DNA recombination as a possible mechanism in declarative memory: a hypothesis

Memory: Synaptic or Cellular, That Is the Question

According to the commonly accepted opinion, memory engrams are formed and stored at the level of neural networks due to a change in the strength of synaptic connections between neurons. This hypothesis of synaptic plasticity (HSP), formulated by Donald Hebb in the 1940s, continues to dominate the directions of experimental studies and the interpretations of experimental results in the field. The universal acceptance of the HSP has transformed it from a hypothesis into an incontrovertible theory. In this article, I show that the entire body of experimental and clinical data obtained in studies of long-term memory in mammals and humans is inconsistent with the HSP. Instead, these data suggest that long-term memory is formed and stored at the intracellular level where it is reliably protected from ongoing synaptic activity, including pathological epileptic activity. It seems that the generally accepted HSP became a serious obstacle to understanding the mechanisms of memory and that progress in this field requires rethinking this doctrine and shifting experimental efforts toward exploring the intracellular mechanisms.

Personality changes following heart transplantation: The role of cellular memory

Personality changes following heart transplantation, which have been reported for decades, include accounts of recipients acquiring the personality characteristics of their donor. Four categories of personality changes are discussed in this article: (1) changes in preferences, (2) alterations in emotions/temperament, (3) modifications of identity, and (4) memories from the donor's life. The acquisition of donor personality characteristics by recipients following heart transplantation is hypothesized to occur via the transfer of cellular memory, and four types of cellular memory are presented: (1) epigenetic memory, (2) DNA memory, (3) RNA memory, and (4) protein memory. Other possibilities, such as the transfer of memory via intracardiac neurological memory and energetic memory, are discussed as well. Implications for the future of heart transplantation are explored including the importance of reexamining our current definition of death, studying how the transfer of memories might affect the integration of a donated heart, determining whether memories can be transferred via the transplantation of other organs, and investigating which types of information can be transferred via heart transplantation. Further research is recommended.

Identification and Characterization of the V(D)J Recombination Activating Gene 1 in Long-Term Memory of Context Fear Conditioning

An increasing body of evidence suggests that mechanisms related to the introduction and repair of DNA double strand breaks (DSBs) may be associated with long-term memory (LTM) processes. Previous studies from our group suggested that factors known to function in DNA recombination/repair machineries, such as DNA ligases, polymerases, and DNA endonucleases, play a role in LTM. Here we report data using C57BL/6 mice showing that the V(D)J recombination-activating gene 1 (RAG1), which encodes a factor that introduces DSBs in immunoglobulin and T-cell receptor genes, is induced in the amygdala, but not in the hippocampus, after context fear conditioning. Amygdalar induction of RAG1 mRNA, measured by real-time PCR, was not observed in context-only or shock-only controls, suggesting that the context fear conditioning response is related to associative learning processes. Furthermore, double immunofluorescence studies demonstrated the neuronal localization of RAG1 protein in amygdalar sections prepared after perfusion and fixation. In functional studies, intra-amygdalar injections of RAG1 gapmer antisense oligonucleotides, given 1 h prior to conditioning, resulted in amygdalar knockdown of RAG1 mRNA and a significant impairment in LTM, tested 24 h after training. Overall, these findings suggest that the V(D)J recombination-activating gene 1, RAG1, may play a role in LTM consolidation.

Increased 5-bromo-2'-deoxyuridine incorporation in various brain structures following passive avoidance training in mice

We studied the effects of training on DNA synthesis intensity in mouse brain. Brain cells where DNA synthesis-associated processes took place under the influence of training were detected by immunohistochemical labeling of DNA molecules with synthetic thymine analogue 5-bromo-2'-deoxyuridine. The number of 5-bromo-2'-deoxyuridine-positive cell increased in various structures of the brain under the influence of training.

