What do phasic cholinergic signals do?

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.

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.

Vesicular acetylcholine transporter in the basal forebrain improves cognitive impairment in chronic cerebral hypoperfusion rats by modulating theta oscillations in the hippocampus

The cholinergic transmission in the medial septum and ventral limb of the diagonal band of broca (MS/VDB)-hippocampal circuit and its associated theta oscillations play a crucial role in chronic cerebral hypoperfusion (CCH)-related cognitive impairment. However, the contribution and mechanism of the vesicular acetylcholine transporter (VAChT), a vital protein that regulates acetylcholine (ACh) release, in CCH-related cognitive impairment are not well understood. To investigate this, we established a rat model of CCH by performing 2-vessel occlusion (2-VO) and overexpressed VAChT in the MS/VDB via stereotaxic injection of adeno-associated virus (AAV). We evaluated the cognitive function of the rats using the Morris Water Maze (MWM) and Novel Object Recognition Test (NOR). We employed enzyme-linked immunosorbent assay (ELISA), Western blot (WB), and immunohistochemistry (IHC) to assess hippocampal cholinergic levels. We also conducted in vivo local field potentials (LFPs) recording experiments to evaluate changes in hippocampal theta oscillations and synchrony. Our findings showed that VAChT overexpression shortened the escape latency in the hidden platform test, increased swimming time in the platform quadrant in probe trains, and increased the recognition index (RI) in NOR. Moreover, VAChT overexpression increased hippocampal cholinergic levels, improved theta oscillations, and improved the synchrony of theta oscillations between CA1 and CA3 in CCH rats. These results suggest that VAChT plays a protective role in CCH-induced cognitive deficits by regulating cholinergic transmission in the MS/VDB-hippocampal circuit and promoting hippocampal theta oscillations. Therefore, VAChT could be a promising therapeutic target for treating CCH-related cognitive impairments.

Vigilance, arousal, and acetylcholine: Optimal control of attention in a simple detection task

Paying attention to particular aspects of the world or being more vigilant in general can be interpreted as forms of ‘internal’ action. Such arousal-related choices come with the benefit of increasing the quality and situational appropriateness of information acquisition and processing, but incur potentially expensive energetic and opportunity costs. One implementational route for these choices is widespread ascending neuromodulation, including by acetylcholine (ACh). The key computational question that elective attention poses for sensory processing is when it is worthwhile paying these costs, and this includes consideration of whether sufficient information has yet been collected to justify the higher signal-to-noise ratio afforded by greater attention and, particularly if a change in attentional state is more expensive than its maintenance, when states of heightened attention ought to persist. We offer a partially observable Markov decision-process treatment of optional attention in a detection task, and use it to provide a qualitative model of the results of studies using modern techniques to measure and manipulate ACh in rodents performing a similar task.

Paying attention to a stimulus is costly, both in terms of energy and the lost opportunity to pay attention to something else. It is also beneficial, providing more information about its target. Thus, whether and when we pay more or less attention may best be considered as a choice of internal action that responds to this trade-off. Furthermore, measurements and manipulation of the neuromodulator acetylcholine have suggested that it is one of the instruments of attention, providing us with a window onto this choice. Here, we build an abstract model of a task in which an animal must look out for a brief visual stimulus that may or may not occur on each trial. We show that optimal attentional choices in the model depend on many factors, including how likely a signal is to occur across time, the balance between the improvement in information possible by paying greater attention and its increased cost, and whether there are also costs associated with switching between different attentional states. We also show that our model can qualitatively match results from experiments involving acetylcholine.

Cognitive control in Parkinson's disease

Cognitive control is the ability to act according to plan. Problems with cognitive control are a primary symptom and a major decrement of quality of life in Parkinson's disease (PD). Individuals with PD have problems with seemingly different controlled processes (e.g., task switching, impulsivity, gait disturbance, apathetic motivation). We review how these varied processes all rely upon disease-related alteration of common neural substrates, particularly due to dopaminergic imbalance. A comprehensive understanding of the neural systems underlying cognitive control will hopefully lead to more concise and reliable explanations of distributed deficits. However, high levels of clinical heterogeneity and medication-invariant control deficiencies suggest the need for increasingly detailed elaboration of the neural systems underlying control in PD.

Temporal dynamics of affect in the brain: Evidence from human imaging and animal models

Emotions are time-varying internal states that promote survival in the face of dynamic environments and shifting homeostatic needs. Research in non-human organisms has recently afforded specific insights into the neural mechanisms that support the emergence, persistence, and decay of affective states. Concurrently, a separate affective neuroscience literature has begun to dissect the neural bases of affective dynamics in humans. However, the circuit-level mechanisms identified in animals lack a clear mapping to the human neuroscience literature. As a result, critical questions pertaining to the neural bases of affective dynamics in humans remain unanswered. To address these shortcomings, the present review integrates findings from humans and non-human organisms to highlight the neural mechanisms that govern the temporal features of emotional states. Using the theory of affective chronometry as an organizing framework, we describe the specific neural mechanisms and modulatory factors that arbitrate the rise-time, intensity, and duration of emotional states.

