In vivo pharmacological characterization of AC-3933, a benzodiazepine receptor partial inverse agonist for the treatment of Alzheimer's disease

Strategies for Treatment of Disease-Associated Dementia Beyond Alzheimer's Disease: An Update

Memory, cognition, dementia, and neurodegeneration are complexly interlinked processes with various mechanistic pathways, leading to a range of clinical outcomes. They are strongly associated with pathological conditions like Alzheimer's disease, Parkinson's disease, schizophrenia, and stroke and are a growing concern for their timely diagnosis and management. Several cognitionenhancing interventions for management include non-pharmacological interventions like diet, exercise, and physical activity, while pharmacological interventions include medicinal agents, herbal agents, and nutritional supplements. This review critically analyzed and discussed the currently available agents under different drug development phases designed to target the molecular targets, including cholinergic receptor, glutamatergic system, GABAergic targets, glycine site, serotonergic targets, histamine receptors, etc. Understanding memory formation and pathways involved therein aids in opening the new gateways to treating cognitive disorders. However, clinical studies suggest that there is still a dearth of knowledge about the pathological mechanism involved in neurological conditions, making the dropouts of agents from the initial phases of the clinical trial. Hence, a better understanding of the disease biology, mode of drug action, and interlinked mechanistic pathways at a molecular level is required.

Epileptic Mechanisms Shared by Alzheimer's Disease: Viewed via the Unique Lens of Genetic Epilepsy

Our recent work on genetic epilepsy (GE) has identified common mechanisms between GE and neurodegenerative diseases including Alzheimer's disease (AD). Although both disorders are seemingly unrelated and occur at opposite ends of the age spectrum, it is likely there are shared mechanisms and studies on GE could provide unique insights into AD pathogenesis. Neurodegenerative diseases are typically late-onset disorders, but the underlying pathology may have already occurred long before the clinical symptoms emerge. Pathophysiology in the early phase of these diseases is understudied but critical for developing mechanism-based treatment. In AD, increased seizure susceptibility and silent epileptiform activity due to disrupted excitatory/inhibitory (E/I) balance has been identified much earlier than cognition deficit. Increased epileptiform activity is likely a main pathology in the early phase that directly contributes to impaired cognition. It is an enormous challenge to model the early phase of pathology with conventional AD mouse models due to the chronic disease course, let alone the complex interplay between subclinical nonconvulsive epileptiform activity, AD pathology, and cognition deficit. We have extensively studied GE, especially with gene mutations that affect the GABA pathway such as mutations in GABA_A receptors and GABA transporter 1. We believe that some mouse models developed for studying GE and insights gained from GE could provide unique opportunity to understand AD. These include the pathology in early phase of AD, endoplasmic reticulum (ER) stress, and E/I imbalance as well as the contribution to cognitive deficit. In this review, we will focus on the overlapping mechanisms between GE and AD, the insights from mutations affecting GABA_A receptors, and GABA transporter 1. We will detail mechanisms of E/I imbalance and the toxic epileptiform generation in AD, and the complex interplay between ER stress, impaired membrane protein trafficking, and synaptic physiology in both GE and AD.

Chronic BACE-1 Inhibitor Administration in TASTPM Mice (APP KM670/671NL and PSEN1 M146V Mutation): An EEG Study

OBJECTIVE

In this exploratory study, we tested whether electroencephalographic (EEG) rhythms may reflect the effects of a chronic administration (4 weeks) of an anti-amyloid β-site amyloid precursor protein (APP) cleaving enzyme 1 inhibitor (BACE-1; ER-901356; Eisai Co., Ltd., Tokyo, Japan) in TASTPM (double mutation in APP KM670/671NL and PSEN1 M146V) producing Alzheimer's disease (AD) amyloid neuropathology as compared to wild type (WT) mice.

METHODS

Ongoing EEG rhythms were recorded from a bipolar frontoparietal and two monopolar frontomedial (prelimbic) and hippocampal channels in 11 WT Vehicle, 10 WT BACE-1, 10 TASTPM Vehicle, and 11 TASTPM BACE-1 mice (males; aged 8/9 months old at the beginning of treatment). Normalized EEG power (density) was compared between the first day (Day 0) and after 4 weeks (Week 4) of the BACE-1 inhibitor (10 mg/Kg) or vehicle administration in the 4 mouse groups. Frequency and magnitude of individual EEG delta and theta frequency peaks (IDF and ITF) were considered during animal conditions of behaviorally passive and active wakefulness. Cognitive status was not tested.

