Hormonal responses to female conspecifics in hyperprolactinemic male rats and mice

Exposure to a female results in an acute release of LH and testosterone (T) in normal male rats and mice. This study was conducted to determine whether these hormonal responses are altered in hyperprolactinemic (hyperPRL) male rats in which copulatory behavior is known to be suppressed and in hyperPRL male mice in which it is not. Adult male CDF (F-344) rats were made hyperPRL either by grafting of three anterior pituitaries under the kidney capsule or by treatment with diethylstilbestrol (DES). Exposure of control males to receptive females for 10-15 min produced the expected two- to fourfold statistically significant elevations in plasma LH levels. In contrast, plasma LH levels in pituitary grafted or DES-treated males were not altered by female exposure. Male mice were pituitary grafted (two pituitaries per recipient) or sham-operated and housed individually with a female for 1 week. The resident females were then replaced with novel females in half of the cages and blood samples were taken from the males after 5 min exposure for determination of LH levels or after 45-60 min exposure for T levels. Female-induced LH and T elevations occurred in both hyperPRL and control groups. Failure of hyperPRL male rats to experience an increase in plasma LH levels in response to a female suggests abnormality of mechanisms controlling LHRH release. Suppression of LHRH release may be involved also in the induction of deficits of sexual behavior in these animals.     

Effects of a long-acting LHRH agonist preparation on plasma gonadotropin and prolactin levels in castrated male rats and on the release of prolactin from ectopic pituitaries

We have examined the effects of a single subcutaneous injection of an LHRH agonist, D-Trp-6-LHRH, in biodegradable microcapsules of poly(DL-lactide-co-glycolide) on plasma gonadotropin and prolactin (PRL) levels in castrated and in castrated-hypophysectomized-pituitary grafted (CAST-APX-GRAFT) male rats. The results were compared to the effects of daily injections of the same LHRH agonist dissolved in saline. In castrated rats, there were no significant alterations in plasma LH or PRL levels during the 10 days following the injection of LHRH agonist microcapsules, while FSH levels were generally reduced. In castrated males given daily injections of 6 micrograms of LHRH agonist in saline, plasma LH levels were significantly reduced while plasma PRL levels were not changed. In CAST-APX-GRAFT rats, both D-Trp-6-LHRH microcapsules and daily LHRH agonist injections appeared to increase plasma PRL levels. The pattern of changes in PRL release in both groups was similar, with levels on day 6 being significantly higher than those measured on days 1, 3 and 10 after onset of treatment. As expected, LH and FSH levels in these animals were extremely low. Immunoreactive D-Trp-6-LHRH was consistently detectable in the plasma of CAST-APX-GRAFT animals after microcapsule administration, whereas in animals given daily injections of this agonist in saline, its plasma concentrations were often below the detectability limit of the employed assay. These findings suggest that the LHRH agonist, D-Trp-6-LHRH, is capable of causing a short term stimulation of PRL release from ectopic pituitaries. Elevation of plasma LH levels is apparently not required for this effect.     

Pituitary luteinizing hormone-releasing hormone receptors in ovariectomized cows after challenge with ovarian steroids

Acute changes of bovine pituitary luteinizing hormone-releasing hormone (LHRH) receptors in response to steroid challenges have not been documented. To investigate these changes 96 ovariectomized (OVX) cows were randomly allotted to one of the following treatments: 1) 1 mg estriol (E3); 2) 1 mg 17 beta-estradiol (E2); or 3) 25 mg progesterone (P) twice daily for 7 days before 1 mg E2 and continuing to the end of the experiment. Serum was collected at hourly intervals from 4 animals in each group for 28 h following estrogen treatment. Four animals from each treatment were killed at 4-h intervals from 0 to 28 h after estrogen injection to recover pituitaries and hypothalami. Treatment with E3 or E2 decreased serum luteinizing hormone (LH) within 3 h and was followed by surges of LH that were temporally and quantitatively similar (P greater than 0.05). Progesterone did not block the decline in serum LH, but did prevent (P less than 0.05) the E2-induced surge of LH. Serum follicle-stimulating hormone (FSH) was unaffected (P less than 0.05) by treatment. Pituitary concentrations of LH and FSH were maximal (P less than 0.001) at 16 h for E3 and 20 h for E2, whereas P prevented (P greater than 0.05) the pituitary gonadotropin increase. Concentrations of LHRH in the hypothalamus were similar (P greater than 0.05) among treatments. Pituitary concentrations of receptors for LHRH were maximal (P less than 0.005) 12 h after estrogen injection (approximately 8 h before the LH surge), even in the presence of P. This study demonstrated that in the OVX cow: 1) E2 and E3 increased the concentration of receptors for LHRH and this increase occurred before the surge of LH; and 2) P did not block the E2-induced increase in pituitary receptors for LHRH but did prevent the surge of LH.     

