Background

Transgenic mice have been widely used to study the cellular and behavioral alterations underlying complex brain disorders, such as Alzheimer’s Disease (AD) (Mahar et al., 2017; Mondragón-Rodríguez et al., 2018a, 2018b and 2019). The current and most accepted hypothesis regarding AD pathophysiology upholds that accumulation of amyloid-β (Aβ) lead to neurotoxicity and abnormally phosphorylated tau (pTau) in the hippocampus (Ittner et al., 2010). Additionally, several studies have suggested that protein aggregation correlates with cell damage and the cognitive deficits that characterize AD development (Selkoe, 1990; Oddo et al., 2003; LaFerla et al., 2007; Ittner et al., 2010; Mondragón-Rodríguez et al., 2014). However, there is evidence demonstrating that Ca²⁺ dysregulation due to presenilin (PS) mutation occurs prior to the formation of Aβ plaques and pTau accumulation (review in Tong et al., 2018; Hashimoto et al., 2018). Thus, electrophysiological data demonstrated that the PS holoprotein itself functions as a passive ER Ca²⁺ leak channel that is impaired in mutant PS leading to Ca²⁺ overload (review in Tong et al., 2018; Hashimoto et al., 2018). Of relevance, the activities of the major kinases involved in tau phosphorylation are Ca²⁺ dependent (review in Tong et al., 2018).

PS is ubiquitously found in cellular membranes, including the ER membrane (review in Tong et al., 2018; Hashimoto et al., 2018). PS mutation disrupts ER Ca²⁺ release through inositol 1,4,5-triphosphate receptors (InsP₃R) and ryanodine receptors (RyR) that elevate cytoplasmic Ca²⁺, therefore, affecting synaptic transmission and brain activity (review in Tong et al., 2018; Hashimoto et al., 2018). Furthermore, recent evidence suggests that changes in brain activity, potentially leading to hippocampal memory alterations, occurs before any clear neurodegenerative signs appear (Mucke and Selkoe., 2012; Peña-Ortega et al., 2012; Mondragón-Rodríguez et al., 2013, 2017, 2018a and 2018b). In this regard, we have reported that brain activity self-generated in the hippocampus is affected before the detection of cognitive dysfunctions (Mondragón-Rodríguez et al., 2018a, 2018b and 2019). Specifically, we found that CA1 pyramidal neurons from an adolescent transgenic mouse model showed less spike accommodation and increased subthreshold membrane oscillations (Mondragón-Rodríguez et al., 2018a).

To achieve these findings, patch-clamp recordings of pyramidal cells were performed (first group), and hippocampal dependent innate behaviors were assessed in a p30 (postnatal day 30) transgenic AD mouse model (second group) (Oddo et al., 2003).

To evaluate how pyramidal cells are challenged during the early stages of AD development, we performed electrophysiological analyses of their intrinsic properties (Mondragón-Rodríguez et al., 2018a). Action potential properties were examined using depolarizing current steps of increasing amplitude. Additionally, subthreshold oscillations in pyramidal cells, which are the main constituents of subthreshold-level activity (Peña et al., 2010; V-Ghaffari et al., 2016) were evaluated.

To address the cognitive state (hippocampal-dependent function) of p30 transgenic mice we performed burrowing and nesting tests (Salgado-Puga et al., 2015; Gjendal et al., 2019). While, typical hippocampal-dependent tests, such as the morris water maze, are not reliable in p30 mice, burrowing and nesting are examples of highly motivated innate behaviors in rodents (Deacon, 2012; Salgado-Puga et al., 2015). Additionally, both paradigms have proven to be a reliable assay for monitoring the development of brain diseases in mice (Deacon et al., 2001; Orta-Salazar et al., 2013; Salgado-Puga et al., 2015). Thus, deterioration in the ability to perform activities of daily living is an early sign of AD development (Lilamand et al., 2019). Although several protocols exist in the literature, here we simplify and explain the procedures used by our research group with a well-defined scoring system applicable to p30 mice.