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

Ellagic acid prevents dementia through modulation of PI3-kinase-endothelial nitric oxide synthase signalling in streptozotocin-treated rats

Ellagic acid (EGA)-enriched dietary supplements are widely acclaimed, owing to its versatile bioactivities. Previously, we reported that chronic administration of EGA prevented the impairment of cognitive abilities in rats using the intracerebroventricular-administered streptozotocin (STZ-ICV) model of Alzheimer's disease. Impairment of phosphoinositide 3 (PI3)-kinase-regulated endothelial nitric oxide synthase (eNOS) activity by central administration of STZ in rodents instigates dementia. The aim of the present study was to delineate the role of PI3-kinase-eNOS activity in the prevention of STZ-ICV-induced memory dysfunctions by EGA. The Morris water maze and elevated plus maze tests were conducted, and brain oxidative stress markers (TBARS, GSH, SOD, CAT), nitrite, acetylcholinesterase (AChE), LDH, TNF-α and eNOS were quantified. Administration of EGA (35 mg/k, p.o.) for 4 weeks daily attenuated the STZ-ICV (3 mg/kg)-triggered increase of brain oxidative stress, nitrite and TNF-α levels; AChE and LDH activity; and decline of brain eNOS activity. The memory restoration by EGA in STZ-ICV-treated rats was conspicuously impaired by N(G)-nitro-L-arginine methyl ester (L-NAME) (20 mg/kg, 28 days) and wortmannin (5 μg/rat; ICV) treatments. Wortmannin (PI3-kinase inhibitor) and L-NAME groups manifested elevated brain oxidative stress, TNF-α content and AChE and LDH activity and diminished nitrite content. L-NAME (arginine-based competitive eNOS inhibitor) enhanced the eNOS expression (not activity) whereas wortmannin reduced the brain eNOS levels in EGA- and STZ-ICV-treated rats. However, the L-NAME group exhibited superior cognitive abilities in comparison to the wortmannin group. It can be concluded that EGA averted the memory deficits by precluding the STZ-ICV-induced loss of PI3-kinase-eNOS signalling in the brain of rats.

Protective effects of a new glutamic acid derivative against stress after nNOS blockade

We studied the effects of a new glutamic acid derivative, glufimet, on oxidative stress, activity of antioxidant enzymes, mitochondrial respiration, endothelial vasodilation and anti-platelet activity in female rats after exposure to 24-hour immobilization pain stress and 7-nitroindazole, a neuronal nitric oxide synthase (nNOS) inhibitor. A single dose administration of glufimet (29 mg/kg intraperitoneally) 10 minutes before stress exposure caused a decrease of NO metabolites in serum (by 27.2%) and heart homogenate (33.5% (p£0.05), respectively, compared with the control group. Administration of 7-nitroindazole with glufimet also decreased the studied parameters by 14.3% in the heart homogenate and by 30,3% in the brain (p£0.05) compared with stress exposed rats receiving only the nNOS inhibitor. Glufimet decreased the levels of primary and secondary products of lipid peroxidation (LPO), conjugated dienes by 20% (p£0.05) and 17.3% (p£0.05), ketodienes by 16% and 13.7%, malondialdehyde by 15% (p£0.05) and 26.6% (p£0.05) in the heart and brain mitochondria of stress exposed rats, respectively, compared with the control group. Glufimet administration also increased SOD activity (by 14.4% and 13.1%, respectively), catalase (by 19% and 26.8%, respectively) and glutathione peroxidase (GPx) activity (by 45.5% (p£0.05) and 7.3%, respectively). The antioxidant effect of glufimet may be also attributed to increased coupling between the processes of mitochondria respiration and oxidative phosphorylation. This was evidenced by an increase in the respiratory control ratio (RCR) (by 46.0% (p£0.05) for malate/glutamate and by 49,7% (p£0.05) for succinate) in the heart mitochondria. A statistically significant increase in RCR (by 37.3% (p£0.05)) was observed in stress exposed female rat brain mitochondria for succinate. RCRs differed significantly for succinate in the heart and brain of rats receiving glufimet after nNOS blockade. RCR increased by 62.3% (p£0.05) in the heart mitochondria and by 72.2% (p£0.05) in the brain mitochondria compared with the RCRs in stress exposed rats receiving 7-nitroindazole.

