Neurotoxicologyand Teratology,Vol. 14, pp. 383-392, 1992 Printed in the U.S.A. All rights reserved.

0892-0362/92 $5.00 + .00 Copyright©1992PergamonPress Ltd.

Premature Decline in Morris Water Maze Performance of Aging Micrencephalic Rats MOON

H. LEE 1 AND AUSMA RABE

N e w York State Institute f o r Basic Research in Developmental Disabilities, Staten Island, N Y 10314 Received 17 D e c e m b e r 1991; A c c e p t e d 7 A u g u s t 1992 LEE, M. H. AND A. RABE. Prematuredecline in Morris water maze performance of aging micrencephalic rats. NEUROTOXICOL TERATOL 14(6) 383-392, 1992.-The rat with mcthylazoxymcthanol-induced micrencephaly is a useful animal model of congenital brain defects and associated cognitive impairment. Born with profound morphological and neurochemicai alterations in the forebrain, it shows impaired ability to learn mazes. In order to determine how an animal with such a developmentally damaged brain would function in old age, Long-Evans rats 6, 15, and 24 months of age were tested for their ability to learn to locate a hidden platform in the Morris water maze, The performance of micrencephalic rats of all ages was impaired on acquisition, retention, and transfer trials. Moreover, the magnitude of their acquisition deficit increased with age. It remains to be determined whether the premature decline of the micrencephalic rat in learning the task simply reflects a greater impact on an already compromised brain by neuron loss characteristic of aging brains or whether the prenatal insult alters some basic processes resulting in premature aging. Congenital brain defects Developmental disabilities

Aging

Methylazoxymcthanol acetate

Micrencephaly

Spatial navigation

for the cerebral cortex and other forebraln regions (1,11,12). The effect is dose dependent; doses in the range of 20 to 30 mg/kg of M A M reduce the mass of the forebraln by 40% to 60% (3,11,39) and produce various neuroanatomical and neurochemical abnormalities (1,3,11,34). Behavioral abnormalities, suggestive of altered motivational/emotional and cognitive functions, accompany the morphological and neurochemical changes. Most consistently reported are hyperactivity (26,32,39) and learning deficits in mazes (8,15,19,26,39). A maze learning deficit has been detected in micrencephalic rats as early as 15 days of age (7). The purpose of the present study was to determine whether aging is associated with a further decline in the already compromised learning ability of the micrencephalic rat. Therefore, age-associated changes were examined by comparing the performance of rats of several different ages, ranging from adulthood to old age. The Morris water maze (21) offered an ideal task for this purpose. It is sensitive to changes in learning ability during development (31) and aging (6,16,22,28) in the normal rat, as well as to the impaired competence in young adult rats with MAM-induced micrencephaly (19).

T H E rat with hypoplasia of the cerebrum, or micrencephaly, induced by prenatal exposure to methylazoxymethanol acetate (MAM), provides a useful model for the study o f congenital brain defects resulting from damage to neuron precursors. Its apparent similarity to certain human conditions involving congenital anomalies of the brain has led some investigators to consider it an animal model not only of the congenital microencephaly syndrome but also of mental retardation in general (1,39). The behavioral deficits of the rat with congenital micrencephaly are similar in many ways to those of another animal model of mental retardation in which multiple cortical and subcortical surgical lesions are made in the adult brain (38). Previous studies have shown that the behavioral deficits, as well as morphological and neurochemical abnormalities characteristic of the micrencephalic brain, appear early in life and persist into adulthood (7,8,15,19,23-26,32,39). However, how the rat, with such a profoundly altered brain functions in old age is unknown. The present study is the first to address this issue. Micrencephaly (Fig. 1) can he produced in the whole litter by a single injection of the pregnant dam with M A M on gestation day (GD) 14 or 15 (8,32). By methylating the bases of DNA and R N A during a short period following its access to the fetal brain, M A M Mils neurons undergoing cell division, while sparing postmitotic neurons (32). Thus, exposure to M A M on the 14th or 15th day of gestation results in selective elimination of many precursor cells predominantly destined

