Long-term potentiation is reduced in mice that are doubly mutant in endothelial and neuronal nitric oxide synthase

Evidence for involvement of the cGMP-protein kinase G signaling system in the induction of long-term depression, but not long-term potentiation, in the dentate gyrus in vitro

The involvement of the cGMP-protein kinase G (PKG) signaling pathway in the induction of long-term depression (LTD) and long-term potentiation (LTP) was investigated in the medial perforant path of the dentate gyrus in vitro. Low-frequency stimulation (LFS)-induced LTD of field EPSPs was inhibited by bath perfusion of the selective soluble guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3, -a]quinoxalin-1-one (ODQ). LFS-induced LTD of EPSPs and whole-cell patch-clamped EPSCs was also blocked by bath perfusion and postsynaptic intracellular injection, respectively, of the selective PKG inhibitor KT5823. Elevation of intracellular cGMP by perfusion of the cGMP phosphodiesterase inhibitor zaprinast resulted in induction of LTD of field EPSPs and EPSCs. Occlusion experiments showed mutual inhibition between LFS-induced LTD and zaprinast-induced LTD. The zaprinast-induced LTD of field EPSPs was inhibited by perfusion of ODQ and KT5823. In addition, zaprinast-induced LTD of EPSCs was inhibited by postsynaptic application of KT5823. Glutamate receptor stimulation, especially that of metabotropic glutamate receptors (mGluRs), was required for zaprinast-induced LTD, because cessation of test stimulation or perfusion with the mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) inhibited zaprinast-induced LTD. No inhibitory effect of ODQ or KT5823 on the induction of LTP of EPSPs or EPSCs was found. These data indicate that the cGMP-guanyly cyclase-PKG signaling pathway in the dentate gyrus is essential for induction of LTD, although not of LTP, in the dentate gyrus.

Nitric oxide mediates N-methyl-D-aspartate receptor-induced activation of p21ras

N-methyl-D-aspartate (NMDA) glutamate receptor-mediated increases in intracellular calcium are thought to play a critical role in synaptic plasticity. The mechanisms by which changes in cytoplasmic calcium transmit the glutamate signal to the nucleus, which is ultimately important for long-lasting neuronal responses, are poorly understood. We show that NMDA receptor stimulation leads to activation of p21(ras) (Ras) through generation of nitric oxide (NO) via neuronal NO synthase. The competitive NO synthase inhibitor, L-nitroarginine methyl ester, prevents Ras activation elicited by NMDA and this effect is competitively reversed by the NO synthase substrate, L-arginine. NMDA receptor stimulation fails to activate Ras in neuronal cultures from mice lacking neuronal NO synthase. NMDA-induced Ras activation occurs through a cGMP-independent pathway as 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ), a potent and selective inhibitor of guanylyl cyclase, has no effect on NMDA receptor-induced activation of Ras, and the cell-permeable cGMP analog, 8Br-cGMP, does not activate Ras. Furthermore, NO directly activates immunoprecipitated Ras from neurons. NMDA also elicits tyrosine phosphorylation of extracellular signal-regulated kinases, a downstream effector pathway of Ras, through a NO/non-cGMP dependent mechanism, thus supporting the physiologic relevance of endogenous NO regulation of Ras. These results suggest that Ras is a physiologic target of endogenously produced NO and indicates a signaling pathway for NMDA receptor activation that may be important for long-lasting neuronal responses.

