Arrest of movement induced by Pedunculopontine-stimulation obstructs hippocampal theta rhythm

Complete movement arrest has recently been reported to be induced in rodents by optogenetic stimulation of a subpopulation within the pedunculopontine nucleus (PPN). This evoked arrest appears conspicuously similar to freezing behavior often seen as a fear response in prey animals but could also be akin to the freezing of gait, which is a symptom of Parkinson's disease. This introduces two perspectives on the functional roles of this sub-region: A hub for orchestrating fear-related responses or an omnipotent halting mechanism devoid of emotional components. To better understand this phenomenon and its cognitive component, we engage the distinct electrical brain activity, the hippocampal theta rhythm. This rhythm has a well-described contextual association between various aspects of cognition and behavior. It is prominent during locomotor activity in rodents and immobile yet aroused states like behavioral freezing. We recorded the electrical activity in the hippocampus of rats while walking and being arrested by PPN stimulation. A clear obstruction of the ongoing theta activity was associated with the motor arrest. The timescale of movement arrest was less than 200 ms, similar to the obstruction of the theta rhythm. Since, anxiety, fear, and behavioral freezing are associated with hippocampal theta rhythm, which we did not see during PPN stimulation, we suggest the induced motor arrest occurs without an associated emotional component.