Physiology&Behavior,Vol. 52, pp. 1197-1200, 1992
0031-9384/92 $5.00 + .00 Copyright © 1992PergamonPress Ltd.
Printed in the USA.
BRIEF COMMUNICATION
Further Characterization of Cortical Polarization-Induced Motor Behavior in Rabbits YUN-FEI LU, YUKIO
HATTORI,
YASUSHI HAYASHI, AKIYOSHI
MORIWAKI
AND YASUO HORI l
Department of Physiology, Okayama University Medical School, 2-5-1 Shikata-cho, Okayama 700, Japan R e c e i v e d 26 N o v e m b e r 1991 LU, Y.-F., Y. HATTORI, Y. HAYASHI, A. MORIWAKI AND Y. HORI. Furthercharacterization of corticalpolarizationinduced motor behavior in rabbits. PHYSIOL BEHAV 52(6) 1197-1200, 1992.--Anodal direct currents of 1 or 10 tLA were unilaterally applied for 30 min once a day to the premotor area of the cerebral cortex in rabbits, in which the current application was repeated 10 times at intervals of 2-3 days. Peripheral motor behavior was observed during and after each polarization trial, and was compared with that before polarization. The motor manifestations were classified into two types: gentle flexion of either forelimb and struggle with violent movement of forelimbs. Flexion of the forelimb contralateral to the polarized cortical side was clearly increased by polarization at an intensity of 1 pA, but not at 10 ttA. Forelimb struggle of both sides decreased only when 10 #A was applied. These results suggest that the passage of 1 #A current activates the polarized local area of the cortex, while l0/zA has an inhibitory effect on cortical activity. Polarizing current
Anodal polarization
Motor behavior
CEREBRAL cortical activity has been shown to be enhanced by the passage of anodal direct current to the cerebral cortex (1,2,10). The excitation focus which is possibly formed in the cortex by the anodal polarization has been called a d o m i n a n t focus (13). An increase of forelimb flexion, especially of the forelimb contralateral to the polarized cortex, has been regarded as one of the behavioral indices for determining whether or not a d o m i n a n t focus is formed in the cortical motor area (11,13). This characteristic behavior has been reported to persist consistently for several hours (11) or even as long as several weeks (7,14,16) once it begins. The p h e n o m e n o n is of considerable interest, both because it produces a long-lasting change in brain activity by a small temporary alteration in the physical environment of the nerve cells (1) and because it helps us to understand the mechanisms of m e m o r y trace formation and learning, and possibly also the process of conditioning (2). Studies have been made to clarify the effects of polarizing currents at intensities from 0.1 (1) to 300 t~A (5) on neuronal and behavioral activity in several species of animals. The optim u m intensity is not yet certain, although it has been found that current intensity is quite critical because establishing the motor manifestations depends upon slight increases or decreases of the current flow (10,13). Therefore, the use of consistent intensities and schedules for anodal polarization is important to avoid the complexity resulting from variations in experimental
Flexion
Struggle
Cerebral cortex
Rabbit
conditions. In this study, direct current at an intensity of 1 or 10 #A was applied 10 times to the premotor area of the rabbit cerebral cortex, and the effects were characterized in relation to peripheral motor behavior that was divided into flexion and struggle.
METHOD
Surgery Male rabbits weighing about 3 kg were used. The surgical procedures were essentially the same as those described previously (7). Briefly, the animals were lightly anesthetized with an intravenous injection of sodium pentobarbital (40 mg/kg). Four silver electrodes (1 m m in diameter) were implanted into the cranial bone so that the tip was set in the bone. Two of them were located 3 m m lateral to the bregma, and the other two were 7 m m rostral and 3 m m lateral to the bregma over the premotor cortex. Six stainless steel electrodes (1.5 m m in diameter) were implanted into the frontal, parietal, and occipital bones to record the electroencephalogram (EEG). All the electrodes were connected to the pins of a miniature socket and fixed with dental resin. The animals were allowed to recover from surgery for at least 10 days before use in the experiments.
i Requests for reprints should be addressed to Yasuo Hori, M.D., D.M.S., Department of Physiology, Okayama University Medical School, 2-51 Shikata-cho, Okayama 700, Japan.
