Anatomical and functional examination of superior colliculus projections to the inferior olivary in mice

The inferior olive (IO) is an important region for motor learning and movement coordination. IO activity carried by the climbing fiber (CF) projection to the Purkinje neurons in the cerebellar cortex drives the complex spike activity, central to theories of cerebellar function. Unlike many other neurons in the olivo-cerebeller system, IO neurons are not spontaneously active but rather spike in response to inputs from various regions of the brain. The superior colliculus (SC), a midbrain structure known for its role in orienting behaviors, is one of the input sources to the IO. Here, we investigate the SC projections to the IO using viral tracers, calcium imaging, and optogenetic stimulation. We reveal that, in addition to the known projections to the medial accessory olive (MAO), the SC axons also project to the ventral principal olive (PO). We show that SC axons terminate on both dendritic shafts and spines of IO neurons, potentially influencing not only spiking probability, but also the network synchronization mediated by gap junction coupling on dendritic spines. As a demonstration of the SC axons' ability to drive IO spiking, we employ in vivo calcium imaging of the IO and show that optogenetic activation of SC inputs can drive spiking and modulate overall synchronization of the IO. This study provides a fundamental basis for studying the behavioral significance of the SC-IO pathway in mice.