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Cortical stimulation and suppression of activity in the rat
Physiology & Behavior, Vol. 23, pp. 1041-1048.PergamonPress and BrainResearch Publ., 1979. Printedin the U.S.A.
Cortical Stimulation and Suppression of Activity in the Rat R. C. W I L C O T T 1
D e p a r t m e n t o f Psychology, Case Western Reserve University, Cleveland, O H 44106 R e c e i v e d 4 M a y 1979 WILCOTT, R. C. Cortical stimulation and suppression of activity in the rat. PHYSIOL. BEHAV. 23(6) 1041-1048, 1979.--Low-frequency electrical stimulation at sites in the frontal cortex of the rat will suppress running in an activity wheel, but will have no effect on general activity measured in a stabilimeter. This confirms a prediction made from the effects of frontal lesions on these two kinds of activity. No evidence of abnormality in the EEG was observed following frontal stimulation. Stimulation in the frontal cortex probably influences the same inhibitory processes that are modified by frontal lesions. Further results indicate that the differential effects of frontal stimulation on running and general activity may be partly due to differences in response effort required. Frontal cortex Rat
Electrical stimulation
Suppression of activity
INCREASED skeletal and autonomic activity, and reactivity, are usually observed following removal of parts of the frontal cortex, including the prefrontal area, in primates, dogs, and cats Ill. In the rat skeletal activity is usually increased following removal of comparable parts of the frontal cortex [9,13]. These results seem to be due to the release of inhibition normally maintained by the frontal cortex. The apparent inhibitory influence of the frontal cortex in the rat has also been demonstrated by electrical stimulation, particularly at a low frequency. Frontal stimulation will suppress the heart rate response to a noxious stimulus [7], and the bar-press response for food [161. Effective cortical sites for suppression of bar-pressing were found in the frontal pole and over most of the frontal dorsolateral cortex. The distribution of these sites generally agrees with the inhibitory areas established in lesion studies [9,13], but inhibitory processes appear to be more widespread in the dorsolateral cortex than these studies indicated. The report describes three experiments which were intended to further explore the nature of behavioral suppression produced by frontal stimulation in the rat. EXPERIMENT 1 Results obtained in studies with frontal lesions and frontal stimulation raise the question of whether these two methods influence the same inhibitory processes in the frontal cortex. One way to test this would be to determine the effects of low frequency stimulation in the frontal cortex on skeletal activity. In the rat frontal lesions consistently increase running in an activity wheel, but have little or no effect on general activity in a stabilimeter [4] or similar situations such as an open field [8]. If frontal stimulation influences the same
Electroencephalogram
Response effort
cortical processes that are altered by frontal lesions in the rat, then frontal stimulation should have a greater suppression effect on running in an activity wheel than activity in a stabilimeter. This prediction was tested in this experiment.
METHOD
Animals Results were obtained from seven Holtzman male rats, weighing 375-400 g at the time of surgery.
Cortical Stimulation Electrodes (Plastic Products Co.) consisted of insulated stainless steel wires, 0.2 mm in diameter, wound together with only the cut tips exposed. Implant procedures were the same as previously described [16]. For three rats electrodes were positioned within the frontal pole, and for four rats they were positioned slightly below the surface of the dorsal frontal cortex (Fig. l). These sites were chosen because stimulation in this area had been found to produce strong suppression of the bar-press response for food [16]. Each rat also had an electrode in an area of the posterior dorsal cortex where stimulation is known to produce little or no response suppression. Three rats were implanted unilaterally, and four rats were implanted bilaterally. For the latter, electrodes were implanted at the same site on each side as closely as possible. Bilateral electrodes in the frontal cortex were in the frontal pole of one rat, and at the dorsal cortex of three rats. At necropsy perfused brains were examined for locations of dorsal cortex electrodes, and the locations of frontal pole electrodes were identified from frozen brain sections. Frozen sections were also prepared through elec-
~Send reprint requests to R. C. Wilcott, Department of Psychology, Case Western Reserve University, Cleveland, OH 44106.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/79/121041-08502.00/1
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WILCOTT Activity Wheel
B
FIG. I. Circles mark stimulation electrode sites within the frontal pole (left) for Experiment 1. The cross section of the frontal pole is about 6 mm anterior to the bregma. All stimulation sites at the frontal or posterior dorsal cortex (right) in Experiment 1 were in shaded areas. In Experiments 2 and 3, stimulation electrode sites were in the frontal shaded area only. Where bilateral stimulation was used electrodes were approximately at the same site on the two sides. Circles (right) mark the locations of EEG electrodes over the anterior and posterior cortex in Experiment 2. The EEG electrode near the bregma (B) was used as a common reference site. The anterior EEG electrode was as close as possible to the anterior stimulation electrode. trode sites at the dorsal cortex of two rats and in each instance it was found that electrode tips at both the frontal and posterior cortex were within cortical gray matter. Stimulation was done with a Grass $44 Stimulator feeding through a Grass SIU5 Stimulus Isolation Unit and a Grass Constant Current Unit. T w o stimulators and associated e q u i p m e n t were used for bilateral stimulation. Square w a v e pulses were 2 msec in duration and at a frequency of 10/sec. F o r unilateral stimulation current was adjusted at each cortical site by the same procedures used earlier [16]. Current was set just below a m a x i m u m level that n e v e r p r o d u c e d m o v e m e n t s or an arousal response. This point was ascertained at each cortical site by gradually increasing stimulus current, usually in steps of 0.01 or 0.02 mA, until m o v e m e n t s or an arousal response w e r e observed. Current was then reduced to a level where it seemed certain that no m o v e m e n t s , etc., would be produced during repeated testing. At some nonsuppression sites m o v e m e n t s , etc., were not o b s e r v e d and a maximum current of 0.25 m A was used. F o r bilateral stimulation current was adjusted separately on each side, and w h e n presented bilaterally, further adjusted to near a m a x i m u m level where it was certain that m o v e m e n t s , etc., would not be produced. To partially control for possible transfer effects, four rats were first tested for suppression of running activity and then tested for suppression of stabilimeter activity, The other three rats were tested in the opposite order.
