The Relationship Between Suicidal Behaviour and Cortical Inhibition

What's the science?

Within a year, 18% of adolescents will consider suicide, and identifying adolescents at risk for suicide or identifying a ‘suicide biomarker’ in the brain is difficult. There is some evidence that adolescents with suicidal behaviours have an imbalance between two major neurotransmitters (chemicals) in the brain: GABA (inhibitory) and glutamate (excitatory). This week in Neuropsychopharmacology, Lewis and colleagues use transcranial magnetic stimulation (TMS) to assess how excitation and inhibition might differ in the brains of these adolescents.

How did they do it?

They recruited 20 healthy adolescents, 37 depressed adolescents and 17 depressed adolescents with a history of suicidal behaviours. They used TMS to apply magnetic pulses to the motor cortex in the brain. These pulses, when applied to a particular region of the motor cortex, temporarily affect activity in certain muscles in the body. They measured the motor evoked potentials (i.e. electrical signal recorded from the muscle) in the thumb after stimulating the motor cortex using a variety of inhibitory/excitatory paradigms, including long-interval intracortical inhibition (LICI) where two TMS pulses are applied and the two motor evoked potential responses are compared. The second response is usually smaller, and this is thought to be mediated by GABA-B. They compared responses a) across the three groups and b) in relation to the severity of the history of suicidal behaviour.

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What did they find?

As expected, the motor evoked potential after the second TMS pulse was much smaller (it was inhibited) compared to the motor evoked potential after the first pulse in healthy and depressed adolescents. In comparison, depressed adolescents with a history of suicidal behavior showed less inhibited (i.e. stronger) motor evoked potentials after the second pulse. These effects were found at inter-stimulus intervals of both 100 and 150 ms. Further, the amplitude of the responses was correlated with the severity of past suicidal behaviour.

What's the impact?

This study suggests that GABA-B activity (involved in inhibition in the brain) may be abnormal in adolescents with past suicidal behaviors. This neurotransmitter could potentially act as a biomarker for suicide risk in the future. However, individuals currently experiencing an acute episode of suicidal behaviour were not studied, and neurotransmitter profiles may be different in this group.

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C.P. Lewis et al., Cortical inhibitory markers of lifetime suicidal behavior in depressed adolescents. Neuropsychopharmacology (2018). Access the original scientific publication here.