BEHAVIORAL BIOLOGY, 14,405-406,(1975), Abstract No. 5110
NOTE
Steady Potential Concomitants of Cortical Spreading Depression: A Reply to Huston
TERRY J. CROW and LEWIS PETRINOVICH
Department of Psychology, University of California, Riverside, California 92502
Huston has suggested that the shifts in cortical steady potential recorded in the hemisphere contralateral to the locus of initiation of spreading depression are an artifact of the recording technique reported by Crow, Petrinovich and Carew (1973). The issue raised by Huston concerns the nature of the placement of the reference electrode. Crow et al., (1973) implanted bilaterally two epidural Ag-AgC1 electrodes 2mm posterior and 3mm lateral to bregma. A Ag-AgC1 reference electrode was placed on the midline 7-10ram anterior to bregma in Expt I and on the neck in Expt II. Steady potentials were recorded differentially between an active electrode on each hemisphere and the common reference electrode. Changes in the cortical steady potential accompanying spreading depression, as initially described by Leao (1947), consist of a negative change which reaches its maximum value within 0.5-1 min. Following the return to normal value the cortex becomes positive with respect to the reference electrode. In the initial reports by Leao the reference electrode was located on bone near the external occipital protruberance. The same results have been obtained with the reference electrode applied to the nasal bone (Leao, 1972). Huston's account of the steady potential changes recorded from depressed cortex is a reasonable explanation for steady potentials accompanying spreading depression recorded between two active cortical electrodes as reported by Huston and Bures (1970) and Megirian and Bures (1970). However this cannot explain such potentials when recording between a single active cortical electrode and an extracerebral reference electrode. Crucial to Huston's argument is the claim that the common reference electrode is recording activity from depressed cortex. The steady potential shifts recorded on the hemisphere treated with KC1 in our study are the result of changes occurring at the active electrode and not at the reference electrode as implied by Huston. This is 405 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
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supported by the following: the waveforms were similar when the shifts were recorded between the cortical electrode and the reference electrode Placed on the neck as in Expt II; the steady potential shifts followed the typical waveform as reported by numerous investigators recording between a cortical electrode and an extracerebral reference electrode. An additional claim of Huston is that the contralateral shifts reported in Expt II are not supported by the figures. The contralateral shifts that we reported were not as evident in the recordings shown in Figs. 3-5 because of a decrease in gain for those records. This was pointed out both in the text and in the figure legend and was the case for recordings using frontal reference as well as the neck reference. Although the methods reported by Huston and Bures (1970) and Crow et al. (1970) are ostensibly similar there are differences between the methods that could account for the differences in the recordings. For example, Huston and Bures reported that the indifferent electrode was screwed i n t o the frontal bone. In our study the reference electrode was placed o n the frontal bone. Screwing the reference electrode into the frontal bone would increase the chances of the reference electrode recording activity from depressed frontal cortex. In summary, the reinterpretation of our results by Huston is the result of a misinterpretation of the methods and basic findings of our study. The contralateral changes that we reported are not without precedent. A number of previous reports have appeared describing contralateral EEG changes following unilateral spreading depression as well as other bioelectric changes remote from the locus of initiation of spreading depression.
REFERENCES Crow, T. J., Petrinovich, L., and Carew, T. J. (1973). Electrophysiological correlates of cortical spreading depression. Behav. Biol. 8,219-226. Huston, J. P., and Bures, J. (1970). Drinking and eating elicited by cortical spreading depression. Science 169, 702-704. Leao, A. A. P. (1947). Further observations on the spreading depression of activity in the cerebral cortex. J. Neurophysiol. 10, 409-414. Leao, A. A. P. (1972). Spreading Depression. In D. P. Purpura, J. K. Peary, D. Tower, D. M. Woodbury, and R. Walter (Eds.), "Experimental Models of Epilepsy," pp. 173-196. New York: Raven Press. Megirian, D., and Bares, J. (1970). Unilateral cortical spreading depression and conditioned eyeblink responses in the rabbit. Exp. NeuroL 27, 34-45.
