A noradrenergic lesion exacerbates neurodegeneration in a Down syndrome mouse model

Individuals with Down syndrome (DS) acquire Alzheimer's-like dementia (AD) and associated neuropathology earlier and at significantly greater rates than age-matched normosomic individuals. However, biological mechanisms have not been discovered and there is currently limited therapy for either DS- or AD-related dementia. Segmental trisomy 16 (Ts65Dn) mice provide a useful model for many of the degenerative changes which occur with age in DS including cognitive deficits, neuroinflammation, and degeneration of basal forebrain cholinergic neurons. Loss of noradrenergic locus coeruleus (LC) neurons is an early event in AD and in DS, and may contribute to the neuropathology. We report that Ts65Dn mice exhibit progressive loss of norepinephrine (NE) phenotype in LC neurons. In order to determine whether LC degeneration contributes to memory loss and neurodegeneration in Ts65Dn mice, we administered the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4; 2 doses of 50 mg/kg, i.p.) to Ts65Dn mice at four months of age, prior to working memory loss. At eight months of age, Ts65Dn mice treated with DSP-4 exhibited an 80% reduction in hippocampal NE, coupled with a marked increase in hippocampal neuroinflammation. Noradrenergic depletion also resulted in accelerated cholinergic neuron degeneration and a further impairment of memory function in Ts65Dn mice. In contrast, DSP-4 had minimal effects on normosomic littermates, suggesting a disease-modulated vulnerability to NE loss in the DS mouse model. These data suggest that noradrenergic degeneration may play a role in the progressive memory loss, neuroinflammation, and cholinergic loss occurring in DS individuals, providing a possible therapeutic avenue for future clinical studies.     

Behavioral comparison of 4 and 6 month-old Ts65Dn mice: age-related impairments in working and reference memory

Ts65Dn mice are partially trisomic for a segment of murine chromosome 16 similar to the gene segment on human chromosome 21 affected in Down's syndrome (DS). These animals display cognitive deficits, neurochemical imbalances, and cholinergic degeneration resembling alterations in DS and early onset Alzheimer's disease. The loss of basal forebrain cholinergic phenotype in Ts65Dn mice begins at approximately 6 months of age and may be due to an improperly functioning neurotrophic system. We compared 4 and 6 month-old Ts65Dn mice in a water-escape radial-arm maze task to investigate working and reference memory before and after the reported onset of cholinergic decline. Both 4 and 6 month-old Ts65Dn mice exhibited impaired performance compared to age-matched controls. However, the younger Ts65Dn mice displayed the capability to learn all working and reference memory measures, while the older Ts65Dn mice did not. Ts65Dn mice failed to maintain performance as working memory load increased, and the ability to handle an increasing working memory load also diminished with age. Collectively, these data suggest that major alterations in cognitive function occur in Ts65Dn mice between the ages of 4 and 6 months.     

Ts65Dn mice, a model for Down syndrome, have deficits in context discrimination learning suggesting impaired hippocampal function

The Ts65Dn mouse is segmentally trisomic for a part of mouse chromosome 16 and is a genetic model for Down syndrome and Alzheimer's disease. Although many studies have examined the learning and memory processes in Ts65Dn mice, it has yet to be determined if Ts65Dn mice are specifically impaired in learning tasks that require an intact hippocampus. Context discrimination learning is dependent on the dorsal hippocampus in mice. In this task, mice learn to discriminate two similar contexts, one of which is associated with foot shock. In the current study, Ts65Dn mice learned almost identically to what has been reported for mice with dorsal hippocampal lesions, while controls behaved similarly to sham lesioned mice. Therefore, Ts65Dn mice have learning deficits in a hippocampal dependent task that may be related to the loss of cholinergic input to the hippocampus, which occurs after 6 months of age.     

Frontal cortex BDNF levels correlate with working memory in an animal model of Down syndrome

Individuals with Down syndrome (DS) develop most neuropathological hallmarks of Alzheimer's disease early in life, including loss of cholinergic markers in the basal forebrain. Ts65Dn mice, an animal model of DS, perform poorly on tasks requiring spatial memory and also exhibit basal forebrain pathology beginning around 6 months of age. We evaluated memory as well as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein levels in basal forebrain, frontal cortex, hippocampus, and striatum in Ts65Dn mice at the age when cholinergic degeneration is first observed, and compared values to normosomic controls. Six-month-old Ts65Dn mice exhibited impairments in working and reference memory as assessed on a water radial-arm maze. The working memory deficit was related to the inability of Ts65Dn mice to successfully sustain performance as the working memory load increased. Coupled with cognitive performance deficiencies, Ts65Dn mice also exhibited lower frontal cortex BDNF protein levels than controls. Further, BDNF levels were negatively correlated with working memory errors during the latter portion of testing in Ts65Dn mice, thereby suggesting that lower BDNF protein levels in the frontal cortex may be associated with the observed working memory impairment.     

