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Amelioration of an age-related deficit in exploratory behavior by preexposure to the test environment
BEHAVIORAL AND NEURAL BIOLOGY 3 4 , 5 5 - 6 2
(1982)
Amelioration of an Age-Related Deficit in Exploratory Behavior by Preexposure to the Test Environment 1 MICHAEL J. BRENNAN, DAVID A . BLIZARD, 2 AND DAVID QUARTERMAIN
Division of Behavioral Neurology, Department of Neurology, New York University School of Medicine, New York, New York 10010 Aged (28-month-old) C57BL/6N Nia mice spent significantly less time exploring a novel stimulus than 4- or 8-month-old mice. The lower level of exploratory behavior in 28-month-old mice was independent of age differences in locomotor activity and the detection of a novel stimulus. Prior exposure to the test apparatus, without the novel stimulus present, produced a significant enhancement of the exploratory behavior in 28-month-old mice and attenuated age differences in exploratory behavior. It is suggested that the reduced exploratory behavior of 28-month-old mice is due to their greater reactivity to the novel stimulus.
Exposure to a novel environment or to changes in the stimulus characteristics of a previously experienced environment will typically elicit exploratory behavior in animals. The nature of the animal's response to the environment is dependent on the novelty or the discrepancy between the stimuli which are present in the test environment and stimuli which have been previously experienced and represented in memory (Berlyne, 1960; O'Keefe & Nadel, 1978). This fundamental form of learning has been found to vary during development (e.g., File, 1978). The basic intent of the present study was to determine whether an animal's response to novel stimuli is likewise altered during aging. Though various studies of open field behavior in rodents (Goodrick, 1967; Sprott & Eleftheriou, 1974; Werboff & Havlena, 1962) have suggested that exploratory behavior declines with age, the effect of aging on exploratory behavior is unclear. This ambiguity stems, in large part, from the almost exclusive reliance on the open field test as a means of assessing the effect of aging on exploratory behavior. Despite its apparent popularity, the open field test has numerous limitations, which have been periodically reviewed (Archer, 1973; Corey, 1978; Walsh & Cummins, 1976). 1 This research was supported by NIH Grants R23 AG02159 (M. J. B.) and RO 1 AG00760 (D. Q.). 2 Present address: Department of Physiology, Bowman-Gray School of Medicine, Winston-Salem, NC 27103. 55 0163 - 1047/82/010055-08 $02.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
56
BRENNAN, BLIZARD, AND QUARTERMAIN
The open-field test does not provide a means of distinguishing clearly between spontaneous locomotor activity and stimulus-directed exploratory behavior. The observed differences in the open-field behavior of aged animals may be merely coincidental to age-related changes in general locomotor activity. Further, it is often unclear whether open-field behavior is reflective of exploratory behavior or escape behavior (Welker, 1959). Although the free-exploration method of open-field testing has been suggested as a means of resolving some of these problems (Goodrick, 1971), this alternative is not entirely satisfactory. Elias, Elias, and Eleftheriou (1975) found that, when free exploration scores were adjusted for differences in general locomotor activity, no age-related differences in exploratory behavior were indicated. The underlying problem in interpreting open field activity as exploratory behavior is the inability to specify the stimuli which elicit activity. Test situations, in which discrete stimuli are present and distinguished from more general background stimuli, should provide a clearer means of studying the response to novelty in different age groups. In the present experiment, we used a modification of the paradigm of Wimer and Fuller (1965). The response to a novel stimulus by young and aged mice was examined under two conditions: (1) upon the subject's initial exposure to the test environment, and (2) following a 20-min exposure to the test apparatus itself. This latter condition was included as an attempt to further enhance the distinctiveness of the novel stimulus by first habituating subjects to general apparatus cues.
METHODS
Subjects The subjects for this experiment were 4-, 8-, and 28-month-old male C57BL/6N Nia mice, obtained from the National Institute of Aging. The mice were adapted to the laboratory for 1 month prior to the start of the experiment. The mice were housed 3-5 to a cage in an insolated vivarium, in which a 12-hr light-dark cycle (0700 hr light onset) was in effect.
