Ethanol inhibits luteinizing hormone-releasing hormone (LHRH) secretion by blocking the response of LHRH neuronal terminals to nitric oxide

The chronic alcohol consumption influences the gonadal axis in men: Results from a meta-analysis

BACKGROUND

Low testosterone concentrations affect 2-13% of adult males, with a direct association between reduction in testosterone (T) concentrations and cardiovascular events. Lifestyle habits have been linked to visceral fat accumulation and endocrine disorders like secondary hypogonadism. Alcohol intake has also been a topic of debate, with studies showing a detrimental effect on sperm production and underlying mechanisms. This meta-analysis aims to comprehensively evaluate the effect of alcohol consumption on T serum concentrations in adult men.

METHODS

The literature search included only controlled clinical trials comparing men who drink alcohol to men who do not, or who assumed placebo or nonalcoholic beverages. The primary outcome was the comparison of total testosterone serum concentrations between the study and control groups. The publications were examined for publication bias using Egger's test.

RESULTS

Twenty-one studies were included in the analysis for a total of 30 trials that examined the effects of alcohol consumption on testosterone level in 10,199 subjects. The meta-analysis showed that alcohol consumption overall is related to significant reduction in circulating concentrations of total testosterone (mean difference [MD] = -4.02; 95% CI -6.30, -1.73), free T (MD = -0.17; 95% CI -0.23, -0.12), sex hormone binding globulin (SHBG) (MD = -1.94; 95% CI -3.37, -0.48), an increase in estradiol (E2) (MD = 7.65; 95% CI 1.06, 14.23) and neutral effect on luteinizing hormone (LH) (MD = -0.15; 95% CI -0.36, 0.06), independently by age, body mass index (BMI), E2, and LH serum concentrations and alcohol intake. However, these results are evident only in healthy men exposed to chronic alcohol consumption and not in those with a recognized diagnosis of alcohol use disorder or after acute alcohol intake.

CONCLUSION

This study suggests how chronic alcohol consumption may inhibit the gonadal axis in healthy men, although the exact pathophysiological mechanisms connecting alcohol exposure and steroidogenesis are still not completely clarified.

Impact of Alcohol Consumption on Male Fertility Potential: A Narrative Review

Alcohol abuse disorder is a serious condition, implicating more than 15 million people aged 12 years and older in 2019 in the United States. Ethanol (or ethyl alcohol) is mainly oxidized in the liver, resulting in the synthesis of acetaldehyde and acetate, which are toxic and carcinogenic metabolites, as well as in the generation of a reductive cellular environment. Moreover, ethanol can interact with lipids, generating fatty acid ethyl esters and phosphatidylethanol, which interfere with physiological cellular pathways. This narrative review summarizes the impact of excessive alcohol consumption on male fertility by describing its metabolism and how ethanol consumption may induce cellular damage. Furthermore, the impact of alcohol consumption on hormonal regulation, semen quality, and genetic and epigenetic regulations is discussed based on evidence from animal and human studies, focusing on the consequences on the offspring. Finally, the limitations of the current evidence are discussed. Our review highlights the association between chronic alcohol consumption and poor semen quality, mainly due to the development of oxidative stress, as well as its genotoxic impact on hormonal regulation and DNA integrity, affecting the offspring’s health. New landscapes of investigation are proposed for the identification of molecular markers for alcohol-associated infertility, with a focus on advanced OMICS-based approaches applied to the analysis of semen samples.

Alterations in sperm-inherited noncoding RNAs associate with late-term fetal growth restriction induced by preconception paternal alcohol use

Using a mouse model, our group recently described an association between chronic paternal alcohol use prior to conception and deficits in offspring growth. Here, we sought to determine the impact of alcohol exposure on male reproductive physiology and the association of sperm-inherited noncoding RNAs with the transmission of the observed growth defects. Alcohol exposure did not appreciably alter male reproductive physiology or fertility. However, chronic alcohol use reproducibly induced late-term fetal growth restriction in the offspring, which correlated with a shift in the proportional ratio of transfer RNA-derived small RNAs to Piwi-interacting RNAs, as well as altered enrichment of microRNAs miR21, miR30, and miR142 in alcohol-exposed sperm. Although our dataset share similarities to prior works examining the impact of paternal stress on offspring phenotype, we were unable to identify any changes in plasma corticosterone, indicating alcohol may alter sperm-inherited noncoding RNAs through distinct mechanisms.

Murine sperm capacitation, oocyte penetration and decondensation following moderate alcohol intake

Male chronic alcohol abuse causes testicular failure and infertility. We analyzed the effects of moderate sub-chronic alcohol intake on sperm morphology, capacitation, fertilization and sperm head decondensation. CF-1 male mice were administered 15% ethanol in drinking water for 15 days; control mice received ethanol-free water. Similar patterns of tyrosine phosphorylation were observed in capacitated spermatozoa of control and treated males. Percentage of hyperactivation (H) and spontaneous (SAR) and progesterone-induced (IAR) acrosome reaction significantly decreased at 120 and 150 min of capacitation in treated males compared to controls (H: 14.1 ± 2.5 vs 23.7 ± 2.6, P < 0.05; SAR-T120 min: 17.9 ± 2.5 vs 32.9 ± 4.1, P < 0.01; IAR-150 min: 43.3 ± 3.5 vs 73.1 ± 1.1, P < 0.001, n = 6). During in vitro fertilization (2.5, 3.5 and 4.5 h post-insemination), there was an increased percentage of fertilized oocytes (with a decondensed sperm head and one or two pronuclei) in treated males (P < 0.001, n = 7). After 60 min of in vitro decondensation with glutathione plus heparin, the percentage of decondensed sperm heads was significantly higher in treated males than in controls (mean ± s.d.: 57.1 ± 5.6 vs 48.3 ± 4.5, P < 0.05, n = 5). The percentage of morphologically normal sperm heads was significantly decreased in treated males with respect to controls (P < 0.001, n = 9). These results show that short-term moderate alcohol consumption in outbred mice affect sperm morphology, hyperactivation, acrosomal exocytosis, and the dynamics of in vitro fertilization and in vitro sperm nuclear decondensation.

Effects of unilaterally microinjecting ethanol in the preoptic-anterior hypothalamic areas of rats on ovulation

BACKGROUND

Intragastric or intraperitoneal ethanol (EtOH) treatment inhibits reproductive functions in females and male rats. The area of the hypothalamus where these effects take place is unknown. As the participations of the preoptic-anterior hypothalamic area (POA-AHA) in regulating ovulation is asymmetric, this study aims to analyze the effects on 17β-estradiol(E2 ), progesterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) serum levels, the messenger ribonucleic acid (mRNA) expression of estrogen receptor alpha (ERα) and beta (ERβ), and ovulation resulting from unilaterally microinjecting water or an EtOH solution into either side of the POA-AHA.

METHODS

The treatment consisted of microinjecting a 8.6 μM EtOH solution into either side of the POA-AHA. The study was performed on groups of adult cyclic rats at 09.00 hours on diestrus-1, and sacrificed on diestrus-2 at 13.00, on proestrus at 09.00 or 17.00 or on estrus at 09.00 hours. Ovulation rates were assessed in rats sacrificed on estrus. Hormonal serum levels were measured using radioimmunoassay, and as a function of ERα and ERβ mRNA expression in each side of the POA-AHA by reverse transcriptase polymerase chain reaction.

RESULTS

EtOH treatment blocked ovulation and the preovulatory release of LH, and lowered E2 levels. Irrespective of the treated POA-AHA side, ERα mRNA expression was consistently lower in the left POA-AHA and higher on the right. EtOH treatment in the left POA-AHA decreased FSH serum levels and lowered ERβ mRNA expression. In turn, EtOH treatment on the right POA-AHA resulted in higher FSH levels and ERβ mRNA expression.

CONCLUSIONS

The present results show that EtOH blocks the preovulatory surge of LH on the POA-AHA. The effects of EtOH treatment of preovulatory FSH surge on the POA-AHA are asymmetric (stimulative on the right and inhibiting in the left). The effects of EtOH treatment on preovulatory LH and FSH surge could be explained by the inhibition of ERα and ERβ mRNA expression, respectively.

