Abstract
Cerebellar granule cells (GrCs) constitute over half of all neurons in the vertebrate brain and are proposed to decorrelate convergent mossy fiber inputs in service of learning. Interneurons within the granule cell layer, Golgi cells (GoCs), are the primary inhibitors of this vast population and therefore play a major role in influencing the computations performed within the layer. Despite this central function for GoCs, few studies have directly examined how GoCs integrate inputs from specific afferents which vary in density to regulate GrC population activity. We used a variety of methods in mice of either sex to study feedforward inhibition recruited by identified MFs, focusing on features that would influence integration by GrCs. Comprehensive 3D reconstruction and quantification of GoC axonal boutons revealed tightly clustered boutons that focus feedforward inhibition in the neighborhood of GoC somata. Acute whole cell patch clamp recordings from GrCs in brain slices showed that despite high GoC bouton density, fast phasic inhibition was very sparse relative to slow spillover mediated inhibition. Dynamic clamp simulating inhibition combined with optogenetic mossy fiber activation at moderate rates supported a predominant role of slow spillover mediated inhibition in reducing GrC activity. Whole cell recordings from GoCs revealed a role for the density of active MFs in preferentially driving them. Thus, our data provide empirical conformation of predicted rules by which MFs activate GoCs to regulate GrC activity levels.
SIGNIFICANCE STATEMENT
A unifying framework in neural circuit analysis is identifying circuit motifs that subserve common computations. Widefield inhibitory interneurons globally inhibit neighbors and have been studied extensively in the insect olfactory system and proposed to serve pattern separation functions. Cerebellar Golgi cells (GoCs), a type of mammalian widefield inhibitory interneuron observed in the granule cell layer, are well suited to perform normalization or pattern separation functions but the relationship between spatial characteristics of input patterns to GoC mediated inhibition has received limited attention. This study provides unprecedented quantitative structural details of GoCs and identifies a role for population input activity levels in recruiting inhibition using in vitro electrophysiology and optogenetics.
Footnotes
The authors declare no competing financial interests.
We thank Ms Samantha Lewis for expert technical support during the project and Dr. Christian Rickert for assistance with dynamic clamp design. This work was supported by the Japan Society for The Promotion of Science (JSPS) Overseas Research Fellowship and The Uehara Memorial Foundation research fellowship to ST; and the Klingenstein Foundation, the Boettcher Foundation Webb-Waring biomedical research award and NS084996 to ALP and the Neuroscience Training Grant (T32NS 099042 to JIG). Imaging experiments were performed in the University of Colorado Anschutz Medical Campus Advance Light Microscopy Core supported in part by Rocky Mountain Neurological Disorders Core Grant Number P30NS048154 and by NIH/NCATS Colorado CTSI Grant Number UL1 TR001082. Engineering support was provided by the Optogenetics and Neural Engineering Core at the University of Colorado Anschutz Medical Campus, funded in part by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under award number P30NS048154.
Jump to comment: