In some of the same brain areas where c-fos is turned on, researchers found evidence that cells are actively producing proteins for an opiate receptor called mu, indicating that they're upping their natural number of these receptors. This may be one way animals become immune to pain during defeat. Other studies indicate that animals without this receptor gene get no pain relief from morphine. These changes are not only large, long-lasting and specific to certain areas of the central nervous system, but these changes are important behaviorally for issues of tolerance and drug abuse. In fact, along with tolerance to morphine, an opposite effect takes place.
Defeated animals become more sensitive to other drugs, such as cocaine.
One, tolerance, a form of neuroadaptation, and the other, sensitization, occurring roughly in the same timeframe. In addition, he said, defeated animals learn to self-administer cocaine roughly twice as fast as undefeated animals. He and his colleagues are using a new technique to look at changes in the brain as they occur. By sticking tiny capillary tubes into rats' brains, they can measure the chemicals active in the brain as the rat moves about. Dopamine and serotonin are of particular interest because high levels of dopamine and low levels of serotonin have been associated with aggression in animals and humans.