Fear Conditioning

Fear Conditioning: Modulation by Drugs

By September 30, 2019 October 4th, 2019 No Comments

Given the implications of the fear conditioning model, a likely correlate of such human conditions as post-traumatic stress disorder (PTSD) and panic disorders,[1] research into the modulation of the fear conditioning time-course and overall character are obvious avenues of interest for translation research. If pharmacological manipulation in laboratory animals allows for the relative erasure or diminishment of the fear-conditioned response, the findings may translate to therapeutic interventions for those suffering from chronic anxiety or PTSD. Aside from this immediate translational value, fear conditioning is an extremely common model for the investigation of the biological underpinnings of learning and memory in the brain.[2] As such, drugs that may modulate this process can provide valuable insight into these mechanisms and further our collective understanding of how, where, and why the brain forms, modulates and eventually rids itself of memories. In fact, many such approaches have been tested with variable results. Here, we will discuss some of the most significant findings in this field, the modulation of fear conditioning and extinction by pharmacological manipulation.

Following the induction of a conditioned response in which a previously neutral stimulus (the conditioned stimulus or CS) is associated with an aversive experience (the unconditioned stimulus or US), animals exhibit signs of fear when presented with the CS alone (i.e. in the absence of an aversive experience). At this point, two relevant processes can proceed. First, in a process known as extinction, repeated exposure to the CS in the absence of the US eventually diminishes this fear responding.[3] In humans, the correlate to this extinction process is commonly known as exposure therapy and is a common treatment for disorders such as PTSD.[4]

Aside from induced extinction, in many cases, the CS-US association often weakens over time when the animal is not repeatedly exposed to the paired protocol. Conversely to the extinction process, animals who have lost the strong CS-US association either via the passage of time or induced extinction may have the pairing renewed or bolstered in a process known as renewal.[5] In a renewal test, the animal is re-exposed to the original CS-US pairing context in order to reinforce the fear response. Many pharmacological manipulations including those which alter the monoamines (serotonin, dopamine, and norepinephrine), those which alter the endocannabinoid system, and those which alter hormones, have been found to modulate both the extinction and the renewal processes in different fashions.[6]

Monoamine Modulators

Monoamine-targeting drugs, such as antidepressants that increase levels of serotonin, have been used in the treatment of stress disorders for several decades. Interestingly, converse results have been found between drugs that, by all respects, act in the same manner. For example, the common antidepressant citalopram, when given in the period between fear conditioning acquisition and the extinction protocol, has been found to diminish the effectiveness of extinction.[7] In contrast, another selective serotonin reuptake inhibitor, fluoxetine, enhances this process.[8] When given in the period between fear conditioning and extinction, fluoxetine has been found to prevent further renewal of the conditioned response. This finding has been repeated with additional US types,[9] and even extends to the acquisition of new CS-US pairings following the extinction of the initial conditioned response.[10]

Another monoamine neurotransmitter, norepinephrine, has also been investigated for its role in modulating fear conditioning. Unlike serotonin, the results from studies manipulating norepinephrine are quite clear: enhancing signaling increases the efficiency of the extinction process, while decreased signaling diminishes it.[11] Yohimbine, a norepinephrine releasing agent, has been found to dose-dependently reduce the freezing response during a CS presentation when given prior to extinction training.[12] On the other hand, when rats are given the beta-adrenergic antagonist propranolol, the extinction procedure becomes markedly less effective.[13]

The final monoamine neurotransmitter which has been found to modulate the fear conditioning process is dopamine, whose role in learning and memory is well-established. When rodents are treated with methylphenidate, a dopamine and norepinephrine reuptake inhibitor, before and after extinction training, the CS responding behavior is substantially diminished.[14] This dopamine-mediated enhancement of extinction is also supported by data showing that treating rodents with dopamine’s precursor molecule, L-Dopa, increases the effectiveness of extinction training.[15] Moreover, this treatment also diminishes the renewal process, suggesting that dopamine’s ability to enhance the extinction of a memory is robust enough to prevent its latent traces in the brain.

