Anticholinergic activity in the nervous system: Consequences for visuomotor function

Muscarinic receptors mediate motivation via preparatory neural activity in humans

Motivation depends on dopamine, but might be modulated by acetylcholine which influences dopamine release in the striatum, and amplifies motivation in animal studies. A corresponding effect in humans would be important clinically, since anticholinergic drugs are frequently used in Parkinson's disease, a condition that can also disrupt motivation. Reward and dopamine make us more ready to respond, as indexed by reaction times (RT), and move faster, sometimes termed vigour. These effects may be controlled by preparatory processes that can be tracked using electroencephalography (EEG). We measured vigour in a placebo-controlled, double-blinded study of trihexyphenidyl (THP), a muscarinic antagonist, with an incentivised eye movement task and EEG. Participants responded faster and with greater vigour when incentives were high, but THP blunted these motivational effects, suggesting that muscarinic receptors facilitate invigoration by reward. Preparatory EEG build-up (contingent negative variation [CNV]) was strengthened by high incentives and by muscarinic blockade, although THP reduced the incentive effect. The amplitude of preparatory activity predicted both vigour and RT, although over distinct scalp regions; frontal activity predicted vigour, whereas a larger, earlier, central component predicted RT. The incentivisation of RT was partly mediated by the CNV, though vigour was not. Moreover, the CNV mediated the drug's effect on dampening incentives, suggesting that muscarinic receptors underlie the motivational influence on this preparatory activity. Taken together, these findings show that a muscarinic blocker impairs motivated action in healthy people, and that medial frontal preparatory neural activity mediates this for RT.

Protective role of acetylcholine and the cholinergic system in the injured heart

This review explores the roles of the cholinergic system in the heart, comprising the neuronal and non-neuronal cholinergic systems. Both systems are essential for maintaining cardiac homeostasis by regulating the release of acetylcholine (ACh). A reduction in ACh release is associated with the early onset of cardiovascular diseases (CVDs), and increasing evidence supports the protective roles of ACh against CVD. We address the challenges and limitations of current strategies to elevate ACh levels, including vagus nerve stimulation and pharmacological interventions such as cholinesterase inhibitors. Additionally, we introduce alternative strategies to increase ACh in the heart, such as stem cell therapy, gene therapy, microRNAs, and nanoparticle drug delivery methods. These findings offer new insights into advanced treatments for regenerating the injured human heart.

The impact of traumatic brain injury on inhibitory control processes assessed using a delayed antisaccade task

It has been well established that traumatic brain injury (TBI) can affect cognitive function such as attention, working memory and executive functions. In the present study, we further investigated TBI-related changes in cognitive functions by investigating the ability to reorient visuospatial attention using a modified antisaccade task. Performing an antisaccade requires disengaging attention, inhibiting a reflexive saccade, and then engaging attention to execute a voluntary saccade in a direction opposite to a peripheral target. Particularly we quantified the time (latency), and accuracy (directional and disinhibition errors) of 26 TBI and 33 normal participants in making an antisaccade after a variable period of delay (0, 0.0625, 0.125, 0.250, 0.500 or 1.0 s). Changing the delay period allowed to systematically quantify the temporal and spatial characteristics of preparing and initiating an antisaccade and whether this process is affected by TBI. TBI participants took longer (approximately 33-66 ms for variable delays) to generate correct delayed antisaccades and showed increased directional errors (2-11 % for variable delays) and increased disinhibition prosaccade errors (2-6 % for variable delays) compared to controls. However, both groups made similar disinhibition antisaccade errors. These findings indicate that TBI participants required a longer time to process information, and a possible poorer response inhibition and poor spatial information processing due to head injury.

Voluntary activation of muscle in humans: does serotonergic neuromodulation matter?

Ionotropic inputs to motoneurones have the capacity to depolarise and hyperpolarise the motoneurone, whereas neuromodulatory inputs control the state of excitability of the motoneurone. Intracellular recordings of motoneurones from in vitro and in situ animal preparations have provided extraordinary insight into the mechanisms that underpin how neuromodulators regulate neuronal excitability. However, far fewer studies have attempted to translate the findings from cellular and molecular studies into a human model. In this review, we focus on the role that serotonin (5-HT) plays in muscle activation in humans. 5-HT is a potent regulator of neuronal firing rates, which can influence the force that can be generated by muscles during voluntary contractions. We firstly outline structural and functional characteristics of the serotonergic system, and then describe how motoneurone discharge can be facilitated and suppressed depending on the 5-HT receptor subtype that is activated. We then provide a narrative on how 5-HT effects can influence voluntary activation during muscle contractions in humans, and detail how 5-HT may be a mediator of exercise-induced fatigue that arises from the central nervous system.

Transient Bilateral Ophthalmoplegia: A Case of a Forgotten Anesthetic Medication Effect

A 58-year-old woman was found to have bilateral ptosis and downward gaze deviation immediately after elective shoulder surgery with general anesthesia and supraclavicular nerve block. A code stroke was activated due to concern for the neurologic process, but neuroimaging did not reveal acute changes or vascular abnormality. Her symptoms gradually resolved in the following hours with supportive care and were ultimately deemed to be related to anesthetic and transdermal scopolamine exposures layered upon her underlying comorbidities. Transient bilateral ophthalmoplegia after general anesthetics has been previously described; drug effect should be considered in the differential of this alarming presentation, which can mimic acute stroke and/or Horner syndrome.

