Synaptic plasticity in human cortical circuits: cellular mechanisms of learning and memory in the human brain?

The role of astrocyte structural plasticity in regulating neural circuit function and behavior

Brain circuits undergo substantial structural changes during development, driven by the formation, stabilization, and elimination of synapses. Synaptic connections continue to undergo experience‐dependent structural rearrangements throughout life, which are postulated to underlie learning and memory. Astrocytes, a major glial cell type in the brain, are physically in contact with synaptic circuits through their structural ensheathment of synapses. Astrocytes strongly contribute to the remodeling of synaptic structures in healthy and diseased central nervous systems by regulating synaptic connectivity and behaviors. However, whether structural plasticity of astrocytes is involved in their critical functions at the synapse is unknown. This review will discuss the emerging evidence linking astrocytic structural plasticity to synaptic circuit remodeling and regulation of behaviors. Moreover, we will survey possible molecular and cellular mechanisms regulating the structural plasticity of astrocytes and their non‐cell‐autonomous effects on neuronal plasticity. Finally, we will discuss how astrocyte morphological changes in different physiological states and disease conditions contribute to neuronal circuit function and dysfunction.

Single-Vesicle Electrochemistry Following Repetitive Stimulation Reveals a Mechanism for Plasticity Changes with Iron Deficiency

Deficiency of iron, the most abundant transition metal in the brain and important for neuronal activity, is known to affect synaptic plasticity, causing learning and memory deficits. How iron deficiency impacts plasticity by altering neurotransmission at the cellular level is not fully understood. We used electrochemical methods to study the effect of iron deficiency on plasticity with repetitive stimulation. We show that during iron deficiency, repetitive stimulation causes significant decrease in exocytotic release without changing vesicular content. This results in a lower fraction of release, opposite to the control group, upon repetitive stimulation. These changes were partially reversible by iron repletion. This finding suggests that iron deficiency has a negative effect on plasticity by decreasing the fraction of vesicular release in response to repetitive stimulation. This provides a putative mechanism for how iron deficiency modulates plasticity.

The Combined Influences of Exercise, Diet and Sleep on Neuroplasticity

Neuroplasticity refers to the brain's ability to undergo structural and functional adaptations in response to experience, and this process is associated with learning, memory and improvements in cognitive function. The brain's propensity for neuroplasticity is influenced by lifestyle factors including exercise, diet and sleep. This review gathers evidence from molecular, systems and behavioral neuroscience to explain how these three key lifestyle factors influence neuroplasticity alone and in combination with one another. This review collected results from human studies as well as animal models. This information will have implications for research, educational, fitness and neurorehabilitation settings.

A Graph Network Model for Neural Connection Prediction and Connection Strength Estimation

Objective. Reconstruction of connectomes at the cellular scale is a prerequisite for understanding the principles of neural circuits. However, due to methodological limits, scientists have reconstructed the connectomes of only a few organisms such as C. elegans, and estimated synaptic strength indirectly according to their size and number. Approach. Here, we propose a graph network model to predict synaptic connections and estimate synaptic strength by using the calcium activity data from C. elegans. Main results. The results show that this model can reliably predict synaptic connections in the neural circuits of C. elegans, and estimate their synaptic strength, which is an intricate and comprehensive reflection of multiple factors such as synaptic type and size, neurotransmitter and receptor type, and even activity dependence. In addition, the excitability or inhibition of synapses can be identified by this model. We also found that chemical synaptic strength is almost linearly positively correlated to electrical synaptic strength, and the influence of one neuron on another is non-linearly correlated with the number between them. This reflects the intrinsic interaction between electrical and chemical synapses. Significance. Our model is expected to provide a more accessible quantitative and data-driven approach for the reconstruction of connectomes in more complex nervous systems, as well as a promising method for accurately estimating synaptic strength.

EphrinB2/ephB2 activation facilitates colonic synaptic potentiation and plasticity contributing to long-term visceral hypersensitivity in irritable bowel syndrome

AIMS

Sustained visceral hypersensitivity is a hallmark of irritable bowel syndrome (IBS) could be partially explained by enteric neural remodeling. Particularly, synaptic plasticity in the enteric nervous system, a form of enteric "memory", has been speculated as a participant in the pain maintenance in IBS. This study aimed to elucidate the role of ephrinB2/ephB2 in enteric synaptic plasticity and visceral pain in IBS.

MATERIALS AND METHODS

EphrinB2/ephB2 expression and synaptic plasticity were assessed in colonic tissues from IBS patients, and rats induced by Trichinella spiralis infection and those treated with ephB2-Fc (an ephB2 receptor blocker) or ifenprodil (a selective NR2B antagonist). Furthermore, abdominal withdrawal reflex scores to colorectal distention and mesenteric afferent firing were assessed. EphrinB2-Fc(an ephB2 receptor activator) induced enteric synaptic plasticity was further evaluated in longitudinal muscle-myenteric plexus(LMMP) cultures and primary cultured myenteric neurons.

KEY FINDINGS

EphrinB2/ephB2 was specifically expressed in colonic nerves and upregulated in IBS patients and rats, which was correlated with pain severity. The functional synaptic plasticity, visceral sensitivity to colorectal distention and colonic mesenteric afferent activity to mechanical and chemical stimulus were enhanced in IBS rats, and were blocked by ephB2-Fc or ifenprodil treatment. EphrinB2-Fc promoted the phosphorylation of NR2B in IBS rats and LMMP cultures, and could mediate sustained neural activation via increased [Ca2+]i and raised expression of synaptic plasticity-related early immediate genes, including c-fos and arc.

SIGNIFICANCE

EphrinB2/ephB2 facilitated NR2B-mediated synaptic potentiation in the enteric nervous system that may be a novel explanation and potential therapeutic target for sustained pain hypersensitivity in IBS.

