Induction of chemical castration in male rats by a new long-acting LHRH-antagonist

Low luteinizing hormone enhances spatial memory and has protective effects on memory loss in rats

Though several studies have suggested that estradiol improves hippocampal-dependent spatial memory, the effects of other hormones in the hypothalamic-pituitary-gonadal axis on memory have largely been ignored. Estradiol and luteinizing hormone (LH) are generally inversely related and LH may significantly affect spatial memory. Ovariectomized (ovx) rats treated with Antide (a gonadotropin releasing hormone receptor antagonist) had low LH levels and showed enhanced spatial memory, comparable to treatment with estradiol. Antide-treated ovx females retained spatial memory longer than estradiol-treated ovx females. Deficits in spatial memory are a primary symptom of neurodegenerative disorders including Alzheimer's disease (AD). Treatment with Antide prevented spatial memory deficits in a neurotoxin-induced model typical of early AD. These data suggest that memory impairments seen in female rats after ovariectomy or women after menopause may be due to high LH levels and that a reduction in LH enhances memory. These results also implicate an LH lowering agent as a potential preventative therapy for AD.

Effect of gonadotropin-releasing hormone antagonist on ovarian and uterine weights in immature female rats

The immature rat uterotrophic assay has been proposed as a screening test method for detecting estrogenic and antiestrogenic chemicals. Although the immature rat uterotrophic assay is advantageous because the test animals are not traumatized by the ovariectomizing process, the effect of endogenous estrogen on ovarian and uterine weight in the immature animals that are used in immature rat uterotrophic assay has not received much attention. In this study, 19-day-old rats were treated with antide, a gonadotropin-releasing hormone antagonist, antide, to block gonadal production of endogenous estrogen. Uterine and ovarian weights of the antide-treated animals were markedly lower than those of control animals. This finding suggests that endogenous gonadal estrogen may already be acting on the uterus and ovaries in immature rats. Blocking endogenous estrogen with a gonadotropin-releasing hormone antagonist may enhance the sensitivity of the immature rat uterotrophic assay; however, the possibility that this protocol may interfere with the ability of the immature rat uterotrophic assay to detect centrally-mediated effects can not be discounted.

Differential regulation of luteinizing hormone and follicle-stimulating hormone in male siberian hamsters by exposure to females and photoperiod

Siberian hamsters have decreased gonadotropin levels and testis size after short-day (SD) exposure. Upon transfer from short to long days, FSH and testis weight increase rapidly, whereas LH and T remain low for much longer. We investigated whether an additional environmental stimulus, specifically a female, could trigger an earlier release of LH and whether the response to the female was dependent on photoperiod. An increase in serum LH was induced in long day (LD), but not SD, males within minutes of female exposure. The ability of SD males to secrete LH upon female exposure was regained within 4 d of photostimulation. FSH was not secreted after female exposure, but varied with photoperiod. Thus, FSH and LH are differentially regulated by photoperiod and female exposure. In subsequent studies melatonin injections and a GnRH antagonist were used to show that photoperiod modulates the endocrine responsiveness of a male to a female via melatonin and that female-induced LH release is GnRH dependent. Collectively, these results suggest separation of gonadotropin signaling pathways by environmental stimuli and provide an excellent model to elucidate the effects of photoperiod on the processing of social and chemosensory inputs to the GnRH neurons of the hypothalamus.

Homologous regulation of the gonadotropin-releasing hormone receptor gene is partially mediated by protein kinase C activation of an activator protein-1 element

Homologous regulation of GnRH receptor (GnRHR) gene expression is an established mechanism for controlling the sensitivity of gonadotropes to GnRH. We have found that expression of the GnRHR gene in the gonadotrope-derived alpha T3-1 cell line is mediated by a tripartite enhancer that includes a consensus activator protein-1 (AP-1) element, a binding site for SF-1 (steroidogenic factor-1), and an element we have termed GRAS (GnRHR-activating sequence). Further, in transgenic mice, approximately 1900 b.p. of the murine GnRHR gene promoter are sufficient for tissue-specific expression and GnRH responsiveness. The present studies were designed to further delineate the molecular mechanisms underlying GnRH regulation of GnRHR gene expression. Vectors containing 600 bp of the murine GnRHR gene promoter linked to luciferase (LUC) were transiently transfected into alpha T3-1 cells and exposed to treatments for 4 or 6 h. A GnRH-induced, dose-dependent increase in LUC expression of the -600 promoter was observed with maximal induction of LUC noted at 100 nM GnRH. We next tested the ability of GnRH to stimulate expression of vectors containing mutations in each of the components of the tripartite enhancer. GnRH responsiveness was lost in vectors containing mutations in AP-1. Gel mobility shift data revealed binding of fos/jun family members to the AP-1 element of the murine GnRHR promoter. Treatment with GnRH or phorbol-12-myristate-13-acetate (PMA) (100 nM), but not forskolin (10 microM), increased LUC expression, which was blocked by the protein kinase C (PKC) inhibitor, GF109203X (100 nM), and PKC down-regulation (10 nM PMA for 20 h). In addition, a specific MEK1/MEK2 inhibitor, PD98059 (60 microM), reduced the GnRH and PMA responses whereas the L-type voltage-gated calcium channel agonist, +/- BayK 8644 (5 microM), and antagonist, nimodipine (250 nM), had no effect on GnRH responsiveness. Furthermore, treatment of alpha T3-1 cells with 100 nM GnRH stimulated phosphorylation of both p42 and p44 forms of extracellular signal-regulated kinase (ERK), which was completely blocked with 60 microM PD98059. We suggest that GnRH regulation of the GnRHR gene is partially mediated by an ERK-dependent activation of a canonical AP-1 site located in the proximal promoter of the GnRHR gene.