Developing grasshopper neurons show variable levels of guanylyl cyclase activity on arrival at their targets

The ability of certain grasshopper neurons to respond to exogenously applied donors of nitric oxide (NO) by producing cyclic GMP (cGMP) depends on their developmental state. ODQ, a selective blocker of NO-sensitive guanylyl cyclase, blocks cGMP production at 10(-5) M, thus confirming the nature of the response. Experiments in which the distal axon is separated from its proximal stump before application of an NO donor show that guanylyl cyclase is distributed uniformly throughout the neuron. In the locust abdomen, where segments are formed sequentially, the pattern of guanylyl cyclase up-regulation is predictable and sequential from anterior to posterior. There are two patterns of innervation by cGMP-expressing motor neurons. In the first, typified by muscle 187, an innervating neuron begins to be NO responsive on arrival at its muscle and continues to be so over most of the remainder of embryonic development, including the formation of motor end plates. In the second, typified by a neuron innervating muscle 191, the neuron extends well along the muscle, apparently laying down a number of sites of contact with it, before it becomes NO responsive. In both patterns, however, NO responsiveness marks the neuron's transition from growth cone elongation to the production of lateral branches. Individual muscles receive innervation from multiple motor neurons, some of which express transient NO sensitivity during development and others which do not. With the exception of the leg motor neuron SETi, the first motor neuron to reach any muscle is usually not NO responsive. We suggest that cGMP plays a role in, or reflects, the early stages of communication between a target and specific innervating neurons.

Molecular characterization of NOS in a mollusc: expression in a giant modulatory neuron

Here we report on the molecular characterization of the first molluscan NOS in the CNS of the pond snail Lymnaea stagnalis. This Lymnaea NOS (Lym-nNOS) which is expressed preferentially in the CNS is most similar to mammalian neuronal NOS but contains tandem repeats of a seven amino acid motif not found in any other known NOS. We have localized Lym-nNOS to the serotonergic cerebral giant cells (CGCs) which modulate synaptic transmission within a neural network that generates feeding behavior. Our results suggest that the CGCs employ both NO and serotonin in the modulation of the central neural network underlying feeding.

Nitric Oxide: Diverse Actions in the Central and Peripheral Nervous Systems

Nitric oxide (NO) has revolutionized our conceptions about neurotransmission. NO is not stored in synaptic vesicles, is not released by exocytosis, and does not mediate its action by binding to cell surface receptors. Instead, NO simply diffuses to its targets, and its actions are mediated through molecules that accept or share its unpaired electron. NO has diverse biological roles, including functions as the nitrergic transmitter of the peripheral nervous system, the major regulator of blood vessel tone, and actions as the cytotoxic agent of activated macrophages. In the CNS, NO function is just beginning to be explored, but it seems to play prominent roles in plasticity and the regulation of complex behaviors. Under conditions of excessive formation. NO has emerged as an important endogenous neurotoxin. Strategies aimed at reducing NO formation may therefore have therapeutic benefit. NEUROSCIENTIST 4:96–112, 1998

Erectile dysfunction in aging: upregulation of endothelial nitric oxide synthase

OBJECTIVES

To evaluate whether alterations in nitric oxide (NO) synthesis or activity contribute to age-related erectile dysfunction and to elucidate the mechanisms causing these alterations using the rabbit as our model of aging.

METHODS

We compared the ability of the rabbit cavernosal smooth muscle to relax in the organ bath in response to acetylcholine (Ach, endothelium-dependent vasodilator), sodium nitroprusside (SNP, an NO donor), and A23187 (a calcium ionophore) in young (6 month old) and aged (2.5 to 3.5 year old) rabbits. In addition, the immunohistochemical expression of endothelial nitric oxide synthase (eNOS) in both young and aged rabbit cavernosal tissue was examined. Endothelial integrity was examined immunohistochemically with JC70.

