In this project we investigated how the decision between escape and freezing is computed at the neuronal and synaptic level in two midbrain circuits that are critical for organizing defensive responses in the mouse. We studied defensive behaviour using a paradigm that was established in the host laboratory, in which mice are faced with either threatening ultrasonic sweep or visual looming stimuli that mimick an approaching predator or object from above. When a shelter is placed in the arena, mice rapidly escape to the shelter when faced with either threatening cue. However, when the shelter is removed, in response to the same threatening cues, mice freeze instead, in order to avoid detection.
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.
