Regulation Of Luteinizing Hormone Release Research Articles

Effects of Galanin and Leptin on Gonadotropin-Releasing Hormone-Stimulated Luteinizing Hormone Release from the Pituitary

Background: A number of peptides regulate luteinizing hormone (LH) release. In some cases, these peptides exert an effect at the pituitary, to directly regulate LH release and/or to modulate the effects of gonadotropin-releasing hormone (GnRH). A link between nutrition and reproductive function is well established. In this study we investigated the effects of two peptides associated with appetite control, galanin and leptin, on the regulation of LH release from the pituitary. Methods: Using perifused anterior pituitary tissue from freely fed rats, we investigated the effect of galanin on basal LH release and GnRH-stimulated LH release from pituitaries in a high-estrogen (proestrus) and a low-estrogen (metestrus) environment. In addition, we examined the effect of galanin on LH release following GnRH self-priming. The effect of inhibiting protein synthesis, with cycloheximide, was also studied under these conditions. Finally, the effects of leptin alone and on a galanin-modulated LH response were investigated. Results: There was no detectable effect of leptin, either alone or in conjunction with galanin. We observed that galanin enhanced GnRH stimulation of LH secretion, an effect that was dependent on protein synthesis and on an estrogenized environment. In addition, importantly, galanin also enhanced the LH response in GnRH self-primed pulses. Conclusion: Our results provide further details on the effect of galanin on the LH surge, particularly the effect on the response to pulsatile GnRH and the effect of protein production, and thereby indicate a means by which appetite- and nutrition-related peptides may act on the ovulatory cycle at the pituitary.

THE PRESENCE AND ROLES OF GONADOTROPINRELEASING HORMONE IN THE REPRODUCTION OF CORALS

In southern Taiwan, the mass spawning of scleractinian corals occurs yearly in lunar mid-March. Our previous studies shown that peak levels of estradiol (E2) and aromatase activity in corals during the spawning period. We hypothesize that the endocrine system, especially gonadotropin- releasing hormone (GnRH) plays an important role as the endogenous factor in the synchronous mass spawning of scleractinian corals, using Euphyllia ancora as a model species. The investigation of the presence, bioactivity in the regulation of luteinizing hormone (LH) release, annual cycle and the possible roles in the reproduction of GnRH were conducted. Coral polyps were extracted with acid solution and purified by a Sep-Pak C-18 cartridge column and HPLC, GnRH was measured by a validated radioimmunoassay. Immunoreactive GnRH was consistently detected in female and male coral tissue throughout 3-year period. Positive GnRH neurons were immunohistochemically stained in the corals. Peak concentrations of GnRH were obtained in the coral tissue just prior to spawning with a 10-fold increase as compared to the non-spawning season. The primary culture of pituitary cells was further developed in black porgy fish to examine the activity of coral GnRH in the regulation of LH release. Coral GnRH extracts and mGnRH analog could similarly stimulate LH release from dispersed black porgy pituitary cells at a dosedependent manner, respectively. GnRH antagonist significantly and dosedependently inhibited the stimulation of LH release in response to coral extracts and mGnRH analog. mGnRH analog could significantly stimulate the increase of aromatase activity, testosterone and E2 concentrations, and steroid glucuronization in corals. Exogenous GnRH further stimulated the oocyte growth of corals. Seasonal variation of coral GnRH, with a dramatic increase during the spawning period, concomitant to that in aromatase and sex steroids, as well as the ability of mGnRH agonist to stimulate coral aromatase, steroidogenesis, and steroid glucuronization suggest that GnRH plays an ancestral role in the control of oocyte growth and mass spawning in corals. (poster)

5-HT 7 receptor subtype as a mediator of the serotonergic regulation of luteinizing hormone release in the zona incerta

