Neuromodulation by glutamate and acetylcholine can change circuit dynamics by regulating the relative influence of afferent input and excitatory feedback

A theory of hippocampal function: New developments

We develop further here the only quantitative theory of the storage of information in the hippocampal episodic memory system and its recall back to the neocortex. The theory is upgraded to account for a revolution in understanding of spatial representations in the primate, including human, hippocampus, that go beyond the place where the individual is located, to the location being viewed in a scene. This is fundamental to much primate episodic memory and navigation: functions supported in humans by pathways that build 'where' spatial view representations by feature combinations in a ventromedial visual cortical stream, separate from those for 'what' object and face information to the inferior temporal visual cortex, and for reward information from the orbitofrontal cortex. Key new computational developments include the capacity of the CA3 attractor network for storing whole charts of space; how the correlations inherent in self-organizing continuous spatial representations impact the storage capacity; how the CA3 network can combine continuous spatial and discrete object and reward representations; the roles of the rewards that reach the hippocampus in the later consolidation into long-term memory in part via cholinergic pathways from the orbitofrontal cortex; and new ways of analysing neocortical information storage using Potts networks.

Muscarinic Modulation of Synaptic Transmission and Short-Term Plasticity in the Dorsal and Ventral Hippocampus

Muscarinic neurotransmission is fundamentally involved in supporting several brain functions by modulating flow of information in brain neural circuits including the hippocampus which displays a remarkable functional segregation along its longitudinal axis. However, how muscarinic neuromodulation contributes to the functional segregation along the hippocampus remains unclear. In this study we show that the nonselective muscarinic receptor agonist carbachol similarly suppresses basal synaptic transmission in the dorsal and ventral CA1 hippocampal field, in a concentration-depended manner. Furthermore, using a ten-pulse stimulation train of varying frequency we found that carbachol changes the frequency filtering properties more in ventral than dorsal hippocampus by facilitating synaptic inputs at a wide range of input frequencies in the ventral compared with dorsal hippocampus. Using the M2 receptor antagonist gallamine and the M4 receptor antagonist tropicamide, we found that M2 receptors are involved in controlling basal synaptic transmission and short-term synaptic plasticity (STSP) in the ventral but not the dorsal hippocampus, while M4 receptors participate in modulating basal synaptic transmission and STSP in both segments of the hippocampus. These results were corroborated by the higher protein expression levels of M2 receptors in the ventral compared with dorsal hippocampus. We conclude that muscarinic transmission modulates excitatory synaptic transmission and short-term synaptic plasticity along the entire rat hippocampus by acting through M4 receptors and recruiting M2 receptors only in the ventral hippocampus. Furthermore, M4 receptors appear to exert a permissive role on the actions of M2 receptors on STSP in the ventral hippocampus. This dorsoventral differentiation of muscarinic modulation is expected to have important implications in information processing along the endogenous hippocampal circuitry.

Subfield-specific interneuron circuits govern the hippocampal response to novelty in male mice

The hippocampus is the brain's center for episodic memories. Its subregions, the dentate gyrus and CA1-3, are differentially involved in memory encoding and recall. Hippocampal principal cells represent episodic features like movement, space, and context, but less is known about GABAergic interneurons. Here, we performed two-photon calcium imaging of parvalbumin- and somatostatin-expressing interneurons in the dentate gyrus and CA1-3 of male mice exploring virtual environments. Parvalbumin-interneurons increased activity with running-speed and reduced it in novel environments. Somatostatin-interneurons in CA1-3 behaved similar to parvalbumin-expressing cells, but their dentate gyrus counterparts increased activity during rest and in novel environments. Congruently, chemogenetic silencing of dentate parvalbumin-interneurons had prominent effects in familiar contexts, while silencing somatostatin-expressing cells increased similarity of granule cell representations between novel and familiar environments. Our data indicate unique roles for parvalbumin- and somatostatin-positive interneurons in the dentate gyrus that are distinct from those in CA1-3 and may support routing of novel information.

M-current modulation of cortical slow oscillations: Network dynamics and computational modeling

The slow oscillation is a synchronized network activity expressed by the cortical network in slow wave sleep and under anesthesia. Waking up requires a transition from this synchronized brain state to a desynchronized one. Cholinergic innervation is critical for the transition from slow-wave-sleep to wakefulness, and muscarinic action is largely exerted through the muscarinic-sensitive potassium current (M-current) block. We investigated the dynamical impact of blocking the M-current on slow oscillations, both in cortical slices and in a cortical network computational model. Blocking M-current resulted in an elongation of Up states (by four times) and in a significant firing rate increase, reflecting an increased network excitability, albeit no epileptiform discharges occurred. These effects were replicated in a biophysical cortical model, where a parametric reduction of the M-current resulted in a progressive elongation of Up states and firing rate. All neurons, and not only those modeled with M-current, increased their firing rates due to network recurrency. Further increases in excitability induced even longer Up states, approaching the microarousals described in the transition towards wakefulness. Our results bridge an ionic current with network modulation, providing a mechanistic insight into network dynamics of awakening.

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

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

Hippocampal spatial view cells for memory and navigation, and their underlying connectivity in humans

Hippocampal and parahippocampal gyrus spatial view neurons in primates respond to the spatial location being looked at. The representation is allocentric, in that the responses are to locations "out there" in the world, and are relatively invariant with respect to retinal position, eye position, head direction, and the place where the individual is located. The underlying connectivity in humans is from ventromedial visual cortical regions to the parahippocampal scene area, leading to the theory that spatial view cells are formed by combinations of overlapping feature inputs self-organized based on their closeness in space. Thus, although spatial view cells represent "where" for episodic memory and navigation, they are formed by ventral visual stream feature inputs in the parahippocampal gyrus in what is the parahippocampal scene area. A second "where" driver of spatial view cells are parietal inputs, which it is proposed provide the idiothetic update for spatial view cells, used for memory recall and navigation when the spatial view details are obscured. Inferior temporal object "what" inputs and orbitofrontal cortex reward inputs connect to the human hippocampal system, and in macaques can be associated in the hippocampus with spatial view cell "where" representations to implement episodic memory. Hippocampal spatial view cells also provide a basis for navigation to a series of viewed landmarks, with the orbitofrontal cortex reward inputs to the hippocampus providing the goals for navigation, which can then be implemented by hippocampal connectivity in humans to parietal cortex regions involved in visuomotor actions in space. The presence of foveate vision and the highly developed temporal lobe for object and scene processing in primates including humans provide a basis for hippocampal spatial view cells to be key to understanding episodic memory in the primate and human hippocampus, and the roles of this system in primate including human navigation.

A spiking computational model for striatal cholinergic interneurons

Cholinergic interneurons in the striatum, also known as tonically active interneurons or TANs, are thought to have a strong effect on corticostriatal plasticity and on striatal activity and outputs, which in turn play a critical role in modulating downstream basal ganglia activity and movement. Striatal TANs can exhibit a variety of firing patterns and responses to synaptic inputs; furthermore, they have been found to display various surges and pauses in activity associated with sensory cues and reward delivery in learning as well as with motor tic production. To help explain the factors that contribute to TAN activity patterns and to provide a resource for future studies, we present a novel conductance-based computational model of a striatal TAN. We show that this model produces the various characteristic firing patterns observed in recordings of TANs. With a single baseline tuning associated with tonic firing, the model also captures a wide range of TAN behaviors found in previous experiments involving a variety of manipulations. In addition to demonstrating these results, we explain how various ionic currents in the model contribute to them. Finally, we use this model to explore the contributions of the acetylcholine released by TANs to the production of surges and pauses in TAN activity in response to strong excitatory inputs. These results provide predictions for future experimental testing that may help with efforts to advance our understanding of the role of TANs in reinforcement learning and in motor disorders such as Tourette's syndrome.

