Radial maze performance in young and aged mice: neurochemical correlates

The Improving Effect of HL271, a Chemical Derivative of Metformin, a Popular Drug for Type II Diabetes Mellitus, on Aging-induced Cognitive Decline

In recent years, as the aging population grows, aging-induced cognitive impairments including dementia and Alzheimer's disease (AD) have become the biggest challenges for global public health and social care. Therefore, the development of potential therapeutic drugs for aging-associated cognitive impairment is essential. Metabolic dysregulation has been considered to be a key factor that affects aging and dementia. Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a primary sensor of cellular energy states and regulates cellular energy metabolism. Metformin (1,1-dimethylbiguanide hydrochloride) is a well-known AMPK activator and has been widely prescribed for type 2 diabetes mellitus (T2DM). Since the incidence of T2DM and dementia increases with aging, metformin has been considered to be one of the most promising drugs to target dementia and its related disorders. To that end, here, we tested the efficacy of metformin and HL271, a novel metformin derivative, in aging-induced cognitive decline. Water (control), metformin (100 mg/kg) or HL271 (50 mg/kg) were orally administered to aged mice for two months; then, the mice were subjected to behavioral tests to measure their cognitive function, particularly their contextual, spatial and working memory. AMPK phosphorylation was also measured in the drug-treated mouse brains. Our results show that oral treatment with HL271 (50 mg/kg) but not metformin (100 mg/kg) improved cognitive decline in aged mice. AMPK activation was correlated with behavior recovery after aging-induced cognitive decline. Taken together, these results suggest that the newly synthesized AMPK activator, HL271, could be a potential therapeutic agent to treat age-related cognitive decline.

Reaction to New Objects and Spatial Changes in Young and Aged Grey Mouse Lemurs (Microcebus murinus)

The exploratory behaviour of young and grey mouse lemurs (Microceb11s murinus) in an experimental enclosure containing various objects was recorded. A new object was added for each test session and could displace an object that had previously been introduced to a new position. For young grey mouse lemurs, there was an obvious preference for new objects and for displaced familiar objects. This indicates that they are able, like other primates, to “map out” and memorize their environment. For the aged grey mouse lemurs, preference for the new objects was very slight. No react ion to changes in the location of familiar objects was noted. Age- related deficits in memory, in particular spatial memory, could account for this lack of response to changes in the environment.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

A functional role for adult hippocampal neurogenesis in spatial pattern separation

The dentate gyrus (DG) of the mammalian hippocampus is hypothesized to mediate pattern separation-the formation of distinct and orthogonal representations of mnemonic information-and also undergoes neurogenesis throughout life. How neurogenesis contributes to hippocampal function is largely unknown. Using adult mice in which hippocampal neurogenesis was ablated, we found specific impairments in spatial discrimination with two behavioral assays: (i) a spatial navigation radial arm maze task and (ii) a spatial, but non-navigable, task in the mouse touch screen. Mice with ablated neurogenesis were impaired when stimuli were presented with little spatial separation, but not when stimuli were more widely separated in space. Thus, newborn neurons may be necessary for normal pattern separation function in the DG of adult mice.

The AMPA modulator S 18986 improves declarative and working memory performances in aged mice

The aim of this study was to further characterize the memory-enhancing profile of S 18986 a positive allosteric modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. S 18986 was studied in two mouse models of age-related memory deficits, using radial maze paradigms involving long-term/declarative memory and short-term/working memory. Aged mice exhibited severe deficits when compared with their younger counterparts in the two behavioural tests. S 18986 at the dose of 0.1 mg/kg selectively improved aged mouse performance in the test of long-term/declarative memory flexibility and exerted a beneficial effect on short-term retention of successive arm-visits in the short-term/working memory test. This study confirms the memory-enhancing properties of S 18986 and, in line with emerging data on multiple AMPA modulators, highlights the relevance of targeting AMPA receptors in the development of new memory enhancers.

Comparative effects of the alpha7 nicotinic partial agonist, S 24795, and the cholinesterase inhibitor, donepezil, against aging-related deficits in declarative and working memory in mice

INTRODUCTION

The comparative effects of a newly described specific alpha7 nAChR partial agonist, S 24795, and a cholinesterase inhibitor, donepezil, currently used as a symptomatic Alzheimer's disease treatment were studied in two mouse models of aging-related memory deficits.

MATERIALS AND METHODS

We employed radial arm-maze paradigms assessing short-term working memory (STWM, experiment A) and mnemonic flexibility, a cardinal property of long-term declarative (LTDM, experiment B). Both compounds were administered daily at 0.3 and 1 mg/kg subcutaneously (~3 weeks).

RESULTS

In the STWM experiment, vehicle-treated aged mice displayed a severe and persistent deficit in the retention of successive arm visits in comparison to younger controls. S 24795 at 1 mg/kg (trends at 0.3 mg/kg) and donepezil at 0.3 mg/kg (but not 1 mg/kg) exerted beneficial effects on this deficit: The performance of aged mice treated with these drugs remarkably increased across the testing days and almost reached young adult performance level. In the critical test trials of memory flexibility (i.e., LTDM), in experiment B, S 24795 at 1 mg/kg (trends at 0.3 mg/kg) and donepezil at the dose of 1 mg/kg (but not 0.3 mg/kg) improved aged mice performance.

CONCLUSION

This preclinical demonstration that S 24795 restored specific age-related memory deficits with as much efficacy as donepezil adds to recent literature in highlighting the potential interest of an alpha7 nAChR drug as a symptomatic AD therapeutic.

