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 controls 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 ACE 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 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 (FA) and higher radial diffusivity (RD) 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.

The CA2 hippocampal subfield in humans: A review.

CA2 is probably the most enigmatic of the hippocampal fields. It is small in size (in humans about 500 µm across the mediolateral axis), and yet, it is involved in important functions, such as in social memory and anxiety. This study offers a glimpse of several significant aspects of the anatomical organization of CA2. We present an overview of the anatomical structure of CA2, imbued in the general organization of the human hippocampal formation. The location and distinctiveness of CA2 is presented in relation with CA3 and CA1, based in a total of 23 human control cases serially sectioned throughout the whole longitudinal axis of the hippocampus, examined every 500 µm in Nissl-stained sections. The longitudinal extent of CA2 is close to 30 mm, starting in the hippocampal head, 2.5 mm caudal to the DG and 3.5 mm caudal to the start of CA3, approximately 10 mm from the hippocampus rostral end. The connectional information of human CA2 is very scarce, thereby we relied on nonhuman primate tract tracing studies of the hippocampal formation, given its resemblance to the human brain. Human CA2 is subject of neuropathological studies, and we chose to present Alzheimer's disease, schizophrenia, and Mesial Temporal Lobe Epilepsy with hippocampal sclerosis in those aspects that impinge directly into CA2.

Study of the human hippocampal formation: a method for histological and magnetic resonance correlation in perinatal cases.

Little information is available on the magnetic resonance imaging (MRI) determination of the hippocampal formation (HF) during the perinatal period. However, this exploration is increasingly used, which requires defining visible HF landmarks on MRI images, validated through histological analysis. This study aims to provide a protocol to identify HF landmarks on MRI images, followed by histological validation through serial sections of the temporal lobe of the samples examined, to assess the longitudinal extent of the hippocampus during the perinatal period. We examined ex vivo MRI images from nine infant control brain samples. Histological validation of the hippocampal formation MRI images was obtained through serial sectioning and examination of Nissl-stained sections at 250 µm intervals along the entire length of the hippocampal formation. Up to six landmarks were identified both in MRI images and the serial histological sections. Proceeding in an anterior to posterior (rostrocaudal) direction, these were as follows: 1) the limen insulae (fronto-temporal junction); 2) the beginning of the amygdaloid complex; 3) the beginning of the lateral ventricle; 4) the caudal limit of the uncus, indicated by the start of the lateral geniculate nucleus (at the level of the gyrus intralimbicus); 5) the end of the lateral geniculate nucleus (beginning of the pulvinar); and 6) the beginning of the fornix. After histological validation of each of these landmarks, the full longitudinal length of the hippocampal formation and distances between landmarks were calculated. No statistically significant differences were found in total length or between landmarks. While the HF is anatomically organized at birth, its annotation is particularly challenging to perform. The histological validation of HF landmarks allows a better understanding of MRI images. The proposed protocol could be useful to assess MRI hippocampal quantification in children and possible variations due to different neurological diseases.

Ex vivo, in situ perfusion protocol for human brain fixation compatible with microscopy, MRI techniques, and anatomical studies.

We present a method for human brain fixation based on simultaneous perfusion of 4% paraformaldehyde through carotids after a flush with saline. The left carotid cannula is used to perfuse the body with 10% formalin, to allow further use of the body for anatomical research or teaching. The aim of our method is to develop a vascular fixation protocol for the human brain, by adapting protocols that are commonly used in experimental animal studies. We show that a variety of histological procedures can be carried out (cyto- and myeloarchitectonics, histochemistry, immunohistochemistry, intracellular cell injection, and electron microscopy). In addition, ex vivo, ex situ high-resolution MRI (9.4T) can be obtained in the same specimens. This procedure resulted in similar morphological features to those obtained by intravascular perfusion in experimental animals, provided that the postmortem interval was under 10 h for several of the techniques used and under 4 h in the case of intracellular injections and electron microscopy. The use of intravascular fixation of the brain inside the skull provides a fixed whole human brain, perfectly fitted to the skull, with negligible deformation compared to conventional techniques. Given this characteristic of ex vivo, in situ fixation, this procedure can probably be considered the most suitable one available for ex vivo MRI scans of the brain. We describe the compatibility of the method proposed for intravascular fixation of the human brain and fixation of the donor's body for anatomical purposes. Thus, body donor programs can provide human brain tissue, while the remainder of the body can also be fixed for anatomical studies. Therefore, this method of human brain fixation through the carotid system optimizes the procurement of human brain tissue, allowing a greater understanding of human neurological diseases, while benefiting anatomy departments by making the remainder of the body available for teaching purposes.

