Nitric oxide synthase in the rat anterior pituitary gland and the role of nitric oxide in regulation of luteinizing hormone secretion

The masculinization of the fetus during pregnancy due to inhalation of diesel exhaust

This study was conducted to determine the impact of diesel exhaust inhalation on the fetus. Seventy-two pregnant rats and 18 nonpregnant rats were divided into three groups: a group exposed to total diesel engine exhaust containing 5.63 mg/m(3) particulate matter, 4.10 ppm nitrogen dioxide, and 8.10 ppm nitrogen oxide; a group exposed to filtered exhaust without particulate matter; and a group exposed to clean air. The exposure period was from day 7 until day 20 of pregnancy. In addition, 15 pregnant rats were treated with aromatase inhibitors or testosterone to clarify the process by which diesel exhaust exerts its toxicity. The anogenital distance was significantly longer in male and female fetuses from both exhaust-exposed groups than in those of the control. Differentiation of the testis, ovary, and thymus was delayed and disturbed. Maternal testosterone and progesterone levels, which increased due to pregnancy whether or not the rats were exposed, were significantly higher and lower, respectively, in the pregnant rats exposed to total exhaust and filtered exhaust. The serum adrenocorticotropic hormone (ACTH) level and urinary excretion of 17-hydroxycorticosteroids (OHCS) did not differ among the pregnant groups. These results indicate that elevated testosterone did not result from elevated maternal adrenal function. The feto-placental-ovarian unit and inhibition of aromatase activity and synthesis caused by diesel exhaust inhalation might have played an essential role in the accumulation of testosterone. Since both exhaust-exposed groups showed almost the same reactions toward the inhalation, the gaseous phase must have included the relevant toxicants.

Nitric oxide and the control of reproduction

The free radical gas, nitric oxide is now known to be an important biological messenger in animals. Signal transmission by a gas that is produced by one cell, penetrates through membranes and regulates the function of another cell, represents new principles for signalling in biological systems. Nitric oxide is synthesised from L-arginine by enzyme nitric oxide synthase, which exists in multiple isoforms in a wide range of mammalian cells. Studies conducted in recent years point at a strong influence of NO in a wide range of reproductive functions. It is implicated in the control of gonadotrophin secretion at both hypothalamic and hypophyseal levels, LH surge mechanism, sexual behaviour, estradiol synthesis, follicle survival and ovulation. While considerable work lies ahead in unravelling the role of NO at the peripheral, cellular and molecular level in the domestic animal reproduction, findings presented in this review provide a general overview of growing appreciation of NO as a vital molecule controlling hypothalamic-pituitary-gonadal (HPG) axis.

Effects of S-nitroso-cysteine on proteins that regulate exocytosis in PC12 cells: inhibitory effects on translocation of synaptophysin and ADP-ribosylation of GTP-binding proteins

S-Nitroso-cysteine (SNC) inhibits Ca2+-induced noradrenaline (NA) release from PC12 cells. Since SNC stimulated Ca2+ mobilization from intracellular Ca2+ pools and SNC-induced inhibition of NA release was not washed-out, SNC may modify exocytosis-related proteins that overcome Ca2+ mobilization. In the present study, we investigated the effects of SNC on exocytosis-related proteins in PC12 cells. Ionomycin stimulated NA release and increased the immunoreactivity of synaptophysin in the cytosol fraction. A 25-kDa synaptosome-associated protein (SNAP-25), which localizes to plasma membranes and vesicles, increased in the cytosol fraction after stimulation. The increases in these proteins by ionomycin were inhibited in PC12 cells treated with 0.6 mM SNC. Synaptobrevin and synapsin-1 in the cytosol fraction, and syntaxin and 43 kDa growth-associated protein in the membrane fraction were not affected by ionomycin or SNC. Incubation of each protein with SNC did not affect antibody immunoreactivity. [32P]ADP-ribosylation of GTP-binding proteins (Gi/Go) by pertussis toxin, but not Gs by cholera toxin, was inhibited in SNC-treated PC12 cells and by co-addition of SNC to the assay mixture. These findings suggest that 1) SNC inhibits translocation of vesicles containing synaptophysin and SNAP-25, and 2) SNC reacts with cysteine residues in Gi/Go, causing inhibition of ADP-ribosylation by pertussis toxin.

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.

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.

C-type natriuretic peptide: an important neuroendocrine regulator?

In the decade since its discovery, C-type natriuretic peptide (CNP), the third member of the natriuretic peptide family, has been shown to be produced by most of the major endocrine glands, including the hypothalamus and anterior pituitary. The relative abundance of its guanylyl cyclase-containing GC-B receptor in these glands suggests that CNP might be a local neuroendocrine regulator. Here, we review this possibility, emphasizing signalling and integration with other regulatory systems in the neuroendocrine control of reproduction.

Intercellular communication in the anterior pituitary

In addition to hypothalamic and feedback inputs, the secretory cells of the anterior pituitary are influenced by the activity of factors secreted within the gland. The list of putative intrapituitary factors has been expanding steadily over the past decade, although until recently much of the work was limited to descriptions of potential interactions. This took the form of evidence of production within the pituitary of factors already known to influence activity of secretory cells, or further descriptions of actions on pituitary cells by such factors when added exogenously. A new phase of discovery has been entered, with extensive efforts being made to delineate the control of the synthesis and secretion of the pituitary factors within the gland, regulation of the receptors and response mechanisms for the factors in pituitary cells, and measurements of the endogenous actions of the factors through the use of specific immunoneutralization, receptor blockade, tissue from transgenic animals, and other means. Taken together, these findings are producing blueprints of the intrapituitary interactions that influence each of the individual types of secretory cells, leading toward an understanding of the physiological significance of the interactions. The purpose of this article is to review the recent literature on many of the factors acting as intrapituitary signals and to present such finding in the context of the physiology of the secretory cells.

Nadph-diaphorase activity in corpus allatum cells of the cockroach, Diploptera punctata

Using the fixation insensitive NADPH-diaphorase reaction as a histochemical marker for the enzyme nitric oxide synthase (NOS), we investigated the possible sites of putatively NOS-related NADPH-diaphorase in the brain and retrocerebral complex of the cockroach, Diploptera punctata. In the cerebral ganglion, NADPH-diaphorase expression was localized in antennal lobes, optic lobes, mushroom bodies and neurosecretory cells. The highest NADPH activity was detected in the corpora allata (CA). Spectrophotometric quantitation indicated that NADPH-diaphorase activity first increased and then decreased (cycled) in the CA of mated females. In addition, during the first ovarian cycle, NADPH-diaphorase activity fluctuated concurrently with cyclic changes in the size of corpus allatum cells. In virgin females, NADPH-diaphorase activity remained at a low level, but it increased if the neural connectives between CA and brain were severed, indicating that the brain inhibited NADPH-diaphorase expression in the CA. Although nerve terminals were abundant in the CA, NADPH-diaphorase was clearly endogenous and synthesized by glandular cells, as was shown by histochemical staining of the cytosol in all dissociated cells of the CA. We have also demonstrated NADPH-diaphorase activity in the CA of the American cockroach Periplaneta americana, the house cricket Acheta domesticus, the lepidopteran Leucania loreyi, and the fruit fly Drosophila melanogaster, suggesting that NOS occurs in the CA of most, if not all insects. It is therefore possible that corpus allatum cells release NO, along with juvenile hormone, which presumably can function as a messenger molecule.