The human orbitofrontal cortex, vmPFC, and anterior cingulate cortex effective connectome: emotion, memory, and action

The human orbitofrontal cortex, ventromedial prefrontal cortex (vmPFC), and anterior cingulate cortex are involved in reward processing and thereby in emotion but are also implicated in episodic memory. To understand these regions better, the effective connectivity between 360 cortical regions and 24 subcortical regions was measured in 172 humans from the Human Connectome Project and complemented with functional connectivity and diffusion tractography. The orbitofrontal cortex has effective connectivity from gustatory, olfactory, and temporal visual, auditory, and pole cortical areas. The orbitofrontal cortex has connectivity to the pregenual anterior and posterior cingulate cortex and hippocampal system and provides for rewards to be used in memory and navigation to goals. The orbitofrontal and pregenual anterior cortex have connectivity to the supracallosal anterior cingulate cortex, which projects to midcingulate and other premotor cortical areas and provides for action-outcome learning including limb withdrawal or flight or fight to aversive and nonreward stimuli. The lateral orbitofrontal cortex has outputs to language systems in the inferior frontal gyrus. The medial orbitofrontal cortex connects to the nucleus basalis of Meynert and the pregenual cingulate to the septum, and damage to these cortical regions may contribute to memory impairments by disrupting cholinergic influences on the neocortex and hippocampus.

Low glutamate diet improves working memory and contributes to altering BOLD response and functional connectivity within working memory networks in Gulf War Illness

Gulf War Illness is a chronic multi-symptom disorder with severe cognitive impairments which may be related to glutamate excitotoxicity and central nervous system dysfunction. The low glutamate diet has been proposed as a comprehensive intervention for Gulf War Illness. We examined the effects of the low glutamate diet on verbal working memory using a fMRI N-back task. Accuracy, whole-brain blood oxygen level dependency (BOLD) response, and task-based functional connectivity were assessed at baseline and after 1 month on the diet (N = 24). Multi-voxel pattern analysis identified regions of whole-brain BOLD pattern differences after the diet to be used as seeds for subsequent seed-to-voxel functional connectivity analyses. Verbal working memory accuracy improved after the diet (+ 13%; p = 0.006). Whole-brain BOLD signal changes were observed, revealing lower activation within regions of the frontoparietal network and default mode network after the low glutamate diet. Multi-voxel pattern analysis resulted in 3 clusters comprising parts of the frontoparietal network (clusters 1 and 2) and ventral attention network (cluster 3). The seed-to-voxel analyses identified significant functional connectivity changes post-diet for clusters 1 and 2 (peak p < 0.001, cluster FDR p < 0.05). Relative to baseline, clusters 1 and 2 had decreased functional connectivity with regions in the ventral attention and somatomotor networks. Cluster 2 also had increased functional connectivity with regions of the default mode and frontoparietal networks. These findings suggest that among veterans with Gulf War Illness, the low glutamate diet improves verbal working memory accuracy, alters BOLD response, and alters functional connectivity within two networks central to working memory.

The hippocampus, ventromedial prefrontal cortex, and episodic and semantic memory

The human ventromedial prefrontal cortex (vmPFC)/anterior cingulate cortex is implicated in reward and emotion, but also in memory. It is shown how the human orbitofrontal cortex connecting with the vmPFC and anterior cingulate cortex provide a route to the hippocampus for reward and emotional value to be incorporated into episodic memory, enabling memory of where a reward was seen. It is proposed that this value component results in primarily episodic memories with some value component to be repeatedly recalled from the hippocampus so that they are more likely to become incorporated into neocortical semantic and autobiographical memories. The same orbitofrontal and anterior cingulate regions also connect in humans to the septal and basal forebrain cholinergic nuclei, thereby helping to consolidate memory, and helping to account for why damage to the vMPFC impairs memory. The human hippocampus and vmPFC thus contribute in complementary ways to forming episodic and semantic memories.

Septotemporal variation in modulation of synaptic transmission, paired-pulse ratio and frequency facilitation/depression by adenosine and GABAB receptors in the rat hippocampus

Short-term synaptic plasticity represents a fundamental mechanism in neural information processing and is regulated by neuromodulators. Here, using field recordings from the CA1 region of adult rat hippocampal slices, we show that excitatory synaptic transmission is suppressed by strong but not moderate activation of adenosine A₁ receptors by 2-Chloro-N⁶-cyclopentyladenosine (CCPA) more in the dorsal than the ventral hippocampus; in contrast, both mild and strong activation of GABA_B receptors by baclofen (1 μM, 10 μM) suppress synaptic transmission more in the ventral than the dorsal hippocampus. Using a 10-pulse stimulation train of variable frequency, we found that CCPA modulates short-term synaptic plasticity independently of the suppression of synaptic transmission in both segments of the hippocampus and at stimulation frequencies greater than 10 Hz. However, specifically regarding the paired-pulse ratio (PPR) and frequency facilitation/depression (FF/D) we found significant drug action before but not after adjusting conditioning responses to control levels. Activation of GABA_BRs by baclofen suppressed synaptic transmission more in the ventral than the dorsal hippocampus. Furthermore, relatively high (10 μM) but not low (1 μM) baclofen concentration enhanced both PPR and FF in both hippocampal segments at stimulation frequencies greater than 1 Hz, independently of the suppression of synaptic transmission by baclofen. These results show that A₁Rs and GABA_BRs control synaptic transmission more effectively in the dorsal and the ventral hippocampus, respectively, and suggest that these receptors modulate PPR and FF/D at different frequency bands of afferent input, in both segments of the hippocampus.

Five-Year PTSD Symptom Remission in Two Patients Following Treatment With Rivastigmine

INTRODUCTION

The beneficial effect of rivastigmine, an acetylcholinesterase inhibitor (AChEi), which increases levels of acetylcholine (ACh), was first reported in 2013. This paper replicates those findings and reports sustained symptom remission.

METHODS AND MATERIALS

The high-frequency (HF) component of heart rate variability (HRV) is a measure of cholinergic withdrawal and was measured using a Zephyr Bioharness HR monitor, pre- and post-commencement of treatment. Data analysis was performed using Kubios HRV software. PTSD symptom severity was assessed using the Post-Traumatic Checklist-Civilian (PCL-C).

RESULTS

Low HF HRV was observed in both patients before rivastigmine treatment and reductions in PCL-C scores paralleled increases in HF HRV values. Follow-up revealed low HF HRV values in both patients despite PCL-C scores indicating remission. Sympathetic nervous system hyperactivity was observed in one patient, just before a suicide attempt. Following rivastigmine treatment, the patient had no further suicidal ideation or attempts. Another patient reported worsening of her PTSD symptoms in the peri-menstrual period, which was abolished by rivastigmine. She also experienced symptom relapse following prolonged infections.

CONCLUSION

Low HF HRV has been reported in PTSD patients, but findings have been inconsistent. Cholinergic withdrawal could explain the disturbances in sleep, learning, and memory seen in PTSD patients. The relapse of symptoms following prolonged infection implicates the immune system as a possible initiator of the disorder. ACh and estrogen have anti-inflammatory properties, supporting a possible role of inflammation in initiating PTSD. The effect of rivastigmine treatment should be tested in properly controlled clinical trials.

Prediction errors disrupt hippocampal representations and update episodic memories

The brain supports adaptive behavior by generating predictions, learning from errors, and updating memories to incorporate new information. Prediction error, or surprise, triggers learning when reality contradicts expectations. Prior studies have shown that the hippocampus signals prediction errors, but the hypothesized link to memory updating has not been demonstrated. In a human functional MRI study, we elicited mnemonic prediction errors by interrupting familiar narrative videos immediately before the expected endings. We found that prediction errors reversed the relationship between univariate hippocampal activation and memory: greater hippocampal activation predicted memory preservation after expected endings, but memory updating after surprising endings. In contrast to previous studies, we show that univariate activation was insufficient for understanding hippocampal prediction error signals. We explain this surprising finding by tracking both the evolution of hippocampal activation patterns and the connectivity between the hippocampus and neuromodulatory regions. We found that hippocampal activation patterns stabilized as each narrative episode unfolded, suggesting sustained episodic representations. Prediction errors disrupted these sustained representations and the degree of disruption predicted memory updating. The relationship between hippocampal activation and subsequent memory depended on concurrent basal forebrain activation, supporting the idea that cholinergic modulation regulates attention and memory. We conclude that prediction errors create conditions that favor memory updating, prompting the hippocampus to abandon ongoing predictions and make memories malleable.

