Dopamine is thought of as the pleasure and reward neurotransmitter. New research is revealing that it is not quite a simple as pleasure drug: 

In the emerging view, discussed in part at the Society for Neuroscience meeting last week in Chicago, dopamine is less about pleasure and reward than about drive and motivation, about figuring out what you have to do to survive and then doing it. “When you can’t breathe, and you’re gasping for air, would you call that pleasurable?” said Nora D. Volkow, a dopamine researcher and director of the National Institute on Drug Abuse. “Or when you’re so hungry that you eat something disgusting, is that pleasurable?”

In both responses, Dr. Volkow said, the gasping for oxygen and the wolfing down of something you would ordinarily spurn, the dopamine pathways of the brain are at full throttle. “The whole brain is of one mindset,” she said. “The intense drive to get you out of a state of deprivation and keep you alive.”

Dopamine is also part of the brain’s salience filter, its get-a-load-of-this device. “You can’t pay attention to everything, but you want to be adept as an organism at recognizing things that are novel,” Dr. Volkow said. “You might not notice a fly in the room, but if that fly was fluorescent, your dopamine cells would fire.”

In addition, our dopamine-driven salience detector will focus on familiar objects that we have imbued with high value, both positive and negative: objects we want and objects we fear. If we love chocolate, our dopamine neurons will most likely start to fire at the sight of a pert little chocolate bean lying on the counter. But if we fear cockroaches, those same neurons may fire even harder when we notice that the “bean” has six legs. The pleasurable taste of chocolate per se, however, or the anxiety of cockroach phobia, may well be the handiwork of other signaling molecules, like opiates or stress hormones. Dopamine simply makes a relevant object almost impossible to ignore.

Should the brain want to ignore what it might otherwise notice, dopamine must be muzzled.

From: A Molecule of Motivation, Dopamine Excels at Its Task – NYTimes.com