Reward system

Addiction and dependence glossary
• addiction – a state characterized by compulsive engagement in rewarding stimuli despite adverse consequences
• addictive behavior – a behavior that is both rewarding and reinforcing
• addictive drug – a drug that is both rewarding and reinforcing
• dependence – an adaptive state associated with a withdrawal syndrome upon cessation of repeated exposure to a stimulus (e.g., drug intake)
• drug sensitization or reverse tolerance – the escalating effect of a drug resulting from repeated administration at a given dose
• drug withdrawal – symptoms that occur upon cessation of repeated drug use
• physical dependence – dependence that involves persistent physical–somatic withdrawal symptoms (e.g., fatigue and delirium tremens)
• psychological dependence – dependence that involves emotional–motivational withdrawal symptoms (e.g., dysphoria and anhedonia)
• reinforcing stimuli – stimuli that increase the probability of repeating behaviors paired with them
• rewarding stimuli – stimuli that the brain interprets as intrinsically positive or as something to be approached
• sensitization – an amplified response to a stimulus resulting from repeated exposure to it
• tolerance – the diminishing effect of a drug resulting from repeated administration at a given dose

The reward system is a group of neural structures that are critically involved in mediating the effects of reinforcement. A reward is an appetitive stimulus given to a human or some other animal to alter its behavior. Rewards typically serve as reinforcers. A reinforcer is something that, when presented after a behavior, causes the probability of that behavior's occurrence to increase. Note that, just because something is labelled as a reward, it does not necessarily imply that it is a reinforcer. A reward can be defined as reinforcer only if its delivery increases the probability of a behavior.

Reward or reinforcement is an objective way to describe the positive value that an individual ascribes to an object, behavioral act or an internal physical state. Primary rewards include those that are necessary for the survival of species, such as food and sexual contact. Secondary rewards derive their value from primary rewards. Money is a good example. They can be produced experimentally by pairing a neutral stimulus with a known reward. Things such as pleasurable touch and beautiful music are often said to be secondary rewards, but such claims are questionable. For example, there is a good deal of evidence that physical contact, as in cuddling and grooming, is an unlearned or primary reward. Rewards are generally considered more desirable than punishment in modifying behavior.

Definition

In neuroscience, the reward system is a collection of brain structures that are involved in the regulation of behaviors that are associated with desirable, approachable, or intrinsically positive stimuli.

Anatomy of the reward system

The brain structures which compose the reward system are primarily contained within the cortico–basal ganglia–thalamic loop; the basal ganglia portion of the loop drives activity within the reward system. Most of the pathways which connect structures within the reward system are glutamatergic interneurons, GABAergic medium spiny neurons, and dopaminergic projection neurons, although other types of projection neurons contribute (e.g., orexinergic projection neurons).

The reward system includes the ventral tegmental area, ventral striatum (primarily the nucleus accumbens, but also the olfactory tubercle), substantia nigra (i.e., the pars compacta and pars reticulata), dorsal striatum (i.e., caudate nucleus and putamen), prefrontal cortex, anterior cingulate cortex, insular cortex, hippocampus, hypothalamus (particularly, the orexinergic nucleus in the lateral hypothalamus), thalamus (multiple nuclei), subthalamic nucleus, globus pallidus (both external and internal), ventral pallidum, amygdala, and the remainder of the extended amygdala.

Among the pathways that connect the structures in the cortico–basal ganglia–thalamic loop, the group of neurons known as the mesolimbic dopamine pathway, which connect the ventral tegmental area (VTA) to the nucleus accumbens (NAcc), along with the associated GABAergic D1-type medium spiny neurons in the nucleus accumbens, is a critical component of the reward system that is directly involved in the immediate perception of reward. Most of the dopamine pathways (i.e., neurons which use the neurotransmitter dopamine to transmit a signal to other structures) that originate in the VTA are part of the reward system; in these neurons, dopamine acts on D1-like receptors or D2-like receptors to either stimulate (D1-like) or inhibit (D2-like) the production of cAMP. The GABAergic medium spiny neurons of the striatum are all components of the reward system. The glutamatergic projection nuclei in the subthalamic nucleus, prefrontal cortex, hippocampus, thalamus, and amygdala connect to other parts of the reward system via glutamate pathways as well.

