Nitric oxide is required for the induction and heterosynaptic spread of long-term potentiation in rat cerebellar slices

The Pathway-Selective Dependence of Nitric Oxide for Long-Term Potentiation in the Anterior Cingulate Cortex of Adult Mice

Nitric oxide (NO) is a key diffusible messenger in the mammalian brain. It has been proposed that NO may diffuse in retrograde into presynaptic terminals, contributing to the induction of hippocampal long-term potentiation (LTP). Here, we present novel evidence that NO is selectively required for the synaptic potentiation of the interhemispheric projection in the anterior cingulate cortex (ACC). Unilateral low-frequency stimulation (LFS) induced a short-term synaptic potentiation on the contralateral ACC through the corpus callosum (CC). The use of the antagonists of the NMDA receptor (NMDAR), or the inhibitor of the L-type voltage-dependent Ca2+ channels (L-VDCCs), blocked the induction of this ACC-ACC potentiation. In addition, the inhibitor of NO synthase, or inhibitors for its downstream signaling pathway, also blocked this ACC-ACC potentiation. However, the application of the NOS inhibitor blocked neither the local electric stimulation-induced LTP nor the stimulation-induced recruitment of silent responses. Our results present strong evidence for the pathway-selective roles of NO in the LTP of the ACC.

Induction of Activity-Dependent Plasticity at Auditory Nerve Synapses

Exposure to nontraumatic noise in vivo drives long-lasting changes in auditory nerve synapses, which may influence hearing, but the induction mechanisms are not known. We mimicked activity in acute slices of the cochlear nucleus from mice of both sexes by treating them with high potassium, after which voltage-clamp recordings from bushy cells indicated that auditory nerve synapses had reduced EPSC amplitude, quantal size, and vesicle release probability (P r). The effects of high potassium were prevented by blockers of nitric oxide (NO) synthase and protein kinase A. Treatment with the NO donor, PAPA-NONOate, also decreased P r, suggesting NO plays a central role in inducing synaptic changes. To identify the source of NO, we activated auditory nerve fibers specifically using optogenetics. Strobing for 2 h led to decreased EPSC amplitude and P r, which was prevented by antagonists against ionotropic glutamate receptors and NO synthase. This suggests that the activation of AMPA and NMDA receptors in postsynaptic targets of auditory nerve fibers drives release of NO, which acts retrogradely to cause long-term changes in synaptic function in auditory nerve synapses. This may provide insight into preventing or treating disorders caused by noise exposure.SIGNIFICANCE STATEMENT Auditory nerve fibers undergo long-lasting changes in synaptic properties in response to noise exposure in vivo, which may contribute to changes in hearing. Here, we investigated the cellular mechanisms underlying induction of synaptic changes using high potassium and optogenetic stimulation in vitro and identified important signaling pathways using pharmacology. Our results suggest that auditory nerve activity drives postsynaptic depolarization through AMPA and NMDA receptors, leading to the release of nitric oxide, which acts retrogradely to regulate presynaptic neurotransmitter release. These experiments revealed that auditory nerve synapses are unexpectedly sensitive to activity and can show dramatic, long-lasting changes in a few hours that could affect hearing.

Guanosine Neuroprotective Action in Hippocampal Slices Subjected to Oxygen and Glucose Deprivation Restores ATP Levels, Lactate Release and Glutamate Uptake Impairment: Involvement of Nitric Oxide

Stroke is a major cause of disability and death worldwide. Oxygen and glucose deprivation (OGD) in brain tissue preparations can reproduce several pathological features induced by stroke providing a valuable ex vivo protocol for studying the mechanism of action of neuroprotective agents. Guanosine, an endogenous guanine nucleoside, promotes neuroprotection in vivo and in vitro models of neurotoxicity. We previously showed that guanosine protective effect was mimicked by inhibition of nitric oxide synthases (NOS) activity. This study was designed to investigate the involvement of nitric oxide (NO) in the mechanisms related to the protective role of guanosine in rat hippocampal slices subjected to OGD followed by reoxygenation (OGD/R). Guanosine (100 μM) and the pan-NOS inhibitor, L-NAME (1 mM) afforded protection to hippocampal slices subjected to OGD/R. The presence of NO donors, DETA-NO (800 μM) or SNP (5 μM) increased reactive species production, and abolished the protective effect of guanosine or L-NAME against OGD/R. Guanosine or L-NAME treatment prevented the impaired ATP production, lactate release, and glutamate uptake following OGD/R. The presence of a NO donor also abolished the beneficial effects of guanosine or L-NAME on bioenergetics and glutamate uptake. These results showed, for the first time, that guanosine may regulate cellular bioenergetics in hippocampal slices subjected to OGD/R injury by a mechanism that involves the modulation of NO levels.

Genetic and molecular basis of epilepsy-related cognitive dysfunction

Epilepsy is a common neurological disease characterized by recurrent seizures. About 70 million people were affected by epilepsy or epileptic seizures. Epilepsy is a complicated complex or symptomatic syndromes induced by structural, functional, and genetic causes. Meanwhile, several comorbidities are accompanied by epileptic seizures. Cognitive dysfunction is a long-standing complication associated with epileptic seizures, which severely impairs quality of life. Although the definitive pathogenic mechanisms underlying epilepsy-related cognitive dysfunction remain unclear, accumulating evidence indicates that multiple risk factors are probably involved in the development and progression of cognitive dysfunction in patients with epilepsy. These factors include the underlying etiology, recurrent seizures or status epilepticus, structural damage that induced secondary epilepsy, genetic variants, and molecular alterations. In this review, we summarize several theories that may explain the genetic and molecular basis of epilepsy-related cognitive dysfunction.

Modulation, Plasticity and Pathophysiology of the Parallel Fiber-Purkinje Cell Synapse

