Factors influencing one-trial passive avoidance behavior: Implications for studies of retrograde amnesia (RA)

Physiology and Behavior, Vol. 10, pp. 817-819. Brain Research Publications Inc., 1973. Printed in the U.S.A.

BRIEF COMMUNICATION Factors Influencing One-Trial Passive Avoidance Behavior: Implications for Studies of Retrograde Amnesia (RA)1,: RICHARD A. KING AND RICHARD L. GLASSER

Neurobiology Program, University o f North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514

(Received 13 October 1972) KING, R. A. AND R. L. GLASSER. Factors influencing one-trial passive avoidance behavior: implicationsfor studies of retrograde amnesia (RA). PHYSIOL. BEHAV. 10(4) 817-819, 1973. -Rats were run in a step-through one-trial passive avoidance task. The effects of preexposure (or familiarization) to the apparatus, ptehandling outside of the apparatus, and postshock delay in the apparatus were studied. Preexposure to the apparatus significantly increased one-trial passive avoidance; with no preexposure little passive avoidance learning was obtained. Prehandling did not significantly increase the strength of passive avoidance behavior. Postshock delay did not increase passive avoidance behavior. The results are discussed in terms of their implications for studies of retrograde amnesia (RA) which use one-trial passive avoidance learning. Retrograde amnesia

One-trial passive avoidance learning

ONE-TRIAL passive avoidance is a learning situation that has provided much of the experimental data on memory consolidation in animals [8]. In the course of conducting experiments on retrograde amnesia (RA) in rats, we have encountered difficulties, at times, in obtaining sufficient one-trial passive avoidance learning to allow for tests of RA. One of the major factors appeared to be pretreatment exposure of the rats to the training apparatus. (In papers previously published by us [6], we have given preexposure trials.) In a preliminary experiment explicity designed to test the effect of preexpsoure on one-trial passive avoidance, significant differences were obtained between a group given preexposure and a group not given preexposure. The experiment described in this paper is an extension of these preliminary findings. In addition to the preexposure variable, several other factors which might have an effect on one-trial passive avoidance are studied. The preexposure variable has been termed "familiarization" in the retrograde amnesia literature [7]. Although agreement has not been reached on the conditions under which familiarization is effective [3,5], the effect of prior familiarization, or preexposure, with the passive avoidance apparatus is sometimes said to decrease the amount of RA obtained after the application of electroconvulsive shock [7 ]. Attenuation of RA with preexposure will be discussed in the light of results obtained in this experiment.

METHOD

A nimals Eighty-four male Spague-Dawley rats in the 2 5 0 - 3 5 0 g weight range were used. They were experimentally naive and were housed in individual cages and fed ad lib.

A ppara tus A two-chamber box similar to that used in other experiments in this laboratory was the apparatus [6]. The two chambers were separated by a horizontally-sliding door which moved across a 1.5 cm high hurdle. The start chamber (12.5x26.0x 12.5 cm high) was made of unpainted plywood and was brightly illuminated; the shock compartment (12.Sx38.5x12.5 cm high) was painted black with 1.25 cm white stripes on the walls, had a grill floor, and was dimly illuminated. Both compartments were covered by a Plexiglas roof. Scrambled footshock (2 mA for 2 sec) was administered from a Grason-Stadler Model 700 shock generator.

Procedure Runs were made several times over a six-month period. In each run squads of rats were run in a number of the experimental conditions. The conditions were as follow: (1) preexposure, no postshock delay (2 PE, N=9); (2) pre-

Thanks ate due to Daniel Hobbs and Stephen Hurst for their help in conducting this experiment. 2Send for copies of reprints to: Dr. Richard A. King, Department of Psychology, U. of North Carolina, Chapel Hill, N. C. 27514. 817

