Brain and cognitive changes in patients with long COVID compared with infection-recovered control subjects.

Between 2.5% and 28% of people infected with SARS-CoV-2 suffer long COVID or persistence of symptoms for months after acute illness. Many symptoms are neurological, but the brain changes underlying the neuropsychological impairments remain unclear. This study aimed to provide a detailed description of the cognitive profile, the pattern of brain alterations in long COVID and the potential association between them. To address these objectives, 83 patients with persistent neurological symptoms after COVID-19 were recruited, and 22 now healthy control subjects chosen because they had suffered COVID-19 but did not experience persistent neurological symptoms. Patients and controls were matched for age, sex and educational level. All participants were assessed by clinical interview, comprehensive standardized neuropsychological tests and structural MRI. The mean global cognitive function of patients with long COVID assessed by Addenbrooke's Cognitive Examination-III screening test [overall cognitive level (OCLz) = -0.39 ± 0.12] was significantly below the infection recovered-controls (OCLz = +0.32 ± 0.16, P < 0.01). We observed that 48% of patients with long COVID had episodic memory deficit, with 27% also with impaired overall cognitive function, especially attention, working memory, processing speed and verbal fluency. The MRI examination included grey matter morphometry and whole brain structural connectivity analysis. Compared to infection recovered controls, patients had thinner cortex in a specific cluster centred on the left posterior superior temporal gyrus. In addition, lower fractional anisotropy and higher radial diffusivity were observed in widespread areas of the patients' cerebral white matter relative to these controls. Correlations between cognitive status and brain abnormalities revealed a relationship between altered connectivity of white matter regions and impairments of episodic memory, overall cognitive function, attention and verbal fluency. This study shows that patients with neurological long COVID suffer brain changes, especially in several white matter areas, and these are associated with impairments of specific cognitive functions.

Cell-type-specific optogenetic stimulation of the locus coeruleus induces slow-onset potentiation and enhances everyday memory in rats.

Memory formation is typically divided into phases associated with encoding, storage, consolidation, and retrieval. The neural determinants of these phases are thought to differ. This study first investigated the impact of the experience of novelty in rats incurred at a different time, before or after, the precise moment of memory encoding. Memory retention was enhanced. Optogenetic activation of the locus coeruleus mimicked this enhancement induced by novelty, both when given before and after the moment of encoding. Optogenetic activation of the locus coeruleus also induced a slow-onset potentiation of field potentials in area CA1 of the hippocampus evoked by CA3 stimulation. Despite the locus coeruleus being considered a primarily noradrenergic area, both effects of such stimulation were blocked by the dopamine D1/D5 receptor antagonist SCH 23390. These findings substantiate and enrich the evidence implicating the locus coeruleus in cellular aspects of memory consolidation in hippocampus.

Novelty-induced memory consolidation is accompanied by increased Agap3 transcription: a cross-species study.

Novelty-induced memory consolidation is a well-established phenomenon that depends on the activation of a locus coeruleus-hippocampal circuit. It is associated with the expression of activity-dependent genes that may mediate initial or cellular memory consolidation. Several genes have been identified to date, however, to fully understand the mechanisms of memory consolidation, additional candidates must be identified. In this cross-species study, we used a contextual novelty-exploration paradigm to identify changes in gene expression in the dorsal hippocampus of both mice and rats. We found that changes in gene expression following contextual novelty varied between the two species, with 9 genes being upregulated in mice and 3 genes in rats. Comparison across species revealed that ArfGAP with a GTPase domain, an ankyrin repeat and PH domain 3 (Agap3) was the only gene being upregulated in both, suggesting a potentially conserved role for Agap3. AGAP3 is known to regulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor trafficking in the synapse, which suggests that increased transcription of Agap3 may be involved in maintaining functional plasticity. While we identified several genes affected by contextual novelty exploration, we were unable to fully reverse these changes using SCH 23390, a dopamine D1/D5 receptor antagonist. Further research on the role of AGAP3 in novelty-induced memory consolidation could lead to better understanding of this process and guide future research.

Execution of new trajectories toward a stable goal without a functional hippocampus.

The hippocampus is a critical component of a mammalian spatial navigation system, with the firing sequences of hippocampal place cells during sleep or immobility constituting a "replay" of an animal's past trajectories. A novel spatial navigation task recently revealed that such "replay" sequences of place fields can also prospectively map onto imminent new paths to a goal that occupies a stable location during each session. It was hypothesized that such "prospective replay" sequences may play a causal role in goal-directed navigation. In the present study, we query this putative causal role in finding only minimal effects of muscimol-induced inactivation of the dorsal and intermediate hippocampus on the same spatial navigation task. The concentration of muscimol used demonstrably inhibited hippocampal cell firing in vivo and caused a severe deficit in a hippocampal-dependent "episodic-like" spatial memory task in a watermaze. These findings call into question whether "prospective replay" of an imminent and direct path is actually necessary for its execution in certain navigational tasks.

