Neuropeptides Revealed as Key Messengers in Fear Response

Researchers at Salk developed techniques to examine neuropeptides, which are messenger proteins in the brain that regulate the fear response in mice. This discovery could lead to the creation of more potent analgesics and therapies for disorders associated with fear, such as anxiety and PTSD.

Pain and a sense of impending danger surge in as humans unintentionally touch the hot cast iron skillet handle. Pain receptors in the finger send sensory signals to the spinal cord and the brainstem. After that, those pain signals are transmitted by a unique class of neurons to the amygdala, a higher brain region that elicits the emotional fear response and helps remember to stay away from hot skillets going forward.

Scientists believe that fast-acting chemicals called neurotransmitters must be involved in this process because they quickly convert pain into a threatening memory. However, when Salk researchers investigated their function, greater, slower-acting molecules known as neuropeptides turned out to be the main messengers in this fear circuit.

Although neuropeptides are known to be important in brain communication, the specifics have not been well understood because researchers have not had the resources to examine them in behaved animals.

The Salk team developed two new instruments that, at last, enable researchers to watch and control neuropeptide release in the brains of living mice, helping them understand neuropeptides' function in this circuit.

The study published in Cell showed that multiple neuropeptides are involved in the danger circuit's primary messenger function, which is neuropeptides rather than neurotransmitters. Their research may result in the creation of stronger painkillers or novel therapies for diseases linked to fear, such as anxiety and PTSD (post-traumatic stress disorder).

There is so much we have left to uncover about neuropeptides, but thankfully at Salk, we have the legacy of Nobel Prize winner Roger Guillemin’s work to highlight their importance and encourage our discovery. To do this, we created two genetically encoded tools for monitoring and silencing neuropeptide release from nerve endings. We believe these new tools will significantly advance the field of neuropeptide research, and our discovery of their role in fear processing is really just the beginning.”

Sung Han, Study Senior Author, Associate Professor and Pioneer Fund Development Chair, Salk Institute

Information must move throughout the body and brain for humans to process and respond to things in the environment.

Neurons transmit and receive these signals to create well-organized circuits that direct information to its proper location. Neurotransmitters and neuropeptides are examples of molecules that neurons send and receive to communicate with one another.

It is widely acknowledged that neuropeptides function as neuromodulators, assisting and modifying the actions of primary neurotransmitters. Nonetheless, early adopters such as Roger Guillemin postulated that neuropeptides could function as primary transmitters on their own.

This concept has not been thoroughly tested because there has been a lack of tools for visualizing and manipulating their release in living animals. The Salk team's investigation into neuropeptides aimed to create new instruments for comprehending their function in brain circuits.

Han and colleagues leveraged a distinct feature of neuropeptides to target them precisely: whereas neurotransmitters are encapsulated in small spheres known as synaptic vesicles, neuropeptides are encapsulated in large dense core vesicles.

The researchers developed biochemical tools, such as neuropeptide sensors and silencers, to target these big vesicles. The sensor, which tags large, dense core vesicles with proteins that glow when the vesicles are released from the nerve ending, allows researchers to watch neuropeptide release in real-time.

By selectively breaking down neuropeptides inside big, dense core vesicles, the silencer reveals what transpires in the brain when neuropeptides are missing.

We have created a novel way to trace neuropeptide travel and function in the brains of living animals. These tools will help further our understanding of the brain’s neuropeptide circuits and enable neuroscientists to explore questions that were previously difficult to address.”

Dong-II Kim, Study First Author and Postdoctoral Researcher, Salk Institute

The researchers examined how neuropeptides and glutamate behaved in live mice as they experienced a mild stimulus - just enough to stimulate the fear circuit - using their newly developed neuropeptide sensor and silencer in addition to the already-existing sensor and silencer tools for glutamate, the neurotransmitter found in the brain in greatest abundance.

The researchers discovered that during the stimulus, neuropeptides were released but not glutamate. Furthermore, the mice's fear behaviors were decreased by blocking neuropeptide release, but glutamate was unaffected.

To Han's surprise and delight, neuropeptides, not glutamate, were the main messenger molecules in this brainstem fear circuit. Moreover, the results corroborate their current research on PACAP, a neuropeptide that regulates panic disorder.

These new tools and discoveries are an important step toward better neurological drug development. We found that multiple neuropeptides are packaged together in a single vesicle and released all at once by a painful stimulus to function in this fear circuit, which made us think, ‘This might be why some drugs that target only one neuropeptide are failing in clinical trials.’ With this new information, we can provide insights to develop new drugs that target multiple neuropeptide receptors at once, which may serve as better painkillers or help treat fear-related disorders like PTSD.”

Sung Han, Study Senior Author, Associate Professor and Pioneer Fund Development Chair, Salk Institute

The team will soon start investigating additional brain circuits and functions with their newly acquired neuropeptide toolkit.

The discovery that it is essential to target multiple neuropeptides simultaneously, along with further research into neuropeptide signaling in other brain regions, should spur the creation of more potent medications to treat a variety of neurological conditions.