The suspected source of pain relief was a cluster of cells in the VMM. In 1979, Howard Fields of the University of California at San Francisco identified two groups of neurons in the brain stem that could enhance or inhibit pain. Neurons that "facilitate" pain, which he labeled "ON cells, fire in bursts when animals are awake and are inactive during slow-wave sleep. "OFF" cells, which suppress pain, are active during slow-wave sleep.
Foo and Mason, who completed her post-doctoral training in Fields' laboratory, found that the pain-enhancing ON cells were inhibited and the pain-suppressing OFF cells were excited during feeding, results that duplicated the effects of morphine. The changes occurred only during selected portions of eating, while the animal was both holding and chewing the food. A similar response took place when the animals drank water or urinated.
This response "protects critical behaviors from disruption," the authors note, "allowing an animal to nourish itself without being distracted." (Animals that don't urinate are at high-risk for infections.)
As a final test the authors used drugs to inactivate the VMM. This completely eliminated the delayed response to pain.
"This is the best evidence so far on the biological significance of OFF cell activity," said UCSF's Fields, who was not connected with the study. "It is a major step in understanding the neurobiological mechanisms of decision making in the face of conflict -- in this case the choice between whether to feed or flee. Off cells help you feed instead of flee."
Mason and Foo also found, however, that in situations that might be more dangerous, the feed-or-flee balance could be tipped the other way. Given just before feeding, a strong puff of air perhaps indicating a nearby predator -- activated
Contact: John Easton
University of Chicago Medical Center