No abstract

In Alzheimer's disease (AD), hippocampus-dependent memories underlie an extensive decline. The neuronal ensemble encoding a memory, termed engram, is partly recapitulated during memory recall. Artificial activation of an engram can restore memory in a mouse model of early AD, but its fate and the factors that render the engram nonfunctional are yet to be revealed. Here we used repeated two-photon in vivo imaging in fosGFP transgenic mice that performed a hippocampus-dependent memory task. We found that the partial reactivation of the CA1 engram during recall is preserved under AD-like conditions. However, we identified a novelty-like ensemble that interfered with the engram and thus compromised recall. Mimicking a novelty-like ensemble in healthy mice was sufficient to affect memory recall. In turn, reducing the novelty-like signal rescued the recall impairment under AD-like conditions. These findings suggest a novel mechanistic process that contributes to the deterioration of memories in AD.

Transgenic Cre-recombinase expressing mouse lines are widely used to express fluorescent proteins and opto-/chemogenetic actuators, making them a cornerstone of modern neuroscience. The investigation of interneurons in particular has benefitted from the ability to genetically target specific cell types. However, the specificity of some Cre driver lines has been called into question. Here, we show that nonspecific expression in a subset of hippocampal neurons can have substantial nonspecific functional effects in a somatostatin-Cre (SST-Cre) mouse line. Nonspecific targeting of CA3 pyramidal cells caused large optogenetically evoked excitatory currents in remote brain regions. Similar, but less severe patterns of nonspecific expression were observed in a widely used SST-IRES-Cre line, when crossed with a reporter mouse line. Viral transduction on the other hand yielded more specific expression but still resulted in nonspecific expression in a minority of pyramidal layer cells. These results suggest that a careful analysis of specificity is mandatory before the use of Cre driver lines for opto-or chemogenetic manipulation approaches.

The mouse visual system recently became the most popular model to study the cellular and circuit mechanisms of sensory processing. However, the importance of eye movements in mice only started to be appreciated recently. Eye movements provide a basis for active sensing and deliver insights into various brain functions and dysfunctions. A plethora of knowledge on the central control of eye movements and their role in perception and behaviour arose from work on primates. However, an overview of the known eye movement types in mice and a comparison to primates is missing.Here, we review the eye movement types described to date in mice and compare them to those observed in primates. We discuss the central neuronal mechanisms for their generation and control. Furthermore, we review the mounting literature on eye movements in mice during head-fixed and freely moving behaviours. Finally, we highlight gaps in our understanding and suggest future directions for research.