Localizations of alpha 1 and beta 1 subunits of soluble guanylate cyclase in the rat brain

The effects of nitric oxide on magnocellular neurons could involve multiple indirect cyclic GMP-dependent pathways

Nitric oxide (NO) is known to regulate the release of arginine-vasopressin (AVP) and oxytocin (OT) by the paraventricular nucleus (PVN) and the supraoptic nucleus (SON). The aim of the current study was to identify in these nuclei the NO-producing neurons and the NO-receptive cells in mice. The determination of NO-synthesizing neurons was performed by double immunohistochemistry for the neuronal form of NO synthase (NOS), and AVP or OT. Besides, we visualized the NO-receptive cells by detecting cyclic GMP (cGMP), the major second messenger for NO, by immunohistochemistry on hypothalamus slices. Neuronal NOS was exclusively colocalized with OT in the PVN and the SON, suggesting that NO is mainly synthesized by oxytocinergic neurons in mice. By contrast, cGMP was not observed in magnocellular neurons, but in GABA-, tyrosine hydroxylase- and glutamate-positive fibers, as well as in GFAP-stained cells. The cGMP-immunostaining was abolished by incubating brain slices with a NOS inhibitor (L-NAME). Consequently, we provide the first evidence that NO could regulate the release of AVP and OT indirectly by modulating the activity of the main afferents to magnocellular neurons rather than by acting directly on magnocellular neurons. Moreover, both the NADPH-diaphorase activity and the mean intensity of cGMP-immunofluorescence were increased in monoamine oxidase A knock-out mice (Tg8) compared to control mice (C3H) in both nuclei. This suggests that monoamines could enhance the production of NO, contributing by this way to the fine regulation of AVP and OT release and synthesis.

Major occurrence of the new alpha2beta1 isoform of NO-sensitive guanylyl cyclase in brain

NO-sensitive guanylyl cyclase (GC) acts as the effector molecule for NO and therefore plays a key role in the NO/cGMP signalling cascade. Besides the long known GC isoform (alpha(1)beta(1)), another heterodimer (alpha(2)beta(1)) has recently been identified to be associated with PSD-95 in brain.Here, we report on the tissue distribution of all known guanylyl cyclase subunits to elucidate the isoform content in different tissues of the mouse. The guanylyl cyclase subunit levels were assessed with quantitative real-time PCR, and the most important results were verified in Western blots. We demonstrate the major occurrence of the alpha(2)beta(1) heterodimer in brain, find a significant amount in lung and lower amounts in all other tissues tested. In brain, the levels of the alpha(2)beta(1) and alpha(1)beta(1) isoforms were comparable; in all other tissues, the alpha(1)beta(1) heterodimer was the predominating isoform. The highest guanylyl cyclase content was found in lung; here the GC amounted to approximately twice as much as in brain. In sum, the major occurrence of the alpha(2)beta(1) heterodimer suggests a special role in synaptic transmission; whether this isoform outside the brain also occurs in neuronal networks has to be addressed in future studies.

Histamine H1-receptor modulation of inter-neuronal coupling among vasopressinergic neurons depends on nitric oxide synthase activation

Inter-neuronal coupling is a relatively recently documented property of a wide variety of cell groups in the mammalian central nervous system. For many of these groups there is evidence that the coupling can be modulated by synaptic inputs. Incidence of dye coupling among vasopressin (VP) neurons of the rat supraoptic nucleus (SON) has been shown to increase in response to either activation of histamine H(1)-receptors or to increased NO production. Both of these effects involve activation of cGMP-dependent pathways. We tested the hypothesis that activation of H(1)-receptors resulted in downstream activation of NO synthase, which then mediated the H(1)-receptor effects. Putative VP neurons were intracellularly recorded and dye-injected in horizontal slices of hypothalamus, in which monosynaptic connections from the tuberomammillary nucleus (TM) were intact and electrically stimulated. Single-pulse TM stimulation evoked EPSPs and repetitive stimulation resulted in a nearly 3-fold increase in coupling incidence over unstimulated slices. TM-stimulated increased coupling was completely blocked by inhibitors of NO synthase (L-NAME) or of soluble guanylyl cyclase (ODQ or methylene blue), or pyrilamine, suggesting that the H(1)-receptor is not directly linked to guanylyl cyclase. Addition of the NO precursor, L-arginine or the NO donor, SNP, in combination with TM stimulation produced increases in coupling that were not significantly larger than those seen with stimulation alone, supporting the idea that a common pathway was used. We conclude that H(1)-receptors activate NO synthase via G-protein-coupled pathways and that NO working though its receptor, induces the downstream cGMP-dependent processes that result in increased inter-neuronal coupling.

Isoforms of NO-sensitive guanylyl cyclase

By the formation of cGMP the NO-sensitive guanylyl cyclase plays a key role within the NO/cGMP signaling cascade involved in vascular regulation and neurotransmission. The prosthetic heme group of the enzyme acts as the NO sensor, and binding of NO induces conformational changes leading to an up to 200-fold activation of the enzyme. The unexpected fast dissociation half-life of NO of a few seconds is fast enough to account for the deactivation of the enzyme in biological systems. YC-1 and its analogues acting as NO sensitizers uncovered a new pharmacologically and conceivably physiologically relevant regulatory principle of the enzyme. Two existing isoforms of the heterodimeric guanylyl cyclase (alpha1beta1, alpha2beta1) are known that are functionally indistinguishable. Up to now, the NO-sensitive guanylyl cyclase has been considered as a soluble enzyme. However, recent evidence about the alpha2beta1 isoform interacting with a PDZ domain of the postsynaptic scaffold protein PSD-95 suggests that the alpha2 subunit directs a membrane association of this isoform. The interaction with PSD-95 locates the alpha2beta1 isoform in close proximity to the NO-generating NO synthase thereby enabling the NO sensor to respond to locally raised NO concentrations.