Impaired social recognition memory in recombination activating gene 1-deficient mice

The recombination activating genes (RAGs) encode two enzymes that play key roles in the adaptive immune system. RAG1 and RAG2 mediate VDJ recombination, a process necessary for the maturation of B- and T-cells. Interestingly, RAG1 is also expressed in the brain, particularly in areas of high neural density such as the hippocampus, although its function is unknown. We tested evidence that RAG1 plays a role in brain function using a social recognition memory task, an assessment of the acquisition and retention of conspecific identity. In a first experiment, we found that RAG1-deficient mice show impaired social recognition memory compared to mice wildtype for the RAG1 allele. In a second experiment, by breeding to homogenize background genotype, we found that RAG1-deficient mice show impaired social recognition memory relative to heterozygous or RAG2-deficient littermates. Because RAG1 and RAG2 null mice are both immunodeficient, the results suggest that the memory impairment is not an indirect effect of immunological dysfunction. RAG1-deficient mice show normal habituation to non-socially derived odors and habituation to an open-field, indicating that the observed effect is not likely a result of a general deficit in habituation to novelty. These data trace the origin of the impairment in social recognition memory in RAG1-deficient mice to the RAG1 gene locus and implicate RAG1 in memory formation.

Weismann, Wittgenstein and the homunculus fallacy

A problem that has troubled both neo-Darwinists and neo-Lamarckians is whether instincts involve knowledge. This paper discusses the contributions to this problem of the evolutionary biologist August Weismann and the philosopher Ludwig Wittgenstein. Weismann discussed an empirical homunculus fallacy: Lamarck's theory mistakenly presupposes a homunculus in the germ cells. Wittgenstein discussed a conceptual homunculus fallacy which applies to Lamarck's theory: it is mistaken to suppose that knowledge is stored in the brain or DNA. The upshot of these two fallacies is that instincts arise through a neo-Darwinian process but are not cognitions in the sense that they involve (the recollection of stored) knowledge. Although neo-Lamarckians have rightly argued that learning processes may contribute to the development of instincts, their ideas about the role of knowledge in the evolution and development of instincts are mistaken.

Two functions of early language experience

The unique human ability of linguistic communication, defined as the ability to produce a practically infinite number of meaningful messages using a finite number of lexical items, is determined by an array of "linguistic" genes, which are expressed in neurons forming domain-specific linguistic centers in the brain. In this review, I discuss the idea that infants' early language experience performs two complementary functions. In addition to allowing infants to assimilate the words and grammar rules of their mother language, early language experience initiates genetic programs underlying language production and comprehension. This hypothesis explains many puzzling characteristics of language acquisition, such as the existence of a critical period for acquiring the first language and the absence of a critical period for the acquisition of additional language(s), a similar timetable for language acquisition in children belonging to families of different social and cultural status, the strikingly similar timetables in the acquisition of oral and sign languages, and the surprisingly small correlation between individuals' final linguistic competence and the intensity of their training. Based on the studies of microcephalic individuals, I argue that genetic factors determine not only the number of neurons and organization of interneural connections within linguistic centers, but also the putative internal properties of neurons that are not limited to their electrophysiological and synaptic properties.

Identification of flap structure-specific endonuclease 1 as a factor involved in long-term memory formation of aversive learning

We previously proposed that DNA recombination/repair processes play a role in memory formation. Here, we examined the possible role of the fen-1 gene, encoding a flap structure-specific endonuclease, in memory consolidation of conditioned taste aversion (CTA). Quantitative real-time PCR showed that amygdalar fen-1 mRNA induction was associated to the central processing of the illness experience related to CTA and to CTA itself, but not to the central processing resulting from the presentation of a novel flavor. CTA also increased expression of the Fen-1 protein in the amygdala, but not the insular cortex. In addition, double immunofluorescence analyses showed that amygdalar Fen-1 expression is mostly localized within neurons. Importantly, functional studies demonstrated that amygdalar antisense knockdown of fen-1 expression impaired consolidation, but not short-term memory, of CTA. Overall, these studies define the fen-1 endonuclease as a new DNA recombination/repair factor involved in the formation of long-term memories.

Immune and nervous systems share molecular and functional similarities: memory storage mechanism

One of the most complex and important features of both the nervous and immune systems is their data storage and retrieval capability. Both systems encounter a common and complex challenge on how to overcome the cumbersome task of data management. Because each neuron makes many synapses with other neurons, they are capable of receiving data from thousands of synaptic connections. The immune system B and T cells have to deal with a similar level of complexity because of their unlimited task of recognizing foreign antigens. As for the complexity of memory storage, it has been proposed that both systems may share a common set of molecular mechanisms. Here, we review the molecular bases of memory storage in neurons and immune cells based on recent studies and findings. The expression of certain molecules and mechanisms shared between the two systems, including cytokine networks, and cell surface receptors, are reviewed. Intracellular signaling similarities and certain mechanisms such as diversity, memory storage, and their related molecular properties are briefly discussed. Moreover, two similar genetic mechanisms used by both systems is discussed, putting forward the idea that DNA recombination may be an underlying mechanism involved in CNS memory storage.

