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Acquisition and retention of different learning tasks in old rats
BEHAVIORAL AND NEURAL BIOLOGY
35, 139-146 (1982)
Acquisition and Retention of Different Learning Tasks in Old Rats HEIDE-LINDE RIJTHRICH, WOLFRAM WETZEL, AND HANSJURGEN MATTHIES
Institute of Pharmacology and Toxicology, Medical Academy, 3090 Magdeburg, Leipziger Strasse 44, German Democratid Republi6 To study learning and memory in old rats, two behavioral methods were used-a brightness discrimination reaction in a Y chamber and an active avoidance training in a shuttle box. Twenty-one-month-old Wistar rats of our breeding stock did not learn the brightness discrimination reaction so well as the 8-week-old control animals and showed a statistically significant impairment of retention performance tested 24 hr after training. In the shuttle-box training the old rats attained the same number of conditioned responses as the young control animals only after 7 training days and showed a considerably faster extinction. The retention deficits observed in old rats should be considered in pharmacological studies.
The systematic development of active agents capable of influencing mental disturbances during senescense requires the use of suitable experimental animal models. It is difficult, however, to demonstrate reliably the existence of age-dependent impairment of memory in experimental animals (Sprott & Stavnes, 1975). Thus, depending on learning method, task difficulty, age, genotype, and other variables, quite divergent results were obtained from learning and memory studies in old animals (Elias & Elias, 1976). But in most cases the acquisition ability rather than retention performance was investigated in experiments with old animals. With increasing degrees of difficulty and complexity of the training tasks, age-dependent alterations were demonstrable in a cogent manner (Elias & Elias, 1976; Michel & Klein, 1978; Barnes, 1979). In the case of maze learning the use of the T maze and other simple tasks showed no agedependent deficit, while deficit in memory was evident when using more complex mazes such as Stone 14-unit maze (Botwinick, Brinley, & Robbin, 1962; Botwinick, Brinley, & Robbin, 1963; Birren, 1962; Dye, 1969; Verzar-McDougall, 1957; Goodrick, 1968, 1972). Contradictory findings were reported concerning shape, pattern, and brightness discrimination in old animals (for review see Elias & Elias, 1976). Most frequently, however, avoidance training tasks were used. Em139 0163-1047/82/060139-08 $02.00/0 Copyright @ 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
140
RI)THRICH, WETZEL, AND MATTHIES
ploying these methods, the results of avoidance-training studies revealing age-dependent disturbances in learning and memory abilities must be regarded with caution (Sprott & Stavnes, 1975). Ray and Barret (1973) observed age-dependent deficit with avoidance learning, but not with discrimination. McLaughlin, Eller, and Korol (1975) reported that in avoidance experiments old rats showed poorer acquisition ability than young animals, but there were no differences in retention performance. Conversely, in avoidance experiments by Gold and McGaugh (1975) the animals showed no differences in acquisition ability but the retention test revealed poorer performance for old rats. In the present experiments two methods were used--a conditioned active avoidance using a shuttle box, and a brightness discrimination task in a Y maze. The active avoidance technique offers advantages for pharmacological studies but is affected by problems concerning changes in locomotor activity and exploratory behavior (Elias & Redgate, 1975; Brizzee, Ordy, Hofer, & Kaack 1978) as well as footshock sensitivity (Pare, 1969) in old animals (Sprott & Stavnes, 1975). Contrary to discrimination learning, in avoidance learning the time-related response to a conditioned signal rather than to spatial orientation is in the fore. The brightness discrimination technique offers the advantage that the learning process can be measured on the basis of a yes-no response. The aim of the present study using two different learning methods was to ascertain if in old rats a retention deficit, suitable for further pharmacological investigations, can be found and at which age such a deficit occurs. Methods Male Wistar rats of our own breeding stock, aged 8 weeks and 6, 12, or 21 months, were used throughout. The controls were 8-week-old rats. Initially, the rats were trained to learn a footshock-motivated brightness discrimination in a semiautomatic Y chamber (Ott & Matthies, 1973). The animals had to leave the start compartment following a 1-mA footshock and run into one of the remaining two alleys of the Y chamber. Each run into the dark alley (error) was punished by footshock, while runs into the illuminated arm of the Y chamber were counted as correct runs. The training session consisted of 31 consecutive runs. Twenty-four hours later, the relearning test was performed under identical conditions. The retention performance was estimated by the calculation savings (%) =
training errors - relearning errors × 100. training errors
After the brightness discrimination experiment the oldest rats (21-monthold animals) and the young controls were used for learning a conditioned avoidance reaction in a shuttle box. An automatic shuttle box (25 x 25 x 60 cm) with a grid floor and buzzer located at the midline was used.
