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

Nitric oxide-sensitive guanylyl cyclase is differentially regulated by nuclear and non-nuclear estrogen pathways in anterior pituitary gland

17β-estradiol (E2) regulates hormonal release as well as proliferation and cell death in the pituitary. The main nitric oxide receptor, nitric oxide sensitive- or soluble guanylyl cyclase (sGC), is a heterodimer composed of two subunits, α and β, that catalyses cGMP formation. α1β1 is the most abundant and widely expressed heterodimer, showing the greater activity. Previously we have shown that E2 decreased sGC activity but exerts opposite effects on sGC subunits increasing α1 and decreasing β1 mRNA and protein levels. In the present work we investigate the mechanisms by which E2 differentially regulates sGC subunits' expression on rat anterior pituitary gland. Experiments were performed on primary cultures of anterior pituitary cells from adult female Wistar rats at random stages of estrous cycle. After 6 h of E2 treatment, α1 mRNA and protein expression is increased while β1 levels are down-regulated. E2 effects on sGC expression are partially dependent on de novo transcription while de novo translation is fully required. E2 treatment decreased HuR mRNA stabilization factor and increased AUF1 p37 mRNA destabilization factor. E2-elicited β1 mRNA decrease correlates with a mRNA destabilization environment in the anterior pituitary gland. On the other hand, after 6 h of treatment, E2-BSA (1 nM) and E2-dendrimer conjugate (EDC, 1 nM) were unable to modify α1 or β1 mRNA levels, showing that nuclear receptor is involved in E2 actions. However, at earlier times (3 h), 1 nM EDC causes a transient decrease of α1 in a PI3k-dependent fashion. Our results show for the first time that E2 is able to exert opposite actions in the anterior pituitary gland, depending on the activation of classical or non-classical pathways. Thus, E2 can also modify sGC expression through membrane-initiated signals bringing to light a new point of regulation in NO/sGC pathway.

RNA splicing in regulation of nitric oxide receptor soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC) is a key protein in the nitric oxide (NO)/-cGMP signaling pathway. sGC activity is involved in a number of important physiological processes including smooth muscle relaxation, neurotransmission and platelet aggregation and adhesion. Regulation of sGC expression and activity emerges as a crucial factor in control of sGC function in normal and pathological conditions. Recently accumulated evidence strongly indicates that the regulation of sGC expression is a complex process modulated on several levels including transcription, post-transcriptional regulation, translation and protein stability. Presently our understanding of mechanisms governing regulation of sGC expression remains very limited and awaits systematic investigation. Among other ways, the expression of sGC subunits is modulated at the levels of mRNA abundance and transcript diversity. In this review we summarize available information on different mechanisms (including transcriptional activation, mRNA stability and alternative splicing) involved in the modulation of mRNA levels of sGC subunits in response to various environmental clues. We also summarize and cross-reference the information on human sGC splice forms available in the literature and in genomic databases. This review highlights the fact that the study of the biological role and regulation of sGC splicing will bring new insights to our understanding of NO/cGMP biology.

Nitric oxide sensitive-guanylyl cyclase subunit expression changes during estrous cycle in anterior pituitary glands

17beta-estradiol (E2) exerts inhibitory actions on the nitric oxide pathway in rat adult pituitary glands. Previously, we reported that in vivo E2 acute treatment had opposite effects on soluble guanylyl cyclase (sGC) subunits, increasing alpha1- and decreasing beta1-subunit protein and mRNA expression and decreasing sGC activity in immature rats. Here we studied the E2 effect on sGC protein and mRNA expression in anterior pituitary gland from adult female rats to address whether the maturation of the hypothalamus-pituitary axis influences its effects and to corroborate whether these effects occur in physiological conditions such as during estrous cycle. E2 administration causes the same effect on sGC as seen in immature rats, and these effects are estrogen receptor dependent. These results suggest that E2 is the main effector of these changes. Since the sGC alpha-subunit increases while the sGC activity decreases, we studied if other less active isoforms of the sGC alpha-subunit are expressed. Here we show for the first time that sGCalpha2 and sGCalpha2 inhibitory (alpha2i) isoforms are expressed in this gland, but only sGCalpha2i mRNA increased after E2 acute treatment. Finally, to test whether E2 effects take place under a physiological condition, sGC subunit expression was monitored over estrous cycle. sGCalpha1, -beta1, and -alpha2i fluctuate along estrous cycle, and these changes are directly related with E2 level fluctuations rather than to NO level variations. These findings show that E2 physiologically regulates sGC expression and highlight a novel mechanism by which E2 downregulates sGC activity in rat anterior pituitary gland.

