Effects of adrenalectomy on photoperiod-induced changes in release of luteinizing hormone and prolactin in ovariectomized ewes

Finnish Landrace x Southdown ewes were ovariectomized (OVX) and subjected to daily photoperiods of 16L:8D (Group I) or 8L:16D (Group II) for 84 days. Ewes were then either adrenalectomized (ADX) (N = 5 for Group I; N = 4 for Group II) or sham ADX (N = 6 for Groups I + II). After surgery, ewes in Group I were subjected to 8L:16D for 91 days and 16L:8D for 91 days whereas ewes in Group II were exposed to 16L:8D for 91 days and 8L:16D for 91 days. Oestradiol implants were inserted into all ewes on Day 148. Sequential blood samples were taken at 28, 56, 91, 119, 147 and 168 days after surgery to determine secretory profiles of LH and prolactin. Photoperiod did not influence LH release in Group I in the absence of oestradiol. Although photoperiod influenced frequency and amplitude of LH pulses in Group II before oestradiol treatment, adrenalectomy did not prevent these changes in patterns of LH release. However, in Group II the increase in LH pulse amplitude during exposure to long days was greater (P less than 0.01) in adrenalectomized ewes than in sham-operated ewes. Mean concentrations of LH increased in ADX ewes on Days 91 (P = 0.07) and 119 (P less than 0.05). Adrenalectomy failed to influence photoperiod-induced changes in mean concentrations of LH, amplitude of LH pulses and frequency of LH pulses in the presence of oestradiol. Concentrations of prolactin were influenced by photoperiod. In Groups I and II concentrations of prolactin increased (P less than 0.01) after adrenalectomy, but the magnitude of this effect decreased over time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma follicle stimulating hormone and luteinizing hormone patterns during the estrous cycle of ewes

Fifteen Suffolk ewes were used in three experiments to compare plasma follicle stimulating hormone (FSH) and luteinizing hormone (LH) patterns during the estrous cycle and to determine whether FSH levels undergo changes in pulse frequency. Luteinizing hormone changed inversely with progesterone levels whereas FSH and progesterone concentrations revealed no obvious relationship. Unlike LH, FSH levels did not pulsate during the follicular phase. Higher FSH levels were detected on days 1, 6 and 12 and lower levels on days 0, 4 and 16. Coincident preovulatory LH and FSH surges were observed and this was the only time FSH and LH levels appeared to be jointly controlled.     

Attenuation of the estradiol-induced surge release of luteinizing hormone by antihistamine in ovariectomized ewes

Ovariectomized ewes received intramuscular (i.m.) injections of an H₁-histamine receptor antagonist, diphenhydramine, or saline during the anestrous and breeding seasons to determine if histamine may regulate the estradiol-induced surge release of LH in ewes. In addition, concentrations of histamine and GnRH in hypothalamic regions and histamine and LH in the pituitary gland were determined during the estradiol-induced surge of LH. Pretreatment mean, basal, and estradiol-induced secretion of LH did not differ (P > 0.05) among seasons. However, the quantity of LH (ng) measured during the estradiol-induced surge of LH was less (P < 0.05) in ewes treated with diphenhydramine (411 ± 104) than saline (747 ± 133). Treatment with diphenhydramine did not (P > 0.05) influence steady-state concentrations of histamine in hypothalamic or pituitary gland tissues, hypothalamic concentrations of GnRH, or anterior pituitary concentrations of LH during the estradiol-induced surge of LH. It is concluded that histamine may modulate the estradiol-induced surge release of LH in ewes by affecting the secretion of GnRH.     

Effect of heat stress on plasma concentrations of prolactin and luteinizing hormone in ewes.

Basal concentrations of prolactin but not luteinizing hormone were elevated in ewes by 8--10 h of heat stress given daily during the first 11 days of their oestrous cycle. However, the prolactin and luteinizing hormone responses to thyrotrophin releasing hormone and gonadotrophin releasing hormone were unaffected.     

Concentrations of luteinizing hormone in ovariectomized ewes and the effect of disturbance, starvation and vascular cannulation

Neither routine experimental procedures, including vascular cannulation and venipuncture, nor starvation or disturbance caused significant fluctuations in luteinizing hormone (LH) concentrations in ovariectomized ewes. Samples obtained at 5-min intervals revealed episodic LH releases occurring every 10-35 min. Hourly sampling over 3 days revealed large LH variations but no distinct diurnal variation.     