Apparatus The test apparatus consisted of 4 equal (9.5 cm 2) quadrants with 12.7cm-high walls, constructed out of Plexiglas. Three of the quadrants were white with a black plastic ball (3.2-cm diameter) placed in the center of each quadrant. The fourth quadrant was removable and will be designated the exchange quadrant. Depending on testing conditions the exchange quadrant was either identical to the three other quadrants or was black with a white wire spiral (3.2 cm diameter, 3.2 cm high) placed in the center of the quadrant.
AGE AND EXPLORATORY BEHAVIOR
57
Procedure Twenty-four hours prior to the start of the experiment, animals were individually housed. Half the mice in each age group were given a 20min exposure to the apparatus, with a black exchange quadrant in place (control condition). The remaining half of the animals (preexposure condition, PE) were given a 20-min exposure to the test apparatus with four identical quadrants. Each animal was placed into the quadrant diagonally opposite to the exchange quadrant. The latency to enter the exchange quadrant, the total number of entries, and the total duration of time spent in the exchange quadrant during the initial 2 min of the exposure period were recorded. An entry into the exchange quadrant was scored when the animal had completely (all 4 paws) crossed into the exchange quadrant. Following the 20-min exposure period, each animal was removed from the apparatus and placed in a holding cage. After a 1-min intertrial interval, half the animals in each condition were given a 2-min test in the apparatus; the remaining animals were tested 24 hr later. On the test trial, all animals were tested with a black exchange quadrant placed in the apparatus. In summary, the design for this study was a 3 (age) x 2 (preexposure condition, control vs PE) x 2 (test interval, 1 min vs 24 hr) factorial, with approximately 7 animals per group. All testing was conducted in a quiet, dimly illuminated room between 1300 and 1600 hr. Throughout the experiment, mice were maintained on food and water ad libitum. RESULTS
Initial Exposure The data for initial exposure period are summarized in Table 1. Analyses of variance performed on each of the behavioral measures revealed a clear effect of stimulus condition. When a black exchange quadrant was in place (control condition), mice in each age group made significantly more entries (F(1, 83) = 4.33, p < .05), had a significantly shorter latency to enter the exchange quadrant (F(I, 83) = 4.11, p < .05), and spent more time in the exchange quadrant (F(1, 83) = 77.81, p < .001) than mice exposed to four identical quadrants (PE condition). No significant differences were noted as a function of age in terms of either the latency to the exchange quadrant or the total number of entries. An age-related decline in exploratory behavior was indicated, however, by a significant age by exposure condition interaction (F(2, 83) = 3,29, p < .05) for the total duration measure; 28-month-old control mice spent significantly less time in the exchange quadrant than either 4- or 8-monthold control mice (p < .05). No significant differences in total duration were observed between 4- and 8-month-old control mice, nor were any
58
BRENNAN, BLIZARD, AND QUARTERMAIN
TABLE 1 The Performance of 4-, 8-, and 28-Month-Old C57BL/6N Nia Mice during the First 2 Min of the Initial Exposure Period, Shown as a Function of Exposure Condition
Exposure condition ~ Control
Preexposed (PE)
Age (months) 4 8 28 4 8 28
Mean latency to enter the exchange quadrant (sec) 13.5 15.2 16.3 25.1 20.1 30.7
(-4-3.56) b (-+3.13) (-+2.20) (-+6.63) (-+6.26) (-+12.05)
Mean total entries
Mean duration of time in exchange quadrant (sec)
6.0 5.3 4.7 4.8 4.5 4.1
59.2 61.5 39.2 19.2 21.2 20.5
(___0.56) (-+0.36) (---0.82) (-+0.56) (-+0.46) (-+0.68)
(---6.40) (-+4.04) (---5.06)c (-+ 1.86) (-+3.62) (-+3.95)
a For control condition the exchange quadrant was black and the remaining quadrants were white. For PE condition all four quadrants were white. b The standard error of the mean is shown in parentheses. c Significantly less time than either 4- or 8-month-old mice (p < .05).
