Cholinergic modulation of cognitive processing: insights drawn from computational models

Intrinsic mechanisms stabilize encoding and retrieval circuits differentially in a hippocampal network model

Acetylcholine regulates memory encoding and retrieval by inducing the hippocampus to switch between pattern separation and pattern completion modes. However, both processes can introduce significant variations in the level of network activity and potentially cause a seizure-like spread of excitation. Thus, mechanisms that keep network excitation within certain bounds are necessary to prevent such instability. We developed a biologically realistic computational model of the hippocampus to investigate potential intrinsic mechanisms that might stabilize the network dynamics during encoding and retrieval. The model was developed by matching experimental data, including neuronal behavior, synaptic current dynamics, network spatial connectivity patterns, and short-term synaptic plasticity. Furthermore, it was constrained to perform pattern completion and separation under the effects of acetylcholine. The model was then used to investigate the role of short-term synaptic depression at the recurrent synapses in CA3, and inhibition by basket cell (BC) interneurons and oriens lacunosum-moleculare (OLM) interneurons in stabilizing these processes. Results showed that when CA3 was considered in isolation, inhibition solely by BCs was not sufficient to control instability. However, both inhibition by OLM cells and short-term depression at the recurrent CA3 connections stabilized the network activity. In the larger network including the dentate gyrus, the model suggested that OLM inhibition could control the network during high cholinergic levels while depressing synapses at the recurrent CA3 connections were important during low cholinergic states. Our results demonstrate that short-term plasticity is a critical property of the network that enhances its robustness. Furthermore, simulations suggested that the low and high cholinergic states can each produce runaway excitation through unique mechanisms and different pathologies. Future studies aimed at elucidating the circuit mechanisms of epilepsy could benefit from considering the two modulatory states separately.

Elevated prefrontal myo-inositol and choline following breast cancer chemotherapy

Breast cancer survivors are at increased risk for cognitive dysfunction, which reduces quality of life. Neuroimaging studies provide critical insights regarding the mechanisms underlying these cognitive deficits as well as potential biologic targets for interventions. We measured several metabolite concentrations using (1)H magnetic resonance spectroscopy as well as cognitive performance in 19 female breast cancer survivors and 17 age-matched female controls. Women with breast cancer were all treated with chemotherapy. Results indicated significantly increased choline (Cho) and myo-inositol (mI) with correspondingly decreased N-acetylaspartate (NAA)/Cho and NAA/mI ratios in the breast cancer group compared to controls. The breast cancer group reported reduced executive function and memory, and subjective memory ability was correlated with mI and Cho levels in both groups. These findings provide preliminary evidence of an altered metabolic profile that increases our understanding of neurobiologic status post-breast cancer and chemotherapy.

Entorhinal cortical defects in Tg2576 mice are present as early as 2-4 months of age

The entorhinal cortex (EC) is one of the first brain areas to display neuropathology in Alzheimer's disease. A mouse model which simulates amyloid-β (Aβ) neuropathology, the Tg2576 mouse, was used to address these early changes. Here, we show EC abnormalities occur in 2- to 4-month-old Tg2576 mice, an age before Aβ deposition and where previous studies suggest that there are few behavioral impairments. First we show, using a sandwich enzyme-linked immunosorbent assay, that soluble human Aβ40 and Aβ42 are detectable in the EC of 2-month-old Tg2576 mice before Aβ deposition. We then demonstrate that 2- to 4-month-old Tg2576 mice are impaired at object placement, an EC-dependent cognitive task. Next, we show that defects in neuronal nuclear antigen expression and myelin uptake occur in the superficial layers of the EC in 2- to 4-month-old Tg2576 mice. In slices from Tg2576 mice that contained the EC, there were repetitive field potentials evoked by a single stimulus to the underlying white matter, and a greater response to reduced extracellular magnesium ([Mg(2+)]o), suggesting increased excitability. However, deep layer neurons in Tg2576 mice had longer latencies to antidromic activation than wild type mice. The results show changes in the EC at early ages and suggest that altered excitability occurs before extensive plaque pathology.

