The role of acetylcholine in cortical synaptic plasticity

Desiderata for Normative Models of Synaptic Plasticity

Normative models of synaptic plasticity use computational rationales to arrive at predictions of behavioral and network-level adaptive phenomena. In recent years, there has been an explosion of theoretical work in this realm, but experimental confirmation remains limited. In this review, we organize work on normative plasticity models in terms of a set of desiderata that, when satisfied, are designed to ensure that a given model demonstrates a clear link between plasticity and adaptive behavior, is consistent with known biological evidence about neural plasticity and yields specific testable predictions. As a prototype, we include a detailed analysis of the REINFORCE algorithm. We also discuss how new models have begun to improve on the identified criteria and suggest avenues for further development. Overall, we provide a conceptual guide to help develop neural learning theories that are precise, powerful, and experimentally testable.

Cell-type-specific cholinergic control of granular retrosplenial cortex with implications for angular velocity coding across brain states

Cholinergic receptor activation enables the persistent firing of cortical pyramidal neurons, providing a key cellular basis for theories of spatial navigation involving working memory, path integration, and head direction encoding. The granular retrosplenial cortex (RSG) is important for spatially-guided behaviors, but how acetylcholine impacts RSG neurons is unknown. Here, we show that a transcriptomically, morphologically, and biophysically distinct RSG cell-type - the low-rheobase (LR) neuron - has a very distinct expression profile of cholinergic muscarinic receptors compared to all other neighboring excitatory neuronal subtypes. LR neurons do not fire persistently in response to cholinergic agonists, in stark contrast to all other principal neuronal subtypes examined within the RSG and across midline cortex. This lack of persistence allows LR neuron models to rapidly compute angular head velocity (AHV), independent of cholinergic changes seen during navigation. Thus, LR neurons can consistently compute AHV across brain states, highlighting the specialized RSG neural codes supporting navigation.

Types of memory, dementia, Alzheimer's disease, and their various pathological cascades as targets for potential pharmacological drugs

Alzheimer's disease (AD) is the most common type of dementia accounting for 90% of cases; however, frontotemporal dementia, vascular dementia, etc. prevails only in a minority of populations. The term dementia is defined as loss of memory which further takes several other categories of memories like working memory, spatial memory, fear memory, and long-term, and short-term memory into consideration. In this review, these memories have critically been elaborated based on context, duration, events, appearance, intensity, etc. The most important part and purpose of the review is the various pathological cascades as well as molecular levels of targets of AD, which have extracellular amyloid plaques and intracellular hyperphosphorylated tau protein as major disease hallmarks. There is another phenomenon that either leads to or arises from the above-mentioned hallmarks, such as oxidative stress, mitochondrial dysfunction, neuroinflammation, cholinergic dysfunction, and insulin resistance. Several potential drugs like antioxidants, anti-inflammatory drugs, acetylcholinesterase inhibitors, insulin mimetics or sensitizers, etc. studied in various previous preclinical or clinical reports were put as having the capacity to act on these pathological targets. Additionally, agents directly or indirectly targeting amyloid and tau were also discussed. This could be further investigated in future research.

Sound-evoked adenosine release in cooperation with neuromodulatory circuits permits auditory cortical plasticity and perceptual learning

Meaningful auditory memories are formed in adults when acoustic information is delivered to the auditory cortex during heightened states of attention, vigilance, or alertness, as mediated by neuromodulatory circuits. Here, we identify that, in awake mice, acoustic stimulation triggers auditory thalamocortical projections to release adenosine, which prevents cortical plasticity (i.e., selective expansion of neural representation of behaviorally relevant acoustic stimuli) and perceptual learning (i.e., experience-dependent improvement in frequency discrimination ability). This sound-evoked adenosine release (SEAR) becomes reduced within seconds when acoustic stimuli are tightly paired with the activation of neuromodulatory (cholinergic or dopaminergic) circuits or periods of attentive wakefulness. If thalamic adenosine production is enhanced, then SEAR elevates further, the neuromodulatory circuits are unable to sufficiently reduce SEAR, and associative cortical plasticity and perceptual learning are blocked. This suggests that transient low-adenosine periods triggered by neuromodulatory circuits permit associative cortical plasticity and auditory perceptual learning in adults to occur.

Acetylcholine facilitates localized synaptic potentiation and location specific feature binding

Forebrain acetylcholine (ACh) signaling has been shown to drive attention and learning. Recent experimental evidence of spatially and temporally constrained cholinergic signaling has sparked interest to investigate how it facilitates stimulus-induced learning. We use biophysical excitatory-inhibitory (E-I) multi-module neural network models to show that external stimuli and ACh signaling can mediate spatially constrained synaptic potentiation patterns. The effects of ACh on neural excitability are simulated by varying the conductance of a muscarinic receptor-regulated hyperpolarizing slow K+ current (m-current). Each network module consists of an E-I network with local excitatory connectivity and global inhibitory connectivity. The modules are interconnected with plastic excitatory synaptic connections, that change via a spike-timing-dependent plasticity (STDP) rule. Our results indicate that spatially constrained ACh release influences the information flow represented by network dynamics resulting in selective reorganization of inter-module interactions. Moreover the information flow depends on the level of synchrony in the network. For highly synchronous networks, the more excitable module leads firing in the less excitable one resulting in strengthening of the outgoing connections from the former and weakening of its incoming synapses. For networks with more noisy firing patterns, activity in high ACh regions is prone to induce feedback firing of synchronous volleys and thus strengthening of the incoming synapses to the more excitable region and weakening of outgoing synapses. Overall, these results suggest that spatially and directionally specific plasticity patterns, as are presumed necessary for feature binding, can be mediated by spatially constrained ACh release.

