Comparative aspects on nitric oxide in brain and its role as a cerebral vasodilator

Recent insights into mechanisms of hypoxia-induced vasodilatation in the human brain

The cerebral vasculature manages oxygen delivery by adjusting arterial blood in-flow in the face of reductions in oxygen availability. Hypoxic cerebral vasodilatation, and the associated hypoxic cerebral blood flow reactivity, involve many vascular, erythrocytic and cerebral tissue mechanisms that mediate elevations in cerebral blood flow via micro- and macrovascular dilatation. This contemporary review focuses on in vivo human work - with reference to seminal preclinical work where necessary - on hypoxic cerebrovascular reactivity, particularly where recent advancements have been made. We provide updates with the following information: in humans, hypoxic cerebral vasodilatation is partially mediated via a - likely non-obligatory - combination of: (1) nitric oxide synthases, (2) deoxygenation-coupled S-nitrosothiols, (3) potassium channel-related vascular smooth muscle hyperpolarization, and (4) prostaglandin mechanisms with some contribution from an interrelationship with reactive oxygen species. And finally, we discuss the fact that, due to the engagement of deoxyhaemoglobin-related mechanisms, reductions in O2 content via haemoglobin per se seem to account for ∼50% of that seen with hypoxic cerebral vasodilatation during hypoxaemia. We further highlight the issue that methodological impediments challenge the complete elucidation of hypoxic cerebral reactivity mechanisms in vivo in healthy humans. Future research is needed to confirm recent advancements and to reconcile human and animal findings. Further investigations are also required to extend these findings to address questions of sex-, heredity-, age-, and disease-related differences. The final step is to then ultimately translate understanding of these mechanisms into actionable, targetable pathways for the prevention and treatment of cerebral vascular dysfunction and cerebral hypoxic brain injury.

Acid-Sensing Ion Channels: Novel Mediators of Cerebral Vascular Responses

RATIONALE

Precise regulation of cerebral blood flow is critical for normal brain function. Insufficient cerebral blood flow contributes to brain dysfunction and neurodegeneration. Carbon dioxide (CO2), via effects on local acidosis, is one of the most potent regulators of cerebral blood flow. Although a role for nitric oxide in intermediate signaling has been implicated, mechanisms that initiate CO2-induced vasodilation remain unclear.

OBJECTIVE

Acid-sensing ion channel-1A (ASIC1A) is a proton-gated cation channel that is activated by extracellular acidosis. Based on work that implicated ASIC1A in the amygdala and bed nucleus of the stria terminalis in CO2-evoked and acid-evoked behaviors, we hypothesized that ASIC1A might also mediate microvascular responses to CO2.

METHODS AND RESULTS

To test this hypothesis, we genetically and pharmacologically manipulated ASIC1A and assessed effects on CO2-induced dilation of cerebral arterioles in vivo. Effects of inhalation of 5% or 10% CO2 on arteriolar diameter were greatly attenuated in mice with global deficiency in ASIC1A (Asic1a-/-) or by local treatment with the ASIC inhibitor, psalmotoxin. Vasodilator effects of acetylcholine, which acts via endothelial nitric oxide synthase were unaffected, suggesting a nonvascular source of nitric oxide may be key for CO2 responses. Thus, we tested whether neurons may be the cell type through which ASIC1A influences microvessels. Using mice in which Asic1a was specifically disrupted in neurons, we found effects of CO2 on arteriolar diameter were also attenuated.

CONCLUSIONS

Together, these data are consistent with a model wherein activation of ASIC1A, particularly in neurons, is critical for CO2-induced nitric oxide production and vasodilation. With these findings, ASIC1A emerges as major regulator of microvascular tone.

