Nitric oxide and carbon monoxide as possible retrograde messengers in hippocampal long-term potentiation

Synapse-specific structural plasticity that protects and refines local circuits during LTP and LTD

Synapses form trillions of connections in the brain. Long-term potentiation (LTP) and long-term depression (LTD) are cellular mechanisms vital for learning that modify the strength and structure of synapses. Three-dimensional reconstruction from serial section electron microscopy reveals three distinct pre- to post-synaptic arrangements: strong active zones (AZs) with tightly docked vesicles, weak AZs with loose or non-docked vesicles, and nascent zones (NZs) with a postsynaptic density but no presynaptic vesicles. Importantly, LTP can be temporarily saturated preventing further increases in synaptic strength. At the onset of LTP, vesicles are recruited to NZs, converting them to AZs. During recovery of LTP from saturation (1-4 h), new NZs form, especially on spines where AZs are most enlarged by LTP. Sentinel spines contain smooth endoplasmic reticulum (SER), have the largest synapses and form clusters with smaller spines lacking SER after LTP recovers. We propose a model whereby NZ plasticity provides synapse-specific AZ expansion during LTP and loss of weak AZs that drive synapse shrinkage during LTD. Spine clusters become functionally engaged during LTP or disassembled during LTD. Saturation of LTP or LTD probably acts to protect recently formed memories from ongoing plasticity and may account for the advantage of spaced over massed learning. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Dentate gyrus is needed for memory retrieval

The hippocampus is crucial for acquiring and retrieving episodic and contextual memories. In previous studies, the inactivation of dentate gyrus (DG) neurons by chemogenetic- and optogenetic-mediated hyperpolarization led to opposing conclusions about DG's role in memory retrieval. One study used Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-mediated clozapine N-oxide (CNO)-induced hyperpolarization and reported that the previously formed memory was erased, thus concluding that denate gyrus is needed for memory maintenance. The other study used optogenetic with halorhodopsin induced hyperpolarization and reported and dentate gyrus is needed for memory retrieval. We hypothesized that this apparent discrepancy could be due to the length of hyperpolarization in previous studies; minutes by optogenetics and several hours by DREADD/CNO. Since hyperpolarization interferes with anterograde and retrograde neuronal signaling, it is possible that the memory engram in the dentate gyrus and the entorhinal to hippocampus trisynaptic circuit was erased by long-term, but not with short-term hyperpolarization. We developed and applied an advanced chemogenetic technology to selectively silence synaptic output by blocking neurotransmitter release without hyperpolarizing DG neurons to explore this apparent discrepancy. We performed in vivo electrophysiology during trace eyeblink in a rabbit model of associative learning. Our work shows that the DG output is required for memory retrieval. Based on previous and recent findings, we propose that the actively functional anterograde and retrograde neuronal signaling is necessary to preserve synaptic memory engrams along the entorhinal cortex to the hippocampal trisynaptic circuit.

Intracellular and extracelluar cyclic GMP in the brain and the hippocampus

Cyclic Guanosine-Monophosphate (cGMP) is implicated as second messenger in a plethora of pathways and its effects are executed mainly by cGMP-dependent protein kinases (PKG). It is involved in both peripheral (cardiovascular regulation, intestinal secretion, phototransduction, etc.) and brain (hippocampal synaptic plasticity, neuroinflammation, cognitive function, etc.) processes. Stimulation of hippocampal cGMP signaling have been proved to be beneficial in animal models of aging, Alzheimer's disease or hepatic encephalopathy, restoring different cognitive functions such as passive avoidance, object recognition or spatial memory. However, even when some inhibitors of cGMP-degrading enzymes (PDEs) are already used against peripheral pathologies, their utility as neurological treatments is still under clinical investigation. Additionally, it has been demonstrated a list of cGMP roles as not second but first messenger. The role of extracellular cGMP has been specially studied in hippocampal function and cognitive impairment in animal models and it has emerged as an important modulator of neuroinflammation-mediated cognitive alterations and hippocampal synaptic plasticity malfunction. Specifically, it has been demonstrated that extracellular cGMP decreases hippocampal IL-1β levels restoring membrane expression of glutamate receptors in the hippocampus and cognitive function in hyperammonemic rats. The mechanisms implicated are still unclear and might involve complex interactions between hippocampal neurons, astrocytes and microglia. Membrane targets for extracellular cGMP are still poorly understood and must be addressed in future studies.

Low-Dose Carbon Monoxide Inhibits Rhinovirus Replication in Human Alveolar and Airway Epithelial Cells

Carbon monoxide (CO) and nitric oxide (NO) exhibit physiological properties that include the activation of guanylate cyclase. NO inhibits replication of rhinovirus (RV), a major cause of the common cold and exacerbation of bronchial asthma and chronic obstructive pulmonary disease. However, the anti-rhinoviral effects of CO remain unclear. This study investigated whether the exogenous application of low-dose CO could inhibit RV replication in human alveolar and airway epithelial cells. A549 human lung carcinoma cells with alveolar epithelial features and primary cultures of human tracheal epithelial (HTE) cells were pretreated with CO (100 ppm) and infected with a major group RV, type 14 RV (RV14). CO exposure reduced RV14 titers in the supernatants and RV RNA levels in A549 and HTE cells. The treatment with a guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, reversed the inhibitory effects of CO exposure on RV14 replication in A549 cells. Pretreatment of A549 cells with 8-Br-cGMP, a cell-permeable cGMP analog, caused the decrease in RV14 replication, while CO exposure increased cGMP production. CO exposure also increased the expression levels of interferon (IFN)-γ mRNA and protein. In contrast, pretreatment with CO did not increase DNA fragmentation and did not reduce the expression of intercellular adhesion molecule-1, the RV14 receptor, or the number of acidic endosomes, through which RV RNA enters the cytoplasm. These findings suggest that low-dose CO may decrease RV14 replication in alveolar and airway epithelial cells. IFN-γ production, which is induced by CO exposure via guanylate cyclase activation-mediated cGMP production, may be involved in RV14 replication inhibition.

