In vitro nitric oxide effects on basal and gonadotropin-releasing hormone-induced gonadotropin secretion by pituitary gland of male crested newt (Triturus carnifex) during the annual reproductive cycle

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.

Oral arginine improves linear growth of long bones and the neuroendocrine mechanism

OBJECTIVE

To investigate the effect of oral administration of arginine on linear growth of long bones in male pubertal rats and the underlying mechanisms, focusing on expression of genes related to the hypothalamus-pituitary growth axis and the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway.

METHODS

Rats were randomly divided into control and intervention groups. In the intervention group, arginine was solved in water (0.045 g L-arginine was mixed with 1 mL water) and administered in rats (10 mL/kg) through gastric perfusion once per day, for totally 28 d. Rats in the control group received normal saline treatment. Bone histomorphometry analysis was used to measure growth plate width and mineral apposition rate of the tibia, as well as trabecular bone volume fraction, osteoblast surface and osteoclast surface of the femur. Serum growth hormone (GH) concentration was determined by radioimmunoassay. Real-time PCR was used to measure the expression of neuronal nitric oxide synthase (nNOS), soluble guanylyl cyclases (sGCα1 and sGCβ1), growth hormone-releasing hormone (Ghrh) and somatostatin (SS) in hypothalamus, as well as Gh in pituitary. Western blot was used to detect the protein levels of nNOS, sGCα1 and sGCβ1 in hypothalamus.

RESULTS

After treatment with arginine, the growth plate width of tibia and osteoblast surface of femur were increased (P < 0.05), and serum GH concentration was elevated (P < 0.05). Besides, mRNA and protein levels of nNOS and sGCα1 (P < 0.05), as well as the expression of Gh mRNA (P < 0.01), were significantly up-regulated, while the expression of SS mRNA was down-regulated (P < 0.05).

CONCLUSION

Oral administration of arginine could improve linear growth of long bones by regulating mRNA expression of SS and Gh and inducing GH secretion, possibly via nNOS-NO-sGC-cGMP signal transduction pathway.

Pituitary adenylate cyclase-activating polypeptide stimulates nitric-oxide synthase type I expression and potentiates the cGMP response to gonadotropin-releasing hormone of rat pituitary gonadotrophs

Nitric-oxide synthase type I (NOS I) is expressed primarily in gonadotrophs and in folliculo-stellate cells of the anterior pituitary. In gonadotrophs, the expression and the activity of NOS I are stimulated by gonadotropin-releasing hormone (GnRH) under both experimental and physiological conditions. In the present study, we show that pituitary adenylate cyclase-activating polypeptide (PACAP) is twice as potent as GnRH at increasing NOS I levels in cultured rat anterior pituitary cells. The action of PACAP is detectable after 4-6 h and maximal at 24 h, this effect is mimicked by 8-bromo-cAMP and cholera toxin and suppressed by H89 suggesting a mediation through the cAMP pathway. Surprisingly, NADPH diaphorase staining revealed that these changes occurred in gonadotrophs exclusively although PACAP and cAMP, in contrast to GnRH, have the potential to target several types of pituitary cells including folliculo-stellate cells. There was no measurable alteration in NOS I mRNA levels after cAMP or PACAP induction. PACAP also stimulated cGMP synthesis, which was maximal within 15 min and independent of cAMP, however, only part resulted from NOS I/soluble guanylate cyclase activation implying that in contrast to GnRH, PACAP has a dual mechanism in cGMP production. Interestingly, induction of NOS I by PACAP markedly enhanced the capacity of gonadotrophs to produce cGMP in response to GnRH. The fact that PACAP may act on gonadotrophs to alter NOS I levels, generate cGMP, and potentiate the cGMP response to GnRH, suggests that cGMP could play important cellular functions.