RESULTS

Ach-mediated relaxation of penile corporal tissue was significantly attenuated from a maximum of 68.39 +/- 6.27 (0.1 mM Ach, n = 4) in young rabbits to 39.02 +/- 4.88 (0.1 mM Ach, n = 6) in aged rabbits (P < 0.04). No statistically significant difference (P > 0.05) was noted between cavernosal relaxation to sodium nitroprusside between young rabbits (97.8%, 0.1 mM SNP, n = 5) and aged rabbits (76.1%, 0.1 mM SNP, n = 5). This suggested that the defect in the Ach-NO pathway was at the level of NO synthesis, not activity. Immunohistochemical staining for eNOS demonstrated upregulation in both the vascular endothelium and corporal smooth muscle of aged rabbit tissue compared with young rabbit cavernosal tissue (n = 5). Anatomic endothelial integrity was demonstrated in the young and aged rabbits by the presence of JC70. This suggested that the defect in the Ach-NO synthetic pathway was not at the level of eNOS and was not due to anatomic endothelial cell disruption. Finally, Ach-mediated cavernosal smooth muscle relaxation in the young rabbit was not significantly augmented (P > 0.05) in the presence of the calcium ionophore A23187 (10 microM). A23187, however, significantly augmented (P < 0.04) Ach-mediated relaxation in the aged rabbit from a maximum of 33.93 +/- 6.58 to 41.55 +/- 6.58 (10 microM Ach, n = 5). This suggested that a potential defect in the Ach-NO synthetic pathway was at the level of intracellular calcium flux and possibly at the level of the calcium-eNOS interaction.

CONCLUSIONS

Endothelium-dependent relaxation is attenuated in the aging rabbit; eNOS is upregulated in the aging rabbit; and no difference is noted in response to direct NO donation between the young and aged rabbit. The endothelium is anatomically intact in both the young and aging rabbit. The calcium ionophore A23187 augmented the attenuated vasorelaxation in the aging rabbit cavernosum (although not to the levels seen in the young rabbit cavernosum) and had no effect on the young rabbit cavernosum. These data suggest that erectile dysfunction in the aging rabbit cavernosum appears to be related to endothelial dysfunction and is characterized by eNOS upregulation and aberrant intracellular calcium fluxes.

Distribution and development of NMDA receptor activities at hippocampal synapses examined using mice lacking the epsilon1 subunit gene

The effects of targeted disruption of the gene encoding N-methyl-D-aspartate (NMDA) receptor epsilon1 subunit were examined in hippocampal CA1 pyramidal cell synapses and compared with the effects in the CA3 region. The mutation resulted in the significant reduction of NMDA receptor activities at the synapses in the CA1 stratum oriens, as had been observed in the CA1 stratum radiatum which we reported before. This result was in sharp contrast to our previous observation that in the CA3 region, the epsilon1 mutation suppressed NMDA receptors at the synapses in the stratum radiatum but not in the stratum oriens. It is suggested that the subunit composition of NMDA receptors may not be determined simply by the location within a pyramidal cell, but by other factors such as properties of synaptic inputs. We also examined the postnatal development of long-term potentiation (LTP) in the CA3 region. The development of LTP at the CA3 stratum radiatum synapses closely followed the development of the epsilon1 subunit, and the epsilon1 mutation strongly suppressed this LTP, suggesting that the targeted disruption of the epsilon1 subunit may not be compensated by other epsilon subunits. The LTP at the CA3 stratum oriens synapses was not significantly affected by the mutation at any age.

Impaired learning in rats in a 14-unit T-maze by 7-nitroindazole, a neuronal nitric oxide synthase inhibitor, is attenuated by the nitric oxide donor, molsidomine

In previous experiments, it was demonstrated that systemic or central administration of the nitric oxide synthase (NO synthase) inhibitor, NG-nitro-L-arginine (N-Arg), produced dose-dependent learning impairments in rats in a 14-unit T-maze; and that sodium nitroprusside, a NO donor, could attenuate the impairment. Since N-Arg is not specific for neuronal NO synthase and produces hypertension, it is possible that effects on the cardiovasculature may have contributed to the impaired maze performance. In the present experiment, we have investigated the maze performance of 3-4 months old male Fischer-344 rats following treatment with 7-nitroindazole, a NO synthase inhibitor that is selective for neuronal NO synthase and does not produce hypertension. In addition, we examined the effects of the NO donor, molsidomine, which is much longer acting than sodium nitroprusside. Rats were pretrained to avoid footshock in a straight runway and received training in a 14-unit T-maze 24 h later. In an initial dose-response study, rats received intraperitoneal (i.p.) injections of either 7-nitroindazole (25, 50, or 65 mg/kg) or peanut oil 30 min prior to maze training. 7-nitroindazole produced significant, dose-dependent maze acquisition deficits, with 65 mg/kg producing the greatest learning impairment. This dose of 7-nitroindazole had no significant effect on systolic blood pressure. Following the dose-response study, rats were given i.p. injections of either 7-nitroindazole (70 mg/kg) plus saline, 7-nitroindazole (70 mg/kg) plus the NO donor, molsidomine (2 or 4 mg/kg), or peanut oil plus saline as controls. Both doses of molsidomine significantly attenuated the learning deficit induced by 7-nitroindazole relative to controls. These findings represent the first evidence that impaired learning produced by inhibition of neuronal NO synthase can be overcome by systemic administration of a NO donor.

