Phosphodiesterase 5 inhibition improves synaptic function, memory, and amyloid-beta load in an Alzheimer's disease mouse model

Behavioral assays with mouse models of Alzheimer's disease: practical considerations and guidelines

In Alzheimer's disease (AD) basic research and drug discovery, mouse models are essential resources for uncovering biological mechanisms, validating molecular targets and screening potential compounds. Both transgenic and non-genetically modified mouse models enable access to different types of AD-like pathology in vivo. Although there is a wealth of genetic and biochemical studies on proposed AD pathogenic pathways, as a disease that centrally features cognitive failure, the ultimate readout for any interventions should be measures of learning and memory. This is particularly important given the lack of knowledge on disease etiology - assessment by cognitive assays offers the advantage of targeting relevant memory systems without requiring assumptions about pathogenesis. A multitude of behavioral assays are available for assessing cognitive functioning in mouse models, including ones specific for hippocampal-dependent learning and memory. Here we review the basics of available transgenic and non-transgenic AD mouse models and detail three well-established behavioral tasks commonly used for testing hippocampal-dependent cognition in mice - contextual fear conditioning, radial arm water maze and Morris water maze. In particular, we discuss the practical considerations, requirements and caveats of these behavioral testing paradigms.

Erectile dysfunction in the elderly: an old widespread issue with novel treatment perspectives

Erectile dysfunction (ED) is one of the most common chronic diseases affecting men and its prevalence increases with aging. It is also the most frequently diagnosed sexual dysfunction in the older male population. A number of different diseases potentially worsening sexual function may occur in elderly people, together with polypharmacy. Related causes of ED are variable and can include arterial, neurogenic, hormonal, cavernosal, iatrogenic, and psychogenic causes. The aim of the present review was to examine the main aspects of erectile dysfunction going through epidemiology and pathophysiology and revise most of ED in elderly disabled men and in those affected with psychiatric disorders. Lastly we tried to focus on the main aspects of nonpharmacological and pharmacological treatments of ED and the recreational use in the elderly. Phosphodiesterase-5 inhibitors (PDE5-I) are commonly used for on-demand or chronic treatment of ED. It is widely known that PDE5-I have lower response rates in older men than in younger patients, but they have the advantages of ease of use and excellent safety profile, also in the elderly. The old and new PDE5-I as well as the alternative treatments for ED are extensively discussed.

Modulating nitric oxide signaling in the CNS for Alzheimer's disease therapy

Nitric oxide (NO)/solube GC (sGC)/cGMP signaling is important for modulating synaptic transmission and plasticity in the hippocampus and cerebral cortex, which are critical for learning and memory. Physiological concentrations of NO also elicit anti-apoptotic/prosurvival effects against various neurotoxic challenges and brain insults through multiple mechanisms. Depression of the NO/sGC pathway is a feature of Alzheimer's disease (AD), attributed to amyloid-β neuropathology, and altered expression and activity of NOS, sGC and PDE enzymes. Different classes of NO-releasing hybrid drugs, including nomethiazoles, NO-NSAIDs and NO-acetylcholinesterase inhibitors were designed to deliver low concentrations of exogenous NO to the CNS while targeting other underlying disease mechanisms, such as excitotoxicity, neuro-inflammation and acetylcholine deficiency, respectively. Incorporating a NO-donating moiety may also reduce gastrointestinal and liver toxicity of the parent drugs. Progress has also been made in targeting downstream sGC and PDE enzymes. The PDE9 inhibitor PF-04447943 has completed Phase II clinical trials for AD. The search for effective NO-donating hybrid drugs, CNS-targeting sGC stimulators/activators and selective PDE inhibitors is an important goal for pharmacotherapy that manipulates NO biochemical pathways involved in cognitive function and neuroprotection. Rigorous preclinical validation of target engagement, and optimization of pharmacokinetic and toxicity profiles are likely to advance more drug candidates into clinical trials for mild cognitive impairment and early stage AD.

Sildenafil mediates blood-flow redistribution and neuroprotection after neonatal hypoxia-ischemia

BACKGROUND AND PURPOSE

The best conceivable treatment for hypoxia-ischemia (HI) is the restoration of blood flow to the hypoxic-ischemic region(s). Our objective was to examine whether boosting NO-cGMP signaling using sildenafil citrate, a phosphodiesterase-type 5 inhibitor, could modify cerebral blood flow and reduce lesions in the developing brain.

METHODS

HI was induced in P7 Sprague-Dawley rats by unilateral carotid artery occlusion and hypoxia, and followed by either PBS or sildenafil. Blood-flow velocities were measured by ultrasound imaging with sequential Doppler recordings to evaluate collateral recruitment. Cell death, blood-brain barrier integrity, and glial activation were analyzed by immunohistochemistry. Motor behavior was evaluated using an open-field device adapted to neonatal animals.

RESULTS

Sildenafil citrate (10 mg/kg) induced collateral patency, reduced terminal dUTP nick-end labeling-positive cells, reactive astrogliosis, and macrophage/microglial activation at 72 hours and 7 days post-HI. Sildenafil also reduced the number of terminal dUTP nick-end labeling-positive endothelial cells within lesion site. Seven days after HI and sildenafil treatment, tissue loss was significantly reduced, and animals recovered motor coordination.

CONCLUSIONS

Our findings strongly indicate that sildenafil citrate treatment, associated with a significant increase in cerebral blood flow, reduces HI damage and improves motor locomotion in neonatal rats. Sildenafil may represent an interesting therapeutic strategy for neonatal neuroprotection.

Effect of sildenafil, a selective phosphodiesterase 5 inhibitor, on the anticonvulsant action of some antiepileptic drugs in the mouse 6-Hz psychomotor seizure model

Sildenafil, a selective phosphodiesterase 5 inhibitor (PDE5), has been recently reported to have both pro- and anticonvulsant action in various experimental models of seizures and epilepsy. Furthermore, it affects anticonvulsant action of some antiepileptic drugs (AEDs) in mice seizure tests and both pharmacodynamic and pharmacokinetic interactions were noted. The present study was carried out to investigate influence of sildenafil on the threshold for 6 Hz-induced psychomotor seizures in mice. Effect of sildenafil on activity of some AEDs, i.e., phenobarbital (PB), clonazepam (CZP), ethosuximide (ETS), valproic acid (VPA), tiagabine (TGB), oxcarbazepine (OXC) and levetiracetam (LEV), in 6 Hz test was also examined. Moreover, combination of sildenafil with LEV was investigated in terms of influence on motor coordination (determined by the chimney test), muscular strength (evaluated in the grip-strength test) and long-term memory (assessed in the passive avoidance task) in mice. To determine the type of pharmacological interaction between sildenafil and LEV, free plasma and total brain concentrations of this AED were determined by LC-MS/MS method. Sildenafil at a dose ranging from 10 to 40 mg/kg statistically increased psychomotor seizure threshold in mice. Moreover, sildenafil enhanced the anticonvulsant action of all the studied AEDs in this test. Interactions between this PDE5 inhibitor and PB, CZP, ETS, TGB and OXC seem to be pharmacodynamic. Since sildenafil increased free plasma and total brain concentration of LEV, interactions between these drugs have pharmacokinetic nature. This kind of interaction was also noted between sildenafil and VPA. Neither LEV (2.32 mg/kg) nor its co-administration with sildenafil (40 mg/kg) produced any significant changes in motor coordination, muscular strength and long-term memory in mice.

