Nano contrast enhancement agents used in the eye for tracing axons: trauma or illumination?

Abstract

Purpose: : Inject various nanomaterials into the eye to increase contrast in the optic nerve and optic tract, to enhance the visualization of regenerating retinal ganglion cell axons in vivo after transection of the brachium of the superior colliculus (SC). A tissue gap caused by deep transections of the optic tract (OT) in the midbrain can completely block the reinnervation of the SC by the retina, even at young ages when the axons have regenerative potential. Previously we demonstrated that a self-assembling peptide nanofiber scaffold (SAPNS) facilitated the reconstruction of a tissue substrate that supports regeneration across the tissue disruption, even if treated 3 months after the original lesion. Here we show that by using a nano contrast agent (NCA) optic tract regeneration can be visualized in vivo in a mammalian chronic injury model.

Methods: : In several groups of adult hamsters (8 wk), we injected several different contrast enhancement agents and measured (1) the rate of transport, (2) the contrast provided and (3) the toxicity in the retina. Each group received a series of injections over 6 weeks with different contrast agents; the impact on both the retina and the lens was then assessed. Intravitreal injections of 2µl of material was injected and compared with saline injected controls. The OT at the brachium of the SC was completely severed with a deep knife wound, extending 1-2 mm below the surface from the midline to a point beyond the lateral margin of SC. Following the transection, the eyes were injected with a NCA and imaged in a 7 Tesla fMRI. This was repeated 3 more times just before the second surgery and SAPNS treatment, then twice following the treatment. During the second OT surgery the animals had a partial scar resection and were injected with 100 µl of 1% SAPNS into the site of injury. The contralateral side of the same animal served as the control.

Results: : One of the contrast agents was found to cause severe trauma, at levels that are safe in humans, while some new agents appeared not to impact the eye in a negative way. Imaging revealed that the first transection was complete. Imaging after the second treatment revealed regenerated axons in the SC of the SAPNS-treated animals.

Conclusions: : Some contrast enhancement agents will cause trauma when used in rodent models, while others provide better contrast without producing the trauma associated with compatible agents in humans. A 7 Tesla fMRI is able to detect axons in the optic tract in hamsters before, during and after regeneration in a chronic injury treatment model.