Impaired cortical plasticity in drug abuse

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Science Translational Medicine  13 Jul 2016:
Vol. 8, Issue 347, pp. 347ec113
DOI: 10.1126/scitranslmed.aah3544

Drug addiction is currently understood as a disorder of maladaptive brain plasticity in response to repeated administration of rewarding substances. In human drug addicts, this is seen in the form of altered brain responses to drug-predicting cues, whereas in experimental animals there is direct evidence for plasticity in reward-processing brain circuits. Indeed, it has been argued from experiments in rodent brain slices that every drug of abuse is characterized by an ability to modulate the capacity for brain plasticity. However, it is unknown whether drug addiction in humans alters the capacity for induction of cortical plasticity.

In a new study, Shen and colleagues used a simple model of human cortical plasticity—repetitive noninvasive transcranial magnetic brain stimulation (rTMS) of the motor cortex of heroin addicts—to investigate this question. The researchers monitored changes evoked in the motor cortex in response to single TMS pulses. They found that a protocol that normally induces long-term potentiation-like changes in healthy individuals (i.e. sustained increased motor cortical responses) failed to do so in a group of heroin addicts. Moreover, this plasticity impairment was related to greater drug craving symptoms. Despite the commonality of plasticity experiments in animals exposed to drugs of abuse, this is surprisingly the first such demonstration in humans. This study paves the way for advancing the understanding and treatment of individuals with drug addiction but also raises a number of questions. For example, is impaired TMS-induced potentiation due to the fact that heroin itself led to a potentiated state? If this were the case, there may be an increased capacity in these individuals for long-term depression-like changes in response to low frequency rTMS. Is the capacity for plasticity altered in the prefrontal cortex of the brain of heroin addicts, where control over drug cravings would likely need to happen? The study by Shen and colleagues points to an exciting future path for using rTMS to test new plasticity-based interventions for treating drug addiction.

Y. Shen et al., Addition impairs human cortical plasticity. Bio. Psychiatry 10.1016/j.biopsych.2016.06.013 (2016). [Abstract]

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