Evidence of Multiple Ethanol Pools in the Brain: An in Vivo Proton Magnetization Transfer Study

Abstract

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.

Magnetic coupling of creatine/phosphocreatine protons in rat skeletal muscle, as studied by1H-magnetization transfer MRS

“…It seems plausible that the ethanol MT phenomenon is due to its well-known membrane partitioning. Interestingly, Meyerhoff et al (11) reported that off-resonance saturation at the water frequency produced an even larger decrease of the ethanol methyl signal, consistent with water molecules being in close proximity to ethanol molecules at the site(s) of restricted motion. Using similar experiments, we also observed a magnetic coupling between mobile tCr and mobile water protons (Fig.…”

“…An MT effect for the signal of the methyl protons of lactate in a C6 glioma tumor in rat brain was described by Gruetter et al (10). Meyerhoff et al (11) reported MT effects for the methyl signal of ethanol in rat brain after intraperitoneal ethanol administration. In the above studies, the MT effect was tentatively ascribed to the chemical exchange of the molecules concerned between a motionally restricted environment, characterized by a signal that is several kHz wide, and a free state in which essentially complete motional averaging of dipolar interactions occurs.…”

Spectroscopic imaging of the uptake kinetics of human brain ethanol

“…Other studies using spin echo methods and refocusing pulses which reduce J-modulation have yielded higher values. For example, Meyerhoff et al (9) reported an ethanol visibility of 66% in a in vivo rat brain model, which is similar to that reported in human gray matter by Spielman et al, 72%, (10). Finally, using a spectral editing sequence at 4.7 T in a rabbit model, Petroff et al (11) reported an ethanol visibility of nearly 100%.…”

1H and31P magnetization transfer studies of hindleg muscle in wild-type and creatine kinase-deficient mice

“…It is important to note that the methyl protons of creatine and phosphocreatine are not resolved in in vivo 1 H-NMR spectra of rat skeletal muscle at 4.7 T. Therefore, in the present discussion of the 1 H-NMR data creatine is synonymous to total creatine, the sum of creatine and phosphocreatine. Several other reports on 1 H MT effects on tissue metabolites have recently appeared, including creatine in rat brain (2)(3)(4) and in human brain (5-7) and muscle (6,8), as well as lactate (3,4,9) and ethanol (10) in animal models. Our previous rat study (1) provided neither information on the mechanism(s) underlying the off-resonance magnetization transfer effects of creatine nor on its possible functional consequences.…”