Dopamine molecule binds to a Dopamine receptor (in pink). After the Dopamine binds, it comes off the receptor and is removed from the synaptic cleft by uptake pumps (also proteins) that reside on the terminal (arrows show the direction of movement). This process is important because it ensures that not too much Dopamine remains in the synaptic cleft at any one time. There are neighboring neurons that release another compound called a neuromodulator. Neuromodulators help to enhance or inhibit neurotransmission that is controlled by neurotransmitters such as Dopamine. In this case, the neuromodulator is an “endorphin” (in red). Endorphins bind to opiate receptors (in yellow) which can reside on the post-synaptic cell (shown here) or, in some cases, on the terminals of other neurons (this is not shown so it must be pointed out). The endorphins are destroyed by enzymes rather than removed by uptake pumps.
Opiates bind to Opiate receptors (yellow) on the neighboring terminal and this sends a signal to the Dopamine terminal to release more dopamine. One theory is that Opiate receptor activation decreases GABA release, which normally inhibits Dopamine release, so Dopamine release is increased.
Cocaine binds to sites in areas of the brain that are rich in dopamine synapses. Dopamine transmission in the close-up of a synapse. Dopamine (inside the terminal) is released into the synaptic space. Dopamine binds to Dopamine receptors and then is taken up by uptake pumps back into the terminal. When cocaine is present (yellow), Cocaine binds to the uptake pumps and prevents them from transporting Dopamine back into the neuron terminal. So more Dopamine builds up in the synaptic space and it is free to activate more Dopamine receptors.
This is the same effect that you saw in the image with Morphine (Opiate), where Morphine increased Dopamine release from the terminal to produce more Dopamine in the synaptic space.
Dopamine has many functions in the brain, including important roles in behavior and cognition, voluntary movement, motivation, punishment and reward, inhibition of prolactin production (involved in lactation and sexual gratification), sleep, mood, attention, working memory, and learning.
Addiction can in theory be derived from any rewarding behaviour, and is believed to be strongly associated with the Dopaminergic (Dopamine) system of the brain’s reward system.
A person who is physically dependent, but not psychologically dependent can have their dose slowly dropped until they are no longer physically dependent. However, if that person is psychologically dependent, they are still at serious risk for relapse into abuse and subsequent physical dependence.
Quoted from & Featured Image Source:
drugabuse.gov – neurobiology – dopamine – neurotransmission
drugabuse.gov – drug-addiction – heroin – morphine – opiates binding to opiate receptor
drugabuse.gov – cocaine – dopamine binding to receptors
wikipedia.org – Substance_Abuse – Substance Dependence
methinsideout.com – animation