Memantine inhibits ATP-dependent K+ conductances in dopamine neurons of the rat substantia nigra pars compacta

Energy sensors in drug addiction: A potential therapeutic target

Addiction is defined as the repeated exposure and compulsive seek of psychotropic drugs that, despite the harmful effects, generate relapse after the abstinence period. The psychophysiological processes associated with drug addiction (acquisition/expression, withdrawal, and relapse) imply important alterations in neurotransmission and changes in presynaptic and postsynaptic plasticity and cellular structure (neuroadaptations) in neurons of the reward circuits (dopaminergic neuronal activity) and other corticolimbic regions. These neuroadaptation mechanisms imply important changes in neuronal energy balance and protein synthesis machinery. Scientific literature links drug-induced stimulation of dopaminergic and glutamatergic pathways along with presence of neurotrophic factors with alterations in synaptic plasticity and membrane excitability driven by metabolic sensors. Here, we provide current knowledge of the role of molecular targets that constitute true metabolic/energy sensors such as AMPK, mTOR, ERK, or K_ATP in the development of the different phases of addiction standing out the main brain regions (ventral tegmental area, nucleus accumbens, hippocampus, and amygdala) constituting the hubs in the development of addiction. Because the available treatments show very limited effectiveness, evaluating the drug efficacy of AMPK and mTOR specific modulators opens up the possibility of testing novel pharmacotherapies for an individualized approach in drug abuse.

The effect of memantine, an antagonist of the NMDA glutamate receptor, in in vitro and in vivo infections by Trypanosoma cruzi

Chagas disease, caused by Trypanosoma cruzi, is a neglected tropical disease that affects 5–6 million people in endemic areas of the Americas. Presently, chemotherapy relies on two compounds that were proposed as trypanocidal drugs four decades ago: nifurtimox and benznidazole. Both drugs are able to eliminate parasitemia and to avoid seroconversion in infected people when used in the acute phase; however, their use in the chronic phase (the time when the majority of cases are diagnosed) is limited due to their serious side effects. Memantine is a glutamate receptor antagonist in the central nervous system of mammals that has been used for the treatment of Alzheimer’s disease. Our group previously reported memantine as a trypanocidal drug that is able to induce apoptosis-like death in T. cruzi. In the present work, we further investigated the effects of memantine on the infection of RAW 264.7 macrophages and in vivo (in BALB/c mice). Here, we showed that memantine is able to diminish NO and Ca²⁺ entry in both LPS-activated and non-activated cells. These results, together with the fact that memantine was also able to reduce the infection of macrophages, led us to propose that this drug is able to activate a pro-oxidant non-NO-dependent cell defense mechanism. Finally, infected mice that were treated with memantine had diminished parasitemia, cardiac parasitic load, and inflammatory infiltrates. In addition, the treated mice had an increased survival rate. Taken together, these results indicate memantine to be a candidate drug for the treatment of Chagas disease.

Chagas disease affects approximately 5 million people and is caused by the protist parasite Trypanosoma cruzi. Until now, there are no vaccines to prevent the human infection, and the therapy relies on the use of two drugs discovered more than 50 years ago, nifurtimox and benznidazole. Both drugs are efficient during the acute phase of the disease, however their efficacy in the chronic phase, when most of patients are diagnosed is controversial. In addition, both drugs are toxic, causing severe side effects during the treatment. For these reasons, new drugs against T. cruzi are urgently needed. In this work, we report a series of experiments supporting the repositioning of memantine, a drug used for treating Alzheimer´s disease, to treat the T. cruzi infection in an experimental infection model. Our data show that infected mice treated with memantine have diminished their parasitemia, cardiac parasitic load and inflammatory infiltrates and more importantly, they have diminished their mortality. Taken together, these results prompt memantine as a promising drug for treating Chagas disease.

