Repeated administration of the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) modulates neuroinflammation and amyloid plaque load in mice bearing amyloid precursor protein and presenilin-1 mutant transgenes

The Contribution of the Locus Coeruleus-Noradrenaline System Degeneration during the Progression of Alzheimer's Disease

Alzheimer's disease (AD), which is characterized by extracellular accumulation of amyloid-beta peptide and intracellular aggregation of hyperphosphorylated tau, is the most common form of dementia. Memory loss, cognitive decline and disorientation are the ultimate consequences of neuronal death, synapse loss and neuroinflammation in AD. In general, there are many brain regions affected but neuronal loss in the locus coeruleus (LC) is one of the earliest indicators of neurodegeneration in AD. Since the LC is the main source of noradrenaline (NA) in the brain, degeneration of the LC in AD leads to decreased NA levels, causing increased neuroinflammation, enhanced amyloid and tau burden, decreased phagocytosis and impairment in cognition and long-term synaptic plasticity. In this review, we summarized current findings on the locus coeruleus-noradrenaline system and consequences of its dysfunction which is now recognized as an important contributor to AD progression.

Noradrenaline in Alzheimer's Disease: A New Potential Therapeutic Target

A growing body of evidence demonstrates the important role of the noradrenergic system in the pathogenesis of many neurodegenerative processes, especially Alzheimer’s disease, due to its ability to control glial activation and chemokine production resulting in anti-inflammatory and neuroprotective effects. Noradrenaline involvement in this disease was first proposed after finding deficits of noradrenergic neurons in the locus coeruleus from Alzheimer’s disease patients. Based on this, it has been hypothesized that the early loss of noradrenergic projections and the subsequent reduction of noradrenaline brain levels contribute to cognitive dysfunctions and the progression of neurodegeneration. Several studies have focused on analyzing the role of noradrenaline in the development and progression of Alzheimer’s disease. In this review we summarize some of the most relevant data describing the alterations of the noradrenergic system normally occurring in Alzheimer’s disease as well as experimental studies in which noradrenaline concentration was modified in order to further analyze how these alterations affect the behavior and viability of different nervous cells. The combination of the different studies here presented suggests that the maintenance of adequate noradrenaline levels in the central nervous system constitutes a key factor of the endogenous defense systems that help prevent or delay the development of Alzheimer’s disease. For this reason, the use of noradrenaline modulating drugs is proposed as an interesting alternative therapeutic option for Alzheimer’s disease.

Heightened Hippocampal β-Adrenergic Receptor Function Drives Synaptic Potentiation and Supports Learning and Memory in the TgF344-AD Rat Model during Prodromal Alzheimer's Disease

The central noradrenergic (NA) system is critical for the maintenance of attention, behavioral flexibility, spatial navigation, and learning and memory, those cognitive functions lost first in early Alzheimer's disease (AD). In fact, the locus coeruleus (LC), the sole source of norepinephrine (NE) for >90% of the brain, is the first site of pathologic tau accumulation in human AD with axon loss throughout forebrain, including hippocampus. The dentate gyrus is heavily innervated by LC-NA axons, where released NE acts on β-adrenergic receptors (ARs) at excitatory synapses from entorhinal cortex to facilitate long-term synaptic plasticity and memory formation. These synapses experience dysfunction in early AD before cognitive impairment. In the TgF344-AD rat model of AD, degeneration of LC-NA axons in hippocampus recapitulates human AD, providing a preclinical model to investigate synaptic and behavioral consequences. Using immunohistochemistry, Western blot analysis, and brain slice electrophysiology in 6- to 9-month-old wild-type and TgF344-AD rats, we discovered that the loss of LC-NA axons coincides with the heightened β-AR function at medial perforant path-dentate granule cell synapses that is responsible for the increase in LTP magnitude at these synapses. Furthermore, novel object recognition is facilitated in TgF344-AD rats that requires β-ARs, and pharmacological blockade of β-ARs unmasks a deficit in extinction learning only in TgF344-AD rats, indicating a greater reliance on β-ARs in both behaviors. Thus, a compensatory increase in β-AR function during prodromal AD in TgF344-AD rats heightens synaptic plasticity and preserves some forms of learning and memory.SIGNIFICANCE STATEMENT The locus coeruleus (LC), a brain region located in the brainstem which is responsible for attention and arousal, is damaged first by Alzheimer's disease (AD) pathology. The LC sends axons to hippocampus where released norepinephrine (NE) modulates synaptic function required for learning and memory. How degeneration of LC axons and loss of NE in hippocampus in early AD impacts synaptic function and learning and memory is not well understood despite the importance of LC in cognitive function. We used a transgenic AD rat model with LC axon degeneration mimicking human AD and found that heightened function of β-adrenergic receptors in the dentate gyrus increased synaptic plasticity and preserved learning and memory in early stages of the disease.

