Evidence for involvement of nitric oxide in the regulation of hypothalamic portal blood flow

Drinking Water Nitrate and Human Health: An Updated Review

Nitrate levels in our water resources have increased in many areas of the world largely due to applications of inorganic fertilizer and animal manure in agricultural areas. The regulatory limit for nitrate in public drinking water supplies was set to protect against infant methemoglobinemia, but other health effects were not considered. Risk of specific cancers and birth defects may be increased when nitrate is ingested under conditions that increase formation of N-nitroso compounds. We previously reviewed epidemiologic studies before 2005 of nitrate intake from drinking water and cancer, adverse reproductive outcomes and other health effects. Since that review, more than 30 epidemiologic studies have evaluated drinking water nitrate and these outcomes. The most common endpoints studied were colorectal cancer, bladder, and breast cancer (three studies each), and thyroid disease (four studies). Considering all studies, the strongest evidence for a relationship between drinking water nitrate ingestion and adverse health outcomes (besides methemoglobinemia) is for colorectal cancer, thyroid disease, and neural tube defects. Many studies observed increased risk with ingestion of water nitrate levels that were below regulatory limits. Future studies of these and other health outcomes should include improved exposure assessment and accurate characterization of individual factors that affect endogenous nitrosation.

Nitrate and nitrite in the diet: how to assess their benefit and risk for human health

Nitrate is a natural constituent of the human diet and an approved food additive. It can be partially converted to nitrogen monoxide, which induces vasodilation and thereby decreases blood pressure. This effect is associated with a reduced risk regarding cardiovascular disease, myocardial infarction, and stroke. Moreover, dietary nitrate has been associated with beneficial effects in patients with gastric ulcer, renal failure, or metabolic syndrome. Recent studies indicate that such beneficial health effects due to dietary nitrate may be achievable at intake levels resulting from the daily consumption of nitrate-rich vegetables. N-nitroso compounds are endogenously formed in humans. However, their relevance for human health has not been adequately explored up to now. Nitrate and nitrite are per se not carcinogenic, but under conditions that result in endogenous nitrosation, it cannot be excluded that ingested nitrate and nitrite may lead to an increased cancer risk and may probably be carcinogenic to humans. In this review, the known beneficial and detrimental health effects related to dietary nitrate/nitrite intake are described and the identified gaps in knowledge as well as the research needs required to perform a reliable benefit/risk assessment in terms of long-term human health consequences due to dietary nitrate/nitrite intake are presented.

Functional importance of blood flow dynamics and partial oxygen pressure in the anterior pituitary

The pulsatile release of hormone is obligatory for the control of a range of important body homeostatic functions. To generate these pulses, endocrine organs have developed finely regulated mechanisms to modulate blood flow both to meet the metabolic demand associated with intense endocrine cell activity and to ensure the temporally precise uptake of secreted hormone into the bloodstream. With a particular focus on the pituitary gland as a model system, we review here the importance of the interplay between blood flow regulation and oxygen tensions in the functioning of endocrine systems, and the known regulatory signals involved in the modification of flow patterns under both normal physiological and pathological conditions.

Nitric oxide as key mediator of neuron-to-neuron and endothelia-to-glia communication involved in the neuroendocrine control of reproduction

Nitric oxide (NO) is a peculiar chemical transmitter that freely diffuses through aqueous and lipid environments and plays a role in major aspects of brain function. Within the hypothalamus, NO exerts critical effects upon the gonadotropin-releasing hormone (GnRH) network to maintain fertility. Here, we review recent evidence that NO regulates major aspects of the GnRH neuron physiology. Far more active than once thought, NO powerfully controls GnRH neuronal activity, GnRH release and structural plasticity at the neurohemal junction. In the preoptic region, neuronal nitric oxide synthase (nNOS) activity is tightly regulated by estrogens and is found to be maximal at the proestrus stage. Natural fluctuations of estrogens control both the differential coupling of this Ca²+-activated enzyme to glutamate N-methyl-D-aspartic acid receptor channels and phosphorylation-mediated nNOS activation. Furthermore, NO endogenously produced by neurons expressing nNOS acutely and directly suppresses spontaneous firing in GnRH neurons, which suggests that neuronal NO may serve as a synchronizing switch within the preoptic region. At the median eminence, NO is spontaneously released from an endothelial source and follows a pulsatile and cyclic pattern of secretion. Importantly, GnRH release appears to be causally related to endothelial NO release. NO is also highly involved in mediating the dialogue set in motion between vascular endothelial cells and tanycytes that control the direct access of GnRH neurons to the pituitary portal blood during the estrous cycle. Altogether, these data raise the intriguing possibility that the neuroendocrine brain uses NO to coordinate both GnRH neuronal activity and GnRH release at key stages of reproductive physiology.

Nitric oxide signaling as a common target of organohalogens and other neuroendocrine disruptors

Organohalogen compounds such as polychlorinated biphenyls (PCB) and polybrominated diphenyl ethers (PBDE) are global environmental pollutants and highly persistent, bioaccumulative chemicals that produce adverse effects in humans and wildlife. Because of the widespread use of these organohalogens in household items and consumer products, indoor contamination is a significant source of human exposure, especially for children. One significant concern with regard to health effects associated with exposure to organohalogens is endocrine disruption. Toxicological studies on organohalogen pollutants primarily focused on sex steroid and thyroid hormone actions, and findings have largely shaped the way one envisions their disruptive effects occurring. Organohalogens exert additional effects on other systems including other complex endocrine systems that may be disregulated at various levels of organization. Over the last 20 years evidence has mounted in favor of a critical role of nitric oxide (NO) in numerous functions ranging from neuroendocrine functions to learning and memory. With its participation in multiple systems and action at several levels of integration, NO signaling has a pervasive influence on nervous and endocrine functions. Like blockers of NO synthesis, PCBs and PBDEs produce multifaceted effects on physiological systems. Based on this unique set of converging information it is proposed that organohalogen actions occur, in part, by hijacking processes associated with this ubiquitous bioactive molecule. The current review examines the emerging evidence for NO involvement in selected organohalogen actions and includes recent progress from our laboratory that adds to our current understanding of the actions of organohalogens within hypothalamic neuroendocrine circuits. The thyroid, vasopressin, and reproductive systems as well as processes associated with long-term potentiation were selected as sample targets of organohalogens that rely on regulation by NO. Information is provided about other toxicants with demonstrated interference of NO signaling. Our focus on the convergence between NO system and organohalogen toxicity offers a novel approach to understanding endocrine and neuroendocrine disruption that is particularly problematic for developing organisms. This new working model is proposed as a way to encourage future study in elucidating common mechanisms of action that are selected with a better operational understanding of the systems affected.