Three-dimensional mapping of neurofibrillary tangle burden in the human medial temporal lobe.

Tau protein neurofibrillary tangles are closely linked to neuronal/synaptic loss and cognitive decline in Alzheimer's disease and related dementias. Our knowledge of the pattern of neurofibrillary tangle progression in the human brain, critical to the development of imaging biomarkers and interpretation of in vivo imaging studies in Alzheimer's disease, is based on conventional two-dimensional histology studies that only sample the brain sparsely. To address this limitation, ex vivo MRI and dense serial histological imaging in 18 human medial temporal lobe specimens (age 75.3 ± 11.4 years, range 45 to 93) were used to construct three-dimensional quantitative maps of neurofibrillary tangle burden in the medial temporal lobe at individual and group levels. Group-level maps were obtained in the space of an in vivo brain template, and neurofibrillary tangles were measured in specific anatomical regions defined in this template. Three-dimensional maps of neurofibrillary tangle burden revealed significant variation along the anterior-posterior axis. While early neurofibrillary tangle pathology is thought to be confined to the transentorhinal region, we found similar levels of burden in this region and other medial temporal lobe subregions, including amygdala, temporopolar cortex, and subiculum/cornu ammonis 1 hippocampal subfields. Overall, the three-dimensional maps of neurofibrillary tangle burden presented here provide more complete information about the distribution of this neurodegenerative pathology in the region of the cortex where it first emerges in Alzheimer's disease, and may help inform the field about the patterns of pathology spread, as well as support development and validation of neuroimaging biomarkers.

Neurological Symptoms of COVID-19: The Zonulin Hypothesis.

The irruption of SARS-CoV-2 during 2020 has been of pandemic proportions due to its rapid spread and virulence. COVID-19 patients experience respiratory, digestive and neurological symptoms. Distinctive symptom as anosmia, suggests a potential neurotropism of this virus. Amongst the several pathways of entry to the nervous system, we propose an alternative pathway from the infection of the gut, involving Toll-like receptor 4 (TLR4), zonulin, protease-activated receptor 2 (PAR2) and zonulin brain receptor. Possible use of zonulin antagonists could be investigated to attenuate neurological manifestations caused by SARS-CoV-19 infection.

Persistent high levels of immune activation and their correlation with the HIV-1 proviral DNA and 2-LTR circles loads, in a cohort of Mexican individuals following long-term and fully suppressive treatment.

OBJECTIVES: The aim of this study was to investigate the correlation between the HIV-1 reservoir and the levels of immune activation in chronic patients under fully suppressive cART. METHODS: We quantified the HIV proviral DNA and 2-LTR circles loads from PBMCs, the levels of CD38+ and Ki-67+ T-cells, and the levels of IL-7 in a cohort of patients with more than 5 years of ART at enrollment and after 1 year. RESULTS: In 29 participants with a median of 8 years (IQR, 6.9-9.4) under suppressive cART we found higher levels of CD8+ CD38+ T-cells after 1-year (P = .000). There was a non-statistically significant poor correlation between the levels of immune activation and the proviral DNA of CD4+ and CD8+ T-cells. Ki-67+ T-cells declined without significant differences, and there was no significant correlation with the proportion of CD38+. IL-7 decreased at the follow-up observation (P = .094), but there was no correlation with the levels of CD38+ and Ki-67+ T-cells. CONCLUSIONS: We found a weak but non-statistically significant correlation of the levels of T-cell activation with the proviral DNA and 2-LTR circles. This suggests the likely occurrence of further mechanisms driving chronic versus early immune activation other than viral replication by itself in chronic patients.

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.

Born Too Early and Too Small: Higher Order Cognitive Function and Brain at Risk at Ages 8-16.