Spatial Pattern Separation Testing Differentiates Alzheimer's Disease Biomarker-Positive and Biomarker-Negative Older Adults With Amnestic Mild Cognitive Impairment

Background: The hippocampus, entorhinal cortex (EC), and basal forebrain (BF) are among the earliest regions affected by Alzheimer’s disease (AD) pathology. They play an essential role in spatial pattern separation, a process critical for accurate discrimination between similar locations.

Objective: We examined differences in spatial pattern separation performance between older adults with amnestic mild cognitive impairment (aMCI) with AD versus those with non-Alzheimer’s pathologic change (non-AD) and interrelations between volumes of the hippocampal, EC subregions and BF nuclei projecting to these subregions (medial septal nuclei and vertical limb of the diagonal band of Broca – Ch1-2 nuclei) with respect to performance.

Methods: Hundred and eighteen older adults were recruited from the Czech Brain Aging Study. Participants with AD aMCI (n = 37), non-AD aMCI (n = 26), mild AD dementia (n = 26), and cognitively normal older adults (CN; n = 29) underwent spatial pattern separation testing, cognitive assessment and brain magnetic resonance imaging.

Results: The AD aMCI group had less accurate spatial pattern separation performance than the non-AD aMCI (p = 0.039) and CN (p < 0.001) groups. The AD aMCI and non-AD groups did not differ in other cognitive tests. Decreased BF Ch1-2 volume was indirectly associated with worse performance through reduced hippocampal tail volume and reduced posteromedial EC and hippocampal tail or body volumes operating in serial.

Conclusion: The study demonstrates that spatial pattern separation testing differentiates AD biomarker positive and negative older adults with aMCI and provides evidence that BF Ch1-2 nuclei influence spatial pattern separation through the posteromedial EC and the posterior hippocampus.

Enhanced Discriminative Abilities of Auditory Cortex Neurons for Pup Calls Despite Reduced Evoked Responses in C57BL/6 Mother Mice

A fundamental task for the auditory system is to process communication sounds according to their behavioral significance. In many mammalian species, pup calls became more significant for mothers than other conspecific and heterospecific communication sounds. To study the cortical consequences of motherhood on the processing of communication sounds, we recorded neuronal responses in the primary auditory cortex of virgin and mother C57BL/6 mice which had similar ABR thresholds. In mothers, the evoked firing rate in response to pure tones was decreased and the frequency receptive fields were narrower. The responses to pup and adult calls were also reduced but the amount of mutual information (MI) per spike about the pup call's identity was increased in mother mice. The response latency to pup and adult calls was significantly shorter in mothers. Despite similarly decreased responses to guinea pig whistles, the response latency, and the MI per spike did not differ between virgins and mothers for these heterospecific vocalizations. Noise correlations between cortical recordings were decreased in mothers, suggesting that the firing rate of distant neurons was more independent from each other. Together, these results indicate that in the most commonly used mouse strain for behavioral studies, the discrimination of pup calls by auditory cortex neurons is more efficient during motherhood.

Prior knowledge promotes hippocampal separation but cortical assimilation in the left inferior frontal gyrus

An adaptive memory system rarely learns information tabula rasa, but rather builds on prior knowledge to facilitate learning. How prior knowledge influences the neural representation of novel associations remains unknown. Here, participants associated pairs of faces in two conditions: a famous, highly familiar face with a novel face or two novel faces while undergoing fMRI. We examine multivoxel activity patterns corresponding to individual faces before and after learning. The activity patterns representing members of famous-novel pairs becomes separated in the hippocampus, that is, more distinct from one another through learning, in striking contrast to paired novel faces that become similar. In the left inferior frontal gyrus, however, prior knowledge leads to integration, and in a specific direction: the representation of the novel face becomes similar to that of the famous face after learning, suggesting assimilation of new into old memories. We propose that hippocampal separation might resolve interference between existing and newly learned information, allowing cortical assimilation. Thus, associative learning with versus without prior knowledge relies on radically different computations.

Prior knowledge strongly impacts new learning, but its influence on the neural representation of novel information is unknown. Here, the authors show multiple neural codes for learning: prior knowledge leads to integrated cortical representations, while promoting hippocampal separation.

Mesoscopic Mapping of Stimulus-Selective Response Plasticity in the Visual Pathways Modulated by the Cholinergic System

The cholinergic potentiation of visual conditioning enhances visual acuity and discrimination of the trained stimulus. To determine if this also induces long-term plastic changes on cortical maps and connectivity in the visual cortex and higher associative areas, mesoscopic calcium imaging was performed in head-fixed awake GCaMP6s adult mice before and after conditioning. The conditioned stimulus (0.03 cpd, 30°, 100% contrast, 1 Hz-drifting gratings) was presented 10 min daily for a week. Saline or Donepezil (DPZ, 0.3 mg/kg, s.c.), a cholinesterase inhibitor that potentiates cholinergic transmission, were injected prior to each conditioning session and compared to a sham-conditioned group. Cortical maps of resting state and evoked response to the monocular presentation of conditioned or non-conditioned stimulus (30°, 50 and 75% contrast; 90°, 50, 75, and 100% contrast) were established. Amplitude, duration, and latency of the peak response, as well as size of activation were measured in the primary visual cortex (V1), secondary visual areas (AL, A, AM, PM, LM, RL), retrosplenial cortex (RSC), and higher cortical areas. Visual stimulation increased calcium signaling in all primary and secondary visual areas, the RSC, but no other cortices. There were no significant effects of sham-conditioning or conditioning alone, but DPZ treatment during conditioning significantly decreased the integrated neuronal activity of superficial layers evoked by the conditioned stimulus in V1, AL, PM, and LM. The activity of downstream cortical areas was not changed. The size of the activated area was decreased in V1 and PM, and the signal-to-noise ratio was decreased in AL and PM. Interestingly, signal correlation was seen only between V1, the ventral visual pathway, and the RSC, and was decreased by DPZ administration. The resting state activity was slightly correlated and rarely affected by treatments, except between binocular and monocular V1 in both hemispheres. In conclusion, cholinergic potentiation of visual conditioning induced change in visual processing in the superficial cortical layers. This effect might be a key mechanism in the establishment of the fine cortical tuning in response to the conditioned visual stimulus.

Mnemonic prediction errors bias hippocampal states

When our experience violates our predictions, it is adaptive to upregulate encoding of novel information, while down-weighting retrieval of erroneous memory predictions to promote an updated representation of the world. We asked whether mnemonic prediction errors promote hippocampal encoding versus retrieval states, as marked by distinct network connectivity between hippocampal subfields. During fMRI scanning, participants were cued to internally retrieve well-learned complex room-images and were then presented with either an identical or a modified image (0-4 changes). In the left hemisphere, we find that CA1-entorhinal connectivity increases, and CA1-CA3 connectivity decreases, with the number of changes. Further, in the left CA1, the similarity between activity patterns during cued-retrieval of the learned room and during the image is lower when the image includes changes, consistent with a prediction error signal in CA1. Our findings provide a mechanism by which mnemonic prediction errors may drive memory updating—by biasing hippocampal states.

When our expectations are violated, it is adaptive to update our internal models to improve predictions in the future. Here, the authors show that during mnemonic violations, hippocampal networks are biased towards an encoding state and away from a retrieval state to potentially update these predictions.

