Escalation of Ethanol Drinking in Mice Is Associated With Neurochemical Changes in the Dorsal Striatum

Prior studies indicate that a bias towards excitation in the excitatory/inhibitory balance occurs in the dorsal striatum as a result of chronic heavy alcohol use. However, investigations of the dorsal striatum by noninvasive means, such as magnetic resonance spectroscopy (MRS) in vivo, have been lacking. In this study, 49 mice (27 female) on a C57BL/6J genetic background were subject to a chronic intermittent ethanol and forced swim stress (CIE-FSS) procedure. For each mouse, MRS data were acquired at two timepoints from a single voxel covering the dorsal striatum bilaterally. The baseline MRS timepoint occurred in an ethanol naïve state. Ethanol drinking for all mice consisted of 1-h/day access to one bottle of 15% ethanol, for 5 days/week. After a 6-week ethanol drinking acclimation period, mice were assigned to four experimental groups: air and no stress, air and stress, EtOH_vapour and no stress, and EtOH_vapour and stress. Mice then underwent four cycles of CIE-FSS, each cycle consisted of 1 week of vapour (air or ethanol) followed by 1 week of either 10 min of FSS swim stress or no swim stress, and 1-h access to 15% ethanol. A follow-up MRS timepoint occurred at the conclusion of the 14-week CIE-FSS protocol, after at least 43 h of abstinence from ethanol. Whole-cohort ethanol drinking increased from a mean ± SEM of 1.51 ± 0.11 g/kg at Week 1, to a mean of 3.15 ± 0.13 g/kg at Week 14. Robust increases in several neurochemicals were observed between baseline and follow up, with the largest increases found for glutamate, glutamine, taurine, and creatine. Two sets of analysis on MRS data were performed: (1) assessing effects of experimental group (CIE, FSS and/or sex) on striatal neurochemistry and (2) investigating correlations between longitudinal changes in ethanol drinking and changes in neurochemical concentrations. Striatal neurochemical concentrations exhibited CIE-FSS group effects (1), notably EtOH_vapour exposure significantly reduced lactate concentrations at follow up. The change in glutamate between baseline and follow up was significantly lower in EtOH_vapour exposed mice. Changes in the glutamate/glutamine ratio were markedly reduced in the EtOH_vapour condition. In our correlational analysis (2) of changes in drinking versus changes in neurochemical concentrations from baseline to follow up, we found that female mice have strong positive linear correlations between the change in ethanol drinking and the changes in concentration of glutamate, glutamine, taurine, myo-inositol and creatine. In male mice, inverse correlations were found between changes in drinking and changes in glutamate, taurine and creatine concentrations. Additionally, the change in cerebral ventricle volume was found to correlate with the change in ethanol drinking, but ventricular volume change was not significantly affected by CIE or FSS. Together, the observed effects of CIE on MRS outcomes in dorsal striatum and the correlations between neurochemical changes and drinking behaviour substantiate the importance of the dorsal striatum in escalating ethanol consumption and are promising evidence for the utility of MRS to detect behaviourally relevant changes in heavy ethanol drinking phenotypes.