Abstracts

Rationale: The cure and treatment of epilepsy is limited by the lack of understanding of the underlying neural substrates and molecular mechanisms of the responses to the anti-epileptic drugs (AEDs). Most AEDs aim to reset the ratio of inhibitory:excitatory neurotransmission, the in vivo responses of the AEDs are not known. Basic questions such as whether the effects are global or region specific have not been answered. Magnetic resonance spectroscopy (MRS) has the potential to measure cellular metabolites and some neurotransmitters, including glutamate and acetylcholine. However, the inhibitory neurotransmitter GABA has posed a challenge. We have developed the imaging algorithms to detect relative GABA levels in the live mouse brain.Methods: In this study, we compared in vivo levels in wildtype B6 strain mice with their siblings that are lacking a single copy of the gene (Gad1) that is the main converting enzyme glutamatic acid decarboxlase 67 (Gad67), B6-Gad1tm11.Bgc mice. Littermate age-matched male mice (3 - 6 months) were obtained from heterozygous breedings and genotyped using PCR. In vivo 1H MRS experiments were performed on a Bruker BioSpec 70/30USR Avance III 7T horizontal bore MR scanner (Bruker Biospin MRI GmbH, Germany) equipped with a BGA12S gradient system and interfaced to a Bruker Paravision 5.1 console. A Bruker four-element 1H surface coil array was used as the receiver and a Bruker 72 mm linear-volume coil as the transmitter. A three-slice (axial, mid-sagittal, and coronal) scout using fast low angle shot (FLASH) sequence was obtained to localize the mouse brain. A fast shimming procedure (FASTMAP) was used to improve the B0 homogeneity in the region of interest. Both Proton-density- and T2- weighted images were obtained using a 2D rapid acquisition with relaxation enhancement (RARE) sequence in the axial plane. A customized short-TE PRESS pulse sequence was used for MRS data acquisition. Results: The in vivo 1H spectra demonstrate excellent spectral resolution and sensitivity in both regions. A number of metabolites including gamma-aminobutyric acid (GABA), glutamate (Glu), glutamine (Gln), glutamate + glutamine (Glx), N-acetylaspartate (NAA), and total Creatine (tCr) were reliability detected. Among the metabolites, GABA (P<0.03), Gln (P< 0.05), Glu (P<0.01) and Gln+Glu (Glx, P< 0.01) concentrations in B6-Gad1tm1.1Bgc mice are significantly lower compared to B6 mice. Regional differences were found between the cerebral cortex, striatum and hippocampus with respect to GABA and glutamate levels and mouse genotype. Cellular localization using immunohistochemical corresponded to the MRS values. Conclusions: Our data shows that we can reliably measure neurotransmitter levels in mice which serve as models for human epilepsy. When used in the context of current AED therapies, the MRS data have the potential provide information about regional and global effects of drugs. Understanding the regional changes in neurometabolites and the behavioral outcomes will aid in the choice of AED for a specific epilepsy and also guide development of new AEDs.