Subthreshold changes at the site of initiation of spreading cortical depression by mechanical stimuli

Abstract

1. 1. The effect of the local application of potassium chloride in concentrations which are subthreshold for the initiation of spreading depression on the cerebral cortex of rats is summed with the effect of subthreshold mechanical stimuli. The percentage of occurrence of SD increases with subthreshold values of both kinds of stimuli. 2. 2. The effect of a subthreshold mechanical pulse is also summed with the effect of application of 2,4-dinitrophenol in subthreshold concentrations. The time effect of DNP is, however, longer than of potassium chloride. 3. 3. Two successive subthreshold mechanical stimuli or a volley of the latter applied at intervals longer than 200 msec do not lead to a summation and initiation of SD. On the contrary an inhibition of suprathreshold mechanical pulse follows a previously applied subthreshold one. This inhibition reaches its maximum (50%) for a testing stimulus 2 mm above threshold applied in the 15 sec after a conditioning stimulus — 1 mm below threshold. A return to normal values is attained after 2 min. 4. 4. Evoked potentials at the site of application of a subthreshold mechanical stimulus are reduced approximately to 25 per cent of their original amplitude when a subthreshold stimulus 2 mm below the threshold is used. The recovery of the amplitude to 50 per cent of the initial value is approximately three times faster than in the case of SD. 5. 5. The lowering of the evoked potentials is limited only to the region of stimulus application and shows a steep spatial decrement. Already 3 mm from the edge of the affected area no reduction of evoked potentials can be observed. 6. 6. The mechanism of the summation of subthreshold stimuli as well as the relative refractoriness following subthreshold mechanical stimulus is discussed.