Purinergic glio-endothelial coupling during neuronal activity: role of P2Y1 receptors and eNOS in functional hyperemia in the mouse somatosensory cortex

Impairment of moment-to-moment adjustment of cerebral blood flow (CBF) via neurovascular coupling is thought to play a critical role in the genesis of cognitive impairment associated with aging and pathological conditions associated with accelerated cerebromicrovascular aging (e.g., hypertension, obesity). Although previous studies demonstrate that endothelial dysfunction plays a critical role in neurovascular uncoupling in these conditions, the role of endothelial NO mediation in neurovascular coupling responses is not well understood. To establish the link between endothelial function and functional hyperemia, neurovascular coupling responses were studied in mutant mice overexpressing or deficient in endothelial NO synthase (eNOS), and the role of P2Y1 receptors in purinergic glioendothelial coupling was assessed. We found that genetic depletion of eNOS (eNOS(-/-)) and pharmacological inhibition of NO synthesis significantly decreased the CBF responses in the somatosensory cortex evoked by whisker stimulation and by administration of ATP. Overexpression of eNOS enhanced NO mediation of functional hyperemia. In control mice, the selective and potent P2Y1 receptor antagonist MRS2179 attenuated both whisker stimulation-induced and ATP-mediated CBF responses, whereas, in eNOS(-/-) mice, the inhibitory effects of MRS2179 were blunted. Collectively, our findings provide additional evidence for purinergic glio-endothelial coupling during neuronal activity, highlighting the role of ATP-mediated activation of eNOS via P2Y1 receptors in functional hyperemia.

7-NI and ODQ Disturbs Memory in the Elevated Plus Maze, Morris Water Maze, and Radial Arm Maze Tests in Mice

Nitric oxide (NO) is an atypical neurotransmitter that causes changes in cognition. Nitric oxide synthase (NOS) and guanylate cyclase (GC) inhibitors have been shown to exert some effects on cognition in previous studies; however, the findings have been controversial. This study was aimed at understanding the effects of an NOS inhibitor, 7-nitroindazole (7-NI), and a guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), on spatial memory in modified elevated plus maze (mEPM), Morris water maze (MWM), and radial arm maze (RAM) tests. Male Balb-c mice were treated via intraperitoneal injections with 7-NI (15 mg/kg), ODQ (3, 10 mg/kg), L-arginine (100 mg/kg) + 7-NI (15 mg/kg), or physiological saline. ODQ (3 mg/kg) and 7-NI (15 mg/kg) significantly increased the second-day latency in the mEPM test. 7-NI (15 mg/kg) and ODQ (10 mg/kg) significantly increased the escape latency in second, third, and fourth sessions, decreased the time spent in the escape platform’s quadrant, and increased the mean distance to the platform in the probe trial of the MWM test. ODQ (3, 10 mg/kg) and 7-NI (15 mg/kg) significantly increased the number of errors, whereas only 7-NI increased the latency in the RAM test. The administration of L-arginine (100 mg/kg) prior to 7-NI inverted the effects of 7-NI, which supports the role of NO on cognition. Our study shows that the NO/cGMP/GS pathway can regulate spatial memory in mice.

Enhancement of tonic and phasic GABAergic currents following nitric oxide synthase inhibition in hippocampal CA1 pyramidal neurons

