The mechanism of the stimulation of LH production by synthetic LH-releasing hormone (LH-RH) in tissue culture

Synthetic LH-RH was found to stimulate production of LH by human female adenohypophysis in monolayer culture. This effect occurs at 0.30 μg/2 ml LH-RH. New messenger-RNA synthesis does not have to occur to stimulate the production of LH by the action of synthetic LH-RH in cultures of under 4 days. In cultures of over 4 days, this synthesis must occur in order for LH to be produced by the action of LH-RH. However, new DNA synthesis does not have to occur to stimulate the production of LH by the action of synthetic LH-RH.     

Plasma LH concentrations after single or double injections of synthetic LH-RH in dairy cows.

Milked Friesian dairy cows were given 1 or 2 i.v. injections of synthetic LH-RH. The second of 2 injections of LH-RH given 1.5 h apart induced a significantly greater release of LH than did the first. The response to this injection was also significantly greater than that caused by injections given 6 h apart. There was no significant difference between the LH responses to the two injections given 6 h apart. The total LH release produced by the two injections of 100 microgram synthetic LH-RH given 1.5 h apart was significantly greater than that produced by single injection of 200 microgram synthetic LH-RH. It is concluded that there is a short-term increase in the responsiveness of the bovine pituitary gland to LH-RH after an initial LH-RH stimulus.     

Pituitary responsiveness to synthetic LH-RH and pituitary LH content at various reproductive stages in the sheep.

In sheep the basal concentration of LH in jugular vein plasma was significantly higher during the first 50 days of gestation in late pregnancy or at parturition. The pituitary response to a single i.v. injection of 200 microng synthetic LH-RH was determined at different stages of gestation and compared with that of anoestrous and cyclic sheep. Pituitary response to LH-RH decreased progressively with advancing gestation: by 56 days after mating the response had declined to 35% and by parturition to 14% of the value in anoestrous sheep. The pituitary response to LH-RH increased after parturition and the pattern of recovery differed in non-lactating and lactating sheep. By 63 days postpartum the response to LH-RH in non-lactating and lactating animals had returned to values similar to those in sheep during anoestrus and sheep during the luteal phase of the oestrous cycle. A decrease in pituitary responsiveness during pregnancy was associated with a decrease in pituitary content of LH. The quantity of LH released in response to a standard injection of LH-RH was linearly related to pituitary LH content.     

The participation of corticosterone in luteinizing hormone releasing hormone (LH-RH) action on luteinizing hormone (LH) release from anterior pituitary cells in vitro

Corticosterone alone was not able to stimulate release of luteinizing hormone (LH) from anterior pituitary cells $\text{in}$$\text{vitro}$, but corticosterone in combination with luteinizing hormone releasing hormone (LHRH) augmented the release of LH into the culture media. These results may indicate that corticosterone may have the capacity to activate membrane receptors for LHRH in the gonadotrophs.     

Fertility in the ewe following cervical insemination with fresh or frozen-thawed semen at a natural or synchronised oestrus

Artificial insemination (AI) in sheep is currently limited by the poor fertility obtained following non-surgical intracervical insemination of frozen-thawed semen. An exception to this general finding is the non-return rate of around 58% reported for large scale on-farm AI in Norway. The objective of the present study was to determine if similar results could be obtained under Irish conditions. Comparisons were made between semen collected, and frozen, from rams in Norway (NOR) and Ireland (IRL). The effects of synchronisation and inseminator were also examined. Parous ewes (n=297) of various breed types were inseminated to a natural (N) or synchronised (S) oestrus with either fresh (from Irish rams) or frozen-thawed (IRL and NOR) semen. Ewes were randomly assigned, within breed, to the following treatment groups: (i) Fresh-N: n=28, (ii) Fresh-S: n=30, (iii) IRL-N: n=62, (iv) IRL-S: n=50, (v) NOR-N: n=68, (vi) NOR-S: n=59. Within each group, ewes were inseminated by an experienced Norwegian or by an Irish inseminator. Pregnancy rate did not differ significantly between ewes inseminated to a natural or synchronised oestrus nor between Norwegian and Irish frozen semen. The proportion of ewes pregnant after insemination with fresh semen was 0.82 and 0.70 (treatments i and ii) compared with 0.40, 0.52, 0.34 and 0.37 (treatments (iii)-(vi)) for frozen semen (P<0.001). Corresponding litter sizes (+/-S.E.), adjusted for ovulation rate, were 2.9+/-0.22, 3.3+/-0.23, 2.2+/-0.21, 1.7+/-0.21, 2.2+/-0.21 and 2.1+/-0.21 (fresh versus frozen; P<0.001). There was an interaction between semen type (fresh or frozen) and oestrus type (N or S) for litter size due to an increased adverse effect of frozen semen on litter size in synchronised ewes (P<0.05). Pregnancy rate was significantly influenced by breed of ewe (P<0.01) and inseminator (P<0.05). These results suggest that ewe breed may be a critical determinant of the potential for the exploitation of cervical insemination of frozen-thawed semen in sheep breeding programmes.     

