By Luciana Gravotta
Professor Akil’s lab bred two types of rat: One type, the high responders, show the same type of behavior as your average risk-taker. When put in a new setting, they’ll run around and explore—normal rats tend to freeze in this situation, vetting the unknown before they venture out. These risk-taking rats display a number of characteristics linked to addiction vulnerability in people. They are highly attentive to cues related to food and cocaine, they are given to impulsive actions and their dopamine reward systems are hypersensitive. They also like feeding on cocaine and readily return to it after a period of sobriety.
The second type of rat bred in the Akil lab, the low responders, show behaviors similar to those of people who are anxious or depressed. Their movement is inhibited in new surroundings, they don’t get addicted easily and they can resist cocaine when it's offered again after a break from regular use.
When rats learn that a light predicts a shot of cocaine, the brains of low responders begin releasing the same levels of dopamine for both the cue and the reward. High responders, on the other hand, release more dopamine in response to the cue than to the actual reward.
Such responsivity may explain the propensity to relapse, says Akil. A burst of dopamine when sighting an old drug-related haunt could make it too alluring to resist. The findings seen in the animals, she believes, are similar to those seen in humans.
But even those animals inclined to resist addiction can succumb to substance abuse in the presence of environmental stressor. While low-responding rats don’t get hooked on cocaine as easily as the high responders, social stress—such as regular social defeat—can boost their interest in drug-taking.
The finding that differences in characteristics related to temperament underlie differences in drug use leads Akil to conclude that "there cannot be one size fits all in terms of treatment." Initiation, self-control, and relapse can be all very different experiences depending on personality features.