Rhythmicity of Prefrontal Local Field Potentials after Nucleus Basalis Stimulation

The action of acetylcholine in the cortex is critical for cognitive functions and cholinergic drugs can improve functions such as attention and working memory. An alternative means of enhancing cholinergic neuromodulation in primates is the intermittent electrical stimulation of the cortical source of acetylcholine, the nucleus basalis (NB) of Meynert. NB stimulation generally increases firing rate of neurons in the prefrontal cortex, however its effects on single neurons are diverse and complex. We sought to understand how NB stimulation affects global measures of neural activity by recording and analyzing local field potentials (LFPs) in monkeys as they performed working memory tasks. NB stimulation primarily decreased power in the alpha frequency range during the delay interval of working memory tasks. The effect was consistent across variants of the task. No consistent modulation in the delay interval of the task was observed in the gamma frequency range, which has previously been implicated in the maintenance of working memory. Our results reveal global effects of cholinergic neuromodulation via deep brain stimulation, an emerging intervention for the improvement of cognitive function.

Nicotine Enhances Goal-Tracking in Ethanol and Food Pavlovian Conditioned Approach Paradigms

Rationale

Nicotine promotes alcohol intake through pharmacological and behavioral interactions. As an example of the latter, nicotine can facilitate approach toward food- and alcohol-associated stimuli ("sign-tracking") in lever-Pavlovian conditioned approach (PavCA) paradigms. However, we recently reported that nicotine can also enhance approach toward locations of reward delivery ("goal-tracking") triggered by ethanol-predictive stimuli when the location of ethanol delivery is non-static (i.e., a retractable sipper bottle).

Objective

To determine whether the non-static nature of the reward location could have biased the development of goal-tracking in our previous study (Loney et al., 2019); we assessed the effect of nicotine in a lever-PavCA paradigm wherein the location of ethanol delivery was static (i.e., a stationary liquid receptacle). Then, to determine whether nicotine's enhancement of goal-tracking is unique to ethanol-predictive stimuli, we assessed the effect of systemic nicotine on approach triggered by food-predictive stimuli in a lever-PavCA paradigm.

Methods

Long-Evans rats were used in two PavCA experiments wherein a lever predicted the receipt of ethanol (15% vol/vol; experiment 1) or food (experiment 2) into a stationary receptacle. Prior to testing, rats were administered nicotine (0.4 mg/kg subcutaneously) or saline systemically.

Results

In both experiments, nicotine increased measures of goal-tracking, but not sign-tracking.

Conclusion

Nicotine can facilitate approach to reward locations without facilitating approach to reward-predictive stimuli. As such, conceptualization of the mechanisms by which nicotine affects behavior must be expanded to explain an enhancement of goal-tracking by nicotine.

Nucleus basalis stimulation enhances working memory by stabilizing stimulus representations in primate prefrontal cortical activity

Acetylcholine plays a critical role in the neocortex. Cholinergic agonists and acetylcholinesterase inhibitors can enhance cognitive functioning, as does intermittent electrical stimulation of the cortical source of acetylcholine, the nucleus basalis (NB) of Meynert. Here we show in two male monkeys how NB stimulation affects working memory and alters its neural code. NB stimulation increases dorsolateral prefrontal activity during the delay period of spatial working memory tasks and broadens selectivity for stimuli but does not strengthen phasic responses to each neuron's optimal visual stimulus. Paradoxically, despite this decrease in neuronal selectivity, performance improves in many task conditions, likely indicating increased delay period stability. Performance under NB stimulation does decline if distractors similar to the target are presented, consistent with reduced prefrontal selectivity. Our results indicate that stimulation of the cholinergic forebrain increases prefrontal neural activity, and this neuromodulatory tone can improve cognitive performance, subject to a stability-accuracy tradeoff.

Excitatory-inhibitory tone shapes decision strategies in a hierarchical neural network model of multi-attribute choice

We are constantly faced with decisions between alternatives defined by multiple attributes, necessitating an evaluation and integration of different information sources. Time-varying signals in multiple brain areas are implicated in decision-making; but we lack a rigorous biophysical description of how basic circuit properties, such as excitatory-inhibitory (E/I) tone and cascading nonlinearities, shape attribute processing and choice behavior. Furthermore, how such properties govern choice performance under varying levels of environmental uncertainty is unknown. We investigated two-attribute, two-alternative decision-making in a dynamical, cascading nonlinear neural network with three layers: an input layer encoding choice alternative attribute values; an intermediate layer of modules processing separate attributes; and a final layer producing the decision. Depending on intermediate layer E/I tone, the network displays distinct regimes characterized by linear (I), convex (II) or concave (III) choice indifference curves. In regimes I and II, each option's attribute information is additively integrated. In regime III, time-varying nonlinear operations amplify the separation between offer distributions by selectively attending to the attribute with the larger differences in input values. At low environmental uncertainty, a linear combination most consistently selects higher valued alternatives. However, at high environmental uncertainty, regime III is more likely than a linear operation to select alternatives with higher value. Furthermore, there are conditions where readout from the intermediate layer could be experimentally indistinguishable from the final layer. Finally, these principles are used to examine multi-attribute decisions in systems with reduced inhibitory tone, leading to predictions of different choice patterns and overall performance between those with restrictions on inhibitory tone and neurotypicals.

Make a Left Turn: Cortico-Striatal Circuitry Mediating the Attentional Control of Complex Movements

BACKGROUND

In movement disorders such as Parkinson's disease (PD), cholinergic signaling is disrupted by the loss of basal forebrain cholinergic neurons, as well as aberrant activity in striatal cholinergic interneurons (ChIs). Several lines of evidence suggest that gait imbalance, a key disabling symptom of PD, may be driven by alterations in high-level frontal cortical and cortico-striatal processing more typically associated with cognitive dysfunction.