Embryo developmental disruption during organogenesis produced by CF-1 murine periconceptional alcohol consumption

The aim was to study the control females (CF)-1 mouse embryo differentiation, growth, morphology on embryonic E- and N-cadherin expression at midgestation after periconceptional moderate alcohol ingestion. Adult female mice were exposed to 10% ethanol in drinking water for 17 days previous to and up to day 10 of gestation (ethanol-exposed females, EF) and were compared with nonexposed CF. EF presented reduced quantities of E10 to E10.5 embryos, greater percentage of embryos at stages less than E7.5, reduced implantation site numbers/female, and increased resorptions compared with CF. EF-embryo growth was significantly affected as evidenced by reduced cephalic and body sizes of E10 and E10.5 embryos (scanning electron microscopy) and decreased protein content of E10.5 embryos vs. CF embryos. A significantly higher percentage of EF-E10-10.5 embryos presented abnormal neural tube (NT) closure vs. the percentage of CF. E10 embryos from EF presented elevated tissue disorganization, pyknosis and nuclear condensation in somites, mesenchymal and neuroepithelial tissue. Immunohistochemical E- and N-cadherin distribution patterns were similar in organic structures of E10 embryos between groups. However, western blot revealed that E- and N-cadherin expression levels were significantly increased in EF-derived embryos vs. controls. Perigestational ethanol consumption by CF-1 mice induced significant damage in the organogenic embryogenesis by producing delayed differentiation, growth deficiencies, and increasing the frequency of NT defects. Ethanol exposure may disrupt cell-cell adhesion leading to upregulation of E- and N-cadherin expression suggesting that deregulation of cell adhesion molecules could be involved in the disruption of embryo development at organogenesis in CF-1 mouse.

Adrenal gland responses to lipopolysaccharide after stress and ethanol administration in male rats

All forms of stress, including restraint stress (RS) and lipopolysaccharide (LPS) administration, activate the hypothalamic-pituitary-adrenal (HPA) axis. LPS binds to a recognition protein (CD14) and toll-like receptor 2/4 in different cells and tissues, including the adrenal gland, to induce the production of cytokines and cause upregulation of cyclooxygenase and nitric oxide synthase (NOS) enzymes. Acute ethanol exposure activates the HPA axis, but in some conditions prolonged administration can dampen this activation as well as decrease the inflammatory responses to LPS. Therefore, this study was designed to evaluate the adrenal response to a challenge dose of LPS (50 μg/kg) injected i.p., after submitting male rats to RS, twice a day (2 h each time) for 5 days and/or ethanol administration (3 g/kg) by gavage also for 5 days, twice daily. At the end of the experiment, plasma corticosterone concentrations and adrenal gland content of prostaglandin E (PGE) and NOS activity were measured as stress mediators. The results showed that repetitive ethanol administration attenuated the adrenal stress response to LPS challenge alone and after RS, by preventing the increase in plasma corticosterone concentrations and by decreasing the PGE content and NOS activity in the adrenal gland. Therefore, we conclude that moderate alcohol consumption could attenuate the effects of psychophysical stress and impair an inflammatory response.

Roles of nitric oxide, carbon monoxide, and hydrogen sulfide in the regulation of the hypothalamic-pituitary-adrenal axis

The importance of stress in modifying human behavior and lifestyle is no longer a matter of debate. Although mild stress enhances the immune response and prevents infections, prolonged stress seems to play pathogenic roles in depression and neurodegenerative disorders. The body has developed an adaptive stress response consisting of cardiovascular, metabolic, and psychological changes, which act in concert to eliminate stressors. One of the major components of this response is the hypothalamic-pituitary-adrenal axis, also known as the stress axis. Over the last 30 years, many studies have documented the integrated stress-axis regulation by neurotransmitters. They have also demonstrated that gaseous neuromodulators, such as NO, CO, and H(2)S, regulate the hypothalamic release of neuropeptides. The specific effects (stimulatory vs. inhibitory) of these gases on the stress axis varies, depending on the type of stress (neurogenic or immuno-inflammatory), its intensity (low or high), and the species studied (rodents or humans). This review examines the complex roles of NO, CO, and H(2)S in modulation of stress-axis activity, with particular emphasis on the regulatory effects they exert at the hypothalamic level.

Acute effect of manganese on hypothalamic luteinizing hormone releasing hormone secretion in adult male rats: involvement of specific neurotransmitter systems

Manganese chloride (MnCl2) is capable of stimulating luteinizing hormone releasing hormone (LHRH) secretion in adult male Sprague-Dawley rats through the activation of the hypothalamic nitric oxide/cyclic guanosine monophosphate (cGMP)/protein kinase G pathway. The present study aimed to determine the involvement of specific neurotransmitters involved in this action. Our results indicate that dopamine, but not glutamic acid and prostaglandins, mediates the MnCl2 stimulated secretion of LHRH from medial basal hypothalami in vitro, as well as increases the activity of nitric oxide synthase. Furthermore, a biphasic response was observed in that gamma aminobutyric acid (GABA) release was also increased, which acts to attenuate the MnCl2 action to stimulate LHRH secretion. Although it is clear that manganese (Mn+2) can acutely induce LHRH secretion in adult males, we suggest that the additional action of MnCl2 to release GABA, a LHRH inhibitor, may ultimately contribute to suppressed reproductive function observed in adult animals following exposure to high chromic levels of Mn+2.

Systemic Administration of Alcohol to Adult Rats Inhibits Leydig Cell Activity: Time Course of Effect and Role of Nitric Oxide

BACKGROUND

Alcohol has been shown to interfere with testosterone (T) release from Leydig cells. However, the mechanisms responsible for this phenomenon, which may include decreased activity of the luteinizing hormone-releasing hormone (LHRH)-LH axis, as well as a direct influence of the drug on the testes, are not fully understood. In this work, we investigated the influence of alcohol, administered intragastrically (i.g.) or delivered via vapors, on Leydig cell activity and T release. Leydig cell function was studied by measuring changes in the levels of the steroidogenic proteins steroidogenic acute regulatory (StAR), the peripheral-type benzodiazepine receptor (PBR), and the cytochrome P450 side-chain cleavage enzyme (P450scc). Testosterone release was studied under basal conditions or in response to human chorionic gonadotropin (hCG). Finally, to identify potential factors mediating the influence of alcohol, we measured the testicular variant of the neuronal nitric oxide (NO) synthase (NOS), TnNOS, in semipurified Leydig cells.

METHODS

Adult male Sprague-Dawley rats were either injected with alcohol i.g. once or exposed to alcohol vapors (4 h/d) for 1 or 5 days. Controls received the vehicle (i.g. model) or were kept in boxes through which no vapors were circulated. Following these treatments, one series of experiments was devoted to investigate Leydig cell responsiveness by measuring plasma T levels before or after the intravenous injection of hCG (1 U/kg). In another series of experiments, we used semipurified Leydig cell preparations to measure StAR, PBR, P450scc, and TnNOS by Western blot analysis.

RESULTS

In the i.g. model, the T response to hCG was blunted for 12 hours following alcohol injection, but showed a rebound at 48 hours. Levels of StAR protein and of PBR, but not of P450scc, were significantly decreased within 10 minutes of drug administration. While StAR then remained depressed for 24 hours, PBR values were variable over this time course. By 48 hours, StAR, PBR, and P450scc levels had increased above control values. Both StAR and PBR levels showed correlations with plasma T levels. In the alcohol vapor models, both regimens of the drug also significantly depressed StAR and PBR protein concentrations, blunted the T response to hCG, and did not alter P450scc. Finally, we observed that alcohol delivered i.g. or via vapors up-regulated TnNOS levels in Leydig cells, but that blockade of NO formation failed to restore a normal T response to hCG.

CONCLUSIONS

Collectively, these results suggest that (a) the ability of Leydig cells to release T does not show a simple correlation with changes in StAR, PBR, and P450scc levels; (b) the time course of the alcohol-induced changes were protein-specific; and (c) despite the ability of alcohol to stimulate TnNOS expression, NO does not appear to mediate the inhibitory influence of this drug on testicular steroidogenesis in the models that we studied.

