Simultaneous activation of mGlu2 and muscarinic receptors reverses MK-801-induced cognitive decline in rodents

Prevention of MK-801-induced amnestic effect with combined activation of 5-HT1A and muscarinic receptors in mice

BACKGROUND

Muscarinic or 5-HT1A receptors are crucial in learning and memory processes, and their expression is evident in the brain areas involved in cognition. The administration of the activators of these receptors prevents the development of cognitive dysfunctions in animal models of schizophrenia induced by MK-801 (N-methyl-d-aspartate receptor antagonist) administration. GABAergic dysfunction is considered as one of the most important causes of MK-801-induced spatial learning deficits.

METHODS

Novel object recognition (NOR) and Morris water maze (MWM) tests were used to study the anti-amnestic effect of the biased 5-HT1A receptor agonist (F15599) alone or in combinations with VU0357017 (M1 receptor allosteric agonist), VU0152100 (M4 receptor positive allosteric modulator), and VU0238429 (M5 receptor positive allosteric modulator) on MK-801-induced dysfunctions. The compounds were administered for 5 consecutive days. Animals tested with the MWM underwent 5-day training. Western blotting was used to study the expressions of 5-HT1A receptors and the level of GAD65 in the frontal cortices (FCs) and hippocampi of the animals.

RESULTS

F15599 prevented the amnestic effect induced by MK-801 in the MWM at a dose of 0.1 mg/kg. The co-administration of the compound with muscarinic receptors activators had no synergistic effect. The additive effect of the combinations was evident in the prevention of declarative memory dysfunctions investigated in NOR. The administration of MK-801 impaired 5-HT1A expression in the hippocampi and decreased GAD65 levels in both the FCs and hippocampi. The administration of muscarinic ligands prevented these MK-801-induced deficits only in the hippocampi of MWM-trained animals. No effects of the compounds were observed in untrained mice.

CONCLUSION

Our results indicate that F15599 prevents schizophrenia-related spatial learning deficits in the MWM; however, the activity of the compound is not intensified with muscarinic receptors activators. In contrast, the combined administration of the ligands is effective in the NOR model of declarative memory. The muscarinic receptors activators reversed MK-801-induced 5-HT1A and GAD65 dysfunctions in the hippocampi of MWM-trained mice, but not in untrained mice.

The Impact of LY487379 or CDPPB on eNOS Expression in the Mouse Brain and the Effect of Joint Administration of Compounds with NO• Releasers on MK-801- or Scopolamine-Driven Cognitive Dysfunction in Mice

The role of endothelial nitric oxide synthase (eNOS) in the regulation of a variety of biological processes is well established, and its dysfunction contributes to brain pathologies, including schizophrenia or Alzheimer's disease (AD). Positive allosteric modulators (PAMs) of metabotropic glutamate (mGlu) receptors were shown to be effective procognitive compounds, but little is known about their impact on eNOS expression and stability. Here, we investigated the influence of the acute and chronic administration of LY487379 or CDPPB (mGlu2 and mGlu5 PAMs), on eNOS expression in the mouse brain and the effect of the joint administration of the ligands with nitric oxide (NO) releasers, spermineNONOate or DETANONOate, in different combinations of doses, on MK-801- or scopolamine-induced amnesia in the novel object recognition (NOR) test. Our results indicate that both compounds provoked eNOS monomer formation, and CDPPB at a dose of 5 mg/kg exaggerated the effect of MK-801 or scopolamine. The coadministration of spermineNONOate or DETANONOate enhanced the antiamnesic effect of CDPPB or LY487379. The best activity was observed for ineffective or moderate dose combinations. The results indicate that treatment with mGluR2 and mGluR5 PAMs may be burdened with the risk of promoting eNOS uncoupling through the induction of dimer dissociation. Administration of the lowest possible doses of the compounds with NO• donors, which themselves have procognitive efficacy, may be proposed for the treatment of schizophrenia or AD.

Muscarinic M1 and M4 receptor agonists for schizophrenia: promising candidates for the therapeutic arsenal

INTRODUCTION

Successful phase 3 trials of KarXT in people with schizophrenia herald a new era of treating the disorder with drugs that do not target the dopamine D2 receptor. The active component of KarXT is xanomeline, a muscarinic (CHRM) M1 and M4 agonist, making muscarinic receptors a viable target for treating schizophrenia.

AREAS COVERED

This review covers the process of taking drugs that activate the muscarinic M1 and M4 receptors from conceptualization to the clinic and details the mechanisms by which activating the CHRM1 and 4 can affect the broad spectrum of symptoms experienced by people with schizophrenia.