Characterization of peristaltic motility in the striated muscle portion of the esophagus using a novel in vivo method in rats

BACKGROUND

Esophageal peristalsis is controlled by the brainstem via vago-vagal reflex. However, the precise regulatory mechanisms in the striated muscle portion are largely unknown. The aim of this study was to characterize peristaltic motility in the portion of the esophagus using a novel in vivo method in rats.

METHODS

A balloon-tipped catheter was placed in the esophagus of a rat anesthetized with urethane. To induce esophageal peristalsis, the balloon was inflated by water injection.

KEY RESULTS

When the balloon was inflated near the bronchial bifurcation, the balloon was transported in the aboral direction. Vagotomy abolished the peristaltic response. The threshold volume for inducing esophageal peristalsis varied according to the velocity of balloon distention; the volume being effective to induce peristalsis at a low inflation speed was smaller than the threshold volume at a rapid inflation speed. Even in the absence of inflation, keeping the balloon inside the esophagus during an interval period prevented subsequent induction of peristaltic motility. In addition, a nitric oxide synthase inhibitor abolished the induction of esophageal peristalsis.

CONCLUSIONS AND INFERENCES

Our findings suggest that (a) in addition to the intensity, the velocity of distention is important for activating the mechanosensory mechanism to induce esophageal peristalsis, (b) tonic inputs from afferent fibers located at the mucosa may reduce the excitability of mechanosensors which is necessary for inducing peristalsis, and (c) nitric oxide plays essential roles in the induction of esophageal peristalsis. These results provide novel insights into the regulatory mechanisms of esophageal motility.

Pharmacokinetics and pharmacodynamics of nitric oxide mimetic agents

Drug discovery focusing on NO mimetics has been hamstrung due to its unconventional nature. Central to these challenges is the fact that direct measurement of molecular NO in biological systems is exceedingly difficulty. Hence, drug development of NO mimetics must rely upon measurement of the NO donating specie (i.e., a prodrug) and a downstream marker of efficacy without directly measuring the molecule, NO, that is responsible for biological effect. The focus of this review is to catalog in vivo attempts to monitor the pharmacokinetics (PK) of the NO donating specie and the pharmacodynamic (PD) readout of NO bioactivity.

Prostaglandin Signaling Governs Spike Timing-Dependent Plasticity at Sensory Synapses onto Mouse Spinal Projection Neurons

Highly correlated presynaptic and postsynaptic activity evokes spike timing-dependent long-term potentiation (t-LTP) at primary afferent synapses onto spinal projection neurons. While prior evidence indicates that t-LTP depends upon an elevation in intracellular Ca²⁺ within projection neurons, the downstream signaling pathways that trigger the observed increase in glutamate release from sensory neurons remain poorly understood. Using in vitro patch-clamp recordings from female mouse lamina I spino-parabrachial neurons, the present study demonstrates a critical role for prostaglandin synthesis in the generation of t-LTP. Bath application of the selective phospholipase A₂ (PLA₂) inhibitor arachidonyl trifluoromethyl ketone (AACOCF₃) or the cyclooxygenase 2 (Cox-2) inhibitor nimesulide prevented t-LTP at sensory synapses onto spino-parabrachial neurons. Similar results were observed following the block of the EP2 subtype of prostaglandin E₂ (PGE₂) receptor with PF 04418948. Meanwhile, perfusion with PGE₂ or the EP2 agonist butaprost potentiated the amplitude of monosynaptic primary afferent-evoked EPSCs while decreasing the paired-pulse ratio, suggesting a presynaptic site of action. Cox-2 was constitutively expressed in both spinal microglia and lamina I projection neurons within the superficial dorsal horn (SDH). Suppression of microglial activation with minocycline had no effect on the production of t-LTP, suggesting the possibility that prostaglandins produced within projection neurons could contribute to an enhanced probability of glutamate release at primary afferent synapses. Collectively, the results suggest that the amplification of ascending nociceptive transmission by the spinal SDH network is governed by PLA₂-Cox-2-PGE₂ signaling.SIGNIFICANCE STATEMENT Long-term potentiation (LTP) of primary afferent synapses contributes to the sensitization of spinal nociceptive circuits and has been linked to greater pain sensation in humans. Prior work has implicated elevated glutamate release in the generation of spike timing-dependent LTP (t-LTP) at sensory synapses onto ascending spinal projection neurons, but the underlying mechanisms remain unknown. Here we provide evidence that the activation of EP2 prostaglandin receptors by prostaglandin E₂, occurring downstream of phospholipase A₂ and cyclooxygenase 2 activation, mediates t-LTP at these synapses via changes in presynaptic function. This suggests that prostaglandins can increase the flow of nociceptive information from the spinal cord to the brain independently of their known ability to suppress synaptic inhibition within the dorsal horn.

Ferulic Acid Improves Cognitive Skills Through the Activation of the Heme Oxygenase System in the Rat

Over the last years, many studies reported on the antioxidant effects of ferulic acid (FA) in preclinical models of dementia through the activation of the heme oxygenase/biliverdin reductase (HO/BVR) system. However, only a few studies evaluated whether FA could improve neurological function under milder conditions, such as psychological stress. The aim of this study was to investigate the effects of FA (150 mg/kg intraperitoneal route) on cognitive function in male Wistar rats exposed to emotional arousal. Animals were randomly assigned to two experimental groups, namely not habituated or habituated to the experimental context, and the novel object recognition test was used to evaluate their cognitive performance. The administration of FA significantly increased long-term retention memory in not habituated rats. Ferulic acid increased the expression of HO-1 in the hippocampus and frontal cortex of not habituated rats only, whereas HO-2 resulted differently modulated in these cognitive brain areas. No significant effects on either HO-1 or HO-2 or BVR were observed in the cerebellum of both habituated and not habituated rats. Ferulic acid activated the stress axis in not habituated rats, as shown by the increase in hypothalamic corticotrophin-releasing hormone levels. Pre-treatment with Sn-protoporphyrin-IX [0.25 μmol/kg, intracerebroventricular route (i.c.v.)], a well-known inhibitor of HO activity through which carbon monoxide (CO) and biliverdin (BV) are generated, abolished the FA-induced improvement of cognitive performance only in not habituated rats, suggesting a role for HO-derived by-products. The CO-donor tricarbonyldichlororuthenium (II) (30 nmol/kg i.c.v.) mimicked the FA-related improvement of cognitive skills only in not habituated rats, whereas BV did not have any effect in any group. In conclusion, these results set the stage for subsequent studies on the neuropharmacological action of FA under conditions of psychological stress.

