Cerebellar cortical molecular layer inhibition is organized in parasagittal zones

Molecular layer inhibitory interneurons generate on-beam and off-beam inhibition in the cerebellar cortex that is hypothesized to control the timing and/or spatial patterning of Purkinje cell discharge. On- and off-beam inhibition has been assumed to be spatially uniform and continuous within a folium. Using flavoprotein autofluorescence optical imaging in the mouse cerebellar cortex in vivo, this study demonstrates that the inhibition evoked by parallel fiber and peripheral stimulation results in parasagittal bands of decreases in fluorescence that correspond to zebrin II-positive bands. The parasagittal bands of decreased fluorescence are abolished by GABA_A antagonists and reflect the activity of molecular layer interneurons on their targets. The same banding pattern was observed using Ca²⁺ imaging. The bands produce spatially specific decreases in the responses to peripheral input. Therefore, molecular layer inhibition is compartmentalized into zebrin II parasagittal domains that differentially modulate the spatial pattern of cerebellar cortical activity.