Effect of castration and steroid treatment on the release of gonadotropins by the rat pituitary-hypothalamus complex in vitro

Early post-natal life stress induces permanent adrenocorticotropin-dependent hypercortisolism in male mice

PURPOSE

It has been hypothesized that specific early-life stress (ES) procedures on CD-1 male mice produce diabetes-like alterations due to the failure of negative feedback of glucocorticoid hormone in the pituitary. The aim of this study is to investigate the possible mechanism that leads to this pathological model, framing it in a more specific clinical condition.

METHODS

Metabolic and hypothalamic-pituitary-adrenal-related hormones of stressed mice (SM) have been analyzed immediately after stress procedures (21 postnatal days, PND) and after 70 days of a peaceful (unstressed) period (90 PND). These data have been compared to parameters from age-matched controls (CTR), and mice treated during ES procedures with oligonucleotide antisense for pro-opiomelanocortin (AS-POMC).

RESULTS

At 21 PND, SM presented an increased secretion of hypothalamic CRH and pituitary POMC-derived peptides, as well as higher plasmatic levels of ACTH and corticosterone vs. CTR. At 90 PND, SM showed hyperglycemia, with suppression of hypothalamic CRH, while pituitary and plasmatic ACTH levels, as well as plasma corticosterone, were constantly higher than in CTR. These values are accompanied by a progressive acceleration in gaining total body weight, which became significant vs. CTR at 90 PND together with a higher pituitary weight. Treatment with AS-POMC prevented all hormonal and metabolic alterations observed in SM, both at 21 and 90 PND.

CONCLUSIONS

These findings show that these specific ES procedures affect the negative glucocorticoid feedback in the pituitary, but not in the hypothalamus, suggesting a novel model of ACTH-dependent hypercortisolism that can be prevented by silencing the POMC gene.

Prolactin and male fertility: the long and short feedback regulation

In the last 20 years, a pituitary-hypothalamus tissue culture system with intact neural and portal connections has been developed in our lab and used to understand the feedback mechanisms that regulate the secretions of adenohypophyseal hormones and fertility of male rats. In the last decade, several in vivo rat models have also been developed in our lab with a view to substantiate the in vitro findings, in order to delineate the role of pituitary hormones in the regulation of fertility of male rats. These studies have relied on both surgical and pharmacological interventions to modulate the secretions of gonadotropins and testosterone. The interrelationship between the circadian release of reproductive hormones has also been ascertained in normal men. Our studies suggest that testosterone regulates the secretion of prolactin through a long feedback mechanism, which appears to have been conserved from rats to humans. These studies have filled in a major lacuna pertaining to the role of prolactin in male reproductive physiology by demonstrating the interdependence between testosterone and prolactin. Systemic levels of prolactin play a deterministic role in the mechanism of chromatin condensation during spermiogenesis.

The role of kisspeptins and GPR54 in the neuroendocrine regulation of reproduction

Neurons that produce gonadotropin-releasing hormone (GnRH) reside in the basal forebrain and drive reproductive function in mammals. Understanding the circuitry that regulates GnRH neurons is fundamental to comprehending the neuroendocrine control of puberty and reproduction in the adult. This review focuses on a family of neuropeptides encoded by the Kiss1 gene, the kisspeptins, and their cognate receptor, GPR54, which have been implicated in the regulation of GnRH secretion. Kisspeptins are potent secretagogues for GnRH, and the Kiss1 gene is a target for regulation by gonadal steroids (e.g., estradiol and testosterone), metabolic factors (e.g., leptin), photoperiod, and season. Kiss1 neurons in the arcuate nucleus may regulate the negative feedback effect of gonadal steroids on GnRH and gonadotropin secretion in both sexes. The expression of Kiss1 in the anteroventral periventricular nucleus (AVPV) is sexually dimorphic, and Kiss1 neurons in the AVPV may participate in the generation of the preovulatory GnRH/luteinizing hormone (LH) surge in the female rodent. Kiss1 neurons have emerged as primary transducers of internal and environmental cues to regulate the neuroendocrine reproductive axis.