LEARNING AND MEMORY IN OLD RATS
141
The box was divided into two equal compartments by a 5-cm hurdle. Each compartment could be illuminated by 40-W bulbs mounted in the lids. Prior to the training session the animals were allowed to explore the box for 5 rain. The conditioned stimulus (buzzer plus light) lasted 4.5 sec and was followed by a 0.5 to 0.8-mA footshock if the animals did not avoid. The intertrial interval was 30 sec. The aim of the training was that both old animals and young controls achieve the same number of conditioned reactions. This was the case after 7 days of training. In the first 3 days, the training session was conditioned until the animal reached a criterion of 5 (first and second day) or 10 (third day) conditioned reactions, respectively; on the following days, the daily training sessions involved 20 runs. Afterwards, extinction was tested during a 10-day period. Each extinction session involved 20 trials. For statistical evaluation, the Mann-Whitney U test was used. Results In the brightness discrimination experiment we found no differences between the 6-month-old rats and the 8-week-old controls (Fig. la). The 12-month-old rats showed a slight decrease in percent savings, but this difference was not statistically significant (Fig. lb). The group of 21-month old animals exhibited a significantly lower acquisition ability, when compared with controls, i.e., more errors were registered for old animals than for the 8-week-old rats (Fig. 2). In order to get a reliable estimation of the differences in retention, we compared groups of 8-week-old and 21-month-old rats showing nearly the same mean numbers of errors during training. The corresponding training and retention values are represented in Fig. lc. As shown in this graph, 21month-old animals exhibited a statistically significant lower performance during the relearning session, compared with the performance of 8-weekold rats, resulting in statistically significant lower percent savings for the old rats. In the shuttle-box experiment, 21-month-old rats needed more runs (statistically significant) than the young animals to achieve the training criterion on both Days I and 2 of training (Fig. 3). While 8-week-old rats needed less runs on the second training day than on the first, the old animals showed approximately the same number of runs on both training days. On the seventh training day both groups attained the same number of conditioned reactions (Fig. 4). During the extinction test it was evident that the old animals already showed a nearly complete extinction after two days, while the young animals exhibited a slow extinction during the period of I0 days (Fig. 4). Discussion The present results show that with both behavioral methods used here, 21-month-old Wistar rats have lower memory retention than young an-
142
RI]THRICH, WETZEL, AND MATTHIES %
100 15
t9 5 a
6 months
100 15 10. 5 b
12 months
IO0 15
p
p
10 5 21 months
C training
retention
savings
I==1 -rats= 8weeks P'= -rats= 6,12 21 months F]d. l. Retention of a brightness discrimination reaction in rats of different ages: number of errors during training and retention sessions (left) and percent savings (right) (/( +__ SE). The numbers inside the columns refer to the number of animals per group. In each of the three separate experiments, 8-week-old rats served as controls. (a) rats, aged 6 months; (b) rats, aged 12 months; (c) rats, aged 21 months. 21 months
total population meanvalue:12.8 p< 0,05 ~
~
m
"o
8weeks
total population meanvalue:|0.9
E~
~
2
4
0
I I II iI - ~
selected population I-~
8
selected population mean value:11.t ns
10 12 14 lb
mean1.7value:lO.7 18errors 20
FIG. 2. Acquisition of a brightness discrimination reaction in rats aged 21 months (number of errors during the training session). Selection of young and old animals according to the acquisition values in order to obtain comparable populations for the evaluation of retention (hatched columns: selected rats)
143
LEARNING AND MEMORY IN OLD RATS trials to criterion
40
30
20
10
M
day 1 |
~<0.05 -.....J
day 2
Fio. 3. Shuttle-box avoidance learning in 21-month-old rats during Days ! and 2 of training (trials to criterion; )( -+ SE). Hatched column, 8-week-old control rats; white column, 21-month-old rats. Numbers inside the columns: animals per group.
