Ryan G. Parsons

Implications of memory modulation for post-traumatic stress and fear disorders

Post-traumatic stress disorder, panic disorder and phobia manifest in ways that are consistent with an uncontrollable state of fear. Their development involves heredity, previous sensitizing experiences, association of aversive events with previous neutral stimuli, and inability to inhibit or extinguish fear after it is chronic and disabling. We highlight recent progress in fear learning and memory, differential susceptibility to disorders of fear, and how these findings are being applied to the understanding, treatment and possible prevention of fear disorders. Promising advances are being translated from basic science to the clinic, including approaches to distinguish risk versus resilience before trauma exposure, methods to interfere with fear development during memory consolidation after a trauma, and techniques to inhibit fear reconsolidation and to enhance extinction of chronic fear. It is hoped that this new knowledge will translate to more successful, neuroscientifically informed and rationally designed approaches to disorders of fear regulation.

Translational control via the mammalian target of rapamycin pathway is critical for the formation and stability of long-term fear memory in amygdala neurons.

The mammalian target of rapamycin kinase (mTOR) regulates protein synthesis in neurons at the translational level through phosphorylation of several intracellular targets. Recent work in invertebrates indicates that mTOR-dependent translational control may be critical for the induction and maintenance of activity-dependent synaptic plasticity underlying memory formation. Here, we report that training rats in a simple fear conditioning procedure evokes a time-dependent increase in the phosphorylation of p70s6 kinase, a major direct downstream target of mTOR. When the activation of mTOR was prevented by posttraining injection of rapamycin into the amygdala, formation of the memory and the increase in p70s6 kinase phosphorylation was attenuated. Furthermore, when rapamycin was applied to the amygdala after the recall of a previously stored fear memory, subsequent retention was disrupted, indicating that local translational control at active synapses is required for the stability as well as the formation of long-term memory in this system.

Long-term stability of fear memory depends on the synthesis of protein but not mRNA in the amygdala

Synaptic modification supporting memory formation is thought to depend on gene expression and protein synthesis. Disrupting either process around the time of learning prevents the formation of long‐term memory. Recent evidence suggests that memory also becomes susceptible to disruption upon retrieval. Whether or not the molecular events involved in the formation of new memory are the same as what is needed for memory to persist after retrieval has yet to be determined. In the present set of experiments, rats were given inhibitors of protein or messenger ribonucleic acid (mRNA) synthesis into the amygdala just after training or retrieval of fear memory. Results showed that blocking mRNA or protein synthesis immediately after learning prevented the formation of long‐term memory, while stability of memory after retrieval required protein, but not mRNA, synthesis. These data suggest that the protein needed for memory reconsolidation after retrieval may be transcribed from pre‐existing stores of mRNA.

Consolidation and reconsolidation of contextual fear memory requires mammalian target of rapamycin-dependent translation in the dorsal hippocampus.

The mammalian target of rapamycin (mTOR) pathway is important for regulating protein translation. The present study characterized the role of mTOR-dependent translation in the dorsal hippocampus (DH) during the consolidation and reconsolidation of contextual fear memory. We first showed that fear conditioning resulted in increased phosphorylation of p70s6 kinase (p70s6K) in the DH and that infusion of the mTOR inhibitor rapamycin (RAP) into the DH immediately after training disrupted formation of long-term contextual fear memory. Additionally we showed that p70s6K was activated after retrieval of a previously stored fear memory, and inhibition of mTOR by DH infusion of RAP blocked the reconsolidation of contextual fear memory. Together these results demonstrate that within the DH translational control through the mTOR pathway is important for consolidation as well as the stability of fear memory after retrieval.

Temporary disruption of fear-potentiated startle following PKMζ inhibition in the amygdala

We examined whether protein kinase M zeta (PKMζ) inhibition in the amygdala permanently disrupts fear memory by testing retention at various intervals after PKMζ blockade. Although the expression of fear memory was disrupted when the inhibitor was applied shortly before testing, it had no effect when rats were tested with longer retention intervals. These results suggest that PKMζ inhibition does not erase memory, but temporarily disrupts expression of memory.

The timing of multiple retrieval events can alter GluR1 phosphorylation and the requirement for protein synthesis in fear memory reconsolidation

Numerous studies have indicated that maintaining a fear memory after retrieval requires de novo protein synthesis. However, no study to date has examined how the temporal dynamics of repeated retrieval events affect this protein synthesis requirement. The present study varied the timing of a second retrieval of an established auditory fear memory and followed this second retrieval with infusions of the protein synthesis inhibitor anisomycin (ANI) into the basolateral amygdala. Results indicated that the memory-impairing effects of ANI were not observed when the second retrieval occurred soon after the first (within 1 h), and that the inhibitor gradually regained effectiveness as the retrieval episodes were spaced further apart. Additionally, if the second of the closely timed retrievals was omitted prior to ANI infusions, long-term memory deficits were observed, suggesting that the altered effectiveness of ANI was due specifically to the second retrieval event. Further experiments revealed that the second retrieval was not associated with a change in Zif268 protein expression but did produce a rapid and persistent dephosphorylation of GluR1 receptors at Ser845, an AMPAR trafficking site known to regulate the stability of GluR2 lacking AMPARs, which have been shown to be important in memory updating. This suggests that the precise timing of multiple CS presentations during the reconsolidation window may affect the destabilization state of the memory trace.