METHOD

Subjects and Treatment A total of 70 Long-Evans hooded rats (originally of Blue Spruce Farms, Altamont, NY stock) of both sexes of three

Requests for reprints should be addressed to Moon H. Lee, Ph.D., New York State Institute for Basic Research in Developmental Disabilities, 1050 Forest Hill Road, Staten Island, NY 10314. 383

384

FIG. I. Brains of young adult Long-Evans rats: normal (a) and with MAM-indueed micrencephaly (b). Scale: ram.

different ages were used. All animals were derived from primiparous female rats impregnated at 3-4 months o f age. Pregnancy was determined by copulatory plug found under a malefemale pair placed together in a wire mesh cage overnight, the day of the plug being GD1. Micrencephalic offspring (M, n = 35) were produced by an IP injection of pregnant dams with 30 mg/kg body weight o f methylazoxymethanol acetate (Sigma) on GDI5. The treatment did not produce any effect on litter size and all offspring appeared normal with no grossly identifiable abnormalities. The control animals (C, n = 35) were the offspring of untreated dams. Litters were culled to 8-10 pups at birth and reared by their biological mothers. Following weaning at 4 weeks of age, all animals were maintained in groups o f 2-3 in 24 x 45 x 21 cm polypropylene cages on a 12L: 12D cycle, the light cycle commencing at 6 a.m. F o o d and water were always available. Testing began at ages of 6 months (13M, 13C), 15 months (11M, 11C), and 24 months (11M, 1IC), with each animal being identified by an individual ear code given at weaning. In order to minimize litter effects, a litter provided not more than two rats (one of each sex) to any o f the three age conditions. Apparatus and procedure. The rat's ability to learn to locate a hidden escape platform and then to remember this location was evaluated in a Morris maze, a circular pool measuring 142 cm in diameter and 60 cm in height. Constructed of galvanized steel and painted white, it was filled with opaque, milk water (26 + 1 °C) to a depth of 40 cm. It was located in a testing room with distinctive distal visual cues including a door, a tall cabinet, a set o f covered windows, and fixed lighting sets around the walls. A clear Plexiglas escape platform, measuring 11 × 11 × 39 cm, was placed 30 cm from the pool wall. Its top surface, covered with a white towel, was 1 cm below the water level. A closed-circuit videocamera (Video Tracking System, San Diego Instruments) mounted on the ceiling 2 m above the pool, monitored and recorded the time it took for each rat to reach the platform, as well as the distance it swam. Pretraining was given in three sessions in 1 day, 3 days prior to acquisition training. In a rectangular Plexiglas tank

LEE AND R_ABE measuring 120 x 42 crn without an escape platform, each rat was allowed to swim for 60 s per session. Acquisition training consisted of a total of 25 trials, given as 5 spaced trials a day for 5 consecutive days. The platform was placed in a fixed position (designated as Northeast), while the start position randomly alternated between two locations, South and West. Upon reaching the platform, the rat was allowed to remain on the platform for 5-s before it was removed, dried with a towel and then placed in a dry cage under a heat lamp until the next trial. If a rat was unable to escape within 3 min, it was gently guided to the platform. The intertrial interval was about 10 rain. A retention test o f five trials was given 10 days after the conclusion of the acquisition test under the same conditions as the acquisition trials, followed on the same day by a 30-s probe trial, during which the platform was removed from the maze and the distance the rat travelled in each of the four quadrants of the pool was recorded. A transfer test was given on the following day, in which the platform location was moved 90 ° from the original position (i.e., to the Southeast quadrant). The measure of performance was the time each rat took to reach the escape platform, except for the probe trial for which the percent distance travelled in the trained quadrant was analyzed. Although most rats swam forward vigorously until reaching the platform, some animals displayed stationary swimming behavior (wall-hugging, floating); in particular, the escape times of the first few acquisition trials were heavily contaminated by such behavior. Swimming speed o f each rat was measured during the pretraining sessions, and at regular intervals during acquisition and retention trials. Upon the conclusion o f the maze test, all animals were killed under an overdose of Nembutal and their brains weighed and inspected to determine the extent of micrencephaly. Brains of 48 rats (4/sex/age/treatment) were dissected to obtain separate weights for three brain divisions, the telencephalon (including the olfactory bulb), di- and mesencephalon, and the hindbrain (cerebellum and posterior brainstem). Data analyses. The typical analysis consisted of a multiway analysis of variance (ANOVA; using primarily Systat 5.0 software) with treatment, age, and sex as the between-subjects factors and trial (nested under training day in the analysis of acquisition data) or brain region as the within-subjects factor. The ANOVAs were supplemented by pairwise multiple comparisons (the Tukey-Kramer test) between treatment groups of the same age, as well as among different age groups of the same treatment. In addition, in the evaluation of retention and transfer trials, covariance analyses were performed to adjust the data for the differences in the final level of acquisition by using the rats' performance on the last trial o f acquisition as a covariate. Because of significant group differences in variance, all behavioral data were submitted to logarithmic transformations before being analyzed. The transformation resulted in homogeneity of group variances in most trials. RESULTS