Interactions of alcohol and nitric-oxide synthase in the brain

Nitric oxide (NO) is an important molecule associated with both physiological and pathological brain events. Three separate genes encode for nitric-oxide synthase (NOS), the rate-limiting enzyme in NO production, all of which are expressed within brain tissue. Effects of ethanol on NO production may be important to ethanol modification of brain function. Existing data indicate that alcohol exposure alters NOS expression and activity in the brain. Modulation of NOS is suggested to be involved in alcohol-induced behavioral modifications. Furthermore, alcohol-induced changes in NOS may alter immunocompetence, response to injury in the central nervous system, and may be involved in ethanol-mediated neurodegeneration and neurotoxicity. The extent and direction of change in NOS expression and activity depends on cell type and length of exposure. The mechanisms underlying these effects are only partially understood. Herein, the current understanding of the interactions of ethanol and NOS in the brain are discussed.

Role of nitric oxide in alcohol alteration of beta-endorphin release from hypothalamic cells in primary cultures

BACKGROUND

Nitric oxide (NO) mediates many pharmacological actions of ethanol. NO's role in regulating ethanol action on hypothalamic beta-endorphin (beta-EP) neurons is not established.

METHODS

In this study, we determined the role of NO in ethanol regulation of beta-EP release from primary cultures of rat fetal mediobasal hypothalamic cells. Real-time polymerase chain reaction was used for messenger RNA (mRNA) detection; radioimmunoassay was used for hormone measurements.

RESULTS

Acute ethanol treatment for 3 hr increased the release of beta-EP but reduced nitrite levels in the media of hypothalamic cells in primary cultures. In contrast, ethanol exposure for 48 hr reduced the release of beta-EP but increased the release of nitrite from these cells. Alcohol treatments altered the expression of neuronal NO synthase mRNA, but not inducible NO synthase mRNA, in a pattern similar to that of nitrite levels. Alcohol treatments blocked sodium nitroprusside-induced increases in the level of cellular cyclic guanidine monophosphate. The nonspecific NO blocker NG-nitro-l-arginine-methyl-esther, but not the inactive isomer N-nitro-d-arginine-methyl-esther (d-NAME), inhibited ethanol inhibitory actions on beta-EP release.

CONCLUSIONS

These results suggest that the cyclic guanidine monophosphate/NO pathway is involved in ethanol alteration of hypothalamic beta-EP release.

Chronic daily ethanol and withdrawal: 4. Long-term changes in plasma testosterone regulation, but no effect on GnRH gene expression or plasma LH concentrations

Although ethanol has been repeatedly demonstrated to inhibit the hypothalamo-pituitary-testes axis by multiple mechanisms, plasma testosterone levels can be normal in alcoholics who do not exhibit severely compromised liver function and even increased in some abstinent alcoholics, suggesting that adaptive changes to chronic alcohol abuse may alter these regulatory mechanisms. To address this variability, we have investigated the effects of chronic ethanol and withdrawal on rat testosterone regulation using a well-characterized liquid diet model that we have previously demonstrated to (1) provide daily oral ethanol consumption that produces behaviorally relevant plasma ethanol levels during the active (awake) stage of the photoperiod; (2) establish physical dependence on ethanol; and (3) produce not only hypothalamo-pituitary-adrenal axis, but also behavioral (anxiety-like behavior, response to novelty, sucrose preference) changes consistent with those of actively drinking and subsequently abstinent alcoholics. The results demonstrate that chronic daily episodes of ethanol consumption and withdrawal by male Sprague-Dawley rats decreased (p < 0.01) plasma testosterone levels late in the afternoon (by 70% relative to ad libitum-fed controls and 63% relative to pair-fed controls), but not in the morning. During gradual cessation of daily ethanol consumption, morning plasma testosterone levels increased, and this 90-115% (p < 0.05) increase was maintained for 3 d after complete cessation of ethanol consumption. Three weeks after cessation of ethanol consumption, plasma testosterone levels were again increased by approx 100% (p < 0.01). Plasma luteinizing hormone (LH) concentrations and anterior hypothalamus/preoptic area gonadotropin-releasing hormone (GnRH) mRNA levels were not altered at any of these time points. Thus, chronic daily ethanol consumption and daily withdrawal induced changes in circulating testosterone regulation that (a) were time of day dependent and (b) included adaptive changes persisting long after consumption of ethanol ceased. Accordingly, resolution of changes in testosterone regulation and their potential roles in alcohol abuse and relapse will require evaluating changes throughout the circadian cycle during, shortly after, and long after active alcohol abuse.

Effect of neurogenic stress and ethanol on nitric oxide synthase and cyclooxygenase activities in rat adrenals

Repeated restraint stress (RRS) in male rats activated the pituitary adrenal system, as indicated by increases in adrenal weight and plasma corticosterone concentration that were accompanied by a decrease in constitutive nitric oxide synthase (cNOS), but not inducible NOS (iNOS). iNOS activated cyclooxgenase, causing elevated prostaglandin E(2) (PGE(2)) and F(2 alpha) in the adrenals, but had no effect on lipoxygenase. Administration of ethanol (ETOH) was also associated with elevated adrenal weight and a slight increase in corticosterone coupled with a decrease in both cNOS and iNOS and PGs in the adrenal. When ETOH was administered together with RRS, a decrease in iNOS and PGE release was noted consequent to a reduction in iNOS. Thus, ETOH probably reduced RRS-induced adrenocorticotropic hormone release. Adrenals were incubated in vitro to further evaluate the role of NO in these processes. Results indicated that NO released by sodium nitroprusside increased corticosterone release presumably by activating guanylyl cyclase with production of cyclic guanosine monophosphate (cGMP), because although NO also increased PGE release, PGE(2) (10(-5)-10(-9) M) decreased corticosterone release, an effect that was highly significant at a concentration of 10(-7) M PGE(2). ETOH (100 mM) had no effect on corticosterone release and did not block the increase in corticosterone caused by NO; however, ETOH reduced PGE release into the medium and blocked PGE(2) release induced by NO. Consequently, NO activated corticosterone release not by PGs, but by activation of guanylyl cyclase and release of cGMP. PGs have a negative feedback to suppress corticosterone release.

Carbon monoxide as a novel central pyrogenic mediator

Carbon monoxide (CO) are produced by heme oxygenase (HO), and HO was detected in hypothalamus. However, the roles of CO produced in hypothalamus was not fully elucidated. So, we tested the effects of CO on body temperature because preoptic-anterior hypothalamus was known as the presumptive primary fever-producing site. CO-saturated aCSF (4 microl, i.c.v.) and hemin (10 microg, i.c.v.) elicited marked febrile response. Pretreatment with indomethacin completely inhibited CO- and hemin-induced fever. Zinc protoporphyrin-IX (10 microg, i.c.v.) or ODQ (50 microg, i.c.v.) partially reduced hemin-induced febrile response. Dibutyryl-cGMP (100 microg, i.c.v.) produced profound febrile response and this febrile response was attenuated by indomethacin. These results indicate that endogenous CO may have a role as a pyrogenic mediator in CNS and CO-mediated pyresis is dependent on prostaglandin production and partially on activation of soluble guanylate cyclase.