Mechanism delineating differential effect of an antiestrogen, tamoxifen, on the serum LH and FSH in adult male rats

Tamoxifen, a synthetic non-steroidal antiestrogen with residual estrogenic activity, administered to adult male rats reduces their fertility. A decrease in the circulating LH and testosterone levels with a transient rise or no change in circulating FSH levels was observed. The present study was carried out to delineate the mechanism causing the differential effect of tamoxifen on circulating gonadotropins by correlating it to changes in the hypothalamic LHRH, pituitary gonadotropins and testicular inhibin/activin. Hypothalamus, pituitary-hypothalamus complex (PHC) and intact pituitary (PI) from control and tamoxifen-treated male rats were superfused in vitro, and pulsatile release of LHRH by hypothalamus and that of LH and FSH by the PHC and PI were studied. Concomitantly, testicular immunoexpression of alpha and betaB subunits of inhibin/activin were studied by immunohistochemistry and enzyme-linked immunosorbent assays (ELISA). At 0.4 mg/kg/day dose of tamoxifen a decrease in mean hypothalamic LHRH and LH pulse frequency from PHC construct was observed. FSH pulse frequency was not affected under the same experimental conditions. At the same dose of tamoxifen, testicular expression of both alpha and betaB subunits of inhibin/activin was upregulated. The study demonstrated that reduced circulating LH levels were due to a decrease in hypothalamic LHRH concentration and in LH pulsatility following tamoxifen treatment. The lack of effect on circulating FSH under the same experimental conditions was likely due to its modulation by inhibin and activin.

Androgens, aging, and Alzheimer's disease

Testosterone depletion is a normal consequence of aging in men that is associated with senescent effects in androgen- responsive tissues. We discuss new evidence that one consequence of testosterone depletion in men is an increased risk for the development of Alzheimer's disease (AD). Furthermore, we discuss two candidate mechanisms by which testosterone may affect AD pathogenesis. First, testosterone has been identified as an endogenous regulator of beta-amyloid, a protein that abnormally accumulates in AD brain and is implicated as a causal factor in the disease. Second, findings from several different paradigms indicate that testosterone has both neurotrophic and neuroprotective functions. These new findings support the clinical evaluation of androgen-based therapies for the prevention and treatment of AD.

Cysteamine-a somatostatin-inhibiting agent-induced growth hormone secretion and growth acceleration in juvenile grass carp (Ctenopharyngodon idellus)

Effects of cysteamine hydrochloride (CSH)-a somatostatin-inhibiting agent on growth hormone (GH) secretion from pituitary fragments (PF) or hypothalamus plus pituitary fragments (HPF) under static incubation conditions, serum GH, 3,5,3(')-triiodothyronine (T(3)) and thyroxine (T(4)) levels, and growth in juvenile grass carp (Ctenopharyngodon idellus) were investigated. CSH (0.1, 1, and 10 mM) had no influences on GH release from PF after 1 and 6h incubation, but was effective in stimulating GH release from HPF in a dose-dependent manner after 1 and 6h incubation. Moreover, prolonged treatment of HPF with CSH decreased the magnitude of enhancement of GH levels in culture medium. CSH and neuropeptides [e.g., human GH-releasing hormone (hGHRH, 100 nM), luteinizing hormone-releasing hormone analog (LHRH-A, [D-Trp(6),Pro(9)]LHRH, 100 nM)], or salmon gonadotropin-releasing hormone analogue (sGnRH-A, [D-Ala(6),Pro(9)]LHRH, 100 nM), alone and in combination during static incubation stimulated GH release from HPF after 1h incubation; in addition, there was an additive, not a synergistic effect of CSH and neuropeptides on stimulation of GH release. Administration of CSH (2.5mg/g diet) in combination with LHRH-A (5 microg/g diet) in diet twice daily for 8 weeks resulted in higher serum GH, T(3), and T(4) levels, ratio of RNA/DNA in muscle, food conversion efficiency, and growth rate than CSH or LHRH-A alone. At trial termination, significant decreases in condition factors and body lipid levels were observed in fish fed with CSH and/or LHRH-A. No significant differences were recorded for viscero-somatic index, hepato-somatic index, and percent body moisture and protein in muscle. These findings, taken as a whole, strongly suggest that the action of CSH stimulating GH release in vitro appears to be mediated through hypothalamic pathways and dietary delivery of CSH directly or indirectly stimulates endogenous GH, T(3), and T(4) secretion, and subsequently leads to a increase in growth rate in grass carp.

