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Longitudinal study on age-related changes of working and reference memory in the rat
17
NeuroscienceLetters, 128 (1991) 17-20 O 1991 Elsevier Scientific Publishers Ireland Ltd. 0304-3940/91/$ 03.50 ADONIS 0304394091003182 NSL 07837
Longitudinal study on age-related changes of working and reference memory in the rat Susumu A n d o I and Yasushi Ohashi 2 1Departmentof MembraneBiochemistry, Tokyo Metropolitan Institute of Gerontology, Tokyo (Japan) and 2Departmentof Psychology, School of Letters, WasedaUniversity, Tokyo (Japan) (Received 23 January 1991; Revised version received 20 March 1991; Accepted 25 March 1991)
Key words: Aging; Rat; Working memory; Reference memory; T-maze task Age-related changes of working memory and reference memory were examined using different age groups of Fischer 344 rats (cross-sectional study) and one group of the same strain which was repeatedly tested at different age points (longitudinal study). Delayed non-matching test in a T-maze revealed 3 points as follows: (1) since motivation by food deprivation in the task performance seemed to decrease with aging, it was needed to adjust the food deprivation level so as to attain a similar degree of motivation in different age groups; (2) scores in reference memory task remained very high in the aged in both cross-sectional and longitudinal studies; and on the other hand (3) performance of working memory task declined with aging to the same degree in both cross-sectional and longitudinal studies, suggesting that working memory progressively deteriorates in advancing age.
The researchers using rats or mice as subjects have suggested age-related declines in learning and memory [2-4, 7-9, 11]. The memory system is usually supposed to be composed of two components, that is, reference memory which means memory of task solution processes and working memory which means active memory processes taking part in task solution [6, 10, 12]. Lowy et al. [9] found some age-related impairment of the two memory systems. But their study seemed to be accompanied by intrinsic variations in the results due to different subjects used, because it was cross-sectional observation. In addition to this kind of study, it would be worthwhile to perform a longitudinal study on memory functions by repeated tests on the same individuals along with aging. It would provide a more reliable conclusion for agerelated changes in the memory system to compare the resuits obtained by cross-sectional and longitudinal studies. Longitudinal studies on memory using animal models in relation to aging have not been found in literature as far as we know. Therefore, a longitudinal study as well as a cross-sectional one was carried out using a delayed non-matching T-maze task in the present study. This seemed to be a suitable task to separately measure working memory and reference memory using stem choice and arm choice paradigms [5, 9]. Correspondence: S. Ando, Department of Membrane Biochemistry, Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashi-ku, Tokyo- 173, Japan.
An apparatus for a delayed non-matching T-maze test was made according to Hepler et al. [5]. A stem path (20 x 70 cm) was connected to an arm path (10 x 90 cm) in a T-shape. A starting platform was separated from the stem by a guillotine door. The stem was divided into two halves by a wooden partition (22 cm long) in front of the arm. Either exit of two halves was blocked by a Plexiglas barrier. A cloth curtain was hung in front of the two ways so that rats could not see the barrier. At the entrance to the arm, another Plexiglas barrier was set on either side of the arm in order to force rats to run into one side of the arms. One food cup each was located at both ends of the arms. The T-maze task consisted of trial-independent memory discrimination (reference memory) task and trial-dependent memory discrimination (working memory) task. These tasks correspond to stem choice and arm choice, respectively. In the reference memory task, getting into one side of the stem blocked by Plexiglas was counted as an error response, and getting through the other side of the stem was regarded as a correct response. The detour was always set in the right hand side during this experiment, and this choice was trial-independent, as considered to measure reference memory. On the other hand, the working memory task consisted of two discrete trials with rewarded alternation in the arm. A rat was first directed at random to the right or left arm in a forced run. After getting a food pellet, the rat was put back into the starting platform. After a 5-s delay, the guillotine door was
18
opened, and the rat was allowed to run down the stem and choose either arm, where the Plexiglas was removed. Choosing the opposite side of the arm as in the forced run was a correct response to get a pellet in the choice run. Since the side of blockade of arm in forced runs was changed at random, this choice was regarded as working memory because of its trial-dependency. Learning performance was motivated by reward of food in this study. Food motivation is usually driven by the reduction of body weight due to food deprivation. The same degree of reduction in body weight was applied to young and aged rats in a reported study [7]. But, it was questionable whether the same food deprivation rate induced the same motivation level in different age groups. As the degree of food motivation might alter with aging, it was necessary to maintain the motivation at a constant level among different age groups of rats. Accordingly, we examined the correlation between the degree of body weight loss and learning performance in mature and aged rats. Female Fischer 344 rats were used: 4 mature groups (each consisted of 15-20 rats) were 7-11 months old and 3 aged groups (each consisted of 9-18 rats) were 24-25 months old. The body weight of mature rats was reduced by 7-19%, and that of aged rats by 15-28%. The same procedure as in the abovementioned delayed non-matching T-maze task was used.