Distribution of nitric oxide synthase immunoreactivity in the mouse brain

Nitric oxide (NO) is a gaseous intercellular messenger with a wide range of neural functions. NO is synthesized by activation of different isoforms of nitric oxide synthases (NOS). At present NOS immunoreactivity has been described in mouse brain in restricted and definite areas and no detailed mapping studies have yet been reported for NOS immunoreactivity. We have studied the distribution of neuronal NOS-containing neurons in the brain of three months male mice, using a specific commercial polyclonal antibody against the neuronal isoform of nitric oxide synthase (nNOS). Neuronal cell bodies exhibiting nNOS immunoreactivity were found in several distinct nuclei throughout the brain. The neurons that were positively stained exhibited different intensities of reaction. In some brain areas (i.e., cortex, striatum, tegmental nuclei) neurons were intensely stained in a Golgi-like fashion. In other regions, immunoreactive cells are moderately stained (i.e., magnocellular nucleus of the posterior commissure, amygdaloid nucleus, interpeduncular nucleus, lateral periaqueductal gray) or weakly stained (i.e., vascular organ of the lamina terminalis, hippocampus, inferior colliculus, reticular nucleus). In the mouse, the NO-producing system appears well developed and widely diffused. In particular, nNOS immunoreactive neurons seem chiefly present in several sensory pathways like all the nuclei of the olfactory system, as well as in many regions of the lymbic system. These data suggest a widespread role for the NO system in the mouse nervous system.

Nitric oxide synthase activity in human pituitary adenomas

OBJECTIVES

The purpose of the present study was to examine human pituitary adenomas for nitric oxide synthase (NOS) activity by immunohistochemical and enzymatic methods.

MATERIALS AND METHODS

Adenomatous tissue from 16 patients were obtained during operation and stained immunohistochemically for hormone production and for the three NOS isoenzymes. Cell types that expressed NOS immunoreactivity (IR) were identified, and the NOS isoform was noted. NOS activity was measured enzymatically by the conversion of L-arginine to L-citrulline in tissue samples.

RESULTS

Endothelial cells of pituitary adenomas showed increase of eNOS IR compared with control tissue. The nNOS and iNOS IR were the same in adenomas and controls. There was no correlation between NOS IR and NOS activity measured enzymatically and the endocrine activity of the tumour or other clinical variables.

CONCLUSION

The observation of increased eNOS IR in endothelial cells of adenomas may suggest that NO plays a role in the regulation of blood flow in pituitary adenomas.

The roles of carbon monoxide and nitric oxide in the control of the neuroendocrine stress response: complementary or redundant

There is widespread evidence in favour of nitric oxide (NO) acting as a gaseous neurotransmitter in the central nervous system, diffusing from its cells of origin and affecting surrounding neuronal tissue in evanescent three-dimensional waves. This is also true of the hypothalamus, where amongst other activities NO inhibits stimulation of corticotrophin-releasing hormone (CRH) and vasopressin release by inflammatory stressors, effects thought to be mediated by binding with soluble guanylate cyclase (sGC). Carbon monoxide is being increasingly recognised as another gaseous neuromodulator, but with principal effects on other hemoproteins such as cyclo-oxygenase, and a distinctly different profile of localisation.NO is predominantly a pro-inflammatory agent in the periphery while CO is often anti-inflammatory. In the hypothalamus, the actions of CO are also distinct from those of NO,with marked antagonistic effects on the inflammatory release of vasopressin, both in vitro and in vivo, but with little involvement in the regulation of CRH. Thus, it would appear that these apparently similar gases exert quite distinct and separate effects, although they cause broadly similar overall changes in the secretion of neuroendocrine stress hormones. We conclude that these two gases may play significant but different roles in the control of the neuroendocrine stress response, but one common feature may be attenuation of inflammation-induced release of stress hormones.

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.

Nitric oxide and the cerebral vascular function

The presence of a cholinergic vasodilator innervation to cerebral circulation is well established. Despite its high endogenous concentration in cerebral blood vessels, acetylcholine (ACh) is not the transmitter for vasodilation. This finding has led to the discovery that nitric oxide (NO), which is coreleased with ACh and neural peptides such as vasoactive intestinal polypeptide (VIP) from the respective cholinergic-nitrergic (nitric oxidergic) nerves and the VIPergic-nitrergic nerves, is the primary transmitter in relaxing smooth muscle. ACh and VIP act presynaptically to inhibit and facilitate, respectively, the release of NO. Release of NO from cerebral vascular endothelial cells is also well established. A similar system for recycling L-citrulline to L-arginine for synthesizing more NO has been demonstrated in both cerebral perivascular nerves and endothelial cells. Neuronal and endothelial NO appears to play an important role in controlling cerebral vascular tone and circulation in health and disease.

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.

Influence of carbon monoxide, and its interaction with nitric oxide, on the adrenocorticotropin hormone response of the normal rat to a physico-emotional stress

We determined whether the gas carbon monoxide (CO) altered the adrenocorticotropin hormone (ACTH) response to mild inescapable electrofootshocks, and whether it interacted with nitric oxide (NO). Peripheral injection of the NO synthase (NOS) inhibitor Nwnitro-L-arginine-methylester (L-NAME), a compound which readily crosses the blood-brain barrier, produced the expected blunting of the ACTH response to the shocks. This effect was mimicked by other arginine analogues such as L-nitroarginine (L-NNA) and NG-methyl-L-arginine (NMMA). The subcutaneous (s.c.) administration of the heme oxygenase (HO) blockers tin mesoporphyrin (SnMP) or tin protoporphyrin (SnPP) significantly decreased brain HO levels, indicating that both compounds had penetrated the brain. Blood pressure showed a modest increase in response to SnMP, and no change after SnPP. SnMP and SnPP both decreased shock-induced ACTH release, though the magnitude of this effect was slightly less than that of L-NAME. The influence of SnPP was further augmented in rats with concomitant blockade of NO formation, which suggests that both NO and CO are necessary for the full response of this axis to electrofootshocks. Finally, the ability of SnPP to significantly blunt the expression of the mRNA for the immediate early gene NGFI-B in the paraventricular nucleus (PVN) of rats exposed to shocks, indicates that the influence of CO was exerted on hypothalamic neuronal activity. Collectively, our results show that NO and CO exert a stimulatory effect on the HPA axis response to mild electrofootshocks, and that at least part of this influence takes place on hypothalamic neurons and/or their afferents.

