The Dopamine Hypothesis and Reward System
The dopamine system involves a complicated network of intercellular and intracellular mechanisms involved in regulating, receiving, and producing signals involved with reward learning. The reward system is the neural network of interactions that allows us to interpret relevant, survival affirming actions. Learning to do a simple action, such as eating, involves a number of small steps. Using this example, one must first learn to predict the reward associated with food, to want food, to take action to obtain it and to eventually consume it. These steps are regulated and promoted by neurotransmitters, such as dopamine, which influence responses to natural rewards such as eating and sexual pleasure (Hyman, 2006). Dopamine is important in the process of learning how to ‘want’ or ‘desire’ an object or action that has previously been determined to be ‘liked’ (Berridge, 1998). Thus, dopamine is important in learning motivational behaviors with the aim of obtaining a given reward (Hyman, 2006). Dopamine is a necessary component of the reward system because it insures that behaviors which promote the obtainment of the reward are learned.
Addictive drugs are diverse in nature but most are analogous in their ability to increase dopamine in key areas of the brain (Hyman 2005, Hyman 2006,Shultz 1998). It is because drugs such as cocaine exploit the dopamine system that the act of taking these drugs is easily learned and difficult to extinguish. By way of its molecular messenger, some addictive drugs have the ability to interact with the dopamine system, which regulates such basic parts of reward learning as evaluation of rewards and reward-associated memories (Everitt et al., 2003). When any environmental condition, such as the ingestion of drugs, affects the dopamine system, the very act of learning itself will be altered in potentially destructive configurations. Continued drug use yields a decrease in dopamine D2 receptors as well as a decrease in the
release of dopamine (Volkow et al., 2004). The possible effects on learning are considerable. The reward circuits are less sensitive to stimulation by naturally occurring rewards, and the addict experiences a decrease in the motivational salience of non-drug related stimuli (Volkow, 2004). The result is an under-stimulated addict with a larger drive to use, in an attempt to activate these desensitized reward circuits (Volkow, 2004). When an addict’s cravings are satiated the learning cycle is complete, the cycle of drug dependence is propagated.
Dopamine is a universal message because it is not specialized to any specific type of reward behavior. Because dopamine does not discriminate between different rewards, anything that affects dopamine levels can affect the learning and the reward system. Addictive drugs are no different. Not only do they send trigger signals in the brain that encode pleasure, they infringe on a system that normally processes relevant stimuli for pavlovian incentive reinforcement learning (Volkow, 2004; Fenu, 2003).
Labels: addiction, biology, dopamine, drugs, science
