Mice deficient in endothelial nitric oxide synthase exhibit a selective deficit in hippocampal long-term potentiation

The molecular memory code and synaptic plasticity: A synthesis

The most widely accepted view of memory in the brain holds that synapses are the storage sites of memory, and that memories are formed through associative modification of synapses. This view has been challenged on conceptual and empirical grounds. As an alternative, it has been proposed that molecules within the cell body are the storage sites of memory, and that memories are formed through biochemical operations on these molecules. This paper proposes a synthesis of these two views, grounded in a computational model of memory. Synapses are conceived as storage sites for the parameters of an approximate posterior probability distribution over latent causes. Intracellular molecules are conceived as storage sites for the parameters of a generative model. The model stipulates how these two components work together as part of an integrated algorithm for learning and inference.

Structural brain abnormalities in endothelial nitric oxide synthase-deficient mice revealed by high-resolution magnetic resonance imaging

INTRODUCTION

Endothelial nitric oxide synthase (eNOS) produces nitric oxide, which is essential for a variety of physiological functions in the brain. Previous work has demonstrated the detrimental effects of eNOS deficiency on brain function in male eNOS knockout (eNOS KO) mice. However, the effect of eNOS deficiency on brain structure and any association between these effects and sex is unknown.

METHODS

This study used three-dimensional high-resolution ex vivo magnetic resonance imaging and behavioral tests of anxiety and cognitive performance to investigate structure-function relationships in the brain of female and male eNOS KO mice in young adulthood.

RESULTS

While there were no differences in anxiety-like behavior or locomotion, there was a sex-specific deficit in contextual fear memory retention in male, but not in female, eNOS mice compared to wild-type controls. Moreover, we found that eNOS deficiency induced changes in multiple brain regions that are involved in learning and fear memory including the hippocampus, amygdala, hypothalamus, and areas of the cortex. Several of these MRI-detectable neuroanatomical changes were dependent on sex.

CONCLUSION

The observation that eNOS deficiency impacts brain structure at an early age demonstrates the importance of eNOS for healthy brain development.

A multilevel prediction of physiological response to challenge: Interactions among child maltreatment, neighborhood crime, endothelial nitric oxide synthase gene (eNOS), and GABA(A) receptor subunit alpha-6 gene (GABRA6)

Physiological response to stress has been linked to a variety of healthy and pathological conditions. The current study conducted a multilevel examination of interactions among environmental toxins (i.e., neighborhood crime and child maltreatment) and specific genetic polymorphisms of the endothelial nitric oxide synthase gene (eNOS) and GABA(A) receptor subunit alpha-6 gene (GABRA6). One hundred eighty-six children were recruited at age 4. The presence or absence of child maltreatment as well as the amount of crime that occurred in their neighborhood during the previous year were determined at that time. At age 9, the children were brought to the lab, where their physiological response to a cognitive challenge (i.e., change in the amplitude of the respiratory sinus arrhythmia) was assessed and DNA samples were collected for subsequent genotyping. The results confirmed that complex Gene × Gene, Environment × Environment, and Gene × Environment interactions were associated with different patterns of respiratory sinus arrhythmia reactivity. The implications for future research and evidence-based intervention are discussed.

Nitric oxide targets oligodendrocytes and promotes their morphological differentiation

In the central nervous system, nitric oxide (NO) transmits signals from one neurone to another, or from neurones to astrocytes or blood vessels, but the possibility of oligodendrocytes being physiological NO targets has been largely ignored. By exploiting immunocytochemistry for cGMP, the second messenger generated on activation of NO receptors, oligodendrocytes were found to respond to both exogenous and endogenous NO in cerebellar slices from rats aged 8 days to adulthood. Atrial natriuretic peptide, which acts on membrane-associated guanylyl cyclase-coupled receptors, also raised oligodendrocyte cGMP in cerebellar slices. The main endogenous source of NO accessing oligodendrocytes appeared to be the neuronal NO synthase isoform, which was active even under basal conditions and in a manner that was independent of glutamate receptors. Oligodendrocytes in brainstem slices were also shown to be potential NO targets. In contrast, in the optic nerve, oligodendrocyte cGMP was raised by natriuretic peptides but not NO. When cultures of cerebral cortex were continuously exposed to low NO concentrations (estimated as 40–90 pM), oligodendrocytes responded with a striking increase in arborization. This stimulation of oligodendrocyte growth could be replicated by low concentrations of 8-bromo-cGMP (maximum effect at 1 µM). It is concluded that oligodendrocytes are probably widespread targets for physiological NO (or natriuretic peptide) signals, with the resulting rise in cGMP serving to enhance their growth and maturation. NO might help coordinate the myelination of axons to the ongoing level of neuronal activity during development and could potentially contribute to adaptive changes in myelination in the adult.

Two sides to long-term potentiation: a view towards reconciliation

Almost since the discovery of long-term potentiation (LTP) in the hippocampus, its locus of expression has been debated. Throughout the years, convincing evidence has accumulated to suggest that LTP can be supported either presynaptically, by an increase in transmitter release, or postsynaptically, by an increase in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor number. However, whereas postsynaptic enhancement appears to be consistently obtained across studies following LTP induction, presynaptic enhancement is not as reliably observed. Such discrepancies, along with the failure to convincingly identify a retrograde messenger required for presynaptic change, have led to the general view that LTP is mainly supported postsynaptically, and certainly, research within the field for the past decade has been heavily focused on the postsynaptic locus. Here, we argue that LTP can be expressed at either synaptic locus, but that pre- and postsynaptic forms of LTP are dissociable phenomena mediated by distinct mechanistic processes, which are sensitive to different patterns of neuronal activity. This view of LTP helps to reconcile discrepancies across the literature and may put to rest a decades-long debate.

