Age-related spatial working memory impairment is caused by prefrontal cortical dopaminergic dysfunction in rats

There is evidence of prefrontal cortex (PFC)–dependent cognitive deficits, such as working memory impairment, during the normal aging process in humans and animals. Although working memory function and the PFC dopaminergic system are thought to be closely related, the relationship between them in aged subjects remains unclear. The present study was aimed to clarify the involvement of PFC dopaminergic activity in age-related working memory impairment. For this purpose, we examined working memory in young (3-month-old) and aged (24-month-old) rats, using the T-maze delayed alternation task. As a result, delayed alternation performance was impaired in aged rats compared to young rats, indicating age-related working memory impairment. In addition, aged rats showed reduced dopaminergic transmission in the prelimbic cortical region of the PFC, concomitant with attenuated tyrosine hydroxylase activity in the PFC, but not in the ventral tegmental area and substantia nigra, which was evaluated immunohistochemically and enzymatically. Moreover, age-related working memory impairment was improved by direct stimulation of the prelimbic cortical region of the PFC with 10 or 30 ng, but not 100 ng, of a D1 receptor agonist, SKF 81297, indicating that the SKF 81297 response was an inverted “U” pattern. The maximum SKF 81297 response (30 ng) was abolished by a D1 receptor antagonist, SCH 23390. Thus, age-related working memory impairment was through reduced PFC dopaminergic transmission caused by decreased dopamine synthesis in the prefrontal termination region, but not at the site where the projections originate. This finding provides direct evidence showing the involvement of dopaminergic dysfunction in the development of PFC cognitive deficits during the normal aging process and would help to understand the aging physiology and pathology of the brain.     

A comparison of discrimination and reversal learning for olfactory and visual stimuli in aged rats

The present study investigated age-related differences in discrimination and reversal learning for olfactory and visual stimuli in 6-month and 24-month-old rats. Rats were trained to discriminate between two pseudo-randomly selected odors or objects. Once each animal reached a criterion on discrimination trials, the reward contingencies were reversed. Young and aged rats acquired the olfactory and visual discrimination tasks at similar rates. However, on reversal trials, aged rats required significantly more trials to reach the learning criterion on both the olfactory and visual reversal tasks than young rats. The deficit in reversal learning was comparable for odors and objects. Furthermore, the results showed that rats acquired the olfactory task more readily than the visual task. The present study represents the first examination of age-related differences in reversal learning using the same paradigm for odors and objects to facilitate cross-modal comparisons. The results may have important implications for the selection of memory paradigms for future research studies on aging.     

Teaching old rats new tricks: age-related impairments in olfactory reversal learning

Recent work suggests that normal aging may be associated with decline in different brain systems. In the present study, young and aged Long-Evans rats were tested in a spatial version of the Morris water maze dependent on medial temporal lobe function and also on an odor discrimination reversal task previously used to investigate orbitofrontal function. Aged rats acquired the odor discrimination problems normally but were impaired in acquiring subsequent reversals of the problems. A subset of the aged rats also exhibited impaired spatial learning in the water maze. There was no correlation between reversal performance and spatial learning in the aged rats, indicating that the reversal learning impairment was not related to decline in medial temporal lobe function. Instead the performance of the aged rats on the odor discrimination task resembled that of young rats with neurotoxic lesions of orbitofrontal cortex. These data indicate that rats show independent decline of different brain systems during normal aging and suggest orbitofrontal cortex as one prefrontal area where changes may be localized for further study.     

Discrimination learning requiring different memory components in rats: age and neurochemical comparisons

The performances of young (8-9 months) and aged (22-24 months) male ACI rats were compared in a T-maze requiring two discriminations, each of which placed different demands on memory processing. A spatial discrimination in the stem of the T-maze required long-term reference memory; a discrete-trial, alternation discrimination in the arms of the T-maze required working memory. Following acquisition training in one maze, rats were also trained in a second maze at a different location in the room. The correct response in the stem of this maze was opposite to that in the first maze. In two experiments with slightly different pretraining procedures, similar results demonstrated that aged rats made more errors in all phases of maze training than did their young counterparts. The results suggest that all components of memory processing were affected equivalently because the age-related impairment was not selectively greater in any component of the task. In a third experiment, aged rats were unimpaired in the ability to perform in a T-maze task involving a brightness discrimination with intramaze cues. This result suggests that the age-related impairment in the two-component T-maze task was restricted to the cognitive demands of the task. Neurochemical analyses were performed to determine whether regional neurotransmitter synthetic enzyme activities could be used to identify neurochemical systems associated with performance in these tasks and with any age-related impairments observed. Choline acetyltransferase and glutamic acid decarboxylase were assayed as markers for cholinergic and GABAergic systems, respectively, in the hippocampi and the following cortical regions: frontal, sensorimotor, auditory, cingulate, occipital, and pyriform-perirhinal. A slight (8%) but significant age-related decline was observed in the activity of glutamic acid decarboxylase but not of choline acetyltransferase. Although the correlation between maze performance and regional enzyme activities generally supported several previous observations, the only significant correlation to emerge was between working memory performance and glutamic acid decarboxylase activity in the cingulate cortex.     

