Removal of the synaptic target permits terminal sprouting of a mature intact axon

When the central nervous system (CNS) develops, neurones send out axons to make contact with appropriate synaptic target cells and then stop growing. If its usual target is missing, an axon may continue to grow until it synapses with a suitable but inappropriate target. This suggests that contact with a synaptic target is important in stopping axonal growth during development. Many classes of neurone in the adult CNS retain a capacity to grow towards denervated targets, but it is not known whether the synaptic contacts established during development continue to regulate the growth of individual mature, intact axons. This has been a difficult problem to investigate; in vertebrates most studies necessarily involve large populations of neurones, and the most direct approach, removal of a synaptic target, usually damages many neurones, including the axons that are to be studied. We report here a demonstration of target cell influences on the growth of a single mature, intact axon in the CNS of the leech by selectively destroying the axon's synaptic target without injuring the axon itself. Target removal, which itself does not trigger sprouting of intact axons, permits the intact axon to grow at its tip in response to injury of other axonal branches of the same cell.