i
E ew
..... t./_ _.
7060Y---4.37X+414 50-
4030i
o
Y---2.52X+245
2010-
"6
100
I
I
9O 8O % of reduced body weight (X)
I
7O
Fig. 1. Percentages of good learners due to different levels of food deprivation. The lines were drawn by the method of least squares. Y = - 4 . 3 7 X + 4 1 4 and Y = - 2 . 5 2 X + 2 4 5 , for mature ((3) and aged groups (O), respectively. The dotted line was drawn at two-thirds of rat population that acquired the arm choice task.
Before the experiment, handling and shaping were carried out. For shaping, rats were allowed to freely explore and access food pellets scattered on the maze, which were gradually reduced to let the animals learn the location of food at both ends of the arms. The conditioned rats were subjected to a delayed non-matching T-maze task as mentioned above. In order to make them learn the arm choice task, 4 trials per day were carried out at intervals of about 5 min for 8 days. During the following 4 days, 8 trials per day were performed for memory test. Each trial was composed of a forced run and a following choice run, and one session consisted of 8 trials. Individuals which did not take a pellet nor enter the arms were excluded as poor learners from the memory test procedure. Rats other than poor learners were regarded as good learners, and they all reached correct response scores above chance level (50%) in memory test. As shown in Fig. 1, the same rates of good learners could be attained for mature and aged rats by employing different degrees of food deprivation. This result coincided with other research [4, 7]. Accordingly, in the following experiment, body weights were reduced by about 80% for the mature group (11 mo), by about 75 % for the middle-aged group (17 mo), and by about 70% for the aged group (25 mo) in cross-sectional study. In longitudinal study, body weights were reduced by about 80 % for mature (10 mo), by about 75% for middle-aged (14 mo), and by about 70% for aged rats (20 mo). Under such controlled conditions, we performed a cross-sectional study using 3 different age groups (each consisted of 16-20 rats) and a longitudinal study using one group (15 rats) at 3 age points (10, 14, 20 mo) on both working memory and reference memory. The procedure was composed of shaping, training and test sessions in the same way as mentioned above. Poor learners were excluded from the test sessions: 13 mature, 12 middle-aged and 10 aged rats were used in cross-sectional study, and 10 rats were tested in longitudinal study. As shown in Fig. 2, some changes in the correct responses were observed between sessions for the same animal groups. Especially the aged group in cross-sectional study appeared to show a trend of improvement. The apparent changes, however, were not statistically significant. Therefore, the averages of the results of 4 sessions were employed to draw Fig. 3. The performance of armchoice task regarded as working memory significantly declined with aging as revealed by both cross-sectional (analysis of variance: F=31.5, df=2, 34, P<0.01) and longitudinal studies (F= 45.9, d f = 2, 27, P < 0.01). Thus, altered working memory function in aged rats that had been observed in reported studies using separate groups of different ages [1] was confirmed by the present longitudinal study. On the other hand, the performance of
19
100
Cross-sectional
100 >. v
i
9o
•
90
so
~ 8o
C O Q.
o
70
0 0
N
6o m
e.