Prematurity presents a risk for higher order cognitive functions. Some of these deficits manifest later in development, when these functions are expected to mature. However, the causes and consequences of prematurity are still unclear. We conducted a longitudinal study to first identify clinical predictors of ultrasound brain abnormalities in 196 children born very preterm (VP; gestational age ≤32 weeks) and with very low birth weight (VLBW; birth weight ≤1500 g). At ages 8-16, the subset of VP-VLBW children without neurological findings (124) were invited for a neuropsychological assessment and an MRI scan (41 accepted). Of these, 29 met a rigorous criterion for MRI quality and an age, and gender-matched control group (n = 14) was included in this study. The key findings in the VP-VLBW neonates were: (a) 37% of the VP-VLBW neonates had ultrasound brain abnormalities; (b) gestational age and birth weight collectively with hospital course (i.e., days in hospital, neonatal intensive care, mechanical ventilation and with oxygen therapy, surgeries, and retinopathy of prematurity) predicted ultrasound brain abnormalities. At ages 8-16, VP-VLBW children showed: a) lower intelligent quotient (IQ) and executive function; b) decreased gray and white matter (WM) integrity; (c) IQ correlated negatively with cortical thickness in higher order processing cortical areas. In conclusion, our data indicate that facets of executive function and IQ are the most affected in VP-VLBW children likely due to altered higher order cortical areas and underlying WM.

Cytoarchitectonic Areas of the Gyrus ambiens in the Human Brain.

The Gyrus ambiens is a gross anatomical prominence in the medial temporal lobe (MTL), associated closely with Brodmann area 34 (BA34). It is formed largely by the medial intermediate subfield of the entorhinal cortex (EC) [Brodmann area 28 (BA28)]. Although the MTL has been widely studied due to its well-known role on memory and spatial information, the anatomical relationship between G. ambiens, BA34, and medial intermediate EC subfield has not been completely defined, in particular whether BA34 is part of the EC or a different type of cortex. In order to clarify this issue, we carried out a detailed analysis of 37 human MTLs, determining the exact location of medial intermediate EC subfield and its extent within the G. ambiens, its cortical thickness, and the histological-MRI correspondence of the G. ambiens with the medial intermediate EC subfield in 10 ex vivo MRI. Our results show that the G. ambiens is limited between two small sulci in the medial aspect of the MTL, which correspond almost perfectly to the extent of the medial intermediate EC subfield, although the rostral and caudal extensions of the G. ambiens may extend to the olfactory (rostrally) and intermediate (caudally) entorhinal subfields. Moreover, the cortical thickness averaged 2.5 mm (1.3 mm for layers I-III and 1 mm for layers V-VI). Moreover, distance among different landmarks visible in the MRI scans which are relevant to the identification of the G. ambiens in MRI are provided. These results suggest that BA34 is a part of the EC that fits best with the medial intermediate subfield. The histological data, together with the ex vivo MRI identification and thickness of these structures may be of use when assessing changes in MRI scans in clinical settings, such as Alzheimer disease.

Hippocampal-caudate nucleus interactions support exceptional memory performance.

Participants of the annual World Memory Championships regularly demonstrate extraordinary memory feats, such as memorising the order of 52 playing cards in 20 s or 1000 binary digits in 5 min. On a cognitive level, memory athletes use well-known mnemonic strategies, such as the method of loci. However, whether these feats are enabled solely through the use of mnemonic strategies or whether they benefit additionally from optimised neural circuits is still not fully clarified. Investigating 23 leading memory athletes, we found volumes of their right hippocampus and caudate nucleus were stronger correlated with each other compared to matched controls; both these volumes positively correlated with their position in the memory sports world ranking. Furthermore, we observed larger volumes of the right anterior hippocampus in athletes. Complementing these structural findings, on a functional level, fMRI resting state connectivity of the anterior hippocampus to both the posterior hippocampus and caudate nucleus predicted the athletes rank. While a competitive interaction between hippocampus and caudate nucleus is often observed in normal memory function, our findings suggest that a hippocampal-caudate nucleus cooperation may enable exceptional memory performance. We speculate that this cooperation reflects an integration of the two memory systems at issue-enabling optimal combination of stimulus-response learning and map-based learning when using mnemonic strategies as for example the method of loci.

