Nitric Oxide-Dependent Mechanisms Underlying MK-801- or Scopolamine-Induced Memory Dysfunction in Animals: Mechanistic Studies

MK-801, an NMDA receptor antagonist, and scopolamine, a cholinergic receptor blocker, are widely used as tool compounds to induce learning and memory deficits in animal models to study schizophrenia or Alzheimer-type dementia (AD), respectively. Memory impairments are observed after either acute or chronic administration of either compound. The present experiments were performed to study the nitric oxide (NO)-related mechanisms underlying memory dysfunction induced by acute or chronic (14 days) administration of MK-801 (0.3 mg/kg, i.p.) or scopolamine (1 mg/kg, i.p.). The levels of L-arginine and its derivatives, L-citrulline, L-glutamate, L-glutamine and L-ornithine, were measured. The expression of constitutive nitric oxide synthases (cNOS), dimethylaminohydrolase (DDAH1) and protein arginine N-methyltransferases (PMRTs) 1 and 5 was evaluated, and the impact of the studied tool compounds on cGMP production and NMDA receptors was measured. The studies were performed in both the cortex and hippocampus of mice. S-nitrosylation of selected proteins, such as GLT-1, APP and tau, was also investigated. Our results indicate that the availability of L-arginine decreased after chronic administration of MK-801 or scopolamine, as both the amino acid itself as well as its level in proportion to its derivatives (SDMA and NMMA) were decreased. Additionally, among all three methylamines, SDMA was the most abundant in the brain (~70%). Administration of either compound impaired eNOS-derived NO production, increasing the monomer levels, and had no significant impact on nNOS. Both compounds elevated DDAH1 expression, and slight decreases in PMRT1 and PMRT5 in the cortex after scopolamine (acute) and MK-801 (chronic) administration were observed in the PFC, respectively. Administration of MK-801 induced a decrease in the cGMP level in the hippocampus, accompanied by decreased NMDA expression, while increased cGMP production and decreased NMDA receptor expression were observed after scopolamine administration. Chronic MK-801 and scopolamine administration affected S-nitrosylation of GLT-1 transport protein. Our results indicate that the analyzed tool compounds used in pharmacological models of schizophrenia or AD induce changes in NO-related pathways in the brain structures involved in cognition. To some extent, the changes resemble those observed in human samples.

The effects of galangin in prepulse inhibition test and experimental schizophrenia models

Objective: Acetylcholinesterase inhibitors are the focus of interest in the management of schizophrenia. We aimed to investigate the effects of acute galangin administration, a flavonoid compound with acetylcholinesterase inhibiting activity, on schizophrenia-associated cognitive deficits in rats and schizophrenia models in mice. Methods: Apomorphine-induced prepulse inhibition (PPI) disruption for cognitive functions, nicotinic, muscarinic and serotonergic mechanism involvement, and brain acetylcholine levels were investigated in Wistar rats. Apomorphine-induced climbing, MK-801-induced hyperlocomotion, and catalepsy tests were used as schizophrenia models in Swiss albino mice. The effects of galangin were compared with acetylcholinesterase inhibitor donepezil, and typical and atypical antipsychotics haloperidol and olanzapine, respectively. Results: Galangin (50,100 mg/kg) enhanced apomorphine-induced PPI disruption similar to donepezil, haloperidol, and olanzapine (p<0.05). This effect was not altered in the combination of galangin with the nicotinic receptor antagonist mecamylamine (1 mg/kg), the muscarinic receptor antagonist scopolamine (0.05 mg/kg), or the serotonin-1A receptor antagonist WAY-100635 (1 mg/kg) (p>0.05). Galangin (50,100 mg/kg) alone increased brain acetylcholine concentrations(p<0.05), but not in apomorphine-injected rats (p>0.05). Galangin (50 mg/kg) decreased apomorphine-induced climbing and MK-801-induced hyperlocomotion similar to haloperidol and olanzapine (p<0.05), but did not induce catalepsy, unlike them. Conclusion: We suggest that galangin may help enhance schizophrenia-associated cognitive deficits, and nicotinic, muscarinic cholinergic and serotonin-1A receptors are not involved in this effect. Galangin also exerted an antipsychotic-like effect without inducing catalepsy and may be considered as an advantageous antipsychotic agent.

The Impact of the Combined Administration of 1MeTIQ and MK-801 on Cell Viability, Oxidative Stress Markers, and Glutamate Release in the Rat Hippocampus

MK-801, as an N-methyl-D-aspartate (NMDA) receptor inhibitor, causes elevation in glutamate release, which may lead to an increase in excitotoxicity, oxidative stress and, consequently, cell death. 1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) shows antioxidant activity. The aim of the present study was to evaluate the effect of combined treatment with 1MeTIQ and MK-801 on cell viability, antioxidant enzyme activity, and glutamate release in the rat hippocampus. Cytotoxicity was measured using lactate dehydrogenase leakage assay (LDH) and the methyl tetrazolium (MTT) assay; antioxidant enzyme activity (glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT)) were measured by ELISA kits. The release of glutamate in the rat hippocampus was measured using in vivo microdialysis methodology. An in vitro study showed that MK-801 induced cell death in a concentration-dependent manner and that 1MeTIQ partially reduced this adverse effect of MK-801. An ex vivo study indicated that MK-801 produced an increase in antioxidant enzyme activity (GPx, GR, and SOD), whereas coadministration of MK-801 and 1MeTIQ restored the activity of these enzymes to the control level. An in vivo microdialysis study demonstrated that combined treatment with both drugs decreased the release of glutamate in the rat hippocampus. The above results revealed that 1MeTIQ shows limited neuroprotective activity under conditions of glutamate-induced neurotoxicity.

Антагонисты NMDA-рецепторов снижают чувствительность кожи при воздействии агониста TRPV1-рецепторов капсаицина

Глутаматные рецепторы широко представлены в центральной нервной системе и на периферии, что определяет их участие в центральной и периферической ноцицепции. В частности, NMDA-рецепторы локализованы в области дермоэпидермального соединения кожи. На иннервирующих кожу немиелинизированных нервных волокнах находятся TRPV1-рецепторы, способные функционально взаимодействовать с NMDA-рецепторами в каскадах кальций/кальмодулин-зависимой протеинкиназы типа II и протеинкиназы С. В данной работе оценено влияние неконкурентных антагонистов NMDA-рецепторов на периферическую сенситизацию кожи, вызванную подкожной инъекцией агониста TRPV1-рецепторов капсаицина, при разных способах введения. Установлено, что и высокоаффинный антагонист NMDA-рецепторов MK-801, и низкоаффинный антагонист NMDA-рецепторов гимантан снижают длительность реакции мышей на подкожную инъекцию раствора капсаицина в область плюсны при накожной аппликации, системном (внутрибрюшинном для гимантана и подкожном для МК-801) и подкожном интраплантарном введении.