Ischemic-LTP in striatal spiny neurons of both direct and indirect pathway requires the activation of D1-like receptors and NO/soluble guanylate cyclase/cGMP transmission

Selective block of adenosine A2A receptors prevents ischaemic-like effects induced by oxygen and glucose deprivation in rat medium spiny neurons

BACKGROUND AND PURPOSE

Ischaemia is known to cause massive neuronal depolarization, termed anoxic depolarization (AD), due to energy failure and loss of membrane ion gradients. The neuromodulator adenosine accumulates extracellularly during ischaemia and activates four metabotropic receptors: A1 , A2A , A2B and A3 . Striatal medium spiny neurons (MSNs) express high levels of A2A receptors and are particularly vulnerable to ischaemic insults. A2A Receptor blockade reduces acute striatal post-ischaemic damage but the cellular mechanisms involved are still unknown.

EXPERIMENTAL APPROACH

We performed patch-clamp recordings of MSNs in rat striatal slices subjected to oxygen and glucose deprivation (OGD) to investigate the effects of A2A receptor ligands or ion channel blockers on AD and OGD-induced ionic imbalance, measured as a positive shift in Erev of ramp currents.

KEY RESULTS

Our data indicate that the A2A receptor antagonist SCH58261 (10 μM) significantly attenuated ionic imbalance and AD appearance in MSNs exposed to OGD. The K+ channel blocker Ba2+ (2 mM) or the Na+ channel blocker tetrodotoxin (1 μM) exacerbated and attenuated, respectively, OGD-induced changes. Spontaneous excitatory post-synaptic current (sEPSC) analysis in MSNs revealed that the A2A receptor agonist CGS21680 (1 μM) prevented OGD-induced decrease of sEPSCs within the first 5 min of the insult, an effect shared by the K+ channel blocker Ba2+ , indicating facilitated glutamate release.

CONCLUSION AND IMPLICATIONS

Adenosine, released during striatal OGD, activates A2A receptors that may exacerbate OGD-induced damage through K+ channel inhibition. Our results could help to develop A2A receptor-selective therapeutic tools for the treatment of brain ischaemia.

Participation of glutamatergic and nitrergic systems in the striatal dopamine release induced by isatin, a MAO inhibitor

Isatin is a biofactor with different biochemical and pharmacological properties whose effects attract much attention because it is an endogenous inhibitor of the monoamine oxidase in the brain. When exogenously administrated, isatin increases dopamine levels in intact and denervated striatum of rats, an effect that could indicate its potential as a therapeutic agent in Parkinson disease. However, the neurochemical mechanisms by which isatin increases dopamine in the striatum are poorly understood. In the present study, we evaluate the role of the glutamatergic and nitrergic systems in the isatin-induced dopamine release from rat striatum. Our findings show that the intrastriatal administration of 10 mM isatin significantly increases the in vivo release of dopamine (1,104.7% ± 97.1%), and the amino acids glutamate (428.7% ± 127%) and taurine (221% ± 22%) from rat striatum measured by brain microdialysis. The pretreatment with MK-801 (500 µM) or AP5 (650 µM) (glutamatergic NMDA receptors antagonists) significantly reduces the effect of isatin on dopamine release by 52% and 70.5%, respectively. The administration of the nitric oxide synthase inhibitors, L-NAME (100 µM) or 7-NI (100 µM) also decreases the isatin-induced dopamine release by 77% and 42%, respectively. These results show that isatin, in addition to increasing dopamine release, also increases glutamate levels, and possibly activates NMDA receptors and nitric oxide production, which can promote a further increase in the dopamine release.

Deep hypothermia prevents striatal alterations produced by perinatal asphyxia: Implications for the prevention of dyskinesia and psychosis

GABAergic medium spiny neurons are the main neuronal population in the striatum. Calbindin is preferentially expressed in medium spiny neurons involved in the indirect pathway. The aim of the present work is to analyze the effect of perinatal asphyxia on different subpopulations of GABAergic neurons in the striatum and to assess the outcome of deep therapeutic hypothermia. The uterus of pregnant rats was removed by cesarean section and the fetuses were exposed to hypoxia by immersion in water (19 min) at 37°C (perinatal asphyxia). The hypothermic group was exposed to 10°C during 30 min after perinatal asphyxia. The rats were euthanized at the age of one month (adolescent/adult rats), their brains were dissected out and coronal sections were immunolabeled for calbindin, calretinin, NeuN, and reelin. Reelin+ cells showed no staining in the striatum besides subventricular zone. The perinatal asphyxia (PA) group showed a significant decrease in calbindin neurons and a paradoxical increase in neurons estimated by NeuN staining. Moreover, calretinin+ cells, a specific subpopulation of GABAergic neurons, showed an increase caused by PA. Deep hypothermia reversed most of these alterations probably by protecting calbindin neurons. Similarly, there was a reduction of the diameter of the anterior commissure produced by the asphyxia that was prevented by hypothermic treatment.