Early postnatal stress alters the extinction of context-dependent conditioned fear in adult rats

Fear extinction is hypothesized to be a learning process based on a new inhibitory memory. The present study was conducted to elucidate the effect of early postnatal stress on the extinction of context-dependent fear memory in adult rats, with a focus on the serotonergic system. Extinction was estimated by the expression of freezing behavior during repeated extinction trials (i.e., repeated exposure to contextual fear conditioning) on consecutive days. The decrease in fear expression was attenuated in adult rats that had been subjected to footshock (FS) at the third postnatal week (3wFS), but not in those exposed to footshock at the second postnatal week (2wFS). The decreased attenuation of freezing behavior observed in 3wFS was abolished by repeated treatment with the partial N-methyl-D-aspartate receptor agonist D-cycloserine (15 mg/kg, i.p., for 4 days), which has been shown to facilitate cue-dependent extinction. Repeated treatment with the serotonin 5-hydroxytryptamine-1A (5-HT(1A)) receptor agonist tandospirone (1 mg/kg, i.p., for 4 days) prevented the expression of freezing behavior in 3wFS, whereas diazepam treatment (1 mg/kg, i.p., for 4 days) in 3wFS did not. These results suggest that exposure to early postnatal stress at the third week is responsible for attenuating extinction of contextual fear conditioning and is mediated by a serotonergic 5-HT(1A) receptor mechanism. In other words, exposure to traumatic events during the early postnatal period might precipitate long-lasting alterations in synaptic function that underlie extinction processes of context-dependent fear memory.

“The seven sins” of the Hebbian synapse: Can the hypothesis of synaptic plasticity explain long-term memory consolidation?

Memorizing new facts and events means that entering information produces specific physical changes within the brain. According to the commonly accepted view, traces of memory are stored through the structural modifications of synaptic connections, which result in changes of synaptic efficiency and, therefore, in formations of new patterns of neural activity (the hypothesis of synaptic plasticity). Most of the current knowledge on learning and initial stages of memory consolidation ("synaptic consolidation") is based on this hypothesis. However, the hypothesis of synaptic plasticity faces a number of conceptual and experimental difficulties when it deals with potentially permanent consolidation of declarative memory ("system consolidation"). These difficulties are rooted in the major intrinsic self-contradiction of the hypothesis: stable declarative memory is unlikely to be based on such a non-stable foundation as synaptic plasticity. Memory that can last throughout an entire lifespan should be "etched in stone." The only "stone-like" molecules within living cells are DNA molecules. Therefore, I advocate an alternative, genomic hypothesis of memory, which suggests that acquired information is persistently stored within individual neurons through modifications of DNA, and that these modifications serve as the carriers of elementary memory traces.

Alzheimer's disease, brain immune privilege and memory: a hypothesis

The most distinctive feature of Alzheimer's disease (AD) is the specific degeneration of the neurons involved in memory consolidation, storage, and retrieval. Patients suffering from AD forget basic information about their past, loose linguistic and calculative abilities and communication skills. Thus, understanding the etiology of AD may provide insights into the mechanisms of memory and vice versa. The brain is an immunologically privileged site protected from the organism's immune reactions by the blood-brain barrier (BBB). All risk factors for AD (both cardiovascular and genetic) lead to destruction of the BBB. Evidence emerging from recent literature indicates that AD may have an autoimmune nature associated with BBB impairments. This hypothesis suggests that the process of memory consolidation involves the synthesis of novel macromolecules recognized by the immune system as "non-self" antigens. The objective of this paper is to stimulate new approaches to studies of neural mechanisms underpinning memory consolidation and its breakdown during AD. If the hypothesis on the autoimmune nature of AD is correct, the identification of the putative antigenic macromolecules might be critical to understanding the etiology and prevention of AD, as well as for elucidating cellular mechanisms of learning and memory.

Somatic DNA recombination in the brain

Possible somatic DNA recombination in the brain has been investigated by attempting to capture direct or indirect evidence of it. Until recently, the biological significance of the DNA event, the genes is involved in the recombination, or even whether the event actually occurs in the brain has remained unclear. The DNA-rearranged locus-oriented approach and the recombination activity-oriented approach have mutually contributed to the elucidation of the biological features of extra-immune system somatic DNA recombination. There have been only 2 loci proposed for the candidate, one is a repetitive sequence and the other DNA recombination is nonrepetitive locus. This review states conventional concepts and discussions chronologically and finally to the newest aspects of DNA rearrangement in the brain.