Acquisition At all three ages examined, the M rats showed a marked deficit in learning to locate the hidden platform. A n ANOVA revealed that, in addition to the expected training day, F(4, 232) = 100.24, p < 0.001, and trial, F(4, 232) = 24.74, p < 0.001, effects, the main effects of both treatment, F(1, 58) = 84.97,p < 0.001, and age, F(2, 58) = 3.43,p < 0.05, were significant. The sex effect was not significant.

F U N C T I O N A L A G I N G IN M I C R E N C E P H A L I C RATS

385 The dally means depicted in Fig. 2 suggest that the M groups reduced their escape times at a faster rate than the C rats over the 5 days of acquisition. However, analyses of the 5 trials of Day 1 (not shown) indicated that the C rats, who on the first trial spent about the same amount of time as the M rats in locating the platform (Fig. 2), improved their performance more than the M rats during the five trials of Day 1. An ANOVA on the 5 trials of the first day of training showed significant effects of treatment, F ( I , 58) = 57.05, p < 0.001, as well as interactions between treatment and trial, F(4, 232) = 2.67, p < 0.05, and treatment, age and trial F(8, 232) = 2.55, p < 0.05, indicating a differential rate of learning on the first day of training by the M and C rats of different ages.

A significant interaction among treatment, age, and training day, F(8, 232) = 2.91, p < 0.01, as well as among treatment, age, training day, and trial, F(32, 928) = 1.741, p < 0.01, indicated a differential rate of task mastery among the three age groups of the M and C rats. The learning curves of Fig. 2 show that all three C groups could master the problem in 2 days of training, whereas M rats, except for the 6-monthold group, were unable to fully learn the task in five days. Furthermore, although all C groups, regardless of age, had a similar rate of learning, the rate of learning of the M groups slowed increasingly at 15 and 24 months of age. Follow-up trend analyses revealed that the linear slope of the acquisition curves varied across ages for the M rats to a greater extent than for the C animals over the five training days, F(2, 58) = 4.61, p < 0.05, as well as over the five trials in each day, F(2, 58) = 5.52, p < 0.01. A significant quartic trend, F(2, 58) = 3.53, p < 0.05, probably reflects the greater degree of irregularity shown by the two old M groups. Post-hoc tests confirmed an age-dependent performance decline in the M groups: the 24-month, F(5, 39) = 2.98, p < 0.05, and 15month, F(5, 39) = 2.75, p < 0.05, groups, were significantly different from the 6-month-old young adults. No age-related differences were detected among the C groups. The learning curves also suggest that of the three age groups of M animals only the 6-month-old group could eventually improve to the control level. Palrwise comparisons confirmed that, although significantly (p < 0.05) poorer than their C age-mates on the first 4 days o f acquisition, they no longer differed on the fifth training day. At the other two ages, the M rats were poorer than controls throughout training ( p < 0.05). A

