Imagine you are cycling home from a party in the city centre, which, due to the tremendously towering Amsterdam city centre rents, is always slightly farther than you wished. Suddenly, you are faced with an unexpected dilemma. Due to road construction your regular route is partly blocked and you are faced with two options: either you go right and cycle through an unlit, murky and rather sinister park or you can go left and cycle over a just renewed road, sided by houses and streetlights. The moment you are weighing your options, your brain is interpreting the safety and anxiety signals from the environment and informing you, hopefully, to take a safe lefty.

As you may deduct from the previous example, the ability to differentiate safety from danger is crucial for survival. However, the underlying neural mechanisms that facilitate safety signalling remained unknown until a New York research group recently published astonishing results (Likhtik et al. 2014). In their study, they elucidate how brain networks provide a safety-signalling mechanism, in which the medial Prefrontal Cortex (mPFC) detects safety and signals to the amygdala to decrease fear. These are intriguing insights from animal studies into how a healthy rodent brain processes fear and safety. This has immense implications for research towards anxiety disorders in humans, as safety and fear signalling plays an important role here.

These implications are especially true for Post Traumatic Stress Disorder (PTSD), with a prevalence of 18,8%, three months after experiencing a traumatic event (Santiago et al. 2013). PTSD is an anxiety disorder that develops after experiencing a trauma, characterised by intrusion symptoms, for example frightening flashbacks, heightened arousal, persistent avoidance of trauma-related stimuli and negative thoughts and mood. Previous studies have shown that PTSD patients suffer from a deficit in processing safety signals and this causes an overgeneralization of fear (Jovanovic, Kazama, et al. 2012; Jovanovic et al. 2010; Etkin 2012). For example, as a combat veteran, the sound of a whirring helicopter might cause a fear response, because it horrifically reminds you of a possible attack by the enemy in Afghanistan. However, after repatriation and several confrontations with the sound of a helicopter in the safe environment of home, amongst your loved ones, this fear response should go extinct. Though, a striking and long established characteristic of PTSD is the reduced extinction of a conditioned fear response (Daskalakis & Yehuda 2014; Etkin 2012; Jovanovic, Kazama, et al. 2012; Jovanovic et al. 2010; Rougemont-Bücking et al. 2011; Likhtik et al. 2008; Likhtik et al. 2014; Shin et al. 2013; Thomaes et al. 2014), which may be caused by a lack of safety signalling.

Fear and safety in the brain

One of the best-known structures of the brain that is involved in fear processing, is the amygdala. In an incredible number of studies, the basolateral amygdala (BLA) has been determined as the structure that is activated in fear and is crucial for fear conditioning. Studies towards the neural correlates of PTSD, have shown that the BLA is hyperactive in PTSD patients (Shin et al. 2013; Jovanovic, Kazama, et al. 2012; Jovanovic et al. 2010; Rougemont-Bücking et al. 2011; Etkin 2012). This structure is connected to several other brain regions, therefore facilitating a behavioural response after a fearful cue in the environment. The signalling of fear is essential for survival, however, when the fearful cue vanishes, the amygdala has to receive a signal to decrease its activity. The big brother that controls this safety signal, appears to be the medial Prefrontal Cortex.

In their animal experiment, Lithik and colleagues looked at long-range communication in the brain after signals of fear and safety. This long range communication is facilitated by the so called theta (4-12 Hz) oscillations (brain-waves).

The keen set-up of the experiment allowed them to look directly and with high temporal resolution at these theta oscillations in the brain, while the rat experienced either a fearful or a safe stimulus. The rats participated in two experiments, a fear conditioning experiment and an open field test.

The Experiment: Communicating fear and safety

In the fear conditioning test, the rats learned to associate an auditory stimulus with an aversive stimulus (CS+), a mild short shock to the paws, or with no other stimulus (CS-). After three days of training, recall of the conditioned responses was tested in a new context. As may be expected, rats showed a fear response (so called freezing) to the CS+, but startlingly varied in their response to the CS-. This led to the formation of two groups, the Generalizers, who showed increased fear responses to both the CS+ and the CS-, and the Discriminators, who showed a significantly higher fear response to the CS+ than the CS-. When the brain data were examined, they found that the associated auditory stimulus yielded greater theta power in the BLA and mPFC, in both groups. In Discriminators, the auditory-evoked increase in theta power was higher during the CS+ than the CS-, while Generalizers showed equal theta power increase in CS+ and CS-.