Cytokines induced by long-term potentiation (LTP) recording: a potential explanation for the lack of correspondence between learning/memory performance and LTP

The relationship between learning/memory performance and long-term potentiation (LTP) induction is ambiguous. Although a large body of data supports a strong correspondence between learning/memory performance and LTP, many studies have also provided evidence to the contrary. In this study, we found that 2-month-old senescence-accelerated mice/prone 8 (SAMP8 mice) displayed both impaired performance in a Morris Water Maze (MWM) and enhanced LTP compared to senescence-accelerated mice/resistance 1 (SAMR1). BALB/c mice challenged with Complete Freund's Adjuvant (CFA) performed better in the shuttle-box test but displayed impaired LTP compared to intact animals. It is interesting that BALB/c mice challenged with Incomplete Freund's Adjuvant (IFA) performed better than intact animals, with no LTP impairment. Cytokine analysis showed no significant differences between the interleukin-6 (IL-6), interleukin-10 (IL-10) or TNF-α content in the intact hippocampal tissues of either the SAMR1 and SAMP8 mice or the immune-challenged BALB/c and intact animals. Further analysis demonstrated that the increase in cytokine content was higher in the hippocampal tissues used for LTP recording in the SAMR1 and CFA-challenged animals compared to the SAMP8 and intact BALB/c mice. A correlation analysis demonstrated that pro-inflammatory cytokines (IL-6 and TNF-α) displayed a negative correlation with LTP, while an anti-inflammatory cytokine (IL-10) displayed a positive correlation with LTP. These results suggest that pro-inflammatory cytokines induced by LTP manipulation in experiments (e.g., via tissue injury caused by electrode insertion) may be one of the factors contributing to the observed lack of correspondence between memory/learning ability and LTP.

Frequent spontaneous seizures followed by spatial working memory/anxiety deficits in mice lacking sphingosine 1-phosphate receptor 2

The diverse physiological effects of sphingosine 1-phosphate (S1P) are mostly mediated by its five cognate G protein-coupled receptors, S1P(1)-S1P(5), which have attracted much attention as future drug targets. To gain insight into S1P(2)-mediated signaling, we analyzed frequent spontaneous seizures in S1P(2)-deficient (S1P(2)(-/-)) mice obtained after several backcrosses onto a C57BL/6N background. Full-time video recording of 120 S1P(2)(-/-) mice identified 420 seizures both day and night between postnatal days 25 and 45, which were accompanied by high-voltage synchronized cortical discharges and a series of typical episodes: wild run, tonic-clonic convulsion, freezing, and, occasionally, death. Nearly 40% of 224 S1P(2)(-/-) mice died after such seizures, while the remaining 60% of the mice survived to adulthood; however, approximately half of the deliveries from S1P(2)(-/-) pregnant mice resulted in neonatal death. In situ hybridization revealed exclusive s1p(2) expression in the hippocampal pyramidal/granular neurons of wild-type mice, and immunohistochemistry/microarray analyses identified enhanced gliosis in the whole hippocampus and its neighboring neocortex in seizure-prone adult S1P(2)(-/-) mice. Seizure-prone adult S1P(2)(-/-) mice displayed impaired spatial working memory in the eight-arm radial maze test and increased anxiety in the elevated plus maze test, whereas their passive avoidance learning memory performance in the step-through test and hippocampal long-term potentiation was indistinguishable from that of wild-type mice. Our findings suggest that blockade of S1P(2) signaling may cause seizures/hippocampal insults and impair some specific central nervous system functions.

The function of NO-sensitive guanylyl cyclase: what we can learn from genetic mouse models

The signaling molecule nitric oxide (NO) acts as physiological activator of NO-sensitive guanylyl cyclase (NO-GC) in the cardiovascular, gastrointestinal and nervous systems. Two isoforms of NO-GC are known to exist on the protein level. The enzyme is a heterodimer consisting of an alpha (alpha(1) or alpha(2)) and a beta subunit (beta(1)). Strategies for the genomic deletion of either subunit have been developed in the recent years. Removal of one of the two isoforms by deletion of one of the alpha subunits allowed the investigation of the specific functions of the respective isoform. The deletion of the beta(1) subunit led to complete knock-out thus completely disrupting the NO/cGMP signaling cascade. The phenotypes of these KO mice have corroborated the already known physiological importance of the NO/cGMP cascade e.g. in the regulation of blood pressure, platelet inhibition, interneuronal communication; yet, they have also given hints to novel functions and mechanisms. In addition, mice lacking both NO-GC isoforms permitted the investigation of possible cGMP-independent signaling pathways of NO. As cell- and tissue-specific knock-out models are beginning to emerge, a more detailed analysis of the importance of the NO receptor in specific tissues will become possible.