Kinetics of nitric oxide-cyclic GMP signalling in CNS cells and its possible regulation by cyclic GMP

Physiologically, nitric oxide (NO) signal transduction occurs through soluble guanylyl cyclase (sGC), which catalyses cyclic GMP (cGMP) formation. Knowledge of the kinetics of NO-evoked cGMP signals is therefore critical for understanding how NO signals are decoded. Studies on cerebellar astrocytes showed that sGC undergoes a desensitizing profile of activity, which, in league with phosphodiesterases (PDEs), was hypothesized to diversify cGMP responses in different cells. The hypothesis was tested by examining the kinetics of cGMP in rat striatal cells, in which cGMP accumulated in neurones in response to NO. Based on the effects of selective PDE inhibitors, cGMP hydrolysis following exposure to NO was attributed to a cGMP-stimulated PDE (PDE 2). Analysis of NO-induced cGMP accumulation in the presence of a PDE inhibitor indicated that sGC underwent marked desensitization. However, the desensitization kinetics determined under these conditions described poorly the cGMP profile observed in the absence of the PDE inhibitor. An explanation shown plausible theoretically was that cGMP determines the level of sGC desensitization. In support, tests in cerebellar astrocytes indicated an inverse relationship between cGMP level and recovery of sGC from its desensitized state. We suggest that the degree of sGC desensitization is related to the cGMP concentration and that this effect is not mediated by (de)phosphorylation.

Regional distribution of protein and activity of the nitric oxide receptor, soluble guanylyl cyclase, in rat brain suggests multiple mechanisms of regulation

Nitric oxide (NO) is an unconventional neuromodulator that signals by intercellular diffusion. Its effects are often mediated by activation of its cytosolic receptor, the hemoprotein soluble guanylyl cyclase (sGC). Regional distribution of heterodimeric (alpha/beta) sGC at both the activity and protein level and its regulation are still unclear. Here, sGC was analyzed in rat brain by Western blot and NO donor-stimulated cyclic GMP accumulation. sGCalpha(1) and sGCbeta(1) immunoreactive protein signals strongly correlated with each other. However, V(max) values depended on the type of NO donor used. Sodium nitroprusside, the most widely used compound and formally an NO(+) donor, was up to 20-fold less effective in stimulating sGC activity than the NO donor diethylamine NONOate. In contrast to the rather even distribution of sGC proteins and SNP-stimulated cGMP accumulation in various regions of rat brain, diethylamine NONOate-stimulated sGC activity varied up to 8-fold between the different brain regions tested. In conclusion, we show that expression of both sGCalpha(1) and sGCbeta(1) subunits is tightly coregulated in rat brain, while yet unknown additional mechanisms affect the V(max) of sGC.

Activation and up-regulation of spinal cord nitric oxide receptor, soluble guanylate cyclase, after formalin injection into the rat hind paw

Nitric oxide synthase is expressed abundantly in the spinal cord, and nitric oxide (NO) has been shown to play important roles in the central mechanism of inflammatory hyperalgesia. However, the expression and function of the NO receptor, soluble guanylate cyclase, is not fully understood in this processing at the spinal cord level. In the present study, we report that the soluble guanylate cyclase alpha(1) subunit but not the beta(1) subunit was expressed in rat spinal cord, particularly in the dorsal horn. We showed that intrathecal administration of a selective inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, produced a significant anti-nociception demonstrated by the decrease in the number of flinches and shakes in the formalin-induced inflammatory pain model. This was accompanied by a marked reduction in formalin-induced c-fos expression in the spinal cord. During formalin-induced long-lasting inflammation, we found that the expression of the alpha(1) subunit of soluble guanylate cyclase was dramatically increased in the lumbar spinal cord on the second and fourth days after formalin injection into the dorsal side of a hind paw. Intraperitoneal pretreatment with an N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine maleate (MK-801), and a neuronal NO synthase inhibitor, 7-nitroindazole, not only significantly blocked formalin-induced secondary thermal hyperalgesia but also suppressed formalin-produced increase in the alpha(1) subunit of soluble guanylate cyclase in the spinal cord. The present results indicate that peripheral inflammation not only initially activates but also later up-regulates soluble guanylate cyclase expression via the NMDA receptor-NO signaling pathway, suggesting that soluble guanylate cyclase might be involved in the central mechanism of formalin-induced inflammatory hyperalgesia in the spinal cord.

Patterns of expression of calcium binding proteins and neuronal nitric oxide synthase in different populations of hippocampal GABAergic neurons in mice

We examined the expression of calcium binding proteins parvalbumin (PV), calretinin (CR), and calbindin D28K (CB), and neuronal nitric oxide synthase (nNOS) in gamma-aminobutyric acid (GABA)ergic neurons of the mouse hippocampus, with particular reference to areal and dorsoventral differences. First, we estimated the colocalization of the calcium binding proteins and nNOS. GABAergic neurons containing both PV and nNOS, i.e., PV-immunoreactive (-IR)/nNOS-IR neurons, were rare in Ammon's horn but frequent in the dentate gyrus (DG). CR-IR/nNOS-IR neurons and CB-IR/nNOS-IR neurons were frequent in Ammon's horn but rare in the DG. In the entire hippocampus, the percentage of CR-IR neurons containing nNOS was significantly higher at the ventral level (44.3%) than at the dorsal level (17.0%). The percentage of CB-IR neurons containing nNOS was also significantly higher at the ventral level (42.3%) than at the dorsal level (29.3%). Next, we estimated the numerical densities (NDs) of calcium binding protein-containing GABAergic neurons. The ND of PV-IR neurons was comparable at the dorsal (1.16 x 10(3)/mm(3)) and ventral levels (1.23 x 10(3)/mm(3)), respectively. The ND of CR-IR neurons was less at the dorsal level (0.52 x 10(3)/mm(3)) than at the ventral level (0.64 x 10(3)/mm(3)). The ND of CB-IR neurons was also less at the dorsal level (0.91 x 10(3)/mm(3)) than at the ventral level (1.57 x 10(3)/mm(3)). Overall, approximately half of the GABAergic neurons contained one of the three calcium binding proteins (45% at the dorsal level and 47% at the ventral level). These data establish a baseline for examining potential roles of GABAergic neurons in hippocampal network activity in mice.