1197
1198
t!( , \ i .
A m~dal Polarization
14
Rabbits were restrained in a wooden pillory on the experimental stand. The hindlimbs were fastened tightly to the stand. The forelimbs were stretched loosely to allow movement or flexing, and connected to a mechanograph for recording flexion and struggle movements. Before polarization, the rabbits were habituated to the experimental environment for 5 days (60 min per day). No current was applied during the habituation period. For polarization, direct current from the anode of a battery was passed through the silver electrode to the cortical point over the left premotor area which has been demonstrated to be appropriate for establishing the target motor behavior (6,16). A silver plate attached to the left ear lobe was connected to the cathode as a counter electrode. Direct current at an intensity of 1 or 10 ~A was applied for 30 min once a day to separate groups of rabbits. In order to avoid an overintensity and a sudden oppositely directed gradient, the current flow was switched on with a m i n i m u m intensity and then turned to the prescribed intensity, and tapered off'. The polarization trial was repeated 10 times at intervals of 2-3 days. The whole polarization schedule including the habituation period was completed within 6 weeks.
A
T
12 10 8
1
6 0 tLO CO tO X d.) LL
4 2 0
B
Behavioral and EEG Recording Motor behavior was observed in the habituation period, as well as during (30 min) and after (60 min) each polarization trial. Forelimb movements, flexion, and struggle, were recorded with the mechanograph. The behavior of the animals was also captured by a videocamera for later replay and analysis. EEGs were recorded continuously throughout the experiments.
Data Ana(v.s'is The various movements of the animals were examined and counted from the mechanogram recordings, and also confirmed with the video recording with lower speed replay. Flexions of the left and right forelimbs were counted separately. Movement of the forelimbs was evaluated as a single episode of flexion when the forelimb was lifted up and replaced. The forelimb struggles were also counted in the same way. Flexions and struggles during polarization (30 min) and after polarization (60 min) were accumulated in 10 experimental trials, in which the values after polarization were converted to those per 5 h. Data were expressed as the means + SEM. The two-tailed M a n n - W h i t n e y U-test was used for statistical analysis and a p value < 0.05 was considered significant. Movements in the habituation period were used as the control. RESULTS The motor behavior observed in these experiments was classified as flexion and struggle. Flexion was defined as gentle, slow, and smooth movements of the forelimbs without an associated change in posture. Struggle was defined as violent, quick, and strong limb movements which were usually accompanied with movement of the head and trunk. Based on these criteria, flexion and struggle were differentiated from both the mechanograms and the video replay. The flexion appeared as single or several episodes with intermission, and the struggle showed a series of highly frequent movements. The frequency of flexion of the left and right forelimbs is shown in Fig. 1. When the left premotor area was polarized at 1 #A, flexion of the fight forelimb (contralateral to the polarized side) was increased significantly during current application, as compared with the habituation (control) period. An increase in flexion of the right forelimb was also detected for 60 min after
Left
Right
FIG. 1. Effects of anodal polarization on flexion of the left and fight forelimbs in rabbits. Polarizing currents of I ~A (A) and 10 ~A (B) were applied to the premotor cortical area. Current application for 30 rain was repeated l0 times at intervals of 2-3 days. The frequency of flexion of the forelimbs was determined in the habituation period before polarization (open column), during (slanted line column), and after (dotted column) each trial of the anodal polarizations. Flexion in the habituation period was taken as the control. The control value was 0 in l0 uA polarization group. Data are expressed as the means _+SEM of the number of flexions per 5 h in five (1 t~A group) or four (10 gA group) different animals. *Significant difference from before polarization, p < 0.05.