Interaction between brain areas is shown by the synchronous firing of neurons in two distinct brain areas and the coherence and timing of this firing is a measure for connectivity. The researchers found that the interactions between the BLA and mPFC were explicitly increased when fear discrimination accurately occurred. Deeper investigation of this data showed that only Discriminators had higher coherence between the BLA and mPFC after the auditory stimulus, during both the CS+ and CS-, compared to Generalizers.
Moreover, the temporal relationship of brain waves between these areas suggest that while Discriminators experience a CS-, the mPFC sends a safety signal to the BLA. This implies that the BLA-mPFC interaction is important for the behavioural relevant signalling of safety. This is facilitated by the mPFC, which initiates a theta frequency oscillation towards the BLA to declare a situation is safe.

Figure 1. Safety & Danger signalling between the Medial Prefrontal Cortex (mPFC) and Basolateral Amygdala (BLA)
(Likhtik et al. 2014)

In the second experiment, the rats were placed in a brightly lit open field, which causes an innate anxiety response. It was shown that when the rats moved towards the relatively safe periphery of the open field, BLA and mPFC power and synchrony was amplified, whereas BLA firing declined. In agreement with the previous experiment, in the safe periphery, the BLA follows the theta frequency oscillations from the mPFC. This test confirms the relationship between the BLA and mPFC, and shows that in both learned and innate anxiety, the mPFC detects and subsequently signals safety to the BLA (Likhtik et al. 2014).
This exciting experiment leads to the conclusion that the mPFC and BLA work together to signal safety in the environment. More specifically, the mPFC detects safety and sends a signal to the BLA that a situation is safe, thereby diminishing fear (Likhtik et al. 2014).

Implications for PTSD in humans

This study is the first to elucidate the neural mechanism behind safety signalling in the rodent brain, interestingly however, previous research found that safety signalling is impaired in human patients with PTSD (Jovanovic et al. 2010; Jovanovic, Kazama, et al. 2012; Rougemont-Bücking et al. 2011). Therefore, a deficit in signalling safety may be an important (and exclusive) biomarker of the disease (Jovanovic, Kazama, et al. 2012). The discovery of a safety signalling mechanism in the brain has large implications in the field of PTSD. Nowadays, therapeutic strategies focus on decreasing feelings of fear, by applying cognitive behavioural therapy or anti-anxiety drugs with several side effects. But what would happen if you would instead increase feelings of safety?
The locus of safety, according to this study of Likthik and colleagues (2014) is situated in the medial prefrontal cortex. The mPFC is a brain structure that has an important role in cognitive functioning, especially in cognitive control and inhibiting impulsive responses. Researchers found that these inhibitory mechanisms in the mPFC underline the inability of PTSD patients to inhibit fear. PTSD patients show decreased activation of the mPFC in an inhibition task. Remarkably, the amount of activation correlated with the ability to inhibit a fear response and generalize safety in a subsequent fear conditioning task. This effect was exclusively established in patients with PTSD and not found in traumatized citizens without the disorder (Jovanovic, Ely, et al. 2012). This is another thread of admirable evidence that the mPFC is essential in signalling safety in the brain and that this ability is adversely affected in post traumatic stress disorder.

Besides PTSD patients, other patients that suffer from for example generalized anxiety disorders could also benefit from an increase in safety signalling. This study shows that not only learned fear is decreased by enhancing safety signalling, the same goes for innate anxiety. Therefore, modulation of the mPFC may likewise contribute positively to anxious people that are naturally more prone to develop an anxiety disorder.