Distinct contributions of ventral CA1/amygdala co-activation to the induction and maintenance of synaptic plasticity.

The amygdala is known to modulate hippocampal synaptic plasticity. One role could be an immediate effect of basolateral amygdala (BLA) in priming synaptic plasticity in the hippocampus. Another role could be through associative synaptic co-operation and competition that triggers events involved in the maintenance of synaptic potentiation. We present evidence that the timing and activity level of BLA stimulation are important factors for the induction and maintenance of long-term potentiation (LTP) in ventral hippocampal area CA1. A 100 Hz BLA co-stimulation facilitated the induction of LTP, whereas 200 Hz co-stimulation attenuated induction. A 100 Hz BLA co-stimulation also caused enhanced persistence, sufficient to prevent synaptic competition. This maintenance effect is likely through translational mechanisms, as mRNA expression of primary response genes was unaffected, whereas protein level of plasticity-related products was increased. Further understanding of the neural mechanisms of amygdala modulation on hippocampus could provide insights into the mechanisms of emotional disorders.

Sleep deprivation and hippocampal ripple disruption after one-session learning eliminate memory expression the next day.

Memory reactivation during non-rapid-eye-movement ripples is thought to communicate new information to a systems-wide network and thus can be a key player mediating the positive effect of sleep on memory consolidation. Causal experiments disrupting ripples have only been performed in multiday training paradigms, which decrease but do not eliminate memory performance, and no comparison with sleep deprivation has been made. To enable such investigations, we developed a one-session learning paradigm in a Plusmaze and show that disruption of either sleep with gentle handling or hippocampal ripples with electrical stimulation impaired long-term memory. Furthermore, we detected hippocampal ripples and parietal high-frequency oscillations after different behaviors, and a bimodal frequency distribution in the cortical events was observed. Faster cortical high-frequency oscillations increased after normal learning, a change not seen in the hippocampal ripple-disruption condition, consistent with these having a role in memory consolidation.

Neuronal signature of spatial decision-making during navigation by freely moving rats by using calcium imaging.

A challenge in spatial memory is understanding how place cell firing contributes to decision-making in navigation. A spatial recency task was created in which freely moving rats first became familiar with a spatial context over several days and thereafter were required to encode and then selectively recall one of three specific locations within it that was chosen to be rewarded that day. Calcium imaging was used to record from more than 1,000 cells in area CA1 of the hippocampus of five rats during the exploration, sample, and choice phases of the daily task. The key finding was that neural activity in the startbox rose steadily in the short period prior to entry to the arena and that this selective population cell firing was predictive of the daily changing goal on correct trials but not on trials in which the animals made errors. Single-cell and population activity measures converged on the idea that prospective coding of neural activity can be involved in navigational decision-making.

Ca2+ imaging of self and other in medial prefrontal cortex during social dominance interactions in a tube test.

The study of social dominance interactions between animals offers a window onto the decision-making involved in establishing dominance hierarchies and an opportunity to examine changes in social behavior observed in certain neurogenetic disorders. Competitive social interactions, such as in the widely used tube test, reflect this decision-making. Previous studies have focused on the different patterns of behavior seen in the dominant and submissive animal, neural correlates of effortful behavior believed to mediate the outcome of such encounters, and interbrain correlations of neural activity. Using a rigorous mutual information criterion, we now report that neural responses recorded with endoscopic calcium imaging in the prelimbic zone of the medial prefrontal cortex show unique correlations to specific dominance-related behaviors. Interanimal analyses revealed cell/behavior correlations that are primarily with an animal's own behavior or with the other animal's behavior, or the coincident behavior of both animals (such as pushing by one and resisting by the other). The comparison of unique and coincident cells helps to disentangle cell firing that reflects an animal's own or the other's specific behavior from situations reflecting conjoint action. These correlates point to a more cognitive rather than a solely behavioral dimension of social interactions that needs to be considered in the design of neurobiological studies of social behavior. These could prove useful in studies of disorders affecting social recognition and social engagement, and the treatment of disorders of social interaction.

Brain region networks for the assimilation of new associative memory into a schema.

Alterations in long-range functional connectivity between distinct brain regions are thought to contribute to the encoding of memory. However, little is known about how the activation of an existing network of neocortical and hippocampal regions might support the assimilation of relevant new information into the preexisting knowledge structure or 'schema'. Using functional mapping for expression of plasticity-related immediate early gene products, we sought to identify the long-range functional network of paired-associate memory, and the encoding and assimilation of relevant new paired-associates. Correlational and clustering analyses for expression of immediate early gene products revealed that midline neocortical-hippocampal connectivity is strongly associated with successful memory encoding of new paired-associates against the backdrop of the schema, compared to both (1) unsuccessful memory encoding of new paired-associates that are not relevant to the schema, and (2) the mere retrieval of the previously learned schema. These findings suggest that the certain midline neocortical and hippocampal networks support the assimilation of newly encoded associative memories into a relevant schema.

