Brain-Derived Neurotrophic Factor and trkB Signaling in Parasympathetic Neurons: Relevance to Regulating α7-Containing Nicotinic Receptors and Synaptic Function

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Abstract
Parasympathetic neurons do not require neurotrophins for survival and are thought to lack high-affinity neurotrophin receptors (i.e., trks). We report here, however, that mRNAs encoding both brain-derived neurotrophic factor (BDNF) and its high-affinity receptor tropomyosin-related kinase B (trkB) are expressed in the parasympathetic chick ciliary ganglion (CG) and that BDNF-like protein is present in the ganglion and in the iris, an important peripheral target of ciliary neurons. Moreover, CG neurons express surface trkB and exogenous BDNF not only initiates trk-dependent signaling, but also alters nicotinic acetylcholine receptor (nAChR) expression and synaptic transmission. In particular, BDNF applied to CG neurons rapidly activates cAMP-dependent response element-binding protein (CREB), and over the long-term selectively upregulates expression of α7-subunit-containing, homomeric nAChRs (α7-nAChRs), increasing α7-subunit mRNA levels, α7-nAChR surface sites, and α7-nAChR-mediated whole-cell currents. At nicotinic synapses formed on CG neurons in culture, brief and long-term BDNF treatments also increase the frequency of spontaneous EPSCs, most of which are mediated by heteromeric nAChRs containing α3, α5, β4, and β2 subunits (α3*-nAChRs) with a minor contribution from α7-nAChRs. Our findings demonstrate unexpected roles for BDNF-induced, trk-dependent signaling in CG neurons, both in regulating expression of α7-nAChRs and in enhancing transmission at α3*-nAChR-mediated synapses. The presence of BDNF-like protein in CG and iris target coupled with that of functional trkB on CG neurons raise the possibility that signals generated by endogenous BDNF similarly influence α7-nAChRs and nicotinic synapsesin vivo.