Substance P downregulates basal and gonadotropin-releasing hormone-induced gonadotropin in vitro secretion by pituitary gland of crested newt, Triturus carnifex

The possible role of Substance P (SP) was studied in the modulation of basal and gonadotopin-releasing hormone (GnRH)-induced gonadotropin secretion in the urodele crested newt, Triturus carnifex. During prereproduction, reproduction (noncourtship and courtship), refractory, recovery and aestivation, male and female pituitaries were incubated with medium-alone, GnRH, SP, GnRH receptor antagonist (antide), and SP receptor antagonist (L-703606). Since antisera raised against gonadotropins are not available for this species, we measured these hormones indirectly through their effects on the secretion of testicular androgens and ovarian progesterone from gonads superfused with the preincubated pituitaries. Pituitaries of both sexes preincubated with medium-alone, GnRH, GnRH plus L-703606, and GnRH plus SP plus L-703606 increased steroid secretion during prereproduction, noncourtship, courtship, and recovery; the increase induced by the pituitaries incubated with medium-alone was lower during prereproduction, noncourtship, and recovery. Pituitaries preincubated with SP, GnRH plus SP, GnRH plus SP plus antide, and SP plus antide did not change basal steroid secretion in any of the reproductive phases considered. Antide, L-703606, GnRH plus antide, GnRH plus SP plus antide plus L-703606, SP plus L-703606, and antide plus L-703606 experimental groups showed the same results as those with medium-alone. These results suggest that SP downregulates gonadotropin release in both Triturus carnifex sexes. In addition, an antagonist role, through receptor-independent mechanisms, exists between GnRH (upregulation) and SP (downregulation) in the modulation of pituitary.

Evidence that nitric oxide is involved in the regulation of growth hormone secretion in goldfish

Whether nitric oxide (NO) plays a role in regulation of growth hormone (GH) secretion from somatotropes in the pituitary of the goldfish Carassius auratus was investigated. Immunocytochemistry with two antibodies against mammalian NO synthase (NOS) revealed the presence of a NOS-like enzyme in primary cultures of dispersed goldfish pituitary cells, including morphologically identified somatotropes. NO donors S-nitroso-N-acetylpenicillamine and sodium nitroprusside (SNP), as well as a cyclic guanosine monophosphate analogue (dibutyryl guanosine 3':5'-cyclic monophosphate), all significantly increased GH secretion from dispersed goldfish pituitary cells in static culture. Somatostatin abolished the response to SNP, and NOS inhibitors aminoguanadine hemisulfate (AGH) and N-(3-aminomethyl)benzylacetamidine, dihydrochloride (1400W) decreased the GH release response to known neuroendocrine factors stimulatory to GH release (gonadotropin-releasing hormone and a dopamine D1 agonist). AGH and 1400W did not alter basal GH secretion. These data suggest that NO plays a role in mediating the GH response to endogenous neuroendocrine factors in goldfish.

Effects of nitric oxide donors on the afferent resting activity in the cephalopod statocyst

The effects of bath applications of the nitric oxide (NO) donors sodium nitroprusside (SNP), diethylamine sodium (DEA), 3-morpholinosydnonimine (SIN-1), and S-nitroso-N-acetyl-penicillamine (SNAP) on the resting activity (RA) of afferent crista fibers were studied in isolated statocysts of the cuttlefish Sepia officinalis. The NO donors had three different effects: inhibition, excitation, and excitation followed by an inhibition. The SNAP analog N-acetyl-DL-penicillamine (xSNAP; with no NO moiety) had no effect. When the preparation was pre-treated with the NO synthase inhibitor N(G)-nitric-L-arginine methyl ester HCl (L-NAME), the NO donors were still effective. When the preparation was pre-treated with the guanylate cyclase inhibitors methylene blue (M-BLU) or cystamine (CYS), NO donors had only excitatory effects, whereas their effects were inhibitory only when pre-treatment was with the adenylate cyclase inhibitors nicotinic acid (NIC-A), 2',3'-dideoxyadenosine (DDA), or MDL-12330A. When pre-treatment was with a guanylate and an adenylate cyclase inhibitor combined, NO donors had no effect; in that situation, the RA of the afferent fibers remained and the preparation still responded to bath applications of GABA. Selective experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus vulgaris gave comparable results. These data indicate that in cephalopod statocysts an inhibitory NO-cGMP and an excitatory NO-cAMP signal transduction pathway exist, that these two pathways are the key pathways for the action of NO, and that they have only modulatory effects on, and are not essential for the generation of, the RA.