GABAergic Transmission in the Basolateral Amygdala Differentially Modulates Plasticity in the Dentate Gyrus and the CA1 Areas

Distinct Neural Representations and Cognitive Behaviors Attributable to Naturally Developed Active Avoidance or Reactive Escape Strategies in the Male Rat

BACKGROUND

The high individual variability in coping with stress is often attributed to genetic background differences, sustained environmental conditions, or a combination of both. However, the neural mechanisms underlying coping style variability are still poorly understood.

METHODS

Here we examined the impact of a single extended emotional challenge on coping style variability and the associated involvement of the hippocampus, medial prefrontal cortex (mPFC), and periaqueductal gray (PAG). Male Sprague-Dawley rats (n = 170) were trained in an extended 2-way shuttle avoidance (eTWSA) task for 7 days, and daily avoidance rates were measured. Forced swim test, elevated plus maze, or Morris water maze was tested before or after eTWSA exposure. Excitotoxic lesion of the hippocampal dentate gyrus (DG) was performed by Ibotenic infusion. Transient pharmacological blocking of DG, mPFC, or PAG was performed by muscimol or CNQX+TTX infusion.

RESULTS

Exposing rats to eTWSA was found to lead to naturally developing dichotomous, not continuous, coping styles, which we termed active avoidance (AA) or reactive escape (RE). Prior emotional responses did not predict the developing coping style. AA was associated with beneficial outcomes, including reduced behavioral despair and improved spatial learning. RE led to impaired spatial retrieval. AA was abolished by lesioning or pharmacological blocking of the DG. RE was prevented by blocking mPFC or PAG.

CONCLUSION

The results indicate that a single exposure to a significant emotional challenge can lead, in otherwise healthy individuals, to dichotomous development of an active or reactive coping style with distinctive neural correlates and subsequent behavioral significance.

Significance of GABAA Receptor for Cognitive Function and Hippocampal Pathology

The hippocampus is a primary area for contextual memory, known to process spatiotemporal information within a specific episode. Long-term strengthening of glutamatergic transmission is a mechanism of contextual learning in the dorsal cornu ammonis 1 (CA1) area of the hippocampus. CA1-specific immobilization or blockade of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor delivery can impair learning performance, indicating a causal relationship between learning and receptor delivery into the synapse. Moreover, contextual learning also strengthens GABA_A (gamma-aminobutyric acid) receptor-mediated inhibitory synapses onto CA1 neurons. Recently we revealed that strengthening of GABA_A receptor-mediated inhibitory synapses preceded excitatory synaptic plasticity after contextual learning, resulting in a reduced synaptic excitatory/inhibitory (E/I) input balance that returned to pretraining levels within 10 min. The faster plasticity at inhibitory synapses may allow encoding a contextual memory and prevent cognitive dysfunction in various hippocampal pathologies. In this review, we focus on the dynamic changes of GABA_A receptor mediated-synaptic currents after contextual learning and the intracellular mechanism underlying rapid inhibitory synaptic plasticity. In addition, we discuss that several pathologies, such as Alzheimer’s disease, autism spectrum disorders and epilepsy are characterized by alterations in GABA_A receptor trafficking, synaptic E/I imbalance and neuronal excitability.

Amygdala-hippocampal interactions in synaptic plasticity and memory formation

Memories of emotionally arousing events tend to endure longer than other memories. This review compiles findings from several decades of research investigating the role of the amygdala in modulating memories of emotional experiences. Episodic memory is a kind of declarative memory that depends upon the hippocampus, and studies suggest that the basolateral complex of the amygdala (BLA) modulates episodic memory consolidation through interactions with the hippocampus. Although many studies in rodents and imaging studies in humans indicate that the amygdala modulates memory consolidation and plasticity processes in the hippocampus, the anatomical pathways through which the amygdala affects hippocampal regions that are important for episodic memories were unresolved until recent optogenetic advances made it possible to visualize and manipulate specific BLA efferent pathways during memory consolidation. Findings indicate that the BLA influences hippocampal-dependent memories, as well as synaptic plasticity, histone modifications, gene expression, and translation of synaptic plasticity associated proteins in the hippocampus. More recent findings from optogenetic studies suggest that the BLA modulates spatial memory via projections to the medial entorhinal cortex, and that the frequency of activity in this pathway is a critical element of this modulation.

Intra-amygdala metaplasticity modulation of fear extinction learning

The amygdala is a key brain region involved in emotional memory formation. It is also responsible for memory modulation in other brain areas. Under extreme conditions, amygdala modulation may lead to the generation of abnormal plasticity and trauma-related psychopathologies. However, the amygdala itself is a dynamic brain region, which is amenable to long-term plasticity and is affected by emotional experiences. These alterations may modify the way the amygdala modulates activity and plasticity in other related brain regions, which in turn may alter the animal's response to subsequent challenges in what could be termed as "Behavioral metaplasticity."Because of the reciprocal interactions between the amygdala and other emotion processing regions, such as the medial prefrontal cortex (mPFC) or the hippocampus, experience-induced intra-amygdala metaplasticity could lead to alterations in mPFC-dependent or hippocampus-dependent behaviors. While initiated by alterations within the basolateral amygdala (BLA), such alterations in other brain regions may come to be independent of BLA modulation, thus establishing what may be termed "Trans-regional metaplasticity." In this article, we review evidence supporting the notions of intra-BLA metaplasticity and how this may develop into "Trans-regional metaplasticity." Future research is needed to understand how such dynamic metaplastic alterations contribute to developing psychopathologies, and how this knowledge may be translated into promoting novel interventions in psychopathologies associated with fear, stress, and trauma.