50 2
3
4
Session 100
Longitudinal
L_
m
70
0
6o m l,r
50
6
12
118
t 24
Age (X, month) Fig. 3. T-maze performance concerning working memory. The lines were drawn by the method of least squares. Y = - 1.33X+ 103 and Y = - 1.60X+ 108, for cross-sectional (El) and longitudinal (11) studies, respectively. Values are means + S.D.
i,° 3
6o
n,
I
f
I
|
1
2
3 Session
4
Fig. 2. Changes of the correct responses in the arm choice task during 4 successive sessions. Cross-sectional studies for mature (O), middleaged (A) and aged ([]); and longitudinal studies for mature ( • ) , middle-aged ( • ) and aged ( • ) rats were performed.
reference memory task as stem-choice remained very high in the course of aging in both the studies (99 + 1% for mature, 99 + 1% for middle-aged and 97-1-2% for aged rats in cross-sectional study; 99+ 1% for 10 mo, 99 -t- 1% for 14 mo, and 99 -I- 1% for 20 mo in longitudinal study). This study using a T-maze task indicated specific deterioration of working memory in rats with aging. It seems to be consistent with the previous observations using T-maze [1] and radial arm maze [2, 7]. On the other hand, Furukawa and Iwasaki [4] reported that learning impairment in aged rats was attributed to deficits in reference memory but not in working memory. The task for reference memory using 8-arm radial maze [4] in which entering in 4 unbaited arms was regarded as error choice seemed much harder than the stem choice task in
this study. In other words, the stem choice might be too simple to reveal probable differences in reference memory retention among mature and aged rats. Concerning the working memory an opposite relationship may be pointed out. Moving of rats is interrupted for a period in the delayed non-matching arm choice test, while traveling of rats is continuously carried out in the 8-arm radial maze, suggesting that the retention of working memory in the latter may be superior to that of the former. Cues might seem more abundant in 8-arm radial maze than in T-maze. These may imply the discrepancies between the study using 8-arm radial maze [4] and the present study. Detailed analysis on age-related changes in working memory that have been found in this study remain to be investigated using other methods or paradigms. 1 Aggleton, J.P., Blindt, H.S. and Candy, J.M., Working memory in aged rats, Behav. Neurosci., 103 (1989) 975-983. 2 Barnes, C.A., Nadel, L. and Honig, W.K., Spatial memory deficit in senescent rats, Can. J. Psychol., 34 (1980) 29-39. 3 Davis, H.P., Idowu, A. and Gibson, G.E., Improvement of 8-arm maze performance in aged Fischer 344 rats with 3,4-diaminopyridine, Exp. Aging. Res., 9 (1983) 211-214. 4 Furukawa, S. and Iwasaki, T., Deficits in radial-arm maze learning in aged rats (in Japanese), Jpn. J. Psychol., 60 (1989) 192-195. 5 Hepler, D.J., Olton, D.S., Wenk, G.L. and Coyle, J.T., Lesions in nucleus basalis magnocellularis and medial septal area of rats produce qualitatively similar memory impairments, J. Neurosci., 5 (1985) 866-873. 6 Honig, W.K., Working memory and the temporal map. In N.E. Spear and R.R. Millear (Eds.), Information processing in animals, Lawrence Erlbaum, 1981, pp. 167-197. 7 Ingram, D., London, E.D. and Goodrick, C.L., Age and neurochemical correlates of radial maze performance in rats, Neurobiol. Aging, 2 (1981) 41-47.
20 8 Leslie, F.M., Loughlin, S.E., Sternberg, D.B., McGaugh, J.L., Young, L.E. and Zornetzer, S.F., Noradrenergic changes and memory loss in aged mice, Brain Res., 359 (1985) 292 299. 9 Lowy, A.M., Ingram, D.K., Olton, D.S., Waller, S.B., Reynolds, M.A. and London, E.D., Discrimination learning requiring different memory components in rats: age and neurochemical comparisons, Behav. Neurosci., 99 (1985) 638-651. 10 Olton, D. and Samuelson, R.J., Remembrance of places passed: spatial memory in rats, J. Exp. Psychol.: Anita. Behav. Proc., 2 (1976) 97 116.
I l Rapp, P.R., Rosenberg, R.A. and Gallagher, M., An evaluation of spatial information processing in aged rats, Behav. Neurosci., 101 (1987) 3 12. 12 Spencer, D.G., Jr., Michael, J.P. and Heise, G.A., Central cholinergic involvement in working memory: effects of scopolamine on continuous nonmatching and discrimination performance in the rat, Behav. Neurosci., 99 (1985) 1049-1065.