RESULTS

Compared with the WT group, the TASTPM group generally showed a significantly lower reactivity in frontoparietal ITF power during the active over the passive condition (p < 0.05). Notably, there was no other statistically significant effect (e.g., additional electrodes, recording time, and BACE-1 inhibitor).

CONCLUSIONS

The above EEG biomarkers reflected differences between the WT and TASTPM groups, but no BACE-1 inhibitor effect. The results suggest an enhanced experimental design with the use of younger mice, longer drug administrations, an effective control drug, and neuropathological amyloid markers.

Recent progress on the role of GABAergic neurotransmission in the pathogenesis of Alzheimer's disease

Despite their possible causative role, targeting amyloidosis, tau phosphorylation, acetylcholine esterase, glutamate, oxidative stress and mitochondrial metabolism have not yet led to the development of drugs to cure Alzheimer's disease (AD). Recent preclinical and clinical reports exhibit a surge in interest in the role of GABAergic neurotransmission in the pathogenesis of AD. The interaction among GABAergic signaling, amyloid-β and acetylcholine is shown to affect the homeostasis between excitation (glutamate) and inhibition (GABA) in the brain. As a consequence, over-excitation leads to neurodegeneration (excitotoxicity) and impairment in the higher level functions. Previously, the glutamate arm of this balance received the most attention. Recent literature suggests that over-excitation is primarily mediated by dysfunctional GABA signaling and can possibly be restored by rectifying anomalous metabolism observed in the GABAergic neurons during AD. Additionally, neurogenesis and synaptogenesis have also been linked with GABAergic signaling. This association may provide a basis for the needed repair mechanism. Furthermore, several preclinical interventional studies revealed that targeting various GABA receptor subtypes holds potential in overcoming the memory deficits associated with AD. In conclusion, the recent scientific literature suggests that GABAergic signaling presents itself as a promising target for anti-AD drug development.

Pharmaco-EEG Studies in Animals: An Overview of Contemporary Translational Applications

The contemporary value of animal pharmaco-electroencephalography (p-EEG)-based applications are strongly interlinked with progress in recording and neuroscience analysis methodology. While p-EEG in humans and animals has been shown to be closely related in terms of underlying neuronal substrates, both translational and back-translational approaches are being used to address extrapolation issues and optimize the translational validity of preclinical animal p-EEG paradigms and data. Present applications build further on animal p-EEG and pharmaco-sleep EEG findings, but also on stimulation protocols, more specifically pharmaco-event-related potentials. Pharmaceutical research into novel treatments for neurological and psychiatric diseases has employed an increasing number of pharmacological as well as transgenic models to assess the potential therapeutic involvement of different neurochemical systems and novel drug targets as well as underlying neuronal connectivity and synaptic function. Consequently, p-EEG studies, now also readily applied in modeled animals, continue to have an important role in drug discovery and development, with progressively more emphasis on its potential as a central readout for target engagement and as a (translational) functional marker of neuronal circuit processes underlying normal and pathological brain functioning. In a similar vein as was done for human p-EEG studies, the contribution of animal p-EEG studies can further benefit by adherence to guidelines for methodological standardization, which are presently under construction by the International Pharmaco-EEG Society (IPEG).

AC-3933, a benzodiazepine partial inverse agonist, improves memory performance in MK-801-induced amnesia mouse model