Spatial learning and psychomotor performance of C57BL/6 mice: age sensitivity and reliability of individual differences

Two tests often used in aging research, the elevated path test and the Morris water maze test, were examined for their application to the study of brain aging in a large sample of C57BL/6JNia mice. Specifically, these studies assessed: (1) sensitivity to age and the degree of interrelatedness among different behavioral measures derived from these tests, (2) the effect of age on variation in the measurements, and (3) the reliability of individual differences in performance on the tests. Both tests detected age-related deficits in group performance that occurred independently of each other. However, analysis of data obtained on the Morris water maze test revealed three relatively independent components of cognitive performance. Performance in initial acquisition of spatial learning in the Morris maze was not highly correlated with performance during reversal learning (when mice were required to learn a new spatial location), whereas performance in both of those phases was independent of spatial performance assessed during a single probe trial administered at the end of acquisition training. Moreover, impaired performance during initial acquisition could be detected at an earlier age than impairments in reversal learning. There were modest but significant age-related increases in the variance of both elevated path test scores and in several measures of learning in the Morris maze test. Analysis of test scores of mice across repeated testing sessions confirmed reliability of the measurements obtained for cognitive and psychomotor function. Power calculations confirmed that there are sufficiently large age-related differences in elevated path test performance, relative to within age variability, to render this test useful for studies into the ability of an intervention to prevent or reverse age-related deficits in psychomotor performance. Power calculations indicated a need for larger sample sizes for detection of intervention effects on cognitive components of the Morris water maze test, at least when implemented at the ages tested in this study. Variability among old mice in both tests, including each of the various independent measures in the Morris maze, may be useful for elucidating the biological bases of different aspects of dysfunctional brain aging.

Maintenance of inhibitory interneurons and boutons in sensorimotor cortex between middle and old age in Fischer 344 X Brown Norway rats

Ultrastructurally identified inhibitory synapses in layer II of rat sensorimotor cortex decline between middle and old age [Poe, B.H., Linville, C., Brunso-Bechtold, J., 2001. Age-related decline of presumptive inhibitory synapses in the sensorimotor cortex as revealed by the physical disector. J. Comp. Neurol. 439, 65-72]. The current study investigated whether a loss or shrinkage of gamma-aminobutyric acid (GABA)ergic interneurons contribute to that decline. Coronal sections from middle-aged (15-17 months) and old (25-29 months) Fischer 344 X Brown Norway male rats were immunoreacted with antibodies to the GABA synthesizing enzyme glutamic acid decarboxylase (GAD); the calcium-binding protein parvalbumin (PV), or the neuronal marker NeuN. The number of GAD-immunoreactive (IR), PV-IR, and NeuN-IR cells were determined stereologically using the optical disector technique and the cross-sectional areas of GAD-IR cells were measured in layers II/III, IV, V and VI of sensorimotor cortex. Neither the number of GAD-IR or NeuN-IR cells, nor the size of GAD-IR cells, declined significantly between middle and old age. A modest decline in the PV-IR subset of inhibitory interneurons was observed, predominantly due to changes in layers V and VI. Stereological analysis of layer II/III GAD-IR boutons revealed a stability of immunocytochemically identified inhibitory terminals. Taken together, these results indicate a general maintenance of overall GABAergic neurons in sensorimotor cortex between middle and old age and the loss of ultrastructurally identified inhibitory synapses may be due to the decline of a subset of GABAergic terminals.

Comparison of Galantamine and Donepezil for Effects on Nerve Growth Factor, Cholinergic Markers, and Memory Performance in Aged Rats

This study was designed to determine 1) whether repeated exposures to the acetylcholinesterase inhibitors (AChEIs) galantamine (GAL) or donepezil (DON) resulted in positive effects on nerve growth factor (NGF) and its receptors, cholinergic proteins, and cognitive function in the aged rat, and 2) whether GAL had any advantages over DON given its allosteric potentiating ligand (APL) activity at nicotinic acetylcholine receptors. Behavioral tests (i.e., water maze and light/dark box) were conducted in aged Fisher 344 rats during 15 days of repeated (subcutaneous) exposure to either GAL (3.0 or 6.0 mg/kg/day) or DON (0.375 or 0.75 mg/kg/day). Forty-eight hours after the last drug injection, cholinergic receptors were measured by [(125)I]-(+/-)-exo-2-(2-iodo-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([(125)I]IPH; epibatidine analog), (125)I-alpha-bungarotoxin ((125)I-BTX), [(3)H]pirenzepine ([(3)H]PRZ), and [(3)H]-5,11-dihydro-11-[((2-(2-((dipropylamino)methyl)-1-piperidinyl)ethyl)amino)carbonyl]-6H-pyrido(2,3-b)(1,4)-benzodiazepin-6-one methanesulfonate ([(3)H]AFDX-384, or [(3)H]AFX) autoradiography. Immunochemical methods were used to measure NGF, high (TrkA and phospho-TrkA)- and low (p75 neurotrophin receptor)-affinity NGF receptors, choline acetyltransferase (ChAT), and the vesicular acetylcholine transporter (VAChT) in memory-related brain regions. Depending on dose, both GAL and DON enhanced spatial learning (without affecting anxiety levels) and increased [(125)I]IPH, [(3)H]PRZ, and [(3)H]AFX (but decreased (125)I-BTX) binding in some cortical and hippocampal brain regions. Neither AChEI was associated with marked changes in NGF, NGF receptors, or VAChT, although DON did moderately increase ChAT in the basal forebrain and hippocampus. The results suggest that repeated exposures to either GAL or DON results in positive (and sustained) behavioral and cholinergic effects in the aged mammalian brain but that the APL activity of GAL may not afford any advantage over acetylcholinesterase inhibition alone.

Cognitive function in young and adult IL (interleukin)-6 deficient mice

Interleukin-6 (IL-6) is a cytokine shown to affect brain function and to be involved in pathological neurodegenerative disorders such as Alzheimer's disease (AD). In the present study we investigated the cognitive function in transgenic mice not expressing IL-6 (IL-6 KO) and in wild type (WT) genotype at 4 and 12 months of age, using a passive avoidance and an eight-arm radial maze tasks. Motor function was quantified using an Animex apparatus. Hippocampal choline acetyltransferase (ChAT) activity was evaluated in both genotypes. No difference was observed in both genotypes for spontaneous motor activity. The mean latency (s) to re-enter the shock box, was similar in both young mutant and WT mice. However, a decreased sensitivity (50%) to scopolamine (1 mg/kg) in mutant compared to WT mice, was obtained. IL-6 KO mice exhibited a facilitation of radial maze learning over 30 days, in terms of a lower number of working memory errors and a higher percentage of animals reaching the criterion as compared with WT genotype tested at both ages. Furthermore, mutant mice, at the age of 12 months, showed a faster acquisition (22 days versus 30 days to reach the criterion). The pattern of arm entry exhibited by IL-6 KO mice showed a robust tendency to enter an adjacent arm at both ages, while WT only at the age of 4 months. ChAT activity was inversely correlated with memory performance. These findings suggest a possible involvement of IL-6 on memory processes, even if the mechanism remains still unclear.

