Amygdala electrical stimulation inducing spatial memory recovery produces an increase of hippocampal bdnf and arc gene expression

DNA methylation inhibition participates in the anterograde amnesia key mechanism through the suppression of the transcription of genes involved in memory formation in grape snails

The study of the amnesia mechanisms is of both theoretical and practical importance. The mechanisms of anterograde amnesia are the least studied, due to the lack of an experimental model that allows studying this amnesia type molecular and cellular mechanisms. Previously, we found that conditional food aversion memory reconsolidation impairment in snails by NMDA glutamate receptor antagonists led to the amnesia induction, in the late stages of which (>10 days) repeated training did not cause long-term memory formation. In the same animals, long-term memory aversion to a new food type was formed. We characterized this amnesia as specific anterograde amnesia. In the present work we studied the role of epigenetic DNA methylation processes as well as protein and mRNA synthesis in the mechanisms of anterograde amnesia and memory recovery. DNMT methyltransferase inhibitors (iDNMT: zebularine, RG108 (N-Phthalyl-1-tryptophan), and 5-AZA (5-Aza-2'-deoxycytidine)) were used to alter DNA methylation. It was found that in amnesic animals the iDNMT administration before or after shortened repeated training led to the rapid long-term conditional food aversion formation (Ebbinghaus saving effect). This result suggests that amnestic animals retain a latent memory, which is the basis for accelerated memory formation during repeated training. Protein synthesis inhibitors administration (cycloheximide) before or immediately after repeated training or administration of RNA synthesis inhibitor (actinomycin D) after repeated training prevented memory formation under iDNMT action. The earlier protein synthesis inhibitor effect suggests that the proteins required for memory formation are translated from the pre-existing, translationally repressed mRNAs. Thus, we have shown for the first time that the anterograde amnesia key mechanism is DNMT-dependent suppression of the transcription of genes involved in memory mechanisms. Inhibition of DNMT during repeated training reversed these genes expression blockade, opening access to them by transcription factors synthesized during training from the pre-existing mRNAs.

Basolateral amygdala stimulation plus water maze training restore dentate gyrus LTP and improve spatial learning and memory

Synaptic plasticity is a key mechanism of neural plasticity involved in learning and memory. A reduced or impaired synaptic plasticity could lead to a deficient learning and memory. On the other hand, besides reducing hipocampal dependent learning and memory, fimbria-fornix lesion affects LTP. However, we have consistently shown that stimulation of the basolateral amygdala (BLA) 15 min after water maze training is able to improve spatial learning and memory in fimbria fornix lesioned rats while also inducing changes in the expression of plasticity-related genes expression in memory associated brain regions like the hippocampus and prefrontal cortex. In this study we test that hypothesis: whether BLA stimulation 15 min after water maze training can improve LTP in the hippocampus of fimbria-fornix lesioned rats. To address this question, we trained fimbria-fornix lesioned rats in water maze for four consecutive days, and the BLA was bilaterally stimulated 15 min after each training session.Our data show that trained fimbria-fornix lesioned rats develop a partially improved LTP in dentated gyrus compared with the non-trained fimbria-fornix lesioned rats. In contrast, dentated gyrus LTP in trained and BLA stimulated fimbria-fornix lesioned rats improved significantly compared to the trained fimbria-fornix lesioned rats, but was not different from that shown by healthy animals. BLA stimulation in non-trained FF lesioned rats did not improve LTP; instead produces a transient synaptic depression. Restoration of the ability to develop LTP by the combination of training and BLA stimulation would be one of the mechanisms involved in ameliorating memory deficits in lesioned animals.

Alzheimer’s Disease, Neural Plasticity, and Functional Recovery

Alzheimer’s disease (AD) is the most common and devastating neurodegenerative condition worldwide, characterized by the aggregation of amyloid-β and phosphorylated tau protein, and is accompanied by a progressive loss of learning and memory. A healthy nervous system is endowed with synaptic plasticity, among others neural plasticity mechanisms, allowing structural and physiological adaptations to changes in the environment. This neural plasticity modification sustains learning and memory, and behavioral changes and is severely affected by pathological and aging conditions, leading to cognitive deterioration. This article reviews critical aspects of AD neurodegeneration as well as therapeutic approaches that restore neural plasticity to provide functional recoveries, including environmental enrichment, physical exercise, transcranial stimulation, neurotrophin involvement, and direct electrical stimulation of the amygdala. In addition, we report recent behavioral results in Octodon degus, a promising natural model for the study of AD that naturally reproduces the neuropathological alterations observed in AD patients during normal aging, including neuronal toxicity, deterioration of neural plasticity, and the decline of learning and memory.

Protein synthesis inhibitor administration before a reminder caused recovery from amnesia induced by memory reconsolidation impairment with NMDA glutamate receptor antagonist

Memory recovery in amnestic animals is one of the most poorly studied processes. In this paper, we examine the role of protein synthesis and a reminder in the mechanisms of amnesia and memory recovery in grape snails trained to conditioned food aversion. Amnesia was induced by the impairment of memory reconsolidation using NMDA (N-methyl d-aspartate) glutamate receptor antagonists. In an early stage of amnesia (day 3), injections of protein synthesis inhibitors into animals combined with a reminder by a conditioned stimulus (CS) led to the recovery of aversive reactions to its presentation. Two types of changes in reactions to CS were revealed. In most animals, a persistent recovery of memory retrieval was found that lasted for at least 10 days. In other snails, aversive responses to CS persisted for 24 h. Isolated injections of inhibitors, injections of inhibitors and a reminder by the learning environment (without presenting a CS), usage of a differentiating stimulus instead of a CS, or inhibitor injections after the reminder did not affect the development of amnesia. The administration of protein synthesis inhibitors and a reminder in the late period after amnesia induction (10 days) did not affect its development or caused a short-term memory recovery. We suggest that amnesia is an active process that develops over time. The reminder induces the reactivation of the amnesia process dependent on protein synthesis, while the administration of protein synthesis inhibitors leads to the impairment of amnesia reactivation and recovery of the state formed before amnesia induction (i.e., recovery of conditioned food aversion memory).