Amygdala gene expression correlates of social behavior in monkeys experiencing maternal separation

Children exposed to early parental loss from death or separation carry a greater risk for developing future psychiatric illnesses, such as major depression and anxiety. Monkeys experiencing maternal separation at 1 week of age show fewer social behaviors and an increase in self-comforting behaviors (e.g., thumb sucking) over development, whereas in contrast, monkeys experiencing maternal separation at 1 month of age show increased seeking of social comfort later in life. We sought to identify neural systems that may underlie these stress-induced behavioral changes by examining changes in mRNA content in amygdala tissue collected from 1 week separated, 1 month separated, and maternally reared infants at 3 months of age. mRNA from the right medial temporal lobe, primarily the amygdala, was analyzed using Affymetrix U133A 2.0 arrays. One gene, guanylate cyclase 1 alpha 3 (GUCY1A3), showed differential expression between the 1 week and maternally reared groups and the 1 week and 1 month groups; these changes were confirmed by in situ hybridization. The expression of this gene was positively correlated with acute social-comforting behavior (r = 0.923; p = 0.001) and longer-term close social behavior (r = 0.708; p = 0.015) and negatively correlated with self-comforting behaviors (r = -0.88; p < 0.001). Additional in situ hybridization studies of GUCY1A3 in normal monkeys showed that this gene is expressed at adult levels by 1 week of age and that its expression is greater in the amygdala than all other brain areas examined. We conclude that GUCY1A3 may contribute to the altered behavioral phenotypes that are differentially displayed depending on the age at which macaque infants experience an early-life stress.

Enhanced locomotor activity in adult rats with neonatal administration ofN-omega-nitro-L-arginine

Nitric oxide (NO) is a neuronal messenger molecule that plays important roles in the development, maintenance, and functional modifications of brain circuits. We investigated whether the NO levels at different postnatal day (P) periods of the brain develop interference with the locomotion in a novel environment during the postpuberal age (P60). First, using the determination of the nitrite accumulation, we evaluated whether treatment with the NO-synthase inhibitor N-nitro-L-arginine (L-NNA) during different neonatal ages (P1 to P3, P4 to P6, and P7 to P9) affected the levels of NO activity in different regions in the neonatal brain of the rat. We then evaluated whether the locomotor activity in the adult rat (P60) is affected by the blocking of the neonatal NO-activity during a specific period of the development of the nervous system. Neonatal rats with L-NNA administration at P4 to P6 and P7 to P9 show a significant decrease in the levels of NO activity in all the brain regions. However, the blocking of NO synthesis during the neonatal period between P4 to P6 produced an increase in the locomotion after puberty. These data suggest that during a specific step in the development of the brain, the NO levels may play a critical role in the structures that control the spontaneous locomotion in a novel environment after puberty.

Expression of soluble guanylyl cyclase in rat cerebral cortex during postnatal development

Soluble guanylyl cyclase (sGC), the principle "receptor" for nitric oxide (NO), catalyzes the formation of cyclic guanosine monophosphate (cGMP), an intracellular second messenger. Studies in invertebrates have shown that the NO/cGMP pathway is involved in several aspects of neural development, including neuronal migration, dendritic and axonal outgrowth, and synaptogenesis. In vitro studies suggest a developmental role also in mammals. To investigate whether the NO/cGMP pathway might mediate these processes in vivo, we performed immunohistochemistry for sGC on sections from postnatal rat cerebral cortex. Early in postnatal development, migrating neurons in the cortical plate were immunonegative, whereas neurons deeper in the cortex that had completed migration were immunopositive. At the subcellular level, sGC preferentially stained dendrites rather than axons, but, at postnatal day 1 (PND1), sGC was found in a large fraction of axonal growth cones, especially those oriented toward the pial surface. At PND10-20 (the period of maximal synaptogenesis), sGC immunostaining was located mainly in dendritic shafts and was only occasionally associated with spines or axon terminals. These results support a role for the NO/cGMP pathway in dendritic development but argue against a major role in neuronal migration and synaptogenesis.

