The brains of rats injected with saline or morphine in a test environment were examined after the animals were returned to the test environment but not injected. Increased levels of Fos expression in the rats that received morphine in the test environment suggest that the association of drug use with an environmental cue produces physiological changes in specific brain regions. Fos expression is indicated by dark spots. PL: prelimbic cortex; CC: corpus callosum.
Environmental cues-the sights, sounds, or physical surroundings that a person associates with drug use-often trigger a nearly irresistible urge to use drugs, even when he or she may not have used drugs for months or years. NIDA-supported researchers seeking an explanation for this phenomenon recently showed that, in rats, reminders of drug use induce powerful physiological responses in brain regions associated with learning, emotion, and decisionmaking.
"These changes in cognitive regions may contribute significantly to the cue-induced craving that is thought to play such an important role in relapse to drug use in humans," says Dr. Ann Kelley of the University of Wisconsin-Madison Medical School.
To detect changes in the rats' brains, Dr. Kelley and Brock Schroeder, along with their University of Wisconsin colleagues, first established an association between morphine and a specific environment. They then evaluated levels in various brain regions of a protein called Fos, which is expressed by genes involved in the regulation of nerve cells and is an indicator of nerve cell activity.
The researchers injected rats with morphine or a saline solution once each day for 10 consecutive days while the rats were in a cage different from the one in which they were normally housed. Another group of rats received daily injections of morphine while in their home cages. Compared with the rats injected with saline solution, rats injected with morphine, either in the test cage or in their home cages, exhibited increased activity. "It is well known that repeated administration of opiate drugs can result in the conditioning of drug effects, including increased locomotion," Mr. Schroeder says.
Following 3, 5, or 7 days with no exposure to either the test cage or morphine, the test-cage animals were returned to the test cages but not injected with the drug or saline. Rats previously exposed to morphine showed levels of increased activity similar to those exhibited when they had received the drug, indicating a conditioned association between the environment and morphine.
Rats received daily injections of morphine or a saline solution in a test cage on 10 consecutive days. Beams of light registered rats' movements: Vertical movements, such as rearing, and side-to-side or horizontal movements crossed a single beam of light. Walking crossed two adjacent light beams. Rats that received morphine exhibited more physical activity than did rats that received saline. This difference in physical activity persisted when rats were returned to the test cage 3, 5, and 7 days after the injections were stopped, suggesting that the pattern of activity is triggered by environmental cues associated with exposure to the drug.
When the brains of the animals conditioned to both the drug and the test environment were examined, the researchers found increased levels of Fos-positive nerve cells in the prefrontal cortex, which is involved in goal-directed behavior; the cingulate cortex, which is involved in conditioning and reward; and the preoptic cortex, which also is involved in reward. The brains of the animals exposed to morphine in their home cages showed no increased Fos expression.
"The increased Fos expression associated with exposure to both the drug and the test environment suggests that the drug-environment association has a direct effect on specific brain regions and circuitry," says Mr. Schroeder.
Drug exposure alone is not sufficient to create the increases in Fos expression because the animals that received morphine in their home cages had no increased Fos activity, Dr. Kelley points out. "Pairing the drug with a drug-related environment is necessary for the increase in Fos expression," she says.
"Results of this study provide a molecular framework for the behavioral and psychological processes associated with drug craving and relapse," says Dr. Roger Brown of NIDA's Division of Neuroscience and Behavioral Research. "Learning more about the molecular basis of cue-induced craving may lead to development of pharmacotherapies that target these specific sites in the brain."
Schroeder, B.E.; Holahan, M.R.; Landry, C.F.; Kelley, A.E. Morphine-associated environmental cues elicit conditioned gene expression. Synapse 37:146-158, 2000. [Abstract]