Endogenous production of nitric oxide and effects of nitric oxide and superoxide on melanotrope functioning in the pituitary pars intermedia of Xenopus laevis

Previous studies have focused on the immunohistochemical detection of a nitric oxide (NO)-cyclic 3',5'-monophosphate (cGMP) pathway in the brain and pituitary of the aquatic toad Xenopus laevis. We here investigate the endogenous production and possible involvement of NO signaling in the regulation of melanotrope cell activity in the pituitary pars intermedia of this amphibian. Using immunohistochemical staining of cultured cells with a polyclonal antiserum against inducible NO synthase (iNOS), immunoreactivity was observed both in melanotropes and in stellate-shaped cells. Part of these stellate-shaped cells is characterized as folliculo-stellate cells by their capacity of beta-Ala-Lys-N(epsilon)-AMCA uptake. Using chemiluminescence detection we demonstrate the presence of NO and reaction products like nitrite (NO(-)(2)) or peroxynitrite (ONOO(-)) in the incubation medium of cultured melanotropes. Bacterial lipopolysaccharide (LPS) stimulates the generation of NO and reaction products, the effect of which was blocked by S-methyl-l-thiocitrulline hydrochloride, a potent general NOS inhibitor. With [(3)H]lysine incorporation and a superfusion technique, it is shown that peptide release from melanotropes is stimulated by administration of superoxide dismutase (SOD), which was added to the superfusion medium to prevent scavenging of NO by superoxide anions. Pretreating the cells with the general NOS inhibitor l-nitroarginine methyl ester for 48 h attenuated the SOD-induced stimulation, but did not affect the stimulation by sodium nitroprusside (SNP) or 3-morpholinylsydnoneimine chloride (SIN-1), whereas hemoglobin blocked the combined effect of SOD plus NO donors. The soluble guanylate cyclase inhibitor 1H-[1,2, 4]oxadiazolo[4,3a]-quinoxaline-1-one did not inhibit but even significantly potentiated the effect of NO donors on peptide release without affecting the SOD-induced stimulation of peptide release. In addition to the previously described neuronal NOS (nNOS) immunoreactivity in nerve fibers in the pars intermedia of Xenopus, the present data reveal iNOS and nNOS as potential sources of endogenous NO production in cultured cells of the pars intermedia. Our study shows that also in nonmammalian vertebrates endogenous NO production may be physiologically relevant under conditions where protection against oxidative damage is needed. The endocrine cells of the pars intermedia themselves, as well as the folliculo-stellate cells, under such conditions may dispose of a protective mechanism against oxidative stress. The sensitivity of the endogenous NO production to LPS suggests that NO may also play a role during systemic inflammation.

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

In recent years nitric oxide (NO) has emerged as an important intra- and intercellular transmitter involved in the control of hypothalamic-pituitary axis. In order to discriminate the potential actions of NO at hypothalamic or pituitary level in the control of PRL and LH release, we have studied PRL and LH secretion by dispersed pituitary cells obtained from males, cycling and lactating females in the presence of 1) sodium nitroprusside (SNP), a NO donor; 2) cyclic guanosine monophosphate (cGMP), the second messenger for a wide range of NO actions; 3) Nw-nitro-L-arginine methyl ester (NAME), a competitive inhibitor of NO synthase (NOS) and 4) oxadialoquinoxalione (OQD) and LY 83,583, antagonists of guanylyl cyclases. We found that SNP (at doses of 100 and 500 micromol) stimulated LH and FSH release and partially blocked the inhibitory action of dopamine (50 and 100 nmol) on prolactin secretion. These effects were not mimicked by cGMP and remained in the presence of OQD and LY 83,583. NAME alone had no significant effect on hormone secretion. These results suggest that NO plays a role in the control of gonadotropins and prolactin secretion acting directly at the pituitary level and that these effects are mediated by mechanisms other than changes in cGMP levels.

Direct evidence of gonadotropin-releasing hormone (GnRH)-stimulated nitric oxide production in the L beta T-2 clonal gonadotropes

An immortal cell line (L beta T-2) with characteristics of gonadotropes, such as LH-containing secretory granules, and LH release responsiveness to GnRH, was used to investigate the effect of GnRH stimulation on nitric oxide (NO) production. RT-PCR analysis showed that mouse nNOS mRNA was expressed in cultured L beta T-2 cells. L beta T-2 cells were treated with the calcium ionophore, A23187, and NO levels were measured as nitrite using the Griess assay. The data clearly showed that NO production was increased dose-dependently by A23187 treatment (0-10(-5) M). Next, changes in the intracellular concentration of ionized calcium ([Ca2+]i) in L beta T-2 cells induced by GnRH were analyzed by quantitative fluorescence microscopy, and [Ca2+]i was found to be increased markedly by GnRH treatment. In fact, exposure of L beta T-2 cells to increasing concentrations of GnRH from 0 to 10(-6) M was found to enhance NO production in a dose dependent manner, with maximal augmentation at 10(-6) M. However, the stimulation of NO production by GnRH at this concentration was significantly attenuated by pretreatment with NG-nitro-L-arginine methyl ester hydrochloride (L-NAME), a NO synthase inhibitor. Taken together, the present results suggest that GnRH treatment results in increased NO production in L beta T-2 clonal gonadotropes, and intracellular calcium augmentation produced by GnRH may be participate in this process. Our findings also indicate that the L beta T-2 cell line is a useful tool for in vitro studies of the autocrine and paracrine roles of NO in the anterior pituitary gland.

Intracerebroventricular administration of copper-zinc superoxide dismutase inhibits pulsatile luteinizing hormone secretion in ovariectomized ewes

Intracerebroventricular (i.c.v.) injection of an inhibitor of nitric oxide synthase (NOS) abolishes pulsatile luteinizing hormone (LH) secretion. It has been demonstrated that structural and functional analogs of copper-zinc superoxide dismutase (Cu,Zn-SOD) inhibit neuronal NOS. The present study examined the ability of Cu,Zn-SOD to affect pulsatile LH release in the ewe. Bovine Cu,Zn-SOD was administrated into the third cerebral ventricle of unanesthetized, freely moving, ovariectomized (OVX) ewes. Jugular blood samples were taken every 15 min for 5 h before and 8 h after i.c.v. injections. In a pilot trial using three OVX ewes, i.c.v. injection of Cu,Zn-SOD at a dose of 0.5, 1.0 or 2.0 microg in 100 microl saline decreased plasma LH levels and abolished LH pulses, without affecting FSH secretion. In the main experiment, i.c.v. injection of 100 microl saline had no effect on mean LH levels and LH pulse frequency, whereas i.c.v. injection of Cu,Zn-SOD at a dose of 1 microg/100 microl saline significantly (P < 0.01) decreased mean LH levels and LH pulse frequency. In conclusion, this study provides the first evidence for the role of Cn,Zn-SOD in the control of LH secretion at the level of the brain in female mammals.