Emerging Trends in Retrograde Signaling

Retrograde signaling is defined as the signaling events leading from the plastids to the nucleus in plants and across the chemical synapse, from the postsynaptic neuron to the presynaptic neuron in animals. The discovery of various retrograde messengers has opened many avenues and clouds of thoughts as to the role of retrograde signaling. They have been implicated particularly in long-term potentiation (LTP) and synaptic plasticity. But the basic assumptions about retrograde signaling have not been studied upon for many years. This review focuses on established facts and hypothesis put forward in retrograde signaling.

Modulation of ATP-induced LTP by cannabinoid receptors in rat hippocampus

Cannabinoids exert powerful action on various forms of synaptic plasticity. These retrograde messengers modulate GABA and glutamate release from presynaptic terminals by acting on presynaptic CB1 receptors. In particular, they inhibit long-term potentiation (LTP) elicited by electrical stimulation of excitatory pathways in rat hippocampus. Recently, LTP of the field excitatory postsynaptic potential (fEPSP) induced by exogenous ATP has been thoroughly explored. The present study demonstrates that cannabinoids inhibit ATP-induced LTP in hippocampal slices of rat. Administration of 10 μM of ATP led to strong inhibition of fEPSPs in CA1/CA3 hippocampal synapses. Within 40 min after ATP removal from bath solution, robust LTP was observed (fEPSP amplitude comprised 130.1 ± 3.8% of control, n = 10). This LTP never appeared when ATP was applied in addition to cannabinoid receptor agonist WIN55,212-2 (100 nM). Selective CB1 receptor antagonist, AM251 (500 nM), completely abolished this effect of WIN55,212-2. Our data indicate that like canonical LTP elicited by electrical stimulation, ATP-induced LTP is under control of CB1 receptors.

Heme oxygenase-1 and chronic hypoxia

A myriad of changes are necessary to adapt to chronic hypoxemia. Key among these changes increases in arterial oxygen carrying capacity, ventilation and sympathetic activity. This requires the induction of several gene products many of which are regulated by the activity of HIF-1α, including HO-1. Induction of HO-1 during chronic hypoxia is necessary for the continued breakdown of heme for the enhanced production of hemoglobin and the increased respiratory and sympathetic responses. Several human HO-1 polymorphisms have been identified that can affect the expression or activity of HO-1. Associations between these polymorphisms and the prevalence of hypertension have recently been assessed in specific populations. There are major gaps in our understanding of the mechanisms of how HO-1 mediates changes in the activity of the hypoxia-sensitive chemosensors and whether HO-1 polymorphisms are an important factor in the integrated response to chronic hypoxia. Understanding how HO-1 mediates cardiorespiratory responses could provide important insights into clinical syndromes such as obstructive sleep apnea.

Nitric oxide metabolism controlled by formaldehyde dehydrogenase (fdh, homolog of mammalian GSNOR) plays a crucial role in visual pattern memory in Drosophila

Nitric oxide (NO) plays an important role in learning and memory which is essential for animals to adapt to the external environment. However, little is known about the role of NO metabolism in this process. S-nitrosoglutathione reductase (GSNOR) is a key protein in the control of NO metabolism and protein S-nitrosation. To study the relationship between NO metabolism and learning and memory, the expression of gene fdh which is homolog to mammalian GSNOR was modulated by the Gal4/UAS system in Drosophila. The over-expression of the fdh in the central nervous system significantly increased GSNOR activity and induced visual pattern memory defects of Drosophila. The role of fdh in learning and memory was independent of development and was neuron-specific: over-expression of the fdh in the fan-shaped body induced memory defect, while over-expression in the mushroom body did not. The visual pattern memory defect could be rescued by co-expression with exogenous cGMP-dependent protein kinase (PKG). Moreover, fdh over-expression resulted in denitrosation of multiple proteins functionally enriched in vesicle-mediated transport, which is important for learning and memory. These results showed that regulation of NO metabolism plays an important role in learning and memory, and the mechanism may involve both NO-cGMP-PKG signaling pathway and S-nitrosation modification.

Superoxide signaling in pain is independent of nitric oxide signaling

Two reactive oxygen species (ROS), nitric oxide (NO(.)) and superoxide ((.)O2), contribute to persistent pain. Using three different animal models where ROS mediate pain, this study examined whether NO(.) and (.)O2 converge to peroxynitrite (ONOO(-)) or whether each has an independent signaling pathway to produce hyperalgesia. The hyperalgesia after spinal nerve ligation was attenuated by removing (.)O2 by TEMPOL or inhibiting NO(.) production by L-NAME, but not by removing peroxynitrite with FeTMPyP. Nitric oxide-induced hyperalgesia was not affected by removing (.)O2 but was reduced by a guanyl cyclase inhibitor. Superoxide-induced hyperalgesia was not affected by inhibiting NO(.) production but was suppressed by a protein kinase C inhibitor. The data suggest that NO(.) and (.)O2 operate independently to generate pain.

