Development of a neural circuit in the superior colliculus: analysis of the propagation of neuronal excitation from intermediate to superficial layers.

The superior colliculus is important for orientation behaviors, in which visuomotor transformation is performed by the pathway from the superficial layer (SGS) to the intermediate layers (SGI). The opposite pathway (from the SGI to the SGS) also exists, raising the possibility of a feedback circuit, although it could be either negative (inhibitory) or positive (excitatory). In this study, we focused on the development of the feedback circuit. We used optical imaging methods that can measure neuronal population responses directly, as the orientation behaviors are determined by large population activities of superior colliculus neurons. We examined the postnatal development of the propagation pattern of neuronal excitation from the SGI to the SGS using a GABA(A) receptor antagonist. The optical response propagated within the SGI, but not to the SGS in infant mice that have not opened their eyes. In contrast, in young mice after eye opening, the optical response propagated initially in the SGI and then to the SGS. The GABA(A) receptor antagonist increased the optical response in the SGS in young mice, as well as that in the SGI in infant mice. Together, these results suggest that axons of SGI neurons terminate to the SGS during development after eye opening.