Topographical relationship between neuronal nitric oxide synthase immunoreactivity and cyclic 3',5'-guanosine monophosphate accumulation in the brain of the adult Xenopus laevis

Immunohistochemical assessment of cyclic guanosine monophosphate (cGMP) and soluble guanylate cyclase (sGC) within the rostral ventrolateral medulla

Functional evidence suggests that nitric oxide (NO) signalling in the rostral ventrolateral medulla (RVLM) is cGMP-dependent and that this pathway is impaired in hypertension. We examined cGMP expression as a marker of active NO signalling in the C1 region of the RVLM, comparing adult (>18 weeks) Wistar-Kyoto (WKY, n = 4) and spontaneously hypertensive rats (SHR, n = 4). Double label immunohistochemistry for cGMP-immunoreactivity (IR) and C1 neurons [as identified by phenylethanolamine N-methyltransferase (PNMT-IR) or tyrosine hydroxylase TH-IR)], or neuronal NO synthase (nNOS) neurones, failed to reveal cGMP-IR neurons in the RVLM of either strain, despite consistent detection of cGMP-IR in the nucleus ambiguus (NA). This was unchanged in the presence of isobutylmethylxanthine (IBMX; 0.5 mM, WKY, n = 4, SHR n = 2) and in young animals (WKY, 10-weeks, n = 3). Incubation of RVLM-slices (WKY, 10-weeks, n = 9) in DETA-NO (100 mum; 10 min) or NMDA (10 muM; 2 min) did not uncover cGMP-IR. In all studies, cGMP was prominent within the vasculature. Soluble guanylate cyclase (sGC)-IR was found throughout neurones of the RVLM, but did not co-localise with PNMT, TH or nNOS-IR neurons (WKY, 10-weeks, n = 6). Results indicate that within the RVLM, cGMP is not detectable using immunohistochemistry in the basal state and cannot be elicited by phosphodiesterase inhibition, NMDA receptor stimulation or NO donor application.

The instructive role of binocular vision in the Xenopus tectum

This review presents the fascinating neurobiology underlying the development of the frog optic tectum, the brain structure where the two separate inputs from the two eye are combined into a single, integrated map. In the species Xenopus laevis, binocular visual information has a dramatic impact on axon growth and connectivity, and the formation of binocular connections in this system provides a rich basis for both theoretical and experimental investigations.

Distinct subpopulations of cyclic guanosine monophosphate (cGMP) and neuronal nitric oxide synthase (nNOS) containing sympathetic preganglionic neurons in spontaneously hypertensive and Wistar-Kyoto rats

The sympathetic preganglionic neurons (SPN) of the intermediolateral cell column (IML) play a critical role in the maintenance of vascular tone. We undertook a comparative neuroanatomical analysis of neuronal nitric oxide synthase (nNOS) expression in the SPN of the mature normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rat (SHR). The anatomical relationship between nNOS and the NO signaling molecule cyclic guanosine monophosphate (cGMP) was also determined. All animals were male, age > 6 months. Fluorogold (FG) retrograde labeling of SPN (detected with immunohistochemistry) was combined with NADPH-diaphorase histochemistry for NOS in the thoracic spinal cord (T1-11, n = 5 WKY, 5 SHR). There was no difference in the total number of FG-labeled SPN (WKY 6,542 +/- 828, SHR 6,091 +/- 820), but the proportion of FG-labeled cells expressing NOS was significantly less in the SHR (WKY 64.4 +/- 5.1 vs. SHR 55.6 +/- 2.1, P < 0.05). Fluorescence immunohistochemistry for nNOS/cGMP (n = 4 WKY, 4 SHR) was also performed. Confocal microscopy revealed that all nNOS-positive SPN contain cGMP and confirmed a strain-specific anatomical arrangement of SPN cell clusters. A novel subpopulation of cGMP-only cells were also identified. Double labeling for cGMP and choline acetyltransferase (n = 3 WKY, 3 SHR), confirmed these cells as SPN in both WKY and SHR. These results suggest that cGMP is a key signaling molecule in SPN, and that a reduced number of NOS neurons in the SHR may play a role in the increase in sympathetic tone associated with hypertension in these animals.

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.