Abnormal social behavior, hyperactivity, impaired remote spatial memory, and increased D1-mediated dopaminergic signaling in neuronal nitric oxide synthase knockout mice

Background

Neuronal nitric oxide synthase (nNOS) is involved in the regulation of a diverse population of intracellular messenger systems in the brain. In humans, abnormal NOS/nitric oxide metabolism is suggested to contribute to the pathogenesis and pathophysiology of some neuropsychiatric disorders, such as schizophrenia and bipolar disorder. Mice with targeted disruption of the nNOS gene exhibit abnormal behaviors. Here, we subjected nNOS knockout (KO) mice to a battery of behavioral tests to further investigate the role of nNOS in neuropsychiatric functions. We also examined the role of nNOS in dopamine/DARPP-32 signaling in striatal slices from nNOS KO mice and the effects of the administration of a dopamine D1 receptor agonist on behavior in nNOS KO mice.

Results

nNOS KO mice showed hyperlocomotor activity in a novel environment, increased social interaction in their home cage, decreased depression-related behavior, and impaired spatial memory retention. In striatal slices from nNOS KO mice, the effects of a dopamine D1 receptor agonist, SKF81297, on the phosphorylation of DARPP-32 and AMPA receptor subunit GluR1 at protein kinase A sites were enhanced. Consistent with the biochemical results, intraperitoneal injection of a low dose of SKF81297 significantly decreased prepulse inhibition in nNOS KO mice, but not in wild-type mice.

Conclusion

These findings indicate that nNOS KO upregulates dopamine D1 receptor signaling, and induces abnormal social behavior, hyperactivity and impaired remote spatial memory. nNOS KO mice may serve as a unique animal model of psychiatric disorders.

Genetic mouse models of the NO receptor 'soluble' guanylyl cyclases

The NO/cGMP signalling cascade has an important role in smooth muscle relaxation, inhibition of platelet aggregation and neuronal transmission. Although the function of the main NO receptor GC (NO-GC) is well established, the particular tasks of the NO receptor isoforms (NO-GC1 and NO-GC2) are unclear and NO targets other than NO-GC have been postulated. Mice deficient in either NO receptor isoform or with a complete lack of NO-GC are now available and allow new insights in NO/cGMP signalling. The first reports about the KO strains show that, outside the neuronal system, the NO-GC isoforms can substitute for each other, and that amazingly low cGMP increases are sufficient to induce smooth muscle relaxation. In the neuronal system, however, the NO-GC isoforms obviously serve distinct functions as both isoforms are required for long term potentiation. Analysis of the complete NO-GC KO provides evidence that the vasorelaxing and platelet-inhibiting effects of NO are solely mediated by NO-GC. Thus, NO-GC appears to be the only NO receptor in these two systems.

cGMP signalling in the mammalian brain: role in synaptic plasticity and behaviour

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays a crucial role in the control of cardiovascular and gastrointestinal homeostastis, but its effects on neuronal functions are less established. This review summarizes recent biochemical and functional data on the role of the cGMP signalling pathway in the mammalian brain, with a focus on the regulation of synaptic plasticity, learning, and other complex behaviours. Expression profiling, along with pharmacological and genetic manipulations, indicates important functions of nitric oxide (NO)-sensitive soluble guanylyl cyclases (sGCs), cGMP-dependent protein kinases (cGKs), and cGMP-regulated phosphodiesterases (PDEs) as generators, effectors, and modulators of cGMP signals in the brain, respectively. In addition, neuronal cGMP signalling can be transmitted through cyclic nucleotide-gated (CNG) or hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels. The canonical NO/sGC/cGMP/cGK pathway modulates long-term changes of synaptic activity in the hippocampus, amygdala, cerebellum, and other brain regions, and contributes to distinct forms of learning and memory, such as fear conditioning, motor adaptation, and object recognition. Behavioural studies indicate that cGMP signalling is also involved in anxiety, addiction, and the pathogenesis of depression and schizophrenia. At the molecular level, different cGK isoforms appear to mediate effects of cGMP on presynaptic transmitter release and postsynaptic functions. The cGKs have been suggested to modulate cytoskeletal organization, vesicle and AMPA receptor trafficking, and gene expression via phosphorylation of various substrates including VASP, RhoA, RGS2, hSERT, GluR1, G-substrate, and DARPP-32. These and other components of the cGMP signalling cascade may be attractive new targets for the treatment of cognitive impairment, drug abuse, and psychiatric disorders.

Concepts of neural nitric oxide-mediated transmission

As a chemical transmitter in the mammalian central nervous system, nitric oxide (NO) is still thought a bit of an oddity, yet this role extends back to the beginnings of the evolution of the nervous system, predating many of the more familiar neurotransmitters. During the 20 years since it became known, evidence has accumulated for NO subserving an increasing number of functions in the mammalian central nervous system, as anticipated from the wide distribution of its synthetic and signal transduction machinery within it. This review attempts to probe beneath those functions and consider the cellular and molecular mechanisms through which NO evokes short- and long-term modifications in neural performance. With any transmitter, understanding its receptors is vital for decoding the language of communication. The receptor proteins specialised to detect NO are coupled to cGMP formation and provide an astonishing degree of amplification of even brief, low amplitude NO signals. Emphasis is given to the diverse ways in which NO receptor activation initiates changes in neuronal excitability and synaptic strength by acting at pre- and/or postsynaptic locations. Signalling to non-neuronal cells and an unexpected line of communication between endothelial cells and brain cells are also covered. Viewed from a mechanistic perspective, NO conforms to many of the rules governing more conventional neurotransmission, particularly of the metabotropic type, but stands out as being more economical and versatile, attributes that presumably account for its spectacular evolutionary success.