A Behavioral Task Modeling 'Everyday Memory' in an Event Arena to Foster Allocentric Representations for Rodents.

The event arena provides an optimal platform to investigate learning and memory. The appetitive everyday memory task described in this paper provides a robust protocol for the investigation of episodic and spatial memory in rodents, which specifically fosters allocentric memory representation. Rats are trained to find and dig for food during the encoding phase and, after a time delay, rats are given a choice to find the reward food pellet in the correct location. There are two key elements that promote the use of an allocentric strategy in this protocol: 1) rats start from different start locations within and between sessions, 2) a stable home-base is deployed where rats have to carry their food to eat. By means of these modifications, we effectively encourage the rodents to use allocentric spatial representations to perform the task. In addition, the task provides a good paradigm for within-subject experimental design and allows experimenters to manipulate different conditions to reduce variability. Used in conjunction with behavioral and physiological techniques, the resulting rodent model provides an effective test-bed for future research into memory formation and retention.

Experiential modulation of social dominance in a SYNGAP1 rat model of Autism Spectrum Disorders.

Advances in the understanding of developmental brain disorders such as autism spectrum disorders (ASDs) are being achieved through human neurogenetics such as, for example, identifying de novo mutations in SYNGAP1 as one relatively common cause of ASD. A recently developed rat line lacking the calcium/lipid binding (C2) and GTPase activation protein (GAP) domain may further help uncover the neurobiological basis of deficits in children with ASD. This study focused on social dominance in the tube test using Syngap+/Δ-GAP (rats heterozygous for the C2/GAP domain deletion) as alterations in social behaviour are a key facet of the human phenotype. Male animals of this line living together formed a stable intra-cage hierarchy, but they were submissive when living with wild-type (WT) cage-mates, thereby modelling the social withdrawal seen in ASD. The study includes a detailed analysis of specific behaviours expressed in social interactions by WT and mutant animals, including the observation that when the Syngap+/Δ-GAP mutants that had been living together had separate dominance encounters with WT animals from other cages, the two higher ranking Syngap+/Δ-GAP rats remained dominant whereas the two lower ranking mutants were still submissive. Although only observed in a small subset of animals, these findings support earlier observations with a rat model of Fragile X, indicating that their experience of winning or losing dominance encounters has a lasting influence on subsequent encounters with others. Our results highlight and model that even with single-gene mutations, dominance phenotypes reflect an interaction between genotypic and environmental factors.

iHELMET: A 3D-printing solution for safe endoscopic Ca2+ recording in social neuroscience.

BACKGROUND: In vivo calcium imaging using a microendoscope is a state-of-the-art technique to study the cellular activity inside the brain of freely moving animals such as mice or rats. A problem that can arise in social behaviour tests in rats, or similar size rodents, is that one animal interferes with or may even damage the miniature endoscopic camera attached to the second animal. NEW METHOD: We outline an inexpensive, lightweight, 3D-printed protector (iHELMET) that surrounds but is not in physical contact with the camera, together with details of its design and construction. RESULTS: Using a simple design, we demonstrate successful protection of the endoscope and recording in a social situation such as the social dominance tube test. COMPARISON WITH EXISTING METHODS: The helmet's 3D-printed dimensions can be readily adjusted to work with various micro-endoscopes, which may be more difficult for the only other system of which we are aware. CONCLUSIONS: In addition to camera protection, features of the design aid camera stability, helping to secure more optimal imaging of calcium transients in specific regions of interest during long recording sessions.

A stable home-base promotes allocentric memory representations of episodic-like everyday spatial memory.

A key issue in neurobiological studies of episodic-like memory is the geometric frame of reference in which memory traces of experience are stored. Assumptions are sometimes made that specific protocols favour either allocentric (map-like) or egocentric (body-centred) representations. There are, however, grounds for suspecting substantial ambiguity about coding strategy, including the necessity to use both frames of reference occasionally, but tests of memory representation are not routinely conducted. Using rats trained to find and dig up food in sandwells at a particular place in an event arena (episodic-like 'action-where' encoding), we show that a protocol previously thought to foster allocentric encoding is ambiguous but more predisposed towards egocentric encoding. Two changes in training protocol were examined with a view to promoting preferential allocentric encoding-one in which multiple start locations were used within a session as well as between sessions; and another that deployed a stable home-base to which the animals had to carry food reward. Only the stable home-base protocol led to excellent choice performance which rigorous analyses revealed to be blocked by occluding extra-arena cues when this was done after encoding but before recall. The implications of these findings for studies of episodic-like memory are that the representational framework of memory at the start of a recall trial will likely include a path direction in the egocentric case but path destination in the allocentric protocol. This difference should be observable in single-unit recording or calcium-imaging studies of spatially-tuned cells.