Estrogen alters amyloid precursor protein as well as dendritic and cholinergic markers in a mouse model of Down syndrome

Individuals with Down syndrome (DS) develop the pathological hallmarks of Alzheimer's disease at an early age, later followed by memory decline and dementia. Women with DS are twice as likely to undergo early menopause, and levels of estradiol correlate with onset of cognitive decline in these women. We have demonstrated that a mouse model of DS, mice with segmental trisomy of chromosome 16 (Ts65Dn), develop a significant deficit in both reference and working memory as young adults (6-10 months of age), coupled with phenotypic loss of cholinergic neurons in the basal forebrain and altered growth factor levels. In the present study we examined cholinergic and dendritic markers in the hippocampal formation and levels of the amyloid precursor protein (APP) in different brain regions of Ts65Dn mice treated with estradiol for 60 days. The density of the dendritic marker Map2 was significantly decreased in the hippocampal formation of middle-aged trisomic mice (9-15 months old), and the density of cholinergic neurites (acetylcholinesterase [AChE] histochemistry) was also decreased in specific layers of the hippocampus. Treatment with 17beta-estradiol alleviated the decreases in Map2 and AChE staining, but had no effect on full-length APP levels in the hippocampus. In contrast, a main effect of treatment on APP levels in the striatum was noted, with significant elevations observed in controls and trisomics. These findings demonstrate that estrogen can alleviate deficits in cholinergic and dendritic elements in the hippocampal formation and further strengthens the rationale to explore estrogen replacement therapy in women with DS.     

Loss of cholinergic phenotype in basal forebrain coincides with cognitive decline in a mouse model of Down's syndrome

Mice with segmental trisomy of chromosome 16 (Ts65Dn) have been used as a model for Down's syndrome. These mice are born with a normal density of basal forebrain cholinergic neurons but, like patients with Down's syndrome, undergo a significant deterioration of these neurons later in life. The time course for this degeneration of cholinergic neurons has not been studied, nor is it known if it correlates with the progressive memory and learning deficits described. Ts65Dn mice that were 4, 6, 8, and 10 months old were sacrificed for evaluation of basal forebrain morphology. Separate groups of mice were tested on visual or spatial discrimination learning and reversal. We found no alterations in cholinergic markers in 4-month-old Ts65Dn mice, but thereafter a progressive decline in density of cholinergic neurons, as well as significant shrinkage of cell body size, was seen. A parallel loss of staining for the high-affinity nerve growth factor receptor, trkA, was observed at all time points, suggesting a biological mechanism for the cell loss involving this growth factor. Other than transient difficulty in learning the task requirements, there was no impairment of trisomic mice on visual discrimination learning and reversal, whereas spatial learning and reversal showed significant deficits, particularly in the mice over 6 months of age. Thus, the loss of ChAT-immunoreactive neurons in the basal forebrain was coupled with simultaneous deficits in behavioral flexibility on a spatial task occurring for the first time around 6 months of age. These findings suggest that the loss of cholinergic function and the simultaneous decrease in trkA immunoreactivity in basal forebrain may directly correlate with cognitive impairment in the Ts65Dn mouse Copyright 2000 Academic Press.     

