Sensory nerve scientists have not known whether the nerves with itchy receptors and pain receptors were actually sending both types of messages to the brain, or just itch messages. What the current study found is that, in nerves with the itchy receptor MrgA3, electrical signals sent in response to both painful and itchy stimuli are interpreted by the brain as itch.
To reach this conclusion, the researchers first used a genetic trick to label the MrgA3 cells in mice with a glowing protein that allowed them to see the cells under the microscope. Aided by the glow, they were able to plug in those tiny electricity monitors and watch nerve cell responses to different stimuli. The cells transmitted electrical signals when the mice were exposed to itch-inducing chloroquine and histamine, as well as pain-inducing capsaicin and heat. Based on this result, the researchers tentatively concluded that the cells could send both pain and itch signals.
In the next experiment, the researchers monitored the behavioral responses of mice to the different stimuli. As expected, when the tails of normal mice were placed in hot water, they quickly pulled them out; when normal mice were given a bit of chloroquine or histamine, they scratched vigorously with their hind legs.
Then, to examine the role of MrgA3 cells in pain and itch, the scientists selectively killed MrgA3 nerve cells in adult mice and retested their responses. Presumably, the researchers noted, because MrgA3 cells are only a small fraction of all pain-sens
|Contact: Catherine Kolf|
Johns Hopkins Medicine