How does gnrh effect prolactin

Previous work on primary pituitary cultures revealed combined suppressive effects of prolactin PRL and dopamine on the gonadotrophin response to GnRH. However, whether these effects occur directly at the level of the gonadotroph and are accompanied by changes in gene expression is still unresolved. The LH secretory response was differentially affected by treatments. This effect was, however, completely blocked by Br.

These results reveal direct effects of PRL and dopamine at the level of the gonadotroph cell, and interactions between these two hormones in the regulation of gonadotrophin secretion. Hyperprolactinaemia is a major cause of amenorrhoea in humans and has been shown to be associated with the suppression of gonadotrophin secretion and impaired fertility in this Delvoye et al.

While the specific mechanisms underlying prolactin PRL effects on gonadotrophin release are still unresolved, it has become increasingly apparent that, in addition to the established actions of PRL at the level of the hypothalamus affecting gonadotrophin-releasing hormone GnRH output, interactions between the gonadotrophic and PRL axes occur at the level of the pituitary gland.

Indeed, close associations between gonadotrophs and lactotrophs were originally identified in the rodent pituitary, where cell-to-cell communication in the form of adherans junctions Horvath et al. These morphological associations between the gonadotroph and lactotroph cells have also been identified in the pituitaries of larger vertebrates, where isolated gonadotrophs were shown to be situated within clusters of lactotrophs Tortonese et al.

Functional observations further support the existence of intrapituitary effects of PRL on gonadotrophin secretion. An additional evidence for a direct pituitary action of PRL on the gonadotrophic axis was revealed by a study reporting the selective expression of PRL receptors in gonadotroph cells of the pars distalis and pars tuberalis of the ovine pituitary, a species that shows overt temporal changes in fertility throughout the year in response to photoperiod Tortonese et al.

More recently, our laboratory demonstrated a seasonal modulation of the combined suppressive effects of PRL and dopamine on the gonadotrophin response to GnRH Gregory et al. Further studies have demonstrated LH secretion via a regulated pathway, and changes in GnRH receptor expression in response to signalling by GnRH and steroid hormones Turgeon et al. The expression of PRL receptors by either of these cell lines is yet to be determined.

Following incubation, the medium was aspirated off prior to cell lysis and RNA extraction. The optimal number of amplification cycles for quantification of the products of each primer set had been previously determined by titration experiments.

For the assessment of the expression of PRL and dopamine receptors, 35 cycles were used. PCR products were separated by electrophoresis on a 1. The amounts of PCR products of each subunit were normalised to those of the PCR products of GAPDH in each sample and the values were converted to a percentage of the control in order to allow a direct comparison between individual experiments Kanasaki et al.

The minimum detectable concentration was 0. For each group, four wells were assigned to each treatment, and the experiments were carried out three times with reproducible results. The effects of treatments on the gonadotrophin subunit mRNA and secretory responses were examined by ANOVA, followed by pairwise comparison using Fisher's test to determine specific differences among treatments.

As shown in Fig. The products were separated on a 1. Citation: Journal of Endocrinology , 2; The results of this study demonstrate the expression of functional PRL and dopamine receptors in gonadotroph cells.

While PRL is known to participate in the central control of fertility with well-documented effects in the hypothalamus, it is becoming increasingly apparent that PRL can also act directly at the level of the pituitary gland to regulate gonadotrophin output. Early morphological studies reporting intercellular interactions between gonadotroph and lactotroph cells in the rodent pituitary Horvath et al. In the current study, the demonstration that both the PRL receptor and the DA receptor are expressed within the gonadotroph has important clinical implications, since it reveals that the gonadotroph itself can be a point for the control of fertility in hyperprolactinaemia-induced amenorrhoea.

Indeed, the suppression of both gonadotrophin subunits mRNA responses to GnRH by PRL and bromocriptine shows that the secretory effects of this treatment originally detected in primary cultures are likely to be accompanied by changes in gene expression.

The common components in the signalling of one or more of these hormones are likely to be mediating these interactions. Simultaneously, GnRH induces intracellular calcium mobilisation and extracellular calcium influx Stojilkovic et al.

