Etude de la plasticité synaptique hippocampique induite par un apprentissage contextuel

La potentialisation à long terme (PLT) est l'augmentation de la force de connexion entre des neurones lorsqu'ils sont activés de manière synchrone et répétée. Ce processus a été très étudié chez les mammifères, particulièrement dans une région cérébrale impliquée dans la mémorisation contextuelle : l'hippocampe. La littérature montre un lien entre mémoire et PLT, mais leurs mécanismes sont discutés et seules quelques études ont montré qu'il y a réellement de la PLT pendant un apprentissage. Dans ce travail, nous avons d'abord montré chez la souris que l'hippocampe peut rapidement former une représentation contextuelle de l'environnement. Ensuite, à l'aide d'outils électrophysiologiques, nous avons pu enregistrer pour la première fois de la PLT induite par l'apprentissage chez la souris. Nous avons montré que toutes les connexions ne sont pas potentialisées ; la PLT est répartie dans l'hippocampe. Cette répartition semble être différente selon l'apprentissage utilisé. Nous avons donc montré que l'hippocampe est nécessaire pour encoder rapidement un nouveau contexte et avons validé une méthode fiable d'enregistrement de la PLT induite par l'apprentissage chez la souris.Long term potentiation (LTP) is a process by which the strength of the connection between neurons is increased when they are synchronously and repeatedly activated. This process has been well studied in mammals, especially in a brain region involved in contextual memory: the hippocampus. Literature tends to show a direct link between memory and LTP, but their mechanisms are not clear and there is only a few studies evidencing that LTP actually occurs during learning. In this work, we evidenced that the hippocampus could quickly form a representation of the context to which mice were exposed, this had never been reported before. Then, using electrophysiological tools, we were able to record for the first time LTP in the hippocampus after a learning task with mice. We showed that not all the connections are potentiated during memory formation; LTP is spread in the hippocampus. This repartition of LTP seems to differ between learning tasks. We showed that the hippocampus is needed to rapidly encode a new context, and we validated a method to reliably record learning-induced LTP in mice

NMDAR plasticity and metaplasticity in early odor preference learning in rats

Early odor preference learning is a classical conditioning behavioral model which can be used to understand the molecular mechanism of learning and memory, and synaptic plasticity. In this thesis the research is focused on the role of N-methyl D-aspartate receptor (NMADR) plasticity and metaplasticity in early odor preference learning. In Chapter 2, we investigated the differential roles of L-type calcium channel (LTCC) and NMDAR in early odor preference learning. The results suggest that the NMDAR is crucial for creating stimulus-specific memory and LTCC is required for maintenance of the memory. Activation of LTCC without NMDAR can cause loss of input specificity and as a result, generalization of the memory. Chapter 3 depicts that, the effect of altered number of NMDARs in the anterior piriform cortex (aPC) can significantly modifies future learning and synaptic plasticity. Here we show that NMDARs are down regulated at 3hr following the early odor preference learning. Repeated training at 3hr leads to unlearning and this unlearning is mediated by NMDAR itself. Inhibition of the NMDAR prior to retraining at 3hr, blocks unlearning. In continuation of Chapter 3, we have characterized the molecular mechanism underlying the NMDAR mediated unlearning in Chapter 4. Calcineurine and metabotropic glutamate receptor (mGluR) plays a vital role in NMDAR downregulation at 3hr following early odor preference training. In Chapter 5, we have investigated whether NMDAR plasticity and its mediated metaplasiticity observed in the early odor preference learning can be induced by stronger trainings that produce prolonged memories. Previous research has shown that infusing trichostatin A (TSA), a histone deacetylation inhibitor, in the olfactory bulb (OB) extends odor preference memory up to 5 days. Our data suggests that OB TSA infusion prevents NMDAR down-regulation and unlearning. These outcomes argue that it is critical to understand the metaplastic effects of training which have implications for learning optimization

The effects of Morris water maze learning on the number, morphology and molecular composition of rat hippocampal dentate gyrus synapses