Frontal and Insular Input to the Dorsolateral Temporal Pole in Primates: Implications for Auditory Memory.

The temporal pole (TP) has been involved in multiple functions from emotional and social behavior, semantic processing, memory, language in humans and epilepsy surgery, to the fronto-temporal neurodegenerative disorder (semantic) dementia. However, the role of the TP subdivisions is still unclear, in part due to the lack of quantitative data about TP connectivity. This study focuses in the dorsolateral subdivision of the TP: area 38DL. Area 38DL main input originates in the auditory processing areas of the rostral superior temporal gyrus. Among other connections, area 38DL conveys this auditory highly processed information to the entorhinal, rostral perirhinal, and posterior parahippocampal cortices, presumably for storage in long-term memory (Muñoz-López et al., 2015). However, the connections of the TP with cortical areas beyond the temporal cortex suggest that this area is part of a wider network. With the aim to quantitatively determine the topographical, laminar pattern and weighting of the lateral TP afferents from the frontal and insular cortices, we placed a total of 11 tracer injections of the fluorescent retrograde neuronal tracers Fast Blue and Diamidino Yellow at different levels of the lateral TP in rhesus monkeys. The results showed that circa 50% of the total cortical input to area 38DL originates in medial frontal areas 14, 25, 32, and 24 (25%); orbitofrontal areas Pro and PAll (15%); and the agranular, parainsular and disgranular insula (10%). This study sets the anatomical bases to better understand the function of the dorsolateral division of the TP. More specifically, these results suggest that area 38DL forms part of the wider limbic circuit that might contribute, among other functions, with an auditory component to multimodal memory processing.

In memoriam: John Lisman - commentaries on CaMKII as a memory molecule.

Shortly before he died in October 2017, John Lisman submitted an invited review to Molecular Brain on 'Criteria for identifying the molecular basis of the engram (CaMKII, PKMζ)'. John had no opportunity to read the referees' comments, and as a mark of the regard in which he was held by the neuroscience community the Editors decided to publish his review as submitted. This obituary takes the form of a series of commentaries on Lisman's review. At the same time we are publishing as a separate article a longer response by Todd Sacktor and André Fenton entitled 'What does LTP tell us about the roles of CaMKII and PKMζ in memory?' which presents the case for a rival memory molecule, PKMζ.

Silent Learning.

We introduce the concept of "silent learning"-the capacity to learn despite neuronal cell-firing being largely absent. This idea emerged from thinking about dendritic computation [1, 2] and examining whether the encoding, expression, and retrieval of hippocampal-dependent memory could be dissociated using the intrahippocampal infusion of pharmacological compounds. We observed that very modest enhancement of GABAergic inhibition with low-dose muscimol blocked both cell-firing and the retrieval of an already-formed memory but left induction of long-term potentiation (LTP) and new spatial memory encoding intact (silent learning). In contrast, blockade of hippocampal NMDA receptors by intrahippocampal D-AP5 impaired both the induction of LTP and encoding but had no effect on memory retrieval. Blockade of AMPA receptors by CNQX impaired excitatory synaptic transmission and cell-firing and both memory encoding and retrieval. Thus, in keeping with the synaptic plasticity and memory hypothesis [3], the hippocampal network can mediate new memory encoding when LTP induction is intact even under conditions in which somatic cell-firing is blocked.

Memory, Novelty and Prior Knowledge.

Over the 40 years that TINS has been in existence, there has been substantial progress in understanding the types, organisation, and neural mechanisms of memory. The selectivity of memory maintenance and retention remains a puzzle, and we here summarise two contributions of our own research to this enigma: the striking impact of the novelty and surprise often of other events happening around the time that a new memory is encoded and how activated prior knowledge guides the updating process that characterises aspects of memory consolidation.

Memory: Looking back and looking forward.

This review brings together past and present achievements in memory research, ranging from molecular to psychological discoveries. Despite some false starts, major advances include our growing understanding of learning-related neural plasticity and the characterisation of different classes of memory. One striking example is the ability to reactivate targeted neuronal ensembles so that an animal will seemingly re-experience a particular memory, with the further potential to modify such memories. Meanwhile, human functional imaging studies can distinguish individual episodic memories based on voxel activation patterns. While the hippocampus continues to provide a rich source of information, future progress requires broadening our research to involve other sites. Related challenges include the need to understand better the role of glial-neuron interactions and to look beyond the synapse as the sole site of experience-dependent plasticity. Unmet goals include translating our neuroscientific knowledge in order to optimise learning and memory, especially among disadvantaged populations.