A standard size activity wheel, 14 in. in diameter and 5 in. wide, was constructed for this study. It was made from 1/4 in. wire mesh and was pivoted at one side by a steel plate mounted onto a shaft which r e v o l v e d on ball bearings. The wheel was housed inside an Industrial Acoustics C h a m b e r fitted with a one-way window. The rat was placed in the wheel through a small door on the lower front side of the wheel. E l e c t r o d e leads, c o v e r e d by a steel spring passed up and out through a small hole in the center of the wheel. They were plugged into a swivel which was mounted just outside of the center hole. The rat was free to run in either direction with minimum interference from the leads. Small round pegs were fitted on the outer surface of the wheel so that as it turned they tripped a microswitch. These pegs were mounted on opposite sides of the wheel such that the switch was tripped at each half turn of the wheel. Tripping of the switch was registered on one channel of a Bechman Type R Dynograph recorder. To insure relatively high activity, body weight was reduced to 80% of its level with ad libitum feeding, and rats were tested just before their daily feeding. They were habituated to the wheel for two 30-min sessions on c o n s e c u t i v e days before experimental testing. Eighteen trials were given with stimulation at a suppression site and 18 trials with stimulation at a nonsuppression site. T h e y were presented in a randomized order along with 18 nonstimulation trials o v e r 4-5 days. Trials were 10 sec in duration. They were presented when the rat had been running at a rate sufficient to produce at least three half turns of the wheel during a 10-sec interval. A trial began immediately after a half turn of the wheel. An all-or-none criterion for response suppression was used. This was where the rat stopped running after trial onset and before the next half turn was registered, and did not start running again during that trial interval. Time between trials was n e v e r less than 1 rain. Stabilimeter A loudspeaker type stabilimeter, originally intended for the cat, was used. A 12-in. speaker was mounted in the center of a 0.5 in.-thick plywood board and this was c e m e n t e d to one side of a 2 4 × 2 4 x l . 5 - i n . wood frame. A sheet metal c o v e r was stretched o v e r the other side of the frame and c e m e n t e d in place. The space b e t w e e n the speaker cone and the sheet metal was air tight so that any vibrations of the sheet metal were transferred to the speaker cone. A standard single rat cage, fitted with an aluminum c o v e r and resting on metal pegs at each corner, was placed in the center of the stabilimeter. The entire unit was housed in the Industrial Acoustics Chamber. The speaker coil was coupled to two channels of the Bechman recorder. One channel was for direct recording, and the second was coupled through a Backman E M G Integrator (Fig. 2). This stabilimeter registered the onset and magnitude of m o v e m e n t s , but it did not accurately display the time a m o v e m e n t ended. The reason was that the system continued to vibrate for short time after the rat stopped moving. But this error was relatively small and constant and did not appear to present a serious problem in the way these stabilimeter data were analyzed in this study. Procedures used during testing were the same as those used with the activity wheel, including body weight, and
S U P P R E S S I O N OF A C T I V I T Y
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FIG. 2. A, examples of the recording of running in the activity wheel with low frequency stimulation (10 sec) at a suppressmn site, left, and with stimulation at a nonsuppression site. B, direct recording and integrator recording of general activity in a stabilimeter with stimulation (10 sec) at a suppression site.
number of trials. Trials were given when activity was clearly registered by the recorder, and when it had been at about the same level for at least 10 sec prior to trial onset. Data were obtained over a 3-4 day period. The magnitude of activity was measured from the integrator recording and specified as the area under the curve in square cm as measured by a planimeter. An adjacent pen marked a baseline (Fig. 2). Activity during the 10-sec trial was compared with activity during the 10-sec interval immediately preceding each trial. Each 10-sec interval was marked on the integrator records by vertical lines extending
from the baseline. These vertical lines were boundaries for measuring the area under the curve.
RESULTS
Activity Wheel Electrode sites at the frontal suppression area and at the posterior nonsuppression area are given in Fig. 1. Activity wheel data are presented in Table 1. F or each rat, unilateral stimulation at a suppression site produced a
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WILCOTT TABLE 1 SUPPRESSION OF RUNNING ACTIVITY
~2
Rat
A Suppression site*
B Nonsuppression site*
C Nonstimulation trials*
A-C
B-C
1 2 3 4 5 6 7
16 (0.07) 15 (0.07) 16 (0.09) 14 (0.06) 13 (0.07) 15 (0.07) 14 (0.08)
6 (0.1)91 6 (0.141 5 (0.25) 4 (0.141 4 (0.10) 7 (0.131 5 (0.141
5 6 4 5 5 7 6
33,50+ 20.25+ 46.29 + 22.43+ 17.72+ 14.95+ 15.99I
0.2775 0.960¢
*Suppression of running activity during stimulation at a suppression site (A), and nonsuppression site (B) or on nonstimulation trials (C), and X'-'between these conditions. Eighteen trials were given under each condition, and values shown are the number of trials where the rat stopped running according to the all-or-none criterion used. Stimulation current in mA at suppression and nonsuppression sites is given in parentheses. +p<0.001. $p>0.050. TABLE 2 SUPPRESSION OF STABILIMETER ACTIVITY Stimulation trials* Before SD
Rat
M
1 2 3 4 5 6 7
1.47 1.51 1.73 1.36 1.45 1.74 1.67
0.417 0.529 0.617 0.427 0.412 0.590 0.616
M
During SD
•40 .31 •71 •30 .39 .70 .65
0.405 0.397 0.592 0.379 0.423 0.598 0.609
Nonstimulation trials* Before SD
t+
M
0.7743 0.8734 0.5682 0.7141 0.7225 0.5819 0.4765
1.50 1.43 1.67 1.41 1.45 1.66 1.59
0.498 0.407 0.555 0.427 0.407 0.588 0.627
M
During SD
1.35 1.46 1.60 1.40 1.41 1.62 1.61
I}.487 11.514 0.582 0.491 0.431 I).529 0.611
r; 0.6551 0.7312 0.3198 0.5213 0.5917
*Means (M) and standard deviations (SD) of stabilimeter activity scores before and during stimulation at a suppression site and for nonstimulation trials, and t-tests between these two conditions. A stabilimeter score was derived from the area under the curve in square cm of the integrator recording. Means were computed from 18 trials. ~p<0.050 for all t values. significantly g r e a t e r s u p p r e s s i o n of r u n n i n g t h a n t h a t f o u n d d u r i n g n o n s t i m u l a t i o n trials. S t i m u l a t i o n at n o n s u p p r e s s i o n sites did n o t h a v e a significant effect. E x a m p l e s of activity w h e e l r e c o r d i n g s are g i v e n in Fig. 2. D u r i n g s u p p r e s s i o n o f r u n n i n g t h e b e h a v i o r of the rat did not a p p e a r to be different t h a n w h e n it s t o p p e d r u n n i n g s p o n t a n e o u s l y . It would turn a r o u n d in the wheel, s t a n d on its hind legs a n d e x p l o r e the wheel, g r o o m itself, etc. R a t s usually r e s u m e d r u n n i n g within 5-15 sec following t e r m i n a tion of s t i m u l a t i o n at a s u p p r e s s i o n site.
Stabilimeter U n i l a t e r a l s t i m u l a t i o n at a s u p p r e s s i o n site did not prod u c e a significant effect on s t a b i l i m e t e r activity with a n y of the rats t e s t e d . T h e s e d a t a are g i v e n in T a b l e 2. B e c a u s e s t i m u l a t i o n did not h a v e a significant effect, only d a t a for s t i m u l a t i o n at a s u p p r e s s i o n site a n d for n o n s t i m u l a t i o n trials are given. A n e x a m p l e of s t a b i l i m e t e r r e c o r d i n g is s h o w n in Fig. 2.
