Cortical choline transporter function measured in vivo using choline-sensitive microelectrodes: clearance of endogenous and exogenous choline and effects of removal of cholinergic terminals

Non-neuronal cell-derived acetylcholine, a key modulator of the vascular endothelial function in health and disease

Vascular endothelial cells play an important role in regulating peripheral circulation by modulating arterial tone in the microvasculature. Elevated intracellular Ca2+ levels are required in endothelial cells to induce smooth muscle relaxation via endothelium-dependent mechanisms such as nitric oxide production, prostacyclin, and endothelial cell hyperpolarization. It is well established that exogenous administration of acetylcholine can increase intracellular Ca2+ concentrations, followed by endothelium-dependent vasodilation. Although endogenous acetylcholine's regulation of vascular tone remains debatable, recent studies have reported that endogenously derived acetylcholine, but not neuronal cell-derived acetylcholine, is a key modulator of endothelial cell function. In this minireview, we summarize the current knowledge of the non-neuronal cholinergic system (NNCS) in vascular function, particularly vascular endothelial cell function, which contributes to blood pressure regulation. We also discuss the possible pathophysiological impact of endothelial NNCS, which may induce the development of vascular diseases due to endothelial dysfunction, and the potential of endothelial NNCS as a novel therapeutic target for endothelial dysfunction in the early stages of metabolic syndrome, diabetes, and hypertension.

Cholinergic Mechanisms in Gastrointestinal Neoplasia

Acetylcholine-activated receptors are divided broadly into two major structurally distinct classes: ligand-gated ion channel nicotinic and G-protein-coupled muscarinic receptors. Each class encompasses several structurally related receptor subtypes with distinct patterns of tissue expression and post-receptor signal transduction mechanisms. The activation of both nicotinic and muscarinic cholinergic receptors has been associated with the induction and progression of gastrointestinal neoplasia. Herein, after briefly reviewing the classification of acetylcholine-activated receptors and the role that nicotinic and muscarinic cholinergic signaling plays in normal digestive function, we consider the mechanics of acetylcholine synthesis and release by neuronal and non-neuronal cells in the gastrointestinal microenvironment, and current methodology and challenges in measuring serum and tissue acetylcholine levels accurately. Then, we critically evaluate the evidence that constitutive and ligand-induced activation of acetylcholine-activated receptors plays a role in promoting gastrointestinal neoplasia. We focus primarily on adenocarcinomas of the stomach, pancreas, and colon, because these cancers are particularly common worldwide and, when diagnosed at an advanced stage, are associated with very high rates of morbidity and mortality. Throughout this comprehensive review, we concentrate on identifying novel ways to leverage these observations for prognostic and therapeutic purposes.

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.

Cholinergic Reinforcement Signaling Is Impaired by Amyloidosis Prior to Its Synaptic Loss

Alzheimer's disease (AD) is associated with amyloidosis and dysfunction of the cholinergic system, which is crucial for learning and memory. However, the nature of acetylcholine signaling within regions of cholinergic-dependent plasticity and how it changes with experience is poorly understood, much less the impact of amyloidosis on this signaling. Therefore, we optically measure the release profile of acetylcholine to unexpected, predicted, and predictive events in visual cortex (VC)-a site of known cholinergic-dependent plasticity-in a preclinical mouse model of AD that develops amyloidosis. We find that acetylcholine exhibits reinforcement signaling qualities, reporting behaviorally relevant outcomes and displaying release profiles to predictive and predicted events that change as a consequence of experience. We identify three stages of amyloidosis occurring before the degeneration of cholinergic synapses within VC and observe that cholinergic responses in amyloid-bearing mice become impaired over these stages, diverging progressively from age- and sex-matched littermate controls. In particular, amyloidosis degrades the signaling of unexpected rewards and punishments, and attenuates the experience-dependent (1) increase of cholinergic responses to outcome predictive visual cues, and (2) decrease of cholinergic responses to predicted outcomes. Hyperactive spontaneous acetylcholine release occurring transiently at the onset of impaired cholinergic signaling is also observed, further implicating disrupted cholinergic activity as an early functional biomarker in AD. Our findings suggest that acetylcholine acts as a reinforcement signal that is impaired by amyloidosis before pathologic degeneration of the cholinergic system, providing a deeper understanding of the effects of amyloidosis on acetylcholine signaling and informing future interventions for AD.SIGNIFICANCE STATEMENT The cholinergic system is especially vulnerable to the neurotoxic effects of amyloidosis, a hallmark of Alzheimer's disease (AD). Though amyloid-induced cholinergic synaptic loss is thought in part to account for learning and memory impairments in AD, little is known regarding how amyloid impacts signaling of the cholinergic system before its anatomic degeneration. Optical measurement of acetylcholine (ACh) release in a mouse model of AD that develops amyloidosis reveals that ACh signals reinforcement and outcome prediction that is disrupted by amyloidosis before cholinergic degeneration. These observations have important scientific and clinical implications: they implicate ACh signaling as an early functional biomarker, provide a deeper understanding of the action of acetylcholine, and inform on when and how intervention may best ameliorate cognitive decline in AD.

Disrupted Choline Clearance and Sustained Acetylcholine Release In Vivo by a Common Choline Transporter Coding Variant Associated with Poor Attentional Control in Humans

Transport of choline via the neuronal high-affinity choline transporter (CHT; SLC5A7) is essential for cholinergic terminals to synthesize and release acetylcholine (ACh). In humans, we previously demonstrated an association between a common CHT coding substitution (rs1013940; Ile89Val) and reduced attentional control as well as attenuated frontal cortex activation. Here, we used a CRISPR/Cas9 approach to generate mice expressing the I89V substitution and assessed, in vivo, CHT-mediated choline transport, and ACh release. Relative to wild-type (WT) mice, CHT-mediated clearance of choline in male and female mice expressing one or two Val89 alleles was reduced by over 80% in cortex and over 50% in striatum. Choline clearance in CHT Val89 mice was further reduced by neuronal inactivation. Deficits in ACh release, 5 and 10 min after repeated depolarization at a low, behaviorally relevant frequency, support an attenuated reloading capacity of cholinergic neurons in mutant mice. The density of CHTs in total synaptosomal lysates and neuronal plasma-membrane-enriched fractions was not impacted by the Val89 variant, indicating a selective impact on CHT function. When challenged with a visual disruptor to reveal attentional control mechanisms, Val89 mice failed to adopt a more conservative response bias. Structural modeling revealed that Val89 may attenuate choline transport by altering conformational changes of CHT that support normal transport rates. Our findings support the view that diminished sustained cholinergic signaling capacity underlies perturbed attentional performance in individuals expressing CHT Val89. The CHT Val89 mouse serves as a valuable model to study heritable risk for cognitive disorders arising from cholinergic dysfunction.SIGNIFICANCE STATEMENT Acetylcholine (ACh) signaling depends on the functional capacity of the neuronal choline transporter (CHT). Previous research demonstrated that humans expressing the common CHT coding variant Val89 exhibit attentional vulnerabilities and attenuated fronto-cortical activation during attention. Here, we find that mice engineered to express the Val89 variant exhibit reduced CHT-mediated choline clearance and a diminished capacity to sustain ACh release. Additionally, Val89 mice lack cognitive flexibility in response to an attentional challenge. These findings provide a mechanistic and cognitive framework for interpreting the attentional phenotype associated with the human Val89 variant and establish a model that permits a more invasive interrogation of CNS effects as well as the development of therapeutic strategies for those, including Val89 carriers, with presynaptic cholinergic perturbations.