Blended Motor-Sensory Nerve Bundles on Diffused Tensor Imaging: Evidence of Brain Plasticity in a Patient with 36-year Sequelae from Encephalitis

BACKGROUND: Brain plasticity refers to the extraordinary ability of the brain to modify its structure and function following changes within the body or in the external environment. However, it is not easy to find it on non-invasive imaging modality. CASE REPORT: In this article, we report the case of a 36-year-old male patient with sequelae of encephalitis. The patient had general epilepsy with multiple hospital admissions. MRI 3.0 Tesla showed his cerebral hemispheres were asymmetrical both morphologically and tractographically; there was a scar at the right temporo-occipital region, and an atrophy of the right temporal lobe, hippocampus and pontine. DTI reconstruction showed asymmetrical cortico-spinal and thalamo-cortical tracts with posterior thalamo-cortical tract was partly damaged by the scar. Blended motor-sensory nerve bundles were observed only on the left side of the patient’s brain but not on the right or healthy subjects. DTI quantification showed the lower line number, lower FA and higher ADC in the patient compared to healthy subjects and within the patient with decreased functionality on the side of the scar. CONCLUSION: Non-invasive DTI with 3D image reconstruction on the patient showed evidence of brain plasticity appeared on cortico-spinal and thalamo-cortical tracts and can inform diagnosis and treatment strategies.

Upregulation of the endogenous peptide antisecretory factor enhances hippocampal long-term potentiation and promotes learning in wistar rats

Antisecretory Factor (AF) is an endogenous peptide known for its powerful antisecretory and anti-inflammatory properties. We have previously shown that AF also acts as a neuromodulator of GABAergic synaptic transmission in rat hippocampus in a way that results in disinhibition of CA1 pyramidal neurons. Disinhibition is expected to facilitate the induction of long-term potentiation (LTP), and LTP is known to play a crucial role in learning and memory acquisition. In the present study we investigated the effect of AF on LTP in CA3-CA1 synapses in rat hippocampus. In addition, endogenous AF plasma activity was upregulated by feeding the rats with specially processed cereals (SPC) and spatial learning and memory was studied in the Morris Water Maze (MWM). We found that LTP was significantly enhanced in the presence of AF, both when added exogenously in vitro as well as when upregulated endogenously by SPC-feeding. In the presence of the GABA_A-receptor antagonist picrotoxin (PTX) there was however no significant enhancement of LTP. Moreover, rats fed with SPC demonstrated enhanced spatial learning and short-term memory, compared with control animals. These results show that the disinhibition of GABAergic transmission in the hippocampus by the endogenous peptide AF enhances LTP as well as spatial learning and memory.

Using Hebbian-Type Stimulation to Rescue Arm Function After Stroke: Study Protocol for a Randomized Clinical Trial

Background

Upper-extremity hemiplegia after stroke remains a significant clinical problem. The supplementary motor area (SMA) is vital to the motor recovery outcomes of chronic stroke patients. Therefore, rebuilding the descending motor tract from the SMA to the paralyzed limb is a potential approach to restoring arm motor function after stroke. Paired associative stimulation (PAS), which is based on Hebbian theory, is a potential method for reconstructing the connections in the impaired motor neural circuits. The study described in this protocol aims to assess the effects of cortico–peripheral Hebbian-type stimulation (HTS), involving PAS, for neural circuit reconstruction to rescue the paralyzed arm after stroke.

Methods

The study is a 4-month double-blind randomized sham-controlled clinical trial. We will recruit 90 post-stroke individuals with mild to moderate upper limb paralysis. Based on a 1:1 ratio, the participants will be randomly assigned to the HTS and sham groups. Each participant will undergo 5-week HTS or sham stimulation. Assessments will be conducted at baseline, immediately after the 5-week treatment, and at a 3-month follow-up. The primary outcome will be the Wolf Motor Function Test (WMFT). The secondary outcomes will be Fugl-Meyer Assessment for Upper Extremity (FMA-UE), Functional Independence Measure (FIM), and functional near-infrared spectroscopy (fNIRS) parameters. The adverse events will be recorded throughout the study.

Discussion

Upper-limb paralysis in stroke patients is due to neural circuit disruption, so the reconstruction of effective motor circuits is a promising treatment approach. Based on its anatomical structure and function, the SMA is thought to compensate for motor dysfunction after focal brain injury at the cortical level. Our well-designed randomized controlled trial will allow us to analyze the clinical efficacy of this novel Hebbian theory-based neuromodulation strategy regarding promoting the connection between the cortex and peripheral limb. The results may have significance for the development and implementation of effective neurorehabilitation treatments.

Clinical Trial Registration

[www.ClinicalTrials.gov], identifier [ChiCTR2000039949].

Transcranial magnetic stimulation as a tool to induce and explore plasticity in humans

Activity-dependent synaptic plasticity is the main theoretical framework to explain mechanisms of learning and memory. Synaptic plasticity can be explored experimentally in animals through various standardized protocols for eliciting long-term potentiation and long-term depression in hippocampal and cortical slices. In humans, several non-invasive protocols of repetitive transcranial magnetic stimulation and transcranial direct current stimulation have been designed and applied to probe synaptic plasticity in the primary motor cortex, as reflected by long-term changes in motor evoked potential amplitudes. These protocols mimic those normally used in animal studies for assessing long-term potentiation and long-term depression. In this chapter, we first discuss the physiologic basis of theta-burst stimulation, paired associative stimulation, and transcranial direct current stimulation. We describe the current biophysical and theoretical models underlying the molecular mechanisms of synaptic plasticity and metaplasticity, defined as activity-dependent changes in neural functions that modulate subsequent synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), in the human motor cortex including calcium-dependent plasticity, spike-timing-dependent plasticity, the role of N-methyl-d-aspartate-related transmission and gamma-aminobutyric-acid interneuronal activity. We also review the putative microcircuits responsible for synaptic plasticity in the human motor cortex. We critically readdress the issue of variability in studies investigating synaptic plasticity and propose available solutions. Finally, we speculate about the utility of future studies with more advanced experimental approaches.

The Effects of Intraoperative Hypothermia on Postoperative Cognitive Function in the Rat Hippocampus and Its Possible Mechanisms

Intraoperative hypothermia is a common complication during operations and is associated with several adverse events. Postoperative cognitive dysfunction (POCD) and its adverse consequences have drawn increasing attention in recent years. There are currently no relevant studies investigating the correlation between intraoperative hypothermia and POCD. The aim of this study was to assess the effects of intraoperative hypothermia on postoperative cognitive function in rats undergoing exploratory laparotomies and to investigate the possible related mechanisms. We used the Y-maze and Morris Water Maze (MWM) tests to assess the rats’ postoperative spatial working memory, spatial learning, and memory. The morphological changes in hippocampal neurons were examined by haematoxylin-eosin (HE) staining and hippocampal synaptic plasticity-related protein expression. Activity-regulated cytoskeletal-associated protein (Arc), cyclic adenosine monophosphate-response element-binding protein (CREB), S133-phosphorylated CREB (p-CREB [S133]), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor 1 (AMPAR1), and S831-phosphorylated AMPAR1 (p-AMPAR1 [S831]) were evaluated by Western blotting. Our results suggest a correlation between intraoperative hypothermia and POCD in rats and that intraoperative hypothermia may lead to POCD regarding impairments in spatial working memory, spatial learning, and memory. POCD induced by intraoperative hypothermia might be due to hippocampal neurons damage and decreased expression of synaptic plasticity-related proteins Arc, p-CREB (S133), and p-AMPAR1 (S831).

