Reactivation-dependent amnesia for object recognition memory is contingent on hippocampal theta-gamma coupling during recall.

Hippocampal dopamine D1/D5 receptor-dependent destabilization is necessary for object recognition memory (ORM) updating through reconsolidation. Dopamine also regulates hippocampal theta and gamma oscillations, which are involved in novelty and memory processing. We found that, in adult male rats, ORM recall in the presence of a novel object, but not in the presence of a familiar one, triggers hippocampal theta-gamma coupling. Hippocampal theta-gamma coupling (hPAC) does not happen when ORM destabilization is prevented by blocking D1/D5 receptors, but artificial hPAC generation during recall in the presence of a familiar object enables the amnesic effect of reconsolidation inhibitors. Therefore, hPAC controls ORM destabilization, and its modulation could increase reconsolidation-based psychotherapy efficacy.

Essential Oils from Colombian Plants: Antiviral Potential against Dengue Virus Based on Chemical Composition, In Vitro and In Silico Analyses.

Currently, there are no therapies to prevent severe dengue disease. Essential oils (EOs) can serve as primary sources for research and the discovery of phytomedicines for alternative therapy. Fourteen EOs samples were obtained by distillation from six plants used in Colombian folk medicine. GC/MS analysis identified 125 terpenes. Cytopathic effect (CPE) reduction assays revealed differences in antiviral activity. EOs of Lippia alba, citral chemotype and carvone-rich fraction; Lippia origanoides, phellandrene chemotype; and Turnera diffusa, exhibited strong antiviral activity (IC50: 29 to 82 µg/mL; SI: 5.5 to 14.3). EOs of Piper aduncum, Ocimum basilicum, and L. origanoides, carvacrol, and thymol chemotypes, exhibited weak antiviral activity (32 to 53% DENV-CPE reduction at 100 µg/mL; SI > 5.0). Cluster and one-way ANOVA analyses suggest that the strong antiviral activity of EOs could be attributed to increased amounts of non-phenolic oxygenated monoterpenes and sesquiterpene hydrocarbons. Docking analyses (AutoDock Vina) predicted binding affinity between the DENV-2 E protein and terpenes: twenty sesquiterpene hydrocarbons (−8.73 to −6.91 kcal/mol), eight oxygenated monoterpenes (−7.52 to −6.98 kcal/mol), and seven monoterpene hydrocarbons (−7.60 to −6.99 kcal/mol). This study reports for the first time differences in the antiviral activity of EOs against DENV, corresponding to their composition of monoterpenes and sesquiterpenes.

Cross-Frequency Phase-Amplitude Coupling between Hippocampal Theta and Gamma Oscillations during Recall Destabilizes Memory and Renders It Susceptible to Reconsolidation Disruption.

Avoidance memory reactivation at recall triggers theta-gamma hippocampal phase amplitude coupling (hPAC) only when it elicits hippocampus-dependent reconsolidation. However, it is not known whether there is a causal relationship between these phenomena. We found that in adult male Wistar rats, silencing the medial septum during recall did not affect avoidance memory expression or maintenance but abolished hPAC and the amnesia caused by the intrahippocampal administration of reconsolidation blockers, both of which were restored by concomitant theta burst stimulation of the fimbria-fornix pathway. Remarkably, artificial hPAC generated by fimbria-fornix stimulation during recall of a learned avoidance response naturally resistant to hippocampus-dependent reconsolidation made it susceptible to reactivation-dependent amnesia. Our results indicate that hPAC mediates the destabilization required for avoidance memory reconsolidation and suggest that the generation of artificial hPAC at recall overcomes the boundary conditions of this process.SIGNIFICANCE STATEMENT Theta-gamma hippocampal phase-amplitude coupling (hPAC) increases during the induction of hippocampus-dependent avoidance memory reconsolidation. However, whether hPAC plays a causal role in this process remains unknown. Using behavioral, electrophysiological, optogenetic, and biochemical tools in adult male Wistar rats, we demonstrate that reactivation-induced hPAC is necessary for avoidance memory destabilization, and that artificial induction of this patterned activity during recall of reconsolidation-resistant aversive memories renders them liable to the amnesic effect of reconsolidation inhibitors.

PKMζ Inhibition Disrupts Reconsolidation and Erases Object Recognition Memory.