spatial long-term memory formation is dependent upon the hippocampus and associated brain structures in mammals. Memory storage is believed to involve changes in the way information is exchanged between neurons, and this is principally governed by their synaptic connections. Changes can occur in the functional properties of individual synapses, but evidence suggests that morphological changes may also occur. Research described in this thesis has used the Morris water maze, a behavioural paradigm that requires rodents to form long-term memories about a spatial environment, and this learning task involves the function of the hippocampus. Electron microscopy was used to investigate the ultrastructural morphology and composition of synapses in the hippocampal dentate gyrus in several groups of animals. Three time- points were investigated, 3, 9 and 24 hours after the start of training, which also corresponded to small, intermediate and large amounts of training, as well as two different types of control, naïve and swim-only. Animals investigated 3 hours after the start of training did not show significant long term memory for the task, whereas animals investigated 9 and 24 hours after the start of learning displayed long-term memory recall when measured by the quadrant analysis test (probe trial). Hippocampal dimensions and dentate granule cell densities were similar between all animal groups. No significant changes to synaptic ultrastructural morphology were evident in the 3 hour group. In the 9 hour group, significant increases in synapse density and synapse to neuron ratio were observed, with a simultaneous decrease in the synapse mean height and average area of PSD (post-synaptie density) per synapse. No significant changes were observed in the exercise-matched swim-only controls, suggesting that the changes were related to long-term memory formation. Morphological changes were not evident in the 24 hour group, despite long term memory recall, suggesting that the morphological changes following spatial learning in the Morris water maze are transient. The total amount of synaptic membrane was not significantly different between any of the groups, suggesting that although new, smaller synapses may be formed as a result of learning, changes also occur to existing synapses, which may result in their re-categorisation or even removal. Analysis of ionotropic glutamate receptors following training proved inconclusive, particularly for NMDA receptors, but did suggest that AMP A receptors are increased in the initial stages of learning, which may be a mechanism of short-term memory storage

Immediate axonal retrograde signaling in amyloid-dependent neurodegeneration

The following dissertation herein discusses the role of axonal protein synthesis in Aβ1-42-dependent neurodegeneration, which has important implications in AD pathogenesis. In Part 1, I provide a brief introduction to relevant topics including neurodegeneration and axonal protein synthesis. In Part 2, I discuss findings that we published in 2014 describing a mechanism by which axonal exposure to Aβ1-42 induces cell death via axonal synthesis and retrograde transport of a transcription factor, ATF4. In Part 3, I discuss a follow-up project that I conducted independently, which is not yet published but is in preparation for submission describing the immediate effect of Aβ1-42 on axonal protein synthesis, which mediates the downstream axonal ATF4 signaling events described in Part 2. In Part 4, I discuss the key findings from these two projects including their significance and potential future directions. In the Appendix, I provide details regarding experimental methods and statistical analyses performed in Part 3

Investigating a Novel Thalamo-Amygdala Circuit for the Regulation of a Drug-Cue-Specific Memory

Addiction is a chronic, progressive disorder caused by repetitive drug use, which leads to long-lasting synaptic alterations in the brain. Drugs of abuse are powerful reinforcers that ‘hijack’ specific brain circuits that control reward-motivated behaviors, creating a maladaptive state that can trigger craving and relapse, even after prolonged periods of abstinence. Environmental contexts and cues that are present during drug use can later evoke drug-related memories that promote drug-seeking conditioned responses. Despite recent attempts to treat addiction with a combination of behavioral and pharmacological tools, success has been limited, mainly because the underlying mechanisms that govern drug-associated memories have not been fully elucidated. Studying the neural circuits and the synaptic and molecular underpinnings of drug-associated memories may uncover a means to reduce their salience, thereby reducing the likelihood of relapse. The amygdala is important for integrating sensory information during cue-dependent learning and is activated by both fear- and drug-related stimuli. Preclinical models have shown that the amygdala activates behavioral responses to fear- or drug-conditioned cues. Neuroadaptations within amygdala-based circuits have been examined in models of aversive learning but have not been well studied during reward-based learning, especially in the context of drugs of abuse. The work presented in this dissertation focuses on understanding the circuit-specific synaptic and molecular changes that occur within the amygdala both during the formation of a drug-cue memory and upon subsequent re-exposure to drug-related cues. Through a combination of electrophysiological, molecular, optogenetic, and behavioral techniques we find that repeated self-administration of cocaine, paired with an audiovisual cue, involves strengthening of auditory thalamic, but not cortical, synapses in the lateral amygdala. We also find that reversing drug-cue induced plasticity in this pathway, either through extensive cue re-exposure or by optical induction of postsynaptic LTD, inhibits cue-elicited relapse-like behavior. Finally, we show that specific pharmacological manipulations of an intracellular signaling pathway involved in bidirectional regulation of synaptic activity can interfere with reconsolidation and promote extinction of a drug-cue memory. Together, these studies suggest a projection-specific mechanistic approach for the inhibition of drug-cue memory, which may be informative for future pharmacobehavioral relapse prevention strategies

PROTEIN SYNTHESIS INHIBITION AND SHORT AND LONG TERM HABITUATION OF THE DORSAL ANTENNAE WITHDRAWAL RESPONSE IN HELIX ASPERSA