Endothelial nitric oxide synthase overexpression by neuronal cells in neurodegeneration: a link between inflammation and neuroprotection

The roles of neuronal and inducible nitric oxide synthases in neurones have been extensively investigated; by contrast, the biological significance of endothelial nitric oxide synthase (eNOS) overexpression that occurs in several pathological conditions has not yet been studied. We have started addressing this issue in a cell model of neurodegeneration, i.e. human SKNBE neuroblastoma cells transfected with a mutant form of alsin, a protein causing an early-onset type of amyotrophic lateral sclerosis, ALS2. We found that eNOS, which is endogenously expressed by these cells, was activated by tumour necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine that plays important roles in ALS2 and several neurodegenerative diseases. The TNF-alpha-dependent eNOS activation occurred through generation, by sphingosine-kinase-1, of sphingosine-1-phosphate, stimulation of its membrane receptors and activation of Akt, as determined using small interference RNA and dominant negative constructs specific for the enzymes and receptors. eNOS activation by TNF-alpha conferred cytoprotection from excitotoxicity and neurotoxic cues such as reactive oxygen species, endoplasmic reticulum stress, DNA damage, and mutated alsin itself. Our results suggest that overexpression of eNOS by neurones is a broad-range protective mechanism activated during damage and establish a link of pathophysiological relevance between this enzyme and inflammation accompanying neurodegenerative diseases. These findings also question the concept that high NO output in the presence of oxidative stress leads always to peroxynitrite formation contributing to neurodegeneration.

Involvement of nitric oxide in insulin induced memory improvement

Although brain was considered as an insulin-insensitive organ, recently it has appeared that insulin has some interesting effects on some brain regions like hippocampus. It has been known that intra-hippocampally administered insulin can improve learning and memory. Knowing that insulin can stimulate nitric oxide (NO) synthesis via eNOS activation and also that NO synthase (NOS) inhibitors can affect learning and memory, the aim of this study was to assess if NO is involved in insulin induced memory improvement. Wistar male rats were intra-CA1 cannulated and the effect of post-training and pre-probe trial intra-hippocampal administration of N-nitro-L-arginine methyl ester (L-NAME) (5, 10, 30 microg), insulin+L-NAME+/-L-arginine were assessed in a single-day testing version of Morris water maze (MWM) task. Our results show that, l-NAME can prevent insulin induced memory improvement. This drug had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous nitric oxide has a role in spatial learning and memory improvement caused by insulin.

NO/cGMP-dependent modulation of synaptic transmission

Nitric oxide (NO) is a multifunctional messenger in the CNS that can signal both in antero- and retrograde directions across synapses. Many effects of NO are mediated through its canonical receptor, the soluble guanylyl cyclase, and the second messenger cyclic guanosine-3',5'-monophosphate (cGMP). An increase of cGMP can also arise independently of NO via activation of membrane-bound particulate guanylyl cyclases by natriuretic peptides. The classical targets of cGMP are cGMP-dependent protein kinases (cGKs), cyclic nucleotide hydrolysing phosphodiesterases, and cyclic nucleotide-gated (CNG) cation channels. The NO/cGMP/cGK signalling cascade has been linked to the modulation of transmitter release and synaptic plasticity by numerous pharmacological and genetic studies. This review focuses on the role of NO as a retrograde messenger in long-term potentiation of transmitter release in the hippocampus. Presynaptic mechanisms of NO/cGMP/cGK signalling will be discussed with recently identified potential downstream components such as CaMKII, the vasodilator-stimulated phosphoprotein, and regulators of G protein signalling. NO has further been suggested to increase transmitter release through presynaptic clustering of a-synuclein. Alternative modes of NO/cGMP signalling resulting in inhibition of transmitter release and long-term depression of synaptic activity will also be addressed, as well as anterograde NO signalling in the cerebellum. Finally, emerging evidence for cGMP signalling through CNG channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels will be discussed.

Long-term potentiation in the visual cortex requires both nitric oxide receptor guanylyl cyclases

The role of nitric oxide (NO)/cGMP signaling in long-term potentiation (LTP) has been a lingering matter of debate. Within the cascade, the NO receptor guanylyl cyclase (GC), the cGMP-forming enzyme that is stimulated by NO, plays a key role. Two isoforms of GC (alpha2-GC, alpha1-GC) exist. To evaluate their contribution to synaptic plasticity, we analyzed knock-out mice lacking either one of the GC isoforms. We found that LTP induced in the visual cortex is NO dependent in the wild-type mice, absent in either of the GC isoform-deficient mice, and restored with application of a cGMP analog in both strains. The requirement of both NO receptor GCs for LTP indicates the existence of two distinct NO/cGMP-mediated pathways, which have to work in concert for expression of LTP.