imals. Concerning the age-related differences in the training performance, it is possible that such differences are due to acquisition ability differences between old and young rats. It must be taken into consideration, however, that the acquisition ability can be influenced to a considerable degree by age-dependent parameters such as body size, body weight, locomotor activity, or footshock sensitivity. Yet the comparison of re%conditioned reactions
70 6O
40 30
10 1
2
:3456 training
1 2 3 4 56 - extinction
78910 ~
FIG. 4. Shuttle-box avoidance behavior in 21-month-old rats during 7 days of training and I0 days of extinction test (percent conditioned reactions per days). Thin curve, 8week-old rats; heavy curve, 21-week-old rats.
144
RIJTHRICH, WETZEL, AND MATTHIES
tention performance of old and young animals showing approximately identical acquisition values in the brightness discrimination, revealed a retention deficit in the 21-month-old rats. The extinction differences found in the shuttle-box experiment after identical levels of conditioned reactions during the last training day are a further indication of an impaired retention performance in the 21-month-old rats. But this statement must be interpreted with caution since extinction can be also regarded as acquisition of a novel behavior. However, the clear difference between old and young rats on the first day of extinction and the inferior acquisition in the old rats during the training procedure seem to rather support the retention impairment interpretation. On the other hand, a number of papers have suggested that old animals are able to show a resistance to extinction as well as increase in perseveration behavior (Elias & Elias, 1976). The high individual variability observed in the present experiments suggests that the group of old rats included animals exhibiting a decreased retention performance but also animals with a good retention performance as compared with that of young. This observation was consistent with data from the literature. Goodrick (1968, 1972) reported that old rats showed generally more errors than young animals, but there were animals both with extremely bad and with good performance, and the latter were as good as young animals. Michel and Klein (1978) also observed, during several days of training, rapidly reacting old rats whose behavior was comparable to that of young animals, and slowly reacting rats making a large number of errors. The retention deficit of old rats could be attributed to various differences between old and young animals such as age-dependent neurophysiological (Landfield & Lynch, 1977; Elias & Elias, 1976), morphological (Berlin & Wallace, 1976; Bondareff & Geinisman, 1976; Geinisman, 1979) or biochemical changes in the central nervous system as decreased catecholamine and acetylcholine turnover (Sun, 1976; Berlin & Wallace, 1976) or decreased RNA synthesis (Novakova, Sandritter, & Schlt~ter, 1971; Berlin & Wallace, 1976; Frolkis, Bezrukov, & Muradian, 1979; Szesz~ik, Coradetti, & Ezs-Nagy, 1977) in old animals. Further studies should be undertaken to examine to what extent the described retention deficit in old rats can be influenced by treatment with drugs, in order to draw a conclusion about the causes of the reduced memory retention which would permit a more precise study of this problem. REFERENCES Barnes, C. A. (1979). Memory deficits associated with senescence: A neurophysiological and behavioral study in the rat. Journal of Comparative and Physiological Psychology, 93, 74-104.