A cortical circuit for gain control by behavioral state

The brain's response to sensory input is strikingly modulated by behavioral state. Notably, the visual response of mouse primary visual cortex (V1) is enhanced by locomotion, a tractable and accessible example of a time-locked change in cortical state. The neural circuits that transmit behavioral state to sensory cortex to produce this modulation are unknown. In vivo calcium imaging of behaving animals revealed that locomotion activates vasoactive intestinal peptide (VIP)-positive neurons in mouse V1 independent of visual stimulation and largely through nicotinic inputs from basal forebrain. Optogenetic activation of VIP neurons increased V1 visual responses in stationary awake mice, artificially mimicking the effect of locomotion, and photolytic damage of VIP neurons abolished the enhancement of V1 responses by locomotion. These findings establish a cortical circuit for the enhancement of visual response by locomotion and provide a potential common circuit for the modulation of sensory processing by behavioral state.

The intrahippocampal infusion of crotamine from Crotalus durissus terrificus venom enhances memory persistence in rats

Previous research has shown that crotamine, a toxin isolated from the venom of Crotalus durissus terrificus, induces the release of acetylcholine and dopamine in the central nervous system of rats. Particularly, these neurotransmitters are important modulators of memory processes. Therefore, in this study we investigated the effects of crotamine infusion on persistence of memory in rats. We verified that the intrahippocampal infusion of crotamine (1 μg/μl; 1 μl/side) improved the persistence of object recognition and aversive memory. By other side, the intrahippocampal infusion of the toxin did not alter locomotor and exploratory activities, anxiety or pain threshold. These results demonstrate a future prospect of using crotamine as potential pharmacological tool to treat diseases involving memory impairment, although it is still necessary more researches to better elucidate the crotamine effects on hippocampus and memory.

Modulation of pilocarpine-induced seizures by cannabinoid receptor 1

Administration of the muscarinic agonist pilocarpine is commonly used to induce seizures in rodents for the study of epilepsy. Activation of muscarinic receptors has been previously shown to increase the production of endocannabinoids in the brain. Endocannabinoids act at the cannabinoid CB1 receptors to reduce neurotransmitter release and the severity of seizures in several models of epilepsy. In this study, we determined the effect of CB1 receptor activity on the induction in mice of seizures by pilocarpine. We found that decreased activation of the CB1 receptor, either through genetic deletion of the receptor or treatment with a CB1 antagonist, increased pilocarpine seizure severity without modifying seizure-induced cell proliferation and cell death. These results indicate that endocannabinoids act at the CB1 receptor to modulate the severity of pilocarpine-induced seizures. Administration of a CB1 agonist produced characteristic CB1-dependent behavioral responses, but did not affect pilocarpine seizure severity. A possible explanation for the lack of effect of CB1 agonist administration on pilocarpine seizures, despite the effects of CB1 antagonist administration and CB1 gene deletion, is that muscarinic receptor-stimulated endocannabinoid production is acting maximally at CB1 receptors to modulate sensitivity to pilocarpine seizures.

Cholinergic blockade reduces theta-gamma phase amplitude coupling and speed modulation of theta frequency consistent with behavioral effects on encoding

Large-scale neural activation dynamics in the hippocampal-entorhinal circuit local field potential, observable as theta and gamma rhythms and coupling between these rhythms, is predictive of encoding success. Behavioral studies show that systemic administration of muscarinic acetylcholine receptor antagonists selectively impairs encoding, suggesting that they may also disrupt the coupling between the theta and gamma bands. Here, we tested the hypothesis that muscarinic antagonists selectively disrupt coupling between theta and gamma. Specifically, we characterized the effects of systemically administered scopolamine on movement-induced theta and gamma rhythms recorded in the superficial layers of the medial entorhinal cortex (MEC) of freely moving rats. We report the novel result that gamma power at the peak of theta was most reduced following muscarinic blockade, significantly shifting the phase of maximal gamma power to occur at later phases of theta. We also characterize the existence of multiple distinct gamma bands in the superficial layers of the MEC. Further, we observed that theta frequency was significantly less modulated by movement speed following muscarinic blockade. Finally, the slope relating speed to theta frequency, a correlate of familiarity with a testing enclosure, increased significantly less between the preinjection and recovery trials when scopolamine was administered during the intervening injection session than when saline was administered, suggesting that scopolamine reduced encoding of the testing enclosure. These data are consistent with computational models suggesting that encoding and retrieval occur during the peak and trough of theta, respectively, and support the theory that acetylcholine regulates the balance between encoding versus retrieval.