A role for cortical interneurons as adversarial discriminators

The brain learns representations of sensory information from experience, but the algorithms by which it does so remain unknown. One popular theory formalizes representations as inferred factors in a generative model of sensory stimuli, meaning that learning must improve this generative model and inference procedure. This framework underlies many classic computational theories of sensory learning, such as Boltzmann machines, the Wake/Sleep algorithm, and a more recent proposal that the brain learns with an adversarial algorithm that compares waking and dreaming activity. However, in order for such theories to provide insights into the cellular mechanisms of sensory learning, they must be first linked to the cell types in the brain that mediate them. In this study, we examine whether a subtype of cortical interneurons might mediate sensory learning by serving as discriminators, a crucial component in an adversarial algorithm for representation learning. We describe how such interneurons would be characterized by a plasticity rule that switches from Hebbian plasticity during waking states to anti-Hebbian plasticity in dreaming states. Evaluating the computational advantages and disadvantages of this algorithm, we find that it excels at learning representations in networks with recurrent connections but scales poorly with network size. This limitation can be partially addressed if the network also oscillates between evoked activity and generative samples on faster timescales. Consequently, we propose that an adversarial algorithm with interneurons as discriminators is a plausible and testable strategy for sensory learning in biological systems.

Impact of Anticholinergic Medications on Speech Perception Performance after Cochlear Implantation

OBJECTIVE

To identify and characterize the impact of anticholinergic medications, which have known adverse effects on cognition in older adults, on speech perception after cochlear implantation.

STUDY DESIGN

Retrospective cohort.

SETTING

Tertiary referral center.

SUBJECT POPULATION

Adult patients who underwent cochlear implantation between January 2010 and September 2020 with speech perception scores at 3, 6, and 12 months.

INTERVENTIONS

Anticholinergic burden of patients' prescribed medications.

MAIN OUTCOME MEASURES

AzBio speech perception scores after implantation.

RESULTS

One hundred twenty-six patients had documented AzBio in quiet speech perception score at all three postactivation time points. Patients were divided into three groups by anticholinergic burden (ACB) score, including ACB = 0 (90 patients), 1 (23 patients), and ≥2 (13 patients). There was no statistically significant difference between ACB groups in audiologic performance at candidacy testing ( p = 0.77) or at 3 months after implantation ( p = 0.13). Beginning at 6 months, a lower mean AzBio was seen in patients with higher ACB scores (68% ACB = 0; 62% ACB = 1; 48.1% ACB ≥ 2; p = 0.03). At 12 months, there were further differences between the groups (71.0% ACB = 0, 69.5% ACB = 1, 48.0% ACB ≥2, p < 0.01). Controlling for the effects of age using multivariate linear regression showed persistent effects of ACB score on learning-related AzBio improvements. Comparatively, the negative impact of a single ACB score point was equivalent to nearly 10 years of aging ( p = 0.03).

CONCLUSIONS

Increased ACB is associated with worse speech perception scores after cochlear implantation, an effect that persists even when accounting for patient age, suggesting that these medications may have cognitive and learning effects that reduce cochlear implant performance.

How uncertain are you? Disentangling expected and unexpected uncertainty in pupil-linked brain arousal during reversal learning

During decision making, we are continuously faced with two sources of uncertainty regarding the links between stimuli, our actions, and outcomes. On the one hand, our expectations are often probabilistic, that is, stimuli or actions yield the expected outcome only with a certain probability (expected uncertainty). On the other hand, expectations might become invalid due to sudden, unexpected changes in the environment (unexpected uncertainty). Several lines of research show that pupil-linked brain arousal is a sensitive indirect measure of brain mechanisms underlying uncertainty computations. Thus, we investigated whether it is involved in disentangling these two forms of uncertainty. To this aim, we measured pupil size during a probabilistic reversal learning task. In this task, participants had to figure out which of two response options led to reward with higher probability, whereby sometimes the identity of the more advantageous response option was switched. Expected uncertainty was manipulated by varying the reward probability of the advantageous choice option, whereas the level of unexpected uncertainty was assessed by using a Bayesian computational model estimating change probability and resulting uncertainty. We found that both aspects of unexpected uncertainty influenced pupil responses, confirming that pupil-linked brain arousal is involved in model updating after unexpected changes in the environment. Furthermore, high level of expected uncertainty impeded the detection of sudden changes in the environment, both on physiological and behavioral level. These results emphasize the role of pupil-linked brain arousal and underlying neural structures in handling situations in which the previously established contingencies are no longer valid.

Noradrenergic and cholinergic systems take centre stage in neuropsychiatric diseases of ageing

Noradrenergic and cholinergic systems are among the most vulnerable brain systems in neuropsychiatric diseases of ageing, including Alzheimer's disease, Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. As these systems fail, they contribute directly to many of the characteristic cognitive and psychiatric symptoms. However, their contribution to symptoms is not sufficiently understood, and pharmacological interventions targeting noradrenergic and cholinergic systems have met with mixed success. Part of the challenge is the complex neurobiology of these systems, operating across multiple timescales, and with non-linear changes across the adult lifespan and disease course. We address these challenges in a detailed review of the noradrenergic and cholinergic systems, outlining their roles in cognition and behaviour, and how they influence neuropsychiatric symptoms in disease. By bridging across levels of analysis, we highlight opportunities for improving drug therapies and for pursuing personalised medicine strategies.

