The protocol for assessing olfactory working memory capacity in mice

The Olfactory Working Memory Capacity Paradigm

Working memory capacity (WMC), a crucial component of working memory (WM), has consistently drawn the attention of researchers. Exploring the underlying neurobiological mechanisms behind it is currently a prominent focus in the field of neuroscience. Previously, we developed a novel behavioral paradigm for rodents called the olfactory working memory capacity (OWMC) paradigm, which serves as an effective tool for quantifying the WMC of rodents. The OWMC task comprises five phases: context adaptation, digging training, rule-learning for nonmatching to a single sample odor (NMSS), rule-learning for nonmatching to multiple sample odors (NMMS), and capacity testing. In the first phase, mice are handled to reduce stress and acclimate to the training cage. The second phase involves training mice to dig in a bowl of unscented sawdust to locate a piece of cheese. In the third phase, mice are trained to locate the cheese pellet in a bowl with a noveal odor. The fourth phase requires mice to distinguish the novel odor among multiple scented bowls to locate the cheese pellet. Finally, in the fifth phase, mice undergo several WMC tests until they achieve a stable level of performance. In this protocol paper, we will provide detailed instructions on how to implement the behavioral paradigm. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Context adaptation Basic Protocol 2: Digging training Basic Protocol 3: Rule-learning for NMSS Basic Protocol 4: Rule-learning for NMMS Basic Protocol 5: Capacity testing.

The olfactory working memory capacity paradigm: A more sensitive and robust method of assessing cognitive function in male 5XFAD mice

The olfactory working memory capacity (OWMC) paradigm is able to detect cognitive deficits in 5XFAD mice (an animal model of Alzheimer's disease [TG]) as early as 3 months of age, while other behavioral paradigms detect cognitive deficits only at 4-5 months of age. Therefore, we aimed to demonstrate that the OWMC paradigm is more sensitive and consistent in the early detection of declines in cognitive function than other commonly used behavioral paradigms. The prefrontal cortex (PFC), retrosplenial cortex (RSC), subiculum (SUB), and amygdala (AMY) of 5XFAD mice were harvested and subjected to immunostaining to detect the expression of β-amyloid (Aβ). Additionally, we compared the performance of 3-month-old male 5XFAD mice on common behavioral paradigms for assessing cognitive function (i.e., the open field [OF] test, novel object recognition [NOR] test, novel object location [NOL] test, Y-maze, and Morris water maze [MWM]) with that on the OWMC task. In the testing phase of the OWMC task, we varied the delay periods to evaluate the working memory capacity (WMC) of wild-type (WT) mice. Significant amyloid plaque deposition was observed in the PFC, RSC, SUB, and AMY of 3-month-old male 5XFAD mice. However, aside from the OWMC task, the other behavioral tests failed to detect cognitive deficits in 5XFAD mice. Additionally, to demonstrate the efficacy of the OWMC task in assessing WMC, we varied the retention delay periods; we found that the WMC of WT mice decreased with longer delay periods. The OWMC task is a sensitive and robust behavioral assay for detecting changes in cognitive function.

Diminished activation of excitatory neurons in the prelimbic cortex leads to impaired working memory capacity in mice

BACKGROUND

Working memory capacity impairment is an early sign of Alzheimer's disease, but the underlying mechanisms remain unclear. Clarifying how working memory capacity is affected will help us better understand the pathological mechanism of Alzheimer's disease. We used the olfactory working memory capacity paradigm to evaluate memory capacity in 3-month-old 5XFAD (an animal model of Alzheimer's disease) mice. Immunofluorescence staining of the prefrontal cortex was performed to detect the number of FOS-positive neurons, calmodulin-dependent protein kinase II-positive neurons, and glutamate decarboxylase-positive neurons in the prelimbic cortex and infralimbic cortex. A chemogenetic method was then used to modulate the inhibition and activation of excitatory neurons in the prelimbic cortex of wild-type and 5XFAD mice and to measure the memory capacity of mice.

RESULTS

Working memory capacity was significantly diminished in 5XFAD mice compared to littermate wild-type mice. Neuronal activation of the prelimbic cortex, but not the infralimbic cortex, was attenuated in 5XFAD mice performing the olfactory working memory capacity task. Subsequently, the FOS-positive neurons were co-localized with both calmodulin-dependent protein kinase II-positive neurons and glutamate decarboxylase-positive neurons. The results showed that the activation of excitatory neurons in the prelimbic cortex was correlated with working memory capacity in mice. Our results further demonstrate that the chemogenetic inhibition of prelimbic cortex excitatory neurons resulted in reduced working memory capacity in wild-type mice, while the chemogenetic activation of prelimbic cortex excitatory neurons improved the working memory capacity of 5XFAD mice.

CONCLUSION

The diminished activation of prelimbic cortex excitatory neurons in 5XFAD mice during task performance is associated with reduced working memory capacity, and activation modulation of excitatory neurons by chemogenetic methods can improve memory capacity impairment in 5XFAD mice. These findings may provide a new direction for exploring Alzheimer's disease therapeutic approaches.