significant age differences in total duration observed between mice in the PE condition. Test Trial The data for the test trial are summarized in Table 2. An effect of preexposure was indicated by the enhancement of the duration of exploration in mice exposed to the novel (black) exchange quadrant 1 min after a 20-min exposure to four homogeneous (white) quadrants. PE mice spent significantly more time in the exchange quadrant than control mice on the test trial (F(1, 39) = 12.62, p < .001). In order to further assess the degree of enhancement, comparisons were made between the test performance of PE mice and the performance of control mice during the initial exposure period. The most pronounced preexposure effect was observed in 28-month-old PE mice, tested after a 1-min intertrial interval. These animals exhibited a significant increase in the duration of exploration (F(1, 16) = 5.86, p < .05). While both 4- and 8-month-old PE mice spent more time in the exchange quadrant on the test trial than did control mice during the initial exposure period, these differences were not statistically significant. More importantly, however, preexposure to the test environment was found to attenuate age-related differences in exploratory behavior. In contrast to the differences observed between control mice during the initial exposure period, no significant age differences in the duration of exploration were observed between PE mice tested at the 1-min interval.
AGE
AND
EXPLORATORY
59
BEHAVIOR
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BRENNAN, BLIZARD, AND QUARTERMAIN
The facilitative effect of preexposure appeared to be relatively shortlived. When the test trial was delayed 24 hr, no enhancement of exploration was observed in PE mice; in fact, a depression of exploration was indicated. Compared to PE mice tested at the l-rain interval, PE mice, in each age group, spent significantly less time in the exchange quadrant (F(1, 37) = 32.38, p < .001) when tested 24 hr after preexposure to the test apparatus• As indicated in Table 2, 4- and 8-month-old control mice exhibited only a slight, nonsignificant, habituation of the duration of exploration on the test trial. In contrast, 28-month-old control mice exhibited an increase in duration of exploration on the test trial. These differences, however, were only found to be marginally significant (F(2, 40) = 2.72, • 10 > p
> .05).
DISCUSSION The present study provides a demonstration of an age-related reduction in exploratory behavior. The absence of age differences in the initial latency to enter the exchange quadrant and in the frequency of entries into the exchange quadrant during the initial exposure period suggest that this difference in exploratory behavior was independent of any agerelated changes in general locomotor activity. The possibility that the reduced duration of exploration by aged mice was related to age differences in brightness preference (e.g., Wax & Goodrick, 1975) is negated by the absence of any significant differences in the test performance of preexposed mice, upon their initial exposure to the black exchange quadrant. The fact that preexposure to the test environment attenuated age differences in exploratory behavior is of particular significance. Goodrick (1971) attempted to alter age-related differences in exploratory behavior by social pair testing, prior handling, and by varying the time of day when animals were tested. While these manipulations were found to affect the behavior of young rats, these manipulations were without any appreciable influence on the behavior of aged rats. In light of Goodrick' s findings, the fact that apparatus preexposure was shown, in the present experiment, to affect the behavior of aged mice suggests some factors underlying age-related differences in exploratory behavior. One possibility is that preexposure to the test environment enhanced the distinctiveness of the novel stimulus. Upon an animal's initial exposure to the test environment, the black exchange quadrant, as well as general apparatus cues, represent novel stimuli. With habituation to the general apparatus cues, mice may be more likely to attend to, and to explore, the novel quadrant. It might be suggested that, in the absence of preexposure to the test apparatus, aged mice were less likely to detect, and to respond to, the presence of the novel quadrant. The fact that no
AGE AND EXPLORATORY BEHAVIOR
61