Central respiratory activity of the tadpole in vitro brain stem is modulated diversely by nitric oxide

Nitric oxide (NO) is a potent central neuromodulator of respiration, yet its scope and site of action are unclear. We used 7-nitroindazole (7-NI), a selective inhibitor of endogenous neuronal NO synthesis, to investigate the neurogenesis of respiration in larval bullfrog (Rana catesbeiana) isolated brain stems. 7-NI treatment (0.0625-0.75 mM) increased the specific frequency of buccal ventilation (BV) events, indicating influence on BV central rhythm generators (CRGs). The drug reduced occurrence, altered burst shape, and disrupted clustering of lung ventilation (LV) events, without altering their specific frequency. LV burst occurrence and clustering also differed between pH conditions. We conclude that NO has diverse effects on respiratory rhythmogenesis, being necessary for the expression of respiratory rhythms, inhibiting the frequency of BV CRG, and affecting both shape and clustering of LV bursts through conditional modulation of LV CRG. We confirm central chemosensitivity in these preparations and demonstrate chemomodulation of LV burst clustering and occurrence but not specific frequency. Results support distinct oscillators underlying LV and BV CRGs.

Effect of spinal cord compression on cyclic 3',5'-guanosine monophosphate in the white matter columns of rabbit

Changes in the level of cyclic 3',5'-guanosine monophosphate (cGMP) were studied one day after a surgically induced spinal cord constriction performed at the Th7 segment level in the dorsal, lateral and ventral white matter columns and in the non-compartmentalized white matter of Th5-Th6 segments, i.e., above the site of the spinal cord constriction and in Th8-Th9 segments, located below the spinal cord constriction. The midthoracic spinal cord constriction caused a significant decrease in the level of cGMP in the ventral column of Th5-Th6 segments and a significant increase in the lateral column of Th8-Th9 segments. The level of cGMP in the dorsal column, located either rostrally or caudally to the site of the spinal cord injury, remained unchanged. In addition, no significant changes in the level of cGMP were found in the non-compartmentalized white matter of Th5-Th6 and Th8-Th9 segments in response to constriction of the Th7 segment.

Role of pH on the calcium ion dependence of the nitric oxide synthase in the carp brain

The role of pH on the calcium dependence of nitric oxide synthase (NOS) of Cyprinus carpio brain was investigated. This fish is known to survive prolonged periods of hypoxia. Under this condition, cerebral blood flow is no longer regulated by nitric oxide (NO). Nitric oxide synthase activity is pH dependent in the range of pH between 7.4 and 6.2 with a decrease when tissue acidifies. At acidic pH, the dependence of the NOS activity on the free Ca(2+) concentrations changes considerably and shows an EC(50) of 0.13 microM at pH 7.1 and of 5.1 microM at pH 6.2 for the soluble enzyme. The variation in the Ca(2+) dependence with acidification is greater for the soluble than for the particulate enzyme. This may be the main factor protecting sudden NO formation mainly during anoxic-normoxic transitions.

Regional changes of cyclic 3',5'-guanosine monophosphate in the spinal cord of the rabbit following brief repeated ischemic insults

The regional distribution of cyclic 3',5'-guanosine monophosphate was studied in the lumbosacral segments of the spinal cord of the rabbit under physiological conditions and following brief repeated sublethal ischemic insults. While the basal cGMP level in the gray matter was about 0.120 nmol cGMP/mg wet. wt., the level of cGMP in non-compartmentalized white matter was about half of this value. The highest level of cGMP in the compartmentalized gray matter was found in the dorsal horns, about 0.180 nmol cGMP/mg wet. wt., whereas the level of cGMP was greatly reduced in the ventral horns, reaching one half of the previous value. Multiple sublethal ischemic insults, repeated at 1-h intervals, caused a statistically significant decrease of cGMP in all gray matter regions. While the post-ischemic and post-reperfusion level of cGMP in the dorsal horns remained relatively high in comparison with the intermediate zone and ventral horns, the changes of cGMP level detected in the white matter columns differed considerably and resulted in a statistically significant cGMP increase in the dorsal and ventral columns and, vice versa, a statistically significant decrease of cGMP was found in the lateral columns.

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

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