polarization. Flexion of the left forelimb (the ipsilateral one) did not change either during or after polarization. When 10 #A was applied, no significant difference was found between polarizing and control period. After polarization with l0 ~tA, there was a tendency for the right forelimb flexion to increase, but it was not significant. The frequency of forelimb struggles is shown in Fig. 2. When l #A current was applied, struggles showed no difference from the control period, either during or after polarization. When l0 uA was applied, struggles were decreased to a significantly lower level both during and after polarization than in the control period. No significant difference was observed between left and right sides in any experimental periods. The EEGs showed low activity during anodal polarization with 10 yA current, but recovered within about 10 m i n after the current was switched off. Such inhibitory effect was observed in the EEGs recorded from both the left and right hemispheres, and there was no obvious difference between them. In the case of polarization with 1 ~tA current, there was no change in the EEG activity even when forelimb flexion was occurring (data not shown). DISCUSSION Various intensities of direct current ranging from 0.1 to 300 uA have been reported to be applied to the cerebral cortex to
POLARIZATION-INDUCED BEHAVIOR IN RABBITS
70
.A
60 50 40 30 c'- 20 LO 10 O9 (D
0 50
Or)
40
B
T
T
30 20 10 0
Left
Right
FIG. 2. Effects of anodal polarization on struggle of the left and right forelimbs in rabbits. Currents of 1 ~tA (A) and 10 t~A (B) were applied as described in the legend to Fig. 1. Forelimb struggle was assessed in the habituation period before polarization(open column), during (slanted line column), and after (dotted column) each polarization trial. Struggle in the habituation period was taken as the control. Data are expressed as the means _+SEM of the number of forelimb strugglesper 5 h in five (l #A group) or four (10 ~A group) different animals. *Significantdifference from before polarization, p < 0.05.
induce the change in peripheral motor behavior (3,5,13). However, the optimum current intensity has not yet been clarified because of wide variations in the experimental conditions, including the animal species used, the electrode locations, and the polarization schedule. In the present study, the experimental procedures were simplified and standardized; we found that flexion of the contralateral forelimb was profoundly increased by anodal polarization with 1/~A current of the premotor cortex. The increase in forelimb flexion lasted for at least 60 min after polarization; no such effect was observed when 10 #A was applied. These results suggest that 1 /~A is a better current level than 10 #A for the formation of a motor dominant focus under the present experimental conditions. Local changes in the activity of the rat cerebral cortex have been reported to be produced by direct current at intensities between 0.1 and 0.5/~A. Further, polarization with the currents
1199 exceeding 0.5 #A has been shown to result in the inhibition of neuronal activity (1). Such an inhibitory effect was also found in rabbits when current was too strong or the polarization trials were too frequent (14). In view of these findings, our present study revealed that polarization reduced motor activity, in particular, struggle movements, when the current was too strong; struggle was decreased by polarization with 10/~A current, but not with 1 #A current. The rabbits were really silent and drowsy, which was clearly confirmed by the video review. The inhibitory effect of 10/~A polarization was also supported by EEG findings. In our separate experiments, flexion of the contralateral forelimb seemed to reach a plateau in the third to fifth polarization trials. In addition, no lateral difference was observed in changes of forelimb struggle and EEG activity induced by 10 ~A, in contrast to the increase in contralateral flexion induced by 1 ~tA. Therefore, the passage of current of that level through the cortex appears to exert an inhibitory effect, although the mechanism remains to be clarified. The cerebral impedance and current density have been studied in several species of animals, like rabbits. Studies using the electrodes on the scalp revealed that the current entering the brain was 42-45% of the total applied current, and its largest density was in the area near the test electrode (8,12,15). Taken together with these findings, it should be noted that the current passing through the brain is approximately one-half or more of the applied current when the polarizing electrode is set in the cranial bone as carried out in the present study. Further, the current density is thought to be largest on the premotor cortex below the polarizing electrode. In this context, it is conceivable that the effects of current on the target cortical area result in the behavioral changes observed in this study. Regarding the mechanism producing a motor dominant focus, attempts have been made to determine the possible involvement of cyclic AMP-generating systems in the cortex (4). Differences in the intensity and duration of the polarizing current have been shown to alter the generation of cyclic AMP. The in vitro noradrenaline-stimulated accumulation of cyclic AMP was increased in cortical tissues polarized with 0.3 ~A current for 90 min or with 3.0 #A for 30 min, while no change was observed with 30 #A for 30 min or more (9). More direct evidence for biphasic effects of polarizing currents was provided in adenosinestimulated cyclic AMP accumulation in the cortex (3). Changes in cyclic AMP generation are likely to be part of the neurochemical basis underlying the cortical and behavioral activity induced by anodal polarization. In conclusion, unilateral anodal polarization of the premotor cortex with 1 ~tA current clearly promoted flexions of the contralateral forelimb, while polarization with 10 #A current resulted in a decrease in struggle. Thus, it is likely that the polarizing current of 1 #A produces a local increase in cortical activity by which peripheral motor activity is subsequently increased, while 10 ~A current inhibits both cortical and motor activity.