Learning in a 14-unit T-maze is impaired in rats following systemic treatment with N omega-nitro-L-arginine

We examined whether inhibition of nitric oxide synthase (NO synthase) impairs learning in male Fischer-344 rats (9 mo) in a shock-motivated 14-unit T-maze. Rats were pretrained in one-way active avoidance of foot shock to a criterion of 13/15 avoidances in a straight runway. The next day, rats received intraperitoneal (i.p.) injections of 0.9% NaCl as controls or Nomega-nitro-L-arginine (N-Arg: 3.0. 4.5, or 6.0 mg/kg) to inhibit NO synthase 30 min before maze training. During 15 trials, rats were required to negotiate each of 5 segments within 10 s to avoid footshock. Performance variables included errors (deviations from the correct pathway), runtime from start to goal, shock frequency and duration. N-Arg treatment impaired performance on all variables in a dose-dependent manner. Specifically, only the 6 mg/kg N-Arg dose significantly increased errors compared to controls over the last 10 trials but not the first 5 trials. Controls and rats treated with 3 or 4.5 mg/kg N-Arg were retested in the maze 7-10 days following training, with half receiving N-Arg (6 mg/kg i.p.) 30 min in advance. In this retention test, maze performance was not significantly affected; thus, these results indicated that NO synthase inhibition primarily impaired acquisition without impacting upon noncognitive aspects of performance. This conclusion was further reinforced by the demonstration that 6 mg/kg N-Arg did not significantly affect sensorimotor performance in a rotarod task. When rats were treated with sodium nitroprusside, an NO donor, at 1 min, but not 30 min, prior to training, the N-Arg induced impairment (6 or 8 mg/kg i.p.) in maze learning was significantly attenuated.

Precise distribution of neuronal nitric oxide synthase mRNA in the rat brain revealed by non-radioisotopic in situ hybridization

Regional distribution of neurons expressing neuronal nitric oxide synthase mRNA in the rat brain was examined by non-radioisotopic in situ hybridization, using digoxigenin-labeled complementary RNA probes. Clustering of intensely positive neurons was observed in discrete areas including the main and accessory olfactory bulbs, the islands of Calleja, the amygdala, the paraventricular nucleus of the thalamus, several hypothalamic nuclei, the lateral geniculate nucleus, the magnocellular nucleus of the posterior commissure, the superior and inferior colliculi, the laterodorsal and pedunculopontine tegmental nuclei, the nucleus of the trapezoid body, the nucleus of the solitary tract and the cerebellum. Strongly-stained isolated neurons were scattered mainly in the cerebral cortex, the basal ganglia and the brain stem, especially the medulla reticular formation. In the hippocampus, an almost uniform distribution of moderately stained neurons was observed in the granular cell layer of the dentate gyrus and in the pyramidal cell layer of the Ammon's horn, while more intensely stained isolated neurons were scattered over the entire hippocampal region. These observations can serve as a good basis for studies on function and gene regulation of neuronal nitric oxide synthase.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

An evaluation of what the mouse knockout experiments are telling us about mammalian behaviour