The effect of heme oxygenase inhibition on visual evoked potentials

This study investigated the effect of heme oxygenase (HO) inhibition on visual evoked potentials (VEPs). HO catalyzes the oxidative degradation of heme. Products of HO reaction are biliverdin, ferrous iron, and carbon monoxide (CO). CO is a signal molecule and is an endogenous modulator in the soluble guanylate cyclase/cyclic guanosine monophosphate signaling pathway. Rats were treated with HO inhibitors tin protoporphyrin IX (SnPP IX) or zinc protoporphyrin IX (ZnPP IX) or HO inducer sodium arsenite (Na-arsenite). Soluble guanylate cyclase is inhibited by 1H-[1,2,3]oxydiazolo[4,3-a]quinoxalin-1-one (ODQ) and induced by 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1). VEPs were recorded under mild ether anesthesia with the help of stainless steel subdermal electrodes and a photic stimulator. SnPP IX, ODQ or SnPP IX + YC-1 injections significantly prolonged latencies of P3; however, Na-arsenite shortened latency of P3. It has been shown that HO affects VEPs.

Postsynaptic regulation of synaptic plasticity by synaptotagmin 4 requires both C2 domains

Analogous to synaptotagmin 1, a calcium-sensitive regulator of presynaptic vesicle fusion, synaptotagmin 4 needs both of its calcium-binding sites to regulate synaptic plasticity via postsynaptic retrograde signaling.

Ca²⁺ influx into synaptic compartments during activity is a key mediator of neuronal plasticity. Although the role of presynaptic Ca²⁺ in triggering vesicle fusion though the Ca²⁺ sensor synaptotagmin 1 (Syt 1) is established, molecular mechanisms that underlie responses to postsynaptic Ca²⁺ influx remain unclear. In this study, we demonstrate that fusion-competent Syt 4 vesicles localize postsynaptically at both neuromuscular junctions (NMJs) and central nervous system synapses in Drosophila melanogaster. Syt 4 messenger RNA and protein expression are strongly regulated by neuronal activity, whereas altered levels of postsynaptic Syt 4 modify synaptic growth and presynaptic release properties. Syt 4 is required for known forms of activity-dependent structural plasticity at NMJs. Synaptic proliferation and retrograde signaling mediated by Syt 4 requires functional C2A and C2B Ca²⁺–binding sites, as well as serine 284, an evolutionarily conserved substitution for a key Ca²⁺-binding aspartic acid found in other synaptotagmins. These data suggest that Syt 4 regulates activity-dependent release of postsynaptic retrograde signals that promote synaptic plasticity, similar to the role of Syt 1 as a Ca²⁺ sensor for presynaptic vesicle fusion.

Nitric oxide and cyclic nucleotide signal transduction modulates synaptic vesicle turnover in human model neurons

The human Ntera2 (NT2) teratocarcinoma cell line can be induced to differentiate into post-mitotic neurons. Here, we report that the human NT2 neurons generated by a spherical aggregate cell culture method express increasing levels of typical pre-synaptic proteins (synapsin and synaptotagmin I) along the neurite depending on the length of in vitro culture. By employing an antibody directed against the luminal domain of synaptotagmin I and the fluorescent dye N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide, we show that depolarized NT2 neurons display calcium-dependent exo-endocytotic synaptic vesicle recycling. NT2 neurons express the neuronal isoform of neuronal nitric oxide synthase and soluble guanylyl cyclase (sGC), the major receptor for nitric oxide (NO). We tested whether NO signal transduction modulates synaptic vesicle turnover in human NT2 neurons. NO donors and cylic guanosine-monophosphate analogs enhanced synaptic vesicle recycling while a sGC inhibitor blocked the effect of NO donors. Two NO donors, sodium nitroprusside, and and N-Ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydrazino) ethanamine evoked vesicle exocytosis which was partially blocked by the sGC inhibitor. The activator of adenylyl cyclase, forskolin, and a cAMP analog induced synaptic vesicle recycling and exocytosis via a parallel acting protein kinase A pathway. Our data from NT2 neurons suggest that NO/cyclic nucleotide signaling pathways may facilitate neurotransmitter release in human brain cells.

Protective effect of cyanidin 3-O-β-d-glucoside on ochratoxin A-mediated damage in the rat

The aim of the present study was to verify whether the oral administration of cyanidin 3-O-β-d-glucoside (C3G) might counteract damage induced by chronic exposure (28 d) to ochratoxin A (OTA) in rats and if its effect may be mediated by haeme oxygenase-1 (HO-1). Forty male Sprague–Dawley rats, individually caged, were divided into four groups of ten animals. A control group received a commercial diet, group C3G received the control diet supplemented with C3G (1 g/kg feed), group OTA received the control diet supplemented with 200 parts per billion of OTA, and group OTA+C3G received the OTA group diet supplemented with C3G (1 g/kg feed). After 4 weeks of treatment animals were killed and the liver, kidneys and brain of each rat were collected and homogenised to evaluate non-proteic thiol groups (RSH), lipid hydroperoxide (LOOH) levels, HO-1 expression and DNA fragmentation. Rats of the OTA group showed a significant (P < 0·001) decrease in RSH content of kidney and liver and a significant (P < 0·001) increase of LOOH in all the examined tissues compared with the control group. In the OTA+C3G group both RSH content and LOOH levels were similar to those observed in the control group, demonstrating that C3G was able to counteract the effects of OTA. A significant (P < 0·001) induction of HO-1 was evident in kidney and liver of both OTA and C3G groups. DNA damage occurred in all the examined tissues of the OTA group, whereas C3G was able to prevent it. The present study confirmed that the effects of OTA are mediated by oxidative stress and demonstrated that C3G efficiently counteracted deleterious effects of OTA because of its antioxidant and HO-1-inducing properties.