Control of gonadotropin secretion by follicle-stimulating hormone-releasing factor, luteinizing hormone-releasing hormone, and leptin

Fractionation of hypothalamic extracts on a Sephadex G-25 column separates follicle-stimulating hormone-releasing factor (FSHRF) from luteinizing hormone-releasing hormone (LHRH). The FSH-releasing peak contained immunoreactive lamprey gonadotropin-releasing hormone (lGnRH) by radioimmunoassay, and its activity was inactivated by an antiserum specific to lGnRH. The identity of lGnRH-III with FSHRF is supported by studies with over 40 GnRH analogs that revealed that this is the sole analog with preferential FSH-releasing activity. Selective activity appears to require amino acids 5-8 of lGnRH-III. Chicken GnRH-II has slight selective FSH-releasing activity. Using a specific lGnRH-III antiserum, a population of lGnRH-III neurons was visualized in the dorsal and ventral preoptic area with axons projecting to the median eminence in areas shown previously to control FSH secretion based on lesion and stimulation studies. Some lGnRH-III neurons contained only this peptide, others also contained LHRH, and still others contained only LHRH. The differential pulsatile release of FSH and LH and their differential secretion at different times of the estrous cycle may be caused by differential secretion of FSHRF and LHRH. Both FSH and LHRH act by nitric oxide (NO) that generates cyclic guanosine monophosphate. lGnRH-III has very low affinity to the LHRH receptor. Biotinylated lGnRH-III (10(-9) M) labels 80% of FSH gonadotropes and is not displaced by LHRH, providing evidence for the existence of an FSHRF receptor. Leptin has equal potency as LHRH to release gonadotropins by NO. lGnRH-III specifically releases FSH, not only in rats but also in cows.

The mechanism of action of cytokines to control the release of hypothalamic and pituitary hormones in infection

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 that 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. 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 PRL release is also mediated by intrahypothlamic action of NO, which inhibits release of the PRL-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 (COX) and lipoxygenase (LOX) 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 ACTH. The adipocyte hormone leptin, a member of the cytokine family, has largely opposite actions to those of the proinflammatory cytokines, stimulating the release of FSHRF and LHRH from the hypothalamus and FSH and LH from the pituitary directly by NO.

Median eminence nitric oxide signaling

It is becoming increasingly clear that nitric oxide (NO), an active free radical formed during the conversion of arginine to citrulline by the enzyme NO synthase (NOS), is a critical neurotransmitter and biological mediator of the neuroendocrine axis. Current evidence suggests that NO modulates the activity of both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal axis. Supporting this hypothesis is the finding that the highest expression of neuronal NOS in the brain is found within the hypothalamus in areas where the cell bodies of the neurons from the different neuroendocrine systems are located. In this regard, the influence of neuronal NO on the regulation of the neuroendocrine neural cell body activity has been well-documented whereas little is known about NO signaling that directly modulates neurohormonal release into the pituitary portal vessels from the neuroendocrine terminals within the median eminence, the common termination field of the adenohypophysiotropic systems. Studies in rat suggest that NO is an important factor controlling both gonadotropin-releasing hormone (GnRH) and corticotropin-releasing hormone (CRH) release at the median eminence. The recent use of amperometric NO detection from median eminence fragments coupled to the use of selective NOS inhibitors demonstrated that a major source of NO at the median eminence might be endothelial in origin rather than neuronal. The present article reviews the recent progress in identifying the origin and the role of the NO produced at the median eminence in the control of neurohormonal release. We also discuss the potential implications of the putative involvement of the median eminence endothelial cells in a neurovascular regulatory process for hypothalamic neurohormonal signaling.

Aproteic diet decreases hypothalamic catecholamine turnover in adult male rats

Previous reports indicate that malnutrition reduces reproductive functions. We have demonstrated that protein deprivation in the diet also causes reproductive dysfunction by reducing hypothalamic GnRH secretion. Noradrenaline and nitric oxide are modulators of GnRH secretion. Noradrenaline stimulates GnRH secretion and nitric oxide inhibits catecholamine release. This work studies the hypothalamic catecholaminergic and nitrergic neuron activity in Wistar adult male rats fed on an aproteic diet (AP) during 21 days; this treatment was started when rats were 70 days old. Our first experiment studied catecholamine turnover rate after inhibition of tyrosine hydroxylase activity by injecting (i.p.) 400 mg/kg alpha-methyl-p-tyrosine. Our second experiment studied in vitro hypothalamic nitric oxide synthase (NOS) activity in animals under the same diet. AP diet significantly decreased both noradrenaline (P<0.05) and dopamine (P<0.05) hypothalamic turnover rate. Noradrenaline turnover in cerebral cortex was not altered by the aproteic diet. However, hypothalamic NOS activity was not affected in animals fed on an AP diet. These results indicate that the lack of protein in the diet reduces catecholaminergic neuron activity in adult male rats by a NO-independent mechanism, thus suggesting that a decrease in noradrenergic activity may be involved in the reduction of GnRH secretion induced by an AP diet.

Inhibitory pathways and the inhibition of luteinizing hormone-releasing hormone release by alcohol

In this research we examined the mechanisms by which ethanol (EtOH) inhibits luteinizing hormone-releasing hormone (LHRH) release from incubated medial basal hypothalamic explants. EtOH (100 mM) stimulated the release of two inhibitory neurotransmitters: gamma-aminobutyric acid (GABA) and beta-endorphin. EtOH also inhibited NO production, indicative of a suppression of nitric oxide synthase (NOS) activity. This inhibition was reversed by naltroxone (10(-8) M), a micro-opioid receptor blocker, indicating that the inhibition of NOS by EtOH is mediated by beta-endorphin. EtOH also blocked N-methyl-d-aspartic acid-induced LHRH release, but the blockade could not be reversed by either the GABA receptor blocker, bicuculline (10(-5) M), naltroxone (10(-8) M), or both inhibitors added together. However, increasing the concentration of naltrexone (10(-6) M) but not bicuculline (10(-4) M) reversed the inhibition. When we lowered the concentration of EtOH (50 mM), the EtOH-induced blockade of LHRH release could be reversed by either bicuculline (10(-5) M), naltroxone (10(-8) M), or the combination of the two blockers. Therefore, GABA is partially responsible for the blockade of N-methyl-d-aspartic acid-induced LHRH release. The block by GABA was exerted by inhibiting the activation of cyclooxygenase by NO, because it was reversed by prostaglandin E(2), the product of activation of cyclooxygenase. Because the inhibition caused by the higher concentration of EtOH could not be reduced by bicuculline (10(-4) M) but was blocked by naltroxone (10(-6) M), the action of alcohol can be accounted for by stimulation of beta-endorphin neurons that inhibit LHRH release by inhibition of activation of NOS and stimulation of GABA release.

The role of nitric oxide in reproduction

Nitric oxide (NO) plays a crucial role in reproduction at every level in the organism. In the brain, it activates the release of luteinizing hormone-releasing hormone (LHRH). The axons of the LHRH neurons project to the mating centers in the brain stem and by afferent pathways evoke the lordosis reflex in female rats. In males, there is activation of NOergic terminals that release NO in the corpora cavernosa penis to induce erection by generation of cyclic guanosine monophosphate (cGMP). NO also activates the release of LHRH which reaches the pituitary and activates the release of gonadotropins by activating neural NO synthase (nNOS) in the pituitary gland. In the gonad, NO plays an important role in inducing ovulation and in causing luteolysis, whereas in the reproductive tract, it relaxes uterine muscle via cGMP and constricts it via prostaglandins (PG).

Ovarian hormone dependence of alpha(1)-adrenoceptor activation of the nitric oxide-cGMP pathway: relevance for hormonal facilitation of lordosis behavior

The ovarian hormones estradiol (E(2)) and progesterone (P) facilitate rat lordosis behavior in part by regulating the expression of and signal transduction by adrenoceptors in the hypothalamus (HYP) and preoptic area (POA). The major adrenoceptor subtype mediating E(2) and P facilitation of lordosis is the alpha(1)-adrenoceptor. In the present studies, we tested the hypotheses that (1) alpha(1)-adrenoceptors in the HYP enhance lordosis responses by activating the nitric oxide (NO)-cGMP signaling pathway, and (2) coupling of alpha(1)-adrenoceptors to this signal transduction pathway is hormone-dependent. Basal levels of cGMP were significantly higher in HYP and POA slices from animals treated with E(2) and P when compared with slices from ovariectomized controls or females treated with only E(2) or P. When slices of HYP and POA from ovariectomized female rats were incubated with norepinephrine or the selective alpha(1)-adrenoceptor agonist phenylephrine, cGMP accumulation was observed only if slices had been derived from females treated with both E(2) and P before experimentation. Moreover, alpha(1)-adrenoceptor stimulation of cGMP synthesis was blocked by an inhibitor of NO synthase, confirming that these receptors act by NO-mediated stimulation of soluble guanylyl cyclase. Behavioral studies demonstrated further that the cell-permeable cGMP analog 8-bromoadenosine-cGMP reverses the inhibitory effects of the alpha(1)-adrenoceptor antagonist prazosin on lordosis behavior in E(2)- and P-treated female rats. Thus, the NO-cGMP pathway mediates the facilitatory effects of alpha(1)-adrenoceptors on lordosis behavior in female rats, and previous exposure of the HYP and POA to both E(2) and P are required to link alpha(1)-adrenoceptors to this pathway.