Antifertility effects of fluphenazine in adult male rats

The underlying mechanisms in human infertility associated with hyperprolactinemia have yet to be established. Hyperprolactinemia is a known side-effect of fluphenazine, a broad spectrum, long-acting phenothiazine known to be D2 dopamine receptor antagonist. Dose-related effects of fluphenazine decanoate were ascertained on the fertility of 60-day treated, adult male rats. Significant increase in the serum levels of prolactin and decrease in the levels of LH and FSH were seen at doses of 1-3 mg/kg/day. No effect was evident on the serum testosterone (T) and estradiol. The tissue levels of Inhibins were not affected. The weights of testes, epididymides, seminal vesicles, ventral prostate, adrenal and pituitary glands were not affected. Testicular histology showed sloughing indicating the sensitivity of this parameter to FSH deficiency. Mating occurred within 10 days of cohabitation in the control and 1-2 mg/kg/day treated groups but delayed in the 3 mg/kg/day treated group with a significant effect on potency. Implantation sites, litter size and fertility index were significantly reduced at 2-3 mg/kg/day doses of fluphenazine. No effects however were seen on sperm counts or motility whereas morphological changes were apparent in the acrosome. Chromatin decondensation in vitro was enhanced and sperm chromatin structure assay revealed DNA denaturation. Hypothalamic tyrosine hydroxylase levels were increased in 1-3 mg/kg/day dose range. Hyperprolactinemic males sired fewer pups as compared to controls. Hypothalamic tyrosine hydroxylase was upregulated at all the doses. The antifertility effects of fluphenazine-induced hyperprolactinemia appeared to be unrelated to testosterone (T). In addition, FSH decrease might have affected the intrinsic sperm quality and thereby reduced litter size.

Effects of castration and chronic steroid treatments on hypothalamic gonadotropin-releasing hormone content and pituitary gonadotropins in male wild-type and estrogen receptor-alpha knockout mice

Testicular androgens are integral components of the hormonal feedback loops that regulate circulating levels of LHbeta and FSH. The sites of feedback include hypothalamic areas regulating GnRH neurons and pituitary gonadotropes. To better define the roles of androgen receptor (AR), estrogen receptor-alpha (ERalpha), and estrogen receptor-beta (ERbeta) in mediating feedback effects of sex steroids on reproductive neuroendocrine function, we have determined the effects of castration and steroid replacement therapy on hypothalamic GnRH content, pituitary LHbeta and FSHbeta messenger RNA (mRNA) levels, and serum gonadotropins in male wild-type (WT) and estrogen receptor-alpha knockout (ERKO) mice. Hypothalami from intact WT and ERKO males contained similar amounts of GnRH, whereas castration significantly reduced GnRH contents in both genotypes. Replacement therapy with estradiol (E2), testosterone (T), or dihydrotestosterone (DHT) restored hypothalamic GnRH content in castrated (CAST) WT mice; only the androgens were effective in CAST ERKOs. Analyses of pituitary function revealed that LHbeta mRNA and serum LHbeta levels in intact ERKOs were 2-fold higher than those in intact WT males. Castration increased levels of LHbeta mRNA (1.5- to 2-fold) and serum LHbeta (4- to 5-fold) in both genotypes. Both E2 and T treatments significantly suppressed LHbeta mRNA and serum LH levels in CAST WT males. However, E2 was completely ineffective, and T was only partially effective in suppressing these two indexes in the CAST ERKO males. DHT treatments stimulated a 50% increase in LHbeta mRNA and serum LH levels in WT males, whereas serum LH was significantly suppressed in DHT-treated ERKO males. Although the pituitaries from intact ERKO males contained similar amounts of FSHbeta mRNA, serum FSH levels were 20% higher than those in the intact WT males. Castration increased FSHbeta mRNA levels only in WT males, but significantly increased serum FSH levels in both genotypes. Both E2 and T treatments significantly suppressed serum FSH in CAST WT males, whereas only E2 suppressed FSHbeta mRNA. DHT treatments of CAST WT mice stimulated a small increase in serum FSH, but failed to alter FSHbeta mRNA levels. None of the steroid treatments exerted any significant effect on FSHbeta mRNA or serum FSH levels in CAST ERKOs. These data suggest that hypothalamic GnRH contents can be maintained solely through AR signaling pathways. However, normal regulation of gonadotrope function requires aromatization of T and activation of ERalpha signaling pathways in the gonadotrope. In addition, serum FSH levels in male ERKOs appear to be regulated largely by nonsteroidal testicular factors such as inhibin. Finally, these data suggest that hypothalamic ERbeta may not be involved in mediating the negative feedback effects of T on serum LH and FSH in male mice.