The vestibulo- and preposito-cerebellar cholinergic neurons of a ChAT-tdTomato transgenic rat exhibit heterogeneous firing properties and the expression of various neurotransmitter receptors

Cerebellar function is regulated by cholinergic mossy fiber inputs that are primarily derived from the medial vestibular nucleus (MVN) and prepositus hypoglossi nucleus (PHN). In contrast to the growing evidence surrounding cholinergic transmission and its functional significance in the cerebellum, the intrinsic and synaptic properties of cholinergic projection neurons (ChPNs) have not been clarified. In this study, we generated choline acetyltransferase (ChAT)-tdTomato transgenic rats, which specifically express the fluorescent protein tdTomato in cholinergic neurons, and used them to investigate the response properties of ChPNs identified via retrograde labeling using whole-cell recordings in brainstem slices. In response to current pulses, ChPNs exhibited two afterhyperpolarisation (AHP) profiles and three firing patterns; the predominant AHP and firing properties differed between the MVN and PHN. Morphologically, the ChPNs were separated into two types based on their soma size and dendritic extensions. Analyses of the firing responses to time-varying sinusoidal current stimuli revealed that ChPNs exhibited different firing modes depending on the input frequencies. The maximum frequencies in which each firing mode was observed were different between the neurons that exhibited distinct firing patterns. Analyses of the current responses to the application of neurotransmitter receptor agonists revealed that the ChPNs expressed (i) AMPA- and NMDA-type glutamate receptors, (ii) GABAA and glycine receptors, and (iii) muscarinic and nicotinic acetylcholine receptors. The current responses mediated by these receptors of MVN ChPNs were not different from those of PHN ChPNs. These findings suggest that ChPNs receive various synaptic inputs and encode those inputs appropriately across different frequencies.

Grid cell spatial tuning reduced following systemic muscarinic receptor blockade

Grid cells of the medial entorhinal cortex exhibit a periodic and stable pattern of spatial tuning that may reflect the output of a path integration system. This grid pattern has been hypothesized to serve as a spatial coordinate system for navigation and memory function. The mechanisms underlying the generation of this characteristic tuning pattern remain poorly understood. Systemic administration of the muscarinic antagonist scopolamine flattens the typically positive correlation between running speed and entorhinal theta frequency in rats. The loss of this neural correlate of velocity, an important signal for the calculation of path integration, raises the question of what influence scopolamine has on the grid cell tuning as a read out of the path integration system. To test this, the spatial tuning properties of grid cells were compared before and after systemic administration of scopolamine as rats completed laps on a circle track for food rewards. The results show that the spatial tuning of the grid cells was reduced following scopolamine administration. The tuning of head direction cells, in contrast, was not reduced by scopolamine. This is the first report to demonstrate a link between cholinergic function and grid cell tuning. This work suggests that the loss of tuning in the grid cell network may underlie the navigational disorientation observed in Alzheimer's patients and elderly individuals with reduced cholinergic tone.

Learning-related translocation of δ-opioid receptors on ventral striatal cholinergic interneurons mediates choice between goal-directed actions

The ability of animals to extract predictive information from the environment to inform their future actions is a critical component of decision-making. This phenomenon is studied in the laboratory using the pavlovian-instrumental transfer protocol in which a stimulus predicting a specific pavlovian outcome biases choice toward those actions earning the predicted outcome. It is well established that this transfer effect is mediated by corticolimbic afferents on the nucleus accumbens shell (NAc-S), and recent evidence suggests that δ-opioid receptors (DORs) play an essential role in this effect. In DOR-eGFP knock-in mice, we show a persistent, learning-related plasticity in the translocation of DORs to the somatic plasma membrane of cholinergic interneurons (CINs) in the NAc-S during the encoding of the specific stimulus-outcome associations essential for pavlovian-instrumental transfer. We found that increased membrane DOR expression reflected both stimulus-based predictions of reward and the degree to which these stimuli biased choice during the pavlovian-instrumental transfer test. Furthermore, this plasticity altered the firing pattern of CINs increasing the variance of action potential activity, an effect that was exaggerated by DOR stimulation. The relationship between the induction of membrane DOR expression in CINs and both pavlovian conditioning and pavlovian-instrumental transfer provides a highly specific function for DOR-related modulation in the NAc-S, and it is consistent with an emerging role for striatal CIN activity in the processing of predictive information. Therefore, our results reveal evidence of a long-term, experience-dependent plasticity in opioid receptor expression on striatal modulatory interneurons critical for the cognitive control of action.