Epitranscriptomic Analysis of N6-methyladenosine in Infant Rhesus Macaques after Multiple Sevoflurane Anesthesia

Clinical investigations to date have proposed the possibility that exposure to anesthetics is associated with neurodevelopmental deficits. Sevoflurane is the most commonly used general anesthetic in pediatric patients. Animal studies have demonstrated that multiple exposures to sevoflurane during the postnatal period resulted in neuropathological brain changes and long-term cognitive deficits. However, the underlying mechanisms remain to be clarified. In this study, methylated RNA immunoprecipitation sequencing (MeRIP-Seq) was performed to acquire genome-wide profiling of RNA N6-methyladenosine (m⁶A) in the prefrontal cortex of infant rhesus macaques. The macaques in the sevoflurane group had more m⁶A peaks than the macaques in the control group (p ≤ 0.05). After sevoflurane treatment, the mRNA levels of YT521-B homology domain family 1 (YTHDF1) and YT521-B homology domain family 3 (YTHDF3) were decreased, and sevoflurane anesthesia dynamically regulated RNA m⁶A methylation. Gene ontology (GO) analysis revealed that after sevoflurane exposure, genes with increased methylation of m⁶A sites were enriched in some physiological processes relevant to neurodevelopment, mainly focused on synaptic plasticity. The female macaques had 18 hypermethylated genes. The males had 35 hypermethylated genes, and some physiological processes related to the regulation of synaptic structure were enriched. Rhesus macaques are genetically closer to human beings. Our findings can help in the study of the mechanism of sevoflurane-relevant neurodevelopmental deficits at the posttranscriptional level and can provide new insights into potential clinical preventions and interventions for the neurotoxicity of neonatal anesthesia exposure.

Glucocorticoid modulation of synaptic plasticity in the human temporal cortex of epilepsy patients: Does chronic stress contribute to memory impairment?

OBJECTIVE

Memory impairment is common in patients with temporal lobe epilepsy and seriously affects life quality. Chronic stress is a recognized cofactor in epilepsy and can also impair memory function. Furthermore, increased cortisol levels have been reported in epilepsy patients. Animal models have suggested that aggravating effects of stress on memory and synaptic plasticity were mediated via glucocorticoids. The aim of this study was, therefore, to investigate the effect of glucocorticoid receptor (GR) modulation on synaptic plasticity in the human cortex of epilepsy patients.

METHODS

We performed field potential recordings in acute slices from the temporal neocortex of patients who underwent surgery for drug-resistant temporal lobe epilepsy. Synaptic plasticity was investigated by a theta-burst stimulation (TBS) protocol for induction of long-term potentiation (LTP) in the presence of GR modulators.

RESULTS

LTP was impaired in temporal cortex from epilepsy patients. Pretreatment of the slices with the GR antagonist mifepristone (RU486) improved LTP induction, suggesting that LTP impairment was due to baseline GR activation in the human cortex. The highly potent GR agonist dexamethasone additionally weakened synaptic strength in an activity-dependent manner when applied after TBS.

SIGNIFICANCE

Our results show a direct negative glucocorticoid effect on synaptic potentiation in the human cortex and imply chronic activation of GRs. Chronic stress may therefore contribute to memory impairment in patients with temporal lobe epilepsy. Furthermore, the activity-dependent acute inhibitory effect of dexamethasone suggests a mechanism of synaptic downscaling by which postictally increased cortisol levels may prevent pathologic plasticity upon seizures.

Sublamina-Specific Dynamics and Ultrastructural Heterogeneity of Layer 6 Excitatory Synaptic Boutons in the Adult Human Temporal Lobe Neocortex

Synapses “govern” the computational properties of any given network in the brain. However, their detailed quantitative morphology is still rather unknown, particularly in humans. Quantitative 3D-models of synaptic boutons (SBs) in layer (L)6a and L6b of the temporal lobe neocortex (TLN) were generated from biopsy samples after epilepsy surgery using fine-scale transmission electron microscopy, 3D-volume reconstructions and electron microscopic tomography. Beside the overall geometry of SBs, the size of active zones (AZs) and that of the three pools of synaptic vesicles (SVs) were quantified. SBs in L6 of the TLN were middle-sized (~5 μm²), the majority contained only a single but comparatively large AZ (~0.20 μm²). SBs had a total pool of ~1100 SVs with comparatively large readily releasable (RRP, ~10 SVs L6a), (RRP, ~15 SVs L6b), recycling (RP, ~150 SVs), and resting (~900 SVs) pools. All pools showed a remarkably large variability suggesting a strong modulation of short-term synaptic plasticity. In conclusion, L6 SBs are highly reliable in synaptic transmission within the L6 network in the TLN and may act as “amplifiers,” “integrators” but also as “discriminators” for columnar specific, long-range extracortical and cortico-thalamic signals from the sensory periphery.

Isoform-Specific Reduction of the Basic Helix-Loop-Helix Transcription Factor TCF4 Levels in Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder with onset of characteristic motor symptoms at midlife, preceded by subtle cognitive and behavioral disturbances. Transcriptional dysregulation emerges early in the disease course and is considered central to HD pathogenesis. Using wild-type (wt) and HD knock-in mouse striatal cell lines we observed a HD genotype-dependent reduction in the protein levels of transcription factor 4 (TCF4), a member of the basic helix-loop-helix (bHLH) family with critical roles in brain development and function. We characterized mouse Tcf4 gene structure and expression of alternative mRNAs and protein isoforms in cell-based models of HD, and in four different brain regions of male transgenic HD mice (R6/1) from young to mature adulthood. The largest decrease in the levels of TCF4 at mRNA and specific protein isoforms were detected in the R6/1 mouse hippocampus. Translating this finding to human disease, we found reduced expression of long TCF4 isoforms in the postmortem hippocampal CA1 area and in the cerebral cortex of HD patients. Additionally, TCF4 protein isoforms showed differential synergism with the proneural transcription factor ASCL1 in activating reporter gene transcription in hippocampal and cortical cultured neurons. Induction of neuronal activity increased these synergistic effects in hippocampal but not in cortical neurons, suggesting brain region-dependent differences in TCF4 functions. Collectively, this study demonstrates isoform-specific changes in TCF4 expression in HD that could contribute to the progressive impairment of transcriptional regulation and neuronal function in this disease.