An immunohistochemical study of temperature-related changes in galanin and nitric oxide synthase immunoreactivity in the hypothalamus of the toad

Galanin (GAL) and nitric oxide synthase (NOS) have been implicated in the control of thermogenesis in mammals. An experimental protocol was designed to determine whether or not the expression of these molecules in the hypothalamus of the toad could be related to environmental temperature changes. Exposure of the animals to low temperature increased the number and intensity of NOS-positive neurons in the magnocellular hypothalamic region, in contrast to a weak immunoreactivity observed in control animals kept in a natural environment at a spring-summer temperature (23-27 degrees C). Also a significantly higher number of GAL-immunoreactive (-IR) cells was observed in the preoptic area as compared to that observed in controls, while no difference in the intensity of GAL immunostaining intensity was detected. These results show a temperature-related expression of GAL and NOS in the hypothalamus and preoptic area of the toad. The results suggest a possible role of GAL and NOS in the regulation of hibernation in these animals.

Morphine and anandamide coupling to nitric oxide stimulates GnRH and CRF release from rat median eminence: neurovascular regulation

Nitric oxide (NO) is involved in neurohormonal secretion from median eminence neuroendocrine nerve terminals. We report that stimulation of NO release from median eminence fragments including vascular tissues occurs by mu3 receptor activation by morphine, or by cannabinoid type 1 receptor activation by anandamide. The released levels of NO are lower after anandamide than after morphine stimulation. These processes can be blocked by L-NAME, a specific nitric oxide synthase inhibitor, by naloxone for the morphine-stimulated NO release, or SR 141716A, a specific CB1 receptor inhibitor, for the anandamide-stimulated NO release. Furthermore, morphine and anandamide, by this NO dependent process, influences neurohormonal release from median eminence nerve terminals within 10 min. Via this NO dependent process, morphine stimulates both GnRH and CRF release, whereas anandamide selectively stimulates GnRH release. These observations together with previous data suggest that morphine and the anandamide-stimulated NO originates from the vascular endothelium of the portal plexus. These results indicate that endothelial cells of the median eminence may be involved in the release of neurohormones.

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.

Nitrergic innervation of the cerebral arterial tree in the bent-winged bat (mammalia: Microchiroptera)

The distribution and origin of cerebrovascular nitrergic nerves were studied immunohistochemically and histochemically in the bent-winged bat. The supply of nitric oxide synthase (NOS)-immunoreactive (IR) and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-positive nerves to the bat major cerebral arteries differs from the general mammalian pattern in that it is preferential for the vertebrobasilar system (VBS) as opposed to the internal carotid system. Interestingly, a few nerve cells with bright NOS immunofluorescence and intense NADPHd activity were localized in the walls of the vertebral artery (VA) and basilar artery (BA) from many individual bats. Cerebral perivascular NOS-IR nerves were generally immunoreactive for vasoactive intestinal polypeptide (VIP). NOS-IR neurons intrinsic to the BA and VA expressed variable degrees of VIP immunoreactivity and showed no acetylcholinesterase (AChE) activity. Most cell bodies of the microganglia (MG) in the carotid canal and tympanic cavity, and those of the cranial and cervical facial ganglia, showed both NOS and VIP immunoreactivities and were stained intensely for NADPHd. From these and other findings, it is suggested that, in the bent-winged bat at least, the BA and VA of the cerebral arterial tree are frequently dually innervated by two neurochemically defined nitrergic neurons, the cranial parasympathetic VIP-IR and AChE-positive neurons, which are derived mainly from the MG via the internal carotid artery, and the intrinsic neurons, either IR or immunonegative for VIP but negative for AChE, which form an outflow tract from some caudally located ganglia projecting to the VBS via the VA.

Nitric oxide synthase, choline acetyltransferase, catecholamine enzymes and neuropeptides and their colocalization in the anterior pelvic ganglion, the inferior mesenteric ganglion and the hypogastric nerve of the male guinea pig

By the indirect immunofluorescence method, the distribution of nitric oxide synthase (NOS)-like immunoreactivity (LI) and its possible colocalization with neuropeptide immunoreactivities, with two enzymes for the catecholamine synthesis pathway, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH), as well as the enzyme for the acetylcholine synthesis pathway, choline acetyltransferase (ChAT) were studied in the anterior pelvic ganglion (APG), the inferior mesenteric ganglion (IMG) and the hypogastric nerve in the male guinea pig. The analyses were performed on tissues from intact animals, as well as after compression/ligation or cut of the hypogastric nerve. In some cases the colonic nerves were also cut. Analysis of the APG showed two main neuronal cell populations, one group containing NOS localized in the caudal part of the APG and one TH-positive group lacking NOS in its cranial part. The majority of the NOS-positive neurons contained ChAT-LI. Some NOS-positive cells did not contain detectable ChAT, but all ChAT-positive cells contained NOS. NOS neurons often contained peptides, including vasoactive intestinal peptide (VIP), neuropeptide tyrosine (NPY), somatostatin (SOM) and/or calcitonin gene-related peptide (CGRP). Some NOS cells expressed DBH, but never TH. The second cell group, characterized by absence of NOS, contained TH, mostly DBH and NPY and occasionally SOM and CGRP. Some TH-positive neurons lacked DBH. In the IMG, the NOS-LI was principally in nerve fibers, which were of two types, one consisting of strongly immunoreactive, coarse, varicose fibers with a patchy distribution, the other one forming fine, varicose, weakly immunoreactive fibers with a more general distribution. In the coarse networks, NOS-LI coexisted with VIP- and DYN-LI and the fibers surrounded mainly the SOM-containing noradrenergic principal ganglion cells. A network of ChAT-positive, often NOS-containing nerve fibers, surrounded the principal neurons. Occasional neuronal cell bodies in the IMG contained both NOS- and ChAT-LI. Accumulation of NOS was observed, both caudal and cranial, to a crush of the hypogastric nerve. VIP accumulated mainly on the caudal side and often coexisted with NOS. NPY accumulated on both sides of the crush, but mainly on the cranial side, and ENK was exclusively on the cranial side. Neither peptide coexisted with NOS. Both substance P (SP) and CGRP showed the strongest accumulation on the cranial side, possibly partly colocalized with NOS. It is concluded that the APG in the male guinea-pig consists of two major complementary neuron populations, the cholinergic neurons always containing NOS and the noradrenergic neurons containing TH and DBH. Some NOS neurons lacked ChAT and could represent truly non-adrenergic, non-cholinergic neurons. In addition, there may be a small dopaminergic neuron population, that is containing TH but lacking DBH. The cholinergic NOS neurons contain varying combinations of peptides. The noradrenergic population often contained NPY and occasionally SOM and CGRP. It is suggested that NO may interact with a number of other messenger molecules to play a role both within the APG and IMG and also in the projection areas of the APG.

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.

