Scientists at Stanford and MIT had targeted the dopamine neurons, which make a chemical thought to elevate mood in a reward circuit located in the midbrain.

WSJ: Scientists Cast Light Onto Roots of Illness Deep in the Brain

By 2003 researchers he had funded were able to turn embryonic stem cells into dopamine-producing neurons, but once they were put into mice, the neurons either reverted to embryonic stem cells or stopped producing dopamine.

FORBES: On The Cover/Top Stories

It's hoped that replacing lost dopamine-producing neurons might help those with Parkinson's disease.

FORBES: Geron, And Stem Cells, Keep On Going

Some of the damage the disease causes to brain neurons that make the neurotransmitter dopamine also occur in the gut neurons, researchers say.

WSJ: A Gut Check for Many Ailments

"We knew two years ago that we could make neurons that make dopamine from these stem cells, " McKay said during a press conference.

FORBES: Stem Cells' Double Breakthrough

In one application of optogenetics, scientists investigated how neurons that make dopamine, a neurotransmitter in the brain, may give rise to feelings of reward and pleasure.

CNN: How to 'take over' a brain