Effects of time after ovariectomy, season and oestradiol on luteinizing hormone and follicle-stimulating hormone secretion in ovariectomized ewes.

During the breeding season, five groups of three ewes were implanted at ovariectomy with 0.36, 0.5, 1.0 and 6.0 cm oestradiol implants or implants containing no steroid. Eleven days after receiving implants, blood samples were taken every 10 min for 6 h; implants were then removed. Treatments were repeated three times during each of two consecutive breeding seasons and four times during the intervening anoestrus. In ovariectomized ewes without steroid treatment, luteinizing hormone (LH) pulse frequency increased from early to mid-breeding season, decreased to a minimum at mid-anoestrus and increased to reach a maximum at the mid-point of the second breeding season, subsequently declining. LH pulse amplitude was inversely related to frequency. Basal serum LH concentrations decreased gradually from the first breeding season to reach a minimum at mid-anoestrus and gradually increased to reach a maximum at the end of the second breeding season. Mean serum LH and follicle-stimulating hormone (FSH) concentrations were higher at the end of the second breeding season compared with the beginning of the first breeding season. All parameters of gonadotrophin secretion were decreased much more by oestradiol during the anoestrus than during the breeding season. LH pulse frequency was decreased during anoestrus and at high oestradiol concentrations during the first breeding season. Apart from LH pulse amplitude, the decreases in all parameters of gonadotrophin secretion were less during the second compared with the first breeding season. The minimum effective dose of oestradiol required to decrease mean and basal serum concentrations of LH during anoestrus was lower than in the breeding season. The minimum effective dose of oestradiol required to decrease mean serum concentrations of FSH was lower in the first compared with the second breeding season. Oestradiol depression of LH pulse amplitude and mean serum concentrations of LH and FSH showed a dose dependency during the breeding season. During anoestrus dose dependency was seen for basal concentrations of LH and mean serum concentrations of LH and FSH. We conclude that significant chronic changes in gonadotrophin secretion occur in the ewe with time after ovariectomy. Sensitivity to oestradiol also changes, and the effects of oestradiol are not always dose dependent. We suggest that the circannual pattern of LH pulse frequency and basal LH secretion are directly linked to the circannual cycle of photoperiod.(ABSTRACT TRUNCATED AT 400 WORDS)     

Simultaneous measurement of luteinizing hormone-releasing hormone and luteinizing hormone during estradiol-induced luteinizing hormone surges in the ovariectomized ewe

Sequential bleeding and push-pull perfusion of the hypothalamus were used to characterize luteinizing hormone (LH) and LH-releasing hormone (LHRH) release in ovariectomized (OVX) ewes after injection of corn oil or estradiol benzoate (EB). Push-pull cannulae were surgically implanted into the stalk median eminences of 24 OVX ewes. Seven to 14 days later each of 20 animals was given an i.m. injection of 50 micrograms EB. Blood samples and push-pull perfusate were collected at 10-min intervals for 6-12 h beginning 12-15 h after EB injection. Four OVX ewes were given i.m. injections of corn oil 7 days after implantation of push-pull cannulae. Blood samples and push-pull perfusate were collected at 10-min intervals for 4 h between 18 and 22 h after injection of corn oil. Luteinizing hormone remained below 2 ng/ml throughout most of the sampling periods in 9 of 20 EB-treated ewes. In 5 of these 9 LHRH also was undetectable, whereas in 4 LHRH was detectable (1.84 +/- 0.29 pg/10 min), but did not increase with time. Preovulatory-like surges of LH occurred in 11 EB-treated ewes, but LHRH was undetectable in 5. In 4 of 6 ewes showing LH surges and detectable LHRH, sampling occurred during the onset of the LH surge.(ABSTRACT TRUNCATED AT 250 WORDS)     

The melanocortin 4 receptor mediates leptin stimulation of luteinizing hormone and prolactin surges in steroid-primed ovariectomized rats