Nitric oxide regulation of gonadotrophin secretion in prepubertal heifers1

Mechanisms responsible for the pulsatile release of gonadotrophin secretion in prepubertal heifers are not fully known. We have shown that an excitatory amino acid agonist, N-Methyl-D,L-aspartic acid (NMA), induces an immediate release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in prepubertal heifers. Nitric oxide (NO) has also emerged as an important regulator of LH release in rats. This study was designed to test the role of NO in the regulation of gonadotrophin release as well as the possible mediation by NO of the effects of NMA and gonadotrophin releasing hormone (GnRH) on gonadotrophin secretion in heifer calves. In experiment 1, four groups of five prepubertal heifers (33 weeks old) received one of the following treatments: (1); N-G-nitro-L-arginine methyl ester (L-NAME, a NO synthase inhibitor, 35 mg/kg, i.v., once); (2) NMA (4.7 mg/kg, i.v., once); (3) L-NAME+NMA (as above); and (4) Vehicle (saline, i.v.). All heifers in all groups were also challenged with a bolus injection of GnRH (10 ng/kg, i.v., once). Blood samples were collected every 15 min for 10 h. L-NAME was injected after the first blood sample, NMA after 2 h and GnRH after 6 h of blood sampling. Administration of L-NAME alone, suppressed the spontaneous pulses of LH (P<0.04). Heifers in the NMA group responded with a significantly greater LH release than did the heifers in the L-NAME+NMA group (P<0.05). Following the GnRH challenge, heifer calves treated with L-NAME or NMA had higher LH pulse responses than the controls (P<0.05). In a second experiment, four groups of five heifer calves (34 weeks old) were given one of the following treatments: (1) L-NAME (as above); (2) L-arginine, a NO precursor (ARG, 100 mg/kg/h, i.v. drip infused for 6 h starting 2 h after first blood sample was taken); (3) L-NAME+ARG (as above); and (4) Vehicle (saline i.v. bolus and drip for 6 h). Blood samples were taken every 10 min for 8 h. Administration of L-NAME suppressed the pulsatile release of LH and FSH (P<0.05). Compared to the control group, infusion of ARG by itself did not change the pattern of LH secretion (P>0.05); however, in heifers given L-NAME, ARG restored a normal pattern of LH pulses, similar to the control values (P>0.05). It was therefore concluded that NO is involved in the regulation of LH, and possibly FSH, secretion and that NO may mediate, at least in part, the stimulatory effects of NMA on LH, and to some extent FSH, release. The responses to GnRH led us to suggest that NO may have inhibitory effects on the pituitary and NMA may have increased pituitary sensitivity to GnRH.

The structure and function of folliculo-stellate cells in the anterior pituitary gland

The folliculo-stellate cells (FS cells) in the anterior pituitary gland are characterized by their star-like appearance and their ability to form follicles. Although FS cells do not produce any pituitary hormones, their special tendency to surrounding endocrine cells with their long cytoplasmic processes suggests that they regulate endocrine cells by intercellular communication. In spite of many morphological and cytophysiological studies recently performed, a precise understanding of the major functions of FS cells in the pituitary gland remains obscure. We review here the morphological characteristics of FS cells and their suspected functions in the anterior pituitary gland. It is well established that the FS cell produces many kinds of growth factors, i.e., fibroblast growth factor, vascular endothelial cell growth factor and interleukin 6. The biological significances of these growth factors in the anterior pituitary gland are also discussed in this paper. The origin and differentiation of FS cells, especially the possibility that the FS cell is a kind of stem cell which has the potential to differentiate into endocrine cells, is also presented.

Hormonal and cellular brain mechanisms regulating the amplexus of male and female water frog (Rana esculenta)

The role of nitric oxide (NO) synthase, prostaglandin E2-9-ketoreductase, and aromatase brain activities in regulating frog amplexus was assessed in the water frog (Rana esculenta). Plasma concentrations of testosterone were higher, and concentrations of 17beta-oestradiol lower, in amplexing males than in unamplexing males; while concentrations of testosterone and PGE2 were lower, and those of 17beta-oestradiol and PGF2alpha higher, in amplexing females compared to unamplexing females. Hormone release rescued from frog brains in vitro mirrored plasma hormone measures. Brain aromatase activity was lower in amplexing males; NO synthase was lower and PGE2-9-ketoreductase and aromatase were higher in amplexing females. In male brains, PGE2-9-ketoreductase inhibitor decreased PGF2alpha release and increased that of PGE2; aromatase inhibitor decreased 17beta-oestradiol and increased testosterone release. In female brains, NO donor and PGE2-9-ketoreductase inhibitor increased testosterone and PGE2 release and decreased that of 17beta-oestradiol and PGF2alpha; NO synthase inhibitor decreased testosterone release and PGE2 and increased 17beta-oestradiol and PGF2alpha release; PGF2alpha decreased testosterone release and increased 17beta-oestradiol release; aromatase inhibitor decreased 17beta-oestradiol release and increased testosterone release. In female brains, NO donor and PGE2-9-ketoreductase inhibitor decreased PGE2-9-ketoreductase and aromatase activities; PGF2alpha increased aromatase activity; NO synthase inhibitor increased PGE2-9-ketoreductase and aromatase activity. The data suggest that, in amplexing female brains, external and/or internal stimuli inhibit NO synthase, decreasing NO and activating PGE2-9-ketoreductase; in turn, PGF2alpha increases aromatase activity and 17beta-oestradiol release; while, in amplexing male brains, stimuli inhibit aromatase activity, thereby increasing testosterone production.

Abnormal estrous cyclicity after disruption of endothelial and inducible nitric oxide synthase in mice

The roles of nitric oxide (NO) and nitric oxide synthase (NOS) in reproduction were studied by examining the estrous cycle of wild-type (WT) mice, inducible NOS (iNOS)-, and endothelial NOS (eNOS)-knockout mice. We observed an average estrous cycle of 4.8 +/- 0.2 days in WT mice. While we observed no significant influence of iNOS deficiency on cycle length, eNOS-knockout females showed a significantly longer estrous cycle (6.6 +/- 0.6 days; p < 0.03) than WT females, due to an extension of diestrus (p < 0.03). There was no influence of iNOS deficiency on ovulation rate compared with that in WT females; however, eNOS-knockout mice showed a significant reduction (p < 0.05) in ovulatory efficiency relative to WT or iNOS-knockout females. In contrast to WT females, in which the highest level of estradiol (E2) was observed at 1500 h of proestrus, iNOS-knockout females reached a peak of E2 at 1830 h of proestrus. In eNOS-knockout females, the peak of E2 occurred at 1830 h, as in iNOS-knockout mice; however, E2 levels were 5-fold and 3-fold higher (p < 0.05) than levels observed in WT and iNOS-knockout females, respectively. There was no effect of genotype on the plasma LH concentrations at proestrus. On the first day of diestrus, eNOS-knockout females showed significantly higher plasma E2 and progesterone levels (p < 0.05) relative to WT and iNOS-knockout females. The dysfunction in cyclicity, ovulation rate, ovarian morphology, and steroidogenesis in eNOS-knockout female mice strongly supports the concept that eNOS/NO plays critical roles in ovulation and follicular development.

