Cognitive impairment in old rats: a comparison of object displacement, object recognition and water maze

Cognitive improvement via a modulated rhythmic pulsed magnetic field in D-galactose-induced accelerated aging mice

Rhythmic physical stimulations have emerged as effective noninvasive intervention strategies in the treatment of pathological cognitive deficits. Transcranial magnetic stimulation (TMS) can regulate neural firing and improve the learning and memory abilities of rodents or patients with cognitive deterioration. However, the effects of elaborate magnetic stimulation with low intensity during aging or other neurological disordering processes on cognitive decline remain unclear. In this study, we developed an elaborate modulated pulsed magnetic field (PMF) stimulation with a complex pattern in the theta repeated frequency and gamma carrier frequency and then determined the effects of this rhythmic PMF on the cognitive function of accelerated aging mice established by chronic subcutaneous injection of D-galactose (D-gal). The results of the Morris water maze (MWM) test showed that mice treated with modulated PMF displayed shorter swimming distance and latency time in the spatial exploration acquisition trial and exhibited a significant preference in the target presumptive platform area in the probe trial, all of which indicated the enhancement in spatial learning and memory abilities upon PMF stimulation of the accelerated aging mice. The novel object recognition (NOR) test results showed a similar tendency as the MWM results although without statistical significance. Further determination of histological structures demonstrated that the cognitive function-related hippocampal CA3 neurons degenerated upon D-gal injection, which could also be partially rescued by PMF application. In comparison with the high-intensity TMS approach, low-intensity magnetic stimulation could be much safer and allow deeper penetration without adverse effects such as seizure. In summary, modulated PMF, even with low intensity, could effectively improve rodent cognitive functions impaired by D-gal-induced accelerated aging, which might provide a new safe therapeutic strategy for cognitive deficits as well as other neurological disorders.

Impact of Gut Microbiota on Radiation-Associated Cognitive Dysfunction and Neuroinflammation in Mice

Radiation-induced brain injury is a common complication of brain irradiation that eventually leads to irreversible cognitive impairment. Evidence has shown that the gut microbiome may play an important role in radiation-induced cognitive function. However, the effects of gut microbiota on radiation-induced brain injury (RIBI) remain poorly understood. Here we studied the link between intestinal microbes and radiation-induced brain injury to further investigate the effects of intestinal bacteria on neuroinflammation and cognitive function. We first verified the differences in gut microbes between male and female mice and administered antibiotics to C57BL/6 male mice to deplete the gut flora and then expose mice to radiation. We found that depletion of intestinal flora after irradiation may act as a protective modulator against radiation-induced brain injury. Moreover, we found that pretreatment with depleted gut microbes in RIBI mice suppressed brain pro-inflammatory factor production, and high-throughput sequencing analysis of mouse feces at 1-month postirradiation revealed microbial differences. Interestingly, a proportion of Verrucomicrobia Akkermansia showed partial recovery. Additionally, short-chain fatty acid treatments increased neuroinflammation in the radiation-induced brain injury model. Although a further increase in cognitive function was not observed, brain injury was aggravated in whole-brain irradiated mice to some extent. The protective effects of depleted intestinal flora and the utilization of the brain-gut axis open new avenues for development of innovative therapeutic strategies for radiation-induced brain injury.

Magnesium acetyltaurate prevents retinal damage and visual impairment in rats through suppression of NMDA-induced upregulation of NF-κB, p53 and AP-1 (c-Jun/c-Fos)