The Human Periallocortex: Layer Pattern in Presubiculum, Parasubiculum and Entorhinal Cortex. A Review.

The cortical mantle is not homogeneous, so that three types of cortex can be distinguished: allocortex, periallocortex and isocortex. The main distinction among those three types is based on morphological differences, in particular the number of layers, overall organization, appearance, etc., as well as its connectivity. Additionally, in the phylogenetic scale, this classification is conserved among different mammals. The most primitive and simple cortex is the allocortex, which is characterized by the presence of three layers, with one cellular main layer; it is continued by the periallocortex, which presents six layers, although with enough differences in the layer pattern to separate three different fields: presubiculum (PrS), parasubiculum (PaS), and entorhinal cortex (EC). The closest part to the allocortex (represented by the subiculum) is the PrS, which shows outer (layers I-III) and inner (V-VI) principal layers (lamina principalis externa and lamina principalis interna), both separated by a cell poor band, parallel to the pial surface (layer IV or lamina dissecans). This layer organization is present throughout the anterior-posterior axis. The PaS continues the PrS, but its rostrocaudal extent is shorter than the PrS. The organization of the PaS shows the layer pattern more clearly than in the PrS. Up to six layers are recognizable in the PaS, with layer IV as lamina dissecans between superficial (layers I-III) and deep (V-VI) layers, as in the PrS. The EC presents even more clearly the layer pattern along both mediolateral and rostrocaudal extent. The layer pattern is a thick layer I, layer II in islands, layer III medium pyramids, layer IV as lamina dissecans (not present throughout the EC extent), layer V with dark and big pyramids and a multiform layer VI. The EC borders laterally the proisocortex (incomplete type of isocortex). Variations in the appearance of its layers justify the distinction of subfields in the EC, in particular in human and nonhuman primates. EC layers are not similar to those in the neocortex. The transition between the periallocortical EC and isocortex is not sharp, so that the proisocortex forms an intervening cortex, which fills the gap between the periallocortex and the isocortex.

Everyday memory: towards a translationally effective method of modelling the encoding, forgetting and enhancement of memory.

The testing of cognitive enhancers could benefit from the development of novel behavioural tasks that display better translational relevance for daily memory and permit the examination of potential targets in a within-subjects manner with less variability. We here outline an optimized spatial 'everyday memory' task. We calibrate it systematically by interrogating certain well-established determinants of memory and consider its potential for revealing novel features of encoding-related gene activation. Rats were trained in an event arena in which food was hidden in sandwells in a different location everyday. They found the food during an initial memory-encoding trial and were then required to remember the location in six alternative choice or probe trials at various time-points later. Training continued daily over a period of 4 months, realizing a stable high level of performance and characterized by delay-dependent forgetting over 24 h. Spaced but not massed access to multiple rewards enhanced the persistence of memory, as did post-encoding administration of the PDE4 inhibitor Rolipram. Quantitative PCR and then genome-wide analysis of gene expression led to a new observation - stronger gene-activation in hippocampus and retrosplenial cortex following spaced than massed training. In a subsidiary study, a separate group of animals replicated aspects of this training profile, going on to show enhanced memory when training was subject to post-encoding environmental novelty. Distinctive features of this protocol include its potential validity as a model of memory encoding used routinely by human subjects everyday, and the possibility of multiple within-subject comparisons to speed up assays of novel compounds.

Hippocampal damage and memory impairment in congenital cyanotic heart disease.

Neonatal hypoxia can lead to hippocampal atrophy, which can lead, in turn, to memory impairment. To test the generalizability of this causal sequence, we examined a cohort of 41 children aged 8-16, who, having received the arterial switch operation to correct for transposition of the great arteries, had sustained significant neonatal cyanosis but were otherwise neurodevelopmentally normal. As predicted, the cohort had significant bilateral reduction of hippocampal volumes relative to the volumes of 64 normal controls. They also had significant, yet selective, impairment of episodic memory as measured by standard tests of memory, despite relatively normal levels of intelligence, academic attainment, and verbal fluency. Across the cohort, degree of memory impairment was correlated with degree of hippocampal atrophy suggesting that even as early as neonatal life no other structure can fully compensate for hippocampal injury and its special role in serving episodic long term memory. © 2017 Wiley Periodicals, Inc.