Effects of chronic intermittent ethanol exposure on orbitofrontal and medial prefrontal cortex-dependent behaviors in mice

In humans, stroke or trauma-induced damage to the orbitofrontal cortex (OFC) or medial prefrontal cortex (mPFC) results in impaired cognitive flexibility. Alcoholics also exhibit similar deficits in cognitive flexibility, suggesting that the OFC and mPFC are susceptible to alcohol-induced dysfunction. The present experiments investigated this issue using an attention set-shifting assay in ethanol dependent adult male C57BL/6J mice. Ethanol dependence was induced by exposing mice to repeated cycles of chronic intermittent ethanol (CIE) vapor inhalation. Behavioral testing was conducted 72 hours or 10 days following CIE exposure to determine whether ethanol-induced changes in OFC-dependent (reversal learning) and mPFC-dependent (set-shifting) behaviors are long lasting. During early ethanol abstinence (72 hrs), CIE mice showed reduced reversal learning performance as compared to controls. Reversal learning deficits were revealed as greater number of trials to criterion, more errors made, and a greater difficulty in performing a reversal learning task relative to baseline performance. Furthermore, the magnitude of the impairment was greater during reversal of a simple discrimination rather than reversal of an intra-dimensional shift. Reversal learning deficits were no longer present when mice were tested 10 days after CIE exposure, suggesting that ethanol-induced changes in OFC function can recover. Unexpectedly, performance on the set-shifting task was not impaired during abstinence from ethanol. These data suggest reversal learning, but not attention set-shifting, is transiently disrupted during short-term abstinence from CIE. Given that reversal learning requires an intact OFC, these findings support the idea that the OFC may be vulnerable to the cognitive impairing actions of ethanol.     

Naltrexone reverses age-induced cognitive deficits in rats

We evaluated young (3-4 months) and aged (22-24 months) male Sprague-Dawley rats in an attentional set-shifting procedure that assessed reversal, intradimensional shift (IDS), and extradimensional shift (EDS) discrimination learning tasks within one test session. These aspects of discrimination learning are sensitive to damage to distinct regions of frontal cortex. Compared to young animals, aged rats were significantly impaired on the EDS task and did not demonstrate significant impairment on the reversal or IDS tasks. The opioid antagonist naltrexone (2mg/kg, ip) was administered to young and aged rats prior to testing to assess possible improvements in aged-related cognitive impairments. Naltrexone (2mg/kg) attenuated the impairments in cognitive function in the EDS task for aged animals, but did not alter any task performance in the younger group. These results suggest that normal aging in rats is associated with impaired medial frontal cortex function as assessed by this attentional set-shifting procedure and opioid mediated mechanisms may represent a therapeutic target for drugs to improve cognitive deficits associated with aging.     

Effects of neonatal dopamine depletion on spatial ability during ontogeny

Evidence suggests that dopaminergic systems are critical for the learning and memory of spatial discriminations in the adult rat. However, the effect that early dopaminergic lesions have on spatial discriminations throughout development has not been examined. Thus the present experiments investigated the effects of neonatal dopamine (DA) depletion on spatial discrimination tasks. Subjects were lesioned at 3 and 6 days of age with intraventricular 6-hydroxydopamine preceded by desipramine. In Experiment 1 subjects were tested before weaning for the ability to spontaneously alternate in a T-maze both in the presence of their home cage shavings and with a screen covering the shavings. Lesioned animals were significantly impaired in their ability to spontaneously alternate. Control and lesioned animals both showed a decrease in performance when the screen prevented full access to the shavings, and the magnitude of the impairment was significantly greater for the lesioned animals. In Experiment 2, subjects were tested after weaning in an appetitively motivated task in T-maze. The task required both a functional working and a reference memory. Lesioned animals were impaired on both the working and the reference component of the task; however, the magnitude of the impairment was greater in the working memory task. These results suggest that early lesions of general DA systems produce deficits in tasks requiring spatial alternation behavior. These deficits can be exaggerated by certain environmental stimuli and are not reversed by food reward.     