Hypothalamic-pituitary-adrenocortical axis changes in a transgenic mouse with impaired glucocorticoid receptor function

Recently, a transgenic mouse with impaired glucocorticoid receptor (GR) function was created to serve as an animal model for the study of neuroendocrine changes occurring in stress-related disorders, such as major depression. Here, we investigated the hypothalamic-pituitary-adrenocortical (HPA) axis changes in these transgenic mice. There were no significant differences between basal early morning plasma ACTH and corticosterone levels in normal and transgenic mice. When animals were exposed to a mild stressor, an enhanced response in plasma ACTH was observed in the transgenic mice, whereas plasma corticosterone responses were not different. In view of these differences in plasma ACTH and corticosterone responses, we directed our studies toward the regulation of ACTH secretion on the hypothalamic-hypophyseal level in vitro. Therefore, an in vitro model, the pituitary-hypothalamic complex (PHc) was developed and its ACTH release profile was compared with that of the pituitary (PI) alone. The basal ACTH release by PHc and PI from normal and transgenic mice was similar. Regardless of the strain under study, the basal ACTH release by PI was significantly lower than the release by PHc. Stimulation of tissues with either high K+ (56 mM) or CRH (10 or 20 nM) produced an enhanced ACTH release from both PHc and PI, whereas the response in PI was larger than that in PHC. Moreover, the responses to these stimuli were markedly enhanced in tissues from transgenic mice. In tissues of normal mice, corticosterone inhibited both basal and CRH-stimulated ACTH release more potently in PHc than in PI. Furthermore, the feedback capacity of corticosterone to restrain both basal and CRH-stimulated ACTH release was highly impaired in tissues of transgenic mice, whereas the feedback in PHc appeared to be more affected than that in the PI of these animals. In conclusion, the in vitro data on PHc and PI revealed intrahypothalamic mechanisms operating 1) to fine-tune stimulus-evoked ACTH responses; and 2) to facilitate the negative feedback action of glucocorticoids. Moreover, in the transgenic tissues, the impaired GR function was found to cause augmented stimulus-evoked ACTH responses and an impaired glucocorticoid feedback efficacy which appeared to be mainly defective at the hypothalamic level. Thus, in the transgenic mice with life-long central GR dysfunction we found impaired negative feedback combined with "normal" (i.e. noncompensated) in vivo plasma corticosterone responses. This is a condition with potentially grave pathophysiological consequences and, therefore, this transgenic animal may be regarded as a valuable model for the study of functional glucocorticoid insufficiency at the central nervous system level.

Modulation of pituitary gonadotropins and prolactin secretion by testosterone in vitro

Investigations were undertaken to study the differential modulation of LH, FSH and PRL secretion by testosterone (T) using whole pituitary (PI) or pituitary-hypothalamus coincubates (PHC) as in vitro constructs. PI and PHC from intact and castrated rats were incubated with or without T thrice, for 24 h each, (24 h x 3, total incubation period 72 h). The spent media was replenished every 24 h. At the end of 72 h, a few of the pituitary glands were challenged with 10 nM LHRH for 4 h. The spent media and pituitary glands were analyzed for LH, FSH and PRL using specific RIAs. Incubation of PI or PHC from intact rats with T stimulated the release of LH and FSH but inhibited the release of PRL. T had no effect on the intrapituitary contents of LH but inhibited intrapituitary contents of FSH and PRL, as compared to controls incubated without T. Castration increased intrapituitary contents of LH and FSH with concomitant decrease in PRL levels. Incubation of PI or PHC from castrated rats with T inhibited intrapituitary contents of LH to intact pituitary levels, while PRL levels were further reduced instead of being ameliorated. It is concluded that PI or PHC can be used as convenient in vitro models to monitor the effect of castration or of T modulation of pituitary and hypothalamus functions. T does not affect the synthesis of LH at the gonadotroph level but facilitates the regulation of intracellular LH and FSH levels. It is postulated that T inhibits the synthesis of FSH/PRL at the gonadotroph/lactotroph levels.