Soluble guanylate cyclase and neuronal nitric oxide synthase colocalize in rat nucleus tractus solitarii

Nitric oxide has been implicated in transmission of cardiovascular signals in the nucleus tractus solitarii (NTS). Pharmacological studies suggest that activation of neurons by nitric oxide in the NTS may involve soluble guanylate cyclase (sGC). However, anatomical data supporting this suggestion have not been available. In this study, we tested the hypothesis that neurons and fibers containing neuronal nitric oxide synthase (nNOS) lie in close proximity to those containing sGC and the two enzymes colocalize in some neurons and fibers in the NTS. We perfused six rats and obtained brain stem sections for double immunofluorescent staining utilizing antibodies selective for sGC and for nNOS combined with confocal microscopy. The distribution and staining intensity of nNOS-immunoreactivity (IR) was similar to our earlier reports. IR of sGC was present in cell bodies, proximal dendrites and fibers of many brain stem regions. Strong sGC-IR was noted in the hypoglossal, dorsal motor nucleus of vagus and gracilis nuclei. The NTS exhibited moderate sGC-IR. Superimposed images showed that many NTS neurons contained both nNOS-IR and sGC-IR. The percentage of sGC-IR positive cells that were also nNOS-IR positive differed among NTS subnuclei. Similarly, the percentage of nNOS-IR positive cells that were also sGC positive differed among NTS subnuclei. Fibers stained for both nNOS-IR and sGC-IR were also present in NTS subnuclei. In addition, we identified fibers that were stained for nNOS-IR or sGC-IR alone and often found such singly labeled fibers apposed to each other. These data support our hypothesis and provide anatomical support for the suggestion that nitroxidergic activation of the NTS involves sGC.

Cerebral expression of the α2-subunit of soluble guanylyl cyclase is linked to cerebral maturation and sensory pathway refinement during postnatal development

Soluble guanylyl cylase (sGC) has been identified for being a receptor for the gaseous transmitters nitric oxide and carbon monoxide. Currently four subunits alpha1, alpha2, beta1, and beta2 have been characterized. Heterodimers of alpha and beta-subunits as well as homodimers of the beta2-subunit are known to constitute functional sGC which use GTP to form cGMP a potent signal molecule in a multitude of second messenger cascades. Since NO-cGMP signaling plays a pivotal role in neuronal development we analyzed the maturational expression pattern of the newly characterized alpha2-subunit of sGC within the brain of Wistar rats by means of RNase protection assay and immunohistochemistry. alpha2-subunit mRNA as well as immunoreactive alpha2-protein increased during postnatal cerebral development. Topographical analysis revealed a selective high expression of the alpha2-subunit in the choroid plexus and within developing sensory systems involving the olfactory and somatosensory system of the forebrain as well as parts of the auditory and visual system within the hindbrain. In cultured cortical neurons the alpha2-subunit was localized to the cell membrane, especially along neuronal processes. During the first 11 days of postnatal development several cerebral regions showed a distinct expression of the alpha2-subunit which was not paralleled by the alpha1/beta1-subunits especially within the developing thalamo-cortical circuitries of the somatosensory system. However, at later developmental stages all three subunits became more homogenously distributed among most cerebral regions, indicating that functional alpha1/beta1 and alpha2/beta1 heterodimers of sGC could be formed. Our findings indicate that the alpha2-subunit is an essential developmentally regulated constituent of cerebral sensory systems during maturation. In addition the alpha2-subunit may serve other functions than forming a functional heterodimer of sGC during the early phases of sensory pathway refinement.