Inhalation of diesel engine exhaust affects spermatogenesis in growing male rats

We conducted experiments to determine whether diesel engine exhaust affects reproductive endocrine function in growing rats. The rats were assigned to three groups: a group exposed to total diesel engine exhaust containing 5.63 mg/m3 particulate matter, 4.10 ppm nitrogen dioxide, and 8.10 ppm nitrogen oxide; a group exposed to filtered exhaust without particulate matter; and a group exposed to clean air. Dosing experiments were performed for 3 months beginning at birth (6 hr/day for 5 days/week). Serum levels of testosterone and estradiol were significantly higher in animals exposed to total diesel exhaust and filtered exhaust (p < 0.05 for each group) as compared to the controls. Follicle-stimulating hormone was significantly decreased in the two groups exposed to diesel exhaust as compared to the control group (p < 0.05). Luteinizing hormone was significantly decreased in the total exhaust-exposed group as compared to the control and filtered groups (p < 0.05). Although testis weight did not show any significant difference among the groups, sperm production and activity of testicular hyaluronidase were significantly reduced in both exhaust-exposed groups as compared to the control group. Histological examination showed decreased numbers of step 18 and 19 spermatids in stage VI, VII, and VIII tubules in the testes of both diesel exhaust-exposed groups. This study suggests that diesel exhaust stimulates hormonal secretion of the adrenal cortex, depresses gonadotropin-releasing-hormone, and inhibits spermatogenesis in rats. Because these effects were not inhibited by filtration, the gaseous phase of the exhaust appears to be more responsible than particulate matter for disrupting the endocrine system.

Effects of L-arginine in rat adrenal cells: involvement of nitric oxide synthase

The effects of L-arginine on corticosterone production, cGMP, and nitrite levels were examined in zona fasciculata adrenal cells. L-Arginine significantly decreased both basal and ACTH-stimulated corticosterone production. This effect was still evident when steroidogenesis was induced by 8-bromo-cAMP and 22(R)-hydroxycholesterol, but not in the presence of exogenously added pregnenolone. L-Arginine increased cGMP and nitrite levels,; these effects were blocked by the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl-ester. Transport of L-[3H]arginine was rapid, saturable, and monophasic, with an apparent Km of 163+/-14 microM and a maximum velocity of 53+/-6 pmol/min x 10(5) cells. The basic amino acids L-lysine and L-ornithine, but not D-arginine or the nitric oxide synthase inhibitors N(G)-nitro-L-arginine methyl-ester and N(G)-nitro-L-arginine, impaired L-arginine uptake. Taken together, these results suggest that steroidogenesis in zona fasciculata adrenal cells may be negatively modulated by L-arginine-derived nitric oxide.

Thyrotropin-releasing hormone stimulates nitric oxide release from GH3 cells

Constitutive nitric oxide synthase (NOS) is expressed in the rat adenohypophysis but the mechanisms regulating its activity at the cellular level remain to be elucidated. The effect of TRH on nitric oxide release from GH3 cells was studied by means of reverse-phase HPLC to measure NO-2 and NO-3 concentrations in the incubation medium, and by polarography using electrodes specific for NO. Medium NO-2 concentrations in the incubation medium were dependent on the incubation time, and were further increased by sodium nitroprusside (SNP) or high potassium. NO-3 was detectable only in the presence of 100 microM SNP. Addition of L-arginine increased medium NO-2 concentrations. Diamino-hydroxypyrimidine decreased medium NO-2 concentrations, which were restored by the addition of (6R)-5, 6, 7, 8-tetrahydro-L-biopterin (THB). TRH elicited dose-related increases in medium NO-2 concentrations and in nitric oxide-specific currents, which were abolished by Nomega-nitro-L-arginine methyl ester. TRH failed to increase medium NO-2 concentrations in cells loaded with an intracellular Ca2+-chelating agent. The findings suggest that mobilization of intracellular Ca2+ by TRH stimulation activates Ca2+-dependent NOS in GH3 cells.

The role of nitric oxide in the control of basal and LHRH-stimulated LH secretion

The gaseous transmitter nitric oxide (NO) appears to be involved in the control of LH secretion and in the modulation of LH responses after stimulation with luteinizing hormone releasing hormone (LHRH), excitatory amino acids (EAAs) and leptin. The regulatory action of NO in the control of LH secretion includes modulation of LHRH release, changes in hypothalamic-pituitary blood flow and direct effects at pituitary level. To determine the net balance of these actions we evaluated (1) the effects of systemic administration of sodium nitroprusside (SNP, a NO donor) and Nw-nitro-L-arginine methyl ester (NAME, a blocker of NO synthase) on basal and LHRH-stimulated LH secretion in intact and ovariectomized females; and (2) the effects of SNP and NAME on LH secreted by dispersed pituitary cells. Finally, since NO is involved in the stimulatory effect of excitatory amino acids (EAAs) on LH secretion, we analyzed the effects of different inhibitors of NO synthase (NOS) in the LH response to kainic acid (KA), an agonist of kainate receptors, in male and female rats, neonatally injected with estradiol that show an increased sensitivity to EAAs. We found that NAME (40 and 60 mg/kg) increases LH secretion in intact and ovariectomized females, while SNP had no effect. The effect of NAME was not mediated through a direct action at pituitary level, since the basal and LHRH-stimulated LH release remained unchanged in presence of NAME. Similarly, basal and LHRH-stimulated LH secretion from dispersed pituitary cells were unaffected by NAME. Finally, the stimulatory effects of KA on LH release were not abolished by NOS inhibitors. In conclusion, our results provide evidence that the global action of NOS inhibitors is an increase in basal LH secretion, through a mechanism that remains to be fully characterized. In addition, our data demonstrate that the KA-stimulated LH secretion is not mediated by an increase in NO generation.

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

The objective of this study was to test the possible nitric oxide (NO) involvement in pituitary gonadotropin secretion in the male crested newt, Triturus carnifex. Pituitaries were incubated in vitro with medium alone, GnRH, NO donor (NOd, sodium nitroprusside), NO synthase inhibitor (NOSi, Nomega-nitro-L-arginine methyl ester), cGMP analogue (cGMPa, 8-bromo-cGMP), soluble guanylate cyclase inhibitor (sGCi, cystamine), GnRH plus NOSi, GnRH plus sGCi, and NOd plus sGCi during the annual reproductive cycle: pre-reproduction, reproduction (noncourtship and courtship), and the refractory, recovery, and estivation periods. To determine pituitary gonadotropin secretion indirectly, newt testes were superfused in vitro with preincubated pituitaries, and androgen release was determined. NO synthase (NOS) activity and cGMP levels were assessed in the preincubated pituitaries. Medium alone- and GnRH-preincubated pituitary increased androgen secretion during pre-reproduction, noncourtship, courtship, and recovery; the GnRH-induced increase was higher than the medium alone-induced increase during pre-reproduction, noncourtship, and recovery. NOd and cGMPa increased androgens in all reproductive phases considered except courtship; the NOd- and cGMP-induced increase was higher than the medium alone-induced increase during pre-reproduction, noncourtship, and recovery. NOS activity was highest during courtship and lowest during the refractory and estivation periods. GnRH increased NOS activity during pre-reproduction, noncourtship, and recovery. Cyclic GMP levels were highest during courtship and lowest during the refractory period and estivation. GnRH increased cGMP levels during pre-reproduction, noncourtship, and recovery, while NOd did so during all reproductive phases considered. These results suggest that basal and GnRH-induced gonadotropin secretion are up-regulated by NO in the pituitary gland of the male Triturus carnifex.

