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How Drugs Affect Communication between Nerve Cells
A typical nerve cell looks a bit like a sapling, or young tree. At one end it has a cell body with protruding branches called dendrites. It has a long, skinny, trunk-like structure called an axon. At the other end, the axon spreads out like roots, each tipped by a knob called an axon terminal button (also sometimes referred to as a synaptic knob).
The terminal button of one cell is separated by a very tiny gap from a dendrite on another cell. This contact point between nerve cells, called a synapse, is where the communication between the cells takes place. Nerve cells communicate using neurotransmitters, which are chemicals released by the terminal buttons of the nerve cell on one side of the synapse. The neurotransmitters are received by receptors on the nerve cell on the other side of the synapse--the postsynaptic cell. Depending on what neurotransmitter is released and what receptors are present on the postsynaptic cell, the neurotransmitters can either activate or inhibit the postsynaptic cell.
Animation of nerve communication at the synapse
A Nerve Cell
Image Credit: The Society for Neuroscience: Brain Facts
(See page 5 for detailed version)
Addictive drugs influence the way nerve cells communicate by altering how neurotransmitters carry their messages from one nerve cell to another. Different drugs affect neurotransmitters in different ways. Some drugs mimic neurotransmitters. Other drugs block them. Still others cause more or less neurotransmitter to be released, or change how quickly the released neurotransmitter is inactivated.
Animations showing how three different drugs (cocaine, alcohol and opiates) affect the action of neurotransmitters
Anatomy of Addiction
Different drugs of abuse produce different changes in different regions of the brain. However, one group of brain structures is common to the actions of all addictive drugs. These structures are part of the reward system in our brains--so called because it is responsible for creating the pleasurable feelings that result when humans and other animals engage in adaptive behaviors.
Key structures in the reward system are the ventral tegmental area (VTA), the nucleus accumbens and the circuit of nerve cells linking these two areas of the brain. Drugs of abuse cause a marked increase in the level of the neurotransmitter dopamine in the VTA. The prefrontal cortex is another area of the brain that is part of the brain's reward system, and is affected by addictive drugs. Brain structures involved in learning and memory also play a role in addiction.
Image Credit: The Society for Neuroscience: Brain Facts
(See page 34 for detailed version)
Addictive drugs alter the way the brain's reward center functions. Causing nerve cells to release high levels of dopamine can overwork these nerve cells and ultimately kill them. Even if the dopamine-releasing nerve cells survive, changes in the cells receiving the dopamine--the postsynaptic nerve cells--alter the way the brain responds to drugs, as well as food and experiences that would normally activate the reward center. Specifically, as a result of being bombarded with dopamine, the postsynaptic cells may reduce the number of dopamine receptors on their surface. This makes them less likely to be activated by dopamine.
These and other changes in the brain, and other parts of the body, lead to drug tolerance--the need to take a higher dose to get a "high"--and withdrawal symptoms. The following links have more information about drug tolerance and withdrawal symptoms.
How casual drug use leads to addiction
How the body learns to anticipate and counteract some physiological effects of drugs
Detailed description of the physiological and psychological mechanisms of drug tolerance
Ongoing research is revealing the various and often subtle effects that drugs have on the brain. For example, we now know that many neurotransmitters play a role in drug addiction. One such transmitter is serotonin. The following links have more information about serotonin's role in addiction.
Serotonin and dopamine pathways in the brain
Serotonin and cocaine addiction
Serotonin and alcoholism
Dopamine
Serotonin
You may have heard of serotonin because of its role in depression. Prozac, a medication used to treat depression, is a serotonin reuptake inhibitor. That means it blocks serotonin from being cleared from the synapse, allowing it to have a greater effect on the postsynaptic cell. In the lesson video, Professor Sejnowski describes how Prozac affects the social status and risk taking behavior of monkeys. Knowing the role of neurotransmitters in normal behavior is essential for understanding how, by interfering with brain processes, addictive drugs ultimately alter behavior.
