Study Of Superior Colliculus-lateral Posterior Nucleus Of Thalamus Pathway In Mediating Innate Fear Responses

Visual threat stimuli can induce innate defensive responses in animals, such as freezing or flight. Previous work from our lab found that overhead looming evoked extended freezing in mice is mediated by a subcortical pathway from the superior colliculus(SC) to the amygdala via the lateral posterior nucleus of thalamus(LP). Here, we aim to further investigate how the LP receive the SC encoded visual inputs and extract the specific threat related information, and identify the difference of neural responses among the LP sub-divisions by looming stimulation.In present study, we use a variety of methods to elucidate these questions in mice, including looming detection behavior and optogenetics stimulation test, immunohistochemistry, in vivo multi-channel electrophysiology and fiber photometry. The results show that: 1) the magnitude of firing rate of excitatory neurons in intermediate layer of medial superior colliculus(ILSCm) may encode different defensive behaviors. Increasing the optical stimulation frequency can induce more flight than freezing behavior, and a possible switch point from freezing to flight is about 60 Hz. 2) LPMC(mediocaudal of lateral posterior thalamic nucleus is the major target received looming threat signal from the SC. First, we find stimulation of SC projecting terminals in the LPMC/LPLR(laterorostral of lateral posterior thalamic nucleus) induced more strong innate freezing response than activating the SC to LPMR(mediorostral of lateral posterior thalamic nucleus)pathway. Second, we record the electrophysiology and fiber photometry in these three different sub-regions respectively responding to the overhead looming stimulation. The LPMC and LPMR, but not LPLR, show responses to the stimuli, and only the LPMC activity correlates to the looming induced innate response.Precisely elucidating the role of SC and LP in the innate defensive behaviors will provide a powerful evidence for the controversially subcortical pathway and further shed some new lights on developing the psychotropic strategy on emotional disorders related abnormal fear.