Ovarian hormone dependence of alpha(1)-adrenoceptor activation of the nitric oxide-cGMP pathway: relevance for hormonal facilitation of lordosis behavior

The ovarian hormones estradiol (E(2)) and progesterone (P) facilitate rat lordosis behavior in part by regulating the expression of and signal transduction by adrenoceptors in the hypothalamus (HYP) and preoptic area (POA). The major adrenoceptor subtype mediating E(2) and P facilitation of lordosis is the alpha(1)-adrenoceptor. In the present studies, we tested the hypotheses that (1) alpha(1)-adrenoceptors in the HYP enhance lordosis responses by activating the nitric oxide (NO)-cGMP signaling pathway, and (2) coupling of alpha(1)-adrenoceptors to this signal transduction pathway is hormone-dependent. Basal levels of cGMP were significantly higher in HYP and POA slices from animals treated with E(2) and P when compared with slices from ovariectomized controls or females treated with only E(2) or P. When slices of HYP and POA from ovariectomized female rats were incubated with norepinephrine or the selective alpha(1)-adrenoceptor agonist phenylephrine, cGMP accumulation was observed only if slices had been derived from females treated with both E(2) and P before experimentation. Moreover, alpha(1)-adrenoceptor stimulation of cGMP synthesis was blocked by an inhibitor of NO synthase, confirming that these receptors act by NO-mediated stimulation of soluble guanylyl cyclase. Behavioral studies demonstrated further that the cell-permeable cGMP analog 8-bromoadenosine-cGMP reverses the inhibitory effects of the alpha(1)-adrenoceptor antagonist prazosin on lordosis behavior in E(2)- and P-treated female rats. Thus, the NO-cGMP pathway mediates the facilitatory effects of alpha(1)-adrenoceptors on lordosis behavior in female rats, and previous exposure of the HYP and POA to both E(2) and P are required to link alpha(1)-adrenoceptors to this pathway.

Effect of alcohol on the proestrous surge of luteinizing hormone (LH) and the activation of LH-releasing hormone (LHRH) neurons in the female rat

Reproduction is adversely affected by alcohol abuse in humans and laboratory animals. In rats, alcohol exposure suppresses both luteinizing hormone (LH) and sex steroid secretion, although consensus is lacking as to which level of the hypothalamic-pituitary-gonadal (HPG) axis is primarily affected. We tested the hypothesis that acute alcohol treatment inhibits the HPG axis by blunting release of LH-releasing hormone (LHRH) in female rats, by examining the effect of this drug on the central reproductive endocrine event; i.e., the proestrous surge of gonadotropins, which triggers ovulation. In a first series of experiments, we injected alcohol at 8 A.M. and 12 P.M. on proestrus and measured plasma levels of LH, estradiol (E2), and progesterone during the afternoons of proestrus and estrus. Alcohol administration blocked the proestrous surge of LH and ovulation. In subsequent experiments, alcohol inhibited the surge of LHRH (measured by push-pull cannulation) and LHRH neuronal activation (measured by Fos labeling in LHRH neurons). Because alcohol also decreased E2 levels, we reasoned that it might have prevented positive feedback; however, alcohol retained its ability to inhibit the LH surge evoked by E2 implantation in ovariectomized females, disproving this hypothesis. Additionally, alcohol does not act via increased corticosteroid secretion, because alcohol also blocked the proestrous surge in adrenalectomized females. Last, exogenous administration of LHRH to alcohol-blocked animals evoked LH secretion and ovulation, indicating that pituitary and/or ovarian function could be restored by mimicking the hypothalamic signal. Collectively, these data indicate that in female rats, alcohol inhibits the gonadotropin surge primarily by decreasing LHRH secretion.

beta-Endorphin blocks luteinizing hormone-releasing hormone release by inhibiting the nitricoxidergic pathway controlling its release

beta-Endorphin blocks release of luteinizing hormone (LH)-releasing hormone (LHRH) into the hypophyseal portal vessels by stimulating mu-opiate receptors, thereby inhibiting secretion of LH. LHRH release is controlled by release of nitric oxide from nitricoxidergic (NOergic) neurons in the basal tuberal hypothalamus. To determine whether beta-endorphin exerts its inhibitory action on this NOergic pathway, medial basal hypothalami (MBH) from male rats were incubated with beta-endorphin (10(-8) M). beta-Endorphin decreased basal secretion of LHRH, and significantly inhibited the release of prostaglandin E2 (PGE2), a known stimulant of LHRH release. Incubation of MBH with beta-endorphin at various concentrations (10(-9)-10(-6) M) in vitro decreased the activity of NO synthase (NOS) (measured by the conversion of [14C]arginine to labeled citrulline). Conversely, the activity of NOS was increased by the mu-receptor antagonist, naltrexone (10(-8) M). Not only was the inhibitory action of beta-endorphin on LHRH and PGE2 release blocked by naltrexone (10(-8) M), but it increased NOS activity and LHRH and PGE2 release. beta-Endorphin also stimulated gamma-aminobutyric acid (GABA) release. Because GABA inhibits both nitroprusside (NP-induced PGE2 and LHRH release by blocking the activation of cyclooxygenase by NO, this is another mechanism by which beta-endorphin inhibits NP-induced PGE2 and LHRH release. The results indicate that beta-endorphin stimulates mu-opioid receptors on NOergic neurons to inhibit the activation and consequent synthesis of NOS in the MBH. beta-Endorphin also blocks the action of NO on PGE2 release and, consequently, on LHRH release, by stimulating GABAergic inhibitory input to LHRH terminals that blocks NO-induced activation of cyclooxygenase and consequent PGE2 secretion.

Alcohol rapidly lowers plasma testosterone levels in the rat: evidence that a neural brain-gonadal pathway may be important for decreased testicular responsiveness to gonadotropin

Alcohol is reported to suppress testosterone (T) secretion in the adult male rat. Decreases in the circulating levels of luteinizing hormone (LH) and/or the activity of testicular steroidogenic enzymes have been proposed as putative mechanisms underlying this inhibitory effect. We have recently provided functional evidence for a neural pathway between the brain and the male gonads that plays an important role in the ability of brain proinflammatory cytokines to blunt testicular responsiveness to human chorionic gonadotropin (hCG). The present work was designed to test the hypothesis that a similar pathway might be implicated in the inhibitory influence of alcohol on T secretion. Alcohol, administered intraperitoneally or intragastrically, significantly prevented the T response to the gonadotropin. This effect was significant within 15 min of drug treatment. In the intragastric model (the only one used for this type of experiment), the effect of alcohol was not altered by prior blockade of LH release, which suggests that it is independent of changes in the activity of the pituitary gonadotrophs. The lowest effective dose of alcohol, delivered intraperitoneally, was 2.0 g/kg. The intracerebroventricular injection of the alpha- and beta-adrenergic receptor antagonists phentolamine and propranolol significantly reversed the inhibitory influence of alcohol when it was administered 15 min, but not 60 min, before hCG. Collectively, our results indicate that (1) alcohol induces a rapid and profound decrease in plasma T levels that is secondary to decreased testicular responsiveness to hCG; and (2) at least part of this acute inhibitory action of alcohol may depend on the activation of a neural, adrenergic-dependent pathway between the brain and the testes.