significant age differences were observed in the initial latency to enter the exchange quadrant nor in the total frequency of entries, however, suggests that there were no age-related differences in the detection of the novel stimulus. Alternatively, the present results may reflect age differences in reactivity to the novel stimulus. In addition to its informational properties, a novel stimulus has arousal- or stress-inducing properties, as suggested by the elevation of plasma corticosterone levels that is observed in rodents following exposure to a novel environment (e.g., Hennessy & Levine, 1979). It is possible that exposure to the novel stimulus and the test environment induced a greater stress response in the aged mice, resulting in a suppression of exploratory behavior. In support of this view, aged C57BL/6J mice have been found to exhibit a greater elevation in plasma corticosterone levels following exposure to a novel open field than do 6- to 9-month-old mice (Eleftheriou, 1974). The enhancement effect of apparatus preexposure may be due to an habituation of a stress response to the test environment during the initial exposure period. The fact that aged control mice exhibited an increase, rather than a decrease, in exploration on the test trial may likewise be a reflection of a stressinduced suppression of exploratory behavior rather than an impairment of habituation, per se. In support of this latter supposition, we have consistently observed that aged C57BL/6 N Nia mice exhibit a significant increase in hole-board exploratory behavior during a 20-min test session. Further, it is of interest to note that one of the manipulations, prior handling, which Goodrick (1971) found to be without effect on the exploratory behavior of aged rats, likewise did not appear to affect the emotionality in aged rats. No differences in open field defecation were observed between handled and nonhandled aged rats. Finally, the apparent temporal fragility of the preexposure effect indicated in the present study was, in part, a function of the brief duration of the test period. In subsequent studies with adult mice (Note 1), we have observed an enhancement of exploratory behavior 24 hr after preexposure to the test apparatus. This enhancement, however, was not observed during the subjects' initial 1- to 2-min reexposure to the test apparatus, but was detected when the duration of the test period was extended. In summary, we have demonstrated an age-related decline in exploratory behavior. Because these differences were attenuated when mice were preexposed to the test apparatus, and in view of related findings, we have suggested age differences in initial exploratory behavior reflected a greater stress-induced suppression of exploratory behavior in aged mice. Additional work is required to further confirm this supposition and to identify its physiological basis. If, however, aged animals are to be characterized by a greater reactivity to novel stimuli, the implications
62
BRENNAN, BLIZARD, AND QUARTERMAIN
of the present findings extend beyond the discussion of age-related differences in exploratory behavior. Greater reactivity to novel environments may contribute to impairments of acquisition and/or retention performance of aged rats in associative learning situations. REFERENCES Archer, J. (1973). Tests for emotionality in rats and mice: A review. Animal Behavior, 21, 205-235. Berlyne, D. E. (1960). Conflict, Arousal, and Curiosity. New York: McGraw-Hill. Brennan, M. J., & Quartermain, D. (1980). Age-related differences in within session habituation: The effect of stimulus complexity. The Gerontologist, 20, 71 (abst.) Corey, D. T. (1978). The determinants of exploration and neophobia. Neuroscience and Biobehavioral Reviews, 2, 235-253. Eleftherion, B. E. (1974). Changes with age in pituitary-adrenal responsiveness and reactivity to mild stress in mice. Gerontologia, 20, 224-230. Elias, P. K., Elias, M. F., & Eleftheriou, B. E. (1975). Emotionality, exploratory behavior, and locomotion in aging inbred strains of mice. Gerontologia, 21, 46-55. File, S. E. (1978). The ontogeny of exploration in the rat: Habituation and the effects of handling. Developmental Psychobiology, 11, 321-328. Goodrick, C. L. (1967). Exploration of nondeprived male Sprague-Dawley rats as a function of age. Psychological Reports, 20, 159-163. Goodrick, C. L. (1971). Variables affecting free exploration responses of male and female Wistar rats as a function of age. Developmental Psychology, 4, 440-446. Hennessy, J. W., & Levine, S. (1979). Stress, arousal, and the pituitary-adrenal system: A psychoendocrine hypothesis. Progress in Psychobiology and Physiological Psychology, 8, 133-178. O'Keefe, J., & Nadel, L. (1978). The Hippocampus as a Cognitive Map. London/New York: Oxford Univ. Press. Sprott, R. L., & Eleftheriou, B. E. (1974). Open-field behavior in aging inbred mice. Gerontologia, 20, 155-162. Walsh, R. N., & Cummins, R. A. (1976). The open-field test: A critical review. Psychological Bulletin, 83, 482-504. Wax, T. M., & Goodrick, C. L. (1975). Voluntary exposure to light by young and aged albino and pigmented inbred mice as a function of light intensity. Developmental Psychobiology, 8, 297-303. Welker, W. I. (1959). Escape, exploratory, and food-seeking responses of rats in a novel situation. Journal of Comparative and Physiological Psychology, 52, 106-111. Werboff, J., & Havlena, J. (1962). Effects of aging on open field behavior. Psychological Reports, 10, 395-398. Wimer, R. E., & Fuller, J. L. (1965). The effects of d-amphetamine sulphate on three exploratory behaviors. Canadian Journal of Psychology, 19, 94-103.