Dendrites help mitigate the plasticity-stability dilemma

With Hebbian learning 'who fires together wires together', well-known problems arise. Hebbian plasticity can cause unstable network dynamics and overwrite stored memories. Because the known homeostatic plasticity mechanisms tend to be too slow to combat unstable dynamics, it has been proposed that plasticity must be highly gated and synaptic strengths limited. While solving the issue of stability, gating and limiting plasticity does not solve the stability-plasticity dilemma. We propose that dendrites enable both stable network dynamics and considerable synaptic changes, as they allow the gating of plasticity in a compartment-specific manner. We investigate how gating plasticity influences network stability in plastic balanced spiking networks of neurons with dendrites. We compare how different ways to gate plasticity, namely via modulating excitability, learning rate, and inhibition increase stability. We investigate how dendritic versus perisomatic gating allows for different amounts of weight changes in stable networks. We suggest that the compartmentalisation of pyramidal cells enables dendritic synaptic changes while maintaining stability. We show that the coupling between dendrite and soma is critical for the plasticity-stability trade-off. Finally, we show that spatially restricted plasticity additionally improves stability.

Tissue-type plasminogen activator induces conditioned receptive field plasticity in the mouse auditory cortex

Tissue-type plasminogen activator (tPA) is a serine protease that is expressed in various compartments in the brain. It is involved in neuronal plasticity, learning and memory, and addiction. We evaluated whether tPA, exogenously applied, could influence neuroplasticity within the mouse auditory cortex. We used a frequency-pairing paradigm to determine whether neuronal best frequencies shift following the pairing protocol. tPA administration significantly affected the best frequency after pairing, whereby this depended on the pairing frequency relative to the best frequency. When the pairing frequency was above the best frequency, tPA caused a best frequency shift away from the conditioned frequency. tPA significantly widened auditory tuning curves. Our data indicate that regional changes in proteolytic activity within the auditory cortex modulate the fine-tuning of auditory neurons, supporting the function of tPA as a modulator of neuronal plasticity.

Basal Forebrain Cholinergic Neurons Selectively Drive Coordinated Motor Learning in Mice

Motor control requires precise temporal and spatial encoding across distinct motor centers that is refined through the repetition of learning. The recruitment of motor regions requires modulatory input to shape circuit activity. Here, we identify a role for the baso-cortical cholinergic pathway in the acquisition of a coordinated motor skill in mice. Targeted depletion of basal forebrain cholinergic neurons results in significant impairments in training on the rotarod task of coordinated movement. Cholinergic neuromodulation is required during training sessions as chemogenetic inactivation of cholinergic neurons also impairs task acquisition. Rotarod learning is known to drive refinement of corticostriatal neurons arising in both medial prefrontal cortex (mPFC) and motor cortex, and we have found that cholinergic input to both motor regions is required for task acquisition. Critically, the effects of cholinergic neuromodulation are restricted to the acquisition stage, as depletion of basal forebrain cholinergic neurons after learning does not affect task execution. Our results indicate a critical role for cholinergic neuromodulation of distant cortical motor centers during coordinated motor learning.SIGNIFICANCE STATEMENT Acetylcholine release from basal forebrain cholinergic neuron terminals rapidly modulates neuronal excitability, circuit dynamics, and cortical coding; all processes required for processing complex sensory information, cognition, and attention. We found that depletion or transient silencing of cholinergic inputs to anatomically isolated motor areas, medial prefrontal cortex (mPFC) and motor cortex, selectively led to significant impairments on coordinated motor learning; disrupting this baso-cortical network after acquisition elicited no effect on task execution. Our results indicate a pivotal role for cholinergic neuromodulation of distant cortical motor centers during coordinated motor learning. These findings support the concept that cognitive components (such as attention) are indispensable in the adjustment of motor output and training-induced improvements in motor performance.

Local activation of α2 adrenergic receptors is required for vagus nerve stimulation induced motor cortical plasticity

Vagus nerve stimulation (VNS) paired with rehabilitation training is emerging as a potential treatment for improving recovery of motor function following stroke. In rats, VNS paired with skilled forelimb training results in significant reorganization of the somatotopic cortical motor map; however, the mechanisms underlying this form of VNS-dependent plasticity remain unclear. Recent studies have shown that VNS-driven cortical plasticity is dependent on noradrenergic innervation of the neocortex. In the central nervous system, noradrenergic α2 receptors (α2-ARs) are widely expressed in the motor cortex and have been critically implicated in synaptic communication and plasticity. In current study, we examined whether activation of cortical α2-ARs is necessary for VNS-driven motor cortical reorganization to occur. Consistent with previous studies, we found that VNS paired with motor training enlarges the map representation of task-relevant musculature in the motor cortex. Infusion of α2-AR antagonists into M1 blocked VNS-driven motor map reorganization from occurring. Our results suggest that local α2-AR activation is required for VNS-induced cortical reorganization to occur, providing insight into the mechanisms that may underlie the neuroplastic effects of VNS therapy.

Visuo-Acoustic Stimulation's Role in Synaptic Plasticity: A Review of the Literature

Brain plasticity is the capacity of cerebral neurons to change, structurally and functionally, in response to experiences. This is an essential property underlying the maturation of sensory functions, learning and memory processes, and brain repair in response to the occurrence of diseases and trauma. In this field, the visual system emerges as a paradigmatic research model, both for basic research studies and for translational investigations. The auditory system remains capable of reorganizing itself in response to different auditory stimulations or sensory organ modification. Acoustic biofeedback training can be an effective way to train patients with the central scotoma, who have poor fixation stability and poor visual acuity, in order to bring fixation on an eccentrical and healthy area of the retina: a pseudofovea. This review article is focused on the cellular and molecular mechanisms underlying retinal sensitivity changes and visual and auditory system plasticity.