Nitric oxide as an anterograde neurotransmitter in the trigeminal motor pool

We demonstrate the presence of nitric oxide synthase containing fibers within the guinea pig trigeminal motor nucleus and describe the effects of nitric oxide (NO) on trigeminal motoneurons. Using immunohistochemical techniques, we observed nitrergic fibers displaying varicosities and giving rise to bouton-like structures in apposition to retrogradely labeled motoneuron processes, most of which were dendrites. NO-donors evoked a membrane depolarization (mean 7.5 mV) and a decrease in rheobase (mean 38%). These substances also evoked an apparent increase in an hyperpolarization-activated cationic current (I(H)). These changes were not accompanied by any modification of the motoneurons' input resistance or time constant. The effects were suppressed by blocking the cytosolic guanlyate cyclase. A membrane-permeant cyclic guanosine 3,5'-monophosphate (cGMP) analogue mimicked the effects of NO. There was a considerable increase in synaptic activity following NO-donors or db-cGMP application. Tetrodotoxin supressed the increase in synaptic activity evoked by NO-donors. The histological and electrophysiological evidence, taken together, indicates the existence of a nitrergic system able to modulate trigeminal motoneurons under yet unknown physiological conditions.

Peripartum interneuronal coupling in the supraoptic nucleus

In the supraoptic nucleus (SON), the incidence of conducting gap junctions (gjs), as indicated by dye coupling, is low in cycling females, but dramatically elevated in nursing mothers. Functionally, this is consistent with the well-established presence of synchronous milk ejection bursts among oxytocin neurons only in the lactating rat. In situ hybridization data, however, revealed elevated gj mRNA expression on the last day of pregnancy, a time when burst firing by putative oxytocin neurons is absent. Using Lucifer Yellow dye coupling, we determined the incidence of high conductance gjs in SONs of proestrous, immediately prepartum, postpartum non-lactating, lactating day 1, and lactating day 9-10 rats. Results indicate that coupling incidence is high only at times when milk ejection bursts are known to occur, and that the elevated gj mRNA expression seen on the last day of pregnancy does not indicate conducting gjs. It is suggested that gj conductance states, but not gj expression, are modulated by plasma estradiol titers.

Substance P and nitric oxide signaling in cerebral cortex: anatomical evidence for reciprocal signaling between two classes of interneurons

Parvalbumin-containing fast-spiking interneurons in the cerebral cortex exhibit widespread electrical coupling, as do somatostatin-containing low-threshold spiking interneurons. Besides the classical neurotransmitter gamma-aminobutyric acid, these cortical interneurons may also release various neuropeptides including substance P (SP), as well as the freely diffusible messenger nitric oxide (NO). To investigate whether these two networks of interneurons might interact via these nonclassical messengers, we performed immunocytochemistry for SP and NO signaling pathways in rat somatic sensory cortex. SP was found in a subset of parvalbumin-positive cells concentrated in layers IV and V, whereas its receptor, NK1, was found in a subset of somatostatin-containing neurons (and also, at much lower levels, in a disjoint subset of parvalbumin-containing neurons). Only 4% of SP-containing axon terminals were apposed to NK1-positive dendrites, suggesting that in the cerebral cortex, SP may act predominantly as a paracrine neuromediator. Nitric oxide synthase-I (NOS-I), the synthetic enzyme for NO, was found almost exclusively in NK1-positive neurons; 95% of intensely somatostatin/NK1-positive neurons were also positive for NOS-I, and 94% of NOS-positive neurons were also positive for NK1. Immunoreactivity for soluble guanylyl cyclase (the NO receptor) was at high levels in the apical dendrites of layer V pyramidal neurons and in parvalbumin/SP-positive neurons. These data point to a novel reciprocal chemical interaction between two inhibitory networks in the rat neocortex.

Expression of cGMP-specific phosphodiesterase 9A mRNA in the rat brain

cGMP has been implicated in the regulation of many essential functions in the brain, such as synaptic plasticity, phototransduction, olfaction, and behavioral state. Cyclic nucleotide phosphodiesterase (PDE) hydrolysis of cGMP is the major mechanism underlying the clearance of cGMP and is likely to be important in any process that depends on intracellular cGMP. PDE9A has the highest affinity for cGMP of any PDE, and here we studied the localization of this enzyme in the rat brain using in situ hybridization. PDE9A mRNA is widely distributed throughout the brain with varying regional expression. The pattern of PDE9A mRNA expression closely resembles that of soluble guanylyl cyclase (sGC) in the rat brain, suggesting a possible functional association or coupling of these two enzymes in the regulation of cGMP levels. Most of the brain areas expressing PDE9A mRNA also contain neuronal nitric oxide synthase (NOS), the enzymatic source of NO and the principal activator of sGC. PDE9A is the only cGMP-specific PDE with significant expression in the forebrain, and as such is likely to play an important role in NO-cGMP signaling.