Long-Term Depression of Striatal DA Release Induced by mGluRs via Sustained Hyperactivity of Local Cholinergic Interneurons

The cellular mechanisms regulating dopamine (DA) release in the striatum have attracted much interest in recent years. By in vitro amperometric recordings in mouse striatal slices, we show that a brief (5 min) exposure to the metabotropic glutamate receptor agonist DHPG (50 μM) induces a profound depression of synaptic DA release, lasting over 1 h from DHPG washout. This long-term depression is sensitive to glycine, which preferentially inhibits local cholinergic interneurons, as well as to drugs acting on nicotinic acetylcholine receptors and to the pharmacological depletion of released acetylcholine. The same DHPG treatment induces a parallel long-lasting enhancement in the tonic firing of presumed striatal cholinergic interneurons, measured with multi-electrode array recordings. When DHPG is bilaterally infused in vivo in the mouse striatum, treated mice display an anxiety-like behavior. Our results demonstrate that metabotropic glutamate receptors stimulation gives rise to a prolonged depression of the striatal dopaminergic transmission, through a sustained enhancement of released acetylcholine, due to the parallel long-lasting potentiation of striatal cholinergic interneurons firing. This plastic interplay between dopamine, acetylcholine, and glutamate in the dorsal striatum may be involved in anxiety-like behavior typical of several neuropsychiatric disorders.

Acetylcholine Regulates Pulmonary Pathology During Viral Infection and Recovery

Introduction

This study was designed to explore the role of acetylcholine (ACh) in pulmonary viral infection and recovery. Inflammatory control is critical to recovery from respiratory viral infection. ACh secreted from non-neuronal sources, including lymphocytes, plays an important, albeit underappreciated, role in regulating immune-mediated inflammation.

Methods

ACh and lymphocyte cholinergic status in the lungs were measured over the course of influenza infection and recovery. The role of ACh was examined by inhibiting ACh synthesis in vivo. Pulmonary inflammation was monitored by Iba1 immunofluorescence, using a novel automated algorithm. Tissue repair was monitored histologically.

Results

Pulmonary ACh remained constant through the early stage of infection and increased during the peak of the acquired immune response. As the concentration of ACh increased, cholinergic lymphocytes appeared in the BAL and lungs. Cholinergic capacity was found primarily in CD4 T cells, but also in B cells and CD8 T cells. The cholinergic CD4⁺ T cells bound to influenza-specific tetramers and were retained in the resident memory regions of the lung up to 2 months after infection. Histologically, cholinergic lymphocytes were found in direct physical contact with activated macrophages throughout the lung. Inflammation was monitored by ionized calcium-binding adapter molecule 1 (Iba1) immunofluorescence, using a novel automated algorithm. When ACh production was inhibited, mice exhibited increased tissue inflammation and delayed recovery. Histologic examination revealed abnormal tissue repair when ACh was limited.

Conclusion

These findings point to a previously unrecognized role for ACh in the transition from active immunity to recovery and pulmonary repair following respiratory viral infection.

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.

Nucleus Accumbens Cholinergic Interneurons Oppose Cue-Motivated Behavior

BACKGROUND

Environmental reward-predictive stimuli provide a major source of motivation for adaptive reward pursuit behavior. This cue-motivated behavior is known to be mediated by the nucleus accumbens (NAc) core. The cholinergic interneurons in the NAc are tonically active and densely arborized and thus well suited to modulate NAc function. However, their causal contribution to adaptive behavior remains unknown. Here we investigated the function of NAc cholinergic interneurons in cue-motivated behavior.

METHODS

We used chemogenetics, optogenetics, pharmacology, and a translationally analogous Pavlovian-to-instrumental transfer behavioral task designed to assess the motivating influence of a reward-predictive cue over reward-seeking actions in male and female rats.

RESULTS

The data show that NAc cholinergic interneuron activity critically opposes the motivating influence of appetitive cues. Chemogenetic inhibition of NAc cholinergic interneurons augmented cue-motivated behavior. Optical stimulation of acetylcholine release from NAc cholinergic interneurons prevented cues from invigorating reward-seeking behavior, an effect that was mediated by activation of β2-containing nicotinic acetylcholine receptors.

CONCLUSIONS

NAc cholinergic interneurons provide a critical regulatory influence over adaptive cue-motivated behavior and therefore are a potential therapeutic target for the maladaptive cue-motivated behavior that marks many psychiatric conditions, including addiction and depression.

Acetylcholine Neuromodulation in Normal and Abnormal Learning and Memory: Vigilance Control in Waking, Sleep, Autism, Amnesia and Alzheimer's Disease

Adaptive Resonance Theory, or ART, is a neural model that explains how normal and abnormal brains may learn to categorize and recognize objects and events in a changing world, and how these learned categories may be remembered for a long time. This article uses ART to propose and unify the explanation of diverse data about normal and abnormal modulation of learning and memory by acetylcholine (ACh). In ART, vigilance control determines whether learned categories will be general and abstract, or specific and concrete. ART models how vigilance may be regulated by ACh release in layer 5 neocortical cells by influencing after-hyperpolarization (AHP) currents. This phasic ACh release is mediated by cells in the nucleus basalis (NB) of Meynert that are activated by unexpected events. The article additionally discusses data about ACh-mediated tonic control of vigilance. ART proposes that there are often dynamic breakdowns of tonic control in mental disorders such as autism, where vigilance remains high, and medial temporal amnesia, where vigilance remains low. Tonic control also occurs during sleep-wake cycles. Properties of Up and Down states during slow wave sleep arise in ACh-modulated laminar cortical ART circuits that carry out processes in awake individuals of contrast normalization, attentional modulation, decision-making, activity-dependent habituation, and mismatch-mediated reset. These slow wave sleep circuits interact with circuits that control circadian rhythms and memory consolidation. Tonic control properties also clarify how Alzheimer's disease symptoms follow from a massive structural degeneration that includes undermining vigilance control by ACh in cortical layers 3 and 5. Sleep disruptions before and during Alzheimer's disease, and how they contribute to a vicious cycle of plaque formation in layers 3 and 5, are also clarified from this perspective.

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.

Optogenetic excitation of cholinergic inputs to hippocampus primes future contextual fear associations

Learning about context is essential for appropriate behavioral strategies, but important contingencies may not arise during initial learning. A variant of contextual fear conditioning, context pre-exposure facilitation, allows us to directly test the relationship between novelty-induced acetylcholine release and later contextual associability. We demonstrate that optogenetically-enhanced acetylcholine during initial contextual exploration leads to stronger fear after subsequent pairing with shock, suggesting that novelty-induced acetylcholine release primes future contextual associations.

Discovery of Compounds that Positively Modulate the High Affinity Choline Transporter

Cholinergic hypofunction is associated with decreased attention and cognitive deficits in the central nervous system in addition to compromised motor function. Consequently, stimulation of cholinergic neurotransmission is a rational therapeutic approach for the potential treatment of a variety of neurological conditions. High affinity choline uptake (HACU) into acetylcholine (ACh)-synthesizing neurons is critically mediated by the sodium- and pH-dependent high-affinity choline transporter (CHT, encoded by the SLC5A7 gene). This transporter is comparatively well-characterized but otherwise unexplored as a potential drug target. We therefore sought to identify small molecules that would enable testing of the hypothesis that positive modulation of CHT mediated transport would enhance activity-dependent cholinergic signaling. We utilized existing and novel screening techniques for their ability to reveal both positive and negative modulation of CHT using literature tools. A screening campaign was initiated with a bespoke compound library comprising both the Pfizer Chemogenomic Library (CGL) of 2,753 molecules designed specifically to help enable the elucidation of new mechanisms in phenotypic screens and 887 compounds from a virtual screening campaign to select molecules with field-based similarities to reported negative and positive allosteric modulators. We identified a number of previously unknown active and structurally distinct molecules that could be used as tools to further explore CHT biology or as a starting point for further medicinal chemistry.