The parallel fiber-Purkinje cell (PF-PC) synapse represents the point of maximal signal divergence in the cerebellar cortex with an estimated number of about 60 billion synaptic contacts in the rat and 100,000 billions in humans. At the same time, the Purkinje cell dendritic tree is a site of remarkable convergence of more than 100,000 parallel fiber synapses. Parallel fiber activity generates fast postsynaptic currents via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and slower signals, mediated by mGlu1 receptors, resulting in Purkinje cell depolarization accompanied by sharp calcium elevation within dendritic regions. Long-term depression (LTD) and long-term potentiation (LTP) have been widely described for the PF-PC synapse and have been proposed as mechanisms for motor learning. The mechanisms of induction for LTP and LTD involve different signaling mechanisms within the presynaptic terminal and/or at the postsynaptic site, promoting enduring modification in the neurotransmitter release and change in responsiveness to the neurotransmitter. The PF-PC synapse is finely modulated by several neurotransmitters, including serotonin, noradrenaline and acetylcholine. The ability of these neuromodulators to gate LTP and LTD at the PF-PC synapse could, at least in part, explain their effect on cerebellar-dependent learning and memory paradigms. Overall, these findings have important implications for understanding the cerebellar involvement in a series of pathological conditions, ranging from ataxia to autism. For example, PF-PC synapse dysfunctions have been identified in several murine models of spino-cerebellar ataxia (SCA) types 1, 3, 5 and 27. In some cases, the defect is specific for the AMPA receptor signaling (SCA27), while in others the mGlu1 pathway is affected (SCA1, 3, 5). Interestingly, the PF-PC synapse has been shown to be hyper-functional in a mutant mouse model of autism spectrum disorder, with a selective deletion of Pten in Purkinje cells. However, the full range of methodological approaches, that allowed the discovery of the physiological principles of PF-PC synapse function, has not yet been completely exploited to investigate the pathophysiological mechanisms of diseases involving the cerebellum. We, therefore, propose to extend the spectrum of experimental investigations to tackle this problem.

Age-dependent changes in the glutamate-nitric oxide pathway in the hippocampus of the triple transgenic model of Alzheimer's disease: implications for neurometabolic regulation

Age-dependent changes in nitric oxide ((•)NO) concentration dynamics may play a significant role in both decaying synaptic and metabolic functions in Alzheimer's disease (AD). This neuromodulator acts presynaptically to increase vesicle release and glutamatergic transmission and also regulates mitochondrial function. Under conditions of altered intracellular redox environment, (•)NO may react and produce reactive species such as peroxynitrite. Using the triple transgenic mouse model of AD (3xTgAD), we investigated age-dependent changes in the glutamate-(•)NO axis in the hippocampus. Direct measurement of (•)NO concentration dynamics revealed a significant increase in N-methyl-D-aspartate type receptor-evoked peak (•)NO in the 3xTgAD model at an early age. Aging produced a decrease in peak (•)NO accompanied by significant decrease in production and decay rates in the transgenic model. Evaluation of energy metabolism revealed age-dependent decrease in basal oxygen consumption rate, a general decrease in mitochondrial oxidative phosphorylation parameters, and loss in mitochondrial sparing capacity in both genotypes. Finally, we observed age-dependent increase in 3-nitrotyrosine residues in the hippocampus, consistent with a putative shift in (•)NO bioactivity toward oxidative chemistry associated with neurotoxicity.

Climbing fibers in spinocerebellar ataxia: A mechanism for the loss of motor control

The spinocerebellar ataxias (SCAs) form an ever-growing group of neurodegenerative disorders causing dysfunction of the cerebellum and loss of motor control in patients. Currently, 41 different genetic causes have been identified, with each mutation affecting a different gene. Interestingly, these diverse genetic causes all disrupt cerebellar function and produce similar symptoms in patients. In order to understand the disease better, and define possible therapeutic targets for multiple SCAs, the field has been searching for common ground among the SCAs. In this review, we discuss the physiology of climbing fibers and the possibility that climbing fiber dysfunction is a point of convergence for at least a subset of SCAs.

Localization of Presynaptic Plasticity Mechanisms Enables Functional Independence of Synaptic and Ectopic Transmission in the Cerebellum

In the cerebellar molecular layer parallel fibre terminals release glutamate from both the active zone and from extrasynaptic “ectopic” sites. Ectopic release mediates transmission to the Bergmann glia that ensheathe the synapse, activating Ca²⁺-permeable AMPA receptors and glutamate transporters. Parallel fibre terminals exhibit several forms of presynaptic plasticity, including cAMP-dependent long-term potentiation and endocannabinoid-dependent long-term depression, but it is not known whether these presynaptic forms of long-term plasticity also influence ectopic transmission to Bergmann glia. Stimulation of parallel fibre inputs at 16 Hz evoked LTP of synaptic transmission, but LTD of ectopic transmission. Pharmacological activation of adenylyl cyclase by forskolin caused LTP at Purkinje neurons, but only transient potentiation at Bergmann glia, reinforcing the concept that ectopic sites lack the capacity to express sustained cAMP-dependent potentiation. Activation of mGluR1 caused depression of synaptic transmission via retrograde endocannabinoid signalling but had no significant effect at ectopic sites. In contrast, activation of NMDA receptors suppressed both synaptic and ectopic transmission. The results suggest that the signalling mechanisms for presynaptic LTP and retrograde depression by endocannabinoids are restricted to the active zone at parallel fibre synapses, allowing independent modulation of synaptic transmission to Purkinje neurons and ectopic transmission to Bergmann glia.

Regulation of brain glutamate metabolism by nitric oxide and S-nitrosylation

Nitric oxide (NO) is a signaling intermediate during glutamatergic neurotransmission in the central nervous system (CNS). NO signaling is in part accomplished through cysteine S-nitrosylation, a posttranslational modification by which NO regulates protein function and signaling. In our investigation of the protein targets and functional impact of S-nitrosylation in the CNS under physiological conditions, we identified 269 S-nitrosocysteine residues in 136 proteins in the wild-type mouse brain. The number of sites was significantly reduced in the brains of mice lacking endothelial nitric oxide synthase (eNOS(-/-)) or neuronal nitric oxide synthase (nNOS(-/-)). In particular, nNOS(-/-) animals showed decreased S-nitrosylation of proteins that participate in the glutamate/glutamine cycle, a metabolic process by which synaptic glutamate is recycled or oxidized to provide energy. (15)N-glutamine-based metabolomic profiling and enzymatic activity assays indicated that brain extracts from nNOS(-/-) mice converted less glutamate to glutamine and oxidized more glutamate than those from mice of the other genotypes. GLT1 [also known as EAAT2 (excitatory amino acid transporter 2)], a glutamate transporter in astrocytes, was S-nitrosylated at Cys(373) and Cys(561) in wild-type and eNOS(-/-) mice, but not in nNOS(-/-) mice. A form of rat GLT1 that could not be S-nitrosylated at the equivalent sites had increased glutamate uptake compared to wild-type GLT1 in cells exposed to an S-nitrosylating agent. Thus, NO modulates glutamatergic neurotransmission through the selective, nNOS-dependent S-nitrosylation of proteins that govern glutamate transport and metabolism.

Caffeine Modulates Vesicle Release and Recovery at Cerebellar Parallel Fibre Terminals, Independently of Calcium and Cyclic AMP Signalling

Background

Cerebellar parallel fibres release glutamate at both the synaptic active zone and at extrasynaptic sites—a process known as ectopic release. These sites exhibit different short-term and long-term plasticity, the basis of which is incompletely understood but depends on the efficiency of vesicle release and recycling. To investigate whether release of calcium from internal stores contributes to these differences in plasticity, we tested the effects of the ryanodine receptor agonist caffeine on both synaptic and ectopic transmission.