We have previously reported that leptin, the product of the obese (ob) gene, may play a physiologically relevant role in the generation of estradiol/progesterone-induced luteinizing hormone (LH) and prolactin (PRL) surges in female rats. In the present study, we examined whether the stimulatory effect of leptin on the hormonal surges is mediated through the melanocortin (MC) 4 receptor in the brain, as is leptin's effect on feeding behavior. We also explored whether the MC4 receptor participates in tonic stimulation of steroid-induced LH and PRL surges. Experiments were performed on both normally fed and 3-day starved rats, which were ovariectomized and primed with estradiol and progesterone. At 11:00 h on the day of the experiments, the normally fed rats received an intracerebroventricular administration of artificial cerebrospinal fluid (vehicle), SHU 9119 (a nonselective MC3/MC4 receptor antagonist, 1.0 nmol), or HS014 (a selective MC4 receptor antagonist, 1.0 nmol). The 3-day starved rats were given vehicle, recombinant mouse leptin (0.3 nmol), leptin (0.3 nmol) + SHU9119 (1.0 nmol), or leptin (0.3 nmol) + HS014 (1.0 nmol). From 11:00 to 18:00 h, blood was collected every 30 min to measure LH and PRL. The 3-day starvation completely abolished both LH and PRL surges, but leptin significantly reinstated these hormonal surges. Both SHU9119 and HS014 significantly decreased the magnitude of LH and PRL surges in normally fed rats and also significantly blocked the leptin stimulation of the hormonal surges in starved rats. These results suggest that the MC4 receptor may be the pivotal subtype of MC receptors mediating the leptin stimulation of LH and PRL surges. The data also suggest that endogenous MC(s) may tonically stimulate the hormonal surges in normally fed rats via the MC4 receptor. This is the first report describing a physiological role of a specific MC receptor in regulating the reproductive axis.     

Simultaneous radioimmunoassay for luteinizing hormone and prolactin

A combined radioimmunoassay (RIA) for the measurement of the anterior pituitary proteins luteinizing hormone (LH) and prolactin (PRL) is described and compared with individual RIAs for these hormones. The standard curves and the sample values for LH and PRL were identical when determined in a combined or in an individual RIA. This technique may prove useful to a number of laboratories where it is desirable to determine levels of more than one hormone in limited sample volumes.     

Effect of N-methyl-d,l-aspartate on luteinizing hormone secretion in ovariectomized ewes in the absence and presence of estradiol

N-methyl-d,l-aspartate (NMA), a potent agonist of the neuroexcitatory amino acids aspartate and glutamate, stimulates release of luteinizing hormone (LH) in rats and nonhuman primates. The objective of the experiments described here was to determine the effect of NMA on LH secretion in ovariectomized ewes, in both the absence and presence of estradiol. In Experiment 1, blood samples were collected from 16 ewes every 12 min for 4 h. At Hour 2, ewes received i.v. injections of either 0, 6, 12, or 24 mg NMA/kg body weight dissolved in 0.9% saline (n = 4 per treatment). Mean LH concentrations were unaltered by any dose of NMA (p greater than 0.3). Immediately after completion of Experiment 1, each ewe received an s.c. Silastic implant designed to maintain circulating concentrations of estradiol of approximately 1 pg/ml. Three weeks later, Experiment 2 was conducted, using the same blood sampling regimen and doses of NMA as Experiment 1. The estradiol implants decreased serum LH concentrations in all animals. Treatment with saline failed to alter mean LH concentrations (p greater than 0.3). In contrast, 6, 12, and 24 mg NMA/kg body weight increased mean LH concentrations by 326% (p less than 0.03), 1125% (p less than 0.02), and 441% (p less than 0.0001), respectively. These results demonstrate that exogenous estradiol suppresses LH release in sheep in a manner antagonized by NMA.     