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.

In vivo functional neurochemistry of human cortical cholinergic function during visuospatial attention

Cortical acetylcholine is involved in key cognitive processes such as visuospatial attention. Dysfunction in the cholinergic system has been described in a number of neuropsychiatric disorders. Levels of brain acetylcholine can be pharmacologically manipulated, but it is not possible to directly measure it in vivo in humans. However, key parts of its biochemical cascade in neural tissue, such as choline, can be measured using magnetic resonance spectroscopy (MRS). There is evidence that levels of choline may be an indirect but proportional measure of acetylcholine availability in brain tissue. In this study, we measured relative choline levels in the parietal cortex using functional (event-related) MRS (fMRS) during performance of a visuospatial attention task, with a modelling approach verified using simulated data. We describe a task-driven interaction effect on choline concentration, specifically driven by contralateral attention shifts. Our results suggest that choline MRS has the potential to serve as a proxy of brain acetylcholine function in humans.

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.

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.

A microelectrochemical biosensor for real-time in vivo monitoring of brain extracellular choline

A first generation Pt-based polymer enzyme composite biosensor developed for real-time neurochemical monitoring was characterised in vivo for sensitive and selective detection of choline.

Interactions between Aβ oligomers and presynaptic cholinergic signaling: age-dependent effects on attentional capacities

Substantial evidence suggests that cerebral deposition of the neurotoxic fibrillar form of amyloid precursor protein, β-amyloid (Aβ), plays a critical role in the pathogenesis of Alzheimer's disease (AD). Yet, many aspects of AD pathology including the cognitive symptoms and selective vulnerability of cortically projecting basal forebrain (BF) cholinergic neurons are not well explained by this hypothesis. Specifically, it is not clear why cognitive decline appears early when the loss of BF cholinergic neurons and plaque deposition are manifested late in AD. Soluble oligomeric forms of Aβ are proposed to appear early in the pathology and to be better predictors of synaptic loss and cognitive deficits. The present study was designed to examine the impact of Aβ oligomers on attentional functions and presynaptic cholinergic transmission in young and aged rats. Chronic intracranial infusions of Aβ oligomers produced subtle decrements in the ability of rats to sustain attentional performance with time on task, irrespective of the age of the animals. However, Aβ oligomers produced robust detrimental effects on performance under conditions of enhanced attentional load in aged animals. In vivo electrochemical recordings show reduced depolarization-evoked cholinergic signals in Aβ-infused aged rats. Moreover, soluble Aβ disrupted the capacity of cholinergic synapses to clear exogenous choline from the extracellular space in both young and aged rats, reflecting impairments in the choline transport process that is critical for acetylcholine (ACh) synthesis and release. Although aging per se reduced the cross-sectional area of BF cholinergic neurons and presynaptic cholinergic proteins in the cortex, attentional performance and ACh release remained unaffected in aged rats infused with the control peptide. Taken together, these data suggest that soluble Aβ may marginally influence attentional functions at young ages primarily by interfering with the choline uptake processes. However, age-related weakening of the cholinergic system may synergistically interact with these disruptive presynaptic mechanisms to make this neurotransmitter system vulnerable to the toxic effects of oligomeric Aβ in robustly impeding attentional capacities.

Prefrontal cholinergic mechanisms instigating shifts from monitoring for cues to cue-guided performance: converging electrochemical and fMRI evidence from rats and humans

We previously reported involvement of right prefrontal cholinergic activity in veridical signal detection. Here, we first recorded real-time acetylcholine release in prefrontal cortex (PFC) during specific trial sequences in rats performing a task requiring signal detection as well as rejection of nonsignal events. Cholinergic release events recorded with subsecond resolution ("transients") were observed only during signal-hit trials, not during signal-miss trials or nonsignal events. Moreover, cholinergic transients were not observed for consecutive hits; instead they were limited to signal-hit trials that were preceded by factual or perceived nonsignal events ("incongruent hits"). This finding suggests that these transients mediate shifts from a state of perceptual attention, or monitoring for cues, to cue-evoked activation of response rules and the generation of a cue-directed response. Next, to determine the translational significance of the cognitive operations supporting incongruent hits we used a version of the task previously validated for use in research in humans and blood oxygenation level-dependent (BOLD)-functional magnetic resonance imaging. Incongruent hits activated a region in the right rostral PFC (Brodmann area 10). Furthermore, greater prefrontal activation was correlated with faster response times for incongruent hits. Finally, we measured tissue oxygen in rats, as a proxy for BOLD, and found prefrontal increases in oxygen levels solely during incongruent hits. These cross-species studies link a cholinergic response to a prefrontal BOLD activation and indicate that these interrelated mechanisms mediate the integration of external cues with internal representations to initiate and guide behavior.

The presynaptic choline transporter imposes limits on sustained cortical acetylcholine release and attention

Functional variation in the gene encoding the presynaptic choline transporter (CHT) has been linked to attention-deficit/hyperactivity disorder. Here, we report that a heterozygous deletion in the CHT gene in mice (CHT(+/-)) limits the capacity of cholinergic neurons to sustain acetylcholine (ACh) release and attentional performance. Cortical microdialysis and amperometric methods revealed that, whereas wild-type and CHT(+/-) animals support equivalent basal ACh release and choline clearance, CHT(+/-) animals exhibit a significant inability to elevate extracellular ACh following basal forebrain stimulation, in parallel with a diminished choline clearance capacity following cessation of stimulation. Consistent with these findings, the density of CHTs in cortical synaptosomal plasma membrane-enriched fractions from unstimulated CHT(+/-) animals matched those observed in wild-type animals despite reductions in CHT levels in total extracts, achieved via a redistribution of CHT from vesicle pools. As a consequence, in CHT(+/-) animals, basal forebrain stimulation was unable to mobilize wild-type quantities of CHT to the plasma membrane. In behavioral studies, CHT(+/-) mice were impaired in performing a sustained attention task known to depend on cortical cholinergic activity. In wild-type mice, but not CHT(+/-) mice, attentional performance increased the density of CHTs in the synaptosomal membrane in the right frontal cortex. Basal CHT levels in vesicle-enriched membranes predicted the degree of CHT mobilization as well as individual variations in performance on the sustained attention task. Our findings demonstrate biochemical and physiological alterations that underlie cognitive impairments associated with genetically imposed reductions in choline uptake capacity.

Diminished trkA receptor signaling reveals cholinergic-attentional vulnerability of aging

The cellular mechanisms underlying the exceptional vulnerability of the basal forebrain (BF) cholinergic neurons during pathological aging have remained elusive. Here we employed an adeno-associated viral vector-based RNA interference (AAV-RNAi) strategy to suppress the expression of tropomyosin-related kinase A (trkA) receptors by cholinergic neurons in the nucleus basalis of Meynert/substantia innominata (nMB/SI) of adult and aged rats. Suppression of trkA receptor expression impaired attentional performance selectively in aged rats. Performance correlated with trkA levels in the nMB/SI. trkA knockdown neither affected nMB/SI cholinergic cell counts nor the decrease in cholinergic cell size observed in aged rats. However, trkA suppression augmented an age-related decrease in the density of cortical cholinergic processes and attenuated the capacity of cholinergic neurons to release acetylcholine (ACh). The capacity of cortical synapses to release ACh in vivo was also lower in aged/trkA-AAV-infused rats than in aged or young controls, and it correlated with their attentional performance. Furthermore, age-related increases in cortical proNGF and p75 receptor levels interacted with the vector-induced loss of trkA receptors to shift NGF signaling toward p75-mediated suppression of the cholinergic phenotype, thereby attenuating cholinergic function and impairing attentional performance. These effects model the abnormal trophic regulation of cholinergic neurons and cognitive impairments in patients with early Alzheimer's disease. This rat model is useful for identifying the mechanisms rendering aging cholinergic neurons vulnerable as well as for studying the neuropathological mechanisms that are triggered by disrupted trophic signaling.

Electrochemical sensing via selective surface modification of iridium microelectrodes to create a platinum black interface

The ability to selectively deposit platinum black (PtB) on iridium microelectrodes and functionalize the surface for the purposes of choline sensing was investigated in this study. Platinum black was deposited by cycling 100-200 times between 0.5 V and -0.7 V in a solution of 1 mM K₂PtCl₆ in 0.1 M KCl. Deposition of PtB showed good chemical stability as well as good adhesion following insertion into agarose gel as a model for brain insertion. Electrode sites were also tested for their oxidative capabilities of hydrogen peroxide during which they showed high current change in response to small concentration changes - attributable to the high surface area of the PtB. Sites were then coated with an enzyme solution containing choline oxidase, and a permselective layer of meta-phenylenediamine was added to filter interferents. Electrode sites yielded a high sensitivity to choline compared to interferents including ascorbic acid and dopamine.

The antinociceptive effects of nicotinic receptors α7-positive allosteric modulators in murine acute and tonic pain models

The α7 nicotinic acetylcholine receptor (nAChR) subtype is abundantly expressed in the central nervous system and in the periphery. Recent evidence suggests that α7 nAChR subtypes, which can be activated by an endogenous cholinergic tone, comprising acetylcholine and the α7 nAChR agonist choline, play an important role in subchronic pain and inflammation. This study's objective was to test whether α7 nAChR positive allosteric modulators (PAMs) produce antinociception in in vivo mouse models of acute and persistent pain. Testing type I [N-(5-chloro-2-hydroxyphenyl)-N'-[2-chloro-5-(trifluoromethyl)phenyl] (NS1738)] and type II [1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl) (PNU-120596)] α7 nAChR PAMs in acute and persistent pain, we found that, although neither reduced acute thermal pain, only PNU-120596 dose-dependently attenuated paw-licking behavior in the formalin test. The long-acting effect of PNU-120596 in this test was in discordance with its pharmacokinetic profile in mice, which suggests the involvement of postreceptor signaling mechanisms. Our results with selective mitogen-activated protein kinase kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene monoethanolate (U0126) argues for an important role of extracellular signal-regulated kinase-1/2 pathways activation in PNU-120596's antinociceptive effects. The α7 antagonist MLA, administered intrathecally, reversed PNU-120596's effects, confirming PNU-120596's action, in part, through central α7 nAChRs. Importantly, tolerance to PNU-120596 was not developed after subchronic treatment of the drug. Surprisingly, PNU-120596's antinociceptive effects were blocked by NS1738. Our results indicate that type II α7 nAChR PAM PNU-120596, but not type I α7 nAChR PAM NS1738, shows significant antinociception effects in persistent pain models in mice.

Synthesis and characterization of luminescent cadmium selenide/zinc selenide/zinc sulfide cholinomimetic quantum dots

Luminescent quantum dots conjugated with highly selective molecular recognition ligands are widely used for targeting and imaging biological structures. In this paper, water soluble cholinomimetic cadmium selenide (core), zinc selenide/zinc sulfide (shell) quantum dots were synthesized for targeting cholinergic sites. Cholinomimetic specificity was incorporated by conjugation of the quantum dots to an aminated analogue of hemicholinium-15, a well known competitive inhibitor of the high affinity choline uptake transporter. Detailed evaluation of the nanocrystal synthesis and characterization of the final product was conducted by (1)H and (31)P NMR, absorption and emission spectroscopy, as well as transmission electron microscopy.

Persistence and storage of activity patterns in spiking recurrent cortical networks: modulation of sigmoid signals by after-hyperpolarization currents and acetylcholine

Many cortical networks contain recurrent architectures that transform input patterns before storing them in short-term memory (STM). Theorems in the 1970's showed how feedback signal functions in rate-based recurrent on-center off-surround networks control this process. A sigmoid signal function induces a quenching threshold below which inputs are suppressed as noise and above which they are contrast-enhanced before pattern storage. This article describes how changes in feedback signaling, neuromodulation, and recurrent connectivity may alter pattern processing in recurrent on-center off-surround networks of spiking neurons. In spiking neurons, fast, medium, and slow after-hyperpolarization (AHP) currents control sigmoid signal threshold and slope. Modulation of AHP currents by acetylcholine (ACh) can change sigmoid shape and, with it, network dynamics. For example, decreasing signal function threshold and increasing slope can lengthen the persistence of a partially contrast-enhanced pattern, increase the number of active cells stored in STM, or, if connectivity is distance-dependent, cause cell activities to cluster. These results clarify how cholinergic modulation by the basal forebrain may alter the vigilance of category learning circuits, and thus their sensitivity to predictive mismatches, thereby controlling whether learned categories code concrete or abstract features, as predicted by Adaptive Resonance Theory. The analysis includes global, distance-dependent, and interneuron-mediated circuits. With an appropriate degree of recurrent excitation and inhibition, spiking networks maintain a partially contrast-enhanced pattern for 800 ms or longer after stimuli offset, then resolve to no stored pattern, or to winner-take-all (WTA) stored patterns with one or multiple winners. Strengthening inhibition prolongs a partially contrast-enhanced pattern by slowing the transition to stability, while strengthening excitation causes more winners when the network stabilizes.

α7 nicotinic ACh receptors as a ligand-gated source of Ca(2+) ions: the search for a Ca(2+) optimum

The spatiotemporal distribution of cytosolic Ca(2+) ions is a key determinant of neuronal behavior and survival. Distinct sources of Ca(2+) ions including ligand- and voltage-gated Ca(2+) channels contribute to intracellular Ca(2+) homeostasis. Many normal physiological and therapeutic neuronal functions are Ca(2+)-dependent, however an excess of cytosolic Ca(2+) or a lack of the appropriate balance between Ca(2+) entry and clearance may destroy cellular integrity and cause cellular death. Therefore, the existence of optimal spatiotemporal patterns of cytosolic Ca(2+) elevations and thus, optimal activation of ligand- and voltage-gated Ca(2+) ion channels are postulated to benefit neuronal function and survival. Alpha7 nicotinic -acetylcholine receptors (nAChRs) are highly permeable to Ca(2+) ions and play an important role in modulation of neurotransmitter release, gene expression and neuroprotection in a variety of neuronal and non-neuronal cells. In this review, the focus is placed on α7 nAChR-mediated currents and Ca(2+) influx and how this source of Ca(2+) entry compares to NMDA receptors in supporting cytosolic Ca(2+) homeostasis, neuronal function and survival.

