Trace Fear Conditioning: Procedure for Assessing Complex Hippocampal Function in Mice

Methylated urolithin A, mitigates cognitive impairment by inhibiting NLRP3 inflammasome and ameliorating mitochondrial dysfunction in aging mice

Effective therapeutic interventions for elderly patients are lacking, despite advances in pharmacotherapy. Methylated urolithin A (mUro A), a modified ellagitannin (ET)-derived metabolite, exhibits anti-inflammatory, antioxidative, and anti-apoptotic effects. Current research has primarily investigated the neuroprotective effects of mUroA in aging mice and explored the underlying mechanisms. Our study used an in vivo aging model induced by d-galactose (D-gal) to show that mUro A notably improved learning and memory, prevented synaptic impairments by enhancing synaptic protein expression and increasing EPSCs, and reduced oxidative damage in aging mice. mUro A alleviated the activation of the NOD-like receptor thermal protein domain-associated protein 3 (NLRP3) inflammasome, leading to reduced glial cell activity and neuroinflammation in both accelerated aging and naturally senescent mouse models. Moreover, mUroA enhanced the activity of TCA cycle enzymes (PDH, CS, and OGDH), decreased 8-OHdG levels, and raised ATP and NAD+ levels within the mitochondria. At the molecular level, mUro A decreased phosphorylated p53 levels and increased the expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), thus enhancing mitochondrial function. In conclusion, mUro A alleviates cognitive impairment in aging mice by suppressing neuroinflammation through NLRP3 inflammasome inhibition and restoring mitochondrial function via the p53-PGC-1α pathway. This suggests its potential therapeutic agent for brain aging and aging-related diseases.

Assessing cognitive decline in the aging brain: lessons from rodent and human studies

As life expectancy continues to increase worldwide, age-related dysfunction will largely impact our societies in the future. Aging is well established to promote the deterioration of cognitive function and is the primary risk factor for the development of prevalent neurological disorders. Even in the absence of dementia, age-related cognitive decline impacts specific types of memories and brain structures in humans and animal models. Despite this, preclinical and clinical studies that investigate age-related changes in brain physiology often use largely different methods, which hinders the translational potential of findings. This review seeks to integrate what is known about age-related changes in the brain with analogue cognitive tests used in humans and rodent studies, ranging from "pen and paper" tests to virtual-reality-based paradigms. Finally, we draw parallels between the behavior paradigms used in research compared to the enrollment into clinical trials that aim to study age-related cognitive decline.

TPL2 kinase activity regulates microglial inflammatory responses and promotes neurodegeneration in tauopathy mice

Tumor progression locus 2 (TPL2) (MAP3K8) is a central signaling node in the inflammatory response of peripheral immune cells. We find that TPL2 kinase activity modulates microglial cytokine release and is required for microglia-mediated neuron death in vitro. In acute in vivo neuroinflammation settings, TPL2 kinase activity regulates microglia activation states and brain cytokine levels. In a tauopathy model of chronic neurodegeneration, loss of TPL2 kinase activity reduces neuroinflammation and rescues synapse loss, brain volume loss, and behavioral deficits. Single-cell RNA sequencing analysis indicates that protection in the tauopathy model was associated with reductions in activated microglia subpopulations as well as infiltrating peripheral immune cells. Overall, using various models, we find that TPL2 kinase activity can promote multiple harmful consequences of microglial activation in the brain including cytokine release, iNOS (inducible nitric oxide synthase) induction, astrocyte activation, and immune cell infiltration. Consequently, inhibiting TPL2 kinase activity could represent a potential therapeutic strategy in neurodegenerative conditions.

Incidental temporal binding in rats: A novel behavioral task

We designed a behavioral task called One-Trial Trace Escape Reaction (OTTER), in which rats incidentally associate two temporally discontinuous stimuli: a neutral acoustic cue (CS) with an aversive stimulus (US) which occurs two seconds later (CS-2s-US sequence). Rats are first habituated to two similar environmental contexts (A and B), each consisting of an interconnected dark and light chamber. Next, rats experience the CS-2s-US sequence in the dark chamber of one of the contexts (either A or B); the US is terminated immediately after a rat escapes into the light chamber. The CS-2s-US sequence is presented only once to ensure the incidental acquisition of the association. The recall is tested 24 h later when rats are presented with only the CS in the alternate context (B or A), and their behavioral response is observed. Our results show that 59% of the rats responded to the CS by escaping to the light chamber, although they experienced only one CS-2s-US pairing. The OTTER task offers a flexible high throughput tool to study memory acquired incidentally after a single experience. Incidental one-trial acquisition of association between temporally discontinuous events may be one of the essential components of episodic memory formation.

Inhibitory circuits in fear memory and fear-related disorders

Fear learning and memory rely on dynamic interactions between the excitatory and inhibitory neuronal populations that make up the prefrontal cortical, amygdala, and hippocampal circuits. Whereas inhibition of excitatory principal cells (PCs) by GABAergic neurons restrains their excitation, inhibition of GABAergic neurons promotes the excitation of PCs through a process called disinhibition. Specifically, GABAergic interneurons that express parvalbumin (PV+) and somatostatin (SOM+) provide inhibition to different subcellular domains of PCs, whereas those that express the vasoactive intestinal polypeptide (VIP+) facilitate disinhibition of PCs by inhibiting PV+ and SOM+ interneurons. Importantly, although the main connectivity motifs and the underlying network functions of PV+, SOM+, and VIP+ interneurons are replicated across cortical and limbic areas, these inhibitory populations play region-specific roles in fear learning and memory. Here, we provide an overview of the fear processing in the amygdala, hippocampus, and prefrontal cortex based on the evidence obtained in human and animal studies. Moreover, focusing on recent findings obtained using genetically defined imaging and intervention strategies, we discuss the population-specific functions of PV+, SOM+, and VIP+ interneurons in fear circuits. Last, we review current insights that integrate the region-specific inhibitory and disinhibitory network patterns into fear memory acquisition and fear-related disorders.