Cannabinoid Modulators

The endocannabinoid system, widely acknowledged for its role in learning and memory, has been repeatedly shown as a source of potential manipulation for the fear conditioning process.[16] Indeed, when humans are given either cannabidiol or ∆9-THC, two active ingredients in cannabis which increase endocannabinoid signal transduction, extinction procedures following fear acquisition are significantly more robust.[17,18]

Similarly, when rats are administered a direct agonist of the cannabinoid receptor CB1, the extinction process is substantially enhanced.[19] This finding is supported by inverse experiments, in which rodents are treated with CB1 receptor antagonists, leading to a reduction in the efficacy of extinction training.[20] Studies with knockout animals, in which the CB1 receptor is selectively deleted from their genome, further confirm the role of the endocannabinoid system in this process. First, mice with deleted CB1 receptors have been shown to exhibit significantly impaired extinction learning.[21] In a more complex model known as conditional knockouts, these receptors may be selectively reintroduced to the brain at a given time and place. When these experiments were conducted in mice, selective expression of the CB1 receptor in telencephalic glutamate neurons efficiently rescued and enhanced extinction learning.[22] Together, these results paint a clear picture of the role of the endocannabinoid system in fear conditioning: CB1 receptors are a necessary component of extinction learning and enhancing or promoting their function subsequently enhances this process while blocking their function has the opposite effect.

Hormone Modulators

It is well known that hormones such as corticosterone/cortisol (known as the “stress hormone”) and the gonadal hormone estrogen interact directly with the brain in regions such as the amygdala and hippocampus, both of which are heavily implicated in learning and, in particular, fear conditioning. Thus, it is of little surprise that modulation of these hormones impacts the fear conditioning process.

In rats, enhancing corticosterone signaling significantly improves extinction learning. In two studies, administration of the agonist dexamethasone was found to facilitate extinction learning[23] while preventing corticosterone synthesis with the drug metyrapone impaired it.[24] The picture in mice, however, is less clear. While corticosterone given immediately after extinction training has been shown to enhance extinction learning in BALB/c mice, this same procedure impairs extinction learning in C57BL/6J mice.[25] Thus, to some degree, the effects of corticosterone modulation on fear extinction remain inconclusive. One study in humans has confirmed the rat findings, however, showing enhanced extinction learning when participants with acrophobia were given cortisol (the human version of rodents’ corticosterone) prior to exposure therapy.[26]

Finally, while few studies have been conducted on the role of gonadal hormones in modulating the fear conditioning process, those which have been published suggest that they effectively enhance extinction learning. For instance, when female rats are given estrogen or progesterone during the low estrogen/progesterone period of their hormone cycle, extinction learning is significantly facilitated.[27] While direct comparisons regarding hormone levels between males and females are difficult to decipher, further experiments support this role for progesterone-mediated enhancement of extinction learning by comparing male rats to females when the latter are tested during the high-progesterone period of their hormone cycle. These pro-estrus female rats exhibit comparatively higher levels of extinction learning, an effect which is not present when the rats are ovariectomized, a process which negates their hormone cycling and therefore controls for innate male/female differences.[28]

Conclusions

Clearly, there exist a plethora of pharmacological manipulations that may bidirectionally modulate the process of fear conditioning, in particular, extinction training. While results on selective serotonin reuptake inhibitors are somewhat mixed, the evidence for norepinephrine and dopamine both confirm that enhancement of monamine transmission is a strong promotor of extinction learning. The endocannabinoid system has additionally been confirmed as playing a crucial role in this process, with drugs that increase or decrease its function subsequently enhancing or impairing the learning, respectively. Finally, the levels of hormones such as cortisol/corticosterone and estrogen are intimately tied to the efficacy of learning, indicating an important avenue of research for scientists and practitioners interested in gaining further insight into potential treatments for such conditions as chronic stress, phobias, and PTSD. As research into these various modulatory drugs continues, so too will the development of effective pharmacotherapeutic manipulations for learning and memory as it pertains to these serious conditions.