Design of novel β-carboline derivatives with pendant 5-bromothienyl and their evaluation as phosphodiesterase-5 inhibitors

New derivatives with the tetrahydro-β-carboline-imidazolidinedione and tetrahydro-β-carboline-piperazinedione scaffolds and a pendant bromothienyl moiety at C-5/C-6 were synthesized and tested for their ability to inhibit PDE5 in vitro. The following SAR can be concluded: The tetracyclic scaffold is essential for PDE5 inhibition; the ethyl group is the most suitable among the adopted N-substituents on the terminal ring (hydantoin/piperazinedione); the appropriate stereochemistry of C-5/C-6 derived from the aldehyde rather than C-11a/C-12a derived from tryptophan appears crucial for inhibition of PDE5; surprisingly, derivatives with the hydantoin terminal ring are more active than their analogs with the piperazinedione ring; the selectivity versus PDE5 relative to PDE11 with cGMP as a substrate is mainly a function of the substitution and stereochemistry pattern of the external ring, in other words of the interaction with the H-loop residues of the isozymes. Thirteen derivatives showed PDE5 inhibitory activity with IC(50) values in the range of 0.16-5.4 µm. Compound 8 was the most potent PDE5 inhibitor and showed selectivity towards PDE5 versus other PDEs, with a selectivity index of 49 towards PDE5 rather than PDE11 with cGMP as the substrate.

New perspectives in cyclic nucleotide-mediated functions in the CNS: the emerging role of cyclic nucleotide-gated (CNG) channels

Cyclic nucleotides play fundamental roles in the central nervous system (CNS) under both physiological and pathological conditions. The impact of cAMP and cGMP signaling on neuronal and glial cell functions has been thoroughly characterized. Most of their effects have been related to cyclic nucleotide-dependent protein kinase activity. However, cyclic nucleotide-gated (CNG) channels, first described as key mediators of sensory transduction in retinal and olfactory receptors, have been receiving increasing attention as possible targets of cyclic nucleotides in the CNS. In the last 15 years, consistent evidence has emerged for their expression in neurons and astrocytes of the rodent brain. Far less is known, however, about the functional role of CNG channels in these cells, although several of their features, such as Ca(2+) permeability and prolonged activation in the presence of cyclic nucleotides, make them ideal candidates for mediators of physiological functions in the CNS. Here, we review literature suggesting the involvement of CNG channels in a number of CNS cellular functions (e.g., regulation of membrane potential, neuronal excitability, and neurotransmitter release) as well as in more complex phenomena, like brain plasticity, adult neurogenesis, and pain sensitivity. The emerging picture is that functional and dysfunctional cyclic nucleotide signaling in the CNS has to be reconsidered including CNG channels among possible targets. However, concerted efforts and multidisciplinary approaches are still needed to get more in-depth knowledge in this field.

Effect of phosphodiesterase-5 inhibition on apoptosis and beta amyloid load in aged mice

Age-related cognitive decline is accompanied by an increase of neuronal apoptosis and a dysregulation of neuroplasticity-related molecules such as brain-derived neurotrophic factor and neurotoxic factors including beta amyloid (Aβ) peptide. Because it has been previously demonstrated that phosphodiesterase-5 inhibitors (PDE5-Is) protect against hippocampal synaptic dysfunction and memory deficits in mouse models of Alzheimer's disease and physiological aging, we investigated the effect of a treatment with the PDE5-I, sildenafil, on cell death, pro- and antiapoptotic molecules, and Aβ production. We demonstrated that chronic intraperitoneal injection of sildenafil (3 mg/kg for 3 weeks) decreased terminal deoxyuridine triphosphate nick end labeling-positive cells in the CA1 hippocampal area of 26-30-month-old mice, downregulating the proapoptotic proteins, caspase-3 and B-cell lymphoma 2-associated X, and increasing antiapoptotic molecules such as B-cell lymphoma protein-2 and brain-derived neurotrophic factor. Also, sildenafil reverted the shifting of amyloid precursor protein processing toward Aβ42 production and the increase of the Aβ42:Aβ40 ratio in aged mice. Our data suggest that PDE5-I might be beneficial to treat age-related detrimental features in a physiological mouse model of aging.

Supplementation of nitric oxide attenuates AβPP and BACE1 protein in cerebral microcirculation of eNOS-deficient mice

Recently, we demonstrated in endothelial nitric oxide synthase deficient (eNOS-/-) mice that loss of endothelial NO led to increased protein levels of amyloid-β protein precursor (AβPP), β-site AβPP cleaving enzyme 1 (BACE1), and amyloid-β (Aβ) peptide. Therefore, our aim was to determine if NO supplementation in vivo would attenuate AβPP and BACE1 protein levels. cGMP levels were significantly increased while AβPP and BACE1 protein levels were statistically lower in cerebral microvessels from nitroglycerin-treated eNOS-/- mice as compared to vehicle-treated mice. Our findings support the concept that preservation of NO/cGMP signaling is an important modulator of expression and processing of AβPP.

Regulation of hippocampal cGMP levels as a candidate to treat cognitive deficits in Huntington's disease

Huntington’s disease (HD) patients and mouse models show learning and memory impairment associated with hippocampal dysfunction. The neuronal nitric oxide synthase/3',5'-cyclic guanosine monophosphate (nNOS/cGMP) pathway is implicated in synaptic plasticity, and in learning and memory processes. Here, we examined the nNOS/cGMP pathway in the hippocampus of HD mice to determine whether it can be a good therapeutic target for cognitive improvement in HD. We analyzed hippocampal nNOS and phosphodiesterase (PDE) 5 and 9 levels in R6/1 mice, and cGMP levels in the hippocampus of R6/1, R6/2 and Hdh^Q7/Q111 mice, and of HD patients. We also investigated whether sildenafil, a PDE5 inhibitor, could improve cognitive deficits in R6/1 mice. We found that hippocampal cGMP levels were 3-fold lower in 12-week-old R6/1 mice, when they show deficits in object recognition memory and in passive avoidance learning. Consistent with hippocampal cGMP levels, nNOS levels were down-regulated, while there were no changes in the levels of PDE5 and PDE9 in R6/1 mice. A single intraperitoneal injection of sildenafil (3 mg/Kg) immediately after training increased cGMP levels, and improved memory in R6/1 mice, as assessed by using the novel object recognition and the passive avoidance test. Importantly, cGMP levels were also reduced in R6/2 mouse and human HD hippocampus. Therefore, the regulation of hippocampal cGMP levels can be a suitable treatment for cognitive impairment in HD.

Involvement of nNOS/NO/sGC/cGMP signaling pathway in cocaine sensitization and in the associated hippocampal alterations: does phosphodiesterase 5 inhibition help to drug vulnerability?

RATIONALE

Repeated cocaine administration induces behavioral sensitization in about 50 % of treated animals. Nitric oxide could be involved in the acquisition and maintenance of behavioral cocaine effects, probably by activation of neuronal nitric oxide synthase (nNOS)/NO/soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP) signaling pathway, since inhibition of the nNOS enzyme attenuates development of sensitization in rats. On the other hand, increased cGMP availability by phosphodiesterase 5 inhibitors has been correlated to the misuse and recreational use of these agents and also to the concomitant use with illicit drugs in humans. Hippocampus is an important brain region for conditioning to general context previously associated to drug availability, influencing drug-seeking behavior and sensitization. Moreover, cocaine and other drugs of abuse can affect the strength of glutamate synapses in this structure, lastly modifying neuronal activity in main regions of the reward circuitry.

