Glial cell line-derived neurotrophic factor administration in postnatal life results in motor unit enlargement and continuous synaptic remodeling at the neuromuscular junction

Overexpression of glial cell line-derived neurotrophic factor (GDNF) in emb ryonic muscle fibers causes dramatic hyperinnervation of neuromuscular junc tions. However, it is not known whether GDNF induces the extra innervation by regulation of axonal branching and/or synaptic maintenance. To address t his issue, high levels of circulating GDNF were established by administerin g subcutaneous injections starting either at birth or later and continuing for up to 40 d. Treatment with exogenous GDNF beginning in the first week, but not later, increased the number of axons converging at neuromuscular ju nctions. The effect of GDNF on the branching pattern of individual motor ax ons was determined by reconstructing labeled axonal arbors from transgenic mice expressing yellow fluorescent protein in subsets of motor neurons. Whe reas, at postnatal day 8 (P8) individual axons in control animals branched to sporadically innervate junctions within circumscribed regions of the mus cle, motor units from GDNF injected animals had significantly more axonal b ranches and exhibited a high degree of localized arborization such that adj acent muscle fibers were often innervated by the same axon. Administration beginning at P0 and continuing through P40 prolonged multiple innervation o f most fibers throughout the period of injection. Between P30 and P40 there was no net change in multiple innervation, although there was evidence of retraction bulbs, suggesting that axon extension and retraction were in equ ilibrium. We conclude that GDNF has a developmentally regulated effect on p resynaptic branching and that sustained administration of GDNF induces a st ate of continuous synaptic remodeling.