There are several strategies that could be implemented to target the mPFC therapeutically, to stimulate safety signalling to the BLA, thus decreasing fear. By tapping into the safety signalling circuit to promote inhibition of the BLA, PTSD patients might be relieved of their recurrent intrusive thoughts, terrible nightmares and fearful flashbacks. Sounds like a perfect plan, doesn’t it? So, what would happen if you use specific stimulation techniques such as state of the art deep brain stimulation or pharmacological substances that specifically target the mPFC in PTSD? The truth is, that we simply do not know yet.

No matter how fruitful the discovery of this novel circuit may seem in the development of new therapeutic targets, drug industries look before they leap: these treatments do not develop overnight. As is always the case in drug development, it takes at least several years before this research could be possibly implemented to treat PTSD patients. Firstly, animal research should confirm and determine a locus in the mPFC as being the safety signalling site. Researchers are making progress on developing animal models for PTSD, which mimic the disease on various aspects (Daskalakis & Yehuda 2014). Secondly, interventions in the brain of a PTSD rodent should be applied to confirm (or of course contradict) that this safety signalling mechanism can indeed be altered.
Besides, it should be confirmed that these changes have the desired effect of generalizing safety and eradicating fear in safe environment. Important to note is, that this safety signalling should be applied specifically for the trauma-related memory, and not provide an overall and continuous feeling of safety. After all, the recognition and discrimination of fear and safety is an essential survival skill. Then, translational studies should be performed (and confirmed and re-confirmed) before these techniques could possibly be applied in clinical trials with human subjects.

A shortcut?

However, there may be a shortcut. A few years ago, Transcranial Magnetic Stimulation (TMS for short), was developed. TMS is a fast and safe technique in which electromagnetic fields are used to non-invasively modulate certain brain areas. Delivery of high frequency pulses is stimulating neural activity in underlying brain regions, while low frequency pulses inhibiting neural activity (Karsen et al. 2014). Recently, in a limited number of studies, TMS has been applied to investigate the effectiveness of this technique in PTSD patients and these show promising results (Karsen et al. 2014; Isserles et al. 2013). One study specifically targeted the medial prefrontal cortex with TMS during exposure to traumatic scenes and found decreased scores on a PTSD symptom scale. Interestingly, this effect lasted for two weeks up till two months after the experiment (Isserles et al. 2013). The authors subscribe this effect of TMS on the mPFC to the fact that the mPFC is an important region in fear extinction. However, when considering the latest discovery of the safety signalling mechanism, one could speculate these researchers were actually, unknowingly, stimulating safety. To further investigate this possibility, more research towards TMS and safety signalling in PTSD should be conducted. If this method succeeds, it would save a lot of time, hassle and money invested in translational research. Therefore, increasing safety signalling by using TMS in PTSD should become a priority research area.

If scientists succeed in developing safety enhancing therapies in which safe cues from the environment that evoke terrifying trauma-related memories, are specifically targeted and retrieve their safe valence, PTSD can be cured. A serious candidate to non-invasively stimulate the mPFC to signal safety, and thereby treat PTSD, is dTMS. This treatment will allow PTSD patients to yet again discriminate between the sound of a helicopter safely at home, and out there in the field and allow them to signal safety and overcome fear in the first situation. This would not only bring an unutterable relief for PTSD patients and their families, the profits for society as a whole are enormous either, considering the billions of dollars that flow towards the treatment of PTSD.

Conclusion

The discovery of the previously unknown underlying neurobiological mechanism of safety signalling in the brain, with a major modulating role for the mPFC towards the amygdala, is magnificent and possibly embodies a tremendously underestimated therapeutic target. However, further research needs to be conducted to confirm the locus of safety found in this study and to investigate how to enhance this safety signalling mechanism. Hopefully, this will eventually lead to the finding of a suitable application of this information to relieve symptoms of PTSD patients.

References

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Likhtik, E. et al., 2014. Prefrontal entrainment of amygdala activity signals safety in learned fear and innate anxiety. Nature neuroscience, 17(1), pp.106–13. Available at: www.pubmedcentral.nih.gov/… [Accessed July 9, 2014].

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