Effects of nitric oxide on neuroendocrine function and behavior

Nitric oxide (NO) is an unusual chemical messenger. NO mediates blood vessel relaxation when produced by endothelial cells. When produced by macrophages, NO contributes to the cytotoxic function of these immune cells. NO also functions as a neurotransmitter and neuromodulator in the central and peripheral nervous systems. The effects on blood vessel tone and neuronal function form the basis for an important role of NO on neuroendocrine function and behavior. NO mediates hypothalamic portal blood flow and, thus, affects oxytocin and vasopression secretion; furthermore, NO mediates neuroendocrine function in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes. NO influences several motivated behaviors including sexual, aggressive, and ingestive behaviors. Learning and memory are also influenced by NO. Taken together, NO is emerging as an important chemical mediator of neuroendocrine function and behavior.

NADPH-diaphorase activity and vasopressin in the paraventricular nucleus of the hypothalamus following adrenalectomy

In order to investigate novel neuroendocrine functions of the nitric oxide synthesizing enzyme a combined histochemical and immunocytochemical study focused on the paraventricular nucleus of the hypothalamus was conducted to check a possible influence of bilateral adrenalectomy on three different neuronal populations, NADPH diaphorase (ND)-positive, vasopressin (VP)-immunoreactive and neurons expressing both markers. In the adrenalectomized animals, a slight increase (P > 0.05) of the number of ND magnocellular neurons was detected, whereas no changes were observed in the ND-parvicellular population and in the neurons showing coexistence (magno- and parvicellular) (P > 0.05). By contrast, following bilateral adrenalectomy, a significant increase (P < 0.05) in the VP-parvicellular population (anterior, medial and periventricular subdivisions) was detected, which was reversed when the animals received daily doses of corticosterone. These results suggest that nitric oxide is not closely related to the hypothalamic regulation of the adenocorticotropin secretion exerted by the paraventricular nucleus.

Pituitary gland receives both central and peripheral neuropeptide Y innervation

Neuropeptide Y (NPY)-containing neural projections to the rat pituitary gland were studied by combining NPY immunohistochemistry with retrograde tracing with Fluorogold as well as central and peripheral denervations. Numerous pituitary-projecting, i.e. Fluorogold-labelled, neurons in the superior cervical ganglion, as well as in the hypothalamic magnocellular nuclei were NPY-immunoreactive (NPY-IR). In contrast, no other hypothalamic NPY-IR neurons, e.g. in the arcuate nucleus or the preoptic area, were observed to be projecting into the pituitary. Within the posterior lobe of the pituitary gland two morphologically distinct NPY-IR fiber populations were discovered, namely thinner parenchymal terminals, distinct from the neurosecretory terminals, and thicker, perivascular fibers. Neurosecretory nerve terminals, in contrast, were devoid of NPY-IR, being consistent with the previous reports on their sensitivity to osmotic stimulation. On the other hand, the anterior and intermediate lobes contained no NPY-IR fibers. Bilateral extirpation of the superior cervical ganglion resulted in disappearance of the perivascular NPY-IR fibers leaving the parenchymal NPY-IR fibers unaffected, while transection of the pituitary stalk abolished all of the parenchymal NPY-IR neurons, leaving the perivascular fibers unaffected. These findings together with the observed colocalization of tyrosine hydroxylase and NPY in the posterior lobe perivascular fibers indicated that they are sympathetic nerve endings. The thin parenchymal terminals, instead, are suggested to stem from central sources other than hypothalamus. Our findings indicate that the pituitary gland receives NPY-containing innervation from at least three distinct sources, and NPY may thus affect pituitary functions in various ways, such as blood flow and vasopressin release.

Selective inhibitors of nitric oxide synthase (NOS) implicate a constitutive isoform of NOS in the regulation of interleukin-1-induced ACTH secretion in rats

Nitric oxide synthase (NOS) exists in at least three distinct isoforms: an inducible NOS (NOS II), and two forms which are constitutively expressed-brain NOS (NOS I) and endothelial NOS (NOS III). We have previously shown that the NOS inhibitor, N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME), markedly potentiates and prolongs the increase in plasma adrenocorticotropin (ACTH) concentrations produced by the intravenous injection of interleukin-1β (IL-1β) in the rat. However, the mechanism of action of L-NAME is unknown. The purpose of the present study was to determine the effects on IL-1β-induced ACTH secretion in the rat, of several NOS inhibitors, whose selectivity for the different NOS isoforms has been well characterized, and which lack the muscarinic receptor antagonist properties that have been reported for L-NAME. Subcutaneous (sc) pretreatment with L-NAME (50-300 μmol/kg) produced the expected pronounced exacerbation of the ACTH response to IL-1β. This effect was mimicked by N(G)-nitro-L-arginine, which preferentially inhibits constitutive forms of NOS. In contrast, aminoguanidine, a selective inducible NOS inhibitor at doses up to 3×1.8 mmol/kg, was without effect, suggesting that it is a constitutive form of NOS that regulates the ACTH response to IL-1β. Selective inhibition of brain NOS using either 7-nitro-indazole (administered intraperitoneally) or L-NAME (administered intracerebroventricularly) did not significantly alter ACTH concentrations after IL-1β. Collectively, these data indicate that NO restrains the ACTH response to IL-1β, and that the NO responsible for this effect is generated by a constitutive, most probably endothelial, isoform of NOS.

Immobilization-induced stress activates neuronal nitric oxide synthase (nNOS) mRNA and protein in hypothalamic-pituitary-adrenal axis in rats

The purpose of this study was to determine whether immobilization stress can cause changes in the enzyme activity and gene expression of neuronal nitric oxide synthase (nNOS) in the hypothalamus, pituitary, and adrenal gland in rats. NOS enzyme activity was measured as the rate of [3H]arginine conversion to citrulline, and the level of nNOS mRNA signal was determined using in situ hybridization and image analysis. NOS-positive cells were also visualized using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-diaphorase) histochemistry and by immunohistochemistry using an anti-nNOS antibody. A significant increase of NOS enzyme activity in the anterior pituitary, adrenal cortex, and adrenal medulla (1.5-, 3.5-, and 2.5-fold) was observed in the stressed animals (immobilization of 6 h) as compared to non-stressed control rats. Up-regulation of nNOS mRNA expression in anterior pituitary and adrenal cortex was already detectable after stress for 2 h with 1.5- and 2-fold increase, respectively. The nNOS mRNA signals in hypothalamic paraventricular nucleus (PVN) significantly increased after the stress for 6 h. This increase in NOS enzyme activity was confirmed using NADPH-diaphorase staining and immunostaining in the PVN and adrenal cortex. An increase of NOS enzyme activity in adrenal medulla after immobilization for 6 h posited by far longer than in the adrenal cortex and anterior pituitary. The present findings suggest that psychological and/or physiological stress causes NO release in hypothalamic-pituitary-adrenal (HPA) axis and in sympatho-adrenal system. It is suggested that NO may modulate a stress-induced activation of the HPA axis and the sympatho-adrenal medullary system. The different duration of stress-induced NOS activity in HPA axis and the adrenal medulla may suggest NO synthesis is controlled by separate mechanism in the two HPA and the sympatho-adrenal systems.

