Extracellular matrix controls neuronal features that mediate the persistence of fear

The Perineuronal Net Protein Brevican Acts in Nucleus Accumbens Parvalbumin-Expressing Interneurons of Adult Mice to Regulate Excitatory Synaptic Inputs and Motivated Behaviors

BACKGROUND

Experience-dependent functional adaptation of nucleus accumbens (NAc) circuitry underlies the development and expression of reward-motivated behaviors. Parvalbumin-expressing GABAergic (gamma-aminobutyric acidergic) interneurons (PVINs) within the NAc are required for this process. Perineuronal nets (PNNs) are extracellular matrix structures enriched around PVINs that arise during development and have been proposed to mediate brain circuit stability. However, their function in the adult NAc is largely unknown. Here, we studied the developmental emergence and adult regulation of PNNs in the NAc of male and female mice and examined the cellular and behavioral consequences of reducing the PNN component brevican in NAc PVINs.

METHODS

We characterized the expression of PNN components in mouse NAc using immunofluorescence and RNA in situ hybridization. We lowered brevican in NAc PVINs of adult mice using an intersectional viral and genetic method and quantified the effects on synaptic inputs to NAc PVINs and reward-motivated learning.

RESULTS

PNNs around NAc PVINs were developmentally regulated and appeared during adolescence. In the adult NAc, PVIN PNNs were also dynamically regulated by cocaine. Transcription of the gene that encodes brevican was regulated in a cell type- and isoform-specific manner in the NAc, with the membrane-tethered form of brevican being highly enriched in PVINs. Lowering brevican in NAc PVINs of adult mice decreased their excitatory inputs and enhanced both short-term novel object recognition and cocaine-induced conditioned place preference.

CONCLUSIONS

Regulation of brevican in NAc PVINs of adult mice modulates their excitatory synaptic drive and sets experience thresholds for the development of motivated behaviors driven by rewarding stimuli.

The effectiveness of extinction training in male rats: Temporal considerations and brain mechanisms

The extinction of conditioned fear is frequently used in laboratories as a model for human exposure therapy and is crucial for studies of posttraumatic stress disorder (PTSD). However, the efficacy of specific protocols can vary greatly, and the underlying brain mechanisms are not sufficiently clarified. To address this issue, variable starting time (one or twenty-eight days after fear conditioning) and extinction protocols were used, and the efficacy and durability of fear extinction were also studied. Changes in the behavior, stress hormone levels and neuronal activation patterns of stressed rats were analyzed. Conditioned fear was rapidly and efficiently extinguished by all the protocols investigated. However, when these extinction protocols were initiated one day after fear training, conditioned fear relapsed spontaneously four weeks later. In contrast, when extinction trials were started 28 days after conditioning, no relapse occurred. Hormone measurements taken by the end of extinction trials indicated that adrenocorticotropin, but not corticosterone responses reflected behavioral extinction without any sign of relapse. The last extinction training increased the activation of the medial prefrontal cortex and decreased the activation of the central and medial amygdala when extinction began one day after fear conditioning. By contrast, the activation of the basolateral amygdala and the entire hippocampus decreased by the last training session when extinction started 28 days after fear conditioning. Our findings show that extinction training can extinguish remote fear memories more effectively than recent ones, and that the brain mechanisms underlying remote and recent fear memory extinction differ. Laboratory models should also focus on a later time point to increase their translational value.

Extrasynaptic therapeutic targets in substance use and stress disorders

Treatments for substance use and stress disorders are based on ameliorating behavioral symptoms, not on reversing the synaptic pathology that has the potential to cure disorders. This failing arises in part from a research focus on how pre- and postsynaptic physiology is changed even though key neuropathology exists in the perisynaptic neuropil that homeostatically regulates synaptic transmission. We explore recent findings from the substance use and stress disorder literature pointing to a key role for perisynaptic astroglia and signaling in the extracellular matrix (ECM) in regulating synaptic pathology. We conclude that drugs and stress initiate long-lasting changes in brain synapses via enduring neuroadaptations in astroglia and the ECM, and that modulating extrasynaptic regulators may be therapeutically useful.

Perineuronal Net Receptor PTPσ Regulates Retention of Memories

Perineuronal nets (PNNs) have an important physiological role in the retention of learning by restricting cognitive flexibility. Their deposition peaks after developmental periods of intensive learning, usually in late childhood, and they help in long-term preservation of newly acquired skills and information. Modulation of PNN function by various techniques enhances plasticity and regulates the retention of memories, which may be beneficial when memory persistence entails negative symptoms such as post-traumatic stress disorder (PTSD). In this study, we investigated the role of PTPσ [receptor-type tyrosine-protein phosphatase S, a phosphatase that is activated by binding of chondroitin sulfate proteoglycans (CSPGs) from PNNs] in retention of memories using Novel Object Recognition and Fear Conditioning models. We observed that mice haploinsufficient for PTPRS gene (PTPσ^+/–), although having improved short-term object recognition memory, display impaired long-term memory in both Novel Object Recognition and Fear Conditioning paradigm, as compared to WT littermates. However, PTPσ^+/– mice did not show any differences in behavioral tests that do not heavily rely on cognitive flexibility, such as Elevated Plus Maze, Open Field, Marble Burying, and Forced Swimming Test. Since PTPσ has been shown to interact with and dephosphorylate TRKB, we investigated activation of this receptor and its downstream pathways in limbic areas known to be associated with memory. We found that phosphorylation of TRKB and PLCγ are increased in the hippocampus, prefrontal cortex, and amygdaloid complex of PTPσ^+/– mice, but other TRKB-mediated signaling pathways are not affected. Our data suggest that PTPσ downregulation promotes TRKB phosphorylation in different brain areas, improves short-term memory performance but disrupts long-term memory retention in the tested animal models. Inhibition of PTPσ or disruption of PNN-PTPσ-TRKB complex might be a potential target for disorders where negative modulation of the acquired memories can be beneficial.

Nonsynaptic plasticity model of long-term memory engrams

Using steady-state electrical properties of non-ohmic dendrite based on cable theory, we derive electrotonic potentials that do not change over time and are localized in space. We hypothesize that clusters of such stationary, local and permanent pulses are the electrical signatures of enduring memories which are imprinted through nonsynaptic plasticity, encoded through epigenetic mechanisms, and decoded through electrotonic processing. We further hypothesize how retrieval of an engram is made possible by integration of these permanently imprinted standing pulses in a neural circuit through neurotransmission in the extracellular space as part of conscious recall that acts as a guiding template in the reconsolidation of long-term memories through novelty characterized by uncertainty that arises when new fragments of memories reinstate an engram by way of nonsynaptic plasticity that permits its destabilization. Collectively, these findings seem to reinforce this hypothesis that electrotonic processing in non-ohmic dendrites yield insights into permanent electrical signatures that could reflect upon enduring memories as fragments of long-term memory engrams.

Removal of perineuronal nets disrupts recall of a remote fear memory

Throughout life animals learn to recognize cues that signal danger and instantaneously initiate an adequate threat response. Memories of such associations may last a lifetime and far outlast the intracellular molecules currently found to be important for memory processing. The memory engram may be supported by other more stable molecular components, such as the extracellular matrix structure of perineuronal nets (PNNs). Here, we show that recall of remote, but not recent, visual fear memories in rats depend on intact PNNs in the secondary visual cortex (V2L). Supporting our behavioral findings, increased synchronized theta oscillations between V2L and basolateral amygdala, a physiological correlate of successful recall, was absent in rats with degraded PNNs in V2L. Together, our findings suggest a role for PNNs in remote memory processing by stabilizing the neural network of the engram.