Molecular Neurobiology Laboratory, Howard Hughes Medical Institute at the Salk Institute, 10010 North Torrey Pines Road, La Jolla, California 92037, USA. hsong@ems.salk.edu
Many diffusible axon guidance cues and their receptors have been identified recently. These cues are often found to be bifunctional, acting as attractants or repellents under different circumstances. Studies of cytoplasmic signaling mechanisms have led to the notion that the response of a growth cone to a particular guidance cue depends on the internal state of the neuron, which, in turn, is under the influence of other coincident signals received by the neuron. Furthermore, many diffusible guidance cues appear to share common cytoplasmic signaling pathways.