Dopamine is important in the reward-driven learning system; every reward studied involves dopamine being released from the brain. Many addictive drugs, including cocaine and amphetamine, act directly on the dopamine system. Dopamine has also been linked to Parkinson's disease, schizophrenia and ADHD.
Dopamine can structurally be classed as a phenethylamine (a monoamine chain attached to a benzene ring) molecule. It contains a catechol group attached to a monoamine chain. A monoamine chain is made up of an amine group attached to an ethane chain. This monoamine chain can be found in many neurotransmitters, including histamine, serotonin, adrenaline and noradrenaline. It's also found in many drugs, examples being tryptamines and phenethylamines.
The dopamine system
Dopamine receptors are found primarily in the central nervous system, as well as the cardio-pulmonary and renal systems. The dopamine receptors can be split into two categories. D1 and D5 receptors are part of the D1-like family, which are responsible for excitatory responses. D2-4 receptors are part of the D2-like family, which are responsible for inhibitory responses.
Drugs targeting the dopamine system
Many stimulant drugs exhibit an excitatory effect upon dopamine receptors. Among these are amphetamine, its analogues, and cocaine. Many psychedelic and entactogenic drugs also indirectly excite dopamine receptors. Drugs that have an inhibitory effect on dopamine receptors include anti-psychotics and medication for restless-legs syndrome (RLS).
Dopamine and the reward system
The reward system is the body's method of regulating and controlling specific behaviors by inducing pleasurable effects. The mesolimbic pathway is a dopaminergic reward pathway in the brain that is associated with drug addiction. Drug addiction and misuse of the main classes of addictive drugs (opiates, stimulants, ethanol and nicotine) are due to increased dopamine transmission in the mesolimbic pathways.
Stimulant psychosis is a psychotic disorder induced by certain psychostimulant drugs, such as amphetamine, cocaine and methylphenidate. It usually occurs in recreational abuse, with large doses, but can occur due to therapeutic use. Symptoms include auditory and visual hallucinations, delusions and catatonia. It is theorized that the dopaminergic mesolimbic pathway is responsible for stimulant psychosis, as the stimulants involved all have agonist effects on dopamine receptors. This is supported by the fact that antipsychotic medication has antagonist effects on dopamine receptors.
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