Effects of ether lipid 1-O-octadecyl-2-methoxy-rac-glycero- 3-phosphocholine and its analogs PAF and CPAF on the release of nitric oxide in primary cultures of rat astrocytes

Ether lipid 1-O-octadecyl-2-O-methoxy-rac-glicero-3-phosphocholine (ET-18-OCH3) is an immunomodulator with antineoplastic activity. Its analog compounds PAF and CPAF share some of its biological effects. In our experiments, even very small amounts of ET-18-OCH3 released a remarkable quantity of nitric oxide (NO) from rat astrocytes cultured in vitro. The NO biosynthesis was inhibited by pretreatment with the antagonist BN 50730. The effect of ET-18-OCH3 was greater than that of the LPS inducer. PAF did not produce NO, even at high doses, while the nonmetabolizable CPAF only induced a significant release of NO from 12 micrograms/ml onwards. These results demonstrate that ET-18-OCH3 is functionally active also in astrocyte cultures. Stimulation of NO biosynthesis is of a great value on account of its the known effect as a neurotransmitter, potentiator of immune defences and possible modulator of cerebral circulation.

Changes in NADPH diaphorase activity in forebrain structures of the laminae terminalis after chronic dehydration

The effect of 3-day chronic dehydration on nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-diaphorase) histochemical staining in forebrain circumventricular structures was investigated in the rat. Increased number and/or intensity of staining of NADPH-diaphorase positive neurons was observed in subfornical organ, organum vasculosum of the laminae terminalis, and median preoptic nucleus. In addition, dense punctate NADPH-diaphorase reaction product was found throughout the internal portion of median eminence. These data suggest the involvement of nitric oxide synthase producing neurons in homeostatic mechanisms controlling body fluid balance and the circulation.

Regional study of the co-localization of neuronal nitric oxide synthase with muscarinic receptors in the rat cerebral cortex

There is increasing evidence that nitric oxide is an important molecular messenger involved in a wide variety of biological processes including the regulation of the cerebral circulation. For instance, it has been implicated in the vascular response to nucleus basalis magnocellularis stimulation, a structure which is widely recognized as the predominant source of cholinergic fibres projecting to the neocortex. The present investigation was carried out to determine if muscarinic receptors are present on cortical neurons expressing neuronal nitric oxide synthase (nitric oxide-producing enzyme). To this aim, double labelling of both neuronal nitric oxide synthase/vessels and neuronal nitric oxide synthase/muscarinic receptors was performed on free-floating cryosections obtained from rat brain. The observations were made by confocal laser scanning microscopy. The double labelling of neuronal nitric oxide synthase with the arterioles demonstrated the presence of nitroxidergic fibres in the wall of intraparenchymal vessels. A rich network of nitroxidergic fibres independent of the vessels was also seen in the parenchyma. Since the maximal surface of a square of tissue without any nitroxidergic fibres corresponded to 1400 +/- 105 microns2, the distance separating any cortical point from its closest neuronal nitric oxide synthase-positive fibre was never higher than 25 microns (half diagonal of square). According to models of the diffusional spread of nitric oxide, it is likely that nitric oxide can reach the whole cortical volume. Our results on the regional study of neuronal nitric oxide synthase/muscarinic receptors showed a high density of neuronal nitric oxide synthase-positive neurons principally in the frontal and perirhinal cortices and a low density in the occipital cortex. These data fit well with the known pattern of cortical projections from the nucleus basalis magnocellularis as revealed by anterogradely transported markers. The double labelling showed that about 10% of neuronal nitric oxide synthase-positive neurons were co-localized with muscarinic receptors in the frontoparietal cortex. In agreement with previous papers, the vascular innervation by nitroxidergic neuronal processes was often found to lie near the branching points of arterioles. Such localization allows neuronal nitric oxide synthase-positive neurons an extensive control of the vascular tree without requiring a large number of neuronal commands. Therefore, despite the low level of neuronal nitric oxide synthase/muscarinic receptor co-localization, this neuronal subpopulation could represent a possible relay implicated in the vascular effects of the nucleus basalis magnocellularis.

Distribution and synaptic relations of NOS neurons in the dorsal raphe nucleus: a comparison to 5-HT neurons

Anti-nitric oxide synthase antibody was used to study the distribution, cytoarchitecture, and synaptic relations of nitric oxide synthase-like immunoreactive neurons in the whole rostral-caudal length of the dorsal raphe nucleus of the rat and compared them with serotonergic neurons. Results showed that the distribution of the nitric oxide synthase in the dorsal raphe nucleus was similar to that of the serotonergic neurons at the rostral part of the dorsal raphe nucleus, including the mediodorsal and the medioventral cell groups, and changed at the middle and caudal parts of the dorsal raphe nucleus. The cytoarchitecture of the nitric oxide synthase-like immunoreactive neurons in the medioventral cell group of the dorsal raphe nucleus was similar to that of the serotonergic neurons. Similar to the serotonergic neurons there, nitric oxide synthase-like immunoreactive neurons also received synapses from axon terminals that contained round, or flattened vesicles, or both kinds. Different to the serotonergic neurons, the few nitric oxide synthase-like immunoreactive axon terminals that were in this area formed synapses.