The formation of auditory fear memory requires the synthesis of protein and mRNA in the auditory thalamus.

The medial geniculate nucleus of the thalamus responds to auditory information and is a critical part of the neural circuitry underlying aversive conditioning with auditory signals for shock. Prior work has shown that lesions of this brain area selectively disrupt conditioning with auditory stimuli and that neurons in the medial geniculate demonstrate plastic changes during fear conditioning. However, recent evidence is less clear as to whether or not this area plays a role in the storage of auditory fear memories. In the current set of experiments rats were given infusions of protein or messenger RNA (mRNA) synthesis inhibitors into the medial geniculate nucleus of the thalamus 30 min prior to auditory fear conditioning. The next day animals were tested to the auditory cue and conditioning context. Results showed that rats infused with either inhibitor demonstrated less freezing to the auditory cue 24 h after training, while freezing to the context was normal. Autoradiography confirmed that the doses used were effective in disrupting synthesis. Taken together with prior work, these data suggest that the formation of fear memory requires the synthesis of new protein and mRNA at multiple brain sites across the neural circuit that supports fear conditioning.

A Metaplasticity-Like Mechanism Supports the Selection of Fear Memories: Role of Protein Kinase A in the Amygdala

How the brain determines which memories are selected for long-term storage is critical for a full understanding of memory. One possibility is that memories are selected based on the history of activity and current state of neurons within a given memory circuit. Many in vitro studies have demonstrated metaplasticity-like effects whereby prior neuronal activity can affect the ability of cells to express synaptic plasticity in the future; however, the significance of these findings to memory is less clear. Here we show in rats that a single pairing of a light with shock, insufficient to support either short- or long-term fear memory, primes future learning such that another trial delivered within a circumscribed time window lasting from ∼60 min to 3 d results in the formation of a long-lasting and robust fear memory. Two adequately spaced training trials support long-term fear memory only if the two trials are signaled by the same cue. Furthermore, although a single training trial does not support formation of an observable fear memory, it does result in the phosphorylation of several targets of protein kinase A (PKA) in the amygdala. Accordingly, blocking PKA signaling in the amygdala before the first training trial completely prevents the ability of that trial to facilitate the formation of long-term fear memory when a second trial is delivered 24 h later. These findings may provide insight into how memories are selected for long-term storage.

Regulation of extinction-related plasticity by opioid receptors in the ventrolateral periaqueductal gray matter.

Recent work has led to a better understanding of the neural mechanisms underlying the extinction of Pavlovian fear conditioning. Long-term synaptic changes in the medial prefrontal cortex (mPFC) are critical for extinction learning, but very little is currently known about how the mPFC and other brain areas interact during extinction. The current study examined the effect of drugs that impair the extinction of fear conditioning on the activation of the extracellular-related kinase/mitogen-activated protein kinase (ERK/MAPK) in brain regions that likely participate in the consolidation of extinction learning. Inhibitors of opioid and N-methyl-d-aspartic acid (NMDA) receptors were applied to the ventrolateral periaqueductal gray matter (vlPAG) and amygdala shortly before extinction training. Results from these experiments show that blocking opioid receptors in the vlPAG prevented the formation of extinction memory, whereas NMDA receptor blockade had no effect. Conversely, blocking NMDA receptors in the amygdala disrupted the formation of fear extinction memory, but opioid receptor blockade in the same brain area did not. Subsequent experiments tested the effect of these drug treatments on the activation of the ERK/MAPK signaling pathway in various brain regions following extinction training. Only opioid receptor blockade in the vlPAG disrupted ERK phosphorylation in the mPFC and amygdala. These data support the idea that opiodergic signaling derived from the vlPAG affects plasticity across the brain circuit responsible for the formation of extinction memory.

Memory accuracy predicts hippocampal mTOR pathway activation following retrieval of contextual fear memory

Prior work suggests that hippocampus‐dependent memory undergoes a systems consolidation process such that recent memories are stored in the hippocampus, while older memories are independent of the hippocampus and instead dependent on cortical areas. One problem with interpreting these studies is that memory for the contextual stimuli weakens as time passes between the training event and testing and older memories are often less detailed, making it difficult to determine if memory storage in the hippocampus is related to the age or to the accuracy of the memory. Activity of the mammalian target of rapamycin (mTOR) signaling pathway is known to be important for controlling protein translation necessary for both memory consolidation after initial learning and for the reconsolidation of memory after retrieval. We tested whether p70s6 kinase (p70s6K), a key component of the mTOR signaling pathway, is activated following retrieval of context fear memory in the dorsal hippocampus (DH) and anterior cingulate cortex (ACC) at 1, 10, or 36 days after context fear conditioning. We also tested whether strengthening memory for the contextual stimuli changed p70s6K phosphorylation in these structures 36 days after training. We show that under standard training conditions retrieval of a recently formed memory is initially precise and involves the DH. Over time it loses detail, becomes independent of the DH and depends on the ACC. In a subsequent experiment, we preserved the accuracy of older memories through pre‐exposure to the training context. We show that remote memory still involved the DH in animals given pre‐exposure. These data support the notion that detailed memories depend on the DH regardless of their age.