The nitric oxide hypothesis of aging

Nitric oxide (NO), generated by endothelial (e) NO synthase (NOS) and neuronal (n) NOS, plays a ubiquitous role in the body in controlling the function of almost every, if not every, organ system. Bacterial and viral products, such as bacterial lipopolysaccharide (LPS), induce inducible (i) NOS synthesis that produces massive amounts of NO toxic to the invading viruses and bacteria, but also host cells by inactivation of enzymes leading to cell death. The actions of all forms of NOS are mediated not only by the free radical oxidant properties of this soluble gas, but also by its activation of guanylate cyclase (GC), leading to the production of cyclic guanosine monophosphate (cGMP) that mediates many of its physiological actions. In addition, NO activates cyclooxygenase and lipoxygenase, leading to the production of physiologically relevant quantities of prostaglandin E2 (PGE2) and leukotrienes. In the case of iNOS, the massive release of NO, PGE2, and leukotrienes produces toxic effects. Systemic injection of LPS causes induction of interleukin (IL)-1 beta mRNA followed by IL-beta synthesis that induces iNOS mRNA with a latency of two and four hours, respectively, in the anterior pituitary and pineal glands, meninges, and choroid plexus, regions outside the blood-brain barrier, and shortly thereafter, in hypothalamic regions, such as the temperature-regulating centers, paraventricular nucleus containing releasing and inhibiting hormone neurons, and the arcuate nucleus, a region containing these neurons and axons bound for the median eminence. We are currently determining if LPS similarly activates cytokine and iNOS production in the cardiovascular system and the gonads. Our hypothesis is that recurrent infections over the life span play a significant role in producing aging changes in all systems outside the blood-brain barrier via release of toxic quantities of NO. NO may be a major factor in the development of coronary heart disease (CHD). Considerable evidence has accrued indicating a role for infections in the induction of CHD and, indeed, patients treated with a tetracycline derivative had 10 times less complications of CHD than their controls. Stress, inflammation, and infection have all been shown to cause induction of iNOS in rats, and it is likely that this triad of events is very important in progression of coronary arteriosclerosis leading to coronary occlusion. Aging of the anterior pituitary and pineal with resultant decreased secretion of pituitary hormones and the pineal hormone, melatonin, respectively, may be caused by NO. The induction of iNOS in the temperature-regulating centers by infections may cause the decreased febrile response in the aged by loss of thermosensitive neurons. iNOS induction in the paraventricular nucleus may cause the decreased nocturnal secretion of growth hormone (GH) and prolactin that occurs with age, and its induction in the arcuate nucleus may destroy luteinizing hormone-releasing hormone (LHRH) neurons, thereby leading to decreased release of gonadotropins. Recurrent infections may play a role in aging of other parts of the brain, because there are increased numbers of astrocytes expressing IL-1 beta throughout the brain in aged patients. IL-1 and products of NO activity accumulate around the plaques of Alzheimer's, and may play a role in the progression of the disease. Early onset Parkinsonism following flu encephalitis during World War I was possibly due to induction of iNOS in cells adjacent to substantia nigra dopaminergic neurons leading to death of these cells, which, coupled with ordinary aging fall out, led to Parkinsonism. The central nervous system (CNS) pathology in AIDS patients bears striking resemblance to aging changes, and may also be largely caused by the action of iNOS. Antioxidants, such as melatonin, vitamin C, and vitamin E, probably play an important acute and chronic role in reducing or eliminating the oxidant damage produced by NO.

Hypothalamic control of gonadotropin secretion by LHRH, FSHRF, NO, cytokines, and leptin

Gonadotropin secretion by the pituitary gland is under the control of luteinizing hormone-releasing hormone (LHRH) and the putative follicle stimulating hormone-releasing factor (FSHRF). Lamprey III LHRH is a potent FSHRF in the rat and seems to be resident in the FSH controlling area of the rat hypothalamus. It is an analog of mammalian LHRH and may be the long sought FSHRF. Gonadal steroids feedback at hypothalamic and pituitary levels to either inhibit or stimulate the release of LH and FSH, which is also affected by inhibin and activin secreted by the gonads. Important control is exercised by acetylcholine, norepinephrine (NE), dopamine, serotonin, melatonin, and glutamic acid (GA). Furthermore, LH and FSH also act at the hypothalamic level to alter secretion of gonadotropins. More recently, growth factors have been shown to have an important role. Many peptides act to inhibit or increase release of LH and the sign of their action is often reversed by estrogen. A number of cytokines act at the hypothalamic level to suppress acutely the release of LH but not FSH. NE, GA, and oxytocin stimulate LHRH release by activation of neural nitric oxide synthase (nNOS). The pathway is as follows: oxytocin and/or GA activate NE neurons in the medial basal hypothalamus (MBH) that activate NOergic neurons by alpha, (alpha 1) receptors. The NO released diffuses into LHRH terminals and induces LHRH release by activation of guanylate cyclase (GC) and cyclooxygenase. NO not only controls release of LHRH bound for the pituitary, but also that which induces mating by actions in the brain stem. An exciting recent development has been the discovery of the adipocyte hormone, leptin, a cytokine related to tumor necrosis factor (TNF) alpha. In the male rat, leptin exhibits a high potency to stimulate FSH and LH release from hemipituitaries incubated in vitro, and increases the release of LHRH from MBH explants. LHRH and leptin release LH by activation of NOS in the gonadotropes. The NO released activates GC that releases cyclic GMP, which induces LH release. Leptin induces LH release in conscious, ovariectomized estrogen-primed female rats, presumably by stimulating LHRH release. At the effective dose of estrogen to activate LH release, FSH release is inhibited. Leptin may play an important role in induction of puberty and control of LHRH release in the adult as well.

Alcohol and nitric oxide production by cells of the brain

The L-arginine-nitric oxide pathway is important to both physiological and pathologic brain events. Brain tissue contains cells able to express all known isoforms of nitric oxide synthase, the rate-limiting enzyme in nitric oxide (NO) production and release. Effects of ethanol on NO production may be important to ethanol modification of brain function. Recent studies support this idea and demonstrate diverse interactions. For example, acute ethanol treatment decreases NMDA- and cytokine-stimulated NO synthesis by cortical neurons and glia, respectively, but enhances cytokine-stimulated NO synthesis in blood-brain barrier endothelial cells and does not affect norepinephrine-stimulated NO synthesis in medial basal hypothalamus. Furthermore, chronic ethanol enhances NMDA-stimulated NO synthesis in cortical neurons, but more potently decreases cytokine-induced NO synthesis in glial cells. The mechanisms underlying these effects are partially understood and include changes in NOS-2 gene expression. These observations illustrate that ethanol selectively affects NO production by brain cells, which may relate to reported behavioral interactions, but the extend and direction of change depends on cell type and length of exposure.

Role of nitric oxide and alcohol on gonadotropin release in vitro and in vivo

Alcohol suppresses reproduction in humans, monkeys, and small rodents by suppressing release of luteinizing hormone (LH). The major action is on the hypothalamus to decrease release of LH-releasing hormone (LHRH). The release of LHRH is controlled by nitric oxide (NO) as determined by in vivo and in vitro experiments. The hypothesized pathway is via norepinephrine (NE)-induced release of NO from NOergic neurons, which activates LHRH release. We have evaluated details of this process in male rats by incubating medial basal hypothalamic (MBH) explants in vitro and examining the release of NO and metabolites generated by NO that control LHRH release. NE increased release of NO as measured by determining the content of the enzyme at the end of the experiment (30 min) by adding [14C]arginine to the homogenate and measuring its conversion to [14C]citrulline since this is formed in equimolar quantities with NO by NO synthase (NOS). Because this increase in content, presumably caused by activation of the enzyme by NE, was blocked by the alpha 1 receptor blocker prazosin, it appears that alpha 1 receptors activate NOS by increasing intracellular free calcium in the NOergic neurons, which combines with calmodulin to activate NOS. The release of LHRH induced by nitroprusside (NP), a donor of NO, is accompanied by an increase in cyclic guanosine monophosphate (cGMP) in the medium supporting the activation of guanylate cyclase by NO. This activation is important in releasing LHRH since addition of 8-monobutyryl cGMP also released the peptide. Ethanol had no effect on the content of NOS or on the increase in content induced by NE, indicating that it did not act on NOS. Earlier experiments indicated that prostaglandin E2 (PGE2) was important in releasing LHRH. PGE2 is produced by activation of cyclooxygenase by NO since this occurred following addition of the NO donor, NP. Not only does NP increase PGE2 release, but it also increases the conversion of [14C]arachidonic acid to its metabolites, particularly PGE2, by activating cyclooxygenase. NP also activated lipoxygenase as indicated by increased release of leukotrienes, which also stimulate LHRH release. Ethanol acts at this step, because it completely blocked the release of PGE2, leukotrienes, and LHRH induced by NP. Therefore, the results support the theory that NE acts to stimulate NO release from NOergic neurons. This NO diffuses to the LHRH terminals, where it activates guanylate cyclase, leading to an increase in cGMP. At the same time, it also activates cyclooxygenase and lipoxygenase. The increase in cGMP increases intracellular free calcium, required for activation of phospholipase A2. Phospholipase A2 converts membrane phospholipids into arachidonic acid, the substrate for conversion by the activated cyclooxygenase and lipoxygenase to PGE2 and leukotrienes that activate the release of LHRH. Because alcohol inhibits conversion of labeled arachidonic acid to PGE2 and leukotrienes, it must act either directly to inhibit cyclooxygenase and lipoxygenase or by some other mechanism which, in turn, inhibits the enzyme. We initially believed that the action of alcohol was exerted directly on the LHRH terminals; however, our recent experiments indicate that alcohol suppresses LHRH release, at least in part, by stimulating beta-endorphinergic neurons that inhibit the release of NE, which drives the NOergic release of LHRH.