Functional role of ascorbic acid in the central nervous system: a focus on neurogenic and synaptogenic processes

Ascorbic acid, a water-soluble vitamin, is highly concentrated in the brain and participates in neuronal modulation and regulation of central nervous system (CNS) homeostasis. Ascorbic acid has emerged as a neuroprotective compound against neurotoxicants and neurodegenerative diseases, including Alzheimer's disease, multiple sclerosis and amyotrophic lateral sclerosis. Moreover, it improves behavioral and biochemical alterations in psychiatric disorders, including schizophrenia, anxiety, major depressive disorder, and bipolar disorder. Some recent studies have advanced the knowledge on the mechanisms associated with the preventive and therapeutic effects of ascorbic acid by showing that they are linked to improved neurogenesis and synaptic plasticity. This review shows that ascorbic acid has the potential to regulate positively stem cell generation and proliferation. Moreover, it improves neuronal differentiation of precursors cells, promotes adult hippocampal neurogenesis, and has synaptogenic effects that are possibly linked to its protective or therapeutic effects in the brain.

Altered motor cortical plasticity in patients with hepatic encephalopathy: A paired associative stimulation study

OBJECTIVE

Hepatic encephalopathy (HE) is a potentially reversible brain dysfunction caused by liver failure. Altered synaptic plasticity is supposed to play a major role in the pathophysiology of HE. Here, we used paired associative stimulation with an inter-stimulus interval of 25 ms (PAS25), a transcranial magnetic stimulation (TMS) protocol, to test synaptic plasticity of the motor cortex in patients with manifest HE.

METHODS

23 HE-patients and 23 healthy controls were enrolled in the study. Motor evoked potential (MEP) amplitudes were assessed as measure for cortical excitability. Time courses of MEP amplitude changes after the PAS25 intervention were compared between both groups.

RESULTS

MEP-amplitudes increased after PAS25 in the control group, indicating PAS25-induced synaptic plasticity in healthy controls, as expected. In contrast, MEP-amplitudes within the HE group did not change and were lower than in the control group, indicating no induction of plasticity.

CONCLUSIONS

Our study revealed reduced synaptic plasticity of the primary motor cortex in HE.

SIGNIFICANCE

Reduced synaptic plasticity in HE provides a link between pathological changes on the molecular level and early clinical symptoms of the disease. This decrease may be caused by disturbances in the glutamatergic neurotransmission due to the known hyperammonemia in HE patients.

The Integrated Information Theory of consciousness: A case of mistaken identity

Giulio Tononi's Integrated Information Theory (IIT) proposes explaining consciousness by directly identifying it with integrated information. We examine the construct validity of IIT's measure of consciousness, phi (Φ), by analyzing its formal properties, its relation to key aspects of consciousness, and its co-variation with relevant empirical circumstances. Our analysis shows that IIT's identification of consciousness with the causal efficacy with which differentiated networks accomplish global information transfer (which is what Φ in fact measures) is mistaken. This misidentification has the consequence of requiring the attribution of consciousness to a range of natural systems and artifacts that include, but are not limited to, large-scale electrical power grids, gene-regulation networks, some electronic circuit boards, and social networks. Instead of treating this consequence of the theory as a disconfirmation, IIT embraces it. By regarding these systems as bearers of consciousness ex hypothesi, IIT is led towards the orbit of panpsychist ideation. This departure from science as we know it can be avoided by recognizing the functional misattribution at the heart of IIT's identity claim. We show, for example, what function is actually performed, at least in the human case, by the cortical combination of differentiation with integration that IIT identifies with consciousness. Finally, we examine what lessons may be drawn from IIT's failure to provide a credible account of consciousness for progress in the very active field of research concerned with exploring the phenomenon from formal and neural points of view.

Role of mTOR-Regulated Autophagy in Synaptic Plasticity Related Proteins Downregulation and the Reference Memory Deficits Induced by Anesthesia/Surgery in Aged Mice

Postoperative cognitive dysfunction increases mortality and morbidity in perioperative patients and has become a major concern for patients and caregivers. Previous studies demonstrated that synaptic plasticity is closely related to cognitive function, anesthesia and surgery inhibit synaptic function. In central nervous system, autophagy is vital to synaptic plasticity, homeostasis of synapticproteins, synapse elimination, spine pruning, proper axon guidance, and when dysregulated, is associated with behavioral and memory functions disorders. The mammalian target of rapamycin (mTOR) negatively regulates the process of autophagy. This study aimed to explore whether rapamycin can ameliorate anesthesia/surgery-induced cognitive deficits by inhibiting mTOR, activating autophagy and rising synaptic plasticity-related proteins in the hippocampus. Aged C57BL/6J mice were used to establish POCD models with exploratory laparotomy under isoflurane anesthesia. The Morris Water Maze (MWM) was used to measure reference memory after anesthesia and surgery. The levels of mTOR phosphorylation (p-mTOR), Beclin-1 and LC3-II were examined on postoperative days 1, 3 and 7 by western blotting. The levels of synaptophysin (SYN) and postsynaptic density protein 95 (PSD-95) in the hippocampus were also examined by western blotting. Here we showed that anesthesia/surgery impaired reference memory and induced the activation of mTOR, decreased the expression of autophagy-related proteins such as Beclin-1 and LC3-II. A corresponding decline in the expression of neuronal/synaptic, plasticity-related proteins such as SYN and PSD-95 was also observed. Pretreating mice with rapamycin inhibited the activation of mTOR and restored autophagy function, also increased the expression of SYN and PSD-95. Furthermore, anesthesia/surgery-induced learning and memory deficits were also reversed by rapamycin pretreatment. In conclusion, anesthesia/surgery induced mTOR hyperactivation and autophagy impairments, and then reduced the levels of SYN and PSD-95 in the hippocampus. An mTOR inhibitor, rapamycin, ameliorated anesthesia/surgery-related cognitive impairments by inhibiting the mTOR activity, inducing activation of autophagy, enhancing SYN and PSD-95 expression.