Deficits across multiple cognitive domains in a subset of aged Fischer 344 rats

Rodent models of cognitive aging routinely use spatial performance on the water maze to characterize medial temporal lobe integrity. Water maze performance is dependent upon this system and, as in the aged human population, individual differences in learning abilities are reliably observed among spatially characterized aged rats. However, unlike human aging in which cognitive deficits rarely occur in isolation, few non-spatial learning deficits have been identified in association with spatial impairment among aged rats. In this study, a subset of male aged Fischer 344 rats was impaired both in water maze and odor discrimination tasks, whereas other aged cohorts performed on par with young adult rats in both settings. The odor discrimination learning deficits were reliable across multiple problems. Moreover, these deficits were not a consequence of anosmia and were specific to olfactory learning, as cognitively impaired aged rats performed normally on an analogous non-olfactory discrimination task. These are among the first data to describe an aging model in which individual variability among aged rat cognition occurs across two independent behavioral domains.     

Effects of selective lesions of fimbria-fornix on learning set in the rat

The effects of selective partial lesions of the Fimbria-Fornix (FiFx) on reversal and place learning sets were investigated in rats by using a T-maze and a semi-circular multiple discrimination apparatus. Lesions restricted to the Fimbria (Fi) produced a significant deficit in reversal and place learning set, whereas lesions to the Fornix (Fx) only disturbed the learning set based on a reversal procedure. Combined Fi + Fx lesions resulted in impairment in the retention of spatial discrimination tested in the two mazes. Ventral Hippocampal Commissure (vhc) had no significant effect on reversal learning set. These results confirm previous data that the hippocampal formation is involved in learning transfer, and suggest that the Fi and the Fx may play a role in learning set. Our data also confirm previous demonstrations of the ability of rats to rapidly acquire place learning set.     

Olfactory sensitivity, learning and cognition in young adult and aged male Wistar rats

Psychophysical experiments with male Wistar rats, ranging from 2 to more than 25 months old, revealed age-related differences in olfactory sensitivity. The highest sensitivities were found in rats 13 months old, and the lowest sensitivity was found in the group aged 25 months and older. Consequently, we considered the hypotheses that young rats will require less time and less trials than aged conspecifics to learn an olfactory discrimination task and that olfactory cognitive abilities will be reduced in older individuals. Rats were initially trained in an olfactometer using operant techniques to discriminate between the odor ethyl acetate (EA) and clean air. Next, young adult and 28-month-old rats were tested on seven different go/no-go odor discrimination tasks. Aged rats performed as well as young adults did on all tasks and we conclude that, for a variety of odor discrimination problems, aged rats show no deterioration in learning ability. This is the first report on olfactory sensitivity, learning ability and cognition in Wistar rats that have passed the normal life span for this strain. Data show that the inability to learn and cognitive deficits do not necessarily develop with age.     

Impairments in acquisition and reversals of two-choice discriminations by aged rhesus monkeys

The ability to learn and perform reversals of two object, two patterns, and one spatial discrimination was examined in eight aged (28–34 years), and four adult (8–13 years) behaviorally naive monkeys. As a group, the aged monkeys demonstrated significant difficulties in learning and reversing some of the visual discrimination problems, but had no difficulty learning or reversing the spatial discrimination. Additional analyses revealed that an impairment in learning an object discrimination by the aged monkeys was characterized by a prolonged period of chance performance, and the impairments in performing visual discrimination reversals was related to difficulties in two distinct stages of reversal learning. Despite age-related differences, there was considerable variability in performance among the aged monkeys. These experiments provide the first evidence of significant impairments in learning and reversing visual discriminations by aged monkeys that have not had prior exposure to complex behavioral tasks.     

Individual differences in aging: behavioral and neurobiological correlates

The goal of this experiment was to determine the correlations among different behavioral and neurobiological measures in aged rats. Aged Sprague-Dawley rats were given a battery of cognitive and sensorimotor tests, followed by electrophysiological assessment of sleep and biochemical measurements of various neurotransmitter systems. The behavioral tests included the following: Activity level in an open field; short-term and long-term memory of a spatial environment as assessed by habituation: spatial navigation, discrimination reversal, and cue learning in the Morris water pool; spatial memory in a T-maze motivated by escape from water; spatial memory and reversal on the Barnes circular platform task; passive avoidance; motor skills. Sleep was assessed by electrographic cortical records. The following neurotransmitter markers were examined: Choline acetyltransferase; the density of nicotinic, benzodiazepine and glutamine receptors in the cortex and caudate nucleus; endogenous levels of norepinephrine, dopamine, and serotonin in the cortex and hippocampus. The duration of bouts of paradoxical sleep was strongly correlated with several cognitive measures and selected serotonergic markers. This finding suggests that changes in sleep patterns and brain biochemistry contribute directly to deficits in learning and memory, or that the same neurobiological defect contributes to age-related impairments in sleep and in learning and memory.     

