Our nervous system has been intricately constructed to both perceive and react to fear, a mechanism necessary to our survival, and quite developed after centuries of eluding predators. We may see fear in a negative light, but it also forces us to remain vigilant and dodge potential dangers before they befall us, whether it’s something as ordinary as the sounds of a quiet, settling house by night, or the snarl of a ferocious leopard hiding nearby. However, should this experience of fear manifest by itself, without the presence of actual danger, it can have a strong and negative impact on our overall well-being. This phenomenon, which the researchers describe as fear generalization, can often plague individuals who suffered from severe stress or recall episodes of trauma. Although we often speak about trigger warnings, and the nature of trauma, the exact underlying mechanisms of generalized fear largely continue to elude us.
A research team of neurobiologists from the University of California San Diego, headed by former Assistant Project Scientist Hui-quan Li alongside Distinguished Professor Nick Spitzer, sought to address this – and they took a considerable step in determining how the mechanisms work. Their new paper, published by the research journal Science, reveals what biochemical changes and neural circuitry take place when our brain is provoked into a generalized fear response. Their research effort not only illuminates a pathway on how our fear responses are activated but even offers some groundwork for how we can produce interventions to better control a generalized fear response.
The primary motivation behind this study was to uncover the cellular and circuit mechanisms responsible for fear generalization. While fear responses are essential for survival, they can become detrimental when generalized to non-threatening situations. Such maladaptive fear responses are common in various stress-related disorders, including PTSD. The researchers aimed to identify the specific neurotransmitters and neural circuits involved in this process, hoping to pave the way for targeted treatments that could mitigate the harmful effects of generalized fear.
The team of researchers conducted this study with laboratory bred mice, honing in on a particular brain region we call the dorsal raphe, which is situated within the brain stem. This area has already been known to partake in weighing our fear responses. The research team investigated how acute stress – the presence of a direct inducer of physical stress – impacted neurotransmitter signals within the neurons of this region, namely looking at a switch from the brain’s excitatory neurotransmitters (glutamate) to inhibitory ones, called gamma-aminobutyric acid (GABA).
To induce the acute stress, the researchers administered foot shocks at varying degrees of intensity to their mice. The researchers then examined the mice’s fear responses in alternate contexts. More specifically, they documented the intervals of time in which the mice spent “frozen,” a fairly typical fear response for rodents, in both their original enclosure where they were given the shocks, and when situated in a new, different location. This gave the researchers a control to determine between the state of conditioned fear (specific to context) in contrast to generalized fear (extending to a newly given context).
The team also utilized advanced medical techniques to help them track any changes in neurotransmitter expression throughout these dorsal raphe neurons. This entailed immunostaining (the use of antibodies to isolate and highlight specific neurotransmitters as well as the enzymes they emitted.) They also made use of genetic signaling tools in order to control the processes of neurotransmitter synthesis, which allowed them to gauge how different fear responses impacted the brain region.
The study concluded that administering strong foot shocks gradually produced a generalized fear response in lab mice. This response happened in tandem with a sizable switch in neurotransmitter signals occurring within the brain’s dorsal raphe neurons, from activating glutamate to the GABA. More specifically, the neurons that first co-expressed glutamate switched to co-expressing GABA instead, and the change persisted over the course of several weeks.
A deeper analysis indicated that this particular neurotransmitter switch was crucial when it came to the brain’s development of generalized fear – a sense of uneasiness that kept the individual regularly on guard – what we might call an anticipation of the other shoe dropping. Whenever the research team utilized genetic tools in order to suppress any production of GABA within the dorsal raphe neurons, the rodents exhibited no signs of generalized fear, even if they had experienced a myriad of strong foot shocks just before the suppression. This observation highlights the essential role for the glutamate-to-GABA switch when it comes to mediating stress-induced fear generalization.
“Our results provide important insights into the mechanisms involved in fear generalization,” says Spitzer, who is a member of UC San Diego’s Department of Neurobiology and Kavli Institute for Brain and Mind. “The benefit of understanding these processes at this level of molecular detail — what is going on and where it’s going on — allows an intervention that is specific to the mechanism that drives related disorders.”
Building upon their findings they documented in mice, the team of researchers then moved to examine postmortem brain samples extracted from individuals who suffered with PTSD when they were alive. They discovered a similar type of switch between glutamate and GABA had occurred in the dorsal raphe neurons of the deceased individuals, suggesting that what mechanisms they recorded in the lab mice are indeed relevant to humans when they experience PTSD – a state in which the mind believes itself to be constantly in a state of attack.
The research team also probed into what potential interventions could be used to stave off the brain’s development of generalized fear. They discovered that the administration of an adeno-associated virus (AAV) was able to suppress the gene instrumental for GABA production in the dorsal raphe before an episode of acute stress effectively averted generalized fear in the mice. Additionally, by treating the lab mice with an antidepressant fluoxetine (more commonly referred to as Prozac) immediately following a stressful experience, the research team could prevent the neurotransmitter switch from activating, and thus avoiding the subsequent initiation of generalized fear.
Although the new study offers some important observations in its data, there are limitations. This research was most prominently conducted with lab mice, and even though similar mechanisms were documented among human PTSD samples, further research is necessary to confirm their conclusions. In addition, further investigation is needed to understand the long-term effects from manipulating neurotransmitter synthesis and even potential side effects from such interventions themselves.
In the near future, researchers could look into the broader implications for these discoveries. For example, determining whether there are similar neurotransmitter switches that happen due to other kinds of stress, in particular, psychological stress, could offer a much clearer picture of what fear generalization looks like in the brain. Furthermore, investigating what specific neural circuits located downstream of the brain’s dorsal raphe are responsible for generalized fear responses could provide us with better targeted and effective treatments when it comes to PTSD.
“Now that we have a handle on the core of the mechanism by which stress-induced fear happens and the circuitry that implements this fear, interventions can be targeted and specific,” says Spitzer.
The new study, titled: “Generalized fear following acute stress is caused by change in co-transmitter identity of serotonergic neurons,” was co-authored by Hui-quan Li, Wuji Jiang, Lily Ling, Vaidehi Gupta, Cong Chen, Marta Pratelli, Swetha K. Godavarthi, and Nicholas C. Spitzer.
