Evidence for increased motor cortical facilitation and decreased inhibition in atypical depression

Major depressive disorder (MDD) is a clinically heterogeneous condition. However, the role of cortical glutamate and gamma-aminobutyric acid (GABA) receptor-mediated activity, implicated in MDD pathophysiology, has not been explored in different MDD subtypes. Our aim was to assess the atypical and melancholic depression subtypes regarding potential differences in GABA and glutamate receptor-mediated activity through established transcranial magnetic stimulation (TMS) neurophysiological measures from the motor cortex.

We evaluated 81 subjects free of antidepressant medication, including 21 healthy controls and 20 patients with atypical, 20 with melancholic, and 20 with undifferentiated MDD. Single and paired-pulse TMS paradigms were used to evaluate intracortical facilitation (ICF), cortical silent period (CSP), and short intracortical inhibition (SICI), which index glutamate, GABAB receptor-, and GABAA receptor-mediated activity respectively.

Patients with MDD demonstrated significantly decreased mean CSP values than healthy controls (Cohen's d = 0.22-0.3, P < 0.01 for all comparisons). Atypical depression presented a distinct cortical excitability pattern of decreased cortical inhibition and increased cortical facilitation, that is, an increased mean ICF and SICI ratios than other depression subtypes (d = 0.22-0.33, P < 0.01 for all comparisons).

Different MDD subtypes may demonstrate different neurophysiology in relation to GABAA and glutamatergic activity. TMS as an investigational tool might be useful to distinguish between different MDD subtypes.