Recombinant interleukin-1β dilates steelhead trout coronary microvessels: effect of temperature and role of the endothelium, nitric oxide and prostaglandins

Interleukin (IL)-1β is associated with hypotension and cardiovascular collapse in mammals during heat stroke, and the mRNA expression of this pro-inflammatory cytokine increases dramatically in the blood of Atlantic cod (Gadus morhua) at high temperatures. These data suggest that release of IL-1β at high temperatures negatively impacts fish cardiovascular function and could be a primary determinant of upper thermal tolerance in this taxa. Thus, we measured the concentration-dependent response of isolated steelhead trout (Oncorhynchus mykiss) coronary microvessels (<150 μm in diameter) to recombinant (r) IL-1β at two temperatures (10 and 20°C). Recombinant IL-1β induced a concentration-dependent vasodilation with vessel diameter increasing by approximately 8 and 30% at 10(-8) and 10(-7) mol l(-1), respectively. However, this effect was not temperature dependent. Both vessel denudation and cyclooxygenase blockade (by indomethacin), but not the nitric oxide (NO) antagonist L-NIO, inhibited the vasodilator effect of rIL-1β. In contrast, the concentration-dependent dilation caused by the endothelium-dependent calcium ionophore A23187 was completely abolished by L-NIO and indomethacin, suggesting that both NO and prostaglandin signaling mechanisms exist in the trout coronary microvasculature. These data: (1) are the first to demonstrate a functional link between the immune and cardiovascular systems in fishes; (2) suggest that IL-1β release at high temperatures may reduce systemic vascular resistance, and thus, the capacity of fish to maintain blood pressure; and (3) provide evidence that both NO and prostaglandins play a role in regulating coronary vascular tone, and thus, blood flow.

Effects of nitric oxide on blood-brain barrier permeability in common carp (Cyprinus carpio L.)

OBJECTIVE

To determine in vivo effects of nitric oxide (NO) on blood-brain barrier (BBB) permeability in common carp (Cyprinus carpio L.).

ANIMALS

148 carp. PROCEDURES Carp received glyceryl trinitrate (1 mg/kg) as an NO donor or received no treatment (control group). Nitrite and nitrate concentrations in carp sera were determined 0.25, 1, 3, 6, 8, 12, and 24 hours after treatment. In control and treatment groups, BBB permeability was analyzed by assessment of leakage of Evans blue dye into various brain areas at 6, 12, and 24 hours after glyceryl trinitrate treatment. Brain edema was determined by means of the wet-dry weight method and assessed with light microscopy on H&E-stained preparations of tissues obtained 6 and 24 hours after glyceryl trinitrate treatment.

RESULTS

Treatment with glyceryl trinitrate induced endogenous synthesis of NO, which was upregulated 6 and 8 hours after treatment. Increased NO synthesis was associated with increased permeability of the BBB, which developed 6 hours after treatment with the NO donor. Although the BBB became impermeable again by 12 hours after glycerol trinitrate treatment, brain edema still persisted 24 hours after treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

In this study, treatment with an NO donor caused reversible opening of the BBB and brain edema in common carp. An intact BBB is important to prevent influx of potentially harmful substances into the brain. This investigation highlighted the possibility of BBB disarrangement caused by NO, a substance found in the CNS of all vertebrates evaluated.

A kinetic approach to assess oxidative metabolism related features in the bivalve Mya arenaria

Electron paramagnetic resonance uses the resonant microwave radiation absorption of paramagnetic substances to detect highly reactive and, therefore, short-lived oxygen and nitrogen centered radicals. Previously, steady state concentrations of nitric oxide, ascorbyl radical (A·) and the labile iron pool (LIP) were determined in digestive gland of freshly collected animals from the North Sea bivalve Mya arenaria. The application of a simple kinetic analysis of these data based on elemental reactions allowed us to estimate the steady state concentrations of superoxide anion, the rate of A· disappearance and the content of unsaturated lipids. This analysis applied to a marine invertebrate opens the possibility of a mechanistic understanding of the complexity of free radical and LIP interactions in a metabolically slow, cold water organism under unstressed conditions. This data can be further used as a basis to assess the cellular response to stress in a simple system as the bivalve M. arenaria that can then be compared to cells of higher organisms.