OBJECTIVE

The objective of this study is to determine whether the pharmacological manipulation of nNOS/NO/sGC/cGMP signaling pathway altered changes induced by repeated cocaine exposure.

RESULTS

The present investigation showed a relationship between behavioral cocaine sensitization, reduced threshold to generate long-term potentiation (LTP) in hippocampal dentate gyrus, and increased nNOS activity in this structure. However, when nNOS or sGC were inhibited, the number of sensitized animals was reduced, and the threshold to generate LTP was increased. The opposite occurred when cGMP availability was increased.

CONCLUSION

We demonstrate a key role of the nNOS activity and NO/sGC/cGMP signaling pathway in the development of cocaine sensitization and in the associated enhancement of hippocampal synaptic transmission.

Neuroprotection by sildenafil: neuronal networks potentiation in acute experimental stroke

AIMS

Sildenafil, a phosphodiesterase type 5 inhibitor, has been found to produce functional recovery in ischemic rats by increasing the cGMP level and triggering neurogenesis. The aim of this study was to investigate further sildenafil mechanisms.

METHODS

Male Sprague-Dawley rats underwent middle cerebral artery occlusion and reperfusion, followed by intraperitoneal or intravenous treatment of sildenafil starting 2 h later. Behavioral tests were performed on day 1 or day 7 after reperfusion, while cerebral infarction, edema, Nissl staining, Fluoro-Jade B staining, and electron microscopy studies were carried out 24 h poststroke. The cGMP-dependent Nogo-66 receptor (Nogo-R) pathway, synaptophysin, PSD-95/neuronal nitric oxide synthases (nNOS), brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB), and nerve growth factor (NGF)/tropomyosin-related kinase A (TrkA) were measured.

RESULTS

Sildenafil enhanced neurological recovery and inhibited infarction, even following delayed administration 4 h after stroke onset. Furthermore, sildenafil reduced the loss of neurons and modulated the expressions of the cGMP-dependent Nogo-R pathway. Moreover, sildenafil protected the structure of synapses and mediated the expressions of synaptophysin, PSD-95/nNOS, BDNF/TrkB, and NGF/TrkA.

CONCLUSIONS

Sildenafil produces significant neuroprotective effects on injured neurons in acute stroke, and these are mediated by the cGMP-dependent Nogo-R pathway, NGF/TrkA, and BDNF/TrkB.

Therapeutics of Alzheimer's disease: Past, present and future

Alzheimer's disease (AD) is the most common cause of dementia worldwide. The etiology is multifactorial, and pathophysiology of the disease is complex. Data indicate an exponential rise in the number of cases of AD, emphasizing the need for developing an effective treatment. AD also imposes tremendous emotional and financial burden to the patient's family and community. The disease has been studied over a century, but acetylcholinesterase inhibitors and memantine are the only drugs currently approved for its management. These drugs provide symptomatic improvement alone but do less to modify the disease process. The extensive insight into the molecular and cellular pathomechanism in AD over the past few decades has provided us significant progress in the understanding of the disease. A number of novel strategies that seek to modify the disease process have been developed. The major developments in this direction are the amyloid and tau based therapeutics, which could hold the key to treatment of AD in the near future. Several putative drugs have been thoroughly investigated in preclinical studies, but many of them have failed to produce results in the clinical scenario; therefore it is only prudent that lessons be learnt from the past mistakes. The current rationales and targets evaluated for therapeutic benefit in AD are reviewed in this article. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Phosphodiesterases: Regulators of cyclic nucleotide signals and novel molecular target for movement disorders

Movement disorders rank among the most common neurological disorders. During the last two decades substantial progress has been made in understanding of the pathological basis of these disorders. Although, several mechanisms have been proposed, downregulation of cyclic nucleotide mediated signaling cascade has consistently been shown to contribute to the striatal dysfunctioning as seen in movement disorders. Thus, counteracting dysregulated cyclic nucleotide signaling has been considered to be beneficial in movement disorders. Cyclic nucleotide phosphodiesterases (PDEs) are the enzymes responsible for the breakdown of cyclic nucleotides and upregulation in PDE activity has been reported in various movement disorders. Thus, PDE inhibition is considered to be a novel strategy to restore cerebral cyclic nucleotide levels and their downstream signalling cascade. Indeed, various PDE inhibitors have been tested pre-clinically and were reported to be neuroprotective in various neurodegenerative disorders associated with movement disabilities. In this review, we have discussed a putative role of PDE inhibitors in movement disorders and associated abnormalities.

Neuroprotective role of PDE4 and PDE5 inhibitors in 3-nitropropionic acid induced behavioral and biochemical toxicities in rats

Phosphodiesterase inhibitors have been reported to be beneficial in cognitive and motor disorders. In the present study, we have investigated the effects of RO 20-1724 (PDE4 inhibitor) and sildenafil (PDE5 inhibitor) in 3-nitropropionic acid (3-NP) induced experimental Huntington's disease in rats. 3-Nitropropionic acid was administered for 14 days (10 mg/kg i.p.) 1h following 3-NP administration, the rats were treated with either vehicle, RO 20-1724 (0.25 and 0.5 mg/kg i.p.) or sildenafil (2 and 4 mg/kg i.p.) for 14 days. Cognitive functions were assessed by using Morris water maze whereas, motor functions were assessed by spontaneous locomotor activity, limb withdrawal and suspended wire test at different time points. Biochemically, markers of oxidative stress and cell damage, such as reduced glutathione, malondialdehyde, nitrite and lactate dehydrogenase levels were assessed terminally in the brain homogenate. Chronic administration of 3-NP produced significant decrease in body weight, showed marked abnormalities in cognitive and motor functions. Further, significant oxidative-nitrosative stress and cell damage was also observed. Chronic administration of RO 20-1724 and sildenafil in 3-NP treated rats significantly and dose dependently attenuated 3-NP induced behavioral and biochemical abnormalities in rats. Both these drugs were equally effective in attenuating 3-NP induced neurotoxicity. These results suggesting that the inhibition of PDE4 and PDE5 would be therapeutic in neurodegenerative disorders associated with cognitive and motor dysfunction.

The effects of the phosphodiesterase type 5 inhibitor vardenafil on cognitive performance in healthy adults: a behavioral-electroencephalography study

Phosphodiesterase type 5 inhibitors (PDE5-Is) improve cognitive performance of rodents, but the few human studies investigating their effects did not systematically investigate cognitive effects and the results have been quite contradictory. Therefore, we examined whether the PDE5-I vardenafil improves memory and executive functioning and affect electroencephalography (EEG) in healthy young adults. Participants were selected out of a group of volunteers, based on their performance on a memory screening and they were orally treated with vardenafil (10-20 mg or placebo). Memory and executive functioning were tested while EEG activity was recorded. Additionally, a simple reaction time task and questionnaires addressing various complaints were presented. No prominent effects of vardenafil on cognition were found: participants only made more mistakes on a reaction time task after 20 mg vardenafil. During encoding of words, the P300 was generally smaller after vardenafil treatment. Furthermore, the N400 was larger after vardenafil 10 mg than placebo treatment in a spatial memory task at Fz. Finally, headache and feeling weak were reported more after vardenafil treatment. Vardenafil did not affect cognitive performance of healthy adults and showed only some incidental effects on ERPs. These findings in humans do not corroborate the cognition-enhancing effects of PDE5-Is in healthy animals.