Melanin-concentrating hormone stimulates the release of luteinizing hormone-releasing hormone and gonadotropins in the female rat acting at both median eminence and pituitary levels

The purpose of this study was to investigate whether melanin-concentrating hormone (MCH) acts directly on the median eminence and on the anterior pituitary of female rats regulating LHRH and gonadotropin release. In addition, immunohistochemistry was used to examine the density and distribution of MCH-immunoreactive fibers in the median eminence of proestrous rats. MCH-immunoreactive fibers were found in both the internal and external layers of the median eminence and in close association with hypophysial portal vessels. In the first series of in vitro experiments, median eminences and anterior pituitaries were incubated in Krebs-Ringer bicarbonate buffer containing two MCH concentrations (10(-10) and 10(-8) M). The lowest MCH concentration (10(-10) M) increased (P < 0.01) LHRH release only from proestrous median eminences. Anterior pituitaries incubated with both MCH concentrations also showed that 10(-10) M MCH increased gonadotropin release only from proestrous pituitaries. In the second series of experiments, median eminences and pituitaries from proestrous rats were incubated with graded concentrations of MCH. MCH (10(-10) and 10(-9) M) increased (P < 0.01) LHRH release from the median eminence, and only 10(-10) M MCH increased (P < 0.01) LH and FSH release from the anterior pituitary. The effect of MCH on the stimulation of both gonadotropins from proestrous pituitaries was similar to the effect produced by LHRH. Simultaneous incubation of pituitaries with MCH and LHRH did not modify LH but increased the FSH release induced by LHRH. The present results suggest that MCH could be involved in the regulation of preovulatory gonadotropin secretion.     

The release of pituitary gonadotrophins by luteinizing hormone releasing hormone in the intact,castrated and aspermatogenic rat

The change in serum gonadotrophin concentration in response to synthetic Luteinizing Hormone Releasing Hormone (LHRH - 400 ng i.v.) was investigated under barbiturate anaesthesia in adult male rats either chronically castrated, rendered aspermatogenic by the administration of α-chlorohydrin 12–16 weeks previously (to remove inhibin), or treated with vehicle. A single injection of LHRH increased serum LH and FSH concentrations similarly in both intact and aspermatogenic rats. In castrated rats the amount of LH released was much greater and the FSH secretion sustained. A second injection produced a similar increase although a second peak of FSH could not be detected in castrated rats as the FSH level was still elevated. The increase in LH levels was two to three times larger in response to the second injection of LHRH than to the first in all groups. The results do not support the hypothesis that the enhanced gonadotropin response to castration in the aspermatogenic rat is due to increased pituitary sensitivity to LHRH.     

Changes in serum luteinizing hormone (LH) concentrations in response to luteinizing hormone releasing hormone (LHRH) in bull calves that attained puberty early or late