Immunohistochemical localization of nitric oxide synthase and soluble guanylyl cyclase in the ventral cochlear nucleus of the rat

The diffusible messenger nitric oxide (NO) is implicated in auditory processing. It acts in the brain largely through activation of soluble guanylyl cyclase (sGC), a heterodimer comprised of alpha and beta subunits. The authors used immunohistochemistry to study the NO/guanosine 3',5'-cyclic monophosphate (cGMP) pathway in the cochlear nucleus of Sprague-Dawley rats. Central fibers of the cochlear nerve were stained for neuronal nitric oxide synthase (NOS-I) but not for sGCbeta. Within the ventral cochlear nucleus, a large fraction of principal cells were immunopositive for both NOS-I and sGCbeta; these cells could be seen at times receiving contacts from NOS-I-positive fibers. sGC staining of somatic cytoplasm extended into the distal dendritic tree. At variance with this pattern, NOS-I was concentrated mainly in somata. Double-labeling experiments showed that most of the principal neurons expressed both antigens. By contrast, in the granule cell domain, small cells that were immunopositive for NOS-I rarely corresponded to those that were immunopositive for sGC. To assess whether NOS-I and sGC immunoreactivities colocalize with their respective catalytic activities, the authors performed multiple labeling with L-citrulline (a by-product of the formation of NO from L-arginine) and cGMP, respectively. L-citrulline was restricted to NOS-I-positive elements, and the large majority of NOS-expressing neurons were positive for citrulline. Multiple labeling revealed that almost all sGC-positive neurons also accumulated cGMP both in the ventral cochlear nucleus and in the granule cell domain. These data suggest that NO is a signaling molecule in the cochlear nucleus, perhaps functioning in both a paracrine manner and an autocrine manner.

Presence of soluble and particulate guanylyl cyclase in the same hippocampal astrocytes

The localisation of particulate and soluble guanylyl cyclase was studied in hippocampal astrocytes. Counting the colocalisation of cGMP immunoreactivity with the astrocytic marker glial fibrillary acidic protein after stimulation of brain slices with sodium nitroprusside (0.1 mM) or atrial natriuretic peptide (100 nM), we were able to show that at least 67% of the hippocampal astrocytes contained both guanylyl cyclase isoforms. In addition, it was shown that a large number of atrial natriuretic peptide, brain-derived natriuretic peptide or sodium nitroprusside responsive cells contain the beta1-subunit of the soluble guanylyl cyclase. The results show that, in at least a subset of hippocampal astrocytes, soluble and particulate guanylyl cyclases are simultaneously present in the same cells.

Subunits of the nitric oxide receptor, soluble guanylyl cyclase, expressed in rat brain

Despite the widespread use of nitric oxide as a signalling molecule in the central nervous system, the molecular makeup of its receptor, soluble guanylyl cyclase (sGC), therein is poorly understood. Accordingly, RT-PCR and in situ hybridization were used to identify sGC subunits expressed in rat brain. In addition to the expected mRNA for alpha 1 and beta1 subunits, message for the beta 2 subunit was detected in the cerebellum at all developmental stages investigated (1--150 days postnatum). The use of degenerate primers allowed the identification of mRNA coding for the rat alpha 2 subunit, which was also expressed at every age studied. All but beta 2 were detected by in situ hybridization in the brains of both 8-day-old and adult rats. The distribution patterns indicated that in some areas, e.g. caudate-putamen and nucleus accumbens, sGC probably exists mainly as the alpha 1 beta 1 heterodimer. In others, e.g. hippocampus and olfactory bulb, alpha 2 beta 1 is likely to be dominant. In the cerebellum, alpha 1 and beta 1 message was strong in the Purkinje cell layer but was not confined to Purkinje cells: smaller cells, presumed to be the Bergmann glia, were also labelled. In contrast, alpha 2 mRNA was concentrated in cerebellar granule cells. Western blotting indicated an excess of alpha 1 over beta 1 protein in the cerebellum, the reverse of what was found in the lung. It is concluded that, in molecular terms, sGC is likely to be more complex and exhibit more regional variation in the brain than previously thought. The functional consequences of this heterogeneity require investigation.

The normal distribution and projections of constitutive NADPH-d/NOS neurons in the brainstem vestibular complex of the rat

The vestibular system is a highly conserved sensory system in vertebrates that is largely responsible for maintenance of one's orientation in space, posture, and balance and for visual fixation of objects during motion. In light of the considerable literature indicating an involvement of nitric oxide (NO) in sensory systems, it is important to determine whether NO is associated with vestibular pathways. To study the relationship of NO to vestibular pathways, we first examined the normal distribution of constitutive NADPH-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), in the vestibular complex (VC) and then examined its association with selected vestibular projection neurons. Survey of the four major vestibular nuclei revealed that only the medial vestibular nucleus contained significant numbers of perikarya stained for NADPH-d/NOS. By contrast, all the vestibular nuclei contained a network of fine processes that stained positive for NADPH-d, although the density of this network varied among the individual nuclei. To determine whether NADPH-d/NOS neurons project to vestibular efferent targets, injections of the retrograde tracer Fluoro-Gold were made into known targets of second-order vestibular neurons. Vestibular neurons containing constitutive NADPH-d/NOS were found to project predominantly to the oculomotor nucleus. A small number of neurons also participate in vestibulothalamic and intrinsic vestibular connections. These results indicate that NADPH-d/NOS neurons are prevalent in the MVN and that a subpopulation of these neurons project to the oculomotor complex. Nitric oxide is probably released locally from axons located throughout the vestibular complex but may play a particularly important role in vestibulo-ocular pathways.

