Neurotrophic factors in the central nucleus of amygdala may be organized to provide substrates for associative learning

The central nucleus of amygdala was examined to identify the ultrastructural distribution of neurotrophins responsible for the complex of neuronal signaling processes which regulate synaptic transmission and neuronal plasticity, and possibly underlie memory formation. We investigated at the electron microscopic level the cellular organization of brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), in the extended amygdala (CE). We also investigated the interaction between cortical inputs to CE and BDNF and TrkB. Our results indicate the presence of pro-BDNF and BDNF in terminals in the CE which show a strong association with immunoreactive postsynaptic densities. TrkB receptor immunoreactivity was localized to postsynaptic densities of asymmetric synapses on dendrites and dendritic spines. Cortical terminals formed asymmetric synapses with dendritic shafts and spines, but were not BDNF immunoreactive. TrkB receptors were observed opposed to cortical terminals. These data also suggest that one potential substrate for associative learning may be the interaction of different cortical inputs with neurotrophin-containing terminals ending on dendritic spines and other neuronal structures of CE.