AddictionHow Drugs Work In The Brain Part 5The Reward Pathway And AddictionLet me introduce the concept of reward. Humans, as well as other organisms engage in behaviors that are rewarding; the pleasurable feelings provide positive reinforcement so that the behavior is repeated. There are natural rewards as well as artificial rewards, such as drugs. Natural RewardsNatural rewards such as food, water, sex and nurturing allow the organism to feel pleasure when eating, drinking, procreating and being nurtured. Such pleasurable feelings reinforce the behavior so that it will be repeated. Each of these behaviors is required for the survival of the species. There is a pathway in the brain that is responsible for rewarding behaviors. The Reward Pathway This is a view of the brain cut down the middle. An important part of the reward pathway is shown and the major structures are highlighted: The VTA is connected to both the nucleus accumbens and the prefrontal cortex via this pathway and it sends information to these structures via its neurons. The neurons of the VTA contain the neurotransmitter dopamine which is released in the nucleus accumbens and in the prefrontal cortex (point to each of these structures).
 This Pathway Is Activated By A Rewarding Stimulus Note: the pathway shown here is not the only pathway activated by rewards, other structures are involved too, but only this part of the pathway is shown for simplicity. Activation Of The Reward Pathway By An Electrical StimulusThe discovery of the reward pathway was achieved with the help of animals such as rats. Rats were trained to press a lever for a tiny electrical jolt to certain parts of the brain. When an electrode is placed in the nucleus accumbens, the rat keeps pressing the lever to receive the small electrical stimulus because it feels pleasurable. This rewarding feeling is also called positive reinforcement.
 Electrode Is Placed In The Nucleus Accumbens When the electrode is placed there, the rat will not press the lever for the electrical stimulus because stimulating neurons in a nearby area that does not connect with the nucleus accumbens does not activate the reward pathway. The importance of the neurotransmitter dopamine has been determined in these experiments because scientists can measure an increased release of dopamine in the reward pathway after the rat receives the reward. And, if the dopamine release is prevented (either with a drug or by destroying the pathway), the rat won't press the bar for the electrical jolt. So with the help of the rats, scientists figured out the specific brain areas as well as the neurochemicals involved in the reward pathway. Books Buzzed: |