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The dual-specificity phosphatase (DUSP) family includes a heterogeneous group of protein phosphatases that dephosphorylate both phospho-tyrosine and phospho-serine/phospho-threonine residues within a single substrate. These protein phosphatases have many substrates and modulate diverse neural functions, such as neurogenesis, differentiation, and apoptosis. DUSP genes have furthermore been associated with mental disorders such as depression and neurological disorders such as Alzheimer's disease. Herein, we review the current literature on the DUSP family of genes concerning mental and neurological disorders. This review i) outlines the structure and general functions of DUSP genes, and ii) overviews the literature on DUSP genes concerning mental and neurological disorders, including model systems, while furthermore providing perspectives for future research., (Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Mood and anxiety disorders are prevalent conditions affecting one out of four people during lifetime. The development of high validity animal models to study these disorders has been a major challenge in the past. When considering experimental approaches for studying affective disorders, the social defeat paradigm has been shown to have etiological, predictive and face validity. Here, we explain the general principle of social defeat stress paradigms, with a strong focus on the resident-intruder model and compare different experimental settings as published to date. We discuss behavioral changes described in defeated animals as well as changes in the animal's physiological parameters. In addition, we provide an overview of the molecular adaptations that are found in animals subjected to defeat stress, with special attention to neural circuits and neuroendocrine signaling. Defeat produces specific behaviors resembling the signs and symptoms of humans with affective disorders, such as anhedonia, social avoidance, despair and anxiety. These can be linked to a wide range of physiological changes-ranging from cardiovascular changes to alterations in the immune system- or by disturbances in specific neurotransmitter systems, in particular serotonin and dopamine. The defeat stress model thus impacts on several functional domains of behavior and may mimic cardinal features of a multitude of psychiatric disorders including depression, post-traumatic stress disorder and schizophrenia. This manuscript critically reviews the core findings, strengths and limitations of the range of animal studies in this field and provides future perspectives., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Epigenetics is a quickly growing field encompassing mechanisms regulating gene expression that do not involve changes in the genotype. Epigenetics is of increasing relevance to neuroscience, with epigenetic mechanisms being implicated in brain development and neuronal differentiation, as well as in more dynamic processes related to cognition. Epigenetic regulation covers multiple levels of gene expression; from direct modifications of the DNA and histone tails, regulating the level of transcription, to interactions with messenger RNAs, regulating the level of translation. Importantly, epigenetic dysregulation currently garners much attention as a pivotal player in aging and age-related neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, where it may mediate interactions between genetic and environmental risk factors, or directly interact with disease-specific pathological factors. We review current knowledge about the major epigenetic mechanisms, including DNA methylation and DNA demethylation, chromatin remodeling and non-coding RNAs, as well as the involvement of these mechanisms in normal aging and in the pathophysiology of the most common neurodegenerative diseases. Additionally, we examine the current state of epigenetics-based therapeutic strategies for these diseases, which either aim to restore the epigenetic homeostasis or skew it to a favorable direction to counter disease pathology. Finally, methodological challenges of epigenetic investigations and future perspectives are discussed., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The enzyme DNA methyltransferase 3a (Dnmt3a) is crucially involved in DNA methylation and recent studies have demonstrated that Dnmt3a is functionally involved in mediating and moderating the impact of environmental exposures on gene expression and behavior. Findings in rodents have suggested that DNA methylation is involved in regulating neuronal proliferation and differentiation. So far, it has been shown that chronic social defeat might influence neurogenesis, while susceptibility