Pain sensation and the emotional experience of pain are not the same. A research team at Stanford University has now identified a group of neurons in mice responsible for the latter.
“We’re looking at what the brain makes of that information. While painful stimuli are detected by nerves, this information doesn’t mean anything emotionally until it reaches the brain, so we set out to find the cells in the brain that are behind the unpleasantness of pain,” said Dr. Grégory Scherrer, co-lead author of the study.
The amygdala, a region of the brain classically associated with emotion and fear, seemed to the researchers a logical place to start.
Within this region, they narrowed their search by looking for neurons in mice that were active during brief pain stimulation — such as a drop of hot, but not scalding, water applied to a paw.
Neurons that are active express more of a specific gene called c-Fos, and indeed, a sea of c-Fos-expressing neurons flared after this stimulus.
“But that really only tells you that those neurons were active at some point, and it’s not specific enough. What we wanted was to look at the neurons of freely moving animals,” Dr. Scherrer said.
To observe the deep-seated wiring of a mouse’s brain, Dr. Scherrer and colleagues used a miniscope — a microscope about the length of a small paper clip, which could be affixed to a mouse’s head to record activity in its brain. They positioned the device strategically to visualize the amygdala.
The mouse, alive and well, could stroll as it pleased, while the miniscope recorded calcium flux in the neurons, a proxy for cell activity.
The scientists monitored the mouse brains with the microscope, watched the mice detect something uncomfortable, observed the aversive reactions and then checked which neurons were active.
“With this setup, we identified a set of neurons in the amygdala that selectively encodes signals related to the emotional aspects of a painful experience,” said Dr. Mark J. Schnitzer, co-lead author of the study.
When the mice touched a drop of uncomfortably hot or cold water, they withdrew, signaling to the scientists that the rodents were not pleased.
Upon this withdrawal, the microscope’s recording showed a bundle of neurons firing in the amygdala — specifically in the basolateral region — suggesting that these neurons were specifically responsible for the emotion of pain.
It was, however, still possible that this basolateral ensemble was simply firing to relay general emotion, rather than specifically the unpleasantness of pain.
So, the team fed the mice sugar water — a sweet treat known to bring joy to any mouse — and kept an eye on the collection of neurons suspected to relay displeasure. As expected, those neurons stayed silent.
“There’s also a difference between experiencing pain and experiencing something annoying, so we further wanted to test if the amygdala neurons active during pain were also associated with overall negative emotion, rather than pain particularly,” Dr. Scherrer said.
What miffs a mouse? The same things that might bother a sibling: tiny puffs of air to the face, an unappetizingly bitter taste or a very bad smell. While bothering the mice, the researchers again monitored the basolateral amygdala pain ensemble, and here, too, the neurons remained subdued.
“After all of that, we concluded that this ensemble of neurons selectively responds during pain,” Dr. Scherrer said.
The findings were published in the journal Science.