Sex differences in neurochemical markers that correlate with behavior in aging mice

Sex differences in neurochemical markers that correlate with behavior in aging mice NEUROBIOL AGING. We examined whether the enzymatic activities of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) were altered similarly with age in male and female mice, and whether these changes were correlated with age-related alterations in memory and anxiety. ChAT and GAD activities were measured in neocortex, hippocampus, and striatum of behaviorally characterized male and female C57BL/6 mice (5, 17, and 25 months). Generally, ChAT activity was increased, and GAD activity decreased, with age. However, disparate changes were revealed between the sexes; activities of both enzymes were decreased in 17-month males, whereas alterations in females were not observed until 25-months. Furthermore, enzyme-behavior correlations differed between the sexes; in males, ChAT activity was related to one behavioral task, whereas in females, activities of both enzymes were correlated with multiple tasks. Significant enzyme-behavior correlations were most evident at 17 months of age, likely the result of behavioral and enzymatic sex differences at this age. These data represent the first comprehensive report illustrating differential alterations of ChAT and GAD activities in aging male and female mice.

Immunohistochemical and neurochemical correlates of learning deficits in aged rats

This study examined whether cholinergic and monoaminergic dysfunctions in the brain could be related to spatial learning capabilities in 26-month-old, as compared to three-month-old, Long-Evans female rats. Performances were evaluated in the water maze task and used to constitute subgroups with a cluster analysis statistical procedure. In the first experiment (histological approach), the first cluster contained young rats and aged unimpaired rats, the second one aged rats with moderate impairment and the third one aged rats with severe impairment. Aged rats showed a reduced number of choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis, and choline acetyltransferase-positive neurons in the striatum. In the second experiment (neurochemical approach), the three clusters comprised young rats, aged rats with moderate impairment and aged rats with severe impairment. Alterations related to aging consisted of reduced concentration of acetylcholine, norepinephrine and serotonin in the striatum, serotonin in the occipital cortex, dopamine and norepinephrine in the dorsal hippocampus, and norepinephrine in the ventral hippocampus. In the first experiment, there were significant correlations between water maze performance and the number of; (i) choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis; (ii) choline acetyltransferase-positive neurons in the striatum and; (iii) p75(NTR)-positive neurons in the medial septum. In the second experiment, water maze performance was correlated with the concentration of; (i) acetylcholine and serotonin in the striatum; (ii) serotonin and norepinephrine in the dorsal hippocampus; (iii) norepinephrine in the frontoparietal cortex and; (iv) with other functional markers such as the 5-hydroxyindoleacetic acid/serotonin ratio in the striatum, 3,4-dihydroxyphenylacetic acid/dopamine ratio in the dorsal hippocampus, 5-hydroxyindoleacetic acid/serotonin and homovanillic acid/dopamine ratios in the frontoparietal cortex, and 3,4-dihydroxyphenylacetic acid/dopamine ratio in the occipital cortex. The results indicate that cognitive deficits related to aging might involve concomitant alterations of various neurochemical systems in several brain regions such as the striatum, the hippocampus or the cortex. It also seems that these alterations occur in a complex way which, in addition to the loss of cholinergic neurons in the basal forebrain, affects dopaminergic, noradrenergic and serotonergic processes.

Reference memory, anxiety and estrous cyclicity in C57BL/6NIA mice are affected by age and sex

Age-related changes in learning and memory are common in rodents. However, direct comparisons of the effects of aging on learning and memory in both males and females are lacking. The present study examined whether memory deteriorates with increasing age in C57BL/6NIA mice, and whether age-related changes in learning and memory are similar in both sexes. Male and female mice (five, 17 and 25 months of age) were tested in a battery of behavioral tasks including the Morris water maze (spatial and non-spatial reference memory), simple odor discrimination (olfactory reference memory), plus maze (anxiety/exploration), locomotor activity, and basic reflexes. Five-month-old mice learned the water maze and odor discrimination tasks rapidly. Relative to five-month-old mice, 25-month-old mice exhibited impaired spatial and olfactory reference memory, but intact non-spatial reference memory. The spatial reference memory of 17-month-old mice was also impaired, but less so than 25-month mice. Seventeen-month-old mice exhibited intact non-spatial (visual and olfactory) reference memory. Five and 25-month-old mice had similar levels of plus maze exploration and locomotor activity, whereas 17-month-old mice were more active than both groups and were slightly less exploratory than five-month-old mice. Although sex differences were not observed in the five- and 25-month groups, 17-month-old females exhibited more impaired spatial reference memory and increased anxiety relative to 17-month-old males. Estrous cycling in females deteriorated significantly with increased age; all 25-month-old females had ceased cycling and 80% of 17-month-old females displayed either irregular or absent estrous cycling. This study is the first to directly compare age-related mnemonic decline in male and female mice. The results suggest that: (i) aged mice exhibit significant deficits in spatial and olfactory reference memory relative to young mice, whereas middle-aged mice exhibit only a moderate spatial memory deficit and; (ii) spatial reference memory decline begins at an earlier age in females than in males, a finding that may be related to the cessation of estrous cycling.

Developing mouse models of aging: a consideration of strain differences in age-related behavioral and neural parameters

Increased interest is emerging for using mouse models to assess the genetics of brain aging and age-related neurodegenerative diseases. Despite this demand, relatively little information is available on aging in behavioral or neuromorphological parameters in various mouse strains that are being used to create transgenic and null mutant mice. We review several issues regarding selection of appropriate strains as follows: (1) Does the behavioral parameter exhibit a significant age by strain interaction? (2) Do the strains differ in lifespan? (3) Are there potential intervening variables, such as strain-specific performance strategies or disease, in the behavioral task being investigated that would confound the desired conclusions? (4) Does the behavioral difference have an underlying neural correlate? In this context we present a conceptual model pertaining to the selection of mouse strains and behavioral parameters for genetic analyses. We also review the importance of applying stereological techniques for determining age-related structural changes in the mouse brain as well as the potential value of a database that would catalog this information. Thus, our intention is to underscore the growing importance of mouse models of brain aging and the concomitant need for additional information about mouse aging in general.