Elements of the nitric oxide/cGMP pathway expressed in cerebellar granule cells: biochemical and functional characterisation

It is known that the nitric oxide (NO)/cGMP pathway affects neuronal development and the expression of the different proteins is developmentally dependent in several brain areas. However, so far there are no data on the expression of the proteins involved in this signalling system during the development of the cerebellar granule cell, one of the most widely used models of neuronal development. This study was accordingly designed to analyse the developmental regulation of neuronal nitric oxide synthase (nNOS), soluble guanylyl cyclase subunits (alpha1, alpha2 and beta1) and cGMP-dependent protein kinases (cGK I and cGK II) in cerebellar granule cells through real time-polymerase chain reaction (RT-PCR) and Western blotting. We were able to detect guanylyl cyclase subunits and cGK I and cGK II in cerebellar granule cells at every stage of development examined (cells freshly isolated from 7-day-old rat pups, and cells cultured for 7 days or 14 days). Expression levels, nevertheless, varied significantly at each stage. nNOS, alpha2 and beta1 and cGK II levels increased during granule cell development, while alpha1 and cGK I showed an opposite behaviour pattern; the levels of these latter proteins diminished as the cells matured. The functionality of this pathway was assessed by stimulating cells kept in culture for 7 days with DEA/NO or with N-methyl-D-aspartate (NMDA). Cells responded by increasing intracellular cGMP and activating cGMP-dependent protein kinase activity, which effectively phosphorylated two well-known substrates of this activity, the vasodilator stimulated phosphoprotein (VASP) and the cAMP response element binding protein (CREB). In summary, through both functional and biochemical tests, this is the first demonstration of a complete NO/cGMP signalling transduction pathway in cerebellar granule cells. Our results also indicate the developmental regulation of the proteins in this system.

Regionally restricted expression of the transcription factor c-myc intron 1 binding protein during brain development

Transcription factors regulate proliferation, differentiation, and regionalization of the central nervous system. In a screen of developing cerebral cortex, we identified the transcription factor c-myc intron 1 binding protein (mibp1) due to its abundant expression. In this study, we analyzed the temporal and spatial expression patterns of mibp1 mRNA in developing mouse brain to address the putative role of this transcription factor in neural differentiation. Northern hybridization studies revealed that mibp1 is expressed first in the mouse dorsal telencephalon at embryonic day (E) 14.5, during peak neuronal production. In situ hybridization experiments revealed that mibp1 expression in the cerebral wall is most abundant in postmitotic cells of the cortical plate and absent from proliferative zones. Moreover, mibp1 is restricted to dorsal telencephalon during embryogenesis with expression only in the cerebral wall, olfactory bulb, and hippocampus. N-myc, a potential target of mibp1 regulation, exhibited complementary, nonoverlapping expression patterns in the telencephalon with greatest expression in proliferating cells of the ventricular zone from E12.5 to E14.5; N-myc was absent from the telencephalon by E15.5. The specificity and timing of mibp1 expression in the cerebral cortex suggests a role in maintaining a state of neuronal differentiation in the dorsal telencephalon.

CCAAT-binding factor regulates expression of the beta1 subunit of soluble guanylyl cyclase gene in the BE2 human neuroblastoma cell line

Soluble guanylyl cyclase (sGC) is a cytosolic enzyme producing the intracellular messenger cyclic guanosine monophosphate (cGMP) on activation with nitric oxide (NO). sGC is an obligatory heterodimer composed of alpha and beta subunits. We investigated human beta1 sGC transcriptional regulation in BE2 human neuroblastoma cells. The 5' upstream region of the beta1 sGC gene was isolated and analyzed for promoter activity by using luciferase reporter constructs. The transcriptional start site of the beta1 sGC gene in BE2 cells was identified. The functional significance of consensus transcriptional factor binding sites proximal to the transcriptional start site was investigated by site deletions in the 800-bp promoter fragment. The elimination of CCAAT-binding factor (CBF) and growth factor independence 1 (GFI1) binding cores significantly diminished whereas deletion of the NF1 core elevated the transcription. Electrophoretic mobility-shift assay (EMSA) and Western analysis of proteins bound to biotinated EMSA probes confirmed the interaction of GFI1, CBF, and NF1 factors with the beta1 sGC promoter. Treatment of BE2 cells with genistein, known to inhibit the CBF binding to DNA, significantly reduced protein levels of beta1 sGC by inhibiting transcription. In summary, our study represents an analysis of the human beta1 sGC promoter regulation in human neuroblastoma BE2 cells and identifies CBF as a critically important factor in beta1 sGC expression.

