Diferenças funcionais das regiões hipocampais na formação da memória do tipo o quê", "quando" e "onde" em ratos

Episodic memory refers to the recollection of what, where and when a specific event occurred. Hippocampus is a key structure in this type of memory. Computational models suggest that the dentate gyrus (DG) and the CA3 hippocampal subregions are involved in pattern separation and the rapid acquisition of episodic memories, while CA1 is involved in memory consolidation. However there are few studies with animal models that access simultaneously the aspects ‗what-where-when . Recently, an object recognition episodic-like memory task in rodents was proposed. This task consists of two sample trials and a test phase. In sample trial one, the rat is exposed to four copies of an object. In sample trial two, one hour later, the rat is exposed to four copies of a different object. In the test phase, 1 h later, two copies of each of the objects previously used are presented. One copy of the object used in sample trial one is located in a different place, and therefore it is expected to be the most explored object.However, the short retention delay of the task narrows its applications. This study verifies if this task can be evoked after 24h and whether the pharmacological inactivation of the DG/CA3 and CA1 subregions could differentially impair the acquisition of the task described. Validation of the task with a longer interval (24h) was accomplished (animals showed spatiotemporal object discrimination and scopolamine (1 mg/kg, ip) injected pos-training impaired performance). Afterwards, the GABA agonist muscimol, (0,250 μg/μl; volume = 0,5 μl) or saline were injected in the hippocampal subregions fifteen minutes before training. Pre-training inactivation of the DG/CA3 subregions impaired the spatial discrimination of the objects (‗where ), while the temporal discrimination (‗when ) was preserved. Rats treated with muscimol in the CA1 subregion explored all the objects equally well, irrespective of place or presentation time. Our results corroborate the computational models that postulate a role for DG/CA3 in spatial pattern separation, and a role for CA1 in the consolidation process of different mnemonic episodes

Antidepressivos modificam a extinção de uma memória aversiva em ratas

Treatment of major depression, posttraumatic stress disorder and other psychopathologies with antidepressants can be associated with improvement of the cognitive deficits related to these disorders. Although the mechanisms of these effects are not completely elucidated, alterations in extinction of aversive memories are believed to be present in these psychopathologies. Moreover, researches with laboratory animals usually focus on male subjects, and we have recently verified that extinction of an aversive task is reduced in female rats when compared to males. In the present study, female rats were long-term treated with clinically used antidepressants (fluoxetine, nortriptyline or mirtazapine) and tested in the plus-maze discriminative avoidance and forced swimming tests in order to evaluate learning, memory, extinction, anxiety and depression-related behaviors. All groups learned the task, but learning was somewhat faster in nortriptyline and mirtazapine-treated animals . Task retrieval was also showed by all experimental groups. Chronic treatment with fluoxetine, but not with the other antidepressants, increased extinction of the discriminative task. In the forced swimming test, animals treated with fluoxetine and mirtazapine showed decreased immobility duration. In conclusion, antidepressants interfere with learning and female rats treated with fluoxetine presented increased extinction of the aversive memory task. On the other hand, both fluoxetine and mirtazapine were effective in the forced swimming test, suggesting dissociation between the antidepressant effects and the extinction of aversive memories