Effects of pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) on prolactin, luteinizing hormone and growth hormone secretion in the ewe

This study was undertaken to investigate the roles of PACAP and VIP in the control of pituitary hormone secretion in the ewe. The first experiment was designed to identify any direct effects at the level of the pituitary and was conducted during the luteal phase of a prostaglandin-synchronized oestrous cycle. PACAP (0.008, 0.04, 0.2 and 1.0 nmol/min) or VIP (0.06, 0.2, 0.6 and 1.8 nmol/min) was infused into the carotid artery over a 10 min period. Blood samples were taken before and after the infusions so that plasma PRL, LH and GH concentrations could be measured. Blood pressure was also monitored to determine if the doses used were biologically active. In no case was an effect on hormone secretion observed. In contrast, the highest dose of each peptide induced an increase in heart rate to almost three-fold the resting value. Although both peptides are active in vivo, this result suggests that neither peptide has a direct effect on hormone release from the pituitary of prostaglandin-synchronized ewes. In a second experiment, we investigated whether the peptides had central effects on hormone secretion. Intracerebroventricular (ICV) injection of PACAP or VIP at the dose 10 nmol was tested in ovariectomized ewes. After injection, PACAP suppressed PRL and GH secretion so that plasma hormone concentrations from 1-3 h after injection were significantly different from the control (P < 0.05 for PRL, P < 0.01 for GH). In addition, PACAP significantly reduced mean LH concentration (P < 0.05) and LH pulse frequency (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide synthase-immunoreactive nerve fibers in the nasal mucosa of the rat

An immunohistochemical study was performed to detect the localization of nitric oxide synthase (NOS) in the rat nasal mucosa by light and electron microscopy. NOS-immunoreactive nerve fibers were observed around blood vessels and seromucous glands. They were found in the subepithelial layer and even within the epithelium. But no NOS-immunoreactivity was found in the olfactory neuroepithelium. Electron microscopy showed that NOS-immunoreactive nerve profiles were in close contact with the cytolemma of respiratory epithelial cells and acinar cells of seromucous glands. NOS-immunoreactive axon varicosities were located at a considerable distance from the smooth muscle of arterioles and small veins as well as the endothelial cells of venules and capillaries. We confirmed that NOS-containing nerves innervated the epithelium, blood vessels and seromucous glands of the nasal mucosa. These findings, collectively, suggested the possibility that nitric oxide participated in the sensory function of the epithelium, the secretory activities of the nasal gland, and the regulation of vascular tone and vascular permeability in the nasal mucosa.

Immunohistochemical demonstration of nitric oxide synthase in the peripheral autonomic nervous system

In the present immunohistochemical study the distribution of nitric oxide synthase (NOS) was studied in various autonomic ganglia and in related peripheral tissues of the rat. For comparison some other neuronal markers including acetylcholinesterase and tyrosine hydroxylase as well as several neuropeptides were analysed on adjacent or the same sections. The distribution of NOS-like immunoreactivity (LI) and of these other markers has been semiquantitatively summarized. In some parasympathetic ganglia such as the sphenopalatine and submandibular ganglia NOS-LI was present in most ganglion cells, at least partly coexisting with peptide histidine isoleucine (PHI), vasoactive intestinal polypeptide (VIP) and neuropeptide tyrosine (NPY). In the pelvic ganglia a comparatively smaller proportion of neurons was NOS-positive and they often contained VIP-LI and less frequently NPY-LI. In the tissues innervated by these ganglia, such as nasal mucosa and salivary glands, NOS-positive fibers were observed around blood vessels and within the glandular parenchyma, although generally less abundant than VIP/PHI nerves, while in the uterus, vas deferens and penis a more close correlation was seen. NOS-positive fibers were also widely distributed in other tissues. In the sympathetic ganglia NOS-LI was mainly present in dense fiber networks, which disappeared after transection of the sympathetic trunc central to the ganglion. Since many cell bodies in the sympathetic lateral column of the spinal cord also were NOS-positive, it is likely that the majority of preganglionic fibers innervating sympathetic ganglia are NOS-positive. VIP-positive cells in stellate ganglia did not contain NOS-LI. The present results suggest that NO may be a messenger molecule both in parasympathetic postganglionic neurons and in preganglionic sympathetic neurons.

Nitric oxide synthase in the pig autonomic nervous system in relation to the influence of NG--nitro-L-arginine on sympathetic and parasympathetic vascular control in vivo

Nitric oxide synthase, the enzyme responsible for the formation of nitric oxide, was demonstrated by an indirect immunofluorescence technique to be present in both the sympathetic and parasympathetic nervous system of the domestic pig. In the sympathetic nervous system, nitric oxide synthase was mainly present in preganglionic neurons projecting to postganglionic neurons, some of which contained neuropeptide Y in the superior cervical, the coeliac and the lumbar ganglia of the sympathetic chain. A minor population of postganglionic sympathetic neurons contained nitric oxide synthase, vasoactive intestinal polypeptide and peptide histidine isoleucine. In the densely sympathetically innervated vascular beds such as the spleen, kidney and skeletal muscle, many neuropeptide Y- but no nitric oxide synthase-positive fibres were found. The nitric oxide synthase inhibitor NG-nitro-L-arginine reduced cardiac output by 40% and caused profound vasoconstriction in a variety of vascular beds. Furthermore, no or minor changes in plasma catecholamines, neuropeptide Y or endothelin-1 were observed up to 20 min after NG-nitro-L-arginine. Milrinone (a phosphodiesterase III inhibitor) prevented this NG-nitro-L-arginine-induced reduction in cardiac output, and the regional vasoconstriction was reduced, whereas some elevation of the blood pressure was still observed. Sympathetic nerve stimulation, with single impulses of 10 Hz for 1 s in the presence of NG-nitro-L-arginine, evoked vasoconstrictor responses which were largely in the same range as in control conditions. Parasympathetic postganglionic neurons to the submandibular salivary gland contained nitric oxide synthase, vasoactive intestinal polypeptide, peptide histidine isoleucine and neuropeptide Y. The vasodilatation evoked by parasympathetic nerve stimulation (10 Hz for 1 s) in the presence as well as in the absence of atropine was, on the other hand, markedly reduced by NG-nitro-L-arginine administration. Milrinone attenuated the inhibitory effect of NG-nitro-L-arginine on the parasympathetic vasodilation. In conclusion, nitric oxide synthase can be demonstrated in preganglionic sympathetic and postganglionic parasympathetic neurons. The main effect of nitric oxide synthase inhibition seems to be related to attenuation of basal endothelial nitric oxide production and parasympathetic transmission. Inhibition of phosphodiesterase counteracts both the haemodynamic and the neuronal effects of NG-nitro-L-arginine.

Nitric oxide regulates peptide release from parasympathetic nerves and vascular reactivity to vasoactive intestinal polypeptide in vivo

The possible involvement of nitric oxide (NO) in the vasodilator response to parasympathetic nerve stimulation in the pig submandibular gland in vivo was studied using the NO synthase inhibitor, NG-nitro-L-arginine. The atropine-resistant vasodilation elicited by parasympathetic stimulation (10 Hz, 30 s) and the response elicited by i.v. injection of vasoactive intestinal polypeptide (VIP) were markedly reduced by NG-nitro-L-arginine. Furthermore, peptide release from the gland elicited by nerve stimulation was attenuated after NG-nitro-L-arginine administration. Addition of the NO donor, nitroprusside, reversed the NG-nitro-L-arginine evoked attenuation of the response to nerve stimulation and VIP. Also the cholinergic parasympathetic component and the vascular effect of acetylcholine were reduced by NG-nitro-L-arginine. Furthermore, the NG-nitro-L-arginine-induced attenuation of the vascular responses was partially prevented by milrinone, an inhibitor of the cyclic GMP-regulated phosphodiesterase III. The present results suggest that NO may be crucial for parasympathetic vasodilatation by regulating peptide release and second messenger systems for VIP and acetylcholine.

