Periodic Reporting for period 1 - defence_SC (Computation of innate threats and defensive behaviour in the mouse)
Reporting period: 2017-09-01 to 2019-08-31
Recent developments in recording and behavioural techniques, which are used throughout this project, provide a unique opportunity for understanding the basic biology of how the brain makes decisions, at the level of individual neurons. This is important for understanding the mechanisms by which the brain generates behaviour, and is also the level of resolution necessary for devising therapeutic strategies for mental health problems, such as anxiety. Anxious individuals often perceive the environment as over-threatening, and engage in defensive actions too often in detriment of other behaviours. Thus, understanding how neuronal activity causes defensive behaviour in the mouse has the potential to have a positive impact in the treatment of anxiety disorders in humans.
We developed a novel cutting edge electrophysiological and behavioural setup to translate the freely-moving innate behviours into head-fixed preparation for the purpose of intracellular recordings of neuronal activity. We built a state-of-the-art set up (Figure 1), in which awake mice are head-fixed and placed on a light-weight platform which contains a shelter, and is floating on an air table (Neurotar Mobile Home Cage). Mice are able to move the platform with their paws, and therefore fictively walk and explore the environment. During exploration, the threatening stimuli are presented, which elicit defensive responses comparable to those observed in freely behaving mice. Two electrophysiological recording techniques were used: high-density Neuropixels silicon probes, to record simultaneously extracellular action potentials from hundreds of neurons, across all layers of the relevant midbrain structures; and whole-cell patch-clamp, to record sub- and supra-threshold events in single neurons during behaviour.
Using this set up we found neurons in the superior colliculus and in the periaqueductal gray that respond to threatening auditory stimuli, visual stimuli, or to both (comprising about 40% of the neurons). We could then divide them further by the nature of the response, which could either be a phasic response at the beginning and at the end of the stimulus, or a persistent response lasting longer then the stimulus presentation. Other neurons were found to respond exclusively during the defensive behavior and not during stimulation that did not cause a behavioural response, indicating that they are behavior neurons. Current efforts are focused on collecting more data to understand the mechanisms underlying the neuronal activity patterns during escape and freezing episodes.
These findings were presented to the scientific community both at the institutional level, and at international workshops and conferences. The researcher will keep on disseminating the results to the wider public through lectures and various outreach activities.
Figure 1: electrophysiological recordings during innate bahaviour in the Neurotar arena. A. The recording setup consists of an air table, a bridge onto which the mouse is head-fixed, and a light-weight carbon cage with a shelter. Two high speed cameras used from above and from the side of the animal, and looming spots or ultrasonic sweeps are played using a monitor and an ultrasonic speaker. B. Left, example of mouse track over 6 minutes of exploration in the arena. Right, example of speed trace when the mouse is escaping from a looming stimulus. C. Example of single unit recordings in the PAG with the Neuropixels probe during presentation of an ultrasonic sweep (grey). Top, peristimulus time histogram. Bottom, raster plots of 5 trials. D. Example of whole cell-recording from a PAG neuron during movement in the arena.