Animals vs humans

Based on data from Kent Berridge, the liking and disliking reaction involving taste shows similarities among human newborns, orangutans, and rats. Most neuroscience studies have shown that dopamine alterations change the level of likeliness toward a reward, which is called the hedonic impact. This is changed by how hard the reward is worked for. Experimenter Berridge modified testing a bit when working with reactions by recording the facial expressions of liking and disliking. Berridge discovered that by blocking dopamine systems there did not seem to be a change of the positive reaction to something sweet; in other words, the hedonic impact remained the same even with this change. It is believed that dopamine is the brain's main pleasure neurotransmitter but, with these results, that did not seem to be the case. Even with more intense dopamine alterations, the data seemed to remain the same. This is when Berridge came up with the incentive salience hypothesis to explain why the dopamine seems to only sometimes control pleasure when in fact that does not prove to be happening at all. This hypothesis dealt with the wanting aspect of rewards. Scientists can use this study done by Berridge to further explain the reasoning of getting such strong urges when addicted to drugs. Some addicts respond to certain stimuli involving neural changes caused by drugs. This sensitization in the brain is similar to the effect of dopamine because wanting and liking reactions occur. Human and animal brains and behaviors experience similar changes regarding reward systems because they both are so prominent.

History

File:Skinner box scheme 01.png

Skinner box

James Olds and Peter Milner were researchers who found the reward system in 1954. They discovered, while trying to teach rats how to solve problems and run mazes, stimulation of certain regions of the brain. Where the stimulation was found seemed to give pleasure to the animals. They tried the same thing with humans and the results were similar.

In a fundamental discovery made in 1954, researchers James Olds and Peter Milner found that low-voltage electrical stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. It seemed that stimulation of those parts of the brain gave the animals pleasure, and in later work humans reported pleasurable sensations from such stimulation. When rats were tested in Skinner boxes where they could stimulate the reward system by pressing a lever, the rats pressed for hours. 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”.

Ivan Pavlov was a psychologist who used the reward system in order to study classical conditioning. Pavlov used the reward system by rewarding dogs with food after they had heard a bell or another stimulus. Pavlov was rewarding the dogs so that the dogs associated food, the reward, with the bell, the stimulus. Edward L. Thorndike used the reward system in order to study operant conditioning. He began by putting cats in a puzzle box and placing food outside of the box so that the cat will want to escape. The cats worked to get out of the puzzle box to get to the food. Although the cats ate the food after they escaped the box, Thorndike learned that the cats attempted to escape the box without the reward of food. Thorndike used the rewards of food and freedom in order to stimulate the reward system of the cats. Thorndike used this in order to see how the cats learned to escape the box.

Drug and behavioral addictions

Main article: ΔFosB

ΔFosB (delta FosB), a gene transcription factor, is the common factor among virtually all forms of addiction (behavioral addictions and drug addictions) that, when overexpressed in D1-type medium spiny neurons in the nucleus accumbens, is necessary and sufficient for many types of addiction-related behavioral and neural plasticity. Examples of behavioral plasticity regulated by ΔFosB include drug self-administration, reward sensitization, and reward cross-sensitization effects among specific addictive drugs and behaviors. Examples of neuroplasticity modulated by ΔFosB include altered trafficking or density of NMDA receptors, AMPA receptors, and dopamine receptors in the striatum.

Addictive drugs and addictive behaviors are rewarding and reinforcing (i.e., are addictive) due to their effects on the dopamine reward pathway.

See also

3

External links

• Scholarpedia Reward
• Scholarpedia Reward signals