Memory components of shock escape reaction in rat

Physiology and Behawor Vol. 3, pp. 773-776. Pergamon Press, 1968 Printed m Great Britain

Memory Components of Shock Escape Reaction in Rat MILOSLAV KUKLETA

Department of Physiology, Medical Faculty, J. E. Purkyn~ University, Brno, Czechoslovakia, Professor V. Kruta (Received 2 M a y 1968) KUKLETA, M. Memory components of shock escapereacnon in rat. PHYSIOL.B~HAV.3 (5) 773-776, 1968.--Avoidance and escape learning under cortical spreading depression (CSD) and the transfer of experience between functionally decorticate and normal states were studied in rats. To assess the character of transferred experience, three experimental tasks were used. The results demonstrate: (a) animals under bilateral CSD are trainable both in escape and avoidance situations; (b) information on the localization of the goal is transferred from a normal to a decorticate state and vice versa. Two separate memory components are postulated to be involved in escape learning. The first, which can be described as the ability to localize the goal in space, is probably stored extracortically in the intact brain. The second component, closely related to motor functions, determines the external form of the escape response. Its extracortical storage is considered a manifestation of the compensatory potencies of the decorticated brain.

Memory

Escape learning

Cortical spreading depression

BUREg and BURE.tiOV~ [3] have studied the memory-disturbing effect of cortical spreading depression (CSD) in rats, using a left-right discrimination task. The bilateral CSD was evoked after to-criterion training of the left side response. Twentyfour hr later in free choice trials 90 per cent of animals went through the left alley. The memory-disturbing effect was evident however, when the original position discrimination was reversed. The criterion of reversal was more rapidly reached in the depressed group, as compared with the control group. As the CSD, evoked by application of 25 per cent KC1, suppresses the spontaneous and evoked cortical activity for several hours, it Is probable that consolidation of experience remaining after the CSD treatment (i.e. the preference for the left alley) takes place before the decortication and/or outside the depressed structures. Several findings support the idea that memory components of the escape response are stored both cortically and extracortically. It has been proved that bilaterally decorticated animals retain a part of the experience acquired with normal cortex [2, 5, 6, 8, 9]. Bureg and Buregovfi [4] and more recently Kukleta and Libouban [7] have demonstrated that intact animals can partially utilize an experience acquired during preliminary training under CSD. Analysis of the above experiments leads us to the conclusion that in the case of escape responses, the information about the situation of the goal is stored extracortically. In the present paper this assumption has been experimentally eval~aated.

Three apparatuses were used (Fig. 1). The first consisted of a 20 x 60 x 40 cm wooden start box with an electrifiable grid floor and a 20 x 20 x 10 cm goal box with an insulated floor. Both boxes were connected by a 10 × 15 cm opening, situated opposite the starting point in one of the shorter walls of the start box. The insulated floor was placed 12 cm above the grid floor. The second apparatus consisted of a 20 x 60 x 40 cm start box with a grid floor, which was connected with two goal boxes situated left and right of the longitudinal axis of the start box. Two 10 x 15 cm connective openings were located at floor level. The third apparatus consisted of a 30 x 60 x 40 cm wooden box which was divided into two equal compartments. The greater part of the partition was formed by transparent organic glass which was lifted out during test sessions. A 6 x 6 cm opening placed at floor level in the fixed part of the partition formed a permanent connection between both compartments. The position and movement of the animal within the experimental box were detected by a modified capacity plethysmograph and were automatically recorded.

Surgery The animals were operated on in the evening prior to the day of experiment. Under ether anesthesia, a trephine opening 5 mm in dia. was made over the frontoparietal region of both hemispheres. The trephine openings were covered with greased gauze and the skin was loosely sutured. After 12-14 hr the suture was cut and the trephine openings were checked. Animals with cut dura and bram edema were discarded. Bilateral CSD was elicited by application of a circle of filter paper (5 mm in dia.), soaked in 25 per cent KCI, onto the exposed cortical surface. Training began 10 min after application of KCI.

METHOD

Subject and apparatus The animals were Wistar albino rats weighing about 200 g. They were housed in groups [6--7 per cage] and had free access to food and water. 773