REFERENCES 1. Bindman, L. J.; Lippold, O. C. J.; Redfearn, J. W. T. Long-lasting changes in the level of the electrical activity of the cerebral cortex produced by polarizing currents. Nature 196:584-585; 1962. 2. Bindman, L. J.; Lippold, O. C. J.; Redfearn, J. W. T. The action of brief polarizingcurrents on the cerebral cortex of the rat (1) during current flow and (2) in the production of long-lastingafter-effects. J. Physiol. 172:369-382; 1964. 3. Hattori, Y.; Moriwaki, A.; Hori, Y. Biphasic effects of polarizing current on adenosine-sensitivegeneration of cyclic AMP in rat cerebral cortex. Neurosci. Lett. 116:320-324; 1990.
4. Hattori, Y.; Moriwaki, A.; Pavlygina, R. A.; Hori, Y. Regional difference in the histamine-elicited accumulation of cyclic AMP in rabbit cerebral cortex with a cortical dominant focus. Brain Res. 279:308-310; 1983. 5. Hayashi, Y.; Hattori, Y.; Moriwaki, A.; Asaki, H.; Hori, Y. Effects of prolonged weak anodal direct current on electrocorticogram in awake rabbit. Acta. Med. Okayama 42:293-296; 1988. 6. Hori, Y.; Yamaguchi, K. On the localization of the motor representation of contralateral forelimb in the motor cortex of rabbit. Osaka Daigaku Igaku Zasshi 26:59-64; 1974 (in Japanese).
1200
7. ttori, Y.; Yamaguchi, K. Prolonged formation of a cortical dominant focus by anodal polarization. Med..I. Osaka Univ. 26:2738: 1975. 8. Jarzembski, W. B.; Larson, S. J.; Sances, A., Jr. Evaluation ofspecitic cerebral impedance and cerebral current density. Ann. NY Acad. Sci. 170:476-490; 1970. 9. Moriwaki, A. Polarizing currents increase noradrenaline-elicited accumulation of cyclic AMP in rat cerebral cortex. Brain Res. 544: 248-252: 1991. 10. Morrell, F. Effect of anodal polarization on the firing pattern of single cortical cells. Ann. NY Acad. Sci. 92:860-876; 1961. 11. Novikova, L. A.; Rusinov, V. S.; Semiokhina, A. F. Electrophysiological analysis of closing function in the cerebral cortex of rabbit
{
12. t3. 14.