Endothelial nitric oxide synthase in the amphibian, Xenopus tropicalis

Nitric oxide (NO) is generated by NO synthase (NOS) of which there are three isoforms: neuronal NOS (nNOS, nos1), inducible NOS (iNOS, nos2), and endothelial NOS (eNOS, nos3). This study utilised the genome of Xenopus tropicalis to sequence a nos3 cDNA and determine if eNOS protein is expressed in blood vessels. A nos3 cDNA was sequenced that encoded a 1177 amino acid protein called XteNOS, which showed closest sequence identity to mammalian eNOS protein. The X. tropicalis nos3 gene and eNOS protein were determined to be an orthologue of mammalian nos3 and eNOS using gene synteny and phylogenetic analyses, respectively. In X. tropicalis, nos3 mRNA expression was highest in lung and skeletal muscle and lower in the liver, gut, kidney, heart and brain. Western analysis of kidney protein using an affinity-purified anti-XteNOS produced a single band at 140kDa. Immunohistochemistry showed XteNOS immunoreactivity in the proximal tubule of the kidney and endocardium of the heart, but not in the endothelium of blood vessels. Thus, X. tropicalis has a nos3 gene that appears not to be expressed in the vascular endothelium.

Mechanisms of vasodilation in the dorsal aorta of the elephant fish, Callorhinchus milii (Chimaeriformes: Holocephali)

This study investigated vasodilator mechanisms in the dorsal aorta of the elephant fish, Callorhinchus milii, using anatomical and physiological approaches. Nitric oxide synthase could only be located in the perivascular nerve fibres and not the endothelium of the dorsal aorta, using NADPH histochemistry and immunohistochemistry. In vitro organ bath experiments demonstrated that a NO/soluble guanylyl cyclase (GC) system appeared to be absent in the vascular smooth muscle, since the NO donors SNP (10(-4) mol l(-1)) and SIN-1 (10(-5) mol l(-1)) were without effect. Nicotine (3 x 10(-4) mol l(-1)) mediated a vasodilation that was not affected by ODQ (10(-5) mol l(-1)), L-NNA (10(-4) mol l(-1)), indomethacin (10(-5) mol l(-1)), or removal of the endothelium. In contrast, the voltage-gated sodium channel inhibitor, tetrodotoxin (10(-5) mol l(-1)), significantly decreased the dilation induced by nicotine, suggesting that it contained a neural component. Pre-incubation of the dorsal aorta with the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37) (10(-6) mol l(-1)) also caused a significant decrease in the nicotine-induced dilation. We propose that nicotine is mediating a neurally-derived vasodilation in the dorsal aorta that is independent of NO, prostaglandins and the endothelium, and partly mediated by CGRP.

Dual mechanisms for nitric oxide control of large arteries in the estuarine crocodile Crocodylus porosus

In reptiles, accumulating evidence suggests that nitric oxide (NO) induces a potent relaxation in the systemic vasculature. However, very few studies have examined the source from which NO is derived. Therefore, the present study used both anatomical and physiological approaches to establish whether NO-mediated vasodilation is via an endothelial or neural NO pathway in the large arteries of the estuarine crocodile Crocodylus porosus. Specific endothelial nitric oxide synthase (NOS) staining was observed in aortic endothelial cells following nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and endothelial NOS immunohistochemistry (IHC), suggesting that an endothelial NO pathway is involved in vascular control. This finding was supported by in vitroorgan bath physiology, which demonstrated that the relaxation induced by acetylcholine (10-5 mol l-1) was abolished in the presence of the NOS inhibitor, N-omega-nitro-l-arginine(l-NNA; 10-4 mol l-1), the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ;10-5 mol l-1), or when the endothelium was removed. Interestingly, evidence for a neural NO pathway was also identified in large arteries of the crocodile. Neural NOS was located in perivascular nerves of the major blood vessels following NADPH-d histochemistry and neural NOS IHC and in isolated aortic rings, l-NNA and ODQ, but not the removal of the endothelium, abolished the relaxation effect of the neural NOS agonist,nicotine (3×10-4 mol l-1). Thus, we conclude that the large arteries of C. porosus are potentially regulated by NO-derived from both endothelial and neural NOS.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Effects of gonadal hormones on central nitric oxide producing systems