Tonic and phasic nitric oxide signals in hippocampal long-term potentiation

Nitric oxide (NO) participates in long-term potentiation (LTP) and other forms of synaptic plasticity in many different brain areas but where it comes from and how it acts remain controversial. Using rat and mouse hippocampal slices, we tested the hypothesis that tonic and phasic NO signals are needed and that they derive from different NO synthase isoforms. NMDA increased NO production in a manner that was potently inhibited by three different neuronal NO synthase (nNOS) inhibitors. Tonic NO could be monitored after sensitizing guanylyl cyclase-coupled NO receptors, allowing the very low ambient NO concentrations to be detected by cGMP measurement. The levels were unaffected by inhibition of NMDA receptors, nNOS, or the inducible NO synthase (iNOS). iNOS was also undetectable in protein or activity assays. Tonic NO was susceptible to agents inhibiting endothelial NO synthase (eNOS) and was missing in eNOS knock-out mice. The eNOS knock-outs exhibited a deficiency in LTP resembling that seen in wild-types treated with a NO synthase inhibitor. LTP in the knock-outs could be fully restored by supplying a low level of NO exogenously. Inhibition of nNOS also caused a major loss of LTP, particularly of late-LTP. Again, exogenous NO could compensate, but higher concentrations were needed compared with those restoring LTP in the eNOS knock-outs. It is concluded that tonic and phasic NO signals are both required for hippocampal LTP and the two are generated, respectively, by eNOS and nNOS, the former in blood vessels and the latter in neurons.

Signaling from blood vessels to CNS axons through nitric oxide

Brain function is usually perceived as being performed by neurons with the support of glial cells, the network of blood vessels situated nearby serving simply to provide nutrient and to dispose of metabolic waste. Revising this view, we find from experiments on a rodent central white matter tract (the optic nerve) in vitro that microvascular endothelial cells signal persistently to axons using nitric oxide (NO) derived from the endothelial NO synthase (eNOS). The endogenous NO acts to stimulate guanylyl cyclase-coupled NO receptors in the axons, leading to a raised cGMP level which then causes membrane depolarization, apparently by directly engaging hyperpolarization-activated cyclic nucleotide-gated ion channels. The tonic depolarization and associated endogenous NO-dependent cGMP generation was absent in optic nerves from mice lacking eNOS, although such nerves responded to exogenous NO, with raised cGMP generation in the axons and associated depolarization. In addition to the tonic activity, exposure of optic nerves to bradykinin, a classical stimulator of eNOS in endothelial cells, elicited reversible NO- and cGMP-dependent depolarization through activation of bradykinin B2 receptors, to which eNOS is physically complexed. No contribution of other NO synthase isoforms to either the action of bradykinin or the continuous ambient NO level could be detected. The results suggest that microvascular endothelial cells participate in signal processing in the brain and can do so by generating both tonic and phasic NO signals.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

Phosphodiesterase type 5 inhibition improves early memory consolidation of object information

The nitric oxide (NO)-cyclic GMP (cGMP) signaling pathway is assumed to play an important role in processes underlying learning and memory. We used phosphodiesterase type 5 (PDE5) inhibitors to study the role of cGMP in object- and spatial memory. Our results and those reported in other studies indicate that elevated hippocampal cGMP levels are required to improve the memory performance of rodents in object recognition and passive avoidance learning, but not in spatial learning. The timing of treatment modulates the effects on memory and strongly supports a role for cGMP in early stages of memory formation. Alternative explanations for the improved memory performance of PDE5 inhibitors are also discussed. Immunocytochemical studies showed that in vitro slice incubations with PDE5 inhibitors increase NO-stimulated cGMP levels mainly in hippocampal varicose fibers. Reviewing the available data on the localization of the different components of the NO-cGMP signaling pathway, indicates a complex interaction between NO and cGMP, which may be independent of each other. It is discussed that further studies are needed, immunocytochemical and behavioral, to better understand the cGMP-mediated molecular mechanisms underlying memory formation.

Species differences in the localization of cGMP-producing and NO-responsive elements in the mouse and rat hippocampus using cGMP immunocytochemistry

The aim of the study was to compare the localization of the nitric oxide (NO)-cGMP pathway in hippocampus of mice and rats using cGMP- and soluble guanylyl cyclase (GC) immunocytochemistry and in situ hybridization of the cGMP-hydrolysing phosphodiesterase types 2, 5 and 9. In vitro incubation of hippocampus slices in the absence of a guanylyl cyclase stimulator or a phosphodiesterase inhibitor resulted in cGMP-positive astrocytes mainly in the CA1 area in mouse slices. In contrast, no cGMP immunoreactivity was observed under these conditions in the rat hippocampus. Treatment with an NO synthase inhibitor or inhibitors of soluble or particulate GC did not abolish cGMP immunoreactivity in astrocytes. Incubation with the NO donors sodium nitroprusside or diethylamino NONOate, or with the NO-independent activators of soluble GC, YC-1 and BAY 41-2272, in combination with phosphodiesterase inhibitors, resulted in an increase in cGMP immunoreactivity in numerous astrocytes throughout the mouse hippocampus. In contrast, under these conditions cGMP immunoreactivity was primarily observed in varicose fibers in rat hippocampus. Comparison of the cellular localization of the beta1 subunit of soluble GC and the mRNAs of PDE2, PDE5 and PDE9 revealed that in both species the beta1 subunit was observed in pyramidal and granule cells, which also expressed the mRNAs of the three phosphodiesterase families. Although the beta1 subunit was observed in astrocytes, none of the phosphodiesterases were detected in these cells. We conclude that, although the expression profiles of the soluble GC beta1 subunit and cGMP-hydrolysing phosphodiesterase mRNAs were identical, the cellular patterns of cGMP immunoreactivity differ between rat and mouse hippocampus.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Species, strain and developmental variations in hippocampal neuronal and endothelial nitric oxide synthase clarify discrepancies in nitric oxide-dependent synaptic plasticity