LEARNING AND MEMORY IN OLD RATS
145
Berlin, M., & Wallace, R. B. (1976). Aging and the central nervous system. Aging Research, 2, t25-164. Birren, J. E. (1962). Age differences in learning a two-choice water maze by rats. Journal of Gerontology, 17, 207-213. Bondareff, W., & Geinisman, Y. (1976). Loss of synapses in the dentata gyrus of the senescent rat. American Journal of Anatomy, 145, 129-136. Botwinick, J., Brinley, J. F., & Robbin, J. S. (1962). Learning a position discrimination and positive reversals by Sprague-Dawley rats of different ages. Journal of Gerontology, 17, 315-319. Botwinick, J., Brinley, J. F., & Robbin, J. S. (1963). Learning and reversing a four-choice multiple Y-maze by rats of three ages. Journal of Gerontology, 18, 279-282. Brizzee, K. R., Ordy, J. M., Hofer, H., & Kaack, B. (1978). Animal models for the study of senile brain disease and aging changes in the brain. In: R. Katzman, R. D. Terry, & K. L. Blick (Eds.), Alzheimer's Disease: Senile Dementia and Related Disorders, Vol. 7, Aging, pp. 515-553. New York: Raven Press. Dye, C. J. (1969). Effects of interruption of initial learning upon retention in young, mature, and old rats. Journal of Gerontology, 24, 12-17. Elias, P. K., & Elias, M. F. (1976). Effects of age on learning ability: Contributions from the animal literature. Experimental Aging Research, 2, 165-186. Elias, P. K., & Redgate, E. (1975). Effects of immobilization stress on open field behavior and plasma corticosterone levels of aging C 57 BL/6J mice. Experimental Aging Research, 1, 127-135. Frolkis, V. V., Bezrukov, V. V., & Muradian, K. K. (1979). Hypothalamic-pituitary-adrenocortical regulation of RNA-synthesis in aging. Experimental Gerontology, 14, 77-85. Geinisman, Y. (1979). Loss of axosomatic synapses in the dentate gyrus of aged rats. Brain Research, 168, 485-492. Gold, P. E., & McGaugh, J. L. (1975). Changes in learning and memory during aging. In: J. M. Ordy & K. R. Brizzee (Eds.), Neurobiology of Aging, pp. 145-158. New York: Plenum. Goodrick, C. L. (1968). Learning, retention, and extinction of a complex maze habit for mature-young and senescent Wistar albino rats Journal of Gerontology, 23, 298-304. Goodrick, C. L. (1972). Learning by mature-young and aged Wistar albino rats as a function of test complexity. Journal of Gerontology, 27, 353-357. Landfield, P. W., & Lynch, G. (1977). Impaired monosynaptic potentiation in vitro hippocampal slices from aged, memory-deficient rats. Journal of Gerontology, 32, 523-533. McLaughlin, L. J., Eller, H. D., & Korol, B. (1975). Acquisition and retention of an avoidance response by rats of different ages. Developmental Psychobiology, 8, 233-239. Michel, M. E., & Klein, A. W. (1978). Performance differences in a complex maze between young and aged rats. Age, 1, 13-16. Novakova, V., Sandritter, W., & Schlfiter, R. (1971). Ribonucleic acid and learning in old rats. Activitas Nervouse Superior, 13, 253-258. Ott, T., & Matthies, H. (1973). Some effects of RNA precursors on development and maintenance of long-term memory: Hippocampal and cortical pre- and post-training application of RNA precursors. Psychopharmacologia 28, 195-204. Pare, W. P. (1969). Age, sex and strain differences in the aversive threshold to grid shock in the rat. Journal of Comparative and Physiological psychology, 69, 214-218, Ray, O. S., & Barrett, R. J. (1973). Interaction of learning and memory with age in the rat. In C. Eisdorfer & W. E. Fann (Eds.), Psychopharmaolcology andAging Primates, pp. 17-39. New York: Plenum.