5-Hydroxytryptamine (5-HT) and the 5-HT 1A/7 receptor agonist (+)-8-hydroxy-2-(di- n-propylamino) tetralinHBr (8-OH-DPAT), injected into the zona incerta (an area in the dorsal hypothalamus) of the female rat, inhibit the release of luteinizing hormone (LH) and the effects of both are blocked by the 5-HT 2/7 receptor antagonist, ritanserin. As both 8-OH-DPAT and ritanserin have moderate activity at the 5-HT 7 receptor subtype, the possibility that this subtype might mediate their effects in the zona incerta has been investigated. Ovariectomised rats were primed with 5 μg oestradiol benzoate followed at 48 h by 0.5 mg progesterone, which induces an LH surge. 5-Carboxamidotryptamine (5-CT), a potent but non-selective agonist at 5-HT 7 receptors, like 5-HT and 8-OH-DPAT, inhibited the LH surge at 5 and 1.25 nmol injected bilaterally into the zona incerta. The non-selective 5-HT 2/7 receptor antagonist ritanserin and the selective 5-HT 7 receptor antagonist, ( R)-3-(2-(2-(4-methyl-piperidin-1-yl)-pyrrolidine-1-sulfonyl)-phenol (SB-269970-A) at 0.5 μg/side blocked all three receptor agonists when injected concurrently into the zona incerta. However, lower (0.2 μg) and higher doses (2 and 5 μg) of SB-269970-A were less effective, indicating a bell-shaped dose–response curve. SB-269970-A was also inhibitory when administered systemically (1 mg/kg intraperitoneally (i.p.)). When LH release was suppressed by 5 μg oestradiol benzoate, SB-269970-A (0.5 and 2 μg) did not elevate levels, indicating it is unlikely that 5-HT 7 receptors mediate a tonic inhibition on release but rather are involved in terminating the pre-ovulatory LH surge. These data demonstrate that 5-HT 7 receptors play a role in the regulation of LH by the zona incerta in rat brain.

Cabergoline Influences Ovarian Stimulation in Hyperprolactinemic Patients With Polycystic Ovary Syndrome

Women with polycystic ovary syndrome (PCOS) typically have abnormal gonadotropin secretion and, in 30% of cases, mild and transient hyperprolactinemia. Elevated prolactin (PRL) is found in the late follicular and luteal phases of natural as well as stimulated cycles. Increased levels of both PRL and luteinizing hormone (LH) conceivably reflect a reduced dopamine inhibitory effect. To clarify the influence of dopaminergic control in PCOS, the investigators retrospectively examined the effects of cabergoline, a potent dopamine agonist that inhibits PRL secretion, on the ovarian response to stimulation with recombinant follicle-stimulating hormone (FSH). Enrolled in the study were 44 couples in which the women were diagnosed as having PCOS, in part because of anovulatory infertility and/or oligomenorrhea or amenorrhea. Eighteen women (group A) whose mean serum PRL was 32. 8 ng/ml received 0.25 mg of cabergoline (Dostinex) per week before and during ovarian stimulation. The remaining 26 women (group B), with a mean PRL of 31.3 ng/ml, received no treatment for hyperprolactinemia. For ovarian stimulation investigators used Triptorelin, a gonadotropin-releasing hormone agonist; FSH in an individualized dose; and human chorionic gonadotropin (hCG). The two groups were similar demographically and clinically. The total of 107 ovulatory-stimulated cycles available for study represented approximately 2 ½ per participant. Cabergoline reduced the mean serum prolactin to 12.5 ng/ml at the time stimulation began. Women not given cabergoline received significantly less recombinant FSH per cycle and were stimulated for a shorter time before hCG was given. Their peak serum estradiol levels were significantly higher than in group A women. Serum progesterone levels on the day of hCG treatment did not differ significantly in the two groups. Ultrasonography showed that hyperprolactinemic women developed significantly more follicles of all sizes on the day hCG was given than did those given cabergoline. Mild ovarian hyperstimulation syndrome (OHSS) developed in 6.2% of group B women and 2.4% of those in group A; all these cycles were canceled. There were 8 pregnancies in group A, 3 of them multiple, and 11 in group B, 5 of them multiple; the differences were not significant. These findings agree with the existence of a dopaminergic component in the regulation of LH release in women with PCOS. Pretreatment with the dopamine agonist cabergoline seems to reduce the ovarian response to recombinant FSH without altering the pregnancy rate or the risk of multiple pregnancy. This might be a helpful means of limiting the risk of OHSS.

The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish

Both glutamate and gamma-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotransmitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarboxylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts more than 400 million years ago. There is good evidence for the involvement of GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA action to stimulate LH release appears to be a combination of effects on GnRH release, potentiation of gonadotropin hormone-releasing hormone (GnRH) action, and in some cases directly at the LH cell. These actions appear to be dependent on such factors as sex or sex steroid levels, and there may also be species differences. Nevertheless, the stimulatory effects of GABA on LH are present in at least 4 fish species. In contrast, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 fish species. The sites and mechanisms of action of amino acid neurotransmitters on LH release have yet to be fully characterized. Both 130N-methyl-D-aspartic acid (NMDA) and S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABA(A) type which mediates the effects of GABA on LH release in fish, at least partially acting on the GnRH neuron, but likely directly acting at the gonadotroph as well. GABA may also be involved in regulating the release of other pituitary hormones in fish, namely follicle stimulating hormone (FSH = GTH-I), prolactin, and growth hormone. Based on the findings described in this review, a working model for the involvement of glutamate and GABA in the regulation of LH release in teleost fish is proposed.

