Reward system

In neuroscience, the reward system is a collection of brain structures which attempts to regulate and control behavior by inducing pleasurable effects. The neurological reward system is part of what makes reinforcement possible.

Psychological reward
A psychological reward is a process that reinforces behavior &mdash; 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. Natural rewards include those that are necessary for the survival of species, such as eating, mating, and fighting.

Secondary rewards derive their value from the primary reward, and include shelter, 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.

Rewards induce learning, approach behavior and feelings of positive emotions.

History of discovery
In a fundamental discovery made in 1954, Canadian researchers James Olds and Peter Milner found that stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. The conclusion from such experiments is that low-voltage electrical stimulation in certain parts of the brain gives the animals pleasure. Animals were tested in Skinner boxes where they could stimulate themselves by pressing a lever, the rats would repeatedly press the lever — even up to 2000 times per hour. Research in the next two decades established that dopamine is one of the main chemicals aiding neural signaling in these regions, and dopamine was suggested to be the brain’s “pleasure chemical.”

Anatomy
The major neurochemical pathway of the reward system in the brain involves the mesolimbic and mesocortical pathway. Of these pathways, the mesolimbic pathway plays the major role, and goes from the ventral tegmental area via the medial forebrain bundle to nucleus accumbens, which is the primary release site for the neurotransmitter dopamine. Dopamine acts on D1 or D2 receptors to either stimulate (D1) or inhibit (D2) the production of cAMP.

Modulation by drugs
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.

Psychological drug tolerance
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.

Sensitization
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.