Fluidity of normal and epileptogenic freeze-lesioned synaptic membranes in the cat.

Electron paramagnetic resonance spectra obtained with a 5-Nitroxylstearate spin label indicate that inhomogeneous lipid domains exist naturally in the bulk lipid matrix of cat synaptic plasma membranes at physiological temperatures, associated with non-cooperative phase transitions spanning broad temperature intervals between about 10 degrees C and 40 degrees C. These properties are qualitatively similar in normal membranes and in membranes isolated from epileptogenic freeze-lesioned brain cortex. The fluidity of matrix lipids isolated from cortex adjacent to such lesions appears to be abnormally increased at physiological temperatures, but membrane polarity, partially reflecting water penetration of the surface of the bilayer, remains normal in membranes from lesioned areas. Both fluidity and polarity of membranes isolated from 'mirror' sites contralateral to lesions appear to remain normal, and thus unaffected by seizure activity per se in these preparations, in which independent 'mirror' foci did not develop. These findings demonstrate that lipid phase inhomogeneities are present and thus may play a role in normal and abnormal functions of synaptic membranes, and that abnormalities of membrane fluidity are in fact closely associated with epileptogenesis in the freeze-lesion model.