Melanin-concentrating hormone, melanocortin receptors and regulation of luteinizing hormone release.

Melanin-concentrating hormone (MCH) is a neuropeptide, identified by its ability to either mimic or antagonize the melanin-dispersing action of alpha-melanocyte stimulating hormone (alphaMSH) on skin melanophores. MCH and alphaMSH also have antagonistic actions in the brain affecting feeding behaviour, aggression, anxiety, arousal and reproductive function through the release of luteinizing hormone (LH). It is not clear, however, how they exert their opposite effects in the central nervous system (CNS). One possibility is that they act via a common receptor. In this study we have examined the effect of a number of MC receptor antagonists, with relative selectivity for the MC3, 4 and 5 subtypes, on the actions of MCH on LH release. We confirmed that bilateral administration of MCH (100 and 200 ng/side) into the medial preoptic area of oestrogen-primed (oestradiol benzoate 5 microgram) ovariectomized anaesthetized rats, stimulated the release of LH. This effect was blocked by the concomitant administration into the medial preoptic area of the MC4/5 antagonist ([D-Arg8]ACTH(4-10) and the MC3/5 antagonist ([Ala6]ACTH(4-10)-both at 500 ng/side-but not by the MC3/4 antagonist, SHU9119 (200 ng/side). Furthermore, the MC3 agonist [Nle3]-gamma2 MSH failed to affect LH release. These results indicate that the MC3 and MC4 receptors are not involved in mediating the action of MCH but are consistent with an action via the MC5 subtype. Preputial glands, which express MC5 receptors, were also stimulated by MCH which is in keeping with this idea. In HEK293 cells transfected with the MC5 receptor MCH increased the production of IP3. However, it was much less potent than alphaMSH and unlike alphaMSH, had no effect on the production of cAMP. MCH (10-10 to 10-5 M) also failed to displace I125NDP-MSH from cells transfected with MC5 receptors indicating that it was not acting as a competitive antagonist and its binding site was distinct from that of alphaMSH. Thus while MCH may function as an agonist at the MC5 receptor, its stimulation of LH release is more likely to be mediated via a specific MCH receptor that has common properties with the MC5 receptor.

Expression of activin and follistatin in the rat hypothalamus: anatomical association with gonadotropin-releasing hormone neurons and possible role of central activin in the regulation of luteinizing hormone release.

The central role of activin in the regulation of the reproductive axis remains largely unexplored. Evidence suggests that activin may play a role in control-linggonadotropin-releasing hormone (GnRH) release. We assessed potential neuroanatomical associations between activin- and GnRH-neuronal systems via examination of the distribution of activin betaA-subunit and activin binding protein (follistatin) protein and mRNA signals relative to GnRH neurons in the adult rat brain. Activin betaA-subunit-immunostained fibers were distributed throughout the hypothalamus and GnRH-positive perikarya, and fibers were in close association with betaA-subunit-immunoreactive fibers. Follistatin mRNA-expressing cells were also identified throughout the hypothalamus with GnRH fibers often observed juxtaposed to follistatin cell bodies. Colocalization of either the betaA-subunit or follistatin within GnRH neurons was not detected. The functional significance of central activin in the regulation of the reproductive axis was also demonstrated. The intracerebroventricular infusion of rh-activin A significantly increased luteinizing hormone, but not follicule-stimulating hormone, serum levels in adult male rats. Taken together, the present results support an interaction between activin and GnRH neuronal systems in the rat hypothalamus, and suggest activin may act within the brain to regulate the reproductive axis.

Differential regulation of luteinizing hormone release by gamma-aminobutyric acid receptor subtypes in the arcuate-ventromedial region of the castrated ram.