to social defeat stress is dependent on gene expression changes in the nucleus accumbens and the mesolimbic dopaminergic system. However, the role of Dnmt3a herein has not been fully characterized. Our earlier immunohistochemical work has revealed the existence of two types of Dnmt3a-immunoreactive cells in the mouse hippocampus, of which one represents a distinct type with intense Dnmt3a-immunoreactivity (Dnmt3a type II cells) co-localizing with a marker of recent proliferation. Based on this, we hypothesize that behavioral susceptibility to chronic social defeat stress is linked to (i) Dnmt3a protein levels in the nucleus accumbens and hippocampus, and (ii) to the density of Dnmt3a type II cells in the hippocampal dentate gyrus. While no differences were found in global levels of Dnmt3a protein expression in the nucleus accumbens and hippocampus, our stereological quantifications indicated a significantly increased density of Dnmt3a type II cells in the dentate gyrus of animals resilient to social defeat stress compared to susceptible and control animals. Further characterization of the Dnmt3a type II cells revealed that these cells were mostly doublecortin (25%) or NeuN (60%) immunopositive, thus defining them as immature and mature neurons. Moreover, negative associations between the density of Dnmt3a type II cells and indices of depressive-like behavior in the sucrose intake and forced swim test were found. These correlational data suggest that DNA methylation via Dnmt3a in the hippocampus co-regulates adaptivity of the behavioral response to chronic social defeat stress, and set the stage for further experimental studies testing a mediating role of Dnmt3a in experience-dependent plasticity, neurogenesis and (mal) adaptation to severe stressors., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Depression during pregnancy and postpartum is a significant health problem and affects up to 20% of women. While selective serotonin reuptake inhibitor (SSRI) medications are the drug of choice for treatment of maternal depression, the combined effect of maternal depression and perinatal SSRI exposure on offspring development is poorly investigated. Our aim was to determine the role of exposure to fluoxetine during development on sexual behavior and sexually dimorphic brain structures in male offspring using a rodent model of maternal adversity. Sprague-Dawley rat dams were stressed during gestation and were chronically treated throughout lactation with either fluoxetine or vehicle beginning on postnatal day 1. Four groups of offspring were used: (1) Control+Vehicle, (2) Control+Fluoxetine, (3) Prenatal Stress+Vehicle, and (4) Prenatal Stress+Fluoxetine. We show here that developmental fluoxetine treatment decreases the anogenital distance in juvenile male offspring. In adult male offspring, maternal fluoxetine treatment results in a decrease in the number of intromissions, a longer latency to the first intromission, and a longer latency to the first ejaculation. Furthermore, developmental fluoxetine and/or prenatal stress decrease the area of the sexually dimorphic nucleus of the preoptic area (SDN-POA). Prenatal stress, but not exposure to developmental fluoxetine, decreases the number of tyrosine hydroxylase (TH)-positive cells in anteroventral periventricular nucleus (AVPv) and the volume of the posterior bed nucleus of the stria terminalis (pBST) in male offspring. These results provide important evidence for the long-term impact of maternal adversity and maternal fluoxetine use on the development of primary endocrinology systems in juvenile and adult male offspring., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Alzheimer's disease (AD) is characterized by progressive cognitive deficits and synaptic dysfunction. Over the last decade phosphodiesterase inhibitors (PDEIs) have received increasing attention as putative cognition enhancers and have been suggested as a novel treatment strategy for AD. Given their ability to prevent hydrolysis of cAMP and/or cGMP, they can stimulate the cAMP/protein kinase A (PKA)/cAMP element-binding protein (CREB) and cGMP/PKG/CREB pathway to enhance synaptic transmission by increasing CREB phosphorylation (pCREB) and brain-derived neurotrophic factor (BDNF) transcription. Based on previous research, we hypothesized that chronic PDE2I treatment would improve AD-related cognitive deficits, by decreasing amyloid-β (Aβ) plaque load, enhancing pCREB and BDNF levels and increasing synaptic density in the hippocampus of 8-month-old APPswe/PS1dE9 mice. Results indicated that chronic PDE2I treatment could indeed improve memory performance in APPswe/PS1dE9 mice, without affecting anxiety, depressive-like behavior or hypothalamus-pituitary-adrenal