Nitric oxide (NO) is involved in synaptic plasticity in the hippocampus through different presynaptic and postsynaptic mechanisms that include the modulation of the GABAergic neurotransmission. Inhibitory synapses on hippocampal pyramidal neurons are known to possess the molecular machinery for retrograde NO-signaling, but the modulation of GABAARs function by NO in these neurons and the mechanisms of action involved have not been fully characterized. Here we show that suppression of the endogenous NO generation by the nitric oxide synthase (NOS) inhibitor L-NAME produces significant and reversible increases in the magnitude of both tonic and phasic GABAergic currents in CA1 hippocampal pyramidal neurons. GABA-evoked chloride currents were measured in the presence or absence of L-NAME using whole-cell patch-clamp recordings in acute hippocampal slices from young adult mice. Enhancement of the tonic GABA responses induced by L-NAME was insensitive to TTX and decreased by co-incubation with the NO donor DEA/NO. Applications of DEA/NO alone did not produce significant effects on tonic GABA responses. L-NAME treatment also increased the amplitude of phasic GABAergic currents evoked by GABA-puffs. Our results indicate that the extent of tonic and phasic inhibition mediated by GABAA receptors in CA1 hippocampal pyramidal neurons is affected by endogenous NO production.

Effects of 7-NI and ODQ on memory in the passive avoidance, novel object recognition, and social transmission of food preference tests in mice

Background

Nitric oxide (NO) is an intercellular messenger that plays a critical role in learning and memory processes. Effects of nitric oxide synthase (NOS) inhibitors and guanylate cyclase (GC) inhibitors on cognitive function remain controversial.

Material/Methods

The aim of this study was to investigate effects of an NOS inhibitor, 7-nitroindazole (7-NI), and a GC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), on different aspects of memory in passive avoidance (PA), novel object recognition (NOR), and social transmission of food preference (STFP) tests. Male Balb-c mice were treated intraperitoneally with 7-NI (15 mg/kg), ODQ (3,10 mg/kg), L-arginine (100 mg/kg) + 7-NI (15 mg/kg), or physiological saline.

Results

ODQ (10 mg/kg) and 7-NI (15 mg/kg) significantly decreased second-day latency in PA test. 7-NI (15 mg/kg) and ODQ (10 mg/kg) significantly decreased the ratio index in the NOR test. 7-NI and ODQ (10 mg/kg) decreased cued/non-cued food eaten in STFP test. Amount of time spent in center zone significantly increased in ODQ (10 mg/kg) and 7-NI (15 mg/kg) groups in open field test, but there was no effect on total distance moved and speed of animals. ODQ (10 mg/kg) significantly increased number of entries into new compartments in exploratory activity apparatus, while 7-NI had no effect. Administration of L-arginine (100 mg/kg) before 7-NI reversed 7-NI-induced effects, supporting the role of NO in cognition.

Conclusions

Our results confirm that inhibition of NO/cGMP/GS pathway might disturb emotional, visual, and olfactory memory in mice. Also, 7-NI and ODQ had anxiolytic effects in open field test, and ODQ also enhanced exploratory activity.

Nitric oxide as a regulator of behavior: new ideas from Aplysia feeding

Nitric oxide (NO) regulates Aplysia feeding by novel mechanisms, suggesting new roles for NO in controlling the behavior of higher animals. In Aplysia, (1) NO helps maintain arousal when produced by neurons responding to attempts to swallow food; (2) NO biases the motor system to reject and reposition food that resists swallowing; (3) if mechanically resistant food is not successfully swallowed, NO mediates the formation and expression of memories of food inedibility; (4) NO production at rest inhibits feeding, countering the effects of food stimuli exciting feeding. At a cellular level, NO-dependent channels contribute to the resting potential of neurons controlling food finding and food consumption. Increases in L-arginine after animals eat act as a post-feeding inhibitory signal, presumably by modulating NO production at rest. NO also signals non-feeding behaviors that are associated with feeding inhibition. Thus, depending on context, NO may enhance or inhibit feeding behavior. The different functions of NO may reflect the evolution of NO signaling from a response to tissue damage that was then elaborated and used for additional functions. These results suggest that in higher animals (1) elicited and background transmitter release may have similar effects; (2) NO may be produced by neurons without firing, influencing adjacent neurons; (3) background NO production may contribute to a neuron's resting potential; (4) circulating factors affecting background NO production may regulate spatially separated neurons; (5) L-arginine can be used to regulate neural activity; (6) L-arginine may be an effective post-ingestion metabolic signal to regulate feeding.