Effects of oestradiol on LH, FSH and prolactin in ovariectomized ewes

This study was conducted to test the hypothesis that the rate (dose/time) at which oestradiol-17 beta (oestradiol) is presented to the hypothalamo-pituitary axis influences secretion of LH, FSH and prolactin. A computer-controlled infusion system was used to produce linearly increasing serum concentrations of oestradiol in ovariectomized ewes over a period of 60 h. Serum samples were collected from ewes every 2 h from 8 h before to 92 h after start of infusion, and assayed for oestradiol, LH, FSH and prolactin. Rates of oestradiol increase were categorized into high (0.61-1.78 pg/h), medium (0.13-0.60 pg/h) and low (0.01-0.12 pg/h). Ewes receiving high rates of oestradiol (N = 11) responded with a surge of LH 12.7 +/- 2.0 h after oestradiol began to increase, whereas ewes receiving medium (N = 15) and low (N = 11) rates of oestradiol responded with a surge of LH at 19.4 +/- 1.7 and 30.9 +/- 2.0 h, respectively. None of the surges of LH was accompanied by a surge of FSH. Serum concentrations of FSH decreased and prolactin increased in ewes receiving high and medium rates of oestradiol, when compared to saline-infused ewes (N = 8; P less than 0.05). We conclude that rate of increase in serum concentrations of oestradiol controls the time of the surge of LH and secretion of prolactin and FSH in ovariectomized ewes. We also suggest that the mechanism by which oestradiol induces a surge of LH may be different from the mechanism by which oestradiol induces a surge of FSH.     

Serum luteinizing hormone follicle-stimulating hormone and prolactin in neonatal-androgenized rats.

Serum levels of LH, FSH, Prolactin and Testosterone of 90 days old male rats androgenized soon after birth were determined by specific radioimmunoassay and were compared to untreated rats. LH and FSH levels were also determined in 90 days old female rats neo-natally treated with testosterone and compared with normal diestrus rats. Androgenization of male rats significantly increased serum FSH and Prolactin levels without producing changes in plasma LH and testosterone concentrations. Similar increase in the FSH levels were found in androgenized female rats although plasma FSH concentrations were lower than in the male groups. These results obtained in male rats give an additional evidence that androgens acting in the first days of life are responsible of the higher levels of FSH and Prolactin that characterize the male or tonic pattern of gonadotrophin secretion.     

Changes in prolactin levels caused by luteinizing hormone releasing hormone

The acute effects of luteinizing hormone releasing hormone (LHRH) on the release of prolactin (PRL) were investigated in 12 normal cycling women and 42 women with various menstrual disorders. LHRH (100 micrograms) was bolusly injected intramuscularly and PRL levels were measured immediately before the injection and at 30 minutes and 60 minutes after the injection. LHRH elicited an increase of more than 25% in PRL levels in 15 cases (27.8%) at both 30 minutes and 60 minutes after the injection, whereas PRL levels were decreased by more than 25% in 7 cases (13.0%). The PRL response to LHRH seemed to be related to basal PRL levels. Especially when the PRL concentration was 20 ng/ml or more, LHRH decreased PRL levels in 7 cases out of 16. On the other hand, LHRH increased PRL levels in the majority of cases with a PRL concentration less than 20 ng/ml. In conclusion, the LHRH injection occasionally alters PRL levels in either a positive or negative manner, depending upon the basal PRL levels.     

Release of luteinizing hormone in prepubertal and middle-aged ovariectomized rats

Concentrations of circulating LH were determined in conscious, free-moving ovariectomized rats. All of the animals had been ovariectomized at 24 days of age. Between 30 and 90 days there was an increase in mean blood LH concentrations; a more vigorous pulsatile release of LH characterized by an increase in amplitude and frequency of LH release; and an elevated responsiveness to LHRH administration. Rats which had been ovariectomized for 1 year still had elevated blood LH levels but had episodic pulses of reduced amplitude and a decrease in responsiveness to LHRH. These data suggest that important alterations occur with age in the neuroendocrine mechanisms responsible for the release of LH.     

Genistein-induced pituitary prolactin gene expression and prolactin release in ovariectomized ewes following a series of intracerebroventricular infusions

The aim of the study was to evaluate whether genistein, a phytoestrogen commonly present in feed plants, affects prolactin release and its gene expression in the pituitary gland. In the experimental model, genistein was infused into the third ventricle (IIIv) of the brain in ewes during the short-daylight period (November-December), when the physiological plasma level of prolactin is low. Animals were ovariectomized six weeks before the experiment, to remove the main source of endogenous estrogens, and three weeks later a stainless steel guide cannula was implanted into IIIv. Genistein (10 ng/100 microl/h, n=5) or vehicle (control, n=5) were infused in a series of four one-hour infusions at 30-min intervals (from 16:30 to 22:00). Plasma samples were collected at 15-min intervals from 14:00 to 22:00 through a catheter inserted into the jugular vein and after the experiment ewes were slaughtered. Northern blot analysis revealed that pituitary prolactin mRNA content increased significantly in response to genistein, compared to the vehicle-infused ewes (p<0.05). Prolactin concentration in plasma rose significantly during the periods of genistein infusion, as compared to the values found before infusion (p<0.05-p<0.01) as well as to the values of the concomitant periods in vehicle-infused ewes (p<0.001). Our results show an effective estrogenic action of genistein on prolactin synthesis and release in ovariectomized ewes that might in part be exerted at the central nervous system level.     

