Evidence for pretreatment LICI deficits among depressed children and adolescents with nonresponse to fluoxetine

Background Research suggests that alterations in gamma-aminobutyric acid receptor functioning have a role in depression. Paired-pulse transcranial magnetic stimulation (TMS) paradigms are noninvasive measures of cortical inhibitory and excitatory circuits. Objective/hypothesis The present study examined pretreatment short-interval intracortical inhibition (SICI), long-interval cortical inhibition (LICI), and intracortical facilitation (ICF) in children and adolescents with major depressive disorder who were initiating fluoxetine treatment. The primary objective was to examine the relationship of these measures with subsequent treatment response. It was hypothesized that alterations in pretreatment GABA and glutamate mediated neurotransmission, would be associated with fluoxetine nonresponse. Methods Sixteen children and adolescents with major depressive disorder underwent paired-pulse TMS testing before beginning fluoxetine treatment. Response was prospectively characterized by scores of 1 or 2 on the Clinical Global Impression Scale and less than 40 on the Children's Depression Rating Scale-Revised after 6 weeks of fluoxetine treatment (20-40 mg/day). Results Eight patients responded to treatment. Least-squares mean LICI values were consistently higher bilaterally for treatment nonresponders. Higher LICI values indicate less inhibition and impaired GABA_B functioning. There was no significant effect of treatment response on the measures of SICI and ICF. Conclusions Our findings suggest that deficits in pretreatment GABA_B may be related to fluoxetine nonresponse in depressed youth. This is congruent with prior work demonstrating that GABA_B interneurons have serotonergic input and antidepressants modulate GABA_B receptors. These findings also show that TMS paradigms have utility in studying the neurophysiology and treatment of childhood mood disorders.