Prefrontal cortex afferents to the anterior temporal lobe in the Macaca fascicularis monkey.

The anatomical organization of the lateral prefrontal cortex (LPFC) afferents to the anterior part of the temporal lobe (ATL) remains to be clarified. The LPFC has two subdivisions, dorsal (dLPFC) and ventral (vLPFC), which have been linked to cognitive processes. The ATL includes several different cortical areas, namely, the temporal polar cortex and rostral parts of the perirhinal, inferotemporal, and anterior tip of the superior temporal gyrus cortices. Multiple sensory modalities converge in the ATL. All of them (except the rostral inferotemporal and superior temporal gyrus cortices) are components of the medial temporal lobe, which is critical for long-term memory processing. We studied the LPFC connections with the ATL by placing retrograde tracer injections into the ATL: the temporal polar (n = 3), perirhinal (areas 35 and 36, n = 6), and inferotemporal cortices (area TE, n = 5), plus one additional deposit in the posterior parahippocampal cortex (area TF, n = 1). Anterograde tracer deposits into the dLPFC (A9 and A46, n = 2), the vLPFC (A46v, n = 2), and the orbitofrontal cortex (OF; n = 2) were placed for confirmation of those projections. The results showed that the vLPFC displays a moderate projection to rostral area TE and the dorsomedial portion of the temporal polar cortex; in contrast, the dLPFC connections with the ATL were weak. By comparison, the OFC and medial frontal cortices (MFC) showed dense connectivity with the ATL, namely, A13 with the temporopolar and perirhinal cortices. All areas of the MFC projected to the temporopolar cortex, albeit with a lower intensity. The functional significance of such paucity of LPFC afferents is unknown.

Past, present, and future in hippocampal formation and memory research.

Over 100 years of research on the hippocampal formation has led us understand the consequences of lesions in humans, the functional networks, anatomical pathways, neuronal types and their local circuitry, receptors, molecules, intracellular cascades, and some of the physiological mechanisms underlying long-term spatial and episodic memory. In addition, complex computational models allow us to formulate sophisticated hypotheses; many of them testable with techniques recently developed unthinkable in the past. Although the neurobiology of the cognitive map is starting to be revealed today, we still face a future with many unresolved questions. The aim of this commentary is twofold. First is to point out some of the critical findings in hippocampal formation research and new challenges. Second, to briefly summarize what the anatomy of memory can tell us about how highly processed sensory information from distant cortical areas communicate with different subareas of the entorhinal cortex, dentate gyrus, and hippocampal subfields to integrate and consolidate unique episodic memory traces.

Anatomical pathways for auditory memory in primates.

Episodic memory or the ability to store context-rich information about everyday events depends on the hippocampal formation (entorhinal cortex, subiculum, presubiculum, parasubiculum, hippocampus proper, and dentate gyrus). A substantial amount of behavioral-lesion and anatomical studies have contributed to our understanding of the organization of how visual stimuli are retained in episodic memory. However, whether auditory memory is organized similarly is still unclear. One hypothesis is that, like the "visual ventral stream" for which the connections of the inferior temporal gyrus with the perirhinal cortex are necessary for visual recognition in monkeys, direct connections between the auditory association areas of the superior temporal gyrus and the hippocampal formation and with the parahippocampal region (temporal pole, perhirinal, and posterior parahippocampal cortices) might also underlie recognition memory for sounds. Alternatively, the anatomical organization of memory could be different in audition. This alternative "indirect stream" hypothesis posits that, unlike the visual association cortex, the majority of auditory information makes one or more synapses in intermediate, polymodal areas, where they may integrate information from other sensory modalities, before reaching the medial temporal memory system. This review considers anatomical studies that can support either one or both hypotheses - focusing on anatomical studies on the primate brain, primarily in macaque monkeys, that have reported not only direct auditory association connections with medial temporal areas, but, importantly, also possible indirect pathways for auditory information to reach the medial temporal lobe memory system.