Endothelial nitric oxide deficiency promotes Alzheimer's disease pathology

Aging and the presence of cerebrovascular disease are associated with increased incidence of Alzheimer's disease. A common feature of aging and cerebrovascular disease is decreased endothelial nitric oxide (NO). We studied the effect of a loss of endothelium derived NO on amyloid precursor protein (APP) related phenotype in late middle aged (LMA) (14-15 month) endothelial nitric oxide synthase deficient (eNOS(-/-) ) mice. APP, β-site APP cleaving enzyme (BACE) 1, and amyloid beta (Aβ) levels were significantly higher in the brains of LMA eNOS(-/-) mice as compared with LMA wild-type controls. APP and Aβ1-40 were increased in hippocampal tissue of eNOS(-/-) mice as compared with wild-type mice. LMA eNOS(-/-) mice displayed an increased inflammatory phenotype as compared with LMA wild-type mice. Importantly, LMA eNOS(-/-) mice performed worse in a radial arm maze test of spatial learning and memory as compared with LMA wild-type mice. These data suggest that chronic loss of endothelial NO may be an important contributor to both Aβ related pathology and cognitive decline. Cardiovascular risk factors are associated with increased incidence of Alzheimer's disease (AD). A common feature of these risk factors is decreased endothelial nitric oxide (NO). We observed, in mice deficient in endothelial nitric oxide synthase, increased amyloid precursor protein (APP), β-site APP cleaving enzyme 1, amyloid beta levels, microglial activation, and impaired spatial memory. This suggests chronic loss of endothelial NO may be an important contributor to the pathogenesis of sporadic AD.

Anandamide-CB1 receptor signaling contributes to postnatal ethanol-induced neonatal neurodegeneration, adult synaptic, and memory deficits

The transient exposure of immature rodents to ethanol during postnatal day 7 (P7), which is comparable with the third trimester in human pregnancy, induces synaptic dysfunctions. However, the molecular mechanisms underlying these dysfunctions are still poorly understood. Although the endocannabinoid system has been shown to be an important modulator of ethanol sensitivity in adult mice, its potential role in synaptic dysfunctions in mice exposed to ethanol during early brain development is not examined. In this study, we investigated the potential role of endocannabinoids and the cannabinoid receptor type 1 (CB1R) in neonatal neurodegeneration and adult synaptic dysfunctions in mice exposed to ethanol at P7. Ethanol treatment at P7, which induces neurodegeneration, increased anandamide (AEA) but not 2-arachidonylglycerol biosynthesis and CB1R protein expression in the hippocampus and cortex, two brain areas that are important for memory formation and storage, respectively. N-Arachidonoyl phosphatidylethanolamine-phospholipase D (NAPE-PLD), glycerophosphodiesterase (GDE1), and CB1R protein expression were enhanced by transcriptional activation of the genes encoding NAPE-PLD, GDE1, and CB1R proteins, respectively. In addition, ethanol inhibited ERK1/2 and AKT phosphorylation. The blockade of CB1Rs before ethanol treatment at P7 relieved ERK1/2 but not AKT phosphorylation and prevented neurodegeneration. CB1R knock-out mice exhibited no ethanol-induced neurodegeneration and inhibition of ERK1/2 phosphorylation. The protective effects of CB1R blockade through pharmacological or genetic deletion resulted in normal adult synaptic plasticity and novel object recognition memory in mice exposed to ethanol at P7. The AEA/CB1R/pERK1/2 signaling pathway may be directly responsible for the synaptic and memory deficits associated with fetal alcohol spectrum disorders.

Selective inhibition of phosphodiesterase 5 enhances glutamatergic synaptic plasticity and memory in mice

Phosphodiesterases (PDEs) belong to a family of proteins that control metabolism of cyclic nucleotides. Targeting PDE5, for enhancing cellular function, is one of the therapeutic strategies for male erectile dysfunction. We have investigated whether in vivo inhibition of PDE5, which is expressed in several brain regions, will enhance memory and synaptic transmission in the hippocampus of healthy mice. We have found that acute administration of sildenafil, a specific PDE5 inhibitor, enhanced hippocampus-dependent memory tasks. To elucidate the underlying mechanism in the memory enhancement, effects of sildenafil on long-term potentiation (LTP) were measured. The level of LTP was significantly elevated, with concomitant increases in basal synaptic transmission, in mice treated with sildenafil (1 mg/kg/day) for 15 days compared to control mice. These results suggest that moderate PDE5 inhibition enhances memory by increasing synaptic plasticity and transmission in the hippocampus.

Phosphodiesterase-5 inhibitor sildenafil prevents neuroinflammation, lowers beta-amyloid levels and improves cognitive performance in APP/PS1 transgenic mice

Memory deficit is a marker of Alzheimer's disease (AD) that has been highly associated with the dysfunction of cyclic GMP (cGMP) signaling and an ongoing inflammatory process. Phosphodiesterase-5 (PDE5) inhibitors prevent the breakdown of cGMP and are currently studied as a possible target for cognitive enhancement. However, it is still unknown whether inhibition of PDE5 reversed β-amyloid peptide (Aβ)-induced neuroinflammation in APP/PS1 transgenic (Tg APP/PS1) mice. The present study evaluated the cognitive behaviors, inflammatory mediators, and cGMP/PKG/pCREB signaling in 15-month-old Tg APP/PS1 mice and age-matched wild-type (WT) mice that were treated with PDE5 inhibitor sildenafil and the inhibitor of cGMP-dependent protein kinase Rp-8-Br-PET-cGMPS. In comparison with WT mice, Tg APP/PS1 mice were characterized by impaired cognitive ability, neuroinflammatory response, and down-regulated cGMP signaling. Sildenafil reversed these memory deficits and cGMP/PKG/pCREB signaling dysfunction; it also reduced both the soluble Aβ1-40 and Aβ1-42 levels in the hippocampus. These effects of sildenafil were prevented by intra-hippocampal infusion of the Rp-8-Br-PET-cGMPS. These results suggest that sildenafil could restore cognitive deficits in Tg APP/PS1 mice by the regulation of PKG/pCREB signaling, anti-inflammatory response and reduction of Aβ levels.

The role of phosphodiesterases in hippocampal synaptic plasticity

Phosphodiesterases (PDEs) degrade cyclic nucleotides, signalling molecules that play important roles in synaptic plasticity and memory. Inhibition of PDEs may therefore enhance synaptic plasticity and memory as a result of elevated levels of these signalling molecules, and this has led to interest in PDE inhibitors as cognitive enhancers. The development of new mouse models in which PDE subtypes have been selectively knocked out and increasing selectivity of PDE antagonists means that this field is currently expanding. Roles for PDE2, 4, 5 and 9 in synaptic plasticity have so far been demonstrated and we review these studies here in the context of cyclic nucleotide signalling more generally. The role of other PDE families in synaptic plasticity has not yet been investigated, and this area promises to advance our understanding of cyclic nucleotide signalling in synaptic plasticity in the future. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Synthesis of quinoline derivatives: discovery of a potent and selective phosphodiesterase 5 inhibitor for the treatment of Alzheimer's disease

Phosphodiesterase type 5 (PDE5) mediates the degradation of cGMP in a variety of tissues including brain. Recent studies have demonstrated the importance of the nitric oxide/cGMP/cAMP-responsive element-binding protein (CREB) pathway to the process of learning and memory. Thus, PDE5 inhibitors (PDE5Is) are thought to be promising new therapeutic agents for the treatment of Alzheimer's disease (AD), a neurodegenerative disorder characterized by memory loss. To explore this possibility, a series of quinoline derivatives were synthesized and evaluated. We found that compound 7a selectively inhibits PDE5 with an IC(50) of 0.27 nM and readily crosses the blood brain barrier. In an in vivo mouse model of AD, compound 7a rescues synaptic and memory defects. Quinoline-based, CNS-permeant PDE5Is have potential for AD therapeutic development.

