Addiction is a neurological disorder that affects the reward system in the brain.
In a healthy person, the reward system reinforces important behaviors that are
essential for survival such as feeding, sexual activity, and social interaction. For example,
the reward system ensures that you reach for food when you are hungry, because you
know that after eating you will feel good. In other words, it makes the activity of eating
pleasurable and memorable, so you would want to do it again and again whenever you feel hungry.
Drugs of abuse hijack this system, turning the person’s natural needs into drug needs.
The brain consists of billions of nerve cells, or neurons, which communicate via
chemical messages, or neurotransmitters. When a neuron is sufficiently stimulated,
an electrical impulse called an action potential is generated and travels down the axon to the
nerve terminal. Here, it triggers the release of a neurotransmitter into the synaptic cleft - a
space between neurons. The neurotransmitter then binds to a receptor on a neighboring
neuron, generating a signal in it, thereby transmitting the information to that neuron.
The major reward pathways involve the transmission of a neurotransmitter called dopamine from the
ventral tegmental area - the VTA - of the midbrain, to the limbic system and
the frontal cortex. Engaging in enjoyable activities generates action potentials in
dopamine-producing neurons of the VTA, causing dopamine release into the synaptic space. Dopamine
then binds to and stimulates dopamine-receptor on the receiving neuron. This stimulation is
believed to produce the pleasurable feeling or rewarding effect. Dopamine molecules are
then removed from the synapse and transported back to the transmitting neuron by a special
protein known as dopamine-transporter. This clearing of dopamine is to ensure that the
postsynaptic neurons are not overstimulated. Most drugs of abuse increase the level of
dopamine in the reward pathway. Some drugs such as alcohol, heroin, and nicotine indirectly excite
the dopamine-producing neurons in the VTA so that they generate more action potentials.
Cocaine acts at the nerve terminal. It binds to dopamine-transporter and blocks the re-uptake of
dopamine. Methamphetamine, a psychostimulant, acts similarly to cocaine in blocking dopamine removal.
In addition, it can enter the transmitting neuron, into dopamine-containing vesicles,
where it triggers dopamine release even in the absence of action potentials.
Different drugs act in different ways but the common outcome is that dopamine
builds-up in the synapse to a much greater amount than normal. This causes a continuous
overstimulation of receiving neurons and is responsible for prolonged and intense
euphoria experienced by drug users. Repeated exposures to dopamine surges caused by drugs
eventually desensitize the reward system. The system no longer responds to everyday stimuli;
the only thing that is rewarding is the drug. That is how drugs change the person’s
life priority. After some time, even the drug loses its ability to reward and higher doses
are required to achieve the same pleasurable effect. This ultimately leads to drug overdose.