Vagus Nerve Stimulation Induced Motor Map Plasticity Does Not Require Cortical Dopamine

Background: Vagus nerve stimulation (VNS) paired with motor rehabilitation is an emerging therapeutic strategy to enhance functional recovery after neural injuries such as stroke. Training-paired VNS drives significant neuroplasticity within the motor cortex (M1), which is thought to underlie the therapeutic effects of VNS. Though the mechanisms are not fully understood, VNS-induced cortical plasticity is known to depend on intact signaling from multiple neuromodulatory nuclei that innervate M1. Cortical dopamine (DA) plays a key role in mediating M1 synaptic plasticity and is critical for motor skill acquisition, but whether cortical DA contributes to VNS efficacy has not been tested.

Objective: To determine the impact of cortical DA depletion on VNS-induced cortical plasticity.

Methods: Rats were trained on a skilled reaching lever press task prior to implantation of VNS electrodes and 6-hydroxydopamine (6-OHDA) mediated DA depletion in M1. Rats then underwent training-paired VNS treatment, followed by cortical motor mapping and lesion validation.

Results: In both intact and DA-depleted rats, VNS significantly increased the motor map representation of task-relevant proximal forelimb musculature and reduced task-irrelevant distal forelimb representations. VNS also significantly increased tyrosine hydroxylase (TH+) fiber density in intact M1, but this effect was not observed in lesioned hemispheres.

Conclusion: Our results reveal that though VNS likely upregulates catecholaminergic signaling in intact motor cortices, DA itself is not required for VNS-induced plasticity to occur. As DA is known to critically support M1 plasticity during skill acquisition, our findings suggest that VNS may engage a unique set of neuromodulatory signaling pathways to promote neocortical plasticity.

Cholinergics contribute to the cellular mechanisms of deep brain stimulation applied in rat infralimbic cortex but not white matter

Deep brain stimulation (DBS) of the subcallosal cingulate gyrus (SCG) is a promising therapy for treatment-resistant depression. Pre-clinical models have been widely used to investigate the neural mechanisms underlying its antidepressant benefit. The ventral division of the medial prefrontal cortex (vmPFC), particularly the infralimbic cortex (IL), is the homologous region in rat and DBS applied to vmPFC shows antidepressant-like effects in the forced swim test. Therefore we investigated the cellular mechanisms of simulated DBS (sDBS) in layer 5 IL neurons, using in vitro whole-cell patch clamp recordings. sDBS in IL layer 5 induced a prolonged after-depolarization (ADP) in both pyramidal and fast spiking neurons, which was dependent on current amplitude and pulse width. In contrast, sDBS applied in the forebrain white matter fibers, although delivered at a higher intensity, failed to induce any persistent depolarization in layer 5 IL pyramidal neurons. Cholinergic blockade (atropine, 2.0 µM) decreased both the ADP amplitude and duration in pyramidal neurons, but left those in fast spiking neurons unchanged. These data suggest that: (i) sDBS in IL gray and white matter produced different cellular effects on pyramidal neurons; (ii) sDBS-induced ADP in pyramidal, but not fast spiking neurons, was mediated by acetylcholine; and (iii) different neuromodulators may contribute to sDBS-induced ADP in IL. In summary, cholinergic mediated ADP in pyramidal neurons may contribute to the antidepressant effects of DBS in IL.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Cortical ChAT+ neurons co-transmit acetylcholine and GABA in a target- and brain-region-specific manner

The mouse cerebral cortex contains neurons that express choline acetyltransferase (ChAT) and are a potential local source of acetylcholine. However, the neurotransmitters released by cortical ChAT⁺ neurons and their synaptic connectivity are unknown. We show that the nearly all cortical ChAT⁺ neurons in mice are specialized VIP⁺ interneurons that release GABA strongly onto other inhibitory interneurons and acetylcholine sparsely onto layer 1 interneurons and other VIP⁺/ChAT⁺ interneurons. This differential transmission of ACh and GABA based on the postsynaptic target neuron is reflected in VIP⁺/ChAT⁺ interneuron pre-synaptic terminals, as quantitative molecular analysis shows that only a subset of these are specialized to release acetylcholine. In addition, we identify a separate, sparse population of non-VIP ChAT⁺ neurons in the medial prefrontal cortex with a distinct developmental origin that robustly release acetylcholine in layer 1. These results demonstrate both cortex-region heterogeneity in cortical ChAT⁺ interneurons and target-specific co-release of acetylcholine and GABA.

Differential effects of the mode of touch, active and passive, on experience-driven plasticity in the S1 cutaneous digit representation of adult macaque monkeys

This study compared the receptive field (RF) properties and firing rates of neurons in the cutaneous hand representation of primary somatosensory cortex (areas 3b, 1, and 2) of 9 awake, adult macaques that were intensively trained in a texture discrimination task using active touch (fingertips scanned over the surfaces using a single voluntary movement), passive touch (surfaces displaced under the immobile fingertips), or both active and passive touch. Two control monkeys received passive exposure to the same textures in the context of a visual discrimination task. Training and recording extended over 1–2 yr per animal. All neurons had a cutaneous receptive field (RF) that included the tips of the stimulated digits (D3 and/or D4). In area 3b, RFs were largest in monkeys trained with active touch, smallest in those trained with passive touch, and intermediate in those trained with both; i.e., the mode of touch differentially modified the cortical representation of the stimulated fingers. The same trends were seen in areas 1 and 2, but the changes were not significant, possibly because a second experience-driven influence was seen in areas 1 and 2, but not in area 3b: smaller RFs with passive exposure to irrelevant tactile inputs compared with recordings from one naive hemisphere. We suggest that added feedback during active touch and higher cortical firing rates were responsible for the larger RFs with behavioral training; this influence was tempered by periods of more restricted sensory feedback during passive touch training in the active + passive monkeys.

