Throughout the nervous system, including the brain, there are billions of specialized cells that are responsible for processing and transmitting information. These cells, called neurons, pass chemical and electrical signals that convey many different types of information related to emotions, senses, and muscle movement, among other things.

The transfer of signals from one neuron to another is called neurotransmission. When a neuron is stimulated, an electrical impulse moves like a wave through the cell until it triggers the release of chemical molecules. These chemical messengers, called neurotransmitters, are released into the gap between neurons, called the synapse. The neurotransmitters are then taken up by receptor sites on the adjoining neuron and the signal continues to propagate through the receiving neuron.

There are hundreds of different neurotransmitters, and each has a specific function. There are also many different types of receptors. Similar to a lock and key system, particular neurotransmitters are paired with particular receptors. In other words, receptors can only receive matching neurotransmitters.

Dopamine is a neurotransmitter that plays an important role in the reward pathway. Also called the "pleasure circuit," the reward pathway is a network of neurons devoted to feelings of pleasure. By releasing the chemical dopamine to make you feel good, the reward pathway ensures that organisms do things that are necessary for survival, such as eating and drinking. The pleasurable feelings reinforce behavior so that the behavior that leads to the release of dopamine is likely to be repeated.

Drugs (chemical substances that affect the function of living things) interfere with natural neurotransmitters and change the communication pathways in the brain. When people use drugs to make them feel "high," they change the way the brain functions, in particular by altering the reward pathway (for example, by altering dopamine receptors and changing the flow of dopamine). When drugs affect the reward pathway, the temporary good feelings can lead to continued abuse (non-medical use) and addiction (uncontrollable and compulsive drug seeking and use, regardless of negative effects). Because the part of the brain that is responsible for assessing situations and making sound decisions—the prefrontal cortex—is still developing during adolescence, young people may be particularly vulnerable to making decisions that lead them to try drugs, thus increasing the risk of substance abuse and potential addiction.

Scientists use mice to perform experiments dealing with addiction that would not be possible or ethical with humans. However, data from animal experiments can be applied to humans because we share similar brain circuitry and chemistry with other mammals. Drugs have very complex effects on many areas of the brain, so while some drugs may have pleasurable effects, it is important to recognize that they also have potentially dangerous effects, such as increased or decreased heart rate, respiratory problems, distorted sensory perception, loss of coordination, and altered thought processes that can lead to permanent physical and emotional damage.

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