Effect of estradiol benzoate and clomiphene on tyrosine hydroxylase activity and on luteinizing hormone and prolactin levels in ovariectomized rats

The effects of estradiol benzoate (EB) on tyrosine hydroxylase (TH) activity in the medial basal hypothalamus (MBH) and on plasma levels of luteinizing hormone (LH) and prolactin were studied in long-term ovariectomized rats. Administration of 10 μg EB produced significant elevation of TH activity on Days 1 and 3 following injection. LH levels were significantly lower than controls throughout the three day treatment period, although there was a significant increase from Day 1 to Day 2. TH activity and LH levels were inversely related throughout the experimental period. Clomiphene (15 μg/rat/day), a purported estrogen antagonist, was administered over a period of three days to control and EB-treated rats to determine whether the effect of EB on plasma LH levels was causally related to changes in TH activity. In rats receiving both EB and clomiphene, TH activity was lower and plasma LH was higher than after EB alone. The results support the hypothesis that the feedback effects of estradiol on LH release involve an action on the tuberoinfundibular dopaminergic (TIDA) neurons of the MBH and that clomiphene can oppose the inhibitory effect of estradiol on LH release by directly inhibiting TIDA neuron activity. Furthermore, EB-induced release of prolactin does not appear to involve detectable changes in the activity of TIDA neurons.     

A nongenomic action of estradiol as the mechanism underlying the acute suppression of secretion of luteinizing hormone in ovariectomized ewes

The objective of the present study was to determine how rapidly estradiol (E2) was able to suppress the secretion of LH in ovariectomized (OVX) ewes and to evaluate the ability of conjugated forms of E2 (E2 conjugated to BSA [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethyloxime:BSA [E2-BSA] and a novel conjugate, E2 conjugated to a small peptide [E2-PEP]) to mimic the actions of E2 on secretion of LH and FSH. Animals (n = 5-6 per group) were given infusions for 4 h of 50 microg of E2 or equimolar concentrations of E2-BSA or E2-PEP. Treatments with E2, E2-BSA, and E2-PEP each induced an acute suppression of LH secretion (<20 min, P < 0.01). In contrast, E2, but not E2-BSA or E2-PEP, induced the characteristic preovulatory-like surge of LH (at 10 h after priming treatment) and decreased secretion of FSH (at 4 h after priming treatment). In conclusion, the acute inhibition of LH secretion induced by E2 in OVX ewes supports the concept of a nongenomic action as the mechanism underlying the sudden suppression in secretion of LH. In addition, the fact that conjugated forms of E2 mimicked only the acute suppression of secretion of LH, without inducing the putative genomic actions on secretion of LH or FSH (i.e., a preovulatory-like surge), suggests that the acute effect of E2 may be mediated via the plasma membrane.     

Characteristics of pulsatile luteinizing hormone secretion in middle-aged ovariectomized rats

Experiments were performed to characterize the pulsatile patterns of circulating luteinizing hormone (LH) in the middle-aged ovariectomized (OVX) rat. Frequent blood samples were taken from OVX rats at 6, 7-8, and 9-10 mo of age, and LH was measured by radioimmunoassay. Rats had been OVX either 2 wk (STO) or 10-20 wk (LTO) previously. Mean LH levels were significantly lower with increasing age, reflecting effects on both pulse amplitude and pulse frequency. Mean LH levels were higher in LTO than STO groups, reflecting primarily an increase in pulse amplitude, but there was also a small, significant decrease in pulse frequency with increased time following OVX. In a second experiment, a random selection of the rats in the STO groups was tested again 10 wk after OVX. A significantly higher number of 9- to 10-mo-old rats had pulsatile patterns at 10 wk than at 2 wk following OVX. Furthermore, mean plasma LH concentrations were higher in all three groups. We conclude that decreases in several parameters of LH secretion are seen in middle-aged OVX rats, at the time when irregularities are first seen in the estrous cycle in the intact rat.