Differences in acetylcholine but not choline in brain tissue fixed by freeze fixation or microwave heating

Among the many rapidly metabolized compounds in the brain, acetylcholine is one of the most challenging to sample effectively due to its rapid synthesis, degradation and sequestration. To ascertain problems that invalidate sampling procedures two methods of tissue fixation, microwave heat inactivation and freeze fixation, were used for obtaining mice and rat brain samples, respectively. The data show that acetylcholine levels obtained by microwave fixation were much higher than those obtained by freeze fixation. Choline levels were not affected by the fixation method used. Microwave fixation results in more accurate assessment of acetylcholine levels than the freeze fixation method, even though the tissue fixation time was less than 1 s in both methods, because tissue integrity is maintained in the microwave fixation, but not during freeze fixation.