Comparison of the rate of decline in plasma progesterone concentrations at a natural and progesterone-synchronized oestrus and its effect on tonic LH secretion in the ewe

The pattern of change in plasma progesterone and LH concentrations was monitored in Clun Forest ewes at a natural oestrus and compared to that observed after removal of progesterone implants. The rate of decline in plasma progesterone concentrations after implant withdrawal (1.8 +/- 0.2 ng/ml h-1) was significantly greater (P less than 0.001) than that observed at natural luteolysis (0.2 +/- 0.1 ng/ml h-1), and this resulted in an abnormal pattern of change in tonic LH secretion up to the time of the preovulatory LH surge. This more rapid rate of progesterone removal was also associated with a shortening of the intervals from the time that progesterone concentrations attained basal values to the onset of oestrus (P less than 0.05) and the onset of the preovulatory LH surge (P less than 0.01). However, there were no significant differences in the duration of the LH peak, preovulatory peak LH concentration, ovulation rate or the pattern of progesterone concentrations in the subsequent cycle. It is suggested that the abnormal patterns of change in progesterone and tonic LH concentrations may be one factor involved in the impairment of sperm transport and abnormal patterns of oestradiol secretion known to occur at a synchronized oestrus.     

Pattern of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in bovine blood plasma after injection of a synthetic gonadotropin-releasing hormone (Gn-RH)

A synthetic gonadotropin-releasing hormone (Gn-RH) was administered to female and male adult bovines in order to study the release of luteinizing and follicle-stimulating hormones into blood by the pituitary gland. Plasma LH and FSH were determined by means of a radioimmunological method. In females as well as in males, increasing doses of Gn-RH (range 50 to 1500 μg) administered i.v. or i.m. caused a linear increase in plasma LH. The release of FSH evidently was curvilinear over the same dosage range.After 2 or 3 injections of Gn-RH every 3 hours, or every 24 hours or more, smaller amounts of LH were released; repeated treatment did not result in reduction of FSH. Thus pituitary depletion of LH occurred more readily than FSH. The effect of Gn-RH on plasma levels of LH and FSH at various stages of the estrous cycle shows a tendency for an increasing release of both gonadotropins on Days 17 – 18 in comparison to Days 4 – 5 or Days 11 – 12.The results suggest that, within the limits allowed by the heterogenous FSH assay and the method of administration used in these experiments, synthetic Gn-RH does not evoke completely normal physiological responses. Therefore, further work is needed to determine its role in improving reproductive function.     

Observations on variability in LH release and fertility during oestrus in the domestic cat (Felis catus)

Hormonal changes, behaviour, ovulation and fertility were examined in response to coitus at two different times during oestrus in the female domestic cat housed in conditions of natural light (N = 13). On Day 2 or Day 4/5 of oestrus females were allowed 1 copulation in 15 min (single matings) or 2-3 copulations in 30 min (multiple matings). Plasma LH, oestradiol-17 beta and progesterone concentrations during the 24-h period after coitus were measured by radioimmunoassay; ovulation was assumed to have occurred if progesterone values were elevated 7-30 days after coitus. With the exception of 2 out of 3 animals receiving single matings on Day 2 of oestrus, all animals showed subsequent elevated progesterone values. Females receiving multiple matings had significantly greater releases of LH as measured by the area under the curve than those receiving single matings. There was significantly greater variability in the LH response of queens on Day 2 of oestrus compared to those on Day 4/5 for peak values and area under the curve; the only failure in release of LH was in queens on Day 2. Oestradiol levels did not differ significantly between Day 2 and Day 4/5 of oestrus. Progesterone values remained less than 1 ng/ml for 24 h after coitus. Both LH peak values and area under the curve were significantly greater for animals that became pregnant. There were also significant differences in coital behaviour between queens on Day 2 and those on Day 4/5 of oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Immunocytochemistry of luteinizing hormone releasing hormone (LHRH) in the sheep hypothalamus during various reproductive stages

Summary Using the labeled and unlabeled immunoperoxidase methods, the distribution and concentration (1) of immunoreactive LHRH-material in the hypothalamus, and (2) of gonadotropic hormones in the adenohypophysis of the ewe were determined during various reproductive stages, including two phases of the estrous cycle, anestrus, lactation, and the state after ovariectomy. The concentration of LHRH-immunoreactive material varied in particular regions of the median eminence (ME) and was closely dependent on the physiological state. The immunoreactive material was most abundant in hypothalami of lactating animals, exceeding gradually the corresponding deposits in ovariectomized animals, in ewes during the 16th day of the estrous cycle (before ovulation), and in anestrous ewes. A severe depletion of LHRH from the rostral and central parts of the ME was observed 24h after ovulation. This depletion was accompanied by a degranulation of LH-and prolactin-producing cells in the adenohypophysis. It is postulated that LHRH is stored in the ME of the sheep in all examined endocrine stages. Only circumscribed regions of the ME take part in the cyclic release of LHRH during the ovulatory phase; this suggests a functional differentiation of the nerve terminals in this neurohemal area.