Electrical Activity and Ultrastructure

Although it is often assumed that failure to record electrical activity from a particular brain slice will be associated with poor ultrastructural preservation, this proposition only appears to have been tested directly by Bak et al. (1980) and Yamamoto et al. (1970). In both series of experiments, the same slice of tissue was used for both the physiological and morphological parts of the experiments and the parallel between functional and ultrastructural preservation of the neuronal elements examined directly. In the earlier study, incubation conditions were optimized to maintain the electrical activity of a slice from the olfactory cortex, and this appeared to ensure the preservation of morphological integrity. More convincingly, the authors showed suppression of electrical activity to be associated with ultrastructural abnormalities. 

In contrast, electron-microscopic studies of the 700- xm slices showed 

Dark pycnotic nuclei surrounded by glial cells were found in considerable numbers in both thin and thick slices, immediately after cutting. Since the numbers did not increase with incubation and similar cells have been seen in animals fixed by perfusion (Cammermeyer, 1978), they are thought to be due to mechanical stress during cutting, and unrelated to the subsequent handling of the tissue. 

Rather surprisingly, the incubation of the 700- xm slices at room temperature at 25°C instead of 36°C did not improve the morphological appearance of the thick slices. This is surprising, since a reduction in temperature has been shown rather convincingly by Harvey and his colleagues (Harvey et al, 1974) to allow the use of thicker slices of olfactory cortex and thus recordings from the deeper and smaller inhibitory interneurons. 

Not only does intact or disrupted morphology of the slices correlate with their electrical activity, but also with their ability to synthesize acetylcholine. Rather surprisingly, 300- xm slices maintained at 36°C for 2 hr contain 10 times as much acetylcholine (ACh) as a similar slice in vivo (Weiler et al., 1979). At room temperature, in vitro levels of acetylcholine are reduced to half. In thick slices, ACh levels never attain these higher values, but they are still in excess of those in vivo. 

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Grossman, R.G. and Williams, V.F. Electrical Activity and Ultrastructure of Cortical Neurones and Synapses in Ischaemia. In "Brain Hypoxia", p. 61o Eds. Brierly, J.B. and Meldrum, B.S. 

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