The new findings by Jonathan Britt, from McGill University, were presented in a lecture titled, "Investigating the Pleasure Centers of the Brain: How reward Signals Are Transmitted" at the 2014 Canadian Neuroscience Meeting, which took place in May of 2014.
The so-called “pleasure center” of the brain was co-discovered in 1954 by James Olds, who was an American psychologist, and Peter Milner while he was a postdoctoral fellow at McGill University. Olds and Milner stumbled on the pleasure center after they implanted electrodes into the septal area of the rat and found that rats became addicted to pushing a lever that was stimulating the nucleus accumbens (NAcc).
What Role Does the Nucleus Accumbens Play In Seeking Pleasure?
Electrical stimulation of the ventral tegmental area (VTA) triggers the release of dopamine in the nucleus accumbens much in the same way that addictive drugs and natural reinforcers, such as sex, drugs, alcohol, food—or anything pleasurable—also triggers the release of dopamine in the nucleus accumbens.
The rats in Olds and Milner's experiment would continue to press this lever incessantly even at the expense of eating and drinking for sustenance. This suggested that the area is the "pleasure center" of the brain and is involved in the reward-driven reinforcement of learning and addiction.
Since the 1950s, multiple research studies on mice and rats have shown that the levels of dopamine in the extracellular fluid of the nucleus accumbens increase when rats are injected with addictive drugs such as cocaine, heroin, nicotine, or alcohol. When dopamine is released into the NAcc, it reinforces addictive behaviors that are driven by a pleasurable reward or the anticipation of a reward.
The same results have been seen in human subjects in brain imaging studies. Increased levels of dopamine have been observed in the extracellular fluid of the nucleus accumbens when human test subjects experience the rewarding rush of sex, drugs, or hitting the jackpot when gambling.
Optogenetics Is a Revolutionary Brain Imaging Technology
In the latest study from McGill, Britt used a new brain imaging technique called “optogenetics” to determine whether there was a positive or negative effect on reward seeking. He also identified specific inputs linked to pleasure and addiction coming from different regions of the brain.
Optogenetics uses light-responsive proteins to study the activation of neural circuits in distinct locations, which allowed him to precisely dissect the roles of different neural circuits in the brain.
Over the years, Britt's research has revealed the brain circuits responsible for habitual behavior and how reward signals—such as those produced by addictive drugs—travel through the brain and how pleasurable experiences modify brain circuits. These findings could lead to new pharmaceutical treatments tailored to treat drug addiction.
Britt’s research confirms the widely held assumption that the most immediate effects of drugs on the brain are linked to an increase in the levels of dopamine in the nucleus accumbens. Until the advent of optogenetics, it was impossible to dissect the precise role of inputs from various brain regions and how these brain connections might result in a specific behavior.
Dr. Britt was able to identify different ways that the nucleus accumbens integrates dopamine-dependent reinforcement signals with environmental stimuli, which depend on a second neurochemical called glutamate. Glutamate-dependent signals to the nucleus accumbens come from an array of brain regions that include the amygdala, hippocampus, thalamus, and the prefrontal cortex.
The pleasurable associations of a reward become hardwired into the synapses with the addition of glutamate into the neurobiological mix. Neurons that fire together generally wire together. Glutamate appears to be the glue that makes these synaptic bonds stick together. In my book, The Athlete’s Way, I describe glutamate saying, “There is a protein called glutamate that sears synapses together ... glutamate bonds are hard to dissolve."
Conclusion: Breaking Glutamate-Dependent Bonds Holds Clues for Treating Addiction
If neuroscientists can figure out how glutamate bonds hardwire neural networks it will greatly advance our understanding of motivation, desire, pleasure-seeking behaviors, and addiction. This research might lead to better cognitive therapies for reward-driven behavior and potential pharmaceutical treatments for addiction.