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Sprott, R. L., & Stavnes, K. (1975). Avoidance learning, behavior genetics, and aging: A critical review and comment on methodology. Experimental Aging Research, 1, 145-168. Sun, A. Y. (1976). Aging and in vivo norepinephrine uptake in mammalian brain. Experimental Aging Research, 2, 207-219. Szesz~k, F., Coradetti, E., & Ezs-Nagy, I. (1977). Age-dependent alterations of the rate of RNA synthesis in rat brain cell nuclei. Acta Gerontologica, 7, 231-237. Verzar-McDougall, E. (1957). Studies in learning and memory in aging rats. Gerontologia, 1, 65-85.
35, 139-146 (1982)
Acquisition and Retention of Different Learning Tasks in Old Rats HEIDE-LINDE RIJTHRICH, WOLFRAM WETZEL, AND HANSJURGEN MATTHIES
Institute of Pharmacology and Toxicology, Medical Academy, 3090 Magdeburg, Leipziger Strasse 44, German Democratid Republi6 To study learning and memory in old rats, two behavioral methods were used-a brightness discrimination reaction in a Y chamber and an active avoidance training in a shuttle box. Twenty-one-month-old Wistar rats of our breeding stock did not learn the brightness discrimination reaction so well as the 8-week-old control animals and showed a statistically significant impairment of retention performance tested 24 hr after training. In the shuttle-box training the old rats attained the same number of conditioned responses as the young control animals only after 7 training days and showed a considerably faster extinction. The retention deficits observed in old rats should be considered in pharmacological studies.
The systematic development of active agents capable of influencing mental disturbances during senescense requires the use of suitable experimental animal models. It is difficult, however, to demonstrate reliably the existence of age-dependent impairment of memory in experimental animals (Sprott & Stavnes, 1975). Thus, depending on learning method, task difficulty, age, genotype, and other variables, quite divergent results were obtained from learning and memory studies in old animals (Elias & Elias, 1976). But in most cases the acquisition ability rather than retention performance was investigated in experiments with old animals. With increasing degrees of difficulty and complexity of the training tasks, age-dependent alterations were demonstrable in a cogent manner (Elias & Elias, 1976; Michel & Klein, 1978; Barnes, 1979). In the case of maze learning the use of the T maze and other simple tasks showed no agedependent deficit, while deficit in memory was evident when using more complex mazes such as Stone 14-unit maze (Botwinick, Brinley, & Robbin, 1962; Botwinick, Brinley, & Robbin, 1963; Birren, 1962; Dye, 1969; Verzar-McDougall, 1957; Goodrick, 1968, 1972). Contradictory findings were reported concerning shape, pattern, and brightness discrimination in old animals (for review see Elias & Elias, 1976). Most frequently, however, avoidance training tasks were used. Em139 0163-1047/82/060139-08 $02.00/0 Copyright @ 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
140
RI)THRICH, WETZEL, AND MATTHIES
ploying these methods, the results of avoidance-training studies revealing age-dependent disturbances in learning and memory abilities must be regarded with caution (Sprott & Stavnes, 1975). Ray and Barret (1973) observed age-dependent deficit with avoidance learning, but not with discrimination. McLaughlin, Eller, and Korol (1975) reported that in avoidance experiments old rats showed poorer acquisition ability than young animals, but there were no differences in retention performance. Conversely, in avoidance experiments by Gold and McGaugh (1975) the animals showed no differences in acquisition ability but the retention test revealed poorer performance for old rats. In the present experiments two methods were used--a conditioned active avoidance using a shuttle box, and a brightness discrimination task in a Y maze. The active avoidance technique offers advantages for pharmacological studies but is affected by problems concerning changes in locomotor activity and exploratory behavior (Elias & Redgate, 1975; Brizzee, Ordy, Hofer, & Kaack 1978) as well as footshock sensitivity (Pare, 1969) in old animals (Sprott & Stavnes, 1975). Contrary to discrimination learning, in avoidance learning the time-related response to a conditioned signal rather than to spatial orientation is in the fore. The brightness discrimination technique offers the advantage that the learning process can be measured on the basis of a yes-no response. The aim of the present study using two different learning methods was to ascertain if in old rats a retention deficit, suitable for further pharmacological investigations, can be found and at which age such a deficit occurs. Methods Male Wistar rats of our own breeding stock, aged 8 weeks and 6, 12, or 21 months, were used throughout. The controls were 8-week-old rats. Initially, the rats were trained to learn a footshock-motivated brightness discrimination in a semiautomatic Y chamber (Ott & Matthies, 1973). The animals had to leave the start compartment following a 1-mA footshock and run into one of the remaining two alleys of the Y chamber. Each run into the dark alley (error) was punished by footshock, while runs into the illuminated arm of the Y chamber were counted as correct runs. The training session consisted of 31 consecutive runs. Twenty-four hours later, the relearning test was performed under identical conditions. The retention performance was estimated by the calculation savings (%) =
training errors - relearning errors × 100. training errors
After the brightness discrimination experiment the oldest rats (21-monthold animals) and the young controls were used for learning a conditioned avoidance reaction in a shuttle box. An automatic shuttle box (25 x 25 x 60 cm) with a grid floor and buzzer located at the midline was used.
