Does alpha-melanocyte-stimulating hormone from the pars intermedia regulate suckling-induced prolactin release? Supportive evidence from morphological and functional studies

Differential action of two prolactin isoforms on ischemia and re-perfusion-induced arrhythmias in rats in vivo

BACKGROUND

The different influences of one of the PRL isoforms (PRL I) on the cardiovascular system have been described in the past.

AIM

Our goal was to establish an appropriate iv dose of 2 PRL isoforms (PRL I and PRL II) in intact rats. After establishing this dose, PRL I (0.01 mg/kg) or PRL II (0.001 mg/kg) was administered in bolus 10 min before left anterior descending coronary artery occlusion (7 min) followed by re-perfusion (15 min). We then aimed to study and compare the effects of these isoforms on ischemia- and re-perfusion-induced arrhythmias in the ischemia and re-perfusion-induced arrhythmias model in rats.

MATERIALS AND METHODS

Mortality index, ventricular fibrillation and tachycardia (VF, VT) incidence and duration, systolic, diastolic, and mean arterial blood pressure, heart rate and myocardial index of oxygen consumption [pressure rate product (PRP)] were measured and calculated.

RESULTS

Both PRL isoforms reduced animal mortality (from 50 to 18.75 and 25%, respectively). PRL II significantly reduced VF incidence (to 25%) as well as VT duration (18.21 ± 3.09) and these effects were markedly different from PRL I and from the control group (p<0.05). Both PRL reduced PRP in the recovery phase (p<0.05).

CONCLUSIONS

We proved that supraphysiological doses of PRL isoforms administered in bolus could protect against sudden cardiac death as well as severe arrhythmias episodes during re-perfusion. Because of PRL's positive influence on the cardiovascular system and as an endogenous, well-tolerated substance, it might be of potential clinical use.

Pivotal roles of alpha-melanocyte-stimulating hormone and the melanocortin 4 receptor in leptin stimulation of prolactin secretion in rats

Leptin, the obese gene product, was reported to stimulate prolactin (PRL) secretion, but the neuroendocrine mechanism underlying this hormonal response is largely unknown. Thus, in this study we examined the involvement of several important PRL regulators in the leptin-induced PRL secretion in male rats. Compared with the values in normally fed rats, food deprivation for 3 days significantly decreased both PRL and leptin levels in the plasma. These changes were reverted to normal by a 3-day constant infusion of 75 microg/kg/day of leptin to the fasted rats, while 225 microg/kg/day of leptin further elevated both PRL and leptin levels. These four groups of animals were used for the following experiments. Results of dopamine and serotonin turnover studies in the brain and the pituitary indicated that neither of these biogenic amines plays a primary role in mediating leptin's effects on PRL. Repeated intracerebroventricular injections over 72 h of neutralizing antibodies against vasoactive intestinal peptide, PRL-releasing peptide, or beta-endorphin, did not significantly suppress the leptin actions. However, both the blockade of the melanocortin (MC) 4 receptor (R) and the immunoquenching of brain alpha-melanocyte-stimulating hormone (alpha-MSH) completely abolished the leptin-induced PRL release, and the stimulation of the MC4-R, but not the MC3-R, significantly elevated PRL levels in the fasted rats. These results suggest that alpha-MSH, a cleaved peptide from pro-opiomelanocortin of which synthesis is stimulated by leptin, may be the pivotal neuropeptide in the brain mediating the leptin's stimulatory influence on PRL secretion. It was also suggested that the MC4-R may be the primary subtype of the MC-Rs mediating this action of alpha-MSH.