Maturational differences in soluble guanylate cyclase activity in ovine carotid and cerebral arteries

Basal cGMP concentrations are greater in immature than in mature cranial arteries, which may help explain why cerebrovascular resistance is lower in neonates than in adults. The present studies explore the hypothesis that this difference derives from age-related differences in soluble guanylate cyclase (sGC) activity. Maturation depressed (p < 0.01) maximal sGC activity (pmol cGMP/mg/min) in both carotid (from 11.10 +/- 0.50 to 3.60 +/- 0.20) and cerebral (from 3.10 +/- 0.31 to 1.45 +/- 0.08) arteries. Western blot analysis of relative sGC abundance (relative to sGC expression in adult kidney) found that sGC abundance was significantly greater (p < 0.05) in newborn carotid (0.38 +/- 0.04) and cerebral arteries (0.37 +/- 0.06) than in adult arteries (0.25 +/- 0.05 and 0.17 +/- 0.03, respectively). Basal Km values in carotid and cerebral arteries did not differ significantly between newborns (3- to 7-d old) and adults. Activation of sGC with nitrosylated heme significantly reduced Km values 3- to 5-fold in both types of artery and in both age groups. Within artery type, maturation had no significant effect on activated Km. Between artery types, activated Km values were greater (p < 0.05) in cerebral (200 +/- 40 microM) than in carotid (80 +/- 10 microM) arteries. Together, these data suggest that variations in sGC substrate affinity contribute to observed differences in sGC activity between artery types but not those between age groups. In contrast, variations in enzyme abundance, and possibly also enzyme-specific activity, appear responsible for differences in sGC activity associated with both age and artery type.

Guanylyl cyclase-B represents the predominant natriuretic peptide receptor expressed at exceptionally high levels in the pineal gland

The generation and function(s) of the signalling molecule cyclic GMP (cGMP) in brain are still poorly understood. One mechanism to raise intracellular cGMP levels is binding of C-type natriuretic peptide (CNP) to a membrane guanylyl cyclase (GC), termed GC-B. Here, we demonstrate an exceptionally strong expression of GC-B in the pineal gland. Crosslinking experiments performed with 125I-Tyr(0)-CNP and membranes from various rat tissues identified the receptor as a 130-kDa protein, expressed at highest levels in pineal membranes. Receptor autoradiography on brain sections confirmed a striking density of CNP binding sites in pineal tissue, whereas binding sites for the related atrial natriuretic peptide (ANP) predominate in other regions of the brain. Incubations of freshly dissected whole pineal glands in either the absence or presence of natriuretic peptides followed by immunohistochemical analyses of cGMP revealed strong accumulations of cGMP in response to CNP but not to ANP in the majority of pinealocytes. Stimulation of soluble GC (sGC) activity by use of sodium nitroprusside (SNP) resulted in a very similar pattern of cGMP immunostaining, indicating a co-expression at high levels of particulate and soluble forms of GC. These findings point to a major role of cGMP signalling in pinealocytes and suggest an important regulatory function for CNP.

Molecular control of locus coeruleus neurotransmission

The locus coeruleus (LC) is the major noradrenergic nucleus in the brain and innervates large segments of the neuraxis. LC neurons are thought to regulate states of attention and vigilance as well as activity of the sympathetic nervous system. These neurons also have been implicated in the actions of stress, antidepressants, and opiates on the brain. Aided in part by the fact that the LC is relatively homogeneous, it has been possible to understand some of the cellular and molecular mechanisms that control their functional state. This review focuses on the role played by the cAMP pathway in regulation of LC neurons, particularly after chronic perturbations. Thus, several components of this intracellular signaling pathway are upregulated in the LC after chronic stress or chronic opiate treatment, but downregulated after chronic antidepressant treatment. LC neurons exhibit a pacemaker activity, which appears to be mediated, at least in part, by a nonspecific cation current that is activated by protein kinase A. As a result, stimuli that upregulate the cAMP pathway after chronic administration (e.g., stress or opiates) increase the excitability of LC neurons, whereas stimuli that downregulate the cAMP pathway (e.g., antidepressants) exert the opposite effect. Such molecular adaptations could contribute to the behavioral plasticity that is associated with these various conditions.

Tissue distribution of the human soluble guanylate cyclases

Soluble guanylate cyclase (sGC) is an important component of the NO signaling pathway. Human sGC isoforms alpha(1), alpha(2), and beta(1) show differential mRNA tissue distributions. alpha(1) and beta(1) are expressed in most tissues; however, the alpha(2) isoform shows a more restricted expression pattern with high levels in brain, placenta, spleen, and uterus only. Both alpha subunits exist as multiple transcripts whereas beta(1) exists as a single message. This study reports for the first time the tissue distribution of human sGC message and demonstrates that sGC isoforms are nonuniformly expressed which may be useful if the enzyme is to be exploited as a therapeutic target.

Localization of the cGMP-dependent protein kinases in relation to nitric oxide synthase in the brain

The distributions of the type I and type II isoforms of cGMP-dependent protein kinase were determined in the rat brain using immunohistochemistry and in situ hybridization, and compared with the localization of NO synthase determined with NADPH-diaphorase histochemistry. The type I cGMP-dependent protein kinase was highly expressed in the Purkinje cells of the cerebellar cortex, where it was closely associated with the NO synthase containing granule and basket cells. This kinase was also found in neurons in the dorsomedial nucleus of the hypothalamus, where it may be regulated by NO or atriopeptides. The type I kinase was not detected in other central neurons. In contrast, the type II kinase was widely distributed in the brain. In particular, it was highly expressed in the olfactory bulb, cortex, septum, thalamus, tectum and various brainstem nuclei. Many regions expressing this kinase also contained, or received innervation from NO synthase positive neurons. These results indicate that type I cGMP-dependent protein kinase may act as a downstream effector for NO only in the cerebellar cortex and the dorsomedial hypothalamus. The type II cGMP-dependent protein kinase appears to be a major mediator of NO actions in the brain.