15. 16.
il
\i
in the presence of a dominant ibcus. Zh, Vyssfi. Ncrv. i)eiat. 2:844 861: 1952. Rush, S.: Driscoll. D. A. Current distribution in the brain fl-om surface electrodes. Anesth. Analg. 47:717-723:1968 Rusinov. V. S. The dominant focus: Electrophysiological investigations. New York: Consultants Bureau; 1973. Sokolova, A. A.; Bu, K. S. Electrophysiological study of the dominant area in the cerebral cortex of a rabbit produced by the action of a continuous current. Zh. Vyssh. Nerv. Deiat. 7:135-145; 1957. Van Harreveld, A.; Murphy, T.; Nobel, K. W. Specific impedance of rabbit's cortical tissue. Am. J. Physiol. 205:203-207: 1963. Yamaguchi, K.; Hori, Y. Long-lasting retention of cortical dominant focus in rabbit. Med. J. Osaka Univ. 26:39-50: 1975.
0031-9384/92 $5.00 + .00 Copyright © 1992PergamonPress Ltd.
Printed in the USA.
BRIEF COMMUNICATION
Further Characterization of Cortical Polarization-Induced Motor Behavior in Rabbits YUN-FEI LU, YUKIO
HATTORI,
YASUSHI HAYASHI, AKIYOSHI
MORIWAKI
AND YASUO HORI l
Department of Physiology, Okayama University Medical School, 2-5-1 Shikata-cho, Okayama 700, Japan R e c e i v e d 26 N o v e m b e r 1991 LU, Y.-F., Y. HATTORI, Y. HAYASHI, A. MORIWAKI AND Y. HORI. Furthercharacterization of corticalpolarizationinduced motor behavior in rabbits. PHYSIOL BEHAV 52(6) 1197-1200, 1992.--Anodal direct currents of 1 or 10 tLA were unilaterally applied for 30 min once a day to the premotor area of the cerebral cortex in rabbits, in which the current application was repeated 10 times at intervals of 2-3 days. Peripheral motor behavior was observed during and after each polarization trial, and was compared with that before polarization. The motor manifestations were classified into two types: gentle flexion of either forelimb and struggle with violent movement of forelimbs. Flexion of the forelimb contralateral to the polarized cortical side was clearly increased by polarization at an intensity of 1 pA, but not at 10 ttA. Forelimb struggle of both sides decreased only when 10 #A was applied. These results suggest that the passage of 1 #A current activates the polarized local area of the cortex, while l0/zA has an inhibitory effect on cortical activity. Polarizing current
Anodal polarization
Motor behavior
CEREBRAL cortical activity has been shown to be enhanced by the passage of anodal direct current to the cerebral cortex (1,2,10). The excitation focus which is possibly formed in the cortex by the anodal polarization has been called a d o m i n a n t focus (13). An increase of forelimb flexion, especially of the forelimb contralateral to the polarized cortex, has been regarded as one of the behavioral indices for determining whether or not a d o m i n a n t focus is formed in the cortical motor area (11,13). This characteristic behavior has been reported to persist consistently for several hours (11) or even as long as several weeks (7,14,16) once it begins. The p h e n o m e n o n is of considerable interest, both because it produces a long-lasting change in brain activity by a small temporary alteration in the physical environment of the nerve cells (1) and because it helps us to understand the mechanisms of m e m o r y trace formation and learning, and possibly also the process of conditioning (2). Studies have been made to clarify the effects of polarizing currents at intensities from 0.1 (1) to 300 t~A (5) on neuronal and behavioral activity in several species of animals. The optim u m intensity is not yet certain, although it has been found that current intensity is quite critical because establishing the motor manifestations depends upon slight increases or decreases of the current flow (10,13). Therefore, the use of consistent intensities and schedules for anodal polarization is important to avoid the complexity resulting from variations in experimental
Flexion
Struggle
Cerebral cortex
Rabbit
conditions. In this study, direct current at an intensity of 1 or 10 #A was applied 10 times to the premotor area of the rabbit cerebral cortex, and the effects were characterized in relation to peripheral motor behavior that was divided into flexion and struggle.