Stimulation of prolactin secretion by chronic, but not acute, administration of leptin in the rat

Leptin, the product of obese (ob) gene, has been reported to affect the secretion of all six anterior pituitary hormones, but data are especially scarce regarding the interplay between leptin and prolactin (PRL). Thus, in this study we examined and compared in vivo the effects of acute and chronic administrations of recombinant mouse leptin on PRL secretion in male rats. Normally-fed and 3-day-fasted rats received an intraperitoneal bolus injection of leptin [1.0 mg/kg body weight (BW)] or vehicle only. The leptin treatment was without effect on plasma PRL levels up to 5 h postadministration. Food deprivation for 3 days significantly decreased both PRL and leptin levels. This decrease in plasma PRL was prevented by a 3-day constant infusion of 75 microg/kg BW/day of leptin, which maintained plasma leptin levels similar to those of normally-fed rats. The administration of three times the higher dose of leptin (225 microg/kg BW/day) to fasted rats led to further increases in both PRL and leptin in the plasma. Thus, a dose-dependent stimulatory effect of chronic leptin treatment on PRL secretion was indicated. This study demonstrates that chronic, but not acute, administration of leptin stimulates PRL secretion in the rat.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Suckling-induced increase in cyclic AMP exclusively in the central region of the rat adenohypophysis

Investigating the cellular events in the pituitary gland, the intracellular cyclic AMP (cAMP) of the neural lobe (NL), intermediate lobe (IL), the inner (IZ-AL) and outer zone (OZ-AL) of the anterior lobe (AL) have been measured during the suckling stimulus. Ten-minutes suckling, parallel to the elevation of plasma PRL, induced a significant increase of cAMP concentration in the IZ-AL. In contrast, 10- and 30-min suckling resulted in a decrease of cAMP level in the NL. Changes in cAMP of the OZ-AL and the IL as well as in the plasma level of alpha-MSH could not be detected. These region-specific changes of cAMP in the pituitary gland during suckling stimulus seems to be related to interacting neuroendocrine signals delivered concomitantly from the hypothalamus and from the NIL to the IZ-AL.

Suckling-induced change in oxytocin but not in alpha-MSH concentrations of the median eminence, the neural-, intermediate- and anterior lobes of the pituitary gland

Previous reports have implicated that pituitary-derived prolactin (PRL) is secreted from two distinct zones of mammotropes within the anterior lobe (AL). The inner zone (AL-IZ), located adjacent to the neuro-intermediate lobe (NIL), is supposed to be involved in the rapid and massive discharge of PRL from the pituitary gland due to suckling stimulus. Whereas the outer-zone (AL-OZ) gives the basal secretion and it does not play a role in the acute secretory response during nursing. Anatomically, the AL-IZ has an intimate contact with the NIL because the blood passing through the short portal vessels (SPV) bathes it first. Based on this fact it would be hypothesized that locally released and/or produced compounds, like OXY and alpha-MSH, can be delivered to the AL-IZ. In conjunction, OXY and alpha-MSH have already been implicated to play a role in the regulation of PRL release during suckling. Therefore, the purpose of this study was to examine the possible local transportation of these hormones into the median eminence and various regions of the pituitary gland of lactating rats. We have measured the concentrations of OXY and alpha-MSH from tissue samples of nonsuckled (NS) and 10 or 30 min after suckling (S) was initiated using specific RIAs. It has been shown that there are no changes in the concentration of OXY and alpha-MSH in theAL-IZ and AL-OZ due to suckling stimulus. In contrast, our data provide compelling evidence that OXY is transported into the IL, which can be further increased by suckling stimulus. These data suggest that blood transfusing NL passes through the IL before it is drained into the cavernous sinus, which opens the road for OXY into the general circulation. In addition, our data have unequivocally shown a lack of local delivery of either alpha MSH or OXY into the AL that raises serious doubt about their possible role in PRL secretion during suckling stimulus.

alpha-MSH potentiates the responsiveness of mammotropes by increasing Ca2+ entry

It is well known that the suckling stimulus renders mammotropes considerably more responsive to prolactin (PRL)-releasing stimuli, and the neurointermediate lobe peptide alpha-melanocyte-stimulating hormone (alpha-MSH) has been proposed to play a pivotal role in this priming. The objectives of the present study were to determine whether alpha-MSH could act directly on pituitary cells to potentiate PRL release in response to two physiologically relevant PRL secretagogues, thyrotropin-releasing hormone (TRH) and ATP, and, if so, to identify the mechanism by which this priming phenomenon is manifested. To this end, we cultured anterior pituitary cells from lactating rats overnight and then subjected them to a reverse hemolytic plaque assay for PRL to evaluate their responses to various test agents. We found that alpha-MSH, which had no effect on PRL export when tested alone, augmented by more than threefold the secretory responses to TRH and ATP. Next, we utilized digital-imaging fluorescence microscopy of fura 2 to evaluate the role of intracellular Ca2+ in this process. We found that PRL export induced by pharmacological activation of L-type voltage-operated calcium channels was also potentiated by alpha-MSH, as was Ca2+ entry induced by TRH. Our results indicate that alpha-MSH acts as a mammotrope-priming agent on a subset of mammotropes by increasing Ca2+ entry induced by PRL secretagogues.

