Evidence of multiple ethanol pools in the brain: an in vivo proton magnetization transfer study

Studies of isolated cell membranes and animal brain extracts have shown that ethanol (EtOH) partitions into cell membranes. We tested the hypothesis that EtOH in the living brain after EtOH administration exists in two or more pools: a free, mobile pool of EtOH and one or more EtOH pools that are restricted in their molecular mobility, possibly because of association with membranes. In vivo brain proton magnetic resonance spectroscopy (1H MRS) routinely detects the methyl protons of the mobile EtOH pool but does not detect motionally restricted EtOH. We used in vivo brain 1H MRS in rat brain (n = 11) after intraperitoneal EtOH administration to measure the signal intensity of methyl EtOH protons in the presence and absence of off-resonance saturation. Off-resonance saturation resulted in a 33 +/- 4% decrease of the EtOH methyl proton signal. We interpret this signal reduction as a magnetization transfer effect. It is consistent with the existence of an MRS-invisible EtOH pool with restricted molecular mobility, which is in exchange with the free EtOH pool. Off-resonance saturation at the water frequency resulted in an even larger decrease of the EtOH methyl signal, consistent with water molecules being in close proximity to EtOH molecules at the restricted motion site(s). These results provide support for the hypothesis that partial MRS-invisibility of brain EtOH is at least to some extent caused by the presence of a (MRS-invisible) pool of motionally restricted EtOH. They also strongly suggest that water suppression, routinely used in in vivo 1H MRS, may reduce the observable EtOH methyl signal intensity through a magnetization transfer mechanism. These studies may provide both a mechanism of, and a means to investigate the alterations of EtOH MRS visibility observed in heavy drinkers.