Using fast visual rhythmic stimulation to control inter-hemispheric phase offsets in visual areas

Spike timing dependent plasticity (STDP) is believed to be important for neural communication and plasticity in human episodic memory, but causal evidence is lacking due to technical challenges. Rhythmic sensory stimulation that has been used to investigate causal relations between oscillations and cognition may be able to address this question. The challenge, however, is that the frequency corresponding to the critical time window for STDP is gamma (~40 Hz), yet the application of rhythmic sensory stimulation has been limited primarily to lower frequencies (<30 Hz). It remains unknown whether this method can be applied to precisely control the activation time delay between distant groups of neurons at a millisecond scale. To answer this question and examine the role of STDP in human episodic memory, we simulated the STDP function by controlling the activation time delay between the left and right visual cortices during memory encoding. This was achieved by presenting flickering (37.5 Hz) movie pairs in the left and right visual fields with a phase lag of either 0, 90, 180 or 270°. Participants were asked to memorize the two movies within each pair and the association was later tested. Behavioral results revealed no significant difference in memory performance across conditions with different degrees of gamma phase synchrony. Yet importantly, our study showed for the first time, that oscillatory activity can be driven with a precision of 6.67 ms delay between neuronal groups. Our method hereby provides an approach to investigate relations between precise neuronal timing and cognitive functions.

A Dynamic Systems Framework for Gender/Sex Development: From Sensory Input in Infancy to Subjective Certainty in Toddlerhood

From birth to 15 months infants and caregivers form a fundamentally intersubjective, dyadic unit within which the infant's ability to recognize gender/sex in the world develops. Between about 18 and 36 months the infant accumulates an increasingly clear and subjective sense of self as female or male. We know little about how the precursors to gender/sex identity form during the intersubjective period, nor how they transform into an independent sense of self by 3 years of age. In this Theory and Hypothesis article I offer a general framework for thinking about this problem. I propose that through repetition and patterning, the dyadic interactions in which infants and caregivers engage imbue the infant with an embodied, i.e., sensori-motor understanding of gender/sex. During this developmental period (which I label Phase 1) gender/sex is primarily an intersubjective project. From 15 to 18 months (which I label Phase 2) there are few reports of newly appearing gender/sex behavioral differences, and I hypothesize that this absence reflects a period of developmental instability during which there is a transition from gender/sex as primarily inter-subjective to gender/sex as primarily subjective. Beginning at 18 months (i.e., the start of Phase 3), a toddler's subjective sense of self as having a gender/sex emerges, and it solidifies by 3 years of age. I propose a dynamic systems perspective to track how infants first assimilate gender/sex information during the intersubjective period (birth to 15 months); then explore what changes might occur during a hypothesized phase transition (15 to 18 months), and finally, review the emergence and initial stabilization of individual subjectivity-the period from 18 to 36 months. The critical questions explored focus on how to model and translate data from very different experimental disciplines, especially neuroscience, physiology, developmental psychology and cognitive development. I close by proposing the formation of a research consortium on gender/sex development during the first 3 years after birth.

All-trans retinoic acid induces synaptic plasticity in human cortical neurons

A defining feature of the brain is the ability of its synaptic contacts to adapt structurally and functionally in an experience-dependent manner. In the human cortex, however, direct experimental evidence for coordinated structural and functional synaptic adaptation is currently lacking. Here, we probed synaptic plasticity in human cortical slices using the vitamin A derivative all-trans retinoic acid (atRA), a putative treatment for neuropsychiatric disorders such as Alzheimer's disease. Our experiments demonstrated that the excitatory synapses of superficial (layer 2/3) pyramidal neurons underwent coordinated structural and functional changes in the presence of atRA. These synaptic adaptations were accompanied by ultrastructural remodeling of the calcium-storing spine apparatus organelle and required mRNA translation. It was not observed in synaptopodin-deficient mice, which lack spine apparatus organelles. We conclude that atRA is a potent mediator of synaptic plasticity in the adult human cortex.

Cortical plasticity is correlated with cognitive improvement in Alzheimer's disease patients after rTMS treatment

OBJECTIVE

Repetitive transcranial magnetic stimulation (rTMS) has been widely used in non-invasive treatments for different neurological disorders. Few biomarkers are available for treatment response prediction. This study aims to analyze the correlation between changes in long-term potentiation (LTP)-like cortical plasticity and cognitive function in patients with Alzheimer's disease (AD) that underwent rTMS treatment.

METHODS

A total of 75 AD patients were randomized into either 20 Hz rTMS treatment at the dorsolateral prefrontal cortex (DLPFC) group (n = 37) or a sham treatment group (n = 38) for 30 sessions over six weeks (five days per week) with a three-month follow-up. Neuropsychological assessments were conducted using the Mini-Mental State Examination (MMSE) and Alzheimer's Disease Assessment-Cognitive Component (ADAS-Cog). The cortical plasticity reflected by the motor-evoked potential (MEP) before and after high-frequency repetitive TMS to the primary motor cortex (M1) was also examined prior to and after the treatment period.

RESULTS

The results showed that the cognitive ability of patients who underwent the MMSE and ADAS-Cog assessments showed small but significant improvement after six weeks of rTMS treatment compared with the sham group. The cortical plasticity improvement correlated to the observed cognition change.

CONCLUSIONS

Cortical LTP-like plasticity could predict the treatment responses of cognitive improvements in AD patients receiving rTMS intervention. This warrants future clinical trials using cortical LTP as a predictive marker.

A Mechanistic Model of NMDA and AMPA Receptor-Mediated Synaptic Transmission in Individual Hippocampal CA3-CA1 Synapses: A Computational Multiscale Approach

Inside hippocampal circuits, neuroplasticity events that individual cells may undergo during synaptic transmissions occur in the form of Long-Term Potentiation (LTP) and Long-Term Depression (LTD). The high density of NMDA receptors expressed on the surface of the dendritic CA1 spines confers to hippocampal CA3-CA1 synapses the ability to easily undergo NMDA-mediated LTP and LTD, which is essential for some forms of explicit learning in mammals. Providing a comprehensive kinetic model that can be used for running computer simulations of the synaptic transmission process is currently a major challenge. Here, we propose a compartmentalized kinetic model for CA3-CA1 synaptic transmission. Our major goal was to tune our model in order to predict the functional impact caused by disease associated variants of NMDA receptors related to severe cognitive impairment. Indeed, for variants Glu413Gly and Cys461Phe, our model predicts negative shifts in the glutamate affinity and changes in the kinetic behavior, consistent with experimental data. These results point to the predictive power of this multiscale viewpoint, which aims to integrate the quantitative kinetic description of large interaction networks typical of system biology approaches with a focus on the quality of a few, key, molecular interactions typical of structural biology ones.