METHODS

Here we describe the corticostriatal circuitry that mediates the cognitive-motor interactions underlying such complex movement control. The ability to navigate dynamic, obstacle-rich environments requires the continuous integration of information about the environment with movement selection and sequencing. The cortical-attentional processing of extero- and interoceptive cues requires modulation by cholinergic activity to guide striatal movement control. Cue-derived information is "transferred" to striatal circuitry primarily via fronto-striatal glutamatergic projections.

RESULT

Evidence from parkinsonian fallers and from a rodent model reproducing the dual cholinergic-dopaminergic losses observed in these patients supports the main hypotheses derived from this neuronal circuitry-guided conceptualization of parkinsonian falls. Furthermore, in the striatum, ChIs constitute a particularly critical node for the integration of cortical with midbrain dopaminergic afferents and thus for cues to control movements.

CONCLUSION

Procholinergic treatments that enhance or rescue cortical and striatal mechanisms may improve complex movement control in parkinsonian fallers and perhaps also in older persons suffering from gait disorders and a propensity for falls. © 2021 International Parkinson and Movement Disorder Society.

Forebrain Cholinergic Signaling: Wired and Phasic, Not Tonic, and Causing Behavior

Conceptualizations of cholinergic signaling as primarily spatially diffuse and slow-acting are based largely on measures of extracellular brain ACh levels that require several minutes to generate a single data point. In addition, most such studies inhibited the highly potent catalytic enzyme for ACh, AChE, to facilitate measurement of ACh. Absent such inhibition, AChE limits the presence of ambient ACh and thus renders it unlikely that ACh influences target regions via slow changes in extracellular ACh concentrations. We describe an alternative view by which forebrain signaling in cortex driving cognition is largely phasic (milliseconds to perhaps seconds), and unlikely to be volume-transmitted. This alternative is supported by new evidence from real-time amperometric recordings of cholinergic signaling indicating a specific function of rapid, phasic, transient cholinergic signaling in attentional contexts. Previous neurochemical evidence may be reinterpreted in terms of integrated phasic cholinergic activity that mediates specific behavioral and cognitive operations; this reinterpretation fits well with recent computational models. Optogenetic studies support a causal relationship between cholinergic transients and behavior. This occurs in part via transient-evoked muscarinic receptor-mediated high-frequency oscillations in cortical regions. Such oscillations outlast cholinergic transients and thus link transient ACh signaling with more sustained postsynaptic activity patterns to support relatively persistent attentional biases. Reconceptualizing cholinergic function as spatially specific, phasic, and modulating specific cognitive operations is theoretically powerful and may lead to pharmacologic treatments more effective than those based on traditional views.Dual Perspectives Companion Paper: Diverse Spatiotemporal Scales of Cholinergic Signaling in the Neocortex, by Anita A. Disney and Michael J. Higley.

Cholinergic Signaling Dynamics and Cognitive Control of Attention

The central cholinergic system is one of the most important modulator neurotransmitter system implicated in diverse behavioral processes. Activation of the basal forebrain cortical cholinergic input system represents a critical step in cortical information processing. This chapter explores recent developments illustrating cortical cholinergic transmission mediate defined cognitive operations, which is contrary to the traditional view that acetylcholine acts as a slowly acting neuromodulator that influences arousal cortex-wide. Specifically, we review the evidence that phasic cholinergic signaling in the prefrontal cortex is a causal mediator of signal detection. In addition, studies that support the neuromodulatory role of cholinergic inputs in top-down attentional control are summarized. Finally, we review new findings that reveal sex differences and hormonal regulation of the cholinergic-attention system.

Rescuing the attentional performance of rats with cholinergic losses by the M1 positive allosteric modulator TAK-071

RATIONALE

Loss of basal forebrain cholinergic neurons contributes to the severity of the cognitive decline in age-related dementia and, in patients with Parkinson's disease (PD), to impairments in gait and balance and the resulting risks for falls. Contrasting with the extensive evidence indicating an essential role of cholinergic activity in mediating cognitive, specifically attentional abilities, treatment with conventional acetylcholinesterase inhibitors (AChEIs) has not fulfilled the promise of efficacy of pro-cholinergic treatments.

OBJECTIVES

Here, we investigated the potential usefulness of a muscarinic M1 positive allosteric modulator (PAM) in an animal model of cholinergic loss-induced impairments in attentional performance. Given evidence indicating that fast, transient cholinergic signaling mediates the detection of cues in attentional contexts, we hypothesized that a M1 PAM amplifies such transient signaling and thereby rescues attentional performance.

RESULTS

Rats performed an operant sustained attention task (SAT), including in the presence of a distractor (dSAT) and during a post-distractor (post-dSAT) period. The post-dSAT period served to assess the capacity for recovering performance following a disruptive event. Basal forebrain infusions of the cholino-specific immunotoxin 192 IgG-saporin impaired SAT performance, and greater cholinergic losses predicted lower post-dSAT performance. Administration of TAK-071 (0.1, 0.3 mg/kg, p.o., administered over 6-day blocks) improved the performance of all rats during the post-dSAT period (main effect of dose). Drug-induced improvement of post-dSAT performance was relatively greater in lesioned rats, irrespective of sex, but also manifested in female control rats. TAK-071 primarily improved perceptual sensitivity (d') in lesioned rats and facilitated the adoption of a more liberal response bias (B˝_D) in all female rats.

CONCLUSIONS

These findings suggest that TAK-071 may benefit the attentional performance of patients with partial cholinergic losses and specifically in situations that tax top-down, or goal-driven, attentional control.