Methods

Whole cell patch clamp recordings from Purkinje neurons and Bergmann glia were carried out in transverse cerebellar slices from juvenile (P16-20) Wistar rats.

Key Results

Caffeine caused complex changes in transmission at both synaptic and ectopic sites. The amplitude of postsynaptic currents in Purkinje neurons and extrasynaptic currents in Bergmann glia were increased 2-fold and 4-fold respectively, but paired pulse ratio was substantially reduced, reversing the short-term facilitation observed under control conditions. Caffeine treatment also caused synaptic sites to depress during 1 Hz stimulation, consistent with inhibition of the usual mechanisms for replenishing vesicles at the active zone. Unexpectedly, pharmacological intervention at known targets for caffeine—intracellular calcium release, and cAMP signalling—had no impact on these effects.

Conclusions

We conclude that caffeine increases release probability and inhibits vesicle recovery at parallel fibre synapses, independently of known pharmacological targets. This complex effect would lead to potentiation of transmission at fibres firing at low frequencies, but depression of transmission at high frequency connections.

Development of nitric oxide synthase inhibitors for neurodegeneration and neuropathic pain

Nitric oxide (NO) is an important signaling molecule in the human body, playing a crucial role in cell and neuronal communication, regulation of blood pressure, and in immune activation. However, overproduction of NO by the neuronal isoform of nitric oxide synthase (nNOS) is one of the fundamental causes underlying neurodegenerative disorders and neuropathic pain. Therefore, developing small molecules for selective inhibition of nNOS over related isoforms (eNOS and iNOS) is therapeutically desirable. The aims of this review focus on the regulation and dysregulation of NO signaling, the role of NO in neurodegeneration and pain, the structure and mechanism of nNOS, and the use of this information to design selective inhibitors of this enzyme. Structure-based drug design, the bioavailability and pharmacokinetics of these inhibitors, and extensive target validation through animal studies are addressed.

Age-associated changes of nitric oxide concentration dynamics in the central nervous system of Fisher 344 rats

The increase in life expectancy is accompanied by an increased risk of developing neurodegenerative disorders and age is the most relevant risk factor for the appearance of cognitive decline. While decreased neuronal count has been proposed to be a major contributing factor to the appearance of age-associated cognitive decline, it appears to be insufficient to fully account for the decay in mental function in aged individuals. Nitric oxide ((•)NO) is a ubiquitous signaling molecule in the mammalian central nervous system. Closely linked to the activation of glutamatergic transmission in several structures of the brain, neuron-derived (•)NO can act as a neuromodulator in synaptic plasticity but has also been linked to neuronal toxicity and degenerative processes. Many studies have proposed that changes in the glutamate-(•)NO signaling pathway may be implicated in age-dependent cognitive decline and that the exact effect of such changes may be region specific. Due to its peculiar physical-chemical properties, namely hydrophobicity, small size, and rapid diffusion properties, the rate and pattern of (•)NO concentration changes are critical determinants for the understanding of its bioactivity in the brain. Here we show a detailed study of how (•)NO concentration dynamics change in the different regions of the brain of Fisher 344 rats (F344) during aging. Using microelectrodes inserted into the living brain of anesthetized F344 rats, we show here that glutamate-induced (•)NO concentration dynamics decrease in the hippocampus, striatum, and cerebral cortex as animals age. performance in behavior testing of short-term and spatial memory, suggesting that the impairment in the glutamate:nNOS pathway represents a functional critical event in cognitive decline during aging.

Oxidative stress induced by epileptic seizure and its attenuation by melatonin

An epileptic seizure and postictal period in addition to well-known features are also characterized by massive consumption of energy. This is thought to lead to oxidative stress and increased generation of free radicals, which is reflected by increased levels of oxidative products. Our previous work described the neuroprotective effects of melatonin in preventing cognitive worsening after a single epileptic seizure. This work was aimed on direct measurement of free radicals in brain tissue using the EPR method 1, 15 and 60 minutes after seizure. The measurement was performed in adult male Wistar rats at the mentioned intervals after a single tonic-clonic seizure induced by flurothyl. In comparison to control animals there was a significant increase in hydroxyl and nitroxyl radicals 60 minutes after the seizure. The levels of hydroxyl radicals were significantly lower in animals that received melatonin 60 minutes before seizure induction compared to animals without preventive treatment. Therefore, melatonin affected the generation of the measured free radicals differently. An important finding was the delayed increase in free radicals after a single seizure in the later phases of recovery.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.

Non-associative potentiation of perisomatic inhibition alters the temporal coding of neocortical layer 5 pyramidal neurons

In the neocortex, the coexistence of temporally locked excitation and inhibition governs complex network activity underlying cognitive functions, and is believed to be altered in several brain diseases. Here we show that this equilibrium can be unlocked by increased activity of layer 5 pyramidal neurons of the mouse neocortex. Somatic depolarization or short bursts of action potentials of layer 5 pyramidal neurons induced a selective long-term potentiation of GABAergic synapses (LTPi) without affecting glutamatergic inputs. Remarkably, LTPi was selective for perisomatic inhibition from parvalbumin basket cells, leaving dendritic inhibition intact. It relied on retrograde signaling of nitric oxide, which persistently altered presynaptic GABA release and diffused to inhibitory synapses impinging on adjacent pyramidal neurons. LTPi reduced the time window of synaptic summation and increased the temporal precision of spike generation. Thus, increases in single cortical pyramidal neuron activity can induce an interneuron-selective GABAergic plasticity effectively altering the computation of temporally coded information.

Neural changes accompanying tinnitus following unilateral acoustic trauma in the guinea pig

Animal models of tinnitus allow us to study the relationship between changes in neural activity and the tinnitus percept. Here, guinea pigs were subjected to unilateral noise trauma and tested behaviourally for tinnitus 8 weeks later. By comparing animals with tinnitus with those without, all of which were noise-exposed, we were able to identify changes unique to the tinnitus group. Three physiological markers known to change following noise exposure were examined: spontaneous firing rates (SFRs) and burst firing in the inferior colliculus (IC), evoked auditory brainstem responses (ABRs), and the number of neurons in the cochlear nucleus containing nitric oxide synthase (NOS). We obtained behavioural evidence of tinnitus in 12 of 16 (75%) animals. Both SFRs and incidences of burst firing were elevated in the IC of all noise-exposed animals, but there were no differences between tinnitus and no-tinnitus animals. There were significant decreases in ipsilateral ABR latencies in tinnitus animals, contrary to what might be expected with a small hearing loss. Furthermore, there was an ipsilateral-contralateral asymmetry in NOS staining in the ventral cochlear nucleus (VCN) that was only apparent in tinnitus animals. Tinnitus animals had a significantly greater number of NOS-containing neurons on the noise-exposed side, whereas no-tinnitus animals did not. These data suggest that measuring NOS in the VCN and recording ABRs supplement behavioural methods for confirming tinnitus in animals, and that nitric oxide is involved in plastic neural changes associated with tinnitus.