LEARNING AND MEMORY IN OLD RATS
141
The box was divided into two equal compartments by a 5-cm hurdle. Each compartment could be illuminated by 40-W bulbs mounted in the lids. Prior to the training session the animals were allowed to explore the box for 5 rain. The conditioned stimulus (buzzer plus light) lasted 4.5 sec and was followed by a 0.5 to 0.8-mA footshock if the animals did not avoid. The intertrial interval was 30 sec. The aim of the training was that both old animals and young controls achieve the same number of conditioned reactions. This was the case after 7 days of training. In the first 3 days, the training session was conditioned until the animal reached a criterion of 5 (first and second day) or 10 (third day) conditioned reactions, respectively; on the following days, the daily training sessions involved 20 runs. Afterwards, extinction was tested during a 10-day period. Each extinction session involved 20 trials. For statistical evaluation, the Mann-Whitney U test was used. Results In the brightness discrimination experiment we found no differences between the 6-month-old rats and the 8-week-old controls (Fig. la). The 12-month-old rats showed a slight decrease in percent savings, but this difference was not statistically significant (Fig. lb). The group of 21-month old animals exhibited a significantly lower acquisition ability, when compared with controls, i.e., more errors were registered for old animals than for the 8-week-old rats (Fig. 2). In order to get a reliable estimation of the differences in retention, we compared groups of 8-week-old and 21-month-old rats showing nearly the same mean numbers of errors during training. The corresponding training and retention values are represented in Fig. lc. As shown in this graph, 21month-old animals exhibited a statistically significant lower performance during the relearning session, compared with the performance of 8-weekold rats, resulting in statistically significant lower percent savings for the old rats. In the shuttle-box experiment, 21-month-old rats needed more runs (statistically significant) than the young animals to achieve the training criterion on both Days I and 2 of training (Fig. 3). While 8-week-old rats needed less runs on the second training day than on the first, the old animals showed approximately the same number of runs on both training days. On the seventh training day both groups attained the same number of conditioned reactions (Fig. 4). During the extinction test it was evident that the old animals already showed a nearly complete extinction after two days, while the young animals exhibited a slow extinction during the period of I0 days (Fig. 4). Discussion The present results show that with both behavioral methods used here, 21-month-old Wistar rats have lower memory retention than young an-
142
RI]THRICH, WETZEL, AND MATTHIES %
100 15
t9 5 a
6 months
100 15 10. 5 b
12 months
IO0 15
p
p
10 5 21 months
C training
retention
savings
I==1 -rats= 8weeks P'= -rats= 6,12 21 months F]d. l. Retention of a brightness discrimination reaction in rats of different ages: number of errors during training and retention sessions (left) and percent savings (right) (/( +__ SE). The numbers inside the columns refer to the number of animals per group. In each of the three separate experiments, 8-week-old rats served as controls. (a) rats, aged 6 months; (b) rats, aged 12 months; (c) rats, aged 21 months. 21 months
total population meanvalue:12.8 p< 0,05 ~
~
m
"o
8weeks
total population meanvalue:|0.9
E~
~
2
4
0
I I II iI - ~
selected population I-~
8
selected population mean value:11.t ns
10 12 14 lb
mean1.7value:lO.7 18errors 20
FIG. 2. Acquisition of a brightness discrimination reaction in rats aged 21 months (number of errors during the training session). Selection of young and old animals according to the acquisition values in order to obtain comparable populations for the evaluation of retention (hatched columns: selected rats)
143
LEARNING AND MEMORY IN OLD RATS trials to criterion
40
30
20
10
M
day 1 |
~<0.05 -.....J
day 2
Fio. 3. Shuttle-box avoidance learning in 21-month-old rats during Days ! and 2 of training (trials to criterion; )( -+ SE). Hatched column, 8-week-old control rats; white column, 21-month-old rats. Numbers inside the columns: animals per group.