Locus Coeruleus Modulates Neuroinflammation in Parkinsonism and Dementia

Locus Coeruleus (LC) is the main noradrenergic nucleus of the central nervous system, and its neurons widely innervate the whole brain. LC is severely degenerated both in Alzheimer’s disease (AD) and in Parkinson’s disease (PD), years before the onset of clinical symptoms, through mechanisms that differ among the two disorders. Several experimental studies have shown that noradrenaline modulates neuroinflammation, mainly by acting on microglia/astrocytes function. In the present review, after a brief introduction on the anatomy and physiology of LC, we provide an overview of experimental data supporting a pathogenetic role of LC degeneration in AD and PD. Then, we describe in detail experimental data, obtained in vitro and in vivo in animal models, which support a potential role of neuroinflammation in such a link, and the specific molecules (i.e., released cytokines, glial receptors, including pattern recognition receptors and others) whose expression is altered by LC degeneration and might play a key role in AD/PD pathogenesis. New imaging and biochemical tools have recently been developed in humans to estimate in vivo the integrity of LC, the degree of neuroinflammation, and pathology AD/PD biomarkers; it is auspicable that these will allow in the near future to test the existence of a link between LC-neuroinflammation and neurodegeneration directly in patients.

Effects of Reducing Norepinephrine Levels via DSP4 Treatment on Amyloid-β Pathology in Female Rhesus Macaques (Macaca Mulatta)

The degeneration in the locus coeruleus associated with Alzheimer's disease suggests an involvement of the noradrenergic system in the disease pathogenesis. The role of depleted norepinephrine was tested in adult and aged rhesus macaques to develop a potential model for testing Alzheimer's disease interventions. Monkeys were injected with the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) or vehicle at 0, 3, and 6 months; brains were harvested at 9 months. Reduced norepinephrine in the locus coeruleus was accompanied by decreased dopamine β-hydroxylase staining and increased amyloid-β load in the aged group, and the proportion of potentially toxic amyloid-β42 peptide was increased. Immunohistochemistry revealed no effects on microglia or astrocytes. DSP4 treatment altered amyloid processing, but these changes were not associated with the induction of chronic neuroinflammation. These findings suggest norepinephrine deregulation is an essential component of a nonhuman primate model of Alzheimer's disease, but further refinement is necessary.

Trigeminal, Visceral and Vestibular Inputs May Improve Cognitive Functions by Acting through the Locus Coeruleus and the Ascending Reticular Activating System: A New Hypothesis

It is known that sensory signals sustain the background discharge of the ascending reticular activating system (ARAS) which includes the noradrenergic locus coeruleus (LC) neurons and controls the level of attention and alertness. Moreover, LC neurons influence brain metabolic activity, gene expression and brain inflammatory processes. As a consequence of the sensory control of ARAS/LC, stimulation of a sensory channel may potential influence neuronal activity and trophic state all over the brain, supporting cognitive functions and exerting a neuroprotective action. On the other hand, an imbalance of the same input on the two sides may lead to an asymmetric hemispheric excitability, leading to an impairment in cognitive functions. Among the inputs that may drive LC neurons and ARAS, those arising from the trigeminal region, from visceral organs and, possibly, from the vestibular system seem to be particularly relevant in regulating their activity. The trigeminal, visceral and vestibular control of ARAS/LC activity may explain why these input signals: (1) affect sensorimotor and cognitive functions which are not directly related to their specific informational content; and (2) are effective in relieving the symptoms of some brain pathologies, thus prompting peripheral activation of these input systems as a complementary approach for the treatment of cognitive impairments and neurodegenerative disorders.

Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system

Aside from its roles in as a classical neurotransmitter involved in regulation of behavior, noradrenaline (NA) has other functions in the CNS. This includes restricting the development of neuroinflammatory activation, providing neurotrophic support to neurons, and providing neuroprotection against oxidative stress. In recent years, it has become evident that disruption of physiological NA levels or signaling is a contributing factor to a variety of neurological diseases and conditions including Alzheimer's disease (AD) and Multiple Sclerosis. The basis for dysregulation in these diseases is, in many cases, due to damage occurring to noradrenergic neurons present in the locus coeruleus (LC), the major source of NA in the CNS. LC damage is present in AD, multiple sclerosis, and a large number of other diseases and conditions. Studies using animal models have shown that experimentally induced lesion of LC neurons exacerbates neuropathology while treatments to compensate for NA depletion, or to reduce LC neuronal damage, provide benefit. In this review, we will summarize the anti-inflammatory and neuroprotective actions of NA, summarize examples of how LC damage worsens disease, and discuss several approaches taken to treat or prevent reductions in NA levels and LC neuronal damage. Further understanding of these events will be of value for the development of treatments for AD, multiple sclerosis, and other diseases and conditions having a neuroinflammatory component. The classical neurotransmitter noradrenaline (NA) has critical roles in modulating behaviors including those involved in sleep, anxiety, and depression. However, NA can also elicit anti-inflammatory responses in glial cells, can increase neuronal viability by inducing neurotrophic factor expression, and can reduce neuronal damage due to oxidative stress by scavenging free radicals. NA is primarily produced by tyrosine hydroxylase (TH) expressing neurons in the locus coeruleus (LC), a relatively small brainstem nucleus near the IVth ventricle which sends projections throughout the brain and spinal cord. It has been known for close to 50 years that LC neurons are lost during normal aging, and that loss is exacerbated in neurological diseases including Parkinson's disease and Alzheimer's disease. LC neuronal damage and glial activation has now been documented in a variety of other neurological conditions and diseases, however, the causes of LC damage and cell loss remain largely unknown. A number of approaches have been developed to address the loss of NA and increased inflammation associated with LC damage, and several methods are being explored to directly minimize the extent of LC neuronal cell loss or function. In this review, we will summarize some of the consequences of LC loss, consider several factors that likely contribute to that loss, and discuss various ways that have been used to increase NA or to reduce LC damage. This article is part of the 60th Anniversary special issue.

Noradrenergic dysfunction in Alzheimer's disease

The brain noradrenergic system supplies the neurotransmitter norepinephrine throughout the brain via widespread efferent projections, and plays a pivotal role in modulating cognitive activities in the cortex. Profound noradrenergic degeneration in Alzheimer's disease (AD) patients has been observed for decades, with recent research suggesting that the locus coeruleus (where noradrenergic neurons are mainly located) is a predominant site where AD-related pathology begins. Mounting evidence indicates that the loss of noradrenergic innervation greatly exacerbates AD pathogenesis and progression, although the precise roles of noradrenergic components in AD pathogenesis remain unclear. The aim of this review is to summarize current findings on noradrenergic dysfunction in AD, as well as to point out deficiencies in our knowledge where more research is needed.

Noradrenergic regulation of glial activation: molecular mechanisms and therapeutic implications

It has been known for many years that the endogenous neurotransmitter noradrenaline (NA) exerts anti-inflammatory and neuroprotective effects both in vitro and in vivo. In many cases the site of action of NA are beta-adrenergic receptors (βARs), causing an increase in intracellular levels of cAMP which initiates a broad cascade of events including suppression of inflammatory transcription factor activities, alterations in nuclear localization of proteins, and induction of patterns of gene expression mediated through activity of the CREB transcription factor. These changes lead not only to reduced inflammatory events, but also contribute to neuroprotective actions of NA by increasing expression of neurotrophic substances including BDNF, GDNF, and NGF. These properties have prompted studies to determine if treatments with drugs to raise CNS NA levels could provide benefit in various neurological conditions and diseases having an inflammatory component. Moreover, increasing evidence shows that disruptions in endogenous NA levels occurs in several diseases and conditions including Alzheimer's disease (AD), Parkinson's disease (PD), Down's syndrome, posttraumatic stress disorder (PTSD), and multiple sclerosis (MS), suggesting that damage to NA producing neurons is a common factor that contributes to the initiation or progression of neuropathology. Methods to increase NA levels, or to reduce damage to noradrenergic neurons, therefore represent potential preventative as well as therapeutic approaches to disease.