F o u r rats with bilateral e l e c t r o d e i m p l a n t s (Rats 4 - 7 in T a b l e 2) w e r e also t e s t e d with bilateral stimulation. Results w e r e similar to t h o s e in T a b l e 2 a n d w e r e n o t significant. EXPERIMENT
2
R e s p o n s e s u p p r e s s i o n p r o d u c e d by low f r e q u e n c y stimulation in the frontal c o r t e x o c c u r s m a i n l y d u r i n g the time s t i m u l a t i o n is p r e s e n t e d and this suggests that the s u p p r e s sion could b e due to d i s r u p t i o n of neural activity in the area of stimulation. B u t no e v i d e n c e for this is a p p a r e n t f r o m the b e h a v i o r of rats d u r i n g b e h a v i o r a l s u p p r e s s i o n . F o r e x a m ple, d u r i n g c o m p l e t e s u p p r e s s i o n of the b a r - p r e s s r e s p o n s e for food rats a p p e a r to m o v e a b o u t n o r m a l l y , g r o o m t h e m selves a n d display a p p r o p r i a t e orienting r e s p o n s e s to alerting stimuli [16]. Similar o b s e r v a t i o n s w e r e m a d e d u r i n g activity s u p p r e s s i o n in E x p e r i m e n t 1. A n o t h e r way to investigate possible d i s r u p t i v e effects o f frontal stimulation is to d e t e r m i n e the effects of frontal stimulation on t h e e l e c t r o e n c e p h a l o g r a m ( E E G ) . While a
SUPPRESSION OF ACTIVITY
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FIG. 3. Examples of EEG recorded from the anterior cortex (A) and posterior cortex (P) before and after low frequency stimulation for 30 sec at a suppression site. Calibration is 1 sec and 50/.LV.
readable EEG record cannot usually be obtained during stimulation, an abnormal EEG in the form of an epileptiform after-discharge has been observed following cortical stimulation [12]. Spikes or spike and wave discharges can be present following stimulation in various parts of the cortex, including the frontal region, in the monkey [5] and cat [6]. An after-discharge consisting of 3/sec slow waves has also been observed following frontal stimulation in the cat [14]. These after-discharge patterns appear to be a continuation of abnormal activity produced during stimulation, and they sometimes can be obtained without observable behavioral effects. After-discharges occur best to stimulation frequencies above 25/sec [12], and this phenomenon seems to be unrelated to the behavioral suppression produced by low frequency stimulation. Stimulation at frequencies of 7-10/sec in the frontal cortex of the cat can produce strong behavioral inhibition, and no abnormality in the EEG has been reported 15,101. The effects of low frequency frontal stimulation on the EEG of the rat have apparently not been investigated; and because of the importance of this problem, the EEG following frontal stimulation was studied in Experiment 2. METHOD
Animals Six male Hoitzman rats, weighing 380--400 g at the time of surgery, were used.
Cortical Stimtdation Electrodes were implanted in the dorsal frontal cortex by the same procedures that were used in Experiment 1. Three rats were implanted unilaterally and three were implanted bilaterally. Stimulation equipment, and procedures used to specify stimulation current intensity, were the same as in Experiment 1.
EEG Recording Electrodes were small stainless steel machine screws mounted in the skull on one side. Three screws were positioned at the same approximate location for each rat. One screw was as close to the stimulation electrodes as possible, a second screw was near the bregma, and a third screw was near the visual area. Two-channel recording was done with the screw near the bregma serving as a common
site. Leads had been soldered to the heads of the screws prior to placement, and these leads were connected to sockets of the type used with the stimulation electrodes. A Beckman Type R Dynograph was used to record the EEG. During EEG recording rats were placed in a standard single cage with a cover, and the cage was housed inside the Industrial Acoustics Chamber. A four-channel mercury swivel was used for both recording and stimulation. Because stimulation train duration seems to be an important factor in the appearance of an EEG after-discharge [12], train duration was varied. Three durations were used: 3, 10, and 25 sec. The 3-sec duration was used because it was effective for the cat, and the 10- and 25-sec durations were tested because they have been used in the production of behavioral suppression in the rat (Experiment 1 and in a previous study, [16]). Ten trials were given with each train duration over a 3-day period. Time between trials was 3-4 rain. All rats were tested with unilateral stimulation, and three rats were later tested with bilateral stimulation. Following EEG recording, rats were tested for suppression of running in the activity wheel to insure that electrodes were at effective sites and to determine whether electrode sites had been damaged by excessive stimulation. Test procedures were the same as in Experiment 1. Suppression of running was clearly observed with each rat over 6--8 trials. This indicated that frontal stimulation was still effective for these rats.
RESULTS
The location of stimulation electrodes in the dorsal frontal cortex, and the location of EEG electrodes are given in Fig. I. Abnormal EEG activity was never observed in these rats following the 3-, 10-, or 25-sec stimulation train durations, and with either unilateral or bilateral stimulation. After each stimulation trial the EEG was not observably different than it was before stimulation. A section of a typical record is given in Fig. 3. Stimulation currents ranged from 0.07-0.09 mA. EXPERIMENT 3 A question raised by results of Experiment 1 is why frontal stimulation suppressed running in an activity wheel but did not suppress general activity in a stabilimeter? A possible answer may have to do with the amount of effort required. Running in an activity wheel probably requires
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WILCOTT
TABLE 3 SUPPRESSION OF BAR PRESSING
12/1 ratio~Rats 1 2 3 4 5 6
Before (0.08) (0.07) (0.05) (0.07) (0.07) (0.09)
2.74 2.65 2.90 2.74 2.83 2.79
(2-3) (2-3) (2-3) (2-3) (2-3) (2-3)
During 0.26 0.34 0.37 0.78 0.53 0.57
(0-1) (0-1) (0-2) (0-2) (0-1) (0-1)
3/1 ratiot Percent difference 91.5 86.4 88.2 75.6 82.6 79.5
Before 3.25 3.41 3.31 3.51 3.00 3.32
(3-4) (3-4) (2-4) (3-4) (2-4) (3-4)
During 0.89 1.51 1.12 1.68 0.98 I. 12
(0-1) (0-2) (0-2) (0-1) (1-2) (1-2)
Percent difference
t
71.5 53.8 67.3 52.1 68.3 66. l
1.829:!: 3.107q~ 2.175§ 2.351§ 1.7951.792~:
*Stimulation current used with each rat is given in parentheses. tMeans and ranges of number of reinforcements for the 25 sec intervals before and during frontal stimulation, and mean percent differences between these two intervals, with the 12/1 and 3/1 ratios, and t-tests between these mean percent differences. Means were computed from 14 trials. ~+p<0.050. §p<0.025. ~qp<0.010.
