Chronic cocaine use may dull responsiveness to brain signals

In a study measuring the brain’s degree of excitability, the brains of cocaine-dependent people show an abnormally low response to signals in the region responsible for muscle movement, according to a recent article in Biological Psychiatry. The authors, led by Nashatt N. Boutros, M.D., associate professor of psychiatry, reported that cocaine addicts and longtime users require significantly more stimulus to the motor cortex in order to cause the muscles of their fingers and hands to move. The signals in this study were delivered on the scalp in the form of transcranial magnetic stimulation, rapid magnetic pulses from a handheld coil. The motor threshold, the minimum amount of stimulation needed to produce movement, ran at a mean of about 41 percent in normal subjects but about 65 percent in chronic cocaine users.

Cocaine itself is well known as a drug that excites the brain’s signaling pathways rather than impeding them, so one might expect that longtime users of the drug would have the most signal-sensitive brains of all. Boutros and his colleagues offered two possible explanations for the higher motor threshold seen in cocaine-dependent brains. It may reflect either “an adaptation to those effects of cocaine intoxication that promote cortical excitability and seizures,” they said, or else tissue damage that has left this brain region less responsive. In other words, by becoming less sensitive to signals or less well able to respond to them, the brain may be attempting to balance out the dangerous hyperresponsiveness that comes with cocaine use. The next step, said Boutros, will be to replicate these results in additional cocaine-dependent subjects, using several different measures of cortical responsiveness.