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.

A potential role of cyclic GMP in the regulation of lordosis behavior of female rats

Nitric oxide (NO) has been suggested to play a crucial role in the regulation of lordosis behavior via stimulation of guanylyl cyclase to synthesize cyclic GMP. Whalen and Lauber (1986, Neurosci. Biobehav. Rev. 10, 47-53) hypothesized that hormones and pharmacological agents known to facilitate lordosis in estrogen-primed rodents act through cyclic GMP. The compound 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) has been shown to selectively inhibit NO-stimulated cyclic GMP production. In the present study, we investigated the effects of ODQ on lordosis behavior. Female rats were implanted with a guide cannula aimed at the lateral or third ventricles by stereotaxic surgery, and their ovaries were bilaterally removed. Five days later, animals were injected subcutaneously with 2 microg estradiol benzoate at 48 and 24 hr, and 200 microg progesterone 4 hr before behavioral testing. ODQ or vehicle (1 microl) was administered at the time of progesterone treatment or 20 min before lordosis testing. ODQ significantly decreased lordosis quotients and the quality of lordosis at both intervals of drug infusion. Locomotor activities, measured by line crossing and rearing, were not affected by ODQ. ODQ also inhibited cyclic GMP accumulation in response to NMDA stimulation in hypothalamic and cerebellar slices in vitro. We conclude that cyclic GMP produced by NO generation is an important modulator of female rat sexual behavior.

Molecular cloning of cyclic nucleotide-gated cation channel subunits from rat pineal gland

We have investigated the subunit composition and the molecular structure of the CNG channel expressed in rat pineal gland. Three types of subunits have been cloned: an alpha-subunit (CNG1), two beta-subunit splice variants (rCNG4.1 and rCNG4.2) and a hydrophilic glutamic acid-rich protein (rGARP). In situ hybridization with sections of rat brain revealed the co-expression of CNG1, CNG4 and GARP in pinealocytes. In addition, CNG4-specific transcripts were detected in the arcuate, periventricular and supraoptic nuclei of the hypothalamus.

Interaction between NO and oxytocin: influence on LHRH release

Nitric oxide synthase (NOS)-containing neurons have been localized in various parts of the CNS. These neurons occur in the hypothalamus, mostly in the paraventricular and supraoptic nuclei and their axons project to the neural lobe of the pituitary gland. We have found that nitric oxide (NO) controls luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus acting as a signal transducer in norepinephrine (NE)-induced LHRH release. LHRH not only releases LH from the pituitary but also induces sexual behavior. On the other hand, it is known that oxytocin also stimulates mating behavior and there is some evidence that oxytocin can increase NE release. Therefore, it occurred to us that oxytocin may also stimulate LHRH release via NE and NO. To test this hypothesis, we incubated medial basal hypothalamic (MBH) explants from adult male rats in vitro. Following a preincubation period of 30 min, MBH fragments were incubated in Krebs-Ringer bicarbonate buffer in the presence of various concentrations of oxytocin. Oxytocin released LHRH at concentrations ranging from 0.1 nM to 1 microM with a maximal stimulatory effect (P < 0.001) at 0.1 microM, but with no stimulatory effect at 10 microM. That these effects were mediated by NO was shown by the fact that incubation of the tissues with NG-monomethyl-L-arginine (NMMA), a competitive inhibitor of NOS, blocked the stimulatory effects. Furthermore, the release of LHRH by oxytocin was also blocked by prazocin, an alpha 1-adrenergic receptor antagonist, indicating that NE mediated this effect. Oxytocin at the same concentrations also increased the activity of NOS (P < 0.01) as measured by the conversion of [14C]arginine to citrulline, which is produced in equimolar amounts with NO by the action of NOS. The release of LHRH induced by oxytocin was also accompanied by a significant (P < 0.02) increase in the release of prostaglandin E2 (PGE2), a mediator of LHRH release that is released by NO. On the other hand, incubation of neural lobes with various concentrations of sodium nitroprusside (NP) (300 or 600 microM), a releaser of NO, revealed that NO acts to suppress (P < 0.01) the release of oxytocin. Therefore, our results indicate that oxytocin releases LHRH by stimulating NOS via NE, resulting in an increased release of NO, which increases PGE2 release that in turn induces LHRH release. Furthermore, the released NO can act back on oxytocinergic terminals to suppress the release of oxytocin in an ultrashort-loop negative feedback.

Effect of alcohol on the proestrous surge of luteinizing hormone (LH) and the activation of LH-releasing hormone (LHRH) neurons in the female rat

Reproduction is adversely affected by alcohol abuse in humans and laboratory animals. In rats, alcohol exposure suppresses both luteinizing hormone (LH) and sex steroid secretion, although consensus is lacking as to which level of the hypothalamic-pituitary-gonadal (HPG) axis is primarily affected. We tested the hypothesis that acute alcohol treatment inhibits the HPG axis by blunting release of LH-releasing hormone (LHRH) in female rats, by examining the effect of this drug on the central reproductive endocrine event; i.e., the proestrous surge of gonadotropins, which triggers ovulation. In a first series of experiments, we injected alcohol at 8 A.M. and 12 P.M. on proestrus and measured plasma levels of LH, estradiol (E2), and progesterone during the afternoons of proestrus and estrus. Alcohol administration blocked the proestrous surge of LH and ovulation. In subsequent experiments, alcohol inhibited the surge of LHRH (measured by push-pull cannulation) and LHRH neuronal activation (measured by Fos labeling in LHRH neurons). Because alcohol also decreased E2 levels, we reasoned that it might have prevented positive feedback; however, alcohol retained its ability to inhibit the LH surge evoked by E2 implantation in ovariectomized females, disproving this hypothesis. Additionally, alcohol does not act via increased corticosteroid secretion, because alcohol also blocked the proestrous surge in adrenalectomized females. Last, exogenous administration of LHRH to alcohol-blocked animals evoked LH secretion and ovulation, indicating that pituitary and/or ovarian function could be restored by mimicking the hypothalamic signal. Collectively, these data indicate that in female rats, alcohol inhibits the gonadotropin surge primarily by decreasing LHRH secretion.