Dorsal-Ventral Differences in Modulation of Synaptic Transmission in the Hippocampus

Functional diversification along the longitudinal axis of the hippocampus is a rapidly growing concept. Modulation of synaptic transmission by neurotransmitter receptors may importantly contribute to specialization of local intrinsic network function along the hippocampus. In the present study, using transverse slices from the dorsal and the ventral hippocampus of adult rats and recordings of evoked field postsynaptic excitatory potentials (fEPSPs) from the CA1 stratum radiatum, we aimed to compare modulation of synaptic transmission between the dorsal and the ventral hippocampus. We found that transient heterosynaptic depression (tHSD, <2 s), a physiologically relevant phenomenon of regulation of excitatory synaptic transmission induced by paired stimulation of two independent inputs to stratum radiatum of CA1 field, has an increased magnitude and duration in the ventral hippocampus, presumably contributing to increased input segregation in this segment of the hippocampus. GABA_B receptors, GABA_A receptors, adenosine A1 receptors and L-type voltage-gated calcium channels appear to contribute differently to tHSD in the two hippocampal segments; GABA_BRs play a predominant role in the ventral hippocampus while both GABA_BRs and A1Rs play important roles in the dorsal hippocampus. Activation of GABA_B receptors by an exogenous agonist, baclofen, robustly and reversibly modulated both the initial fast and the late slow components of excitatory synaptic transmission, expressed by the fEPSPslope and fEPSP decay time constant (fEPSP_τ), respectively. Specifically, baclofen suppressed fEPSP slope more in the ventral than in the dorsal hippocampus and enhanced fEPSP_τ more in the dorsal than in the ventral hippocampus. Also, baclofen enhanced paired-pulse facilitation in the two hippocampal segments similarly. Blockade of GABA_B receptors did not affect basal paired-pulse facilitation in either hippocampal segment. We propose that the revealed dorsal-ventral differences in modulation of synaptic transmission may provide a means for specialization of information processing in the local neuronal circuits, thereby significantly contributing to diversifying neuronal network functioning along the dorsal-ventral axis of hippocampus.

Spatial Pattern Separation in Early Alzheimer's Disease

BACKGROUND

The hippocampus, entorhinal cortex, and basal forebrain are among the first brain structures affected by Alzheimer's disease (AD). They play an essential role in spatial pattern separation, a process critical for accurate encoding of similar spatial information.

OBJECTIVE

Our aim was to examine spatial pattern separation and its association with volumetric changes of the hippocampus, entorhinal cortex, and basal forebrain nuclei projecting to the hippocampus (the medial septal nuclei and vertical limb of the diagonal band of Broca - Ch1-2 nuclei) in the biomarker-defined early clinical stages of AD.

METHODS

A total of 98 older adults were recruited from the Czech Brain Aging Study cohort. The participants with amnestic mild cognitive impairment (aMCI) due to AD (n = 44), mild AD dementia (n = 31), and cognitively normal older adults (CN; n = 23) underwent spatial pattern separation testing, comprehensive cognitive assessment, and MRI brain volumetry.

RESULTS

Spatial pattern separation accuracy was lower in the early clinical stages of AD compared to the CN group (p < 0.001) and decreased with disease severity (CN > aMCI due to AD > AD dementia). Controlling for general memory and cognitive performance, demographic characteristics and psychological factors did not change the results. Hippocampal and Ch1-2 volumes were directly associated with spatial pattern separation performance while the entorhinal cortex operated on pattern separation indirectly through the hippocampus.

CONCLUSION

Smaller volumes of the hippocampus, entorhinal cortex, and basal forebrain Ch1-2 nuclei are linked to spatial pattern separation impairment in biomarker-defined early clinical AD and may contribute to AD-related spatial memory deficits.

Dentate gyrus circuits for encoding, retrieval and discrimination of episodic memories

The dentate gyrus (DG) has a key role in hippocampal memory formation. Intriguingly, DG lesions impair many, but not all, hippocampus-dependent mnemonic functions, indicating that the rest of the hippocampus (CA1-CA3) can operate autonomously under certain conditions. An extensive body of theoretical work has proposed how the architectural elements and various cell types of the DG may underlie its function in cognition. Recent studies recorded and manipulated the activity of different neuron types in the DG during memory tasks and have provided exciting new insights into the mechanisms of DG computational processes, particularly for the encoding, retrieval and discrimination of similar memories. Here, we review these DG-dependent mnemonic functions in light of the new findings and explore mechanistic links between the cellular and network properties of, and the computations performed by, the DG.

Neurobiological effects of aerobic exercise, with a focus on patients with schizophrenia

Schizophrenia is a severe neuropsychiatric disease that is associated with neurobiological alterations in multiple brain regions and peripheral organs. Negative symptoms and cognitive deficits are present in about half of patients and are difficult to treat, leading to an unfavorable functional outcome. To investigate the impact of aerobic exercise on various neurobiological parameters, we conducted a narrative review. Add-on aerobic exercise was shown to be effective in improving negative and general symptoms, cognition, global functioning, and quality of life in schizophrenia patients. Based on findings in healthy individuals and animal models, this qualitative review gives an overview of different lines of evidence on how aerobic exercise impacts brain structure and function and molecular mechanisms in patients with schizophrenia and how its effects could be related to clinical and functional outcomes. Structural magnetic resonance imaging studies showed a volume increase in the hippocampus and cortical regions in schizophrenia patients and healthy controls after endurance training. However, results are inconsistent and individual risk factors may influence neuroplastic processes. Animal studies indicate that alterations in epigenetic mechanisms and synaptic plasticity are possible underlying mechanisms, but that differentiation of glial cells, angiogenesis, and possibly neurogenesis may also be involved. Clinical and animal studies also revealed effects of aerobic exercise on the hypothalamus-pituitary-adrenal axis, growth factors, and immune-related mechanisms. Some findings indicate effects on neurotransmitters and the endocannabinoid system. Further research is required to clarify how individual risk factors in schizophrenia patients mediate or moderate the neurobiological effects of exercise on brain and cognition. Altogether, aerobic exercise is a promising candidate in the search for pathophysiology-based add-on interventions in schizophrenia.

Short-term dynamics of input and output of CA1 network greatly differ between the dorsal and ventral rat hippocampus

Background

The functional heterogeneity of the hippocampus along its longitudinal axis at the level of behavior is an established concept; however, the neurobiological mechanisms are still unknown. Diversifications in the functioning of intrinsic hippocampal circuitry including short-term dynamics of synaptic inputs and neuronal output, that are important determinants of information processing in the brain, may profoundly contribute to functional specializations along the hippocampus. The objectives of the present study were the examination of the role of the GABA_A receptor-mediated inhibition, the μ-opioid receptors and the effect of stimulation intensity on the dynamics of both synaptic input and neuronal output of CA1 region in the dorsal and ventral hippocampus. We used recordings of field potentials from adult rat hippocampal slices evoked by brief repetitive activation of Schaffer collaterals.

Results

We find that the local CA1 circuit of the dorsal hippocampus presents a remarkably increased dynamic range of frequency-dependent short-term changes in both input and output, ranging from strong facilitation to intense depression at low and high stimulation frequencies respectively. Furthermore, the input–output relationship in the dorsal CA1 circuit is profoundly influenced by frequency and time of presynaptic activation. Strikingly, the ventral hippocampus responds mostly with depression, displaying a rather monotonous input–output relationship over frequency and time. Partial blockade of GABA_A receptor-mediated transmission (by 5 μM picrotoxin) profoundly influences input and output dynamics in the dorsal hippocampus but affected only the neuronal output in the ventral hippocampus. M-opioid receptors control short-term dynamics of input and output in the dorsal hippocampus but they play no role in the ventral hippocampus.

Conclusion

The results demonstrate that information processing by CA1 local network is highly diversified between the dorsal and ventral hippocampus. Transient detection of incoming patterns of activity and frequency-dependent sustained signaling of amplified neuronal information may be assigned to the ventral and dorsal hippocampal circuitry respectively. This disparity should have profound implications for the functional roles ascribed to distinct segments along the long axis of the hippocampus.

Electronic supplementary material

The online version of this article (10.1186/s12868-019-0517-5) contains supplementary material, which is available to authorized users.

Amplifying the redistribution of somato-dendritic inhibition by the interplay of three interneuron types

GABAergic interneurons play an important role in shaping the activity of excitatory pyramidal cells (PCs). How the various inhibitory cell types contribute to neuronal information processing, however, is not resolved. Here, we propose a functional role for a widespread network motif consisting of parvalbumin- (PV), somatostatin- (SOM) and vasoactive intestinal peptide (VIP)-expressing interneurons. Following the idea that PV and SOM interneurons control the distribution of somatic and dendritic inhibition onto PCs, we suggest that mutual inhibition between VIP and SOM cells translates weak inputs to VIP interneurons into large changes of somato-dendritic inhibition of PCs. Using a computational model, we show that the neuronal and synaptic properties of the circuit support this hypothesis. Moreover, we demonstrate that the SOM-VIP motif allows transient inputs to persistently switch the circuit between two processing modes, in which top-down inputs onto apical dendrites of PCs are either integrated or cancelled.