The objectives of this study were to determine if the response to luteinizing hormone releasing hormone (LHRH) could be used to select bull calves capable of early sexual maturation and to establish the optimum route and dose of LHRH. In Trial 1, at 4, 10 and 20 week of age, 20 calves were treated iv with 2 microg/kg body weight of LHRH 1 and 5h after commencing a 9-h period of blood sampling. Bulls were separated into early and late maturing (n=10), based on age at puberty (scrotal circumference (SC) of >or=28 cm). At 4 and 20 week of age, peak serum LH concentrations and area under the LH response curve in response to LHRH were lower (P<0.05) in early- versus late-maturing bulls. In Trial 2, calves at 20 week of age were given LHRH as follows: 2 microg/kg body weight iv (n=6), im (n=6) or sc (n=6); 5 microg/kg im (n=6), or ischio-rectally (ir, n=6) or sc (n=6); and 10 microg/kg im (n=6) or sc (n=6). Serum LH concentrations were at a plateau from 30 to 165 min after treatment with 5 microg/kg of LHRH (im or ir; P>0.05). We concluded that the LH responses to LHRH in calves at 4 and 20 week of age could facilitate the development of a simple test (one blood sample prior to treatment with LHRH and a second during the period of sustained response to LHRH) to select early-maturing bulls.     

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.     

Effects of ethanol on rat hypothalamic luteinizing hormone releasing hormone. A study utilizing radioimmunoassay

To further understand the mechanism of action by which ethanol (ETOH) decreases plasma luteinizing hormone (LH) levels, the effects of multiple i.p. injections of EOH (1.0--1.5 g/kg) or saline on hypothalamic luteinizing hormone releasing hormone (LHRH) and plasma LH concentrations were evaluated in intact and castrate male rats. After injections, animals were decapitated, brains rapidly removed, and blocks containing the hypothalamus [with median eminence (ME)] were isolated. Hypothalami were subjected to acetic acid extraction and LHRH content quantitated via radioimmunoassay (RIA). Hypothalamic LHRH was found to be inversely correlated with plasma LH. In response to castration, both saline and ETOH-treated rats showed a decrease in hypothalamic LHRH content with a concomitant increase in plasma LH; however, the ETOH-treated animals retained significantly greater concentrations of LHRH and showed significantly lower plasma LH levels when compared to saline-treated controls. Likewise, ETOH-treated intact animals showed significant increases in LHRH content, with LH levels remaining significantly lower than the saline-treated intact controls. Thus, these data from both intact and castrate rats provide evidence to support the hypothesis that alcohol-induced decreases in LH levels are due to a diminished release rate of hypothalamic LHRH.     

Endocrine and ovulatory responses of the gilt to exogenous gonadotropins and estradiol during sexual maturation

Three studies were conducted to investigate the endocrine and ovulatory responses of the prepubertal gilt to exogenous estradiol and gonadotropins. In Study One, prepubertal gilts of 190 days of age were injected s.c. with pregnant mare's serum gonadotropin (PMSG) or physiological saline (SAL). Following PMSG injection, circulating levels of estradiol-17 beta (E2) increased. This increase was followed by a surge of luteinizing hormone (LH), estrus, a rise in progesterone (P4) levels, and ovulation. None of the gilts given SAL had increased levels of E2, LH or P4, and none ovulated. In Study Two, prepubertal gilts of 165 days of age were treated with varying doses of PMSG. A positive correlation was observed between dose of PMSG and peak levels of E2 (r = 0.83, P less than 0.001) and between dose of PMSG and number of corpora lutea (r = 0.96, P less than 0.001). In Study Three, gilts were treated at ages of 70 to 190 days with estradiol benzoate (EB), PMSG, or corn oil plus saline (CO/SAL) followed in 72 to 96 h by human chorionic gonadotropin (hCG) or SAL. All gilts treated with EB at 100 to 175 days of age had two surges of LH at an approximately 24-h interval. Gilts responding to EB at 70 and 190 days had only one surge of LH. Gilts of 100 days of age or older responded to PMSG with a single surge or two surges of LH. Ovulation in response to treatment was observed in gilts of 100 days of age or greater but not at 70 days. The conclusions drawn from these studies are that 1) PMSG-induced ovulation is preceded by an increase in circulating levels of E2 and in some gilts by a surge of LH, and 2) prepubertal gilts are able to respond to exogenous endocrine stimulation with either a single surge or multiple surges of LH at 70 to 190 days but are unable to ovulate in response to exogenous gonadotropins until 100 days of age.     