tDCS over posterior parietal cortex increases cortical excitability but decreases learning: An ERPs and TMS-EEG study

The application of anodal transcranial direct current stimulation (AtDCS) is generally associated with increased neuronal excitability and enhanced cognitive functioning. Nevertheless, previous work showed that applying this straight reasoning does not always lead to the desired results at behavioural level. Here, we investigated electrophysiological markers of AtDCS-mediated effects on visuo-spatial contextual learning (VSCL). In order to assess cortical excitability changes after 3 mA AtDCS applied over posterior parietal cortex, event-related potentials (ERPs) were collected during task performance. Additionally, AtDCS-induced effects on cortical excitability were explored by measuring TMS-evoked potentials (TEPs) collected before AtDCS, after AtDCS and after AtDCS and VSCL interaction. Behavioural results revealed that the application of AtDCS induced a reduction of VSCL. At the electrophysiological level, ERPs showed enhanced cortical response (P2 component) in the group receiving Real-AtDCS as compared to Sham-AtDCS. Cortical responsiveness at rest as measured by TEP, did not indicate any significant difference between Real- and Sham-tDCS groups, albeit a trend was present. Overall, our results suggest that AtDCS increases cortical response to incoming visuo-spatial stimuli, but with no concurrent increase in learning. Detrimental effects on behaviour could result from the interaction between AtDCS- and task-mediated cortical activation. This interaction might enhance cortical excitability and hinder normal task-related neuroplastic phenomena subtending learning.

GRIP1 regulates synaptic plasticity and learning and memory

Hebbian plasticity is a key mechanism for higher brain functions, such as learning and memory. This form of synaptic plasticity primarily involves the regulation of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) abundance and properties, whereby AMPARs are inserted into synapses during long-term potentiation (LTP) or removed during long-term depression (LTD). The molecular mechanisms underlying AMPAR trafficking remain elusive, however. Here we show that glutamate receptor interacting protein 1 (GRIP1), an AMPAR-binding protein shown to regulate the trafficking and synaptic targeting of AMPARs, is required for LTP and learning and memory. GRIP1 is recruited into synapses during LTP, and deletion of Grip1 in neurons blocks synaptic AMPAR accumulation induced by glycine-mediated depolarization. In addition, Grip1 knockout mice exhibit impaired hippocampal LTP, as well as deficits in learning and memory. Mechanistically, we find that phosphorylation of serine-880 of the GluA2 AMPAR subunit (GluA2-S880) is decreased while phosphorylation of tyrosine-876 on GluA2 (GluA2-Y876) is elevated during chemically induced LTP. This enhances the strength of the GRIP1-AMPAR association and, subsequently, the insertion of AMPARs into the postsynaptic membrane. Together, these results demonstrate an essential role of GRIP1 in regulating AMPAR trafficking during synaptic plasticity and learning and memory.

Challenges in Physiological Phenotyping of hiPSC-Derived Neurons: From 2D Cultures to 3D Brain Organoids

Neurons derived from human induced pluripotent stem cells (hiPSC-derived neurons) offer novel opportunities for the development of preclinical models of human neurodegenerative diseases (NDDs). Recent advances in the past few years have increased substantially the potential of these techniques and have uncovered new challenges that the field is facing. Here, we outline and discuss challenges related to the functional characterization of hiPSC-derived neurons and propose ways to overcome current difficulties. In particular, the enormous variability among studies in the electrical properties of hiPSC-derived neurons and broad differences in cell maturation are factors that impair reproducibility. Furthermore, we discuss how the use of 3D brain organoids are of help in resolving some difficulties posed by 2D cultures. Finally, we elaborate on recent and future advances that may help to overcome the discussed challenges and speed-up progress in the field.

Synaptic Organization of the Human Temporal Lobe Neocortex as Revealed by High-Resolution Transmission, Focused Ion Beam Scanning, and Electron Microscopic Tomography

Modern electron microscopy (EM) such as fine-scale transmission EM, focused ion beam scanning EM, and EM tomography have enormously improved our knowledge about the synaptic organization of the normal, developmental, and pathologically altered brain. In contrast to various animal species, comparably little is known about these structures in the human brain. Non-epileptic neocortical access tissue from epilepsy surgery was used to generate quantitative 3D models of synapses. Beside the overall geometry, the number, size, and shape of active zones and of the three functionally defined pools of synaptic vesicles representing morphological correlates for synaptic transmission and plasticity were quantified. EM tomography further allowed new insights in the morphological organization and size of the functionally defined readily releasable pool. Beside similarities, human synaptic boutons, although comparably small (approximately 5 µm), differed substantially in several structural parameters, such as the shape and size of active zones, which were on average 2 to 3-fold larger than in experimental animals. The total pool of synaptic vesicles exceeded that in experimental animals by approximately 2 to 3-fold, in particular the readily releasable and recycling pool by approximately 2 to 5-fold, although these pools seemed to be layer-specifically organized. Taken together, synaptic boutons in the human temporal lobe neocortex represent unique entities perfectly adapted to the “job” they have to fulfill in the circuitry in which they are embedded. Furthermore, the quantitative 3D models of synaptic boutons are useful to explain and even predict the functional properties of synaptic connections in the human neocortex.

The role of Hebbian learning in human perception: a methodological and theoretical review of the human Visual Long-Term Potentiation paradigm

Long-term potentiation (LTP) is one of the most widely studied forms of neural plasticity, and is thought to be the principle mechanism underlying long-term memory and learning in the brain. Sensory paradigms utilising electroencephalography (EEG) and sensory stimulation to induce LTP have allowed translation from rodent and primate invasive research to non-invasive human investigations. This review focusses on visual sensory LTP induced using repetitive visual stimulation, resulting in changes in the visually evoked response recorded at the scalp with EEG. Across 15 years of use and replication in humans several major paradigm variants for eliciting visual LTP have emerged. The application of different paradigms, and the broad implementation of visual LTP across different populations combines to provide a rich and sensitive account of Hebbian LTP, and potentially non-Hebbian plasticity mechanisms. This review will conclude with a discussion of how these findings have advanced existing theories of perceptual learning by positioning Hebbian learning both alongside and within other major theories such as Predictive Coding and The Free Energy Principle.