774

KUKLETA

Training Procedure and Design

TABLE 1

The animals were trained to run from start box to the goal box. Intermittent electric shocks (0.8 mA) were applied to the grid floor simultaneously with the placing animal on start point in the escape training and 15 sec after the placing in the avoidance training. For an intertrial period of approximately I min the animal was placed in the home cage. The training ended when the animal attained the criterion of learned responses or when the conventional limit of trials was reached. Five successive avoidance responses were the criterion of avoidance training and five successive responses in which the animal reached the goal box by the shortest path within 5 sec were the criterion of escape training. The avoidance and escape learning of functionally decorticated animals and the transfer of learning from a decorticate state to a normal state were studied in apparatus one and two. Group A was given avoidance training under bilateral CSD in apparatus one. A maximum of 35 trials was overpassed in a part of the group (4 animals received 55 trials). Group B was given avoidance training under bilateral CSD in apparatus two. Groups C and E received 20 escape trials under CSD (C in apparatus one, E in apparatus two) and 24 hr later the retention testing with nornaal cortex. Control groups D and F underwent the same procedure as groups C and E, but received no trials under CSD. Only the right goal box was available during the training of decorticated animals in apparatus two. Animals trained with a normal cortex had the possibility of free choice between the two goal boxes. The transfer of learning from normal to functionally decorticate state was studied in apparatus three. Group G was given 6 preliminary training sessions (10 trials in each session) with normal hemispheres. The sessions were separated by about 10 hr intervals. The seventh session was performed under CSD. Control group H was given test session under CSD only. The organic glass part of the partition was removed during the fifth and the sixth preliminary sessions and during the session under CSD. Only those animals whose performance of the learned response was not changed after the removal of the partition, formed group G.

AVOIDANCE BEHAVIOR OF FUNCTIONALLY DECORTICATED ANIMALS AS A FUNCTION OF TIIE LOCOMOTOR DEMAND OF THE EXPERIMENTAL SITUATION

Group

Total number of animals

Number of animals reaching the criterion of avoidance

A (task 1)

10

0

B (task 2)

6

5

of avoidance responses wlthm the conventional limit of training. The avoidance responses of group A formed 7 per cent of all responses. None of the animals of this group reached the criterion of avoidance learning, in spite of the fact that 55 mstead of the usual 35 trials were used in a part of the group (4 animals). Nevertheless approximately 30 per cent of responses can be designated as an incomplete avoidance--the animal runs towards the goal box within the

C

I

IC

RESULTS

1

Traming in Functionally Decorticate State Table 1 presents the main results of this experiment. Most animals of group B trained under CSD reached the criterion

2

100

2OO

%

.

G

3

1oo

t~ oc

"-.....

Q..

I,.,.

5o

f

1.5

6.10

1 1 . f 5 fO'20

/

"

j

t

TRIAL NO

3

FIG. 1. Floor plans of apparatus 1. (goal 12 cm above the grid floor) 2 and 3. Broken line represents the orgamc glass portion of the partition, S--starting point, G--goal with insulated floor.

'=

1.5

6"10

",

11.15 10"20

FIG. 2. Course of the escape training of task one under bilateral CSD. Mean sums of escape latenoes and percentage of "direct reactions" in groups of five trials---experimental group C

MEMORY COMPONENTS OF ESCAPE REACTION

775 Transfer o f Experience from a Functionally Decorticate State to a Normal State

preshock period and at its vicinity awaits stimulation. Escape was successful in nearly all trials i.e. the animal reached the goal within the 60 see stimulation period. The results presented in Fig. 2 demonstrate the course of the escape training in the apparatus one. The difference of mean values of escape latencies at the beginning and at the end of the training is statistically significant (p < 0.01, t-tes0. The decrease of escape latencies and the increased incidence of "direct responses" testified that even in the case of training in apparatus one, learning takes place. In "direct response" the animal runs the way start-goal only once, within 10 see.

The results in Table 2 show that preliminary training under CSD does not facilitate the subsequent escape training with the normal cortex. The differences in the mean escape latencies of the first trial and of the first five trials and in the average number of to-criterion trials between experimental groups (C and E) and corresponding control groups (D and F) are not statistically significant (p > 0.05, t-test). Experiments performed in apparatus two make it possible to evaluate

TABLE 2 EFFECT OF TRAINING UNDER BILATERAL C S D ON THE SUBSEQUENT ESCAPE TRAINING WITH NORMAL CORTEX

Group

C D E F

(task (task (task (task

1) 1) 2) 2)

N

Preliminary trials

10 10 I0 10

20 0 20 0

Training with normal cortex Escape latencies Escape latencies -first trial -first five trials 484-17.3 664-12.4 114-3.2 18.5=]:7.5

764-16.5 97+13.3 6 6 ~ 12.8 1044- 15.3

Number of to-criterion trials* 74-0.1 7.64-0.4 144-1.2 16.34-3.4

* five criterion escape trials included

TABLE 3 TRANSFER OF EXPERIENCE ACQUIRED DURING A FUNCTIONALLY DECORTICATE STATE TO A NORMAL STATE

Group

N

Total number of reactions

E F

10 10

139 143

Right in 114 69

82 48

25 74

18 52

another indicator: the choice of one of two goals. The results presented in Table 3 clearly demonstrate that animals of the experimental group E prefer the right goal box i.e. the box which was available during the preliminary training under CSD.

100. %

c~

50,

o

c.j

TRIAL NO

Orientation of reactions opening chosen Left openmg chosen % in %

1"2

3"4

5-6

7 8

9" 10

FIG. 3. Percentage of unsuccessful reactions during a test session under bilateral CSD. Dark circles represent the results of group G (N = 10) prehminary trained with normal cortex, open circles represent the results of control group H (N ~ 10).

