Nitric oxide plays an important role in the diurnal change of tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in ovariectomized, estrogen/progesterone-treated rats

17β-oestradiol acts as a negative modulator of insulin-induced lactotroph cell proliferation through oestrogen receptor α, via nitric oxide/guanylyl cyclase/cGMP

OBJECTIVES

17β-oestradiol interacts with growth factors to modulate lactotroph cell population. However, contribution of isoforms of the oestrogen receptor in these activities is not fully understood. In the present study, we have established participation of α and β oestrogen receptors in effects of 17β-oestradiol on lactotroph proliferation induced by insulin and shown involvement of the NO/sGC/cGMP pathway.

MATERIALS AND METHODS

Cell cultures were prepared from anterior pituitaries of female rats to evaluate lactotroph cell proliferation using bromodeoxyuridine (BrdUrd) detection, protein expression by western blotting and cGMP by enzyme immunoassay.

RESULTS

In serum-free conditions, 17β-oestradiol and α and β oestrogen receptor agonists (PPT and DPN) failed to increase numbers of lactotroph cells undergoing mitosis. Co-incubation of 17β-oestradiol/insulin and PPT/insulin significantly decreased lactotroph mitogenic activity promoted by insulin alone. Both ICI 182780 and NOS inhibitors (L-NMMA and L-NAME) induced reversal of the anti-proliferative effect promoted by 17β-oestradiol/insulin and PPT/insulin. Moreover, 17β-oestradiol, PPT and insulin increased sGC α1 protein expression and inhibited β1, whereas co-incubation of 17β-oestradiol/insulin or PPT/insulin induced increases of the two isoforms α1 and β1. 17β-oestradiol and insulin reduced cGMP production, while 17β-oestradiol/insulin co-incubation increased this cyclic nucleotide.

CONCLUSIONS

Our results suggest that 17β-oestradiol is capable of arresting lactotroph proliferation induced by insulin through ER α with participation of the signalling NO/sGC/cGMP pathway.

24-Hour variation in gene expression of redox pathway enzymes in rat hypothalamus: effect of melatonin treatment

The 24-h changes in medial basal hypothalamic (MBH) gene expression of redox pathway enzymes nitric oxide synthase (NOS)-1 and NOS-2, heme oxygenase (HO)-1 and HO-2, Cu/Zn- and Mn-superoxide dismutases (SOD) and catalase were examined in adult male Wistar rats kept under an alternating regimen of light/dark. Half of the animals received melatonin (approximately 60 microg/day) in the drinking water. After 1 month, rats were killed at six different time intervals, throughout a 24-h cycle. MBH mRNA levels were measured by real-time PCR analysis. In controls, gene expression of NOS-2 and HO-2 peaked at the early light phase while that of HO-1 showed a maximum at the middle of the dark phase. None of MBH mRNAs encoding NOS-1, Cu/Zn-SOD, Mn-SOD and catalase exhibited significant 24-h variations in control rats. Melatonin administration decreased significantly mRNAs for NOS-1, NOS-2, HO-1 and HO-2 as well as changed their 24-h profile. Melatonin augmented gene expression of the antioxidant enzymes Cu/Zn-SOD, Mn-SOD or catalase at certain time intervals only. The results are compatible with the view that the principal indirect (i.e. gene expression of redox pathway enzymes) effect of melatonin on redox pathway in the hypothalamus is mainly exerted via down-regulation of pro-oxidant enzyme mRNAs.

24-Hour rhythm in gene expression of nitric oxide synthase and heme-peroxidase in anterior pituitary of ethanol-fed rats

Chronic exposure of rats to ethanol results in significant changes in pituitary hormone secretion. However, identification of the site(s) and mechanism of action of ethanol to induce these effects remains elusive. Free radical damage at the adenohypophyseal level may play a role in the decline in serum gonadotropin levels in ethanol-fed rats. Since 24-h changes in redox state occurred, we analyzed the 24-h changes in pituitary gene expression of the prooxidant enzymes nitric oxide synthase (NOS) 1 and 2, and of heme oxygenase-1 (HO-1) enzyme, and in plasma NO(2)(-) and NO(3)(-) (NO(x)) levels, in ethanol and control rats. Male rats, 35-day-old, received a liquid diet for 4 weeks. The ethanol-fed group received a similar diet to controls except for that maltose was isocalorically replaced by ethanol. Animals were killed at six time intervals during a 24-h cycle. Anterior pituitary mRNA levels encoding NOS1, NOS2 and HO-1 were measured by real-time PCR analysis. Plasma NO(x) concentration was determined by the Griess reaction. Ethanol feeding of prepubertal rats changed significantly the 24-h pattern of expression of NOS1, NOS2 and HO-1 in the adenohypophysis and augmented NOS2 and HO-1 mRNA levels. Peak values for the three enzymes in ethanol-fed rats occurred at the beginning of the scotophase (i.e., at 21:00 h). Ethanol feeding augmented mean values plasma NO(x) levels with a maximum at 13:00 h while in controls a biphasic pattern was observed, with peaks at 09:00 h and 17:00-21:00 h. One of the mechanisms by which ethanol augments oxidative damage in the adenohypophysis may include overproduction of nitric oxide and carbon monoxide.

Toxic effects of methoxychlor on the episodic prolactin secretory pattern: possible mediated effects of nitric oxide production

BACKGROUND

This work addresses the issue of whether methoxychlor (MTX) exposure may modify the ultradian secretion of prolactin through changes in the synthesis of nitric oxide (NO) induced by Nomega-nitro-L-arginine methyl ester (L-NAME) in the hypothalamic-pituitary axis. Associated changes in dopamine (DA) content in the anterior (AH), mediobasal (MBH) and posterior hypothalamus (PH) and median eminence (ME) were evaluated.

