A study published on Thursday in the journal Cell has shown that the brain has a way to suppress chronic pain when an animal is hungry, allowing it to go and look for food.
Neuroscientists from University of Pennsylvania have found that a tiny population of 300 brain cells are responsible for the ability to prioritize hunger over chronic pain, which may offer targets for novel pain therapies.
"If you're an animal, it doesn't matter if you have an injury, you need to be able to overcome that in order to go find the nutrients you need to survive," said J. Nicholas Betley, an assistant professor of biology in Penn's School of Arts and Sciences.
The researchers observed how mice that hadn't eaten for 24 hours responded to either acute pain or longer-term inflammatory pain, which is thought to involve sensitization of neural circuits in the brain.
They found that hungry mice still responded to sources of acute pain but seemed less responsive to inflammatory pain than their well-fed counterparts.
Their behavior was similar to that of mice that had been given an anti-inflammatory painkiller, according to the researchers.
Then they turned on a group of neurons known to be activated by a group of hunger, agouti-related protein (AgRP) neurons, and found that chronic pain responses subsided, while acute pain responses stayed intact.
Betley's team activated each AgRP neuron subpopulation one at a time, and found that stimulation of only a few hundred AgRP neurons that project to the parabrachial nucleus significantly suppressed inflammatory pain.
"Out of a brain of billions of neurons, this specific behavior is mediated by 300 or so neurons," Betley said.
Further experiments pinpointed a neurotransmitter called NPY molecule, responsible for selectively blocking inflammatory pain responses. The researchers found that blocking receptors for NPY reversed the effects of hunger, and pain returned.
"We've initiated a new way of thinking about how behavior is prioritized," Betley said. "It's not that all the information is funneled up to your higher thinking centers in the brain but that there's a hierarchy, a competition that occurs between different drives, that occurs before something like pain is even perceived."