Influência do ciclo estral no efeito do diazepam na ansiedade e memória de ratas

Memory and anxiety are related phenomena. Several evidences suggest that anxiety is fundamental for learnining and may facilitate or impair the memory formation process depending of the context. The majority of animal studies of anxiety and fear use only males as experimental subjects, while studies with females are rare in the literature. However, the prevalence in phobic and anxiety disorders is greater in women than in men. Moreover, it is known that gender maybe influence benzodiazepine effects, the classic drugs used for anxiety disorders treatment. In this respect, to further investigate if fear/anxiety aspects related to learning in female subjects would contribute to the study of phobic and anxiety disorders and their relationship with learning/memory processes, the present work investigates (a) the effects of benzodiazepine diazepam on female rats performance in a aversive memory task that assess concomitantly anxiety/emotionality, as the interaction between both; (b) the influence of estrous cycle phases of female rats on diazepam effects at aversive memory and anxiety/emotionality, and the interaction between both and (c) the role of hormonal fluctuations during estrous cycle phases in absence of diazepam effects in proestrus, because female rats in this phase received or not mifepristone, the antagonist of progesterone receptor, previously to the diazepam treatment. For this purpose, the plus maze discriminative avoidance task, previously validated for studies of anxiety concomitantly to learning/memory, was used. The apparatus employed is an adaptation of a conventional plus maze, with two opens arms and two closed arms, one of which presenting aversive stimulation (noise and light). The parameters used were: time in non-aversive arm compared to time in aversive and percentage of time in aversive arm on several temporal divisions, in order to evaluate memory; percentage of time in open arms, risk assessment, head dipping and end exploring to evaluate anxiety ; and distance traveled for locomotion. In experiment I, we found anxiolytic effect of diazepam only for 4 mg/kg dose, however the amnestic effect appear at a dose of 2 mg/kg. In second experiment, rats were divided in groups according estrous cycle phase (metaestrus/diestrus, proestrus e estrus). In this experiment, when we considered estrous cycle phase or diazepam treatment, the results did not demonstrate any differences in anxiety/emotionality parameters. The amnestic effects of diazepam occur in female rats in metestrus/diestrus and estrus and is absent in proestrous rats. Proestrous female rats that received mifepristone exhibited the amnestic effect of diazepam and also anxiolytic effects, that it was not previously observed in this dose. The results have demonstrated dissociation of anxiolytic and amnestic diazepam effects, not previously observed in males; the absence of amnestic effect of diazepam in proestrous phase; and the possible role of progesterone in aversive memory over diazepam effect, because the mifepristone, associated with diazepam, caused amnestic effect in proestrus

Morphological and functional study of cerebral aging in rat

Aging is a physiological process characterized by a progressive decline of the “cellular homeostatic reserve”, refereed as the capability to respond suitably to exogenous and endogenous stressful stimuli. Due to their high energetic requests and post-mitotic nature, neurons are peculiarly susceptible to this phenomenon. However, the aged brain maintains a certain level of adaptive capacities and if properly stimulated may warrant a considerable functional recovery. Aim of the present research was to verify the plastic potentialities of the aging brain of rats subjected to two kind of exogenous stimuli: A) the replacement of the standard diet with a ketogenic regimen (the change forces the brain to use ketone bodies (KB) in alternative to glucose to satisfy the energetic needs) and B) a behavioural task able to induce the formation of inhibitory avoidance memory. A) Fifteen male Wistar rats of 19 months of age were divided into three groups (average body weight pair-matched), and fed for 8 weeks with different dietary regimens: i) diet containing 10% medium chain triglycerides (MCT); ii) diet containing 20% MCT; iii) standard commercial chow. Five young (5 months of age) and five old (26-27 months of age) animals fed with the standard diet were used as further controls. The following morphological parameters reflecting synaptic plasticity were evaluated in the stratum moleculare of the hippocampal CA1 region (SM CA1), in the outer molecular layer of the hippocampal dentate gyrus (OML DG), and in the granule cell layer of the cerebellar cortex (GCL-CCx): average area (S), numeric density (Nvs), and surface density (Sv) of synapses, and average volume (V), numeric density (Nvm), and volume density (Vv) of synaptic mitochondria. Moreover, succinic dehydrogenase (SDH) activity was cytochemically determined in Purkinje cells (PC) and V, Nvm, Vv, and cytochemical precipitate area/mitochondrial area (R) of SDH-positive mitochondria were evaluated. In SM CA1, MCT-KDs induced the early appearance of the morphological patterns typical of old animals: higher S and V, and lower Nvs and Nvm. On the contrary, in OML DG, Sv and Vv of MCT-KDs-fed rats were higher (as a result of higher Nvs and Nvm) vs. controls; these modifications are known to improve synaptic function and metabolic supply. The opposite effects of MCT-KDs might reflect the different susceptibility of these brain regions to the aging processes: OML DG is less vulnerable than SM CA1, and the reactivation of ketone bodies uptake and catabolism might occur more efficiently in this region, allowing the exploitation of their peculiar metabolic properties. In GCL-CCx, the results described a new scenario in comparison to that found in the hippocampal formation: 10%MCT-KD induced the early appearance of senescent patterns (decreased Nvs and Nvm; increased V), whereas 20%MCT-KD caused no changes. Since GCL-CCx is more vulnerable to age than DG, and less than CA1, these data further support the hypothesis that MCT-KDs effects in the aging brain critically depend on neuronal vulnerability to age, besides MCT percentage. Regarding PC, it was decided to evaluate only the metabolic effect of the dietetic regimen (20%MCT-KD) characterized by less side effects. KD counteracted age-related decrease in numeric density of SDH-positive mitochondria, and enhanced their energetic efficiency (R was significantly higher in MCT-KD-fed rats vs. all the controls). Since it is well known that Purkinje and dentate gyrus cells are less vulnerable to aging than CA1 neurons, these results corroborate our previous hypothesis. In conclusion, the A) experimental line provides the first evidence that morphological and functional parameters reflecting synaptic plasticity and mitochondrial metabolic competence may be modulated by MCT-KDs in the pre-senescent central nervous system, and that the effects may be heterogeneous in different brain regions. MCT-KDs seem to supply high energy metabolic intermediates and to be beneficial (“anti-aging”) for those neurons that maintain the capability to exploit them. This implies risks but also promising potentialities for the therapeutic use of these diets during aging B) Morphological parameters of synapses and synaptic mitochondria in SM CA1 were investigated in old (26-27 month-old) female Wistar rats following a single trial inhibitory avoidance task. In this memory protocol animals learn to avoid a dark compartment in which they received a mild, inescapable foot-shock. Rats were tested 3 and 6 or 9 hours after the training, divided into good and bad responders according to their performance (retention times above or below 100 s, respectively) and immediately sacrificed. Nvs, S, Sv, Nvm, V, and Vv were evaluated. In the good responder group, the numeric density of synapses and mitochondria was significantly higher and the average mitochondrial volume was significantly smaller 9 hours vs. 6 hours after the training. No significant differences were observed among bad responders. Thus, better performances in passive avoidance memory task are correlated with more efficient plastic remodeling of synaptic contacts and mitochondria in hippocampal CA1. These findings indicate that maintenance of synaptic plastic reactivity during aging is a critical requirement for preserving long-term memory consolidation.