REFERENCE NOTE 1. Unpublished findings.
(1982)
Amelioration of an Age-Related Deficit in Exploratory Behavior by Preexposure to the Test Environment 1 MICHAEL J. BRENNAN, DAVID A . BLIZARD, 2 AND DAVID QUARTERMAIN
Division of Behavioral Neurology, Department of Neurology, New York University School of Medicine, New York, New York 10010 Aged (28-month-old) C57BL/6N Nia mice spent significantly less time exploring a novel stimulus than 4- or 8-month-old mice. The lower level of exploratory behavior in 28-month-old mice was independent of age differences in locomotor activity and the detection of a novel stimulus. Prior exposure to the test apparatus, without the novel stimulus present, produced a significant enhancement of the exploratory behavior in 28-month-old mice and attenuated age differences in exploratory behavior. It is suggested that the reduced exploratory behavior of 28-month-old mice is due to their greater reactivity to the novel stimulus.
Exposure to a novel environment or to changes in the stimulus characteristics of a previously experienced environment will typically elicit exploratory behavior in animals. The nature of the animal's response to the environment is dependent on the novelty or the discrepancy between the stimuli which are present in the test environment and stimuli which have been previously experienced and represented in memory (Berlyne, 1960; O'Keefe & Nadel, 1978). This fundamental form of learning has been found to vary during development (e.g., File, 1978). The basic intent of the present study was to determine whether an animal's response to novel stimuli is likewise altered during aging. Though various studies of open field behavior in rodents (Goodrick, 1967; Sprott & Eleftheriou, 1974; Werboff & Havlena, 1962) have suggested that exploratory behavior declines with age, the effect of aging on exploratory behavior is unclear. This ambiguity stems, in large part, from the almost exclusive reliance on the open field test as a means of assessing the effect of aging on exploratory behavior. Despite its apparent popularity, the open field test has numerous limitations, which have been periodically reviewed (Archer, 1973; Corey, 1978; Walsh & Cummins, 1976). 1 This research was supported by NIH Grants R23 AG02159 (M. J. B.) and RO 1 AG00760 (D. Q.). 2 Present address: Department of Physiology, Bowman-Gray School of Medicine, Winston-Salem, NC 27103. 55 0163 - 1047/82/010055-08 $02.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
56
BRENNAN, BLIZARD, AND QUARTERMAIN
The open-field test does not provide a means of distinguishing clearly between spontaneous locomotor activity and stimulus-directed exploratory behavior. The observed differences in the open-field behavior of aged animals may be merely coincidental to age-related changes in general locomotor activity. Further, it is often unclear whether open-field behavior is reflective of exploratory behavior or escape behavior (Welker, 1959). Although the free-exploration method of open-field testing has been suggested as a means of resolving some of these problems (Goodrick, 1971), this alternative is not entirely satisfactory. Elias, Elias, and Eleftheriou (1975) found that, when free exploration scores were adjusted for differences in general locomotor activity, no age-related differences in exploratory behavior were indicated. The underlying problem in interpreting open field activity as exploratory behavior is the inability to specify the stimuli which elicit activity. Test situations, in which discrete stimuli are present and distinguished from more general background stimuli, should provide a clearer means of studying the response to novelty in different age groups. In the present experiment, we used a modification of the paradigm of Wimer and Fuller (1965). The response to a novel stimulus by young and aged mice was examined under two conditions: (1) upon the subject's initial exposure to the test environment, and (2) following a 20-min exposure to the test apparatus itself. This latter condition was included as an attempt to further enhance the distinctiveness of the novel stimulus by first habituating subjects to general apparatus cues.
METHODS
Subjects The subjects for this experiment were 4-, 8-, and 28-month-old male C57BL/6N Nia mice, obtained from the National Institute of Aging. The mice were adapted to the laboratory for 1 month prior to the start of the experiment. The mice were housed 3-5 to a cage in an insolated vivarium, in which a 12-hr light-dark cycle (0700 hr light onset) was in effect.
