The challenges of modulating the ‘rest and digest’ system: acetylcholine receptors as drug targets

Emerging drugs for the treatment of irritability associated with autism spectrum disorder

INTRODUCTION

Autism spectrum disorder (ASD) is an early-onset disorder with a prevalence of 1% among children and reported disability-adjusted life years of 4.31 million. Irritability is a challenging behavior associated with ASD, for which medication development has lagged. More specifically, pharmacotherapy effectiveness may be limited against high adverse effects (considering side effect profiles and patient medication sensitivity); thus, the possible benefits of pharmacological interventions must be balanced against potential adverse events in each patient.

AREAS COVERED

After reviewing the neuropathophysiology of ASD-associated irritability, the benefits and tolerability of emerging medications in its treatment based on randomized controlled trials were detailed in light of mechanisms and targets of action.

EXPERT OPINION

Succeeding risperidone and aripiprazole, monotherapy with memantine may be beneficial. In addition, N-acetylcysteine, galantamine, sulforaphane, celecoxib, palmitoylethanolamide, pentoxifylline, simvastatin, minocycline, amantadine, pregnenolone, prednisolone, riluzole, propentofylline, pioglitazone, and topiramate, all adjunct to risperidone, and clonidine and methylphenidate outperformed placebo. These effects were through glutamatergic, γ-aminobutyric acidergic, inflammatory, oxidative, cholinergic, dopaminergic, and serotonergic systems. All medications were reported to be safe and tolerable. Considering sample size, follow-up, and effect size, further studies are necessary. Along with drug development, repositioning and combining existing drugs supported by the mechanism of action is recommended.

The optimization of a novel selective antagonist for human M2 muscarinic acetylcholine receptor

Muscarinic acetylcholine receptors (mAChRs) comprise five distinct subtypes denoted M₁ to M₅. The antagonism of M₂ subtype could increase the release of acetylcholine from vesicles into the synaptic cleft and improve postsynaptic functions in the hippocampus via M₁ receptor activation, displaying therapeutic potentials for Alzheimer's disease. However, drug development for M₂ antagonists is still challenged among different receptor subtypes. In this study, by optimizing a scaffold from virtual screening, we synthesized two focused libraries and generated up to 50 derivatives. By measuring potency and binding selectivity, we discovered a novel M₂ antagonist, ligand 47, featuring submicromolar IC₅₀, high M₂/M₄ selectivity (~30-fold) and suitable lipophilicity (cLogP = 4.55). Further study with these compounds also illustrates the structure-activity relationship of this novel scaffold. Our study could not only provide novel lead structure, which was easy to synthesize, but also offer valuable information for further development of selective M₂ ligands.

Cardiovascular Disorders Mediated by Autonomic Nervous System Dysfunction

Cardiovascular disorders, such as orthostatic hypotension and supine hypertension, are common in patients with neurodegenerative synucleinopathies such as Parkinson disease (PD), and may also occur in other conditions, such as peripheral neuropathies, that result in autonomic nervous system (ANS) dysfunction. Dysfunction and degeneration of the ANS are implicated in the development of orthostatic and postprandial hypotension and impaired thermoregulation. Neurogenic orthostatic hypotension (nOH) results from sympathetic failure and is a common autonomic disorder in PD. Supine hypertension may also occur as a result of both sympathetic and parasympathetic dysfunction in conjunction with nOH in the majority of patients with PD. Management of supine hypertension in the setting of nOH can be counterintuitive and challenging. Additionally, the presence of other noncardiovascular comorbidities, such as diabetes mellitus and peripheral edema, may further contribute to the burden of disease. ANS dysfunction thus presents major healthcare implications and challenges for neurology and cardiovascular practices, necessitating an integrated neurology and cardiology management approach.

Mnemonic and behavioral effects of biperiden, an M1-selective antagonist, in the rat

RATIONALE

There is a persistent pressing need for valid animal models of cognitive and mnemonic disruptions (such as seen in Alzheimer's disease and other dementias) usable for preclinical research.

OBJECTIVES

We have set out to test the validity of administration of biperiden, an M1-acetylcholine receptor antagonist with central selectivity, as a potential tool for generating a fast screening model of cognitive impairment, in outbred Wistar rats.

METHODS

We used several variants of the Morris water maze task: (1) reversal learning, to assess cognitive flexibility, with probe trials testing memory retention; (2) delayed matching to position (DMP), to evaluate working memory; and (3) "counter-balanced acquisition," to test for possible anomalies in acquisition learning. We also included a visible platform paradigm to reveal possible sensorimotor and motivational deficits.

RESULTS

A significant effect of biperiden on memory acquisition and retention was found in the counter-balanced acquisition and probe trials of the counter-balanced acquisition and reversal tasks. Strikingly, a less pronounced deficit was observed in the DMP. No effects were revealed in the reversal learning task.

CONCLUSIONS

Based on our results, we do not recommend biperiden as a reliable tool for modeling cognitive impairment.

Multitarget drug design strategy in Alzheimer's disease: focus on cholinergic transmission and amyloid-β aggregation

Aim: Alzheimer pathogenesis has been associated with a network of processes working simultaneously and synergistically. Over time, much interest has been focused on cholinergic transmission and its mutual interconnections with other active players of the disease. Besides the cholinesterase mainstay, the multifaceted interplay between nicotinic receptors and amyloid is actually considered to have a central role in neuroprotection. Thus, the multitarget drug-design strategy has emerged as a chance to face the disease network. Methods: By exploiting the multitarget approach, hybrid compounds have been synthesized and studied in vitro and in silico toward selected targets of the cholinergic and amyloidogenic pathways. Results: The new molecules were able to target the cholinergic system, by joining direct nicotinic receptor stimulation to acetylcholinesterase inhibition, and to inhibit amyloid-β aggregation. Conclusion: The compounds emerged as a suitable starting point for a further optimization process.

Drugs Interfering with Muscarinic Acetylcholine Receptors and Their Effects on Place Navigation

Muscarinic acetylcholine receptors (mAChRs) have been found to regulate many diverse functions, ranging from motivation and feeding to spatial navigation, an important and widely studied type of cognitive behavior. Systemic administration of non-selective antagonists of mAChRs, such as scopolamine or atropine, have been found to have adverse effects on a vast majority of place navigation tasks. However, many of these results may be potentially confounded by disruptions of functions other than spatial learning and memory. Although studies with selective antimuscarinics point to mutually opposite effects of M1 and M2 receptors, their particular contribution to spatial cognition is still poorly understood, partly due to a lack of truly selective agents. Furthermore, constitutive knock-outs do not always support results from selective antagonists. For modeling impaired spatial cognition, the scopolamine-induced amnesia model still maintains some limited validity, but there is an apparent need for more targeted approaches such as local intracerebral administration of antagonists, as well as novel techniques such as optogenetics focused on cholinergic neurons and chemogenetics aimed at cells expressing metabotropic mAChRs.