did not vary with time of exposure to short days. These findings indicate that refractoriness of the Suffolk ewe to an inductive photoperiod is not caused by an inappropriate secretory pattern of melatonin.

Animals↗

PubMedDate not supplied

Control of pulsatile LH secretion during seasonal anoestrus in the ewe.

The seasonality of reproductive activity in the ewe in temperate latitudes is controlled by photoperiod. Its annual variations control the temporal organization of the sexual cycle by changing the activity of the gonadotrophic axis. Cyclic oestrous behaviour usually appears in the ewe at the end of summer or the beginning of autumn and finishes in winter or at the very beginning of spring. Seasonal anoestrus is characterized by the absence of ovulation and sexual behaviour. During seasonal anoestrus, a decrease in LH pulse frequency is observed. The inhibition of pulsatile LH secretion is maintained throughout the anoestrous season and is responsible for the low reproductive activity during this period. Variation in the seasonal inhibition of LH pulsatility results from an increase in the negative feedback by oestradiol on LH pulse frequency during the long days of spring and summer. The inhibition of LH secretion involves increased action of dopamine in the hypothalamus on the chain of nervous elements which controls gonadotrophic activity. Among the various dopaminergic structures, the retrochiasmatic A15 nucleus is involved in the inhibitory control of LH pulsatility by oestradiol during the long day period. Oestradiol increases the dopaminergic tone of the A15 nucleus in ovariectomized ewes during the long day period. In this structure, the effect of oestradiol on the dopaminergic metabolism probably results from a direct, local activation. In the sheep, dopamine might also participate in the inhibition of gonadotrophin activity during other periods of reproductive life.

Anestrus↗

PubMedDate not supplied

Melatonin and the seasonal control of reproduction.

Many mammalian species from temperate latitudes exhibit seasonal variations in breeding activity which are controlled by the annual photoperiodic cycle. Photoperiodic information is conveyed through several neural relays from the retina to the pineal gland where the light signal is translated into a daily cycle of melatonin secretion: high at night, low in the day. The length of the nocturnal secretion of melatonin reflects the duration of the night and it regulates the pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Changes in GnRH release induce corresponding changes in luteinising hormone secretion which are responsible for the alternating presence or absence of ovulation in the female, and varying sperm production in the male. It is not yet known where and how this pineal indoleamine acts to exert this effect. Although melatonin binding sites are preferentially localised in the pars tuberalis (PT) of the adenohypophysis, the hypothalamus contains the physiological target sites of melatonin for its action on reproduction. Melatonin does not seem to act directly on GnRH neurons; rather it appears to involve a complex neural circuit of interneurons that includes at least dopaminergic, serotoninergic and excitatory aminoacidergic neurons.

Animals↗