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Biomedical subjects

J Pelletier

Publications and source records attributed to J Pelletier.

At least 235 records · Page 13Linked to original sources
PubMed1979

LH and testosterone release in developing bulls following LH-RH treatment. Effect of gonadectomy and chronic testosterone propionate pre-treatment.

LH and testosterone release after LH-RH treatment was studied in male calves from 1 week to 8 months of age. Total LH release, LH peak values and duration of the discharge increased from 1 week to 4 months of age by 406, 453 and 110%, respectively. Significant decreases of 54, 48 and 24% were observed at 6 months of age compared to 4 months of age. This was followed by steep increases in those variables of 344, 129 and 69% at 8 months compared to 4 months. A testosterone response following an LH increase was first seen at 4 months of age when a weak discharge occurred in 2 out of 5 calves. In older calves strong and LH-correlated testosterone responses were always observed. In a second experiment the LH response to LH-RH was studied in 6 months old calves castrated 2 months previously. The total LH response was 61% greater than in intact controls. When animals were castrated and pre-treated briefly with testosterone propionate the response was increased by 183% and the time to reach the LH peak was increased by 360%. From these and other results a synthesis of endocrine events during the first year of life is proposed. In particular it seems that the period between 4 and 8 months of age is crucial since interactions between steroid feedback and pituitary sensitivity to LH-RH change and eventually reach an equilibrium.

Age Factors↗
PubMed1979

Periovulatory gonadotrophin and ovarian steroid patterns in sheep of breeds with differing fecundity.

Plasma hormone concentrations before and during luteolysis (induced by injection of a prostaglandin analogue on Day 10 or 11 of the cycle), during the period of preovulatory follicle growth and ovulation were examined in sheep with known differences in ovulation rate (Romanov, Préalpes, Romanov x Préalpes cross, Ile de France). The number of CL at the time of treatment and the ovulation rate in the ensuing cycle were established by endoscopy. Plasma concentrations of FSH, LH, progesterone and total oestrogen were measured by radioimmunoassays in the 3 days before PG injection, then hourly for the 24 h after PG injection and 2-hourly for a further period up to about 100 h after PG injection. The onset and duration of oestrus were also recorded. Although breed differences were observed for many of the features studied, only the intervals between oestrus and the LH peak and between PG injection and the LH peak were significantly correlated with ovulation rate.

Animals↗
PubMed1979

Short-term variations in plasma LH and testosterone in bull calves from birth to 1 year of age.

LH and testosterone levels in bull calves were studied in the plasma samples collected sequentially at 15-min intervals every month during the first year of life. An episodic pattern of LH release occurred after birth and the frequency and magnitude of the LH peaks increased up to 4 months of age and decreased thereafter. A testicular response was not observed before this age. It is suggested that this episodic LH activity is responsible for the testicular development which then initiates puberty.

Aging↗
PubMed1976

Seasonal variations in the response of the testis and LH levels to hemicastration of adult rams.

The weight and histology of the testis and plasma LH levels were analysed after hemicastration of adult Ile-de-France rams in spring or in autumn. After hemicastration, the remaining testes were significantly heavier than those of entire animals measured at the same time of year. At 4 or 6 months after hemicastration performed in spring, the remaining testes were hypertrophied by nearly 40% as compared to the testes of entire sexually active animals, assessed in autumn. The variations of intertubular tissue volume, total seminiferous tubule length, stem cell stocks, daily production of round spermatids, and cellular volume of primary spermatocytes paralleled the variations in testis weight. The annual decrease of the area of the Sertoli cell nuclei and of the yield of meiosis and beginning of spermiogenesis during the non-breeding season was prevented by hemicastration performed in autumn. Plasma LH levels were consistently elevated till autumn after hemicastration performed in spring. A positive and significant correlation was observed between LH levels and yields of spermatogonial divisions.

Animals↗
PubMed1975

[Plasma testosterone and LH levels in the male lamb from birth to puberty].

In peripheral plasma of male lambs testosterone increase was linear from birth to 100 days; after it reached a plateau without clear relationships with puberty. Plasma LH pattern was also linear during the first 70 days. During this period testosterone and LH were significantly correlated (r equals 0,85).

Age Factors↗
PubMed1975

Influence of the presence of rams on the timing of ovulation and discharge of LH in ewes.

The influence of the presence of the male on the ovulation process was investigated in ewes after oestrus control by progestagens (intravaginal fluorogestone acetate) and PMSG. Permanent contact with the rams throughout oestrus accelerates the ovulation and the appearance of the LH surge. The results suggest that the action of the presence of the male on ovulation is mediated by way of the ovulatory surge of LH.