Magnesium acetyltaurate (MgAT) has been shown to have a protective effect against N-methyl-D-aspartate (NMDA)-induced retinal cell apoptosis. The current study investigated the involvement of nuclear factor kappa-B (NF-κB), p53 and AP-1 family members (c-Jun/c-Fos) in neuroprotection by MgAT against NMDA-induced retinal damage. In this study, Sprague-Dawley rats were randomized to undergo intravitreal injection of vehicle, NMDA or MgAT as pre-treatment to NMDA. Seven days after injections, retinal ganglion cells survival was detected using retrograde labelling with fluorogold and BRN3A immunostaining. Functional outcome of retinal damage was assessed using electroretinography, and the mechanisms underlying antiapoptotic effect of MgAT were investigated through assessment of retinal gene expression of NF-κB, p53 and AP-1 family members (c-Jun/c-Fos) using reverse transcription-polymerase chain reaction. Retinal phospho-NF-κB, phospho-p53 and AP-1 levels were evaluated using western blot assay. Rat visual functions were evaluated using visual object recognition tests. Both retrograde labelling and BRN3A immunostaining revealed a significant increase in the number of retinal ganglion cells in rats receiving intravitreal injection of MgAT compared with the rats receiving intravitreal injection of NMDA. Electroretinography indicated that pre-treatment with MgAT partially preserved the functional activity of NMDA-exposed retinas. MgAT abolished NMDA-induced increase of retinal phospho-NF-κB, phospho-p53 and AP-1 expression and suppressed NMDA-induced transcriptional activity of NF-κB, p53 and AP-1 family members (c-Jun/c-Fos). Visual object recognition tests showed that MgAT reduced difficulties in recognizing the visual cues (i.e. objects with different shapes) after NMDA exposure, suggesting that visual functions of rats were relatively preserved by pre-treatment with MgAT. In conclusion, pre-treatment with MgAT prevents NMDA induced retinal injury by inhibiting NMDA-induced neuronal apoptosis via downregulation of transcriptional activity of NF-κB, p53 and AP-1-mediated c-Jun/c-Fos. The experiments were approved by the Animal Ethics Committee of Universiti Teknologi MARA (UiTM), Malaysia, UiTM CARE No 118/2015 on December 4, 2015 and UiTM CARE No 220/7/2017 on December 8, 2017 and Ethics Committee of Belgorod State National Research University, Russia, No 02/20 on January 10, 2020.

Neuroprotective effects of brain-derived neurotrophic factor against amyloid beta 1-40-induced retinal and optic nerve damage

Brain-derived neurotrophic factor (BDNF) could be considered a potential neuroprotective therapy in amyloid beta (Aβ)-associated retinal and optic nerve degeneration. Hence, in this study we investigated the neuroprotective effect of BDNF against Aβ1-40-induced retinal and optic nerve injury. In this study, exposure to Aβ1-40 was associated with retinal and optic nerve injury. TUNEL staining showed significant reduction in the apoptotic cell count in the BDNF-treated group compared with Aβ1-40 group. H&E-stained retinal sections also showed a striking reduction in neuronal cells in the ganglion cell layer (GCL) of retinas fourteen days after Aβ1-40 exposure. By contrast, number of retinal cells was preserved in the retinas of BDNF-treated animals. After Aβ1-40 exposure, visible axonal swelling was observed in optic nerve sections. However, the BDNF-treated group showed fewer changes in optic nerve; axonal swelling was less frequent and less marked. In the present study, exposure to Aβ was associated with oxidative stress, whereas levels of retinal glutathione (GSH), superoxide dismutase (SOD) and catalase were significantly increased in BDNF-treated than in Aβ1-40-treated rats. Both visual object recognition tests using an open-field arena and a Morris water maze showed that BDNF improved rats' ability to recognise visual cues (objects with different shapes) after Aβ1-40 exposure, thus demonstrating that the visual performance of rats was relatively preserved following BDNF treatment. In conclusion, intravitreal treatment with BDNF prevents Aβ1-40-induced retinal cell apoptosis and axon loss in the optic nerve of rats by reducing retinal oxidative stress and restoring retinal BDNF levels.