Inhibition of the enzymatic activity of heme oxygenases by azole-based antifungal drugs

Ketoconazole (KTZ) and other azole antifungal agents are known to have a variety of actions beyond the inhibition of sterol synthesis in fungi. These drugs share structural features with a series of novel heme oxygenase (HO) inhibitors designed in our laboratory. Accordingly, we hypothesized that therapeutically used azole-based antifungal drugs are effective HO inhibitors. Using gas chromatography to quantify carbon monoxide formation in vitro and in vivo, we have shown that azole-containing antifungal drugs are potent HO inhibitors. Terconazole, sulconazole, and KTZ were the most potent drugs with IC(50) values of 0.41 +/- 0.01, 1.1 +/- 0.4, and 0.3 +/- 0.1 microM for rat spleen microsomal HO activity, respectively. Kinetic characterization revealed that KTZ was a noncompetitive HO inhibitor. In the presence of KTZ (2.5 and 10 microM), K(m) values for both rat spleen and brain microsomal HO were not altered; however, a significant decrease in the catalytic capacity (V(max)) was observed (P < 0.005). KTZ was also found to weakly inhibit nitric-oxide synthase with an IC(50) of 177 +/- 2 microM but had no effect on the enzymatic activity of NADPH cytochrome P450 reductase. Because these drugs were effective within the concentration range observed in humans, it is possible that inhibition of HO may play a role in some of the pharmacological actions of these antimycotic drugs.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

A mechanism underlying AMPA receptor trafficking during cerebellar long-term potentiation

Long-term potentiation (LTP) is mediated by the activity-driven delivery of GluR1 glutamate receptors via Ca2+/calmodulin-dependent protein kinase II activity in various brain regions. Recently, postsynaptic LTP was shown to be induced at parallel fiber-Purkinje cell synapses by stimulating the parallel fibers at 1 Hz or applying a NO donor. Here, we demonstrate that NO-evoked postsynaptic LTP in mice cerebellum was blocked by botulinum toxin and enhanced by prior treatment with phorbol ester, which is known to induce GluR2 endocytosis. Interestingly, such LTP was not affected by a Ca2+/calmodulin-dependent protein kinase II inhibitor or a peptide binding to a protein interacting with C kinase 1, but was blocked by a peptide binding to N-ethylmaleimide-sensitive factor, which specifically binds to GluR2. Therefore, although the synaptic incorporation of GluR2 has been reported to be a constitutive pathway, NO-induced postsynaptic LTP in Purkinje cells is likely mediated by a pathway involving N-ethylmaleimide-sensitive factor-dependent GluR2 trafficking.

30 some years of heme oxygenase: from a "molecular wrecking ball" to a "mesmerizing" trigger of cellular events

In the beginning, the microsomal HO system was presumed to be made of one isozymes, now known as HO-1, which was cytP450-dependent; and, was thought to be of physiological significance solely in the context of catalysis of hemoglobin heme to bile pigments and CO. A succession of discoveries including characterization of the system as an independent mono-oxygenase, identification of a second form, called HO-2, free radical quenching activity of bile pigments, analogous function of CO in cell signaling to NO, and characterization of the system as HSP32 cognates has led to such an impressive expansion in the number of reports dealing with the HO system that surpass anyone's expectation. This review is a compilation of certain older findings and recent events that together ensure placement of the HO system in the mainstream research for decades to come.

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.

Alterations in spinal cord gene expression after hindpaw formalin injection

Heme oxygenase type 2 (HO-2) is an enzyme that uses heme as a substrate to produce iron, biliverdin, and carbon monoxide (CO). This enzyme participates in regulation of nociceptive signal transmission in spinal cord tissue. We set out to identify genes undergoing alterations in expression in a model of inflammatory pain and to determine whether HO-2 participates in that regulation. After the hindpaw injection of formalin in mice, we measured changes in expression of immediate early genes including c-fos, c-jun, jun B, nerve growth factor induced genes (NGFI-A and NGFI-B) and activity-related cytoskeletal protein (ARC) using real-time PCR. The mRNA corresponding to these genes increased in abundance in the first hour after formalin injection and then slowly declined. Changes in the abundance of prodynorphin, extracellular signal related kinases (ERK1 and ERK2) and N-methyl-D-aspartate (NMDA) receptor R1 subunit mRNA generally peaked between 8 and 12 hr after formalin injection. In HO-2 null mutant mice, the enhancement of expression was less for all genes studied. We went on to quantify gene expression in superficial dorsal horn tissue using laser capture microdissection followed by RNA amplification and real-time PCR. The results confirmed that the changes in gene expression were occurring in regions of the spinal cord involved in nociceptive processing. We conclude that the hindpaw injection of formalin leads to enhanced early and late expression of many genes in spinal cord dorsal horn tissue, and that this enhancement of expression relies to a degree on the presence of HO-2.

Role of nitric oxide in the regulation of neuronal proliferation, survival and differentiation

Nitric oxide (NO), an important cellular messenger, has been linked to both neurodegenerative and neuroprotective actions. In the present review, we focus on recent data establishing a survival and differentiation role for NO in several neural in vitro and in vivo models. Nitric oxide has been found to be essential for survival of neuronal cell lines and primary neurons in culture under various death challenges. Furthermore, its lack may aggravate some neuropathological conditions in experimental animals. Several cellular pathways and signaling systems subserving this neuroprotective role of NO are considered in the review. Survey of recent data related to the developmental role of NO mainly focus on its action as a negative regulator of neuronal precursor cells proliferation and on its role of promotion of neuronal differentiation. Discussion on discrepancies arising from the literature is focused on the Janus-faced properties of the molecule and it is proposed that most controversial results are related to the intrinsic property of NO to compensate among functionally opposed effects. As an example, the increased proliferation of neural cell precursors under conditions of NO shortage may be, later on in the development, compensated by increased elimination through programmed cell death as a consequence of the lack of the survival-promoting action of the molecule. To elucidate these complex, and possibly contrasting, effects of NO is indicated as an important task for future researches.