Histological and functional studies on the nitroxidergic nerve innervating monkey cerebral, mesenteric and temporal arteries

Nitroxidergic nerves and their functional role were determined in a variety of monkey arteries. Nitric oxide synthase-immunoreactive nerve fibers innervating the monkey arterial wall were histochemically determined by the use of nitric oxide synthase antiserum. Thin nitric oxide synthase-immunoreactive fibers were consistently found in the outer media of monkey cerebral, mesenteric and temporal arteries, in addition to many thicker fibers and nerve bundles in the adventitia. In the monkey pterygopalatine ganglion, the immunoreactivity was clearly seen in nerve cells, bundles and fibers. Helical strips of monkey arteries were exposed to the bathing media for tension recordings and were stimulated by electrical square pulses. In helical strips of the cerebral artery denuded of the endothelium, transmural electrical stimulation produced relaxations that were abolished by tetrodotoxin or NG-nitro-L-arginine, a nitric oxide synthase inhibitor. In the monkey mesenteric and temporal arterial strips treated with alpha-adrenoceptor antagonists, the relaxation caused by electrical stimulation was also abolished by the nitric oxide synthase inhibitor, and it was restored by L-arginine. Nitroxidergic perivascular nerves, histologically demonstrated, appear to play an important role in dilating the monkey cerebral artery and in counteracting a vasoconstriction associated with noradrenergic nerve activation in the mesenteric and temporal arteries.

Upregulation of nitric oxide synthase and galanin message-associated peptide in hypothalamic magnocellular neurons after hypophysectomy. Immunohistochemical and in situ hybridization studies

The expression of several bioactive molecules in magnocellular hypothalamic neurons is modified when the axons of these cells are transected. In this study we have evaluated by means of immunocytochemistry and in situ hybridization the effect of hypophysectomy on the expression of nitric oxide synthase (NOS)- and of galanin message-associated peptide (GMAP)-like immunoreactivities (-LIs) as well as on their respective mRNAs in hypothalamic magnocellular neurosecretory neurons. The results show a transient increase in NOS- and GMAP-LIs in magnocellular neurons of both the paraventricular and supraoptic nuclei when compared to normal animals. The maximal increase in staining was observed between 5 and 7 days, and by 14 days NOS-LI was back to normal levels, whereas strong GMAP-LI could still be detected in a few cells. A similar picture was observed for the NOS and GMAP mRNAs. The functional significance of the present findings is unclear, but they indicate a possible role of nitric oxide and GMAP in neurosecretory neurons after injury.

Neuronal nitric oxide synthase is localized in extrinsic nerves regulating perireceptor processes in the chemosensory nasal mucosae of rats and humans

Nitric oxide synthase is the enzyme responsible for the production of the free radical gas nitric oxide, which has been implicated as an intercellular messenger in both the central and peripheral nervous systems. Immunoreactivity for nitric oxide synthase is often coincident with the histochemical demonstration of NADPH-diaphorase activity. Using an antibody to the neuronal form of nitric oxide synthase and a histochemical technique for NADPH-diaphorase, we have compared the localization of immunoreactivity and histochemical reaction product in the nasal mucosae of rats and humans. Immunoreactivity for neuronal nitric oxide synthase was localized in the extrinsic perivascular innervation of the olfactory and vomeronasal mucosae of rats and in the olfactory mucosa of humans. In the rat nasal mucosa, specific groups of glands were also innervated; the density of nitrinergic innervation varied among them, with vomeronasal glands and posterior glands of the nasal septum being the most densely innervated. In contrast, NADPH-diaphorase activity was present in olfactory, vomeronasal, and septal organ receptor neurons in rats and in olfactory receptor neurons in humans as well as in numerous nerve fibers, glands, and surface epithelial cells. The localization of neuronal nitric oxide synthase in extrinsic perivascular and periglandular nerve fibers suggests that nitric oxide may modulate the perireceptor processes of local blood flow and mucus secretion that influence the access to and clearance of chemical stimuli from rat and human chemosensory mucosae.

Projections of nitric oxide synthase-containing fibers from the sphenopalatine ganglion to cerebral arteries in the rat

The origin and distribution of cerebral perivascular nerves containing nitric oxide, a short-acting messenger or neurotransmitter, have been studied in the rat by histochemistry for reduced nicotinamide adenine dinucleotide phosphate-diaphorase activity, a specific marker for neuronal nitric oxide synthase. Positively stained nerve fibers were distributed throughout the major vessels of the cerebral arteries, though the fiber density was higher in the anterior circulation, including the circle of Willis, than in the posterior arteries. Examination using axonal transport methods indicated that nitric oxide-containing neurons in the sphenopalatine ganglion innervate the cerebral arteries bilaterally. Nitric oxide synthase in these ganglionic cells often co-existed with vasoactive intestinal polypeptide. The anatomical information obtained is discussed in terms of non-adrenergic, non-cholinergic neuronal transmission in the cerebral arteries.

Nitric oxide synthase increases in hypothalamic magnocellular neurons after salt loading in the rat. An immunohistochemical and in situ hybridization study

Magnocellular hypothalamic neurons of the paraventricular (PVN) and supraoptic (SON) nuclei have been shown to contain a wide variety of messenger molecules in addition to vasopressin and oxytocin, including the nitric oxide (NO)-synthesizing enzyme (NOS). In this paper we have investigated the effects of salt loading on the expression of NOS by means of immunohistochemistry and in-situ hybridization. The results show an increase in the number of NOS-immunoreactive (IR) neurons both in the PVN and the SON after 5 and 14 days of salt loading. Several of these neurons were double labelled with vasopressin antiserum. In situ hybridization showed a marked increase in the number of neurons expressing NOS mRNA and a stronger signal in individual neurons. The present results suggest a role for NO in the magnocellular hypothalamic system after salt loading.

