Given the implications of the fear conditioning model, a likely correlate of such human conditions as post-traumatic stress disorder (PTSD) and panic disorders, 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. 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. In humans, the correlate to this extinction process is commonly known as exposure therapy and is a common treatment for disorders such as PTSD.
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. 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.
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. In contrast, another selective serotonin reuptake inhibitor, fluoxetine, enhances this process. 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, and even extends to the acquisition of new CS-US pairings following the extinction of the initial conditioned response.
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. 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. On the other hand, when rats are given the beta-adrenergic antagonist propranolol, the extinction procedure becomes markedly less effective.
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. 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. 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.
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. 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. 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. 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. 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. 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.
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 while preventing corticosterone synthesis with the drug metyrapone impaired it. 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. 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.
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. 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.
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.
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