Prenatal exposure of rats to Ginkgo biloba extract (EGb 761) increases neuronal survival/growth and alters gene expression in the developing fetal hippocampus

Hippocampal neuron survival/growth and gene expression have been examined after prenatal (in utero) exposure of rats to EGb 761, a leaf extract of Ginkgo biloba. Oral administration of EGb 761 (100 or 300 mg/kg/day) to pregnant dams for 5 days increased the number of hippocampal neurons (maintained in culture) of their fetuses, indicating a neurotrophic effect of the extract. Using large-scale oligonucleotide microarrays containing over 8000 combined rat genes and expressed sequence tag clusters, it was shown that treatment of pregnant dams with EGb 761 (25, 50 or 100 mg/kg/day for 5 days) altered the expression of 187 genes in the hippocampi of male fetuses and 160 genes in those of female fetuses. Using gene-cluster analysis, these genes were grouped into 18 distinct clusters for males and 17 distinct clusters for females. Among these clusters, 35 genes shared a common expression pattern in male and female hippocampal development. Of these genes, the changes observed in insulin growth factor II, insulin growth factor binding protein 2, testosterone repressed prostate message-2, glutathione-dependent dehydroascorbate reductase, lipoprotein lipase, guanylate cyclase and DNA binding protein Brn-2 were confirmed by real-time quantitative polymerase chain reaction. These findings, which have provided the first genetic profile of the effects of EGb 761 on the developing rat hippocampus, increase our understanding of the molecular and genetic programs that are activated by the extract. These effects of EGb 761 may underlie its neuroprotective properties.

Differential expression of NO-sensitive guanylyl cyclase subunits during the development of rat cerebellar granule cells: regulation via N-methyl-D-aspartate receptors

In primary cultures of rat cerebellar granule cells with a functional network of glutamatergic neurons, the expression pattern of the different subunits of nitric-oxide (NO)-sensitive guanylyl cyclase changes during cell differentiation. These cells express the alpha1, alpha2 and beta1 subunits of NO-sensitive guanylyl cyclase and synthesize cyclic guanosine monophosphate (cGMP) in response to exogenous or endogenous nitric oxide. In this study, we determined the protein content of the alpha1 and beta1 subunits and quantified alpha1, alpha2 and beta1 mRNA by reverse transcription coupled to a polymerase chain reaction (RT-PCR). Expression of the beta1 subunit increased with the degree of cell differentiation, although most marked changes occurred at the alpha subunit level. In cells freshly isolated from rat pups on postnatal day 7 (P7) the most abundant alpha subunit was alpha1, while alpha2 appeared as the predominant subunit of this type in cultured cells. N-methyl-D-aspartate (NMDA) receptor stimulation in 7- or 14-day-cultured cells led to the upregulation of guanylyl cyclase subunit mRNAs; alpha2 mRNA levels undergoing most significant change. This enhanced subunit expression was accompanied by an increase in the amount of cGMP synthesized in response to NO. Thus, it seems that alpha2 subunits are increasingly expressed as granule cells mature. The presence of this subunit in the guanylyl cyclase heterodimer facilitates its localization at synaptic membranes, where the enzyme acts as a sensor for NO formed by the postsynaptic protein 95 (PSD-95)-associated neuronal NO synthase.

Molecular aspects of soluble guanylyl cyclase regulation

Soluble guanylyl cyclase (sGC) is a heterodimeric enzyme (comprised of alpha and beta subunits) that generates the intracellular second messenger cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP). cGMP is subsequently important for the regulation of protein kinases, ion channels, and phosphodiesterases. Since recent evidence has demonstrated that heterodimerization of the alpha/beta subunits is essential for basal and stimulated enzymatic activity, the existence of several types of isoforms for each of the two subunits, along with their varying degrees of expression in different tissues, implies that multiple regulatory mechanisms exist for sGC. Yet, progress in studying and clarifying the regulatory processes that can alter sGC expression and activity has only slowly started being elucidated. In the following paper, we elaborate on sGC structure, function, and distribution along with recently described signaling pathways that modulate sGC gene expression.