Neuronal-type NO synthase: transcript diversity and expressional regulation

Of the three established isoforms of NO synthase, the gene for the neuronal-type enzyme (NOS I) is by far the largest and most complicated one. The genomic locus of the human NOS I gene is located on chromosome 12 and distributed over a region greater than 200 kb. The nucleotide sequence corresponding to the major neuronal mRNA transcript is encoded by 29 exons. The full-length open reading frame codes for a protein of 1434 amino acids with a predicted molecular weight of 160.8 kDa. However, both in rodents and in humans, multiple, tissue-specific or developmentally regulated NOS I mRNA transcripts have been reported. They arise from the initiation by different transcriptional units containing alternative promoters (at least eight in the human gene), cassette exon deletions or insertions, and/or the usage of alternate polyadenylation signals. Depending on the insertions and deletions, translation results in functional or nonfunctional proteins. The use of alternative promoters can influence gene expression by various means. Indeed, NOS I is not a static, constitutively expressed enzyme, but subject to expressional regulation by various compounds and conditions. The molecular mechanisms underlying these regulations are currently being studied in several laboratories including our own.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Sodium nitroprusside inhibits the tyrosine hydroxylase activity of the median eminence in the rat

Nitric oxide (NO) has been involved in the modulation of various neuroendocrine responses. This work is a study of dose-response and time-course of the effect of intracerebroventricular (i.c.v.) administration of (NO) generator sodium nitroprusside (SNP), on tyrosine hydroxylase (TH) activity of the median eminence (ME) and serum prolactin levels, performed on conscious male rats. SNP (1, 5 and 10 microg) inhibited the TH activity of the ME, 15 min following injection in a dose-dependent way, although the effect was only significant with the highest dose, and also increased in a dose-dependent manner the serum prolactin levels. Both actions were transient but vanished at different times following injection of 10 microg of SNP. These results suggest that NO, released from SNP, inhibits the tuberoinfundibular dopaminergic neurons of the basal hypothalamus to stimulate prolactin secretion.

GnRH-dependent up-regulation of nitric oxide synthase I level in pituitary gonadotrophs mediates cGMP elevation during rat proestrus

We have recently provided evidence that the concentration and activity of the enzyme nitric oxide synthase type I (NOS I) was stimulated in gonadotrophs by GnRH, suggesting a role for the NOS I/NO pathway in the GnRH control of cell functions. To further establish the GnRH regulation of pituitary NOS I under physiological circumstances, we have examined the expression of NOS I during the 4-day estrus cycle in rats. Western blot analysis demonstrated that NOS I was present in the anterior pituitary at low levels during diestrus I (DI) and diestrus II, then was subject to a significant increase on the afternoon of proestrus to reach a maximal 3-fold increase between 19:00 and 20:00 h, after which NOS I decreased to return, during estrus, to levels within the range detected in diestrus. No such variation was apparent in the posterior pituitary lobe. NADPH-diaphorase histochemistry combined with immuno-identification of the cells revealed that active NOS I was expressed in both the gonadotrophs and the folliculo-stellate cells throughout the whole cycle but only the gonadotrophs showed an up-regulation during proestrus. A high temporal correlation was observed in the profiles of NOS I, pituitary cGMP and serum luteinizing hormone (LH) suggesting an implication of GnRH. Consistently, the administration of a potent GnRH antagonist to rats totally abolished the rise in pituitary NOS I and cGMP, in addition to suppressing, as expected, the surge in the secretion of LH. A role of NOS I as a mediator in the GnRH-induced augmentation in cGMP was further established in vitro by incubating anterior pituitaries in the presence of the NOS inhibitor, L-NMMA (1 mM). Altogether, these data demonstrate that the level and activity of NOS I is up-regulated in gonadotrophs during proestrus, in a manner consistent with a major implication of GnRH over a period during which its release from the hypothalamus, as well as gonadotroph responsiveness, are at maximum. This effect is accompanied by a NOS/NO-mediated rise in cGMP. In the absence of obvious effect on gonadotropin release, the roles of NO and cGMP in the regulation of gonadotroph functions, especially during proestrus, remain to be established.

Purification, characterisation and distribution of ovine neuronal nitric oxide synthase

Nitric oxide (NO) is an ubiquitous intercellular messenger molecule synthesised from the amino acid L-arginine by the enzyme nitric oxide synthase (NOS). A number of NOS iso-enzymes have been identified, varying in molecular size, tissue distribution and possible biological role. To further understand the role of NO in the regulation of neuroendocrine function in the sheep, we have purified and characterised ovine neuronal NOS (nNOS) using anion exchange, affinity and size-exclusion chromatography. SDS-PAGE reveals that ovine nNOS has an apparent denatured molecular weight of 150 kDa which correlates well with the other purified nNOS forms such as rat, bovine and porcine. The native molecular weight predicted by size-exclusion chromatography was 200 kD which is in close agreement with that found for porcine and rat nNOS. Internal amino acid sequences generated from tryptic digests of the purified ovine nNOS are highly homologous to rat nNOS. There was no significant difference in the cofactor dependence and kinetic characteristics of ovine nNOS when compared to rat and bovine nNOS, (K(m) for L-arginine 2.8, 2.0 and 2.3 microM respectively). A polyclonal anti-peptide antibody directed toward the C-terminal end of the rat nNOS sequence showed full cross-reactivity with the purified ovine nNOS. Immunohistochemical and Western analysis using this antiserum demonstrate the expression of nNOS in the cortex, cerebellum, hypothalamus and pituitary of the sheep. The lack of staining in the neural and anterior lobes of the pituitary seems to suggest that NOS plays a varied role in the control of endocrine systems between species.

Neuronal nitric oxide synthase gene expression in human pituitary tumours: a possible association with somatotroph adenomas and growth hormone-releasing hormone gene expression

OBJECTIVE

Nitric oxide (NO) has been implicated in the control of the secretory response to growth hormone-releasing hormone (GHRH) and may also modify GH release in response to excitatory aminoacids. Although rat and mouse pituitary cell lines have been shown to express neuronal NO synthase (nNOS), there has until now been no information on nNOS gene expression in human pituitary adenomas. Our objective was to provide such data and correlate the presence of nNOS transcripts with GHRH transcripts.

PATIENTS

Pituitary adenoma tissue was obtained from a random selection of 32 patients with somatotrophadenomas, 16 patients with corticotroph adenomas, 39 patients with endocrinologically inactive adenomas and nine patients with macroprolactinomas undergoing transsphenoidal hypophysectomy.

MEASUREMENTS

Transcripts for nNOS and GHRH were identified in frozen tissue sections by in situ hybridization histochemistry using synthetic 35S-labelled oligodeoxynucleotide probes with 100% homology to the target transcript.

RESULTS

Neuronal NOS transcripts were identified in one of 16 corticotroph adenomas (6%), one of nine macroprolactinomas (11%), six of 39 endocrinologically inactive adenomas (15%) and 13 of 32 somatotroph adenomas (41%). GHRH transcripts were found in a similar distribution to nNOS transcripts in 10 of the 13 nNOS-expressing somatotroph adenomas, and in three of the four remaining adenomas from which suitable tissue was available. Cross-hybridization of the nNOS and GHRH probes to the same target was excluded by including rat brain sections cut through the arcuate nucleus as hybridization controls. Furthermore, two different nNOS oligodeoxynucleotide probes complementary to different regions of the target transcript produced identical results.

CONCLUSIONS

These results suggest that there is a close correlation between nNOS gene expression and 'ectopic' expression of GHRH in human pituitary tumours, especially somatotroph adenomas. The relevance of these findings from a functional or pathologenic viewpoint remains unclear, but the data again emphasize that it is not just GH secretion that distinguishes somatotroph adenomas from other pituitary tumours.