Phosphodiesterases as therapeutic targets for Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia among the elderly. In AD patients, memory loss is accompanied by the formation of beta-amyloid plaques and the appearance of tau in a pathological form. Given the lack of effective treatments for AD, the development of new management strategies for these patients is critical. The continued failure to find effective therapies using molecules aimed at addressing the anti-beta amyloid pathology has led researchers to focus on other non-amyloid-based approaches to restore memory function. Promising non-amyloid related candidate targets include phosphosdiesterases (PDEs), and indeed, Rolipram, a specific PDE4 inhibitor, was the first compound found to effectively restore cognitive deficits in animal models of AD. More recently, PDE5 inhibitors have also been shown to effectively restore memory function. Accordingly, inhibitors of other members of the PDE family may also improve memory performance in AD and non-AD animal models. Hence, in this review, we will summarize the data supporting the use of PDE inhibitors as cognitive enhancers and we will discuss the possible mechanisms of action underlying these effects. We shall also adopt a medicinal chemistry perspective that leads us to propose the most promising PDE candidates on the basis of inhibitor selectivity, brain distribution, and mechanism of action.

Metallothioneins I/II are involved in the neuroprotective effect of sildenafil in focal brain injury

We recently reported that administration of the non-selective cyclic GMP-phosphodiesterase (cGMP-PDE) inhibitor zaprinast to cortically cryoinjured rats results three days post-lesion in reduced neuronal cell death that was associated to decreased macrophage/microglial activation and oxidative stress and increased astrogliosis and angiogenesis. Similar effects have been observed in cryoinjured animals overexpressing metallothioneins I/II (MT-I/II), metal-binding cysteine-rich proteins that are up-regulated in response to injury. In this work we have examined the effect of administration of the selective PDE5 inhibitor sildenafil (10mg/kg, sc) 2h before and 24 and 48h after induction of cortical cryolesion in wild-type and MT-I/II-deficient mice. Our results show that in wild-type animals sildenafil induces similar changes in glial reactivity, angiogenesis and antioxidant and antiapoptotic effects in the cryolesioned cortex as those observed in rats with zaprinast, indicating that inhibition of PDE5 is responsible for the neuroprotective actions. However, these effects were not observed in mice deficient in MT-I/II. We further show that sildenafil significantly increases MT-I/II protein levels in homogenates of lesioned cortex and MT-I/II immunostaining in glial cells around the lesion. Taken together these results indicate that cGMP-mediated pathways regulate expression of MT-I/II and support the involvement of these proteins in the neuroprotective effects of sildenafil in focal brain lesion.

Inhibition of phosphodiesterase-5 rescues age-related impairment of synaptic plasticity and memory

Aging is characterized by a progressive cognitive decline that leads to memory impairment. Because the cyclic nucleotide cascade is essential for the integrity of synaptic function and memory, and it is down-regulated during aging and in neurodegenerative disorders, we investigated whether an increase in cGMP levels might rescue age-related synaptic and memory deficits in mice. We demonstrated that acute perfusion with the phosphodiesterase-5 inhibitor sildenafil (50 nM) ameliorated long-term potentiation in hippocampal slices from 26-30-month-old mice. Moreover, chronic intraperitoneal injection of sildenafil (3mg/kg for 3 weeks) improved age-related spatial learning and reference memory as tested by the Morris Water Maze, and recognition memory as tested by the Object Recognition Test. Finally, sildenafil restored central cAMP responsive element-binding protein (CREB) phosphorylation, which is crucial for synaptic plasticity and memory. Our data suggest that inhibition of phosphodiesterase-5 may be beneficial to treat age-related cognitive dysfunction in a physiological mouse model of aging.

Animal models of Alzheimer disease

Significant insights into the function of genes associated with Alzheimer disease and related dementias have occurred through studying genetically modified animals. Although none of the existing models fully reproduces the complete spectrum of this insidious human disease, critical aspects of Alzheimer pathology and disease processes can be experimentally recapitulated. Genetically modified animal models have helped advance our understanding of the underlying mechanisms of disease and have proven to be invaluable in the preclinical evaluation of potential therapeutic interventions. Continuing refinement and evolution to yield the next generation of animal models will facilitate successes in producing greater translational concordance between preclinical studies and human clinical trials and eventually lead to the introduction of novel therapies into clinical practice.

Sildenafil enhances neurogenesis and oligodendrogenesis in ischemic brain of middle-aged mouse

Adult neural stem cells give rise to neurons, oligodendrocytes and astrocytes. Aging reduces neural stem cells. Using an inducible nestin-CreER^T2/R26R-yellow fluorescent protein (YFP) mouse, we investigated the effect of Sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor, on nestin lineage neural stem cells and their progeny in the ischemic brain of the middle-aged mouse. We showed that focal cerebral ischemia induced nestin lineage neural stem cells in the subventricular zone (SVZ) of the lateral ventricles and nestin expressing NeuN positive neurons and adenomatous polyposis coli (APC) positive mature oligodendrocytes in the ischemic striatum and corpus callosum in the aged mouse. Treatment of the ischemic middle-aged mouse with Sildenafil increased nestin expressing neural stem cells, mature neurons, and oligodendrocytes by 33, 75, and 30%, respectively, in the ischemic brain. These data indicate that Sildenafil amplifies nestin expressing neural stem cells and their neuronal and oligodendrocyte progeny in the ischemic brain of the middle-aged mouse.

Sildenafil influences the anticonvulsant activity of vigabatrin and gabapentin in the timed pentylenetetrazole infusion test in mice

Sildenafil, a selective phosphodiesterase 5 (PDE5) inhibitor, has recently been reported to affect convulsant activity in some animal models of seizures and epilepsy. Moreover, its influence on the protective activity of some antiepileptic drugs (AEDs) was also noted. The aim of the present study was to investigate the effect of sildenafil on the anticonvulsant potential of gabapentin (GBP) and vigabatrin (VGB) in the timed intravenous (i.v.) pentylenetetrazole (PTZ) test in mice. The chimney test, the passive avoidance task and the grip strength test were used to estimate some possible side effects caused by the studied AEDs and their combinations with sildenafil. Total brain and free plasma concentrations of GBP and VGB were determined to evaluate the characteristics of interactions. Our studies revealed that GBP (25-100 mg/kg) increases the threshold for the forelimb tonic extension, whereas VGB raises thresholds both, for myoclonic (200-600 mg/kg) and generalized clonic (400-600 mg/kg) seizures in the used model of seizures. GBP at sub-effective dose of 12.5 mg/kg co-administered with sildenafil at doses of 10 and 20 mg/kg significantly increases the threshold for tonic seizures in the i.v. PTZ test in mice. Combination of sub-effective dose of VGB (200 mg/kg) with sildenafil at a dose of 5mg/kg also showed significant anticonvulsant activity against clonic seizures. The studied AEDs and their combinations with sildenafil did not produce any changes in the motor coordination, long-term memory and muscular strength in mice. Sildenafil did not influence total brain and free plasma concentrations of GBP and VGB. Interactions between the studied AEDs and sildenafil were pharmacodynamic in nature and for that reason they are worthy of consideration in the clinical practice.