imals. Concerning the age-related differences in the training performance, it is possible that such differences are due to acquisition ability differences between old and young rats. It must be taken into consideration, however, that the acquisition ability can be influenced to a considerable degree by age-dependent parameters such as body size, body weight, locomotor activity, or footshock sensitivity. Yet the comparison of re%conditioned reactions
70 6O
40 30
10 1
2
:3456 training
1 2 3 4 56 - extinction
78910 ~
FIG. 4. Shuttle-box avoidance behavior in 21-month-old rats during 7 days of training and I0 days of extinction test (percent conditioned reactions per days). Thin curve, 8week-old rats; heavy curve, 21-week-old rats.
144
RIJTHRICH, WETZEL, AND MATTHIES
tention performance of old and young animals showing approximately identical acquisition values in the brightness discrimination, revealed a retention deficit in the 21-month-old rats. The extinction differences found in the shuttle-box experiment after identical levels of conditioned reactions during the last training day are a further indication of an impaired retention performance in the 21-month-old rats. But this statement must be interpreted with caution since extinction can be also regarded as acquisition of a novel behavior. However, the clear difference between old and young rats on the first day of extinction and the inferior acquisition in the old rats during the training procedure seem to rather support the retention impairment interpretation. On the other hand, a number of papers have suggested that old animals are able to show a resistance to extinction as well as increase in perseveration behavior (Elias & Elias, 1976). The high individual variability observed in the present experiments suggests that the group of old rats included animals exhibiting a decreased retention performance but also animals with a good retention performance as compared with that of young. This observation was consistent with data from the literature. Goodrick (1968, 1972) reported that old rats showed generally more errors than young animals, but there were animals both with extremely bad and with good performance, and the latter were as good as young animals. Michel and Klein (1978) also observed, during several days of training, rapidly reacting old rats whose behavior was comparable to that of young animals, and slowly reacting rats making a large number of errors. The retention deficit of old rats could be attributed to various differences between old and young animals such as age-dependent neurophysiological (Landfield & Lynch, 1977; Elias & Elias, 1976), morphological (Berlin & Wallace, 1976; Bondareff & Geinisman, 1976; Geinisman, 1979) or biochemical changes in the central nervous system as decreased catecholamine and acetylcholine turnover (Sun, 1976; Berlin & Wallace, 1976) or decreased RNA synthesis (Novakova, Sandritter, & Schlt~ter, 1971; Berlin & Wallace, 1976; Frolkis, Bezrukov, & Muradian, 1979; Szesz~ik, Coradetti, & Ezs-Nagy, 1977) in old animals. Further studies should be undertaken to examine to what extent the described retention deficit in old rats can be influenced by treatment with drugs, in order to draw a conclusion about the causes of the reduced memory retention which would permit a more precise study of this problem. REFERENCES Barnes, C. A. (1979). Memory deficits associated with senescence: A neurophysiological and behavioral study in the rat. Journal of Comparative and Physiological Psychology, 93, 74-104.
LEARNING AND MEMORY IN OLD RATS
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