GABAA Receptor Availability in Relation to Cortical Excitability in Depressed and Healthy: A Positron Emission Tomography and Transcranial Magnetic Stimulation Study

Introduction: Gamma-aminobutyric acid (GABA) deficiency is suggested in depressive disorders, along with alterations in cortical excitability. However, whether these excitability changes are related to GABA(A) receptor availability is largely unknown. Our aim was to assess the correlation between these measures in depressed patients and healthy controls.Methods: Twenty-eight patients with a major depressive episode, measured before and after participating in a clinical trial with repetitive transcranial magnetic stimulation (TMS), and 15 controls underwent [C-11]flumazenil positron emission tomography to assess GABA(A) receptor availability and paired pulse TMS (ppTMS) to evaluate cortical excitability. Both whole-brain voxel-wise GABA(A) receptor availability and mean values from left hand motor cortex and left paracentral lobule were correlated to the ppTMS outcomes: short-interval intracortical inhibition reflecting GABA(A) receptor activity, long-interval intracortical inhibition representing GABA(B) receptor activity, intracortical facilitation reflecting glutamate N-methyl-D-aspartate-receptor activity, as well as the resting motor threshold (rMT), considered a global measure of corticospinal excitability.Results: No significant differences in baseline GABA(A) receptor availability or cortical excitability were found between patients and controls. Additionally, no correlations were observed between baseline measurements of GABA(A) receptor availability and TMS outcomes. Changes in GABA(A) receptor availability in the hand motor cortex, between pre- and post-assessments, were inversely related to pre-post changes in hand rMT.Conclusion: We found that a change in GABA(A) receptor availability was inversely related to a change in rMT, suggesting a link between GABA deficiency and increased rMT previously observed in depressive episodes. The results highlight the complex mechanisms governing cortical excitability measures and offer new insight into their properties during the depressive state.