more effort than general activity, and behavior requiring a particular amount of effort may be more easily suppressed. Some support for this explanation is provided by the observation that during complete suppression of bar pressing for food, rats will still eat at the feeder when it is operated externally [16]. Bar-pressing probably requires more effort than eating food. This hypothesis was tested by determining the effects of frontal stimulation on the bar-press response for food where the effort required was varied by changing the ratio between bar pressing and reinforcement. Ratios of 3/1 and 12/1 were tested. If the hypothesis is correct, frontal stimulation should produce greater suppression of bar-pressing with the 12/1 ratio than with the 3/1 ratio. METHOD
Animals Eight Holtzman male rats were prepared for unilateral stimulation at a frontal suppression site in the same way as in Experiment 1. However, data were obtained from only six rats because two rats failed to respond consistently at the 12/1 ratio. Procedttre Rats were trained to bar-press for food (Noyes pellets) in a Gerbrand's Skinner box by procedures previously used [16]. Training was continued until a relatively stable response rate was obtained at the 12/1 ratio. Suppression of bar-pressing by frontal stimulation (Fig. 1) was then tested with stimulation presented for 25 sec. Currently intensity was raised to a level where it would produce at least a 7(F/b reduction in bar pressing without producing movements, etc. Seven trials were given at this current intensity. Next, the ratio was reduced to 3/1 and sufficient training given to assure a relatively stable rate of responding. Then 14 trials with frontal stimulation were given over a 2-day period at the same current intensity used with the 12/1 ratio. Finally, rats were returned to the 12/1 ratio, and after responding was stable, seven stimulation trials were again presented. Because a fixed ratio was used, the rat of bar-pressing was
measured as the number of reinforcements. At each test trial the magnitude of suppression was determined by comparing the number of reinforcements during the 25-sec stimulation interval with the number of reinforcements during the 25-sec interval immediately proceeding the stimulation interval. The mean magnitude of response suppression with the 14 trials at the 12/1 ratio was then compared with the mean magnitude of response suppression during the 14 trials at the 3/1 ratio. RESULTS
Electrodes were in the dorsal frontal cortex as shown in Fig. 1. Results are presented in Table 3. For each rat there was significantly greater response suppression with the 12/1 ratio than with the 3/1 ratio. Examples of response suppression at the 12/1 and 3/1 ratios are given in Fig. 4. DISCUSSION In Experiment I it was demonstrated that low frequency stimulation in the frontal cortex of the rat will suppress running in an activity wheel, but have no effect on general activity measured in a stabilimeter. This confirms the prediction made from the effects of frontal lesions on these two kinds of activity. This suggests that stimulation in the frontal cortex probably influences the same inhibitory processes that are modified by frontal lesions. The apparently normal EEG pattern recorded following frontal stimulation in Experiment 2 suggests that the response suppression produced was not due to production of abnormal cortical activity. These observations are not conclusive because EEG could not be recorded during stimulation, but they are in agreement with E E G data from the cat [10,15]. This problem needs further study in the rat. It is relevant to note that a rhythmical slow electrical activity in the hippocampus has been found to be related to running in the rat [2], and stimulation in the dorsal hippocampus that blocks this type of electrical activity will also suppress running [3]. The effects of frontal stimulation on hippocampal activity should also be investigated.
SUPPRESSION O F ACTIVITY
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FIG. 4. Examples of suppression of the bar-press response for food with a 12/1 (A) of 3/I (B) ratio by low frequency stimulation for 25 sec at a frontal suppression site. In each section of the figure the upper trace registers bar-pressing, the middle trace registers reinforcement, and the lower trace registers stimulation.
Results comparing the magnitudes of suppression of bar-pressing with the 12/1 and 3/1 ratios in Experiment 3 demonstrates that response effort can be an important factor determining the amount of response suppression produced by frontal stimulation. It seems likely, therefore, that the difference in the effects of frontal stimulation on running and general activity can be at least partly explained by the difference in effort required for these two types of activity. It should be noted, however, that frontal stimulation in Experiment 3 still produced a substantial suppression of barpressing with the 3/1 ratio even though this ratio required much less effort than the 12/1 ratio. This leads to the question of whether the difference in effort required for running and general activity can entirely account for the large difference in the effects of frontal stimulation on these two types of activity. Some additional variable may be involved. Response effort could also be a factor in the different effects of frontal lesions on running and general activity. According to this hypothesis, responses requiring the greatest effort would normally be the most inhibited, and following release of inhibition these responses would show the greatest increase. Following frontal lesions in the rat, however, stabilimeter activity as well as running in an activity wheel are sharply increased during starvation [4].
This does not invalidate the response effort explanation, but it does demonstrate that the effects of extreme food deprivation are more potent than other factors that may influence general activity in the frontal rat. It has been found that lesions in the inferior thalamic penduncle produced a greater increase in general activity in a stabilimeter than in running in an activity wheel when rats are tested during starvation [11]. This seems to suggest that the neural processes inhibiting stabilimeter activity are partially separate from those inhibiting running activity. But this would not rule out an inhibitory influence of the frontal cortex on both types of activity. In the absence of extreme conditions, such as starvation, the inhibitory effects of the frontal cortex may be strong enough to suppress running but not strong enough to suppress stabilimeter activity in the rat. It has been suggested that the frontal cortex serves to inhibit the ascending influence of the reticular formation, and thereby suppress arousal [4]. Increased activity in the frontal animal is thought to be due to increased arousal, and to increased reactivity to internal (hunger) and external stimuli. Data reported here are in accord with this hypothesis. But these data are also not opposed to the hypothesis that the frontal cortex has widespread inhibitory influences in the brain [151
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REFERENCES 1. Brutowski, S. Functions of the prefrontal cortex in animals. Physiol. Rev. 45: 721-746, 1965. 2. Bland, B. H. and C. H. Vanderwolf. Diencephalic and hippocampal mechanisms of motor activity in the rat: Effects of posterior hypothalamic stimulation on behavior and hippocampal slow wave activity. Brain Res. 43: 67-88, 1972. 3. Bland, B. H. and C. H. Vanderwolf. Electrical stimulation of the hippocampal formation: Behavioral and bioelectrical effects. Brain Res. 43: 8%105, 1972. 4. Campbell, B. A. and G. S. Lynch. Cortical modulation of spontaneous activity during hunger and thirst. J. comp. physiol. Psychol. 67: 15-22, 1%9. 5. French, J. D., B. E. Gernandt and R. B. Livingston. Regional differences in seizure susceptibility in monkey cortex. Am. Med. Ass. Archs Nettrol. Psychiat. 75: 260-274, 1956. 6. Garner, J. and J. D. French. Regional differences in seizure susceptibility in cat cortex. Am. Med. Ass. Archs Neurol. Psychiat. 80: 675-681, 1958. 7. Glasser, E. M. and J. P. Griffin. Influences of the cerebral cortex on habituation. J. Physiol.. Lond. 160: 42%445, 1%2. 8. Glickman, S. E., R. Scroges and J. Hunt. Brain lesions and locomotor exploration in the albino rat. J. comp. physiol. Psychol. 58: 93-100, 1964. 9. Kolb, B. Dissociation of the effects of lesions of the orbital or medial aspect of the prefrontal cortex of the rat with respect to activity. Behav. Biol. 10: 32%343, 1974.
10. Lineberry, C. G. and J. Siegel. EEG synchronization, behavioral inhibition, and mesencephalic unit effects produced by stimulation of orbital cortex, basal forebrain, and caudate nucleus. Brain Res. 34: 143-161, 1971. 11. Lynch, G. S. Separable forebrain systems controlling different manifestations of spontaneous activity..I, comp. physiol. Psychol. 70: 48-49, 1970. 12. Marsan, C. A. Focal electrical stimulation. In: Experimental Models ql'Epilepsy, edited by D. P. Purpura, J. K. Penry, D. B. Tower, D. M. Woodbury and R. D. Walker. New York: Raven Press, 1972. 13. Richter, C. P. and C. D. Hawkes. Increased spontaneous activity and food intake produced in rats by removal of the frontal poles of the brain..I. Nettrol. Psychiat. 2:231-240, 1939. 14. Siegel, A., H. Edinger and D. Miles. Effects of electrical stimulation of the lateral aspect of the prefrontal cortex on attack behavior in cats. Brain Res. 93" 473-484, 1975. 15. Siegel, J. and R. V. Wang. Electroencephalographic, behavioral and single unit effects produced by stimulation of forebrain inhibitory structures in cats. Expl Nenrol. 42: 28-50, 1974. 16. Wilcott, R. C., B. Sabol and R. P. Yurcheshen. Frontal cortex and response suppression in the rat. Brain Behav. Evol. 13: 116-124, 1976.