Nitric oxide (NO) has been implicated in long-term potentiation (LTP) in pyramidal neurons in cellular area 1 (CA1) of the hippocampus. However, considerable confusion exists about the exact role of NO, and the contribution of the endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) isoforms of NO synthase to NO-dependent LTP (NO-LTP), with results often varying, depending on the organism and experimental paradigm used. Using immunohistochemistry and in situ hybridization, we contrast NO synthase expression and activity in rat, mouse, and human hippocampus. nNOS is prominently expressed in all CA1 pyramidal cells of C57B6 mice and humans, while in rats and SV129 mice, its levels are much lower and restricted to the caudal hippocampus. By contrast, eNOS is restricted to endothelial cells. We observe N-methyl-D-aspartate-dependent citrulline production in pyramidal cells of mouse hippocampus, which is absent in nNOS(Delta/Delta) animals. Finally, we observe robust nNOS expression in human CA1 pyramidal cells.The considerable axial, developmental, strain and species-dependent variations in nNOS expression in CA1 pyramidal neurons can explain much of the variation observed in reports of NO-dependent LTP. Moreover, our data suggest that NO produced by eNOS in endothelial cells may play a paracrine role in modulating LTP.

Hippocampal cGMP-dependent protein kinase I supports an age- and protein synthesis-dependent component of long-term potentiation but is not essential for spatial reference and contextual memory

cGMP-dependent protein kinase I (cGKI) is expressed in the hippocampus, but its role in hippocampal long-term potentiation (LTP) is controversial. In addition, whether cGKI is involved in spatial learning has not been investigated. To address these issues, we generated mice with a hippocampus-specific deletion of cGKI (cGKIhko mice). Unlike conventional cGKI knock-out mice, cGKIhko mice lack gastrointestinal and cardiovascular phenotypes and have a normal life expectancy, which enables us to analyze hippocampal synaptic plasticity and learning in young and adult animals. Hippocampal LTP after repetitive episodes of theta burst stimulation was impaired in adult (12-14 weeks of age) but not in juvenile (3-4 weeks of age) cGKIhko mice. The difference in LTP between adult control and cGKIhko mice was abolished by the protein synthesis inhibitor anisomycin, suggesting that the impairment of LTP in adult cGKIhko mice reflects a defect in late-phase LTP. Despite their deficit in LTP, adult cGKIhko mutants showed normal performance in a discriminatory water maze and had intact contextual fear conditioning. These results suggest that hippocampal cGKI supports an age- and protein synthesis-dependent form of hippocampal LTP, whereas it is dispensable for hippocampus-dependent spatial reference and contextual memory.

Post-ischemic administration [correction of administeration] but not pre-ischemic administration [correction of administeration] of NG-nitro-L-arginine prevents spatial memory impairments and apoptosis by an inhibition of a delayed increase in NOx- in the hippocampus following repeated cerebral ischemia

In the present study, we investigated the effects of N(G)-nitro-L-arginine (L-NAME), an inhibitor of nitric oxide synthase, on repeated cerebral ischemia-induced impairment of spatial memory of the 8-arm radial maze in rats. Repeated ischemia (10 min ischemia x 2 times with 1 h interval) impaired the spatial memory in the 8-arm radial maze test and produced apoptosis in the hippocampus 7 days after final occlusion, and gradually increased the NO(x)(-) levels approximately 30-180 min after the second reperfusion. Post-ischemic administration of L-NAME at a dose of 50 mg/kg, i.p. 30 min following the second occlusion, significantly attenuated the repeated ischemia-induced impairment of spatial memory in the 8-arm radial maze test and suppressed apoptosis in the hippocampus, and also significantly suppressed a delayed increase in the NO(x)(-) levels induced by repeated ischemia. However, pre-ischemic administration of L-NAME at a dose of 50 mg/kg, i.p. 30 min before the first occlusion, caused about 90% mortality (the mortality rate of vehicle-treated group was 10%). These results suggest that the delayed generation of NO(x)(-) may cause spatial memory impairment and induction of apoptosis in the hippocampus in rats subjected to repeated ischemia.

Cortico-striatal synaptic plasticity in endothelial nitric oxide synthase deficient mice

Nitric oxide (NO) is a retrograde messenger involved in the processes of learning and memory. The role of the endothelial isoform of nitric oxide synthase (eNOS) in striatal synaptic plasticity was investigated in eNOS-deficient (eNOS(-/-)) and wild type (WT) mice. Tetanic stimulation of cortical afferents in WT mice evoked either long-term potentiation (LTP), or long-term depression (LTD) of cortico-striatal transmission. Both these plasticity related phenomena were NMDA-receptor-dependent; LTD was blocked by sulpiride, a dopamine D2-receptor antagonist. LTP occurrence in slices from eNOS(-/-) mice was significantly reduced when compared with WT mice. The NOS inhibitor NL-ARG reduced the occurrence of LTP and increased the occurrence of LTD in WT mice, resembling the balance of LTP/LTD in eNOS(-/-) mice. Impairment of NO-synthesis thus shifts striatal plasticity towards LTD. This indicates a possible involvement of eNOS from endothelia in neuronal modulation.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.

