Abstract
Aims/hypothesis: Sympathetic nerves influence islet hormone levels in the circulation. Insights into the islet sympathetic innervation and its remodeling in diabetes impact future therapeutics. However, standard immunohistochemistry and microtome-based microscopy cannot provide an integral view of the islet neurovascular complex. We prepared transparent islet specimens to investigate the spatial relationship between sympathetic nerves, blood vessels, and islet cells in normal, streptozotocin-injected, and nonobese diabetic (NOD) mice. Methods: Cardiac perfusion of the fluorescent lectin was used to label the blood vessels of pancreas. Tyrosine hydroxylase and nuclear staining were used to reveal the islet sympathetic innervation and microstructure. Optical clearing -- use of immersion solution to reduce scattering -- was applied to enable 3-dimensional confocal microscopy of islets to visualize the sympathetic neurovascular complex in space. Results: Unlike the previously reported morphology, we observe perfusive intra-islet, perivascular sympathetic innervation, in addition to the peri-islet contacts of sympathetic nerves with the alpha cells and the sympathetic fibers encircling the adjacent arterioles. The intra-islet axons become markedly prominent in the streptozotocin-injected mice (two weeks after the injection). In the NOD mice, the lymphocytic infiltration remodels the peri-islet sympathetic axons in early insulitis. Conclusions/interpretation: We establish an imaging approach to reveal the spatial features of the mouse islet sympathetic innervation. The neurovascular complex and the sympathetic nerve-alpha cell contact suggest that sympathetic nerves modulate islet hormone secretion through blood vessels in addition to acting directly onto the alpha cells. In islet injuries, sympathetic nerves undergo different remodeling in response to different pathophysiological cues.