Properties of 4 Hz stimulation-induced parallel fiber-Purkinje cell presynaptic long-term plasticity in mouse cerebellar cortex in vivo

Cerebellar parallel fiber-Purkinje cell (PF-PC) long-term synaptic plasticity is important for the formation and stability of cerebellar neuronal circuits, and provides substrates for motor learning and memory. We previously reported both presynaptic long-term potentiation (LTP) and long-term depression (LTD) in cerebellar PF-PC synapses in vitro. However, the expression and mechanisms of cerebellar PF-PC synaptic plasticity in the cerebellar cortex in vivo are poorly understood. In the present study, we studied the properties of 4 Hz stimulation-induced PF-PC presynaptic long-term plasticity using in vivo the whole-cell patch-clamp recording technique and pharmacological methods in urethane-anesthetised mice. Our results demonstrated that 4 Hz PF stimulation induced presynaptic LTD of PF-PC synaptic transmission in the intact cerebellar cortex in living mice. The PF-PC presynaptic LTD was attenuated by either the N-methyl-D-aspartate receptor antagonist, D-aminophosphonovaleric acid, or the group 1 metabotropic glutamate receptor antagonist, JNJ16259685, and was abolished by combined D-aminophosphonovaleric acid and JNJ16259685, but enhanced by inhibition of nitric oxide synthase. Blockade of cannabinoid type 1 receptor activity abolished the PF-PC LTD and revealed a presynaptic PF-PC LTP. These data indicate that both endocannabinoids and nitric oxide synthase are involved in the 4 Hz stimulation-induced PF-PC presynaptic plasticity, but the endocannabinoid-dependent PF-PC presynaptic LTD masked the nitric oxide-mediated PF-PC presynaptic LTP in the cerebellar cortex in urethane-anesthetised mice.

Sex-influence of nicotine and nitric oxide on motor coordination and anxiety-related neurophysiological responses

RATIONALE

Nitric oxide (NO) is a messenger synthesized in both the neuronal and glial populations by nitric oxide synthase type 1 (NOS1). Nicotine regulates NO production in a sex-dependent manner, both molecules being involved in motor function.

OBJECTIVE

The present study evaluates sex differences in motor coordination, general movement, and anxiety-related responses resulting from both constant and continuous nicotine treatment and the genetic depletion of NOS1 activity.

METHODS

Male and female mice were analyzed with the open-field and the rotarod tests. To understand the role of NO, knockout mice for NOS1 (NOS1-/-) were analyzed. Nicotine was administered continuously at a dose of 24 mg/kg/day via osmotic mini-pumps over 14 days because the behavioral effects elicited are similar to those observed with discontinuous administration.

RESULTS

Data analyses revealed noteworthy sex differences derived from NOS1 depletion. Control NOS1-/- males exhibited an exacerbated anxiety-related response in relation to control NOS1-/- females and control wild-type (WT) males; these differences disappeared in the nicotine-administered NOS1-/- males. Additionally, nicotine administration differentially affected the horizontal movements of NOS1-/- females with respect to WT animals. NO depletion affected male but not female motor coordination improvement along the test days. However, the drug affected female motor coordination only at the end of the administration period.

CONCLUSIONS

We show for the first time that NO affects motor and anxiety behaviors in a sex-dependent manner. Moreover, the behavioral effects of constant nicotine administration are dimorphic and dependent on NO production.

Nitric oxide signaling exerts bidirectional effects on plasticity inductions in amygdala

It has been well known that long-term potentiation (LTP) of synaptic transmission in the lateral nucleus of the amygdala (LA) constitutes an essential cellular mechanism contributing to encoding of conditioned fear. Nitric oxide (NO), produced by activation of the postsynaptic N-methyl-D-aspartate receptors (NMDAR) in thalamic input to the LA, has been thought to promote LTP, contributing to the establishment of conditioned fear. However, it is not known whether and how NO, released from cortical input to the LA, plays the role on the plasticity induction and fear memory. Here we report that the diffusion of NO, released in response to activation of presynaptic NMDAR on cortical afferent fibers in the LA, could suppress heterosynaptically a form of presynaptic kainate receptor (KAR) dependent LTP (pre-LTP) in thalamic input, which was induced by low-frequency presynaptic stimuli without postsynaptic depolarization. We also confirmed that NO, produced by activation of postsynaptic NMDAR in thalamic input, can promote postsynaptic NMDAR-dependent LTP (post-LTP), which was induced by pairing protocol. These LTPs were occluded following fear conditioning, indicating that they could contribute to encoding of conditioned fear memory. However, their time courses are different; Post-LTP was more rapidly formed than pre-LTP in the course of fear conditioning. NO, produced by activation of presynaptic NMDAR in cortical input and postsynaptic NMDAR in thalamic input, may control conditioned fear by suppressing pre-LTP and promoting post-LTP, respectively, in thalamic input to the LA.

Role of nitric oxide in cerebellar development and function: focus on granule neurons

More than 20 years of research have firmly established important roles of the diffusible messenger molecule, nitric oxide (NO), in cerebellar development and function. Granule neurons are main players in every NO-related mechanism involving cerebellar function and dysfunction. Granule neurons are endowed with remarkable amounts of the Ca(2+)-dependent neuronal isoform of nitric oxide synthase and can directly respond to endogenously produced NO or induce responses in neighboring cells taking advantage of the high diffusibility of the molecule. Nitric oxide acts as a negative regulator of granule cell precursor proliferation and promotes survival and differentiation of these neurons. Nitric oxide is neuroprotective towards granule neurons challenged with toxic insults. Nitric oxide is a main regulator of bidirectional plasticity at parallel fiber-Purkinje neuron synapses, inducing long-term depression (LTD) or long-term potentiation (LTP) depending on postsynaptic Ca(2+) levels, thus playing a central role in cerebellar learning related to motor control. Granule neurons cooperate with glial cells, in particular with microglia, in the regulation of NO production through the respective forms of NOS present in the two cellular types. Aim of the present paper is to review the state of the art and the improvement of our understanding of NO functions in cerebellar granule neurons obtained during the last two decades and to outline possible future development of the research.