Nitric oxide via cGMP-dependent mechanisms increases dye coupling and excitability of rat supraoptic nucleus neurons

Unlike many neuron populations, supraoptic nucleus (SON) neurons are rich in both nitric oxide synthase (NOS) and the NO receptor-soluble guanylyl cyclase (GC), the activation of which leads to cGMP accumulation. Elevations in cGMP result in increased coupling among SON neurons. We investigated the effect of NO on dye coupling in SONs from male, proestrus virgin female, and lactating rats. In 167 slices 263 SON neurons were recorded; 210 of these neurons were injected intracellularly (one neuron per SON) with Lucifer yellow (LY). The typically minimal coupling seen in virgin females was increased nearly fourfold by the NO precursor, L-arginine, or the NO donor, sodium nitroprusside (SNP). L-Arginine-induced coupling was abolished by a NOS inhibitor. In slices from male and lactating rats who have a higher basal incidence of coupling, SNP increased coupling by approximately twofold over control (p < 0.03). SNP effects were prevented by the NO scavenger hemoglobin (20 microM) and by the selective blocker of NO-activated GC, ODQ (10 microM). These results suggest that NO released from cells within the SON can expand the coupled network of neurons and that this action occurs via cGMP-dependent processes. Because increased coupling is associated with elevated SON neuronal excitability, we also studied the effects of 8-bromo-cGMP on excitability. In both phasically and continuously firing neurons 8-bromo-cGMP (1-2 mM), but not cGMP, produced membrane depolarizations accompanied by membrane conductance increases. Conductance increases remained when depolarizations were eliminated by current-clamping the membrane potential. Thus, NO-induced cGMP increases SON neuronal coupling and excitability.

A critical role of the nitric oxide/cGMP pathway in corticostriatal long-term depression

High-frequency stimulation (HFS) of corticostriatal glutamatergic fibers induces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of LTD can be mimicked by zaprinast, a selective inhibitor of cGMP phosphodiesterases (PDEs). Biochemical analysis shows that most of the striatal cGMP PDE activity is calmodulin-dependent and inhibited by zaprinast. The zaprinast-induced LTD occludes further depression by tetanic stimulation and vice versa. Both forms of synaptic plasticity are blocked by intracellular 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ), a selective inhibitor of soluble guanylyl cyclase, indicating that an increased cGMP production in the spiny neuron is a key step. Accordingly, intracellular cGMP, activating protein kinase G (PKG), also induces LTD. Nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and 7-nitroindazole monosodium salt (7-NINA) block LTD induced by either HFS or zaprinast, but not that induced by cGMP. LTD is also induced by the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine. SNAP-induced LTD occludes further depression by HFS or zaprinast, and it is blocked by intracellular ODQ but not by L-NAME. Intracellular application of PKG inhibitors blocks LTD induced by HFS, zaprinast, and SNAP. Electron microscopy immunocytochemistry shows the presence of NOS-positive terminals of striatal interneurons forming synaptic contacts with dendrites of spiny neurons. These findings represent the first demonstration that the NO/cGMP pathway exerts a feed-forward control on the corticostriatal synaptic plasticity.

Monitoring neuronal NO release in vivo in cerebellum, thalamus and hippocampus

A variety of methods has been developed based on in vivo microdialysis which allow one to examine the NO/cGMP signal transduction system in action in behaving animals. The extracellular levels of cGMP, the NO oxidative products nitrate and nitrite, and NO itself can all be determined. Using these methods changes in NO and cGMP production in response to pharmacological manipulations can be examined in vivo. In addition, it has been discovered that the activity of this system varies with the behavioral state of the animal. NO and cGMP appear to act via distinct downstream effectors in different brain regions. This opens up the possibility of selectively manipulating NO and cGMP signaling in discrete neuronal populations.

Distribution of nitric oxide synthase and nitric oxide-receptive, cyclic GMP-producing structures in the rat brain

The structures capable of synthesizing cyclic GMP in response to nitric oxide in the rat brain were compared relative to the anatomical localization of neuronal nitric oxide synthase. In order to do this, we used brain slices incubated in vitro, where cyclic GMP-synthesis was stimulated using sodium nitroprusside as a nitric oxide-donor compound, in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine. Nitric oxide-stimulated cyclic GMP synthesis was found in cells and fibers, but was especially prominent in varicose fibers throughout the rat brain. Fibers containing the nitric oxide-stimulated cyclic GMP production were present in virtually every area of the rat brain although there were large regional variations in the density of the fiber networks. When compared with the localization of nitric oxide synthase, it was observed that although nitric oxide-responsive and the nitric oxide-producing structures were found in similar locations in general this distribution was complementary. Only occasionally was nitric oxide-mediated cyclic GMP synthesis observed in structures which also contained nitric oxide synthase. We conclude that the nitric oxide-responsive soluble guanylyl cyclase and nitric oxide synthase are usually juxtaposed at very short distances in the rat brain. These findings very strongly support the proposed role of nitric oxide as an endogenous activator of the soluble guanylyl cyclase in the central nervous system and convincingly demonstrate the presence of the nitric oxide-cyclic GMP signal transduction pathway in virtually every area of the rat brain.

