Investigating the long-term stability and neurochemical substrates of TMS and MRS

Transcranial magnetic stimulation (TMS) and magnetic resonance spectroscopy (MRS) are non-invasive techniques that allow the measurement of GABAergic and glutamatergic activity in the brain. TMS and MRS can be used to assess inhibitory and excitatory mechanisms, treatment response or disease presence and progression in vivo. However, despite their growing use in research and medical settings, ambiguity remains regarding their neurochemical substrates and long-term reproducibility. The goal of the present thesis is twofold. First, the long-term stability and reliability of various MRS and TMS measurements, obtained in the motor cortex, was investigated. Second, to better understand which aspects of the GABAergic network are targeted by the two techniques, TMS and MRS measures reflecting cortical inhibition and excitation were obtained following lorazepam administration using a placebo-controlled, double-blind, randomized, crossover design.   Two articles comprise this thesis. The first article is a longitudinal assessment of the stability and reliability of MRS-GABA and Glx (glutamate + glutamine) and TMS measures of cortical inhibition and facilitation in the sensorimotor (SMC) cortex of healthy adults. It was determined that MRS-GABA and MRS-Glx are stable over a three-month interval. TMS measures of resting motor threshold (rMT), cortical excitability (% maximum stimulator output; MSO) and cortical silent period (CSP) were also found to be stable and reliable. However, paired-pulse TMS measures such as short-interval cortical inhibition (SICI), long-interval cortical inhibition (LICI) and intracortical facilitation (ICF) had greater variability.   The second article aims to understand the differential sensitivity of TMS and MRS with respect to GABAergic activity in the primary motor cortex. It is based on the results and conclusions of a placebo-controlled, double-blind, randomized, crossover study, where benzodiazepine lorazepam was given to healthy adult volunteers. Magnetic resonance spectroscopy (GABA and Glx) was performed in the sensorimotor cortex and occipital cortex (OC). TMS measurements were acquired in the motor cortex only. MRS and TMS measures of cortical inhibition and excitability (rMT, input/output (I/O) curve, SICI, LICI, ICF, CSP) were obtained following lorazepam or placebo administration. Lorazepam was found to decrease occipital GABA concentration, increase motor cortical inhibition and decrease cortical excitability. Lorazepam administration had no effect on other neurometabolites or TMS measurements. The effect of Lorazepam on short-interval cortical inhibition was found to depend on endogenous GABA levels in the SMC; higher GABA concentrations predicted a greater increase in SICI following drug intake.  Taken together, the studies presented in this thesis indicate that MRS neurometabolite levels are stable over time and may thus potentially serve as markers for the monitoring of disease progression and treatment response. However, while some TMS measures have good long-term stability (rMT, %MSO, CSP), others are not as reliable nor stable (SICI, LICI, ICF); care must be taken in clinical settings. Furthermore, the differential effects of lorazepam on MRS and TMS measures support the idea that the two techniques probe different aspects of the GABAergic system. Whereas TMS measures of cortical inhibition reflect phasic GABAA receptor activity, MRS-GABA primarily reflects intracellular, non-neurotransmitter metabolic GABA.

Keyphrases: Magnetic resonance spectroscopy, transcranial magnetic stimulation, GABA, glutamate, lorazepam, motor cortex, GABA, glutamate