A psychological reward is a process that reinforces behavior — something that, when offered, causes a behavior to increase in intensity. Reward is an operational concept for describing the positive value an individual ascribes to an object, behavioral act or an internal physical state. Primary rewards include those that are necessary for the survival of the species, such as food, water, and sex. Some people include shelter in primary reward. Secondary rewards derive their value from the primary reward and include money, pleasant touch, beauty, music etc. The functions of rewards are based directly on the modification of behavior and indirectly on the sensory properties of rewards. For instance, altruism may induce a larger psychological reward, although it doesn't cause sensations. Rewards are generally considered more effective than punishment in enforcing positive behavior.
The major neurochemical pathway of the reward system in the brain involves the mesolimbic and mesocortical pathway. Of these pathways, the mesolimbic pathway probably plays the major role, and goes from the ventral tegmental area via the medial forebrain bundle to nucleus accumbens, where mainly dopamine is released. This acts on D3 receptors to inhibit the production of cAMP.
Almost all drugs causing drug addiction increase the dopamine release in the mesolimbic pathway, e.g. opioids, nicotine, amphetamine, ethanol and cocaine. After prolonged use, psychological drug tolerance and sensitization arises.
The reward system is partly responsible for the psychological part of drug tolerance. One explanation of this is a sustained activation of the CREB protein, causing a larger dose to be needed to reach the same effect. In addition, it leaves the user feeling generally depressed and dissatisfied, often leading to a return to the drug for an additional "fix".
Sensitization is an increase in the user's sensitivity to the effects of the substance, counter to the effects of CREB. A transcription factor, known as delta FosB, is thought to be involved by activating genes that causes sensitization. The hypersensitivity that it causes is thought to be responsible for the intense cravings associated with drug addiction, and is often extended to even the peripheral cues of drug use, such as related behaviors or the sight of drug paraphernalia.