NEW & NOTEWORTHY We studied experience-dependent sensory cortical plasticity in relation to tactile discrimination of texture using active and/or passive touch. We showed that neuronal receptive fields in primary somatosensory cortex, especially area 3b, are largest in monkeys trained with active touch, smallest in those trained with passive touch, and intermediate in those trained using both modes of touch. Prolonged, irrelevant tactile input had the opposite influence in areas 1 and 2, favoring smaller receptive fields.

Transcranial direct-current stimulation combined with attention increases cortical excitability and improves motor learning in healthy volunteers

Background

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has the potential to induce motor cortical plasticity in humans. It is well known that motor cortical plasticity plays an essential role in motor learning and recovery in patients with stroke and neurodegenerative disorders. However, it remains unclear how cognitive function influences motor cortical plasticity induced by tDCS. The present study aimed to investigate whether anodal tDCS combined with attention to a target muscle could enhance motor cortical plasticity and improve motor learning in healthy individuals.

Methods

Thirty-three healthy volunteers were assigned to two experiments. In experiment 1, there were three interventional conditions: 1) anodal tDCS was applied while participants paid attention to the first dorsal interosseous (FDI) muscle, 2) anodal tDCS was applied while participants paid attention to the sound, and 3) anodal tDCS was applied without the participants paying attention to the FDI muscle or the sound. Anodal tDCS (2 mA, 10 min) was applied over the primary motor cortex (M1). Changes in motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were assessed before and immediately after (0 min), and then 10 min, 30 min, and 60 min after each intervention. In experiment 2, we investigated whether the combination of anodal tDCS and attention to the abductor pollicis brevis (APB) muscle could facilitate the learning of a ballistic thumb movement.

Results

Anodal tDCS increased cortical excitability in all conditions immediately after the stimulation. Significant increases in MEPs and significant decreases in SICI were observed for at least 60 min after anodal tDCS, but only when participants paid attention to the FDI muscle. In contrast, no significant changes in ICF were observed in any condition. In experiment 2, the combination of tDCS and attention to the APB muscle significantly enhanced the acquisition of a ballistic thumb movement. The higher performance was still observed 7 days after the stimulation.

Conclusions

This study shows that anodal tDCS over M1 in conjunction with attention to the target muscle enhances motor cortex plasticity and improves motor learning in healthy adults. These findings suggest that a combination of attention and tDCS may be an effective strategy to promote rehabilitation training in patients with stroke and neurodegenerative disorders.

Trial registration

Retrospectively registered (UMIN000036848).

T-495, a novel low cooperative M1 receptor positive allosteric modulator, improves memory deficits associated with cholinergic dysfunction and is characterized by low gastrointestinal side effect risk

M₁ muscarinic acetylcholine receptor (M₁ R) activation can be a new therapeutic approach for the treatment of cognitive deficits associated with cholinergic hypofunction. However, M₁ R activation causes gastrointestinal (GI) side effects in animals. We previously found that an M₁ R positive allosteric modulator (PAM) with lower cooperativity (α-value) has a limited impact on ileum contraction and can produce a wider margin between cognitive improvement and GI side effects. In fact, TAK-071, a novel M₁ R PAM with low cooperativity (α-value of 199), improved scopolamine-induced cognitive deficits with a wider margin against GI side effects than a high cooperative M₁ R PAM, T-662 (α-value of 1786), in rats. Here, we describe the pharmacological characteristics of a novel low cooperative M₁ R PAM T-495 (α-value of 170), using the clinically tested higher cooperative M₁ R PAM MK-7622 (α-value of 511) as a control. In rats, T-495 caused diarrhea at a 100-fold higher dose than that required for the improvement of scopolamine-induced memory deficits. Contrastingly, MK-7622 showed memory improvement and induction of diarrhea at an equal dose. Combination of T-495, but not of MK-7622, and donepezil at each sub-effective dose improved scopolamine-induced memory deficits. Additionally, in mice with reduced acetylcholine levels in the forebrain via overexpression of A53T α-synuclein (ie, a mouse model of dementia with Lewy bodies and Parkinson's disease with dementia), T-495, like donepezil, reversed the memory deficits in the contextual fear conditioning test and Y-maze task. Thus, low cooperative M₁ R PAMs are promising agents for the treatment of memory deficits associated with cholinergic dysfunction.

Cortical Synaptic AMPA Receptor Plasticity during Motor Learning

Modulation of synaptic strength through trafficking of AMPA receptors (AMPARs) is a fundamental mechanism underlying synaptic plasticity, learning, and memory. However, the dynamics of AMPAR trafficking in vivo and its correlation with learning have not been resolved. Here, we used in vivo two-photon microscopy to visualize surface AMPARs in mouse cortex during the acquisition of a forelimb reaching task. Daily training leads to an increase in AMPAR levels at a subset of spatially clustered dendritic spines in the motor cortex. Surprisingly, we also observed increases in spine AMPAR levels in the visual cortex. There, synaptic potentiation depends on the availability of visual input during motor training, and optogenetic inhibition of visual cortex activity impairs task performance. These results indicate that motor learning induces widespread cortical synaptic potentiation by increasing the net trafficking of AMPARs into spines, including in non-motor brain regions.

