In vivo glutamate spectroscopy in a 9.4 T scanner combined with microdialysis in rodents

Coordinator:		Dr. Alexander Sartorius
Institution:		ZI Mannheim
Homepage:		www.zi-mannheim.de/transl_imaging

Within SP 15 we were able to establish glutamate spectroscopy at highest levels of resolution. With our high-field 9.4 T animal scanner we quantified inter alia glutamate, glutamine and GABA within voxels of a volume of 12-16 microliters in prefrontal and hippocampal regions. For that we had to establish an automated segmentation algorithm to distinguish between white and grey matter, as well as cerebrospinal fluid. We then transferred the gold standard software protocol for absolute quantification in clinical studies (LCModel) into our animal model. Animals (8 exposed, 9 controls) were made dependent by exposure to 45 daily exposure cycles with peak levels up to 4 g/l blood alcohol concentration. Five in vivo MRS recordings were obtained in a repeated measurement design, i.e. at baseline, during alcohol intoxication, during withdrawal (12 and 60 hr after the last exposure cycle) and after 3 weeks of abstinence. Non-exposed control subjects were assessed in a similar manner. Absolute quantification was done by fitting the in vivo spectra to phantom data of 16 different metabolites. First results confirm findings from clinical studies of addicted patients, i.e. reduced myoinositol and N-acetylaspartate levels as well as increased total choline-containing compounds during alcohol intoxication. Raised glutamate levels are found during early withdrawal, which is in line with the well-established hyperexcitability during alcohol withdrawal. Interestingly, with the exception of taurine, all metabolites returned to control levels in the animals after 3 weeks of abstinence. Whether taurine is an informative marker for the ‘post-dependent’ state needs further exploration. Furthermore, alterations in the metabolic profile appear more pronounced in the medial prefrontal cortex compared to hippocampus confirming region-specific transcriptome analysis from this model.

The glutamate hypothesis of alcohol addiction was firstly and most directly tested in a translational study bringing together animal data and data collected from alcohol addicted patients from the same NGFN-Plus project at the Central Institute of Mental Health. Patients as well as animals from our model of alcohol addiction showed significantly elevated glutamate levels during acute withdrawal (in patients correlating with the severeness of withdrawal symptoms) within the anterior cingulate and the prefrontal cortex, respectively. This result -nicely confirming the neuronal-glial glutamate-glutamine shuttle system- was even more pronounced in the glutamate/glutamine ratio and normalized within the first 3 weeks of abstinence.

Thus our results corroborate in a most direct and translational manner the glutamate hypothesis of alcohol addiction and envisage the option to monitor therapeutic drug effects via MRS marker. Finally, valid and true translational MRS technology allows a better understanding of fundamental metabolic basics of psychiatric disorders.