Sustained pharmacological inhibition of nitric oxide synthase does not affect the survival of intrastriatal rat fetal mesencephalic transplants

The objective of the present study was to investigate the potential role of the free radical nitric oxide (NO) in the development of fetal rat mesencephalic neurons grafted in a 6-hydroxydopamine (6-OHDA) lesioned rat model of Parkinson's disease. First, using nitric oxide synthase (NOS)-immunocytochemistry and reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry, we investigated the presence of the neuronal isoform of NOS (nNOS) in intrastriatal mesencephalic grafts. During the course of the experiment (16 weeks) an increase in the staining intensity and the number of nNOS/NADPH-d positive cells within the grafts was observed, as well as a gradual maturation of dopaminergic neurons. In addition, within both the host striatal and grafted mesencephalic tissue, a NO-dependent accumulation of cyclic guanosine monophosphate (cGMP) was detected, indicating the presence of guanylate cyclase, i.e., the target-enzyme for NO. Secondly, to determine the impact of NO on the survival of grafted dopaminergic neurons, 6-OHDA lesioned rats received mesencephalic grafts and were subsequently treated with the competitive NOS-inhibitor Nomega-nitro-l-arginine methylester (l-NAME). After chronic treatment for 4 weeks, tyrosine hydroxylase immunocytochemistry revealed no apparent differences between the survival of grafted dopaminergic neurons in control- or l-NAME treated animals, respectively. As the maturation of grafted dopaminergic neurons coincides with a gradual increase in the expression of nNOS within the graft and since dopaminergic cell numbers are not changed upon administration of l-NAME, it is concluded that endogenously produced and potentially toxic NO does not affect the survival of grafted fetal dopaminergic neurons.

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.

Soluble guanylate cyclase: the forgotten sibling

Despite widespread distribution in most mammalian cells, the role of soluble guanylate cyclase has, until recently, been poorly defined, especially when compared with its more illustrious sibling, adenylate cyclase. In this review Adrian Hobbs outlines some of the reasons why the soluble guanylate cyclase-cGMP pathway has remained outside the signalling spotlight for much of the past 30 years. He goes on to describe how new molecular biological and biochemical approaches have facilitated a characterization of soluble guanylate cyclase and how this enzyme has acquired a profound physiological significance, and much research attention, as the intracellular 'receptor' for nitric oxide.

Formation of olfactory memories mediated by nitric oxide

Sheep learn to recognize the odours of their lambs within two hours of giving birth, and this learning involves synaptic changes within the olfactory bulb. Specifically, mitral cells become increasingly responsive to the learned odour, which stimulates release of both glutamate and GABA (gamma-aminobutyric acid) neurotransmitters from the reciprocal synapses between the excitatory mitral cells and inhibitory granule cells. Nitric oxide (NO) has been implicated in synaptic plasticity in other regions of the brain as a result of its modulation of cyclic GMP levels. Here we investigate the possible role of NO in olfactory learning. We find that the neuronal enzyme nitric oxide synthase (nNOS) is expressed in both mitral and granule cells, whereas the guanylyl cyclase subunits that are required for NO stimulation of cGMP formation are expressed only in mitral cells. Immediately after birth, glutamate levels rise, inducing formation of NO and cGMP, which potentiate glutamate release at the mitral-to-granule cell synapses. Inhibition of nNOS or guanylyl cyclase activity prevents both the potentiation of glutamate release and formation of the olfactory memory. The effects of nNOS inhibition can be reversed by infusion of NO into the olfactory bulb. Once memory has formed, however, inhibition of nNOS or guanylyl cyclase activity cannot impair either its recall or the neurochemical release evoked by the learned lamb odour. Nitric oxide therefore seems to act as a retrograde and/or intracellular messenger, being released from both mitral and granule cells to potentiate glutamate release from mitral cells by modulating cGMP concentrations. We propose that the resulting changes in the functional circuitry of the olfactory bulb underlie the formation of olfactory memories.

Monitoring nitric oxide (NO) in rat locus coeruleus: differential effects of NO synthase inhibitors

A porphyrinic microsensor combined with in vivo voltammetry was used to monitor extracellular nitric oxide (NO) in the locus coeruleus (LC) of anaesthetized rats. Administration of N omega-nitro-L-arginine p-nitro-anilide (100 mg/kg, i.p) or 7-nitro indazole (30 mg/kg, i.p.), which both inhibit preferentially neuronal NO synthase (NOS), induced a marked decrease in the NO oxidation peak height. On the other hand, N omega-nitro-L-arginine methyl ester (L-NAME) (200 mg/kg, i.p.), a less selective NOS inhibitor, failed to decrease the NO signal. Moreover, intra LC administration of NMDA, known to activate LC noradrenergic neurones, increased the NO signal. This study demonstrates the usefulness of in vivo voltammetry to monitor basal levels of NO and their changes in the LC. Differential effects of NOS inhibitors show that their central activity need to be assessed through in situ measurement of NO before using these inhibitors as neuropharmacological tools.

Cortical afferents modulate striatal gap junction permeability via nitric oxide

Nitric oxide is a ubiquitous cellular messenger that plays a role in a variety of biological mechanisms. Within the central nervous system, it is formed in glia and neuronal cells and can diffuse away from its site of origin to modulate membrane conductances, neurotransmitter release, behavior, or to control the blood flow within its region of action. Nitric oxide has been shown to modulate gap junction conductance in the retina and cortex, an action it shares with a number of neurotransmitters. In this study, we found that the activation of cortical afferents increased dye coupling between rat neostriatal neurons recorded in vitro. This effect was mimicked by a nitric oxide donor and prevented by a nitric oxide synthase inhibitor, suggesting that activation of corticostriatal fibers may open putative gap junctions in the striatum via release of nitric oxide. This is the first report showing that synaptic modulation of gap junctions can be mediated by nitric oxide, and may provide a mechanism to explain cortical modulation of subcortical pattern formation within this highly integrative structure.