Apparatus The test apparatus consisted of 4 equal (9.5 cm 2) quadrants with 12.7cm-high walls, constructed out of Plexiglas. Three of the quadrants were white with a black plastic ball (3.2-cm diameter) placed in the center of each quadrant. The fourth quadrant was removable and will be designated the exchange quadrant. Depending on testing conditions the exchange quadrant was either identical to the three other quadrants or was black with a white wire spiral (3.2 cm diameter, 3.2 cm high) placed in the center of the quadrant.
AGE AND EXPLORATORY BEHAVIOR
57
Procedure Twenty-four hours prior to the start of the experiment, animals were individually housed. Half the mice in each age group were given a 20min exposure to the apparatus, with a black exchange quadrant in place (control condition). The remaining half of the animals (preexposure condition, PE) were given a 20-min exposure to the test apparatus with four identical quadrants. Each animal was placed into the quadrant diagonally opposite to the exchange quadrant. The latency to enter the exchange quadrant, the total number of entries, and the total duration of time spent in the exchange quadrant during the initial 2 min of the exposure period were recorded. An entry into the exchange quadrant was scored when the animal had completely (all 4 paws) crossed into the exchange quadrant. Following the 20-min exposure period, each animal was removed from the apparatus and placed in a holding cage. After a 1-min intertrial interval, half the animals in each condition were given a 2-min test in the apparatus; the remaining animals were tested 24 hr later. On the test trial, all animals were tested with a black exchange quadrant placed in the apparatus. In summary, the design for this study was a 3 (age) x 2 (preexposure condition, control vs PE) x 2 (test interval, 1 min vs 24 hr) factorial, with approximately 7 animals per group. All testing was conducted in a quiet, dimly illuminated room between 1300 and 1600 hr. Throughout the experiment, mice were maintained on food and water ad libitum. RESULTS
Initial Exposure The data for initial exposure period are summarized in Table 1. Analyses of variance performed on each of the behavioral measures revealed a clear effect of stimulus condition. When a black exchange quadrant was in place (control condition), mice in each age group made significantly more entries (F(1, 83) = 4.33, p < .05), had a significantly shorter latency to enter the exchange quadrant (F(I, 83) = 4.11, p < .05), and spent more time in the exchange quadrant (F(1, 83) = 77.81, p < .001) than mice exposed to four identical quadrants (PE condition). No significant differences were noted as a function of age in terms of either the latency to the exchange quadrant or the total number of entries. An age-related decline in exploratory behavior was indicated, however, by a significant age by exposure condition interaction (F(2, 83) = 3,29, p < .05) for the total duration measure; 28-month-old control mice spent significantly less time in the exchange quadrant than either 4- or 8-monthold control mice (p < .05). No significant differences in total duration were observed between 4- and 8-month-old control mice, nor were any
58
BRENNAN, BLIZARD, AND QUARTERMAIN
TABLE 1 The Performance of 4-, 8-, and 28-Month-Old C57BL/6N Nia Mice during the First 2 Min of the Initial Exposure Period, Shown as a Function of Exposure Condition
Exposure condition ~ Control
Preexposed (PE)
Age (months) 4 8 28 4 8 28
Mean latency to enter the exchange quadrant (sec) 13.5 15.2 16.3 25.1 20.1 30.7
(-4-3.56) b (-+3.13) (-+2.20) (-+6.63) (-+6.26) (-+12.05)
Mean total entries
Mean duration of time in exchange quadrant (sec)
6.0 5.3 4.7 4.8 4.5 4.1
59.2 61.5 39.2 19.2 21.2 20.5
(___0.56) (-+0.36) (---0.82) (-+0.56) (-+0.46) (-+0.68)
(---6.40) (-+4.04) (---5.06)c (-+ 1.86) (-+3.62) (-+3.95)
a For control condition the exchange quadrant was black and the remaining quadrants were white. For PE condition all four quadrants were white. b The standard error of the mean is shown in parentheses. c Significantly less time than either 4- or 8-month-old mice (p < .05).