Adenosine acts by A1 receptors to stimulate release of prolactin from anterior-pituitaries in vitro

Adenosine has been identified in the anterior pituitary gland and is secreted from cultured folliculostellate (FS) cells. To determine whether adenosine controls the secretion of anterior pituitary hormones in vitro, adenosine was incubated with anterior pituitaries. It stimulated prolactin (PRL) release at the lowest concentration used (10(-10) M); the stimulation peaked at 10(-8) M with a threefold increase in release and declined to minimal stimulation at 10(-4) and 10(-3) M. Follicle-stimulating hormone release was maximally inhibited at 10(-8) M, whereas luteinizing hormone release was not significantly inhibited. Two selective A1 adenosine receptor antagonists (10(-7) or 10(-5) M) had no effect on basal PRL release, but either antagonist completely blocked the response to the most effective concentration of adenosine (10(-8) M). In contrast, a highly specific A2 receptor antagonist (10(-7) or 10(-5) M) had no effect on basal PRL release or the stimulation of PRL release induced by adenosine (10(-8) M). We conclude that adenosine acts to stimulate PRL release in vitro by activating A1 receptors. Since the A1 receptors decrease intracellular-free calcium, this would decrease the activation of nitric oxide synthase in the FS cells, resulting in decreased release of nitric oxide (NO). NO inhibits PRL release by activating guanylate cyclase that synthesizes cGMP from GTP; cGMP concentrations increase in the lactotrophs leading to inhibition of PRL release. In the case of adenosine, NO release from the FS cells decreases, resulting in decreased concentrations of NO in the lactotrophs, consequent decreased cGMP formation, and resultant increased PRL release.

Nitric oxide mediates gonadotropin-releasing hormone effects on frog pituitary

We studied the possible role of nitric oxide (NO) in GnRH-induced gonadotropin secretion in the female water frog, Rana esculenta. During pre-reproduction, pre-ovulation, ovulation, post-ovulation, refractory, recovery and hibernation, pituitaries were incubated with medium-alone, GnRH, NO donor (NOd), NO synthase inhibitor (NOSi), cyclic GMP analogue (cGMPa), soluble guanylate cyclase inhibitor (sGCi), GnRH plus NOSi, GnRH plus sGCi, and NOd plus sGCi. Because antisera raised against gonadotropins are not available for this species, we measured these hormones indirectly through their effects on ovarian progesterone secretion. The ovaries were superfused with the pituitaries pre-incubated as reported above. In addition, NOS activity and cGMP levels were determined in the pre-incubated pituitaries. Those pre-incubated with medium-alone and with GnRH increased progesterone secretion during pre-reproduction, pre-ovulation, ovulation and recovery; the increase induced by GnRH was higher than that induced by medium-alone during pre-reproduction, pre-ovulation and recovery. NOd and cGMPa increased progesterone in all considered reproductive phases except ovulation; the increase induced by NOd and cGMP was higher than that induced by medium-alone during pre-reproduction, pre-ovulation and recovery. NOS activity was highest during ovulation and lowest during post-ovulation, refractory and hibernation. GnRH increased NOS activity during pre-reproduction, pre-ovulation and recovery. Cyclic GMP levels were highest during ovulation and lowest during post-ovulation, refractory and hibernation. GnRH increased cGMP levels during pre-reproduction, pre-ovulation and recovery, NOd during all considered reproductive phases. These results suggest that NO mediates basal and GnRH-induced gonadotropin secretion in female Rana esculenta.

Role of nitric oxide in the regulation of gonadotropin-releasing hormone and tyrosine hydroxylase gene expression in the male rat brain

It has been recently demonstrated that nitric oxide (NO), a free radical gas which may act as neurotransmitter in the brain, can stimulate the in vivo release of luteinizing hormone as well as the in vitro hypothalamic release of gonadotropin-releasing hormone (GnRH). In order to study the influence of NO on GnRH mRNA expression, two inhibitors of NO synthase (NOS) NG-monomethyl-l-arginine (NMMA) and HP-228, were microinjected into the left lateral ventricle of sham-operated and castrated male rats 4 h before sacrifice. Since the dopaminergic system can positively influence GnRH gene expression, we have also measured in the same animals tyrosine hydroxylase (TH) mRNA in tuberoinfundibular dopamine (TIDA) neurons. GnRH and TH mRNA levels were measured at the cellular level by quantitative in situ hybridization. The injection of HP-228 or NMMA induced a similar decrease (-19.5%) in GnRH mRNA. In castrated animals, the hybridization signal was 88% higher than that observed in sham-operated animals. Both HP-228 and NMMA produced in castrated animals a 39% decrease in GnRH mRNA. In contrast the injection of NOS inhibitors resulted in an increase in the amount of TH mRNA in TIDA neurons. The stimulating effect was more striking in HP-228-treated (+60%) than in NMMA-treated (+32%) animals. Castration did not induce any changes in the number of silver grains overlying TIDA neurons, while the administration of either HP-228 or NMMA induced a 43% increase in castrated animals. These results together with previous ones on GnRH release in vitro suggest that NO exerts a positive influence not only on the secretion but also on the biosynthesis of GnRH. Since NO appears to play a role in the negative regulation of dopamine, it is likely that the increase in GnRH mRNA expression is not mediated by the TIDA system.

Role of nitric oxide in the neuroendocrine responses to cytokines

During infection, bacterial and viral products, such as bacterial lipopolysaccharide (LPS), cause the release of cytokines from immune cells. These cytokines can reach the brain by several routes. Furthermore, cytokines, such as interleukin-1 (IL-1), are induced in neurons within the brain by systemic injection of LPS. These cytokines determine the pattern of hypothalamic-pituitary secretion which characterizes infection. IL-2, by stimulation of cholinergic neurons, activates neural nitric oxide synthase (nNOS). The nitric oxide (NO) released diffuses into corticotropin-releasing hormone (CRH)-secreting neurons and releases CRH. IL-2 also acts in the pituitary to stimulate adrenocorticotropic hormone (ACTH) secretion. On the other hand, IL-1 alpha blocks the NO-induced release of luteinizing hormone-releasing hormone (LHRH) from LHRH neurons, thereby blocking pulsatile LH but not follicle-stimulating hormone (FSH) release and also inhibiting sex behavior that is induced by LHRH. IL-1 alpha and granulocyte macrophage colony-stimulating factor (GMCSF) block the response of the LHRH terminals to NO. The mechanism of action of GMCSF to inhibit LHRH release is as follows. It acts on its receptors on gamma-aminobutyric acid (GABA)ergic neurons to stimulate GABA release. GABA acts on GABAa receptors on the LHRH neuronal terminal to block NOergic stimulation of LHRH release. This concept is supported by blockade of GMCSF-induced suppression of LHRH release from medial basal hypothalamic explants by the GABAa receptor blocker, bicuculline. IL-1 alpha inhibits growth hormone (GH) release by inhibiting GH-releasing hormone (GHRH) release, which is mediated by NO, and stimulating somatostatin release, also mediated by NO. IL-1 alpha-induced stimulation of prolactin release is also mediated by intrahypothalamic action of NO, which inhibits release of the prolactin-inhibiting hormone dopamine. The actions of NO are brought about by its combined activation of guanylate cyclase-liberating cyclic guanosine monophosphate (cGMP) and activation of cyclooxygenase and lipoxygenase with liberation of prostaglandin E2 and leukotrienes, respectively. Thus, NO plays a key role in inducing the changes in release of hypothalamic peptides induced in infection by cytokines. Cytokines, such as IL-1 beta, also act in the anterior pituitary gland, at least in part via induction of inducible NOS. The NO produced inhibits release of anterior pituitary hormones.