Oxytocin stimulates the release of luteinizing hormone-releasing hormone from medial basal hypothalamic explants by releasing nitric oxide

Oxytocin induces mating behavior in rats of both sexes. Previous experiments revealed that progesterone-induced sex behavior in ovariectomized, estrogen-primed rats was caused by release of NO from NOergic neurons that stimulated the release of luteinizing hormone-releasing hormone (LHRH). The LHRH activated brain-stem neurons that initiated the lordosis reflex. We hypothesized that oxytocin might similarly release NO in the medial basal hypothalamic region that would stimulate release of LHRH into the hypophyseal portal vessels to release luteinizing hormone. To investigate this hypothesis, medial basal hypothalamic explants were preincubated in Krebs-Ringer bicarbonate buffer for 30 min, followed by a 30-min incubation in fresh Krebs-Ringer bicarbonate buffer containing the compounds to be tested. Oxytocin stimulated LHRH release 3- to 4-fold at the lowest concentration tested (10(-10) M). Values remained at a plateau as the concentration was increased to 10(-7) M and then declined in a concentration-dependent manner, so that there was no stimulation with a concentration of 10(-5) M. Oxytocin (10(-7) M) stimulated release of prostaglandin E2 into the medium, a finding consistent with a role of NO in the response. That NO indeed mediated the action of oxytocin was supported by blockade of the action of oxytocin by the competitive inhibitor of NO synthase (NOS), N(G)-monomethyl-L-arginine (300 microM). Furthermore, oxytocin (10(-9) to 10(-7) M) activated NOS as measured at the end of the experiments. Oxytocin appeared to act to stimulate norepinephrine terminals in the medial basal hypothalamus, which activated NOS by alpha1-adrenergic receptors, because prazocine, an alpha1 receptor blocker, inhibited the LHRH-releasing action of oxytocin. Finally, incubation of neural lobe explants with sodium nitroprusside, a NO releasor, revealed that nitroprusside (300-600 microM, but not 900 microM) inhibited oxytocin release. Therefore, the NO released by oxytocin also diffuses into the oxytocin neuronal endings and inhibits oxytocin release, forming a negative feedback loop. The results indicate that oxytocin is important not only in induction of mating, but also in stimulating LHRH release with subsequent luteinizing hormone discharge that plays a crucial role in reproduction.

Dependence of intracellular signaling and neurosecretion on phospholipase D activation in immortalized gonadotropin-releasing hormone neurons

The excitability of gonadotropin-releasing hormone (GnRH) neurons is essential for episodic neuropeptide release, but the mechanism by which electrical activity controls GnRH secretion is not well characterized. The role of phospholipase D (PLD) in mediating the activity-dependent secretory pathway was investigated in immortalized GT1 neurons, which both secrete GnRH and express GnRH receptors. Activation of these Ca2+-mobilizing receptors was associated with transient hyperpolarization of GT1 cells, followed by sustained firing of action potentials. This was accompanied by an increase in PLD activity, as indicated by elevated phosphatidylethanol (PEt) production. GnRH-induced PEt production was reduced by inhibition of phospholipase C-dependent phosphoinositide hydrolysis by U73122 and neomycin, suggesting that signaling from phospholipase C led to activation of PLD. The intermediate role of protein kinase C (PKC) in this process was indicated by the ability of phorbol 12-myristate 13-acetate to induce time- and dose-dependent increases in PEt and diacylglycerol, but not inositol trisphosphate, and by reduction of GnRH-induced PEt accumulation in PKC-depleted cells. Consistent with the role of action potential-driven Ca2+ entry in this process, agonist-induced PLD activity was also reduced by nifedipine and low extracellular Ca2+. Inhibition of the PLD pathway by ethanol and propranolol reduced diacylglycerol production and caused a concomitant fall in GnRH release. These data indicate that voltage-gated Ca2+ entry and PKC act in an independent but cooperative manner to regulate PLD activity, which contributes to the secretory response in GT1 cells. Thus, the electrical activity of the GnRH-secreting neuron participates in the functional coupling between GnRH receptors and PLD pathway.

Role of leptin in hypothalamic-pituitary function

A defect in the structure of the obese gene is responsible for development of obesity in the ob/ob mouse. The product of expression of the gene is the protein hormone leptin. Leptin causes weight loss in ob/ob and normal mice, it is secreted by adipocytes, and it is an important controller of the size of fat stores by inhibiting appetite. The ob/ob mouse is infertile and has a pattern of gonadotropin secretion similar to that of prepubertal animals. Consequently, we hypothesized that leptin might play a role in the control of gonadotropin secretion and initiated studies on its possible acute effects on hypothalamic-pituitary function. After a preincubation period, hemi-anterior pituitaries of adult male rats were incubated with leptin for 3 hr. Leptin produced a dose-related increase in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release, which reached peaks with 10(-9) and 10(-11) M leptin, respectively. Gonadotropin release decreased at higher concentrations of leptin to values indistinguishable from that of control pituitaries. On the other hand, prolactin secretion was greatly increased in a dose-related manner but only with leptin concentrations (10(-7)-10(-5) M). Incubation with leptin of median eminence-arcuate nuclear explants from the same animals produced significant increases in LH-releasing hormone (LHRH) release only at the lowest concentrations tested (10(-12)-10(-10) M). As the leptin concentration was increased, LHRH release decreased and was significantly less than control release at the highest concentration tested (10(-6) M). To determine if leptin can also release gonadotropins in vivo, ovariectomized females bearing implanted third ventricle cannulae were injected with 10 microg of estradiol benzoate s.c., followed 72 hr later by microinjection into the third ventricle of leptin (0.6 nmol in 5 microl) or an equal volume of diluent. There was a highly significant increase in plasma LH, which peaked 10-50 min after injection of leptin. Leptin had no effect on plasma FSH concentrations, and the diluent had no effect on either plasma FSH or LH. Thus, leptin at very low concentrations stimulated LHRH release from hypothalamic explants and FSH and LH release from anterior pituitaries of adult male rats in vitro and released LH, but not FSH, in vivo. The results indicate that leptin plays an important role in controlling gonadotropin secretion by stimulatory hypothalamic and pituitary actions.

Nitric oxide synthase-cyclo-oxygenase pathways in organum vasculosum laminae terminalis: possible role in pyrogenic fever in rabbits

1. Fever was induced in rabbits by administration of Escherichia coli endotoxin (lipopolysaccharide; LPS; 0.001-10 micrograms) into the organum vasculosum laminae terminalis (OVLT). Deep body temperature was evaluated over a period of 7 h. 2. The LPS-induced febrile response was mimicked by intra-OVLT injection of the nitric oxide (NO) donors, S-nitroso-acetylpenicillamine (SNAP, 1-10 micrograms), sodium nitroprusside (SNP, 50 micrograms), or hydroxylamine (10 micrograms), the cyclic GMP analogue 8-bromo-cyclic GMP (8-Br-cyclic GMP, 10-100 micrograms), or prostaglandin E2 (PGE2, 0.2 micrograms). 3. Dexamethasone (Dex, a potent inhibitor of the transcription of inducible NO synthase, iNOS, 10 micrograms), anisomycin (a protein synthesis inhibitor, 100 micrograms), L-N5-(1-iminoethyl)ornithine (L-NIO; an irreversible NOS inhibitor, 10-200 micrograms), aminoguanidine (a specific iNOS inhibitor, 1000 micrograms), or NG-methyl-L-arginine acetate (L-NMMA, a NOS inhibitor, 100 micrograms) inhibited fever induced by LPS when injected into the OVLT 1 h before LPS injection. An intra-OVLT dose of 1000 micrograms of NG-nitro-L-arginine methyl ester (L-NAME, a potent inhibitor of constitutive NOS) did not exhibit antipyretic effects. 4. Methylene blue (an inhibitor of NOS and soluble guanylate cyclase, 1-10 micrograms), 6-(phenylamino)-5,8-quinolinedione (LY-83583; an inhibitor of soluble guanylate cyclase and NO release, 20 micrograms), or indomethacin (an inhibitor of cyclo-oxygenase, COX, 400 micrograms) inhibited fever induced by LPS when injected into the OVLT 1 h before LPS injection. Pretreatment with methylene blue or haemoglobin (a NO scavenger, 100 micrograms) attenuated the fever induced by intra-OVLT injection of SNAP. 5. The PGE2-induced fever was potentiated, rather then attenuated, by pretreatment with an intra-OVLT dose of animoguanidine (1000 micrograms), L-NMMA (100 micrograms) or L-NIO (200 micrograms). 6. These results suggest that iNOS-COX pathways in the OVLT represent an important mechanism for modulation of pyrogenic fever in rabbits.