METHODS

Two groups of animals (MTX and MTX+L-NAME treated) received subcutaneous (sc) injections of MTX at a dose of 25 mg/kg/day for one month. The other two groups of animals (control and L-NAME treated) received sc vehicle injections (0.5 mL/day of sesame oil), during the same period of time to be used as controls. Forty hours before the day of the experiment, animals were anaesthetized with intrapritoneal injections of 2.5% tribromoethanol in saline and atrial cannulas were implanted through the external jugular vein. Plasma was continuously extracted in Hamilton syringes coupled to a peristaltic bomb in tubes containing phosphate-gelatine buffer (to increase viscosity). The plasma was obtained by decantation and kept every 7 minutes for the measurement of plasma prolactin levels through a specific radioimmnunoassay and DA concentration by high-pressure liquid chromatography (HPLC).

RESULTS

Prolactin release in animals from all experimental groups analyzed was episodic. Mean plasma prolactin levels during the bleeding period, and the absolute pulse amplitude were increased after MTX or Nomega-nitro-L-arginine methyl ester (L-NAME) administration. However MTX and L-NAME did not modify any other parameter studied with the exception of relative pulse amplitude in MTX treated rats. L-NAME administration to rats treated with the pesticide reduced mean plasma prolactin levels and the absolute amplitude of prolactin peaks. Peak duration, frequency and relative amplitude of prolactin peaks were not changed in the group of rats treated with MTX plus L-NAME as compared to either control or MTX treated rats. Whereas MTX decreased DA content in the ME and increased it in the AH, its content did not change in the MBH or PH, as compared to the values found in controls. Also, L-NAME administration decreased DA content in the ME as compared to controls. However, L- NAME administration to MTX exposed rats, markedly increased DA content in the ME as compared to either MTX treated or control rats. L-NAME administration increased DA content in the AH as compared to the values found in non-treated rats. However L-NAME administration to MTX exposed rats did not modify DA content as compared to either MTX treated or control rats. L-NAME administration did not modify DA content at the MBH nor in saline treated nor in MTX treated rats. However, the values of DA in the MBH in MTX plus L-NAME treated animals were statistically decreased as compared to L-NAME treated rats. In the PH, L-NAME administration increased DA content as compared to the values found in non-treated animals. L-NAME administration to MTX exposed rats also increased DA content as compared to either MTX treated or control rats.

CONCLUSION

The results suggest the existence of an interaction between MTX and L-NAME in the modulation of the ultradian prolactin secretion at the pituitary levels. The possibility of an indirect effect mediated by changes in DA content at the ME requires further examination.

Estradiol and progesterone modulate the nitric oxide/cyclic gmp pathway in the hypothalamus of female rats and in GT1-1 cells

Considerable evidence suggests that the nitric oxide (NO)/cGMP signaling pathway plays an important role in the expression of reproductive behavior and in gonadotropin-releasing hormone (GnRH) release from the hypothalamus The effects of the NO/cGMP pathway on GnRH release and gene expression have also been examined in GT1 cells. However, it is still controversial whether NO/cGMP signaling facilitates or inhibits GnRH release in these cells. The current study examined the effects of estradiol and progesterone on neuronal NO synthase (nNOS), soluble guanylyl cyclase (sGC), and NO-dependent cGMP production in the preoptic area (POA) and hypothalamus (HYP) as well as in GT1-1 cells. Ovariectomized female rats received vehicle, estradiol benzoate (48 h) and/or progesterone (3-4 h) before preparation of brain slices. GT1-1 cells were incubated with vehicle, estradiol (48 h), progesterone (3-4 h), or with both hormones. The combination of estradiol and progesterone increased the expression of nNOS protein in the POA and HYP. Hormones had little effect on the abundance of sGC. Estradiol and progesterone together greatly enhanced NO-stimulated sGC activity in HYP-POA slices. In GT1-1 cells, NO-stimulated sGC activity was significantly increased by estradiol and progesterone, alone or in combination, but sGC expression was not altered by hormones.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Prostaglandins play an important role in diurnal changes of tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in ovariectomized, estrogen-treated rats

Intracerebroventricular injection of prostaglandins (PGs) has long been shown to stimulate the secretion of prolactin (PRL). The action of specific PG and the underlying mechanism, however, remain inconclusive. In this study, we evaluated the involvement of PGs in the diurnal changes of the tuberoinfundibular dopaminergic (TIDA) neurons and PRL secretion in ovariectomized (OVX) rats treated with estrogen (E2). Prior treatment of indomethacin (50 mg/kg, subcutaneous) for 24 h significantly prevented the afternoon decrease of TIDA neuronal activity and blocked the PRL surge. Intracerebroventricular injection of PGE(1) (5 microg/3 microl/rat) at 1000 h significantly lowered the activity of TIDA neurons, while similar treatment of PGE2 was without effect. In OVX + E(2) rats pretreated with indomethacin, PGE(1) given at 15, but not 30 or 180 min before decapitation at 1500 h significantly reversed the effects of indomethacin on TIDA neuronal activity, while the blocked PRL surge was not reversed. The PGs also exhibited a stimulatory effect on nigrostriatal dopamine (DA) neurons with PGE2 being the most effective agent. These results indicate that endogenous PGs play a significant role in the control of central DA neurons, especially the diurnal changes of TIDA neuronal activity and PRL secretion in OVX + E2 rats.

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.