Is the amyloid hypothesis of Alzheimer's disease therapeutically relevant?

The conventional view of AD (Alzheimer's disease) is that much of the pathology is driven by an increased load of β-amyloid in the brain of AD patients (the 'Amyloid Hypothesis'). Yet, many therapeutic strategies based on lowering β-amyloid have so far failed in clinical trials. This failure of β-amyloid-lowering agents has caused many to question the Amyloid Hypothesis itself. However, AD is likely to be a complex disease driven by multiple factors. In addition, it is increasingly clear that β-amyloid processing involves many enzymes and signalling pathways that play a role in a diverse array of cellular processes. Thus the clinical failure of β-amyloid-lowering agents does not mean that the hypothesis itself is incorrect; it may simply mean that manipulating β-amyloid directly is an unrealistic strategy for therapeutic intervention, given the complex role of β-amyloid in neuronal physiology. Another possible problem may be that toxic β-amyloid levels have already caused irreversible damage to downstream cellular pathways by the time dementia sets in. We argue in the present review that a more direct (and possibly simpler) approach to AD therapeutics is to rescue synaptic dysfunction directly, by focusing on the mechanisms by which elevated levels of β-amyloid disrupt synaptic physiology.

Caffeine increases mitochondrial function and blocks melatonin signaling to mitochondria in Alzheimer's mice and cells

Caffeine and melatonin have been shown to protect the Swedish mutant amyloid precursor protein (APP(sw)) transgenic mouse model of Alzheimer's disease from cognitive dysfunction. But their mechanisms of action remain incompletely understood. These Alzheimer's mice have extensive mitochondrial dysfunction, which likely contributes to their cognitive decline. To further explore the mechanism through which caffeine and melatonin protect cognitive function in these mice, we monitored the function of isolated mitochondria from APP(sw) mice treated with caffeine, melatonin, or both in their drinking water for one month. Melatonin treatment yielded a near complete restoration of mitochondrial function in assays of respiratory rate, membrane potential, reactive oxygen species production, and ATP levels. Caffeine treatment by itself yielded a small increase in mitochondrial function. However, caffeine largely blocked the large enhancement of mitochondrial function provided by melatonin. Studies with N2a neuroblastoma cells stably expressing APP(sw) showed that specific inhibition of cAMP-dependent phosphodiesterase (PDE) 4 or cGMP-dependent PDE5 also blocked melatonin protection of mitochondrial function, but A(2a) and A₁ adenosine receptor antagonists were without effect. Melatonin or caffeine at the concentrations used to modulate mitochondrial function in the cells had no effect on cAMP-dependent PDE activity or cellular cAMP or cGMP levels. Therefore, caffeine and increased cyclic nucleotide levels likely block melatonin signaling to mitochondria by independent mechanisms that do not involve adenosine receptor antagonism. The results of this study indicate that melatonin restores mitochondrial function much more potently than caffeine in APP(sw) transgenic mouse and cell models of Alzheimer's disease.

Targeting synaptic dysfunction in Alzheimer's disease therapy

In the past years, major efforts have been made to understand the genetics and molecular pathogenesis of Alzheimer's disease (AD), which has been translated into extensive experimental approaches aimed at slowing down or halting disease progression. Advances in transgenic (Tg) technologies allowed the engineering of different mouse models of AD recapitulating a range of AD-like features. These Tg models provided excellent opportunities to analyze the bases for the temporal evolution of the disease. Several lines of evidence point to synaptic dysfunction as a cause of AD and that synapse loss is a pathological correlate associated with cognitive decline. Therefore, the phenotypic characterization of these animals has included electrophysiological studies to analyze hippocampal synaptic transmission and long-term potentiation, a widely recognized cellular model for learning and memory. Transgenic mice, along with non-Tg models derived mainly from exogenous application of Aβ, have also been useful experimental tools to test the various therapeutic approaches. As a result, numerous pharmacological interventions have been reported to attenuate synaptic dysfunction and improve behavior in the different AD models. To date, however, very few of these findings have resulted in target validation or successful translation into disease-modifying compounds in humans. Here, we will briefly review the synaptic alterations across the different animal models and we will recapitulate the pharmacological strategies aimed at rescuing hippocampal plasticity phenotypes. Finally, we will highlight intrinsic limitations in the use of experimental systems and related challenges in translating preclinical studies into human clinical trials.

Amyloid-β acts as a regulator of neurotransmitter release disrupting the interaction between synaptophysin and VAMP2

BACKGROUND

It is becoming increasingly evident that deficits in the cortex and hippocampus at early stages of dementia in Alzheimer's disease (AD) are associated with synaptic damage caused by oligomers of the toxic amyloid-β peptide (Aβ42). However, the underlying molecular and cellular mechanisms behind these deficits are not fully understood. Here we provide evidence of a mechanism by which Aβ42 affects synaptic transmission regulating neurotransmitter release.

METHODOLOGY/FINDINGS

We first showed that application of 50 nM Aβ42 in cultured neurones is followed by its internalisation and translocation to synaptic contacts. Interestingly, our results demonstrate that with time, Aβ42 can be detected at the presynaptic terminals where it interacts with Synaptophysin. Furthermore, data from dissociated hippocampal neurons as well as biochemical data provide evidence that Aβ42 disrupts the complex formed between Synaptophysin and VAMP2 increasing the amount of primed vesicles and exocytosis. Finally, electrophysiology recordings in brain slices confirmed that Aβ42 affects baseline transmission.

CONCLUSIONS/SIGNIFICANCE

Our observations provide a necessary and timely insight into cellular mechanisms that underlie the initial pathological events that lead to synaptic dysfunction in Alzheimer's disease. Our results demonstrate a new mechanism by which Aβ42 affects synaptic activity.

Design and synthesis of neuroprotective methylthiazoles and modification as NO-chimeras for neurodegenerative therapy

Learning and memory deficits in Alzheimer's disease (AD) result from synaptic failure and neuronal loss, the latter caused in part by excitotoxicity and oxidative stress. A therapeutic approach is described that uses NO-chimeras directed at restoration of both synaptic function and neuroprotection. 4-Methylthiazole (MZ) derivatives were synthesized, based upon a lead neuroprotective pharmacophore acting in part by GABA(A) receptor potentiation. MZ derivatives were assayed for protection of primary neurons against oxygen-glucose deprivation and excitotoxicity. Selected neuroprotective derivatives were incorporated into NO-chimera prodrugs, coined nomethiazoles. To provide proof of concept for the nomethiazole drug class, selected examples were assayed for restoration of synaptic function in hippocampal slices from AD-transgenic mice, reversal of cognitive deficits, and brain bioavailability of the prodrug and its neuroprotective MZ metabolite. Taken together, the assay data suggest that these chimeric nomethiazoles may be of use in treatment of multiple components of neurodegenerative disorders, such as AD.