Effects of DSP4 on the noradrenergic phenotypes and its potential molecular mechanisms in SH-SY5Y cells

Dopamine β-hydroxylase (DBH) and norepinephrine (NE) transporter (NET) are the noradrenergic phenotypes for their functional importance to noradrenergic neurons. It is known that in vivo N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment induces degeneration of noradrenergic terminals by interacting with NET and depleting intracellular NE. However, DSP4's precise mechanism of action remains unclear. In this study various biochemical approaches were employed to test the hypothesis that DSP4 down-regulates the expression of DBH and NET, and to determine molecular mechanisms that may be involved. The results showed that treatment of SH-SY5Y neuroblastoma cells with DSP4 significantly decreased mRNA and protein levels of DBH and NET. DSP4-induced reduction of DBH mRNA and proteins, as well as NET proteins showed a time- and concentration-dependent manner. Flow cytometric analysis demonstrated that DSP4-treated cells were arrested predominantly in the S-phase, which was reversible. The arrest was confirmed by several DNA damage response markers (phosphorylation of H2AX and p53), suggesting that DSP4 causes replication stress which triggers cell cycle arrest via the S-phase checkpoints. Moreover, the comet assay verified that DSP4 induced single-strand DNA breaks. In summary, the present study demonstrated that DSP4 down-regulates the noradrenergic phenotypes, which may be mediated by its actions on DNA replication, leading to replication stress and cell cycle arrest. These action mechanisms of DSP4 may account for its degenerative consequence after systematic administration for animal models.

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.

The noradrenaline precursor L-DOPS reduces pathology in a mouse model of Alzheimer's disease

Damage to noradrenergic neurons in the locus coeruleus (LC) is a hallmark of Alzheimer's disease (AD) and may contribute to disease progression. In 5xFAD transgenic mice, which accumulate amyloid burden at early ages, the LC undergoes stress as evidenced by increased astrocyte activation, neuronal hypertrophy, reduced levels of LC-enriched messenger RNAs (mRNAs), and increased inflammatory gene expression. Central nervous system (CNS) noradrenaline (NA) levels in 5-month-old male 5xFAD mice were increased using the NA precursor L-threo-3,4-dihydroxyphenylserine (L-DOPS). After 1 month, L-DOPS treatment improved learning in the Morris water maze test compared with vehicle-treated mice. L-DOPS increased CNS NA levels, and average latency times in the water maze test were inversely correlated to NA levels. L-DOPS reduced astrocyte activation and Thioflavin-S staining; increased mRNA levels of neprilysin and insulin degrading enzyme, and of several neurotrophins; and increased brain-derived neurotrophic factor protein levels. These data demonstrate the presence of LC stress in a robust mouse model of AD, and suggest that raising CNS NA levels could provide benefit in AD.

Locus coeruleus damage and noradrenaline reductions in multiple sclerosis and experimental autoimmune encephalomyelitis

The endogenous neurotransmitter noradrenaline exerts anti-inflammatory and neuroprotective effects in vitro and in vivo. Several studies report that noradrenaline levels are altered in the central nervous system of patients with multiple sclerosis and rodents with experimental autoimmune encephalomyelitis, which could contribute to pathology. Since the major source of noradrenaline are neurons in the locus coeruleus, we hypothesized that alterations in noradrenaline levels are a consequence of stress or damage to locus coeruleus neurons. In C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein peptide 35-55 to develop chronic disease, cortical and spinal cord levels of noradrenaline were significantly reduced versus control mice. Immunohistochemical staining revealed increased astrocyte activation in the ventral portion of the locus coeruleus in immunized mice. The immunized mice showed neuronal damage in the locus coeruleus detected by a reduction of average cell size of tyrosine hydroxylase stained neurons. Analysis of the locus coeruleus of multiple sclerosis and control brains showed a significant increase in astrocyte activation, a reduction in noradrenaline levels, and neuronal stress indicated by hypertrophy of tyrosine hydroxylase stained cell bodies. However, the magnitude of these changes was not correlated with extent of demyelination or of cellular infiltrates. Together these findings demonstrate the presence of inflammation and neuronal stress in multiple sclerosis as well as in experimental autoimmune encephalomyelitis. Since reduced noradrenaline levels could be permissive for increased inflammation and neuronal damage, these results suggest that methods to raise noradrenaline levels or increase locus coeruleus function may be of benefit in treating multiple sclerosis.