Aging and experience in the recognition of musical transpositions

The authors examined the effects of age, musical experience, and characteristics of musical stimuli on a melodic short-term memory task in which participants had to recognize whether a tune was an exact transposition of another tune recently presented. Participants were musicians and nonmusicians between ages 18 and 30 or 60 and 80. In 4 experiments, the authors found that age and experience affected different aspects of the task, with experience becoming more influential when interference was provided during the task. Age and experience interacted only weakly, and neither age nor experience influenced the superiority of tonal over atonal materials. Recognition memory for the sequences did not reflect the same pattern of results as the transposition task. The implications of these results for theories of aging, experience, and music cognition are discussed.

Metamemory judgments for familiar and unfamiliar tunes

Metamemory is an important skill that allows humans to monitor their own memory abilities; however, little research has concerned what perceptual information influences metamemory judgments. A series of experiments assessed the accuracy of metamemory judgments for music as well as determined if metamemory judgments are affected by ease of processing of musical features. A recognition memory task inconjunction with metamemory judgments (Judgments of Learning, or JOLs) were used to determine actual and predicted memory performance. We found that changing the ease of processing of the volume and timbre of unfamiliar tunes affected metamemory judgments, but not memory performance, for unfamiliar tunes. Manipulating the ease ofprocessing of the timbre and tempo of familiar tunes did not affect metamemory judgments or memory performance although metamemory accuracy on an item-by-item basis was better for familiar tunes as compared to unfamiliar tunes. Thus, metamemory judgments for unfamiliar tunes are more sensitive to ease of processing changes ascompared to familiar tunes, suggesting that different types of information are processed in different ways.

The Role of Features and Context in Recognition of Novel Melodies

WE INVESTIGATED HOW WELL STRUCTURAL FEATURES such as note density or the relative number of changes in the melodic contour could predict success in implicit and explicit memory for unfamiliar melodies. We also analyzed which features are more likely to elicit increasingly confident judgments of "old" in a recognition memory task. An automated analysis program computed structural aspects of melodies, both independent of any context, and also with reference to the other melodies in the testset and the parent corpus of pop music. A few features predicted success in both memory tasks, which points to a shared memory component. However, motivic complexity compared to a large corpus of pop music had different effects on explicit and implicit memory. We also found that just a few features are associated with different rates of "old" judgments, whether the items were old or new. Rarer motives relative to the testset predicted hits and rarer motives relative to the corpus predicted false alarms. This data-driven analysis provides further support for both shared and separable mechanisms in implicit and explicit memory retrieval, as well as the role of distinctiveness in true and false judgments of familiarity.

