Cocaine causes memory drain

U. PITTSBURGH (US) — Using cocaine contributes directly to a host of cognitive deficits, including loss of visual memory and the ability to adapt to rule changes.

It’s not clear if the connection reflects pre-existing traits that make cocaine users more susceptible to drug abuse or to the drug itself, says Charles W. Bradberry, associate professor of psychiatry at the University of Pittsburgh.

“It’s well known that the level of cognitive deficit can predict how successful treatment is likely to be,” he says. “If we understand what the problems are and whether the drug itself was the cause, then we might be able to design treatments that have a better chance of working.”

The findings are reported in the Journal of Neuroscience.

Researchers assessed cognitive skills in 14 rhesus monkeys that learned to touch an abstract shape on a touch screen to receive either a sip of water or a cocaine infusion via a special access port into a blood vessel. Within a week, the cocaine group was regularly self-administering the maximum of six doses daily from Tuesdays to Fridays.

The monkeys’ associative learning skills were then assessed on Monday, after three days without receiving drugs. Each animal was presented with two consistent stimuli on the touch screen with a correct answer recorded for touching the one associated with a higher volume of water reward.

But once a threshold number of correct answers was achieved, the rules were reversed, with the high stimulus becoming low and the low becoming high.

The cocaine group learned the initial rules as quickly and at first responded as accurately as the water-only group, but only five of the eight achieved the threshold of correct responses. The difference between high and low rewards was then increased, which allowed the remaining three monkeys in the cocaine group to get to threshold.

“That tells us the cocaine users have a hard time maintaining focus and attention,” Bradberry says. “But if we increase the reward value of these things they’re trying to learn, we can overcome that cognitive deficit.”

Once the stimuli were reversed, the cocaine group had much greater difficulty learning and adapting to the change in rules, indicating a deficit in cognitive control processes or executive brain functions that require focus and guide goal-directed behavior.

In another task, the animals saw a single image that when touched resulted in a supplemental water reward. Then they were shown the same image along with a random one, and if they chose the correct one after a delay of up to 40 seconds they got the reward. The cocaine group’s performance was less accurate as the delay increased.

“This experiment reveals that cocaine use causes impairments of visual working memory,” Bradberry says. “Like the inability to maintain consistent levels of accuracy in the other task, this could reflect problems with attention in addition to the impairment in cognitive control.”

The study was funded by the National Institute on Drug Abuse, part of the National Institutes of Health; and the Veterans Affairs Medical Research Service.

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