Abstract
Background
Deficits of brain glutathione (GSH), the primary antioxidant in living tissue, and associated redox imbalance are postulated to be implicated in schizophrenia. However, due to poor blood-brain barrier permeability of GSH, direct supplementation is ineffective in restoring its brain levels. Therefore, there has been great interest in investigating N-acetylcysteine (NAC), as a prodrug that can be deacetylated to supply cysteine (Cys), which crosses the BBB and is the rate-limiting substrate in brain GSH synthesis and restoration. Once inside the cell, Cys combines with glutamate (Glu) to initiate GSH synthesis in a reaction that is catalyzed by gamma-glutamyl cysteine ligase (GCL), the rate-limiting enzyme. Likely due to the in situ reaction of NAC-supplied Cys with Glu to spur GSH synthesis, NAC is believed to have anti-glutamatergic properties. However, direct in vivo evidence of NAC as an anti-glutamatergic agent is currently lacking. In this study, we used 1H MRS to monitor changes in brain levels of both GSH and Glu in response to 4 weeks of daily supplementation with NAC in healthy volunteer subjects. We postulated that 4 weeks of NAC treatment would elevate GSH and decrease Glu levels.
Methods
Subjects: Participants recruited for this study consisted of 12 mentally and physically healthy human volunteer (HV) subjects.
NAC Supplementation: To investigate the effects of dietary NAC supplementation on cortical GSH and Glu levels, each HV subject underwent 1H MRS scans at baseline. Then a 4-week supplement of 900mg NAC tablets was provided, to be taken 2 per day for a daily NAC dose of 1800mg. Finally, each subject was brought back after 4 weeks for the post-NAC 1H MRS scans to assess the effect of the treatment on cortical GSH and Glu levels.
Brain 1H MRS: In vivo cortical GSH spectra were recorded in 15 min on a 3.0 T GE MRI system from a 3cmx3cmx2cm occipital cortex voxel, using the standard J-editing technique, with TE/TR 68/1500ms and 290 interleaved excitations (580 total). Levels of Glu, uncontaminated with glutamine, were obtained from the same occipital cortex voxel in 6 min using the CT-PRESS method, with TE/TR 139ms/1500ms, and 129 chemical shift encoding steps in increments of 0.8ms in t1 dimension. Peak areas for both GSH and Glu were derived by frequency-domain fitting of the recorded spectra. The resulting peak areas were then expressed as ratios relative to the area of the unsuppressed tissue water signal in the voxel.
Results
GSH Levels: The effect of 4 weeks of NAC supplementation on GSH levels was a numerical increase that did not reach statistical significance relative to baseline (p=0.33).
Glu Levels
Following 4 weeks of NAC supplementation cortical Glu levels decreased significantly compared to baseline (p=0.04).
Discussion
This study has shown that following 4 weeks of taking NAC, brain Glu levels in HV decreased significantly, even though a numerical increase in GSH levels did not reach statistical significance. The observed Glu decrease at 4 weeks suggests a net increase in the consumption of intracellular Glu reserves in the GCL-catalyzed reaction that combines NAC-supplied Cys with Glu in GSH synthesis. The failure of GSH levels to increase significantly after NAC likely reflected the established “feedback inhibition” mechanism whereby GSH synthesis is shut off in a tissue with normal levels, with de novo synthesis due to NAC occurring only in a subset of subjects with “low” GSH levels (i.e., below the sample mean) at baseline. In summary, the results of this study support the role of NAC as an anti-glutamatergic agent, which seems to modulate Glu levels by increasing the availability of intracellular Cys that then combines with Glu to initiate GSH synthesis, leading to a lowering of total Glu levels as measured with MRS
Correlation of CGS 19755 Neuroprotection against in vitro Excitotoxicity and Focal Cerebral Ischemia