Mini-review: Aging of the neuroendocrine system: Insights from nonhuman primate models

The neuroendocrine system (NES) plays a crucial role in synchronizing the physiology and behavior of the whole organism in response to environmental constraints. The NES consists of a hypothalamic-pituitary-target organ axis that acts in coordination to regulate growth, reproduction, stress and basal metabolism. The growth (or somatotropic), hypothalamic-pituitary-gonadal (HPG), hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-thyroid (HPT) axes are therefore finely tuned by the hypothalamus through the successive release of hypothalamic and pituitary hormones to control the downstream physiological functions. These functions rely on a complex set of mechanisms requiring tight synchronization between peripheral organs and the hypothalamic-pituitary complex, whose functionality can be altered during aging. Here, we review the results of research on the effects of aging on the NES of nonhuman primate (NHP) species in wild and captive conditions. A focus on the age-related dysregulation of the master circadian pacemaker, which, in turn, alters the synchronization of the NES with the organism environment, is proposed. Finally, practical and ethical considerations of using NHP models to test the effects of nutrition-based or hormonal treatments to combat the deterioration of the NES are discussed.

Activity-dependent tuning of intrinsic excitability in mouse and human neurogliaform cells

The ability to modulate the efficacy of synaptic communication between neurons constitutes an essential property critical for normal brain function. Animal models have proved invaluable in revealing a wealth of diverse cellular mechanisms underlying varied plasticity modes. However, to what extent these processes are mirrored in humans is largely uncharted thus questioning their relevance in human circuit function. In this study, we focus on neurogliaform cells, that possess specialized physiological features enabling them to impart a widespread inhibitory influence on neural activity. We demonstrate that this prominent neuronal subtype, embedded in both mouse and human neural circuits, undergo remarkably similar activity-dependent modulation manifesting as epochs of enhanced intrinsic excitability. In principle, these evolutionary conserved plasticity routes likely tune the extent of neurogliaform cell mediated inhibition thus constituting canonical circuit mechanisms underlying human cognitive processing and behavior.

Human Cerebrospinal Fluid Induces Neuronal Excitability Changes in Resected Human Neocortical and Hippocampal Brain Slices

Human cerebrospinal fluid (hCSF) has proven advantageous over conventional medium for culturing both rodent and human brain tissue. In addition, increased activity and synchrony, closer to the dynamic states exclusively recorded in vivo, were reported in rodent slices and cell cultures switching from artificial cerebrospinal fluid (aCSF) to hCSF. This indicates that hCSF possesses properties that are not matched by the aCSF, which is generally used for most electrophysiological recordings. To evaluate the possible significance of using hCSF as an electrophysiological recording medium, also for human brain tissue, we compared the network and single-cell firing properties of human brain slice cultures during perfusion with hCSF and aCSF. For measuring the overall activity from a majority of neurons within neocortical and hippocampal human slices, we used a microelectrode array (MEA) recording technique with 252 electrodes covering an area of 3.2 × 3.2 mm2. A second CMOS-based MEA with 4225 sensors on a 2 × 2 mm2 area was used for detailed mapping of action potential waveforms and cell identification. We found that hCSF increased the number of active electrodes and neurons and the firing rate of the neurons in the slices and induced an increase in the numbers of single channel and population bursts. Interestingly, not only an increase in the overall activity in the slices was observed, but a reconfiguration of the network could also be detected with specific activation and inactivation of subpopulations of neuronal ensembles. In conclusion, hCSF is an important component to consider for future human brain slice studies, especially for experiments designed to mimic parts of physiology and disease observed in vivo.

Functional Consequences of Calcium-Dependent Synapse-to-Nucleus Communication: Focus on Transcription-Dependent Metabolic Plasticity

In the nervous system, calcium signals play a major role in the conversion of synaptic stimuli into transcriptional responses. Signal-regulated gene transcription is fundamental for a range of long-lasting adaptive brain functions that include learning and memory, structural plasticity of neurites and synapses, acquired neuroprotection, chronic pain, and addiction. In this review, we summarize the diverse mechanisms governing calcium-dependent transcriptional regulation associated with central nervous system plasticity. We focus on recent advances in the field of synapse-to-nucleus communication that include studies of the signal-regulated transcriptome in human neurons, identification of novel regulatory mechanisms such as activity-induced DNA double-strand breaks, and the identification of novel forms of activity- and transcription-dependent adaptations, in particular, metabolic plasticity. We summarize the reciprocal interactions between different kinds of neuroadaptations and highlight the emerging role of activity-regulated epigenetic modifiers in gating the inducibility of signal-regulated genes.

Human in vitro systems for examining synaptic function and plasticity in the brain

The human brain shows remarkable complexity in its cellular makeup and function, which are distinct from nonhuman species, signifying the need for human-based research platforms for the study of human cellular neurophysiology and neuropathology. However, the use of adult human brain tissue for research purposes is hampered by technical, methodological, and accessibility challenges. One of the major problems is the limited number of in vitro systems that, in contrast, are readily available from rodent brain tissue. With recent advances in the optimization of protocols for adult human brain preparations, there is a significant opportunity for neuroscientists to validate their findings in human-based systems. This review addresses the methodological aspects, advantages, and disadvantages of human neuron in vitro systems, focusing on the unique properties of human neurons and synapses in neocortical microcircuits. These in vitro models provide the incomparable advantage of being a direct representation of the neurons that have formed part of the human brain until the point of recording, which cannot be replicated by animal models nor human stem-cell systems. Important distinct cellular mechanisms are observed in human neurons that may underlie the higher order cognitive abilities of the human brain. The use of human brain tissue in neuroscience research also raises important ethical, diversity, and control tissue limitations that need to be considered. Undoubtedly however, these human neuron systems provide critical information to increase the potential of translation of treatments from the laboratory to the clinic in a way animal models are failing to provide.