Co-treatment with rivastigmine and idalopirdine reduces the propensity for falls in a rat model of falls in Parkinson's disease

RATIONALE

Falls in patients with Parkinson's disease (PD) are associated with cognitive, specifically attentional impairments and with losses in cholinergic projection systems. We previously established an animal model of the combined basal forebrain cholinergic-striatal dopaminergic losses of PD fallers (Dual Lesioned, DL, rats) and demonstrated that treating DL rats with an acetylcholinesterase inhibitor (AChEI), donepezil, together with a 5HT₆ receptor antagonist, idalopirdine, reduced fall frequency and improved associated aspects of the performance of DL rats traversing rotating rods.

OBJECTIVES

Here, we employed a longer and more taxing rotating beam apparatus to determine the potential therapeutic efficacy of idalopirdine when combined with the pseudo-irreversible, and thus relatively long-acting, AChE- and butyrylcholinesterase- (BuChE) inhibitor rivastigmine.

RESULTS

As before, vehicle-treated DL rats fell more frequently, committed more slips, and exhibited more movement stoppages than intact control rats. Repeated intermittent administration of rivastigmine and idalopirdine significantly improved the performance of DL rats. Rivastigmine alone also produced strong trends for reducing falls and slips. The combination treatment was more effective than rivastigmine alone in reducing stoppages and stoppage-associated falls. As before, idalopirdine treatment alone was ineffective.

CONCLUSIONS

These results extend the prediction that the combined treatment with idalopirdine and an AChEI improves complex movement control and reduces the propensity for falls in patients with movement disorders. Because of the importance of finding better treatments for gait and balance deficits in PD, the present results may further motivate a clinical exploration of the usefulness of this combination treatment.

Repetitive mild concussion in subjects with a vulnerable cholinergic system: Lasting cholinergic-attentional impairments in CHT+/- mice

Previous research emphasized the impact of traumatic brain injury on cholinergic systems and associated cognitive functions. Here we addressed the converse question: Because of the available evidence indicating cognitive and neuronal vulnerabilities in humans expressing low-capacity cholinergic systems or with declining cholinergic systems, do injuries cause more severe cognitive decline in such subjects, and what cholinergic mechanisms contribute to such vulnerability? Using mice heterozygous for the choline transporter (CHT+/- mice) as a model for a limited cholinergic capacity, we investigated the cognitive and neuronal consequences of repeated, mild concussion injuries (rmCc). After five rmCc, and compared with wild type (WT) mice, CHT+/- mice exhibited severe and lasting impairments in sustained attention performance, consistent with effects of cholinergic losses on attention. However, rmCc did not affect the integrity of neuronal cell bodies and did not alter the density of cortical synapses. As a cellular mechanism potentially responsible for the attentional impairment in CHT+/- mice, we found that rmCc nearly completely attenuated performance-associated, CHT-mediated choline transport. These results predict that subjects with an already vulnerable cholinergic system will experience severe and lasting cognitive-cholinergic effects after even relatively mild injuries. If confirmed in humans, such subjects may be excluded from, or receive special protection against, activities involving injury risk. Moreover, the treatment and long-term outcome of traumatic brain injuries may benefit from determining the status of cholinergic systems and associated cognitive functions. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Basal forebrain chemogenetic inhibition disrupts the superior complex movement control of goal-tracking rats

Sign- and goal-tracking behavior signifies the influence of opposed cognitive-motivational styles, with the former being characterized by a tendency for approaching and contacting reward cues, including a readiness for attending, bottom-up, to salient cues, and a relatively greater vulnerability for developing and maintaining addiction-like behaviors. We previously demonstrated that these styles also impact the cognitive-motor interactions that are taxed during traversal of dynamic surfaces, with goal-trackers (GTs) making less movement errors and falling less frequently than sign-trackers (STs). The present experiment tested the hypothesis that complex movement control in GTs, but not STs, depends on activation of the basal forebrain projection system to telencephalic regions. Chemogenetic inhibition of the basal forebrain increased movement errors and falls in GTs during traversal of a rotating zigzag rod but had no significant effect on the relatively lower performance of STs. Neurochemical evidence confirmed the efficacy of the inhibitory designer receptor exclusively activated by designer drug (DREADD). Administration of clozapine-N-oxide (CNO) had no significant effect in GTs not expressing the DREADD. These results indicate that GTs, but not STs, activate the basal forebrain projection system to mediate their relatively superior ability for complex movement control. STs may also serve as an animal model in research on the role of basal forebrain systems in aging- and Parkinson's disease-associated falls. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Behavioural and neural signatures of perceptual decision-making are modulated by pupil-linked arousal

The timing and accuracy of perceptual decision-making is exquisitely sensitive to fluctuations in arousal. Although extensive research has highlighted the role of various neural processing stages in forming decisions, our understanding of how arousal impacts these processes remains limited. Here we isolated electrophysiological signatures of decision-making alongside signals reflecting target selection, attentional engagement and motor output and examined their modulation as a function of tonic and phasic arousal, indexed by baseline and task-evoked pupil diameter, respectively. Reaction times were shorter on trials with lower tonic, and higher phasic arousal. Additionally, these two pupil measures were predictive of a unique set of EEG signatures that together represent multiple information processing steps of decision-making. Finally, behavioural variability associated with fluctuations in tonic and phasic arousal, indicative of neuromodulators acting on multiple timescales, was mediated by its effects on the EEG markers of attentional engagement, sensory processing and the variability in decision processing.

Driving along a busy street requires you to constantly monitor the behavior of other road users. You need to be able to spot and avoid the car that suddenly changes lane, or the pedestrian who steps out in front of you. How fast you can react to such events depends in part on your brain's level of alertness, or 'arousal'. This in turn depends on chemicals within the brain called neuromodulators.