Hippocampal hypocellularity in the Ts65Dn mouse originates early in development

Ts65Dn, a well-characterized animal model for Down syndrome, has three copies of the distal end of mouse chromosome 16 and therefore has segmental trisomy for orthologs for nearly half of the genes located on human chromosome 21. Ts65Dn mice have learning and memory impairments, especially in tasks involving the hippocampus. Previous studies have shown that older adult Ts65Dn mice have structural abnormalities in the hippocampus including fewer granule cells in dentate gyrus and more pyramidal cells in the CA3 subfield of cornus ammonis. However, it is not clear whether those changes are secondary to the age-related neurodegeneration of the basal forebrain cholinergic neurons that project to the hippocampus or if they originate earlier during hippocampal development. To address this question, we performed a quantitative study of the hippocampal volume and the numbers of granule cell and pyramidal neurons in young (postnatal day 6, P6) and adult (3-month-old) mice using the optical fractionator method. At P6, Ts65Dn mice had 20% fewer granule cells in dentate gyrus than did euploid littermates. Similarly, compared to euploid, P6 trisomic mice showed an 18% reduction in mitotic cells in the granule cell layer and the hilus, where granule cell precursors divide to generate the internal granule cell layer. Granule cell hypocellularity persists in 3-month-old Ts65Dn mice before the onset of cholinergic atrophy. The hypocellularity seen in the trisomic adult hippocampus originates early in development and may contribute to specific cognitive deficits in these mice.     

Genetic influence on the working memory circuitry: behavior, structure, function and extensions to illness

Memantine is a partial NMDA receptor antagonist that has been shown to improve learning and memory in several animal models, and is approved for the treatment of Alzheimer's disease (AD). Chronic treatments using memantine in animal models of Alzheimer's disease show disease-modifying effects and suggest a potential neuroprotective function. The present study assessed the effects of both short- and long-term memantine treatment in a mouse model of Down syndrome (DS), the Ts65Dn mouse. The Ts65Dn mouse contains a partial trisomy of murine chromosome 16, and exhibits hippocampal-dependent memory deficits, as well as progressive degeneration of basal forebrain cholinergic neurons (BCFNs). Ts65Dn mice were treated with memantine for a period of 6 months, beginning at 4 months of age. At the end of treatment the mice underwent memory testing using novel object recognition and water radial arm maze tasks, and then histologically analyzed for markers of neurodegeneration. Memantine treatment improved spatial and recognition memory performance in the Ts65Dn mice, though not to the level of normosomic littermate controls. Despite these memory improvements, histological analysis found no morphological signs of neuroprotection of basal forebrain cholinergic or locus coeruleus neurons in memantine-treated Ts65Dn mice. However, memantine treatment of Ts65Dn mice gave rise to elevated brain-derived neurotrophic factor expression in the hippocampus and frontal cortex, suggesting a mechanism of behavioral modification. Thus, our findings provide further evidence for memory facilitation of memantine, but suggest pharmacological rather than neuroprotective effects of memantine both after acute and chronic treatment in this mouse model.     

Maternal choline supplementation differentially alters the basal forebrain cholinergic system of young‐adult Ts65Dn and disomic mice

Down syndrome (DS), trisomy 21, is a multifaceted condition marked by intellectual disability and early presentation of Alzheimer's disease (AD) neuropathological lesions including degeneration of the basal forebrain cholinergic neuron (BFCN) system. Although DS is diagnosable during gestation, there is no treatment option for expectant mothers or DS individuals. Using the Ts65Dn mouse model of DS that displays age‐related degeneration of the BFCN system, we investigated the effects of maternal choline supplementation on the BFCN system in adult Ts65Dn mice and disomic (2N) littermates at 4.3–7.5 months of age. Ts65Dn dams were maintained on a choline‐supplemented diet (5.1 g/kg choline chloride) or a control, unsupplemented diet with adequate amounts of choline (1 g/kg choline chloride) from conception until weaning of offspring; post weaning, offspring were fed the control diet. Mice were transcardially perfused with paraformaldehyde, and brains were sectioned and immunolabeled for choline acetyltransferase (ChAT) or p75‐neurotrophin receptor (p75^NTR). BFCN number and size, the area of the regions, and the intensity of hippocampal labeling were determined. Ts65Dn‐unsupplemented mice displayed region‐ and immunolabel‐dependent increased BFCN number, larger areas, smaller BFCNs, and overall increased hippocampal ChAT intensity compared with 2N unsupplemented mice. These effects were partially normalized by maternal choline supplementation. Taken together, the results suggest a developmental imbalance in the Ts65Dn BFCN system. Early maternal‐diet choline supplementation attenuates some of the genotype‐dependent alterations in the BFCN system, suggesting this naturally occurring nutrient as a treatment option for pregnant mothers with knowledge that their offspring is trisomy 21. J. Comp. Neurol. 522:1390–1410, 2014. © 2013 Wiley Periodicals, Inc.     

