Physiology&Behavior,Vol. 53, pp. 349-352, 1993
0031-9384/93 $6.00 + .00 Copyright © 1993 Pergamon Press Ltd.
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Facilitation of Shuttle-Box Avoidance by the Platform Method: Effects of Conditioned Stimulus Duration MARGALIDA COLL-ANDREU, MARGARITA ISABEL PORTELL-CORTES AND IGNACIO
MARTI-NICOLOVIUS, MORGADO-BERNAL ~
Area de Psicobiologia, Universitat Autdnoma de Barcelona, Ap. 46, 08193 Bellaterra, Barcelona, Spain R e c e i v e d 21 F e b r u a r y 1992 COLL-ANDREU, M., M. MARTI-NICOLOVIUS, I. PORTELL-CORTES AND 1. MORGADO-BERNAL. Facilitation of shuttle-box avoidance by the platform metho& Effects of conditioned stimulus duration. PHYSIOL BEHAV 53(2) 349-352, 1993.--To evaluate whether the duration of the conditioned stimulus (CS) influences the facilitatory effect of posttraining platform treatment upon the acquisition and long-term retention (LTR) of a shuttle-box conditioning, rats were assigned to one of the four following treatments: P-3 group rats were subjected to a 5-h treatment on 16 cm diameter platforms immediately after each of 5 training sessions (10 trials each separated by 24-h intervals) in which the CS consisted of a tone lasting 3 s; control-3 rats were trained the same way but were not subjected to the platform treatment; P- 10 rats did also receive the immediate 5-h treatment on platforms, but the CS was a tone lasting 10 s; finally, control-10 rats did not receive any treatment and were also trained with a 10-s tone. Ten days after training, all rats were also tested for LTR (1 session of 10 trials). When the CS duration was 3 s, the platform treatment improved both the acquisition and LTR of the task, compared to control subjects, but the same treatment had no effect when the CS lasted 10 s. With the use of a 10-s CS, the level of learning achieved by both treated and untreated subjects was similar to the final level of acquisition reached by treated subjects trained with a 3-s CS. We conclude that the facilitatory effects of the platform method treatment upon the acquisition and LTR of a distributed shuttle-box avoidance depend on the difficulty of the task. Learning and memory facilitation
Platform method
Shuttle-box avoidance
IN our laboratory we have shown that, under certain conditions, an immediate posttraining treatment on 16 cm diameter platforms surrounded by water improves the acquisition of a distributed shuttle-box avoidance task in rats (5,10). This effect has been shown to depend on the duration of treatment, because both a 5- and 6-h treatment facilitates this task, although no facilitation is observed when the treatment lasts 3 h (5). The effects of the time of day when it is applied seem to be of little importance, because no statistical differences were observed when the treatment was applied at the beginning of the light cycle than when it was applied at the beginning of the dark cycle (5). As we have reported in our previous works, the moderate degree of stress induced by that treatment and the subsequent enhancement of arousal (CNS excitability) induced during a putative period of m e m o r y consolidation has been proposed as the cause of this facilitatory effect. Therefore, the effect of the platform method upon learning and m e m o r y might share some c o m m o n features with natural processes, such as paradoxical sleep (3,4,8), or with a wide variety of treatments, such as electrical brain stimulation (3,4,6,7), stimulant drugs or stress hormones (9,12), or with other behavioral treatments, such as a
flashing light (7), all of which are accompanied by an increase of CNS arousal and which are capable of improving learning and memory. Having studied some of the temporal parameters influencing the effects of the platform method upon shuttle-box avoidance, we are now interested in investigating whether some conditioning parameters can also have an influence. Because several works have shown that other modulatory treatments can facilitate the acquisition and/or retention of certain tasks when the level achieved by control animals is low but not when it is high (1214), the possibility is raised that a similar finding might be true for the treatment with the platform method. In our previous works the following training parameters were used: the conditioned stimulus (CS) consisted of a tone lasting 3 s, during which time the rat had to make a response (avoidance) in order to avoid an electrical footshock. Five training sessions (10 trials each) separated by 24-h intervals were administered. In a pilot study (unpublished data) we have observed that the duration of the CS affects the basal level of acquisition achieved by untreated subjects. Thus, this level is significantly higher in those animals trained with a 10-s CS than in animals trained with a shorter
Requests for reprints should be addressed to I. Morgado-Bernal.