Common medial frontal mechanisms of adaptive control in humans and rodents

In this report, we describe how common brain networks within the medial frontal cortex facilitate adaptive behavioral control in rodents and humans. We demonstrate that low frequency oscillations below 12 Hz are dramatically modulated after errors in humans over mid-frontal cortex and in rats within prelimbic and anterior cingulate regions of medial frontal cortex. These oscillations were phase-locked between medial frontal cortex and motor areas in both rats and humans. In rats, single neurons that encoded prior behavioral outcomes were phase-coherent with low-frequency field oscillations particularly after errors. Inactivating medial frontal regions in rats led to impaired behavioral adjustments after errors, eliminated the differential expression of low frequency oscillations after errors, and increased low-frequency spike-field coupling within motor cortex. Our results describe a novel mechanism for behavioral adaptation via low-frequency oscillations and elucidate how medial frontal networks synchronize brain activity to guide performance.

Developmental neurotoxicity of cadmium on enzyme activities of crucial offspring rat brain regions

Cadmium (Cd) is an environmental contaminant known to exert significant neurotoxic effects on both humans and experimental animals. The aim of this study was to shed more light on the effects of gestational (in utero) and lactational maternal exposure to Cd (50 ppm of Cd as Cd-chloride in the drinking water) on crucial brain enzyme activities in important rat offspring brain regions (frontal cortex, hippocampus, hypothalamus, pons and cerebellum). Our study provides a brain region-specific view of the changes in the activities of three crucial brain enzymes as a result of the developmental neurotoxicity of Cd. Maternal exposure to Cd during both gestation and lactation results into significant changes in the activities of acetylcholinesterase and Na(+),K(+)-ATPase in the frontal cortex and the cerebellum of the offspring rats, as well as in a significant increase in the hippocampal Mg(2+)-ATPase activity. These brain-region-specific findings underline the need for further research in the field of Cd-induced developmental neurotoxicity. Deeper understanding of the mechanisms underlying the neurodevelopmental deficits taking place due to in utero and early age exposure to Cd could shed more light on the causes of its well-established cognitive implications.

Effect of cholinergic neurotransmission modulation on visual spatial paired associate learning in healthy human adults

RATIONALE

Use of cross-species neuropsychological paradigms such as visual-spatial paired associate learning (PAL) may allow for a better understanding of underlying neural substrates of memory. Such paradigms, which are often used to guide models of memory in animals, can then be carried forward into humans to provide a basis for evaluation of pharmacologic compounds designed to ameliorate learning and memory impairments in neurologic and psychiatric morbidities.

OBJECTIVES

This double-blind, randomized, crossover trial investigated effects of donepezil, an acetylcholinesterase (AChE) inhibitor, in attenuating scopolamine-induced cognitive impairment using a novel, "process-based" computerized measure of visual-spatial PAL.

RESULTS

In healthy male volunteers, scopolamine (0.6 mg) induced a time-dependent reduction in visual-spatial PAL, with the greatest impairment (Cohen's d = 1.37) observed 2 h after dosing. Cotreatment with donepezil (10 mg) significantly ameliorated scopolamine-induced impairment at the 2-h time point (Cohen's d = 0.66). Process-based analyses revealed a significant impairment in both memory (Cohen's d = 1.37 to 0.50) and executive (Cohen's d = 1 .21 to 0.62) aspects of visual-spatial PAL performance following acute scopolamine challenge, and these reductions were ameliorated by donepezil.

CONCLUSIONS

Acute scopolamine challenge can produce large and robust deficits in visual-spatial PAL, which reflect impairments in both memory and executive processes. Coadministration of a single dose of donepezil can ameliorate these deficits. These results provide support for the use of a visual-spatial PAL test as a pharmacodynamic cognitive marker of central nervous system (CNS)-mediating compounds in humans.