Harnessing ionic mechanisms to achieve disease modification in neurodegenerative disorders

Progressive neuronal death is the key pathogenic event leading to clinical symptoms in neurodegenerative disorders (NDDs). Neuroprotective treatments are virtually unavailable, partly because of the marked internal heterogeneity of the mechanisms underlying pathology. Targeted neuroprotection would require deep mechanistic knowledge across the entire aetiological spectrum of each NDD and the development of tailored treatments. Although ideal, this strategy appears challenging, as it would require a degree of characterization of both the disease and the patient that is currently unavailable. The alternate strategy is to search for commonalities across molecularly distinct NDD forms and exploit these for the development of drugs with broad-spectrum efficacy. In this view, mounting evidence points to ionic mechanisms (IMs) as targets with potential therapeutic efficacy across distinct NDD subtypes. The scope of this review is to present clinical and preclinical evidence supporting the link between disruption of IMs and neuronal death in specific NDDs and to critically revise past and ongoing attempts of harnessing IMs for the development of neuroprotective treatments.

Memantine has no effect on KATP channels in pancreatic β cells

Objective

Memantine, a drug for Alzheimer’s disease, is considered to suppress excessive stimulation of N-methyl-d-aspartic acid receptors and to prevent neuronal death. However, a recent report indicated that the neuronal K_ATP channel also can become a target of memantine. The K_ATP channel is a key regulator of insulin secretion in pancreatic β cells. Therefore, if memantine could inhibit the K_ATP channel in pancreatic β cells, it would be an effective drug for both Alzheimer’s disease and diabetes. However, there is no report on the effect of memantine on the K_ATP channel in pancreatic β cells. Therefore, we investigated whether memantine affect the blood glucose level, insulin secretion and K_ATP channel activity in pancreatic β cells.

Results

An intraperitoneal glucose tolerance test was performed with or without memantine (1 mg/kg) injection in intact mice. Insulin secretion from isolated islets was measured under low (2 mM) and high (20 mM) glucose concentrations with or without memantine (1 μM). The effect of memantine (1 μM) on K_ATP channel currents in isolated pancreatic β cells was recorded using the whole-cell patch-clamp technique. Memantine had no effect on the blood glucose level, insulin secretion from isolated islets or K_ATP channel current in pancreatic β cells.

Memantine selectively blocks extrasynaptic NMDA receptors in rat substantia nigra dopamine neurons

Recent studies suggest that selective block of extrasynaptic N-methyl-d-aspartate (NMDA) receptors might protect against neurodegeneration. We recorded whole-cell currents with patch pipettes to characterize the ability of memantine, a low-affinity NMDA channel blocker, to block synaptic and extrasynaptic NMDA receptors in substantia nigra zona compacta (SNC) dopamine neurons in slices of rat brain. Pharmacologically isolated NMDA receptor-mediated EPSCs were evoked by electrical stimulation, whereas synaptic and extrasynaptic receptors were activated by superfusing the slice with NMDA (10 µM). Memantine was 15-fold more potent for blocking currents evoked by bath-applied NMDA compared to synaptic NMDA receptors. Increased potency for blocking bath-applied NMDA currents was shared by the GluN2C/GluN2D noncompetitive antagonist DQP-1105 but not by the high-affinity channel blocker MK-801. Our data suggest that memantine causes a selective block of extrasynaptic NMDA receptors that are likely to contain GluN2C/2D subunits. Our results justify further investigations on the use of memantine as a neuroprotective agent in Parkinson's disease.

Cell proliferation in the hippocampus and in the heart is modified by exposure to repeated stress and treatment with memantine

The present studies were aimed to verify the hypothesis that treatment with memantine, a low affinity NMDA glutamate receptor antagonist, can reduce possible stress-induced alterations in cell proliferation in the hippocampus and in the heart and has consequences on stress hormone release. Adult male Wistar rats were exposed to repeated hypokinesis (movement restraint, 2 h daily) or remained undisturbed and they were treated with memantine (5 mg/kg/day, s.c.) or vehicle for 8 days. On the day 7, all animals were injected with 5-bromo-2'-deoxyuridine (BrdU), a marker of cell proliferation. The mild form of chronic stress used resulted only in moderate decrease in BrdU incorporation into DNA in the hippocampus, while the same stimulus caused a pronounced reduction of the new cells formed in left heart ventricle. In both tissues, stress-induced reduction in cell proliferation was more evident in memantine-treated rats. Memantine failed to modify hormones of the hypothalamic-pituitary-adrenocortical axis, while the treatment increased plasma renin activity. The present study demonstrates that treatment with memantine potentiated rather than prevented stress-induced reduction of cell proliferation. We have shown that stress exposure may induce a reduction in cell proliferation in the heart, even in a higher extent than that in the hippocampus. Effects of memantine under stress conditions might be relevant with respect to clinical use of memantine, which is being used in the treatment of neurodegenerative diseases.

