N-methyl-D-aspartate receptors and protein synthesis are necessary for reinstatement of conditioned fear

N-acetylcysteine facilitates extinction of cued fear memory in rats via reestablishing basolateral amygdala glutathione homeostasis

Individual differences in the development of uncontrollable fear in response to traumatic stressors have been observed in clinic, but the underlying mechanisms remain unknown. In the present study we first conducted a meta-analysis of published clinical data and found that malondialdehyde, an oxidative stress biomarker, was significantly elevated in the blood of patients with fear-related anxiety disorders. We then carried out experimental study in rats subjected to fear conditioning. We showed that reestablishing redox homeostasis in basolateral amygdale (BLA) after exposure to fear stressors determined the capacity of learned fear inhibition. Intra-BLA infusion of buthionine sulfoximine (BSO) to deplete the most important endogenous antioxidant glutathione (GSH) blocked fear extinction, whereas intra-BLA infusion of dithiothreitol or N-acetylcysteine (a precursor of GSH) facilitated extinction. In electrophysiological studies conducted on transverse slices, we showed that fear stressors induced redox-dependent inhibition of NMDAR-mediated synaptic function, which was rescued by extinction learning or reducing agents. Our results reveal a novel pharmacological strategy for reversing impaired fear inhibition and highlight the role of GSH in the treatment of psychiatric disorders.

[Histamine signaling restores retrieval of forgotten memories]

Histamine is a biological amine that functions as a neurotransmitter in the brain to regulate arousal, appetite, and cognitive functions. Many pharmacological studies using histamine receptor agonists and antagonists have found that histamine promotes memory consolidation and retrieval. More recently, we have revealed that the activation of the brain histaminergic system by H₃R antagonists/inverse agonists restores retrieval of forgotten long-term memory in mice and humans. The recovery of memory retrieval may involve histamine-induced excitatory effects. Histamine may increase neuronal excitability throughout the neural circuit, including both neurons that are and are not recruited into the memory trace, similar to noise added to the neural circuits for memory retrieval. Stochastic resonance can explain how adding noise to the circuit enhances memory retrieval. Memory is processed not only by consolidation and retrieval, but also by various processes such as maintenance, reconsolidation, extinction, and reinstatement. Further studies that separately analyze the memory processes are needed to elucidate the whole picture of the effects of histamine on learning and memory. Regarding the human histaminergic system, alterations in histamine signaling have been reported in several neuropsychiatric disorders, and these changes have been suggested to be involved in cognitive dysfunction in patients with the neuropsychiatric disorders. Therefore, the drugs that modulate histamine signaling, including H₃R antagonists/inverse agonists, may be effective in the treatment of cognitive dysfunction, including Alzheimer's disease.

Histamine: A Key Neuromodulator of Memory Consolidation and Retrieval

In pharmacological studies conducted on animals over the last four decades, histamine was determined to be a strong modulator of learning and memory. Activation of histamine signaling enhances memory consolidation and retrieval. Even long after learning and forgetting, it can still restore the retrieval of forgotten memories. These findings based on animal studies led to human clinical trials with histamine H₃ receptor antagonists/inverse agonists, which revealed their positive effects on learning and memory. Therefore, histamine signaling is a promising therapeutic target for improving cognitive impairments in patients with various neuropsychiatric disorders, including Alzheimer's disease. While the memory-modulatory effects of histamine receptor agonists and antagonists have been confirmed by several research groups, the underlying mechanisms remain to be elucidated. This review summarizes how the activation and inhibition of histamine signaling influence memory processes, introduces the cellular and circuit mechanisms, and discusses the relationship between the human histaminergic system and learning and memory.

Excitatory VTA to DH projections provide a valence signal to memory circuits

The positive or negative value (valence) of past experiences is normally integrated into neuronal circuits that encode episodic memories and plays an important role in guiding behavior. Here, we show, using mouse behavioral models, that glutamatergic afferents from the ventral tegmental area to the dorsal hippocampus (VTA→DH) signal negative valence to memory circuits, leading to the formation of fear-inducing context memories and to context-specific reinstatement of fear. To a lesser extent, these projections also contributed to opioid-induced place preference, suggesting a role in signaling positive valence as well, and thus a lack of dedicated polarity. Manipulations of VTA terminal activity were more effective in females and paralleled by sex differences in glutamatergic signaling. By prioritizing retrieval of negative and positive over neutral memories, the VTA→DH circuit can facilitate the selection of adaptive behaviors when current and past experiences are valence congruent.

The neuronal pathway that signals the positive or negative value of memories is not well understood. Here, the authors report that an excitatory projection from the ventral tegmental area to the dorsal hippocampus carries the valence information, contributing, especially in females, to the recurrence of fear and to drug seeking behavior.

Late Arc/Arg3.1 expression in the basolateral amygdala is essential for persistence of newly-acquired and reactivated contextual fear memories

A feature of fear memory is its persistence, which could be a factor for affective disorders. Memory retrieval destabilizes consolidated memories, and then rapid molecular cascades contribute to early stabilization of reactivated memories. However, persistence of reactivated memories has been poorly understood. Here, we discover that late Arc (also known as Arg3.1) expression in the mouse basolateral amygdala (BLA) is involved in persistence of newly-acquired and reactivated fear memories. After both fear learning and retrieval, Arc levels increased at 2 h, returned to basal levels at 6 h but increased again at 12 h. Inhibiting late Arc expression impaired memory retention 7 d, but not 2 d, after fear learning and retrieval. Moreover, blockade of NR2B-containing N-methyl-D-aspartate receptors (NMDARs) prevented memory destabilization and inhibited late Arc expression. These findings indicate that NR2B-NMDAR and late Arc expression plays a critical role in the destabilization and persistence of reactivated memories.

Prefrontal dopamine regulates fear reinstatement through the downregulation of extinction circuits

Prevention of relapses is a major challenge in treating anxiety disorders. Fear reinstatement can cause relapse in spite of successful fear reduction through extinction-based exposure therapy. By utilising a contextual fear-conditioning task in mice, we found that reinstatement was accompanied by decreased c-Fos expression in the infralimbic cortex (IL) with reduction of synaptic input and enhanced c-Fos expression in the medial subdivision of the central nucleus of the amygdala (CeM). Moreover, we found that IL dopamine plays a key role in reinstatement. A reinstatement-inducing reminder shock induced c-Fos expression in the IL-projecting dopaminergic neurons in the ventral tegmental area, and the blocking of IL D1 signalling prevented reduction of synaptic input, CeM c-Fos expression, and fear reinstatement. These findings demonstrate that a dopamine-dependent inactivation of extinction circuits underlies fear reinstatement and may explain the comorbidity of substance use disorders and anxiety disorders.