In vitro, nitric oxide (NO) stimulates LH secretion and partially prevents the inhibitory effect of dopamine on PRL release

Arginine promotes testicular development in boars through nitric oxide and putrescine

The present work aimed to explore the influence and underlying mechanisms involving arginine in testicular development in boars. To this end, thirty 30-day-old male Duroc piglets (7.00 ± 0.30 kg) were randomly sorted into two groups, maintained on either a basal diet (CON, n = 15) or a diet supplemented with 0.8% arginine (ARG, n = 15). Blood and testicular samples were collected during the experimental period to analyse amino acid composition and arginine metabolite levels. The results showed that dietary supplementation with arginine increased number of spermatogonia and height of the seminiferous epithelium (p < 0.05). Sperm density, total number and effective number of sperm of the boars in the ARG group increased significantly compared with those in the CON group (p < 0.05). Although arginine supplementation did not affect plasma amino acid levels, testicular arginine levels in 150-day-old boars exhibited a significant increase (p < 0.05). The level of serum nitric oxide (NO) and activity of nitric oxide synthase (NOS) also increased in 150-day-old boars in the ARG group (p < 0.05). Interestingly, dietary supplementation with arginine increased testicular levels of putrescine in 150-day-old boars (p < 0.05). These results indicated that arginine supplementation increased serum NO levels and testicular arginine and putrescine abundance, thereby improving testicular development and semen quality in boars.

Proteomic analysis reveals key proteins involved in arginine promotion of testicular development in boars

In order to study the effect of l-arginine on testicular development in boars and its underlying mechanism, thirty 30-day-old Duroc boars (7.0 ± 0.3 kg) were randomly allocated to either a basal diet (CON, n = 15) or a 0.8% arginine diet (ARG, n = 15). Blood samples were collected and weight measurements were done regularly to every pig during the experimental period. Testes were collected for histological and proteomic analysis from 150-day-old boars. Results showed that dietary supplementation with arginine significantly increased testicular weight, the number of spermatogonia, and the height of the seminiferous epithelium (P < 0.05). The serum levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH) were also significantly increased in the ARG group (P < 0.05). A total of 154 differentially expressed proteins (DEPs) were identified, 76 of them were upregulated and 78 were downregulated. Mainly enriched metabolic pathways were the mTOR and Wnt signaling pathways. Functional annotation suggested that 18 DEPs were related to male reproduction. Western blotting results further validated the results from proteomics. This study may provide new insights into the molecular mechanisms by which arginine promotes male testicular development.

Nitric oxide and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.

Sustained gonadotropin-releasing hormone stimulation mobilizes the cAMP/PKA pathway to induce nitric oxide synthase type 1 expression in rat pituitary cells in vitro and in vivo at proestrus

Previous in vivo studies have established that pituitary nitric oxide synthase type 1 (NOS1) is regulated by gonadotropin-releasing hormone (GnRH). The aim of our study was to elucidate the mechanisms of NOS1 regulation by GnRH in rat pituitary cells. Using a perifused cell system, we demonstrated that NOS1 induction was sensitive to GnRH pulse frequency and was maximally induced under continuous GnRH stimulation. In primary cultures of rat pituitary cells, sustained stimulation with the GnRH agonist triptorelin (GnRHa) increased NOS1 protein levels, whereas NOS2 and NOS3 levels were unaffected. NOS1 up-regulation occurred in gonadotroph cells only, in a time-dependent and concentration-dependent manner (maximum increase, 2.5-fold; half-maximal concentration, 0.17 nM). GnRHa effect was mimicked by cAMP pathway activators and, most importantly, was blocked by disruption of the protein kinase A (PKA) pathway using pharmacological inhibitors such as Rp-cAMP or drug phosphatase technology-protein kinase inhibitor (DPT-PKI), a cell-permeant PKI peptide. In contrast, modulation of the PKC pathway and inhibition of the MAPK cascade were ineffective. Overall, these experiments demonstrated that GnRH-induced up-regulation of pituitary NOS1 is mediated notably by the cAMP/PKA pathway. Last, in vivo administration of a GnRH antagonist markedly inhibited the pituitary cAMP rise at proestrus in addition to suppressing NOS1 increase. Altogether, our data suggest that the cAMP/PKA signaling pathway is preferentially recruited under sustained GnRH stimulation in vivo during proestrus, allowing the expression of a specific set of PKA-regulated proteins, including NOS1, in gonadotroph cells.

