ReviewBridging the Gap: Towards a cell-type specific understanding of neural circuits underlying fear behaviors
Introduction
Disorders whose major symptoms relate to the dysregulation of fear responses are usually characterized by over-generalization of fear and inability to extinguish fearful responses. Such dysregulation leads to a pathological expression of fear behaviors that can be quite debilitating, leading to a range of intrusive, hyperarousal, avoidance, cognitive, and depression symptoms. The treatment of fear-related disorders often involves cognitive-behavioral therapies, in particular exposure therapy, which mirrors behavioral extinction processes used in rodent models, relying on the repeated and non-reinforced presentation of cues previously associated with noxious stimulus.
Advances in cognitive-behavioral therapy approaches targeting traumatic memories have been made using cognitive enhancers, for example by targeting emotion-related synaptic plasticity via the NMDA, Dopamine, and Cannabinoid receptors (Singewald, Schmuckermair, Whittle, Holmes, & Ressler, 2015). Pharmacological interventions may be used to generally enhance plasticity within neural circuitry including that responsible for behavioral extinction. Across several fear- and anxiety-related disorders, the administration of cognitive enhancers, such as d-cycloserine, in conjunction with exposure-based psychotherapy has been shown to enhance the beneficial effects of behavioral therapy sessions in a rapid and long-lasting manner (Rodrigues et al., 2014, Singewald et al., 2015). Despite these advances, insufficient knowledge of the underlying molecular and cellular mechanisms mediating fear acquisition, expression, and extinction continues to limit the specificity and effectiveness of further therapeutic breakthroughs. Therefore, a greater understanding of the neural circuitry mediating fear processing will catalyze further progress in the development of more selective treatments for fear- and anxiety-related disorders.
In this review, we will begin by discussing the understanding of the circuitry governing the acquisition and extinction of classically conditioned fear behaviors. We will continue by discussing the advent of optogenetic approaches and the contributions this technique has made to our knowledge of fear circuits. We will discuss the use of genetic techniques to determine which and how cell populations are recruited into memory traces. With a special focus on studies that involve behavioral manipulations, we will examine recent advances in the manipulation of identified cellular sub-populations housed within canonical fear and emotional learning related circuitries. Finally, we will provide a brief review of methods for cell-type specific isolation of RNA for sequencing.
As the basic neural circuitry governing fear behaviors continues to be elucidated at a rapid pace, it is necessary to act prospectively by applying these findings towards the discovery of applicable treatments for patients suffering from fear and anxiety related disorders. By uncovering cell-type specific markers for neural circuitry governing fear and anxiety behaviors in rodent models modern researchers have an opportunity to concurrently open avenues for more targeted pharmacological therapies in humans. Cell type specific markers may be conserved across species and targeting these convergences will maximize translational value of discoveries. This review is meant to highlight the need for further cell-type specific approaches in order to make rapid progress towards more selective and targetable pharmacological treatments of fear-related disorders in humans.
Section snippets
Pavlovian conditioning
Pavlovian fear conditioning is a popular and powerful technique for studying learning and memory in animal models. This is primarily due to it being a rapidly acquired behavior with consistent and easily measured behavioral outputs that rely on a well-characterized core neural circuit. Fear conditioning, also discussed as threat conditioning (LeDoux, 2014), occurs through the pairing of an initially innocuous conditioned stimulus (CS, e.g., an auditory tone during auditory fear conditioning or
Optogenetic tracing of fear circuitry
The dawn of modern genetic tools has allowed for remote control of genetically defined cellular sub-populations and has thus greatly enhanced the specificity of manipulations delineating the role of specific nuclei or connections between nuclei involved in fear responses.
Optogenetics is based upon the use of genetically encoded, optically responsive ion channels or pumps. Initially discovered by Negel and colleagues, and greatly expanded by Boyden, Deisseroth, Zhang and others, channelrhodopsin
Search for the memory engram
While the studies described above confirm the basic circuitries involved in fear responses and fear learning, many fundamental questions about these processes remain. As it appears select ensembles of neurons, not entire nuclei, are involved in the encoding of distinct memories; one major area of investigation has been to discover how these ensembles are recruited and whether they are static over time. This line of research, when combined with next cell-type specific techniques, may prove to be
Cell type specific targeting of behavioral processes
An understanding of the neural circuits underlying behavior is clearly valuable for the study of the biology of learning and memory as highlighted in the above sections. However, without translationally tractable strategies for identifying targets to modulate fear responses and learning in humans, the value of further dissection of this circuitry will remain somewhat esoteric. One promising strategy is the manipulation of genetically defined neuronal populations whose global modulation may have
Cell type specific transcriptome sequencing
In the case of several cell-type specific markers mentioned above, direct manipulation of the protein product of the identifier gene is possible; however, in most cases this is either impossible or translationally impractical. In these cases it is necessary to identify additional pharmacologically tractable targets for remote control of these populations in a closed system. To efficiently molecularly phenotype these populations the most expedient route is cell-type specific RNA sequencing.
Summary
Cell-type specific interrogation of the behavioral and molecular profiles of select neuronal populations within the brain is likely the most expedient avenue towards the identification of selective compounds that modulate distinct circuitries involved in fear and anxiety related behaviors and associated disorders. In rodent models, optogenetics has rapidly confirmed and expanded the known neural circuitries underlying fear related behaviors. By identifying and manipulating genetically marked
Acknowledgements and Disclosures
The authors declare no competing financial interests. Support was provided by NIH (R01MH096764) and by an NIH/NCRR base grant (P51RR000165) to Yerkes National Primate Research Center.
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2021, Progress in Neuro-Psychopharmacology and Biological PsychiatryCitation Excerpt :Due to the complexity of connections within brain regions important for fear and extinction, manipulating behavior by selective targeting can be challenging. Therefore, there is a need to identify specific neuronal populations and sub-populations of neurons that are characterized by distinct cell types with unique expression of genes and proteins (see review for cell-type specific analysis of fear circuitry (McCullough et al., 2016)). In last years, the endocannabinoid system has emerged as a critical neuromodulator component of stress and fear regulation.