The pharmacological disruption of memory by various protein synthesis inhibitory drugs (PSIs) has implicated protein synthesis as a requirement for long term learning but not for short. However, evidence derived from PSI research remains equivocal, with the apparent amnesic effects of PSIs being attributed to drug side-effects and general behavioural debilitations. Research reported in this thesis investigates the behavioural effects on short and long term habituation of the dorsal antennae withdrawal reflex in the snail (Helix aspersa) of three antibiotic drugs known to reversibly inhibit protein synthesis; anisomycin, actinomycin D, and puromycin. Initially, habituation was established as true learning in the snail and was demonstrated to be capable of retention for over 24 hours from one training session and over 6 months from a series of training sessions. The parametric characteristics of both short and long term habituation in the snail was established and found to be identical to those demonstrated in vertebrate habituation. Such characteristics were found to be different for short and long term habituation. Injection of PSIs showed no effect on short term habituation but disrupted long term habituation if PSI was active within a 'critical time window' during or for approximately 40 minutes after training. Later injections had no amnesic effect, and neither did injections 2 hours prior to training. The amnesic effects were demonstrated not to be attributable to drug side-effects by the development and application of a 'behavioural test battery' to screen general snail behaviour for drug induced debilitations at a variety of doses. Dose/amnesic effect relationships are also reported. Potentially confounding effects, such as, state dependent learning, and drug performance effects, were controlled out. The effects of the PSIs on short and long term habituation are then reported in terms of their effects on the established short and long term parametric characteristics of the learning. Drug injected snails showed normal short term parametric characteristics in training. However, in a long term retest drug treated snails also showed the parametric characteristics of short term habituation which demonstrated the degree of induced amnesia. The results are discussed in terms of a gene expression model of long term habituation and suggest that short and long term habituation are mediated by different processes. Short term habituation is protein synthesis independent and long term habituation is protein synthesis dependent

Effects of sex steroids on spatial cognition in the zebra finch (Taeniopygia guttata)

It is well established in mammals that chronic, long-term elevations in sex steroids are associated with improvements in spatial cognition. It is less clear the extent to which short to medium term elevations in sex steroids improve spatial cognition and change hippocampal morphology, particularly in birds. The avian hippocampus expresses both androgen receptors (AR) and oestrogen receptor alpha (ERα) and high levels of the enzyme aromatase that converts testosterone to oestrogen. I began by comparing spatial cognition, hippocampal sex steroid receptor and aromatase expression between males and females. There were no differences in spatial or visual cognition or in hippocampal sex steroid receptor expression between the sexes, although hippocampal aromatase mRNA expression was higher in males. I then addressed the effects of acute and medium-term sex steroid treatment on spatial cognition and hippocampal aromatase and sex steroid receptor expression. A single treatment of testosterone 30 minutes or four hours prior to cognitive testing improved spatial performance. Additionally, when testosterone and oestrogen were given daily for five days spatial cognition in both sexes was improved. The testosterone-induced improvement was blocked when testosterone was administered in conjunction with the aromatase inhibitor fadrozole but not when administered with saline. These findings suggest that spatial cognition is improved by an oestrogenic effect. Thirty minutes following acute testosterone treatment, plasma testosterone levels, hippocampal AR and ERα mRNA expression all increased. Five days of oestrogen treatment increased plasma oestrogen levels, hippocampal ERα mRNA and Nmethyl- D-aspartate (NMDA) receptor levels in males and females; all were positively correlated with enhanced spatial cognition on day five of treatment. Finally, I determined which genes were differentially expressed as a result of five days of oestrogen treatment. Nineteen genes, identified as being involved in learning and memory were differentially expressed in the hippocampus, eleven of which were up-regulated and eight were down-regulated. Taken together these results demonstrate that oestrogen can improve spatial cognition in birds. It is plausible that oestrogen acts to improve spatial memory in the hippocampus through upregulation of genes that control neurotransmitter release, reuptake and receptor levels

The Role of Synaptic Tagging and Capture for Memory Dynamics in Spiking Neural Networks