Tadalafil crosses the blood-brain barrier and reverses cognitive dysfunction in a mouse model of AD

Previous studies have demonstrated that cognitive function can be restored in mouse models of Alzheimer's disease (AD) following administration of sildenafil, a specific PDE5 inhibitor (Puzzo et al., 2009; Cuadrado-Tejedor et al.). Another very potent PDE5 inhibitor with a longer half-life and safe in chronic treatments, tadalafil, may represent a better alternative candidate for AD therapy. However, tadalafil was proven unable to achieve similar benefits than those of sildenafil in AD animal models (Puzzo et al., 2009). The lack of efficacy was attributed to inability to cross the blood-brain barrier (BBB). In this paper we first measured the blood and brain levels of tadalafil to prove that the compound crosses BBB and that chronic treatment leads to accumulation in the brain of the J20 transgenic mouse model of AD. We demonstrated the presence of PDE5 mRNA in the brain of the mice and also in the human brain. After a 10 week treatment with either of these PDE5 inhibitors, the performance of the J20 mice in the Morris water maze test improved when compared with the transgenic mice that received vehicle. Biochemical analysis revealed that neither sildenafil nor tadalafil altered the amyloid burden, although both compounds reduced Tau phosphorylation in the mouse hippocampus. This study provides evidence of the potential benefits of a chronic tadalafil treatment in AD therapy. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Chronic phosphodiesterase type 2 inhibition improves memory in the APPswe/PS1dE9 mouse model of Alzheimer's disease

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'.

Sildenafil promotes neuroprotection of the pelvic ganglia neurones after bilateral cavernosal nerve resection in the rat

OBJECTIVE

To determine the gene expression profile of pelvic ganglia neurones after bilateral cavernosal nerve resection (BCNR) and subsequent treatment with sildenafil in relation to neurotrophic-related pathways.

MATERIALS AND METHODS

Fisher rats aged 5 months were subjected to BCNR or sham operation and treated with or without sildenafil (20 mg/kg body-weight in drinking water) for 7 days. Total RNA isolated from pelvic ganglia was subjected to reverse transcription and then to quantitative reverse transcriptase-polymerase chain reaction (PCR) with the RAT-neurotrophic array. Results were corroborated by real-time PCR and western blotting. Another set of animals were injected with a fluorescent tracer at the base of the penis, 7 days before BCNR or sham operation, and were sacrificed 7 days after surgery. Sections of pelvic ganglia were used for immunohistochemistry with antibodies against neurturin, neuronal nitric oxide synthase, tyrosine hydroxylase and glial cell line-derived neurotrophic factor receptor α2.

RESULTS

A down-regulation of the expression of neuronal nitric oxide synthase accompanied by changes in the level of cholinergic neurotrophic factors, such as neurturin and its receptor glial cell line-derived neurotrophic factor receptor α2, artemin, neurotrophin-4 and cilliary neurotrophic factor, was observed 7 days after BCNR in pelvic ganglia neurones. Treatment with sildenafil, starting immediately after surgery, reversed all these changes at a level similar to that in sham-operated animals.

CONCLUSIONS

Sildenafil treatment promotes changes in the neurotrophic phenotype, leading to a regenerative state of pelvic ganglia neurones. The present study provides a justification for the use of phosphodiesterase 5 inhibitors as a neuroprotective agent after BCNR.

Furoxans (1,2,5-oxadiazole-N-oxides) as novel NO mimetic neuroprotective and procognitive agents

Furoxans (1,2,5-oxadiazole-N-oxides) are thiol-bioactivated NO-mimetics that have not hitherto been studied in the CNS. Incorporation of varied substituents adjacent to the furoxan ring system led to modulation of reactivity toward bioactivation, studied by HPLC-MS/MS analysis of reaction products. Attenuated reactivity unmasked the cytoprotective actions of NO in contrast to the cytotoxic actions of higher NO fluxes reported previously for furoxans. Neuroprotection was observed in primary neuronal cell cultures following oxygen glucose deprivation (OGD). Neuroprotective activity was observed to correlate with thiol-dependent bioactivation to produce NO(2)(-), but not with depletion of free thiol itself. Neuroprotection was abrogated upon cotreatment with a sGC inhibitor, ODQ, thus supporting activation of the NO/sGC/CREB signaling cascade by furoxans. Long-term potentiation (LTP), essential for learning and memory, has been shown to be potentiated by NO signaling, therefore, a peptidomimetic furoxan was tested in hippocampal slices treated with oligomeric amyloid-β peptide (Aβ) and was shown to restore synaptic function. The novel observation of furoxan activity of potential therapeutic use in the CNS warrants further studies.

Sildenafil restores cognitive function without affecting β-amyloid burden in a mouse model of Alzheimer's disease

BACKGROUND AND PURPOSE

Inhibitors of phosphodiesterase 5 (PDE5) affect signalling pathways by elevating cGMP, which is a second messenger involved in processes of neuroplasticity. In the present study, the effects of the PDE5 inhibitor, sildenafil, on the pathological features of Alzheimer's disease and on memory-related behaviour were investigated.

EXPERIMENTAL APPROACH

Sildenafil was administered to the Tg2576 transgenic mouse model of Alzheimer's disease and to age-matched negative littermates (controls). Memory function was analysed using the Morris water maze test and fear conditioning tasks. Biochemical analyses were performed in brain lysates from animals treated with saline or with sildenafil.

KEY RESULTS

Treatment of aged Tg2576 animals with sildenafil completely reversed their cognitive impairment. Such changes were accompanied in the hippocampus by a reduction of tau hyperphosphorylation and a decrease in the activity of glycogen synthase kinase 3β (GSK3β) and of cyclin-dependent kinase 5 (CDK5) (p25/p35 ratio). Moreover, sildenafil also increased levels of brain-derived neurotrophic factor (BDNF) and the activity-regulated cytoskeletal-associated protein (Arc) in the hippocampus without any detectable modification of brain amyloid burden.

CONCLUSIONS AND IMPLICATIONS

Sildenafil improved cognitive functions in Tg2576 mice and the effect was not related to changes in the amyloid burden. These data further strengthen the potential of sildenafil as a therapeutic agent for Alzheimer's disease.

Phosphodiesterase 9A regulates central cGMP and modulates responses to cholinergic and monoaminergic perturbation in vivo

Cyclic nucleotides are critical regulators of synaptic plasticity and participate in requisite signaling cascades implicated across multiple neurotransmitter systems. Phosphodiesterase 9A (PDE9A) is a high-affinity, cGMP-specific enzyme widely expressed in the rodent central nervous system. In the current study, we observed neuronal staining with antibodies raised against PDE9A protein in human cortex, cerebellum, and subiculum. We have also developed several potent, selective, and brain-penetrant PDE9A inhibitors and used them to probe the function of PDE9A in vivo. Administration of these compounds to animals led to dose-dependent accumulation of cGMP in brain tissue and cerebrospinal fluid, producing a range of biological effects that implied functional significance for PDE9A-regulated cGMP in dopaminergic, cholinergic, and serotonergic neurotransmission and were consistent with the widespread distribution of PDE9A. In vivo effects of PDE9A inhibition included reversal of the respective disruptions of working memory by ketamine, episodic and spatial memory by scopolamine, and auditory gating by amphetamine, as well as potentiation of risperidone-induced improvements in sensorimotor gating and reversal of the stereotypic scratching response to the hallucinogenic 5-hydroxytryptamine 2A agonist mescaline. The results suggested a role for PDE9A in the regulation of monoaminergic circuitry associated with sensory processing and memory. Thus, PDE9A activity regulates neuronal cGMP signaling downstream of multiple neurotransmitter systems, and inhibition of PDE9A may provide therapeutic benefits in psychiatric and neurodegenerative diseases promoted by the dysfunction of these diverse neurotransmitter systems.