Involvement of nitric oxide in learning & memory processes

Nitric oxide (NO), synthesized from the amino acid, L-arginine by nitric oxide synthase (NOS) has received attention as a neurotransmitter in the brain. NO has been found to induce cognitive behaviour in experimental animals. In order to show evidence for the involvement of NO in learning and memory processes, the reports indicating the effects of its precursor, donors, and inhibitors of its synthesis in mammals, birds, fishes and invertebrates have been reviewed. Further, learning and memory impairment occurring in man and animals due to defective NO activity in the brain due to pathological conditions such as epilepsy, stress, diabetes and side effects of therapeutic agents and reversal of this condition by L-arginine and NO donors have been included. In addition, the reports that indicate ageing-induced impairment of cognition that is known to occur in Alzheimer's disease due to deposition of the toxic protein, beta amyloid and the effect of L-arginine and NO donors in preventing dementia in these patients have been reviewed.

Effects of Selective iNOS Inhibitor on Spatial Memory in Recovered and Non-Recovered Ketamine Induced Anesthesia

Nitric oxide (NO) is thought to be involved in spatial learning and memory in several brain areas such as hippocampus. This study examined the effects of post-training intrahippocampal microinjections of 1400W as a selective iNOS inhibitor on spatial memory, in anesthetized and non-anesthetized situations in rats. In the present work, 4-day training trials of animals were conducted. Spatial memory was tested 48 hours after the drug infusions. For microinjection of 1400W into CA1 region of the hippocampus in conscious animals, guide cannula was implanted into the CA1 area and 1400W was infused after recovery from surgical anesthesia. In anesthetized animals, 1400W was microinjected directly into CA1 region by Hamilton syringe during anesthesia. After completion of training, 1400W (10, 50 and 100 μM/side) were microinjected bilaterally (1 μL/side) and testing trials were performed 48 h after drug infusions in both groups of cannulated and non-cannulated rats. Significant reduction was observed in escape latency and traveled distance in animals that received 1400W (100 μM/side, *p < 0.05) via cannula after recovery in comparison with control group. Also, microinjection of 1400W (100 μM/side) in post recovery phase caused a significant (***p < 0.001) reduction in time and distance of finding the hidden platform in comparison with anesthetized situation. These findings suggest that 1400W has a significant improvement on spatial memory and memory enhancement induced by iNOS inhibitor can be affected by anesthesia in a period of time.

Effects of Selective iNOS Inhibitor on Spatial Memory in Recovered and Non-recovered Ketamine Induced-anesthesia in Wistar Rats

Nitric oxide (NO) is thought to be involved in spatial learning and memory in several brain areas such as hippocampus. This study examined the effects of post-training intrahippocampal microinjections of 1400W as a selective inducible nitric oxide synthase (iNOS) inhibitor on spatial memory, in both anesthetized and non-anesthetized situations in rats. In the present work, 4-day training trials of animals were conducted. Spatial memory was tested 48 h after the drug infusions. For microinjection of 1400W into CA1 region of the hippocampus in conscious animals, guide cannula was implanted into the CA1 area and 1400W was infused after recovery from surgical anesthesia. In anesthetized animals, 1400W was microinjected directly into CA1 region by Hamilton syringe during anesthesia. After completion of training, 1400W (10, 50 and 100 μM/side) were microinjected bilaterally (1 μL/side) and testing trials were performed 48 h after drug infusions in both groups of cannulated and non-cannulated rats. Significant reduction was observed in escape latency and traveled distance in animals that received 1400W (100 μM/side, * P < 0.05) via cannula after recovery in comparison with control group. Moreover, microinjection of 1400W (100 μM/side) in post recovery phase also caused a significant (*** P < 0.001) reduction in time and distance of finding the hidden platform in comparison with anesthetized situation. These results suggest that 1400W has a significant improvement on spatial memory, and memory enhancement induced by iNOS inhibitor can be affected by anesthesia in a period of time.