Effects of melatonin on luteinizing hormone secretion in anestrous ewes following dopamine and opiate receptor blockade

In the present investigation we have examined the ability of melatonin to modify the pulsatile LH secretion induced by treatment with a DA antagonist (sulpiride, SULP) or opioid antagonist (naloxone, NAL) in intact mid-anestrous ewes. The experimental design comprised the following treatments-in experiment 1: (1) intracerebroventricular (i.c.v.) infusion of vehicle (control I); (2) pretreatment with SULP (0.6 mg/kg subcutaneously) and then i.c.v. infusion of vehicle (SULP + veh); (3) pretreatment with SULP and then i.c.v. infusion of melatonin (SULP + MLT, 100 microg per 100 microl/h, total 400 microg). In experiment 2: (4) i.c.v. infusion of vehicle (control II); (5) i.c.v. infusion of NAL (NAL-alone, 100 microg per 100 microl/h, total 300 microg); (6) i.c.v. infusion of NAL in combination with MLT (NAL + MLT, 100 microg + 100 microg per 100 microl/h). All infusions were performed during the afternoon hours. Pretreatment with SULP induced a significant (P < 0.01) increase in LH pulse frequency, but not in mean LH concentration, compared with control I. In SULP + MLT-treated animals, the LH concentration was significantly (P < 0.01) higher during MLT infusion, but due to highly increased LH secretion in only one ewe. The significant changes in the SULP + MLT group occurred in LH pulse frequency. A few LH pulses were noted after melatonin administration compared with the number during the infusion (P < 0.05) and after vehicle infusion in the SULP + MLT group (P < 0.05). The i.c.v. infusion of NAL evoked a significant increase in the mean LH concentration (P < 0.001) and amplitude of LH pulses (P < 0.01) compared with these before the infusion. The enhanced secretion of LH was also maintained after i.c.v. infusion of NAL (P < 0.01) with a concomitant decrease in LH pulse frequency (P < 0.05). In NAL + MLT-treated ewes, mean plasma LH concentrations increased significantly during and after the infusion compared with that noted before ( P < 0.001). No difference in the amplitude of LH pulses was found in the NAL + MLT group, but this parameter was significantly higher in ewes during infusion of both drugs than during infusion of the vehicle (P < 0.01). The LH pulse frequency differed significantly (p < 0.05), increasing slightly during NAL + MLT administration and decreasing after the infusion. In conclusion, these results demonstrate that: (1) in mid-anestrous ewes EOPs, besides DA, are involved in the inhibition of the GnRH/LH axis; (2) brief administration of melatonin in long-photoperiod-inhibited ewes suppresses LH pulse frequency after the elimination of the inhibitory DA input, but seems to not affect LH release following opiate receptor blockade.     

Changes in the dynamics of luteinizing hormone-releasing hormone-stimulated secretion of luteinizing hormone during sexual maturation of female rats

Our aim was to identify age-related changes in the dynamics of luteinizing hormone (LH) release that may contribute to the decline in pituitary sensitivity to luteinizing hormone-releasing hormone (LHRH) during sexual maturation of female rats. We studied LHRH-stimulated LH secretion curves of superfused pituitaries from rats ranging in age from 10 days to the first estrous cycle. Pituitary fragments were exposed for 10 min to medium alone or to medium plus LHRH; incubation continued in medium alone for 130 min and effluent was collected for LH analysis. Secretion curves were compared on the basis of total secretion (area under the curve), maximal change in LH secretion rate, and rates of rise and decay of the curves. The data show that total LH secretion in response to LHRH is greatest in 15-, 20-day-old and first-proestrus animals. Also, the maximal change in LH secretion rate was greater, and the increase in LH secretion rate faster in younger animals than in 30-day-old animals. Analysis of secretory granules in LH-containing gonadotropes of 15- and 30-day-old animals revealed changes in he granule population with age. We conclude that younger animals respond faster with a greater LH secretion response to LHRH than do 30-day-old or first-estrus animals, and that these age-related changes in the dynamics of LH secretion may be due in part to maturation of the LH secretory granules.     