EXPERT OPINION

Schizophrenia is a syndrome which means drugs that activate muscarinic M1 and M4 receptors, as was the case for antipsychotic drugs acting on the dopamine D2 receptor, will not give optimal outcomes in everyone within the syndrome. Thus, it would be ideal to identify people who are responsive to drugs activating the CHRM1 and 4. Given knowledge of the actions of these receptors, it is possible treatment non-response could be restricted to sub-groups within the syndrome who have deficits in cortical CHRM1 or those with one of the cognitive endophenotypes that may be identifiable by changes in the blood transcriptome.

Activation of Metabotropic Glutamate Receptor (mGlu2) and Muscarinic Receptors (M1, M4, and M5), Alone or in Combination, and Its Impact on the Acquisition and Retention of Learning in the Morris Water Maze, NMDA Expression and cGMP Synthesis

The Morris water maze (MWM) is regarded as one of the most popular tests for detecting spatial memory in rodents. Long-term potentiation and cGMP synthesis seem to be among the crucial factors involved in this type of learning. Muscarinic (M₁, M₄, and M₅ receptors) and metabotropic glutamate (mGlu) receptors are important targets in the search for antipsychotic drugs with the potency to treat cognitive disabilities associated with the disorder. Here, we show that muscarinic receptor activators (VU0357017, VU0152100, and VU0238429) and an mGlu₂ receptor activator, LY487379, dose-dependently prevented the development of cognitive disorders as a result of MK-801 administration in the MWM. The dose-ranges of the compounds were as follows: VU0357017, 0.25, 0.5, and 1 mg/kg; VU0152100, 0.05, 0.25, and 1 mg/kg; VU0238429, 1, 5, and 20 mg/kg; and LY487379, 0.5, 3, and 5 mg/kg. The co-administration of LY487379 with each of the individual muscarinic receptor ligands showed no synergistic effect, which contradicts the results obtained earlier in the novel object recognition (NOR) test. MWM learning resulted in increased cGMP synthesis, both in the cortex and hippocampi, when compared to that in intact animals, which was prevented by MK-801 administration. The investigated compounds at the highest doses reversed this MK-801-induced effect. Neither the procedure nor the treatment resulted in changes in GluN2B-NMDA expression.

Clinical investigations of compounds targeting metabotropic glutamate receptors

Pharmacological modulation of glutamate has long been considered to be of immense therapeutic utility. The metabotropic glutamate receptors (mGluRs) are potential targets for safely altering glutamate-driven excitation. Data support the potential therapeutic use of mGluR modulators in the treatment of anxiety, depression, schizophrenia, and other psychiatric disorders, pain, epilepsy, as well as neurodegenerative and neurodevelopmental disorders. For each of the three mGluR groups, compounds have been constructed that produce either potentiation or functional blockade. PET ligands for mGlu5Rs have been studied in a range of patient populations and several mGlu5R antagonists have been tested for potential efficacy in patients including mavoglurant, diploglurant, basimglurant, GET 73, and ADX10059. Efficacy with mGlu5R antagonists has been reported in trials with patients with gastroesophageal reflux disease; data from patients with Parkinson's disease or Fragile X syndrome have not been as robust as hoped. Fenobam was approved for use as an anxiolytic prior to its recognition as an mGlu5R antagonist. mGlu2/3R agonists (pomaglumated methionil) and mGlu2R agonists (JNJ-40411813, AZD 8529, and LY2979165) have been studied in patients with schizophrenia with promising but mixed results. Antagonists of mGlu2/3Rs (decoglurant and TS-161) have been studied in depression where TS-161 has advanced into a planned Phase 2 study in treatment-resistant depression. The Group III mGluRs are the least developed of the mGluR receptor targets. The mGlu4R potentiator, foliglurax, did not meet its primary endpoint in patients with Parkinson's disease. Ongoing efforts to develop mGluR-targeted compounds continue to promise these glutamate modulators as medicines for psychiatric and neurological disorders.

Procognitive activity of nitric oxide inhibitors and donors in animal models

Nitric oxide is a small gaseous molecule that plays important roles in the majority of biological functions. Impairments of NO-related pathways contribute to the majority of neurological disorders, such as Alzheimer's disease (AD), and mental disorders, such as schizophrenia. Cognitive decline is one of the most serious impairments accompanying both AD and schizophrenia. In the present study, the activities of NO donors, slow (spermine NONOate) or fast (DETANONOate) releasers, and selective inhibitor of neuronal nitric oxide synthase N(ω)-propyl-l-arginine (NPLA) were investigated in pharmacological models of schizophrenia and AD. Cognitive impairments were induced by administration of MK-801 or scopolamine and were measured in novel object recognition (NOR) and Y-maze tests. The compounds were investigated at doses of 0.05-0.5 mg/kg. The dose-dependent effectiveness of all the compounds was observed in the NOR test, while only the highest doses of spermine NONOate and NPLA were active in the Y-maze test. DETANONOate was not active in the Y-maze test. The impact of the investigated compounds on motor coordination was tested at doses of 0.5 and 1 mg/kg. Only NPLA at a dose of 1 mg/kg slightly disturbed motor coordination in animals.