Object recognition memory (ORM) confers the ability to discriminate the familiarity of previously encountered items. Reconsolidation is the process by which reactivated memories become labile and susceptible to modifications. The hippocampus is specifically engaged in reconsolidation to integrate new information into the original ORM through a mechanism involving activation of brain-derived neurotrophic factor (BDNF) signaling and induction of LTP. It is known that BDNF can control LTP maintenance through protein kinase Mζ (PKMζ), an atypical protein kinase C isoform that is thought to sustain memory storage by modulating glutamatergic neurotransmission. However, the potential involvement of PKMζ in ORM reconsolidation has never been studied. Using a novel ORM task combined with pharmacological, biochemical, and electrophysiological tools, we found that hippocampal PKMζ is essential to update ORM through reconsolidation, but not to maintain the inactive recognition memory trace stored over time, in adult male Wistar rats. Our results also indicate that hippocampal PKMζ acts downstream of BDNF and controls AMPAR synaptic insertion to elicit reconsolidation and suggest that blocking PKMζ activity during this process deletes active ORM.SIGNIFICANCE STATEMENT Object recognition memory (ORM) is essential to remember facts and events. Reconsolidation integrates new information into ORM through changes in hippocampal plasticity and brain-derived neurotrophic factor (BDNF) signaling. In turn, BDNF enhances synaptic efficacy through protein kinase Mζ (PKMζ), which might preserve memory. Here, we present evidence that hippocampal PKMζ acts downstream of BDNF to regulate AMPAR recycling during ORM reconsolidation and show that this kinase is essential to update the reactivated recognition memory trace, but not to consolidate or maintain an inactive ORM. We also demonstrate that the amnesia provoked by disrupting ORM reconsolidation through PKMζ inhibition is due to memory erasure and not to retrieval failure.

Dengue in Santander State, Colombia: fluctuations in the prevalence of virus serotypes are linked to dengue incidence and genetic diversity of the circulating viruses.

OBJECTIVE: To investigate the link between fluctuations in the prevalence of dengue virus (DENV) serotypes and the number of dengue cases in the metropolitan area of Bucaramanga, Santander State, Colombia, in the 2007-2010 and 2014-2017 periods. METHOD: Viruses were isolated from febrile patient samples by direct application to C6/36-HT cells and typed using monoclonal antibodies. We performed autocorrelation and cross-correlation analyses to determine whether fluctuations in the prevalence of DENV serotypes and dengue cases were correlated. Full envelope (E) gene sequences were employed to examine the genetic diversity of serotypes circulating by using a phylogenetic approach. RESULTS: All four dengue virus serotypes were detected. DENV-1 was the dominant serotype in both periods followed by DENV-3 or DENV-2 depending on the period; DENV-4 was the least prevalent virus in both periods. Cross-correlation analyses suggest a temporal relation between the fluctuations in the prevalence of DENV serotypes, which were almost simultaneous (lag = 0) or related to recent past fluctuations (lag > 1.0) in the number of dengue cases. Data suggest that a sustained predominance of DENV-1, an increase of the DENV-4 prevalence, and a switch from DENV-3 to DENV-2 could be linked to an outbreak. Circulating viruses were grouped into Genotype V, Asia/American III and II for DENV-1, -2, -3 and -4, respectively; intragenotypic diversity was detected. CONCLUSIONS: The present work highlights the need of comprehensive studies on dynamics of DENV in Colombia to understand transmission of dengue and evaluate the effectiveness of a vaccination programme.

OBJECTIF: Etudier le lien entre les fluctuations de la prévalence des sérotypes du virus de la dengue (DENV) et le nombre de cas de dengue dans la région métropolitaine de Bucaramanga, dans l'Etat de Santander, en Colombie, au cours des périodes 2007-2010 et 2014-2017. MÉTHODE: Les virus ont été isolés à partir d'échantillons de patients fébriles par application directe sur des cellules C6/36-HT et typés à l'aide d'anticorps monoclonaux. Nous avons effectué des analyses d'autocorrélation et de corrélation croisée afin de déterminer si les fluctuations de la prévalence des sérotypes du DENV et des cas de dengue étaient corrélées. Des séquences de gènes d'enveloppe complète (E) ont été utilisées pour examiner la diversité génétique des sérotypes en circulation en utilisant une approche phylogénétique. RÉSULTATS: Tous les quatre sérotypes du virus de la dengue ont été détectés. DENV-1 était le sérotype dominant dans les deux périodes, suivi de DENV-3 ou DENV-2, selon la période; le virus DENV-4 était le moins prévalent au cours des deux périodes. Les analyses de corrélation croisée suggèrent une relation temporelle entre les fluctuations de la prévalence des sérotypes de DENV, qui étaient presque simultanées (lag = 0) ou liées aux fluctuations passées récentes (lag > 1,0) du nombre de cas de dengue. Les données suggèrent qu'une prédominance durable de DENV-1, qu'une augmentation de la prévalence de DENV-4 et qu'un passage de DENV-3 à DENV-2 pourraient être liés à une éclosion. Les virus en circulation ont été regroupés dans les génotypes V, Asie/Amérique III et II pour DENV-1, -2, -3 et -4, respectivement; une diversité intra-génotypique a été détectée. CONCLUSIONS: Le présent travail souligne la nécessité d'études approfondies sur la dynamique du DENV en Colombie afin de comprendre la transmission de la dengue et évaluer l'efficacité d'un programme de vaccination.