Nitric oxide-containing neurons are widely distributed within the CNS, including regions involved in the control of reproduction and sexual behavior. The expression of neuronal nitric oxide synthase is influenced by testosterone in male rat, and by estrogens in female. Moreover, nitric oxide synthase may co-localize with gonadal hormones' receptors. Gonadal hormones may influence nitric oxide synthase expression in adulthood as well as during the development. In fact, in mice knockout for estrogen receptor alpha, the nitric oxide synthase-expressing population is deeply reduced in specific regions. In physiological conditions, the female in mammalian species is exposed to short-term changes of gonadal hormones levels (estrous cycle). Our recent studies, performed in the rat vomeronasal system and in mouse hypothalamic and limbic systems reveal that, in rodents, the expression of nitric oxide synthase-producing elements within regions relevant for the control of sexual behavior is under the control of gonadal hormones. The expression of nitric oxide synthase may vary according to the rapid variations of hormonal levels that take place during the estrous cycle. This seems in accordance with the hypothesis that gonadal hormone activation of nitric oxide-cyclic guanosine-monophosphate pathway is important for lordosis behavior, as well as that this system is activated during mating behavior. Finally, comparative data available for other vertebrates suggest that class-specific and species-specific differences occur in the nitric oxide synthase system of hypothalamus and limbic structures. Therefore, particular caution is needed to generalize data obtained from studies in rodents.

The role of nitric oxide in regulation of the cardiovascular system in reptiles

The roles that nitric oxide (NO) plays in the cardiovascular system of reptiles are reviewed, with particular emphasis on its effects on central vascular blood flows in the systemic and pulmonary circulations. New data is presented that describes the effects on hemodynamic variables in varanid lizards of exogenously administered NO via the nitric oxide donor sodium nitroprusside (SNP) and inhibition of nitric oxide synthase (NOS) by l-nitroarginine methyl ester (l-NAME). Furthermore, preliminary data on the effects of SNP on hemodynamic variables in the tegu lizard are presented. The findings are compared with previously published data from our laboratory on three other species of reptiles: pythons (), rattlesnakes () and turtles (). These five species of reptiles possess different combinations of division of the heart and structural complexity of the lungs. Comparison of their responses to NO donors and NOS inhibitors may reveal whether the potential contribution of NO to vascular tone correlates with pulmonary complexity and/or with blood pressure. All existing studies on reptiles have clearly established a potential role for NO in regulating vascular tone in the systemic circulation and NO may be important for maintaining basal systemic vascular tone in varanid lizards, pythons and turtles, through a continuous release of NO. In contrast, the pulmonary circulation is less responsive to NO donors or NOS inhibitors, and it was only in pythons and varanid lizards that the lungs responded to SNP. Both species have a functionally separated heart, so it is possible that NO may exert a larger role in species with low pulmonary blood pressures, irrespective of lung complexity.

Nitric oxide control of lower vertebrate blood vessels by vasomotor nerves

In mammals, much is understood about the endothelial and neural NO control mechanisms in the vasculature. In contrast, NO control of blood vessels in lower vertebrates is poorly understood, with the majority of research focusing on the presence of an endothelial NO system; however, its presence remains controversial. This study examined the mechanisms by which NO regulates the large blood vessels of non-mammalian vertebrates. In all species examined, the arteries and veins contained a plexus of NOS-positive perivascular nerves that included nerve bundles and fine, varicose nerve terminals. However, in the large arteries and veins of various species of fishes and amphibians, no anatomical evidence was found for endothelial NOS using both NADPH-diaphorase and eNOS immunohistochemistry. In contrast, perinuclear NOS staining was readily apparent in blue-tongue lizard, pigeon and rat, which suggested that eNOS first appeared in reptiles. Physiological analysis of NO signalling in the vascular smooth muscle of short-finned eel and cane toad could not find any evidence for endothelial NO signalling. In contrast, it appears that activation of the nitrergic vasomotor nerves is responsible for NO control of the blood vessels.