Neurons in the brain can be classified as excitatory or inhibitory based on whether they activate or deactivate the cells to whom they send signals. Compared to their excitatory counterpart, inhibitory neurons present themselves as a wild diversity of cell classes. It is broadly believed that these classes serve different purposes, but as of now, those are poorly understood. In this article, we suggest how an intricate interplay of three inhibitory cell classes can control whether internal signals—such as predictions, memory signals or motor commands—are taken into account when sensory signals are interpreted. Using a mathematical model and computer simulations, we show that such internal signals can be shut down by regulating which inhibitory cell types are active, and that the interaction of different cell classes allows weak control signals to do so.

The Neuroprotective Effects of Exercise: Maintaining a Healthy Brain Throughout Aging

Physical activity plays an essential role in maintaining a healthy body, yet it also provides unique benefits for the vascular and cellular systems that sustain a healthy brain. While the benefit of exercise has been observed in humans of all ages, the availability of preclinical models has permitted systematic investigations into the mechanisms by which exercise supports and protects the brain. Over the past twenty-five years, rodent models have shown that increased physical activity elevates neurotrophic factors in the hippocampal and cortical areas, facilitating neurotransmission throughout the brain. Increased physical activity (such as by the voluntary use of a running wheel or regular, timed sessions on a treadmill) also promotes proliferation, maturation and survival of cells in the dentate gyrus, contributing to the process of adult hippocampal neurogenesis. In this way, rodent studies have tremendous value as they demonstrate that an 'active lifestyle' has the capacity to ameliorate a number of age-related changes in the brain, including the decline in adult neurogenesis. Moreover, these studies have shown that greater physical activity may protect the brain health into advanced age through a number of complimentary mechanisms: in addition to upregulating factors in pro-survival neurotrophic pathways and enhancing synaptic plasticity, increased physical activity promotes brain health by supporting the cerebrovasculature, sustaining the integrity of the blood-brain barrier, increasing glymphatic clearance and proteolytic degradation of amyloid beta species, and regulating microglia activation. Collectively, preclinical studies demonstrate that exercise initiates diverse and powerful neuroprotective pathways that may converge to promote continued brain health into old age. This review will draw on both seminal and current literature that highlights mechanisms by which exercise supports the functioning of the brain, and aids in its protection.

Nuclear organization and morphology of cholinergic neurons in the brain of the rock cavy (Kerodon rupestris) (Wied, 1820)

The aim of this study was to conduct cytoarchitectonic studies and choline acetyltransferase (ChAT) immunohistochemical analysis to delimit the cholinergic groups in the encephalon of the rock cavy (Kerodon rupestris), a crepuscular Caviidae rodent native to the Brazilian Northeast. Three young adult animals were anesthetized and transcardially perfused. The encephala were cut in the coronal plane using a cryostat. We obtained 6 series of 30-μm-thick sections. The sections from one series were subjected to Nissl staining. Those from another series were subjected to immunohistochemistry for the enzyme ChAT, which is used in acetylcholine synthesis, to visualize the different cholinergic neural centers of the rock cavy. The slides were analyzed using a light microscope and the results were documented by description and digital photomicrographs. ChAT-immunoreactive neurons were identified in the telencephalon (nucleus accumbens, caudate-putamen, globus pallidus, entopeduncular nucleus and ventral globus pallidus, olfactory tubercle and islands of Calleja, diagonal band of Broca nucleus, nucleus basalis, and medial septal nucleus), diencephalon (ventrolateral preoptic, hypothalamic ventrolateral, and medial habenular nuclei), and brainstem (parabigeminal, laterodorsal tegmental, and pedunculopontine tegmental nuclei). These findings are discussed through both a functional and phylogenetic perspective.

The storage and recall of memories in the hippocampo-cortical system

A quantitative computational theory of the operation of the hippocampus as an episodic memory system is described. The CA3 system operates as a single attractor or autoassociation network (1) to enable rapid one-trial associations between any spatial location (place in rodents or spatial view in primates) and an object or reward and (2) to provide for completion of the whole memory during recall from any part. The theory is extended to associations between time and object or reward to implement temporal order memory, which is also important in episodic memory. The dentate gyrus performs pattern separation by competitive learning to create sparse representations producing, for example, neurons with place-like fields from entorhinal cortex grid cells. The dentate granule cells generate, by the very small number of mossy fibre connections to CA3, a randomizing pattern separation effect that is important during learning but not recall and that separates out the patterns represented by CA3 firing as being very different from each other. This is optimal for an unstructured episodic memory system in which each memory must be kept distinct from other memories. The direct perforant path input to CA3 is quantitatively appropriate for providing the cue for recall in CA3 but not for learning. The CA1 recodes information from CA3 to set up associatively learned backprojections to the neocortex to allow the subsequent retrieval of information to the neocortex, giving a quantitative account of the large number of hippocampo-neocortical and neocortical-neocortical backprojections. Tests of the theory including hippocampal subregion analyses and hippocampal NMDA receptor knockouts are described and support the theory.

Factors affecting post-stroke motor recovery: Implications on neurotherapy after brain injury

Neurological disorders are a major cause of chronic disability globally among which stroke is a leading cause of chronic disability. The advances in the medical management of stroke patients over the past decade have significantly reduced mortality, but at the same time increased numbers of disabled survivors. Unfortunately, this reduction in mortality was not paralleled by satisfactory therapeutics and rehabilitation strategies that can improve functional recovery of patients. Motor recovery after brain injury is a complex, dynamic, and multifactorial process in which an interplay among genetic, pathophysiologic, sociodemographic and therapeutic factors determines the overall recovery trajectory. Although stroke recovery is the most well-studied form of post-injury neuronal recovery, a thorough understanding of the pathophysiology and determinants affecting stroke recovery is still lacking. Understanding the different variables affecting brain recovery after stroke will not only provide an opportunity to develop therapeutic interventions but also allow for developing personalized platforms for patient stratification and prognosis. We aim to provide a narrative review of major determinants for post-stroke recovery and their implications in other forms of brain injury.

Altered neurite morphology and cholinergic function of induced pluripotent stem cell-derived neurons from a patient with Kleefstra syndrome and autism

The aim of the present study was to establish an in vitro Kleefstra syndrome (KS) disease model using the human induced pluripotent stem cell (hiPSC) technology. Previously, an autism spectrum disorder (ASD) patient with Kleefstra syndrome (KS-ASD) carrying a deleterious premature termination codon mutation in the EHMT1 gene was identified. Patient specific hiPSCs generated from peripheral blood mononuclear cells of the KS-ASD patient were differentiated into post-mitotic cortical neurons. Lower levels of EHMT1 mRNA as well as protein expression were confirmed in these cells. Morphological analysis on neuronal cells differentiated from the KS-ASD patient-derived hiPSC clones showed significantly shorter neurites and reduced arborization compared to cells generated from healthy controls. Moreover, density of dendritic protrusions of neuronal cells derived from KS-ASD hiPSCs was lower than that of control cells. Synaptic connections and spontaneous neuronal activity measured by live cell calcium imaging could be detected after 5 weeks of differentiation, when KS-ASD cells exhibited higher sensitivity of calcium responses to acetylcholine stimulation indicating a lower nicotinic cholinergic tone at baseline condition in KS-ASD cells. In addition, gene expression profiling of differentiated neuronal cells from the KS-ASD patient revealed higher expression of proliferation-related genes and lower mRNA levels of genes involved in neuronal maturation and migration. Our data demonstrate anomalous neuronal morphology, functional activity and gene expression in KS-ASD patient-specific hiPSC-derived neuronal cultures, which offers an in vitro system that contributes to a better understanding of KS and potentially other neurodevelopmental disorders including ASD.

Feature Detection in Visual Cortex during Different Functional States

Cortical activity exhibits distinct characteristics in different functional states. In awake behaving animals it shows less synchrony, while in rest or sleeping state cortical activity is most synchronous. Previous studies showed that switching between functional states can change the efficiency of flowing sensory information. Switching between functional states can be triggered by releasing neuromodulators which affect neurotransmitter release probability and depolarization of cortical neurons. In this work we focus on studying primary visual area V1, by using firing rate ring model with short-term synaptic depression (STD). We show that reconstruction of visual features from V1 activity depends on the functional state, with best precision achieved at the state with intermediate release probability. We suggest that this regime corresponds to the state of maximal visual attention.