Evidence that gonadotropin-releasing hormone stimulates gene expression and levels of active nitric oxide synthase type I in pituitary gonadotrophs, a process altered by desensitization and, indirectly, by gonadal steroids

To determine the site and mechanism of action of gonadal steroids on pituitary nitric oxide synthase type I (NOS I), present in both gonadotrophs and folliculo-stellate cells, the effects of castration and steroids were examined in male rats, in the presence of a GnRH antagonist (Antarelix). Western analysis showed a rapid and substantial increase with time, after orchidectomy, of NOS I protein, the concentration doubling in 24 h and reaching a maximal 4- to 5-fold increase after 3-7 days, followed by a progressive decline after 2 weeks. Testosterone or estradiol replacement, or administration of GnRH antagonist, totally abolished the effects of castration, demonstrating a mediation of the steroid effects via GnRH. In noncastrated rats, steroids and the GnRH antagonist also caused a reduction in the levels of NOS I (by 50-60%), consistent with inhibition of endogenous GnRH stimulation. In marked contrast, administration of a potent GnRH agonist (Triptorelin) to intact rats increased the levels of NOS I. A time-course study with a long-lasting formulation showed that rise in NOS I developed rapidly after a lag of approximately 5 h, with a 2-fold increase detectable after 8 h and a maximal 4.5-fold after 48 h. The level declined afterwards in a manner consistent with homologous desensitization that may occur in the continuous presence of GnRH; however, the profile was different and delayed compared with those of gonadotropin release. As observed for NOS I protein, NOS I messenger RNA concentration was increased by castration or GnRH agonist and reduced by steroids or GnRH antagonist. Taken together, these data demonstrate that steroids indirectly regulate NOS I messenger RNA and protein levels, through the hypothalamic modulation of GnRH, which represents the primary regulator of NOS I. No effect of steroids on NOS I was seen in the posterior lobe. NADPH-diaphorase histochemistry coupled to immuno-identification of the cells revealed that the treatments affecting the concentration of NOS I concomitantly altered the activity but exclusively in gonadotrophs and not in folliculo-stellate cells (which do not respond to GnRH), reinforcing the idea that GnRH played a major regulatory role. Expression in gonadotrophs of a GnRH-dependent NOS I and the ensuing production of nitric oxide represents a potentially novel signaling pathway for the neuropeptide in the anterior pituitary, consistent with the previously reported GnRH-induced cGMP production, the role of which remains to be evaluated.

Perivascular localization of nitric oxide synthase in the rat adenohypophysis: potential implications for function and cell-cell interaction

The possible localization of nitric oxide (NO) synthase (NOS) in proximity to the microvasculature was examined in the rat adenohypophysis using immunohistochemistry and nicotinamide adenine dinucleotide phosphate diaphorase histochemistry. A population of NOS-positive cells was localized in very close contact with the sinusoidal capillaries. The pattern of this perivascular localization was either unicellular, bicellular or multicellular. These observations suggest that, at least, some actions of NO in the adenohypophysis can be accounted for by a local regulation of the glandular microvasculature.

The actions of NO in the central nervous system and in thymocytes

Nitrogen monoxide (NO) has been suggested to be involved in many physiological and pathological functions. In rat hippocampus, chemical NO donors stimulated noradrenaline release in the presence of thiols such as dithiothreitol and L-cysteine. S-Nitrosocysteine, which is proposed to be a stable and endogenous S-nitrosothiol molecule, stimulated noradrenaline release by itself. The effect of S-nitrosothiol on noradrenaline release was calmodulin-dependent and cyclic GMP-independent. S-Nitrosocysteine was incorporated into the slice via the L-type-like amino acid transporter. These findings suggest the physiological significance of S-nitrosocysteine on neurotransmitter release and propose the existence of a specific uptake system of S-nitrosothiols in neuronal tissues. In rat thymocytes, chemical NO donors inhibited DNA synthesis. Hydrocortisone treatment in vivo inhibited DNA synthesis via the expression of the inducible NO synthase protein, and the accumulation of NO and cyclic GMP. Although it is known that glucocorticoids regulate inducible NO synthase expression negatively in several types of cells in vitro, glucocorticoid treatment in vivo regulates the expression positively. In primary cultured rat glial cells, a combination of cytokines stimulated production of nitrite via expression of inducible NO synthase. In these cells, simultaneous addition of endothelin decreased inducible NO synthase expression induced by cytokines. On the other hand, pretreatment with endothelin for 24 hr enhanced the inducible NO synthase expression. Endothelin has two effects on inducible NO synthase expression, positive and negative, depending on treatment time. The actions of NO on the hippocampus and thymocytes and the regulation of inducible NO synthase expression in glial cells are discussed.

Castration differentially regulates nitric oxide synthase in the hypothalamus and pituitary

Mammalian reproductive function is under control of the integrated hypothalamic-pituitary-gonadal (HPG) axis. Castration in male rats has been utilized as an effective tool to investigate hormonal interactions in the mammalian HPG axis. Recently, nitric oxide (NO) has been suggested to play a role in HPG hormonal regulation. In order to gain further insight into the function of the NO-NOS system in reproductive neuroendocrine control, particularly in the gonadal feedback regulation of the hypothalamic-pituitary unit, we examined steady state levels of nNOS mRNA, nNOS protein, and the important physiological index, NOS enzyme activity, of the intrinsic NOergic system in both hypothalamus and pituitary in castrated male rats and their sham-operated counterparts one week after surgery. In the pituitary, we found a significant four-fold increase in nNOS mRNA, p < 0.0003 compared to sham. Castration also resulted in a four-fold rise in pituitary nNOS protein, p < 0.02 compared to sham. Pituitary NOS enzyme activity was stimulated 2 fold, p < 0.003 after castration. In the hypothalamus, conversely, we observed no significant castration-modulated difference in either nNOS mRNA, nNOS protein or NOS enzyme activity. Thus, it appears that the hypothalamic NO-NOS system is either not required for hypothalamic adaptations to castration, although important in the release of LHRH under normal physiological conditions, or alternatively, the hypothalamus may become more sensitive to the effects of NO in the castrated state. In the pituitary, NO may attenuate the gonadotropin response to castration as a local balancing mediator.

