Evidence for impaired cortical inhibition in patients with unipolar major depression

Background: Several lines of evidence suggest that central cortical inhibitory mechanisms, especially asssociated with aminobutyric acid (GABA) neurotransmission, may play a role in the pathophysiology of major depression. Transcranial magnetic stimulation is a useful tool for investigating central cortical inhibitory mechanisms associated GABAergic neurotransmission in pyschiatric and neurological disorders. Methods: By means of transcranial magnetic stimulation, different parameters of cortical excitability, including motor threshold, the cortical silent period, and intracortical inhibition/facilitation, were investigated in 20 medication-free depressed patients and 20 age and gender-matched healthy volunteers. Results: Silent period and intracortical inhibition were reduced in depressed patients, consistent with a reduced GABAergic tone. Moreover, patients showed a significant hemispheric asymmetry in motor threshold. Conclusions: This study provides evidence of reduced GABAergic tone and motor threshold asymmetry in patients with major depression.