Cortical Stimulation and Suppression of Activity in the Rat R. C. W I L C O T T 1
D e p a r t m e n t o f Psychology, Case Western Reserve University, Cleveland, O H 44106 R e c e i v e d 4 M a y 1979 WILCOTT, R. C. Cortical stimulation and suppression of activity in the rat. PHYSIOL. BEHAV. 23(6) 1041-1048, 1979.--Low-frequency electrical stimulation at sites in the frontal cortex of the rat will suppress running in an activity wheel, but will have no effect on general activity measured in a stabilimeter. This confirms a prediction made from the effects of frontal lesions on these two kinds of activity. No evidence of abnormality in the EEG was observed following frontal stimulation. Stimulation in the frontal cortex probably influences the same inhibitory processes that are modified by frontal lesions. Further results indicate that the differential effects of frontal stimulation on running and general activity may be partly due to differences in response effort required. Frontal cortex Rat
Electrical stimulation
Suppression of activity
INCREASED skeletal and autonomic activity, and reactivity, are usually observed following removal of parts of the frontal cortex, including the prefrontal area, in primates, dogs, and cats Ill. In the rat skeletal activity is usually increased following removal of comparable parts of the frontal cortex [9,13]. These results seem to be due to the release of inhibition normally maintained by the frontal cortex. The apparent inhibitory influence of the frontal cortex in the rat has also been demonstrated by electrical stimulation, particularly at a low frequency. Frontal stimulation will suppress the heart rate response to a noxious stimulus [7], and the bar-press response for food [161. Effective cortical sites for suppression of bar-pressing were found in the frontal pole and over most of the frontal dorsolateral cortex. The distribution of these sites generally agrees with the inhibitory areas established in lesion studies [9,13], but inhibitory processes appear to be more widespread in the dorsolateral cortex than these studies indicated. The report describes three experiments which were intended to further explore the nature of behavioral suppression produced by frontal stimulation in the rat. EXPERIMENT 1 Results obtained in studies with frontal lesions and frontal stimulation raise the question of whether these two methods influence the same inhibitory processes in the frontal cortex. One way to test this would be to determine the effects of low frequency stimulation in the frontal cortex on skeletal activity. In the rat frontal lesions consistently increase running in an activity wheel, but have little or no effect on general activity in a stabilimeter [4] or similar situations such as an open field [8]. If frontal stimulation influences the same
Electroencephalogram
Response effort
cortical processes that are altered by frontal lesions in the rat, then frontal stimulation should have a greater suppression effect on running in an activity wheel than activity in a stabilimeter. This prediction was tested in this experiment.
METHOD
Animals Results were obtained from seven Holtzman male rats, weighing 375-400 g at the time of surgery.
Cortical Stimulation Electrodes (Plastic Products Co.) consisted of insulated stainless steel wires, 0.2 mm in diameter, wound together with only the cut tips exposed. Implant procedures were the same as previously described [16]. For three rats electrodes were positioned within the frontal pole, and for four rats they were positioned slightly below the surface of the dorsal frontal cortex (Fig. l). These sites were chosen because stimulation in this area had been found to produce strong suppression of the bar-press response for food [16]. Each rat also had an electrode in an area of the posterior dorsal cortex where stimulation is known to produce little or no response suppression. Three rats were implanted unilaterally, and four rats were implanted bilaterally. For the latter, electrodes were implanted at the same site on each side as closely as possible. Bilateral electrodes in the frontal cortex were in the frontal pole of one rat, and at the dorsal cortex of three rats. At necropsy perfused brains were examined for locations of dorsal cortex electrodes, and the locations of frontal pole electrodes were identified from frozen brain sections. Frozen sections were also prepared through elec-
~Send reprint requests to R. C. Wilcott, Department of Psychology, Case Western Reserve University, Cleveland, OH 44106.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/79/121041-08502.00/1
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WILCOTT Activity Wheel
B
FIG. I. Circles mark stimulation electrode sites within the frontal pole (left) for Experiment 1. The cross section of the frontal pole is about 6 mm anterior to the bregma. All stimulation sites at the frontal or posterior dorsal cortex (right) in Experiment 1 were in shaded areas. In Experiments 2 and 3, stimulation electrode sites were in the frontal shaded area only. Where bilateral stimulation was used electrodes were approximately at the same site on the two sides. Circles (right) mark the locations of EEG electrodes over the anterior and posterior cortex in Experiment 2. The EEG electrode near the bregma (B) was used as a common reference site. The anterior EEG electrode was as close as possible to the anterior stimulation electrode. trode sites at the dorsal cortex of two rats and in each instance it was found that electrode tips at both the frontal and posterior cortex were within cortical gray matter. Stimulation was done with a Grass $44 Stimulator feeding through a Grass SIU5 Stimulus Isolation Unit and a Grass Constant Current Unit. T w o stimulators and associated e q u i p m e n t were used for bilateral stimulation. Square w a v e pulses were 2 msec in duration and at a frequency of 10/sec. F o r unilateral stimulation current was adjusted at each cortical site by the same procedures used earlier [16]. Current was set just below a m a x i m u m level that n e v e r p r o d u c e d m o v e m e n t s or an arousal response. This point was ascertained at each cortical site by gradually increasing stimulus current, usually in steps of 0.01 or 0.02 mA, until m o v e m e n t s or an arousal response w e r e observed. Current was then reduced to a level where it seemed certain that no m o v e m e n t s , etc., would be produced during repeated testing. At some nonsuppression sites m o v e m e n t s , etc., were not o b s e r v e d and a maximum current of 0.25 m A was used. F o r bilateral stimulation current was adjusted separately on each side, and w h e n presented bilaterally, further adjusted to near a m a x i m u m level where it was certain that m o v e m e n t s , etc., would not be produced. To partially control for possible transfer effects, four rats were first tested for suppression of running activity and then tested for suppression of stabilimeter activity, The other three rats were tested in the opposite order.