References

  1. Bouton ME, Mineka S, Barlow DH. A modern learning theory perspective on the etiology of panic disorder. Psychol Rev. 2001;108:4–32.
  2. Phelps EA, LeDoux JE. Contributions of the Amygdala to Emotion Processing: From Animal Models to Human Behavior. Neuron. 2005;48:175–187.
  3. Pavlov IP. Conditioned reflexes: an investigation of the physiological activity of the cerebral cortex. Oxford University Press; 1927.
  4. Parsons TD, Rizzo AA. Affective outcomes of virtual reality exposure therapy for anxiety and specific phobias: A meta-analysis. Journal of Behavior Therapy and Experimental Psychiatry. 2008;39:250–261.
  5. Bouton ME, Westbrook RF, Corcoran KA, Maren S. Contextual and Temporal Modulation of Extinction: Behavioral and Biological Mechanisms. Biological Psychiatry. 2006b;60:352–360.
  6. Fitzgerald, P. J., Seemann, J. R., & Maren, S. (2014). Can fear extinction be enhanced? A review of pharmacological and behavioral findings. Brain Research Bulletin, 0, 46–60.
  7. Burghardt NS, Sigurdsson T, Gorman JM, McEwen BS, LeDoux JE. Chronic Antidepressant Treatment Impairs the Acquisition of Fear Extinction. Biological Psychiatry. 2013;73:1078–1086.
  8. Deschaux O, Spennato G, Moreau J-L, Garcia R. Chronic treatment with fluoxetine prevents the return of extinguished auditory-cued conditioned fear. Psychopharmacology. 2011;215:231–237.
  9. Deschaux O, Zheng X, Lavigne J, Nachon O, Cleren C, Moreau J-L, Garcia R. Post-extinction fluoxetine treatment prevents stress-induced reemergence of extinguished fear. Psychopharmacology. 2012;225:209–216.
  10. Spennato G, Zerbib C, Mondadori C, Garcia R. Fluoxetine protects hippocampal plasticity during conditioned fear stress and prevents fear learning potentiation. Psychopharmacology. 2008;196:583–589.
  11. Cain CK. Adrenergic Transmission Facilitates Extinction of Conditional Fear in Mice. Learning & Memory. 2004;11:179–187.
  12. Mueller D, Olivera-Figueroa LA, Pine DS, Quirk GJ. The effects of yohimbine and amphetamine on fear expression and extinction in rats. Psychopharmacology. 2009;204:599–606.
  13. Do-Monte FH, Kincheski GC, Pavesi E, Sordi R, Assreuy J, Carobrez AP. Role of beta-adrenergic receptors in the ventromedial prefrontal cortex during contextual fear extinction in rats. Neurobiology of Learning and Memory. 2010;94:318–328.
  14. Abraham AD, Cunningham CL, Lattal KM. Methylphenidate enhances extinction of contextual fear. Learn. Mem. 2012;19:67–72.
  15. Haaker J, Gaburro S, Sah A, Gartmann N, Lonsdorf T, Meier K, Singewald N, Pape H, Morellini F, Kalisch R. Single dose of L-dopa makes extinction memories context-independent and prevents the return of fear. Proceedings of the National Academy of Science. 2013:E2428–E2436.
  16. Lafenêtre P, Chaouloff F, Marsicano G. The endocannabinoid system in the processing of anxiety and fear and how CB1 receptors may modulate fear extinction. Pharmacological Research. 2007;56:367–381.
  17. Das RK, Kamboj SK, Ramadas M, Yogan K, Gupta V, Redman E, Curran HV, Morgan CJA. Cannabidiol enhances consolidation of explicit fear extinction in humans. Psychopharmacology. 2013;226:781–792.
