The effects of target derived factors on sympathetic neuron function and synaptic plasticity

The development of the nervous system is influenced by cell intrinsic genetic programs and by cell extrinsic factors that together regulate cellular differentiation, migration, innervation and function. The neurotrophins, a family of target-derived factors that includes nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF), not only affect the survival of developing neurons, but can also influence target innervation, synapse formation and synaptic function. The Trk receptors are activated by specific neurotrophins, while the p75 receptor is activated by all neurotrophins. The Trk receptors have been implicated in mediating activity-dependent synaptic plasticity, however the role of p75 activation in modulating synaptic activity has not been clearly established. I therefore investigated the function of p75 in modulating activity-induced cholinergic sympathetic synaptic transmission. Co-cultures of neonatal sympathetic neurons and cardiac myocytes are an amenable system in which to study the effects of added factors on the neurotransmitter phenotype and synaptic function of the cultured neurons. The neurons in this system are plastic with regard to neurotransmitter phenotype existing as noradrenergic, cholinergic or dual transmitter neurons depending on the presence of particular exogenous factors. The myocytes in this system contract spontaneously but the presynaptic release of neurotransmitter can alter the rate of myocyte contraction thereby acting as a functional readout for changes in synaptic transmission. In this system NGF acts presynaptically through TrkA to potentiate activity-dependent noradrenergic transmission. I hypothesized that addition of BDNF to the culture system would result in the reversible activity-dependent release of acetylcholine. I found that the p75 receptor was necessary for the reversible BDNF-mediated cholinergic transmission. BDNF and ciliary neurotrophic factor (CNTF) modulated sympathetic co-transmission in this system using distinct but likely converging signaling pathways. Lastly, I identified CaMKII as a presynaptic kinase necessary for mediating BDNF-induced activity-dependent sympathetic cholinergic transmission. The results of these studies provide insights into the modulation of sympathetic neuronal activity by targets of innervation, and the mechanisms that may control co-transmission of distinctly packaged neurotransmitters.