An inhibitor of DNA recombination blocks memory consolidation, but not reconsolidation, in context fear conditioning

Genomic recombination requires cutting, processing, and rejoining of DNA by endonucleases, polymerases, and ligases, among other factors. We have proposed that DNA recombination mechanisms may contribute to long-term memory (LTM) formation in the brain. Our previous studies with the nucleoside analog 1-beta-D-arabinofuranosylcytosine triphosphate (ara-CTP), a known inhibitor of DNA ligases and polymerases, showed that this agent blocked consolidation of conditioned taste aversion without interfering with short-term memory (STM). However, because polymerases and ligases are also essential for DNA replication, it remained unclear whether the effects of this drug on consolidation were attributable to interference with DNA recombination or neurogenesis. Here we show, using C57BL/6 mice, that ara-CTP specifically blocks consolidation but not STM of context fear conditioning, a task previously shown not to require neurogenesis. The effects of a single systemic dose of cytosine arabinoside (ara-C) on LTM were evident as early as 6 h after training. In addition, although ara-C impaired LTM, it did not impair general locomotor activity nor induce brain neurotoxicity. Importantly, hippocampal, but not insular cortex, infusions of ara-C also blocked consolidation of context fear conditioning. Separate studies revealed that context fear conditioning training significantly induced nonhomologous DNA end joining activity indicative of DNA ligase-dependent recombination in hippocampal, but not cortex, protein extracts. Finally, unlike inhibition of protein synthesis, systemic ara-C did not block reconsolidation of context fear conditioning. Our results support the idea that DNA recombination is a process specific to consolidation that is not involved in the postreactivation editing of memories.

Evidence that DNA (cytosine-5) methyltransferase regulates synaptic plasticity in the hippocampus

DNA (cytosine-5) methylation represents one of the most widely used mechanisms of enduring cellular memory. Stable patterns of DNA methylation are established during development, resulting in creation of persisting cellular phenotypes. There is growing evidence that the nervous system has co-opted a number of cellular mechanisms used during development to subserve the formation of long term memory. In this study, we examined the role DNA (cytosine-5) methyltransferase (DNMT) activity might play in regulating the induction of synaptic plasticity. We found that the DNA within promoters for reelin and brain-derived neurotrophic factor, genes implicated in the induction of synaptic plasticity in the adult hippocampus, exhibited rapid and dramatic changes in cytosine methylation when DNMT activity was inhibited. Moreover, zebularine and 5-aza-2-deoxycytidine, inhibitors of DNMT activity, blocked the induction of long term potentiation at Schaffer collateral synapses. Activation of protein kinase C in the hippocampus decreased reelin promoter methylation and increased DNMT3A gene expression. Interestingly, DNMT activity is required for protein kinase C-induced increases in histone H3 acetylation. Considered together, these results suggest that DNMT activity is dynamically regulated in the adult nervous system and that DNMT may play a role in regulating the induction of synaptic plasticity in the mature CNS.

Somatic DNA recombination in a mouse genomic region, BC-1, in brain and non-brain tissueThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

The genomic region BC-1 (GenBank acc. No. AB075899 ) on mouse chromosome 16 has been reported as a genomic region undergoing somatic DNA recombination producing circular DNA and genomic deletion in brain during late embryogenesis. The present study shows that the BC-1 circular DNA production had already started on the 13th day of embryonic age, earlier than the previous observation that the circular DNA production started on the 15th through 17th embryonic day. The BC-1 deletion was also observed in the spleen and ocular lens. In situ hybridization analysis indicated that a human-homologous region in the BC-1 sequence was expressed in the lens at a perinatal period. These data suggest that the somatic DNA recombination in the BC-1 region is not restricted to brain tissue, and that the BC-1 DNA recombination relates to lens development.

Stuck in time without a nucleus: theoretical comment on Sangha et Al. (2005)

Is forgetting caused by the passage of time or by interference from new learning? S. Sangha et al. (2005) offer strong support for the latter idea by using the sea snail Lymnaea. Memory for inhibitory avoidance was prolonged from 3 days to 7 days by preventing the snails from making unreinforced conditioned responses (extinction) following training. Similar effects were obtained with posttraining ablation of the soma of the right pedal dorsal 1, the neuron necessary for consolidation, reconsolidation, and extinction in this task. Without the soma, Lymnaea was unable to retain any new learning or forget old learning, hence remaining "stuck in time." These findings elegantly demonstrate that transcriptional regulation of gene expression is essential for memory consolidation: Local protein synthesis is not sufficient. Furthermore, memory for conditioning and extinction can coexist in the same neuron.