Nitric oxide in control of luminescence from hatchetfish(Argyropelecus hemigymnus) photophores

Nitric oxide synthase-like immunoreactivity (NOS-LI IR) was detected by immunohistochemistry in ventral light organs of the mesopelagic fish, Argyropelecus hemigymnus. Strong NOS-LI IR was present in nerve fibres and in other cells central for production or modulation of light:immunoreactive fibres surrounded the photophores, and were also present in the filter area. Filter cells, particularly in the outer layers, showed strong IR throughout the cytoplasm. Pharmacological studies suggested that nitric oxide(NO) modulates adrenaline-stimulated light emission, and that the modulation is correlated to the ability of the light organ to respond to adrenaline. Adrenaline is known to produce two different types of light response in isolated photophores from Argyropelecus: a slow, long-lasting, high intensity response, or a fast and weak response of short duration. Incubation of photophores in the NO donors sodium nitroprusside or S-nitroso-N-acetylpenicillamine prior to adrenaline stimulation reduced the intensity of the strong and long-lasting type of response, but had little or even a potentiating effect on the weakly responding photophores. Hydroxylamine, which is converted to NO if catalase activity is present in the tissue, reduced the duration and the intensity of the adrenaline response in all tested organs. The NOS-inhibitor l-thiocitrulline potentiated the adrenaline response in the weakly responding organs; the weaker the adrenaline effect, the stronger the potentiation caused by l-thiocitrulline. The strongly responding organs were instead inhibited by l-thiocitrulline. The results suggest that NO has an important role in the control of light emission from Argyropelecus hemigymnus photophores. The cGMP analogue dibutyryl cGMP, the guanylate cyclase inhibitor ODQ and the phosphodiesterase inhibitor pentoxiphylline had no effect, indicating that the NO effect does not involve cGMP.

Role of nitric oxide in larval and juvenile fish

Fish are known to express the three isoforms of nitric oxide synthase (NOS), the constitutive forms endothelial or eNOS, neuronal or nNOS and the inducible form iNOS. Most studies in fish have focussed on possible roles for NO in cardiovascular physiology although there has been recent attention on the role of nNOS in embryonic development. However compared to mammalian studies there have been relatively few studies on effects of nitric oxide (NO) on fish. Studies on heart and blood vessel preparations from various fish species appear to show results specific to the species or to the particular preparation. Possible roles of NO in the in vivo biology of adult fish or larval fish have received little attention. This article reviews effects of nitric oxide on cardiovascular physiology in fish with special emphasis on larval fish. It introduces some experimental work on possible signaling pathways in larval fish and introduces the possibility that NO could be an important environmental influence for some aquatic organisms. In higher vertebrates LPS (lipopolysaccharide) is known to activate the cytokine signaling system and stimulate increased expression of iNOS and increased production of NO, but this remains less investigated in fish. The effects of LPS on cardiovascular and osmoregulatory physiology of larval and juvenile salmonids are discussed and a possible role of NO in stress-induced drinking is suggested.

Formation of reactive species and induction of antioxidant defence systems in polar and temperate marine invertebrates and fish

High oxygen solubility at cold-water temperature is frequently considered to be responsible for an apparently elevated level of antioxidant protection in marine ectotherms from polar environments. However, tissue oxidative stress is in most cases a function of elevated or variable pO2, rather than of an elevated tissue oxygen concentration. This review summarizes current knowledge on pro- and antioxidant processes in marine invertebrates and fish, and relates reactive oxygen species (ROS) formation in polar ectotherms to homeoviscous adaptations of membrane and storage lipids, as well as to tissue hypoxia and re-oxygenation during physiological stress.