Animals↗
PubMed1975

Photoperiodic control of LH release in the ram. I. Influence of increasing and decreasing light photoperiods.

The influence of variations in the duration of light photoperiod on LH release has been studied in intact and castrated rams submitted to a 6 months "annual" light rhythm. Blood was collected once a month and the plasma LH levels were determined by radioimmunoassay. In intact rams, the LH level is decreased when the light photoperiod is increased, but as soon as this photoperiod begins to decrease, a sharp increase (about 80%) in the LH level is observed. Thereafter a slow decline in plasma LH occurs until the photoperiod reaches a minimum. In the castrated animals, the plasma LH level is three to four times higher than in the intact animals but the pattern of the variations with regard to the light photoperiod duration, is essentially the same. Thus an increase of 59 and 43% in plasma LH occurs in the two breeds used when the light photoperiod begins to decrease. These results indicate that hypothalamo-hypophyseal activity is modulated by gradual changes in the duration of light photoperiods either in the presence or absence of androgens.

Animals↗
PubMed1975

Photoperiodic control of LH release in the ram. II. Light-androgens interaction.

The intensity of negative feedback of androgens on hypothalamo-hypophyseal activity was compared in two groups of castrated rams under 8.00 or 16.00 h of daily illumination respectively following a conditioning period. In experiment I, two groups of 15 rams under each light photoperiod were slaughtered at different times after a 200 mg testosterone propionate (TP) intramuscular injection. Animals receiving the vehicle only acted as controls. The hypophyseal LH concentration, which increases when there is temporary inhibition of the release into the blood, was taken as a criterion of the inhibition due to TP. In both groups of rams the LH concentration increased in the pituitary following TP treatment. A return to the pre-injection level was observed 48 h later only in animals under a 8.00 h light photoperiod, indicating a shorter negative effect of the injection than in the case of rams under 16.00 h light photoperiod. In experiment II, the inhibitory effect of TP was assessed at two doses, i. e. 300 and 600 mg, by the decrease in the plasma LH level following an intramuscularly injection. In both cases the decrease in LH release was more pronounced in rams under 16.00 h of daily illumination than in those under 8.00 h (mean decrease in LH release: 21.2 and 33.2% respectively in 300 and 600 mg treated rams under 16.00 h daylight as against 9.2 and 14.8% in rams under 8.00 h). Thus it appears that to the intrinsic action of the photoperiod is superposed on a modulatory effect of steroid action on the hypothalamo-hypophyseal activity. The hypothesis is therefore presented that, in rams a decreasing light photoperiod acts in two ways: 1) by stimulating the activity of the hypothalamo-hypophyseal system, 2) by decreasing the intensity of the negative feedback effect of steroids.

Animals↗
PubMed1975

Luteinizing hormone release in entire and castrated rams following injection of synthetic luteinizing hormone releasing hormone, and effect of testosterone propionate pre-treatment.

Plasma luteinizing hormone (LH) levels and LH responses to intravenous administration of 100 mug luteinizing hormone releasing hormone (LH-RH) were studied in entire rams, long-term castrated animals (operation performed six months previously), long-term castrated animals treated with testosterone for the two prededing weeks and short-term castrated animals (castrated 3 h before LH-RH injection). LH was measured by radioimmunoassay in samples taken at 5 or 15 min intervals. Basal LH levels were lower in entire rams (0-9 ng/ml) than in long-term castrated animals (6-0 ng/nl). After LH-RH treatment the LH response was much smaller (peak level 9-6 ng/ml), total response 13-3 ng/ml/1 h) and slower (120 min to peak) in entire than in long-term castrated animals (peak level 61-8 ng/ml, total response 141-2 ng/ml/1 h, 29 min to peak). Testosterone treatment after long-term castration depressed the basal LH level and delayed the peak LH response after LH-RH to values similar to those for entire rams. After short-term castration the response to LH-RH was already as great (peak level 70-1 ng/ml, total response 133-6 ng/ml/1 h) as after long-term castration. The latency to peak LH level (82 min) was intermediate between that for untreated and testosterone-treated long-term castrated animals (130 min). Testosterone treatment was considered to have acted on the hypothalamus to depress basal levels. The results provided evidence for the presence of two inhibitory actions of the testis at the pituitary level in the ram: a qualitative delaying action of testosterone and a quantitative inhibitory action of the testis on LH release after LH-RH injection. The latter may also be related to plasma testosterone levels.

Animals↗