Hippocampal neuron and synaptophysin-positive bouton number in aging C57BL/6 mice

A loss of hippocampal neurons and synapses had been considered a hallmark of normal aging and, furthermore, to be a substrate of age-related learning and memory deficits. Recent stereological studies in humans have shown that only a relatively minor neuron loss occurs with aging and that this loss is restricted to specific brain regions, including hippocampal subregions. Here, we investigate these age-related changes in C57BL/6J mice, one of the most commonly used laboratory mouse strains. Twenty-five mice (groups at 2, 14, and 28-31 months of age) were assessed for Morris water-maze performance, and modern stereological techniques were used to estimate total neuron and synaptophysin-positive bouton number in hippocampal subregions at the light microscopic level. Results revealed that performance in the water maze was largely maintained with aging. No age-related decline was observed in number of dentate gyrus granule cells or CA1 pyramidal cells. In addition, no age-related change in number of synaptophysin-positive boutons was observed in the molecular layer of the dentate gyrus or CA1 region of hippocampus. We observed a significant correlation between dentate gyrus synaptophysin-positive bouton number and water-maze performance. These results demonstrate that C57BL/6J mice do not exhibit major age-related deficits in spatial learning or hippocampal structure, providing a baseline for further study of mouse brain aging.

Water version of the radial-arm maze: learning in three inbred strains of mice

The conventional land radial-arm maze has several disadvantages, including requiring a complicated automated apparatus, the elimination of odors as cues, and the use of food deprivation. We have created a water version of the maze, based on the principles of the land version, which maintains the advantages and excludes some of the disadvantages. In our maze, BXSB and C57BL/6 mice significantly reduced the number of working and reference memory errors committed over sessions, while NZB mice did not. For each strain, as the working memory 'load' increased during a session, the number of errors increased. However, with practice the BXSB and C57BL/6 strains were able to handle this memory load more effectively. Mice were able to learn the maze without extensive adaptation, training, or testing and they did not exhibit 'chaining'. This maze can also be considered to be an example of a water win-shift task that mice can easily learn. Therefore, the water version of the radial-arm maze can be a simple and useful tool for studying rodent learning and memory.

Inhibitory avoidance and appetitive learning in aged normal mice: comparison with transgenic mice having elevated plasma growth hormone levels

Groups of 25-month-old ("old") B6C3 hybrid male mice, 6-month-old ("young") normal males, and their age-matched transgenic (TG) siblings overexpressing the bovine growth hormone gene were given an inhibitory avoidance training trial (0.20-mA electric shock, 1.0-s duration). The old B6C3 hybrids and the young TG mice displayed poorer retention (shorter latencies to enter the shock compartment) 24 h and 42 days after training than did the young normal mice. In a subsequent multiple-trial acquisition test, young TG and old normal mice required more trials to reach the criterion of complete inhibition of step-through responding for 300 s than did young normal mice. Young normal and young TG mice did not differ in trials to extinction, but TG mice met the extinction criterion sooner than did old normal mice, suggesting poorer longterm retention. In tests of T-maze appetitive learning, young normal, old normal, and young TG mice did not differ in acquisition or 24-h retention. Contrary to expectation, TG mice acquired T-maze reversal learning in fewer trials than did young normal or old normal mice. The TG and young normal mice did not differ in retention when retested 44 days after initial training, but old normal mice showed poorer retention than did the young normals. Results of locomotor activity and shock response tests suggested that learning impairments were not due to differences in locomotor activity or shock response thresholds in these animals. Tests in an elevated plus maze indicated that young TG mice were less anxious in a novel environment than their normal siblings, which may contribute to their impaired inhibitory avoidance learning. These findings suggest that 6-month-old TG mice overexpressing the bovine growth hormone gene display alterations in inhibitory avoidance (but not appetitive) learning similar to those occurring in 25-month-old normal mice. The neurobiological mechanisms mediating inhibitory avoidance and T-maze appetitive learning in these animals may be largely dissociated.

Caloric restriction and spatial learning in old mice

Spatial learning in old mice (19 or 24 months old), some of which had been calorically restricted beginning at 14 weeks of age, was compared to that of young mice, in two separate experiments using a Morris water maze. In the first experiment, only old mice reaching criterion performance on a cued learning task were tested in a subsequent spatial task. Thus, all old mice tested for spatial learning had achieved escape latencies equivalent to those of young controls. Despite equivalent swimming speeds, only about half the old mice in each diet group achieved criterion performance in the spatial task. In the second experiment, old and young mice all received the same number of training trials in a cued task and then in a spatial task. Immediately following spatial training, they were given a 60-s probe trial, with no platform in the pool. Both groups of old mice spent significantly less time in the quadrant where the platform had been and made significantly fewer direct crosses over the previous platform location than did the young control group. As in Experiment 1, calorie restriction failed to provide protection against aging-related deficits. However, in both experiments, some individual old mice evidenced performance in spatial learning indistinguishable from that of young controls. Separate comparisons of "age-impaired" and "age-unimpaired" old mice with young controls may facilitate the identification of neurobiological mechanisms underlying age-related cognitive decline.

Impaired synaptic functions with aging as characterized by decreased calcium influx and acetylcholine release

Age-related alterations of presynaptic functions were studied in terms of acetylcholine (ACh) synthesis and release using synaptosomes isolated from mouse brain cortices. The following three findings were obtained: 1) Choline acetyltransferase activity and ACh production rate remained constant throughout all ages tested. This observation, obtained with synaptosomes, was not consistent with data reported for brain slices (Gibson GE, Peterson C: J Neurochem 37:978-984, 1981). Various conditions, such as low glucose or membrane depolarization, modulated ACh synthesis to similar extents in young and aged synaptosomes. 2) Depolarization-induced release of ACh from synaptosomes significantly decreased in the senescent stage. The fraction of ACh released from aged synaptosomes was less than that released from young synaptosomes, although the ACh contents in the synaptosomes did not change with age. 3) Calcium influx induced by depolarization was lower in the synaptosomal preparations from aged mice than in those from young mice. A strong positive correlation was observed between the amounts of ACh released and the increased calcium levels when the data for all preparations, both from young and aged mice, were plotted. This indicates that diminished calcium influx may cause the reduced ACh release by aged synapses. The present study provides evidence for an age-related decrease in presynaptic functions, that is, a reduction in calcium influx via voltage-dependent calcium channels followed by a decreased ACh release from synapses despite an abundance of ACh within the synapses.