The Effects of Acute Exercise on Mood, Cognition, Neurophysiology, and Neurochemical Pathways: A Review

A significant body of work has investigated the effects of acute exercise, defined as a single bout of physical activity, on mood and cognitive functions in humans. Several excellent recent reviews have summarized these findings; however, the neurobiological basis of these results has received less attention. In this review, we will first briefly summarize the cognitive and behavioral changes that occur with acute exercise in humans. We will then review the results from both human and animal model studies documenting the wide range of neurophysiological and neurochemical alterations that occur after a single bout of exercise. Finally, we will discuss the strengths, weaknesses, and missing elements in the current literature, as well as offer an acute exercise standardization protocol and provide possible goals for future research.

Cholinergic, But Not Dopaminergic or Noradrenergic, Enhancement Sharpens Visual Spatial Perception in Humans

The neuromodulator acetylcholine modulates spatial integration in visual cortex by altering the balance of inputs that generate neuronal receptive fields. These cholinergic effects may provide a neurobiological mechanism underlying the modulation of visual representations by visual spatial attention. However, the consequences of cholinergic enhancement on visuospatial perception in humans are unknown. We conducted two experiments to test whether enhancing cholinergic signaling selectively alters perceptual measures of visuospatial interactions in human subjects. In Experiment 1, a double-blind placebo-controlled pharmacology study, we measured how flanking distractors influenced detection of a small contrast decrement of a peripheral target, as a function of target-flanker distance. We found that cholinergic enhancement with the cholinesterase inhibitor donepezil improved target detection, and modeling suggested that this was mainly due to a narrowing of the extent of facilitatory perceptual spatial interactions. In Experiment 2, we tested whether these effects were selective to the cholinergic system or would also be observed following enhancements of related neuromodulators dopamine or norepinephrine. Unlike cholinergic enhancement, dopamine (bromocriptine) and norepinephrine (guanfacine) manipulations did not improve performance or systematically alter the spatial profile of perceptual interactions between targets and distractors. These findings reveal mechanisms by which cholinergic signaling influences visual spatial interactions in perception and improves processing of a visual target among distractors, effects that are notably similar to those of spatial selective attention.SIGNIFICANCE STATEMENT Acetylcholine influences how visual cortical neurons integrate signals across space, perhaps providing a neurobiological mechanism for the effects of visual selective attention. However, the influence of cholinergic enhancement on visuospatial perception remains unknown. Here we demonstrate that cholinergic enhancement improves detection of a target flanked by distractors, consistent with sharpened visuospatial perceptual representations. Furthermore, whereas most pharmacological studies focus on a single neurotransmitter, many neuromodulators can have related effects on cognition and perception. Thus, we also demonstrate that enhancing noradrenergic and dopaminergic systems does not systematically improve visuospatial perception or alter its tuning. Our results link visuospatial tuning effects of acetylcholine at the neuronal and perceptual levels and provide insights into the connection between cholinergic signaling and visual attention.

Dose-dependent effect of donepezil administration on long-term enhancement of visually evoked potentials and cholinergic receptor overexpression in rat visual cortex

Stimulation of the cholinergic system tightly coupled with periods of visual stimulation boosts the processing of specific visual stimuli via muscarinic and nicotinic receptors in terms of intensity, priority and long-term effect. However, it is not known whether more diffuse pharmacological stimulation with donepezil, a cholinesterase inhibitor, is an efficient tool for enhancing visual processing and perception. The goal of the present study was to potentiate cholinergic transmission with donepezil treatment (0.5 and 1mg/kg) during a 2-week visual training to examine the effect on visually evoked potentials and to profile the expression of cholinergic receptor subtypes. The visual training was performed daily, 10min a day, for 2weeks. One week after the last training session, visual evoked potentials were recorded, or the mRNA expression level of muscarinic (M1-5) and nicotinic (α/β) receptors subunits was determined by quantitative RT-PCR. The visual stimulation coupled with any of the two doses of donepezil produced significant amplitude enhancement of cortical evoked potentials compared to pre-training values. The enhancement induced by the 1mg/kg dose of donepezil was spread to neighboring spatial frequencies, suggesting a better sensitivity near the visual detection threshold. The M3, M4, M5 and α7 receptors mRNA were upregulated in the visual cortex for the higher dose of donepezil but not the lower one, and the receptors expression was stable in the somatosensory (non-visual control) cortex. Therefore, higher levels of acetylcholine within the cortex sustain the increased intensity of the cortical response and trigger the upregulation of cholinergic receptors.

A Cognitive Model Based on Neuromodulated Plasticity

Associative learning, including classical conditioning and operant conditioning, is regarded as the most fundamental type of learning for animals and human beings. Many models have been proposed surrounding classical conditioning or operant conditioning. However, a unified and integrated model to explain the two types of conditioning is much less studied. Here, a model based on neuromodulated synaptic plasticity is presented. The model is bioinspired including multistored memory module and simulated VTA dopaminergic neurons to produce reward signal. The synaptic weights are modified according to the reward signal, which simulates the change of associative strengths in associative learning. The experiment results in real robots prove the suitability and validity of the proposed model.

Presynaptic D1 heteroreceptors and mGlu autoreceptors act at individual cortical release sites to modify glutamate release

The aim of this work was to study release of glutamic acid (GLU) from one-axon terminal or bouton at-a-time using cortical neurons grown in vitro to study the effect of presynaptic auto- and heteroreceptor stimulation. Neurons were infected with release reporters SypHx2 or iGluSnFR at 7 or 3 days-in-vitro (DIV) respectively. At 13-15 DIV single synaptic boutons were identified from images obtained from a confocal scanning microscope before and after field electrical stimulation. We further stimulated release by raising intracellular levels of cAMP with forskolin (10µM). Forskolin-mediated effects were dependent on protein kinase A (PKA) and did not result from an increase in endocytosis, but rather from an increase in the size of the vesicle readily releasable pool. Once iGluSnFR was confirmed as more sensitive than SypHx2, it was used to study the participation of presynaptic auto- and heteroreceptors on GLU release. Although most receptor agonizts (carbamylcholine, nicotine, dopamine D2, BDNF) did not affect electrically stimulated GLU release, a significant increase was observed in the presence of metabotropic D1/D5 heteroreceptor agonist (SKF38393 10µM) that was reversed by PKA inhibitors. Interestingly, stimulation of group II metabotropic mGLU2/3 autoreceptors (LY379268 50nM) induced a decrease in GLU release that was reversed by the specific mGLU2/3 receptor antagonist (LY341495 1µM) and also by PKA inhibitors (KT5720 200nM and PKI14-22 400nM). These changes in release probability at individual release sites suggest another level of control of the distribution of transmitter substances in cortical tissue.

Calcium regulation of HCN channels supports persistent activity in a multiscale model of neocortex

Neuronal persistent activity has been primarily assessed in terms of electrical mechanisms, without attention to the complex array of molecular events that also control cell excitability. We developed a multiscale neocortical model proceeding from the molecular to the network level to assess the contributions of calcium (Ca(2+)) regulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in providing additional and complementary support of continuing activation in the network. The network contained 776 compartmental neurons arranged in the cortical layers, connected using synapses containing AMPA/NMDA/GABAA/GABAB receptors. Metabotropic glutamate receptors (mGluR) produced inositol triphosphate (IP3) which caused the release of Ca(2+) from endoplasmic reticulum (ER) stores, with reuptake by sarco/ER Ca(2+)-ATP-ase pumps (SERCA), and influence on HCN channels. Stimulus-induced depolarization led to Ca(2+) influx via NMDA and voltage-gated Ca(2+) channels (VGCCs). After a delay, mGluR activation led to ER Ca(2+) release via IP3 receptors. These factors increased HCN channel conductance and produced firing lasting for ∼1min. The model displayed inter-scale synergies among synaptic weights, excitation/inhibition balance, firing rates, membrane depolarization, Ca(2+) levels, regulation of HCN channels, and induction of persistent activity. The interaction between inhibition and Ca(2+) at the HCN channel nexus determined a limited range of inhibition strengths for which intracellular Ca(2+) could prepare population-specific persistent activity. Interactions between metabotropic and ionotropic inputs to the neuron demonstrated how multiple pathways could contribute in a complementary manner to persistent activity. Such redundancy and complementarity via multiple pathways is a critical feature of biological systems. Mediation of activation at different time scales, and through different pathways, would be expected to protect against disruption, in this case providing stability for persistent activity.