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.

Serotonergic-Muscarinic Interaction within the Prefrontal Cortex as a Novel Target to Reverse Schizophrenia-Related Cognitive Symptoms

Recent studies revealed that the activation of serotonergic 5-HT_1A and muscarinic M₁, M₄, or M₅ receptors prevent MK-801-induced cognitive impairments in animal models. In the present study, the effectiveness of the simultaneous activation of 5-HT_1A and muscarinic receptors at preventing MK-801-induced cognitive deficits in novel object recognition (NOR) or Y-maze tests was investigated. Activators of 5-HT_1A (F15599), M₁ (VU0357017), M₄ (VU0152100), or M₅ (VU0238429) receptors administered at top doses for seven days reversed MK-801-induced deficits in the NOR test, similar to the simultaneous administration of subeffective doses of F15599 (0.05 mg/kg) with VU0357017 (0.15 mg/kg), VU0152100 (0.05 mg/kg), or VU0238429 (1 mg/kg). The compounds did not prevent the MK-801-induced impairment when administered acutely. Their activity was less evident in the Y-maze. Pharmacokinetic studies revealed high brain penetration of F15599 (brain/plasma ratio 620%), which was detected in the frontal cortex (FC) up to 2 h after administration. Decreases in the brain penetration properties of the compounds were observed after acute administration of the combinations, which might have influenced behavioral responses. This negative effect on brain penetration was not observed when the compounds were administered repeatedly. Based on our results, prolonged administration of a 5-HT_1A activator with muscarinic receptor ligands may be effective at reversing cognitive decline related to schizophrenia, and the FC may play a critical role in this interaction.

Group II metabotropic glutamate receptor (mGlu2 and mGlu3 ) roles in thalamic processing

BACKGROUND AND PURPOSE

As the thalamus underpins almost all aspects of behaviour, it is important to understand how the thalamus operates. Group II metabotropic glutamate (mGlu₂ /mGlu₃ ) receptor activation reduces inhibition in thalamic nuclei originating from the surrounding thalamic reticular nucleus (TRN). Whilst an mGlu₂ component to this effect has been reported, in this study, we demonstrate that it is likely, largely mediated via mGlu₃ .

EXPERIMENTAL APPROACH

The somatosensory ventrobasal thalamus (VB) is an established model for probing fundamental principles of thalamic function. In vitro slices conserving VB-TRN circuitry from wild-type and mGlu₃ knockout mouse brains were used to record IPSPs and mIPSCs. In vivo extracellular recordings were made from VB neurons in anaesthetised rats. A range of selective pharmacological agents were used to probe Group II mGlu receptor function (agonist, LY354740; antagonist, LY341495; mGlu₂ positive allosteric modulator, LY487379 and mixed mGlu₂ agonist/mGlu₃ antagonist LY395756).

KEY RESULTS

The in vitro and in vivo data are complementary and suggest that mGlu₃ receptor activation is largely responsible for potentiating responses to somatosensory stimulation by reducing inhibition from the TRN.

CONCLUSIONS AND IMPLICATIONS

mGlu₃ receptor activation in the VB likely enables important somatosensory information to be discerned from background activity. These mGlu₃ receptors are likely to be endogenously activated via 'glutamate spillover'. In cognitive thalamic nuclei, this mechanism may be of importance in governing attentional processes. Positive allosteric modulation of endogenous mGlu₃ receptor activation may therefore enhance cognitive function in pathophysiological disease states, such as schizophrenia, thus representing a highly specific therapeutic target.

Regulation of Glutamatergic Activity via Bidirectional Activation of Two Select Receptors as a Novel Approach in Antipsychotic Drug Discovery

Schizophrenia is a mental disorder that affects approximately 1-2% of the population and develops in early adulthood. The disease is characterized by positive, negative, and cognitive symptoms. A large percentage of patients with schizophrenia have a treatment-resistant disease, and the risk of developing adverse effects is high. Many researchers have attempted to introduce new antipsychotic drugs to the clinic, but most of these treatments failed, and the diversity of schizophrenic symptoms is one of the causes of disappointing results. The present review summarizes the results of our latest papers, showing that the simultaneous activation of two receptors with sub-effective doses of their ligands induces similar effects as the highest dose of each compound alone. The treatments were focused on inhibiting the increased glutamate release responsible for schizophrenia arousal, without interacting with dopamine (D₂) receptors. Ligands activating metabotropic receptors for glutamate, GABA_B or muscarinic receptors were used, and the compounds were administered in several different combinations. Some combinations reversed all schizophrenia-related deficits in animal models, but others were active only in select models of schizophrenia symptoms (i.e., cognitive or negative symptoms).