Effect of chronic piracetam on age-related changes of cross-maze exploration in mice

Normal aging is known to deteriorate memory, spatial orientation, and perceptual recognition. Experiment 1 examined behavioral manifestations of aging by using a cross-maze exploration test in 2-, 6-, and 10-month-old hybrid mice (CBA x C57BL). A decrease in explorative patrolling and an increase in arm reentries, a latency to start and a total time of exploration were found in 10-month-old mice. In Experiment 2, administration of the cognition enhancer piracetam (2-oxo-1-pirrolidone acetamide) (400 mg/kg, IP, once a day for 10 days) enhanced arm patrolling and decreased reentries in 10-month-old mice to the level displayed by the 2-month-old animals. The results suggest that the cross-maze test may be useful for a preliminary screening of antisenescent drugs.

NS-3(CG3703), a TRH analog, ameliorates scopolamine-induced memory disruption in rats

The effects of a metabolically stable TRH analog, N-[[(3R, 6R)-6-methyl-5-oxo-3-thiomorpholinyl]carbonyl]-L-histidyl-L- prolinamide tetrahydrate (NS-3, CG3703) on the scopolamine-induced memory disruption in maze performance tests were investigated in rats. a) In the delayed nonmatching-to-sample (DNMS) task using a T-maze, NS-3 (0.3 mg/kg) produced a significant reversal of the marginal disruption of choice accuracy induced by scopolamine (0.3 mg/kg) at the short (5 s) and long (120, 480 s) interval delays. Physostigmine (0.5 mg/kg) produced a significant reversal only at a 5-s interval delay. b) In the eight-arm radial maze task, NS-3 (0.3 mg/kg) significantly reversed the deficit of choice accuracy induced by scopolamine (0.3 mg/kg), whereas neither TRH (3-30 mg/kg) nor physostigmine (0.1-1 mg/kg) had any effect. The consistent reversal of these maze-learning performances by NS-3, but not by TRH or physostigmine, may be due to its potent enhancement of cholinergic and noradrenergic neuronal activities.

Behavioral impairment in radial-arm maze learning and acetylcholine content of the hippocampus and cerebral cortex in aged mice

Age-related changes in spatial learning performance were studied in relation to acetylcholine (ACh) content of brain regions in male aged (28-month-old) and young (5-month-old) mice of BDF1 strain. As there were large individual differences in the spatial performance of aged mice, the aged mice were divided into two subgroups, old A and old B. The old A group included the six best performers out of the 12 aged mice and the old B group included the remaining 6 worst performers. In a radial-arm maze task with 8 baited arms, aged mice in the old B group showed a marked deficit in acquisition performance and habituation to the apparatus. In the more difficult maze task with only 4 baited arms, the aged mice in the old B group exhibited marked impairment both in working memory and reference memory throughout training, whereas the aged mice in the old A group showed deficits in reference memory during the first 20 days of training and working memory during the last 20 days relative to young mice. Neurochemical analysis revealed significant decreases in the ACh content of the hippocampus and striatum in both aged groups, and in the frontal cortex and posterior cortex of the old B group as compared to the young group. Correlational analysis showed significant correlations between learning performance in the spatial task and ACh levels in the hippocampus, frontal cortex, and posterior cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related fibrillar deposits in brains of C57BL/6 mice. A review of localization, staining characteristics, and strain specificity

The present article reviews findings regarding the age-related occurrence of clusters of unusual granules in the brains of C57BL/6 (B6) mice and discusses the potential relevance of this phenomenon as a model of specific aspects of brain aging in humans. The granules occur predominantly in the hippocampus of B6 mice and represent aggregations of fibrillar material that are mostly associated with astrocytes. The deposits become evident at about 4 to 6 mo of age, and increase markedly in both number and size thereafter. Similar structures have been observed in adult senescence accelerated mice (SAM) and have been noted, although very rarely, in older mice from other strains. The deposits appear to manifest dominant genetic heritability. Heparan sulfate proteoglycan and laminin or related molecules have been identified as components of the granular material. Although the deposits do not represent senile plaques with beta-amyloid deposition, they might mimic the deposition of extracellular matrix molecules that is thought to be an early event in amyloidogenesis in the aged brain and in Alzheimer's disease.

Age-related deposition of glia-associated fibrillar material in brains of C57BL/6 mice

With advancing age, clusters of unusual granules appear in the brains of C57BL/6 (B6) mice. At the light, confocal laser and electron microscopic levels, the granules represent aggregations of fibrillar material often associated with astrocytes. The fibrillar material is largely free of normal organelles and has been located within astrocytic somata and processes, although in many cases the material is found in the neuropil and is surrounded by a discontinuous membrane. The deposits occur predominantly in hippocampus, but also in piriform cortex, cerebellum and less frequently in some other brain regions. They become evident about six months of age and increase markedly in both number and size thereafter. Incidence of the deposits varies greatly among inbred mouse strains. At six to 12 months of age, granules are abundant in male and female B6, and are absent in BALB/c, CBA, DBA/2 and A mice. In hybrid strains with a B6 background the deposits are also present and thus appear to manifest dominant genetic heritability. Similar granular structures have been described in adult brains of the senescence accelerated mouse and have been noted, albeit very rarely, in aged mice from other strains. While immunostaining of the granules with several polyclonal antisera was found by preabsorption with antigens to be non-specific, immunolabeling with monoclonal antibodies to heparan sulfate proteoglycan core protein and to laminin suggest these or related molecules as components of the fibrillar material. The presence of glycosaminoglycans is supported by staining with periodic acid-Schiff and Gomori's methenamine silver methods. The functional significance of the murine deposits is not yet clear. The deposits do not represent senile plaques with beta-amyloid deposition, but they might mimic the deposition of extracellular matrix molecules that is hypothesized to be a precursor condition for plaque formation and cerebral amyloidosis. Furthermore, the genetic differences in the incidence of the fibrillar deposits has potential to model aspects of familial neurodegenerative diseases.