The localization of neuronal nitric oxide synthase may influence its role in neuronal precursor proliferation and synaptic maintenance

Neuronal nitric oxide synthase (nNOS) is implicated in some developmental processes, including neuronal survival, differentiation, and precursor proliferation. To define the roles of nNOS in neuronal development, we utilized the olfactory system as a model. We hypothesized that the role of nNOS may be influenced by its localization. nNOS expression was developmentally regulated in the olfactory system. During early postnatal development, nNOS was expressed in developing neurons in the olfactory epithelium (OE), while in the adult its expression was restricted to periglomerular (PG) cells in the olfactory bulb (OB). At postnatal week 1 (P1W), loss of nNOS due to targeted gene deletion resulted in a decrease in immature neurons in the OE due to decreased proliferation of neuronal precursors. While the pool of neuronal precursors and neurogenesis normalized in the nNOS null mouse by P6W, there was an overgrowth of mitral or tufted cells dendrites and a decreased number of active synapses in the OB. Cyclic GMP (cGMP) immunostaining was reduced in the OE and in the glomeruli of the OB at early postnatal and adult ages, respectively. Our results suggest that nNOS appears necessary for neurogenesis in the OE during early postnatal development and for glomerular organization in the OB in the adult. Thus, the location of nNOS, either within cell bodies or perisynaptically, may influence its developmental role.

Formalin-induced spinal cord calcium/calmodulin-dependent protein kinase II alpha expression is modulated by heme oxygenase in mice

The injection of formalin into the hindpaws of rats and mice is widely used as a model of inflammatory pain. The allodynia observed in this model is due in part to sensitization of spinal cord dorsal horn neurons, a form of neuroplasticity similar to long-term potentiation in the hippocampus. Ca(2+)/calmodulin-dependent kinase type IIalpha (CaMKIIalpha) is a key component of long-term potentiation. Here we report alterations in CaMKIIalpha mRNA and protein expression in spinal cord tissue from wild-type and heme oxygenase type 2 (HO-2) null mutant mice after formalin injection. Behavioral experiments demonstrated a long lived allodynia in wild-type C57Bl/6J mice after hindpaw formalin injection, but less in null mutant mice. Both CaMKIIalpha mRNA and protein expression were increased in a time-dependent manner in the spinal cords of wild-type mice after formalin injection. Confocal microscopy localized the increased expression to the superficial laminae of the spinal cord dorsal horn. In the HO-2 null mutant mice no significant change in CaMKIIalpha mRNA expression and only a small increase in protein were noted. These findings suggest that time-dependent CaMKIIalpha expression may underlie central sensitization and allodynia induced by hindpaw formalin injection, and that this process is modulated by HO-2.

Carbon monoxide: to boldly go where NO has gone before

The discovery that nitric oxide (NO) has powerful vasoactive properties identical to those of endothelial-derived relaxing factor spawned a vast body of research investigating the physiological actions of small gas molecules. NO, which arises endogenously through the action of nitric oxide synthase (NOS) enzymes, is a highly reactive gas that plays important roles in the regulation of vascular and immune function. Carbon monoxide (CO), a similar yet much more chemically stable gas, occurs in nature as a product of the oxidation or combustion of organic materials. CO also arises in cells and tissues as a byproduct of heme oxygenase (HO) activity, which degrades heme to biliverdin-IXalpha. Like NO, CO acts as a vasorelaxant and may regulate other vascular functions such as platelet aggregation and smooth muscle proliferation. CO has also been implicated as a neurotransmitter in the central nervous system. HO-1, the inducible form of HO, confers cytoprotection against oxidative stress in vitro and in vivo. CO, when applied at low concentration, exerts potent cytoprotective effects mimicking those of HO-1 induction, including down-regulation of inflammation and suppression of apoptosis. Many of the effects of CO depend on the activation of guanylate cyclase, which generates guanosine 3',5'-monophosphate (cGMP), and the modulation of mitogen-activated protein kinase (MAPK) signaling pathways. This review highlights new advances in the interaction of CO with cellular signaling processes.

Contribution of cGMP but not peroxynitrite to negative feedback regulation of penile erection elicited by nitric oxide in the hippocampal formation of the rat

We established previously that nitric oxide (NO) in the hippocampal formation (HF) participates actively in negative feedback regulation of penile erection. This study further evaluated whether this process engaged soluble guanylyl cyclase (sGC)/cGMP cascade or peroxynitrite in the HF. Intracavernous pressure (ICP) recorded from the penis in adult, male Sprague-Dawley rats anesthetized with chloral hydrate was employed as our experimental index for penile erection. Microinjection bilaterally of a NO-independent sGC activator, YC-1 (0.1 or 1 nmol) or a cGMP analog, 8-Bromo-cGMP (0.1 or 1 nmol), into the HF elicited a significant reduction in baseline ICP. Bilateral application into the HF of equimolar doses (0.5 or 1 nmol) of a sGC inhibitor, LY83583 or a NO-sensitive sGC inhibitor, ODQ significantly antagonized the decrease in baseline ICP induced by co-administration of the NO precursor, L-arginine (5 nmol), along with significant enhancement of the magnitude of papaverine-induced elevation in ICP. In contrast, a peroxynitrite scavenger, L-cysteine (50 or 100 pmol), or an active peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis-(N-methyl-4'-pyridyl)-porphyrinato iron (III) (10 or 50 pmol), was ineffective in both events. These results suggest that NO may participate in negative feedback regulation of penile erection by activating the sGC/cGMP cascade in the HF selectively.

Transmission of Arterial Baroreflex Signals Depends on Neuronal Nitric Oxide Synthase

Because inhibition of neuronal nitric oxide synthase in the nucleus tractus solitarii blocks cardiovascular responses to activation of local glutamate receptors, and because glutamate is a neurotransmitter of baroreceptor afferent nerves, we sought to test the hypothesis that neuronal nitric oxide synthase inhibition would block baroreflex transmission and cause hypertension. We determined reflex heart rate responses to intravenous phenylephrine and sodium nitroprusside in 5 anesthetized rats before and after bilateral microinjection (100 nL) of the neuronal nitric oxide synthase inhibitor AR-R 17477 (7.5 nmol) into the nucleus tractus solitarii. The inhibitor significantly increased mean arterial pressure without affecting heart rate, and it significantly reduced the gain of the baroreflex. After administration of the inhibitor, reflex responses of heart rate to changes in mean arterial pressure were always less than those responses to the same, or less, change in mean arterial pressure in the same animal without administration of the inhibitor. Microinjection of saline (100 nL) bilaterally into the nucleus tractus solitarii did not lead to hypertension or change baroreflex responses. These data support the hypothesis and suggest that neuronal nitric oxide synthase is critical to transmission of baroreflex signals through the nucleus tractus solitarii.