significant age differences in total duration observed between mice in the PE condition. Test Trial The data for the test trial are summarized in Table 2. An effect of preexposure was indicated by the enhancement of the duration of exploration in mice exposed to the novel (black) exchange quadrant 1 min after a 20-min exposure to four homogeneous (white) quadrants. PE mice spent significantly more time in the exchange quadrant than control mice on the test trial (F(1, 39) = 12.62, p < .001). In order to further assess the degree of enhancement, comparisons were made between the test performance of PE mice and the performance of control mice during the initial exposure period. The most pronounced preexposure effect was observed in 28-month-old PE mice, tested after a 1-min intertrial interval. These animals exhibited a significant increase in the duration of exploration (F(1, 16) = 5.86, p < .05). While both 4- and 8-month-old PE mice spent more time in the exchange quadrant on the test trial than did control mice during the initial exposure period, these differences were not statistically significant. More importantly, however, preexposure to the test environment was found to attenuate age-related differences in exploratory behavior. In contrast to the differences observed between control mice during the initial exposure period, no significant age differences in the duration of exploration were observed between PE mice tested at the 1-min interval.
AGE
AND
EXPLORATORY
59
BEHAVIOR
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60
BRENNAN, BLIZARD, AND QUARTERMAIN
The facilitative effect of preexposure appeared to be relatively shortlived. When the test trial was delayed 24 hr, no enhancement of exploration was observed in PE mice; in fact, a depression of exploration was indicated. Compared to PE mice tested at the l-rain interval, PE mice, in each age group, spent significantly less time in the exchange quadrant (F(1, 37) = 32.38, p < .001) when tested 24 hr after preexposure to the test apparatus• As indicated in Table 2, 4- and 8-month-old control mice exhibited only a slight, nonsignificant, habituation of the duration of exploration on the test trial. In contrast, 28-month-old control mice exhibited an increase in duration of exploration on the test trial. These differences, however, were only found to be marginally significant (F(2, 40) = 2.72, • 10 > p
> .05).
DISCUSSION The present study provides a demonstration of an age-related reduction in exploratory behavior. The absence of age differences in the initial latency to enter the exchange quadrant and in the frequency of entries into the exchange quadrant during the initial exposure period suggest that this difference in exploratory behavior was independent of any agerelated changes in general locomotor activity. The possibility that the reduced duration of exploration by aged mice was related to age differences in brightness preference (e.g., Wax & Goodrick, 1975) is negated by the absence of any significant differences in the test performance of preexposed mice, upon their initial exposure to the black exchange quadrant. The fact that preexposure to the test environment attenuated age differences in exploratory behavior is of particular significance. Goodrick (1971) attempted to alter age-related differences in exploratory behavior by social pair testing, prior handling, and by varying the time of day when animals were tested. While these manipulations were found to affect the behavior of young rats, these manipulations were without any appreciable influence on the behavior of aged rats. In light of Goodrick' s findings, the fact that apparatus preexposure was shown, in the present experiment, to affect the behavior of aged mice suggests some factors underlying age-related differences in exploratory behavior. One possibility is that preexposure to the test environment enhanced the distinctiveness of the novel stimulus. Upon an animal's initial exposure to the test environment, the black exchange quadrant, as well as general apparatus cues, represent novel stimuli. With habituation to the general apparatus cues, mice may be more likely to attend to, and to explore, the novel quadrant. It might be suggested that, in the absence of preexposure to the test apparatus, aged mice were less likely to detect, and to respond to, the presence of the novel quadrant. The fact that no
AGE AND EXPLORATORY BEHAVIOR
61
