The Effect of Visual Stimulation on GABA and Macromolecule Levels in the Human Brain in vivo

—Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain; it plays an important role in various types of synaptic plasticity and pathology. In the present work, proton magnetic resonance spectroscopy (1H-MRS) and two modified MEGA-PRESS pulse sequences, that is, –GABAMEGA-PRESS and +GABAMEGA-PRESS, were used to investigate the effect of visual stimulation on the GABA level in the human visual cortex in vivo. With –GABAMEGA-PRESS, it was possible to acquire the 1H-NMR signal of methylated protons of GABA with the chemical shift δ = 3.01 ppm without the signals from macromolecules. When +GABAMEGA-PRESS was used, the GABA signal was a superposition of resonances of methylene protons of GABA and macromolecules. The effect of constant visual stimulation on the level of N-acetylaspartate and total level of glutamate and glutamine was also estimated. Constant visual stimulation had no effect on the levels of N-acetylaspartate and glutamine in the visual cortex. The –GABA signal intensity decreased with a statistically significant decrease in the level of GABA signal intensity leading to inactivation of GABA synthesis. No statistically significant changes in the intensity of the +GABA resonance were found, probably due to the effect of visual stimulation on macromolecules. In order to test this hypothesis, a signal from macromolecules was acquired using a specifically designed inversion-recovery pulse sequence. It was shown that the intensity of this signal is unaffected by visual stimulation and the absence of changes in the intensity of the +GABA signal during the stimulation has to be considered as the masking effect of macromolecular signal.