Impaired heat discrimination learning after capsaicin treatment

A heat discrimination experiment has been designed with the intention of deciding whether rats treated with capsaicin are insensitive to heat or their thermolytic reactions are impaired. The animals were trained in a T-maze on an 11°C discrimination. The capsaicin-treated rats were slow in learning the task, and their running time was significantly longer than that of the controls even when they had mastered the problem. No such differences were found in the visual discrimination experiment in the same T-maze. It is suggested that the deficiency of the thermolytic reactions in the capsaicin treated rats may be due to an impaired heat sensitivity.     

Age effect on olfactory discrimination in a non-human primate, Microcebus murinus

In order to characterize age-related cognitive changes, olfactory discrimination was assessed in Microcebus murinus, a prosimian primate. We compared young (n = 10) and old (n = 8) animals for individual performance on three olfactory tasks. Animals had to perform a detection, a transfer, and a reversal learning task using a go, no go conditioning procedure. No differences were observed between the two groups, indicating that aging is not inevitably associated with a decline in cognitive function. We did, however, observe two aged animals showing altered behavior. One animal displayed impairments in the reversal learning task, and the other showed impairments in both the transfer and reversal tasks. Transfer impairment may be due to a hippocampal alteration, whereas the perseverative tendency noted in the reversal task may be associated with frontal lobe dysfunction. Because some aged M. murinus display lesions that are pathognomonic of Alzheimer's disease, our observations highlight its potential utility as a primate model for studying cognitive deficits in relation to age and associated pathologies.     

Guidance of instrumental behavior under reversal conditions requires dopamine D1 and D2 receptor activation in the orbitofrontal cortex

The orbitofrontal cortex (OFC) plays a critical role in learning a reversal of stimulus-reward contingencies. Dopamine (DA) neurons probably support reversal learning by emitting prediction error signals that indicate the discrepancy between the actually received reward and its prediction. However, the role of DA receptor-mediated signaling in the OFC to adapt behavior to changing stimulus-reward contingencies is largely unknown. Here we examined the effects of a selective D1 or D2 receptor blockade in the OFC on learning a reversal of previously acquired stimulus-reward magnitude contingencies. Rats were trained on a reaction time (RT) task demanding conditioned lever release with discriminative visual stimuli signaling in advance the upcoming reward magnitude (one or five food pellets). After acquisition, RTs were guided by stimulus-associated reward magnitudes, i.e. RTs of responses were significantly shorter for expected high versus low reward. Thereafter, stimulus-reward magnitude contingencies were reversed and learning was tested under reversal conditions for three blocks after pre-trial infusions of the selective D1 or D2 receptor antagonists R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepinhydrochloride (SCH23390), eticlopride, or vehicle. For comparisons, we included intra-OFC infusions of the selective N-methyl-d-aspartate receptor antagonist AP5. Results revealed that in animals subjected to intra-OFC infusions of SCH23390 or eticlopride learning a reversal of previously acquired stimulus reward-magnitude contingencies was impaired. Thus, in a visual discrimination task as used here, D1 and D2 receptor-mediated signaling in the OFC seems to be necessary to update the reward-predictive significance of stimuli.     

Age-related working memory deficits in the allocentric place discrimination task: possible involvement in cholinergic dysfunction

It is well known that learning and memory ability declines with aging. Age-related long-term changes in learning and memory ability in rats were investigated with the place navigation task and the allocentric place discrimination task (APDT) in a water maze using the same animals for each task. In a working memory place navigation task, aged animals could learn the location of the platform as well as when they were young, although strategy shifts were observed. In contrast, accuracy in the APDT significantly declined from 90% to 65% with aging. This impairment was ameliorated by an acetylcholine esterase inhibitor physostigmine at 22–23 months old. No amelioration was, however, detected in the same animals tested when they further aged to 26–27 months old. These results suggest that the APDT performance is sensitive to age-related memory deficits and that this may be due to the cholinergic dysfunction.     