Radial maze performance and open-field behaviours in aged C57BL/6 mice: further evidence for preserved cognitive abilities during senescence

C57BL/6 mice, aged 2 or 24 months, were tested in a radial maze and observed for an 8-min period, repeated on 3 consecutive days, in an open-field situation with a novel object. In the eight-arm maze, the number of unrepeated path choices made by old mice does not significantly increase with training, whereas it does in young mice. Older animals also take significantly longer to solve the task but the two age groups do not differ with respect to how many paths they run before making the first error or in the strategies used to solve the task. In the open-field situation, the two age groups differ with regard to grooming and rearing behaviour, while in the novelty situation, older animal show a higher level of locomotor activity, perform less freezing, and interact more with the novel object. Habituation curves for all parameters, except grooming in the open field, do not differ between the two groups, thus indicating that this form of nonassociative learning does not vary substantially with increasing age. Results are discussed in terms of preserved cognitive abilities during senescence in that strain.

The aged mouse as a model of cognitive decline with special emphasis on studies in NMRI mice

The use of the aged mouse as an integrated model of age-related cognitive decline is reviewed, with special emphasis on experiments covering the life span of NMRI mice, using different age-groups ranging from 3 through to 22 months. Age-related changes in the sensorimotor profile, spontaneous behaviour and performance in learning and memory tasks are considered. The data provide evidence for cognitive impairment and decreases in spontaneous activity and exploration from middle age onwards. Chronologically, this age depends on the longevity of the strain selected; in NMRI mice, middle age corresponds to 11-12 months. Complex learning tasks, such as the Morris water maze for spatial learning, appear to be the most sensitive to age-related changes, as are tests requiring prolonged retention of acquired information, for example, using passive avoidance. Cued and simple discrimination learning are only impaired in the oldest animals. Age-related changes in non-cognitive variables, including sensorimotor capacity, pain sensitivity, emotionality, or locomotor activity, do not account for the learning impairments, although deficits in visual acuity cannot be excluded in the very old animals. Detailed analysis of the individual data for middle aged and old mice, using discriminant and correlation studies highlight a marked heterogeneity between animals of any given chronological age. Furthermore, individual aged mice do not exhibit similar degrees of impairment across all the behavioural variables, showing that aging is not a uniform process. The possible relationship between age-related behavioural decline and neurochemical changes is an area as yet unexplored apart from a few isolated investigations, including a study on ChAT and AChE in NMRI mice. The studies in the NMRI mice illustrate the value of investigating the full age-range to detect an age group which shows cognitive decline dissociable from physical or emotional changes and which is representative of the population as a whole.

A study of behavioral and sensorial bases of radial maze learning in mice

In order to analyze the various sensorial and behavioral modes implied in learning on a radial maze, three isogenic mice groups (BALB/C, C57BL/6, and CB6F1) were subjected to four different learning procedures, each ending with a probe test. These four procedures examined the use of radial strategies and allowed to dissociate the use of olfactory and spatial cues. Results showed that all mice preferred to use a radial strategy. When the confinement procedure rendered the use of a radial strategy impossible, BALB/C mice were incapable of establishing spatial orientation but were able to learn the task by using olfactory cues. C57BL/6 mice, on the other hand, seemed to use spatial cues exclusively, while the CB6F1 hybrids showed a high degree of plasticity, using either type of information. These strain-specific differences point out the heterogeneity of the processes called into play during radial maze learning and show that unless olfactory cues are carefully controlled they can account for choice accuracy in some mice.

Age-related changes in radial-arm maze learning and basal forebrain cholinergic systems in senescence accelerated mice (SAM)

Age-related changes in learning performance and the brain cholinergic system were studied in a senescence accelerated mice-prone series (SAM-P/8) and a senescence accelerated mice-resistant series (SAM-R/1, control) bred under specific pathogen-free conditions. In a radial-arm maze task, SAM-P/8 mice at 4 and 12 months of age showed virtually no significant impairment in working memory or reference memory compared with SAM-R/1 mice at the same age, although they needed more time to complete a trial than SAM-R/1. In contrast, in a passive avoidance task, SAM-P/8 showed a marked age-accelerated deficit in acquisition performance relative to SAM-R/1. Also, SAM-P/8 showed an age-accelerated decrease in locomotion and rearing in an open-field box. At the end of these behavioral tasks, neurochemical analyses showed that there were no differences in the concentrations of acetylcholine (ACh) in the cortex, hippocampus, striatum, midbrain, or cerebellum between SAM-P/8 and SAM-R/1. Although SAM-P/8 mice did not demonstrate any age-accelerated decline in radial-arm maze performance, they showed a normal age-related decline particularly in working memory, equal to that observed in SAM-R/1. Also, ACh levels in the aged groups of SAM-P/8 showed a significant decrease related to normal aging in the hippocampus and striatum, and a slight decrease in the cortex compared to the young group of the same strain. Thus, we found that SAM-P/8 show dissociative effects of aging in spatial learning and passive avoidance performance.

Alterations in the properties of hippocampal pyramidal neurons in the aged rat

The electrophysiological and pharmacological properties of CA1 hippocampal pyramidal neurons were studied in slices from young (three to four months) and aged (25-32 months) Sprague-Dawley rats having previously performed two behavioral tasks. About 20% of the aged rats were impaired in either the spontaneous alternation task or the water maze task. Electrophysiological parameters were measured and compared in young and aged animals using intracellular recordings. No age-related differences were observed in membrane potential, input resistance, amplitude of action potentials or amplitude of calcium spikes. The amplitude and duration of individual afterhyperpolarizations following a single spike were unchanged. In contrast, the neuronal excitability was significantly decreased and the spike duration significantly enhanced in aged rats as compared to young rats. The comparison of afterhyperpolarizations (which follow a burst of spikes) between young and aged rats was more complex. An increase in the amplitude and duration of afterhyperpolarizations generally occurred in aged animals. However, this increase was not consistent among animals and was dependent on the holding potential of the neuron and on the number of action potentials used to trigger the afterhyperpolarization. The depolarizing effect of bath-applied carbachol, as well as the associated increase in membrane resistance were reduced in neurons from aged rats. In contrast, the effects of carbachol on the depression of synaptic events and the blockade of the afterhyperpolarizations were similar in young and aged animals. In addition, the amplitude of the slow cholinergic excitatory postsynaptic potential induced by stimulation of cholinergic afferents in the presence of physostigmine was also decreased in aged rats. Excitatory postsynaptic potentials and inhibitory postsynaptic potentials following electrical stimulation of stratum radiatum were compared. The amplitude and duration of excitatory postsynaptic potentials were increased in aged rats. The amplitude and duration of the fast inhibitory postsynaptic potential were not significantly affected in aged animals. In contrast, the duration of the slow inhibitory postsynaptic potential was decreased in aged rats. Since the mean baclofen-induced hyperpolarization was only slightly reduced in aged rats, the most likely explanation is a decrease in the release of GABA rather than an alteration in the postsynaptic response mediated by GABAB receptors. A statistically significant correlation was found between the degree of impairment in the spontaneous alternation task and the amplitude of the carbachol-induced depolarization.