The thalamostriatal pathway and cholinergic control of goal-directed action: interlacing new with existing learning in the striatum

The capacity for goal-directed action depends on encoding specific action-outcome associations, a learning process mediated by the posterior dorsomedial striatum (pDMS). In a changing environment, plasticity has to remain flexible, requiring interference between new and existing learning to be minimized, yet it is not known how new and existing learning are interlaced in this way. Here we investigated the role of the thalamostriatal pathway linking the parafascicular thalamus (Pf) with cholinergic interneurons (CINs) in the pDMS in this process. Removing the excitatory input from Pf to the CINs was found to reduce the firing rate and intrinsic activity of these neurons and produced an enduring deficit in goal-directed learning after changes in the action-outcome contingency. Disconnection of the Pf-pDMS pathway produced similar behavioral effects. These data suggest that CINs reduce interference between new and existing learning, consistent with claims that the thalamostriatal pathway exerts state control over learning-related plasticity.

Urea cycle defects and hyperammonemia: effects on functional imaging

The urea-cycle disorders (UCDs) are a group of congenital enzyme and carrier deficiencies predisposing to hyperammonemia (HA). HA causes changes in the central nervous system (CNS) including alterations of neurotransmitter function, cell volume, and energy deprivation ultimately leading to cerebral edema. Neuropathological findings of UCDs primarily reflect changes in astrocyte morphology. Neurological features accompanying acute HA include changes in behavior and consciousness in the short term, and potential for impairments in memory and executive function as long-term effects. Plasma measures of ammonia and glutamine, although useful for clinical monitoring, prove poor markers of CNS function. Multimodal neuroimaging has potential to investigate impact on cognitive function by interrogating neural networks, connectivity and biochemistry. As neuroimaging methods become increasingly sophisticated, they will play a critical role in clinical monitoring and treatment of metabolic disease. We describe our findings in UCDs; with focus on Ornithine Transcarbamylase deficiency (OTCD) the only X linked UCD.

In vivo cholinergic modulation of the cellular properties of medial entorhinal cortex neurons

Extensive in vitro data and modeling studies suggest that intrinsic properties of medial entorhinal cortex (MEC) neurons contribute to the spiking behaviour of functional cell types of MEC neurons, such as grid cells, recorded in behaving animals. It remains unclear, however, how intrinsic properties of MEC neurons influence cellular dynamics in intact networks in vivo. In order to begin to bridge the gap between electrophysiological data sets from brain slices and behaving animals, in the present study we performed intracellular recordings using sharp electrodes in urethane-anaesthetized rats to elucidate the cellular dynamics of MEC neurons in vivo. We focused on the h-current-dependent sag potential during hyperpolarizing current steps, subthreshold resonance in response to oscillatory frequency sweeps (chirp stimuli), persistent spiking in response to brief depolarizing inputs and the relationship between firing frequency and input (f-I curve), each of which is sensitive to cholinergic modulation in vitro. Consistent with data from in vitro studies, cholinergic activation by systemic application of the acetylcholinesterase inhibitor, physostigmine, resulted in decreased sag amplitude, increased sag time constant and a decrease of the peak resonance frequency. The f-I curve was also modulated by physostigmine in many neurons, but persistent spiking was not observed in any of our recordings, even when picrotoxin, a GABAA blocker, was included in the internal solution of the recording pipette to reduce possible effects of network inhibition. These results suggest that intrinsic oscillatory and rate-coding mechanisms, but not intrinsic bistability, are significantly modulated by acetylcholine in the intact entorhinal network.