AC-3933, a novel benzodiazepine receptor partial inverse agonist, is a drug candidate for cognitive disorders including Alzheimer's disease. We have previously reported that AC-3933 enhances acetylcholine release in the rat hippocampus and ameliorates scopolamine-induced memory impairment and age-related cognitive decline in both rats and mice. In this study, we further evaluated the procognitive effect of AC-3933 on memory impairment induced by MK-801, an N-methyl-d-aspartate receptor antagonist, in mice. Unlike the acetylcholinesterase inhibitor donepezil and the benzodiazepine receptor inverse agonist FG-7142, oral administration of AC-3933 significantly ameliorated MK-801-induced memory impairment in the Y-maze test and in the object location test. Interestingly, the procognitive effects of AC-3933 on MK-801-induced memory impairment were not affected by the benzodiazepine receptor antagonist flumazenil, although this was not the case for the beneficial effects of AC-3933 on scopolamine-induced memory deficit. Moreover, the onset of AC-3933 ameliorating effect on scopolamine- or MK-801-induced memory impairment was different in the Y-maze test. Taken together, these results indicate that AC-3933 improves memory deficits caused by both cholinergic and glutamatergic hypofunction and suggest that the ameliorating effect of AC-3933 on MK-801-induced memory impairment is mediated by a mechanism other than inverse activation of the benzodiazepine receptor.

Differential modulation of GABAA and NMDA receptors by α7-nicotinic receptor desensitization in cultured rat hippocampal neurons

AIM

To explore the modulatory effect of desensitized α7-containing nicotinic receptors (α7-nAChRs) on excitatory and inhibitory amino acid receptors in cultured hippocampal neurons and to identify the mechanism underlying this effect.

METHODS

Whole-cell patch-clamp recordings were performed on cultured rat hippocampal neurons to measure α7-nAChR currents and to determine the role of desensitized α7-nAChRs on brain amino acid receptor activity.

RESULTS

Pulse and perfusion applications of the α7-nAChR agonist choline were applied to induce different types of α7-nAChR desensitization in cultured hippocampal neurons. After a brief choline pulse, α7-nAChR was desensitized as a result of receptor activation, which reduced the response of the A type γ-aminobutyric acid (GABAA) receptor to its agonist, muscimol, and enhanced the response of the NMDA receptor to its agonist NMDA. By contrast, the responses of glycine or AMPA receptors to their agonists, glycine or AMPA, respectively, were not affected. Pretreatment with the α7-nAChR antagonist methyllycaconitine (MLA, 10 nmol/L) blocked the choline-induced negative modulation of the GABAA receptor and the positive modulation of the NMDA receptor. The regulation of the GABAA and NMDA receptors was confirmed using another type of α7-nAChR desensitization, which was produced by a low concentration of choline perfusion. The negative modulation of the GABAA receptor was characterized by choline-duration dependency and intracellular calcium dependency, but the positive modulation of the NMDA receptor was not associated with cytoplasmic calcium.

CONCLUSION

Brain GABAA and NMDA receptors are modulated negatively and positively, respectively, by desensitized α7-nAChR as a result of choline pretreatment in cultured hippocampal neurons.

The Benzodiazepine-Dementia Disorders Link: Current State of Knowledge

The short-term effects of benzodiazepines on memory are well established and are suspected in the long term. Eleven studies have been published so far concerning benzodiazepine use and the risk of dementia disorders; nine of these studies concluded these drugs have a deleterious effect, one found a protective effect, and one (the most recently published) observed no effect. The positive association found in some studies could be due to a reverse causation bias since the main indications for benzodiazepines (e.g. sleep disorders, anxiety) can also be prodromes of dementia disorders. This bias is less likely for treatments started more than 10 years before the diagnosis. Among others, three mechanisms could underlie the potential influence of benzodiazepines on the development of dementia disorders. First, benzodiazepines can decrease beta-site amyloid precursor protein-cleaving enzyme 1 (BACE-1) and γ-secretase activity and slow down the accumulation of Aβ oligomers in the brain. This potential positive effect has never been confirmed; the same is true for the prevention of excitotoxicity through benzodiazepine anti-glutamatergic action. Second, since astrocytes located in the area of amyloid plaques could have gamma-aminobutyric acid (GABA)-secreting activity, patients with pre-dementia lesions could be at increased risk of presenting with more pronounced deleterious cognitive effects of benzodiazepines. Finally, owing to the neural compensation and cognitive reserve concepts, some subjects could cope with initial lesions by using/developing alternative networks. By lowering the brain activation level, benzodiazepines could limit this capacity. In conclusion, it is essential that animal studies explore the mechanistic hypotheses of this association found by most of the pharmacoepidemiological studies conducted on this topic.