Pro-Cognitive Properties of the Immunomodulatory Polypeptide Complex, Yolkin, from Chicken Egg Yolk and Colostrum-Derived Substances: Analyses Based on Animal Model of Age-Related Cognitive Deficits

The study aimed to assess the effect of the polypeptide Y complex (Yolkin), isolated from chicken egg yolk, on behavioural and cognitive functions. It also aimed to compare this activity with colostrum-derived substances (Colostrinin, Coloco), which have a confirmed impact on learning and memory. In the study, the effect of Yolkin, administered to rats of different ages, who performed various tasks involving spatial and episodic memory, motor functions and exploratory behavior, was assessed. The experiment was carried out in rats which were 6 and 12 months old. Two different doses of the studied specimens based on previous comparative studies and two different routes of administration (oral and retroperitoneal) were used. A series of behavioural tests were carried out, including an open field test, a novel object recognition test and a Morris water maze. They were used to evaluate the impact of the studied specimen on improving locomotor function and exploratory behaviour, preventing their decline and assess the functioning of episodic and spatial memory in aging rats. The administration of Yolkin gave distinct effects compared to colostrum-derived substances, although confirmed its suggested pro-cognitive action. Therefore, it may be used to enhance cognitive functions and inhibit the progression of dementia in the course of neurodegenerative disorders.

Deficits of psychomotor and mnesic functions across aging in mouse lemur primates

Owing to a similar cerebral neuro-anatomy, non-human primates are viewed as the most valid models for understanding cognitive deficits. This study evaluated psychomotor and mnesic functions of 41 young to old mouse lemurs (Microcebus murinus). Psychomotor capacities and anxiety-related behaviors decreased abruptly from middle to late adulthood. However, mnesic functions were not affected in the same way with increasing age. While results of the spontaneous alternation task point to a progressive and widespread age-related decline of spatial working memory, both spatial reference and novel object recognition (NOR) memory tasks did not reveal any tendency due to large inter-individual variability in the middle-aged and old animals. Indeed, some of the aged animals performed as well as younger ones, whereas some others had bad performances in the Barnes maze and in the object recognition test. Hierarchical cluster analysis revealed that declarative-like memory was strongly impaired only in 7 out of 25 middle-aged/old animals. These results suggest that this analysis allows to distinguish elder populations of good and bad performers in this non-human primate model and to closely compare this to human aging.

A behavioral paradigm to evaluate hippocampal performance in aged rodents for pharmacological and genetic target validation

Aged-related cognitive ability is highly variable, ranging from unimpaired to severe impairments. The Morris water maze (a reliable tool for assessing memory) has been used to distinguish aged rodents that are superior learners from those that are learning impaired. This task, however, is not practical for pre- and post-pharmacological treatment, as the memory of the task is long lasting. In contrast, the object location memory task, also a spatial learning paradigm, results in a less robust memory that decays quickly. We demonstrate for the first time how these two paradigms can be used together to assess hippocampal cognitive impairments before and after pharmacological or genetic manipulations in rodents. Rats were first segregated into superior learning and learning impaired groups using the object location memory task, and their performance was correlated with future outcome on this task and on the Morris water maze. This method provides a tool to evaluate the effect of treatments on cognitive impairment associated with aging and neurodegenerative disorders.

Dissecting the age-related decline on spatial learning and memory tasks in rodent models: N-methyl-D-aspartate receptors and voltage-dependent Ca2+ channels in senescent synaptic plasticity

In humans, heterogeneity in the decline of hippocampal-dependent episodic memory is observed during aging. Rodents have been employed as models of age-related cognitive decline and the spatial water maze has been used to show variability in the emergence and extent of impaired hippocampal-dependent memory. Impairment in the consolidation of intermediate-term memory for rapidly acquired and flexible spatial information emerges early, in middle-age. As aging proceeds, deficits may broaden to include impaired incremental learning of a spatial reference memory. The extent and time course of impairment has been be linked to senescence of calcium (Ca²⁺) regulation and Ca²⁺-dependent synaptic plasticity mechanisms in region CA1. Specifically, aging is associated with altered function of N-methyl-D-aspartate receptors (NMDARs), voltage-dependent Ca²⁺ channels (VDCCs), and ryanodine receptors (RyRs) linked to intracellular Ca²⁺ stores (ICS). In young animals, NMDAR activation induces long-term potentiation of synaptic transmission (NMDAR-LTP), which is thought to mediate the rapid consolidation of intermediate-term memory. Oxidative stress, starting in middle-age, reduces NMDAR function. In addition, VDCCs and ICS can actively inhibit NMDAR-dependent LTP and oxidative stress enhances the role of VDCC and RyR-ICS in regulating synaptic plasticity. Blockade of L-type VDCCs promotes NMDAR-LTP and memory in older animals. Interestingly, pharmacological or genetic manipulations to reduce hippocampal NMDAR function readily impair memory consolidation or rapid learning, generally leaving incremental learning intact. Finally, evidence is mounting to indicate a role for VDCC-dependent synaptic plasticity in associative learning and the consolidation of remote memories. Thus, VDCC-dependent synaptic plasticity and extrahippocampal systems may contribute to incremental learning deficits observed with advanced aging.

