Spatial coding defects of hippocampal neural ensemble calcium activities in the triple-transgenic Alzheimer's disease mouse model.

Alzheimer's disease (AD) causes progressive age-related defects in memory and cognitive function and has emerged as a major health and socio-economic concern in the US and worldwide. To develop effective therapeutic treatments for AD, we need to better understand the neural mechanisms by which AD causes memory loss and cognitive deficits. Here we examine large-scale hippocampal neural population calcium activities imaged at single cell resolution in a triple-transgenic Alzheimer's disease mouse model (3xTg-AD) that presents both amyloid plaque and neurofibrillary pathological features along with age-related behavioral defects. To measure encoding of environmental location in hippocampal neural ensembles in the 3xTg-AD mice in vivo, we performed GCaMP6-based calcium imaging using head-mounted, miniature fluorescent microscopes ("miniscopes") on freely moving animals. We compared hippocampal CA1 excitatory neural ensemble activities during open-field exploration and track-based route-running behaviors in age-matched AD and control mice at young (3-6.5 months old) and old (18-21 months old) ages. During open-field exploration, 3xTg-AD CA1 excitatory cells display significantly higher calcium activity rates compared with Non-Tg controls for both the young and old age groups, suggesting that in vivo enhanced neuronal calcium ensemble activity is a disease feature. CA1 neuronal populations of 3xTg-AD mice show lower spatial information scores compared with control mice. The spatial firing of CA1 neurons of old 3xTg-AD mice also displays higher sparsity and spatial coherence, indicating less place specificity for spatial representation. We find locomotor speed significantly modulates the amplitude of hippocampal neural calcium ensemble activities to a greater extent in 3xTg-AD mice during open field exploration. Our data offer new and comprehensive information about age-dependent neural circuit activity changes in this important AD mouse model and provide strong evidence that spatial coding defects in the neuronal population activities are associated with AD pathology and AD-related memory behavioral deficits.

Ultraflexible electrodes for recording neural activity in the mouse spinal cord during motor behavior.

Implantable electrode arrays are powerful tools for directly interrogating neural circuitry in the brain, but implementing this technology in the spinal cord in behaving animals has been challenging due to the spinal cord's significant motion with respect to the vertebral column during behavior. Consequently, the individual and ensemble activity of spinal neurons processing motor commands remains poorly understood. Here, we demonstrate that custom ultraflexible 1-µm-thick polyimide nanoelectronic threads can conduct laminar recordings of many neuronal units within the lumbar spinal cord of unrestrained, freely moving mice. The extracellular action potentials have high signal-to-noise ratio, exhibit well-isolated feature clusters, and reveal diverse patterns of activity during locomotion. Furthermore, chronic recordings demonstrate the stable tracking of single units and their functional tuning over multiple days. This technology provides a path for elucidating how spinal circuits compute motor actions.

Monitoring Astrocytic Ca2+ Activity in Freely Behaving Mice.

Monitoring astrocytic Ca2+ activity is essential to understand the physiological and pathological roles of astrocytes in the brain. However, previous commonly used methods for studying astrocytic Ca2+ activities can be applied in only anesthetized or head-fixed animals, which significantly affects in vivo astrocytic Ca2+ dynamics. In the current study, we combined optic fiber recordings with genetically encoded Ca2+ indicators (GECIs) to monitor astrocytic activity in freely behaving mice. This approach enabled selective and reliable measurement of astrocytic Ca2+ activity, which was verified by the astrocyte-specific labeling of GECIs and few movement artifacts. Additionally, astrocytic Ca2+ activities induced by locomotion or footshock were stably recorded in the cortices and hippocampi of freely behaving mice. Furthermore, this method allowed for the longitudinal study of astrocytic activities over several weeks. This work provides a powerful approach to record astrocytic activity selectively, stably, and chronically in freely behaving mice.

Miniscope GRIN Lens System for Calcium Imaging of Neuronal Activity from Deep Brain Structures in Behaving Animals.

