Abstract
Chronic electrophysiological recordings of neuronal activity combined with two-photon Ca2+ imaging give access to high resolution and cellular specificity. In addition, awake drug-free experimentation is required for investigating the physiological mechanisms that operate in the brain. Here, we developed a simple head fixation platform, which allows simultaneous chronic imaging and electrophysiological recordings to be obtained from the hippocampus of awake mice. We performed quantitative analyses of spontaneous animal behaviour, the associated network states and the cellular activities in the dorsal hippocampus as well as estimated the brain stability limits to image dendritic processes and individual axonal boutons. Ca2+ imaging recordings revealed a relatively stereotyped hippocampal activity despite a high inter-animal and inter-day variability in the mouse behavior. In addition to quiet state and locomotion behavioural patterns, the platform allowed the reliable detection of walking steps and fine speed variations. The brain motion during locomotion was limited to ~1.8 μm, thus allowing for imaging of small sub-cellular structures to be performed in parallel with recordings of network and behavioural states. This simple device extends the drug-free experimentation in vivo, enabling high-stability optophysiological experiments with single-bouton resolution in the mouse awake brain.
Highlights
We took advantage of the hippocampal CA1 area as a model because its cellular and network activities have been characterized extensively in freely behaving rodents[25,26,27,28,29,30,31]
The optical encoder was supplied with power (Fig. 1c) and the analog signals were digitized to reliably track the instantaneous speed at up to 120 cm s−1 with millisecond precision (Fig. 1d,e)
The optical encoder signal can be used to derive the step length, frequency and speed acceleration patterns. Such data are usually obtained using motion analysis techniques combined with high-speed video recordings of the behaviour[42], or even more advanced four-axis robotic system equipped with leg-guidance linkages, motors and series of optical encoders that record the rotational position of the motors[43,44]
Summary
We took advantage of the hippocampal CA1 area as a model because its cellular and network activities have been characterized extensively in freely behaving rodents[25,26,27,28,29,30,31]. There is a recognized precise relationship between the CA1 activity and animal speed[32,33]. In addition to well-defined behavioural patterns consisting of immobility and locomotion, the head fixation platform developed here allows to analyse the animal habituation rate and to count walking steps. It enables high-precision analysis of Ca2+ signals in single neurons, neuronal dendrites and individual axonal boutons with a superior stability in locomoting mice. We used the platform to analyse the individual, inter-session and intra-session variability in animal behaviour as well as cell-to-network recruitment. Our data reveal the stereotyped activity patterns of CA1 neurons in parallel with variability in motor activity
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