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.

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.

A delayed role for nitric oxide-sensitive guanylate cyclases in a migratory population of embryonic neurons

Neuronal differentiation requires a coordinated intracellular response to diverse extracellular stimuli, but the role of specific signaling mechanisms in regulating this process is still poorly understood. Soluble guanylate cyclases (sGCs), which can be stimulated by diffusible free radical gasses such as nitric oxide (NO) and carbon monoxide (CO) to produce the intracellular messenger cGMP, have recently been found to be expressed within a variety of embryonic neurons and implicated in the control of both neuronal motility and differentiation. Using the enteric nervous system (ENS) of the moth, Manduca sexta, we examined the role of NO and NO-sensitive sGCs during the migration and differentiation of an identified set of migratory neurons (the EP cells). Shortly after the onset of their migration, a subset of EP cells began to express NO-sensitive sGC activity (visualized with an anti-cGMP antiserum). Unlike many neurons in the central nervous system, the expression of sGC activity in the EP cells was not transient but persisted throughout subsequent periods of axon elongation and terminal branch formation on the gut musculature. In contrast, nitric oxide synthase activity (visualized using NADPH-diaphorase histochemistry) was undetectable in the vicinity of the EP cells until the period of synapse formation. Manipulations designed to alter sGC and NOS activity in an in vivo embryonic culture preparation had no discernible effect on either the migration or axonal outgrowth of the EP cells. In contrast, inhibition of both of these enzymes resulted in a significant reduction in terminal synaptic branch formation within the postmigratory neurons. These results indicate that while NO-sensitive sGC activity is expressed precociously within the EP cells during their initial migratory dispersal, a role for this signaling pathway can only be demonstrated well after migration is complete, coincident with the formation of mature synaptic connections.

Behavioural, neurochemical and neuroanatomical effects of chronic postnatal N-nitro-L-arginine methyl ester treatment in neonatal and adult rats

In the present study we evaluated the consequences of interference with nitric oxide synthesis during development on brain function and behaviour in later life. Rat pups received a daily injection of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME, 25 mg/kg, s.c.) from postnatal day 0 to 24. At postnatal day 8 L-NAME-treated rats had enlarged and heavier stomachs, while body weights appeared to be reduced. The stomachs were not affected in size and weight anymore at postnatal day 24, whereas the body weights were still reduced by the L-NAME treatment, although they soon recovered after termination of the treatment. At four months-of-age, rats were tested in non-cognitive (open field) and cognitive (Morris water escape, two-way active avoidance) tasks. Open field behaviour of adult rats postnatally treated with L-NAME was not affected. In the water escape task there were no differences between the saline and L-NAME-treated rats in spatial discrimination learning and spatial reversal learning. Furthermore, postnatal L-NAME treatment did not have an effect on the acquisition of the two-way active avoidance task. Subsequently, we tested rat pups during the L-NAME treatment at postnatal day 19 through 24 in the open field and the two-way active avoidance task. L-NAME treatment appeared to increase the behavioural activity in the open field. There was no difference in behaviour in the active avoidance task between saline and L-NAME-treated rats. Biochemical and immunocytochemical studies showed that at postnatal day 8 the basal cyclic GMP level was reduced, while the cyclic GMP formation due to incubation with the nitric oxide donor sodium nitroprusside appeared to be increased in the hippocampus, striatum and frontal cortex of L-NAME-treated rats. Hence, nitric oxide synthase was inhibited whereas the soluble guanylyl cyclase activity may be increased in sensitivity. At postnatal day 24 basal cyclic GMP levels and nitric oxide-mediated cyclic GMP formation in the brain structures of L-NAME-treated rats had normal values again. Taken together, the findings of this study suggest that postnatal inhibition of nitric oxide synthase has profound neurochemical effects during development and may have short-lasting effects on non-cognitive behaviour, but it does not affect behaviour and brain function in later life.

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.