Role of amygdala astrocytes in different phases of contextual fear memory.

Growing evidence indicates a critical role of astrocytes in learning and memory. However, little is known about the role of basolateral amygdala complex (BLA-C) astrocytes in contextual fear conditioning (CFC), a paradigm relevant to understand and generate treatments for fear- and anxiety-related disorders. To get insights on the involvement of BLA-C astrocytes in fear memory, fluorocitrate (FLC), a reversible astroglial metabolic inhibitor, was applied at critical moments of the memory processing in order to target the acquisition, consolidation, retrieval and reconsolidation process of the fear memory. Adult Wistar male rats were bilaterally cannulated in BLA-C. Ten days later they were infused with different doses of FLC (0.5 or 1 nmol/0.5 µl) or saline before or after CFC and before or after retrieval. FLC impaired fear memory expression when administered before and shortly after CFC, but not one hour later. Infusion of FLC prior and after retrieval did not affect the memory. Our findings suggest that BLA-C astrocytes are critically involved in the acquisition/early consolidation of fear memory but not in the retrieval and reconsolidation. Furthermore, the extinction process was presumably not affected (considering that peri-retrieval administration could also affect this process).

Effect of intra-BLA overexpression of IGF-1 on the expression of a contextual fear memory trace.

Growth factors, such as insulin-like growth factor 1 (IGF-1), among others are known for their critical involvement in learning and memory processes. IGF-1 regulates cognitive functions, synapse density, neurotransmission, and adult neurogenesis and induces structural and synaptic plasticity-specific changes. Although IGF-1 has been suggested to participate in different memory processes, its role in memories associated with negative emotional experiences still remains to be elucidated. The principal aim of the present study was to test whether IGF-1 overexpression using adenoviral vectors in basolateral amygdala (BLA) influences both the expression and formation of contextual fear memory, as well as the hippocampal structural plasticity associated with such memory trace. We found that IGF-1 overexpression promotes the formation and expression of a specific contextual fear memory trace, and such effect persisted at least 7 days after recall. Moreover, the overexpression of this growth factor in BLA upregulates the activation of the ERK/MAPK pathway in this brain structure. In addition, intra-BLA IGF-1 overexpression causes dorsal hippocampus (DH) structural plasticity modifications promoting changes in the proportion of mature dendritic spines in the CA1 region, after a weak conditioning protocol. The present findings contribute to the knowledge underlying BLA-DH trace memory of fear and reveal important new insights into the neurobiology and neurochemistry of fear acquisition modulated by IGF-1 overexpression. The understanding of how IGF-1 modulates the formation of a fear contextual trace may pave the way for the development of novel therapeutic strategies focused on fear, anxiety, and trauma-related disorders.

Minocycline prevents chronic restraint stress-induced vulnerability to developing cocaine self-administration and associated glutamatergic mechanisms: a potential role of microglia.

Stressful experience-induced cocaine-related behaviors are associated with a significant impairment of glutamatergic mechanisms in the Nucleus Accumbens core (NAcore). The hallmarks of disrupted glutamate homeostasis following restraint stress are the enduring imbalance of glutamate efflux after a cocaine stimulus and increased basal concentrations of extracellular glutamate attributed to GLT-1 downregulation in the NAcore. Glutamate transmission is tightly linked to microglia functioning. However, the role of microglia in the biological basis of stress-induced addictive behaviors is still unknown. By using minocycline, a potent inhibitor of microglia activation with anti-inflammatory properties, we determined whether microglia could aid chronic restraint stress (CRS)-induced glutamate homeostasis disruption in the NAcore, underpinning stress-induced cocaine self-administration. In this study, adult male rats were restrained for 2 h/day for seven days (day 1-7). From day 16 until completing the experimental protocol, animals received a vehicle or minocycline treatment (30 mg/Kg/12h i.p.). On day 21, animals were assigned to microscopic, biochemical, neurochemical or behavioral studies. We confirm that the CRS-induced facilitation of cocaine self-administration is associated with enduring GLT-1 downregulation, an increase of basal extracellular glutamate and postsynaptic structural plasticity in the NAcore. These alterations were strongly related to the CRS-induced reactive microglia and increased TNF-α mRNA and protein expression, since by administering minocycline, the impaired glutamate homeostasis and the facilitation of cocaine self-administration were prevented. Our findings are the first to demonstrate that minocycline suppresses the CRS-induced facilitation of cocaine self-administration and glutamate homeostasis disruption in the NAcore. A role of microglia is proposed for the development of glutamatergic mechanisms underpinning stress-induced vulnerability to cocaine addiction.