Transfer o f Experience from a Normal State to a Functionally Decorticate State The main result of the experiment, in which the character of the information transferred from a normal to a functionally deeorticate state is studied, is presented in Fig. 3. The na'fve animals (control group H) failed in 62 percent of cases to reach the goal box within the 60 see shock period. In the preliminary trained group G, failure occurred in only 18 percent of trials during the test session under CSD. The difference between these groups is statistically significant (t9 < 0.01), t-test. The results indicate that the information on the location of the safe box, acquired with normal cortex was retained even under CSD. The performance of the learned

776

KUKLETA

response is considerably changed, however. The time changes (escape latencies 10 times longer under CSD) seem to be less suitable for the definition of memory loss due to CSD than the analysis of the external form of responses of decorticated animals. The results in Table 4 demonstrate the performance of the learned escape reaction by experimental group G. The response of intact animals (form 1) i.e. the U shaped run start-opening-goal is practically absent during CSD. The deformed version of this reaction (the animal runs from the start point towards the opposite wall and back to the goal box) is present m 21 per cent. Most frequently encountered is

confirm the posslbdlty of avoidance learning under CSD. The inhibition of avoidance behavior in an apparatus with an elevated goal floor level deserves further experimental analysis. The results demonstrate that the escape response established in a functionally decorticate state is partially transferred to the normal state. In the intact animal the choice between left or right goal boxes is controlled by experience acquired under CSD. This experience, however, does not significantly accelerate the learning in the normal state. The analysis of the effect of bilateral CSD on the perform-

TABLE 4 EFFECT OF BILATERAL CSD ON PERFORMANCE OF THE SKILLED ESCAPE REACTION

State of animals

Form of reaction observed during 10 test trmls 1

Normal After operation Under CSD

100 ~ 100 ~ 2~

2

. . 21 ~

3

. .

. . 46~o

4

5*

. . 13 o~,

18oo

*Explanation in text

response 3, consisting of trials, in which the animal shortens the start-goal distance. In most cases the animals used the shortest possible way. Number 4 designates other, hardly definable reactions and form 5 unsuccessful reactions.

DISCUSSION

Avoidance learning m functionally decortlcated rats was demonstrated in apparatuses where the start and goal were situated on a straight line and on the same floor level [1, 9, 10]. The propulsive spontaneous somatomotor activity of depressed animals, frequently observed during training reinforced by foot shocks, could reduce the value of these experimental findings. It seems, however, that the results of training in an L shaped runway (apparatus two) sufficiently

ance of a learned escape response shows that information on the situation of the goal remains avadable after the functional ehmination of the cortex. Contrary to this fact, the acquired form of response i.e. the way by which the animal reaches the goal is severely impaired by CSD. The results mentioned support the assumption that two memory components are involved in the studied reaction. The first, which is transferred between normal and decorticate states, is stored extracortically in the intact brain. This component can be described as the ability to localize the goal in space. The second component, which determines the smooth mannerisms of welltrained animals and ~s closely related to motor functions, is not transferred between decorticate and normal states. The extracortical storage of this component in depressed animals can be considered as a manifestation of the compensatory abilities of the decorticated brain.

REFERENCES l. Bureg, J. Reversible decortlcatlon and behavmr. In' The Central Nervous System and Behavior, edited by M. A. B. Brazier, New York: Josiah Macy, Jr., Foundatton, 1959, 207248. 2. Bureg, J., O. Buregov~tand A. Z~ihorov~l.Condltmned reflexes and Le~.o's spreading cortical depression. J. comp. physiol. Psychol. 51: 350-358, 1958. 3. Buret, J. and O. Bure~ov~t. Cortical spreading depression as a memory disturbing factor. J. comp. physiol. Psychol. 56: 268-272, 1963. 4. Bureg, J. and O. Buregov~i. Vliv informace ziskan6 v obdobi funk6ni dekortikace na pozd6jgi Onnost norm~dniho mozku. ~s. fysiol. 13: 197, 1964. 5. Kukleta, M. Preuve de la localisation sous-corticale des traces de m6moire darts le cerveau du rat. Scripta medica fac. reed. Brun. 38: 19-24, 1965.

6. Kukleta, M. Application de la d6presslon envahlssante ,2 l'6tude de la localisation des traces de m6moire. Physiol. Behav. 1 : 229-232, 1966. 7. Kukleta, M. and S. Llbouban. Acqms~tion du conditlonnement instrumental d6fenslf chez les rats fonctionnellement d6cortiqu6s. J. Physiol. 59: 442, 1967. 8. Tapp, J. T. Reversible corttcal depression and avoidance behavior m the rat. J. cutup, physiol. Psychol. 55: 306-308, 1962. 9. Thompson, R. W. Transfer of avoidance learning between normal and functionally decorticate states. J. cutup, physiol. Psychol. 57: 321-325, 1964. 10. Thompson, R. W. and L. A. Hjelle. Effects of stimulus and response complexity on learning under bilateral spreading depression. J. comp. phystol. Psychol. 59: 122-124, 1965.