Heme oxygenase type 2 modulates behavioral and molecular changes during chronic exposure to morphine

The heme oxygenase (HO) enzyme system has been shown to participate in nociceptive signaling in a number of different models of pain. In these experiments we investigated the role of the HO type 2 (HO-2) isozyme in tolerance to the analgesic effects of morphine, and the hyperalgesia and allodynia which are measurable upon cessation of administration. Wild type C57Bl/6 wild type mice or HO-2 null mutants in that background strain were treated with morphine for 5 days. The morphine administration protocol consisted of either twice daily repeated s.c. boluses of 15 mg/kg or s.c. implantation of a morphine pellet. At the end of the treatment period wild type mice treated by either protocol exhibited tolerance, but the HO-2 null mutants did not. The HO-2 null mutants also exhibited less mechanical allodynia following cessation of morphine administration, though only modest differences in thermal hyperalgesia were noted. There was no correlation between the degree of tolerance obtained in the bolus and pellet protocols and the degree of hyperalgesia and allodynia observed after cessation of morphine administration in the wild type mice. Our final experiments analyzed increases in expression of mRNA for nitric oxide synthase type 1, N-methyl-D-aspartate (NMDA) receptor NMDAR1 subunit and prodynorphin in spinal cord tissue. In pellet-treated mice two- to three-fold increases were observed in the abundance of these species, but very little change was observed in the null-mutant mice. Taken together our results indicate that HO-2 participates in the acquisition of opioid tolerance, the expression of mechanical allodynia after cessation of opioid administration and in gene regulation occurring in the setting of treatment with morphine. Furthermore, these studies suggest that the mechanisms underlying analgesic tolerance and opioid-induced hypersensitivity are at least somewhat distinct.

Immunocytochemical evidence for nitric oxide- and carbon monoxide-producing neurons in the stomatogastric nervous system of the crayfishCherax quadricarinatus

Nitric oxide (NO) and carbon monoxide (CO) have been shown to serve neuromodulatory roles in both vertebrates and invertebrates. Here, we use antibodies to their respective biosynthetic enzymes, nitric oxide synthase (NOS) and heme oxygenase 2 (HO-2), to map the distribution of putative gas-producing neurons in the stomatogastric nervous system (STNS) of the crayfish Cherax quadricarinatus. In this species, NOS immunolabeling is found in the neuropil of the stomatogastric ganglion (STG). This staining originates from two immunopositive axons that project to the STG through the superior oesophageal and stomatogastric nerves, presumably from cell bodies located in the commissural ganglia (CoGs). HO-2 immunoreactivity is present in small diameter fibers and varicosities in the periphery of nerves located in the anterior portion of the STNS. This labeling originates from approximately 12 somata in each CoG. Transmission electron microscopy done on the nerves of the anterior STNS shows they contain a neuroendocrine plexus. Collectively, our results indicate that NO- and CO-producing neurons are likely to exist in the crayfish STNS. Moreover, these gases appear to be produced by distinct subsets of the neurons present there. The localization of NO to the STG neuropil suggests that it serves as a locally released modulator or is involved in the local release of other substances within this ganglion. The presence of CO in the neurohemal plexus of the anterior STNS suggests that it serves as a circulating hormone or is involved in the control of neuroendocrine release from this plexus.

Heme oxygenase-1 and heme oxygenase-2 have distinct roles in the proliferation and survival of olfactory receptor neurons mediated by cGMP and bilirubin, respectively

Heme oxygenase (HO) is implicated in protection against oxidative stress, proliferation and apoptosis in many cell types, including neurons. We utilized olfactory receptor neurons (ORNs) as a model to define the roles of HO-1 and HO-2 in neuronal development and survival, and to determine the mediators of these effects. The olfactory system is a useful model as ORNs display neurogenesis post-natally and do not contain nitric oxide synthase (NOS) activity, which could confound results. HO isoforms were expressed in ORNs during embryogenesis and post-natally. Mice null for either HO-1 or HO-2 displayed decreased proliferation of neuronal precursors. However, apoptosis was increased only in HO-2 null mice. Cyclic GMP immunostaining was reduced in ORNs in both genotypes, providing direct evidence that HO mediates cGMP production in vivo. Bilirubin immunostaining was reduced only in HO-2 null mice. These roles for HO-1 and HO-2 were confirmed using detergent ablation of the epithelium to observe increased neurogenesis of ORNs after target disruption in HO null mice. Primary cultures of ORNs revealed that proliferative and survival effects of HO were mediated through cGMP and bilirubin, respectively. These results support a role for HO, the CO-cGMP signaling system and bilirubin in neurodevelopment and in response to injury.