Excitatory/Inhibitory Balance and Circuit Homeostasis in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) and related neurological disorders are associated with mutations in many genes affecting the ratio between neuronal excitation and inhibition. However, understanding the impact of these mutations on network activity is complicated by the plasticity of these networks, making it difficult in many cases to separate initial deficits from homeostatic compensation. Here we explore the contrasting evidence for primary defects in inhibition or excitation in ASDs and attempt to integrate the findings in terms of the brain's ability to maintain functional homeostasis.

Pattern separation, completion, and categorisation in the hippocampus and neocortex

The mechanisms for pattern completion and pattern separation are described in the context of a theory of hippocampal function in which the hippocampal CA3 system operates as a single attractor or autoassociation network to enable rapid, one-trial, associations between any spatial location (place in rodents, or spatial view in primates) and an object or reward, and to provide for completion of the whole memory during recall from any part. The factors important in the pattern completion in CA3 and also a large number of independent memories stored in CA3 include: a sparse distributed representation, representations that are independent due to the randomizing effect of the mossy fibres, heterosynaptic long-term depression as well as long-term potentiation in the recurrent collateral synapses, and diluted connectivity to minimize the number of multiple synapses between any pair of CA3 neurons which otherwise distort the basins of attraction. Recall of information from CA3 is implemented by the entorhinal cortex perforant path synapses to CA3 cells, which in acting as a pattern associator allow some pattern generalization. Pattern separation is performed in the dentate granule cells using competitive learning to convert grid-like entorhinal cortex firing to place-like fields, and in the dentate to CA3 connections that have diluted connectivity. Recall to the neocortex is achieved by a reverse hierarchical series of pattern association networks implemented by the hippocampo-cortical backprojections, each one of which performs some pattern generalization, to retrieve a complete pattern of cortical firing in higher-order cortical areas. New results on competitive networks show which factors contribute to their ability to perform pattern separation, pattern clustering, and pattern categorisation, and how these apply in different hippocampal and neocortical systems.

Impact of basal forebrain cholinergic inputs on basolateral amygdala neurons

In addition to innervating the cerebral cortex, basal forebrain cholinergic (BFc) neurons send a dense projection to the basolateral nucleus of the amygdala (BLA). In this study, we investigated the effect of near physiological acetylcholine release on BLA neurons using optogenetic tools and in vitro patch-clamp recordings. Adult transgenic mice expressing cre-recombinase under the choline acetyltransferase promoter were used to selectively transduce BFc neurons with channelrhodopsin-2 and a reporter through the injection of an adeno-associated virus. Light-induced stimulation of BFc axons produced different effects depending on the BLA cell type. In late-firing interneurons, BFc inputs elicited fast nicotinic EPSPs. In contrast, no response could be detected in fast-spiking interneurons. In principal BLA neurons, two different effects were elicited depending on their activity level. When principal BLA neurons were quiescent or made to fire at low rates by depolarizing current injection, light-induced activation of BFc axons elicited muscarinic IPSPs. In contrast, with stronger depolarizing currents, eliciting firing above ∼ 6-8 Hz, these muscarinic IPSPs lost their efficacy because stimulation of BFc inputs prolonged current-evoked afterdepolarizations. All the effects observed in principal neurons were dependent on muscarinic receptors type 1, engaging different intracellular mechanisms in a state-dependent manner. Overall, our results suggest that acetylcholine enhances the signal-to-noise ratio in principal BLA neurons. Moreover, the cholinergic engagement of afterdepolarizations may contribute to the formation of stimulus associations during fear-conditioning tasks where the timing of conditioned and unconditioned stimuli is not optimal for the induction of synaptic plasticity.

Transition between encoding and consolidation/replay dynamics via cholinergic modulation of CAN current: A modeling study

Hippocampal place cells that are activated sequentially during active waking get reactivated in a temporally compressed (5-20 times) manner during slow-wave-sleep and quiet waking. The two-stage model of the hippocampus suggests that neural activity during awaking supports encoding function while temporally compressed reactivation (replay) supports consolidation. However, the mechanisms supporting different neural activity with different temporal scales during encoding and consolidation remain unclear. Based on the idea that acetylcholine modulates functional transition between encoding and consolidation, we tested whether the cholinergic modulation may adjust intrinsic network dynamics to support different temporal scales for these two modes of operation. Simulations demonstrate that cholinergic modulation of the calcium activated non-specific cationic (CAN) current and the synaptic transmission may be sufficient to switch the network dynamics between encoding and consolidation modes. When the CAN current is active and the synaptic transmission is suppressed, mimicking the high acetylcholine condition during active waking, a slow propagation of multiple spikes is evident. This activity resembles the firing pattern of place cells and time cells during active waking. On the other hand, when CAN current is suppressed and the synaptic transmission is intact, mimicking the low acetylcholine condition during slow-wave-sleep, a time compressed fast (∼10 times) activity propagation of the same set of cells is evident. This activity resembles the time compressed firing pattern of place cells during replay and pre-play, achieving a temporal compression factor in the range observed in vivo (5-20 times). These observations suggest that cholinergic system could adjust intrinsic network dynamics suitable for encoding and consolidation through the modulation of the CAN current and synaptic conductance in the hippocampus.

Stochastic cortical neurodynamics underlying the memory and cognitive changes in aging

The relatively random spiking times of individual neurons provide a source of noise in the brain. We show how this noise interacting with altered depth in the basins of attraction of networks involved in short-term memory, attention, and episodic memory provide an approach to understanding some of the cognitive changes in normal aging. The effects of the neurobiological changes in aging that are considered include reduced synaptic modification and maintenance during learning produced in part through reduced acetylcholine in normal aging, reduced dopamine which reduces NMDA-receptor mediated effects, reduced noradrenaline which increases cAMP and thus shunts excitatory synaptic inputs, and the effects of a reduction in acetylcholine in increasing spike frequency adaptation. Using integrate-and-fire simulations of an attractor network implementing memory recall and short-term memory, it is shown that all these changes associated with aging reduce the firing rates of the excitatory neurons, which in turn reduce the depth of the basins of attraction, resulting in a much decreased probability in maintaining in short-term memory what has been recalled from the attractor network. This stochastic dynamics approach opens up new ways to understand and potentially treat the effects of normal aging on memory and cognitive functions.

Activation of muscarinic receptors by ACh release in hippocampal CA1 depolarizes VIP but has varying effects on parvalbumin-expressing basket cells

KEY POINTS

Optogenetically released acetylcholine (ACh) from medial septal afferents activates muscarinic receptors on both vasoactive intestinal peptide-expressing (VIP) and parvalbumin-expressing (PV) basket cells (BCs) in mouse hippocampal CA1. ACh release depolarized VIP BCs whereas PV BCs depolarized, hyperpolarized or produced biphasic responses. Depolarizing responses in VIP or PV BCs resulted in increased amplitudes and frequencies of spontaneous inhibitory postsynaptic currents (sIPSCs) in CA1 pyramidal neurons. The instantaneous frequency of sIPSCs that result from excitation of VIP or PV BCs primarily occurred within the low gamma frequency band (25-50 Hz). We investigated the effect of acetylcholine release on mouse hippocampal CA1 perisomatically projecting interneurons. Acetylcholine was optogenetically released in hippocampal slices by expressing the excitatory optogenetic protein oChIEF-tdTomato in medial septum/diagonal band of Broca cholinergic neurons using Cre recombinase-dependent adeno-associated virally mediated transfection. The effect of optogenetically released acetylcholine was assessed on interneurons expressing Cre recombinase in vasoactive intestinal peptide (VIP) or parvalbumin (PV) interneurons using whole cell patch clamp methods. Acetylcholine released onto VIP interneurons that innervate pyramidal neuron perisomatic regions (basket cells, BCs) were depolarized by muscarinic receptors. Although PV BCs were also excited by muscarinic receptor activation, they more frequently responded with hyperpolarizing or biphasic responses. Muscarinic receptor activation resulting from ACh release increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in downstream hippocampal CA1 pyramidal neurons with peak instantaneous frequencies occurring in both the gamma and theta bandwidths. Both PV and VIP BCs contributed to the increased sIPSC frequency in pyramidal neurons and optogenetic suppression of PV or VIP BCs inhibited sIPSCs occurring in the gamma range. Therefore, we propose acetylcholine release in CA1 has a complex effect on CA1 pyramidal neuron output through varying effects on perisomatically projecting interneurons.

