Drugs and the Brain: An Introduction to Neuropharmacology. Pamphlet Series.

The thousands of different drugs on the market can be separated into two categories: drugs that affect behavior, called psychoactive drugs, and drugs that do not affect behavior. Drugs get into the body by mouth, inhalation into the lungs, by injection into a vein, muscle or under the skin, and by absorption through mucous membranes. Regardless of the route most drugs must get into the circulatory system in order to travel to the target organ and exert an effect. Once in the circulation a drug must still pass several "barriers" before reaching its site of action and producing a change in behavior. Three of these barriers are the blood capillaries, the blood-brain barrier, and the placental barrier. The brain receives, integrates, and responds to sensory information it receives from peripheral organs and their receptors, and in addition is responsible for all cognitive functions. Changes in brain cells are responsible for all behavior, including those changes in behavior produced by psychoactive drugs. All psychoactive drugs produce their effects by altering the functional activity of neurotransmitters. Psychoactive drugs can increase or decrease the functional activity of various neurotransmitters. There are five major classifications of psychoactive drugs: stimulants, anti-psychotics, sedatives/hypnotics, opiates or narcotics, and psychedelics or hallucinogens. The termination of drug action is accomplished primarily by changing it to another molecular structure so that it is no longer an active agent or so that it can be removed from the body. Many psychoactive drugs can produce tolerance or dependence. (ABL)