Retention As shown in Fig. 3, the performance of the M groups remained inferior to that of the C groups at all ages during the retention trials. As compared to the last acquisition trial, the escape time of the first retention trial was increased in all groups: in general, the increase was greater for the M rats than the C rats, F(1, 57) = 14.75, p < 0.01, and for older than younger rats of both treatment conditions F(1, 57) = 4.36, p < 0.05. An analysis on all five retention trials with the scores o f the last acquisition trial as a covariate again showed a significant treatment effect, F(1, 57) = 25.37, p < 0.001, indicating a greater forgetting of the platform position by the M animals than the C rats. The effect of age was also significant, F(2, 57) = 3.78, p < 0.05. There, however, was no significant interaction between treatment and age, indicating no differential effect of age on retention of the two treatment groups. The sex effect was not significant. B

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386

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Probe Trial When the platform was removed from the pool after training, the M rats showed a lesser degree o f preference than the C rats for the quadrant where the platform had been previously placed, again indicating deficient learning. The percentage o f distance travelled in the trained quadrant (Q1) is plotted in Fig. 4. A three-way A N O V A on the quadrant preference reveaied only a significant treatment effect, F(1, 57) = 9.44, p < 0.01. While the C rats recorded more than 51% o f the total distance in Q1, the M animals showed only 37% on average. Pair-wise comparisons showed that the difference was statistically significant for the 15- and ?A-month groups (p < 0.05) but not for the 6-month groups.

Transfer When trained to locate the escape platform in a new position a day after the retention test, as Fig. 5 shows, the performance of M rats of all ages was again significantly different from the C animals [treatment, F ( I , 57) = 13.35,p < 0.001]. An analysis on the five transfer trials with the scores of the last acquisition trial as a covariate also showed that, in general, the rats improved performance over the five trials [trial, F(4, 228) = 3.04, p < 0.05]. A significant interaction between trial and treatment, F(4, 228) = 2.92, p < 0.05, with a significant linear component, F(1, 57) = 8.23, p < 0.01, indicated a differential rate of transfer learning by M and C rats. This appears to be related to the poor performance of the 24-month-old M rats who showed little change in escape time over the five transfer trials. The transfer learning curves

show that all but the oldest M group improved their performance over the 5 transfer trials, returning essentially to their pre-transfer (the last acquisition trial) level of performance by the fifth trial. In contrast, the ?A-month-old M group showed no transfer learning at all (i.e., neither the initial rise in escape time on the first transfer trial nor any improvement on the task). The sex effect was not significant.

Body Weight Figure 6 summarizes the body weight at the beginning o f the spatial navigation test. A three-way A N O V A revealed significant effects of all three main factors, treatment, F ( I , 58) = 35.82, p < 0.001, age, F(2, 58) = 22.92, p < 0.001, and sex, F(1, 58) = 250.06, p < 0.001. For interactions, those between treatment and sex, F ( I , 58) = 4.67, p < 0.05, as well as age and sex, F(2, 58) = 3.77, p < 0.05, were significant. Multiple comparison tests revealed that between or within treatment group differences were shown only in males: M males weighed significantly less than the C males of at 6 (/7 < 0.001) and 15 (/7 < 0.05) months o f age but not at 24 months. Within the same treatment, the body weights of the 6-month-old male rats tended to be lower than those in the 15-month groups (p < 0.001 for both M and C). There were no significant between- or within-group differences for females. As expected, males of each age group were always heavier than their female counterparts (p < 0.05).

Swimming Speed A three-way A_NOVA showed significant effects o f treatment, F(1, 57) = 4.12, p < 0.01, and age, F(2, 57) = 8.20,

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FUNCTIONAL AGING IN MICRENCEPHALIC RATS

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p < 0.001, while the effects of sex and interactions among these factors were not significant. As a group, the M rats tended to swim slightly faster than the C rats. The mean (+SEM) swimming speeds of 6-, 15- and 24-month groups were31.1 + 1.2,30.8 + 1.7, and29.4 + 2.0 cm/s for the M rats, and 27.2 + 1.2, 27.6 + 0.8, and 27.1 + 3.9 cm/s for C rats, respectively. None of the pairwise comparisons within or between treatment groups were significant.