Unaltered radial maze performance and brain acetylcholine of the endothelial nitric oxide synthase knockout mouse

Proceeding from previous findings of a beneficial effect of endothelial nitric oxide synthase (eNOS) gene inactivation on negatively reinforced water maze performance, we asked whether this improvement in place learning capacities also holds for a positively reinforced radial maze task. Unlike its beneficial effects on the water maze task, eNOS gene inactivation did not facilitate radial maze performance. The acquisition performance over the days of place learning did not differ between eNOS knockout (eNOS-/-) and wild-type mice (eNOS+/+). eNOS-/- mice displayed a slight and eNOS+/+ mice a more severe working memory deficit in the place learning version of the radial maze compared to the genetic background C57BL/6 strain. Possible differential effects of eNOS inactivation, related to differences in reinforcement contingencies between the Morris water maze and radial maze tasks, behavioral strategy requirements, or to different emotional and physiological concomitants inherent in the two tasks are discussed. These task-unique characteristics might be differentially affected by the reported anxiogenic and hypertensional effects of eNOS gene inactivation. Post-mortem determination of acetylcholine concentrations in diverse brain structures revealed that acetylcholine and choline contents were not different between eNOS-/- and eNOS+/+ mice, but were increased in eNOS+/+ mice compared to C57BL/6 mice in the frontal cortex. Our findings demonstrate that phenotyping of learning and memory capacities should not rely on one learning task only, but should include tasks employing both negative and positive reinforcement contingencies in order to allow valid statements regarding differences in learning capacities between rodent strains.

Spatial nitric oxide imaging using 1,2-diaminoanthraquinone to investigate the involvement of nitric oxide in long-term potentiation in rat brain slices

Long-term potentiation (LTP), a model of activity-dependent synaptic plasticity, involves the persistent enhancement of excitatory neurotransmission. Several recent studies have suggested a critical role for nitric oxide (NO) production in hippocampal LTP. However, increase in NO production in living tissue has not yet been directly demonstrated. We used 1,2-diaminoanthraquinone (DAQ) to demonstrate NO production in rat brain slices in relation to induction of LTP. DAQ was found to be without neurotoxic effects and it neither influenced normal evoked field potential amplitudes nor did it affect induction of LTP in comparison to controls. We found that DAQ-induced fluorescence is elevated within a limited area of about 40,000 microm(2) during LTP induction in the hippocampal area CA1. Furthermore, we could demonstrate that application of the NO-synthetase inhibitor l-NAME inhibits the induction of LTP in area CA1 and causes a strong reduction of DAQ induced fluorescence. Our results are consistent with the hypothesis that NO can serve as a retrograde messenger during induction of LTP in the hippocampus.

Defective hippocampal mossy fiber long-term potentiation in endothelial nitric oxide synthase knockout mice

Mossy fiber long-term potentiation (mfLTP) was compared in hippocampal slices prepared from wild-type mice and mice lacking functional endothelial nitric oxide synthase (eNOS(-/-) mice) using field potential recording. In the presence of D-2-amino-5-phosphonovaleric acid (AP5, 50 microM), the mfLTP induced by tetanic stimulation (100 Hz, 1 sec) was significantly reduced in knockouts (n = 8) in comparison with wild-type (n = 8). Similarly, potentiation induced by forskolin (30 microM) or 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP, 100 microM) was less pronounced in knockouts. However, in wild-types the mfLTP-induced in the presence of the nonselective pharmacological inhibitor of NOS (N-nitro-L-Arginine, 100 microM, n = 6) was not significantly different from control (n = 8). Thus, eNOS is not directly involved in mfLTP, but lack of eNOS during development leads to a deficit downstream of adenylyl cyclase.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Disruption of retinogeniculate pattern formation by inhibition of soluble guanylyl cyclase

During development of the visual system of the ferret, the terminals of retinal ganglion cell axons first segregate to form eye-specific layers and subsequently On-center and Off-center sublayers within the dorsal lateral geniculate nucleus (dLGN). Sublamination requires the activity of the afferent fibers, NMDA receptors, and nitric oxide synthase (NOS). We here report that soluble guanylyl cyclase (sGC), which in turn produces cGMP, is critically involved in the process of sublamination. cGMP expression is upregulated in both retinal terminals and postsynaptic dLGN cells during sublamination, and this expression is controlled by the activity of both NMDA receptors and NOS. Furthermore, the infusion of specific inhibitors of sGC or protein kinase G (PKG), a target of cGMP, prevents sublamination in vivo. We conclude that the sGC-cGMP-PKG pathway acts downstream of NMDA receptors and nitric oxide as an effector of the activity-dependent refinement of connections at this level of the mammalian visual system.

Retrograde signalling with nitric oxide at neocortical synapses

Long-term changes of synaptic transmission in slices of rat visual cortex were induced by intracellular tetanization: bursts of short depolarizing pulses applied through the intracellular electrode without concomitant presynaptic stimulation. Long-term synaptic changes after this purely postsynaptic induction were associated with alterations of release indices, thus providing a case for retrograde signalling at neocortical synapses. Both long-term potentiation and long-term depression were accompanied by presynaptic changes, indicating that retrograde signalling can achieve both up- and down-regulation of transmitter release. The direction and the magnitude of the amplitude changes induced by a prolonged intracellular tetanization depended on the initial properties of the input. The inputs with initially high paired-pulse facilitation (PPF) ratio, indicative of low release probability, were most often potentiated. The inputs with initially low PPF ratio, indicative of high release probability, were usually depressed or did not change. Thus, prolonged postsynaptic activity can lead to normalization of the weights of nonactivated synapses. The dependence of polarity of synaptic modifications on initial PPF disappeared when plastic changes were induced with a shorter intracellular tetanization, or when the NO signalling pathway was interrupted by inhibition of NO synthase activity or by application of NO scavengers. This indicates that the NO-dependent retrograde signalling system has a relatively high activation threshold. Long-term synaptic modifications, induced by a weak postsynaptic challenge or under blockade of NO signalling, were nevertheless associated with presynaptic changes. This suggests the existence of another retrograde signalling system, additional to the high threshold, NO-dependent system. Therefore, our data provide a clear case for retrograde signalling at neocortical synapses and indicate that multiple retrograde signalling systems, part of which are NO-dependent, are involved.

