Latency to startle is reduced in the 5xFAD mouse model of Alzheimer’s disease

Age dependent path integration deficit in 5xFAD mice

Alzheimer's disease (AD) is a severe neurodegenerative disorder and the most common form of dementia in elderly individuals, characterized by memory deficits, cognitive decline, and neuropathology. The identification of preclinical markers for AD remains elusive. We employed an ultrasound-evoked spatial memory assay to investigate path integration (PI) in wild type C57BL/6 J and 5xFAD mice. We observed significant recruitment of the mammillary bodies (MB) and subiculum (Sub) - core regions of the Papez circuit during PI, as indicated by increased expression of the immediate early gene c-Fos in C57BL/6 J mice. In 5xFAD mice, amyloid-beta (Aβ) vulnerability in the MB and Sub was evident at 3-months of age, preceding widespread pathology at 5-months of age. In parallel, we detected significant behavioral deficits in PI in the 5XFAD mice at 5- but not 3-months of age. Sex based analysis revealed a more profound deficit in males compared to females at 5-months of age. Our data suggest PI may be as an early indicator of AD, potentially associated with dysfunction within the Papez circuit.

Sensory processing deficits and related cortical pathological changes in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder primarily affecting cognitive functions. However, sensory deficits in AD start to draw attention due to their high prevalence and early onsets which suggest that they could potentially serve as diagnostic biomarkers and even contribute to the disease progression. This literature review examines the sensory deficits and cortical pathological changes observed in visual, auditory, olfactory, and somatosensory systems in AD patients, as well as in various AD animal models. Sensory deficits may emerge at the early stages of AD, or even precede the cognitive decline, which is accompanied by cortical pathological changes including amyloid-beta deposition, tauopathy, gliosis, and alterations in neuronal excitability, synaptic inputs, and functional plasticity. Notably, these changes are more pronounced in sensory association areas and superficial cortical layers, which may explain the relative preservation of basic sensory functions but early display of deficits of higher sensory functions. We propose that sensory impairment and the progression of AD may establish a cyclical relationship that mutually perpetuates each condition. This review highlights the significance of sensory deficits with or without cortical pathological changes in AD and emphasizes the need for further research to develop reliable early detection and intervention through sensory systems.

Decreased nesting behavior, selective increases in locomotor activity in a novel environment, and paradoxically increased open arm exploration in Neurogranin knockout mice

Aims

Neurogranin (NRGN) is a postsynaptic protein kinase substrate that binds calmodulin in the absence of calcium. Recent studies suggest that NRGN is involved in neuropsychiatric disorders, including schizophrenia, ADHD, and Alzheimer's disease. Previous behavioral studies of Nrgn knockout (Nrgn KO) mice identified hyperactivity, deficits in spatial learning, impaired sociability, and decreased prepulse inhibition, which suggest that these mice recapitulate some symptoms of neuropsychiatric disorders. To further validate Nrgn KO mice as a model of neuropsychiatric disorders, we assessed multiple domains of behavioral phenotypes in Nrgn KO mice using a comprehensive behavioral test battery including tests of homecage locomotor activity and nesting behavior.

Methods

Adult Nrgn KO mice (28‐54 weeks old) were subjected to a battery of comprehensive behavioral tests, which examined general health, nesting behavior, neurological characteristics, motor function, pain sensitivity, locomotor activity, anxiety‐like behavior, social behavior, sensorimotor gating, depression‐like behavior, and working memory.

Results

The Nrgn KO mice displayed a pronounced decrease in nesting behavior, impaired motor function, and elevated pain sensitivity. While the Nrgn KO mice showed increased locomotor activity in the open field test, these mice did not show hyperactivity in a familiar environment as measured in the homecage locomotor activity test. The Nrgn KO mice exhibited a decreased number of transitions in the light‐dark transition test and decreased stay time in the center of the open field test, which is consistent with previous reports of increased anxiety‐like behavior. Interestingly, however, these mice stayed on open arms significantly longer than wild‐type mice in the elevated plus maze. Consistent with previous studies, the mutant mice exhibited decreased prepulse inhibition, impaired working memory, and decreased sociability.

Conclusions

In the current study, we identified behavioral phenotypes of Nrgn KO mice that mimic some of the typical symptoms of neuropsychiatric diseases, including impaired executive function, motor dysfunction, and altered anxiety. Most behavioral phenotypes that had been previously identified, such as hyperlocomotor activity, impaired sociability, tendency for working memory deficiency, and altered sensorimotor gating, were reproduced in the present study. Collectively, the behavioral phenotypes of Nrgn KO mice detected in the present study indicate that Nrgn KO mice are a valuable animal model that recapitulates a variety of symptoms of neuropsychiatric disorders, such as schizophrenia, ADHD, and Alzheimer's disease.