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Phenotypic characterization and functional correlation of alpha-MSH binding to pituitary cells

It is clear that alpha-melanocyte-stimulating hormone (alpha-MSH), released by the hypophysial neurointermediate lobe, is a mediator of suckling-induced prolactin release, but several questions surrounding its role remain unresolved. Accordingly, the objectives of the present study were 1) to establish whether alpha-MSH could bind in a reversible manner to a specific secretory type cell within the adenohypophysis (AP), 2) to resolve the issue of whether the peptide could compete with dopamine for the same receptor binding site, and 3) to seek a functional signaling correlate for alpha-MSH binding. In pursuit of these objectives, we subjected pituitary cells from lactating rats to alpha-MSH receptor autoradiography, AP hormone immunocytochemistry, or digital imaging fluorescence microscopy with fura 2 as a calcium-sensitive probe. Our results show that alpha-MSH binding is restricted to mammotropes and that a specific subpopulation of these express functional alpha-MSH receptors that are coupled to a Ca2+ signaling pathway. Moreover, alpha-MSH does not compete with dopamine antagonists/agonists for the same binding site.

Effect of posterior pituitary lobectomy on in vivo and in vitro secretion of prolactin in lactating rats

The effect of removing the posterior and neuro-intermediate lobes (PLX) of the pituitary gland of lactating rats was determined on both suckling-induced release and transformation of prolactin (PRL), and on regionalization of PRL release. Sixteen hours, or 1 or 4 d after either PLX or sham surgery, acute (15-min) suckling was applied. Also, regionalization of PRL release was analyzed by incubating the central and peripheral regions of APs from nonsuckled rats. Plasma PRL was analyzed by radioimmunoanalysis (RIA), whereas anterior pituitary (AP) PRL content and in vitro released PRL were determined by polyacrylamide gel electrophoresis. Plasma PRL increased 25- to 30-fold after suckling in intact and sham, and 10- to 15-fold in 1- and 4-d PLX rats, but no change occurred on either 16-h PLX nonsuckled and suckled rats. Also, PRL transformation occurred in intact, sham, and 4-d PLX suckled rats, but not in 16-h sham, or in 16-h and 1-d PLX suckled rats. Finally, the higher secretion of PRL shown in vitro by the central region of APs from intact and sham was not observed in APs from PLX rats. These results show that PLX transiently depresses the suckling-regulated PRL transformation and release. Likewise, influences from the posterior and/or neuro-intermediate lobes may determine regionalization of PRL release.

Regulation of GFAP expression in glial-like cells of the rat pituitary intermediate lobe by lactation, salt-loading, and adrenalectomy

Glial-like cells in rat pituitary intermediate lobe were localized and characterized by immunohistochemistry with antisera against glial fibrillary acidic protein (GFAP) and S-100. Individual GFAP immunoreactive (IR) cells possessed several processes that often branched into secondary and tertiary processes, terminating with end-feet. The GFAP-immunopositive cell population was distributed in specific rostrocaudal and dorsoventral patterns. The distribution and numbers of cells differed between male and female rats. Examination of altered physiological states, e.g., adrenalectomy, lactation, and salt-loading, revealed state-specific changes in the appearance and distribution of GFAP-IR cells. Adrenalectomy and lactation increased GFAP-IR glial-like cell numbers, whereas salt-loading decreased their numbers and the typical pattern of distribution. By contrast, S-100-expressing cells were evenly distributed in male and female rats, and its expression was not affected by the experimental conditions. Double-label immunocytochemistry indicated that GFAP-IR cells are a subpopulation of S-100-IR cells. These results suggest that cells normally expressing only S-100 may be induced to express GFAP under altered physiologic conditions.