December 5, 2018
Whole-brain imaging of mice during behavior
In a study published in Neuron, Emilie Macé – from the group of Botond Roska – and collaborators have demonstrated how functional ultrasound imaging can yield high-resolution, unbiased, brain-wide activity maps of behaving mice. These can lead to a brain-wide understanding of how brain activity relates to specific behavior – in healthy mice and in mouse models of neurologic or psychiatric diseases.
Large numbers of brain regions are active during behaviors and there is a need to develop high-resolution, brain-wide activity maps, which identify brain regions involved in specific behaviors. However, current whole-brain functional imaging technologies, such as functional magnetic resonance imaging, are limited in resolution and difficult to apply to awake and behaving mice.
An international team of scientists led by Emilie Macé and Botond Roska, from the Institute of Molecular and Clinical Ophthalmology Basel, the FMI, and the Neuro-Electronics Research Flanders, has developed the emerging technology of high-resolution functional ultrasound imaging to record activity in the whole brain of mice during behavior. The team was particularly interested in the brain regions involved in the optokinetic reflex.
Studying the optokinetic reflex
The optokinetic reflex stabilizes images drifting on the retina both horizontally and vertically by moving the eye in the direction of image drift – we all experienced that our eyes reflexively move to follow the passing landscape as we look out of a train window. This innate reflex is well conserved across species, from mice to humans. Understanding the neuronal circuitry underlying such sensorimotor integration is challenging because this process requires the recruitment of many brain regions interconnected and distributed across the brain. Both the input of this reflex – a specific type of cells in the retina – and the motor output – eye movements – can be manipulated in mice, which makes this reflex an ideal candidate to study sensorimotor integration in the whole-brain using this new approach.
In their study, the researchers found out that of the 181 brain regions consistently identified in all animals, 87 regions – distributed across the whole brain – were modulated during the optokinetic reflex. First author Emilie Macé, a postdoctoral fellow in the group of Botond Roska who developed the concept of functional ultrasound imaging while working in Paris, commented: “We were surprised how precisely we could map activity across the brain and how many brain regions became active during this simple reflex. For example, we discovered that the amygdala – a region usually associated with fear processing – was inhibited during the reflex.”
What happens when the reflex is disrupted?
In the next step, to study the function of these brain regions in sensorimotor integration, the team compared brain activity in healthy mice with mice who lack the optokinetic reflex because of two different induced perturbations – either because of a genetic perturbation affecting the retina that makes them incapable of generating the reflex, or because eye motion was mechanically blocked. The results show that the majority of brain regions active upon eye movement in normal mice become inactive in mice with the genetic perturbation, showing that they are involved in generating the reflex. Among those regions, some regions in the thalamus are particularly interesting: they still respond in normal mice whose eye movements are blocked, but not in mice with the genetic perturbation, showing that they are independent of the motor output of the reflex.
Macé concluded: “Our brain-wide approach revealed new regions that can now be studied more precisely in attempts to understand the logic of sensorimotor transformations at the level of microcircuits.” Roska also highlights the value of the technology developed: “The simplicity, low cost, and ease of use of whole-brain functional ultrasound imaging, together with the ability to precisely identify brain regions, provides a system for obtaining an unbiased view of brain activity in other types of behavior, in wild-type mice as well as in animal models of neurologic or psychiatric diseases.”
Emilie Mace, Gabriel Montaldo, Stuart Trenholm, Cameron Cowan, Alexandra Brignall, Alan Urban, Botond Roska (2018) Whole-brain functional ultrasound imaging reveals brain modules for visuomotor integration. Neuron, 100:1241-1251