We investigated the effects on LH secretion of infusing gamma-aminobutyric acid (GABA) agonists muscimol and baclofen (GABAA and GABAB receptor agonists, respectively) into either the medial preoptic area (mPOA) or the arcuate-ventromedial region (ARC-VMR) of the hypothalamus of castrated rams during the nonbreeding season. Bilateral microdialysis of artificial cerebrospinal fluid for 4 h followed by treatment with artificial cerebrospinal fluid, baclofen (1 mM), or muscimol (1 mM in the ARC-VMR, 250 microM in the mPOA) for 4 h was carried out on three separate occasions in random order. Simultaneously, jugular venous blood was collected at 10-min intervals, and plasma later was assayed for LH. The estimated dose of baclofen delivered to each unilateral microdialysis site was 7.9 micrograms; for muscimol, it was 1.1 micrograms for the mPOA and 4.5 micrograms for the ARC-VMR. In the mPOA, baclofen had no detectable effect, whereas muscimol had a delayed suppressive effect on mean LH (P < 0.01). In the ARC-VMR muscimol rapidly reduced mean LH (P < 0.01). In contrast, baclofen increased mean LH (P = 0.01) and LH pulse amplitude (P = 0.05) without altering the LH interpulse interval (P > 0.10). These results support the involvement of GABAA receptors in the mPOA in regulating LH secretory patterns. More importantly, both GABAA and GABA(B) receptors in the ARC-VMR appear to differentially modulate LH and, presumably, GnRH release. Whether GABA acts directly on the GnRH neuron or indirectly through other neural systems remains to be determined.

Differential regulation of luteinizing hormone release by gamma- aminobutyric acid receptor subtypes in the arcuate-ventromedial region of the castrated ram

Differential regulation of luteinizing hormone release by gamma- aminobutyric acid receptor subtypes in the arcuate-ventromedial region of the castrated ram

Leydig Cell Hyperplasia and Adenomas in Mice Treated with Finasteride, a 5α-Reductase Inhibitor: A Possible Mechanism

Leydig Cell Hyperplasia and Adenomas in Mice Treated with Finasteride, a 5α-Reductase Inhibitor: A Possible Mechanism. Prahalada, S., Majka, J. A., Soper, K. A., Nett, T. M., Bagdon, W. J., Peter, C. P, Burek, J. D., MacDonald, J. S., and van Zwieten, M. J. (1994). Fundam. Appl. Toxicol. 22, 211-219.Finasteride is a selective inhibitor of the enzyme 5α-reductase which is responsible for the conversion of testosterone (T) to dihydrotestosterone (DHT). Finasteride is indicated for the treatment of benign prostatic hyperplasia in man (∼ 0.1 mg/kg/day). The effect of long-term treatment was studied in mice given high doses (2.5, 25, and 250 mg/kg/day) of finasteride for 83 weeks. In finasteride-treated mice, increased incidences of testicular Leydig cell hyperplasia (52% compared to 24% in control group) at doses equal to or greater than 25 mg/kg/day and Leydig cell adenomas (32% compared to 0.5% in control group) at 250 mg/kg/day were observed. There were no drug-related effects on the seminiferous tubules. Since luteinizing hormone (LH) is a trophic hormone for Leydig cells, short-term studies (5 to 14 weeks) were clone to investigate the relationship between Leydig cell hyperplasia and serum LH levels in finasteride-treated mice. In these studies, there was a positive correlation between the drug-related increased incidence of Leydig cell hyperplasia and a statistically significant (p ⩽ 0.05) increase in serum LH levels in finasteride-treated (250 mg/kg/day) mice. Furthermore, studies in castrated male mice showed that the suppression of serum LH levels by T is reversible by inhibition of conversion of T to DHT with finasteride (250 mg/kg/day), supporting the hypothesis that DHT is involved in the regulation of LH release in mice. The data presented support the conclusion that the effects of finasteride on Leydig cells appear to be secondary to increased serum LH levels and that they occur only at very high doses when compared to the therapeutic dose (approximately 0.1 mg/kg/day) in man.

