Psychoactive Drugs and the Brain

Not until the early 1900's did science discover there was a distinct partition between the central nervous system (brain and spinal cord) and the blood. When dyes were injected into the blood, they did not stain the brain. Similarly, when dyes were injected into cerebrospinal fluid, it did not travel through the blood to other organs. The separation between the nervous system and the blood is termed the "blood-brain barrier." Dyes and other water-soluble substances do not travel well between the brain and the blood due to the capillaries that serve the brain. The capillary walls are formed by cells packed tightly together, which means chemicals--unless the molecules are extremely small--cannot pass in between the cells into the brain. To view an illustration of these tightly-packed cells forming a capillary wall, travel to the Irish Scientist website (R). In order for chemicals to access brain tissue, they must travel through the cell membranes. Fat-soluble substances can pass more easily through the cell membrane's lipid (fat) bilayer than do water-soluble substances. Chemical substances can also penetrate the blood-brain barrier if they are transported across the membrane via a specific carrier or with the help of a specific receptor. Another blood-brain barrier illustration is available at the National Institute on Drug Abuse website (O). Interestingly, nanotechnology may play an increasingly important role in medicine, with researchers trying to create very small particles to help carry medicines into the brain.

The basic cell of the nervous system is the neuron. One neuronal structure, the dendrite, receives information from one or more other neurons, sends the information to the soma (cell body) of the neuron, where the electrical information is processed. An electrical impulse is then propagated down the axon of the neuron. The National Institute of Drug Abuse shows an illustration of a nerve cell (R).

Communication between one neuron and another occurs at the space between the two neurons, called the "synapse." Travel to a server on the NIDA website to see the connection between two neurons; you should see a very small space (the synapse) between the axon (alongside the red arrow) and one of the other neuron's dendrites (R). When an electrical impulse is conducted down the axon of one neuron (the presynaptic neuron), it releases a substance called a neurotransmitter, which is released into the synapse and binds to receptor sites on the other neuron (postsynaptic neuron). The capture of neurotransmitter into the postsynaptic neuron allows electrical communication to continue. There are many different types of neurotransmitters, as well as many different receptor sites that are specific to various neurotransmitters, chemicals, and other substances. Some examples include:

Travel to the NIDA website to view an illustration of dopamine (a neurotransmitter)  ready to be released from one neuron to the receptor sites of another neuron (R). Following neurotransmitter release, there may be transport of the chemical back to the presynaptic terminals for reutilization, termed "reuptake." Travel to brainexplorer.org to view an animation about neurotransmission (R--Macromedia Flash is required; you can download for free if you do not have the software; if you do not wish to download Flash at this time, be sure to study the next link from Auburn University).  Dr. Tom Riley, Auburn University, presents a diagram of the steps involved in chemical neurotransmission (R). PBS' "Moyers on Addiction" online program shows a Flash animation of how the neurotransmitter "dopamine" is transmitted from one neuron to another, both with and without cocaine. (R--requires Flash; if you do not wish to download Flash at this time, be sure to study Dr. Tom Riley's diagram, above). With Flash, you can view a similar animation of how dopamine is transmitted, with and without amphetamines (O--The Brain from Top to Bottom website)

Psychoactive drugs act with neurotransmitters in the body. They can mimic the actions of specific neurotransmitters or even block reuptake of the neurotransmitter, causing the effects to last for a longer period of time. Cocaine, for example, works not only by increasing the release of the neurotransmitter dopamine but also by blocking its reuptake. Dopamine is a neurotransmitter which is often associated with feelings of euphoria or heightened mood. The Multimedia Neuroscience Education Projects presents more information about Synaptic Transmission: A Four-Step Process (O). Read about nicotine, cocaine, and addiction at a McGraw-Hill College Division website (R). Many feel these brain chemistry changes occurring with drug abuse is experienced as addiction. Subsequently, much current research is focusing on the manipulation of reducing the effects of certain neurotransmitters, such as dopamine. One example provided by Brookhaven National Laboratory in a Quicktime movie format, focuses on how a specific substance can alter cocaine and nicotine addiction by reducing the effects of dopamine (O).  

By increasing or decreasing the actions of neurotransmitters, psychoactive substances can excite a neuron or inhibit a neuron from firing. The resulting behavior and/or physiological effect from neuron excitation or inhibition further depends on the neurotransmitter(s) involved in the process. Unfortunately, the abuse of some substances can cause changes in the amount of neurotransmitter available at certain sites in the brain. If release of a neurotransmitter (after drug use) causes euphoria, for example, the brain may chronically adapt by reducing the amount of the neurotransmitter or by reducing the number or function of receptors specific to the neurotransmitter. Subsequently, a person will need to take more of the substance to feel euphoric. There are also instances when the brain may never return to normal, even after drug use has stopped, so the user's ability to feel normal pleasure would be negatively impacted. 

Educational Enrichment

Psychoactive Drugs and Abuse

Marijuana and Medical Usage

Drug Testing

International Journal of Drug Testing

Psychoactive Drugs and the Law

Last Revised: 5-27-09