Electrical impedance changes in many sites of brain in paradoxical sleep, anesthesia, and activity

Abstract

The electrical impedance of a part of brain approximately 1-mm dimensions was measured with a four-electrode, very low current method in 61 male rats. The testing frequency was usually only 1000 Hz, and only the magnitude of impedance was measured. Impedance increased in paradoxical sleep in 42 of the 61 sites and decreased at three sites in the pons. The greatest changes were in subiculum and presubiculum with changes usually more than 10% and up to 25-30%. Intermediate changes of 2-10% were found in parasubiculum and entorhinal cortex. Most other changes were less than 4%, and there is a suggestion of greater changes in the pretectal area. All sites with changes greater than 4% were within 1 mm of a pial or ependymal surface. During anesthesia with pentobarbital in 23 rats impedance increased in two, eight showed no change, and 13 decreased. During unrestrained spontaneous activity in a small familiar cage the impedance usually became either more variable or decreased generally to a maximum of 1-10%, or both, but at a single site the response was not always the same. No tests beyond simple observation were used, and with this limited basis no clearer relation of impedance to behavior than simply to motor activity was apparent. In a change from quiet arousal to slow-wave sleep, or vice versa, there were no impedance changes. But at all sites activity usually had an effect. This was particularly marked in entorhinal cortex--parasubiculum and brain stem. Almost no other impedance changes were seen than of these three types. Impedance changes in brain are thus widespread and occur frequently in the usual behavior of rats. These results generally corroborate and expand the results of the Adey-Kado group with a different method with some advantages.