Superior water maze performance and increase in fear-related behavior in the endothelial nitric oxide synthase-deficient mouse together with monoamine changes in cerebellum and ventral striatum

Nitric oxide (NO) has been implicated in the control of emotion, learning, and memory. We have examined endothelial NO synthase-deficient mice (eNOS-/-) in terms of habituation to an open field, elevated plus-maze behavior, Morris water maze performance, and changes in cerebral monoamines. In the open field, eNOS-/- animals were less active than wild-type controls but showed unimpaired habituation. In the plus-maze, an anxiogenic effect was observed. Proceeding from previous findings of deficits in hippocampal and neocortical long-term potentiation (LTP) in our eNOS-/- mice, we investigated whether these animals also express deficits in learning tasks that have been linked to hippocampal function and LTP. Unexpectedly, eNOS gene disruption led to accelerated place learning in the water maze. Furthermore, during long-term retention and reversal learning, eNOS-/- mice showed improved performance. In a cued version of the water maze task, eNOS-/- and control mice did not differ, implying that the superior performance of eNOS-/- animals on the former tasks cannot be attributed solely to differences in sensorimotor capacities. The neurochemical evaluation of the eNOS-/- mice revealed increases in the concentrations of the serotonin metabolite 5-HIAA in the cerebellum, together with an accelerated serotonin turnover in the frontal cortex. Furthermore, eNOS-/- mice had a higher dopamine turnover in the ventral striatum. These findings are discussed in terms of possible concomitant effects on physiological parameters, such as a decreased reactivity of GABAergic neurotransmission or changes in vascular functions, and effects on behavioral processes related to reinforcement, learning, and emotion.

A working model of CaM kinase II activity in hippocampal long-term potentiation and memory

Recent advances in molecular genetics provide strong evidence for a relationship between hippocampal long-term potentiation (LTP) and hippocampus-dependent memory. The alpha-CaM kinase II knock-out mouse and transgenic mice expressing a mutant form of CaM kinase II clearly demonstrate that CaM kinase II plays a prominent role in hippocampal LTP and hippocampus-dependent memory. Furthermore, the observation that there is a diversity of silent as well as functional synapses has shed light on the molecular basis of learning and memory during development as well as in adult brain. Here we present a working model of CaM kinase II activity as a memory molecule in hippocampal LTP and describe molecular targets of CaM kinase II involved in the establishment of functional synapses following LTP induction.

Maternal aggression in endothelial nitric oxide synthase-deficient mice

Lactating female rodents protect their pups by expressing fierce aggression, termed maternal aggression, toward intruders. Mice lacking the neuronal nitric oxide synthase gene (nNOS-/-) exhibit significantly impaired maternal aggression, but increased male aggression, suggesting that nitric oxide (NO) produced by nNOS has opposite actions in maternal and male aggression. In contrast, mice lacking the endothelial nitric oxide synthase gene (eNOS-/-) exhibit almost no male aggression, suggesting that NO produced by eNOS facilitates male aggression. In the present study, maternal aggression in eNOS-/- mice was examined and found to be normal relative to wild-type (WT) mice in terms of the percentage displaying aggression, the average number of attacks against a male intruder, and the total amount of time spent attacking the male intruder. The eNOS-/- females also displayed normal pup retrieval behavior. Because a significant elevation of citrulline, an indirect marker of NO synthesis, occurs in neurons of the hypothalamus of lactating WT mice in association with maternal aggression, we examined the brains of eNOS-/- females for citrulline immunoreactivity following an aggressive encounter. The aggressive eNOS-/- females exhibited a significant elevation of citrulline in the medial preoptic nucleus and the subparaventricular zone of the hypothalamus relative to unstimulated lactating eNOS-/- females. Taken together, these results suggest that NO produced by eNOS neither facilitates nor inhibits maternal aggression and that NO produced by eNOS has a different role in maternal and male aggression.

Inhibition of the endothelial isoform of nitric oxide synthase impairs long-term memory formation in the chick

Previous studies with general inhibitors of nitric oxide synthase have yielded variable and contradictory results with respect to their effects on memory. This may have been due to differential effects of blocking the various isoforms of this enzyme. We show that day-old chicks trained on a single-trial passive-avoidance task suffered significant memory loss from approximately 40 min post-training following post-training intracranial administration of a potent inhibitor of eNOS. Administration of a specific nNOS or iNOS inhibitor at the same time had no effect on retention, although a role for either of these isoforms when administered at a different time after learning has yet to be fully investigated. The onset of memory loss following eNOS inhibition is the same as observed following general NOS inhibition, which suggests that amnestic effects observed in previous studies using nonspecific inhibitors may be attributable to blocking the function of eNOS. The findings indicate that eNOS may play a role in memory formation for this task, which is at least distinct from any role that may be played by nNOS.