EFFECTS OF NITRIC OXIDE ON PASSIVE AVOIDANCE LEARNING IN RATS

To investigate the effects of nitric oxide (NO) on passive avoidance learning, L-NAME, D-NAME, and L-arginine were administered i.p. 30 min prior to learning trial; the effects of these substances were tested 24 h later using a passive avoidance apparatus in rats. To reveal the effect of NO on consolidation of acquired memory, L-NAME, D-NAME, and L-arginine were administered i.p. immediately after learning trials and animals were tested 24 h later. Effect of NO on retention was also investigated by injecting L-NAME, D-NAME, and L-arginine (same dosages) 30 min prior to 24 h testing (retrieval). L-NAME administered 30 min before and 24 h after learning trial significantly decreased the avoidance latency but there was no significant effect on consolidation. L-Arginine appeared to enhance the retention of acquired memory significantly, whereas D-NAME had no effect on any testing regime. The results suggest that NO may be involved in learning and retention of passive avoidance.

Neuronal nitric oxide synthase: structure, subcellular localization, regulation, and clinical implications

Nitric oxide (NO), a free gaseous signaling molecule, is involved in the regulation of the cardiovascular, nervous and immune system. The neurotransmitter function of nitric oxide is dependent on dynamic regulation of its biosynthetic enzyme, nitric oxide synthase (NOS). There are three types of NOS, neuronal nitric oxide synthase (nNOS), endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS). Of the three NOS, we focus on nNOS in the present review. Brain nNOS exists in particulate and soluble forms and the differential subcellular localization of nNOS may contribute to its diverse functions. Proteins bearing PDZ domains can interact directly with the PDZ domain of nNOS, influencing the subcellular distribution and/or activity of the enzyme. During the past several years, an increasing number of reports have demonstrated the importance of nNOS in a variety of synaptic signaling events. nNOS has been implicated in modulating physiological functions such as learning, memory, and neurogenesis, as well as being involved in a number of human diseases. In this review we concentrate on recent findings regarding the structural features, subcellular localization and factors regulating nNOS function. In particular, we conclude with a section discussing the role of nNOS in a wide range of physiological and pathological conditions.

Xiphophorus: carving a niche towards a broader understanding of aggression and dominance

A review of some exciting new directions for Xiphophorus research in behavioral ecology and integrative animal behavior is presented. The primary objective of this review is to identify areas of research in this teleost genus that, if studied further, could yield insights into the manifold causes and consequences of dominance encounters that will have broad relevance in the behavioral biology community. First described is the phenomenon of social eavesdropping--the ability of animals to extract information from signaling interactions between others-as it applies to Xiphophorus systems, and how exploring the context--dependency of individual responses to watching fights will be of benefit. A brief discussion follows of the overwhelming tendency for research on Xiphophorus to focus on individual responses to visual cues available in their social environment, and to promote advancements towards a multimodal approach to understanding social dynamics. Lastly, historical studies on neuroendocrinology in this genus are reviewed, and recent advances in the molecular realm are highlighted that might serve as a springboard for integrative research addressing the behavioral impacts of direct (overt fighting) and indirect (eavesdropping) experiences in Xiphophorus.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Inhibition of endogenous carbon monoxide production induces transient retention losses in the day-old chick when trained using a single trial passive avoidance learning task

Carbon monoxide (CO) is most often thought of as an exogenous toxin rather than as a possible endogenous nootrope. However, a limited number of studies have suggested that CO is necessary in memory processing for at least some tasks. While nitric oxide (NO) and CO are known activators of guanylyl cyclase (GC), only the effect of NO on GC has been extensively investigated as a mechanism underlying memory processing. The aim of the present study was to determine if inhibition of CO production would have an effect on memory processing. Using chicks trained on a single trial passive avoidance task, inhibition of CO production using zinc (II) deuteroporphyrin IX 2,4-bis ethylene glycol (ZnBG; 5 microM) resulted in two transient retention losses occurring at around 40 and 130 min post-training. The timing of these transient retention losses was similar to those observed following inhibition of GC, using the same species and task in a previous study. This supports the notion that CO is necessary in memory processing for this task and may act through a GC-dependent mechanism. As ZnBG also directly inhibits GC or nitric oxide synthase (NOS) at high concentrations, a second experiment was carried-out to confirm the specificity of ZnBG for heme oxygenase (HO) at the concentration used. The action of ZnBG was challenged with the HO agonist hemin (100 microM) and the transient deficits were abolished. This confirmed that the action of ZnBG on memory was through a CO-related mechanism rather than directly on GC or NOS. In this way the specificity of ZnBG (5 microM) for HO could be confirmed. The results support a role for endogenous CO in memory processing, possibly through activation of GC. In addition, the transient retention losses observed following administration of ZnBG suggest that CO may be necessary for memory retrieval and not formation as previously thought.

