Surgery in the rat during electrical analgesia induced by focal brain stimulation.

Chronic monopolar electrodes were implanted in the region of the midbrain central gray in eight rats. In three rats, continuous 60 cycle-per-second sine-wave stimulation resulted in an electrical analgesia defined by the elimination of responses to aversive stimulation while general motor responsiveness was retained. Exploratory laparotomy was carried out in these animals during continuous brain stimulation without the use of chemical anesthetics. Following surgery, brain stimulation was terminated, and responses to aversive stimuli returned. Electrodes effective in inducing electrical analgesia at the lowest currents were located at the dorsolateral perimeter of the midbrain central gray. It was concluded that focal brain stimulation in this region can induce analgesia in the absence of diffusely applied "whole brain" stimulation.