Elena Puerta

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 Alzheimers disease and on memory‐related behaviour were investigated.

Sildenafil protects against 3-nitropropionic acid neurotoxicity through the modulation of calpain, CREB, and BDNF.

In this study we tested whether phosphodiesterase 5 (PDE5) inhibitors, sildenafil and vardenafil, would afford protection against 3-nitropropionic acid (3NP), which produces striatal lesions that closely mimic some of the neuropathological features of Huntingtons Disease (HD). The neurotoxin was given over 5 days by constant systemic infusion using osmotic minipumps. Animals treated with PDE5 inhibitors (sildenafil or vardenafil) showed improved neurologic scores, reduced the loss of striatal DARPP-32 protein levels and lesion volumes, and decreased calpain activation produced by 3NP. This protective effect was independent of changes in 3NP-induced succinate dehydrogenase inhibition. Furthermore, striatal p-CREB levels along with the expression of BDNF were significantly increased in sildenafil-treated rats. In summary, PDE5 inhibitors protected against 3NP-induced striatal degeneration by reducing calpain activation and by promoting survival pathways. These data encourage further evaluation of PDE5 inhibitors in transgenic mouse models of HD.

Phosphodiesterase 5 inhibitors prevent 3,4-methylenedioxymethamphetamine-induced 5-HT deficits in the rat.

Phosphodiesterase 5 (PDE5) inhibitors are often used in combination with club drugs such as 3,4‐methylenedioxymethamphetamine (MDMA or ecstasy). We investigated the consequences of such combination in the serotonergic system of the rat. Oral administration of sildenafil citrate (1.5 or 8 mg/kg) increased brain cGMP levels and protected in a dose‐dependent manner against 5‐hydroxytryptamine depletions caused by MDMA (3 × 5 mg/kg, i.p., every 2 h) in the striatum, frontal cortex and hippocampus without altering the acute hyperthermic response to MDMA. Intrastriatal administration of the protein kinase G (PKG) inhibitor, KT5823 [(9S, 10R, 12R)‐2,3,9,10,11,12‐Hexahydro‐10‐methoxy‐2,9‐dimethyl‐1‐oxo‐9,12‐epoxy‐1H‐diindolo[1,2,3‐fg:3′,2′,1′‐kl]pyrrolo[3,4‐i][1,6]benzodiazocine‐10‐carboxylic acid, methyl ester)], suppressed sildenafil‐mediated protection. By contrast, the cell permeable cGMP analogue, 8‐bromoguanosine cyclic 3′,5′‐monophosphate, mimicked sildenafil effects further suggesting the involvement of the PKG pathway in mediating sildenafil protection. Because mitochondrial ATP‐sensitive K+ channels are a target for PKG, we next administered the specific mitochondrial ATP‐sensitive K+ channel blocker, 5‐hydroxydecanoic acid, 30 min before sildenafil. 5‐hydroxydecanoic acid completely reversed the protection afforded by sildenafil, thereby implicating the involvement of mitochondrial ATP‐sensitive K+ channels. Sildenafil also increased Akt phosphorylation, and so the possible involvement of the Akt/endothelial nitric oxide synthase (eNOS)/sGC signalling pathway was analysed. Neither the phosphatidylinositol 3‐kinase inhibitor, wortmannin, nor the selective eNOS inhibitor, l‐N5‐(1‐iminoethyl)‐l‐ornithine dihydrochloride, reversed the protection afforded by sildenafil, suggesting that Akt/eNOS/sGC cascade does not participate in the protective mechanisms. Our data also show that the protective effect of sildenafil can be extended to vardenafil, another PDE5 inhibitor. In conclusion, sildenafil protects against MDMA‐induced long‐term reduction of indoles by a mechanism involving increased production of cGMP and subsequent activation of PKG and mitochondrial ATP‐sensitive K+ channel opening.

Methylenedioxymethamphetamine inhibits mitochondrial complex I activity in mice: a possible mechanism underlying neurotoxicity

Background and purpose:  3,4‐methylenedioxymethamphetamine (MDMA) causes a persistent loss of dopaminergic cell bodies in the substantia nigra of mice. Current evidence indicates that such neurotoxicity is due to oxidative stress but the source of free radicals remains unknown. Inhibition of mitochondrial electron transport chain complexes by MDMA was assessed as a possible source.

Sildenafil ameliorates cognitive deficits and tau pathology in a senescence-accelerated mouse model

Aging is associated with a deterioration of cognitive performance and with increased risk of neurodegenerative disorders. In the present study we tested whether the specific phosphodiesterase 5 inhibitor sildenafil could ameliorate the age-dependent cognitive impairments shown by the senescence-accelerated mouse prone-8 (SAMP8). Sildenafil administration (7.5 mg/kg for 4 weeks) to 5-month-old SAMP8 mice attenuated spatial learning and memory impairments shown by these mice in the Morris Water Maze. Tau hyperphosphorylation (AT8 but not PHF-1 epitope) shown by SAMP8 mice at this age was also decreased in the hippocampus of sildenafil-treated mice, an effect probably related to a decrease in cyclin-dependent kinase 5 protein expression and activity (p25/p35 ratio). Interestingly, sildenafil also phosphorylated Akt, which was associated with an increase of glycogen synthase kinase-3β phosphorylation, providing a plausible explanation for the reductions in tau hyperphosphorylation (AT8 and PHF-1 epitopes) and attenuation of cognitive deficits shown by 9-month-old SAMP8 mice. Overall, sildenafil might be beneficial in age-related brain dysfunction and could be an emerging candidate for the treatment of other neurodegenerative diseases.