Controlled ozone therapy modulates the neurodegenerative changes in the frontal cortex of the aged albino rat

Aging is a normal process associated with neurodegenerative changes resulting in decline of cognitive and motor functions. Oxidative stress plays an important role. Controlled ozone (O₃) therapy has been proved to induce oxidative preconditioning thus reversing oxidative stress. To the best of our knowledge, this research is the first attempt to investigate whether the antioxidant properties of O₃ can ameliorate age-associated structural alterations of the cerebral cortex. Ozone administration (at a dose of 0.7mg/kg intraperitonially, three times a week for eight weeks) produced significant downregulation of tissue malondialdehyde (MDA) and upregulation of glutathione, superoxide dismutase (SOD) and catalase (CAT) within the frontal cortex of aged rats. Sections of the frontal cortex from adult and aged rats were stained with hematoxylin and eosin and analyzed using light microscopy. In addition, quantitative immunohistochemical assessments of the expression of inducible nitric oxide synthase (iNOS), caspase-3, glial fibrillary acidic protein (GFAP), Ki67 and acetylcholinesterase (AChE) were performed. Our results revealed the beneficial effect of O₃ in improving the neurodegenerative changes of the cerebral cortex of aged rats. Moreover, this study clarified that O₃ exerted its effects via reducing oxidative stress, apoptosis, gliosis as well as improving neurogenesis and cholinergic plasticity. This work added to the previously proved aging - associated neurodegenerative effects and provided a new insight into the promising role of O₃ to ameliorate these effects.

Whole-Brain Mapping of Monosynaptic Afferent Inputs to Cortical CRH Neurons

Corticotropin-releasing hormone (CRH) is a critical neuropeptide modulating the mammalian stress response. It is involved in many functional activities within various brain regions, among which there is a subset of CRH neurons occupying a considerable proportion of the cortical GABAergic interneurons. Here, we utilized rabies virus-based monosynaptic retrograde tracing system to map the whole-brain afferent presynaptic partners of the CRH neurons in the anterior cingulate cortex (ACC). We find that the ACC CRH neurons integrate information from the cortex, thalamus, hippocampal formation, amygdala, and also several other midbrain and hindbrain nuclei. Furthermore, our results reveal that ACC CRH neurons receive direct inputs from two neuromodulatory systems, the basal forebrain cholinergic neurons and raphe serotoninergic neurons. These findings together expand our knowledge about the connectivity of the cortical GABAergic neurons and also provide a basis for further investigation of the circuit function of cortical CRH neurons.

Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

Neural Effects of Cognitive Training in Schizophrenia: A Systematic Review and Activation Likelihood Estimation Meta-analysis

BACKGROUND

Cognitive dysfunction is a core feature of schizophrenia and a strong predictor of functional outcome. There is growing evidence for the effectiveness of behaviorally based cognitive training programs, although the neural basis of these benefits is unclear. To address this, we reviewed all published studies that have used neuroimaging to measure neural changes following cognitive training in schizophrenia to identify brain regions most consistently affected.

METHODS

We searched PubMed for all neuroimaging studies examining cognitive training in schizophrenia published until December 2018. An activation likelihood estimation meta-analysis was conducted on a subset of functional magnetic resonance imaging studies to examine whether any brain regions showed consistent effects across studies.

RESULTS

In total, 31 original neuroimaging studies of cognitive training were retrieved. Of these studies, 16 were functional neuroimaging studies, and 15 of these studies reported increased neural activation following cognitive training, with increased left prefrontal activation being the most frequently observed finding. However, activation likelihood estimation meta-analysis did not reveal any specific brain regions showing consistent effects across studies but rather suggested a broader, more distributed pattern of effects resulting from the interventions tested.

CONCLUSIONS

Although several studies reported increased left prefrontal cortical activation after cognitive training, the lack of statistically significant overlap of brain regions affected by training across studies suggests broad effects of training on brain activation, possibly due to the variety of training programs used.

Depressed by Learning-Heterogeneity of the Plasticity Rules at Parallel Fiber Synapses onto Purkinje Cells

Climbing fiber-driven long-term depression (LTD) of parallel fiber synapses onto cerebellar Purkinje cells has long been investigated as a putative mechanism of motor learning. We recently discovered that the rules governing the induction of LTD at these synapses vary across different regions of the cerebellum. Here, we discuss the design of LTD induction protocols in light of this heterogeneity in plasticity rules. The analytical advantages of the cerebellum provide an opportunity to develop a deeper understanding of how the specific plasticity rules at synapses support the implementation of learning.

Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the teleost Cyprinus carpio

Cholinergic systems play a role in basic cerebral functions and its dysfunction is associated with deficit in neurodegenerative disease. Mechanisms involved in human brain diseases, are often approached by using fish models, especially cyprinids, given basic similarities of the fish brain to that of mammals. In the present paper, the organization of central cholinergic systems have been described in the cyprinid Cyprinus carpio, the common carp, by using specific polyclonal antibodies against ChAT, the synthetic enzyme of acetylcholine, that is currently used as a specific marker for cholinergic neurons in all vertebrates.  In this work, serial transverse sections of the brain and the spinal cord were immunostained for ChAT. Results showed that positive neurons are present in several nuclei of the forebrain, the midbrain, the hindbrain and the spinal cord. Moreover, ChAT-positive neurons were detected in the synencephalon and in the cerebellum. In addition to neuronal bodies, afferent varicose fibers were stained for ChAT in the ventral telencephalon, the preoptic area, the hypothalamus and the posterior tuberculum. No neuronal cell bodies were present in the telencephalon. The comparison of cholinergic distribution pattern in the Cyprinus carpio central nervous system has revealed similarities but also some interesting differences with other cyprinids. Our results provide additional information on the cholinergic system from a phylogenetic point of view and may add new perspectives to physiological roles of cholinergic system during evolution and the neuroanatomical basis of neurological diseases.