PRL receptors are members of the cytokine family Kelly et al. The activation of these receptors leads to phosphorylation, and thus activation of specific transcription factors from the signal transducer and activator of transcription Stat protein family Ihle The activation of the dopamine receptor results in an immediate increase in intracellular potassium and a consequent reduction in the intracellular free calcium, leading to an inhibition of release from secretory granules Gregerson Additionally, dopamine acts to suppress PLC, thus reducing calcium release from the endoplasmic reticulum Caccavelli et al.

The dopamine and GnRH receptors therefore share many components involved in signal transduction. Consequently, by preventing the activation of PKC and inhibiting the increase in MAPK activity, dopamine may be able to decrease gonadotrophin subunit gene transcription.

The results of this study demonstrate an uncoupling between gonadotrophin synthesis and release in response to treatments, with the LH secretory response to GnRH not being inhibited by PRL or Br; in fact, PRL alone resulted in an enhancement of this response. Given that GnRH activates PKC, and that the activation of this enzyme increases the affinity of intracellular calcium for hormone exocytosis Zhu et al. If a significant H-statistic was detected, the Mann-Whitney U test was used to compare between groups.

The total numbers of GnRH neurons detected varied according to anatomical location, with peak numbers of Neither the distribution nor total number of GnRH neurons counted was changed in any of the treatment groups data not shown. Three independent experiments were undertaken. For the first experiment, intact diestrous mice were used.

Basal expression of pCREB was slightly higher Again, control animals had very low levels of pCREB expression 2. A, Low-power image showing typical distribution of GnRH neurons in the rostral hypothalamus at the level of the organum vasculosum of the lamina terminalis. We used the same single-cell RNA extraction technique used previously by our laboratory 38 , 40 followed by a two-step nested PCR amplification for detection of transcripts.

Representative gels are depicted in Fig. Other controls included using water in place of template or mock harvest in which the patch pipette was lowered into the slice and withdrawn. None of the controls yielded a PCR product. Negative controls, processed in parallel, included either reactions without reverse transcriptase A, upper and lower , lane 5 or water in place of template A, upper and lower , lane 7.

Negative controls, processed in parallel, included RT reaction without reverse transcriptase enzyme B, upper and lower , lane 2 , mock harvests B, upper and lower , lane 3 , or water in place of template B, upper and lower , lane 7. Lane 8 all panels contains molecular weight markers, with sizes indicated as needed. These data provide evidence that hyperprolactinemia suppresses LH secretion in female mice through a central nervous system site of action.

Chronic prolactin administration was found to suppress LH secretion and both acute and chronic prolactin treatments were observed to alter CREB phosphorylation in a small subpopulation of GnRH neurons.

These observations are consistent with the hypothesis that prolactin inhibits LH secretion through an action mediated at least in part through the direct suppression of GnRH neurons. Previous studies provided inconsistent data regarding the ability of prolactin to suppress of fertility in mice 26 — Although it is well established that hyperprolactinemia causes infertility in many mammalian species, including humans, one study 26 found that high prolactin did not inhibit LH secretion in mice.

This is in contrast to our present result and may result from the use of different mouse models in the studies. Although it has not been examined in mice, the PRL-R is positively regulated by estrogen in rats 41 , 42 , so we used an OVX mouse model in which low levels of estrogen were present, but in which LH levels were not completely suppressed. In this model, chronic icv prolactin clearly suppressed LH secretion, suggesting a central site of action for prolactin and demonstrating that the mouse is indeed like other mammals in this respect.

There is considerable evidence in rats, sheep, and humans that prolactin can also act in the pituitary gland to inhibit LH secretion 43 — 49 , and this is likely to also contribute to the infertility caused by elevated prolactin.

However, the fact that hyperprolactinemia-induced infertility can be reversed by exogenous GnRH 4 — 6 suggests that lack of GnRH might be the primary cause of prolactin-induced infertility.

At present, the effects of prolactin on mouse gonadotrophs are unknown. Prolactin receptors are members of the cytokine receptor superfamily 50 and activate multiple signal transduction pathways. There are several isoforms of the prolactin receptor in mice: a long form and at least three short forms with truncated cytoplasmic domains 51 , although only the long form is able to initiate full signaling.