Serotonin, a neurotransmitter involved in the regulation of luteinizing hormone release

Serotonin, a neurotransmitter involved in the regulation of luteinizing hormone release

Serotonin, a Neurotransmitter Involved in the Regulation of Luteinizing Hormone Release*

The involvement of serotonin (5-HT) in the regulation of LH secretion is discussed on the basis of experimental and physiological models. The role of 5-HT on low amplitude pulsatile LH release in male rats is not yet clear, in spite of the fact that recent results suggest a weak permissive role. In ovariectomized rats, 5-HT expresses a negative influence on the increased rate of LH release, which is converted into a stimulatory effect by pretreatment of the animals with E2. Moreover, in castrated female rats there is a morning/afternoon oscillatory pattern in 5-HT metabolism of brain areas associated with the control of LH secretion including the hypothalamus. In this area, the fluctuation is modulated by E2 in such a way that the peak of 5-HT neural activity occurs simultaneously with the induced afternoon discharge of LH. On the other hand, P enhances the amplitude of the oscillation of hypothalamic 5-HT metabolism concomitantly with a potentiation of the induced LH surge. This facilitatory role of 5-HT upon phasic LH discharge is also evident in intact female rats. The preovulatory surge of LH is accompanied by an increased hypothalamic and, more precisely, ME 5-HT turnover. Furthermore, 5-HT stimulates in vitro LHRH release from the ME. Serotonergic nuclei located in the brain stem seem to mediate this effect.

Regulation of luteinizing hormone release by pulsatile and continuous administration of gonadotropin-releasing hormone to superfused rat and hamster pituitary cells.

Regulation of LH release by GnRH was studied in superfused anterior pituitary cells from 30-day-old female rats or hamsters. Dispersed cells were cultured 4-6 days on Cytodex beads, then loaded into water-jacketed columns, and perfused with medium (0.5 ml/min) at 37 C. Three-minute fractions of effluent were assayed for LH by RIA. LH release was dose related between 10(-10) and 10(-7) M GnRH. Rat and hamster cells released LH at peak rates of 11.3 and 12.5 ng/(min X 10(6) cells), respectively, when first exposed to 10(-8) M GnRH. Short pulses (6 min) of 10(-8) M GnRH given at 30-min intervals had little effect on the rate of LH release by rat pituitary cells; however, if the interpulse interval was reduced to 12 min, release declined 72% by the fifth pulse. In contrast, pulses of 10(-6) M GnRH at 30-min intervals desensitized rat cells. Hamster cells were desensitized by 10(-8) M GnRH after a single pulse regardless of whether a second pulse was given 30 min or 2.5 h later. Similar desensitization also occurred at other doses (10(-9) and 10(-6) M). After five pulses at 30-min intervals, the LH release rate in hamster cells was depressed 65%. Release was depressed 80% by pulses at 12-min intervals. Thus, in rats, desensitization is both frequency and dose dependent, whereas in hamsters, it is independent of frequency and dose. Stimulation with 10(-6) M GnRH pulses completely overcame desensitization in both species. Continuous exposure of anterior pituitary cells to 10(-8) M GnRH caused an initial rapid LH release, followed by a steady decline in the rate of release from peak rates to baseline levels by 2.5 h in both species. A 6-min, 10(-6) M GnRH pulse given immediately after a 3-h 10(-8) M GnRH exposure rapidly stimulated the cells to release LH at rates up to 192% of initial rates. When these pulses were continued at 30-min intervals, additional desensitization occurred. This overcoming of desensitization shows that desensitized anterior pituitary cells are not refractory to GnRH and suggests that the GnRH regulation of LH release may involve more than one GnRH receptor-mediated phenomenon.

Effect of exogenous progesterone on the preovulatory progesterone secretion in the rat.

A subcutaneous single injection of progesterone at certain early hours on the day of proestrus caused an advanced increase in ovarian progesterone secretion 2 to 3 hrs. before the time of its spontaneous preovulatory increase in 4-day cyclic rats. The secretion of 20α-hydroxypregn-4-en-3-one (20α-OH-P), however, was not significantly altered by exogenous progesterone. Such an effect of the exogenous progesterone on the preovulatory secretion of progesterone was not seen when rats were hypophysectomized or when treated with phenobarbital just before progesterone injection or they were at the stage of diestrus. The results indicate that the progesterone effect is mediated through neural factors controlling luteinizing hormone (LH) release from the pituitary. Our previous works have demonstrated that a preovulatory increase in progesterone secretion occurs on the evening of the day of proestrus and is entirely dependent upon the release of LH from the pituitary. Other endogenous steroids such as 20α-OH-P, estradiol, estriol, testosterone, and 5-androstenediol produced no such a promoting effect on the preovulatory progesterone secretion. It is suggested, therefore, that 20α-OH-P plays no physiological role in the regulation of LH release in the rat. The interdependency between LH release and progesterone secretion was discussed, and neither negative nor positive feed-back control mechanism was accepted for the preovulatory secretions of LH and progesterone in the cyclic rat.