Prior Learning of Relevant Nonaversive Information Is a Boundary Condition for Avoidance Memory Reconsolidation in the Rat Hippocampus.

Reactivated memories can be modified during reconsolidation, making this process a potential therapeutic target for posttraumatic stress disorder (PTSD), a mental illness characterized by the recurring avoidance of situations that evoke trauma-related fears. However, avoidance memory reconsolidation depends on a set of still loosely defined boundary conditions, limiting the translational value of basic research. In particular, the involvement of the hippocampus in fear-motivated avoidance memory reconsolidation remains controversial. Combining behavioral and electrophysiological analyses in male Wistar rats, we found that previous learning of relevant nonaversive information is essential to elicit the participation of the hippocampus in avoidance memory reconsolidation, which is associated with an increase in theta- and gamma-oscillation power and cross-frequency coupling in dorsal CA1 during reactivation of the avoidance response. Our results indicate that the hippocampus is involved in memory reconsolidation only when reactivation results in contradictory representations regarding the consequences of avoidance and suggest that robust nesting of hippocampal theta-gamma rhythms at the time of retrieval is a specific reconsolidation marker.SIGNIFICANCE STATEMENT Posttraumatic stress disorder (PTSD) is characterized by maladaptive avoidance responses to stimuli or behaviors that represent or bear resemblance to some aspect of a traumatic experience. Disruption of reconsolidation, the process by which reactivated memories become susceptible to modifications, is a promising approach for treating PTSD patients. However, much of what is known about fear-motivated avoidance memory reconsolidation derives from studies based on fear conditioning instead of avoidance-learning paradigms. Using a step-down inhibitory avoidance task in rats, we found that the hippocampus is involved in memory reconsolidation only when the animals acquired the avoidance response in an environment that they had previously learned as safe and showed that increased theta- and gamma-oscillation coupling during reactivation is an electrophysiological signature of this process.

Motion contrast in primary visual cortex: a direct comparison of single neuron and population encoding.

Features from outside the classical receptive field (CRF) can modulate the stimulus-driven activity of single cells in the primary visual cortex. This modulation, mediated by horizontal and feedback networks, has been extensively described as a variation of firing rate and is considered the basis of processing features as, for example, motion contrast. However, surround influences have also been identified in pairwise spiking or local field coherence. Yet, evidence about co-existence and integration of different neural signatures remains elusive. To compare multiple signatures, we recorded spiking and LFP activity evoked by stimuli exhibiting a motion contrast in the CRFs surround in anesthetized cat primary visual cortex. We chose natural-like scenes over gratings to avoid predominance of simple visual features, which could be easily represented by a rate code. We analyzed firing rates and phase-locking to low-gamma frequency in single cells and neuronal assemblies. Motion contrast was reflected in all measures but in semi-independent populations. Whereas activation of assemblies accompanied single neuron rates, their phase relations were modulated differently. Interestingly, only assembly phase relations mirrored the direction of movement of the surround and were selectively affected by thermal deactivation of visual interhemispheric connections. We argue that motion contrast can be reflected in complementary and superimposed neuronal signatures that can represent different surround features in independent neuronal populations.

Selective interhemispheric circuits account for a cardinal bias in spontaneous activity within early visual areas.

Ongoing brain activity exhibits patterns resembling neural ensembles co-activated by stimulation or task performance. Such patterns have been attributed to the brain's functional architecture, e.g. selective long-range connections. Here, we directly investigate the contribution of selective connections between hemispheres to spontaneous and evoked maps in cat area 18 close to the 17/18 border. We recorded voltage-sensitive dye imaging maps and spiking activity while manipulating interhemispheric input by reversibly deactivating corresponding contralateral areas. During deactivation, spontaneous maps continued to be generated with similar frequency and quality as in the intact network but a baseline cardinal bias disappeared. Consistently, neurons preferring either horizontal (HN) or vertical (VN), as opposed to oblique contours, decreased their resting state activity. HN decreased their rates also when stimulated visually. We conclude that structured spontaneous maps are primarily generated by thalamo- and/or intracortical connectivity. However, selective long-range connections through the corpus callosum - in perpetuation of the long-range intracortical network - contribute to a cardinal bias, possibly, because they are stronger or more frequent between neurons preferring horizontal and/or cardinal contours. As those contours are easier perceived and appear more frequently in natural environment, long-range connections might provide visual cortex with a grid for probabilistic grouping operations in a larger visual scene.