Age-related alteration of activity and gene expression of endothelial nitric oxide synthase in different parts of the brain in rats

Nitric oxide (NO) plays important roles in aging and neurodegeneration. Our previous results indicated that aging differently affects NOS isoforms. Expression of nNOS mRNA was lower while iNOS was absent at any age. However, total NO synthesis increased in aged cerebral cortex and cerebellum as a consequence of changes of nNOS phosphorylation state. The question arise how aging influences activity and expression of eNOS in different parts of adult and aged brain. The levels of eNOS mRNA, protein and activity were measured using RT-PCR, immuno- and radiochemical methods, respectively. Our studies indicated that after inhibition of nNOS with 7-nitroindazole (7-NI) NO synthesis is lower in all parts of aged brain comparing to adults. However, eNOS activity significantly decreases only in cerebellum. The expression of eNOS determined on mRNA level was enhanced in all investigated aged brain parts to 140-190% of adult value and the data were statistically significant for cerebral cortex and cerebellum. The higher level of mRNA is probably the adaptive response to lower NOS activity. However, the Western-blot signal of eNOS protein was unchanged in aged brain parts comparing to adults suggesting age-related disturbances of protein synthesis and its function. It is also possible that a post-translational modification of the enzyme occurs in the aged rat brain. The lower eNOS activity in aged brain may significantly affects the signal transduction processes on the pathway NO/cGMP/PKG.

Insulin and the insulin receptor in experimental models of learning and memory

Insulin is best known for its action on peripheral insulin target tissues such as the adipocyte, muscle and liver to regulate glucose homeostasis. In the central nervous system (CNS), insulin and the insulin receptor are found in specific brain regions where they show evidence of participation in a variety of region-specific functions through mechanisms that are different from its direct glucose regulation in the periphery. While the insulin/insulin receptor associated with the hypothalamus plays important roles in regulation of the body energy homeostasis, the hippocampus- and cerebral cortex-distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. Emerging evidence has suggested that insulin signaling plays a role in synaptic plasticity by modulating activities of excitatory and inhibitory receptors such as glutamate and GABA receptors, and by triggering signal transduction cascades leading to alteration of gene expression that is required for long-term memory consolidation. Furthermore, deterioration of insulin receptor signaling appears to be associated with aging-related brain degeneration such as the Alzheimer's dementia and cognitive impairment in aged subjects suffering type 2 diabetes mellitus.

Hemispheric dissociation of the involvement of NOS isoforms in memory for discriminated avoidance in the chick

Previous research has indicated a role for both the neuronal (nNOS) and endothelial (eNOS) nitric oxide isoforms in memory formation. In addition, two distinct periods of activity of nitric oxide activity, dissociated by hemispheric localization, are implicated following passive avoidance training in the chick. In the present study, we trained black Australorp-white Leghorn chicks on a color discrimination avoidance task. Diphenyleneiodonium chloride (1 microM) or N-propyl-l-arginine (50 microM) was administered into either the left or right hemisphere of the chick brain in an attempt to differentiate the effects of inhibiting eNOS or nNOS, respectively. The memory loss previously observed following administration of diphenyleneiodonium chloride between 10 and 20 min posttraining was found to be lateralized to the right hemisphere, although administration of this agent into the left hemisphere around the time of training was also amnestic. In contrast, N-propyl-l-arginine caused memory loss only when administered to the left hemisphere around the time of training. These findings suggest that activation of both eNOS and nNOS isoforms may be essential for long-term memory consolidation of this task. Further, these two periods of activity are defined temporally and by hemisphere localization, although confirmation with more selective inhibitors when they become available is advised.