Single-dose administration of the gonadotropin-releasing hormone antagonist, Nal-Lys (antide) to healthy men

OBJECTIVE

To evaluate the ability the Nal-Lys GnRH antagonist ([N-Ac-Nal (2)1, 4ClDPhe2, D3Pal3, Lys (Nic)5, D-Lys(Nic)6, Lys (iPr)8, D-Ala10] to suppress gonadotropins and T in humans and to assess its duration of action and its local effects.

DESIGN

Placebo-controlled clinical study.

SETTING

A university community.

SUBJECTS

Seven normal male volunteers.

INTERVENTIONS

We administered single injections of Nal-Lys (0, 10, 25, and 50 micrograms/kg body weight). Blood samples were collected before and at frequent time intervals after injection.

RESULTS

Nal-Lys caused only minor local effects. At the higher doses (25 and 50 micrograms/kg), serum LH and T levels were suppressed to 50% to 70% of baseline; serum FSH levels were suppressed to 70% to 80% of baseline, and levels of all three hormones returned to basal values within 24 hours after injection.

CONCLUSIONS

In humans, Nal-Lys has similar potency and duration of action to other antagonists and produces fewer local side effects. However, the utility of Nal-Lys is limited by formulation difficulties; current efforts are directed at improving the formulation in order to explore the potential clinical uses of this peptide.

Comparison of biological effects of a sustained delivery system and nonencapsulated LH-RH antagonist SB-75 in rats

Recently, we developed long-acting microcapsules and microgranules of the LH-RH antagonist SB-75. In this study, we compared the inhibitory effects of a single injection of encapsulated and nonencapsulated LH-RH antagonist SB-75 on gonadotropin and testosterone secretion. The resulting serum SB-75 levels were also measured by RIA. Microgranules containing 4% of this antagonist in poly(DL-lactide-co-glycolide) were administered IM at two different doses (30 and 60 mg/rat) to male rats. Other groups of rats were injected SC with equivalent doses of nonencapsulated SB-75 (1.25 and 2.5 mg/rat). The administration of microgranules at a dose of 60 mg/rat produced a significant elevation of serum SB-75 until day 76, and serum testosterone and LH levels were suppressed below the detection limit of the RIA for a period of 70 days. An equivalent dose of nonencapsulated SB-75 acetate (2.5 mg/rat) produced a significant elevation of SB-75 levels for 20 days and decreased testosterone to castration values and LH levels for merely 21 days. In rats treated with 30 mg microgranules of SB-75 or an equivalent dose of SB-75 acetate (1.25 mg/rat), serum testosterone and LH were suppressed to a similar extent, but for only 2 weeks. In another study, the effect of a single SC injection of 1.25 mg/rat of antagonist SB-75 on pituitary LH-RH receptors was determined, 7 and 60 days after administration. SB-75 produced a significant (p < 0.01) downregulation of membrane receptors for LH-RH 7 days after administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Histological changes in Dunning prostate tumors and testes of rats treated with LH-RH antagonist SB-75

Rats bearing Dunning R-3327 hormone-dependent prostate tumors were treated with LH-RH antagonist SB-75 in the form of microcapsules for sustained delivery administered every 3 weeks and which released 24, 48, 72 micrograms/day respectively. The effects were compared with those of microcapsules of the agonist D-Trp-6-LH-RH releasing 25 micrograms/day. Both types of LH-RH analogs significantly inhibited tumor growth over a period of treatment lasting 8 weeks. The effect of SB-75 was dose-dependent. The total inhibition of spermatogenesis, as well as atrophic signs in the prostate and seminal vesicles, demonstrated a marked suppression of the pituitary-gonadal system by these analogs. The histological signs of tumor regression were analyzed. The vascular content of tumors did not change after the treatments, but an increased amount of connective tissue was found in the treated tumors, especially after administration of SB-75. Both the agonist and the antagonist caused a moderate decrease of the number of mitotic cells and a marked increase of apoptosis in the tumors. The apoptotic index, i.e. the percentage of tumorous glands showing signs of apoptosis, reached 40-50% in treated groups, compared to only 15% in controls. An apoptotic index of 60% was noted in a separate group of rats treated with 200 micrograms SB-75/animal/day for 3 days. The signs of enhanced apoptosis disappeared 1 week after the short-term treatment. The induction of apoptosis by LH-RH analogs seemed to be of greater importance in tumor growth inhibition than their antimitotic effect. These results extend our previous observations on the efficacy of LH-RH antagonists in inhibition of various cancers. This histopathologic evaluation clearly supports our contention that modern antagonists of LH-RH, free of edematogenic effects, inhibit the growth of Dunning prostate tumors. Because of the immediate inhibitory effects, the use of LH-RH antagonists might lead to an improvement in the clinical response in patients with prostate cancer.