BRAIN ACTIVATION AND CONNECTIVITY IN NON-DISABLED MULTIPLE SCLEROSIS PATIENTS

Multiple sclerosis (MS) is the most common demyelinating disease affecting the central nervous system. There is no cure for MS and current therapies have limited efficacy. While the majority of individuals with MS develop significant clinical disability, a subset experiences a disease course with minimal impairment even in the presence of significant apparent tissue damage on magnetic resonance imaging (MRI). The current studies combined functional MRI and diffusion tensor imaging (DTI) to elucidate brain mechanisms associated with lack of clinical disability in patients with MS. Recent evidence has implicated cortical reorganization as a mechanism to limit the clinical manifestation of the disease. Functional MRI was used to test the hypothesis that non-disabled MS patients (Expanded Disability Status Scale ≤ 1.5) show increased recruitment of cognitive control regions (dorsolateral prefrontal and anterior cingulate cortex) while performing sensory, motor and cognitive tasks. Compared to matched healthy controls, patients increased activation of cognitive control brain regions when performing non-dominant hand movements and the 2-back working memory task. Using dynamic causal modeling, we tested whether increased cognitive control recruitment is associated with alterations in connectivity in the working memory functional network. Patients exhibited similar network connectivity to that of control subjects when performing working memory tasks. We subsequently investigated the integrity of major white matter tracts to assess structural connectivity and its relation to activation and functional integration of the cognitive control system. Patients showed substantial alterations in callosal, inferior and posterior white matter tracts and less pronounced involvement of the corticospinal tracts and superior longitudinal fasciculi (SLF). Decreased structural integrity within the right SLF in patients was associated with decreased performance, and decreased activation and connectivity of the cognitive control system when performing working memory tasks. These studies suggest that patient with MS without clinical disability increase cognitive control system recruitment across functional domains and rely on preserved functional and structural connectivity of brain regions associated with this network. Moreover, the current studies show the usefulness of combining brain activation data from functional MRI and structural connectivity data from DTI to improve our understanding of brain adaptation mechanisms to neurological disease.

First evidence for differential and sequential efferent effects of stimulus relevance and goal conduciveness appraisal.

In the context of a memory task, participants were presented with pictures displaying biological and cultural threat stimuli or neutral stimuli (stimulus relevance manipulation) with superimposed symbols signaling monetary gains or losses (goal conduciveness manipulation). Results for heart rate and facial electromyogram show differential efferent effects of the respective appraisal outcomes and provide first evidence for sequential processing, as postulated by Scherer's component process model of emotion. Specifically, as predicted, muscle activity over the brow and cheek regions marking the process of relevance appraisal occurred significantly earlier than facial muscle activity markers of goal conduciveness appraisal. Heart rate, in contrast, was influenced by the stimulus relevance manipulation only.

Integration versus segregation in functional brain networks

We propose a new methodology to evaluate the balance between segregation and integration in functional brain networks by using singular value decomposition techniques. By means of magnetoencephalography, we obtain the brain activity of a control group of 19 individuals during a memory task. Next, we project the node-to-node correlations into a complex network that is analyzed from the perspective of its modular structure encoded in the contribution matrix. In this way, we are able to study the role that nodes play I/O its community and to identify connector and local hubs. At the mesoscale level, the analysis of the contribution matrix allows us to measure the degree of overlapping between communities and quantify how far the functional networks are from the configuration that better balances the integrated and segregated activity

Functional hubs in mild cognitive impairment

We investigate how hubs of functional brain networks are modified as a result of mild cognitive impairment (MCI), a condition causing a slight but noticeable decline in cognitive abilities, which sometimes precedes the onset of Alzheimer's disease. We used magnetoencephalography (MEG) to investigate the functional brain networks of a group of patients suffering from MCI and a control group of healthy subjects, during the execution of a short-term memory task. Couplings between brain sites were evaluated using synchronization likelihood, from which a network of functional interdependencies was constructed and the centrality, i.e. importance, of their nodes was quantified. The results showed that, with respect to healthy controls, MCI patients were associated with decreases and increases in hub centrality respectively in occipital and central scalp regions, supporting the hypothesis that MCI modifies functional brain network topology, leading to more random structures.