Nitric oxide control of the dorsal aorta and the intestinal vein of the Australian short-finned eel Anguilla australis

This study investigated the mechanisms by which nitric oxide (NO) regulates the dorsal aorta and the intestinal vein of the Australian short-finned eel Anguilla australis. NADPH diaphorase histochemistry and immunohistochemistry using a mammalian endothelial nitric oxide synthase (NOS)antibody could not demonstrate NOS in the endothelium of either blood vessel;however, NOS could be readily demonstrated in the endothelium of the rat aorta that was used as a control. Both blood vessels contained NADPH diaphorase positive nerve fibres and nerve bundles, and immunohistochemistry using a neural NOS antibody showed a similar distribution of neural NOS immunoreactivity in the perivascular nerves. In vitro organ bath physiology showed that a NO/soluble guanylyl cyclase (GC) system is present in the dorsal aorta and the intestinal vein, since the soluble GC inhibitor oxadiazole quinoxalin-1 (ODQ; 10–5 mol l–1)completely abolished the vasodilatory effect of the NO donor, sodium nitroprusside (SNP; 10–4 mol l–1). In addition, nicotine (3×10–4 mol l–1)mediated a vasodilation that was not affected by removal of the endothelium. The nicotine-mediated dilation was blocked by the NOS inhibitor, Nω-nitro-l-arginine (l-NNA;10–4 mol l–1), and ODQ(10–5 mol l–1). More specifically, the neural NOS inhibitor, Nω-propyl-l-arginine(10–5 mol l–1), significantly decreased the dilation induced by nicotine (3×10–4 mol l–1). Furthermore, indomethacin (10–5 mol l–1) did not affect the nicotine-mediated dilation,suggesting that prostaglandins are not involved in the response. Finally, the calcium ionophore A23187 (3×10–6 mol l–1) caused an endothelium-dependent dilation that was abolished in the presence of indomethacin. We propose the absence of an endothelial NO system in eel vasculature and suggest that neurally derived NO contributes to the maintenance of vascular tone in this species. In addition,we suggest that prostaglandins may act as endothelially derived relaxing factors in A. australis.

Nitric oxide synthase sequences in the marine fish Stenotomus chrysops and the sea urchin Arbacia punctulata, and phylogenetic analysis of nitric oxide synthase calmodulin-binding domains

The phylogenetic distribution and structural diversity of the nitric oxide synthases (NOS) remain important and issues that are little understood. We present sequence information, as well as phylogenetic analysis, for three NOS cDNAs identified in two non-mammalian species: the vertebrate marine teleost fish Stenotomus chrysops (scup) and the invertebrate echinoderm Arbacia punctulata (sea urchin). Partial gene sequences containing the well-conserved calmodulin (CaM)-binding domain were amplified by RT-PCR. Identical 375-bp cDNAs were amplified from scup brain, heart, liver and spleen; this sequence shares 82% nucleic acid and 91% predicted amino acid identity with the corresponding region of human neuronal NOS. A 387-bp cDNA was amplified from sea urchin ovary and testes; this sequence shares 72% nucleic acid identity and 65% deduced amino acid identity with human neuronal NOS. A second cDNA of 381 bp was amplified from sea urchin ovary and it shares 66% nucleic acid and 57% deduced amino acid identity with the first sea urchin sequence. Together with earlier reports of neuronal and inducible NOS sequences in fish, these data indicate that multiple NOS isoforms exist in non-mammalian species. Phylogenetic analysis of these sequences confirms the conserved nature of NOS, particularly of the calmodulin-binding domains.

Regional distribution of nitric oxide synthase and NADPH-diaphorase activities in the central nervous system of teleosts

Neuronal nitric oxide synthase (nNOS) and NADPH-diaphorase activities were investigated in discrete areas of the central nervous system of goldfish and brown trout. Both species showed a similar distribution pattern of nNOS activity with regional differences in all examined areas. Telencephalon and hypothalamus showed the highest nNOS values, while in the goldfish cerebellum and its valvula nNOS was not detectable. In both species, NADPH-diaphorase activity showed a lower regional variability, compared to nNOS. The highest activity was measured in the olfactory bulbs where, conversely, low levels of nNOS activity were present. The non close correspondence between NOS and NADPH-diaphorase activities confirms the discrepancies indicated by morphological data. Western blot analysis revealed the presence of a nNOS isoform of about 150 kDa mol. wt. corresponding to that of mammals. The pattern of nNOS expression in the considered brain regions of the goldfish and trout was comparable to the levels of the nNOS activity.