A second wind for the cholinergic system in Alzheimer's therapy

Notwithstanding tremendous research efforts, the cause of Alzheimer's disease (AD) remains elusive and there is no curative treatment. The cholinergic hypothesis presented 35 years ago was the first major evidence-based hypothesis on the etiology of AD. It proposed that the depletion of brain acetylcholine was a primary cause of cognitive decline in advanced age and AD. It relied on a series of observations obtained in aged animals, elderly, and AD patients that pointed to dysfunctions of cholinergic basal forebrain, similarities between cognitive impairments induced by anticholinergic drugs and those found in advanced age and AD, and beneficial effects of drugs stimulating cholinergic activity. This review revisits these major results to show how this hypothesis provided the drive for the development of anticholinesterase inhibitor-based therapies of AD, the almost exclusively approved treatment in use despite transient and modest efficacy. New ideas for improving cholinergic therapies are also compared and discussed in light of the current revival of the cholinergic hypothesis on the basis of two sets of evidence from new animal models and refined imagery techniques in humans. First, human and animal studies agree in detecting signs of cholinergic dysfunctions much earlier than initially believed. Second, alterations of the cholinergic system are deeply intertwined with its reactive responses, providing the brain with efficient compensatory mechanisms to delay the conversion into AD. Active research in this field should provide new insight into development of multitherapies incorporating cholinergic manipulation, as well as early biomarkers of AD enabling earlier diagnostics. This is of prime importance to counteract a disease that is now recognized to start early in adult life.

Deep brain stimulation of the nucleus basalis of Meynert attenuates early EEG components associated with defective sensory gating in patients with Alzheimer disease - a two-case study

Alzheimer's disease (AD) is associated with deterioration of memory and cognitive function and a degeneration of neurons of the nucleus basalis of Meynert (NBM). The NBM is the major input source of acetylcholine (ACh) to the cortex. The decreasing cholinergic innervation of the cortex due to degeneration of the NBM might be the cause of loss of memory function. NBM-Deep brain stimulation (NBM-DBS) is considered to serve as a potential therapeutic option for patients with AD by supporting residual cholinergic transmission to stabilize oscillatory activity in memory-relevant circuits. However, whether DBS could improve sensory memory functions in patients with AD is not clear. Here, in a passive auditory oddball paradigm, patients with AD (N = 2) listened to repetitive background tones (standard tones) randomly interrupted by frequency deviants in two blocks with NBM-DBS OFF and then NBM-DBS ON, while age-matched healthy controls (N = 6) repeated the experiment twice. The mismatch negativity in NBM-DBS OFF significantly differed from controls in both blocks, but not under NBM-DBS, which was likely due to a pronounced P50 increase overlapping with the N1 in NBM-DBS OFF. This early complex of EEG components recovered under stimulation to a normal level as defined by responses in controls. In this temporal interval, we found in patients with NBM-DBS ON (but not with NBM-DBS OFF) and in controls a strong repetition suppression effect to standard tones - with more attenuated responses to frequently repeated standard tones. This highlights the role of NBM-DBS for sensory gating of familiar auditory information into sensory memory.

Postnatal choline supplementation selectively attenuates hippocampal microRNA alterations associated with developmental alcohol exposure

Prenatal alcohol exposure can result in a range of physical, neuropathological, and behavioral alterations, collectively termed fetal alcohol spectrum disorders (FASD). We have shown that supplementation with the nutrient choline reduces the severity of developmental alcohol-associated deficits in hippocampal-dependent behaviors and normalizes some aspects of hippocampal cholinergic development and DNA methylation patterns. Alcohol's developmental effects may also be mediated, in part, by altering microRNAs (miRNAs) that serve as negative regulators of gene translation. To determine whether choline supplementation alters ethanol's long-lasting effects on miRNAs, Sprague-Dawley rats were exposed to 5.25 g/kg/day ethanol from postnatal days (PD) 4-9 via intubation; controls received sham intubations. Subjects were treated with choline chloride (100 mg/kg/day) or saline vehicle subcutaneously (s.c.) from PD 4-21. On PD 22, subjects were sacrificed, and RNA was isolated from the hippocampus. MiRNA expression was assessed with TaqMan Human MicroRNA Panel Low-Density Arrays. Ethanol significantly increased miRNA expression variance, an effect that was attenuated with choline supplementation. Cluster analysis of stably expressed miRNAs that exceeded an ANOVA p < 0.05 criterion indicated that for both male and female offspring, control and ethanol-exposed groups were most dissimilar from each other, with choline-supplemented groups in between. MiRNAs that expressed an average 2-fold change due to ethanol exposure were further analyzed to identify which ethanol-sensitive miRNAs were protected by choline supplementation. We found that at a false discovery rate (FDR)-adjusted criterion of p < 0.05, miR-200c was induced by ethanol exposure and that choline prevented this effect. Collectively, our data show that choline supplementation can normalize disturbances in miRNA expression following developmental alcohol exposure and can protect specific miRNAs from induction by ethanol. These findings have important implications for the mechanisms by which choline may serve as a potential treatment for FASD.