Early discrimination reversal learning impairment and preserved spatial learning in a longitudinal study of Tg2576 APPsw mice

To understand the relationship between amyloid-beta and cognitive decline in Alzheimer's disease, we evaluated cortical and hippocampal function in a transgenic mouse model of amyloid over-expression in Alzheimer's disease, the Tg2576 mouse. Tg2576 mice and their non-transgenic littermates were assessed at both 6 and 14 months of age in a battery of cognitive tests: attentional set-shifting, water maze spatial reference memory and T-maze working memory. Spatial reference memory was not affected by Tg status at either age. Working memory was only affected by age, with 6-month-old mice performing better than 14-month-old ones. Older mice were also significantly impaired on reversal learning and on the intra- and extra-dimensional shift in attentional set-shifting. A significant transgene effect was apparent in reversal learning, with Tg2576 mice requiring more trials to reach criterion at 6 months old. These data indicate that the effects of normal aging in C57B6xSJL F1 mice are most pronounced on putative frontal cortex-dependent tasks and that increasing Abeta load only affects discrimination reversal learning in our study.     

Parietal cortex lesions do not impair retention performance of rats in a 14-unit T-maze unless hippocampal damage is present

Young male F-344 rats, pretrained in a straight runway to avoid shock, were then trained in a shock-motivated 14-unit T-maze. One day after maze acquisition, extensive parietal cortex lesions (PC) or sham operations (CON) were performed to assess possible involvement of parietal cortex in the age-related impairment previously observed in this task. Twelve days after surgery, a first 10-trial retention session in the 14-unit T-maze was conducted. One day later the vibrissae of half the rats in each group were clipped to examine involvement of the damaged barrel cortex field in maze performance of rats with PC lesions. The following day a second 10-trial retention session occurred. Finally, retention of the straight runway avoidance response was tested. Histological verification revealed a group with consistent parietal damage but also a subgroup with relatively small lesions to dorsal or lateral hippocampus in addition to parietal damage (PC + HIP). Behavioral results revealed virtually perfect maze retention for CON and PC rats. In contrast, PC + HIP rats were severely impaired in maze retention performance. Retention of the straight runway avoidance response was perfect in CON and PC rats but was impaired in PC + HIP rats. Vibrissae clipping did not affect error performance in the maze but led to a transitory increase in runtime. Overall, the results indicate that parietal lobe damage shortly after acquisition does not impair retention performance of young rats in the 14-unit T-maze, unless hippocampal damage is also evident. Thus, parietal lobe dysfunction alone would not appear to be involved in the age-related retention impairment previously observed in this task.     

Effects of rat medial prefrontal cortex lesions on olfactory serial reversal and delayed alternation tasks

When reward reinforcement in a two-choice discrimination task is regularly changed from one stimulus to another immediately after one learning acquisition session, the learning efficiency of a rat increases as if the rat has come to recognize this regularity of reversal. To investigate how the rat medial prefrontal cortex (mPFC) is involved in such improvement, we examined the performance of mPFC-lesioned rats in a serial reversal task of olfactory discrimination. The performance of other mPFC-lesioned rats in a delayed alternation task was also analyzed using the same apparatus to evaluate the contribution of the mPFC to working memory. The mPFC-lesioned rats demonstrated selective difficulty in the second reversal session in the serial reversal task and also showed performance impairment in the delayed alternation task. These results suggest that the rat mPFC mediating working memory is involved in early progress in learning efficiency during experiences of multiple reversals, which may be relevant to cognitive operations in reversal learning beyond a one-time reversal of stimulus response associations.     

NMDA receptor antagonism impairs reversal learning in developing rats

Four experiments examined the effect of dizocilpine maleate (MK-801), a noncompetitive N-methyl-Daspartate (NMDA) receptor antagonist, on reversal learning during development. On postnatal days (PND) 21, 26, or 30, rats were trained on spatial discrimination and reversal in a T-maze. When MK-801 was administered (intraperitoneally) before both acquisition and reversal, 0.18 mg/kg generally impaired performance, whereas doses of 0.06 mg/kg and 0.10 mg/kg, but not 0.03 mg/kg, selectively impaired reversal learning (Experiments 1 and 3). The selective effect on reversal was not a result of sensitization to the second dose of MK-801 (Experiment 2) and was observed when the drug was administered only during reversal in an experiment addressing state-dependent learning (Experiment 4). Spatial reversal learning is more sensitive to NMDA-receptor antagonism than is acquisition. No age differences in sensitivity to MK-801 were found between PND 21 and 30.