The early gene knockout studies with a neurobiological focus were directed at fairly obvious target genes and added very little to our knowledge of behavioural neuroscience. On the contrary, since the behavioural consequences were often predictable, this helped confirm that the technology was working. However, a substantial number of knockouts of genes expressed in the brain have been without obvious behavioural consequences, supporting the concept of genetic canalisation and redundancy. Others have produced a behavioural deficit for which there is no obvious explanation. Many cells of different tissue types have a capacity for memory, and in the brain, cells of the hippocampus are important for spatial learning and memory. Deleting genes that are expressed in the hippocampus has received considerable attention in this behavioural context. Although the initial studies experienced problems of interpretation, considerable advances have since been made. Knockout mice are now subject to tests of different forms of learning, multicellular hippocampal recordings, and restricted gene deletion specific to cells of component regions. This multi-level approach is proving more informative. Nevertheless, there is still a need to recognise that behavioural expression is several steps removed from gene expression, and that the relationship between genes and behaviour can be reciprocal.

Biosynthesis and action of nitric oxide in mammalian cells

Nitric oxide (NO) can act as a vasorelaxant, a modulator of neurotransmission and a defence against pathogens. However, under certain conditions, NO can also have damaging effects to cells. Whether NO is useful or harmful depends on its chemical fate, and on the rate and location of its production. Here, we discuss progress in NO chemistry and the enzymology of NO synthases, and we will also attempt to explain its actions in the cardiovascular, nervous and immune systems.

Memory formation: the sequence of biochemical events in the hippocampus and its connection to activity in other brain structures

Recent data have demonstrated a biochemical sequence of events in the rat hippocampus that is necessary for memory formation of inhibitory avoidance behavior. The sequence initially involves the activation of three different types of glutamate receptors followed by changes in second messengers and biochemical cascades led by enhanced activity of protein kinases A, C, and G and calcium-calmodulin protein kinase II, followed by changes in glutamate receptor subunits and binding properties and increased expression of constitutive and inducible transcription factors. The biochemical events are regulated early after training by hormonal and neurohumoral mechanisms related to alertness, anxiety, and stress, and 3-6 h after training by pathways related to mood and affect. The early modulation is mediated locally by GABAergic, cholinergic, and noradrenergic synapses and by putative retrograde synaptic messengers, and extrinsically by the amygdala and possibly the medial septum, which handle emotional components of memories and are direct or indirect sites of action for several hormones and neurotransmitters. The late modulation relies on dopamine D1, beta-noradrenergic, and 5HT1A receptors in the hippocampus and dopaminergic, noradrenergic, and serotoninergic pathways. Evidence indicates that hippocampal activity mediated by glutamate AMPA receptors must persist during at least 3 h after training in order for memories to be consolidated. Probably, this activity is transmitted to other areas, including the source of the dopaminergic, noradrenergic, and serotoninergic pathways, and the entorhinal and posterior parietal cortex. The entorhinal and posterior parietal cortex participate in memory consolidation minutes after the hippocampal chain of events starts, in both cases through glutamate NMDA receptor-mediated processes, and their intervention is necessary in order to complete memory consolidation. The hippocampus, amygdala, entorhinal cortex, and parietal cortex are involved in retrieval in the first few days after training; at 30 days from training only the entorhinal and parietal cortex are involved, and at 60 days only the parietal cortex is necessary for retrieval. Based on observations on other forms of hippocampal plasticity and on memory formation in the chick brain, it is suggested that the hippocampal chain of events that underlies memory formation is linked to long-term storage elsewhere through activity-dependent changes in cell connectivity.