Induction of the self-priming effect of luteinizing hormone releasing hormone during the sexual maturation of the male rat

Pubertal and young adult male rats release more luteinizing hormone (LH) in response to luteinizing hormone releasing hormone (LHRH) if pretreated with LHRH than if pretreated with saline. Immature male rats do not show this self-priming effect. In order to examine the role of acute changes in testicular steroids in this process, immature (29-30 days old) or pubertal (50-51 days old) male rats were castrated or sham operated under ketamine HCl anesthesia. Beginning immediately after completion of the surgery, they were given three priming injections of 10 ng LHRH/100 g body wt or saline at 30-min intervals. Thirty minutes after the third priming injection, a blood sample was obtained by cardiac puncture followed immediately by a challenge injection of 50 ng LHRH/100 g body wt given to both saline and LHRH primed groups. Ten minutes after the challenge injection a final blood sample was obtained by heart puncture. Serum was assayed for LH concentration by radioimmunoassay. Sham-operated pubertal rats showed a typical self-priming effect. Animals pretreated with LHRH released significantly (P less than 0.01) more LH in response to the challenge injection than did rats pretreated with saline. Acute castration also resulted in a significant (P less than 0.001) self-priming effect in pubertal rats. As anticipated, sham castrated immature males did not show a self-priming effect. Acutely castrated immature rats however, showed a significant (P less than 0.05) self-priming effect. These data provide support for the hypothesis that, prior to puberty, increases in testosterone during the priming process inhibit the expression of the self-priming effect.     

An acute increase in serum prolactin does not modify gonadotropin release in female rats

Attempts were made to find out whether hyperprolactinemia has an effect on the hypothalamo-pituitary response to estrogen feedback and LHRH stimulation. Adult female rats of Wistar strain were ovariectomized and received subcutaneous injection of 20 micrograms estradiol benzoate (EB) 3-4 weeks later (day-0). A second injection of 20 micrograms EB, when administered at noon on day-3, induced a highly significant increase in serum LH (p less than 0.001 vs. basal values), but not FSH, estimated at 1800 h on the same day. This EB-promoted LH release was not altered by pretreatment with rat PRL (5 micrograms/day), which was administered subcutaneously daily in the morning (1100 h) between day-1 and day-3. No statistical difference in the serum LH concentration was found when compared with the values for the control animals pretreated with 0.9% saline alone. Serum gonadotropins 15 min after LHRH administration (100 ng/100 g BW) in 32-day-old female rats were not statistically different between the animals pretreated with 5 micrograms PRL, which was given subcutaneously daily (at 0800 h) for 3 days, and the controls pretreated with 0.9% saline. These results suggest that an acute increase in serum PRL may not exert a negative effect on the gonadotropin release induced by estrogen feedback and LHRH stimulation.     

Administration of human pancreatic growth hormone-releasing factor (GRF) analogs enhances responsiveness of cultured rat pituitary cells to GRF

The aim of this study was to investigate whether anterior pituitary responsiveness to human pancreatic growth hormone-releasing factor containing 29 amino acids (GRF-29) can be modulated by GRF-29 itself. Male rats were injected (sc) daily for 3 days with 50 ug of GRF-29, or were treated twice daily for 14 days with 5 ug of [D-Ala-2]-GRF-29 (a potent GRF agonist). Control animals were injected with saline. After the last injection, pituitaries were removed, dispersed, cultured for 96 h and then challenged with either GRF-29 or [D-Trp-6]-LHRH (a LHRH agonist). Cultured cells from analog-treated rats were more responsive to GRF-29 stimulation than were cells obtained from controls. In contrast, neither treatment altered the response to [D-Trp-6]-LHRH. These studies indicate that periodic administration of GRF analogs can increase hypophyseal GRF responsiveness. Such control may be an important component in the physiological regulation of GH secretion and has important implications for potential therapeutic uses of GRF analogs.     