Nicotinic α7 receptor activation selectively potentiates the function of NMDA receptors in glutamatergic terminals of the nucleus accumbens

We here provide functional and immunocytochemical evidence supporting the co-localization and functional interaction between nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartic acid receptors (NMDARs) in glutamatergic terminals of the nucleus accumbens (NAc). Immunocytochemical studies showed that a significant percentage of NAc terminals were glutamatergic and possessed GluN1 and α7-containing nAChR. A short-term pre-exposure of synaptosomes to nicotine (30 µM) or choline (1 mM) caused a significant potentiation of the 100 µM NMDA-evoked [³H]D-aspartate ([³H]D-Asp) outflow, which was prevented by α-bungarotoxin (100 nM). The pre-exposure to nicotine (100 µM) or choline (1 mM) also enhanced the NMDA-induced cytosolic free calcium levels, as measured by FURA-2 fluorescence imaging in individual NAc terminals, an effect also prevented by α-bungarotoxin. Pre-exposure to the α4-nAChR agonists 5IA85380 (10 nM) or RJR2429 (1 µM) did not modify NMDA-evoked ([³H]D-Asp) outflow and calcium transients. The NMDA-evoked ([³H]D-Asp) overflow was partially antagonized by the NMDAR antagonists MK801, D-AP5, 5,7-DCKA and R(-)CPP and unaffected by the GluN2B-NMDAR antagonists Ro256981 and ifenprodil. Notably, pre-treatment with choline increased GluN2A biotin-tagged proteins. In conclusion, our results show that the GluN2A-NMDA receptor function can be positively regulated in NAc terminals in response to a brief incubation with α7 but not α4 nAChRs agonists. This might be a general feature in different brain areas since a similar nAChR-mediated bolstering of NMDA-induced ([³H]D-Asp) overflow was also observed in hippocampal synaptosomes.

Modulation of neuronal microcircuit activities within the medial prefrontal cortex by mGluR5 positive allosteric modulator

Suppressing anxiety and fear memory relies on bidirectional projections between the medial prefrontal cortex and the amygdala. Positive allosteric modulators of mGluR5 improve cognition in animal models of schizophrenia and retrieval of newly formed associations such as extinction of fear-conditioned behaviour. The increase in neuronal network activities of the medial prefrontal cortex is influenced by both mGluR1 and mGluR5; however, it is not well understood how they modulate network activities and downstream information processing. To map mGluR5-mediated network activity in relation to its emergence as a viable cognitive enhancer, we tested group I mGluR compounds on medial prefrontal cortex network activity via multi-electrode array neuronal spiking and whole-cell patch clamp recordings. Results indicate that mGluR5 activation promotes feed-forward inhibition that depends on recruitment of neuronal activity by carbachol-evoked up states. The rate of neuronal spiking activity under the influence of carbachol was reduced by the mGluR5 positive allosteric modulator, N-(1,3-Diphenyl-1H-pyrazolo-5-yl)-4-nitrobenzamide (VU-29), and enhanced by the mGluR5 negative allosteric modulator, 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride (MTEP). Spontaneous inhibitory post-synaptic currents were increased upon application of carbachol and in combination with VU-29. These results emphasize a bias towards tonic mGluR5-mediated inhibition that might serve as a signal-to-noise enhancer of sensory inputs projected from associated limbic areas onto the medial prefrontal cortex neuronal microcircuit.

Acetylcholine release and inhibitory interneuron activity in hippocampal CA1

Acetylcholine release in the central nervous system (CNS) has an important role in attention, recall, and memory formation. One region influenced by acetylcholine is the hippocampus, which receives inputs from the medial septum and diagonal band of Broca complex (MS/DBB). Release of acetylcholine from the MS/DBB can directly affect several elements of the hippocampus including glutamatergic and GABAergic neurons, presynaptic terminals, postsynaptic receptors, and astrocytes. A significant portion of acetylcholine's effect likely results from the modulation of GABAergic inhibitory interneurons, which have crucial roles in controlling excitatory inputs, synaptic integration, rhythmic coordination of principal neurons, and outputs in the hippocampus. Acetylcholine affects interneuron function in large part by altering their membrane potential via muscarinic and nicotinic receptor activation. This minireview describes recent data from mouse hippocampus that investigated changes in CA1 interneuron membrane potentials following acetylcholine release. The interneuron subtypes affected, the receptor subtypes activated, and the potential outcome on hippocampal CA1 network function is discussed.

Acetylcholine functionally reorganizes neocortical microcircuits

Sensory information is processed and transmitted through the synaptic structure of local cortical circuits, but it is unclear how modulation of this architecture influences the cortical representation of sensory stimuli. Acetylcholine (ACh) promotes attention and arousal and is thought to increase the signal-to-noise ratio of sensory input in primary sensory cortices. Using high-speed two-photon calcium imaging in a thalamocortical somatosensory slice preparation, we recorded action potential activity of up to 900 neurons simultaneously and compared local cortical circuit activations with and without bath presence of ACh. We found that ACh reduced weak pairwise relationships and excluded neurons that were already unreliable during circuit activity. Using action potential activity from the imaged population, we generated functional wiring diagrams based on the statistical dependencies of activity between neurons. ACh pruned weak functional connections from spontaneous circuit activations and yielded a more modular and hierarchical circuit structure, which biased activity to flow in a more feedforward fashion. Neurons that were active in response to thalamic input had reduced pairwise dependencies overall, but strong correlations were conserved. This coincided with a prolonged period during which neurons showed temporally precise responses to thalamic input. Our results demonstrate that ACh reorganizes functional circuit structure in a manner that may enhance the integration and discriminability of thalamic afferent input within local neocortical circuitry.

Muscarinic acetylcholine receptors are expressed by most parvalbumin-immunoreactive neurons in area MT of the macaque

BACKGROUND

In the mammalian neocortex, cells that express parvalbumin (PV neurons) comprise a dominant class of inhibitory neuron that substantially overlaps with the fast/narrow-spiking physiological phenotype. Attention has pronounced effects on narrow-spiking neurons in the extrastriate cortex of macaques, and more consistently so than on their broad-spiking neighbors. Cortical neuromodulation by acetylcholine (ACh) is a candidate mechanism for aspects of attention and in the primary visual cortex (V1) of the macaque, receptors for ACh (AChRs) are strongly expressed by inhibitory neurons. In particular, most PV neurons in macaque V1 express m1 muscarinic AChRs and exogenously applied ACh can cause the release of γ-aminobutyric acid. In contrast, few PV neurons in rat V1 express m1 AChRs. While this could be a species difference, it has also been argued that macaque V1 is anatomically unique when compared with other cortical areas in macaques.

AIMS

The aim of this study was to better understand the extent to which V1 offers a suitable model circuit for cholinergic anatomy in the macaque occipital lobe, and to explore cholinergic modulation as a biological basis for the changes in circuit behavior seen with attention.

MATERIALS AND METHODS

We compared expression of m1 AChRs by PV neurons between area V1 and the middle temporal visual area (MT) in macaque monkeys using dual-immunofluorescence confocal microscopy.

RESULTS AND CONCLUSION

We find that, as in V1, most PV neurons in MT express m1 AChRs but, unlike in V1, it appears that so do most excitatory neurons. This provides support for V1 as a model of cholinergic modulation of inhibition in macaque visual cortex, but not of cholinergic modulation of visual cortical circuits in general. We also propose that ACh acting via m1 AChRs is a candidate underlying mechanism for the strong effects of attention on narrow-spiking neurons observed in behaving animals.