Dysfunctional hippocampal inhibition in the Ts65Dn mouse model of Down syndrome

GABAergic dysfunction is implicated in hippocampal deficits of the Ts65Dn mouse model of Down syndrome (DS). Since Ts65Dn mice overexpress G-protein coupled inward-rectifying potassium (GIRK2) containing channels, we sought to evaluate whether increased GABAergic function disrupts the functioning of hippocampal circuitry. After confirming that GABA(B)/GIRK current density is significantly elevated in Ts65Dn CA1 pyramidal neurons, we compared monosynaptic inhibitory inputs in CA1 pyramidal neurons in response to proximal (stratum radiatum; SR) and distal (stratum lacunosum moleculare; SLM) stimulation of diploid and Ts65Dn acute hippocampal slices. Synaptic GABA(B) and GABA(A) mediated currents evoked by SR stimulation were generally unaffected in Ts65Dn CA1 neurons. However, the GABA(B)/GABA(A) ratios evoked by stimulation within the SLM of Ts65Dn hippocampus were significantly larger in magnitude, consistent with increased GABA(B)/GIRK currents after SLM stimulation. These results indicate that GIRK overexpression in Ts65Dn has functional consequences which affect the balance between GABA(B) and GABA(A) inhibition of CA1 pyramidal neurons, most likely in a pathway specific manner, and may contribute to cognitive deficits reported in these mice.     

Cholinergic degeneration and memory loss delayed by vitamin E in a Down syndrome mouse model

Down syndrome (DS) individuals develop several neuropathological hallmarks seen in Alzheimer's disease, including cognitive decline and the early loss of cholinergic markers in the basal forebrain. These deficits are replicated in the Ts65Dn mouse, which contains a partial trisomy of murine chromosome 16, the orthologous genetic segment to human chromosome 21. Oxidative stress levels are elevated early in DS, and may contribute to the neurodegeneration seen in these individuals. We evaluated oxidative stress in Ts65Dn mice, and assessed the efficacy of long-term antioxidant supplementation on memory and basal forebrain pathology. We report that oxidative stress was elevated in the adult Ts65Dn brain, and that supplementation with the antioxidant vitamin E effectively reduced these markers. Also, Ts65Dn mice receiving vitamin E exhibited improved performance on a spatial working memory task and showed an attenuation of cholinergic neuron pathology in the basal forebrain. This study provides evidence that vitamin E delays onset of cognitive and morphological abnormalities in a mouse model of DS, and may represent a safe and effective treatment early in the progression of DS neuropathology.     

Neonatal mice of the down syndrome model, Ts65Dn, exhibit upregulated VIP measures and reduced responsiveness of cortical astrocytes to VIP stimulation

The Ts65Dn segmental mouse model of Down syndrome (DS) possesses a triplication of the section of chromosome 16 that is most homologous to the human chromosome 21 that is trisomic in DS. This model exhibits many of the characteristics of DS including small size, developmental delays, and a decline of cholinergic systems and cognitive function with age. Recent studies have shown that vasoactive intestinal peptide (VIP) systems are upregulated in aged Ts65Dn mice and that VIP dysregulation during embryogenesis is followed by the hypotonia and developmental delays as seen in both DS and in Ts65Dn mice. Additionally, astrocytes from aged Ts65Dn brains do not respond to VIP stimulation to release survival-promoting substances. To determine if VIP dys-regulation is age-related in Ts65Dn mice, the current study examined VIP and VIP receptors (VPAC-1 and VPAC-2) in postnatal day 8 Ts65Dn mice. VIP and VPAC-1 expression was significantly increased in the brains of trisomic mice compared with wild-type mice. VIP-binding sites were also significantly increased in several brain areas of young Ts65Dn mice, especially in the cortex, caudate/putamen, and hippocampus. Further, in vitro treatment of normal neurons with conditioned medium from VIP-stimulated Ts65Dn astrocytes from neonatal mice did not enhance neuronal survival. This study indicates that VIP anomalies are present in neonatal Ts65Dn mice, a defect occurs in the signal transduction mechanism of the VPAC-1 VIP receptor, cortical astrocytes from neonatal brains are dysfunctional, and further, that VIP dysregulation may play a significant role in DS.     