Immunohistochemical demonstration of the synthesis enzyme for nitric oxide and of comediators in neurons and chromaffin cells of the human adrenal medulla

Within the human adrenal medulla immunoreactivity for the nitric oxide (NO)-generating enzyme nitric oxide synthase (NOS) was demonstrated in neurons, nerve fibres and chromaffin cells. Correlation of NOS-immunoreactivity with immunostaining for the peptides neuropeptide Y, somatostatin, substance P or vasoactive intestinal polypetide and for the catecholamine synthesis-enzyme tyrosine hydroxylase, respectively, in nerve cell bodies revealed colocalization of NOS only with substance P. Sparse intramedullary NOS-immunoreactive varicose nerve fibres associated with blood vessels or with chromaffin tissue were devoid of immunoreactivities for tyrosine hydroxylase or for the investigated peptides. Small NOS-immunolabeled cells belonged to the catecholamine-containing chromaffin cell population and costored VIP, but were distinct from the somatostatin- or neuropeptide Y- immunostained chromaffin subpopulations. The localization of NOS in distinct structural components of the human adrenal medulla indicates that NO is produced in different cell types and may reflect a differential role of this messenger system in autonomic control of adrenal gland function.

Nitric oxide synthase-immunoreactive nerve fibers in dog cerebral and peripheral arteries

Nitric oxide synthase (NOS)-immunoreactive fibers innervating the dog arterial wall were histochemically determined by the use of NOS antiserum. NOS-immunoreactive fibers were consistently found in every arterial wall examined. In a whole-mount preparation, NOS-positive fibers were detectable in the small pial artery having a diameter of about 100 microns as well as the proximal middle cerebral artery. Further detailed analyses in thin cryostat sections indicated that in middle cerebral, basilar, temporal, mesenteric and femoral arteries, fine NOS-positive fibers were detected in outer zones of the media in addition to many thicker fibers in the adventitia. However, in the coronary artery, many thick fibers were situated in the adventitia, and fine NOS-positive fibers were not found in the media. Injection of ethanol to the pterygopalatine ganglion markedly decreased or abolished the NOS immunoreactivity in nerve cells and fibers and abolished the innervation of NOS-positive fibers in the wall of middle cerebral artery of the ipsilateral side. Together with findings in our previous publications concerning the functional role of nitroxidergic nerve in the control of arterial tone, we conclude that perivascular nerves containing NOS are crucial in eliciting the neurally induced, NO-mediated arterial relaxation.

Histochemical localization of NADPH-dependent diaphorase (nitric oxide synthase) activity in vascular endothelial cells in the rat brain

This study investigated the localization of NADPH-dependent diaphorase activity within vascular endothelial cells in the rat brain. Light microscope observations showed that in addition to neurons and neuronal processes stained histochemically for NADPH-dependent diaphorase activity, endothelial cells in many medium to large diameter (20-100 microns) blood vessels were also stained. These vessels were either attached to the pial surface or contained within the substance of the tissue. In vascular endothelia, the formazan end-product of the diaphorase reaction was deposited as discrete clusters of darkly stained punctae that were located around the nucleus of these cells. Correlated light- and electron-microscopical examination revealed that the sites of formazan deposition occurred in regions of endothelial cytoplasm devoid of smooth and rough endoplasmic reticulum and of mitochondria. Since endothelial NADPH dependent diaphorase activity co-localizes with the activity of nitric oxide synthase (the synthetic enzyme for nitric oxide) these observations suggest that in vascular endothelial cells nitric oxide synthase may be a highly localized soluble cytosolic enzyme not structurally associated with any subcellular organelle. In addition, specific regions of the smooth muscle cells encircling the larger diameter blood vessels clearly demonstrated NADPH dependent diaphorase activity. Unmyelinated fibres and fibre-plexi surrounding blood vessels on the pial surface were also stained. The results of this study show specific NADPH dependent diaphorase activity in vascular endothelial cells in the rat brain. Therefore, together with neurons, endothelial cells may control nitric oxide-dependent vasodilation thereby regulating local blood flow in the brain.

Local NADPH-diaphorase neurons innervate pial arteries and lie close or project to intracerebral blood vessels: a possible role for nitric oxide in the regulation of cerebral blood flow

Electrical stimulation of perivascular nerves induced a relaxation of endothelium-denuded cat pial arteries that was significantly reduced by nitric oxide (NO) synthase inhibition, indicating that NO was involved in the neurogenic relaxation of these vessels. Histochemical staining of the pial arteries for NADPH-diaphorase (NADPH-d), used as a marker for NO synthase, showed positive nerve fibers in the adventitial layer. Interestingly, in some restricted areas stained neuronal cell bodies were also observed. These neurons were scattered or distributed in small groups in a ganglion-like manner, and they sent fibers to the vessel wall. No NADPH-d-positive nerve fibers or cell bodies were detected in forelimb, pulmonary, or coronary arteries. Within the brain parenchyma, blood vessels also showed positive fibers around their walls. These fibers were organized in a branching pattern and presented varicosities. NADPH-d-positive neurons were found in the proximity of the intracerebral vascular profiles, sending processes to the vessels and/or being directly apposed to their wall. The neurovascular contacts were preferentially located close to the interface between the cerebral cortex and white matter. The anatomical relationship between NADPH-d-positive neurons and fibers and the cerebral blood vessels, together with the participation of NO in the neurogenic relaxation of pial arteries, suggests that NO is involved in the regulation of cerebral blood flow.

The effect of lactation on nitric oxide synthase gene expression

Nitric oxide-synthesizing (NOS) enzyme has been identified in several neural populations, including the hypothalamic paraventricular nucleus (PVN). The PVN plays a major role in regulating milk ejection. In the present study, using in situ hybridization, the effect of lactation on NOS mRNA in the PVN was investigated. A significant increase was seen in the PVN of lactating rats, indicating a possible involvement of nitric oxide in the hypothalamic-pituitary regulation of milk ejection.

Endothelium-derived vasoactive factors and regulation of the cerebral circulation

Vasoactive factors produced and released by endothelium exert a powerful influence on vascular tone in the cerebral circulation. Endothelium-derived relaxing factor (EDRF), which has been identified as nitric oxide (NO) or an NO-containing compound, is produced under basal conditions in cerebral blood vessels. EDRF mediates endothelium-dependent relaxation in response to a number of stimuli in the cerebral circulation. The influence of NO on the cerebral circulation appears to be particularly important and complex because both neurons and glia, in addition to endothelium, produce NO in response to some stimuli. Neuronally derived NO may mediate local vasodilation in response to increased neuronal activity. In addition to EDRF, cerebral endothelium may produce other relaxing factors, including prostacyclin, endothelium-derived hyperpolarizing factor, and oxygen-derived free radicals. Several pathophysiological conditions are associated with impaired endothelium-dependent responses that may involve the decreased production of EDRF and release of endothelium-derived contracting factors, such as the cyclooxygenase products of arachidonic acid and endothelin. The release of endothelin, an extremely potent and long-lasting vasoconstrictor peptide, may contribute to vasospasm after subarachnoid hemorrhage.