Visualizing neural activity from deep brain regions in freely behaving animals through miniature fluorescent microscope (miniscope) systems is becoming more important for understanding neural encoding mechanisms underlying cognitive functions. Here we present our custom-designed miniscope GRadient INdex (GRIN) lens system that enables simultaneously recording from hundreds of neurons for months. This article includes miniscope design, the surgical procedure for GRIN lens implantation, miniscope mounting on the head of a mouse, and data acquisition and analysis. First, a target brain region is labeled with virus expressing GCaMP6; second, a GRIN lens is implanted above the target brain region; third, following mouse surgical recovery, a miniscope is mounted on the head of the mouse above the GRIN lens; and finally, neural activity is recorded from the freely behaving mouse. This system can be applied to recording the same population of neurons longitudinally, enabling the elucidation of neural mechanisms underlying behavioral control. © 2018 by John Wiley & Sons, Inc.

Direct Imaging of Hippocampal Epileptiform Calcium Motifs Following Kainic Acid Administration in Freely Behaving Mice.

Prolonged exposure to abnormally high calcium concentrations is thought to be a core mechanism underlying hippocampal damage in epileptic patients; however, no prior study has characterized calcium activity during seizures in the live, intact hippocampus. We have directly investigated this possibility by combining whole-brain electroencephalographic (EEG) measurements with microendoscopic calcium imaging of pyramidal cells in the CA1 hippocampal region of freely behaving mice treated with the pro-convulsant kainic acid (KA). We observed that KA administration led to systematic patterns of epileptiform calcium activity: a series of large-scale, intensifying flashes of increased calcium fluorescence concurrent with a cluster of low-amplitude EEG waveforms. This was accompanied by a steady increase in cellular calcium levels (>5 fold increase relative to the baseline), followed by an intense spreading calcium wave characterized by a 218% increase in global mean intensity of calcium fluorescence (n = 8, range [114-349%], p < 10(-4); t-test). The wave had no consistent EEG phenotype and occurred before the onset of motor convulsions. Similar changes in calcium activity were also observed in animals treated with 2 different proconvulsant agents, N-methyl-D-aspartate (NMDA) and pentylenetetrazol (PTZ), suggesting the measured changes in calcium dynamics are a signature of seizure activity rather than a KA-specific pathology. Additionally, despite reducing the behavioral severity of KA-induced seizures, the anticonvulsant drug valproate (VA, 300 mg/kg) did not modify the observed abnormalities in calcium dynamics. These results confirm the presence of pathological calcium activity preceding convulsive motor seizures and support calcium as a candidate signaling molecule in a pathway connecting seizures to subsequent cellular damage. Integrating in vivo calcium imaging with traditional assessment of seizures could potentially increase translatability of pharmacological intervention, leading to novel drug screening paradigms and therapeutics designed to target and abolish abnormal patterns of both electrical and calcium excitation.

A system to measure the pupil response to steady lights in freely behaving mice.

BACKGROUND: Transgenic mice are widely used for the study of basic visual function and retinal disease, including in psychophysical tests. Mice have a robust pupillary light reflex that controls the amount of light that enters the eye, and the attenuating effects of the pupil must be considered during such tests. Measurement of the size of pupils at various luminance levels requires that mice remain stable over prolonged periods of time; however, sedation of mice with anesthesia and/or manual restraint can influence the size of their pupils. NEW METHOD: We present a system to measure the pupillary light response to steady lights of freely behaving mice using a custom-built, portable device that automatically acquires close-up images of their eyes. The device takes advantage of the intrinsic nature of mice to inspect objects of interest and can be used to measure pupillary responses in optomotor or operant behavior testing chambers. RESULTS: The size of the pupils in freely behaving mice decreased gradually with luminance from a maximal area in the dark of 3.8mm2 down to a minimum 0.14mm2 at 80 scotopic cd/m2. The data was well fit with a Hill equation with Lo equal to 0.21cd/m2 and coefficient h=0.48. COMPARISON WITH EXISTING METHODS: These values agree with prior measurements of the pupillary response of unrestrained mice that use more laborious and time consuming approaches. CONCLUSIONS: Our new method facilitates practical, straightforward and accurate measurements of pupillary responses made under the same experimental conditions as those used during psychophysical testing.