Memory serves to process and store information about experiences such that this information can be used in future situations. The transfer from transient storage into long-term memory, which retains information for hours, days, and even years, is called consolidation. In brains, information is primarily stored via alteration of synapses, so-called synaptic plasticity. While these changes are at first in a transient early phase, they can be transferred to a late phase, meaning that they become stabilized over the course of several hours. This stabilization has been explained by so-called synaptic tagging and capture (STC) mechanisms. To store and recall memory representations, emergent dynamics arise from the synaptic structure of recurrent networks of neurons. This happens through so-called cell assemblies, which feature particularly strong synapses. It has been proposed that the stabilization of such cell assemblies by STC corresponds to so-called synaptic consolidation, which is observed in humans and other animals in the first hours after acquiring a new memory. The exact connection between the physiological mechanisms of STC and memory consolidation remains, however, unclear. It is equally unknown which influence STC mechanisms exert on further cognitive functions that guide behavior. On timescales of minutes to hours (that means, the timescales of STC) such functions include memory improvement, modification of memories, interference and enhancement of similar memories, and transient priming of certain memories. Thus, diverse memory dynamics may be linked to STC, which can be investigated by employing theoretical methods based on experimental data from the neuronal and the behavioral level. In this thesis, we present a theoretical model of STC-based memory consolidation in recurrent networks of spiking neurons, which are particularly suited to reproduce biologically realistic dynamics. Furthermore, we combine the STC mechanisms with calcium dynamics, which have been found to guide the major processes of early-phase synaptic plasticity in vivo. In three included research articles as well as additional sections, we develop this model and investigate how it can account for a variety of behavioral effects. We find that the model enables the robust implementation of the cognitive memory functions mentioned above. The main steps to this are: 1. demonstrating the formation, consolidation, and improvement of memories represented by cell assemblies, 2. showing that neuromodulator-dependent STC can retroactively control whether information is stored in a temporal or rate-based neural code, and 3. examining interaction of multiple cell assemblies with transient and attractor dynamics in different organizational paradigms. In summary, we demonstrate several ways by which STC controls the late-phase synaptic structure of cell assemblies. Linking these structures to functional dynamics, we show that our STC-based model implements functionality that can be related to long-term memory. Thereby, we provide a basis for the mechanistic explanation of various neuropsychological effects.2021-09-0

Psychopharmacology of memory and emotion in humans

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Psicología, Departamento de Psicología Básica I (Procesos Cognitivos), leída el 16-06-2017El principal propósito de esta tesis ha sido ahondar en el conocimiento del mecanismo neural subyacente a la influencia de la emoción y el sistema motor en la memoria. El primero de los estudios presentados incide sobre la posibilidad de disminuir la memoria episódica, mientras que el segundo estudio presentado incide sobre la viabilidad de aumentarla. En el primer capítulo, se resumen las principales teorías sobre los procesos de memoria, desde las teorías clásicas a las más actuales. El segundo capítulo repasa los principales sistemas de neurotransmisores del sistema nervioso central y su relación con la emoción y la memoria, presentando los principales estudios en humanos o en animales. En el tercer y cuarto capítulos, se presentan dos estudios novedosos, que son una réplica y ampliación de dos estudios previos. Las teorías clásicas han considerado la memoria como una facultad monolítica e inmutable de la mente. Sin embargo, las nuevas actualizaciones provenientes de las investigaciones realizadas en las últimas décadas, abren una puerta a la modificación de memorias previamente consolidadas. La teoría de la reconsolidación establece la posibilidad de reactivar de nuevo una memoria mediante la presentación de una “clave” relacionada con la misma. Una vez la memoria es reactivada es susceptible de ser modificada dentro de una ventana de tiempo, mediante la administración de diferentes tipos de manipulaciones, tanto de carácter conductual como farmacológico; necesitando ser consolidada de nuevo después de la reactivación. La reconsolidación de la memoria ha sido observada en diferentes especies animales y en humanos, así como con diferentes tareas y tipos de memoria...The main objective of this thesis has been to deepen the knowledge of the neural mechanism underlying the influence of emotion as well as the motor system in memory. The first of the studies presented focuses on the possibility of decreasing episodic memory, while the second study refers to the feasibility of increasing it. In the first chapter, the main theories about the processes of memory are summarized, from the classic theories to the more actual ones. The second chapter reviews the main neurotransmitter systems of the central nervous system and its relation to emotion and memory, presenting the main studies in humans or animals. In the third and fourth chapters, two novel studies are presented, which are a replication and extension of two previous studies. Classical theories have considered memory as a monolithic and immutable faculty of mind. However, new updates from the research carried out in the last decades, opens a door to the modification of previously consolidated memories. The theory of reconsolidation postulates that upon reactivation, memories can become labile and susceptible to manipulation, requiring a new restabilization process in order to maintain them. The reconsolidation of memory has been observed in different animal species and in humans, as well as with different tasks and types of memory. The first study presented (Chapter 3) is based on the hypothesis of the implication of the GABAergic system in the deterioration of the reconsolidation of an episodic emotional memory. For the accomplishment of this experiment it was counted on the participation of patients who were going to undergo an endoscopy procedure and therefore, to receive a scheduled sedation. The results show the impairment of the emotional memory by the administration of propofol, an anesthetic agent, immediately after the reactivation. A significant proportion of the population is affected by psychiatric disorders that have at their core a traumatic or maladaptive emotional memory. The possibility of modifying this type of memory opens a range to new treatments and adjuvant therapies to those already existing...Depto. de Psicología Experimental, Procesos Cognitivos y LogopediaFac. de PsicologíaTRUEunpu