Compartmentalization of signaling in neurons: Evolution and deployment

The localization of signal transduction machinery at synapses is a fundamen tal organizational feature of the nervous system that allows for highly com plex integration of information coding processes. Synaptic communication ev olved as multicellular organisms became more complex, and as selection pres sures were placed on such organisms such that those capable of responding r apidly and specifically to environmental demands survived. Two obvious adva ntages of synaptic transmission las oppossed to endocrine or paracrine sign aling) are that it provides for rapid intercellular communication over grea t distances and that it provides a high level of spatial specificity. There are several structural and functional aspects of synapses that set them ap art from other cellular compartments, with many of the specializations subs erving roles in synaptic signal transduction (e.g., neurotransmitter releas e from the presynaptic terminal and postsynaptic receptor activation and se cond messenger production). However, studies of developing nervous systems have shown that many synaptic signaling mechanisms are operative prior to s ynaptogenesis and play important roles in regulating growth cone behaviors, synaptogenesis, and even programmed cell death. Indeed, the concept that " ontogeny recapitulates phylogeny" can be effectively applied to the evoluti on of the synapse, As the embryo rapidly grows, neurons must elaborate axon s and dendrites, establish functional synaptic connections, and maintain an d adjust those connections as the organism matures. The purpose of this int roductory article is to set the stage for the following articles by briefly reviewing fundamental aspects of the molecular and cellular biology of syn apses in an evolutionary context. (C) 1999 Wiley-Liss, Inc.