AN ALTERNATIVE NEURAL PATHWAY FOR ESCAPE BEHAVIOUR: THE PARABIGEMINAL–SUPERIOR COLLICULUS CIRCUIT​

Innate escape behaviour triggered by threatening stimuli plays a critical role in animal survival. The neural basis underlying this evolutionarily conserved response resides in the midbrain. In the canonical pathway, the superior colliculus (SC) integrates retinal visual input and mediates the execution of defensive responses through projections to the periaqueductal gray (PAG). However, at least one alternative circuits through the parabigeminal nucleus (PBG) has been suggested to also mediate escape in various species including mice and cats; although its role in initiating behaviour has been disputed. In addition, dorsal PAG activity does not mediate escape to visual threat in all tested species. In the beach mouse Peromyscus polionotus optogenetic activation of the PAG activity does not induce escape, while in its sister-species this canonical pathway exists. Together, these findings highlight parallel escape pathways, potentially shaped by evolution. To test whether the SC-PBG circuit is indeed a second escape pathway, we first assessed neural activation in the PBG during escape behaviour by evaluating the expression of cFOS in Peromyscus polionotus and Mus musculus. Ongoing retrograde tracing experiments aim to address previous concerns about collateral projections to the PAG and PBG by mapping the projections of specific neuronal subpopulations in the SC. We will use Neuropixels recordings and optogenetic manipulations to determine whether the activity of the PBG and its inputs correlate with and induce escape behaviour. This work aims to establish the SC-PBG circuit as a second escape circuit and as a locus of up- and down-regulation depending on ecological needs