Botulinum neurotoxin A impairs neurotransmission following retrograde transynaptic transport.

The widely used botulinum neurotoxin A (BoNT/A) blocks neurotransmission via cleavage of the synaptic protein SNAP-25 (synaptosomal-associated protein of 25 kDa). Recent evidence demonstrating long-distance propagation of SNAP-25 proteolysis has challenged the idea that BoNT/A remains localized to the injection site. However, the extent to which distant neuronal networks are impacted by BoNT/A retrograde trafficking remains unknown. Importantly, no studies have addressed whether SNAP-25 cleavage translates into structural and functional changes in distant intoxicated synapses. Here we show that the BoNT/A injections into the adult rat optic tectum result in SNAP-25 cleavage in retinal neurons two synapses away from the injection site, such as rod bipolar cells and photoreceptors. Retinal endings displaying cleaved SNAP-25 were enlarged and contained an abnormally high number of synaptic vesicles, indicating impaired exocytosis. Tectal injection of BoNT/A in rat pups resulted in appearance of truncated-SNAP-25 in cholinergic amacrine cells. Functional imaging with calcium indicators showed a clear reduction in cholinergic-driven wave activity, demonstrating impairments in neurotransmission. These data provide the first evidence for functional effects of the retrograde trafficking of BoNT/A, and open the possibility of using BoNT/A fragments as drug delivery vehicles targeting the central nervous system.

Loss of retinal capillary vasoconstrictor response to Endothelin-1 following pressure increments in living isolated rat retinas.

Increased intraocular pressure (IOP) is a major risk factor for glaucoma, and its contribution to neuronal damage appears multi-factorial. An open issue is whether pressure effects on blood vessels contribute to neuronal damage. In particular, little is known about pressure effects on capillaries, which are the site of most metabolic exchange in the retina, but cannot be easily visualized in vivo. To address this issue, here we have imaged retinal capillaries in acutely isolated living rat retinas, and measured alterations in capillary viability, caliber and response to vasoactive stimuli after controlled pressure stimuli. We found that capillary viability, diameter and response to vasodilator stimulation are not affected after pressure increments; yet, a prolonged lack of capillary response to the vasoconstrictor Endothelin-1 (Et-1) is observed. Considering that Et-1 is a major component of the endogenous control of retinal blood flow the present data lead to the hypothesis that prolonged or repeated IOP elevation could induce capillary disregulation contributing to neuronal damage over time.

The genesis of retinal architecture: an emerging role for mechanical interactions?

Patterns in nature have always fascinated human beings. They convey the idea of order, organization and optimization, and, to the enquiring mind, the alluring promise that understanding their building rules may uncover the forces that shaped them. In the retina, two patterns are outstanding: the stacking of cells in layers and, within the layers, the prevalent arrangement of neurons of the same type in orderly arrays, often referred to as mosaics for the crystalline-like order that some can display. Layers and mosaics have been essential keys to our present understanding of retinal circuital organization and function. Now, they may also be a precious guide in our exploration of how the retina is built. Here, we will review studies addressing the mechanisms controlling the formation of retinal mosaics and layers, illustrating common themes and unsolved problems. Among the intricacies of the building process, a world of physical forces is making its appearance. Cells are extremely complex to model as "physical entities", and many aspects of cell mechanotransduction are still obscure. Yet, recent experiments, focusing on the mechanical aspects of growth and differentiation, suggest that adopting this viewpoint will open new ways of understanding retinal formation and novel possibilities to approach retinal pathologies and repair.