Neuromodulators are a type of neurotransmitter. But whereas other neurotransmitters enable brain cells to signal to each other, neuromodulators turn the volume of these signals up or down. The activity of brain regions that produce neuromodulators varies over time, leading to changes in brain arousal. These changes take place over different time scales. Sudden unexpected events, such as those on the busy street above, trigger sub-second changes in arousal. But arousal levels also show spontaneous fluctuations over minutes to hours. We can follow these changes in real-time by looking into a participant’s eyes. This is because the brain regions that produce neuromodulators also control pupil size.

Van Kempen et al. have now combined measurements of pupil size with recordings of electrical brain activity. Healthy volunteers learned to press a button as soon as a target appeared on a screen. The larger a volunteer’s pupils were before the target appeared, the more slowly the volunteer responded on that trial. Large baseline pupil size is thought to indicate a high baseline level of brain arousal. By contrast, the larger the increase in pupil size in response to the target, the faster the volunteer responded on that trial. This increase in pupil size is thought to reflect an increase in brain arousal.

The recordings of brain activity provided clues to the underlying mechanisms. In trials with large baseline pupil size – and therefore high baseline arousal – the volunteers’ brains showed more variable responses to the target. But in trials with a large increase in pupil size – and a large increase in arousal – the volunteers’ brains showed less variable responses, as well as stronger signals related to attention.

Neuromodulators thus act on different timescales to influence different aspects of cognitive performance, including attention and target detection. Fluctuating levels of neuromodulator activity may help explain the variability in our behavior. Monitoring pupil size is one way to gain insights into the mechanisms that bring about these changes in neuromodulator activity.

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

Cholinergic double duty: cue detection and attentional control

Cholinergic signaling in the cortex involves fast or transient signaling as well as a relatively slower neuromodulatory component. These two components of cholinergic activity mediate separate yet interacting aspects of cue detection and attentional control. The transient component appears to support the activation of cue-associated task or response sets, whereas the slower modulatory component stabilizes task-set and context representations, therefore potentially facilitating top-down control. Evidence from humans expressing genetic variants of the choline transporter as well as from patients with degenerating cholinergic systems supports the hypothesis that attentional control capacities depend on levels of cholinergic neuromodulation. Deficits in cholinergic-attentional control impact diverse cognitive functions, including timing, working memory, and complex movement control.

Structure and function of dual-source cholinergic modulation in early vision

Behavioral states such as arousal and attention have profound effects on sensory processing, determining how-even whether-a stimulus is perceived. This state-dependence is believed to arise, at least in part, in response to inputs from subcortical structures that release neuromodulators such as acetylcholine, often nonsynaptically. The mechanisms that underlie the interaction between these nonsynaptic signals and the more point-to-point synaptic cortical circuitry are not well understood. This review highlights the state of the field, with a focus on cholinergic action in early visual processing. Key anatomical and physiological features of both the cholinergic and the visual systems are discussed. Furthermore, presenting evidence of cholinergic modulation in visual thalamus and primary visual cortex, we explore potential functional roles of acetylcholine and its effects on the processing of visual input over the sleep-wake cycle, sensory gain control during wakefulness, and consider evidence for cholinergic support of visual attention.

Vocal learning in songbirds requires cholinergic signaling in a motor cortex-like nucleus

Cholinergic inputs to cortex modulate plasticity and sensory processing, yet little is known about their role in motor control. Here, we show that cholinergic signaling in a songbird vocal motor cortical area, the robust nucleus of the arcopallium (RA), is required for song learning. Reverse microdialysis of nicotinic and muscarinic receptor antagonists into RA in juvenile birds did not significantly affect syllable timing or acoustic structure during vocal babbling. However, chronic blockade over weeks reduced singing quantity and impaired learning, resulting in an impoverished song with excess variability, abnormal acoustic features, and reduced similarity to tutor song. The demonstration that cholinergic signaling in a motor cortical area is required for song learning motivates the songbird as a tractable model system to identify roles of the basal forebrain cholinergic system in motor control. NEW & NOTEWORTHY Cholinergic inputs to cortex are evolutionarily conserved and implicated in sensory processing and synaptic plasticity. However, functions of cholinergic signals in motor areas are understudied and poorly understood. Here, we show that cholinergic signaling in a songbird vocal motor cortical area is not required for normal vocal variability during babbling but is essential for developmental song learning. Cholinergic modulation of motor cortex is thus required for learning but not for the ability to sing.

Olfactory bulb acetylcholine release dishabituates odor responses and reinstates odor investigation

Habituation and dishabituation modulate the neural resources and behavioral significance allocated to incoming stimuli across the sensory systems. We characterize these processes in the mouse olfactory bulb (OB) and uncover a role for OB acetylcholine (ACh) in physiological and behavioral olfactory dishabituation. We use calcium imaging in both awake and anesthetized mice to determine the time course and magnitude of OB glomerular habituation during a prolonged odor presentation. In addition, we develop a novel behavioral investigation paradigm to determine how prolonged odor input affects odor salience. We find that manipulating OB ACh release during prolonged odor presentations using electrical or optogenetic stimulation rapidly modulates habituated glomerular odor responses and odor salience, causing mice to suddenly investigate a previously ignored odor. To demonstrate the ethological validity of this effect, we show that changing the visual context can lead to dishabituation of odor investigation behavior, which is blocked by cholinergic antagonists in the OB.

Preferential cholinergic excitation of corticopontine neurons

KEY POINTS

Phasic activation of M1 muscarinic receptors generates transient inhibition followed by longer lasting excitation in neocortical pyramidal neurons. Corticopontine neurons in the mouse prefrontal cortex exhibit weaker cholinergic inhibition, but more robust and longer lasting excitation, than neighbouring callosal projection neurons. Optogenetic release of endogenous ACh in response to single flashes of light (5 ms) preferentially enhances the excitability of corticopontine neurons for many tens of seconds. Cholinergic excitation of corticopontine neurons involves at least three ionic mechanisms: suppression of K_V 7 currents, activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Preferential cholinergic excitation of prefrontal corticopontine neurons may facilitate top-down attentional processes and behaviours.

ABSTRACT

Pyramidal neurons in layer 5 of the neocortex comprise two broad classes of projection neurons: corticofugal neurons, including corticopontine (CPn) neurons, and intratelencephalic neurons, including commissural/callosal (COM) neurons. These non-overlapping neuron subpopulations represent discrete cortical output channels contributing to perception, decision making and behaviour. CPn and COM neurons have distinct morphological and physiological characteristics, and divergent responses to modulatory transmitters such as serotonin and acetylcholine (ACh). To better understand how ACh regulates cortical output, in slices of mouse prefrontal cortex (PFC) we compared the responsivity of CPn and COM neurons to transient exposure to exogenous or endogenous ACh. In both neuron subtypes, exogenous ACh generated qualitatively similar biphasic responses in which brief hyperpolarization was followed by longer lasting enhancement of excitability. However, cholinergic inhibition was more pronounced in COM neurons, while excitatory responses were larger and longer lasting in CPn neurons. Similarly, optically triggered release of endogenous ACh from cholinergic terminals preferentially and persistently (for ∼40 s) enhanced the excitability of CPn neurons, but had little impact on COM neurons. Cholinergic excitation of CPn neurons involved at least three distinct ionic mechanisms: suppression of K_V 7 channels (the 'M-current'), activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Our findings demonstrate projection-specific selectivity in cholinergic signalling in the PFC, and suggest that transient release of ACh during behaviour will preferentially promote corticofugal output.

The neuroscience of cognitive-motivational styles: Sign- and goal-trackers as animal models

Cognitive-motivational styles describe predominant patterns of processing or biases that broadly influence human cognition and performance. Here we focus on the impact of cognitive-motivational styles on the response to cues predicting the availability of food or addictive drugs. An individual may preferably conduct an analysis of the motivational significance of reward cues, with the result that such cues per se are perceived as rewarding and worth approaching and working for. Alternatively, a propensity for a "cold" analysis of the behavioral utility of a reward cue may yield search behavior for food or drugs but not involve cue approach. Animal models for studying the neuronal mechanisms mediating such styles have originated from research concerning behavioral indices that predict differential vulnerability to addiction-like behaviors. Rats classified as sign- or goal-trackers (STs, GTs) were found to have opposed attentional biases (bottom-up or cue-driven attention vs. top-down or goal-driven attentional control) that are mediated primarily via relatively unresponsive versus elevated levels of cholinergic neuromodulation in the cortex. The capacity for cholinergic neuromodulation in STs is limited by a neuronal choline transporter (CHT) that fails to support increases in cholinergic activity. Moreover, in contrast to STs, the frontal dopamine system in GTs does not respond to the presence of drug cues and, thus, biases against cue-oriented behavior. The opponent cognitive-motivational styles that are indexed by sign- and goal-tracking bestow different cognitive-behavioral vulnerabilities that may contribute to the manifestation of a wide range of neuropsychiatric disorders. (PsycINFO Database Record

Opposing Cholinergic and Serotonergic Modulation of Layer 6 in Prefrontal Cortex

Prefrontal cortex is a hub for attention processing and receives abundant innervation from cholinergic and serotonergic afferents. A growing body of evidence suggests that acetylcholine (ACh) and serotonin (5-HT) have opposing influences on tasks requiring attention, but the underlying neurophysiology of their opposition is unclear. One candidate target population is medial prefrontal layer 6 pyramidal neurons, which provide feedback modulation of the thalamus, as well as feed-forward excitation of cortical interneurons. Here, we assess the response of these neurons to ACh and 5-HT using whole cell recordings in acute brain slices from mouse cortex. With application of exogenous agonists, we show that individual layer 6 pyramidal neurons are bidirectionally-modulated, with ACh and 5-HT exerting opposite effects on excitability across a number of concentrations. Next, we tested the responses of layer 6 pyramidal neurons to optogenetic release of endogenous ACh or 5-HT. These experiments were performed in brain slices from transgenic mice expressing channelrhodopsin in either ChAT-expressing cholinergic neurons or Pet1-expressing serotonergic neurons. Light-evoked endogenous neuromodulation recapitulated the effects of exogenous neurotransmitters, showing opposing modulation of layer 6 pyramidal neurons by ACh and 5-HT. Lastly, the addition of 5-HT to either endogenous or exogenous ACh significantly suppressed the excitation of pyramidal neurons in prefrontal layer 6. Taken together, this work suggests that the major corticothalamic layer of prefrontal cortex is a substrate for opposing modulatory influences on neuronal activity that could have implications for regulation of attention.

Basal Forebrain Cholinergic Circuits and Signaling in Cognition and Cognitive Decline

Recent work continues to place cholinergic circuits at center stage for normal executive and mnemonic functioning and provides compelling evidence that the loss of cholinergic signaling and cognitive decline are inextricably linked. This Review focuses on the last few years of studies on the mechanisms by which cholinergic signaling contributes to circuit activity related to cognition. We attempt to identify areas of controversy, as well as consensus, on what is and is not yet known about how cholinergic signaling in the CNS contributes to normal cognitive processes. In addition, we delineate the findings from recent work on the extent to which dysfunction of cholinergic circuits contributes to cognitive decline associated with neurodegenerative disorders.

Cognitive enhancement and antipsychotic-like activity following repeated dosing with the selective M4 PAM VU0467154

Although selective activation of the M₁ muscarinic acetylcholine receptor (mAChR) subtype has been shown to improve cognitive function in animal models of neuropsychiatric disorders, recent evidence suggests that enhancing M₄ mAChR function can also improve memory performance. Positive allosteric modulators (PAMs) targeting the M₄ mAChR subtype have shown therapeutic potential for the treatment of multiple symptoms observed in schizophrenia, including positive and cognitive symptoms when assessed in acute preclinical dosing paradigms. Since the cholinergic system has been implicated in multiple stages of learning and memory, we evaluated the effects of repeated dosing with the highly selective M₄ PAM VU0467154 on either acquisition and/or consolidation of learning and memory when dosed alone or after pharmacologic challenge with the N-methyl-d-aspartate subtype of glutamate receptors (NMDAR) antagonist MK-801. MK-801 challenge represents a well-documented preclinical model of NMDAR hypofunction that is thought to underlie some of the positive and cognitive symptoms observed in schizophrenia. In wildtype mice, 10-day, once-daily dosing of VU0467154 either prior to, or immediately after daily testing enhanced the rate of learning in a touchscreen visual pairwise discrimination task; these effects were absent in M₄ mAChR knockout mice. Following a similar 10-day, once-daily dosing regimen of VU0467154, we also observed 1) improved acquisition of memory in a cue-mediated conditioned freezing paradigm, 2) attenuation of MK-801-induced disruptions in the acquisition of memory in a context-mediated conditioned freezing paradigm and 3) reversal of MK-801-induced hyperlocomotion. Comparable efficacy and plasma and brain concentrations of VU0467154 were observed after repeated dosing as those previously reported with an acute, single dose administration of this M₄ PAM. Together, these studies are the first to demonstrate that cognitive enhancing and antipsychotic-like activity are not subject to the development of tolerance following repeated dosing with a selective M₄ PAM in mice and further suggest that activation of M₄ mAChRs may modulate both acquisition and consolidation of memory functions.

Hemicholinium-3 sensitive choline transport in human T lymphocytes: Evidence for use as a proxy for brain choline transporter (CHT) capacity

The synaptic uptake of choline via the high-affinity, hemicholinium-3-dependent choline transporter (CHT) strongly influences the capacity of cholinergic neurons to sustain acetylcholine (ACh) synthesis and release. To advance research on the impact of CHT capacity in humans, we established the presence of the neuronal CHT protein in human T lymphocytes. Next, we demonstrated CHT-mediated choline transport in human T cells. To address the validity of T cell-based choline uptake as a proxy for brain CHT capacity, we isolated T cells from the spleen, and synaptosomes from cortex and striatum, of wild type and CHT-overexpressing mice (CHT-OXP). Choline uptake capacity in T cells from CHT-OXP mice was two-fold higher than in wild type mice, mirroring the impact of CHT over-expression on synaptosomal CHT-mediated choline uptake. Monitoring T lymphocyte CHT protein and activity may be useful for estimating human CNS cholinergic capacity and for testing hypotheses concerning the contribution of CHT and, more generally, ACh signaling in cognition, neuroinflammation and disease.

Acetylcholine Release in Prefrontal Cortex Promotes Gamma Oscillations and Theta-Gamma Coupling during Cue Detection

The capacity for using external cues to guide behavior ("cue detection") constitutes an essential aspect of attention and goal-directed behavior. The cortical cholinergic input system, via phasic increases in prefrontal acetylcholine release, plays an essential role in attention by mediating such cue detection. However, the relationship between cholinergic signaling during cue detection and neural activity dynamics in prefrontal networks remains unclear. Here we combined subsecond measures of cholinergic signaling, neurophysiological recordings, and cholinergic receptor blockade to delineate the cholinergic contributions to prefrontal oscillations during cue detection in rats. We first confirmed that detected cues evoke phasic acetylcholine release. These cholinergic signals were coincident with increased neuronal synchrony across several frequency bands and the emergence of theta-gamma coupling. Muscarinic and nicotinic cholinergic receptors both contributed specifically to gamma synchrony evoked by detected cues, but the effects of blocking the two receptor subtypes were dissociable. Blocking nicotinic receptors primarily attenuated high-gamma oscillations occurring during the earliest phases of the cue detection process, while muscarinic (M1) receptor activity was preferentially involved in the transition from high to low gamma power that followed and corresponded to the mobilization of networks involved in cue-guided decision making. Detected cues also promoted coupling between gamma and theta oscillations, and both nicotinic and muscarinic receptor activity contributed to this process. These results indicate that acetylcholine release coordinates neural oscillations during the process of cue detection.SIGNIFICANCE STATEMENT The capacity of learned cues to direct attention and guide responding ("cue detection") is a key component of goal-directed behavior. Rhythmic neural activity and increases in acetylcholine release in the prefrontal cortex contribute to this process; however, the relationship between these neuronal mechanisms is not well understood. Using a combination of in vivo neurochemistry, neurophysiology, and pharmacological methods, we demonstrate that cue-evoked acetylcholine release, through distinct actions at both nicotinic and muscarinic receptors, triggers a procession of neural oscillations that map onto the multiple stages of cue detection. Our data offer new insights into cholinergic function by revealing the temporally orchestrated changes in prefrontal network synchrony modulated by acetylcholine release during cue detection.

Unresponsive Choline Transporter as a Trait Neuromarker and a Causal Mediator of Bottom-Up Attentional Biases

Some rats [sign-trackers (STs)] are prone to attribute incentive salience to reward cues, which can manifest as a propensity to approach and contact pavlovian cues, and for addiction-like behavior. STs also exhibit poor attentional performance, relative to goal-trackers (GTs), which is associated with attenuated acetylcholine (ACh) levels in prefrontal cortex (Paolone et al., 2013). Here, we demonstrate a cellular mechanism, linked to ACh synthesis, that accounts for attenuated cholinergic capacity in STs. First, we found that electrical stimulation of the basal forebrain increased cortical choline transporter (CHT)-mediated choline transport in GTs, paralleled by a redistribution of CHTs to the synaptic plasma membrane. Neither increases in choline uptake nor translocation of CHTs occurred in STs. Second, and consistent with uptake/translocation alterations, STs demonstrated a reduced ability to support cortical ACh release in vivo compared with GTs after reverse-dialysis to elevate extracellular potassium levels. Third, rats were significantly more likely to develop sign-tracking behavior if treated systemically before pavlovian conditioned approach training with the CHT inhibitor VU6001221. Consistent with its proposed mechanisms, administration of VU6001221 attenuated potassium-evoked ACh levels in prefrontal cortex measured with in vivo microdialysis. We propose that loss of CHT-dependent activation of cortical cholinergic activity in STs degrades top-down executive control over behavior, producing a bias for bottom-up or stimulus-driven attention. Such an attentional bias contributes to nonadaptive reward processing and thus identifies a novel mechanism that can support psychopathology, including addiction.SIGNIFICANCE STATEMENT The vulnerability for addiction-like behavior has been associated with psychological traits, such as the propensity to attribute incentive salience to reward cues that is modeled in rats by sign-tracking behavior. Sign-trackers tend to approach and contact cues associated with reward, whereas their counterparts, the goal-trackers, have a preference for approaching the location of the reward. Here, we show that the capacity of presynaptic cholinergic synapses to respond to stimulation by elevating presynaptic choline uptake and releasing acetylcholine is attenuated in sign-trackers. Furthermore, pharmacological inhibition of choline transport induced sign-tracking behavior. Our findings suggest that reduced levels of cholinergic neuromodulation can mediate an attentional bias toward reward-related cues, thereby allowing such cues to exert relatively greater control over behavior.

Reducing falls in Parkinson's disease: interactions between donepezil and the 5-HT6 receptor antagonist idalopirdine on falls in a rat model of impaired cognitive control of complex movements

Falls are a leading cause of death in the elderly and, in a majority of patients with Parkinson's disease (PD), the leading levodopa-insensitive cause of hospitalization and long-term care. Falling in PD has been attributed to degeneration of forebrain cholinergic neurons that, in interaction with striatal dopamine losses, impairs the cognitive control of balance, gait, and movement. We previously established an animal model of these dual cholinergic-dopaminergic losses ("DL rats") and a behavioral test system (Michigan Complex Motor Control Task, MCMCT) to measure falls associated with traversing dynamic surfaces and distractors. Because the combined treatment of the acetylcholinesterase inhibitor donepezil and the 5-HT₆ receptor antagonist idalopirdine (Lu AE58054) was reported to exhibit synergistic pro-cholinergic activity in rats and improved cognition in patients with moderate Alzheimer's disease, here we assessed the effects of this treatment on MCMCT performance and attention in DL rats. Compared with the vehicle-treated group, the combined treatment greatly reduced (Cohen's d = 0.96) falls in DL rats when traversing dynamic surfaces and when exposed to a passive distractor. However, falls associated with a dual task distractor and sustained attentional performance did not benefit from this treatment. Analyses of the behavior in fall-prone moments suggested that this treatment improved the efficacy and speed of re-instating forward movement after relatively short stoppages. This treatment may reduce fall propensity in PD patients via maintaining planned movement sequences in working memory and improving the vigor of executing such movements following brief periods of freezing of gait.

Cholinergic genetics of visual attention: Human and mouse choline transporter capacity variants influence distractibility

The basal forebrain cholinergic projection system to the cortex mediates essential aspects of visual attention performance, including the detection of cues and the response to performance challenges (top-down control of attention). Higher levels of top-down control are mediated via elevated levels of cholinergic neuromodulation. The neuronal choline transporter (CHT) strongly influences the synthesis and release of acetylcholine (ACh). As the capacity of the CHT to import choline into the neuron is a major, presynaptic determinant of cholinergic neuromodulation, we hypothesize that genetically-imposed CHT capacity variation impacts the balance of bottom-up versus top-down control of visual attention. Following a brief review of the cognitive concepts relevant for this hypothesis, we describe the key results from our research in mice and humans that possess genetically-imposed changes in choline uptake capacity. CHT subcapacity is associated with poor top-down attentional control and attenuated (cholinergic) activation of right frontal regions. Conversely, mice overexpressing the CHT, and humans expressing a CHT variant hypothesized to enhance choline transporter function, are relatively resistant to challenges of visual attention performance. Genetic or environmental modulation of CHT expression and function may be associated with vulnerabilities for cognitive disorders.