The alpha 1 subunit of soluble guanylyl cyclase is expressed prenatally in the rat brain

The mRNA encoding the alpha 1 subunit of soluble guanylyl cyclase (alpha 1sGC) was identified in a different-display screening for genes spatially and temporally regulated during the development of fetal rat brain. The initially isolated fragment of the 3' untranslated region was used for in situ hybridization and to produce full-length cDNA clones by hybridization screening of cDNA libraries and by RACE (rapid amplification of cDNA ends), respectively. In situ hybridization analysis that alpha 1sGC was absent at embryonic day 12 (E12), but by E14-E15, the forebrain exhibited dense expression in the developing striatum, medial cerebral wall containing the presumptive hippocampus, cerebellar neuroepithelium, and roof plate. Weaker expression was observed in the septum, epithalamus, ventral thalamus, pineal gland and retina. This pattern is largely maintained and refined at E18, with additional expression domains in the olfactory tubercle, nucleus accumbens, zona incerta and neocortex. During early postnatal development, the adult pattern is expressed, as previously reported. The unexpected, early expression of alpha 1sGC, in conjunction with the known absence of its heterodimeric partner, the beta subunit of sGC, from the developing rodent brain during fetal ages raises potentially novel functional roles of the alpha 1 subunit during ontogeny, and might imply the existence of an alternative beta subunit specific for the prenatal brain.

Nitric oxide synthase inhibitors attenuate acute and chronic morphine withdrawal response in the rat locus coeruleus: an in vivo voltammetric study

Previous studies have demonstrated that activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors contributes to the hyperactivity of noradrenergic neurons of the locus coeruleus (LC) associated with opioid and non-opioid drug withdrawal syndromes. Using an in vivo voltammetric approach, we have examined the role of nitric oxide (NO), which mediates NMDA receptor function, in this withdrawal-induced LC hyperactivity. In the anaesthetized rat, acute morphine treatment (10 micrograms, i.c.v.) suppressed (55.7 +/- 4.4% of baseline) the catechol oxidation current (CA-OC) recorded from the LC using differential normal pulse voltammetry (DNPV). A subsequent intravenous injection of naloxone (2 mg/kg, i.v.) reversed the drug-induced inhibition of LC response and produced an increase (118.9 +/- 2.3% of baseline) in CA-OC above baseline, indicative of an acute withdrawal response. Systemic (100 mg/kg) and intracerebroventricular (i.c.v.) (100 micrograms) pretreatment of animals with the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) blocked the naloxone-induced LC withdrawal response without influencing the inhibitory effect of morphine on LC activity. In animals chronically infused with morphine (15 micrograms/h, i.c.v., 5 days) a naloxone challenge (2 mg/kg, i.v.) produced significant increase (253.7 +/- 19.3% of baseline) in CA-OC signal. This LC withdrawal response was significantly reduced by pretreatment with L-NAME (100 micrograms, i.c.v.) or N omega-nitro-L-arginine (L-NOARG 10 micrograms, i.c.v.). In unanaesthetized animals pretreated with chronic morphine, systemic (100 mg/kg) and central L-NAME (100 micrograms) pretreatment suppressed some of the behavioural signs of withdrawal precipitated by naloxone (10 mg/kg) injection. As doses of the NOS inhibitors used in this study have previously been reported to produce significant inhibition of brain NOS activity, their effect on opioid withdrawal response most likely is due to NOS inhibition. The results of this study indicate that NO plays an intermediary role in the LC neuronal hyperactivity associated with both acute and chronic morphine withdrawal.

Long-term induction of haem oxygenase-1 (HSP-32) in astrocytes and microglia following transient focal brain ischaemia in the rat

Haem oxygenase-1 (HO-1) is a stress protein and a rate-limiting enzyme in haem degradation, generating ferrous iron, carbon monoxide and bile pigments. HO-1 has been suggested to be protective against oxidative stress. In the normal rodent brain the enzyme is localized in selected neuron populations, but heat shock, glutathione depletion in vivo and oxidative stress in vitro induce HO-1 predominantly in glial cells. We studied HO-1 expression in the brain following transient occlusion of the middle cerebral artery, and found increased mRNA levels in the ischaemic region from 4 h to 7 days after 90 min of ischaemia. The mRNA levels peaked at 12 h, and were localized perifocally. HO-1-immunoreactive astrocytes and microglial cells were seen in the perifocal area, in the ipsilateral and occasionally in the contralateral hippocampus. Some perifocal neurons were also HO-1-immunoreactive. In the infarcted area HO-1-positive microglia/macrophages were detected in double-labelling experiments. A microassay measuring the conversion of [14C]haem to [14C]bilirubin showed a two-fold increase in haem oxygenase activity in the infarcted core. These observations show a long-term induction of HO-1 protein and its activity following ischaemia-reperfusion brain injury, and indicate increased capacity for haem degradation and the generation of biologically active bile products, carbon monoxide and iron in astrocytes and some microglia/macrophages during focal brain ischaemia.

Long-term depression in rat cerebellum requires both NO synthase and NO-sensitive guanylyl cyclase

Long-term depression (LTD) of synaptic transmission between parallel fibres and Purkinje cells is a well-known example of synaptic plasticity taking place in the cerebellum. Nitric oxide (NO) has been implicated in synaptic plasticity in other brain areas, but its function in cerebellar LTD is controversial. Even when an involvement is suggested, the NO signal transduction pathway is unclear. One candidate is the cyclic GMP-synthesizing enzyme, soluble guanylyl cyclase, whose activity in the brain and elsewhere is powerfully stimulated by NO. By recording intracellularly from Purkinje cells in cerebellar slices, we demonstrate that blockade of NO synthase completely inhibits LTD induced by pairing parallel fibre stimulation with postsynaptic Ca2+ spike firing. LTD was also blocked by intracellular application of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a recently identified potent and selective inhibitor of soluble guanylyl cyclase. These findings indicate that soluble guanylyl cyclase is required for cerebellar LTD and suggest that this enzyme, located within Purkinje cells, transduces the NO signal in this form of synaptic plasticity.