A comparison of the working memory performances of young and aged mice combined with parallel measures of testing and drug-induced activations of septo-hippocampal and nbm-cortical cholinergic neurones

The spatial working memory performances of young (2 months) and aged (24-26 months) mice of the C57BL/6 strain were compared using a delayed nonmatching to place (DNMTP) protocol in an automated 8-arm radial maze. The aged mice were observed to exhibit a selective and interference-related memory deficit. Parallel neurochemical analysis of the activity of septo-hippocampal and nbm-cortical cholinergic neurones in vivo was conducted using measures of sodium-dependent high-affinity choline uptake. Results showed that whereas the level of cholinergic activity in both brain regions varied less than 10% between young and aged mice in quiet conditions (basal) the activation usually observed at 30-sec posttest (+20-25%) in young mice was greatly attenuated in the frontal cortex and almost totally absent in the hippocampus of aged mice. In view of these results a complementary experiment was carried out in order to test the intrinsic ability of septo-hippocampal cholinergic neurones to activate using acute injection of scopolamine (1 mg/kg IP 20 min) to both young and aged mice in quiet conditions. The drug injection resulted in a very large (+70%) increase in hippocampal high-affinity choline uptake and with amplitudes which did not vary significantly between young and aged subjects. These observations attest to a relatively well-preserved state of central cholinergic neurones and an intact capacity to activate normally when challenged pharmacologically in aged mice. The results strongly suggest that the loss of cholinergic activation and associated memory deficit in aged mice might rather be related to a hypofunction of phasically active transsynaptic processes which normally mediate the activation of these cholinergic pathways during memory testing.

Hippocampal mossy fibers and radial-maze learning in the mouse: a correlation with spatial working memory but not with non-spatial reference memory

One hundred and eight male mice from nine different inbred strains were tested for two aspects of learning in an eight-arm radial maze. In the first experimental arrangement of the maze, measuring spatial working memory, clear strain differences were found on the fifth day of training. Furthermore, this type of learning showed a high positive correlation with the size of the intra- and infrapyramidal hippocampal mossy fiber terminal field as revealed with Timm's staining. In the second experiment, in which non-spatial reference memory was tested, significant strain differences were found for the learning variables, but there were no significant covariations with the sizes of the intra- and infrapyramidal mossy fiber terminal fields. These results, combined with previous data, suggest that heritable variations of the hippocampal intra- and infrapyramidal mossy fiber projection influence processes determining spatial learning capabilities in mice.

Loss of NGF receptor immunoreactivity in basal forebrain neurons of aged rats: correlation with spatial memory impairment

Nerve growth factor (NGF) has recently been implicated as a trophic agent in the survival and maintenance of basal forebrain cholinergic neurons. To test the hypothesis that NGF may play a role in the age-related decline of cerebral cholinergic function and loss of cognitive ability, we investigated the possible correlation between the loss of basal forebrain neurons that stain for NGF receptor, and impairment of spatial reference memory performance in aged rats. Our results suggest that NGF receptor-positive basal forebrain neurons undergo marked cell atrophy and loss of neuropil staining in aged rats exhibiting impaired spatial learning and memory performance. Conversely, numerous, densely immunoreactive perikarya and a profuse neuritic plexus within the basal forebrain nuclei was consistently observed in behaviorally intact rats. Overall, the mean number of NGF receptor-positive basal forebrain neurons both in the nucleus of the diagonal band and nucleus basalis correlated with retention of the spatial task (r = 0.84 and r = 0.67, respectively; P less than 0.01). Our results support the view that progressive failure of retrograde trophic support due to the age-related loss of NGF receptors may promote degenerative changes in basal forebrain cholinergic neurons, and contribute to deterioration of cognitive ability in senescence.

Dietary restriction: effects on radial maze learning and lipofuscin pigment deposition in the hippocampus and frontal cortex

Previous studies have shown increased life span and decreased lipofuscin deposition in brain structures when both premature and young adult animals are calorically restricted of an otherwise nutritionally adequate diet. Three-months-old C57BL/6 mice were subjected to 12 months of caloric restriction (2 g/day), and subsequently exposed to a radial maze learning paradigm. Mice in the diet restriction group showed faster learning and higher asymptotic performance on the radial maze task, as well as lower lipofuscin deposition in the neurons of hippocampus and frontal cortex relative to control mice fed ad libitum. The results suggest that dietary restriction has effects on radial maze learning, and this improved behavioral performance was associated with significant reduction in lipofuscin pigment deposition in the neurons of hippocampus and frontal cortex.