Effect of memantine on resting state default mode network activity in Alzheimer's disease

BACKGROUND

Memantine is an approved symptomatic treatment for moderate to severe Alzheimer's disease that reduces the excitotoxic effects of hyperactive glutamatergic transmission. However, the exact mechanism of the effect of memantine in Alzheimer's disease patients is poorly understood. Importantly, the default mode network (DMN), which plays a key role in attention, is hypoactive in Alzheimer's disease and is under glutamatergic control.

OBJECTIVE

To assess the effect of memantine on the activity of the DMN in moderate to severe Alzheimer's disease.

METHODS

Functional magnetic resonance imaging (MRI) data from 15 patients with moderate to severe Alzheimer's disease, seven treated with memantine (mean ± SD age 77 ± 8 years, mean ± SD Mini-Mental State Examination [MMSE] score 16 ± 5) and eight with placebo (mean ± SD age 76 ± 6 years, mean ± SD MMSE score 13 ± 1), were acquired at baseline (T0) and after 6 months of treatment (T6). Resting state components were extracted after spatial normalization in individual patients with independent component analysis. The consistency of the components was assessed using ICASSO and the DMN was recognized through spatial correlation with a pre-defined template. Voxel-based statistical analyses were performed to study the change in DMN activity from T0 to T6 in the two groups.

RESULTS

At T0, the two groups showed similar DMN activity except in the precuneus and cuneus, where the patients who started treatment with memantine had slightly greater activity (p < 0.05 corrected for familywise error [FWE]). The prospective comparison between T0 and T6 in the treated patients showed increased DMN activation mapping in the precuneus (p < 0.05, FWE corrected), while the prospective comparison in the untreated patients did not show significant changes. The treatment × time interaction term was significant at p < 0.05, FWE corrected.

CONCLUSIONS

The results suggest a positive effect of memantine treatment in patients with moderate to severe Alzheimer's disease, resulting in an increased resting DMN activity in the precuneus region over 6 months. Future studies confirming the present findings are required to further demonstrate the beneficial effects of memantine on the DMN in Alzheimer's disease.

Inhibitory effect of memantine, an NMDA-receptor antagonist, on electroporation-induced inward currents in pituitary GH3 cells

The membrane electroporation-induced inward current (IMEP) in pituitary tumor (GH3) cells was characterized. This current emerges irregularly when membrane hyperpolarizations to -200 mV with a holding potential of -80 mV were elicited. Neither E-4031 (10 μM), glibenclamide (30 μM), nor ZD7288 (30 μM) caused any effects on IMEP. The single-channel conductance and pore radius were estimated to be around 1.12 nS and 1.7 nm, respectively. LaCl3- and memantidine (MEM)-induced block of this current was also examined. The IC50 value for LaCl3- and MEM-induced inhibition of IMEP was 35 and 75 μM, respectively. However, unlike LaCl3, MEM (300 μM) did not exert any effect on voltage-gated Ca2+ current. In inside-out configuration, MEM applied to either external or internal surface of the excised patch did not suppress the activity of ATP-sensitive K+ channels expressed in GH3 cells, although glibenclamide significantly suppressed channel activity. This study provides the first evidence to show that MEM, a non-competitive antagonist of N-methyl D-aspartate receptors, directly inhibits the amplitude of IMEP in pituitary GH3 cells. MEM-mediated block of IMEP in these cells is unlinked to its inhibition of glutamate-induced currents or ATP-sensitive K+ currents. The channel-suppressing properties of MEM might contribute to the underlying mechanisms by which it and its structurally related compounds affect neuronal or neuroendocrine function.

Tetrahydroberberine blocks ATP-sensitive potassium channels in dopamine neurons acutely-dissociated from rat substantia nigra pars compacta

Tetrahydroberberine (THB) exhibits neuroprotective effects but its targets and underlying mechanisms are largely unknown. Emerging evidence indicates that ATP-sensitive potassium (K(ATP)) channels in the substantia nigra pars compacta (SNc) promote Parkinson disease (PD) pathogenesis, thus blocking K(ATP) channels may protect neurons against neuronal degeneration. In the present study, we tested a hypothesis that THB blocks K(ATP) channels in dopaminergic (DA) neurons acutely dissociated from rat SNc. Using perforated patch-clamp recording in current-clamp mode, the functional K(ATP) channels can be opened by persistent perfusion of rotenone, an inhibitor of complex I of the mitochondrial respiratory chain. Bath-application of THB reversibly blocks opened K(ATP) channels in a concentration-dependent manner, which is comparable to a classical K(ATP) channel blocker, Tol. Compared to THB analogs, l-stepholidine (l-SPD) or l-tetrahydropalmatine (l-THP), THB exhibits more profound blockade in K(ATP) channels. In addition, exposure of THB alone to the recorded neuron increases action potential firing, and THB also restores rotenone-induced membrane hyperpolarization in the presence of dopamine D2 receptor antagonist (sulpiride), suggesting that THB exhibits an excitatory effect on SNc DA neurons through the block of K(ATP) channels. Collectively, the blockade of neuronal K(ATP) channels by THB in SNc DA neurons is a novel pharmacological mechanism of THB, which may contribute to its neuroprotective effects in PD.

Firing properties and functional connectivity of substantia nigra pars compacta neurones recorded with a multi-electrode array in vitro

Dopamine (DA) neurones of the substantia nigra pars compacta (SNc) are involved in a wide variety of functions, including motor control and reward-based learning. In order to gain new insights into the firing properties of neuronal ensembles in the SNc, we recorded extracellular single units from spontaneously active neurones, using a multi-electrode array (MEA) device in midbrain slices. The majority of neurones (50.21%) had a low firing frequency (1-3 Hz) and a stable pacemaker-like pattern, while others (44.84%) were irregular, but still firing at a low rate. The remaining population (4.95%) comprised neurones with a regular higher firing rate (5-10 Hz). High rate neurones, on the whole, were insensitive to DA (30 mum), while low rate neurones were mostly inhibited by DA, although responding either with a prominent or a weak inhibition. However, we recorded low rate regular neurones that were insensitive to DA, or irregular low rate neurones excited by DA. Interestingly, we found pairs of active neurones (12.10 +/- 3.14%) with a significant proportion of spikes occurring synchronously. Moreover, the crosscorrelation probability in each pair tended to increase in response to DA. In conclusion, MEA recordings in midbrain slices reveal a much more complex picture than previously reported with regard to the firing pattern and DA sensitivity of spontaneously active SNc neurones. Moreover, the study opens new prospectives for the in vitro investigation of functional connectivity in the midbrain dopaminergic system, thus proposing new targets for the pharmacological treatment of DA-dependent neurological disorders.

Ca2+ influx through NMDA-gated channels activates ATP-sensitive K+ currents through a nitric oxide-cGMP pathway in subthalamic neurons

Excessive burst firing of action potentials in subthalamic nucleus (STN) neurons has been correlated with the bradykinesia and rigidity seen in Parkinson's disease. Consequently, there is much interest in characterizing mechanisms that promote burst firing, such as the regulation of NMDA receptor function. Using whole-cell recording techniques in rat brain slices, we report that inward currents evoked by NMDA are greatly potentiated by ATP-sensitive K(+) (K-ATP) channel blocking agents in STN neurons but not in dopamine neurons in the substantia nigra. Moreover, we found that the ability of NMDA to evoke K-ATP current was blocked by inhibitors of nitric oxide synthase, guanylyl cyclase, and calcium/calmodulin. By altering firing patterns of STN neurons, this NMDA/K-ATP interaction may exert an important influence on basal ganglia output and thereby affect the clinical expression of Parkinson's disease.

Adjunctive glycine in the treatment of obsessive-compulsive disorder in adults

BACKGROUND

Recent preclinical findings, case reports and non-blinded studies have suggested that glutamatergic interventions may be efficacious for Obsessive-Compulsive Disorder (OCD).