Possible role of nitric oxide-cyclic GMP pathway in object recognition memory: effects of 7-nitroindazole and zaprinast

The effects of 7-nitroindazole, a putative selective inhibitor of neuronal nitric oxide (NO) synthase and zaprinast, a cGMP-selective phosphodiesterase inhibitor, were evaluated on recognition memory of rats in the object recognition test. This test is based on the differential exploration of a new and a familiar object. Two doses of 7-nitroindazole (10 and 30 mg/kg) and zaprinast (3 and 10 mg/kg) were used. The substances were administered i.p. immediately after the exposure to two identical objects, i.e., at the start of the delay interval. After a delay interval of 1 h, control rats spent more time exploring the new object which demonstrates that they recognized the familiar one. Both doses of 7-nitroindazole impaired the discrimination between the two objects after the 1 h interval. After a 4 h interval, control rats did not discriminate between the objects. The highest dose of zaprinast facilitated object recognition after the 4 h interval. In addition, this dose of zaprinast (10 mg/kg) reversed the recognition memory deficit induced by 7-nitroindazole (10 mg/kg) at the 1 h interval. The highest dose of 7-nitroindazole slightly increased mean arterial blood pressure 1 h after its administration. 4 h after administration of zaprinast (10 mg/kg), mean arterial blood pressure was also slightly increased, but not after 1 h after zaprinast administration. However, these effects on blood pressure do not explain the differential effects on object recognition memory. These results therefore suggest that NO-cGMP signal transduction is involved in object recognition memory independently of its cardiovascular role. Finally, since 7-nitroindazole affected mean arterial blood pressure it can not be regarded as a selective inhibitor of neuronal NO synthase.

Effects of nitric oxide on neuroendocrine function and behavior

Nitric oxide (NO) is an unusual chemical messenger. NO mediates blood vessel relaxation when produced by endothelial cells. When produced by macrophages, NO contributes to the cytotoxic function of these immune cells. NO also functions as a neurotransmitter and neuromodulator in the central and peripheral nervous systems. The effects on blood vessel tone and neuronal function form the basis for an important role of NO on neuroendocrine function and behavior. NO mediates hypothalamic portal blood flow and, thus, affects oxytocin and vasopression secretion; furthermore, NO mediates neuroendocrine function in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes. NO influences several motivated behaviors including sexual, aggressive, and ingestive behaviors. Learning and memory are also influenced by NO. Taken together, NO is emerging as an important chemical mediator of neuroendocrine function and behavior.

Nitric oxide in renal health and disease

Nitric oxide (NO) is a labile radical gas that is widely acclaimed as one of the most important molecules in biology. Through covalent modifications of target proteins and redox reactions with oxygen and superoxide radical and transition metal prosthetic groups, NO plays a critical role in many vital biological processes, including the control of vascular tone, neurotransmission, ventilation, hormone secretion, inflammation, and immunity. Moreover, NO has been shown to influence a host of fundamental cellular functions, such as RNA synthesis, mitochondrial respiration, glycolysis, and iron metabolism. NO is formed from L-arginine by NO synthases (NOSs), a family of related enzymes encoded by separate unlinked genes. The different NOS isozymes exhibit disparate tissue and intrarenal distributions and are governed by unique regulatory mechanisms. In the kidney, NO participates in several vital processes, including the regulation of glomerular and medullary hemodynamics, the tubuloglomerular feedback response, renin release, and the extracellular fluid volume. While NO serves beneficial roles as a messenger and host defense molecule, excessive NO production can be cytotoxic, the result of NO's reaction with reactive oxygen and nitrogen species, leading to peroxynitrite anion formation, protein tyrosine nitration, and hydroxyl radical production. Indeed, NO may contribute to the evolution of several commonly encountered renal diseases, including immune-mediated glomerulonephritis, postischemic renal failure, radiocontrast nephropathy, obstructive nephropathy, and acute and chronic renal allograft rejection. Moreover, impaired NO production has been implicated in the pathogenesis of volume-dependent hypertension. This duality of NO's beneficial and detrimental effects has created extraordinary interest in this molecule and the need for a detailed understanding of NO biosynthesis.