NeuroscienceLetters, 128 (1991) 17-20 O 1991 Elsevier Scientific Publishers Ireland Ltd. 0304-3940/91/$ 03.50 ADONIS 0304394091003182 NSL 07837
Longitudinal study on age-related changes of working and reference memory in the rat Susumu A n d o I and Yasushi Ohashi 2 1Departmentof MembraneBiochemistry, Tokyo Metropolitan Institute of Gerontology, Tokyo (Japan) and 2Departmentof Psychology, School of Letters, WasedaUniversity, Tokyo (Japan) (Received 23 January 1991; Revised version received 20 March 1991; Accepted 25 March 1991)
Key words: Aging; Rat; Working memory; Reference memory; T-maze task Age-related changes of working memory and reference memory were examined using different age groups of Fischer 344 rats (cross-sectional study) and one group of the same strain which was repeatedly tested at different age points (longitudinal study). Delayed non-matching test in a T-maze revealed 3 points as follows: (1) since motivation by food deprivation in the task performance seemed to decrease with aging, it was needed to adjust the food deprivation level so as to attain a similar degree of motivation in different age groups; (2) scores in reference memory task remained very high in the aged in both cross-sectional and longitudinal studies; and on the other hand (3) performance of working memory task declined with aging to the same degree in both cross-sectional and longitudinal studies, suggesting that working memory progressively deteriorates in advancing age.
The researchers using rats or mice as subjects have suggested age-related declines in learning and memory [2-4, 7-9, 11]. The memory system is usually supposed to be composed of two components, that is, reference memory which means memory of task solution processes and working memory which means active memory processes taking part in task solution [6, 10, 12]. Lowy et al. [9] found some age-related impairment of the two memory systems. But their study seemed to be accompanied by intrinsic variations in the results due to different subjects used, because it was cross-sectional observation. In addition to this kind of study, it would be worthwhile to perform a longitudinal study on memory functions by repeated tests on the same individuals along with aging. It would provide a more reliable conclusion for agerelated changes in the memory system to compare the resuits obtained by cross-sectional and longitudinal studies. Longitudinal studies on memory using animal models in relation to aging have not been found in literature as far as we know. Therefore, a longitudinal study as well as a cross-sectional one was carried out using a delayed non-matching T-maze task in the present study. This seemed to be a suitable task to separately measure working memory and reference memory using stem choice and arm choice paradigms [5, 9]. Correspondence: S. Ando, Department of Membrane Biochemistry, Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashi-ku, Tokyo- 173, Japan.
An apparatus for a delayed non-matching T-maze test was made according to Hepler et al. [5]. A stem path (20 x 70 cm) was connected to an arm path (10 x 90 cm) in a T-shape. A starting platform was separated from the stem by a guillotine door. The stem was divided into two halves by a wooden partition (22 cm long) in front of the arm. Either exit of two halves was blocked by a Plexiglas barrier. A cloth curtain was hung in front of the two ways so that rats could not see the barrier. At the entrance to the arm, another Plexiglas barrier was set on either side of the arm in order to force rats to run into one side of the arms. One food cup each was located at both ends of the arms. The T-maze task consisted of trial-independent memory discrimination (reference memory) task and trial-dependent memory discrimination (working memory) task. These tasks correspond to stem choice and arm choice, respectively. In the reference memory task, getting into one side of the stem blocked by Plexiglas was counted as an error response, and getting through the other side of the stem was regarded as a correct response. The detour was always set in the right hand side during this experiment, and this choice was trial-independent, as considered to measure reference memory. On the other hand, the working memory task consisted of two discrete trials with rewarded alternation in the arm. A rat was first directed at random to the right or left arm in a forced run. After getting a food pellet, the rat was put back into the starting platform. After a 5-s delay, the guillotine door was
18
opened, and the rat was allowed to run down the stem and choose either arm, where the Plexiglas was removed. Choosing the opposite side of the arm as in the forced run was a correct response to get a pellet in the choice run. Since the side of blockade of arm in forced runs was changed at random, this choice was regarded as working memory because of its trial-dependency. Learning performance was motivated by reward of food in this study. Food motivation is usually driven by the reduction of body weight due to food deprivation. The same degree of reduction in body weight was applied to young and aged rats in a reported study [7]. But, it was questionable whether the same food deprivation rate induced the same motivation level in different age groups. As the degree of food motivation might alter with aging, it was necessary to maintain the motivation at a constant level among different age groups of rats. Accordingly, we examined the correlation between the degree of body weight loss and learning performance in mature and aged rats. Female Fischer 344 rats were used: 4 mature groups (each consisted of 15-20 rats) were 7-11 months old and 3 aged groups (each consisted of 9-18 rats) were 24-25 months old. The body weight of mature rats was reduced by 7-19%, and that of aged rats by 15-28%. The same procedure as in the abovementioned delayed non-matching T-maze task was used.