METHOD
Surgery Male rabbits weighing about 3 kg were used. The surgical procedures were essentially the same as those described previously (7). Briefly, the animals were lightly anesthetized with an intravenous injection of sodium pentobarbital (40 mg/kg). Four silver electrodes (1 m m in diameter) were implanted into the cranial bone so that the tip was set in the bone. Two of them were located 3 m m lateral to the bregma, and the other two were 7 m m rostral and 3 m m lateral to the bregma over the premotor cortex. Six stainless steel electrodes (1.5 m m in diameter) were implanted into the frontal, parietal, and occipital bones to record the electroencephalogram (EEG). All the electrodes were connected to the pins of a miniature socket and fixed with dental resin. The animals were allowed to recover from surgery for at least 10 days before use in the experiments.
i Requests for reprints should be addressed to Yasuo Hori, M.D., D.M.S., Department of Physiology, Okayama University Medical School, 2-51 Shikata-cho, Okayama 700, Japan.
1197
1198
t!( , \ i .
A m~dal Polarization
14
Rabbits were restrained in a wooden pillory on the experimental stand. The hindlimbs were fastened tightly to the stand. The forelimbs were stretched loosely to allow movement or flexing, and connected to a mechanograph for recording flexion and struggle movements. Before polarization, the rabbits were habituated to the experimental environment for 5 days (60 min per day). No current was applied during the habituation period. For polarization, direct current from the anode of a battery was passed through the silver electrode to the cortical point over the left premotor area which has been demonstrated to be appropriate for establishing the target motor behavior (6,16). A silver plate attached to the left ear lobe was connected to the cathode as a counter electrode. Direct current at an intensity of 1 or 10 ~A was applied for 30 min once a day to separate groups of rabbits. In order to avoid an overintensity and a sudden oppositely directed gradient, the current flow was switched on with a m i n i m u m intensity and then turned to the prescribed intensity, and tapered off'. The polarization trial was repeated 10 times at intervals of 2-3 days. The whole polarization schedule including the habituation period was completed within 6 weeks.
A
T
12 10 8
1
6 0 tLO CO tO X d.) LL
4 2 0
B
Behavioral and EEG Recording Motor behavior was observed in the habituation period, as well as during (30 min) and after (60 min) each polarization trial. Forelimb movements, flexion, and struggle, were recorded with the mechanograph. The behavior of the animals was also captured by a videocamera for later replay and analysis. EEGs were recorded continuously throughout the experiments.
Data Ana(v.s'is The various movements of the animals were examined and counted from the mechanogram recordings, and also confirmed with the video recording with lower speed replay. Flexions of the left and right forelimbs were counted separately. Movement of the forelimbs was evaluated as a single episode of flexion when the forelimb was lifted up and replaced. The forelimb struggles were also counted in the same way. Flexions and struggles during polarization (30 min) and after polarization (60 min) were accumulated in 10 experimental trials, in which the values after polarization were converted to those per 5 h. Data were expressed as the means + SEM. The two-tailed M a n n - W h i t n e y U-test was used for statistical analysis and a p value < 0.05 was considered significant. Movements in the habituation period were used as the control. RESULTS The motor behavior observed in these experiments was classified as flexion and struggle. Flexion was defined as gentle, slow, and smooth movements of the forelimbs without an associated change in posture. Struggle was defined as violent, quick, and strong limb movements which were usually accompanied with movement of the head and trunk. Based on these criteria, flexion and struggle were differentiated from both the mechanograms and the video replay. The flexion appeared as single or several episodes with intermission, and the struggle showed a series of highly frequent movements. The frequency of flexion of the left and right forelimbs is shown in Fig. 1. When the left premotor area was polarized at 1 #A, flexion of the fight forelimb (contralateral to the polarized side) was increased significantly during current application, as compared with the habituation (control) period. An increase in flexion of the right forelimb was also detected for 60 min after
Left
Right
FIG. 1. Effects of anodal polarization on flexion of the left and fight forelimbs in rabbits. Polarizing currents of I ~A (A) and 10 ~A (B) were applied to the premotor cortical area. Current application for 30 rain was repeated l0 times at intervals of 2-3 days. The frequency of flexion of the forelimbs was determined in the habituation period before polarization (open column), during (slanted line column), and after (dotted column) each trial of the anodal polarizations. Flexion in the habituation period was taken as the control. The control value was 0 in l0 uA polarization group. Data are expressed as the means _+SEM of the number of flexions per 5 h in five (1 t~A group) or four (10 gA group) different animals. *Significant difference from before polarization, p < 0.05.