Alterations in brain leptin signalling in spite of unchanged CSF leptin levels in Alzheimer's disease

Several studies support the relation between leptin and Alzheimers disease (AD). We show that leptin levels in CSF are unchanged as subjects progress to AD. However, in AD hippocampus, leptin signalling was decreased and leptin localization was shifted, being more abundant in reactive astrocytes and less in neurons. Similar translocation of leptin was found in brains from Tg2576 and apoE4 mice. Moreover, an enhancement of leptin receptors was found in hippocampus of young Tg2576 mice and in primary astrocytes and neurons treated with Aβ1‐42. In contrast, old Tg2576 mice showed decreased leptin receptors levels. Similar findings to those seen in Tg2576 mice were found in apoE4, but not in apoE3 mice. These results suggest that leptin levels are intact, but leptin signalling is impaired in AD. Thus, Aβ accumulation and apoE4 genotype result in a transient enhancement of leptin signalling that might lead to a leptin resistance state over time.

Combination of Apolipoprotein E4 and High Carbohydrate Diet Reduces Hippocampal BDNF and Arc Levels and Impairs Memory in Young Mice

The presence of the E4 allele of apolipoprotein E (apoE) is the strongest known genetic risk factor for sporadic Alzheimers disease (AD). Other risk factors for developing AD have been identified, including lifestyle such as dietary habits. The present study was designed to explore the impact of the interaction between variant human apoE isoforms and a high carbohydrate diet (HCD) on mechanisms behind learning and memory retention. As an investigative model, we compared young apoE3 and apoE4 target replacement mice fed on a HCD for 6 months. Our results indicate that HCD compromises memory processes in apoE4 mice. ApoE4 mice on HCD showed decreased activity-regulated cytoskeletal-associated protein (Arc) and brain derived neurotrophic factor (BDNF) levels, as well as decreased BDNF signaling in the hippocampus. In contrast, apoE3 mice were resistant to the deleterious effects of HCD on both behavior and memory-related proteins. Our results support the hypothesis that already in mid-life, genetic, and environmental risk factors act together on the mechanisms behind cognitive impairment.

Is it possible to improve memory function by upregulation of the cholesterol 24S-hydroxylase (CYP46A1) in the brain?

We previously described a heterozygous mouse model overexpressing human HA-tagged 24S-hydroxylase (CYP46A1) utilizing a ubiquitous expression vector. In this study, we generated homozygotes of these mice with circulating levels of 24OH 30–60% higher than the heterozygotes. Female homozygous CYP46A1 transgenic mice, aged 15 months, showed an improvement in spatial memory in the Morris water maze test as compared to the wild type mice. The levels of N-Methyl-D-Aspartate receptor 1, phosphorylated-N-Methyl-D-Aspartate receptor 2A, postsynaptic density 95, synapsin-1 and synapthophysin were significantly increased in the hippocampus of the CYP46A1 transgenic mice as compared to the controls. The levels of lanosterol in the brain of the CYP46A1 transgenic mice were significantly increased, consistent with a higher synthesis of cholesterol. Our results are discussed in relation to the hypothesis that the flux in the mevalonate pathway in the brain is of importance in cognitive functions.

Minoxidil prevents 3,4-methylenedioxymethamphetamine- induced serotonin depletions: role of mitochondrial ATP-sensitive potassium channels, Akt and ERK

Preconditioning has emerged as a valid strategy against different neurotoxic insults. Although the mechanisms underlying preconditioning are not fully understood, the activation of ATP‐sensitive potassium (KATP) channels has been proposed to play a pivotal role in neuronal preconditioning. In the present work we examine whether minoxidil a KATP channel activator protects against the long‐term toxicity caused by the amphetamine derivative 3,4‐methylenedioxymethamphetamine (MDMA) in rats. Our data show that intrastriatal administration of minoxidil prevents MDMA‐induced long‐term indole depletions in the rat striatum. This effect was not related to an effect on core temperature, as pre‐treatment with minoxidil did not significantly alter MDMA‐induced hyperthermia. Taking into account that minoxidil opens both sarcolemmal and mitochondrial KATP channels, we examined the role of each type of channels in the protective effects of minoxidil using specific inhibitors. The administration of HMR‐1098, a blocker of the sarcolemmal KATP channels, along with minoxidil did not affect the protection afforded by the latter. On the contrary the selective mitochondrial KATP channel blocker 5‐hydroxydecanoic acid completely reversed the protection afforded by minoxidil, thereby implicating the involvement of mitochondrial (but not sarcolemmal) KATP channels. Furthermore our data show the participation of Akt and extracellular signal‐regulated kinases in minoxidil‐afforded protection. Intrastriatal administration of wortmannin or PD98059 (inhibitors of phosphatidylinositol‐3‐kinase and mitogen‐activated protein kinase/extracellular regulated protein kinase, respectively), along with minoxidil abolished the protective effect of minoxidil against the serotonergic toxicity caused by MDMA. These results demonstrate that minoxidil by opening mitochondrial KATP channels completely prevents MDMA toxicity and that Akt and MAP kinases are involved in minoxidil‐afforded neuroprotection.

On the mechanisms underlying 3,4-methylenedioxymethamphetamine toxicity: the dilemma of the chicken and the egg.

Administration of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) to various experimental animals has been shown to induce a selective damage to serotonergic axon terminals. While a great consensus appears to exist regarding the causative role of reactive oxygen species (ROS) in the mechanisms underlying MDMA toxicity, the source of free radicals is still a matter of debate. While some authors support dopamine metabolism/oxidation inside 5-hydroxytryptamine (5-HT) terminals as the key factor responsible for ROS formation and final 5-HT terminal degeneration, others believe it is MDMA metabolism into pro-oxidant compounds. Although at first sight both hypotheses appear to contend with each other, it may not be the case. This mini-review was therefore undertaken to try to reconcile both hypotheses and to address the dilemma of the causality of MDMA neurotoxicity.