Comparison of some CNS effects of luteinizing hormone-releasing hormone and progesterone

Luteinizing hormone-releasing hormone (LHRH) has been reported to facilitate lordotic behavior in estrogen-primed ovariectomized (OVX) female rats in a manner similar to progesterone (P). This study compared P and LHRH with respect to their behavioral effects and site of action within the brain. The hormones were compared using two different components of sexual behavior, receptivity and proceptivity. To test for receptivity, OVX females were given behaviorally ineffective estradiol benzoate (EB) injections sc 48 hr before testing. They were then treated with either P, LHRH, or vehicle by various routes. Two and/or four hours later, receptivity (LQ) was measured. Treatments for the proceptivity test were similar except that a larger EP-priming dose, which facilitates preceptive behavior, was used. Four hours later, LQ and hopping, darting, and earwiggling were scored. In the receptivity test, sc administration of 1 mg P or 1 μg LHRH (but not 0.5 or 5.0 μg) significantly elevated LQ with respect to vehicle injection 4 hr after treatment. In the proceptivity test, 0.5, 1.0, and 5.0 μg of LHRH given sc failed to alter significantly either LQ or soliciting behavior. Progesterone facilitated both parameters. Implantation of crystalline P into the midbrain reticular formation (MRF) has been shown to elicit both the receptive and preceptive effects of the steroid. Microinjection of as much as 100 ng of LHRH in 1.0 μl saline into the same region failed to enhance lordotic behavior compared to saline injection alone, while a 200-ng intracerebroventricular dose significantly facilitated lordosis at 4 hr. The data indicate that LHRH does not induce proceptive behavior. The effects of peripherally administered LHRH on receptive behavior are similar but less pronounced than those of P. The two hormones elicit this effect from different sites in the brain.     

Effects of chronic administration of LH releasing hormone on age-dependent activities of testicular 4-ene-5 alpha-reductase and 17 beta-ol-dehydrogenase in mice

Male (WB X C57BL/6)F1 hybrid mice of 16, 26 and 66 days of age, 4 in each group, were injected daily with 0.2 micrograms/10 g body weight of LH releasing hormone (LHRH) or saline for 14 days. Testicular homogenates were incubated with [14C]4-androstene-3,17-dione and enzyme activities were examined. In mice treated with saline, testicular 17 beta-ol-dehydrogenase activity increased with age but 4-ene-5 alpha-reductase (5 alpha-reductase) activity decreased with age. LHRH treatment for 14 days starting from day 26 resulted in a delay in sexual maturation, as evidence by significant decreases (P less than 0.05) in seminal vesicle weight and testicular 17 beta-ol-dehydrogenase activity and by a significant increase (P less than 0.05) in 5 alpha-reductase activity. However, LHRH treatment starting from day 66 had no significant effect on these testicular enzyme activities.     

Immunohistochemical study of the responsiveness of LH cells of fetal rats to synthetic LHRH in vitro

Summary Fetal rat pituitaries on days 17–19 of gestation were maintained in serum-free Medium 199 for 24 h in the presence of 1, 10 and 100 ng/ml of synthetic LHRH. Immunohistochemical examination of such stimulated tissue reveals a complete depletion of immunoreactive material in most of the LH cells, irrespective of the LHRH concentrations tested, though some cells remain weakly immunopositive in the pituitaries of later developmental stages. Once discharge has occurred, there is little reaccumulation of secretory material in LH cells during prolonged incubation for 48 h in LHRH-free medium containing 10% calf serum. The LHRH treatment causes no immunohistochemical change in TSH cells.It is concluded that in fetal rats recently differentiated LH cells can release the secretory product if they are stimulated by hypothalamic LHRH.     

Modulation by sex steroids and [D-Trp6, Des-Gly-NH2(10)]luteinizing hormone (LH)-releasing hormone ethylamide of alpha-subunit and LH beta messenger ribonucleic acid levels in the rat anterior pituitary gland

