Small-molecule trkB agonists promote axon regeneration in cut peripheral nerves

Treatments with two-small molecule tropomyosin receptor kinase B (trkB) ligands, 7,8 dihydroxyflavone (7,8 DHF) and deoxygedunin, were evaluated for their ability to promote the regeneration of cut axons in injured peripheral nerves in mice in which sensory and motor axons are marked by YFP. Peripheral nerves were cut and repaired with grafts from strain-matched, nonfluorescent donors and secured in place with fibrin glue. Lengths of profiles of regenerating YFP ⁺ axons were measured 2 wk later from confocal images. Axon regeneration was enhanced when the fibrin glue contained dilutions of 500-nM solution of either small-molecule trkB agonist. In mice in which the neurotrophin receptor trkB is knocked out selectively in neurons, axon regeneration is very weak, and topical treatment with 7,8 DHF had no effect on axon regeneration. Similar treatments with deoxygedunin had only a modest effect. In conditional BDNF knockout mice, topical treatments with either 7,8 DHF or deoxygedunin resulted in a reversal of the poor regeneration found in controls and produced significant enhancement of regeneration. In WT mice treated with 2 wk of daily i.p. injections of either 7,8 DHF or deoxygedunin (5 mg/kg), regenerating axon profiles were nearly twice as long as in controls. Restoration of direct muscle responses evoked by sciatic nerve stimulation to pretransection levels over an 8-wk survival period was found only in the treated mice. Treatments with either small-molecule trkB agonist enhanced axon regeneration and muscle reinnervation after peripheral nerve injuries.