i
E ew
..... t./_ _.
7060Y---4.37X+414 50-
4030i
o
Y---2.52X+245
2010-
"6
100
I
I
9O 8O % of reduced body weight (X)
I
7O
Fig. 1. Percentages of good learners due to different levels of food deprivation. The lines were drawn by the method of least squares. Y = - 4 . 3 7 X + 4 1 4 and Y = - 2 . 5 2 X + 2 4 5 , for mature ((3) and aged groups (O), respectively. The dotted line was drawn at two-thirds of rat population that acquired the arm choice task.
Before the experiment, handling and shaping were carried out. For shaping, rats were allowed to freely explore and access food pellets scattered on the maze, which were gradually reduced to let the animals learn the location of food at both ends of the arms. The conditioned rats were subjected to a delayed non-matching T-maze task as mentioned above. In order to make them learn the arm choice task, 4 trials per day were carried out at intervals of about 5 min for 8 days. During the following 4 days, 8 trials per day were performed for memory test. Each trial was composed of a forced run and a following choice run, and one session consisted of 8 trials. Individuals which did not take a pellet nor enter the arms were excluded as poor learners from the memory test procedure. Rats other than poor learners were regarded as good learners, and they all reached correct response scores above chance level (50%) in memory test. As shown in Fig. 1, the same rates of good learners could be attained for mature and aged rats by employing different degrees of food deprivation. This result coincided with other research [4, 7]. Accordingly, in the following experiment, body weights were reduced by about 80% for the mature group (11 mo), by about 75 % for the middle-aged group (17 mo), and by about 70% for the aged group (25 mo) in cross-sectional study. In longitudinal study, body weights were reduced by about 80 % for mature (10 mo), by about 75% for middle-aged (14 mo), and by about 70% for aged rats (20 mo). Under such controlled conditions, we performed a cross-sectional study using 3 different age groups (each consisted of 16-20 rats) and a longitudinal study using one group (15 rats) at 3 age points (10, 14, 20 mo) on both working memory and reference memory. The procedure was composed of shaping, training and test sessions in the same way as mentioned above. Poor learners were excluded from the test sessions: 13 mature, 12 middle-aged and 10 aged rats were used in cross-sectional study, and 10 rats were tested in longitudinal study. As shown in Fig. 2, some changes in the correct responses were observed between sessions for the same animal groups. Especially the aged group in cross-sectional study appeared to show a trend of improvement. The apparent changes, however, were not statistically significant. Therefore, the averages of the results of 4 sessions were employed to draw Fig. 3. The performance of armchoice task regarded as working memory significantly declined with aging as revealed by both cross-sectional (analysis of variance: F=31.5, df=2, 34, P<0.01) and longitudinal studies (F= 45.9, d f = 2, 27, P < 0.01). Thus, altered working memory function in aged rats that had been observed in reported studies using separate groups of different ages [1] was confirmed by the present longitudinal study. On the other hand, the performance of
19
100
Cross-sectional
100 >. v
i
9o
•
90
so
~ 8o
C O Q.
o
70
0 0
N
6o m
e.