polarization. Flexion of the left forelimb (the ipsilateral one) did not change either during or after polarization. When 10 #A was applied, no significant difference was found between polarizing and control period. After polarization with l0 ~tA, there was a tendency for the right forelimb flexion to increase, but it was not significant. The frequency of forelimb struggles is shown in Fig. 2. When l #A current was applied, struggles showed no difference from the control period, either during or after polarization. When l0 uA was applied, struggles were decreased to a significantly lower level both during and after polarization than in the control period. No significant difference was observed between left and right sides in any experimental periods. The EEGs showed low activity during anodal polarization with 10 yA current, but recovered within about 10 m i n after the current was switched off. Such inhibitory effect was observed in the EEGs recorded from both the left and right hemispheres, and there was no obvious difference between them. In the case of polarization with 1 ~tA current, there was no change in the EEG activity even when forelimb flexion was occurring (data not shown). DISCUSSION Various intensities of direct current ranging from 0.1 to 300 uA have been reported to be applied to the cerebral cortex to
POLARIZATION-INDUCED BEHAVIOR IN RABBITS
70
.A
60 50 40 30 c'- 20 LO 10 O9 (D
0 50
Or)
40
B
T
T
30 20 10 0
Left
Right
FIG. 2. Effects of anodal polarization on struggle of the left and right forelimbs in rabbits. Currents of 1 ~tA (A) and 10 t~A (B) were applied as described in the legend to Fig. 1. Forelimb struggle was assessed in the habituation period before polarization(open column), during (slanted line column), and after (dotted column) each polarization trial. Struggle in the habituation period was taken as the control. Data are expressed as the means _+SEM of the number of forelimb strugglesper 5 h in five (l #A group) or four (10 ~A group) different animals. *Significantdifference from before polarization, p < 0.05.
induce the change in peripheral motor behavior (3,5,13). However, the optimum current intensity has not yet been clarified because of wide variations in the experimental conditions, including the animal species used, the electrode locations, and the polarization schedule. In the present study, the experimental procedures were simplified and standardized; we found that flexion of the contralateral forelimb was profoundly increased by anodal polarization with 1/~A current of the premotor cortex. The increase in forelimb flexion lasted for at least 60 min after polarization; no such effect was observed when 10 #A was applied. These results suggest that 1 /~A is a better current level than 10 #A for the formation of a motor dominant focus under the present experimental conditions. Local changes in the activity of the rat cerebral cortex have been reported to be produced by direct current at intensities between 0.1 and 0.5/~A. Further, polarization with the currents
1199 exceeding 0.5 #A has been shown to result in the inhibition of neuronal activity (1). Such an inhibitory effect was also found in rabbits when current was too strong or the polarization trials were too frequent (14). In view of these findings, our present study revealed that polarization reduced motor activity, in particular, struggle movements, when the current was too strong; struggle was decreased by polarization with 10/~A current, but not with 1 #A current. The rabbits were really silent and drowsy, which was clearly confirmed by the video review. The inhibitory effect of 10/~A polarization was also supported by EEG findings. In our separate experiments, flexion of the contralateral forelimb seemed to reach a plateau in the third to fifth polarization trials. In addition, no lateral difference was observed in changes of forelimb struggle and EEG activity induced by 10 ~A, in contrast to the increase in contralateral flexion induced by 1 ~tA. Therefore, the passage of current of that level through the cortex appears to exert an inhibitory effect, although the mechanism