  18. Rabinak CA, Angstadt M, Sripada CS, Abelson JL, Liberzon I, Milad MR, Phan KL. Cannabinoid facilitation of fear extinction memory recall in humans. Neuropharmacology. 2013;64:396–402.
  19. Pamplona FA, Bitencourt RM, Takahashi RN. Short- and long-term effects of cannabinoids on the extinction of contextual fear memory in rats. Neurobiology of Learning and Memory. 2008;90:290–293.
  20. Lin H-C, Mao S-C, Su C-L, Gean P-W. The role of prefrontal cortex CB1 receptors in the modulation of fear memory. Cereb Cortex. 2009;19:165–175.
  21. Marsicano G, Wotjak CT, Azad SC, Bisogno T, Rammes G, Cascio MG, Hermann H, Tang J, Hofmann C, Zieglgänsberger W, Di Marzo V, Lutz B. The endogenous cannabinoid system controls extinction of aversive memories. Nature. 2002;418:530–534.
  22. Ruehle S, Remmers F, Romo-Parra H, Massa F, Wickert M, Wortge S, Haring M, Kaiser N, Marsicano G, Pape HC, Lutz B. Cannabinoid CB1 Receptor in Dorsal Telencephalic Glutamatergic Neurons: Distinctive Sufficiency for Hippocampus-Dependent and Amygdala-Dependent Synaptic and Behavioral Functions. Journal of Neuroscience. 2013;33:10264–10277.
  23. Yang Y-L, Chao P-K, Lu K-T. Systemic and Intra-Amygdala Administration of Glucocorticoid Agonist and Antagonist Modulate Extinction of Conditioned Fear. Neuropsychopharmacology. 2006;31:912–924.
  24. Yang Y-L, Chao P-K, Ro L-S, Wo Y-YP, Lu K-T. Glutamate NMDA Receptors within the Amygdala Participate in the Modulatory Effect of Glucocorticoids on Extinction of Conditioned Fear in Rats. Neuropsychopharmacology. 2007;32:1042–1051.
  25. Brinks V, de Kloet ER, Oitzl MS. Corticosterone facilitates extinction of fear memory in BALB/c mice but strengthens cue related fear in C57BL/6 mice. Experimental Neurology. 2009;216:375–382.
  26. de Quervain D, Bentz D, Michael T, Bolt O, Wiederhol B, Margraf J, Wilhelm F. Glucocorticoids enhance extinction-based psychotherapy. Proceedings of the National Academy of Science. 2011;108:6621–6625.
  27. Milad MR, Igoe SA, Lebron-Milad K, Novales JE. Estrous cycle phase and gonadal hormones influence conditioned fear extinction. Neuroscience. 2009;164:887–895.
  28. Chang Y-J, Yang C-H, Liang Y-C, Yeh C-M, Huang C-C, Hsu K-S. Estrogen modulates sexually dimorphic contextual fear extinction in rats through estrogen receptor β. Hippocampus. 2009;19:1142–1150.

About Andrew Scheyer

Andrew Scheyer is a postdoctoral fellow working at the Institut de Neurobiologie de la Méditerranée in Marseille, France. He specializes in electrophysiology and synaptic network development of the endocannabinoid system. Andrew began his studies at Pitzer College, in California (USA) where he acquired his bachelor's degree in Neuroscience before moving to Rosalind Franklin University in North Chicago, Illinois, where he completed his PhD in Neuroscience working on synaptic mechanisms underlying cocaine addiction and withdrawal. In addition to working as a synaptic physiologist, Andrew has contributed as an author in publications ranging from The Scientist Magazine to textbooks such as Endocannabinoids and Lipid Mediators in Brain Functions. He has additionally been working as a freelance scientific writer and editor since 2018. In his free time, Andrew is an ultra-endurance cyclist and avid reader.