X-linked imprinting: effects on brain and behaviour

Imprinted genes are monoallelically expressed in a parent-of-origin-dependent manner and can affect brain and behavioural phenotypes. The X chromosome is enriched for genes affecting neurodevelopment and is donated asymmetrically to male and female progeny. Hence, X-linked imprinted genes could potentially influence sexually dimorphic neurobiology. Consequently, investigations into such loci may provide new insights into the biological basis of behavioural differences between the sexes and into why men and women show different vulnerabilities to certain mental disorders. In this review, we summarise recent advances in our knowledge of X-linked imprinted genes and the brain substrates that they may act upon. In addition, we suggest strategies for identifying novel X-linked imprinted genes and their downstream effects and discuss evolutionary theories regarding the origin and maintenance of X-linked imprinting.

Consolidation of fear extinction requires protein synthesis in the medial prefrontal cortex

Extinction of conditioned fear is thought to form a long-term memory of safety, but the neural mechanisms are poorly understood. Consolidation of extinction learning in other paradigms requires protein synthesis, but the involvement of protein synthesis in extinction of conditioned fear remains unclear. Here, we show that rats infused intraventricularly with the protein synthesis inhibitor anisomycin extinguished normally within a session but were unable to recall extinction the following day. Anisomycin-treated rats showed no savings in the rate of re-learning of extinction, consistent with amnesia for extinction training. The identical effect was observed when anisomycin was microinfused into the medial prefrontal cortex (mPFC) but not the insular cortex. Furthermore, we observed that extinction training increased c-Fos levels in the mPFC but not in the insular cortex, consistent with extinction-induced gene expression in the mPFC. These findings extend previous lesion and unit-recording data by demonstrating that the mPFC is a critical storage site for extinction memory, rather than simply a pathway for expression of extinction. Understanding consolidation of fear extinction could lead to new treatments for anxiety disorders in which fear extinction is thought to be compromised.

Lead (Pb(+2)) impairs long-term memory and blocks learning-induced increases in hippocampal protein kinase C activity

The long-term storage of information in the brain known as long-term memory (LTM) depends on a variety of intracellular signaling cascades utilizing calcium (Ca2+) and cyclic adenosine monophosphate as second messengers. In particular, Ca(+2)/phospholipid-dependent protein kinase C (PKC) activity has been proposed to be necessary for the transition from short-term memory to LTM. Because the neurobehavioral toxicity of lead (Pb(+2)) has been associated to its interference with normal Ca(+2) signaling in neurons, we studied its effects on spatial learning and memory using a hippocampal-dependent discrimination task. Adult rats received microinfusions of either Na+ or Pb(+2) acetate in the CA1 hippocampal subregion before each one of four training sessions. A retention test was given 7 days later to examine LTM. Results suggest that intrahippocampal Pb(+2) did not affect learning of the task, but significantly impaired retention. The effects of Pb(+2) selectively impaired reference memory measured in the retention test, but had no effect on the general performance because it did not affect the latency to complete the task during the test. Finally, we examined the effects of Pb(+2) on the induction of hippocampal Ca(+2)/phospholipid-dependent PKC activity during acquisition training. The results showed that Pb(+2) interfered with the learning-induced activation of Ca(+2)/phospholipid-dependent PKC on day 3 of acquisition. Overall, our results indicate that Pb(+2) causes cognitive impairments in adult rats and that such effects might be subserved by interference with Ca(+2)-related signaling mechanisms required for normal LTM.

