UVM Theses and Dissertations
Format:
Print
Author:
Simpson, Julie
Dept./Program:
Anatomy and Neurobiology
Year:
2007
Degree:
PhD
Abstract:
Neuronal activity is essential in defining and remodeling axonal innervation during embryonic development. Silencing cholinergic signaling at the neuromuscular junction results in sprouting of motor axons in both developmental and adult systems. It is thought that sprouting serves a compensatory mechanism to reestablish connectivity and function at the neuromuscular junction. These experiments use the innervation of the striated muscle in the iris by ciliary neurons to characterize axonal branching behaviors and to identify molecules that orchestrate branching after blockade of neuromuscular transmission with the nicotinic acetylcholine receptor antagonist d-tubocurarine (dTC). At embryonic day 13 (E13) and E15, programmed cell death is complete and functional synapses have been established. At these embryonic ages dTC treatment does not induce an increase in axonal branching in the iris, quantified as an increase in the percentage of axonal bundles. In contrast, at E17 after the iris has completely transitioned into a striated muscle dTC treatment induces axonal branching. The increase in branching occurs despite a decrease in the survival of ciliary neurons. To identify candidate branch inducing molecules, this study tests the ability of these molecules to induce neurite branching of E8 and El3 ciliary ganglion neurons in culture.
GDNF is the likely c-Ret ligand mediating these events. Expression of a dn trkB retroviral construct blocks BDNF signaling in vivo and induces a two fold increase in axonal branching by E17 demonstrating the importance of BDNF signaling in modulating branching events in older ciliary neurons. Blocking BDNF signaling increases cell death of ciliary neurons between E14 and E17 also establishing a survival component of BDNF signaling in the ganglion. These results suggest that disrupting neuromuscular transmission with dTC results in a dual mechanism to mediate axonal branching. GDNF expression increases in the iris to promote axonal branching whereas trkB expression decreases to allow for a permissive environment for GDNF induced axonal branching.
GDNF is the likely c-Ret ligand mediating these events. Expression of a dn trkB retroviral construct blocks BDNF signaling in vivo and induces a two fold increase in axonal branching by E17 demonstrating the importance of BDNF signaling in modulating branching events in older ciliary neurons. Blocking BDNF signaling increases cell death of ciliary neurons between E14 and E17 also establishing a survival component of BDNF signaling in the ganglion. These results suggest that disrupting neuromuscular transmission with dTC results in a dual mechanism to mediate axonal branching. GDNF expression increases in the iris to promote axonal branching whereas trkB expression decreases to allow for a permissive environment for GDNF induced axonal branching.