Prolactin binding to the long form of the receptor induces activation of the janus kinase-signal transducer and activator of transcription STAT pathway, in particular inducing phosphorylation of STAT5a and 5b. Phosphorylated STAT proteins translocate to the nucleus and bind to specific sequences in the promoters of target genes, regulating transcription.

We have shown in both rats and mice that STAT5b is specifically required for prolactin action in hypothalamic dopamine neurons 52 , Prolactin can also act through either the long or short forms of the receptor to influence a range of other pathways, which may mediate rapid actions of prolactin in other neurons 50 , The activation of any of these pathways would be expected to result in elevated levels of CREB phosphorylation As such, we chose to examine the phosphorylation status of CREB to evaluate whether prolactin may impact intracellular signaling within GnRH neurons.

This result indicates that changes in circulating prolactin levels result in altered intracellular signaling within GnRH neurons. Although the effects of altered CREB phosphorylation in GnRH neurons are unknown, it is interesting to note that both prolactin present data and estrogen 32 enhance CREB phosphorylation in these cells and that this is associated in both cases with a reduction in plasma LH levels. Such heterogeneity is common in adult GnRH neurons 65 , although the functional consequences are not clear.

Although the present results provide evidence for a direct action of prolactin on GnRH neurons, it is important to recognize that indirect mechanisms may also exist. The network of neuronal inputs to GnRH neurons in the mouse has started to be unraveled 66 , and prolactin receptors are known to be expressed in several areas, such as the anteroventral periventricular and arcuate nuclei 16 , 67 , that contain neurons directly innervating rPOA GnRH neurons.

Thus, as is apparent for the many effects of estrogen on GnRH neurons 65 , the final actions of any one hormone may result from a combination of direct and indirect actions. In conclusion, we report here that hyperprolactinemia results in the suppression of LH secretion and that this action is likely to be mediated at least in part through direct actions of prolactin on GnRH neurons.

We also show that estrogen-treated ovariectomized female mice provide a suitable model for investigating the mechanisms of hyperprolactinemia-induced infertility, providing an opportunity to use transgenic models to investigate further prolactin actions on GnRH neurons in the adult brain. Frontiers in neuroendocrinology. New York : Raven Press ; 77 — McNeilly AS Lactational control of reproduction. Reprod Fertil Dev 13 : — Google Scholar.

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Kisspeptin administration restored gonadotropin secretion and ovarian cyclicity, suggesting that kisspeptin neurons play a major role in hyperprolactinemic anovulation.

Our studies indicate that administration of kisspeptin may serve as an alternative therapeutic approach to restore the fertility of hyperprolactinemic women who are resistant or intolerant to dopamine agonists.

Clinical vignette: A year-old woman is referred for management of infertility. After menarche at age 12, menses occurred irregularly for a year and then became regular. She initiated use of oral contraceptive pills at the age of 18, then stopped at age 27 to try to conceive. Other pituitary function tested was normal. Therapy was initiated with bromocriptine, but it was poorly tolerated, with fatigue, nausea, and lightheadedness to the point of syncopal events during her work as a hairdresser.

Treatment was changed to cabergoline, with similar difficulties. Prolactin levels declined to the 30s—40s, but she was never able to tolerate the medication sufficiently to attain normal prolactin levels, and menses were sporadic and infrequent, with only 2—3 occurring per year. She and her husband had not conceived despite regular unprotected intercourse.

She asks whether other medical treatment options might be available for her infertility. Prolactin-secreting adenomas are the most common functioning pituitary tumor 1 , 2.

Similarly, men may also have hypogonadism and infertility. Infertility results from suppression of gonadotropin secretion, manifest as hypogonadotropic hypogonadism HH 3. The goals of therapy are to normalize prolactin, reduce tumor size, reverse hypogonadism, and restore fertility. Adverse effects include nausea, orthostatic hypotension, headache, and fatigue.

The more selective D2 receptor agonist, cabergoline, is more effective and better tolerated than bromocriptine. While hyperprolactinemia is a well-established cause of HH and infertility, the mechanisms of these effects are not well understood.