significant age differences were observed in the initial latency to enter the exchange quadrant nor in the total frequency of entries, however, suggests that there were no age-related differences in the detection of the novel stimulus. Alternatively, the present results may reflect age differences in reactivity to the novel stimulus. In addition to its informational properties, a novel stimulus has arousal- or stress-inducing properties, as suggested by the elevation of plasma corticosterone levels that is observed in rodents following exposure to a novel environment (e.g., Hennessy & Levine, 1979). It is possible that exposure to the novel stimulus and the test environment induced a greater stress response in the aged mice, resulting in a suppression of exploratory behavior. In support of this view, aged C57BL/6J mice have been found to exhibit a greater elevation in plasma corticosterone levels following exposure to a novel open field than do 6- to 9-month-old mice (Eleftheriou, 1974). The enhancement effect of apparatus preexposure may be due to an habituation of a stress response to the test environment during the initial exposure period. The fact that aged control mice exhibited an increase, rather than a decrease, in exploration on the test trial may likewise be a reflection of a stressinduced suppression of exploratory behavior rather than an impairment of habituation, per se. In support of this latter supposition, we have consistently observed that aged C57BL/6 N Nia mice exhibit a significant increase in hole-board exploratory behavior during a 20-min test session. Further, it is of interest to note that one of the manipulations, prior handling, which Goodrick (1971) found to be without effect on the exploratory behavior of aged rats, likewise did not appear to affect the emotionality in aged rats. No differences in open field defecation were observed between handled and nonhandled aged rats. Finally, the apparent temporal fragility of the preexposure effect indicated in the present study was, in part, a function of the brief duration of the test period. In subsequent studies with adult mice (Note 1), we have observed an enhancement of exploratory behavior 24 hr after preexposure to the test apparatus. This enhancement, however, was not observed during the subjects' initial 1- to 2-min reexposure to the test apparatus, but was detected when the duration of the test period was extended. In summary, we have demonstrated an age-related decline in exploratory behavior. Because these differences were attenuated when mice were preexposed to the test apparatus, and in view of related findings, we have suggested age differences in initial exploratory behavior reflected a greater stress-induced suppression of exploratory behavior in aged mice. Additional work is required to further confirm this supposition and to identify its physiological basis. If, however, aged animals are to be characterized by a greater reactivity to novel stimuli, the implications
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of the present findings extend beyond the discussion of age-related differences in exploratory behavior. Greater reactivity to novel environments may contribute to impairments of acquisition and/or retention performance of aged rats in associative learning situations. REFERENCES Archer, J. (1973). Tests for emotionality in rats and mice: A review. Animal Behavior, 21, 205-235. Berlyne, D. E. (1960). Conflict, Arousal, and Curiosity. New York: McGraw-Hill. Brennan, M. J., & Quartermain, D. (1980). Age-related differences in within session habituation: The effect of stimulus complexity. The Gerontologist, 20, 71 (abst.) Corey, D. T. (1978). The determinants of exploration and neophobia. Neuroscience and Biobehavioral Reviews, 2, 235-253. Eleftherion, B. E. (1974). Changes with age in pituitary-adrenal responsiveness and reactivity to mild stress in mice. Gerontologia, 20, 224-230. Elias, P. K., Elias, M. F., & Eleftheriou, B. E. (1975). Emotionality, exploratory behavior, and locomotion in aging inbred strains of mice. Gerontologia, 21, 46-55. File, S. E. (1978). The ontogeny of exploration in the rat: Habituation and the effects of handling. Developmental Psychobiology, 11, 321-328. Goodrick, C. L. (1967). Exploration of nondeprived male Sprague-Dawley rats as a function of age. Psychological Reports, 20, 159-163. Goodrick, C. L. (1971). Variables affecting free exploration responses of male and female Wistar rats as a function of age. Developmental Psychology, 4, 440-446. Hennessy, J. W., & Levine, S. (1979). Stress, arousal, and the pituitary-adrenal system: A psychoendocrine hypothesis. Progress in Psychobiology and Physiological Psychology, 8, 133-178. O'Keefe, J., & Nadel, L. (1978). The Hippocampus as a Cognitive Map. London/New York: Oxford Univ. Press. Sprott, R. L., & Eleftheriou, B. E. (1974). Open-field behavior in aging inbred mice. Gerontologia, 20, 155-162. Walsh, R. N., & Cummins, R. A. (1976). The open-field test: A critical review. Psychological Bulletin, 83, 482-504. Wax, T. M., & Goodrick, C. L. (1975). Voluntary exposure to light by young and aged albino and pigmented inbred mice as a function of light intensity. Developmental Psychobiology, 8, 297-303. Welker, W. I. (1959). Escape, exploratory, and food-seeking responses of rats in a novel situation. Journal of Comparative and Physiological Psychology, 52, 106-111. Werboff, J., & Havlena, J. (1962). Effects of aging on open field behavior. Psychological Reports, 10, 395-398. Wimer, R. E., & Fuller, J. L. (1965). The effects of d-amphetamine sulphate on three exploratory behaviors. Canadian Journal of Psychology, 19, 94-103.
REFERENCE NOTE 1. Unpublished findings.