Brain Weight The brains of all M rats were confLrmed to be severely micrencephalic. As compared to C rats, their total brain weight was reduced on the average by 44°70 (ranging from 39070 for males at 7 months to 47070 for males at 25 months). The brains at 7 and 16 months of age tended to be heavier than those of the 25-month groups and, as expected, the brains of male rats were slightly heavier than those of the females. An ANOVA of the weights of the dissected telencephalon, di- and mes-encephalon, and hindbrain with treatment, age and sex as between-subjects factors and brain division as a withinsubjects factor showed that the main effects of treatment, F(I, 36) = 843.69, p < 0.001, age, F(2, 36) = 3.54, p < 0.05 and sex, F(1, 36) = 29.54, p < 0.001, were all significant,

whereas none of the interactions among the main factors reached statistical significance. In addition to the expected brain division effect, F(2, 72) = 3747.41, p < 0.001, there was a significant interaction between brain region and treatment, F(2, 72) = 1546.99, p < 0.001, indicating that the exposure to 30 mg/kg of MAM on GD15 produced an increasing degree of hypoplasia in a caudal to rostral gradient (Fig. 7). Separate analyses of the three brain regions indicated a significant age-related brain weight changes in the M animals: F (2, 18) = 7.43, p < 0.01, 5.10, p < 0.05, and 4,37, p < 0.05, for telencephalon, di- and mes-encephalon and hindbrain, respectively. The telenccphalic weights (Fig. 7A) of the micreneephalic brains show a greater (22070) decline between 16 and 25 months of age than the controls (6070). For the control brains, no age comparison yielded a statistical significance. DISCUSSION

The results clearly support two conclusions about the ability of rats with prenatally induced micrencephaly to learn a spatial navigation task. First, they were markedly and permanently impaired. This replicates and extends previously reported findings from another laboratory (19). Second, their already impaired competence in learning the maze deterio-

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rated further in old age, in contrast to the unimpaired performance of aging control rats. This is a new finding. We consider each o f these two findings separately.

Learning Deficit As compared to normal rats, the micrencephalic rats of all ages were impaired in learning to find the escape platform. Whereas the 6-month-old M group improved with training to the level o f the normal animal, the two older groups, although improving, remained markedly impaired. This was particularly striking with the oldest group of M rats. After a 10-day interval, the M rats showed a greater degree o f forgetting than the C rats. Surprisingly, the oldest M rats, while they continued to perform poorly, showed no significant additional loss. It appears that their performance, which was extremely variable, had reached its limit and therefore no additional deficits could be demonstrated. The interpretation that the performance o f the oldest M group had reached its upper limit and that it had learned relatively little finds strong support in the transfer data. The 24-month-old M group was the only group that did not increase its escape time when it had to find the escape platform in a new position and did not improve with practice during the 5 transfer trials. The two younger M groups, on the other hand, showed greater increases in escape time than control rats during the initial transfer trials, possibly reflecting an enhanced perseverative tendency (25,32,39). Their performance, however, eventually returned to their previous level of mastery. Our findings confirm and extend those of a study by Mohammed et al. (19) in which male offspring of rats injected