remains to be clarified. The cerebral impedance and current density have been studied in several species of animals, like rabbits. Studies using the electrodes on the scalp revealed that the current entering the brain was 42-45% of the total applied current, and its largest density was in the area near the test electrode (8,12,15). Taken together with these findings, it should be noted that the current passing through the brain is approximately one-half or more of the applied current when the polarizing electrode is set in the cranial bone as carried out in the present study. Further, the current density is thought to be largest on the premotor cortex below the polarizing electrode. In this context, it is conceivable that the effects of current on the target cortical area result in the behavioral changes observed in this study. Regarding the mechanism producing a motor dominant focus, attempts have been made to determine the possible involvement of cyclic AMP-generating systems in the cortex (4). Differences in the intensity and duration of the polarizing current have been shown to alter the generation of cyclic AMP. The in vitro noradrenaline-stimulated accumulation of cyclic AMP was increased in cortical tissues polarized with 0.3 ~A current for 90 min or with 3.0 #A for 30 min, while no change was observed with 30 #A for 30 min or more (9). More direct evidence for biphasic effects of polarizing currents was provided in adenosinestimulated cyclic AMP accumulation in the cortex (3). Changes in cyclic AMP generation are likely to be part of the neurochemical basis underlying the cortical and behavioral activity induced by anodal polarization. In conclusion, unilateral anodal polarization of the premotor cortex with 1 ~tA current clearly promoted flexions of the contralateral forelimb, while polarization with 10 #A current resulted in a decrease in struggle. Thus, it is likely that the polarizing current of 1 #A produces a local increase in cortical activity by which peripheral motor activity is subsequently increased, while 10 ~A current inhibits both cortical and motor activity.
REFERENCES 1. Bindman, L. J.; Lippold, O. C. J.; Redfearn, J. W. T. Long-lasting changes in the level of the electrical activity of the cerebral cortex produced by polarizing currents. Nature 196:584-585; 1962. 2. Bindman, L. J.; Lippold, O. C. J.; Redfearn, J. W. T. The action of brief polarizingcurrents on the cerebral cortex of the rat (1) during current flow and (2) in the production of long-lastingafter-effects. J. Physiol. 172:369-382; 1964. 3. Hattori, Y.; Moriwaki, A.; Hori, Y. Biphasic effects of polarizing current on adenosine-sensitivegeneration of cyclic AMP in rat cerebral cortex. Neurosci. Lett. 116:320-324; 1990.
4. Hattori, Y.; Moriwaki, A.; Pavlygina, R. A.; Hori, Y. Regional difference in the histamine-elicited accumulation of cyclic AMP in rabbit cerebral cortex with a cortical dominant focus. Brain Res. 279:308-310; 1983. 5. Hayashi, Y.; Hattori, Y.; Moriwaki, A.; Asaki, H.; Hori, Y. Effects of prolonged weak anodal direct current on electrocorticogram in awake rabbit. Acta. Med. Okayama 42:293-296; 1988. 6. Hori, Y.; Yamaguchi, K. On the localization of the motor representation of contralateral forelimb in the motor cortex of rabbit. Osaka Daigaku Igaku Zasshi 26:59-64; 1974 (in Japanese).
1200
7. ttori, Y.; Yamaguchi, K. Prolonged formation of a cortical dominant focus by anodal polarization. Med..I. Osaka Univ. 26:2738: 1975. 8. Jarzembski, W. B.; Larson, S. J.; Sances, A., Jr. Evaluation ofspecitic cerebral impedance and cerebral current density. Ann. NY Acad. Sci. 170:476-490; 1970. 9. Moriwaki, A. Polarizing currents increase noradrenaline-elicited accumulation of cyclic AMP in rat cerebral cortex. Brain Res. 544: 248-252: 1991. 10. Morrell, F. Effect of anodal polarization on the firing pattern of single cortical cells. Ann. NY Acad. Sci. 92:860-876; 1961. 11. Novikova, L. A.; Rusinov, V. S.; Semiokhina, A. F. Electrophysiological analysis of closing function in the cerebral cortex of rabbit
{
12. t3. 14.
15. 16.
il
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