Purification and characterization of heme oxygenase

High-yield expression and purification of human heme oxygenase isozyme 1 provided the breakthrough in characterizing the protein from mechanistic and structural standpoints. This unit provides a protocol for high-level expression and subsequent purification of HO-1. The commentary includes a discussion of subsequent biochemical and biophysical characterizations.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

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

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

Expression of heme oxygenase in the oxygen-sensing regions of the rostral ventrolateral medulla

Recently, unique regions in the rostral ventrolateral medulla (RVLM) have been found to be oxygen sensitive. However, the mechanism of sensing oxygen in these RVLM regions is unknown. Because heme oxygenase (HO) has been shown to be involved in the hypoxic responses of the carotid body and pulmonary artery, the aim of this study was to determine whether HO is present in the RVLM and whether expression of HO is altered by chronic hypoxia. Adult rats were exposed to hypoxia (10% O(2)) or normoxia (21% O(2)) for 10 days, and the mRNA for HO-1 and HO-2 was examined in the RVLM by using RT-PCR. Expression of HO-2 mRNA was seen in the RVLM of both control and hypoxic samples, whereas expression of HO-1 mRNA was only seen in the RVLM of hypoxic samples. HO-2 was immunocytochemically localized in brain sections (40 microm) to the C1 region and pre-Bötzinger complex of the RVLM. Together, these results indicate that HO-2 is present in the RVLM under control conditions and that HO-1 is induced in the RVLM during chronic hypoxia, consistent with a potential role for HO in the oxygen-sensing function of these cardiorespiratory RVLM regions.

Nitroxidergic influences on cardiovascular control by NTS: a link with glutamate

Glutamate (GLU) receptor activation, which is important in cardiovascular reflex transmission through the nucleus tractus solitarii (NTS), leads to release of nitric oxide (NO.) from central nitroxidergic neurons. Therefore, we hypothesized that GLU and NO. are linked in cardiovascular control by NTS. We first sought to determine if NO. released into NTS led to cardiovascular changes like those produced by GLU and found that the nitrosothiol S-nitrosocysteine, but not NO. itself or other NO. donors, elicited such responses in anesthetized rats. The responses were dependent on activation of soluble guanylate cyclase but, not being affected by a scavenger of NO., likely did not depend on release of NO. into the extracellular space. Responses to ionotropic GLU agonists in NTS, like those to S-nitrosocysteine, were inhibited by inhibition of soluble guanylate cyclase. Inhibition of neuronal NO. synthase (nNOS) also inhibited responses to ionotropic GLU agonists. The apparent physiologic link between GLU and NO. mechanisms in NTS was further supported by anatomical studies that demonstrated frequent association between GLU-containing nerve terminals and neurons containing nNOS. Furthermore, GLU receptors were often found on NTS neurons that were immunoreactive for nNOS. The anatomical relationships between GLU and nNOS and GLU receptors and nNOS were more pronounced in some subnuclei of NTS than in others. While seen in subnuclei that are known to receive cardiovascular afferents, the association was even more prominent in subnuclei that receive gastrointestinal afferents. These studies support a role for nitroxidergic neurons in mediating cardiovascular and other visceral reflex responses that result from release of GLU into the NTS.

Aminoguanidine prevented the impairment of learning behavior and hippocampal long-term potentiation following transient cerebral ischemia

The present study was performed to investigate whether increases in nitric oxide (NO) production via inducible nitric oxide synthase (iNOS) were involved in impairment of learning behavior and hippocampal long-term potentiation (LTP) following transient ischemia. Rats with four-vessel occlusion (4-VO) were used as an ischemic model. One week after permanent occlusion of the vertebral arteries, the common carotid arteries were clamped for 10 min under halothane anesthesia. Aminoguanidine (10 mg/kg i.p.), a relatively selective iNOS inhibitor, or saline was administered 30 min before common carotid arteries occlusion, and every 24-h for 4 days. We investigated whether hippocampal NO production was increased by ischemic insult using microdialysis on days 1, 4 and 7 after 4-VO. On days 1 and 4 after 4-VO, increases in NO production were observed, and this effect was inhibited by aminoguanidine. Four days after 4-VO, rats were subjected to the Y-maze test and contextual fear conditioning. Ischemic insults impaired learning behavior, and aminoguanidine ameliorated the impairment induced by 4-VO. Four days after 4-VO, the changes in the population spike amplitude were recorded as an index of LTP in Schaffer collateral-CA1 (carotid artery 1), the mossy fiber-CA3 and the perforant path-dentate gyrus synapses. LTP was significantly inhibited by 4-VO, except in mossy fiber-CA3 synapses. Pretreatment with aminoguanidine prevented the reduction of LTP in perforant path-dentate gyrus, but not in Schaffer collateral-CA1 synapses. These results suggest that post-ischemic increase in NO production via iNOS impaired the learning behavior. There is an association between behavioral performance and LTP formation in perforant path-dentate gyrus synapses, but neither in Schaffer collateral-CA1 nor in mossy fiber-CA3 synapses.

A biologically plausible model of associative memory which uses disinhibition rather than long-term potentiation

Mammalian memory is commonly "explained" in terms of long-term potentiation (LTP) of excitatory synapses. However, depotentiation of inhibitory pathways (disinhibition) is also a known phenomenon in the brain. Artificial neural networks which are offered as partial models of the cerebrum traditionally encode memory by the potentiation of excitatory "synapses" in a manner which is thought of as analogous to LTP. Analysis shows that such models have seriously limited storage capacities. The models also depend on mechanisms which do not appear to be biologically plausible. This paper demonstrates that these difficulties are avoided by encoding memory by means of disinhibition rather than LTP. The resulting models are simple and plausible, though unconventional.

Ethanol neurobehavioral teratogenesis and the role of the hippocampal glutamate-N-methyl-D-aspartate receptor-nitric oxide synthase system

The purpose of this review is to evaluate a proposed mechanism for ethanol neurobehavioral teratogenesis in the hippocampus, involving suppression of the glutamate-N-methyl-D-aspartate (NMDA) receptor-nitric oxide synthase (NOS) system. It is postulated that suppression of this signal transduction system in the fetus by chronic maternal consumption of ethanol plays a key role in hippocampal dysmorphology and dysfunction in postnatal life. This mechanism is evaluated critically based on the current literature and our research findings. In view of the apparent time course for loss of CA1 pyramidal cells in the hippocampus produced by chronic prenatal ethanol exposure that manifests in early postnatal life, it is proposed that therapeutic intervention, which targets the glutamate-NMDA receptor-NOS system, may prevent or lessen the magnitude of postnatal hippocampal dysfunction.