Temporal changes in the serum levels of gonadotrophins and testosterone in male rats bearing subcutaneous implants of 5 alpha-dihydrotestosterone

This study has assessed the effect of s.c. implants of 5 alpha-dihydrotestosterone (DHT) on the blood levels of testosterone and gonadotrophins in intact and castrated adult male rats. The rats were bled via cardiac puncture at 1, 3, 7, 14, 21, 28, 35, 42, 49, 56, 63 and 70 days after DHT implantation. On days 28 and 49 post-implantation, rats were injected with LHRH (25 ng) and bled 15 min later. In intact rats bearing DHT implants, the serum levels of LH and testosterone were suppressed significantly with no significant changes in FSH levels. Ventral prostate, seminal vesicles and the pituitary were reduced significantly in weight when compared with controls with empty implants. DHT significantly inhibited LHRH-induced release of FSH in intact rats. In castrated rats, DHT implants inhibited the secretion of both LH and FSH, with a rise in serum DHT levels. DHT stimulated the LHRH-induced release of LH but inhibited FSH. DHT implants increased the weight of the seminal vesicles and ventral prostate but inhibited the weight of the pituitary when compared to castrated rats bearing empty implants. This study demonstrates specific inhibition of serum LH and testosterone by DHT implants in intact adult rats.

Effect of reserpine on the inhibition of prolactin released from different pituitary constructs in vitro by dopamine, bromocriptine and apomorphine

Hourly release of Prolactin by pituitary constructs 1 whole pituitary (PI), adenohypophysis (P-N) and pituitary-hypothalamus co-incubate (PHC) were compared. Adenohypophysis secreted significantly more prolactin than PI and PHC, while PHC secreted significantly less than PI. Co-incubation of (P-N) with posterior pituitary reduced the elevated secretion of prolactin. Addition of dopamine (10(-7) M), bromocriptine (10(-7) M) and apomorphine (5 x 10(-8) M) to these constructs did not affect the release of prolactin from PI but inhibited the same from (P-N) and PHC. Treatment with reserpine increased serum prolactin levels but intrapituitary prolactin contents were decreased. Hourly release of prolactin from pituitary constructs derived from reserpine-treated rats was significantly reduced as compared to ascorbic acid--treated controls. Inclusion of dopamine (10(-7) M), bromocriptine (10(-7) M) and apomorphine (5 x 10(-8) M) in these constructs inhibited prolactin secretion further. In vitro addition of perphenazine stimulated the release of prolactin by PHC but was without any effect on PI and (P-N). The data are interpreted to suggest that dopamine in posterior pituitary may be an important determinant of hypothalamic modulation of prolactin secretion.

The choice of a model for studying the hypothalamus-pituitary interactions in vitro

Comparative studies on the release of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (Prl) by the whole pituitary, pituitary plus hypothalamus and pituitary-hypothalamus complex (PHC) were undertaken to choose an appropriate model for studying the hypothalamus-pituitary interactions in vitro and to relate the importance of the intact neural connections between pituitary and hypothalamus on hypothalamus-pituitary interactions. Also the effect of including dopamine (DA) at 1 X 10(-7) mol/1 in these different in vitro systems on the release of LH, FSH and Prl was investigated. The pituitary released increasing amounts of LH and FSH at 2, 4 and 6 h but the amount of Prl released remained unchanged. The rates of release of LH, FSH and Prl by the pituitary were different and were characteristic of each hormone. Co-incubation of pituitary with hypothalamus stimulated the release of LH and FSH but inhibited the release of Prl. Pituitary-hypothalamus complex behaved almost identical to behaved almost identical to pituitary plus hypothalamus system. Inclusion of 1 X 10(-7) M DA in the incubation medium stimulated the release of LH (80%) but inhibited the release of Prl (71%) by PHC. FSH was unaffected. DA had no significant effect on the release of LH, FSH and Prl by pituitary and pituitary plus hypothalamus systems. It is suggested that PHC is the system of choice for studying hypothalamus-pituitary interactions in vitro.