The antimetabolite ara-CTP blocks long-term memory of conditioned taste aversion

We examined the hypothesis that processes related to DNA recombination and repair are involved in learning and memory. Rats received intracerebroventricular (i.c.v.) infusions of the antimetabolite 1-beta-D-arabinofuranosylcytosine triphosphate (ara-CTP) or its precursor cytosine arabinoside (ara-C) 30 min prior to conditioned taste aversion (CTA) training. Both ara-CTP and ara-C caused significant impairments in long-term memory (LTM) of CTA. Control experiments indicate that the effect of ara-CTP on CTA memory is related to interference with learning. Furthermore, as it was previously demonstrated for the protein synthesis inhibitor anisomycin, ara-CTP had no effect on CTA memory when it was injected 1 h after training. Importantly, although both ara-CTP and anisomycin significantly blocked LTM in the task, short-term memory (STM) measured 1 h after training was not affected by either of the drugs. Finally, ara-CTP had no effect on in vitro transcription, but it did effectively block nonhomologous DNA end joining (NHEJ) activity of brain protein extracts. We suggest that DNA ligase-mediated DNA recombination and repair processes are necessary for the expression of LTM in the brain.

Experience-dependent expression of terminal deoxynucleotidyl transferase in mouse brain

Terminal deoxynucleotidyl transferase (TdT), a template-independent DNA polymerase, contributes to antigen receptor diversity in lymphocytes. Using in situ hybridization, we found that tdt is expressed within neurons of the adult mouse brain. tdt mRNA was localized within pyramidal neurons in the hippocampus, granule and polymorphic cells in the dentate gyrus, Purkinje neurons in the cerebellum, and cortical cells. Increased levels of tdt mRNA in the hippocampus, neocortex, and cerebellum were associated with rearing C57BL/6 mice, but not DBA/2 mice, in enriched environments. Unlike wild types (WT), tdt (-/-) mice did not show improvement in spatial learning and memory as a result of rearing in enriched environments. These results suggest that tdt may be involved in learning and memory saving.

Diversity of the cadherin-related neuronal receptor/protocadherin family and possible DNA rearrangement in the brain

Both the brain and the immune systems are complex. The complexity is generated by enormously diversified single cells. In the immune system, extensive cell death, gene regulation of immunoglobulin (Ig) and T-cell receptor (TCR) gene expression, and somatic rearrangement and mutations are known to generate an enormous diversity of lymphocytes. In this process, double-strand DNA breaks (DSBs) and DSB repair play significant roles. These processes at a DNA level are also physiologically significant in the nervous system during neurogenesis, and chromosomal variations have been detected in the nucleus of differentiated neurones. In another parallel with the immune system, cadherin-related neuronal receptors (CNRs) are diversified synaptic proteins. The CNR genes belong to protocadherin (Pcdh) gene clusters. Genomic organizations of CNR/Pcdh genes are similar to that of the Ig and TCR genes. Somatic mutations in and combinatorial gene regulation of CNR/Pcdh transcripts during neurogenesis have been reported. This review focuses on the diversity of the CNR/Pcdh genes and possible DNA diversification in the nervous system.

Embryonic expression and regulation of the large zinc finger protein KRC

KRC fusion proteins bind to the kappaB enhancer motif and to the signal sequences of V(D)J recombination. Here we have characterized endogenous KRC in mouse embryos and lymphoma cell lines. Starting from midgestation, neuronal- and lymphoid-restricted expression of KRC was observed from the dorsal root ganglia, trigeminal ganglion, thymus, and cerebral cortex. Several B-cell lines produced an alternatively spliced KRC transcript of 4.5 kb and a 115-kDa DNA-binding protein isoform. Additionally, that KRC transcript was induced by lipopolysaccharide, a potent activator of cells in immunity and inflammation. In genetic-engineered B cells stably transfected with inducible expression vectors for the recombination activating genes RAG1, RAG2, or both, the avidity of KRC to DNA was markedly decreased when RAG1 and RAG2 were overexpressed. We hypothesize that KRC may function in developing thymocytes and neurons, where its role might be transcription regulation or DNA recombination.

Role of individual neurons and neural networks in cognitive functioning of the brain: a new insight

The prevailing concept in modern neuroscience is that neuron networks play a dominant role in the functioning of the nervous system, whereas the role of individual neurons is rather insignificant. This concept suggests that "individuality" of single neurons is primarily determined by their place in a network rather than their intrinsic properties. Here I argue that individual neurons may play an important, if not decisive, role in performing cognitive functions of the brain. This tentative viewpoint is supported by experimental and clinical insights into disorders of cognitive functions and by genetic studies of cognitive abilities and disabilities. The results obtained in these studies indicate that many specific cognitive functions are carried out by groups of highly specialized neurons whose roles in performing these functions are genetically predetermined and their activity could not be substituted by the activity of other neurons. In this context, the main role of neural networks and intercellular interactions is to form dynamic ensembles of neurons involved in performing a given cognitive function.