Recently, researchers have discovered why meth addiction is so powerful. In the study, the scientists use mice to find out how the action of becoming a meth addict takes place. Repeatedly stimulating the brain of a mouse with methamphetamine depresses important areas of the brain. These changes in the brain can only be undone by re-introducing the drug, according to research at the University of Washington and other institutions. The study, which appears in the April 10 issue of the journal Neuron, provides one of the most in-depth views of the mechanisms of meth addiction. It suggests that withdrawal from the drug may not be sufficient to undo the changes in the user's brain due to use of the drug.
The researchers set out to determine what changes take place in the brain as a result of repeated use and abuse of meth. They also wanted to better understand addiction-related behaviors like drug craving and relapse. Methamphetamine, also known as simply meth, is one of the most popular illegal drugs in the United States. Abuse of this drug can cause severe addiction.
Scientists have believed that abuse of drugs like meth may cause changes to the neurons in the brain and the synapses and terminals that control transmission of information in the brain. In this project, researchers focused on the mouse brain and how it was affected by methamphetamine over ten days. Ten days is considered to be the mouse equivalent of chronic use in humans.
Researchers found that the long administration and withdrawal of the drug depressed the neural terminals controlling the flow of signals between two areas of the brain, the cortex and striatum. Even a long period of withdrawal, the equivalent of years in humans, did not return the terminals to normal activity level. Re-introducing the drug, however, reversed the changes to the brain.
The areas affected by the drug are called pre-synaptic terminals which are related to the flow of information from the cortex to the striatum. When a person sees something new in their environment, the scientists explained, they focus attention on that item. At the neuron level, that process stimulates the release of dopamine, a chemical involved in transmitting signals in the brain. As the person sees the new item over and over again, the dopamine response drops and synapses in the brain adapt to the no-longer-new item.
What happens with meth addiction is that the drug makes the nervous system release dopamine, which helps a user focus a lot of attention on a particular goal. Scientists believe that meth allows dopamine in the striatum to filter information coming from the cortex through the pre-synaptic terminals. The filtering of the terminals would help someone ignore other things and focus on that one goal or task.
After chronic meth addiction sets in, the filtering process eventually becomes a permanent depression in the activity of those terminals in the brain. The only thing that can help the pre-synaptic terminals recover in mice, they found, was re-administering the drug.
"What we found is that the repeated use of methamphetamine causes adaptations in the brain, and that only re-introducing the drug can reverse that," said Dr. Nigel Bamford, UW assistant professor of neurology and pediatrics and a physician at Seattle Children's Hospital. "We think these changes in the brain may account for at least some of the physiological components of meth addiction."
If the mechanism turns out to be similar in people, Bamford said, this could have big effects on the treatment and management of meth addiction. One treatment for drug addiction is to give people smaller and smaller amounts of the drug to wean them from it and reduce the effects of withdrawal. Unfortunately, that method would not affect the adaptation of the neural terminals in the brain.
"Now that we have some understanding of the mechanism through which meth addiction occurs, we may be able to develop other approaches to treating addiction," explained Bamford. "We might be able to target some of the chemical receptors in the brain to reset the system and get rid of this depressed state in the pre-synaptic terminals."
Though scientists and researchers believe that other stimulants, like methylphenidate, may have similar effects on the brain, they caution against applying these findings to other situations. These synaptic changes may not occur in patients with underlying conditions that require treatment with stimulants, the scientists noted.