The presynaptic compartment: signals and targets

Synaptic terminals are key elements in the functional and structural organization of the nervous system. Release of neurotransmitters, i.e. the activity specifically localized at the terminals, not only sustains the transfer of information among adjacent cells, but also contributes significantly to directing the non-random distribution of macromolecules in the plasmalemma of postsynaptic neurons, with major consequences in their general architecture (assembly of postsynaptic densities, dendritic spines, etc.). In order for these specific functions to be carried out, synaptic terminals need to be specialized in a variety of aspects with respect to the rest of the neuron. This minireview is specifically focused on two such aspects, the generation of transduction signals and their mechanism of action on intraterminal targets. In either aspect nerve terminals are by no means fully homogeneous, yet they certainly share a number of common features. These include the predominant role of Ca2+, collaborating however with other second messengers (cAMP, IP3, diacylglycerol) in the control of processes such as transmitter release and its modulation.