METHODS

We enrolled 24 adult outpatients with OCD on stabilized treatment regimens in a double-blind trial of adjunctive glycine, an NMDA glutamate receptor agonist. Participants were randomly assigned 1:1 to either placebo or glycine titrated to 60g/day, with follow-up visits scheduled at 4, 8 and 12 weeks. Yale-Brown Obsessive Compulsive Scale (Y-BOCS) was the principal outcome measure.

RESULTS

Regimen non-adherence, principally related to complaints about the taste and/or nausea, resulted in only 14 individuals who were evaluable by predetermined criteria. Those receiving glycine (n=5) experienced a mean decrease of 6.04 points in Y-BOCS score, compared with a 1.00 point decrease for those receiving placebo (n=9). Using a hierarchical linear model, compared with placebo, individuals who received glycine had an average 0.82 decrease in Y-BOCS score for each week they remained in the study, not quite reaching statistical significance (p=0.053). Two of those receiving glycine were responders, versus none receiving placebo (p=0.11, ns, Fisher exact). Despite the dropouts, two participants were known to have subsequently continued taking glycine through their regular treating psychiatrist for over a year.

CONCLUSIONS

The glycine condition approached efficacy for treatment of OCD in this study, with the high dropout rate related to problems with palatability and small sample size the principal caveats. This may indicate a new strategy for treatment of OCD, although confirmatory studies are clearly needed. (ClinicalTrials.gov NCT00405535.).

Blockade of the translocation and activation of c-Jun N-terminal kinase 3 (JNK3) attenuates dopaminergic neuronal damage in mouse model of Parkinson's disease

Increasing evidence suggests that c-Jun N-terminal kinase (JNK) is an important kinase mediating neuronal death in Parkinson's disease (PD) model induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). JNK3, the only neural-specific isoform, may play an important role in mediating the neurotoxic effects of MPTP in dopaminergic neuronal injury. To analyze the variation in JNK3 activation, the levels of phospho-JNK3 were measured at the various time points of occurrence of MPTP-induced lesions. In our study, we observed that during MPTP intoxication, two peaks of JNK3 activation appeared at 8 and 24h. To further define the mechanism of JNK3 activation and translocation, the antioxidant N-acetylcysteine (NAC), the N-methyl-D-aspartate (NMDA) receptor antagonist ketamine, and the alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate (KA) receptor antagonist 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX) were administered to the mice 30 min after each of the four MPTP injections. The results revealed that NAC clearly inhibited JNK3 activation during the early intoxication, whereas ketamine preferably attenuated JNK3 activation during the latter intoxication. DNQX had no significant effects on JNK3 activation during intoxication. Consequently, reactive oxygen species (ROS) and the NMDA receptor were closely associated with JNK3 activation following MPTP intoxication. NAC and ketamine exerted a preventive effect against MPTP-induced loss of tyrosine hydroxylase-positive neurons and suppressed the nuclear translocation of JNK3, suggesting that NAC and ketamine can prevent MPTP-induced dopaminergic neuronal death by suppressing JNK3 activation.

Memantine reduces neuronal dysfunctions triggered by in vitro ischemia and 3-nitropropionic acid

Memantine, a low-affinity uncompetitive NMDA receptor antagonist, has been widely utilized for the treatment of Alzheimer's disease. A possible neuroprotective role of this drug in pathophysiological conditions involving an altered energetic metabolism of the basal ganglia has never been addressed. Thus, we have characterized the electrophysiological effect of memantine on striatal spiny neurons recorded under control conditions and after in vitro ischemia (oxygen and glucose deprivation). Memantine reduced in a dose-dependent manner (EC(50)=5 microM) the irreversible loss of field potential amplitude induced by in vitro ischemia. The neuroprotective effect of memantine against in vitro ischemia was even more potent (EC(50)=3.2 microM) in the absence of external magnesium, a condition enhancing NMDA-mediated glutamatergic transmission. Memantine was also able to block long-term potentiation recorded from spiny neurons following a brief ischemic episode. Moreover, memantine showed protection against irreversible field potential loss induced by 3-nitropropionic acid (3-NP), an inhibitor of the mitochondrial complex II, without influencing toxicity induced by rotenone, a complex I inhibitor. Memantine could represent a potential neuroprotective agent in pathophysiological conditions involving an altered energy metabolism of basal ganglia.