Effects of nitric oxide inhibition on avoidance learning in the chick are lateralized and localized

Bilateral administration of nitric oxide synthase inhibitors into the intermediate medial hyperstriatal (IMHV) region of the chick brain impairs memory formation for an avoidance task. The aim of the current study was to determine whether this effect was restricted to a particular location in the brain, and whether inhibition was equally effective in both hemispheres. White Leghorn x black Australorp chicks were administered 0.5 mM N(omega)-Nitro-L-arginine methyl ester bilaterally into the lobus parolfactorius (LPO), or unilaterally into the IMHV. Injections into the LPO between 5 min pre-training and 40 min post-training had no effect on retention. In contrast, unilateral injections into the IMHV impaired retention and memory loss occurred from 40 min post-training. The effective administration time was hemisphere-dependent, requiring left hemisphere administration around the time of training and right hemisphere administration between 15 and 25 min post-training. These data suggest that localized nitric oxide activity in each hemisphere of the chick brain is necessary for the consolidation of memory for this task.

Nitric oxide is necessary for multiple memory processes after learning that a food is inedible in aplysia

Nitric oxide (NO) signaling was inhibited via N(omega)-nitro-L-arginine methyl ester (L-NAME) during and after training Aplysia that a food is inedible. Treating animals with L-NAME 10 min before the start of training blocked the formation of three separable memory processes: (1) short-term, (2) intermediate-term, and (3) long-term memory. The treatment also attenuated, but did not block, a fourth memory process, very short-term memory. L-NAME had little or no effect on feeding behavior per se or on most aspects of the animals' behavior while they were being trained, indicating that the substance did not cause a pervasive modulation or poisoning of many aspects of feeding and other behaviors. Application of L-NAME within 1 min after the training had no effect on short- or long-term memory, indicating that NO signaling was not needed during memory consolidation. Treating animals with the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazdine-1-oxy-3-oxide before training also blocked long-term memory. Memory was not blocked by D-NAME, or by the simultaneous treatment with L-NAME and the NO donor S-nitroso-N-acetyl-penicillamine, confirming that the effect of L-NAME is attributable to its effect as a competitive inhibitor of L-arginine for NO synthase in the production of NO rather than to possible effects at other sites. These data indicate that NO signaling during training plays a critical role in the formation of multiple memory processes.

Nitric oxide is necessary for multiple memory processes after learning that a food is inedible in aplysia

Nitric oxide (NO) signaling was inhibited via N(omega)-nitro-L-arginine methyl ester (L-NAME) during and after training Aplysia that a food is inedible. Treating animals with L-NAME 10 min before the start of training blocked the formation of three separable memory processes: (1) short-term, (2) intermediate-term, and (3) long-term memory. The treatment also attenuated, but did not block, a fourth memory process, very short-term memory. L-NAME had little or no effect on feeding behavior per se or on most aspects of the animals' behavior while they were being trained, indicating that the substance did not cause a pervasive modulation or poisoning of many aspects of feeding and other behaviors. Application of L-NAME within 1 min after the training had no effect on short- or long-term memory, indicating that NO signaling was not needed during memory consolidation. Treating animals with the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazdine-1-oxy-3-oxide before training also blocked long-term memory. Memory was not blocked by D-NAME, or by the simultaneous treatment with L-NAME and the NO donor S-nitroso-N-acetyl-penicillamine, confirming that the effect of L-NAME is attributable to its effect as a competitive inhibitor of L-arginine for NO synthase in the production of NO rather than to possible effects at other sites. These data indicate that NO signaling during training plays a critical role in the formation of multiple memory processes.