A standard size activity wheel, 14 in. in diameter and 5 in. wide, was constructed for this study. It was made from 1/4 in. wire mesh and was pivoted at one side by a steel plate mounted onto a shaft which r e v o l v e d on ball bearings. The wheel was housed inside an Industrial Acoustics C h a m b e r fitted with a one-way window. The rat was placed in the wheel through a small door on the lower front side of the wheel. E l e c t r o d e leads, c o v e r e d by a steel spring passed up and out through a small hole in the center of the wheel. They were plugged into a swivel which was mounted just outside of the center hole. The rat was free to run in either direction with minimum interference from the leads. Small round pegs were fitted on the outer surface of the wheel so that as it turned they tripped a microswitch. These pegs were mounted on opposite sides of the wheel such that the switch was tripped at each half turn of the wheel. Tripping of the switch was registered on one channel of a Bechman Type R Dynograph recorder. To insure relatively high activity, body weight was reduced to 80% of its level with ad libitum feeding, and rats were tested just before their daily feeding. They were habituated to the wheel for two 30-min sessions on c o n s e c u t i v e days before experimental testing. Eighteen trials were given with stimulation at a suppression site and 18 trials with stimulation at a nonsuppression site. T h e y were presented in a randomized order along with 18 nonstimulation trials o v e r 4-5 days. Trials were 10 sec in duration. They were presented when the rat had been running at a rate sufficient to produce at least three half turns of the wheel during a 10-sec interval. A trial began immediately after a half turn of the wheel. An all-or-none criterion for response suppression was used. This was where the rat stopped running after trial onset and before the next half turn was registered, and did not start running again during that trial interval. Time between trials was n e v e r less than 1 rain. Stabilimeter A loudspeaker type stabilimeter, originally intended for the cat, was used. A 12-in. speaker was mounted in the center of a 0.5 in.-thick plywood board and this was c e m e n t e d to one side of a 2 4 × 2 4 x l . 5 - i n . wood frame. A sheet metal c o v e r was stretched o v e r the other side of the frame and c e m e n t e d in place. The space b e t w e e n the speaker cone and the sheet metal was air tight so that any vibrations of the sheet metal were transferred to the speaker cone. A standard single rat cage, fitted with an aluminum c o v e r and resting on metal pegs at each corner, was placed in the center of the stabilimeter. The entire unit was housed in the Industrial Acoustics Chamber. The speaker coil was coupled to two channels of the Bechman recorder. One channel was for direct recording, and the second was coupled through a Backman E M G Integrator (Fig. 2). This stabilimeter registered the onset and magnitude of m o v e m e n t s , but it did not accurately display the time a m o v e m e n t ended. The reason was that the system continued to vibrate for short time after the rat stopped moving. But this error was relatively small and constant and did not appear to present a serious problem in the way these stabilimeter data were analyzed in this study. Procedures used during testing were the same as those used with the activity wheel, including body weight, and
S U P P R E S S I O N OF A C T I V I T Y
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FIG. 2. A, examples of the recording of running in the activity wheel with low frequency stimulation (10 sec) at a suppressmn site, left, and with stimulation at a nonsuppression site. B, direct recording and integrator recording of general activity in a stabilimeter with stimulation (10 sec) at a suppression site.
number of trials. Trials were given when activity was clearly registered by the recorder, and when it had been at about the same level for at least 10 sec prior to trial onset. Data were obtained over a 3-4 day period. The magnitude of activity was measured from the integrator recording and specified as the area under the curve in square cm as measured by a planimeter. An adjacent pen marked a baseline (Fig. 2). Activity during the 10-sec trial was compared with activity during the 10-sec interval immediately preceding each trial. Each 10-sec interval was marked on the integrator records by vertical lines extending
from the baseline. These vertical lines were boundaries for measuring the area under the curve.
RESULTS
Activity Wheel Electrode sites at the frontal suppression area and at the posterior nonsuppression area are given in Fig. 1. Activity wheel data are presented in Table 1. F or each rat, unilateral stimulation at a suppression site produced a
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WILCOTT TABLE 1 SUPPRESSION OF RUNNING ACTIVITY
~2
Rat
A Suppression site*
B Nonsuppression site*
C Nonstimulation trials*
A-C
B-C
1 2 3 4 5 6 7
16 (0.07) 15 (0.07) 16 (0.09) 14 (0.06) 13 (0.07) 15 (0.07) 14 (0.08)
6 (0.1)91 6 (0.141 5 (0.25) 4 (0.141 4 (0.10) 7 (0.131 5 (0.141
5 6 4 5 5 7 6
33,50+ 20.25+ 46.29 + 22.43+ 17.72+ 14.95+ 15.99I
0.2775 0.960¢
*Suppression of running activity during stimulation at a suppression site (A), and nonsuppression site (B) or on nonstimulation trials (C), and X'-'between these conditions. Eighteen trials were given under each condition, and values shown are the number of trials where the rat stopped running according to the all-or-none criterion used. Stimulation current in mA at suppression and nonsuppression sites is given in parentheses. +p<0.001. $p>0.050. TABLE 2 SUPPRESSION OF STABILIMETER ACTIVITY Stimulation trials* Before SD
Rat
M
1 2 3 4 5 6 7
1.47 1.51 1.73 1.36 1.45 1.74 1.67
0.417 0.529 0.617 0.427 0.412 0.590 0.616
M
During SD
•40 .31 •71 •30 .39 .70 .65
0.405 0.397 0.592 0.379 0.423 0.598 0.609
Nonstimulation trials* Before SD
t+
M
0.7743 0.8734 0.5682 0.7141 0.7225 0.5819 0.4765
1.50 1.43 1.67 1.41 1.45 1.66 1.59
0.498 0.407 0.555 0.427 0.407 0.588 0.627
M
During SD
1.35 1.46 1.60 1.40 1.41 1.62 1.61
I}.487 11.514 0.582 0.491 0.431 I).529 0.611
r; 0.6551 0.7312 0.3198 0.5213 0.5917
*Means (M) and standard deviations (SD) of stabilimeter activity scores before and during stimulation at a suppression site and for nonstimulation trials, and t-tests between these two conditions. A stabilimeter score was derived from the area under the curve in square cm of the integrator recording. Means were computed from 18 trials. ~p<0.050 for all t values. significantly g r e a t e r s u p p r e s s i o n of r u n n i n g t h a n t h a t f o u n d d u r i n g n o n s t i m u l a t i o n trials. S t i m u l a t i o n at n o n s u p p r e s s i o n sites did n o t h a v e a significant effect. E x a m p l e s of activity w h e e l r e c o r d i n g s are g i v e n in Fig. 2. D u r i n g s u p p r e s s i o n o f r u n n i n g t h e b e h a v i o r of the rat did not a p p e a r to be different t h a n w h e n it s t o p p e d r u n n i n g s p o n t a n e o u s l y . It would turn a r o u n d in the wheel, s t a n d on its hind legs a n d e x p l o r e the wheel, g r o o m itself, etc. R a t s usually r e s u m e d r u n n i n g within 5-15 sec following t e r m i n a tion of s t i m u l a t i o n at a s u p p r e s s i o n site.
Stabilimeter U n i l a t e r a l s t i m u l a t i o n at a s u p p r e s s i o n site did not prod u c e a significant effect on s t a b i l i m e t e r activity with a n y of the rats t e s t e d . T h e s e d a t a are g i v e n in T a b l e 2. B e c a u s e s t i m u l a t i o n did not h a v e a significant effect, only d a t a for s t i m u l a t i o n at a s u p p r e s s i o n site a n d for n o n s t i m u l a t i o n trials are given. A n e x a m p l e of s t a b i l i m e t e r r e c o r d i n g is s h o w n in Fig. 2.
