a healthy person, the reward system reinforces important behaviors that are essential for

survival such as eating, drinking, sex, 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 neurons, or nerve cells, 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 transmission of the neurotransmitter 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. This causes dopamine release from the neurons into the synaptic space. Dopamine

then binds to and stimulates dopamine-receptor on the receiving neuron. This stimulation

by dopamine is believed to produce the pleasurable feelings or rewarding effect. Dopamine molecules

are then removed from the synaptic space and transported back in to the transmitting neuron

by a special protein called dopamine-transporter. 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 neuron, into the dopamine-containing vesicles

where it triggers dopamine release even in the absence of action potentials.

Different drugs act different way but the common outcome is that dopamine builds-up

in the synapse to a much greater amount than normal. This causes a continuous stimulation,

maybe over-stimulation of receiving neurons and is responsible for prolonged and intense

euphoria experienced by drug users. Repeated exposure to dopamine surges caused by drugs

eventually de-sensitizes the reward system. The system is no longer responsive 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 rewarding effect. This ultimately leads to

drug overdose.