Abstract
Microglia have classically been viewed as the endogenous phagocytes of the brain, however, emerging evidence suggests roles for microglia in the healthy, mature nervous system. We know little of the contribution microglia make to ongoing processing in sensory systems. To explore Iba1+ microglial structural and synaptic diversity, we employed the inferior colliculi as model nuclei, as they are characterized by sub-regions specialized for differing aspects of auditory processing. We discovered extensive Iba1+ ramifications interacting with homer1+ and synaptophysin+ puncta (putative synapses) throughout the parenchyma. There was a greater density of these puncta in dorsal cortex, a sub-region specialized for mediating top-down corticofugal plasticity, than other sub-regions more specialized for processing ascending auditory or polymodal information. Cluster analyses revealed two novel sub-types of GAD67+ neurons which can be distinguished solely based on the quantity of axo-somatic Iba1+ contacts they receive. These data demonstrate Iba1+ microglia exhibit specialized synaptic and cellular adaptations to ongoing processing in the mature, healthy auditory system. These specializations appear to relate to the afferent drive, plastic nature and inhibitory basis of local processing. Taken together, these findings suggest significant heterogeneity amongst microglia in sensory systems, related to the functional demands of their spatial niche.