Enhanced control of attention by stimulating mesolimbic-corticopetal cholinergic circuitry

Sustaining and recovering attentional performance requires interactions between the brain's motivation and attention systems. The first experiment demonstrated that in rats performing a sustained attention task (SAT), presentation of a distractor (dSAT) augmented performance-associated increases in cholinergic neurotransmission in prefrontal cortex. Because stimulation of NMDA receptors in the shell of the nucleus accumbens activates PFC cholinergic neurotransmission, a second experiment demonstrated that bilateral infusions of NMDA into the NAc shell, but not core, improved dSAT performance to levels observed in the absence of a distractor. A third experiment demonstrated that removal of prefrontal or posterior parietal cholinergic inputs, by intracortical infusions of the cholinotoxin 192 IgG-saporin, attenuated the beneficial effects of NMDA on dSAT performance. Mesolimbic activation of cholinergic projections to the cortex benefits the cognitive control of attentional performance by enhancing the detection of cues and the filtering of distractors.

After-hyperpolarization currents and acetylcholine control sigmoid transfer functions in a spiking cortical model

Recurrent networks are ubiquitous in the brain, where they enable a diverse set of transformations during perception, cognition, emotion, and action. It has been known since the 1970's how, in rate-based recurrent on-center off-surround networks, the choice of feedback signal function can control the transformation of input patterns into activity patterns that are stored in short term memory. A sigmoid signal function may, in particular, control a quenching threshold below which inputs are suppressed as noise and above which they may be contrast enhanced before the resulting activity pattern is stored. The threshold and slope of the sigmoid signal function determine the degree of noise suppression and of contrast enhancement. This article analyses how sigmoid signal functions and their shape may be determined in biophysically realistic spiking neurons. Combinations of fast, medium, and slow after-hyperpolarization (AHP) currents, and their modulation by acetylcholine (ACh), can control sigmoid signal threshold and slope. Instead of a simple gain in excitability that was previously attributed to ACh, cholinergic modulation may cause translation of the sigmoid threshold. This property clarifies how activation of ACh by basal forebrain circuits, notably the nucleus basalis of Meynert, may alter the vigilance of category learning circuits, and thus their sensitivity to predictive mismatches, thereby controlling whether learned categories code concrete or abstract information, as predicted by Adaptive Resonance Theory.

Choline modulation of the aβ p1-40 channel reconstituted into a model lipid membrane

Nicotinic acetylcholine receptors (AChRs), implicated in memory and learning, in subjects affected by Alzheimer's disease result altered. Stimulation of α7-nAChRs inhibits amyloid plaques and increases ACh release. β-amyloid peptide (AβP) forms ion channels in the cell and model phospholipid membranes that are retained responsible in Alzheimer disease. We tested if choline, precursor of ACh, could affect the AβP1-40 channels in oxidized cholesterol (OxCh) and in palmitoyl-oleoyl-phosphatidylcholine (POPC):Ch lipid bilayers. Choline concentrations of 5 × 10⁻¹¹ M–1.5 × 10⁻⁸ M added to the cis- or trans-side of membrane quickly increased AβP1-40 ion channel frequency (events/min) and ion conductance in OxCh membranes, but not in POPC:Ch membranes. Circular Dichroism (CD) spectroscopy shows that after 24 and 48 hours of incubation with AβP1-40, choline stabilizes the random coil conformation of the peptide, making it less prone to fibrillate. These actions seem to be specific in that ACh is ineffective either in solution or on AβP1-40 channel incorporated into PLMs.

Localization of pre- and postsynaptic cholinergic markers in rodent forebrain: a brief history and comparison of rat and mouse

Rat and mouse models are widely used for studies in cognition and pathophysiology, among others. Here, we sought to determine to what extent these two model species differ for cholinergic and cholinoceptive features. For this purpose, we focused on cholinergic innervation patterns based on choline acetyltransferase (ChAT) immunostaining, and the expression of muscarinic acetylcholine receptors (mAChRs) detected immunocytochemically. In this brief review we first place cholinergic and cholinoceptive markers in a historic perspective, and then provide an overview of recent publications on cholinergic studies and techniques to provide a literature survey of current research. Next, we compare mouse (C57Bl/J6) and rat (Wistar) cholinergic and cholinoceptive systems simultaneously stained, respectively, for ChAT (analyzed qualitatively) and mAChRs (analyzed qualitatively and quantitatively). In general, the topographic cholinergic innervation patterns of both rodent species are highly comparable, with only considerable (but region specific) differences in number of detectable cholinergic interneurons, which are more numerous in rat. In contrast, immunolabeling for mAChRs, detected by the monoclonal antibody M35, differs markedly in the forebrain between the two species. In mouse brain, basal levels of activated and/or internalized mAChRs (as a consequence of cholinergic neurotransmission) are significantly higher. This suggests a higher cholinergic tone in mouse than rat, and hence the animal model of choice may have consequences for cholinergic drug testing experiments.

Activation of functional α7-containing nAChRs in hippocampal CA1 pyramidal neurons by physiological levels of choline in the presence of PNU-120596

Background

The level of expression of functional α7-containing nicotinic acetylcholine receptors (nAChRs) in hippocampal CA1 pyramidal neurons is believed to be very low compared to hippocampal CA1 interneurons, and for many years this expression was largely overlooked. However, high densities of expression of functional α7-containing nAChRs in CA1 pyramidal neurons may not be necessary for triggering important cellular and network functions, especially if activation of α7-containing nAChRs occurs in the presence of positive allosteric modulators such as PNU-120596.

Methodology/Principal Findings

An approach previously developed for α7-containing nAChRs expressed in tuberomammillary neurons was applied to investigate functional CA1 pyramidal α7-containing nAChRs using rat coronal hippocampal slices and patch-clamp electrophysiology. The majority (∼71%) of tested CA1 pyramidal neurons expressed low densities of functional α7-containing nAChRs as evidenced by small whole-cell responses to choline, a selective endogenous agonist of α7 nAChRs. These responses were potentiated by PNU-120596, a novel positive allosteric modulator of α7 nAChRs. The density of functional α7-containing nAChRs expressed in CA1 pyramidal neurons (and thus, the normalized net effect of activation, i.e., response net charge per unit of membrane capacitance per unit of time) was estimated to be ∼5% of the density observed in CA1 interneurons. The results of this study demonstrate that despite low levels of expression of functional pyramidal α7-containing nAChRs, physiological levels of choline (∼10 µM) are sufficient to activate these receptors and transiently depolarize and even excite CA1 pyramidal neurons in the presence of PNU-120596. The observed effects are possible because in the presence of 10 µM choline and 1–5 µM PNU-120596, a single opening of an individual pyramidal α7-containing nAChR ion channel appears to transiently depolarize (∼4 mV) the entire pyramidal neuron and occasionally trigger action potentials.

Conclusions

1) The majority of hippocampal CA1 pyramidal neurons express functional α7-containing nAChRs. In the absence of PNU-120596, a positive allosteric modulator of α7 nAChRs, a lack of responsiveness of some hippocampal CA1 pyramidal neurons to focal application of 0.5–1 mM choline does not imply a lack of expression of functional α7-containing nAChRs in these neurons. Rather, it may indicate a lack of detection of α7-containing nAChR-mediated currents by patch-clamp electrophysiology. 2) PNU-120596 can serve as a powerful tool for detection and enhancement of responsiveness of low densities of functional α7-containing nAChRs such as those present in hippocampal CA1 pyramidal neurons. 3) In the presence of PNU-120596, physiological concentrations of choline activate functional CA1 pyramidal α7-containing nAChRs and produce step-like currents that cause repetitive step-like depolarizations, occasionally triggering bursts of action potentials in CA1 pyramidal neurons. Therefore, the results of this study suggest that in the presence of PNU-120596 and possibly other positive allosteric modulators, endogenous choline may persistently activate CA1 pyramidal α7-containing nAChRs, enhance the excitability of CA1 pyramidal neurons and thus act as a potent therapeutic agent with potential neuroprotective and cognition-enhancing properties.