We found that Neurogranin knockout mice exhibit decreased nesting behavior, selective increases in locomotor activity in a novel environment, and paradoxically increased open arm exploration. Considering the behavioral phenotypes that had been previously identified, we propose that Neurogranin KO mice are a valuable animal model that recapitulates a variety of symptoms of neuropsychiatric disorders, such as schizophrenia, ADHD, and Alzheimer's disease.

Prepulse inhibition of the acoustic startle reflex and P50 gating in aging and alzheimer's disease

Inhibition plays a crucial role in many functional domains, such as cognition, emotion, and actions. Studies on cognitive aging demonstrate changes in inhibitory mechanisms are age- and pathology-related. Prepulse inhibition (PPI) is the suppression of an acoustic startle reflex (ASR) to an intense stimulus when a weak prepulse stimulus precedes the startle stimulus. A reduction of PPI is thought to reflect dysfunction of sensorimotor gating which normally suppresses excessive behavioral responses to disruptive stimuli. Both human and rodent studies show age-dependent alterations of PPI of the ASR that are further compromised in Alzheimer's disease (AD). The auditory P50 gating, an index of repetition suppression, also is characterized as a putative electrophysiological biomarker of prodromal AD. This review provides the latest evidence of age- and AD-associated impairment of sensorimotor gating based upon both human and rodent studies, as well as the AD-related disruption of P50 gating in humans. It begins with a concise review of neural networks underlying PPI regulation. Then, evidence of age- and AD-related dysfunction of both PPI and P50 gating is discussed. The attentional/ emotional aspects of sensorimotor gating and the neurotransmitter mechanisms underpinning PPI and P50 gating are also reviewed. The review ends with conclusions and research directions.

Life-Course Contribution of Prenatal Stress in Regulating the Neural Modulation Network Underlying the Prepulse Inhibition of the Acoustic Startle Reflex in Male Alzheimer's Disease Mice

The prepulse inhibition (PPI) of the acoustic startle reflex (ASR), as an index of sensorimotor gating, is one of the most extensively used paradigms in the field of neuropsychiatric disorders. Few studies have examined how prenatal stress (PS) regulates the sensorimotor gating during the lifespan and how PS modifies the development of amyloid-beta (Aβ) pathology in brain areas underlying the PPI formation. We followed alternations in corticosterone levels, learning and memory, and the PPI of the ASR measures in APPNL-G-F/NL-G-F offspring of dams exposed to gestational noise stress. In-depth quantifications of the Aβ plaque accumulation were also performed at 6 months. The results indicated an age-dependent deterioration of sensorimotor gating, long-lasting PS-induced abnormalities in PPI magnitudes, as well as deficits in spatial memory. The PS also resulted in a higher Aβ aggregation predominantly in brain areas associated with the PPI modulation network. The findings suggest the contribution of a PS-induced hypothalamic-pituitary-adrenal (HPA) axis hyperactivity in regulating the PPI modulation substrates leading to the abnormal development of the neural protection system in response to disruptive stimuli. The long-lasting HPA axis dysregulation appears to be the major underlying mechanism in precipitating the Aβ deposition, especially in brain areas contributed to the PPI modulation network.

Reduced Acoustic Startle Response and Prepulse Inhibition in the Tg4-42 Model of Alzheimer's Disease

Sensorimotor deficits have been described in several neuropsychiatric disorders including Alzheimer’s disease. The aim of the present study was to evaluate possible sensorimotor gating deficits in the Tg4-42 mouse model of Alzheimer’s disease using the prepulse inhibition task (PPI). Previous studies indicated that the hippocampus is essentially involved in the regulation of PPI. We analyzed 7-month-old homozygous Tg4-42 mice as mice at this age display severe neuron loss especially in the CA1 region of the hippocampus. Our results revealed a reduced startle response and PPI in Tg4-42 mice. The observed deficits in startle response and PPI are likely due to altered sensory processing abilities rather than hearing deficits as Tg4-42 displayed intact hearing in the fear conditioning task. The present study demonstrates for the first time that sensorimotor gating is impaired in Tg4-42 mice. Analyzing startle response as well as the PPI may offer valuable measurements to assess the efficacy of therapeutic strategies in the future in this Alzheimer’s disease model.