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{3-s) CS. Similarly, in a parametric study a 10-s duration of CS has been reported to be optimal for shuttle-box acquisition when a single acquisition session consisting of 40-60 trials is used (1). The aim of the present study, therefore, was to investigate the influence of the platform method treatment upon acquisition and long-term retention (LTR) of a distributed shuttle-box conditioning task under two different durations of the CS (3 and 10 s), which give rise to different levels of learning in untreated animals. METHODS
Subjects
Fifty-eight naive male Wistar rats, obtained from our laboratory breeding stock, with a mean age of 95.69 days [standard deviation (SD) = 5.1 ] and mean weight of 454.85 g (SD = 51.50) at the beginning of the experiment, were used as subjects (Ss). All animals were housed individually, always kept under conditions of controlled temperature (20-25 °C) and humidity (4070%), and subjected to an artificial light/darkness cycle of 12/ 12 h (lights on at 0800 h). Food and water were available ad lib. Procedure
The animals were trained in a two-way shuttle-box avoidance conditioning apparatus (Campden Instruments, Ltd.) along 5 consecutive days. Before the acquisition phase, and in order to habituate the animals to the platform method, they were given two adaptation sessions (one daily), each consisting of being placed for 2 h on the platforms. The platforms were inverted flowerpots (16 cm diameter) placed in watertanks, with undersides projecting 1 cm above the surface of the water. The watertanks were covered with wire mesh lids to prevent the animals from escaping and to allow the placement of food and water. To familiarize the Ss with the conditioning box and reduce both the number of pseudoavoidance responses and the inverted relationship that seems to exist between shock intensity and performance because of fear ( 11 ), every animal was given two shuttle-box adaptation sessions, one per day, each consisting of 10 min of free ambulation, under the same lighting and soundisolation conditions that would later be used during the avoidance training, but without either the unconditioned or the conditioned stimuli. The number of crossings made in these shuttle-box adaptation sessions was considered as an index of basic locomotor activity for each animal. The adaptation sessions to the platform and to the shuttle-box were conducted along the same days. At every phase of the experiment all the experimental procedures were carried out during the light cycle. Prior to the training sessions, the Ss were randomly distributed into four groups: 1:'-3 group (n = 16). The rats were placed for 5 h on the platforms immediately after each training session, in which a 3s CS was used. Control-3 group (n = 15). The rats were given no treatment after the training sessions, in which a 3-s conditioned stimulus was also used. P-IO group (n = 14). The rats were placed for 5 h on the platforms immediately after each training sessions, in which a 10-s CS was used. Control-lO group (n = 13). The rats were given no treatment after the training sessions, in which a 10-s CS was used. The conditioning paradigm consisted of a two-way active avoidance task (five acquisition sessions, one per day, 10 trials each). The CS was an 80 dB and I kHz tone of 3- or 10-s duration. When the CS was on, the animal had to cross to the other side
of the shuttle-box apparatus (avoidance response) in order to set it offand to avoid the appearance of the unconditioned stimulus (US). The latter was a 1 mA electrical footshock that was set offwhen the animal made an escape response. If the animal did not escape, the US lasted 30 s. The trials in a session tbllowed a variable interval schedule of I min. In addition to the number of avoidance responses made in each conditioning session (considered as the level of performance), intertrial crossings were also scored. Immediately before the first acquisition session the Ss were given another adaptation session to the shuttle box identical to the previous mentioned ones. Ten days after the last training session all Ss were given another 10-trial session in order to evaluate the level of LTR of the learned response in each of the four experimental groups. RESUI.TS