50 2
3
4
Session 100
Longitudinal
L_
m
70
0
6o m l,r
50
6
12
118
t 24
Age (X, month) Fig. 3. T-maze performance concerning working memory. The lines were drawn by the method of least squares. Y = - 1.33X+ 103 and Y = - 1.60X+ 108, for cross-sectional (El) and longitudinal (11) studies, respectively. Values are means + S.D.
i,° 3
6o
n,
I
f
I
|
1
2
3 Session
4
Fig. 2. Changes of the correct responses in the arm choice task during 4 successive sessions. Cross-sectional studies for mature (O), middleaged (A) and aged ([]); and longitudinal studies for mature ( • ) , middle-aged ( • ) and aged ( • ) rats were performed.
reference memory task as stem-choice remained very high in the course of aging in both the studies (99 + 1% for mature, 99 + 1% for middle-aged and 97-1-2% for aged rats in cross-sectional study; 99+ 1% for 10 mo, 99 -t- 1% for 14 mo, and 99 -I- 1% for 20 mo in longitudinal study). This study using a T-maze task indicated specific deterioration of working memory in rats with aging. It seems to be consistent with the previous observations using T-maze [1] and radial arm maze [2, 7]. On the other hand, Furukawa and Iwasaki [4] reported that learning impairment in aged rats was attributed to deficits in reference memory but not in working memory. The task for reference memory using 8-arm radial maze [4] in which entering in 4 unbaited arms was regarded as error choice seemed much harder than the stem choice task in
this study. In other words, the stem choice might be too simple to reveal probable differences in reference memory retention among mature and aged rats. Concerning the working memory an opposite relationship may be pointed out. Moving of rats is interrupted for a period in the delayed non-matching arm choice test, while traveling of rats is continuously carried out in the 8-arm radial maze, suggesting that the retention of working memory in the latter may be superior to that of the former. Cues might seem more abundant in 8-arm radial maze than in T-maze. These may imply the discrepancies between the study using 8-arm radial maze [4] and the present study. Detailed analysis on age-related changes in working memory that have been found in this study remain to be investigated using other methods or paradigms. 1 Aggleton, J.P., Blindt, H.S. and Candy, J.M., Working memory in aged rats, Behav. Neurosci., 103 (1989) 975-983. 2 Barnes, C.A., Nadel, L. and Honig, W.K., Spatial memory deficit in senescent rats, Can. J. Psychol., 34 (1980) 29-39. 3 Davis, H.P., Idowu, A. and Gibson, G.E., Improvement of 8-arm maze performance in aged Fischer 344 rats with 3,4-diaminopyridine, Exp. Aging. Res., 9 (1983) 211-214. 4 Furukawa, S. and Iwasaki, T., Deficits in radial-arm maze learning in aged rats (in Japanese), Jpn. J. Psychol., 60 (1989) 192-195. 5 Hepler, D.J., Olton, D.S., Wenk, G.L. and Coyle, J.T., Lesions in nucleus basalis magnocellularis and medial septal area of rats produce qualitatively similar memory impairments, J. Neurosci., 5 (1985) 866-873. 6 Honig, W.K., Working memory and the temporal map. In N.E. Spear and R.R. Millear (Eds.), Information processing in animals, Lawrence Erlbaum, 1981, pp. 167-197. 7 Ingram, D., London, E.D. and Goodrick, C.L., Age and neurochemical correlates of radial maze performance in rats, Neurobiol. Aging, 2 (1981) 41-47.
20 8 Leslie, F.M., Loughlin, S.E., Sternberg, D.B., McGaugh, J.L., Young, L.E. and Zornetzer, S.F., Noradrenergic changes and memory loss in aged mice, Brain Res., 359 (1985) 292 299. 9 Lowy, A.M., Ingram, D.K., Olton, D.S., Waller, S.B., Reynolds, M.A. and London, E.D., Discrimination learning requiring different memory components in rats: age and neurochemical comparisons, Behav. Neurosci., 99 (1985) 638-651. 10 Olton, D. and Samuelson, R.J., Remembrance of places passed: spatial memory in rats, J. Exp. Psychol.: Anita. Behav. Proc., 2 (1976) 97 116.
I l Rapp, P.R., Rosenberg, R.A. and Gallagher, M., An evaluation of spatial information processing in aged rats, Behav. Neurosci., 101 (1987) 3 12. 12 Spencer, D.G., Jr., Michael, J.P. and Heise, G.A., Central cholinergic involvement in working memory: effects of scopolamine on continuous nonmatching and discrimination performance in the rat, Behav. Neurosci., 99 (1985) 1049-1065.