Spreading EEG depression elicited by controlled deformation of the rat cerebral cortex

The elicitation of cortical spreading depression (SD) by electronically generated mechanical stimuli was studied in 52 pentobarbital-anaesthetized rats. The stimuli were applied with a rod (target area 7.065 mm2) immersed at a constant velocity (3 X 10(-6) to 2 X 10(-1) m/sec) to varying depths (0 to 2.5 mm) from the surface of the cerebral cortex at the level of the bregma. SD elicitation depended on the velocity as well as the amplitude of deformation. The relationship between the velocity, v (m/sec), and amplitude, A (m), of threshold stimuli for SD elicitation is described by the equation 6.4 log A + log v = -21.08 in the given measuring range. The mean threshold velocity at a depth of 0.9 mm was 2.85 X 10(-2) m/sec; at 2.2 mm it was 8.54 X 10(-5) m/sec. If the rod was immersed to a given depth at a subthreshold velocity, to elicit SD by plunging it in a further 0.5 mm it was necessary to use velocities of a higher order than the one needed to obtain the same total deformation with a single threshold pulse. In extensive deformation, this velocity difference disappeared. Comparison of the results obtained by a free fall and electronically controlled deformation showed that the threshold of the mechanical stimulus eliciting spreading EEG depression could be expressed by the deformation velocity and amplitude better than by the amount of kinetic energy absorbed.