Four Ss (two in P-3 group; one in control-3 group and one in P-10 group) were excluded from the experiment because of having displayed abnormal shuttle-box behavior (they made null responses, i.e., did not escape the shock and, thus, in most trials they received the US during the 30 s in which it was delivered), therefore leaving 54 Ss (14 in P-3 and control-3 groups and 13 in P-10 and control-10 groups). Rat weight differences among groups throughout the experiment were found to be nonsignificant (MANOVA). Regarding the basal activity in the shuttle-box apparatus (crossings during the adaptation sessions), no differences existed among groups on any session. On the other hand, a decrease in the mean number of crossings made by the Ss during the second adaptation shuttle-box session in comparison with the first one was observed taking the sample as a whole, F(50, 1) = 77.15; p < 0.001, suggesting the animals' initial adaptation to the experimental box. Thus, the mean number of crossings during the first adaptation session for the whole sample was 37.94 (SD = 12.12), although in the second adaptation session this value decreased to 24.44 (SD = 10.79). No significant differences existed in the mean number of crossings made by the Ss when comparing the second adaptation session with the third one (carried out immediately before the first training session). The mean value of the latter session was 20.27 (SD = 9.13). Moreover, no differences existed between groups regarding the number of intertrial crossings during the acquisition and LTR sessions. No significant correlation was found between the number of avoidances made by the subjects on any training sessions and the level of motor activity (intertrial crossings) on the same sessions. As shown in Fig. 1, and according to what was expected, the animals in group control-10 showed a higher performance than animals in control-3 group throughout the training and LTR. On the other hand, the performance of the animals treated on platforms, compared to that of their respective controls, was better both during acquisition and LTR only when the CS consisted of a tone lasting 3 s, but not when the CS lasted 10 s. That is, the facilitatory effect of the platform method was only observed using a 3-s CS. The statistical analyses confirmed those effects. A mixed multivariate analysis of variance (MANOVA) for all the Ss included in the experiment indicated that both group, F(50, 3) = 10.26; p < 0.001, and session, F(250, 5) = 66.37; p < 0.001, factors were significant, the interaction between group and session being also significant, F(250, 15) = 3.36; p < 0.001. Therefore, further analyses were made in order to determine the specific differences which existed among groups on each session. Those analyses (MANOVA, simple contrast) showed that control-3 group differed from control-10 and P-10 groups on all sessions
CS D U R A T I O N A N D P L A T F O R M M E T H O D EFFECTS
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FIG. 1. Acquisition and LTR of avoidance conditioning for each experimental group. Each point in the figure represents the mean score (_+SEM) of the group (n = 14 in P-3 and control-3 groups and n = 13 in P-10 and control-10 groups) in each of the shuttle-box sessions. The right side of the figure depicts the performance of the two groups trained with a 10s CS, while the performance of the two groups trained with a 3-s CS is depicted in the left side. The platform method treatment (solid lines) only improved the acquisition and LTR of the task, compared with control subjects (dashed lines), when the CS lasted 3 s, although no improvement was observed when the CS lasted l0 s. Under the latter conditions, both treated and untreated subjects showed a high level of performance.