axis regulation. However, no treatment effects were observed on Aβ plaque load, pCREB or BDNF concentrations, or presynaptic density in the hippocampus, suggesting that other signaling pathways and/or effector molecules might be responsible for its cognition-enhancing effects. Presynaptic density in the stratum lucidum of the CA3 subregion was significantly higher in APPswe/PS1dE9 mice compared to WT controls, possibly reflecting a compensatory mechanism. In conclusion, PDEs in general, and PDE2 specifically, could be considered as promising therapeutic targets for cognition enhancement in AD, although the underlying mechanism of action remains to be elucidated. This article is part of a Special Issue entitled 'Cognitive Enhancers'., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The mutant ubiquitin UBB(+1) is a substrate as well as an inhibitor of the ubiquitin-proteasome system (UPS) and accumulates in the neuropathological hallmarks of Alzheimer's disease (AD). A role for the UPS has been suggested in the generation of amyloid β (Aβ) plaques in AD. To investigate the effect of UBB(+1) expression on amyloid pathology in vivo, we crossed UBB(+1) transgenic mice with a transgenic line expressing AD-associated mutant amyloid precursor protein (APPSwe) and mutant presenilin 1 (PS1dE9), resulting in APPPS1/UBB(+1) triple transgenic mice. In these mice, we determined the Aβ levels at 3, 6, 9 and 11 months of age. Surprisingly, we found a significant decrease in Aβ deposition in amyloid plaques and levels of soluble Aβ(42) in APPPS1/UBB(+1) transgenic mice compared to APPPS1 mice at 6 months of age, without alterations in UBB(+1) protein levels or proteasomal chymotrypsin activity. These lowering effects of UBB(+1) on Aβ deposition were transient, as this relative decrease in plaque load was not significant in APPPS1/UBB(+1) mice at 9 and 11 months of age. We also show that APPPS1/UBB(+1) mice exhibit astrogliosis, indicating that they may not be improved functionally compared to APPPS1 mice despite the Aβ reduction. The molecular mechanism underlying this decrease in Aβ deposition in APPPS1/UBB(+1) mice is more complex than previously assumed because UBB(+1) is also ubiquitinated at K63 opening the possibility of additional effects of UBB(+1) (e.g. kinase activation)., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Chronic stress and stress-related disorders, such as major depression (MD), have been shown to increase the risk for developing Alzheimer's disease (AD). Brain-derived neurotrophic factor (BDNF) has been postulated as a neurophysiological link between these illnesses. Our previous research has indicated that exposing the APPswe/PS1dE9 mouse model of AD to prenatal maternal stress (PS) induced a depressive-like phenotype, specifically in female mice. Considering the role of BDNF in depressive-like behavior and its interactions with amyloid-β (Aβ), our aim was to explore whether these mice would also exhibit alterations in soluble Aβ, mature BDNF (mBDNF), proBDNF, and the receptors TrkB and p75(NTR) in comparison to non-stressed animals. Our results demonstrate that female APPswe/PS1dE9 mice have higher levels of hippocampal proBDNF and soluble Aβ as compared to their male littermates. Additionally, a tendency was observed for PS to lower mBDNF protein levels in the hippocampus, but only in female mice, while receptor levels remained unaltered by sex or PS exposure. Given that female mice both have higher proBDNF and Aβ levels, these findings suggest an underlying role for BDNF signaling and Aβ production in the selective vulnerability of women for MD and AD development., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The interaction of brain-derived neurotrophic factor (BDNF) with its tropomyosin-related kinase receptor B (TrkB) is involved in fundamental cellular processes including neuronal proliferation, differentiation and survival as well as neurotransmitter release and synaptic plasticity. TrkB signaling has been widely associated with beneficial, trophic effects and many commonly used psychotropic drugs aim to increase BDNF levels in the brain. However, it is likely that a prolonged increased TrkB activation is observed in many pathological conditions, which may underlie the development and course of clinical symptoms. Interestingly, genetic and pharmacological studies aiming at decreasing TrkB activation in rodent models mimicking human pathology have demonstrated a promising therapeutic landscape for TrkB inhibitors in the treatment of various diseases, e.g. central nervous system (CNS) disorders and several types of cancer. Up to date, only a few selective and potent TrkB inhibitors have been developed. As such, the use of