Prolactin-inhibiting activity of GnRH associated peptide in cultured human pituitary cells

Gonadotropin-releasing hormone and adipokinetic hormone/corazonin-related peptide in the female prawn

Crustacean neuropeptides (NPs) play important roles in the regulation of most physiological activities, including growth, molting and reproduction. In this study, we have performed an in silico analysis of female prawn (Macrobrachium rosenbergii) neural transcriptomes to identify NPs not previously identified. We predict that approximately 1309 proteins are destined for the secretory pathway, many of which are likely post-translationally processed to generate active peptides. Within this neural secretome, we identified a gene transcript that encoded a precursor protein with striking similarity to a gonadotropin-releasing hormone (GnRH). We additionally identified another GnRH NP superfamily member, the adipokinetic hormone/corazonin-related peptide (ACP). M. rosenbergii GnRH and ACP were widespread throughout the nervous tissues, implicating them as potential neuromodulators. Furthermore, GnRH was found in non-neural tissues, including the stomach, gut, heart, testis and ovary, in the latter most prominently within secondary oocytes. The GnRH/corazonin receptor-like gene is specific to the ovary, whereas the receptor-like gene expression is more widespread. Administration of GnRH had no effect on ovarian development and maturation, nor any effect on total hemolymph lipid levels, while ACP administration decreased oocyte proliferation (at high dose) and stimulated a significant increase in total hemolymph lipids. In conclusion, our targeted analysis of the M. rosenbergii neural secretome has revealed the decapod GnRH and ACP genes. We propose that ACP in crustaceans plays a role in the lipid metabolism and the inhibition of oocyte proliferation, while the role of the GnRH remains to be clearly defined, possibly through experiments involving gene silencing.

Role of transient hyperprolactinemia in the late follicular phase of the gonadotropin-stimulated cycle

Background: Serum prolactin (PRL) concentration is known to transiently increase in rats; however, its change is obscure and the role of it is also unclear in women. We studied the relationship between estradiol (E2) and PRL production and the role of transient hyperprolactinemia in the late follicular phase of the gonadotropin-stimulated cycle. Methods: (1) Serum E2 and PRL concentrations were measured on an early follicular day and immediately before a human chorionic gonadotropin (hCG) injection in 60 patients with normoprolactinemia. Twelve of the 60 patients also received a gonadotropin injection with bromocriptine, and serum hormone levels were compared with those without bromocriptine. (2) Preovulatory serum E2 and PRL concentrations were compared between the natural and clomiphene treatment cycles in 14 hormonally normal women. (3) Changes of serum PRL concentrations were measured before and after E2 loading in five premature ovarian failure (POF) patients. (4) The E2 production by granulosa cells in the presence of PRL was measured. Results and Conclusion: Serum E2 and PRL concentrations were significantly increased by the gonadotropin injection. Bromocriptine treatment completely inhibited the PRL increase, but further increased serum E2 concentration on the late follicular day. The E2 loading increased serum PRL levels in POF patients. The clomiphene treatment increased serum E2 but decreased PRL concentrations. Prolactin significantly decreased E2 production by granulosa cells. A feedback loop may exist between E2 and PRL to control the excess E2 production induced by gonadotropin injection. (Reprod Med Biol 2002; 1: 69-74).

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Thyrotropin-releasing hormone and gonadotropin-releasing hormone-associated peptide modulation of [Ca2+]i in human lactotrophs

The effect of thyrotropin-releasing hormone (TRH) and gonadotropin-releasing hormone-associated peptide (GAP) was studied on both secretion and intracellular free Ca2+ concentrations ([Ca2+]i) in human pituitary cells cultured from prolactin (PRL)-secreting tumors. Secretion was measured during a 30-min incubation period and we used a microspectrofluorimetric method in individual cells and indo-1 as the fluorescent probe. TRH (10(-8) M) significantly increased PRL release in five out of the six cell populations. In these five cases, more than 68% of individual cells responded to TRH by an increase in [Ca2+]i. No significant increase in PRL secretion was found in another culture in which TRH increased [Ca2+]i in only 37% of the cells. The effect of GAP (10(-7) M) was studied in five cell populations. In three of them, a decrease of 20% to 51% of the PRL basal secretory rate was observed under GAP. GAP inhibited [Ca2+]i in respectively 59%, 46% and 94% of the cells from these cultures. The inhibitory effect of GAP was blocked by a pertussis toxin (PT) pretreatment which demonstrates the involvement of a PT-sensitive G-protein in GAP action. In two other cultures, GAP did not significantly alter PRL secretion or individual cell [Ca2+]i. These observations suggest that GAP might play a role in the control of PRL secretion in the human.

Inhibition of prolactin release by gonadotropin-releasing hormone-associated Peptide in benign, dopamine-sensitive and in malignant, dopamine-resistant pituitary tumors

Since the gonadotropin-releasing hormone-associated peptide (GAP) has been reported to be capable of inhibiting prolactin release from normal lactotrophs, with the present study we have examined the in vitro effects of GAP on prolactin release in an estrone-induced, dopamine-sensitive rat pituitary adenoma and two malignant, transplantable and dopamine-resistant rat pituitary tumors, 7315a and MtTW15. Enzymatically dispersed cells obtained from the three types of tumor were cultured in multiwell dishes for 4 days. On the fifth day, the cells were exposed for 4 h to human GAP 1-56 or to the analog GAP 42-56 or to rat GAP 1-53, at various concentrations. In some experiments, the effect of a pretreatment of the cells for 16 h with pertussis toxin before exposure to human GAP was also evaluated. In the three tissues, rat GAP was able to inhibit prolactin release in a dose-dependent manner. Human GAP 1-56 and GAP 42-56 were able to inhibit prolactin release in a dose-dependent manner in all cells except those of the MtTW15 tumor. Furthermore, in adenomatous cells, the inhibitory effects of these peptides were suppressed by pretreatment of the cells with pertussis toxin. These findings indicate that GAP is capable of inhibiting prolactin release even in dopamine-resistant pituitary tumors. This inhibition is exerted through a pertussis toxin-sensitive G-protein-dependent signaling mechanism in adenomatous cells.

The gene encoding GnRH and its associated peptide GAP: some insights into hypogonadism

The hypogonadal (hpg) mouse represents a unique animal model for hypogonadism. In this mutant the truncation of the gene encoding GnRH and its associated peptide GAP leads to drastically lowered gonadotropin levels and increased circulating prolactin. This deficiency in turn leads to a failure of testes and ovaries to develop normally. Using gene therapy we have restored the reproductive functions of the hpg mouse. The success of this therapy uniquely underscores the importance of the gene encoding the GnRH precursor and lends credence to the hypothesis that no other gene in mammals can replace it. As a consequence, defects in the control and/or structural properties of the human GnRH are expected to result in hypogonadism in humans.