Axonal Regeneration Induced by Xylazine Injection in Crushed Optic Nerve of the Adult Rats

Abstract

Abstract: : Purpose:Many previous studies have revealed that alpha–2 adrenergic agonists have neuroprotective effects on degenerated retinal ganglion cells (RGCs) in various animal models. However, no reports have been focused on damaged optic nerve (ON) axons. In this study, we examined whether xylazine, an alpha–2 adrenergic agonist, has a potency to regenerate the damaged ON axons. Methods:Under anesthesia, the left ON of an adult Wistar rat was crushed by microforceps. xylazine (6mg/kg) was intraperitoneally injected into three groups of rats in different protocols: a single injection immediately after the ON crush (day 1), five intermittent injections (day 1, 2, 5, 8, 11), and thirteen everyday injections (day 1–13). PBS was alone as a vehicle injected into control animals. On day 13, we intraocularly injected 1% cholera toxin subunit B (CTB) to anterogradely label RGC axons. Immunohistochemistry for CTB were performed for visualizing the labeled axons. Under a fluorescent microscope, we counted the CTB–labeled axons at three points of 250, 500 and 1,000 micrometer distant from the crush site. To assess how axonal regeneration was regulated, we performed immunohistochemistry for GAP–43 on the ON and retina. Results:In the crushed ON of control animals, CTB–labeled axons rarely overcame the crush site. In contrast, the everyday injections significantly increased the average number of regenerated axons at each point. However, the intermittent injections regenerated much fewer axons, and even fewer the single injection did. In the protocol of everyday injections, GAP–43 immunoreactivity was markedly up–regulated in the RGC layer and the ON axons. Conclusions:These results strongly suggest that xylazine, an alpha–2 adrenergic agonist, can stimulate the axonal regeneration in vivo.

Keywords: regeneration • retina: proximal (bipolar, amacrine, and ganglion cells) • trauma