F o u r rats with bilateral e l e c t r o d e i m p l a n t s (Rats 4 - 7 in T a b l e 2) w e r e also t e s t e d with bilateral stimulation. Results w e r e similar to t h o s e in T a b l e 2 a n d w e r e n o t significant. EXPERIMENT
2
R e s p o n s e s u p p r e s s i o n p r o d u c e d by low f r e q u e n c y stimulation in the frontal c o r t e x o c c u r s m a i n l y d u r i n g the time s t i m u l a t i o n is p r e s e n t e d and this suggests that the s u p p r e s sion could b e due to d i s r u p t i o n of neural activity in the area of stimulation. B u t no e v i d e n c e for this is a p p a r e n t f r o m the b e h a v i o r of rats d u r i n g b e h a v i o r a l s u p p r e s s i o n . F o r e x a m ple, d u r i n g c o m p l e t e s u p p r e s s i o n of the b a r - p r e s s r e s p o n s e for food rats a p p e a r to m o v e a b o u t n o r m a l l y , g r o o m t h e m selves a n d display a p p r o p r i a t e orienting r e s p o n s e s to alerting stimuli [16]. Similar o b s e r v a t i o n s w e r e m a d e d u r i n g activity s u p p r e s s i o n in E x p e r i m e n t 1. A n o t h e r way to investigate possible d i s r u p t i v e effects o f frontal stimulation is to d e t e r m i n e the effects of frontal stimulation on t h e e l e c t r o e n c e p h a l o g r a m ( E E G ) . While a
SUPPRESSION OF ACTIVITY
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A
f--
--1
FIG. 3. Examples of EEG recorded from the anterior cortex (A) and posterior cortex (P) before and after low frequency stimulation for 30 sec at a suppression site. Calibration is 1 sec and 50/.LV.
readable EEG record cannot usually be obtained during stimulation, an abnormal EEG in the form of an epileptiform after-discharge has been observed following cortical stimulation [12]. Spikes or spike and wave discharges can be present following stimulation in various parts of the cortex, including the frontal region, in the monkey [5] and cat [6]. An after-discharge consisting of 3/sec slow waves has also been observed following frontal stimulation in the cat [14]. These after-discharge patterns appear to be a continuation of abnormal activity produced during stimulation, and they sometimes can be obtained without observable behavioral effects. After-discharges occur best to stimulation frequencies above 25/sec [12], and this phenomenon seems to be unrelated to the behavioral suppression produced by low frequency stimulation. Stimulation at frequencies of 7-10/sec in the frontal cortex of the cat can produce strong behavioral inhibition, and no abnormality in the EEG has been reported 15,101. The effects of low frequency frontal stimulation on the EEG of the rat have apparently not been investigated; and because of the importance of this problem, the EEG following frontal stimulation was studied in Experiment 2. METHOD
Animals Six male Hoitzman rats, weighing 380--400 g at the time of surgery, were used.
Cortical Stimtdation Electrodes were implanted in the dorsal frontal cortex by the same procedures that were used in Experiment 1. Three rats were implanted unilaterally and three were implanted bilaterally. Stimulation equipment, and procedures used to specify stimulation current intensity, were the same as in Experiment 1.
EEG Recording Electrodes were small stainless steel machine screws mounted in the skull on one side. Three screws were positioned at the same approximate location for each rat. One screw was as close to the stimulation electrodes as possible, a second screw was near the bregma, and a third screw was near the visual area. Two-channel recording was done with the screw near the bregma serving as a common
site. Leads had been soldered to the heads of the screws prior to placement, and these leads were connected to sockets of the type used with the stimulation electrodes. A Beckman Type R Dynograph was used to record the EEG. During EEG recording rats were placed in a standard single cage with a cover, and the cage was housed inside the Industrial Acoustics Chamber. A four-channel mercury swivel was used for both recording and stimulation. Because stimulation train duration seems to be an important factor in the appearance of an EEG after-discharge [12], train duration was varied. Three durations were used: 3, 10, and 25 sec. The 3-sec duration was used because it was effective for the cat, and the 10- and 25-sec durations were tested because they have been used in the production of behavioral suppression in the rat (Experiment 1 and in a previous study, [16]). Ten trials were given with each train duration over a 3-day period. Time between trials was 3-4 rain. All rats were tested with unilateral stimulation, and three rats were later tested with bilateral stimulation. Following EEG recording, rats were tested for suppression of running in the activity wheel to insure that electrodes were at effective sites and to determine whether electrode sites had been damaged by excessive stimulation. Test procedures were the same as in Experiment 1. Suppression of running was clearly observed with each rat over 6--8 trials. This indicated that frontal stimulation was still effective for these rats.
RESULTS
The location of stimulation electrodes in the dorsal frontal cortex, and the location of EEG electrodes are given in Fig. I. Abnormal EEG activity was never observed in these rats following the 3-, 10-, or 25-sec stimulation train durations, and with either unilateral or bilateral stimulation. After each stimulation trial the EEG was not observably different than it was before stimulation. A section of a typical record is given in Fig. 3. Stimulation currents ranged from 0.07-0.09 mA. EXPERIMENT 3 A question raised by results of Experiment 1 is why frontal stimulation suppressed running in an activity wheel but did not suppress general activity in a stabilimeter? A possible answer may have to do with the amount of effort required. Running in an activity wheel probably requires
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WILCOTT
TABLE 3 SUPPRESSION OF BAR PRESSING
12/1 ratio~Rats 1 2 3 4 5 6
Before (0.08) (0.07) (0.05) (0.07) (0.07) (0.09)
2.74 2.65 2.90 2.74 2.83 2.79
(2-3) (2-3) (2-3) (2-3) (2-3) (2-3)
During 0.26 0.34 0.37 0.78 0.53 0.57
(0-1) (0-1) (0-2) (0-2) (0-1) (0-1)
3/1 ratiot Percent difference 91.5 86.4 88.2 75.6 82.6 79.5
Before 3.25 3.41 3.31 3.51 3.00 3.32
(3-4) (3-4) (2-4) (3-4) (2-4) (3-4)
During 0.89 1.51 1.12 1.68 0.98 I. 12
(0-1) (0-2) (0-2) (0-1) (1-2) (1-2)
Percent difference
t
71.5 53.8 67.3 52.1 68.3 66. l
1.829:!: 3.107q~ 2.175§ 2.351§ 1.7951.792~:
*Stimulation current used with each rat is given in parentheses. tMeans and ranges of number of reinforcements for the 25 sec intervals before and during frontal stimulation, and mean percent differences between these two intervals, with the 12/1 and 3/1 ratios, and t-tests between these mean percent differences. Means were computed from 14 trials. ~+p<0.050. §p<0.025. ~qp<0.010.