Favorable Effects of Tacrolimus Monotherapy on Myasthenia Gravis Patients

Background and Purpose: Tacrolimus (TAC) has been proven to be a rapid-acting, steroid-sparing agent for myasthenia gravis (MG) therapy. However, evidence related to the effectiveness of TAC alone is rare. Therefore, this study was performed to investigate the effect of TAC monotherapy in MG patients.

Methods: Forty-four MG patients who received TAC monotherapy were retrospectively analyzed. A mixed effect model was used to analyze improvements in MG-specific activities of daily living scale (MG-ADL), quantitative MG score (QMG) and MG-ADL subscores. Kaplan-Meier analysis was used to estimate the cumulative probability of minimal manifestations (MM) or better. Adverse events (AEs) were recorded for safety analyses.

Results: Of the patients receiving TAC monotherapy, MG-ADL scores were remarkably improved at 3, 6 and 12 months compared with scores at baseline (mean difference and 95% CIs: −3.29 [−4.94, −1.64], −3.97 [−5.67, −2.27], and −4.67 [−6.48, −2.85], respectively). QMG scores significantly decreased at 6 and 12 months, with mean differences and 95% CIs of −4.67(−6.88, −2.45) and −5.77 (−7.55, −4.00), respectively. Estimated median period to achieve “MM or better” was 5.0 (95% CIs, 2.8, 7.2) months. Ocular MG (OMG) and generalized MG (GMG) showed similar therapeutic effects in cumulative probabilities of “MM or better” (P-value = 0.764). A better response was observed in MG-ADL subscores for ptosis and bulbar symptoms. AEs occurred in 37.5% of patients and were generally mild and reversible.

Conclusions: TAC monotherapy is a promising option to rapidly alleviate all symptoms of MG, especially for ptosis and bulbar symptoms.

Importance of protein dynamics in the structure-based drug discovery of class A G protein-coupled receptors (GPCRs)

Demand for novel GPCR modulators is increasing as the association between the GPCR signaling pathway and numerous diseases such as cancers, psychological and metabolic disorders continues to be established. In silico structure-based drug design (SBDD) offers an outlet where researchers could exploit the accumulating structural information of GPCR to expedite the process of drug discovery. The coupling of structure-based approaches such as virtual screening and molecular docking with molecular dynamics and/or Monte Carlo simulation aids in reflecting the dynamics of proteins in nature into previously static docking studies, thus enhancing the accuracy of rationally designed ligands. This review will highlight recent computational strategies that incorporate protein flexibility into SBDD of GPCR-targeted ligands.

Structure-based discovery of selective positive allosteric modulators of antagonists for the M2 muscarinic acetylcholine receptor

The orthosteric binding sites of the five muscarinic acetylcholine receptor (mAChR) subtypes are highly conserved, making the development of selective antagonists challenging. The allosteric sites of these receptors are more variable, allowing one to imagine allosteric modulators that confer subtype selectivity, which would reduce the major off-target effects of muscarinic antagonists. Accordingly, a large library docking campaign was prosecuted seeking unique positive allosteric modulators (PAMs) for antagonists, ultimately revealing a PAM that substantially potentiates antagonist binding leading to subtype selectivity at the M₂ mAChR. This study supports the feasibility of discovering PAMs that can convert an armamentarium of potent but nonselective G-protein–coupled receptor (GPCR) antagonist drugs into subtype-selective reagents.

Subtype-selective antagonists for muscarinic acetylcholine receptors (mAChRs) have long been elusive, owing to the highly conserved orthosteric binding site. However, allosteric sites of these receptors are less conserved, motivating the search for allosteric ligands that modulate agonists or antagonists to confer subtype selectivity. Accordingly, a 4.6 million-molecule library was docked against the structure of the prototypical M₂ mAChR, seeking molecules that specifically stabilized antagonist binding. This led us to identify a positive allosteric modulator (PAM) that potentiated the antagonist N-methyl scopolamine (NMS). Structure-based optimization led to compound ’628, which enhanced binding of NMS, and the drug scopolamine itself, with a cooperativity factor (α) of 5.5 and a K_B of 1.1 μM, while sparing the endogenous agonist acetylcholine. NMR spectral changes determined for methionine residues reflected changes in the allosteric network. Moreover, ’628 slowed the dissociation rate of NMS from the M₂ mAChR by 50-fold, an effect not observed at the other four mAChR subtypes. The specific PAM effect of ’628 on NMS antagonism was conserved in functional assays, including agonist stimulation of [³⁵S]GTPγS binding and ERK 1/2 phosphorylation. Importantly, the selective allostery between ’628 and NMS was retained in membranes from adult rat hypothalamus and in neonatal rat cardiomyocytes, supporting the physiological relevance of this PAM/antagonist approach. This study supports the feasibility of discovering PAMs that confer subtype selectivity to antagonists; molecules like ’628 can convert an armamentarium of potent but nonselective GPCR antagonist drugs into subtype-selective reagents, thus reducing their off-target effects.