with 25 mg/kg of MAM on GDI5 also showed an acquisition deficit in the Morris maze when tested at about 3 months of age. As we observed with our 6-month-old M rats, they also reported significant differences in escape time on the first 4 days but not on the fifth day of training. Thus, as a young adult, the rat with severe micrencephaly could eventually master this task to the control level. Overall, the learning deficit in the Morris maze is also consistent with the many previous studies that have demonstrated that micrencephalic rats are deficient in a variety of maze tasks (8,15,19,26). Because the ability of the rat to perform successfully on the Morris spatial navigation task has been shown to require the integrity of the neocortex (4,14,42) and hippocampus (20,35,36), it is tempting to relate the poor performance o f micrencephalic rats on the spatial navigation task to the severe hypoplasia o f the telencephalon (63% loss) and the associated changes in the microstructure and neurochemistry of the neocortex (3,11,43) and the hippocampus (2,3,34). Nevertheless, on the basis of our data, it cannot be concluded that the forebrain dysplasia impaired spatial abilities per se. Even though the testing procedure allowed the use o f a spatial strategy (i.e., locating the hidden platform by distal visual extramaze cues), it also permitted the animals to employ other strategies (e.g., praxis, taxis, spatial localization). Several other functional changes, previously demonstrated to occur in micrencephalic rats but never adequately analyzed, could have been responsible at least in part for the observed learning deficit. Defective visual function could have interfered with the perception of distant visual cues (and hence the use of a spatial strategy). The visual system of rats with MAM-induced micrencephaly is damaged at all levels (1), and their ability to discriminate visual patterns is impaired (23,27).

FUNCTIONAL AGING IN MICRENCEPHALIC RATS As suggested by Mohammed et al. (19), impaired ability to attend could also have contributed toward a poor performance. Hyperactivity (26,32,39) and increased emotionality (24), which also characterize micrencephalic rats, also could have interfered with effective learning in the Morris maze. The faster swimming speed of the micrencephalic rats would be consistent with both hyperactivity and altered emotionality. It is unlikely, on the other hand, that the poor performance of the micrencephalic rats was due to their smaller body size or impaired motor functions. As has been shown previously (5,13,25,39) and again in the present study (Fig. 6), micrencephalic rats tend to be lighter in weight than the age- and sex-matched controls. Nevertheless, their swim speeds were slightly faster, not slower, than those of the normal rats. Moreover, they showed no visible motor impairment, which is consistent with a previous report failing to show any motor deficits (24). The lack of male-female difference in performance also argues against the relevance of body size.

AgingEffect The second finding of this study, the age-related progressive deterioration of the already impaired performance of micrencephalic rats, is new and its implications are provocative.

389 No such decline was seen in the normal aged rats, which is consistent with a previous finding that Long-Evans rats are resistant to age-related behavioral impairment under test conditions similar to those used in our study (16). As compared to the young adults, the micrencephalic rats tested at 24 months of age showed a pronounced decline in performance on acquisition trials. The great variability of their scores and their atypical behavior on the retention and transfer trials suggested that they had learned little, which also demarcated them from the younger micrencephalic animals. The lesser degree of mastery and increased variability of the 15-monthold micrencephalic group suggest that the decline had already begun at this age. The increased variability of the two older micrencephalic groups may have reflected individual differences in the age-associated functional decline. The life-long functional effects of teratogens and developmental neurotoxins have received little attention in behavioral teratology. Although the scarcity of experimental data on the potential interaction of developmental neurotoxins and aging makes our finding particularly interesting, it also means that much of our discussion of factors potentially responsible for this phenomenon must be speculative. If the dysplastic neoeortex and hippocampus are responsible for the poor learning by micrencephalic rats, the further

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FIO. 7. Mean ( + SEM) brain weights of telencephalon, di/mesencephalon, and hindbrain of micrencephalic (C)) and control (O) male rats at three different ages (n = 4/sex/age/treatment). (Brain weights of female rats followed the same pattern but are not shown.) On the average, the telencephalic weight was reduced by 63% and that of the di/mesencephalon by 38~/0. The hindbrain weight was reduced only by 8'70. The differential loss of brain mass changed the relative contribution of each division to the whole brain weight. While the telencephaion represented 59% of the total brain weight in the C rats, it accounted for only 40% in the M animals. In contrast, the ratio of the hindbrain to the total brain weight was increased to 44% for the M rats from 26% in the C group.