Impaired spatial working and reference memory in segmental trisomy (Ts65Dn) mice

To evaluate the cognitive phenotype of the segmental trisomy 16 (Ts65Dn) mouse, a model of Down Syndrome (DS, trisomy 21), we assessed spatial working and reference memory using a 12-arm radial maze (RAM). Ts65Dn mice made a greater number of reference memory errors across trials compared to control mice. Both genotypes showed improvement across trials, although improvement was slower in Ts65Dn mice. Ts65Dn mice also made a greater number of working memory errors on the RAM, and in contrast to control mice, did not improve across trials, always performing at near-chance levels. These results provide evidence for both spatial working and reference memory deficits in Ts65Dn mice, characteristics of cognitive dysfunction.     

Deficits of neuronal density in CA1 and synaptic density in the dentate gyrus, CA3 and CA1, in a mouse model of Down syndrome

Ts65Dn mice are partially trisomic for the distal region of MMU16, which is homologous with the obligate segment of HSA21 triplicated in Down syndrome (DS). Ts65Dn mice are impaired in learning tasks that require an intact hippocampus. In order to investigate the neural basis of these deficits in this mouse model of Down syndrome, quantitative light and electron microscopy were used to compare the volume densities of neurons and synapses in the hippocampus of adult Ts65Dn (n=4) and diploid mice (n=4). Neuron density was significantly lower in the CA1 of Ts65Dn compared to diploid mice (p<0.01). Total synapse density was significantly lower in the dentate gyrus (DG; p<0.001), CA3 (p<0.05) and CA1 (p<0.001) of Ts65Dn compared to diploid mice. The synapse-to-neuron ratio was significantly lower in the DG (p<0.001), CA3 (p<0.01) and CA1 (p<0.001) of Ts65Dn compared to diploid mice. When the data were broken down by synapse type, asymmetric synapse density was found to be significantly lower in the DG (p<0.001), CA3 (p<0.05) and CA1 (p<0.001) of Ts65Dn compared to diploid mice, while such a difference in symmetric synapse density was only present in the DG (p<0.01). The asymmetric synapse-to-neuron ratio was significantly lower in the DG (p<0.001), CA3 (p<0.01) and CA1 (p<0.001) of Ts65Dn compared to diploid mice, but there were no such significant differences in symmetric synapse-to-neuron ratios. These results suggest that impaired synaptic connectivity in the hippocampus of Ts65Dn mice underlies, at least in part, their cognitive impairment.     

Minocycline prevents cholinergic loss in a mouse model of Down's syndrome

Individuals with Down's syndrome develop Alzheimer's-like pathologies comparatively early in life, including progressive degeneration of basal forebrain cholinergic neurons (BFCNs). Cholinergic hypofunction contributes to dementia-related cognitive decline and remains a target of therapeutic intervention for Alzheimer's disease. In light of this, partial trisomy 16 (Ts65Dn) mice have been developed to provide an animal model of Down's syndrome that exhibits progressive loss of BFCNs and cognitive ability. Another feature common to both Down's syndrome and Alzheimer's disease is neuroinflammation, which may exacerbate neurodegeneration, including cholinergic loss. Minocycline is a semisynthetic tetracycline with antiinflammatory properties that has demonstrated neuroprotective properties in certain disease models. Consistent with a role for inflammatory processes in BFCN degeneration, we have shown previously that minocycline protects BFCNs and improves memory in mice with acute, immunotoxic BFCN lesions. We now report that minocycline treatment inhibits microglial activation, prevents progressive BFCN decline, and markedly improves performance of Ts65Dn mice on a working and reference memory task. Minocycline is an established antiinflammatory and neuroprotective drug and may provide a novel approach to treat specific AD-like pathologies.     

Fluoxetine rescues deficient neurogenesis in hippocampus of the Ts65Dn mouse model for Down syndrome

The Ts65Dn mouse, an adult model of Down syndrome displays behavioral deficits consistent with a dysfunctional hippocampus, similar to that seen with DS. In looking for mechanisms underlying these performance deficits, we have assessed adult neurogenesis in the dentate gyrus of Ts65Dn. Under untreated conditions, Ts65Dn mice (2-5 months old) showed markedly fewer BrdU-labeled cells than euploid animals. Chronic antidepressant treatment for over 3 weeks with the serotonin selective reuptake inhibitor, fluoxetine, increased neurogenesis in the Ts65Dn to comparable levels seen in the euploid by augmenting both proliferation and survival of BrdU-labeled cells in the subgranular layer and granule cell layer of the hippocampus, respectively.     