crystallography and in silico approaches to model BDNF-TrkB interaction and to generate relevant pharmacophores represent powerful tools to develop novel compounds targeting the TrkB receptor., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is the neurosurgical therapy of choice for the management of motor deficits in patients with advanced Parkinson's disease, but this treatment can elicit disabling mood changes. Our recent experiments show that in rats, HFS of the STN both inhibits the firing of 5-HT (5-hydroxytryptamine; serotonin) neurons in the dorsal raphe nucleus (DRN) and elicits 5-HT-dependent behavioral effects. The neural circuitry underpinning these effects is unknown. Here we investigated in the dopamine-denervated rat the effect of bilateral HFS of the STN on markers of neuronal activity in the DRN as well as DRN input regions. Controls were sham-stimulated rats. HFS of the STN elicited changes in two 5-HT-sensitive behavioral tests. Specifically, HFS increased immobility in the forced swim test and increased interaction in a social interaction task. HFS of the STN at the same stimulation parameters, increased c-fos immunoreactivity in the DRN, and decreased cytochrome C oxidase activity in this region. The increase in c-fos immunoreactivity occurred in DRN neurons immunopositive for the GABA marker parvalbumin. HFS of the STN also increased the number of c-fos immunoreactive cells in the lateral habenula nucleus, medial prefrontal cortex but not significantly in the substantia nigra. Collectively, these findings support a role for circuitry involving DRN GABA neurons, as well as DRN afferents from the lateral habenula nucleus and medial prefrontal cortex, in the mood effects of HFS of the STN., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Pre-clinical and clinical studies on dimebon (dimebolin or latrepirdine) have demonstrated its use as a cognitive enhancer. Here, we show that dimebon administered to 3-month-old C57BL6N mice 15 min prior to training in both appetitive and inhibitory learning tasks via repeated (0.1 mg/kg) and acute (0.5 mg/kg) i.p. injections, respectively, increases memory scores. Acute treatment with dimebon was found to enhance inhibitory learning, as also shown in the step-down avoidance paradigm in 7-month-old mice. Bolus administration of dimebon did not affect the animals' locomotion, exploration or anxiety-like behaviour, with the exception of exploratory behaviour in older mice in the novel cage test. In a model of appetitive learning, a spatial version of the Y-maze, dimebon increased the rate of correct choices and decreased the latency of accessing a water reward after water deprivation, and increased the duration of drinking behaviour during training/testing procedures. Repeated treatment with dimebon did not alter the behaviours in other tests or water consumption. Acute treatment of water-deprived and non-water-deprived mice with dimebon also did not affect their water intake. Our data suggest that dimebon enhances hippocampus-dependent learning in both appetitive and inhibitory tasks in mice., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The multifaceted concept of impulsivity implies that different impulsivity aspects, mediated by different neural processes, influence behavior at different levels. The nucleus accumbens (NAc) is a key component of the neural processes regulating impulsivity. In this review, we discuss the findings of lesion studies in animals and functional imaging studies in humans focusing on the role of the NAc in impulsivity. Evidence supports that the extent and pattern of involvement of the NAc, and its subregions, the core and the shell, vary among different facets of impulsivity. Data from imaging studies reviewed in this article suggest the involvement of the ventral striatum/NAc in impulsive choice. Findings of animal studies indicate that lesions of the NAc core subregion facilitated impulsivity in tasks involving intertemporal choice, and promoted a risk-averse, less impulsive, tendency in tasks involving options with probability differences. Modification of neurotransmitter activity, especially of dopamine, which is proposed to underlie the changes observed in functional imaging studies, has been shown to influence afferent input pattern in the NAc and the generation of the behavioral output. Parameters of behavioral tasks reflecting response inhibition function are altered by neurochemical interventions and local electrical stimulation in both the core and the shell subregions. In toto, NAc's pattern of neuronal activity, either genetically determined or acquired, has a critical impact on the interindividual variation in the expression of impulsivity. Nevertheless, the NAc