Cognitive reserve is associated with the functional organization of brain networks in healthy aging: a MEG study

The proportion of elderly people in the population has increased rapidly in the last century and consequently "healthy aging" is expected to become a critical area of research in neuroscience. Evidence reveals how healthy aging depends on three main behavioral factors: social lifestyle, cognitive activity and physical activity. In this study, we focused on the role of cognitive activity, concentrating specifically on educational and occupational attainment factors, which were considered two of the main pillars of cognitive reserve. 21 subjects with similar rates of social lifestyle, physical and cognitive activity were selected from a sample of 55 healthy adults. These subjects were divided into two groups according to their level of cognitive reserve; one group comprised subjects with high cognitive reserve (9 members) and the other contained those with low cognitive reserve (12 members). To evaluate the cortical brain connectivity network, all participants were recorded by Magnetoencephalography (MEG) while they performed a memory task (modified version of the Sternberg¿s Task). We then applied two algorithms (Phase Locking Value & Phase-Lag Index) to study the dynamics of functional connectivity. In response to the same task, the subjects with lower cognitive reserve presented higher functional connectivity than those with higher cognitive reserve. These results may indicate that participants with low cognitive reserve needed a greater 'effort' than those with high cognitive reserve to achieve the same level of cognitive performance. Therefore, we conclude that cognitive reserve contributes to the modulation of the functional connectivity patterns of the aging brain.

Anomalous Consistency in Mild Cognitive Impairment: A Complex Networks Approach

Increased variability in performance has been associated with the emergence of several neurological and psychiatric pathologies. However, whether and how consistency of neuronal activity may also be indicative of an underlying pathology is still poorly understood. Here we propose a novel method for evaluating consistency from non-invasive brain recordings. We evaluate the consistency of the cortical activity recorded with magnetoencephalography in a group of subjects diagnosed with Mild Cognitive Impairment (MCI), a condition sometimes prodromal of dementia, during the execution of a memory task. We use metrics coming from nonlinear dynamics to evaluate the consistency of cortical regions. A representation known as parenclitic networks is constructed, where atypical features are endowed with a network structure, the topological properties of which can be studied at various scales. Pathological conditions correspond to strongly heterogeneous networks, whereas typical or normative conditions are characterized by sparsely connected networks with homogeneous nodes. The analysis of this kind of networks allows identifying the extent to which consistency is affected in the MCI group and the focal points where MCI is especially severe. To the best of our knowledge, these results represent the first attempt at evaluating the consistency of brain functional activity using complex networks theory.

Impaired long-term potentiation induction in dentate gyrus of calretinin-deficient mice

Calretinin (Cr) is a Ca2+ binding protein present in various populations of neurons distributed in the central and peripheral nervous systems. We have generated Cr-deficient (Cr−/−) mice by gene targeting and have investigated the associated phenotype. Cr−/− mice were viable, and a large number of morphological, biochemical, and behavioral parameters were found unaffected. In the normal mouse hippocampus, Cr is expressed in a widely distributed subset of GABAergic interneurons and in hilar mossy cells of the dentate gyrus. Because both types of cells are part of local pathways innervating dentate granule cells and/or pyramidal neurons, we have explored in Cr−/− mice the synaptic transmission between the perforant pathway and granule cells and at the Schaffer commissural input to CA1 pyramidal neurons. Cr−/− mice showed no alteration in basal synaptic transmission, but long-term potentiation (LTP) was impaired in the dentate gyrus. Normal LTP could be restored in the presence of the GABAA receptor antagonist bicuculline, suggesting that in Cr−/− dentate gyrus an excess of γ-aminobutyric acid (GABA) release interferes with LTP induction. Synaptic transmission and LTP were normal in CA1 area, which contains only few Cr-positive GABAergic interneurons. Cr−/− mice performed normally in spatial memory task. These results suggest that expression of Cr contributes to the control of synaptic plasticity in mouse dentate gyrus by indirectly regulating the activity of GABAergic interneurons, and that Cr−/− mice represent a useful tool to understand the role of dentate LTP in learning and memory.

Increased discrimination of “false memories” in autism spectrum disorder

Individuals with autism spectrum disorder (ASD) have impaired ability to use context, which may manifest as alterations of relatedness within the semantic network. However, impairment in context use may be more difficult to detect in high-functioning adults with ASD. To test context use in this population, we examined the influence of context on memory by using the “false memory” test. In the false memory task, lists of words were presented to high-functioning subjects with ASD and matched controls. Each list consists of words highly related to an index word not on the list. Subjects are then given a recognition test. Positive responses to the index words represent false memories. We found that individuals with ASD are able to discriminate false memory items from true items significantly better than are control subjects. Memory in patients with ASD may be more accurate than in normal individuals under certain conditions. These results also suggest that semantic representations comprise a less distributed network in high-functioning adults with ASD. Furthermore, these results may be related to the unusually high memory capacities found in some individuals with ASD. Research directed at defining the range of tasks performed superiorly by high-functioning individuals with ASD will be important for optimal vocational rehabilitation.