Effects of protein kinase A and G inhibitors on hippocampal cholinergic markers expressions in rolipram- and sildenafil-induced spatial memory improvement

Although there are number of studies showing that phosphodiesterase (PDE) 4 and 5 inhibitors affect different kinds of memory, their effects on spatial memory consolidation in conjunction with the cholinergic activity in the hippocampus have not been studied before. In the present study firstly, rats were evaluated for the effects of different doses of the PDE4 inhibitor rolipram and the PDE5 inhibitor sildenafil on spatial memory consolidation in the water maze task. Rolipram or sildenafil was daily administered intraperitoneally 3 or 0 h after the last trial of training, respectively. Then in a separate related experiment the effect of the most efficient doses of rolipram or sildenafil accompanied by an intrahippocampally injected protein kinase A (PKA) or protein kinase G (PKG) inhibitor, respectively, was examined. Finally for determination of the hippocampal cholinergic activity the protein expression of hippocampal vesicular acetylcholine transporter (VAChT) and cholineacetyltransferase (ChAT) was measured. Rolipram at 0.03 mg/kg as well as sildenafil at 3 mg/kg increased spatial memory and their enhancing effect was completely blocked following inhibition of PKA and PKG, respectively. Furthermore, none of the treatments had a significant effect on the hippocampal ChAT and VAChT levels. Our data showed that rolipram and sildenafil enhanced spatial memory consolidation in an inverted U-shaped dose-response curve. This effect is dependent on the activity of cAMP/PKA- and cGMP/PKG-mediated pathways, respectively in the hippocampus. However, we did not find evidence for a chronic increase of cholinergic activity in the observed PDE inhibitor-induced memory improvement.

Sildenafil improves epicenter vascular perfusion but not hindlimb functional recovery after contusive spinal cord injury in mice

Nitric oxide (NO) is an important regulator of vasodilation and angiogenesis in the central nervous system (CNS). Signaling initiated by the membrane receptor CD47 antagonizes vasodilation and angiogenesis by inhibiting synthesis of cyclic guanosine monophosphate (cGMP). We recently found that deletion of CD47 led to significant functional locomotor improvements, enhanced angiogenesis, and increased epicenter microvascular perfusion in mice after moderate contusive spinal cord injury (SCI). We tested the hypothesis that improving NO/cGMP signaling within the spinal cord immediately after injury would increase microvascular perfusion, angiogenesis, and functional recovery, with an acute, 7-day administration of the cGMP phosphodiesterase 5 (PDE5) inhibitor sildenafil. PDE5 expression is localized within spinal cord microvascular endothelial cells and smooth muscle cells. While PDE5 antagonism has been shown to increase angiogenesis in a rat embolic stroke model, sildenafil had no significant effect on angiogenesis at 7 days post-injury after murine contusive SCI. Sildenafil treatment increased cGMP concentrations within the spinal cord and improved epicenter microvascular perfusion. Basso Mouse Scale (BMS) and Treadscan analyses revealed that sildenafil treatment had no functional consequence on hindlimb locomotor recovery. These data support the hypothesis that acutely improving microvascular perfusion within the injury epicenter by itself is an insufficient strategy for improving functional deficits following contusive SCI.

Involvement of the cGMP pathway in the osthole-facilitated glutamate release in rat hippocampal nerve endings

Osthole, an active constituent isolated from Cnidium monnieri (L.) Cusson, has previously been shown to have the capacity to increase depolarization-evoked glutamate release in rat hippocampal nerve terminals. As cGMP-dependent signaling cascade has been found to modulate glutamate release at the presynaptic level, the aim of this study was to further examine the role of cGMP signaling pathway in the regulation of osthole on glutamate release in hippocampal synaptosomes. Results showed that osthole dose-dependently increased intrasynaptosomal cGMP levels. The elevation of cGMP levels by osthole was prevented by the phosphodiesterase 5 inhibitor sildenafil but was insensitive to the guanylyl cyclase inhibitor ODQ. In addition, osthole-induced facilitation of 4-aminopyridine (4-AP)-evoked glutamate release was completely prevented by the cGMP-dependent protein kinase (PKG) inhibitors, KT5823, and Rp-8-Br-PET-cGMPS. Direct activation of PKG with 8-Br-cGMP or 8-pCPT-cGMP also occluded the osthole-mediated facilitation of 4-AP-evoked glutamate release. Furthermore, sildenafil exhibited a dose-dependent facilitation of 4-AP-evoked release of glutamate and occluded the effect of osthole on the 4-AP-evoked glutamate release. Collectively, our findings suggest that osthole-mediated facilitation of glutamate release involves the activation of cGMP/PKG-dependent pathway.

Cyclic AMP response element-binding protein (CREB) phosphorylation: a mechanistic marker in the development of memory enhancing Alzheimer's disease therapeutics

CREB-mediated transcription can be initiated by membrane receptor stimulation and subsequent activation of intracellular pathways to the cell nucleus, and has been described as a molecular switch required for learning and memory. While CREB dimers are thought to be constitutively bound to response elements on DNA under basal conditions, it is CREB phosphorylation that is believed to be responsible for transcriptional activation leading to gene products such as BDNF that play a key role in synaptic plasticity and cognitive function. Conversely, preclinical and clinical findings now suggest that impaired CREB phosphorylation may be a pathological component in neurodegenerative disorders, in particular Alzheimer's disease (AD). In this regard, pharmacological-induced CREB phosphorylation in brain regions associated with cognition, i.e. cortex and hippocampus may represent a mechanistic basis for the development of novel AD therapeutics. The purpose of this commentary is to describe an experimental strategy to biochemically characterize the pharmacological induction of CREB phosphorylation as a mechanistic marker across different pharmacological classes of compounds for the potential treatment of AD that include: α7 nicotinic agonists, H3 antagonists and 11β HSD1 inhibitors.

Multi-faced neuroprotective effects of Ginsenoside Rg1 in an Alzheimer mouse model

There has been no extensive characterization of the effects of Ginsenoside Rg1, a pharmacological active component purified from the nature product ginseng, in an Alzheimer's disease mouse model. The well-characterized transgenic Alzheimer disease (AD) mice over expressing amyloid precursor protein (APP)/Aβ (Tg mAPP) and nontransgenic (nonTg) littermates at age of 6 and 9 months were treated with Rg 1 for three months via intraperitoneal injection. Mice were then evaluated for changes in amyloid pathology, neuropathology and behavior. Tg mAPP treated with Rg1 showed a significant reduction of cerebral Aβ levels, reversal of certain neuropathological changes, and preservation of spatial learning and memory, as compared to vehicle-treated mice. Rg1 treatment inhibited activity of γ-secretase in both Tg mAPP mice and B103-APP cells, indicating the involvement of Rg1 in APP regulation pathway. Furthermore, administration of Rg1 enhanced PKA/CREB pathway activation in mAPP mice and in cultured cortical neurons exposed to Aβ or glutamate-mediated synaptic stress. Most importantly, the beneficial effects on attenuation of cerebral Aβ accumulation, improvement in neuropathological and behavioral changes can be extended to the aged mAPP mice, even to 12-13 months old mice that had extensive amyloid pathology and severe neuropathological and cognitive malfunction. These studies indicate that Rg1 has profound multi-faced and neuroprotective effects in an AD mouse model. Rg1 induces neuroprotection through ameliorating amyloid pathology, modulating APP process, improving cognition, and activating PKA/CREB signaling. These findings provide a new perspective for the treatment of AD and demonstrate potential for a new class of drugs for AD treatment.