Prefrontal beta2 subunit-containing and alpha7 nicotinic acetylcholine receptors differentially control glutamatergic and cholinergic signaling

One-second-long increases in prefrontal cholinergic activity ("transients") were demonstrated previously to be necessary for the incorporation of cues into ongoing cognitive processes ("cue detection"). Nicotine and, more robustly, selective agonists at alpha4beta2* nicotinic acetylcholine receptors (nAChRs) enhance cue detection and attentional performance by augmenting prefrontal cholinergic activity. The present experiments determined the role of beta2-containing and alpha7 nAChRs in the generation of prefrontal cholinergic and glutamatergic transients in vivo. Transients were evoked by nicotine, the alpha4beta2* nAChR agonist ABT-089 [2-methyl-3-(2-(S)-pyrrolindinylmethoxy) pyridine dihydrochloride], or the alpha7 nAChR agonist A-582941 [2-methyl-5-(6-phenyl-pyridazin-3-yl)-octahydro-pyrrolo[3,4-c]pyrrole]. Transients were recorded in mice lacking beta2 or alpha7 nAChRs and in rats after removal of thalamic glutamatergic or midbrain dopaminergic inputs to prefrontal cortex. The main results indicate that stimulation of alpha4beta2* nAChRs evokes glutamate release and that the presence of thalamic afferents is necessary for the generation of cholinergic transients. ABT-089-evoked transients were completely abolished in mice lacking beta2* nAChRs. The amplitude, but not the decay rate, of nicotine-evoked transients was reduced by beta2* knock-out. Conversely, in mice lacking the alpha7 nAChR, the decay rate, but not the amplitude, of nicotine-evoked cholinergic and glutamatergic transients was attenuated. Substantiating the role of alpha7 nAChR in controlling the duration of release events, stimulation of alpha7 nAChR produced cholinergic transients that lasted 10- to 15-fold longer than those evoked by nicotine. alpha7 nAChR-evoked cholinergic transients are mediated in part by dopaminergic activity. Prefrontal alpha4beta2* nAChRs play a key role in evoking and facilitating the transient glutamatergic-cholinergic interactions that are necessary for cue detection and attentional performance.

Physiological concentrations of choline activate native alpha7-containing nicotinic acetylcholine receptors in the presence of PNU-120596 [1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea]

The use of PNU-120596 [1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea], a positive allosteric modulator of alpha7 nicotinic acetylcholine receptor (nAChR), may be beneficial for enhancing cholinergic therapies. However, the effects of PNU-120596 on activation of native alpha7-containing nAChRs by physiological concentrations of choline are not known and were investigated in this study using patch-clamp electrophysiology and histaminergic tuberomammillary neurons in hypothalamic slices. In the presence of PNU-120596, subthreshold (i.e., inactive) physiological concentrations of choline ( approximately 10 microM) elicited repetitive step-like whole-cell responses reminiscent of single ion channel openings that were reversibly blocked by 20 nM methyllycaconitine, a selective alpha7 nAChR antagonist. The effects of choline and PNU-120596 were synergistic as administration of 10 to 40 microM choline or 1 to 4 muM PNU-120596 alone did not elicit responses. In voltage clamp at -60 mV, the persistent activation of alpha7-containing nAChRs by 10 microM choline plus 1 microM PNU-120596 was estimated to produce a sustained influx of Ca(2+) ions at a rate of 8.4 pC/min ( approximately 0.14 pA). In the presence of PNU-120596 in current clamp, transient step-like depolarizations ( approximately 5 mV) enhanced neuronal excitability and triggered voltage-gated conductances; a single opening of an alpha7-containing nAChR channel appeared to transiently depolarize the entire neuron and facilitate spontaneous firing. Therefore, this study tested and confirmed the hypothesis that PNU-120596 enhances the effects of subthreshold concentrations of choline on native alpha7-containing nAChRs, allowing physiological levels of choline to activate these receptors and produce whole-cell responses in the absence of exogenous nicotinic agents. In certain neurological disorders, this activation may be therapeutically beneficial, more efficacious, and safer than treatments with nAChR agonists.

Impairment of bidirectional synaptic plasticity in the striatum of a mouse model of DYT1 dystonia: role of endogenous acetylcholine

DYT1 dystonia is a severe form of inherited dystonia, characterized by involuntary twisting movements and abnormal postures. It is linked to a deletion in the dyt1 gene, resulting in a mutated form of the protein torsinA. The penetrance for dystonia is incomplete, but both clinically affected and non-manifesting carriers of the DYT1 mutation exhibit impaired motor learning and evidence of altered motor plasticity. Here, we characterized striatal glutamatergic synaptic plasticity in transgenic mice expressing either the normal human torsinA or its mutant form, in comparison to non-transgenic (NT) control mice. Medium spiny neurons recorded from both NT and normal human torsinA mice exhibited normal long-term depression (LTD), whereas in mutant human torsinA littermates LTD could not be elicited. In addition, although long-term potentiation (LTP) could be induced in all the mice, it was greater in magnitude in mutant human torsinA mice. Low-frequency stimulation (LFS) can revert potentiated synapses to resting levels, a phenomenon termed synaptic depotentiation. LFS induced synaptic depotentiation (SD) both in NT and normal human torsinA mice, but not in mutant human torsinA mice. Since anti-cholinergic drugs are an effective medical therapeutic option for the treatment of human dystonia, we reasoned that an excess in endogenous acetylcholine could underlie the synaptic plasticity impairment. Indeed, both LTD and SD were rescued in mutant human torsinA mice either by lowering endogenous acetylcholine levels or by antagonizing muscarinic M1 receptors. The presence of an enhanced acetylcholine tone was confirmed by the observation that acetylcholinesterase activity was significantly increased in the striatum of mutant human torsinA mice, as compared with both normal human torsinA and NT littermates. Moreover, we found similar alterations of synaptic plasticity in muscarinic M2/M4 receptor knockout mice, in which an increased striatal acetylcholine level has been documented. The loss of LTD and SD on one hand, and the increase in LTP on the other, demonstrate that a 'loss of inhibition' characterizes the impairment of synaptic plasticity in this model of DYT1 dystonia. More importantly, our results indicate that an unbalanced cholinergic transmission plays a pivotal role in these alterations, providing a clue to understand the ability of anticholinergic agents to restore motor deficits in dystonia.

Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

nAChR agonist-induced cognition enhancement: integration of cognitive and neuronal mechanisms

The identification and characterization of drugs for the treatment of cognitive disorders has been hampered by the absence of comprehensive hypotheses. Such hypotheses consist of (a) a precisely defined cognitive operation that fundamentally underlies a range of cognitive abilities and capacities and, if impaired, contributes to the manifestation of diverse cognitive symptoms; (b) defined neuronal mechanisms proposed to mediate the cognitive operation of interest; (c) evidence indicating that the putative cognition enhancer facilitates these neuronal mechanisms; (d) and evidence indicating that the cognition enhancer facilitates cognitive performance by modulating these underlying neuronal mechanisms. The evidence on the neuronal and attentional effects of nAChR agonists, specifically agonists selective for alpha4beta2* nAChRs, has begun to support such a hypothesis. nAChR agonists facilitate the detection of signals by augmenting the transient increases in prefrontal cholinergic activity that are necessary for a signal to gain control over behavior in attentional contexts. The prefrontal microcircuitry mediating these effects include alpha4beta2* nAChRs situated on the terminals of thalamic inputs and the glutamatergic stimulation of cholinergic terminals via ionotropic glutamate receptors. Collectively, this evidence forms the basis for hypothesis-guided development and characterization of cognition enhancers.

Cholinergic mediation of attention: contributions of phasic and tonic increases in prefrontal cholinergic activity

Contrary to the classic description of acetylcholine (ACh) as a slowly acting neuromodulator that influences arousal states, results from experiments that employed enzyme-selective microelectrodes for the real-time monitoring of ACh release in the cortex of attentional task-performing rats indicate that cholinergic signals manifesting on multiple timescales (seconds, tens of seconds, and minutes) support, and are necessary for, the mediation of defined cognitive operations. Specifically, in the prefrontal cortex, second-based cholinergic signals support the detection of behaviorally significant cues. In contrast to these prefrontal cholinergic transients, performance-associated cholinergic activity that manifested at lower temporal resolution also was observed elsewhere in the cortex. Although tonic cholinergic signal levels were correlated with the amplitudes of cue-evoked cholinergic transients, and the latter with response latencies, the interrelationships and interactions between the multiple cholinergic signaling modes remains unclear. Hypotheses concerning the afferent circuitry contributing to the regulation of second- versus minute-based cholinergic signals are discussed. The discovery of cholinergic transients and their crucial role in cue detection and attentional performance form the basis for new hypotheses about the nature of cholinergic dysfunction in cognitive disorders and offer new targets for the development of treatments for the cognitive symptoms of neuropsychiatric and neurodegenerative disorders.

Loss of muscarinic autoreceptor function impairs long-term depression but not long-term potentiation in the striatum

Muscarinic autoreceptors regulate cholinergic tone in the striatum. We investigated the functional consequences of genetic deletion of striatal muscarinic autoreceptors by means of electrophysiological recordings from either medium spiny neurons (MSNs) or cholinergic interneurons (ChIs) in slices from single M(4) or double M(2)/M(4) muscarinic acetylcholine receptor (mAChR) knock-out (-/-) mice. In control ChIs, the muscarinic agonist oxotremorine (300 nM) produced a self-inhibitory outward current that was mostly reduced in M(4)(-/-) and abolished in M(2)/M(4)(-/-) mice, suggesting an involvement of both M(2) and M(4) autoreceptors. In MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, muscarine caused a membrane depolarization that was prevented by the M(1) receptor-preferring antagonist pirenzepine (100 nM), suggesting that M(1) receptor function was unaltered. Acetylcholine has been involved in striatal long-term potentiation (LTP) or long-term depression (LTD) induction. Loss of muscarinic autoreceptor function is predicted to affect synaptic plasticity by modifying striatal cholinergic tone. Indeed, high-frequency stimulation of glutamatergic afferents failed to induce LTD in MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, as well as in wild-type mice pretreated with the M(2)/M(4) antagonist AF-DX384 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,1 1-dihydro-6H-pyrido[2,3b][1,4] benzodiazepin-6-one). Interestingly, LTD could be restored by either pirenzepine (100 nM) or hemicholinium-3 (10 microM), a depletor of endogenous ACh. Conversely, LTP induction did not show any difference among the three mouse strains and was prevented by pirenzepine. These results demonstrate that M(2)/M(4) muscarinic autoreceptors regulate ACh release from striatal ChIs. As a consequence, endogenous ACh drives the polarity of bidirectional synaptic plasticity.

Aberrant trafficking of the high-affinity choline transporter in AP-3-deficient mice

The high-affinity choline transporter (CHT) is expressed in cholinergic neurons and efficiently transported to axon terminals where it controls the rate-limiting step in acetylcholine synthesis. Recent studies have shown that the majority of CHT is unexpectedly localized on synaptic vesicles (SV) rather than the presynaptic plasma membrane, establishing vesicular CHT trafficking as a basis for activity-dependent CHT regulation. Here, we analyse the intracellular distribution of CHT in the adaptor protein-3 (AP-3)-deficient mouse model mocha. In the mocha mouse, granular structures in cell bodies are intensely labelled with CHT antibody, indicating possible deficits in CHT trafficking from the cell body to the axon terminal. Western blot analyses reveal that CHT on SV in mocha mice is decreased by 30% compared with wild-type mice. However, no significant difference in synaptosomal choline uptake activity is detected, consistent with the existence of a large reservoir pool for CHT. To further characterize CHT trafficking, we established a PC12D-CHT cell line. In this line, CHT is found associated with a subpopulation of synaptophysin-positive synaptic-like microvesicles (SLMV). The amounts of CHT detected on SLMV are greatly reduced by treating the cell with agents that halt AP-dependent membrane trafficking. These results demonstrate that APs have important functions for CHT trafficking in neuronal cells.

Glutamatergic contributions to nicotinic acetylcholine receptor agonist-evoked cholinergic transients in the prefrontal cortex

Because modulation of cortical cholinergic neurotransmission has been hypothesized to represent a necessary mechanism mediating the beneficial cognitive effects of nicotine and nicotinic acetylcholine receptor (nAChR) subtype-selective agonists, we used choline-sensitive microelectrodes for the real-time measurement of ACh release in vivo, to characterize cholinergic transients evoked by nicotine and the alpha4beta2*-selective nAChR partial agonist 2-methyl-3-(2-(S)-pyrrolindinylmethoxy)pyridine dihydrochloride (ABT-089), a clinically effective cognition enhancer. In terms of cholinergic signal amplitudes, ABT-089 was significantly more potent than nicotine in evoking ACh cholinergic transients. Moreover, cholinergic signals evoked by ABT-089 were characterized by faster signal rise time and decay rate. The nAChR antagonist mecamylamine attenuated the cholinergic signals evoked by either compound. Cholinergic signals evoked by ABT-089 were more efficaciously attenuated by the relatively beta2*-selective nAChR antagonist dihydro-beta-erythroidine. The alpha7 antagonist methyllycaconitine did not affect choline signal amplitudes but partly attenuated the relatively slow decay rate of nicotine-evoked cholinergic signals. Furthermore, the AMPA receptor antagonist DNQX as well as the NMDA receptor antagonist APV more potently attenuated cholinergic signals evoked by ABT-089. Using glutamate-sensitive microelectrodes to measure glutamatergic transients, ABT-089 was more potent than nicotine in evoking glutamate release. Glutamatergic signals were highly sensitive to tetrodotoxin-induced blockade of voltage-regulated sodium channels. Together, the present evidence indicates that compared with nicotine, ABT-089 evokes more potent and sharper cholinergic transients in prefrontal cortex. Glutamatergic mechanisms necessarily mediate the cholinergic effects of nAChR agonists in the prefrontal cortex.

Increases in cholinergic neurotransmission measured by using choline-sensitive microelectrodes: enhanced detection by hydrolysis of acetylcholine on recording sites?