Percoll Density Gradient-Enriched Populations of Rat Pituitary Cells: Interleukin 6 Secretion, Proliferative Activity, and Nitric Oxide Synthase Expression

We used a discontinuous Percoll density gradient centrifugation to prepare enriched populations of prolactin (PRL), growth hormone (GH), and folliculo-stellate (FS) cells from rat anterior pituitaries in order to characterize these various cell populations. After cell dissociation and centrifugation, enriched PRL cells (55% of total cells as determined by immunocytochemistry [ICC] were present in Fraction 1 (Fr1) (density ([d]) = 1.059). Fr2 (d=1.071) had enriched S100-positive FS cells (31% of total cells), but enriched GH cell (60% of total cells) were present in Fr3 (d=1.094). Interleukin 6 (IL-6) was secreted mainly by enriched PRL cells in Fr1 (350 pg/mL/106 cells) and Fr2 (194 pg/mL/106 cells), and much less by the enriched GH cells inFr3 (16 pg/mL/106). Proliferation studies with combined 3H-thymidine and ICC for pituitary hormones showed that only the PRL cell had significant prolifereative activity. Immunostaining showed that immediately after separation, all three isoforms of nitric oxide synthase (NOS) were expressed anterior pituitary cells. After 3 d of culture, there was a marked increase in nuclear staining for neuronal NOS (nNOS) in all three fractions, whereas inducible NOS (iNOS) and endothelial NOS (eNOS) rexpression did not change significantly. These results indicate that: 1. Enriched populations of PRL, FS, and GH pituitary cells can be readily obtained with a rapid discontinuous percoll density separation procedure. 2. PRL cells from different fractions of the gradient show differenet proliferation rates and IL-6 secretion varied in different enriched cell populations. 3. Although all three isoforms of NOS were expressed in rat pituitary cells, nNOS is the prindipal isoform in anterior pituitary cells, and its expression was icreased after 3 d of culture of anterior pitutuitary cells.

Nitric oxide synthase and background adaptation in Xenopus laevis

Adaptation of the skin colour to the background light condition in the amphibian Xenopus laevis is achieved by migration of pigment granules in the skin melanophores, a process regulated by alpha-MSH secretion from melanotrope cells in the pituitary pars intermedia (PI). alpha-MSH secretion in turn, is regulated by various stimulatory and inhibitory messengers synthesized in brain nuclei, especially the hypothalamic suprachiasmatic and magnocellular nuclei and the locus coeruleus in the hindbrain. In the present study, the roles in background adaptation of nitric oxide (NO) and NO synthase (NOS) enzyme activity were evaluated. In situ, using both immunohistochemistry with anti-human brain NOS (bNOS) serum in paraffin-embedded material and using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry in cryo-sections, we showed NOS in neurons in the optic tectum and in the locus coeruleus. NADPH-d reactivity was also found in neurons in the lateral amygdala, the ventral hypothalamic nucleus and in fibers in the median eminence. Using a Western blot stained with an anti-human bNOS serum, we demonstrated a 150 kDa band in Xenopus hindbrain lysates, which is similar to the NOS protein present in the rat anterior pituitary, but which was not detectable in the lysates from both the neurointermediate and distal lobes in Xenopus. No differences in histochemical staining pattern or on Western blotting were observed between animals adapted to a black or a white background. Paraffin sections of the endocrine PI and pars distalis did not reveal bNOS-like immunoreactivity. NADPH-d reactivity was observed in the endothelia of this gland. However, using a new procedure of thin cryo-sections of pituitary neurointermediate lobes, we observed bNOS-immunoreactive fibers as well as cyclic 3',5' guanosine monophosphate (cGMP)-accumulating fibers in the PI. The PI may be regulated by NOergic neurons from higher brain centers. The possibility that NOergic neurons in the locus coeruleus are involved in the innervation of the PI needs further investigation. The latter neurons are probably not noradrenergic because double labeling studies show no co-localization of NADPH-d reactivity and tyrosine hydroxylase immunoreactivity in locus coeruleus neurons.

Expression and plasticity of NO synthase in the neuroendocrine system

The expression of the nitric oxide (NO)-synthase enzyme (NOS) was analyzed in several hypothalamic nuclei and in the pituitary gland using in situ hybridization and immunohistochemistry. The effects of physiological and experimental stimuli on the expression of NOS was also investigated. Moreover, the role of NO in the secretion of anterior pituitary hormone luteinizing hormone (LH) was studied using primary culture of pituitary cells. The findings indicate that the expression of neuronal NOS is hormonally regulated and suggest that NO plays a role in hormone secretion.

Modulatory effect of L-NAME, a specific nitric oxide synthase (NOS) inhibitor, on stress-induced changes in plasma adrenocorticotropic hormone (ACTH) and corticosterone levels in rats: physiological significance of stress-induced NOS activation in hypothalamic-pituitary-adrenal axis

We investigated whether NG-nitro-L-arginine methyl ester (L-NAME), a specific inhibitor of nitric oxide synthase (NOS), can modify the stress-induced adrenocorticotropic hormone (ACTH) and corticosterone responses, because we found that immobilization-induced stress increases NOS mRNA and protein levels and enzyme activity in the adrenal cortex. The physiological significance of these phenomena, however, remains unknown. Plasma ACTH and corticosterone levels were determined by radioimmunoassay (RIA) of systemic blood samples and NOS enzyme activity was measured as the rate of [3H]arginine conversion to [3H]citrulline in the presence of tissue homogenate of adrenal cortex separated from the adrenal gland. The NOS enzyme activity in the adrenal cortex of rats pre-injected with saline at 2 h after the 2-h immobilization was significantly higher (P < 0.01) than that in the non-stressed controls. Pre-injection of L-NAME (100 mg/kg, s.c.) almost completely abolished the activity. This dose of L-NAME maintained a significantly elevated plasma corticosterone level (P < 0.05, compared with basal level) even 2 h after the 2-h stress, whereas the plasma corticosterone level in rats pre-injected with saline returned to the basal level at the same time point. Plasma ACTH level in L-NAME-pre-treated rats was higher than that in those pre-treated with saline 2 h after the stress, but the difference was not significant. This dose of L-NAME did not influence plasma ACTH or corticosterone levels under resting conditions without stress. These findings suggest that the stress-induced increase in NO synthesis in the adrenal cortex can modify the stress-induced corticosterone response to facilitate the recovery from the elevated corticosterone secretion by stress in the adrenal cortex to the resting basal level.

New aspects of Leydig cell function

Previous studies indicated that the Leydig cells of the human testes show similarities to neuroendocrine cells. In this context, the local synthesis of two neuroactive signaling molecules, namely nitric oxide (NO) and C-type natriuretic peptide (CNP), both acting via the second messenger, cyclic guanosine monophosphate (cGMP), might be of physiological relevance. By immunoblotting, immunohistochemical analyses and affinity crosslinking experiments, respectively, the presence of soluble guanylate cyclase (sGC), the NO receptor, and of guanylate cyclase B (GC-B), representing the CNP receptor, was demonstrated in Leydig cells, seminiferous tubules and blood vessels of the human testis. Moreover, cGMP and its binding protein cGMP-dependent protein kinase type I (GK I) were found in these structures. The functional activity of the two receptors was proved by generation of cGMP in response to treatments with the NO donor, sodium nitroprusside (SNP), and with CNP, respectively. As indicated by immunohistochemical analyses and by treatments of cells with either SNP or CNP, human Leydig tumour cells and MA10 cells, representing a mouse Leydig tumour cell line, were found to be distinguished by a reduced expression of the receptors for NO and CNP. Furthermore, expression levels of the components of the two cGMP-generating systems were found to be widely unchanged in Leydig cells during different ontogenetic stages. Though cGMP has been shown to influence testosterone release, the constant developmental expression patterns of NO and CNP apparently independent of differences in androgen production, the down-regulation of their receptors in tumorous cells, and the presence of GK I, may point to additional autocrine functions of these factors and of cGMP in Leydig cells. Moreover, possible paracrine actions of NO and CNP may include relaxation of seminiferous tubules and blood vessels in order to modulate sperm transport and testicular blood flow, respectively. These findings suggest that Leydig cell-derived factors may exert activities different from or in addition to those involved in the regulation of testosterone production.