Throughout human evolution, activities that brought a release of dopamine were the ones that also kept us alive or contributed to reproduction, but now addictive drugs have the power to hijack the brain’s natural dopamine balance. Take a look at how synapses in the nervous system respond to different classes of drugs.
While the substances considered here have effects on diverse regions of the brain, their addictive potential is heavily dependant on the effect they have on dopamine pathways.
These animations depict the mechanism by which different substances of abuse cause an unnatural dopamine response that can lead to addiction.
Dopamine is a neurotransmitter that plays an important role in both pleasure and motivation. When dopamine is released in our reward pathways, the resulting sense of satisfaction reinforces whatever behavior caused that satisfaction. Most of the time, such behaviors contribute to our survival and reproduction, but for the case of addictive drugs, the reward pathway is stimulated directly and to such an extent that drug-taking behavior is reinforced to the point of addiction.
This causes use to continue despite negative consequences.
Presynaptic neuron receives a signal causing the release of dopamine into the synaptic cleft.
Dopamine binds to dopamine receptors on the postsynaptic neuron, causing a reward sensation.
Some dopamine re-enters the presynaptic neuron, some binds/rebinds to dopamine receptors until all is back in the presynaptic neuron.
Amphetamines work by mimicking dopamine. In doing so, their effect is twofold: They both stimulate dopamine receptors directly and compete with dopamine at dopamine transporters. Because it’s harder for dopamine to re-enter the presynaptic neuron, more of it remains present in the synaptic cleft. In a regular amphetamine user who stops taking the drug, the amount of dopamine is reduced greatly and is unable to be replenished properly for up to a few weeks.
Amphetamines are shaped like dopamine and imitate dopamine in the neuron.
Amphetamines bind to dopamine receptors, initiating the same response as dopamine.
They compete with dopamine at cellular transporters causing dopamine to remain outside the cell longer.
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While opiates have an effect on diverse regions of the body and brain, addiction to opiates is motivated by the pleasure these substances induce and their stimulation of the brain’s reward pathways. Unlike other drugs which may mimic dopamine, or prevent its reuptake, opiates stimulate the presynaptic neuron to release more of it than normal through a complex process that is prohibitively difficult to depict fully.
An opiate binds to an opiate-receptor on the presynaptic neuron.
Opiate binding initiates a cascade of neurochemical activity (not shown).
This activity signals a massive efflux of dopamine into the synaptic cleft.
Your synapse with
Cocaine affects reward pathways by preventing the reuptake of dopamine and, as a result, keeping more dopamine active in the synaptic cleft where it continually stimulates dopamine receptors. When cocaine and alcohol are used in combination, a third substance, cocaethylene (shown below), is produced by the liver as a result of their reaction. Cocaethylene has an even higher affinity for dopamine transporters than cocaine and is thought to be more toxic than cocaine alone.
Cocaine binds to dopamine transporters, blocking dopamine’s re-entry into the presynaptic neuron.
Blocking of cellular transporters increases dopamine levels in the synaptic cleft.
Increased dopamine in the synaptic cleft allows more dopamine to bind to dopamine receptors, increasing reward sensation.
Your synapse with
SOME SUBSTANCES MIMIC DOPAMINE, WHILE OTHERS CAUSE MORE DOPAMINE TO BE PRESENT.
High-calorie foods and sexual relations are examples of activities that give a pleasurable release of dopamine. Such activities were once vital to survival and reproduction, but in our modern era, stimulation of dopamine pathways can be brought about artificially. As we see in the above animations, the dopamine responses caused by certain substances reinforce drug-abusing behavior through the pleasure elicited.
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