Running in place, mice lead us to neuronal circuits underlying threat responses
The brain takes in a tremendous amount of information requiring processing, decision making and action on very short time scales. Correlating neuronal activity with this rapid sequence of events is best done with experiments that combine some type of recording or measurement with alert animals performing a controlled behavioural task. However, this makes the complex even more complicated. Funding of a Marie Skłodowska-Curie Individual Fellowship enabled Yaara Lefler of the Sainsbury Wellcome Centre for Neural Circuits and Behaviour, UCL to make this possible. Within the scope of the defence_SC project, Lefler developed and implemented a novel virtual escape paradigm with in vivo neuronal measurements.
Running for shelter – in place
Neuronal signalling is largely accomplished through the movement of charged ions generating currents and voltages. Electrophysiological recordings are the gold standard for getting a handle on neuronal excitations. However, tiny devices measuring very small electrical fluctuations in single neurons on very fast time scales are subject to significant interference in moving animals. Lefler and her team developed a unique way around this by having mice move their environment rather than themselves to run for cover. She explains: “To create a realistic paradigm with minimal head motion, we used a floating arena consisting of a light weight platform with a shelter floating on an air table. The mice were head-restrained but able to move the platform with their paws while walking. They explored the environment as the platform moved around them, including entering the shelter when desired.”
A virtual ‘haunted house’ of mouse horrors
The midbrain was the region of interest, long recognised as important for defensive behaviours. Different subregions of the midbrain are involved in the sensory processing of threat, in the escape response and in the freeze response. However, it is unclear how sensory integration takes place to signal threat or what the neuronal substrates are in the two opposing, mutually exclusive, escape or freeze responses. To address this, the team presented the mice with either a ‘looming stimulus’ – an expanding dark disk mimicking an approaching predator (visual) – or an ultrasonic stimulus mimicking a predatory rat call (auditory). High density silicon probes recorded extracellular action potentials from hundreds of neurons simultaneously, and whole-cell patch clamp recordings captured synaptic events in single neurons.
Should I stay or should I go now
With all the data at hand, analyses are underway and already beginning to shed light on neuronal circuits that subserve the threat response. Lefler expands: “We found neurons in specific midbrain regions that respond to threatening auditory stimuli, visual stimuli, or both. We also found neurons that respond exclusively during the defensive behaviour, and not with a threatening stimulus that did not cause a behavioural response, indicating that they are active as part of the behavioural mechanism.” Current efforts are focused on analysing the electrophysiological recordings to elucidate the synaptic mechanisms that lead to the decision between escape and freezing. Lefler adds: “This unique setup provides an exciting opportunity to explore the neural mechanisms of decision making at the level of individual neurons. Applications can easily be extended to other behaviours, other brain areas and other species.” Mice spinning a horizontal wheel of fortune lead the way – while running in place.
Keywords
defence_SC, neurons, virtual, decision making, threat response, defensive behaviour, electrophysiological, midbrain, synaptic, action potential