Protective effects of intranasal losartan in the APP/PS1 transgenic mouse model of Alzheimer disease

The local renin-angiotensin system (RAS) in the brain is a multitasking system controlling a plethora of essential functions such as neurogenic hypertension, baroreflexes, and sympathetic activity. Aside from its vasoactive actions, brain angiotensin II (AT-II) has also been implicated in the pathogenesis of cognitive decline, and beneficial effects of angiotensin receptor blockers (ARBs) in Alzheimer (AD) and Parkinson diseases (PD) are suggested. However, the use of ARBs at antihypertensive dosages would lead to unwanted hypotensive reactions in AD patients. Here we treated the APP/PS1 transgenic mouse model of AD with the ARB losartan (10 mg/kg body weight) to determine whether blockade of the AT-II receptor subtype 1 (AT1-R) with intranasal losartan, using at a dosage far below its systemic antihypertensive dose, could maintain its neuroprotective effects independent of its systemic vasoactive action. Intranasal losartan treatment (10 mg/kg every other day for 2 months) of APP/PS1 mice decreased amyloid beta (Abeta) plaques 3.7-fold. Blood serum levels of interleukin-12 (IL-12)p40/p70, IL-1beta, and granulocyte-macrophage colony-stimulating factor (GM-CSF) were increased in the vehicle-treated APP/PS1 mice. Intranasal losartan not only decreased IL-12p40/p70, IL-1beta, and GM-CSF, but also increased IL-10, which suppresses inflammation. Furthermore, losartan markedly increased tyrosine hydroxylase expression in the striatum and locus coeruleus. In conclusion, losartan exerts direct neuroprotective effects via its Abeta-reducing and antiinflammatory effects in the central nervous system (CNS). Therefore, intranasal losartan and potentially other ARBs, at concentrations below their threshold for altering systemic blood pressure, offer a new approach for the treatment of AD.

Neurochemical changes in a double transgenic mouse model of Alzheimer's disease fed a pro-oxidant diet

Oxidative stress is implicated in the pathogenesis of Alzheimer's disease (AD) causing neurodegeneration and decreased monoamine neurotransmitters. We investigated the effect of administration of a pro-oxidant diet on the levels of monoamines and metabolites in the brains of wildtype and transgenic mice expressing mutant APP and PS-1 (TASTPM mice). Three-month-old TASTPM and wildtype (C57BL6/J) mice were fed either normal or pro-oxidant diet for 3 months. The neocortex, cerebellum, hippocampus and striatum were assayed for their monoamine and monoamine metabolite content using HPLC with electrochemical detection. Striatal tyrosine hydroxylase (TOH) levels were analysed by Western blotting. In the striatum, female TASTPM mice had higher levels of DOPAC and male TASTPM mice had higher levels of 5-HIAA compared to wildtype mice. Administration of pro-oxidant diet increased striatal MHPG, turnover of NA and 5-HT levels in female TASTPM mice compared to TASTPM mice fed control diet. The pro-oxidant diet also decreased DOPAC levels in female TASTPM mice compared to those fed control diet. Striatal TOH did not depend on diet, gender or genotype. In the neocortex, the TASTPM genotype increased levels of 5-HIAA in male mice fed control diet compared to wildtype mice. In the cerebellum, the TASTPM genotype led to decreased levels of HVA (male mice only) and also decreased turnover of DA (female mice only) compared to wildtype mice. These data suggest a sparing of monoaminergic neurones in the cortex, striatum and hippocampus of TASTPM mice fed pro-oxidant diet and could be indicative of increased activity in corticostriatal circuits. The decreased cerebellar levels of HVA and turnover of DA in TASTPM mice hint at possible axonal degeneration within this subregion.