Gestational B-vitamin supplementation alleviates PM2.5-induced autism-like behavior and hippocampal neurodevelopmental impairment in mice offspring

Gestational exposure to PM_2.5 is a worldwide environmental issue associated with long-lasting behavior abnormalities and neurodevelopmental impairments in the hippocampus of offspring. PM_2.5 may induce hippocampus injury and lead to autism-like behavior such as social communication deficits and stereotyped repetitive behavior in children through neuroinflammation and neurodegeneration. Here, we investigated the preventive effect of B-vitamin on PM_2.5-induced deleterious effects by focusing on anti-inflammation, antioxidant, synaptic remodeling and neurodevelopment. Pregnant mice were randomly divided into three groups including control group (mice subject to PBS only), model group (mice subject to both 30 μL PM_2.5 of 3.456 μg/μL and 10 mL/(kg·d) PBS), and intervention group (mice subject to both 30 μL PM_2.5 of 3.456 μg/μL and 10 mL/(kg·d) B-vitamin supplementation (folic acid, vitamin B6 and vitamin B12 with concentrations at 0.06, 1.14 and 0.02 mg/mL, respectively)). In the current study B-vitamin significantly alleviated neurobehavioral impairment reflected in reduced social communication disorders, stereotyped repetitive behavior, along with learning and spatial memory impairment in PM_2.5-stimulated mice offspring. Next, B-vitamin corrected synaptic loss and reduced mitochondrial damage in hippocampus of mice offspring, demonstrated by normalized synapse quantity, synaptic cleft, postsynaptic density (PSD) thickness and length of synaptic active area. Furthermore, significantly down-regulated expression of pro-inflammatory cytokines including NF-κB, TNF-α and IL-1β, and lipid peroxidation were found. We observed elevated levels of oxidant-related genes (SOD, GSH and GSH-Px). Moreover, decreased cleaved caspase-3 and TUNEL-positive cells suggested inhibited PM_2.5-induced apoptosis by B-vitamin. Furthermore, B-vitamin increased neurogenesis by increasing EdU-positive cells in the subgranular zone (SGZ) of offspring. Collectively, our results suggest that B-vitamin supplementation exerts preventive effect on autism-like behavior and neurodevelopmental impairment in hippocampus of mice offspring gestationally exposed to PM_2.5, to which alleviated mitochondrial damage, increased anti-inflammatory and antioxidant capacity and synaptic efficiency, reduced neuronal apoptosis and improved hippocampal neurogenesis may contribute.

Ultrastructural heterogeneity of layer 4 excitatory synaptic boutons in the adult human temporal lobe neocortex

Synapses are fundamental building blocks controlling and modulating the ‘behavior’ of brain networks. How their structural composition, most notably their quantitative morphology underlie their computational properties remains rather unclear, particularly in humans. Here, excitatory synaptic boutons (SBs) in layer 4 (L4) of the temporal lobe neocortex (TLN) were quantitatively investigated. Biopsies from epilepsy surgery were used for fine-scale and tomographic electron microscopy (EM) to generate 3D-reconstructions of SBs. Particularly, the size of active zones (AZs) and that of the three functionally defined pools of synaptic vesicles (SVs) were quantified. SBs were comparatively small (~2.50 μm²), with a single AZ (~0.13 µm²); preferentially established on spines. SBs had a total pool of ~1800 SVs with strikingly large readily releasable (~20), recycling (~80) and resting pools (~850). Thus, human L4 SBs may act as ‘amplifiers’ of signals from the sensory periphery, integrate, synchronize and modulate intra- and extracortical synaptic activity.

Synaptogenic pathways

During synaptogenesis, presynaptic and postsynaptic assembly are driven by diverse molecular mechanisms, mediated by intrinsic as well as extrinsic factors. How these processes are initiated and coordinated are open questions. Synapse specificity, or synaptic partner selection, is widely understood to be determined by the trans-synaptic binding of cell adhesion molecules. However, in vivo evidence that cell adhesion molecules subsequently function to initiate synapse assembly, as initially proposed, is lacking. Here, we present a summary of our current understanding of synaptogenic pathways that mediate presynaptic and postsynaptic assembly and the coordination of these processes.

3'-Daidzein Sulfonate Sodium Protects Against Chronic Cerebral Hypoperfusion-Mediated Cognitive Impairment and Hippocampal Damage via Activity-Regulated Cytoskeleton-Associated Protein Upregulation

The learning and memory impairment caused by chronic cerebral hypoperfusion (CCH) is permanent and seriously affects the daily life of patients and their families. The compound 3'-daidzein sulfonate sodium (DSS) protects against CCH-mediated memory impairment and hippocampal damage in a rat model. In the present study, we further investigated the underlying mechanisms of this effect in the rat two-vessel occlusion (2VO) and the oxygen and glucose deprivation (OGD) primary hippocampal neuron models. The hippocampal expression of the activity-regulated cytoskeleton associated protein (Arc) following DSS administration was detected in vivo and in vitro and behavioral testing was used to investigate the role of Arc in the DSS-mediated rescue of CCH-induced neurotoxicity. DSS increased hippocampal Arc expression both in vivo and in vitro. Arc overexpression increased and Arc knockdown decreased hippocampal neuronal densities in rat 2VO model, when compared to DSS treatment alone. Arc overexpression decreased and Arc knockdown increased apoptotic hippocampal neurons in rat 2VO and OGD primary hippocampal neuron models, when compared to DSS treatment alone. Arc overexpression enhanced and Arc knockdown inhibited the beneficial effect of DSS on 2VO-induced cognitive impairment. DSS restored the neuronal OGD-mediated phosphorylation decrease in protein kinase alpha (PKA), extracellular signal-regulated protein kinases 1/2 (ERK1/2) and cAMP response element binding protein (CREB), in vitro. PKA and ERK1/2 inhibition blocked the DSS-mediated effects on neuronal apoptosis and OGD-induced Arc downregulation. In conclusion, DSS protects against CCH-mediated cognitive impairment and hippocampal damage via Arc upregulation, which is activated by the PKA/CREB and ERK/CREB signaling pathways. Our study further confirms the potential use of DSS as an effective treatment for CCH-associated diseases.

Plasticity versus chronicity: Stable performance on category fluency 40 years post-onset

What is the long-term trajectory of semantic memory deficits in patients who have suffered structural brain damage? Memory is, per definition, a changing faculty. The traditional view is that after an initial recovery period, the mature human brain has little capacity to repair or reorganize. More recently, it has been suggested that the central nervous system may be more plastic with the ability to change in neural structure, connectivity, and function. The latter observations are, however, largely based on normal learning in healthy subjects. Here, we report a patient who suffered bilateral ventro-medial damage after presumed herpes encephalitis in 1971. He was seen regularly in the eighties, and we recently had the opportunity to re-assess his semantic memory deficits. On semantic category fluency, he showed a very clear category-specific deficit performing better that control data on non-living categories and significantly worse on living items. Recent testing showed that his impairments have remained unchanged for more than 40 years. We suggest cautiousness when extrapolating the concept of brain plasticity, as observed during normal learning, to plasticity in the context of structural brain damage.