Tetracosahexaenoylethanolamide, a novel N-acylethanolamide, is elevated in ischemia and increases neuronal output

N-acylethanolamines (NAEs) are endogenous lipid-signaling molecules derived from fatty acids that regulate numerous biological functions, including in the brain. Interestingly, NAEs are elevated in the absence of fatty acid amide hydrolase (FAAH) and following CO₂-induced ischemia/hypercapnia, suggesting a neuroprotective response. Tetracosahexaenoic acid (THA) is a product and precursor to DHA; however, the NAE product, tetracosahexaenoylethanolamide (THEA), has never been reported. Presently, THEA was chemically synthesized as an authentic standard to confirm THEA presence in biological tissues. Whole brains were collected and analyzed for unesterified THA, total THA, and THEA in wild-type and FAAH-KO mice that were euthanized by either head-focused microwave fixation, CO₂ + microwave, or CO₂ only. PPAR activity by transient transfection assay and ex vivo neuronal output in medium spiny neurons (MSNs) of the nucleus accumbens by patch clamp electrophysiology were determined following THEA exposure. THEA in the wild-type mice was nearly doubled (P < 0.05) following ischemia/hypercapnia (CO₂ euthanization) and up to 12 times higher (P < 0.001) in the FAAH-KO compared with wild-type. THEA did not increase (P > 0.05) transcriptional activity of PPARs relative to control, but 100 nM of THEA increased (P < 0.001) neuronal output in MSNs of the nucleus accumbens. Here were identify a novel NAE, THEA, in the brain that is elevated upon ischemia/hypercapnia and by KO of the FAAH enzyme. While THEA did not activate PPAR, it augmented the excitability of MSNs in the nucleus accumbens. Overall, our results suggest that THEA is a novel NAE that is produced in the brain upon ischemia/hypercapnia and regulates neuronal excitation.

Icariside II, a phosphodiesterase 5 inhibitor, attenuates cerebral ischaemia/reperfusion injury by inhibiting glycogen synthase kinase-3β-mediated activation of autophagy

Background and Purpose

Cerebral ischaemia/reperfusion causes exacerbated neuronal damage involving excessive autophagy and neuronal loss. The present study was designed to investigate the effect of icariside II, one of main active ingredients of Herba Epimedii on this loss and whether this is related to its PDE 5 inhibitory action.

Experimental Approach

Focal cerebral ischaemia was induced in the rat by transient middle cerebral artery occlusion over 2 hr, followed by reperfusion with icariside II, 3‐methylamphetamine or rapamycin. The effect of icariside II was determined measuring behaviour changes and the size of the infarction. The expressions of PDE 5, autophagy‐related proteins and the level of phosphorylation of glycogen synthase kinase‐3β (GSK‐3β) were determined. Cultured primary cortical neurons were subjected to oxygen and glucose deprivation followed by reoxygenation in the presence and absence of icariside II. A surface plasmon resonance assay and molecular docking were used to explore the interactions of icariside II with PDE 5 or GSK‐3β.

Key Results

Icariside II not only protected against induced ischaemic reperfusion injury in rats but also attenuated such injury in primary cortical neurons. The neuroprotective effects of icariside II on such injury were attributed to interfering with the PKG/GSK‐3β/autophagy axis by directly bounding to PDE 5 and GSK‐3β.

Conclusions and Implications

These findings indicate that icariside II attenuates cerebral I/R‐induced injury via interfering with PKG/GSK‐3β/autophagy axis. This study raises the possibility that icariside II and other PDE 5 inhibitors maybe effective in the treatment ischaemia stroke.

Microglial activation and the nitric oxide/cGMP/PKG pathway underlie enhanced neuronal vulnerability to mitochondrial dysfunction in experimental multiple sclerosis

During multiple sclerosis (MS), a close link has been demonstrated to occur between inflammation and neuro-axonal degeneration, leading to the hypothesis that immune mechanisms may promote neurodegeneration, leading to irreversible disease progression. Energy deficits and inflammation-driven mitochondrial dysfunction seem to be involved in this process. In this work we investigated, by the use of striatal electrophysiological field-potential recordings, if the inflammatory process associated with experimental autoimmune encephalomyelitis (EAE) is able to influence neuronal vulnerability to the blockade of mitochondrial complex IV, a crucial component for mitochondrial activity responsible of about 90% of total cellular oxygen consumption. We showed that during the acute relapsing phase of EAE, neuronal susceptibility to mitochondrial complex IV inhibition is markedly enhanced. This detrimental effect was counteracted by the pharmacological inhibition of microglia, of nitric oxide (NO) synthesis and its intracellular pathway (involving soluble guanylyl cyclase, sGC, and protein kinase G, PKG). The obtained results suggest that mitochondrial complex IV exerts an important role in maintaining neuronal energetic homeostasis during EAE. The pathological processes associated with experimental MS, and in particular the activation of microglia and of the NO pathway, lead to an increased neuronal vulnerability to mitochondrial complex IV inhibition, representing promising pharmacological targets.

