"It's one step more subtle" said Deisseroth, "but this is something that optogenetics was very well-suited to resolve."
An optogenetic method called projection targeting allowed the scientists to work backward from the brain stem and find the exact pathway from neurons in the prefrontal cortex that signal motivation.
The researchers first introduced their light-sensitive protein into cells in the prefrontal cortex. The light sensitivity then spread out like the branches of a tree through all the outgoing connections and eventually made its way to the brain stem, making those regions light sensitive, too.
Then, illuminating the newly light-sensitive regions of the brain stem thought to control motivational movement, Deisseroth and Warden watched the behavioral effects as a subgroup of neurons in the prefrontal cortex that sent connections to brain stem were activated. They could see not only which cells are possibly involved in motivation, but the way motivation moves from one brain region to another.
The researchers suspected that one part of the brain stem in particular, the dorsal raphe nucleus, might be crucial to behaviors that control effort. This cluster of cells is a production hub for serotonin a chemical messenger that changes the firing behavior of other cells. Serotonin is associated with mood modulation; many antidepressant drugs, for instance, may act by increasing serotonin concentration in the brain.
When the pathway between the prefrontal cortex and the dorsal raphe nucleus was stimulated, rodents facing a challenge in the lab showed an immediate and dramatic surge in motivation.
Curiously, however, when the rodents were relaxing in their home enviro
|Contact: Andrew Myers|
Stanford University Medical Center