Nitric oxide, the enigmatic neuronal messenger: its role in synaptic plasticity

The discovery of the intercellular messenger nitric oxide (NO) stimulated new concepts of how synaptic plasticity could be induced in the nervous system. While initial reports found evidence that NO is of importance for the formation of long-term potentiation of synaptic transmission (LTP) and spatial learning in rats, later reports failed to confirm these results. Novel approaches such as deletion of the gene that encodes NO synthase in mice showed that the neuronal and the endothelial isoforms are expressed in neurones. Deletion of both isoforms reduced the inducibility of LTP. Furthermore, novel selective inhibitors of NO synthase impaired spatial learning. These results support the hypothesis that NO plays an important role in synaptic transmission and explain some but not all previously contradictory results.

Cellular and molecular bases of memory: synaptic and neuronal plasticity

Discoveries made during the past decade have greatly improved our understanding of how the nervous system functions. This review article examines the relation between memory and the cellular mechanisms of neuronal and synaptic plasticity in the central nervous system. Evidence indicating that activity-dependent short- and long-term changes in strength of synaptic transmission are important for memory processes is examined. Focus is placed on one model of synaptic plasticity called long-term potentiation, and its similarities with memory processes are illustrated. Recent studies show that the regulation of synaptic strength is bidirectional (e.g., synaptic potentiation or depression). Mechanisms involving intracellular signaling pathways that regulate synaptic strength are described, and the specific roles of calcium, protein kinases, protein phosphatases, and retrograde messengers are emphasized. Evidence suggests that changes in synaptic ultrastructure, dendritic ultrastructure, and neuronal gene expression may also contribute to mechanisms of synaptic plasticity. Also discussed are recent findings about postsynaptic mechanisms that regulate short-term synaptic facilitation and neuronal burst-pattern activity, as well as evidence about the subcellular location (presynaptic or postsynaptic) of mechanisms involved in long-term synaptic plasticity.

Two pathways of nitric oxide production through glutamate receptors in the rat cerebellum in vivo

The effects of N-methyl-D-aspartate (NMDA), (+)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and trans-(+/-)-1-amino-(1S,3R)-cyclopentanedicarboxylic acid (ACPD) on nitric oxide (NO) production in the cerebellum of conscious rats were investigated by measuring the levels of total NO metabolites (nitrite plus nitrate, NOx-) in dialysates obtained by in vivo microdialysis. All glutamate receptor agonists dose-dependently increased NOx- levels. Pharmacological characterization with various glutamate receptor antagonists indicated that the effects of NMDA, AMPA and ACPD are mediated by NMDA, non-NMDA, and L(+)-2-amino-3-phosphonopropionic acid (L(+)-AP-3)-sensitive metabotropic glutamate receptors, respectively. The NO synthase (NOS) inhibitors, including NG-nitro-L-arginine methyl ester (L-NAME), NG-nitro-L-arginine (L-NA), 7-nitroindazole (7-NI), and NG-monomethyl-L-arginine, inhibited NMDA-induced, but not AMPA- or ACPD-induced, increase in NOx- levels. L-Arginine enhanced NMDA-induced, but not AMPA- or ACPD-induced, increase in NOx- levels. Cytochrome P-450 inhibitors, SKF525A and erythromycin, inhibited the effect of NMDA, but not AMPA or ACPD. These results suggest that AMPA and ACPD may induce NO production through a NOS-independent pathway although NMDA receptor-mediated NO production is dependent on NOS activity in the rat cerebellum in vivo.

Synaptic signaling by nitric oxide

Endogenous nitric oxide (NO) mediates certain aspects of synaptic plasticity and neurotoxicity associated with NMDA-type glutamate receptors. Neuronal NO synthase contains a modular protein-protein interaction motif, termed the PDZ domain, that links the synthase to a synaptic protein complex containing postsynaptic density protein PSD-95 and NMDA receptors. Characterization of this pathway has provided new insights into the role of NO in brain physiology and disease.