Distributed synergistic plasticity and cerebellar learning

Studies on synaptic plasticity in the context of learning have been dominated by the view that a single, particular type of plasticity forms the underlying mechanism for a particular type of learning. However, emerging evidence shows that many forms of synaptic and intrinsic plasticity at different sites are induced conjunctively during procedural memory formation in the cerebellum. Here, we review the main forms of long-term plasticity in the cerebellar cortex that underlie motor learning. We propose that the different forms of plasticity in the granular layer and the molecular layer operate synergistically in a temporally and spatially distributed manner, so as to ultimately create optimal output for behaviour.

Nitric Oxide and the Biological Cascades Underlying Increased Neurogenesis, Enhanced Learning Ability, and Academic Ability as an Effect of Increased Bouts of Physical Activity

The consummate principle underlying all physiological research is corporeal adaptation at every level of the organism observed. With respect to humans, the body learns to function based on the external stimuli from the environment, beginning in the womb, throughout the developmental stages of life. Nitric Oxide (NO) appears to be the governor of the plasticity of several systems in mammals implicit in their proper development. It is the purpose of this review to describe the physiological pathways that lead to plasticity of not only the vasculature but also of the brain and how physical activity plays a key role in those alterations by initiating the mechanism that triggers NO production. Further, this review hopes to show a connection between these changes and learning, comprising both motor learning and cognitive learning. This review will show how NO plays a significant role in vascularization and neurogenesis, necessary to enhance the mind-body connection and comprehensive physical performance and adaptation. It is our belief that this review effectively demonstrates, using a multidisciplinary approach, the causal mechanisms underlying the increases in neurogenesis as related to improved learning and academic performance as a result of adequate bouts of physical activity of a vigorous nature.

G-substrate: the cerebellum and beyond

The discovery of nitric oxide (NO) as an activator of soluble guanylate cyclase (sGC) has stimulated extensive research on the NO-sGC-3':5'-cyclic guanosine monophosphate (cGMP)-cGMP-dependent protein kinase (PKG) pathway. However, the restricted localization of pathway components and the lack of information on PKG substrates have hindered research seeking to examine the physiological roles of the NO-sGC-cGMP-PKG pathway. An excellent substrate for PKG is the G-substrate, which was originally discovered in the cerebellum. The role of G-substrate in the cerebellum and other brain structures has been revealed in recent years. This review discusses the relationship between the G-substrate and other components of the NO-sGC-cGMP-PKG pathway and describes the characteristics of the G-substrate gene and protein related to diseases. Finally, we discuss the physiological role of G-substrate in the cerebellum, where it regulates cerebellum-dependent long-term memory, and its role in the ventral tegmental area and retina, where it acts as an effective neuroprotectant.

Post-translational modifications and lipid binding profile of insect cell-expressed full-length mammalian synaptotagmin 1

Synaptotagmin 1 (Syt1) is a Ca(2+) sensor for SNARE-mediated, Ca(2+)-triggered synaptic vesicle fusion in neurons. It is composed of luminal, transmembrane, linker, and two Ca(2+)-binding (C2) domains. Here we describe expression and purification of full-length mammalian Syt1 in insect cells along with an extensive biochemical characterization of the purified protein. The expressed and purified protein is properly folded and has increased α-helical content compared to the C2AB fragment alone. Post-translational modifications of Syt1 were analyzed by mass spectrometry, revealing the same modifications of Syt1 that were previously described for Syt1 purified from brain extract or mammalian cell lines, along with a novel modification of Syt1, tyrosine nitration. A lipid binding screen with both full-length Syt1 and the C2AB fragments of Syt1 and Syt3 isoforms revealed new Syt1-lipid interactions. These results suggest a conserved lipid binding mechanism in which Ca(2+)-independent interactions are mediated via a lysine rich region of the C2B domain while Ca(2+)-dependent interactions are mediated via the Ca(2+)-binding loops.

Nitric oxide signaling in brain function, dysfunction, and dementia

Nitric oxide (NO) is an important signaling molecule that is widely used in the nervous system. With recognition of its roles in synaptic plasticity (long-term potentiation, LTP; long-term depression, LTD) and elucidation of calcium-dependent, NMDAR-mediated activation of neuronal nitric oxide synthase (nNOS), numerous molecular and pharmacological tools have been used to explore the physiology and pathological consequences for nitrergic signaling. In this review, the authors summarize the current understanding of this subtle signaling pathway, discuss the evidence for nitrergic modulation of ion channels and homeostatic modulation of intrinsic excitability, and speculate about the pathological consequences of spillover between different nitrergic compartments in contributing to aberrant signaling in neurodegenerative disorders. Accumulating evidence points to various ion channels and particularly voltage-gated potassium channels as signaling targets, whereby NO mediates activity-dependent control of intrinsic neuronal excitability; such changes could underlie broader mechanisms of synaptic plasticity across neuronal networks. In addition, the inability to constrain NO diffusion suggests that spillover from endothelium (eNOS) and/or immune compartments (iNOS) into the nervous system provides potential pathological sources of NO and where control failure in these other systems could have broader neurological implications. Abnormal NO signaling could therefore contribute to a variety of neurodegenerative pathologies such as stroke/excitotoxicity, Alzheimer's disease, multiple sclerosis, and Parkinson's disease.

A homolog of the vertebrate pituitary adenylate cyclase-activating polypeptide is both necessary and instructive for the rapid formation of associative memory in an invertebrate

Similar to other invertebrate and vertebrate animals, cAMP-dependent signaling cascades are key components of long-term memory (LTM) formation in the snail Lymnaea stagnalis, an established experimental model for studying evolutionarily conserved molecular mechanisms of long-term associative memory. Although a great deal is already known about the signaling cascades activated by cAMP, the molecules involved in the learning-induced activation of adenylate cyclase (AC) in Lymnaea remained unknown. Using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy in combination with biochemical and immunohistochemical methods, recently we have obtained evidence for the existence of a Lymnaea homolog of the vertebrate pituitary adenylate cyclase-activating polypeptide (PACAP) and for the AC-activating effect of PACAP in the Lymnaea nervous system. Here we first tested the hypothesis that PACAP plays an important role in the formation of robust LTM after single-trial classical food-reward conditioning. Application of the PACAP receptor antagonist PACAP6-38 around the time of single-trial training with amyl acetate and sucrose blocked associative LTM, suggesting that in this "strong" food-reward conditioning paradigm the activation of AC by PACAP was necessary for LTM to form. We found that in a "weak" multitrial food-reward conditioning paradigm, lip touch paired with sucrose, memory formation was also dependent on PACAP. Significantly, systemic application of PACAP at the beginning of multitrial tactile conditioning accelerated the formation of transcription-dependent memory. Our findings provide the first evidence to show that in the same nervous system PACAP is both necessary and instructive for fast and robust memory formation after reward classical conditioning.