Optogenetic inactivation of the medial septum impairs long-term object recognition memory formation.

Theta is one of the most prominent extracellular synchronous oscillations in the mammalian brain. Hippocampal theta relies on an intact medial septum (MS) and has been consistently recorded during the training phase of some learning paradigms, suggesting that it may be implicated in hippocampus-dependent long-term memory processing. Object recognition memory (ORM) allows animals to identify familiar items and is essential for remembering facts and events. In rodents, long-term ORM formation requires a functional hippocampus but the involvement of the MS in this process remains controversial. We found that training adult male Wistar rats in a long-term ORM-inducing learning task involving exposure to two different, but behaviorally equivalent novel stimuli objects increased hippocampal theta power, and that suppressing theta via optogenetic MS inactivation caused amnesia. Importantly, the amnesia was specific to the object the animals were exploring when the MS was inactivated. Taken together, our results indicate that the MS is necessary for long-term ORM formation and suggest that hippocampal theta activity is causally linked to this process.

Spatial clustering of orientation preference in primary visual cortex of the large rodent agouti.

All rodents investigated so far possess orientation-selective neurons in the primary visual cortex (V1) but - in contrast to carnivores and primates - no evidence of periodic maps with pinwheel-like structures. Theoretical studies debating whether phylogeny or universal principles determine development of pinwheels point to V1 size as a critical constraint. Thus, we set out to study maps of agouti, a big diurnal rodent with a V1 size comparable to cats'. In electrophysiology, we detected interspersed orientation and direction-selective neurons with a bias for horizontal contours, corroborated by homogeneous activation in optical imaging. Compatible with spatial clustering at short distance, nearby neurons tended to exhibit similar orientation preference. Our results argue against V1 size as a key parameter in determining the presence of periodic orientation maps. They are consistent with a phylogenetic influence on the map layout and development, potentially reflecting distinct retinal traits or interspecies differences in cortical circuitry.

Callosal Influence on Visual Receptive Fields Has an Ocular, an Orientation-and Direction Bias.

One leading hypothesis on the nature of visual callosal connections (CC) is that they replicate features of intrahemispheric lateral connections. However, CC act also in the central part of the binocular visual field. In agreement, early experiments in cats indicated that they provide the ipsilateral eye part of binocular receptive fields (RFs) at the vertical midline (Berlucchi and Rizzolatti, 1968), and play a key role in stereoscopic function. But until today callosal inputs to receptive fields activated by one or both eyes were never compared simultaneously, because callosal function has been often studied by cutting or lesioning either corpus callosum or optic chiasm not allowing such a comparison. To investigate the functional contribution of CC in the intact cat visual system we recorded both monocular and binocular neuronal spiking responses and receptive fields in the 17/18 transition zone during reversible deactivation of the contralateral hemisphere. Unexpectedly from many of the previous reports, we observe no change in ocular dominance during CC deactivation. Throughout the transition zone, a majority of RFs shrink, but several also increase in size. RFs are significantly more affected for ipsi- as opposed to contralateral stimulation, but changes are also observed with binocular stimulation. Noteworthy, RF shrinkages are tiny and not correlated to the profound decreases of monocular and binocular firing rates. They depend more on orientation and direction preference than on eccentricity or ocular dominance of the receiving neuron's RF. Our findings confirm that in binocularly viewing mammals, binocular RFs near the midline are constructed via the direct geniculo-cortical pathway. They also support the idea that input from the two eyes complement each other through CC: Rather than linking parts of RFs separated by the vertical meridian, CC convey a modulatory influence, reflecting the feature selectivity of lateral circuits, with a strong cardinal bias.

Neuronal assembly detection and cell membership specification by principal component analysis.

In 1949, Donald Hebb postulated that assemblies of synchronously activated neurons are the elementary units of information processing in the brain. Despite being one of the most influential theories in neuroscience, Hebb's cell assembly hypothesis only started to become testable in the past two decades due to technological advances. However, while the technology for the simultaneous recording of large neuronal populations undergoes fast development, there is still a paucity of analytical methods that can properly detect and track the activity of cell assemblies. Here we describe a principal component-based method that is able to (1) identify all cell assemblies present in the neuronal population investigated, (2) determine the number of neurons involved in ensemble activity, (3) specify the precise identity of the neurons pertaining to each cell assembly, and (4) unravel the time course of the individual activity of multiple assemblies. Application of the method to multielectrode recordings of awake and behaving rats revealed that assemblies detected in the cerebral cortex and hippocampus typically contain overlapping neurons. The results indicate that the PCA method presented here is able to properly detect, track and specify neuronal assemblies, irrespective of overlapping membership.