LHRH and sex steroids play a major and direct regulatory role in the secretion of LH by the anterior pituitary gland. The aim of the present study was to investigate the interactions between sex steroids, more especially the potentiating effect of progesterone (P) in the presence or absence of a low dose of 17 beta-estradiol (E2) and/or dihydrotestosterone (D) on mRNA levels encoding the alpha- and beta-subunits of LH in both female and male rats. We also studied the effect of 2-week treatment with the LHRH agonist [D-Trp6, des-Gly-NH2(10)]LHRH ethylamide on the same parameters. After treatment with the LHRH agonist (5 micrograms daily), the accumulation of mRNA encoding the alpha-subunit was stimulated by approximately 3-fold while the LH beta mRNA concentration remained unchanged. Ovariectomy performed 14 days earlier, increased pituitary alpha and LH beta mRNA levels by 3.7- and 8.8-fold, respectively, while orchiectomy performed 14 days earlier increased alpha and LH beta mRNA levels by 6- and 6.5-fold, respectively. The present data demonstrate that although P alone exerts no effect on alpha and LH beta mRNA levels in castrated animals, treatment with P markedly potentiates the inhibitory effect of E2 on both mRNA levels in female as well as male rats. In addition, P potentiates the inhibitory effect of D on LH beta mRNA levels in castrated female rats. Furthermore, the present study illustrates the importance of the cumulative inhibitory effects of relatively low doses of E2 and D on mRNAs encoding both LH subunits. Moreover, the present observation of a differential modulation of alpha-subunit and LH beta mRNA levels after chronic treatment with an LHRH agonist offers an explanation for the high plasma levels of free alpha-subunit found in patients treated with LHRH agonists.     

Changes in luteinizing hormone-releasing hormone content of discrete hypothalamic areas associated with spontaneous and induced preovulatory luteinizing hormone surges in the domestic hen

It is widely assumed that luteinizing hormone-releasing hormone (LHRH) neuronal activation is involved in the preovulatory surge of LH in the hen. In addition, this LH surge may be initiated by ovarian progesterone (P4) release. Thus, spontaneous and P4-induced LH surges should be associated with acute changes in LHRH content of discrete hypothalamic areas associated with LHRH cell bodies and/or LHRH axon terminals. Medial preoptic area (mPOA) and infundibulum (INF) LHRH content was measured by radioimmunoassay at intervals before, at, and following peak LH levels of a spontaneous preovulatory surge of LH, as well as when this surge was advanced by P4 administration in laying hens. Nonlaying birds served as additional controls. Levels of serum LH, P4, 17 beta-estradiol and pituitary LH were also measured. Increased (P less than 0.05) LHRH content in mPOA without changes in the INF are associated with peak serum LH levels of the spontaneous LH surge. By contrast, decreased (P less than 0.05) LHRH content in both mPOA and INF is associated with peak serum LH levels when the spontaneous surge was advanced 8 h by P4 administration to laying hens. Medial preoptic area and INF LHRH contents were significantly lower (P less than 0.05) in nonlaying than in laying hens.(ABSTRACT TRUNCATED AT 250 WORDS)     

Release of LHRH in vitro and anterior pituitary responsiveness to LHRH in vivo during sexual maturation in pullets (Gallus domesticus)

An in-vitro superfusion technique was used to study basal and depolarization-induced (32 mmol K+/l) release of LHRH from the mediobasal hypothalamus (MBH) of pullets at 8-25 weeks of age. Plasma LH concentrations and the incremental change (delta LH) after an i.v. injection of 1 or 15 micrograms synthetic ovine LHRH/kg body weight were also determined. Between 8 and 25 weeks of age, significant (P less than 0.01) increases in basal and depolarization-induced release of LHRH (93 and 330%, respectively) were accompanied by a significant (P less than 0.01) rise in the residual LHRH content of MBH tissue (152%), observations which suggest that the ability of the hypothalamus to synthesize and secrete LHRH increases as sexual maturation proceeds. However, plasma LH, which reached a maximum concentration of 2.05 +/- 0.43 micrograms/l at 15 weeks, fell significantly (P less than 0.05) to 1.14 +/- 0.05 micrograms/l at 25 weeks. Since delta LH in response to exogenous LHRH showed a marked and progressive decline between 12 and 20 weeks of age, the low plasma concentration of LH typical of the mature hen is probably attributable to a direct negative-feedback action of ovarian steroids on the anterior pituitary gland rather than to an impaired secretion of LHRH from the median eminence. It is suggested that a dramatic increase in the responsiveness of LHRH nerve terminals in the MBH to depolarization by 32 mmol K+/l between 20 and 25 weeks of age (mean age at onset of lay 21.9 weeks; range 19-25 weeks) may reflect the development of hypothalamic responsiveness to the positive feedback action of progesterone.