Inhibition of presynaptic release-facilitatory kainate autoreceptors by extracellular cyclic GMP

We found that both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate autoreceptors were present on the glutamate-releasing terminals of cerebellar parallel/climbing fibers and that they functioned as facilitatory autoreceptors. Extracellular cGMP inhibited the neurotransmitter release evoked by presynaptic kainate receptor activation; the inhibitory effect of extracellular cGMP was selective for the kainate autoreceptor-mediated response and did not affect the AMPA autoreceptor-mediated response. Endogenously synthesized cGMP might be the physiological source for the extracellular cGMP modulating the response to kainate autoreceptor activation.

Depression of parallel and climbing fiber transmission to Bergmann glia is input specific and correlates with increased precision of synaptic transmission

In the cerebellar cortex, Bergmann glia enclose the synapses of both parallel and climbing fiber inputs to the Purkinje neuron. The glia express Ca(2+)-permeable AMPA receptors, and the GLAST and GLT-1 classes of glutamate transporter, which are activated by glutamate released during synaptic transmission. We have previously reported that parallel fiber to Bergmann glial transmission in rat cerebellar slices exhibits a form of frequency-dependent plasticity, namely long-term depression, following repetitive stimulation at 0.1-1 Hz. Here, we report that this form of plasticity is also present at the climbing fiber input, that climbing and parallel fibers can be depressed independently, that discrete parallel fiber inputs can also be depressed independently, and that depression is maintained when a distributed array of parallel fibers are stimulated (in contrast to several forms of synaptic plasticity at the Purkinje neuron). Depression of glutamate transporter currents does not correlate with a decrease in the stringency with which Purkinje neuron synapses are isolated. Rather, postsynaptic currents in Purkinje neurons decay more rapidly and perisynaptic metabotropic glutamate receptors are activated less effectively after stimulation at 0.2 and 1 Hz, suggesting that depression arises from a decrease in extrasynaptic glutamate concentration and not from impairment of glutamate clearance in and around the synapse. These results indicate that neuron-glial plasticity is activity dependent, input specific and does not require spillover between adjacent synapses to manifest. They also argue against a withdrawal of the glial sheath from synaptic regions as the putative mechanism of plasticity.

Nitric oxide as an endogenous peripheral modulator of visceral sensory neuronal function

Nitric oxide (NO) plays important roles in CNS and smooth muscle function. Here we reveal an additional function in peripheral sensory transmission. We hypothesized that endogenous NO modulates the function of gastrointestinal vagal afferent endings. The nonselective NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester hydrochloride increased responses to tactile mechanical stimuli of mucosal afferent endings in two species, in some cases severalfold. This was mimicked by a neuronal NOS inhibitor but not an endothelial NOS inhibitor. NOS inhibitors did not affect the responsiveness of smooth muscle afferent endings, suggesting that the endogenous source of NO is exclusively accessible to mucosal receptors. The role of the NO-soluble guanylyl cyclase (sGC)-cGMP pathway was confirmed using the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one and the cGMP phosphodiesterase 5' inhibitor sildenafil. The first enhanced and the second inhibited mechanosensory function. Exogenous NO, from the donor S-nitroso-N-acetylpenicillamine, significantly reduced mechanosensitivity of both types of ending. Up to one-third of stomach-projecting afferent neurons in the nodose ganglia expressed neuronal NOS (nNOS). However, anterograde-traced vagal endings were nNOS negative, indicating NOS is not transported peripherally and there are alternative sources of NO for afferent modulation. A subpopulation of enteroendocrine cells in the gut mucosa were nNOS positive, which were found anatomically in close apposition with mucosal vagal afferent endings. These results indicate an inhibitory neuromodulatory role of epithelial NO, which targets a select population of vagal afferents. This interaction is likely to play a role in generation of symptoms and behaviors from the upper gastrointestinal system.

Dual involvement of G-substrate in motor learning revealed by gene deletion

In this study, we generated mice lacking the gene for G-substrate, a specific substrate for cGMP-dependent protein kinase uniquely located in cerebellar Purkinje cells, and explored their specific functional deficits. G-substrate-deficient Purkinje cells in slices obtained at postnatal weeks (PWs) 10-15 maintained electrophysiological properties essentially similar to those from WT littermates. Conjunction of parallel fiber stimulation and depolarizing pulses induced long-term depression (LTD) normally. At younger ages, however, LTD attenuated temporarily at PW6 and recovered thereafter. In parallel with LTD, short-term (1 h) adaptation of optokinetic eye movement response (OKR) temporarily diminished at PW6. Young adult G-substrate knockout mice tested at PW12 exhibited no significant differences from their WT littermates in terms of brain structure, general behavior, locomotor behavior on a rotor rod or treadmill, eyeblink conditioning, dynamic characteristics of OKR, or short-term OKR adaptation. One unique change detected was a modest but significant attenuation in the long-term (5 days) adaptation of OKR. The present results support the concept that LTD is causal to short-term adaptation and reveal the dual functional involvement of G-substrate in neuronal mechanisms of the cerebellum for both short-term and long-term adaptation.

Long-term potentiation of the responses to parallel fiber stimulation in mouse cerebellar cortex in vivo

Long-term potentiation (LTP) of parallel fiber-Purkinje cell (PF-PC) synapses in the cerebellum has been suggested to underlie aspects of motor learning. Previous in vitro studies have primarily used low frequency PF stimulation conditioning paradigms to generate either presynaptic PF-PC LTP (4-8 Hz) or postsynaptic PF-PC LTP (1 Hz). Little is known about the conditions that evoke PF-PC LTP in vivo. High frequency stimulation in vivo increases PC responsiveness to peripheral stimuli; however, neither the site of action nor the signaling pathways involved have been examined. Using flavoprotein autofluorescence optical imaging in the FVB mouse in vivo, this report describes that a conditioning stimulation consisting of a high frequency burst of PF stimulation (100 Hz, 15 pulse trains every 3 s for 5 min) evokes a long-term increase in the response to PF stimulation. Following the conditioning stimulation, the response to PF stimulation increases over 20 min to approximately 130% above baseline and this potentiation persists for at least 2 h. Field potential recordings of the responses to PF stimulation show that the postsynaptic component is potentiated but the presynaptic, parallel fiber volley is not. Paired-pulse facilitation does not change after the conditioning stimulation, suggesting the potentiation occurs postsynaptically. Blocking non-NMDA (N-methyl-d-aspartic acid) ionotropic glutamate receptors with DNQX (6,7-dinitroquinoxaline-2,3-dione disodium salt, 50 muM, bath application) during the conditioning stimulation has no effect on the long-term increase in fluorescence. However, blocking subtype I metabotropic glutamate receptors (mGLuR(1)) with LY367385 (200 muM) during the conditioning stimulation abolishes the long-term increase in fluorescence. Blocking GABAergic neurotransmission is not required to evoke this long-term potentiation. Blocking GABA(A) receptors reduces but does not eliminate the long-term potentiation. Therefore, this study demonstrates that high frequency PF stimulation generates long-term potentiation of PF-PC synapses in vivo. This novel form of LTP is generated primarily postsynaptically and is mediated by mGluR(1) receptors.