Experimental parameters affecting the Morris water maze performance of a mouse model of Down syndrome

The Ts65Dn mouse is the most studied and genetically the most complete animal model of Down syndrome (DS) available. These mice display many DS-like features, including performance deficits in different behavioral tasks, motor dysfunction, and age-dependent loss of cholinergic markers in the basal forebrain. At present, the only robust data demonstrating a behavioral deficit potentially associated with learning and memory in Ts65Dn mice less than 6 months old have come from studies that used some variation of the Morris water maze task. However, the specific features of the water maze deficits seen in these animals are still poorly defined. This study is an initial attempt to bridge this knowledge gap. We investigated three major factors potentially influencing the performance of Ts65Dn mice in the water maze: (1) order in which the test is executed; (2) age of the animals; and (3) levels of aversiveness associated with the test. Measurements of plasma corticosterone levels and core body temperature after swimming were also carried out in additional subsets of mice. Overall, we found that the behavioral phenotype of Ts65Dn mice was milder than previously described in the literature. Additionally, Ts65Dn mice were significantly more responsive to potential stressors and more prone to swim-induced hypothermia than euploid control animals. More studies are needed to tease out further the potential effects of confounding factors on the performance of Ts65Dn mice.     

Deficits in cognition and synaptic plasticity in a mouse model of Down syndrome ameliorated by GABAB receptor antagonists

Cognitive impairment in Down syndrome (DS) is characterized by deficient learning and memory. Mouse genetic models of DS exhibit impaired cognition in hippocampally mediated behavioral tasks and reduced synaptic plasticity of hippocampal pathways. Enhanced efficiency of GABAergic neurotransmission was implicated in those changes. We have recently shown that signaling through postsynaptic GABA(B) receptors is significantly increased in the dentate gyrus of Ts65Dn mice, a genetic model of DS. Here we examined a role for GABA(B) receptors in cognitive deficits in DS by defining the effect of selective GABA(B) receptor antagonists on behavior and synaptic plasticity of adult Ts65Dn mice. Treatment with the GABA(B) receptor antagonist CGP55845 restored memory of Ts65Dn mice in the novel place recognition, novel object recognition, and contextual fear conditioning tasks, but did not affect locomotion and performance in T-maze. The treatment increased hippocampal levels of brain-derived neurotrophic factor, equally in 2N and Ts65Dn mice. In hippocampal slices, treatment with the GABA(B) receptor antagonists CGP55845 or CGP52432 enhanced long-term potentiation (LTP) in the Ts65Dn DG. The enhancement of LTP was accompanied by an increase in the NMDA receptor-mediated component of the tetanus-evoked responses. These findings are evidence for a contribution of GABA(B) receptors to changes in hippocampal-based cognition in the Ts65Dn mouse. The ability to rescue cognitive performance through treatment with selective GABA(B) receptor antagonists motivates studies to further explore the therapeutic potential of these compounds in people with DS.     

Working memory in the aged Ts65Dn mouse, a model for Down syndrome

The Ts65Dn mouse displays several phenotypic abnormalities that parallel characteristics found in Down syndrome. One important characteristic associated with Down syndrome is an increased incidence of early-onset Alzheimer's disease. Since Alzheimer's disease is characterized largely by progressive memory loss, it is of interest to study working memory in the Ts65Dn mouse. Previous research in our lab using a titrating, delayed matching-to-position schedule of reinforcement has demonstrated that young, adult male Ts65Dn mice do not display a working memory deficit when compared to age-matched littermate controls. However, there have been no studies examining the working memory of these mice as they age. Due to the correlation between Down syndrome and Alzheimer's disease, and as part of a larger effort to further characterize the phenotype of the Ts65Dn mouse, the purpose of this study was to determine whether aged Ts65Dn mice possess a working memory deficit when compared to age-matched littermate controls. In order to study working memory, two groups of mice were trained under a titrating, delayed matching-to-position schedule of reinforcement. The first group was trained beginning at 3 months of age, and the second group began training at 15 months of age. Both groups were studied to 24 months of age. Initially, both groups of Ts65Dn mice performed at a lower level of accuracy than the control mice; however, this difference disappeared with further practice. The results from these lifespan studies indicate that the aged Ts65Dn mouse does not possess a working memory deficit when compared to age-matched controls.