Developmental regulation of plasticity along neurite shafts.

Although it is becoming increasingly clear that structural dynamics on neurite shafts play important roles in establishing neuronal architecture, the underlying mechanisms are unknown. The present study investigates local induction of filopodia along the shafts of neurites, a process that, by analogy to the growth cone, can represent the first stage in the generation of a new neuronal process. We show that filopodia can be induced reliably along the neurite shaft in response to a localized electric field stimulus that evokes large local intracellular calcium increases. Neither induction of filopodia nor a local rise in intracellular calcium occurred in calcium-free medium. Although calcium induction of neurite filopodia is highly reliable, forming in response to more than 90% of attempts, it is developmental state-dependent, since neurite filopodia could not be induced in neurons previously defined as "stable state." We have found two distinct changes in stable state neurons that can decrease the ability to induce new neurites. The first is a reduced calcium response: Field stimulation produced large local rises (280 nM) in intracellular calcium in growing neurons, whereas the identical stimulus produced smaller changes (148 nM) in stable state neurons. Second, stable state neurons change so that even when the stimulus intensity was increased to elicit a calcium response that would have been sufficient to induce filopodia in growing neurites, neurite filopodia were still not induced. Thus, intracellular calcium plays a key role in structural changes along the shafts of neurites. Furthermore, developmental changes in both calcium homeostatic components and in calcium responsiveness (i.e., the sensitivity of cellular components that modulate neurite morphology) underlie shifts from plasticity to stability of neuronal architecture in this system.