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KING AND GLASSER

exposure, 165 sec postshock delay (2 PE-165 sec; N=I2); (3) no preexposure, no postshock delay (O PE; N=8); (4) no preexposure, t65 sec postshock delay (O PE-165 sec; N = l l ) ; (5) handling PE, no postshock delay (Hand PE; N=9); (6) handling preexposure, with 165 sec postshock delay (Hand PE-165 sec; N=I 1 ); (7) no shock with no delay (NS; N=15); and (8) no shock with 165 sec delay in grill box (NS-165 sec; N=9). (A postshock delay period of 165 sec was used to make these results compatible with other experiments being done in our laboratory.) In the 2 PE condition rats were given two preexposure trials in the apparatus. On each of these trials the animals were placed in the starting compartment, the sliding door was opened, and latencies of crossing into the shock compartment were recorded; no shock was given on these trials. A minimum of 30 min intervened between all trials and the animals were held in their home cages between trials. On the third trial rats were placed in the start box as usual, but they were shocked (2mA for 2 sec) 2 sec after crossing to the grill box. Animals were removed immediately after the shock in this condition. In the 2 PE-165 sec condition, the rats were run as in the 2 PE condition but were not removed from the apparatus until 165 sec after termination of the shock. In the O PE condition, the animals were not given any preexposure; the first trial was a shock trial and the animals were removed immediately after the shock. In the O PE-165 see condition, animals were given no preexposure and were removed 165 sec after shock termination. The handling PE groups were handled by the experimenter near the avoidance test box. The handling was done on two trials and was about the same duration as the preexposure trials of the PE a n i m a l s - a b o u t 3 0 - 6 0 sec. The handling activity was similar to the PE conditions, with the exception that the animals were not put into the apparatus. After the two handling trials, the animals were run for one trial in the apparatus and removed immediately after shock (Hand PE) or 165 sec after shock (Hand PE-165 sec). Two no-shock groups (NS) also were run. These animals received the preexposure trials, but no shock was given on the third trial. They were removed from the apparatus immediately after crossing on the third trial (NS) or 165 sec after crossing (NS-165 sec). On all trials, the door between the compartments was closed while shock was being given, during the delay period, and while the animal was being removed from the apparatus. Test trials were conducted 48 hr and one week after the experimental trials. Test trials were the same for all groups. On a test trial, the rat was placed in the start chamber, the door was opened, and latency to cross into the grill chamber was measured. (A crossing response was counted when the four feet of the rat were in the grill box.) If an animal did not cross within 300 sec, it was removed from the apparatus. Animals which did cross were removed immediately after crossing. RESULTS On the preexposure trials, no significant differences between groups were obtained. Mean and median test latencies for the 2-day test and the one-week test are shown in Fig. 1. Group medians are shown by bar height; group means are indicated by lines. For both tests the KruskalWallis H was significant (p<0.001). Comparisons of individual groups were done by the Mann-Whitney U test; two-tailed p values less than 0.05 were used. In both tests

the 2 PE and 2 PE-165 sec groups had significantly longer latencies than all but one of the other groups; the exception was the Hand PE group on the 2-day test. Comparisons of the 165-sec delay and nondelay conditions for the handling, the PE preexposure conditions, and the no shock iNS) condition revealed no significant differences. Comparisons between the Hand PE and the O PE, and between the Hand PE and NS groups, were not significant.

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FIG. 1. Mean and median test latencies for the 2-day and o ~ w e e k tests. Group medians are shown by bar height; group means are indicated by lines. The ratio above each bar indicates the number of animals with maximum latencies of 300 sec in the numerator and the total number of animals in the group in the d e r t ~ i n a t o r .

DISCUSSION Preexposure resulted in significantly longer one-trial passive avoidance latencies. One interpretation is t h a t rats are better able to identify the place i n t h e apparatus where shock occurred after preexposure; in other words, this effect might be interpreted in perceptual learning terms. In experiments on dual mechanisms in passive avoidance learning, Blanchard and Blanchard [1,2] obtained the longest latencies in a condition in which discriminability of the shock source was low. But the conditions of these experiments were such that animals in the low discriminability situation received several shocks as they explored the apparatus and were, in our terms, familiarized with the apparatus. Thus there is no discrepancy between these results and those reported in this paper.