Nitric oxide is a volume transmitter regulating postsynaptic excitability at a glutamatergic synapse

Neuronal nitric oxide synthase (nNOS) is broadly expressed in the brain and associated with synaptic plasticity through NMDAR-mediated calcium influx. However, its physiological activation and the mechanisms by which nitric oxide (NO) influences synaptic transmission have proved elusive. Here, we exploit the unique input-specificity of the calyx of Held to characterize NO modulation at this glutamatergic synapse in the auditory pathway. NO is generated in an activity-dependent manner by MNTB principal neurons receiving a calyceal synaptic input. It acts in the target neuron and adjacent inactive neurons to modulate excitability and synaptic efficacy, inhibiting postsynaptic Kv3 potassium currents (via phosphorylation), reducing EPSCs and so increasing action potential duration and reducing transmission fidelity. We conclude that NO serves as a volume transmitter and slow dynamic modulator, integrating spontaneous and evoked neuronal firing, thereby providing an index of global activity and regulating information transmission across a population of active and inactive neurons.

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.

Effects of anti-glutamic acid decarboxylase antibodies associated with neurological diseases

OBJECTIVE

Glutamic acid decarboxylase (GAD) catalyzes the conversion of glutamic acid into GABA. GAD autoantibodies (GAD-Ab) have been described in diabetes mellitus and in diseases involving the central nervous system such as stiff-person syndrome and cerebellar ataxia. However, the pathogenic role of GAD-Ab in neurological diseases remains a matter of debate.

METHODS

Using neurophysiological and neurochemical methods, we analyzed the effects of intracerebellar and paraspinal administration of GAD-Ab in rats.

RESULTS

Intracerebellar administration of IgG from patients with GAD-Ab and neurological involvement (IgG-GAD) blocked the potentiation of the corticomotor response normally associated with trains of repetitive peripheral nerve stimulation. When injected in the lumbar paraspinal region, IgG-GAD induced continuous motor activity with repetitive discharges, abnormal exteroceptive reflexes, and increased excitability of anterior horn neurons, as assessed by F/M ratios. Furthermore, IgG-GAD significantly reduced the N-methyl-D-aspartate-mediated production of nitric oxide in cerebellar nuclei and impaired the synaptic regulation of glutamate after N-methyl-D-aspartate administration. These effects were not observed after administration of IgG from the following groups: (1) patients with GAD-Ab, diabetes mellitus, and without neurological complications; and (2) control patients.

INTERPRETATION

These results indicate that stiff-person syndrome and cerebellar ataxia are the direct consequence of antibody-mediated neuronal dysfunction.

Splice-isoform specific immunolocalization of neuronal nitric oxide synthase in mouse and rat brain reveals that the PDZ-complex-building nNOSalpha beta-finger is largely exposed to antibodies

Knock out mice deficient for the splice-isoform alphaalpha of neuronal nitric oxide synthase (nNOSalphaalpha) display residual nitric oxide synthase activity and immunosignal. To attribute this signal to the two minor neuronal nitric oxide synthase splice variants, betabeta and gammagamma, we generated isoform-specific anti-peptide antibodies against the nNOSalphaalpha specific betabeta-finger motif involved in PDZ domain scaffolding and the nNOSbetabeta specific N-terminus. The nNOSalphaalpha betabeta-finger-specific antibody clearly recognized the 160-kDa band of recombinant nNOSalphaalpha on Western blots. Using immunocytochemistry, this antibody displayed, in rats and wild-type mice, a labeling pattern similar to but not identical with that obtained using a commercial pan-nNOS antibody. This similarity indicates that the majority of immunocytochemically detectable nNOS is not likely to be complexed with PDZ-domain proteins via the betabeta-finger motif. This conclusion was confirmed by the inhibition of PSD-95/nNOS interaction by the nNOSalphaalpha betabeta-finger antibody in pull-down assays. By contrast, nNOSalphaalpha betabeta-finger labeling was clearly reduced in hippocampal and cortical neuropil areas enriched in NMDA receptor complex containing spine synapses. In nNOSalphaalpha knock out mice, nNOSalphaalpha was not detectable, whereas the pan-nNOS antibody showed a distinct labeling of cell bodies throughout the brain, most likely reflecting betabeta/gammagamma-isoforms in these cells. The nNOSbetabeta antibody clearly detected bacterial expressed nNOSbetabeta fusion protein and nNOSbetabeta in overexpressing HEK cells by Western blotting. Immunocytochemically, individual cell bodies in striatum, cerebral cortex, and in some brain stem nuclei were labeled in knock out but not in wild-type mice, indicating an upregulation of nNOSbetabeta in nNOSalphaalpha deficient animals.

neuroConstruct: a tool for modeling networks of neurons in 3D space

Conductance-based neuronal network models can help us understand how synaptic and cellular mechanisms underlie brain function. However, these complex models are difficult to develop and are inaccessible to most neuroscientists. Moreover, even the most biologically realistic network models disregard many 3D anatomical features of the brain. Here, we describe a new software application, neuroConstruct, that facilitates the creation, visualization, and analysis of networks of multicompartmental neurons in 3D space. A graphical user interface allows model generation and modification without programming. Models within neuroConstruct are based on new simulator-independent NeuroML standards, allowing automatic generation of code for NEURON or GENESIS simulators. neuroConstruct was tested by reproducing published models and its simulator independence verified by comparing the same model on two simulators. We show how more anatomically realistic network models can be created and their properties compared with experimental measurements by extending a published 1D cerebellar granule cell layer model to 3D.

An NMDA receptor/nitric oxide cascade in presynaptic parallel fiber-Purkinje neuron long-term potentiation

Plasticity of synaptic transmission between parallel fiber (PF) and Purkinje neurons (PNs) is widely accepted as a cellular model for certain forms of cerebellar learning. Whereas the signaling cascades involved in postsynaptically expressed bidirectional long-term changes at PF-PN synapses are well investigated, data on presynaptically expressed long-term potentiation (LTP) are incomplete and controversial. Here we used transgenic mice that express a fluorescent protein Ca2+ sensor in PFs to demonstrate LTP of PF presynaptic Ca2+ transients after PF stimulation with 120 pulses at 4 Hz. Potentiation of the presynaptic Ca2+ transients correlated with the expression of simultaneously recorded LTP of PF-PN synaptic transmission and was suppressed by a protein kinase A inhibitor. Moreover, this presynaptically expressed form of LTP clearly required activation of an NMDA receptor/nitric oxide pathway, in contrast with the majority of previous reports. Blockade of NMDA receptors did not affect the PF Ca2+ transients induced during 4 Hz stimulation, indicating that the NMDA receptors required for the induction of presynaptic PF LTP are not localized in PFs.

