People with addictions―whether to heroin, nicotine or even caffeine―can attest to the palpable influence their cravings can have on their behaviors and actions. An addict will go to great lengths to get a fix. Understanding the neural mechanisms underlying this kind of behavior is coming together, thanks to advanced technologies empowering contemporary neuroscience research. Such understanding provides crucial leads for those seeking to develop more effective forms of addiction treatment, as well as depression, a condition in which motivation to seek rewards is impaired.
A team of scientists led by 2009 and 2012 NARSAD Young Investigator Grantee Saleem M. Nicola, Ph.D., of Albert Einstein College of Medicine, and including 2008 NARSAD Young Investigator Grantee Sharif A. Taha, Ph.D., of the University of Utah, have published important research that reveals previously unknown functions within a part of the brain long known to play a major role in “reward-seeking” behavior. That area, called the nucleus accumbens (NAc), contains many neurons activated by the neurotransmitter dopamine. When neuronal receptors for dopamine are blocked in the NAc of rodents, it has been noticed that it takes them longer to respond to cues that predict reward.
Drs. Nicola, Taha and colleagues reported in the June 5, 2013 issue of the journal Neuron the results of sophisticated behavioral experiments in which they demonstrated a more complex type of neural response to rewards. They wanted to see what happened in the NAc when a sound cue was given, signaling the possibility of getting a food reward. The question was whether the response of NAc neurons predicted the animal’s reward-seeking behavioral response.
The researchers found that NAc neuronal activity in the tiny sliver of time―small fractions of a second―between the sounding of the cue and the animals’ first movement toward the reward depended on where they stood in their cages at the moment they heard the cue, relative to where the reward was to be given. This kind of neural reckoning also predicted the vigor of the reward-seeking response―the time it took to initiate movement and the speed with which the animals moved to obtain the reward. Remarkably, the neurons made these predictions only when the animal was free to move about the cage before hearing the cue, indicating that their firing may activate a specific computation: what path to take to get to the reward.
The researchers do not believe the NAc neurons participate directly in this computation, only that by activating the computation in “downstream” brain areas NAc neurons determine whether and how vigorously the animal seeks reward. In humans, craving is often triggered by reward-predictive stimuli. These experiments reveal one of many layers of neural computation by which a craving drives reward-seeking action.
Read an abstract of this research.