Nitric oxide signaling in invertebrates

Nitric oxide (NO) is an unconventional neurotransmitter and neuromodulator molecule that is increasingly found to have important signaling functions in animals from nematodes to mammals. NO signaling mechanisms in the past were identified largely through experiments on mammals, after the discovery of NO's vasodilatory functions. The use of gene knock out mice has been particularly important in revealing the functions of the several isoforms of nitric oxide synthase (NOS), the enzyme that produces NO. Recent studies have revealed rich diversity in NO signaling. In addition to the well-established pathway in which NO activates guanylyl cyclase and cGMP production, redox mechanisms involving protein nitrosylation are important contributors to modulation of neurotransmitter release and reception. NO signaling studies in invertebrates are now generating a wealth of comparative information. Invertebrate NOS isoforms have been identified in insects and molluscs, and the conserved and variable amino acid sequences evaluated. Calcium-calmodulin dependence and cofactor requirements are conserved. NADPH diaphorase studies show that NOS is found in echinoderms, coelenterates, nematodes, annelids, insects, crustaceans and molluscs. Accumulating evidence reveals that NO is used as an orthograde transmitter and cotransmitter, and as a modulator of conventional transmitter release. NO appears to be used in diverse animals for certain neuronal functions, such as chemosensory signaling, learning, and development, suggesting that these NO functions have been conserved during evolution. The discovery of NO's diverse and unconventional signaling functions has stimulated a plethora of enthusiastic investigations into its uses. We can anticipate the discovery of many more interesting and some surprising NO signaling functions.

Absence of cerebellar long-term depression in mice lacking neuronal nitric oxide synthase

Extensive pharmacological evidence suggests that nitric oxide (NO) is a crucial transmitter for cerebellar long-term depression (LTD), a long-lasting decrease in efficacy of the synapses from parallel fibers onto Purkinje neurons, triggered by coincident presynaptic activity and postsynaptic depolarization. We now show that LTD cannot be induced in Purkinje neurons under whole-cell patch clamp in cerebellar slices from young adult mice genetically lacking neuronal nitric oxide synthase (nNOS). This genetic evidence confirms the essentiality of NO and nNOS for LTD in young adult rodents. Surprisingly, LTD in cells from nNOS knockout mice cannot be rescued by photolytic uncaging of NO and cGMP inside Purkinje neurons, although such stimuli circumvent acute pharmacological inhibition of nNOS and soluble guanylate cyclase in normal rodents. Also slices from knockout mice show no deficit in cGMP elevation in response to exogenous NO. Therefore, prolonged absence of nNOS allows atrophy of the signaling pathway downstream of cGMP.

Neuronal nitric oxide synthase alternatively spliced forms: prominent functional localizations in the brain

Neuronal nitric-oxide synthase (nNOS) is subject to alternative splicing. In mice with targeted deletions of exon 2 (nNOS(delta/delta)), two alternatively spliced forms, nNOS beta and gamma, which lack exon 2, have been described. We have compared localizations of native nNOS alpha and nNOS beta and gamma by in situ hybridization and immunohistochemistry in wild-type and nNOS(delta/delta) mice. To assess nNOS catalytic activity in intact animals we localized citrulline, which is formed stoichiometrically with NO, by immunohistochemistry. nNOS beta is prominent in several brain regions of wild-type animals and shows 2-to 3-fold up-regulation in the cortex and striatum of nNOS(delta/delta) animals. The persistence of much nNOS mRNA and protein, and distinct citrulline immunoreactivity (cit-IR) in the ventral cochlear nuclei and some cit-IR in the striatum and lateral tegmental nuclei, indicate that nNOS beta is a major functional form of the enzyme in these regions. Thus, nNOS beta, and possibly other uncharacterized splice forms, appear to be important physiological sources of NO in discrete brain regions and may account for the relatively modest level of impairment in nNOS(delta/delta) animals.

Synaptic plasticity: a role for nitric oxide in LTP

Nitric oxide is back in the spotlight with a new series of studies showing that it plays an important role in long-term potentiation, the best-studied type of synaptic plasticity in the central nervous system thought likely to play an important role in learning and memory.