O N E - T R A I L PASSIVE AVOIDANCE AND RA

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Handling seemed to result in increased latencies, but they were not significantly greater than those of the appropriate comparison groups. Although, not significant, the small increases in latency due to handling might, within a perceptual learning framework, be interpreted as due to exposure to extra-apparatus cues that served to provide some guidance for the animals in the apparatus. One alternative explanation for the preexposure effect involves possible decreases in activity. Denti and Epstein [4] have obtained a relationship between activity and passive avoidance learning; male rats with low levels of activity were good passive avoidance learners. If the effect of preexposure was to decease exploratory activity, passive avoidance latency may have been increased by this means. A n o t h e r possible alternative explanation for the preexposure effect is in terms of the "limbic warm-up" effect postulated by Posluns and Vanderwolf [9]. They found that preexposure to the apparatus immediately prior to active avoidance learning led to faster active avoidance learning; perhaps a similar p h e n o m e n o n occurs in passive avoidance learning. It should be emphasized that the preexposure effect obtained in the present experiment has not been found in several other passive avoidance learning studies which used slightly different tasks and different subject species. F o r instance, in a step-down task, familiarization did not increase passive avoidance latencies in the control groups of

an experiment by Lewis, Miller, and Misanin [7]. Dawson and McGaugh [3], using mice in a step-though passive avoidance task, found a decrease in latency for familiarized control animals in one experiment and no effect of familiarization on control groups in another. Furthermore adequate passive avoidance is often obtained with no familiarization in other laboratories. The reason for the large preexposure effect obtained in the present experiment is not clear. However, in studies of retrograde amnesia, it is clear that, when preexposure enhances passive avoidance learning, the amount of RA might be lessened. This is so because avoidance tendencies are increased and there is some evidence that factors, such as strong footshock, which increase avoidance tendencies decrease ECS-produced RA [ 10]. The results of this study thus indicate the importance of taking the possible strengthening effect of preexposure into account when establishing baselines from which RA is to be evaluated. On the one hand, preexposure may give a more stable baseline by strengthening avoidance tendencies; on the other hand, it may increase avoidance tendencies to the point that they may be resistant to amnesic agents. The results of this study do not explain why some studies [7] obtain attentuation of RA with familiarization and others [3] do not. In neither of these studies was there an enhancing effect of familiarization on passive avoidance learning by control groups.

REFERENCES 1. Bianchard, R. J. and D. Caroline Blanchard. Dual mechanisms of passive avoidance: I.Psychonom. Sci. 19: 1-2, 1970. 2. Bianchard, R. J. and D. Caroline Blanchard. Dual mechanisms of passive avoidance: II. Psychonom. Sci. 1 9 : 3 - 4 , 1970. 3. Dawson, R. G. and J. L. McGaugh. Electroconvulsive shockproduced retrograde amnesia: Analysis of the familiarization effect. Communs behav. Biol. A: 4 : 9 1 - 9 5 , 1969. 4. Denti, A. and A. Epstein. Sex differences in the acquisition of two kinds of avoidance behavior in rats. Physiol. Behav. 8: 611-615, 1972. 5. Galosy, R. A. and R. W. Thompson. A further investigation of familiarization effects on ECS-produced retrograde amnesia. Psychonom. Sci. 22: 147-148, 1971.

6. King, R. A. and R. L. Glasser. Duration of electroconvulsive shock-induced retrograde amnesia in rats. Physiol. Behav. 5: 335-339, 1970. 7. Lewis, D. J., R. R. Miller, and J. R. Misanin. Control of retrograde amnesia. J. comp. physiol. Psychol. 66: 48-52, 1968. ,8. McGaugh, J. L. and M. J. Herz. Memory Consolidation. San Francisco: Albion, 1972. 9. Posluns, D. and C. H. Vanderwolf. Improved avoidance performance following exploratory movement. Can. J. Psychol. 24: 499-504, 1970. 10. Ray, O. S. and L. W. Bivens. Reinforcement magnitude as a determinant of performance decrement after electroconvulsive shock. Science 160: 330-332, 1968.