Enhanced fidelity of diffusive nitric oxide signalling by the spatial segregation of source and target neurones in the memory centre of an insect brain

The messenger molecule nitric oxide (NO) is a key mediator of memory formation that can diffuse in the brain over tens of micrometres. It would seem therefore that NO derived from many individual neurones may merge into a volume signal that is inevitably ambiguous, relatively unspecific and thus unreliable. Here we report on the neuronal architecture that supports the NO-cyclic GMP signalling pathway in the mushroom body of an insect brain, the key centre for associative learning. We show that, in the locust (Schistocerca gregaria), parallel axons of intrinsic neurones (Kenyon cells) form tubular NO-producing zones surrounding central cores of NO-receptive Kenyon cell axons, which do not produce NO. This segregated architecture requires NO to spread at physiological concentrations up to 60 microm from the tube walls into the central NO-receptive cores. By modelling NO diffusion we show that a segregated architecture, which requires NO to act at a distance, affords significant advantages over a system where the same sources and targets intermingle. Segregation enhances the precision of NO volume signals by reducing noise and ambiguity, achieving a reliable integration of the activity of thousands of NO-source neurones. In a neural structure that forms NO-dependent associations, these properties of the segregated architecture may reduce the likelihood of forming spurious memories.

Synaptic memories upside down: bidirectional plasticity at cerebellar parallel fiber-Purkinje cell synapses

Information storage in neural circuits depends on activity-dependent alterations in synaptic weights, such as long-term potentiation (LTP) and long-term depression (LTD). Bidirectional synaptic plasticity endows synapses with mechanisms for rapid reversibility, but it remains unclear how it correlates with reversibility in behavioral learning and whether there is a universal synaptic memory mechanism that operates similarly at all types of synapses. A recently discovered postsynaptic form of LTP at cerebellar parallel fiber (PF)-Purkinje cell (PC) synapses provides a reversal mechanism for PF-LTD and enables a fresh look at the implications of bidirectional plasticity in a brain structure that is particularly suitable to correlate cellular to behavioral learning events. Here, we will review recent studies that reveal unique properties of bidirectional cerebellar plasticity and suggest that the induction cascades for cerebellar LTP and LTD provide a mirror image of their counterparts at hippocampal synapses. We will also discuss how PF-LTP helps to explain reversibility observed in cerebellar motor learning.

New perspectives on the mechanisms through which nitric oxide may affect learning and memory processes

Nitric oxide (NO) has been well established as a molecule necessary for memory consolidation. Interestingly, the majority of research has focused on only a single mechanism through which NO acts, namely the up-regulation of guanylate cyclase (GC). However, since NO and NO-derived reactive nitrogen species are capable of interacting with a broad array of enzymes, ion channels and receptors, a singular focus on GC appears short-sighted. Although NO inhibits the action of a number of molecules there are four, in addition to GC, which are up-regulated by the direct presence of NO, or NO-derived radicals, and implicated in memory processing. They are: cyclic nucleotide-gated channels; large conductance calcium-activated potassium channels; ryanodine receptor calcium release (RyR) channels; and the enzyme mono(ADP-ribosyl) transferase. This review presents evidence that not only are these four molecules worthy of investigation as GC-independent mechanisms through which NO may act, but that behavioural evidence already exists suggesting a relationship between NO and the RyR channel.

Influence of nitric oxide on the activity of cuneate neurons in the rat

Some neurons of main and external cuneate nuclei are immunoreactive for nitric oxide (NO) synthase, suggesting a role for endogenous NO in the early stages of somatosensory processing. We tested this hypothesis by investigating the possibility that NO modulates cuneate discharge. We observed that both spontaneous and N-methyl-D-aspartate-evoked activities of cuneate neurons were decreased by NO precursor L-arginine. The inhibition of NO synthase, by application of N-nitro-L-arginine methyl ester, instead, abolished the depressant effect induced by L-arginine. Our data suggest a NO modulation of cuneate neurons and provide support for a physiologic role not only in increasing the signal-to-noise ratio in the excited cells but also in a form of surround inhibition.

Organ systems dependent on nitric oxide and the potential for nitric oxide-targeted therapies in related diseases

Nitric oxide (NO) is a universal messenger molecule that plays diverse and essential physiologic roles in multiple organ systems, including the vasculature, bone, muscle, heart, kidney, liver, and central nervous system. NO is produced by 3 known isoforms-endothelial, neuronal, and inducible NO synthase-each of which perform distinct functions. Impairment of NO bioactivity may be an important factor in the pathogenesis of a wide range of conditions, including preeclampsia, osteoporosis, nephropathy, liver disease, and neurodegenerative diseases. Although increased levels of NO synthase or NO bioactivity have been associated with some of these disease states, research increasingly suggests that preservation or promotion of normal NO bioactivity may be beneficial in reducing the risks and perhaps reversing the underlying pathophysiology. Based on this rationale, studies investigating the use of NO-donating or NO-promoting agents in some of these diseases have produced positive results, at least to some degree, in either animal or human studies. Further investigation of NO-targeted therapies in these diverse diseases is clearly mandated.

Inactivation of nitric oxide by rat cerebellar slices

Nitric oxide (NO) functions as an intercellular messenger throughout the brain. For this role to be performed efficiently, there must be a mechanism for neutralizing NO, but whether an active biological process exists, or whether NO is lost mainly through diffusion is unclear. To investigate this issue, rat cerebellar slices were exposed to constant levels of NO and the cGMP generated within the slice used as an indicator of NO concentrations therein. NO was about 1000-fold less potent in slices (EC50, 1 microM) than in separated cells from the same tissue (EC50, 1.6 nM), consistent with access of NO to the slice interior being greatly hindered by inactivation. Supporting this interpretation, immunohistochemical analysis indicated a marked concentration gradient of cGMP across the thickness of slices exposed to subsaturating NO concentrations, signifying a marked NO gradient. Several known NO-degrading processes, including reaction with lipid peroxyl radicals, erythrocytes and superoxide ions, were eliminated as contributing factors, indicating a novel mechanism. A diffusion-inactivation model was used to estimate the kinetics of NO consumption by the slices. The best fits to experimental data indicated a Michaelis-Menten-type reaction having a Vmax of 1-2 microM s-1 and a Km of around 10 nM. The rates predict that inactivation would impose a very short half-life (<10 ms) on NO in physiological concentrations (up to 10 nM) and that it would play an important role in shaping the NO concentration profiles when it is synthesized by multiple nearby sites.