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On the Use of Microwave Radiation Energy for Brain Tissue Fixation

Abstract: Focused microwave irradiation (MWR) is an increasingly accepted method of sacrifice of laboratory animals such as the mouse or rat. By fixing the brain within a fraction of a second with heat inactivation, the investigation of fast neurochemical events may be obtained. Even though the technique is widely utilized, its application is inconsistent. This report illustrates some of the requirements necessary for the proper application of MWR for the sacrifice of animals, particularly those related to the length of time MWR is applied and the efficiency with which generated MWR power is coupled to the brain tissue. Studies were performed on the mouse, using either a 2.5 KW or 6.3 KW generator with a focused, closed system waveguide at time intervals of 350 or 500 ms or 1.4 s. During each of these intervals MWR was varied so that core brain temperatures for all groups were held between 83 and 95°C. In contrast with reported studies that used full animal restraint, all animals were minimally restrained for less than 1 s before sacrifice. Tissue content of cyclic AMP, an index of neuronal activity grossly affected by subtle changes in the activity of adenylate cyclase and/or phosphodiesterases, was monitored. No differences in tissue cyclic AMP content in any of 12 brain regions were detected after MWR, either at 350 or 500 ms. A substantial increase in cyclic AMP content occurred in 8 of 12 brain regions examined following microwave irradiation for 1.4 s. On the basis of these experiments, accurate determination of cyclic AMP in rodent brain requires that the maximum time interval of MWR exposure should not exceed 500 ms.