Previous experiments demonstrated that second-based transient increases in choline concentrations measured by electrodes coated with choline oxidase (ChOx) and the amperometric detection of hydrogen peroxide validly indicate the depolarization-dependent release of acetylcholine (ACh) and its hydrolysis by endogenous acetylcholinesterase (AChE). Therefore, choline-sensitive microelectrodes have become valuable tools in neuropharmacological and behavioral research. The present experiments were designed to test the possibility that co-immobilization of ChOx plus AChE on recording sites increases the level of detection for evoked ACh release in the brain. If newly released ACh is not completely hydrolyzed by endogenous AChE and capable of reaching the extracellular space, currents recorded via sites equipped with both enzymes should be greater when compared with sites coated with ChOx only. Pairs of platinum-recording sites were coated either with AChE plus ChOx or ChOx alone. Potassium or nicotine-evoked currents were recorded throughout the entire dorsal-ventral extent of the medial prefrontal cortex (mPFC). The amplitudes of evoked cholinergic signals did not differ significantly between AChE+ChOx and ChOx-only coated recording sites. Additional experiments controlling for several potential confounds suggested that, in vivo, ACh levels > or =150fmol were detected by recordings sites featuring dual enzyme coating. Collectively, these results indicate that co-coating of microelectrodes with AChE does not enhance the detection of cholinergic activity in the cortex compared with measurements via recording sites coated only with ChOx.

Prefrontal acetylcholine release controls cue detection on multiple timescales

Cholinergic neurons originating from the basal forebrain innervate the entire cortical mantle. Choline-sensitive microelectrodes were used to measure the synaptic release of cortical acetylcholine (ACh) at a subsecond resolution in rats performing a task involving the detection of cues. Cues that were detected, defined behaviorally, evoked transient increases in cholinergic activity (at the scale of seconds) in the medial prefrontal cortex (mPFC), but not in a nonassociational control region (motor cortex). In trials involving missed cues, cholinergic transients were not observed. Cholinergic deafferentation of the mPFC, but not motor cortex, impaired cue detection. Furthermore, decreases and increases in precue cholinergic activity predicted subsequent cue detection or misses, respectively. Finally, cue-evoked cholinergic transients were superimposed over slower (at the timescale of minutes) changes in cholinergic activity. Cortical cholinergic neurotransmission is regulated on multiple timescales to mediate the detection of behaviorally significant cues and to support cognitive performance.

Choline availability and acetylcholine synthesis in the hippocampus of acetylcholinesterase-deficient mice

Mice deficient for acetylcholinesterase (AChE) have strongly increased extracellular levels of acetylcholine (ACh) in the dorsal hippocampus [Hartmann, J., Kiewert, C., Duysen, E.G., Lockridge, O., Greig, N.H., Klein, J., 2007. Excessive hippocampal acetylcholine levels in acetylcholinesterase-deficient mice are moderated by butyrylcholinesterase activity. J. Neurochem. 100, 1421-1429]. Using microdialysis, we found that increased ACh levels are accompanied by decreased levels of extracellular choline which were 1.60 microM in AChE-deficient mice and 4.36 microM in wild-type mice. Addition of choline (10 microM) to the perfusion fluid, while ineffective in wild-type animals, more than doubled extracellular ACh levels in AChE-deficient mice. High-affinity choline uptake (HACU), as measured ex vivo in corticohippocampal synaptosomes, was more than doubled in AChE-deficient mice. Inhibition of HACU by hemicholinium-3 (HC-3) in vivo reduced extracellular levels of ACh by 60% in wild-type mice but by more than 90% in AChE-deficient mice. Decreased ACh levels caused by infusion of HC-3 or tetrodotoxin (TTX) were accompanied by increased levels of free choline. Infusion of scopolamine (1 microM) caused a fivefold increase of ACh levels in wild-type animals but only a 50% increase in AChE-deficient mice. In conclusion, absence of AChE causes dynamic changes in the ratio of choline to ACh. High levels of extracellular ACh are accompanied by reduced levels of extracellular choline, and ACh release becomes strongly dependent on choline availability. Similar changes may take place in patients chronically exposed to AChE inhibitors.

Abnormal neurotransmitter release underlying behavioral and cognitive disorders: toward concepts of dynamic and function-specific dysregulation

Abnormalities in the regulation of neurotransmitter release and/or abnormal levels of extracellular neurotransmitter concentrations have remained core components of hypotheses on the neuronal foundations of behavioral and cognitive disorders and the symptoms of neuropsychiatric and neurodegenerative disorders. Furthermore, therapeutic drugs for the treatment of these disorders have been developed and categorized largely on the basis of their effects on neurotransmitter release and resulting receptor stimulation. This perspective stresses the theoretical and practical implications of hypotheses that address the dynamic nature of neurotransmitter dysregulation, including the multiple feedback mechanisms regulating synaptic processes, phasic and tonic components of neurotransmission, compartmentalized release, differentiation between dysregulation of basal vs activated release, and abnormal release from neuronal systems recruited by behavioral and cognitive activity. Several examples illustrate that the nature of the neurotransmitter dysregulation in animal models, including the direction of drug effects on neurotransmitter release, depends fundamentally on the state of activity of the neurotransmitter system of interest and on the behavioral and cognitive functions recruiting these systems. Evidence from evolving techniques for the measurement of neurotransmitter release at high spatial and temporal resolution is likely to advance hypotheses describing the pivotal role of neurotransmitter dysfunction in the development of essential symptoms of major neuropsychiatric disorders, and also to refine neuropharmacological mechanisms to serve as targets for new treatment approaches. The significance and usefulness of hypotheses concerning the abnormal regulation of the release of extracellular concentrations of primary messengers depend on the effective integration of emerging concepts describing the dynamic, compartmentalized, and activity-dependent characteristics of dysregulated neurotransmitter systems.

Prenatal choline deficiency increases choline transporter expression in the septum and hippocampus during postnatal development and in adulthood in rats

Supplementation of maternal diet with the essential nutrient, choline, during the second half of pregnancy in rats causes long-lasting improvements in spatial memory in the offspring and protects them from the memory decline characteristic of old age. In contrast, prenatal choline deficiency is associated with poor performance in certain cognitive tasks. The mechanism by which choline influences learning and memory remains unclear; however, it may involve changes to the hippocampal cholinergic system. Previously, we showed that the hippocampi of prenatally [embryonic days (E) 11-17] choline-deficient animals have increased synthesis of acetylcholine (ACh) from choline transported by the high-affinity choline transporter (CHT) and reduced ACh content relative to the control and to the E11-17 choline-supplemented rats. In the current study, we found that, during postnatal period [postnatal days (P) 18-480], prenatal choline deficiency increased the expression of CHT mRNA in the septum and CHT mRNA and protein levels in the hippocampus and altered the pattern of CHT immunoreactivity in the dentate gyrus. CHT immunoreactivity was more prominent in the inner molecular layer in prenatally choline-deficient rats compared to controls and prenatally choline-supplemented animals. In addition, in all groups, we observed a population of hilar interneurons that were CHT-immunoreactive. These neurons are the likely source of the hippocampal CHT mRNA as their number correlated with the levels of this mRNA. The abundance of hippocampal CHT mRNA rose between P1 and P24 and then declined reaching 60% of the P1 value by P90. These data show that prenatal availability of choline alters its own metabolism (i.e., CHT expression). While the upregulated CHT expression during the period of prenatal choline deficiency may be considered as a compensatory mechanism that could enhance ACh synthesis when choline supply is low, the persistent upregulation of CHT expression subsequent to the brief period of prenatal deprivation of choline in utero might be beneficial during choline deficiency in adulthood.