Cell-type-specific modulation of neocortical activity by basal forebrain input

Activation of the cholinergic neurons in the basal forebrain (BF) desynchronizes cortical activity and enhances sensory processing during arousal and attention. How the cholinergic input modulates the activity of different subtypes of cortical neurons remains unclear. Using in vivo two-photon calcium imaging of neurons in layers 1 and 2/3 of mouse visual cortex, we show that electrical stimulation of the BF bi-directionally modulates the activity of excitatory neurons as well as several subtypes of inhibitory interneurons. While glutamatergic activity contributed to the activation of both excitatory and inhibitory neurons, the contribution of acetylcholine (ACh) was more complex. Excitatory and parvalbumin-positive (PV+) neurons were activated through muscarinic ACh receptors (mAChRs) at low levels of cortical desynchronization and suppressed through nicotinic ACh receptors (nAChRs) when cortical desynchronization was strong. In contrast, vasoactive intestinal peptide-positive (VIP+) and layer 1 interneurons were preferentially activated through nAChRs during strong cortical desynchronization. Thus, cholinergic input from the BF causes a significant shift in the relative activity levels of different subtypes of cortical neurons at increasing levels of cortical desynchronization.

Cholinergic enhancement reduces orientation-specific surround suppression but not visual crowding

Acetylcholine (ACh) reduces the spatial spread of excitatory fMRI responses in early visual cortex and receptive field size of V1 neurons. We investigated the perceptual consequences of these physiological effects of ACh with surround suppression and crowding, two phenomena that involve spatial interactions between visual field locations. Surround suppression refers to the reduction in perceived stimulus contrast by a high-contrast surround stimulus. For grating stimuli, surround suppression is selective for the relative orientations of the center and surround, suggesting that it results from inhibitory interactions in early visual cortex. Crowding refers to impaired identification of a peripheral stimulus in the presence of flankers and is thought to result from excessive integration of visual features. We increased synaptic ACh levels by administering the cholinesterase inhibitor donepezil to healthy human subjects in a placebo-controlled, double-blind design. In Experiment 1, we measured surround suppression of a central grating using a contrast discrimination task with three conditions: (1) surround grating with the same orientation as the center (parallel), (2) surround orthogonal to the center, or (3) no surround. Contrast discrimination thresholds were higher in the parallel than in the orthogonal condition, demonstrating orientation-specific surround suppression (OSSS). Cholinergic enhancement decreased thresholds only in the parallel condition, thereby reducing OSSS. In Experiment 2, subjects performed a crowding task in which they reported the identity of a peripheral letter flanked by letters on either side. We measured the critical spacing between the targets and flanking letters that allowed reliable identification. Cholinergic enhancement with donepezil had no effect on critical spacing. Our findings suggest that ACh reduces spatial interactions in tasks involving segmentation of visual field locations but that these effects may be limited to early visual cortical processing.

Malignant synaptic growth and Alzheimer's disease

In this article, we will describe the malignant synaptic growth hypothesis of Alzheimer's disease. Originally presented in 1994, the hypothesis remains a viable model of the functional and biophysical mechanisms underlying the development and progression of Alzheimer's disease. In this article, we will refresh the model with references to relevant empirical support that has been generated in the intervening two decades since it's original presentation. We will include discussion of its relationship, in terms of points of alignment and points of contention, to other models of Alzheimer's disease, including the cholinergic hypothesis and the tau and β-amyloid models of Alzheimer's disease. Finally, we propose several falsifiable predictions made by the malignant synaptic growth hypothesis and describe the avenues of treatment that hold the greatest promise under this hypothesis.

Effects of acetylcholine on neuronal properties in entorhinal cortex

The entorhinal cortex (EC) receives prominent cholinergic innervation from the medial septum and the vertical limb of the diagonal band of Broca (MSDB). To understand how cholinergic neurotransmission can modulate behavior, research has been directed toward identification of the specific cellular mechanisms in EC that can be modulated through cholinergic activity. This review focuses on intrinsic cellular properties of neurons in EC that may underlie functions such as working memory, spatial processing, and episodic memory. In particular, the study of stellate cells (SCs) in medial entorhinal has resulted in discovery of correlations between physiological properties of these neurons and properties of the unique spatial representation that is demonstrated through unit recordings of neurons in medial entorhinal cortex (mEC) from awake-behaving animals. A separate line of investigation has demonstrated persistent firing behavior among neurons in EC that is enhanced by cholinergic activity and could underlie working memory. There is also evidence that acetylcholine plays a role in modulation of synaptic transmission that could also enhance mnemonic function in EC. Finally, the local circuits of EC demonstrate a variety of interneuron physiology, which is also subject to cholinergic modulation. Together these effects alter the dynamics of EC to underlie the functional role of acetylcholine in memory.