Hydroxysafflor Yellow A Protects Neurons From Excitotoxic Death through Inhibition of NMDARs

Excessive glutamate release causes overactivation of N-methyl d-aspartate receptors (NMDARs), leading to excitatory neuronal damage in cerebral ischemia. Hydroxysafflor yellow A (HSYA), a compound extracted from Carthamus tinctorius L., has been reported to exert a neuroprotective effect in many pathological conditions, including brain ischemia. However, the underlying mechanism of HSYA's effect on neurons remains elusive. In the present study, we conducted experiments using patch-clamp recording of mouse hippocampal slices. In addition, we performed Ca²⁺ imaging, Western blots, as well as mitochondrial-targeted circularly permuted yellow fluorescent protein transfection into cultured hippocampal neurons in order to decipher the physiological mechanism underlying HSYA's neuroprotective effect.

Through the electrophysiology experiments, we found that HSYA inhibited NMDAR-mediated excitatory postsynaptic currents without affecting α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor and γ-aminobutyric acid A-type receptor-mediated currents. This inhibitory effect of HSYA on NMDARs was concentration dependent. HSYA did not show any preferential inhibition of either N-methyl d-aspartate receptor subtype 2A- or N-methyl d-aspartate receptor subtype 2B- subunit-containing NMDARs. Additionally, HSYA exhibits a facilitatory effect on paired NMDAR-mediated excitatory postsynaptic currents. Furthermore, HSYA reduced the magnitude of NMDAR-mediated membrane depolarization currents evoked by oxygen-glucose deprivation, and suppressed oxygen-glucose deprivation–induced and NMDAR-dependent ischemic long-term potentiation, which is believed to cause severe reperfusion damage after ischemia. Through the molecular biology experiments, we found that HSYA inhibited the NMDA-induced and NMDAR-mediated intracellular Ca²⁺ concentration increase in hippocampal cultures, reduced apoptotic and necrotic cell deaths, and prevented mitochondrial damage. Together, our data demonstrate for the first time that HSYA protects hippocampal neurons from excitotoxic damage through the inhibition of NMDARs. This novel finding indicates that HSYA may be a promising pharmacological candidate for the treatment of brain ischemia.

Expression and therapeutic targeting of dopamine receptor-1 (D1R) in breast cancer

Patients with advanced breast cancer often fail to respond to treatment, creating a need to develop novel biomarkers and effective therapeutics. Dopamine (DA) is a catecholamine that binds to five G protein-coupled receptors. We discovered expression of DA type-1 receptors (D1Rs) in breast cancer, thereby identifying these receptors as novel therapeutic targets in this disease. Strong to moderate immunoreactive D1R expression was found in 30% of 751 primary breast carcinomas, and was associated with larger tumors, higher tumor grades, node metastasis and shorter patient survival. DA and D1R agonists, signaling through the cGMP/protein kinase G (PKG) pathway, suppressed cell viability, inhibited invasion and induced apoptosis in multiple breast cancer cell lines. Fenoldopam, a peripheral D1R agonist that does not penetrate the brain, dramatically suppressed tumor growth in two mouse models with D1R-expressing xenografts by increasing both necrosis and apoptosis. D1R-expressing primary tumors and metastases in mice were detected by fluorescence imaging. In conclusion, D1R overexpression is associated with advanced breast cancer and poor prognosis. Activation of the D1R/cGMP/PKG pathway induces apoptosis in vitro and causes tumor shrinkage in vivo. Fenoldopam, which is FDA (Food and Drug Administration) approved to treat renal hypertension, could be repurposed as a novel therapeutic agent for patients with D1R-expressing tumors.

Thrombin induces ischemic LTP (iLTP): implications for synaptic plasticity in the acute phase of ischemic stroke

Acute brain ischemia modifies synaptic plasticity by inducing ischemic long-term potentiation (iLTP) of synaptic transmission through the activation of N-Methyl-D-aspartate receptors (NMDAR). Thrombin, a blood coagulation factor, affects synaptic plasticity in an NMDAR dependent manner. Since its activity and concentration is increased in brain tissue upon acute stroke, we sought to clarify whether thrombin could mediate iLTP through the activation of its receptor Protease-Activated receptor 1 (PAR1). Extracellular recordings were obtained in CA1 region of hippocampal slices from C57BL/6 mice. In vitro ischemia was induced by acute (3 minutes) oxygen and glucose deprivation (OGD). A specific ex vivo enzymatic assay was employed to assess thrombin activity in hippocampal slices, while OGD-induced changes in prothrombin mRNA levels were assessed by (RT)qPCR. Upon OGD, thrombin activity increased in hippocampal slices. A robust potentiation of excitatory synaptic strength was detected, which occluded the ability to induce further LTP. Inhibition of either thrombin or its receptor PAR1 blocked iLTP and restored the physiological, stimulus induced LTP. Our study provides important insights on the early changes occurring at excitatory synapses after ischemia and indicates the thrombin/PAR1 pathway as a novel target for developing therapeutic strategies to restore synaptic function in the acute phase of ischemic stroke.