The effects of aging on hippocampal and cortical projections of the forebrain cholinergic system

It has been proposed that disruption of cholinergic input to the hippocampus and cortex contributes to the learning and memory deficits associated with aging. The data reviewed here, however, suggest that the oft-stated generalization that normal aging is characterized by disruption of cholinergic input to the hippocampus and cortex is not entirely correct. Instead it appears that age-related changes are not consistently found on measures such as the activity of ChAT or the content of ACh in these regions, basal levels of ACh release in cortex, and the number of cholinergic neurons in the basal forebrain (source of cholinergic input to the hippocampus and cortex). These observations suggest that unlike Alzheimer's disease, normal aging does not reliably produce a degeneration of the cholinergic innervation of the hippocampus and cortex. The responsivity of the cholinergic system, however, is altered during normal aging. ACh synthesis and stimulation-induced release of ACh are diminished in aged animals. Further, the electrophysiological response of postsynaptic neurons to ACh is reduced during aging. Although some regional differences in these age-related changes may be present, the generalization that the functioning of the cholinergic system is impaired during aging is probably accurate. Thus, investigation of these changes in the dynamic properties of cholinergic input to the hippocampus and cortex during aging may provide clarification of the relationship between cholinergic dysfunction and age-related decline in learning and memory and may also provide a more reasonable rationale for treatment approaches.

Differences in the acquisition process and the effect of scopolamine on radial maze performance in three strains of rats

The acquisition process and the effect of scopolamine (SCOP) on the radial maze task were studied in 3 strains of male rats, Fischer 344 (F344), Sprague-Dawley (SD) and Wistar. The pretraining level of locomotor activity was measured and performance was quantitatively and qualitatively evaluated. The highest pretraining locomotor activity was observed in Wistar rats. In this experiment, rats were allowed to select each arm successively. The changes in the number of correct choices during the first eight selections, error choice in a trial and the total duration of a trial differed with the strain in the first 5 training sessions. Acquisition curve for F344 rats gradually rose. Wistar rats made many error choices. However, during the final 5 sessions, only the total duration of a trial differed with the strain. Wistar rats took the shortest time to finish a trial, but the number of sessions taken to acquire this task was the fewest with F344 rats, and the most with Wistar rats. The effect of SCOP differed among strains in all the above 3 indexes. Generally, the Wistar rat was the most affected by the injection of SCOP. Moreover, the change of the choice accuracy and the spatial strategy by the administration of SCOP were investigated. SD and Wistar rats showed a dose-dependent decrease in choice accuracy in the earlier selection. The spatial strategy was changed in every strain by the injection of SCOP. These findings do not support the previous finding that a high level of locomotor activity yields fast acquisition of this task or that Wistar rats are better in learning than SD, but indicates that SCOP affects not only the working memory but also the motivational factor.

Increased electrotonic coupling in aged rat hippocampus: a possible mechanism for cellular excitability changes

The effects of aging on the intercellular transfer of the low molecular weight fluorescent dye 5,6-carboxyfluorescein was studied in subfields fascia dentata, CA1, and CA3 of rat hippocampal slices maintained in vitro. All three areas exhibited a statistically significant increase in dye coupling with age. The increased dye coupling was accompanied by an apparent increase in postsynaptic excitability as assessed by the ratio of the population spike to EPSP components of the extracellulary recorded field potential. The possibility that artifactual dye coupling due to cell fusion contributed significantly to these results was ruled out by the demonstrations that a high molecular weight, dextran-coupled fluorescein compound did not produce multiple fills and that dye coupling with carboxyfluorescein could be prevented by prior intracellular loading with Ca++, a procedure that decouples gap junctions in other tissue. The increase in extent of electrical coupling suggested by these data may largely account for the apparent increase in cellular excitability of this tissue with age and may reflect the mechanism by which the senescent hippocampus compensates for the loss of afferent input during the course of normal aging. The additional possibility is raised that increased electrical coupling may reflect a mechanism for permanent associative storage of information in this system.

Measurement of cognitive abilities in senescent animals

The behavioral paradigms used in investigating for differences in the cognitive abilities of young and aged animals are critically reviewed with regard to their power to discriminate between young and aged animals' mental capacity. Irrespective of the kind of task, geronto-behavioral research especially is afflicted with difficulties in controlling motivational and emotional influences on cognitive processes. It is hypothesized-somewhat provocatively-that most of the findings indicating an age-related decline are better attributed to the altered motivational status and/or emotional reactiveness than to impaired cognitive processes of senescent animals. Of the common tasks used in this field, it is concluded that complex mazes and different delayed response tasks seem to represent appropriate paradigms in order to study changed capacities in short- and long-term memory (working- and reference-memory, respectively).

Degeneration of hippocampal fibers and spatial memory deficit in the aged rat

Old and young Fisher 344 rats were compared for their ability to learn a delayed alternation task. The old animals displayed significant impairment of alternation learning, and were slower than the young animals. The brains of these animals were examined using a silver degeneration stain, and among old rats there was conspicuous degeneration. The greatest density of degenerating fibers was seen in the hippocampus and in anatomically related tracts, but there was substantial fiber staining in the corpus callosum, anterior commissure, and internal capsule. Examination of the young brains revealed only an occasional fiber. There were no signs of cortical atrophy in the old animals. The histopathology of the aged animals' hippocampus and fiber tracts supports the possibility that the delayed alternation impairment shown by these animals was a result of age related degenerative changes.

Deficits in spatial-memory tasks following lesions of septal dopaminergic terminals in the rat

The behavioral effects of 6-hydroxydopamine, injected bilaterally into the lateral septum, were investigated in two tests of spatial memory (radial 8-arm and T-maze). Three different experiments were conducted in the radial maze. In experiment I, rats were permitted to learn the task with food reinforcement in all arms of the maze. In experiment II, retention of the spatial information (working memory) learned in experiment I was tested by interposing various time intervals between choice 4 and 5 of each trial. In experiment III, reference and working memory were simultaneously assessed by only reinforcing 4 choices in the radial maze. Performances were compared in spaced versus massed trials. In the T-maze, the rats were first tested for learning a spatial discrimination between the two arms of the maze, and subsequently for reversal of the previously learned response. The results showed that the rats with lesions were impaired in all experiments. This impairment was particularly marked in some aspects of the procedures used: (1) in the search for the last 4 pellets in experiment I, (2) in the first presentations of various intervals interposed between choices 4 and 5, (3) in the search for food in the baited arms when the trials were massed in experiment III and (4) in the reversal of previously learned spatial discrimination in the T-maze. These behavioral deficits in the rats with septal dopaminergic lesions were interpreted as an increased susceptibility to interference. The lesions were shown to have selectively depleted dopamine concentrations in the septum without damaging noradrenergic terminals or cholinergic cell bodies. It was concluded that dopaminergic neurons could have a modulatory influence on memory processes.