Epimedii Herba: A Promising Herbal Medicine for Neuroplasticity

Epimedii Herba (EH) is an herbal medicine originating from several plants of the genus Epimedium. It is a major therapeutic option for kidney yang deficiency syndrome, which is closely related to androgen hormones and also has been used to treat hemiplegia following a stroke in traditional medicine of Korea and PR China. To date, many clinical and basic researches of EH have shown the activities on functional recovery from brain diseases. Recently, neuroplasticity, which is the spontaneous reaction of the brain in response to diseases, has been shown to accelerate functional recovery. In addition, androgen hormones including testosterone are known to be the representative of neuroplasticity factors in the brain recovery processes. In this review, we described the neuro-pharmacological activities of EH, focusing on neuroplasticity. Thirty-three kinds of papers from MEDLINE/PubMed, EMBASE, and CNKI were identified and analyzed. We categorized the results into five types based on neuroplasticity mechanisms and presented the definition of each category and briefly described the results of these papers. Altogether, we can suggest that neuroplasticity is a novel viewpoint for guiding future brain research of EH and provide the evidence for the development of new clinical applications using EH in the treatment of brain diseases. Copyright © 2017 John Wiley & Sons, Ltd.

Cholinergic Mechanisms in the Cerebral Cortex: Beyond Synaptic Transmission

Functional overviews of cholinergic mechanisms in the cerebral cortex have traditionally focused on the release of acetylcholine with modulator and transmitter effects. Recently, however, data have emerged that extend the role of acetylcholine and cholinergic innervations to a range of housekeeping and metabolic functions. These include regulation of amyloid precursor protein (APP) processing with production of amyloid β (Aβ) and other APP fragments and control of the phosphorylation of microtubule-associated protein (MAP) tau. Evidence has been also presented for receptor-ligand like interactions of cholinergic receptors with soluble Aβ peptide and MAP tau, with modulator and signaling effects. Moreover, high-affinity binding of Aβ to the neurotrophin receptor p75 (p75NTR) enriched in basalo-cortical cholinergic projections has been implicated in clearance of Aβ and nucleation of amyloid plaques. Here, we critically evaluate these unorthodox cholinergic mechanisms and discuss their role in neuronal physiology and the biology of Alzheimer's disease.

Increased olfactory bulb acetylcholine bi-directionally modulates glomerular odor sensitivity

The glomerular layer of the olfactory bulb (OB) receives heavy cholinergic input from the horizontal limb of the diagonal band of Broca (HDB) and expresses both muscarinic and nicotinic acetylcholine (ACh) receptors. However, the effects of ACh on OB glomerular odor responses remain unknown. Using calcium imaging in transgenic mice expressing the calcium indicator GCaMP2 in the mitral/tufted cells, we investigated the effect of ACh on the glomerular responses to increasing odor concentrations. Using HDB electrical stimulation and in vivo pharmacology, we find that increased OB ACh leads to dynamic, activity-dependent bi-directional modulation of glomerular odor response due to the combinatorial effects of both muscarinic and nicotinic activation. Using pharmacological manipulation to reveal the individual receptor type contributions, we find that m2 muscarinic receptor activation increases glomerular sensitivity to weak odor input whereas nicotinic receptor activation decreases sensitivity to strong input. Overall, we found that ACh in the OB increases glomerular sensitivity to odors and decreases activation thresholds. This effect, along with the decreased responses to strong odor input, reduces the response intensity range of individual glomeruli to increasing concentration making them more similar across the entire concentration range. As a result, odor representations are more similar as concentration increases.

Neuromodulated Spike-Timing-Dependent Plasticity, and Theory of Three-Factor Learning Rules

Classical Hebbian learning puts the emphasis on joint pre- and postsynaptic activity, but neglects the potential role of neuromodulators. Since neuromodulators convey information about novelty or reward, the influence of neuromodulators on synaptic plasticity is useful not just for action learning in classical conditioning, but also to decide "when" to create new memories in response to a flow of sensory stimuli. In this review, we focus on timing requirements for pre- and postsynaptic activity in conjunction with one or several phasic neuromodulatory signals. While the emphasis of the text is on conceptual models and mathematical theories, we also discuss some experimental evidence for neuromodulation of Spike-Timing-Dependent Plasticity. We highlight the importance of synaptic mechanisms in bridging the temporal gap between sensory stimulation and neuromodulatory signals, and develop a framework for a class of neo-Hebbian three-factor learning rules that depend on presynaptic activity, postsynaptic variables as well as the influence of neuromodulators.

Cholinergic systems are essential for late-stage maturation and refinement of motor cortical circuits

Previous studies reported that early postnatal cholinergic lesions severely perturb early cortical development, impairing neuronal cortical migration and the formation of cortical dendrites and synapses. These severe effects of early postnatal cholinergic lesions preclude our ability to understand the contribution of cholinergic systems to the later-stage maturation of topographic cortical representations. To study cholinergic mechanisms contributing to the later maturation of motor cortical circuits, we first characterized the temporal course of cortical motor map development and maturation in rats. In this study, we focused our attention on the maturation of cortical motor representations after postnatal day 25 (PND 25), a time after neuronal migration has been accomplished and cortical volume has reached adult size. We found significant maturation of cortical motor representations after this time, including both an expansion of forelimb representations in motor cortex and a shift from proximal to distal forelimb representations to an extent unexplainable by simple volume enlargement of the neocortex. Specific cholinergic lesions placed at PND 24 impaired enlargement of distal forelimb representations in particular and markedly reduced the ability to learn skilled motor tasks as adults. These results identify a novel and essential role for cholinergic systems in the late refinement and maturation of cortical circuits. Dysfunctions in this system may constitute a mechanism of late-onset neurodevelopmental disorders such as Rett syndrome and schizophrenia.