is not the only substrate responsible for impulsivity and it is not involved in each facet of impulsivity to the same extent., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Electroporation is a widely used technique for enhancing the efficiency of DNA delivery into cells. Application of electric pulses after local injection of DNA temporarily opens cell membranes and facilitates DNA uptake. Delivery of plasmid DNA by electroporation to alter gene expression in tissue has also been explored in vivo. This approach may constitute an alternative to viral gene transfer, or to transgenic or knock-out animals. Among the most frequently electroporated target tissues are skin, muscle, eye, and tumors. Moreover, different regions in the central nervous system (CNS), including the developing neural tube and the spinal cord, as well as prenatal and postnatal brain have been successfully electroporated. Here, we present a comprehensive review of the literature describing electroporation of the CNS with a focus on the adult brain. In addition, the mechanism of electroporation, different ways of delivering the electric pulses, and the risk of damaging the target tissue are highlighted. Electroporation has been successfully used in humans to enhance gene transfer in vaccination or cancer therapy with several clinical trials currently ongoing. Improving the knowledge about in vivo electroporation will pave the way for electroporation-enhanced gene therapy to treat brain carcinomas, as well as CNS disorders such as Alzheimer's disease, Parkinson's disease, and depression., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The central cholinergic system is involved in several cognitive functions such as attention, consciousness, learning and memory. Functional imaging of this neurotransmitter system may provide novel opportunities in the diagnosis and evaluation of cognitive disorders. The aim of this study was to investigate the spatial and temporal activation patterns of muscarinic acetylcholine receptor (mAChR) stimulation in rat brain with pharmacological magnetic resonance imaging (phMRI). We performed blood oxygenation level-dependent (BOLD) MRI and contrast-enhanced cerebral blood volume (CBV)-weighted MRI combined with injection of pilocarpine, a non-selective mAChR agonist. BOLD and CBV responses were assessed after pretreatment with methyl-scopolamine in order to block peripheral muscarinic effects. Region-of-interest analysis in individual animals and group-level independent component analysis failed to show significant BOLD signal changes following pilocarpine injection. However, with contrast-enhanced CBV-weighted MRI, positive CBV responses were detected in the cerebral cortex, thalamus, and hippocampus whereas a negative CBV response was observed in the striatum. Thus, pilocarpine-induced significant activation responses in brain regions that are known to have a high density of muscarinic receptors. Our study demonstrates that phMRI of mAChR stimulation in rats allows functional assessment of the cholinergic system in vivo., (Copyright 2010 Elsevier Ltd. All rights reserved.)

With the aging of the population, the growing incidence and prevalence of Alzheimer's disease (AD) increases the burden on individuals and society as a whole. To date, the pathophysiology of AD is not yet fully understood. Recent studies have suggested that epigenetic mechanisms may play a pivotal role in its course and development. The most frequently studied epigenetic mechanisms are DNA methylation and histone modifications, and investigations relevant to aging and AD are presented in this review. Various studies on human postmortem brain samples and peripheral leukocytes, as well as transgenic animal models and cell culture studies relevant to AD will be discussed. From those, it is clear that aging and AD are associated with epigenetic dysregulation at various levels. Moreover, data on e.g. twin studies in AD support the notion that epigenetic mechanisms mediate the risk for AD. Conversely, it is still not fully clear whether the observed epigenetic changes actually represent a cause or a consequence of the disease. This is mainly due to the fact that most clinical investigations on epigenetics in AD are conducted in samples of patients already in an advanced stage of the disease. Evidently, more research is needed in order to clarify the exact role of epigenetic regulation in the course and development of AD. Research on earlier stages of the disease could provide more insight into its underlying pathophysiology, possibly contributing to the establishment of early diagnosis and the development of more effective treatment strategies., (Copyright 2010 Elsevier Ltd. All rights reserved.)