Neural control of muscle relaxation in echinoderms

Smooth muscle relaxation in vertebrates is regulated by a variety of neuronal signalling molecules, including neuropeptides and nitric oxide (NO). The physiology of muscle relaxation in echinoderms is of particular interest because these animals are evolutionarily more closely related to the vertebrates than to the majority of invertebrate phyla. However, whilst in vertebrates there is a clear structural and functional distinction between visceral smooth muscle and skeletal striated muscle, this does not apply to echinoderms, in which the majority of muscles, whether associated with the body wall skeleton and its appendages or with visceral organs, are made up of non-striated fibres. The mechanisms by which the nervous system controls muscle relaxation in echinoderms were, until recently, unknown. Using the cardiac stomach of the starfish Asterias rubens as a model, it has been established that the NO-cGMP signalling pathway mediates relaxation. NO also causes relaxation of sea urchin tube feet, and NO may therefore function as a ‘universal’ muscle relaxant in echinoderms. The first neuropeptides to be identified in echinoderms were two related peptides isolated from Asterias rubens known as SALMFamide-1 (S1) and SALMFamide-2 (S2). Both S1 and S2 cause relaxation of the starfish cardiac stomach, but with S2 being approximately ten times more potent than S1. SALMFamide neuropeptides have also been isolated from sea cucumbers, in which they cause relaxation of both gut and body wall muscle. Therefore, like NO, SALMFamides may also function as ‘universal’ muscle relaxants in echinoderms. The mechanisms by which SALMFamides cause relaxation of echinoderm muscle are not known, but several candidate signal transduction pathways are discussed here. The SALMFamides do not, however, appear to act by promoting release of NO, and muscle relaxation in echinoderms is therefore probably regulated by at least two neuronal signalling systems acting in parallel. Recently, other neuropeptides that influence muscle tone have been isolated from the sea cucumber Stichopus japonicus using body wall muscle as a bioassay, but at present SALMFamide peptides are the only ones that have been found to have a direct relaxing action on echinoderm muscle. One of the Stichopus japonicus peptides (holothurin 1), however, causes a reduction in the magnitude of electrically evoked muscle contraction in Stichopus japonicus and also causes ‘softening’ of the body wall dermis, a ‘mutable connective tissue’. It seems most likely that this effect of holothurin 1 on body wall dermis is mediated by constituent muscle cells, and the concept of ‘mutable connective tissue’ in echinoderms may therefore need to be re-evaluated to incorporate the involvement of muscle, as proposed recently for the spine ligament in sea urchins.

Development of the pteridine pathway in the zebrafish, Danio rerio

In the zebrafish, the peripheral neurons and the pigment cells are derived from the neural crest and share the pteridine pathway, which leads either to the cofactor tetrahydrobiopterin or to xanthophore pigments. The components of the pteridine pattern were identified as tetrahydrobiopterin, sepiapterin, 7-oxobiopterin, isoxanthopterin, and 2,4,7-trioxopteridine. The expression of GTP cyclohydrolase I activity during the first 24-h postfertilization, followed by 6-pyruvoyl-5,6,7,8-tetrahydropterin synthase and sepiapterin reductase, suggest an early supply of tetrahydrobiopterin for neurotransmitter synthesis in the neurons and for tyrosine supply in the melanophores. At 48-h postfertilization, sepiapterin formation branches off the de novo pathway of tetrahydrobiopterin synthesis. Sepiapterin, via 7,8-dihydrobiopterin and biopterin, serves as a precursor for the formation of 7-oxobiopterin, which may be further catabolized to isoxanthopterin and 2,4,7-trioxopteridine. Neither 7, 8-dihydrobiopterin nor biopterin is a substrate for xanthine oxidoreductase. In contrast, both of these compounds are oxidized at C-7 by a xanthine oxidase variant form, which is inactivated by KCN, but is insensitive to allopurinol. The oxidase and the dehydrogenase form of xanthine oxidoreductase as well as the xanthine oxidase variant have specific developmental patterns. It follows that GTP cyclohydrolase I, the formation of sepiapterin, and the xanthine oxidoreductase family control the pteridine pathway in the zebrafish.