DIFFERENTIAL EFFECTS OF GENERAL ANESTHETICS ON SPONTANEOUS ELECTRICAL ACTIVITY OF NEOCORTICAL AND RHINENCEPHALIC BRAIN SYSTEMS OF THE DOG

¹ Department of Pharmacology, University of Michigan, Ann Arbor, Michigan

The effects of various general anesthetics including diethyl ether, divinyl ether, halothane, cyclopropane, nitrous oxide, ethylene and pentobarbital were determined on the spontaneous electrical activity of the neocortical and rhinencephalic structures of dogs. Both acute preparations as well as animals with chronically implanted electrodes were used. The changes in the spontaneous electrical activity were correlated with the stages and planes of general anesthesia.

In neocortical structures diethyl ether and related inhalation anesthetics produced the following EEG changes: (a) low voltage, fast frequency desynchronization, (b) some low voltage, slow wave activity of approximately 3 to 6 cps, (c) marked flattening of all spontaneous electrical activity.

Diethyl ether and related inhalation anesthetics produced in the amygdala, olfactory bulb, posterior hypothalamus and related rhinencephalic structures the following EEG changes: (a) inhibition of respiratory burst discharges, (b) spikelike 40 cps activity which decreased in frequency during light plane 1 of surgical anesthesia, (c) marked slowing of the frequency of the spontaneous asynchronous activity, (d) increasing hypersynchrony and amplitude of electrical activity during the lower level of plane 1 or in plane 2 of surgical anesthesia, (e) persistent maximal hypersynchrony of 10 to 18 cps lasting from 10 to 30 seconds or more depending upon the level of anesthesia (occasionally such activity bore a striking resemblance to seizure-like discharges; this was especially true with trichlorethylene which was observed to produce grand mal seizures during recovery from anesthesia), (f) loss of synchrony and decreasing amplitude and frequency of the spontaneous activity in plane 3, stage III, and finally, (g) flattening of the amplitude of spontaneous activity in plane 4, stage III of anesthesia. Although the majority of these effects were readily produced by diethyl ether and most other inhalation anesthetics during induction, such phenomena were usually best seen during recovery from the deeper planes of surgical anesthesia.

The actions of pentobarbital on the spontaneous EEG activity were in some respects in striking contrast to those produced by the inhalation anesthetics. Both neocortical as well as the rhinencephalic structures seemed to be affected more in the same direction.