Many studies with chronic stress, a common depression paradigm, lead to inconsistent behavioral results. We are introducing a new model of stress-induced anhedonia, which provides more reproducible induction and behavioral measuring of depressive-like phenotype in mice. First, a 4-week stress procedure induces anhedonia, defined by decreased sucrose preference, in the majority of but not all C57BL/6 mice. The remaining 30-50% non-anhedonic animals are used as an internal control for stress effects that are unrelated to anhedonia. Next, a modified sucrose test enables the detection of inter-individual differences in mice. Moreover, testing under dimmed lighting precludes behavioral artifacts caused by hyperlocomotion, a major confounding factor in stressed mice. Finally, moderation of the stress load increases the reproducibility of anhedonia induction, which otherwise is difficult to provide because of inter-batch variability in laboratory mice. We believe that our new mouse model overcomes some major difficulties in measuring behavior with chronic stress depression models., (Copyright 2009 Elsevier Inc. All rights reserved.)

Acute tryptophan depletion is extensively used to investigate the implication of serotonin in the onset of depressive disorders. In rats, it lowers peripheral tryptophan and decreases central serotonin concentrations. We aimed to establish the rat model of acute tryptophan depletion in the mouse for potential application as serotonin challenge tool in genetic mouse models of depression. Pharmacokinetic and behavioural effects of a tryptophan-free diet were examined in Swiss and C57BL/6 mice. Peripheral amino acids were measured and central tryptophan and serotonin concentrations were compared with anxiety and depression-like behaviour in the elevated zero-maze, forced swimming test or tail suspension test. While acute tryptophan depletion resulted in a 74% reduction of the plasma ratio tryptophan to the sum of other large neutral amino acids in Swiss mice 1h after administration (2x10 ml/kg, 30 min interval), there was only a 40% reduction in C57BL/6 mice. The latter did not show anxiety in the elevated zero-maze or increased immobility in the forced swimming test or tail suspension test. A higher dose (2x20 ml/kg) with a longer interval (60 min) reduced the ratio with 68% in C57BL/6 mice, lowered hippocampal serotonin turnover and had no functional effect when tested in the elevated zero-maze and forced swimming test. These findings have important implications for the use of acute tryptophan depletion in general and in particular for its application in mice. Although in healthy mice no clear central serotonin or functional effects were observed, further research is indicated using mice with pre-existing serotonin dysfunction, as they might be more vulnerable to acute tryptophan depletion., (2009 Elsevier Ltd. All rights reserved.)

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

Once it was believed that the subthalamic nucleus (STN) was no more than a relay station serving as a "gate" for ascending basal ganglia-thalamocortical circuits. Nowadays, the STN is considered to be one of the main regulators of motor function related to the basal ganglia. The role of the STN in the regulation of associative and limbic functions related to the basal ganglia has generally received little attention. In the present review, the functional role of the STN in the control of cortico-basal ganglia-thalamocortical associative and limbic circuits is discussed. In the past 20 years the concepts about the functional role of the STN have changed dramatically: from being an inhibitory nucleus to a potent excitatory nucleus, and from being involved in hyperkinesias to hypokinesias. However, it has been demonstrated only recently, mainly by reports on the behavioral (side-) effects of STN deep brain stimulation (DBS), which is a popular surgical technique in the treatment of patients suffering from advanced Parkinson Disease (PD), that the STN is clinically involved in associative and limbic functions. These findings were confirmed by results from animal studies. Experimental studies applying STN DBS or STN lesions to investigate the neuronal mechanisms involved in these procedures found profound effects on cognitive and motivational parameters. The anatomical, electrophysiological and behavioral data presented in this review point towards a potent regulatory function of the STN in the processing of associative and limbic information towards cortical and subcortical regions. In conclusion, it can be stated that the STN has anatomically a central position within the basal ganglia thalamocortical associative and limbic circuits and is functionally a potent regulator of these pathways.

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