Amygdala stimulation ameliorates memory impairments and promotes c-Fos activity in fimbria-fornix-lesioned rats

Recently we provided data showing that amygdala stimulation can ameliorate spatial memory impairments in rats with lesion in the fimbria-fornix (FF). The mechanisms for this improvement involve early gene expression and synthesis of BDNF, MAP-2, and GAP43 in the hippocampus and prefrontal cortex. Now we have studied which brain structures are activated by the amygdala using c-Fos as a marker of neural activation. First, we studied neuronal activation after tetanic stimulation to the amygdala in intact rats. We then carried out a second study in FF-lesioned rats in which the amygdala was stimulated 15 min after daily spatial memory training in the water maze. Our results showed that amygdala stimulation produces widespread brain activation, that includes cortical, thalamic, and brain stem structures. Activation was particularly intense in the dentate gyrus and the prefrontal cortex. Training in the water maze increased c-Fos positive nuclei in the dentate gyrus of the hippocampus and in medial prefrontal cortex. Amygdala stimulation to trained FF-lesioned rats induced an increase of neural activity in the dentate gyrus and medial prefrontal cortex relative to the FF-lesioned, but not stimulated group, like the c-Fos activity seen in trained control rats. Based on these and previous results we explain the mechanisms of amygdala reinforcement of neural plasticity and the partial recovery of spatial memory deficits.

Protein synthesis inhibitors induce both memory impairment and its recovery

The involvement of protein synthesis in the mechanisms of conditioned food aversion memory impairment and recovery in grape snails was studied. It was found that protein synthesis inhibitor (cycloheximide) injections before a reminder by the conditioned stimulus (CS) caused amnesia development. Three days after amnesia induction, injections of cycloheximide or another protein synthesis inhibitor, anisomycin, combined with a reminder by four CSs resulted in memory retrieval, which was saved for 24 h. Cycloheximide injections and the administration of one CS as a reminder to an amnestic animals caused the memory expression only in response to this CS, while it was absent the next day. The isolated administration of a reminder or inhibitor injections without a reminder was not effective. It is suggested that amnesia is an active process and that one of its mechanisms may be a protein-dependent amnesia reactivation caused by a reminder. The administration of protein synthesis inhibitors led to impairment of amnesia reactivation and to recovery of the state formed before amnesia induction and thus to the recovery of conditioned food aversion memory.

Amygdala stimulation promotes recovery of behavioral performance in a spatial memory task and increases GAP-43 and MAP-2 in the hippocampus and prefrontal cortex of male rats

The relationships between affective and cognitive processes are an important issue of present neuroscience. The amygdala, the hippocampus and the prefrontal cortex appear as main players in these mechanisms. We have shown that post-training electrical stimulation of the basolateral amygdala (BLA) speeds the acquisition of a motor skill, and produces a recovery in behavioral performance related to spatial memory in fimbria-fornix (FF) lesioned animals. BLA electrical stimulation rises bdnf RNA expression, BDNF protein levels, and arc RNA expression in the hippocampus. In the present paper we have measured the levels of one presynaptic protein (GAP-43) and one postsynaptic protein (MAP-2) both involved in synaptogenesis to assess whether structural neuroplastic mechanisms are involved in the memory enhancing effects of BLA stimulation. A single train of BLA stimulation produced in healthy animals an increase in the levels of GAP-43 and MAP-2 that lasted days in the hippocampus and the prefrontal cortex. In FF-lesioned rats, daily post-training stimulation of the BLA ameliorates the memory deficit of the animals and induces an increase in the level of both proteins. These results support the hypothesis that the effects of amygdala stimulation on memory recovery are sustained by an enhanced formation of new synapses.

Erythropoietin improves object placement recognition memory in a time dependent manner in both, uninjured animals and fimbria-fornix-lesioned male rats

An increasing number of reports sustain a possible role of erythropoietin (EPO) as neuroprotective agent. In two previous articles we have evaluated EPO as plasticity promoting agent, and to contribute the restoration of brain function affected by acquired damage. We have shown that EPO is able to induce an increased synaptic efficacy in vivo along with a plasticity promoting effect. In the Morris water maze EPO administration to fimbria-fornix lesioned male rats induces a significant improvement of their spatial memory, affected by the lesion. Singularly, EPO was only effective when administered shortly after training (10 min) but not after several hours (5 h), suggesting a specific EPO effect on time dependent plasticity process. In the present paper we have expanded this line of evidence using a low stress paradigm of object placement recognition in lesioned and healthy male rats. The memory trace in this model is short-lasting; animals could recognize the change in object position when tested 24 h after, but not 48 or 72 h after the acquisition session. EPO administration 10 min after acquisition significantly prolongs retention to, at least, 72 h in healthy rats. No effect was seen if EPO was administered 5 h after training, suggesting a specific EPO modulatory effect on the consolidation process. Remarkably, early EPO treatment to fimbria fornix lesioned animals reverts the memory deficit caused by the lesion. An increased expression of the plasticity related gene arc, was also confirmed in the hippocampus and the prefrontal cortex, that is likely to be involved in the behavioral improvement observed.