Abstract
Synaptogenesis is encoded by multiple genes that program the assembly of neural networks in the immature brain during development and later in life, in the experienced brain that must respond to changes of the external world and of proprioception. Processing of neural network activity cannot solely rely on the activity of established synapses as their plasticity can saturate. For this reason, synaptogenesis is reversible. Assembly and disassembly of synapses depend on the exchange of signals between the pre- and postsynaptic terminals. Synaptic trophic factors are paramount for neural network remodeling, homeostasis, and survival because they are present in limited supply. Neurotrophins present the necessary attributes to operate as synaptic trophin. It is speculated that many diverse and pleiotropic actions of neurotrophins on the synapse depend on the sources of neurotrophin ligands, its modes of secretion, and signaling, all influenced by their locations at the synapse. What information is encoded when synaptic neurotrophin signaling is retrograde or anterograde, paracrine or autocrine? Answers emerged from the characterization of the cells that secrete neurotrophins, the cells that respond to neurotrophins, the various modes of secretion, receptor–signaling pathways, and the temporal constraints imposed by synaptic transmission. Failure to communicate the appropriate synaptic trophic signals impairs neuronal networks maintenance and updated wiring.