Temporal dynamic of the hippocampal structural plasticity associated with the fear memory destabilization/reconsolidation process.

Reconsolidation of a contextual fear memory is a protein synthesis-dependent process in which a previously destabilized memory returns to a stable state. This process has become the subject of many studies due to its importance in memory processing, maintenance and updating, and its potential role as a therapeutical target in fear memory disorders such as phobias and post-traumatic stress disorder. In this sense, understanding the underlying mechanisms of memory reconsolidation is paramount in developing potential treatments for such memory dysfunctions. In the present work, we studied the interaction between two key neural structures involved in the reconsolidation process: the basolateral amygdala complex of the amygdala (BLA) and the dorsal hippocampus (DH). Our results show changes in the structural plasticity of the CA1 region of the DH in the form of dendritic spines density changes associated with the destabilization/reconsolidation process. Furthermore, we demonstrate a modulatory role of BLA over such structural plasticity by infusing different drugs such as ifenprodil, a destabilization blocker, and propranolol, a reconsolidation disruptor, in this brain structure. Altogether our work shows a particular temporal dynamic in the CA1 region of DH that accompanies the destabilization/reconsolidation process and aims to provide new information on the underlying mechanisms of this process that potentially contributes for a better understanding of memory storage, maintenance, expression and updating, and its potential medical applications.

Resistance to fear memory destabilization triggers exaggerated emotional-like responses following memory reactivation.

Fear memory reactivation does not always lead to memory destabilization-reconsolidation. For instance, fear memories formed following withdrawal from chronic ethanol consumption or a stressful event are less likely to become destabilized after reactivation, with the effect of recall of these memories on the affective state still requiring elucidation. Here, we investigated the negative emotional-like responses following fear memory reactivation in ethanol-withdrawn (ETOH) rats by focusing on the possible role played by destabilization. Our findings indicated that ETOH rats displayed an increased freezing in a novel context and an anxiogenic-like response in the elevated plus maze (EPM) following memory reactivation, whereas the behavior of CON animals was not affected. The destabilization blockade by pre-reactivation nimodipine (16 mg/kg, s.c) administration promoted in CON animals a similar behavior in the EPM and in a novel environment as that exhibited by ETOH rats after the reminder. Moreover, facilitating destabilization by pre-reactivation d-cycloserine (5 mg/kg, i.p) administration prevented the emotional-like disturbances observed in ETOH rats. Finally, using restraint stress, which is also an inductor of a fear memory resistant to destabilization, an increased fear response in an unconditioned environment and an anxiogenic-like state was also found after the presentation of the fear reminder in stressed rats. Our results suggest that, in the context of resistant fear memories, the occurrence of destabilization influences how animals respond to subsequent environmental challenges following reactivation.

Stress influences the dynamics of hippocampal structural remodeling associated with fear memory extinction.

Fear extinction is defined as a decline in fear-conditioned responses following non-reinforced exposure to a fear conditioned stimulus, therefore the conditioned stimulus gains new predictive properties. Patients with anxiety related disorders (e.g.: PTSD) subjected to extinction-like exposure treatments often experience a relapse of symptoms. Stress is a risk factor for those psychiatric disorders and a critical modulator of fear learning that turns the memory resistant to the extinction process. Dendritic spines are the anatomical sites where neuronal activity reshapes brain networks during learning and memory processes. Thus, we planned to characterize the dynamics of synaptic remodeling before and after contextual fear extinction in the dorsal hippocampus (DH), and how this process is affected by a previous stress experience. Animals with or without previous stress were contextually fear conditioned and one day later trained in an extinction paradigm. Rats were sacrificed one day after conditioning (pre-extinction) or one day after extinction for spine density analysis in the DH. We confirmed that stress exposure induced a deficit in extinction learning. Further, a higher density of dendritic spines, particularly mature ones, was observed in the DH of non-stressed conditioned animals at pre-extinction. Interestingly, after extinction, the spine levels returned to the control values. Conversely, stressed animals did not show such spines boost (pre-extinction) or any other change (post-extinction). In contrast, such standard dynamics of dendritic changes as well as the behavioral extinction was recovered when stressed animals received an intra-basolateral amygdala infusion of midazolam prior to stress. Altogether, these findings suggest that stress hinders the normal dynamic of dendritic remodeling after fear extinction and this could be part of the neurobiological substrate that makes those memories resistant to be extinguished.