decline in competence must reflect aging-related changes in these systems. As a matter o f fact, at 25 months of age the micrencephalic telencephalon showed the biggest (both absolute and proportional), and statistically the only significant decline in telencephalic weight. Mohammed et al. (19), in an attempt to explain their neurochemical findings with micrencephalic rats, suggested that there may be an accelerated age-dependent degeneration o f catecholamine neurons in the cerebral cortex and hippocampus in young adult (4-5 months old) animals. Perturbations during early brain development (5) might have facilitated the emergence of multiple erosive events leading to what appears to be premature brain aging in the micrencephalic rat. In human microencephalies, for example, the volume of the brain begins to decrease soon after the initial growth in early childhood, and the amount o f reduction in brain size is directly related to the decrease in their life expectancy (9). However, the micrencephalic rats do not conform to the pattern reported for human microencephalies: their brain weights do not decline appreciably until very old age, and they do not appear to die prematurely (unpublished observation in our lab). Perhaps the answer lies in the neuroendocrine system. A recent report of Rodier and associates (30) demonstrated that exposure to M A M on gestation days 12-14 and 16 (i.e., 1-3 days before and 1 day after our treatment) kills hypothalamic neurons that regulate the release o f growth-controlling hormones and thus affects body growth. The neuroendocrine sys-

tem may play an important role in the aging process and evidence indicates an enhanced age-related loss o f growth hormone, which is essential for protein synthesis throughout life (18). Even if the rate of aging is not accelerated, the miereneephalic rat may be more vulnerable to the consequences of the subtle brain changes associated with aging. For example, if aging involves loss o f neurons, the same rate of neuron loss in the micrencephalic as in normal neocortex and hippocampus could constitute a greater relative loss for the micrencephalic rat, and consequently, it could suffer a greater functional decline than the normal animal of the same age. It appears as if the micrencephalic rat were an animal with a smaller margin of reserve, resulting in the animal reaching "event threshold" earlier in its life (29). The premature decline in function with advancing age that we have demonstrated in the micrencephalic rat, may involve processes similar to those responsible for the reappearance with age of dysfunction from which an organism had previously recovered, although the critical mechanisms for the age-associated changes are unknown, and the similarities between these situations and the micrencephalic rats may turn out to be superficial. For example, irradiation of the cerebellum in the neonate rat resulted in dose-related reduction of cerebellar microneurons and associated tremor, ataxia, and changes in activity level in infant and adolescent animals. Considerable functional recovery occurred as the animals matured (40). However, the motor dysfunctions and activity

FUNCTIONAL AGING IN MICRENCEPHALIC RATS changes reappeared in many of the animals when they were 24-30 months of age (41). Similarly, a learning deficit of active avoidance observed at 1 month of age following prenatal carbon monoxide exposure, was no longer displayed by 3month-old rats. This deficit, however, was again detected in animals at 1 year of age (17). In another study, unilateral lesions in the postero-lateral hypothalamus of 11-month-old rats impaired orienting to tactile stimuli on the contralateral side. Within a month, the animals recovered their normal orienting behavior, but 11 months later, when they were 22 months old, the orienting deficit reappeared and remained to the end of their lives 4 months later (33). Finally, a similar phenomenon has also been observed with cortical ChAT activity after unilateral ibotenic acid induced lesions in the nucleus basalis. Following production of the lesion in 2-month-old rats, the enzyme level in the contralateral frontoparietal cortex decreased markedly within 1 week but then recovered to a normal level in the next 3 months. However, the enzyme deft-

391 cit reappeared 9 months postoperatively and progressed further in the next 3 months (10). CONCLUSION In conclusion, the earlier-than-normal decline in the learning competence of the aging micrencephalic rat suggests premature aging. Thus this rat, with cerebral hypoplasia induced during fetal development, may provide a useful and convenient animal model for the study of the relationship between early brain lesions and aging. Such information may be especially pertinent for the aging populations with mental retardation and other developmental disabilities. ACKNOWLEDGEMENTS The present study was supported by New York State Office of Mental Retardation and DevelopmentalDisabilities. We thank Wayne Silverman and Eugene Sersen for helpful comments on data analysis and the manuscript, and Peter Wang, Agnes Heaney, and the staff of Biomedical Photography for their technical assistance.

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