Our nervous system is intricately designed to sense and respond to fear, a crucial survival mechanism. Fear helps us stay alert and avoid potential dangers, whether it’s the disturbing sounds we hear alone at night or the looming threat of a growling animal. However, when fear occurs in the absence of real danger, it can have serious consequences for our well-being. This phenomenon, known as fear generalization, often affects people who have experienced severe stress or trauma, leading to conditions such as post-traumatic stress disorder (PTSD). Despite its prevalence, the mechanisms underlying widespread fear remain largely elusive.
A team of neurobiologists at the University of California, San Diego, led by former deputy project scientist Hui-quan Li and distinguished professor Nick Spitzer, has made significant progress in understanding these mechanisms. Their study, published in the journal Science, reveals the biochemical changes and neural circuits involved in stress-induced generalized fear. This research not only sheds light on how fear responses are triggered, but also opens new avenues for potential interventions.
The main motivation for this study was to uncover the cellular mechanisms and circuits responsible for fear generalization. Although 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 alleviate the harmful effects of generalized fear.
The researchers conducted their study on mice, focusing on a region of the brain known as the dorsal raphe, located in the brainstem. This area plays a crucial role in regulating fear reactions. The team examined how acute stress affected neurotransmitter signals in neurons in this region, focusing particularly on the shift from excitatory neurotransmitters (glutamate) to inhibitory neurotransmitters (GABA).
To induce stress, mice were subjected to foot shocks of varying intensities. The researchers then measured the mice’s fear responses in different contexts. Specifically, they observed how long the mice spent “freezing,” a common fear response, both in the original context where the shock was administered and in a new and different context. This allowed them to distinguish between conditioned fear (specific to the original context) and generalized fear (extending to the new context).
The team also used advanced techniques to track changes in neurotransmitter expression in dorsal raphe neurons. This involved immunostaining to identify the presence of specific neurotransmitters and their synthetic enzymes. Additionally, they used genetic tools to manipulate the synthesis of neurotransmitters, allowing them to assess the impact of these changes on fear responses.
The study found that strong foot shocks led to generalized fear responses in mice. This was accompanied by a notable change in neurotransmitter signals in dorsal raphe neurons, from glutamate to GABA. Specifically, neurons that initially co-expressed glutamate began to co-express GABA, a change that persisted for several weeks.
Further investigation showed that this neurotransmitter change was essential for the development of generalized fear. When researchers used genetic tools to suppress GABA synthesis in dorsal raphe neurons, the mice did not exhibit generalized fear, even after experiencing strong foot shocks. This finding highlights the central role of the shift from glutamate to GABA in mediating stress-induced fear generalization.
“Our findings provide important insights into the mechanisms involved in fear generalization,” said Spitzer, a member of the UC San Diego Department of Neurobiology and the Kavli Institute for Brain and Mind. “The benefit of understanding these processes at this level of molecular detail – what is happening and where it is happening – allows for mechanism-specific intervention causing the associated disorders.”
Building on their findings in mice, the researchers examined post-mortem brain samples from individuals suffering from PTSD. They found a similar switch from glutamate to GABA in the dorsal raphe neurons of these individuals, suggesting that the mechanisms observed in mice are relevant to human PTSD.
The team also explored potential interventions to prevent the development of generalized fear. They found that administering an adeno-associated virus (AAV) to suppress the gene responsible for GABA synthesis in the dorsal raphe before experiencing acute stress effectively prevented generalized fear in mice. Additionally, treating mice with the antidepressant fluoxetine (commonly known as Prozac) immediately after a stressful event also prevented the neurotransmitter shift and subsequent onset of generalized fear.
Although the study provides valuable information, it also has limitations. The research has been primarily conducted in mice, and although similar mechanisms have been observed in human PTSD samples, additional studies are needed to confirm these findings. Additionally, the long-term effects of manipulating neurotransmitter synthesis and the potential side effects of such interventions require further research.
Future research could explore the broader implications of these findings. For example, understanding whether similar neurotransmitter changes occur in response to other forms of stress, such as psychological stress, could provide insight into the generalization of fear. Additionally, studying the specific neural circuits downstream of the dorsal raphe that mediate generalized fear responses could lead to more targeted and effective treatments.
“Now that we have mastered the core mechanism by which stress-induced fear occurs and the circuits that implement that fear, interventions can be targeted and specific,” Spitzer said.
The study, “Generalized fear following acute stress is caused by a change in the cotransmitter identity of serotonergic neurons,” was authored by Hui-quan Li, Wuji Jiang, Lily Ling, Vaidehi Gupta, Cong Chen, Marta Pratelli, Swetha K. Godavarthi. and Nicholas C. Spitzer.