NOTE
Steady Potential Concomitants of Cortical Spreading Depression: A Reply to Huston
TERRY J. CROW and LEWIS PETRINOVICH
Department of Psychology, University of California, Riverside, California 92502
Huston has suggested that the shifts in cortical steady potential recorded in the hemisphere contralateral to the locus of initiation of spreading depression are an artifact of the recording technique reported by Crow, Petrinovich and Carew (1973). The issue raised by Huston concerns the nature of the placement of the reference electrode. Crow et al., (1973) implanted bilaterally two epidural Ag-AgC1 electrodes 2mm posterior and 3mm lateral to bregma. A Ag-AgC1 reference electrode was placed on the midline 7-10ram anterior to bregma in Expt I and on the neck in Expt II. Steady potentials were recorded differentially between an active electrode on each hemisphere and the common reference electrode. Changes in the cortical steady potential accompanying spreading depression, as initially described by Leao (1947), consist of a negative change which reaches its maximum value within 0.5-1 min. Following the return to normal value the cortex becomes positive with respect to the reference electrode. In the initial reports by Leao the reference electrode was located on bone near the external occipital protruberance. The same results have been obtained with the reference electrode applied to the nasal bone (Leao, 1972). Huston's account of the steady potential changes recorded from depressed cortex is a reasonable explanation for steady potentials accompanying spreading depression recorded between two active cortical electrodes as reported by Huston and Bures (1970) and Megirian and Bures (1970). However this cannot explain such potentials when recording between a single active cortical electrode and an extracerebral reference electrode. Crucial to Huston's argument is the claim that the common reference electrode is recording activity from depressed cortex. The steady potential shifts recorded on the hemisphere treated with KC1 in our study are the result of changes occurring at the active electrode and not at the reference electrode as implied by Huston. This is 405 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
406
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supported by the following: the waveforms were similar when the shifts were recorded between the cortical electrode and the reference electrode Placed on the neck as in Expt II; the steady potential shifts followed the typical waveform as reported by numerous investigators recording between a cortical electrode and an extracerebral reference electrode. An additional claim of Huston is that the contralateral shifts reported in Expt II are not supported by the figures. The contralateral shifts that we reported were not as evident in the recordings shown in Figs. 3-5 because of a decrease in gain for those records. This was pointed out both in the text and in the figure legend and was the case for recordings using frontal reference as well as the neck reference. Although the methods reported by Huston and Bures (1970) and Crow et al. (1970) are ostensibly similar there are differences between the methods that could account for the differences in the recordings. For example, Huston and Bures reported that the indifferent electrode was screwed i n t o the frontal bone. In our study the reference electrode was placed o n the frontal bone. Screwing the reference electrode into the frontal bone would increase the chances of the reference electrode recording activity from depressed frontal cortex. In summary, the reinterpretation of our results by Huston is the result of a misinterpretation of the methods and basic findings of our study. The contralateral changes that we reported are not without precedent. A number of previous reports have appeared describing contralateral EEG changes following unilateral spreading depression as well as other bioelectric changes remote from the locus of initiation of spreading depression.
REFERENCES Crow, T. J., Petrinovich, L., and Carew, T. J. (1973). Electrophysiological correlates of cortical spreading depression. Behav. Biol. 8,219-226. Huston, J. P., and Bures, J. (1970). Drinking and eating elicited by cortical spreading depression. Science 169, 702-704. Leao, A. A. P. (1947). Further observations on the spreading depression of activity in the cerebral cortex. J. Neurophysiol. 10, 409-414. Leao, A. A. P. (1972). Spreading Depression. In D. P. Purpura, J. K. Peary, D. Tower, D. M. Woodbury, and R. Walter (Eds.), "Experimental Models of Epilepsy," pp. 173-196. New York: Raven Press. Megirian, D., and Bares, J. (1970). Unilateral cortical spreading depression and conditioned eyeblink responses in the rabbit. Exp. NeuroL 27, 34-45.