with exception of the first acquisition session [differences between control-3 and control-10 groups: 2nd session: F(50, 1) = 22.42, p < 0.000 i; 3rd session: F(50, 1) = 20.50, p < 0.0001; 4th session: F(50, 1) = 13.35, p < 0.0001; 5th session: F(50, 1) = 12.12, p = 0.001; LTR: F(50, 1) = 30.48, p < 0.0001; differences between control-3 and P-10 groups: 2nd session: F(50, 1) = 22.42, p < 0.001; 3rd session: F(50, 1) = 14.15, p < 0.0001; 4th session: F(50, 1) = 7.79, p = 0.007; 5th session: F(50, 1) = 12.12, p = 0.0001; LTR: F(50, 1) = 29.48, p < 0.0001]. Although no statistical differences existed between P-3 and control-3 groups on the first, second, and third acquisition sessions, a tendency to significant differences between those groups was already observed on the fourth acquisition session (p = 0.06). Those differences reached statistical significance on the fifth session, F(50, 1) = 4.79, p = 0.03, and were maintained on the L T R test, F(50, 1) = 16.75, p < 0.0001. In those sessions, the performance of P-3 Ss was significantly higher than that of control-3 Ss. On the other hand, no significant differences existed between P-10 and control-10 groups on any of the sessions. Regarding the differences between P-3 group and those groups of subjects trained with a 10-s CS, it was shown that the performance of the former group was significantly lower than that of the other two groups on the first, second, and third acquisition sessions [differences between P-3 and P-10 groups: 1st session: F(50, 1) = 5.25, p = 0.026; 2nd session: F(50, 1) = 12.42, p < 0.001; 3rd session: F(50, 1) = 3.93, p = 0.05; differences between P-3 and control-10 groups: 1st session: F(50, 1) = 4.05, p = 0.05; 2nd session: F(50, 1) = 12.42, p < 0.001; 3rd session: F(50, 1) = 7.55, p = 0.008]. However, no significant differences existed
among P-3, P-10, and control-10 groups on the 4th and 5th acquisition sessions or during LTR. The development of learning followed the expected upward linear tendency in all groups. This tendency was confirmed by a multivariate analysis of variance [ M A N O V A (polynomial contrast, 1st degree) for all groups pooled: F(50, 1); p < 0.001]. The differences among groups seemed, therefore, attributable to different learning slopes, not to different kinds of algorithmieal functions. DISCUSSION AS in our previous experiments (5,10), when the CS consisted of a tone of 3-s duration, a 5-h treatment on 16 cm platforms surrounded by water has shown to improve both the acquisition and L T R (10 days) of a distributed shuttle-box avoidance conditioning, compared to an untreated control group. However, the same treatment has shown no effect upon either acquisition or L T R of the same task when the CS consisted of a tone lasting l0 s. Under the latter conditions, both treated and untreated groups of Ss show a similar high level o f acquisition and LTR. Thus, according to what was expected, the longer duration of the CS (10 s) seems to diminish the difficulty level of two-way avoidance in such a way that no platform treatment is needed for the subjects to achieve a high performance level, although the same treatment can improve learning when the CS has a shorter duration (3 s) and the level of learning of the control untreated group is low. A somewhat comparable effect has been observed using other modulatory treatments, such as stress hormones. Thus, it has
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been reported that, at certain doses, the administration of epinephrine or arginine vasopressin:_4, an analog of vasopressin devoid of endocrine actions, can improve memory' under conditions which result in a low level of performance in untreated subjects, but no improvement is observed when other conditions are used that result in a high level of performance in control animals (12,14). Moreover, the effects of the latter h o r m o n e upon m e m o r y are also dependent on the proficiency of each individual animal, because this substance does not affect the performance of the more proficient subjects while i m p r o v i n g m e m o r y in the less adept animals (13). This effect has also been found c o m p a r i n g the performance of the Ss under different tasks (2). As a general rule, the differential effects of the same m o d u l a t o r seem to depend on the difficulty of the task. Therefore, it seems that the effects of a given treatment upon learning and memory depend
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2. on some features of the task, such as the specific conditioning parameters used which, in turn, determine its difficulty level for a given animal. In the present work, when the acquisition of the learning task has been improved by prolonging the duration of the CS, the enhancement of arousal level by treating the subjects on the platform method has not been capable of further improving the performance of the subjects. This is presumably due to the fact that the task was already easy enough for the animals to learn. It seems, therefore, that under learning conditions, both conditioning parameters and arousal level might interact in a complementary way. This fact suggests that any work intended to investigate the effects of any modulatory, treatment upon learning and memory should, therefore, take into account this interaction. It also might explain the conflicting results observed with similar treatments by different researchers. ACKNOWI.EDGEMENTS
1. on the interaction between the arousal level induced by training and treatment, and
This work was supported by a DGICYT grant (PB89-0315) and a DGU grant (lnfraestructura 1991).
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