Alan McStravick for redOrbit.com – Your Universe Online
Addiction is a serious disorder of the brain that is characterized by a chronic and compulsive need to take a substance, typically with an increase in dosage and frequency. Someone suffering from addiction will continue their behavior even in the face of negative consequences. The root cause of addiction is not specifically known, however. A new study out of the University of Texas at Austin (UT) may just point to one indicator of future substance abuse and addiction.
Researchers at UT published their findings this week in the journal Neuron, where they detail their findings after having created a social isolation scenario with laboratory rats. They were able to determine that the isolation, presented during the critical period of adolescence, led to an increased vulnerability to addiction of both alcohol and amphetamines. They also determined that an addiction to amphetamine was more difficult to eradicate in the population of rats that experienced the adolescent social isolation and that the addiction effect persisted well after the rats had been reintroduced to the community of other rats.
“Basically the animals become more manipulatable,” said Hitoshi Morikawa, associate professor of neurobiology in the College of Natural Sciences. “They’re more sensitive to reward, and once conditioned the conditioning takes longer to extinguish. We’ve been able to observe this at both the behavioral and neuronal level.”
Traits such as anxiety, aggression, cognitive rigidity and special learning have long been known to be resultant from social isolation during adolescence, according to Morikawa. Until now, however, there was no data supporting the specific kind of behaviors and brain activity related to addiction as a result of isolation during the crucial period of adolescence.
“Isolated animals have a more aggressive profile,” said Leslie Whitaker, a former doctoral student in Morikawa’s lab and now a researcher at the National Institute on Drug Abuse. “They are more anxious. Put them in an open field and they freeze more. We also know that those areas of the brain that are more involved in conscious memory are impaired. But the kind of memory involved in addiction isn’t conscious memory. It’s an unconscious preference for the place in which you got the reward. You keep coming back to it without even knowing why. That kind of memory is enhanced by the isolation.”
The method employed in the experimentation in Morikawa´s UT lab involved isolating the selected rats from their peers for the period of one month from 21 days of age. This age in a rat is comparable to early/middle adolescence in humans. After one month, the subject rats were tested to see how they might respond to different levels of exposure to both amphetamine and alcohol.
According to Mickaël Degoulet, a postdoctoral researcher in Morikawa’s lab, the results were striking. The team found the subject rats were much more apt to form a preference for the small, distinctive box from which amphetamine and alcohol were received than were their peers in the control group who did not experience isolation. In fact, the isolated rats presented a preference after just one exposure to either drug while the control rats required repeated exposure to the substances to display a conditioned behavior response.
The researchers, according to Morikawa, believed this kind of preference for the environmental context in which the reward was received provided them with a more useful way of understanding addiction rather than seeing it as a desire for more of the addictive substance.
“When you drink or take addictive drugs, that triggers the release of dopamine,” he said. “People commonly think of dopamine as a happy transmitter or a pleasure transmitter, which may or may not be true, but it is becoming increasingly clear that it is also a learning transmitter. It strengthens those synapses that are active when dopamine is released. It tells our brain that what we’re doing at that moment is rewarding and thus worth repeating.”
Morikawa cited the import of the constellation of environmental, behavioral and physiological cues that are reinforced when the substance triggers the release of dopamine in the brain over the idea that the individual is actually addicted to the experience of pleasure or relief from the substance.
Neuronal changes were able to be documented by the research team. According to both Morikawa and Whitaker, the social isolation affected the dopamine neurons in the rats´ brain to be able to quickly learn to generate spikes in response to inputs from other brain areas. They claim this implies the dopamine neurons are then able to learn to respond to the context more quickly.
The rats in the control group were not, themselves, immune to the perils of addiction. The control group was able to, upon repeated exposure to amphetamine, achieve the same degree of addiction as the socially isolated rats were. Though, even with a comparable addiction level, the team was able to recognize there were differences between the two groups. Perhaps most interestingly, with both groups being exposed to the same substance extinction protocols, the isolated rats required a much longer period of time to overcome their amphetamine addiction.
“So the social isolation leads to addiction more quickly, and it’s harder to extinguish,” said Whitaker.
The implications of these findings, according to Whitaker, as they apply to humans, are obvious. This is due to the wealth of literature that documents the negative effects of social isolation in humans. Add to that the evidence that points to the functional similarity of addiction between humans and rats at the neurological level.
“It’s not a one-to-one correlation, but there are socially impoverished human environments,” she said. “There are children who are neglected, who have less social input. It’s reasonable to make guesses about what the impact of that is going to be.”
Morikawa points out that their findings may also have implications for how social isolation during adolescence affects conditionability when it comes to other kinds of rewards.
“We think that maybe what’s happening is that the brain reacts to the impoverished environment, to a lack of opportunities to be reinforced by rewarding stimuli, by increasing its sensitivity to reward-based conditioning,” said Morikawa. “The deprived brain may be overinterpreting any reward it encounters. And if that’s the case, it’s likely that you are more conditionable not only to drugs but to any sort of reward, including food reward. One interesting possibility is that it might also make adolescents more prone to food ‘addiction,’ and then to obesity.”
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