Induced LC degeneration in APP/PS1 transgenic mice accelerates early cerebral amyloidosis and cognitive deficits

Degeneration of locus ceruleus neurons and subsequent reduction of norepinephrine concentration in locus ceruleus projection areas represent an early pathological indicator of Alzheimer's disease. In order to model the pathology of the human disease and to study the effects of norepinephrine-depletion on amyloid precursor protein processing, behaviour, and neuroinflammation, locus ceruleus degeneration was induced in mice coexpressing the swedish mutant of the amyloid precursor protein and the presenilin 1 DeltaExon 9 mutant (APP/PS1) using the neurotoxin N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (dsp4) starting treatment at 3 months of age. Norepinephrine transporter immunolabelling demonstrated severe loss of locus ceruleus neurons and loss of cortical norepinephrine transporter starting as early as 4.5 months of age and aggravating over time. Of note, dsp4-treated transgenic mice showed elevated amyloid beta levels and impaired spatial memory performance at 6.5 months of age compared to control-treated APP/PS1 transgenic mice, indicating an accelerating effect on cerebral amyloidosis and cognitive deficits. Likewise, norepinephrine-depletion increased neuroinflammation compared to transgenic controls as verified by macrophage inflammatory protein-1alpha and -1beta gene expression analysis. Exploratory activity and memory retention was compromised by age in APP/PS1 transgenic mice and further aggravated by induced noradrenergic deficiency. In contrast, novel object recognition was not influenced by norepinephrine deficiency, but by the APP/PS1 transgene at 12 months. Overall, our data indicate that early loss of noradrenergic innervation promotes amyloid deposition and modulates the activation state of inflammatory cells. This in turn could have had impact on the acceleration of cognitive deficits observed over time.

The occurrence of a deficit in contextual fear extinction in adult amyloid-over-expressing TASTPM mice is independent of the strength of conditioning but can be prevented by mild novel cage stress

In the amyloid over-expressing TASTPM mouse model of Alzheimer's disease, impaired contextual fear memory occurs early, and is preceded, at 4 months of age, by a deficit in extinction of contextual fear that is resistant to improvement by repeated mild novel cage stress. The first aim of this study was thus to establish whether the extinction deficit could be prevented if the novel cage procedure was applied prior to its onset. The second aim was to establish whether the occurrence of the extinction deficit was dependent on the robustness of the conditioning protocol. We first compared 3-month-old wild-type and TASTPM mice for acquisition, retention and extinction of contextual fear and then, looked at the impact of 5 weeks of novel cage stress (4 x 1 h/week) applied from 3 months onwards, on age-related changes in these behaviours evaluated at 4.5 months of age. In another experiment, we compared 4-month-old TASTPM and wild-type mice for the impact of a 2 and 5-pairing conditioning procedure on the three phases of contextual fear conditioning. In 4.5-month-old TASTPM mice, the deficit in extinction was alleviated by repeated novel cage stress, applied from prior to its onset at 3 months. At 4 months of age, the occurrence of an extinction deficit was independent of the strength of the conditioning procedure, in TASTPM mice, which even showed an increase in aversive memory under the 2-pairing condition. The robust early impairment in the extinction of contextual fear seen in adult TASTPM mice suggests that a deficit in cognitive flexibility is the first sign of behavioural pathology in this model of Alzheimer's disease.

Repeated novel cage exposure-induced improvement of early Alzheimer's-like cognitive and amyloid changes in TASTPM mice is unrelated to changes in brain endocannabinoids levels

Environmental factors (e.g. stress, exercise, enrichment) are thought to play a role in the development of Alzheimer's disease later in life. We investigated the influence of repeated novel cage exposure on the development of early Alzheimer's-like pathology in adult (4 months old) double transgenic mice over-expressing the amyloid precursor protein and presenilin-1 genes (TASTPM mouse line). The procedure involves the repeated placement of the animal into a novel clean cage, a manipulation which induces a stress response and exploratory activity and, as such, can also be seen as a mild form of enrichment. Before and after exposure to the novel cage procedure, separate groups of mice were evaluated for locomotor performance and short-term contextual memory in the fear-conditioning test. Repeated novel cage exposure prevented the onset of a short-term memory deficit that was apparent in 5.5- but not 4-month-old TASTPM mice, without reversing the deficit in extinction already evident at 4 months of age. Brain regional levels of soluble and insoluble amyloid and of endocannabinoids were quantified. Novel cage exposure attenuated soluble and insoluble amyloid accumulation in the hippocampus and frontal cortex, without affecting the age-related increases in regional brain endocannabinoids levels. These beneficial effects are likely to be the consequence of the increase in physical and exploratory activity induced by novel cage exposure and suggest that the impact of environmental factors on Alzheimer's-like changes may be dependent on the degree of activation of stress pathways.