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bioRxiv2022

Intrinsically photosensitive retinal ganglion cells evade temporal filtering to encode environmental light intensity

The retina encodes environmental light intensity to drive innate physiological responses. The synaptic basis of such coding remains obscure. Intrinsically photosensitive retinal ganglion cells (ipRGCs) are the only retinal output neurons stably encoding intensity. They do so even without their melanopsin photopigment, so specializations in their synaptic drive from bipolar cells (BCs) must also contribute. Here, we shed new light on mechanisms responsible for this unique intensity-coding drive. By ultrastructural reconstruction, we show that specific BC types and unusual ribbon synapses carry photoreceptor signals to ipRGCs. By glutamate imaging and electrophysiology, we show that their light responses are unusually persistent. Still, we find that virtually all BCs encode intensity. Intensity coding becomes restricted to ipRGCs primarily because other RGCs filter out steady-state intensity signals postsynaptically. Thus, neural "pinholes" in global, persistent neural "masking" allow intensity signals to be encoded by ipRGCs and sent to specific centers of the visual brain.

neuroscience↗

bioRxiv2021

Chronic CNS Pathology is Associated with Abnormal Collagen Deposition and Fibrotic-like Changes

Multiple sclerosis is a chronic debilitating disease of the CNS. The relapsing remitting form of the disease is driven by CNS directed inflammation. However, in the progressive forms of the disease, inflammation has abated and the underlying pathology is less well understood. In this paper, we show that chronic lesions in progressive MS are associated with fibrotic changes, a type of pathology that has previously not thought to occur in the CNS. In an animal model of chronic MS, late stage disease contains no inflammatory infiltrates and is instead characterized by collagen deposition that is histologically similar to fibrosis. In human MS samples, chronic, but not acute lesions, are devoid of inflammatory infiltrates and instead contain significant collagen deposition. Furthermore, we demonstrate that both mouse and human astrocytes are the cellular source of collagen. These results suggest that anti-fibrotic therapy may be beneficial in the treatment of progressive MS.

neuroscience↗