PACAP stimulation of maturational gonadotropin secretion in goldfish involves extracellular signal-regulated kinase, but not nitric oxide or guanylate cyclase, signaling

In goldfish, nitric oxide synthase (NOS) immunoreactivity is present in gonadotropes and extracellular signal-regulated protein kinase (ERK) mediates GnRH stimulation of gonadotropin release and synthesis. In this study, we tested the possible involvement of nitric oxide (NO) and ERK in mediating PACAP-stimulated maturational gonadotropin (GTH-II) release from primary cultures of dispersed goldfish pituitary cells. In static incubation experiments, PACAP-induced GTH-II release was unaffected by two inhibitors of NOS synthase, AGH and 1400W; whereas addition of a NO donor, SNAP, elevated GTH-II secretion. In perifusion experiments, neither NOS inhibitors (AGH, 1400W and 7-Ni) nor NO scavengers (PTIO and rutin hydrate) attenuated the GTH-II response to pulse applications of PACAP. In addition, the GTH-II responses to PACAP and the NO donor SNP were additive while PTIO blocked SNP action. Although dibutyryl cGMP increased GTH-II secretion in static incubation, inhibition of guanylate cyclase (GC), a known down-stream target for NO signaling, did not reduce the GTH-II response to pulse application of PACAP. On the other hand, GTH-II responses to PACAP in perifusion were attenuated in the presence of two inhibitors of ERK kinase (MEK), U 0126 and PD 98059. These results suggest that although increased availability of NO and cGMP can lead to increased GTH-II secretion, MEK/ERK signaling, rather than NOS/NO/GC activation, mediates PACAP action on GTH-II release in goldfish.

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.

Estrogen modulates the action of nitric oxide in the medial preoptic area on luteinizing hormone and prolactin secretion

Several substances work as neuromediators of the estrogen direct and indirect (through glial cells or interneurons) action on luteinizing hormone- releasing hormone (LH-RH) neurons in medial basal hypothalamus and medial preoptic area (MPOA). Angiotensin II (AII) in the MPOA stimulates the LH and it inhibits PRL secretion in some situations. On the other hand, the effect of excitatory amino acids on LH and PRL surges during proestrus as well LH surge induced by steroids depend on nitric oxide (NO). In the present study we investigated the participation of MPOA endogenous NO on gonadotropin and PRL secretion mediated by estrogen and AII. Plasma LH, FSH and PRL was determinated in estrogen primed and unprimed ovariectomized Wistar rats that received microinjection of AII or saline into the MPOA, associated or not with a previous microinjection of an inhibitor for NOS. Our results show the following: 1 - there was no change in plasma FSH in estrogen- primed or unprimed ovarictomized related with microinjections of AII or NO antagonist in the MPOA; 2- the increase in LH secretion after ovariectomy depends on, at least in part, NO activity in the MPOA; 3- estrogen may have an indirect negative feedback action on LH-RH neurons in the MPOA through NO; 4- the stimulatory action of AII in the MPOA on LH secretion in ovariectomized rats treated with estrogen depends on NO; 5 - NO in the MPOA stimulates or inhibits PRL secretion depending on the absence or presence of estrogen, respectively; 6- the inhibitory action of AII into the MPOA on PRL secretion does not seem to depend on NO.

Effect of nitric oxide on prolactin secretion and hypothalamic biogenic amine contents

Involvement of nitric oxide (NO) in the episodic secretion of prolactin was studied in conscious freely moving adult rats. Prolactin secretion was pulsatile in all animals of either group during the bleeding period (from 10:30 h to 13:30 h). Administration of N(omega)-nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, increased mean plasma levels of prolactin, and the absolute amplitude of prolactin peaks during the whole bleeding period as compared to values found in the control group. L-NAME increased norepinephrine (170%), dopamine (58.27%) and serotonin contents (30%) in the anterior hypothalamus. In the median eminence, dopamine and serotonin contents decreased (19.79% and 33.9% respectively) after L-NAME as compared to the values found in controls. In addition, norepinephrine content increased in mediobasal hypothalamus (79.6%) of rats treated with L-NAME. The results indicate that changes in NO production may modify the episodic secretion of prolactin. These effects were associated with changes in hypothalamic and median eminence biogenic amines.

Involvement of PI3'-K, mitogen-activated protein kinase and protein kinase B in the up-regulation of the expression of nNOSalpha and nNOSbeta splicing variants induced by PRL-receptor activation in GH3 cells

It is well known that GH-PRL secreting GH3 cells express constitutive neuronal nitric oxide synthase (nNOS) and produce nitric oxide (NO*). In addition, these cells possess plasma membrane prolactin (PRL) receptors which can be responsible for an autocrine 'short-loop' feedback. The aim of the present study was to investigate whether the activation of PRL receptors modulates the expression of the different spliced forms of nNOS gene, and the transductional mechanisms involved in this action. In GH3 cells, both exon 2-containing nNOSalpha and exon 2-lacking nNOSbeta were time-dependently expressed, whereas the other two isoforms eNOS and iNOS were not. The antibodies directed against the residues 53-68 of the external domain common to both the long and short form of rat PRL receptors, and the selective D2 agonist cabergoline (1 nm) reduced both basal and exogenous PRL-induced expressions of nNOSalpha and nNOSbeta, but to a greater extent for the beta splicing form. In line with these results, oPRL (1 and 10 microm) added to the incubation medium increased to a greater extent the expression of nNOSbeta form than of the nNOSalpha. The receptor and non-receptor protein tyrosine kinase (PTK) inhibitors, genistein (10 microm), the Src-specific tyrosine kinase inhibitor PP2 (100 microm), the MAPK inhibitor PD 098059 (50 nm) and the two PI3'-K inhibitors, wortmannin (300 nm) and LY-294002 (25 microm) prevented both basal and exogenous PRL-induced expression of nNOSalpha and nNOSbeta isoforms. In addition, exogenous PRL induced a phosphorylation of protein kinase B (PKB) (Akt) that was prevented both by the two MAPK inhibitors PD 098059 and U 0126, and by the PI3'-K inhibitors wortmannin and LY-294002. Up-regulation of the expression of the two splicing forms of nNOS elicited by PRL-receptor activation was mirrored by the increased synthesis of NO*. In conclusion, PRL receptor activation up-regulated the expression of both nNOSalpha and nNOSbeta proteins via a PTK, PI3'-K, MAPK and PKB signalling transduction components. This action may represent the molecular mechanism by which PRL exerts the 'short-loop' feedback on its own secretion.

Involvement of nitric oxide on prolactin release induced by immobilization stress in rats

The nitric oxide (NO) has important participation in the control of hypothalamic-pituitary axis. The authors investigated the effect of NO donor, isosorbide dinitrate (ISDN), on prolactin (PRL) release induced by immobilization stress (IS) in male rats. Pretreatment with the NO donor, ISDN (2.5; 5 and 10 mg/kg), inhibited about 60-85% of the PRL response to IS. It is concluded that NO does participate in the regulation of PRL response to IS.

Gender differences and the effect of different endocrine situations on the NOS expression pattern in the anterior pituitary gland

The presence of neuronal nitric oxide synthase (nNOS) in two populations of pituitary cells, gonadotrophs (LH) and folliculostellate (FS) cells, suggests that pituitary nitric oxide (NO) is involved in the control of hormone secretion. We have used single and double immunostaining and quantitative procedures to investigate possible gender-related differences in the nNOS expression pattern in the anterior pituitary lobe and its possible alterations in different endocrine situations. Our results reveal a sexual dimorphism in the pattern of nNOS expression. In males, nNOS is mainly found in FS cells, whereas only a few LH cells express nNOS. Conversely, in females, nNOS is mainly found in LH cells. After gonadectomy, paralleling an increase in LH cell size and serum luteinizing hormone (LH) levels, there is nNOS upregulation in LH cells and nNOS downregulation in FS cells. After testoterone replacement, LH cells become nNOS-immunonegative again. In lactating rats, LH cells overexpress nNOS, but LH cell size and serum LH levels are low. This suggests that, depending on its cellular source, pituitary NO can exert either an inhibitory or a stimulatory effect on hormone secretion. When released from FS cells, NO exerts a paracrine inhibitory effect, and when released from gonadotrophs it exerts an autocrine or paracrine stimulatory effect on LH or prolactin secretion, respectively.