more effort than general activity, and behavior requiring a particular amount of effort may be more easily suppressed. Some support for this explanation is provided by the observation that during complete suppression of bar pressing for food, rats will still eat at the feeder when it is operated externally [16]. Bar-pressing probably requires more effort than eating food. This hypothesis was tested by determining the effects of frontal stimulation on the bar-press response for food where the effort required was varied by changing the ratio between bar pressing and reinforcement. Ratios of 3/1 and 12/1 were tested. If the hypothesis is correct, frontal stimulation should produce greater suppression of bar-pressing with the 12/1 ratio than with the 3/1 ratio. METHOD
Animals Eight Holtzman male rats were prepared for unilateral stimulation at a frontal suppression site in the same way as in Experiment 1. However, data were obtained from only six rats because two rats failed to respond consistently at the 12/1 ratio. Procedttre Rats were trained to bar-press for food (Noyes pellets) in a Gerbrand's Skinner box by procedures previously used [16]. Training was continued until a relatively stable response rate was obtained at the 12/1 ratio. Suppression of bar-pressing by frontal stimulation (Fig. 1) was then tested with stimulation presented for 25 sec. Currently intensity was raised to a level where it would produce at least a 7(F/b reduction in bar pressing without producing movements, etc. Seven trials were given at this current intensity. Next, the ratio was reduced to 3/1 and sufficient training given to assure a relatively stable rate of responding. Then 14 trials with frontal stimulation were given over a 2-day period at the same current intensity used with the 12/1 ratio. Finally, rats were returned to the 12/1 ratio, and after responding was stable, seven stimulation trials were again presented. Because a fixed ratio was used, the rat of bar-pressing was
measured as the number of reinforcements. At each test trial the magnitude of suppression was determined by comparing the number of reinforcements during the 25-sec stimulation interval with the number of reinforcements during the 25-sec interval immediately proceeding the stimulation interval. The mean magnitude of response suppression with the 14 trials at the 12/1 ratio was then compared with the mean magnitude of response suppression during the 14 trials at the 3/1 ratio. RESULTS
Electrodes were in the dorsal frontal cortex as shown in Fig. 1. Results are presented in Table 3. For each rat there was significantly greater response suppression with the 12/1 ratio than with the 3/1 ratio. Examples of response suppression at the 12/1 and 3/1 ratios are given in Fig. 4. DISCUSSION In Experiment I it was demonstrated that low frequency stimulation in the frontal cortex of the rat will suppress running in an activity wheel, but have no effect on general activity measured in a stabilimeter. This confirms the prediction made from the effects of frontal lesions on these two kinds of activity. This suggests that stimulation in the frontal cortex probably influences the same inhibitory processes that are modified by frontal lesions. The apparently normal EEG pattern recorded following frontal stimulation in Experiment 2 suggests that the response suppression produced was not due to production of abnormal cortical activity. These observations are not conclusive because EEG could not be recorded during stimulation, but they are in agreement with E E G data from the cat [10,15]. This problem needs further study in the rat. It is relevant to note that a rhythmical slow electrical activity in the hippocampus has been found to be related to running in the rat [2], and stimulation in the dorsal hippocampus that blocks this type of electrical activity will also suppress running [3]. The effects of frontal stimulation on hippocampal activity should also be investigated.
SUPPRESSION O F ACTIVITY
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A ,
i
II B
i
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I
IIIII
i
i
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i
r
_
....
I
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i
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i
FIG. 4. Examples of suppression of the bar-press response for food with a 12/1 (A) of 3/I (B) ratio by low frequency stimulation for 25 sec at a frontal suppression site. In each section of the figure the upper trace registers bar-pressing, the middle trace registers reinforcement, and the lower trace registers stimulation.
Results comparing the magnitudes of suppression of bar-pressing with the 12/1 and 3/1 ratios in Experiment 3 demonstrates that response effort can be an important factor determining the amount of response suppression produced by frontal stimulation. It seems likely, therefore, that the difference in the effects of frontal stimulation on running and general activity can be at least partly explained by the difference in effort required for these two types of activity. It should be noted, however, that frontal stimulation in Experiment 3 still produced a substantial suppression of barpressing with the 3/1 ratio even though this ratio required much less effort than the 12/1 ratio. This leads to the question of whether the difference in effort required for running and general activity can entirely account for the large difference in the effects of frontal stimulation on these two types of activity. Some additional variable may be involved. Response effort could also be a factor in the different effects of frontal lesions on running and general activity. According to this hypothesis, responses requiring the greatest effort would normally be the most inhibited, and following release of inhibition these responses would show the greatest increase. Following frontal lesions in the rat, however, stabilimeter activity as well as running in an activity wheel are sharply increased during starvation [4].
This does not invalidate the response effort explanation, but it does demonstrate that the effects of extreme food deprivation are more potent than other factors that may influence general activity in the frontal rat. It has been found that lesions in the inferior thalamic penduncle produced a greater increase in general activity in a stabilimeter than in running in an activity wheel when rats are tested during starvation [11]. This seems to suggest that the neural processes inhibiting stabilimeter activity are partially separate from those inhibiting running activity. But this would not rule out an inhibitory influence of the frontal cortex on both types of activity. In the absence of extreme conditions, such as starvation, the inhibitory effects of the frontal cortex may be strong enough to suppress running but not strong enough to suppress stabilimeter activity in the rat. It has been suggested that the frontal cortex serves to inhibit the ascending influence of the reticular formation, and thereby suppress arousal [4]. Increased activity in the frontal animal is thought to be due to increased arousal, and to increased reactivity to internal (hunger) and external stimuli. Data reported here are in accord with this hypothesis. But these data are also not opposed to the hypothesis that the frontal cortex has widespread inhibitory influences in the brain [151
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REFERENCES 1. Brutowski, S. Functions of the prefrontal cortex in animals. Physiol. Rev. 45: 721-746, 1965. 2. Bland, B. H. and C. H. Vanderwolf. Diencephalic and hippocampal mechanisms of motor activity in the rat: Effects of posterior hypothalamic stimulation on behavior and hippocampal slow wave activity. Brain Res. 43: 67-88, 1972. 3. Bland, B. H. and C. H. Vanderwolf. Electrical stimulation of the hippocampal formation: Behavioral and bioelectrical effects. Brain Res. 43: 8%105, 1972. 4. Campbell, B. A. and G. S. Lynch. Cortical modulation of spontaneous activity during hunger and thirst. J. comp. physiol. Psychol. 67: 15-22, 1%9. 5. French, J. D., B. E. Gernandt and R. B. Livingston. Regional differences in seizure susceptibility in monkey cortex. Am. Med. Ass. Archs Nettrol. Psychiat. 75: 260-274, 1956. 6. Garner, J. and J. D. French. Regional differences in seizure susceptibility in cat cortex. Am. Med. Ass. Archs Neurol. Psychiat. 80: 675-681, 1958. 7. Glasser, E. M. and J. P. Griffin. Influences of the cerebral cortex on habituation. J. Physiol.. Lond. 160: 42%445, 1%2. 8. Glickman, S. E., R. Scroges and J. Hunt. Brain lesions and locomotor exploration in the albino rat. J. comp. physiol. Psychol. 58: 93-100, 1964. 9. Kolb, B. Dissociation of the effects of lesions of the orbital or medial aspect of the prefrontal cortex of the rat with respect to activity. Behav. Biol. 10: 32%343, 1974.
10. Lineberry, C. G. and J. Siegel. EEG synchronization, behavioral inhibition, and mesencephalic unit effects produced by stimulation of orbital cortex, basal forebrain, and caudate nucleus. Brain Res. 34: 143-161, 1971. 11. Lynch, G. S. Separable forebrain systems controlling different manifestations of spontaneous activity..I, comp. physiol. Psychol. 70: 48-49, 1970. 12. Marsan, C. A. Focal electrical stimulation. In: Experimental Models ql'Epilepsy, edited by D. P. Purpura, J. K. Penry, D. B. Tower, D. M. Woodbury and R. D. Walker. New York: Raven Press, 1972. 13. Richter, C. P. and C. D. Hawkes. Increased spontaneous activity and food intake produced in rats by removal of the frontal poles of the brain..I. Nettrol. Psychiat. 2:231-240, 1939. 14. Siegel, A., H. Edinger and D. Miles. Effects of electrical stimulation of the lateral aspect of the prefrontal cortex on attack behavior in cats. Brain Res. 93" 473-484, 1975. 15. Siegel, J. and R. V. Wang. Electroencephalographic, behavioral and single unit effects produced by stimulation of forebrain inhibitory structures in cats. Expl Nenrol. 42: 28-50, 1974. 16. Wilcott, R. C., B. Sabol and R. P. Yurcheshen. Frontal cortex and response suppression in the rat. Brain Behav. Evol. 13: 116-124, 1976.