Validation of a 2-day water maze protocol in mice

We present a 2-day water maze protocol that addresses some of potential confounds present in the water maze when using the aged subjects typical of studies of neurodegenerative disorders, such as Alzheimer's disease. This protocol is based on an initial series of training trials with a visible platform, followed by a memory test with a hidden platform 24h later. We validated this procedure using aged (15-18m) mice expressing three Alzheimer's disease-related transgenes, PS1(M146 V), APP(Swe), and tau(P301L). We also tested these triple transgenic mice (3xTG) and age and sex-matched wild-type (WT) in a behavioral battery consisting of tests of motor coordination (balance beam), spatial memory (object displacement task) visual acuity (novel object recognition task) and locomotor activity (open field). 3xTG mice had significantly longer escape latencies in the memory trial of the 2-day water maze test than WT and than their own baseline performance in the last visible platform trial. In addition, this protocol had improved sensitivity compared to a typical probe trial, since no significant differences between genotypes were evident in a probe trial conducted 24h after the final training trial. The 2-day procedure also resulted in good reliability between cohorts, and controlled for non-cognitive factors that can confound water maze assessments of memory, such as the significantly lower locomotor activity evident in the 3xTG mice. A further benefit of this method is that large numbers of animals can be tested in a short time.

Behavior on the water maze platform: Relationship to learning and open field exploration in aged and adult rats

In the present study we examined whether age-related deficits in the water maze could be accounted for by changes in the behavior on the platform, and whether platform behavior represents some form of exploration, akin to that seen in the open field. Twenty-seven aged and 8 adult rats (26 and 3 months old Wistar rats, respectively) were tested in an empty open field and spatial object exploration task, followed by 9 days of escape learning in a water maze. The aged as compared to the adults exhibited lower levels of open field activity and were deficient in the object displacement task. Escape deficits in the water maze and reduced activity levels on the platform were also found. In the aged, water maze performance was correlated with behavior on the platform, including quiescence, orienting-like activity and turning behavior, a form of axial rotation. In both age groups, turning behavior was also correlated with exploratory activity in the open field. In conclusion, the results support the hypothesis that age-related impairments in the water maze relate to changes in platform behavior, which, in turn, might reflect exploratory activity.

“Despair” induced by extinction trials in the water maze: Relationship with measures of anxiety in aged and adult rats

We have previously reported that extinction of escape behavior in the water maze due to the removal of the platform coincided with the development of behavioral "despair" in aged and adult rats, as assessed by immobility. The present study examines further predictions derived from the hypothesis that the withholding of reinforcement induces behaviors akin to depression. We tested for correlations between extinction performance and immobility, as well as between immobility and measures of anxiety in aged and adult rats. Age comparisons were also performed on these variables. Forty aged and 29 adult male Wistar rats (24 and 3 months old, respectively) were examined in the open field, black/white box and elevated-plus maze followed by 6 days of training in the water maze hidden platform task and 8 days of extinction without the platform. Indices of immobility increased over trials of extinction, with the aged showing higher levels, earlier onsets and larger slope increases of immobility than the adults. A lower resistance-to-extinction was predictive of more "despair" in both age groups. Between-group differences in the open field, black/white box and elevated-plus maze indicated that the aged showed more anxiety-like behavior than the adults and/or explored these environments less. Within the aged group, indicators of fearfulness in the three tests were predictive of higher levels of "despair". The extinction-despair model is held to provide the promise of a conceptual and empirical model of human depression that is the consequence of withdrawal of reinforcement.