Inhibition of monoADP-ribosylation prevents long-term memory consolidation of a single-trial passive avoidance task in the day-old chick

The cytosolic posttranslational protein-modifying mechanism of monoADP-ribosylation has been implicated in long-term potentiation, a synaptic model of memory formation. The current study investigated the effect of inhibiting mono(ADP-ribosyl) transferase on memory for the passive avoidance task in day-old chicks (white Leghorn-black Australorp). Various doses of novobiocin or menadione sodium bisulfite were administered intracranially at different times before or after training. Control chicks were administered saline at matched times. Novobiocin (650 microM) or menadione sodium bisulfite (250 microM) administered between 5.0 min pretraining and 2.5 min posttraining was found to cause a persistent loss of retention from 120 min posttraining. These data provide the first demonstration that monoADP-ribosylation is required for the maintenance of long-term memory. Furthermore, the temporal characteristics of the memory loss caused by monoADP-ribosylation inhibition appears to exclude this mechanism as a downstream effect of the well-established nitric oxide activity previously shown to occur within 40 min of passive avoidance training.

Inhibition of guanylate cyclase and protein kinase G impairs retention for the passive avoidance task in the day-old chick

Nitric oxide (NO) is a highly labile chemical messenger which has previously been implicated in memory processes in a variety of learning paradigms and species. However, there is only limited evidence to suggest which enzymes are acted upon by NO during the formation of memory. The present study investigates the role of guanylate cyclase (GC) and protein kinase G (PKG) in a form of passive avoidance learning known to be dependent on nitric oxide activity. It was determined that in vivo pharmacological inhibition of GC using either 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one or 6-anilino-5,8-quinolinedione resulted in two transitory memory retention deficits centred around 40 and 120 min posttraining, respectively. In contrast, inhibition of PKG with N-[2-(methylamino)ehtyl]-5-isoquinoline-sulfornamide hydrochloride (H-8) resulted in a single temporary retention loss centered at 120 min posttraining. These temporary retention losses appear to be specific to memory since they were dose-dependent and could not be explained by nonspecific performance effects. Further, these results suggest that these agents inhibit memory retrieval rather than formation, since memory is subsequently available. The current findings indicate that guanylyl cyclase mediates two memory retrieval processes, the latter of which appears to be PKG-dependent. In contrast, since inhibition of NO results in a permanent retention loss, it is suggested that NO is required for memory formation through GC-independent processes.

Signaling and gene expression in the neuron-glia unit during brain function and dysfunction: Holger Hydén in memoriam

Holger Hydén demonstrated almost 40 years ago that learning changes the base composition of nuclear RNA, i.e. induces an alteration in gene expression. An equally revolutionary observation at that time was that a base change occurred in both neurons and glia. From these findings, Holger Hydén concluded that establishment of memory is correlated with protein synthesis, and he demonstrated de novo synthesis of several high-molecular protein species after learning. Moreover, the protein, S-100, which is mainly found in glial cells, was increased during learning, and antibodies towards this protein inhibited memory consolidation. S-100 belongs to a family of Ca(2+)-binding proteins, and Holger Hydén at an early point realized the huge importance of Ca(2+) in brain function. He established that glial cells show more marked and earlier changes in RNA composition in Parkinson's disease than neurons. Holger Hydén also had the vision and courage to suggest that "mental diseases could as well be thought to depend upon a disturbance of processes in glia cells as in the nerve cells", and he showed that antidepressant drugs cause profound changes in glial RNA. The importance of Holger Hydén's findings and visions can only now be fully appreciated. His visionary concepts of the involvement of glia in neurological and mental illness, of learning being associated with changes in gene expression, and of the functional importance of Ca(2+)-binding proteins and Ca(2+) are presently being confirmed and expanded by others. This review briefly summarizes highlights of Holger Hydén's work in these areas, followed by a discussion of recent research, confirming his findings and expanding his visions. This includes strong evidence that glial dysfunction is involved in the development of Parkinson's disease, that drugs effective in mood disorders alter gene expression and exert profound effects on astrocytes, and that neuronal-astrocytic interactions in glutamate signaling, NO synthesis, Ca(2+) signaling, beta-adrenergic activity, second messenger production, protein kinase activities, and transcription factor phosphorylation control the highly programmed events that carry the memory trace through the initial, signal-mediated short-term and intermediate memory stages to protein synthesis-dependent long-term memory.