Neural regeneration in gastropod molluscs.

Snails recover function following a variety of neural injuries. They grow new tentacles with associated tentacle ganglia, selectively reinnervate peripheral targets, repair central connections and may even replace lost neurons and ganglia. The plasticity revealed in their responses to neural injury is an extreme expression of the adaptability observed in studies of learning and age-related changes in the nervous system. Recent information on neurogenesis in gastropods is providing a basis for comparing developmental events with neural regeneration. Studies of neural regeneration in gastropods have capitalized on our ability to identify many gastropod neurons individually and characterize the cellular properties and network properties that generate output patterns that underlie behaviors. The robustness of the model systems formed by cultured gastropod neurons is apparent in the similarity of the activity patterns in circuits formed in vitro and in vivo. Cell membrane repair, activation of an altered pattern of protein synthesis, and observation of the searching action of the growth cones can be studied under defined conditions that promote or inhibit the processes. Basic properties of growth cones, the molecular binding and second messenger systems underlying adhesion, sprouting and pathfinding, and events in synaptogenesis are accessible to analysis. Rules that govern selection of synaptic partners are being evaluated on the basis of cellular characteristics such as transmitter and receptor expression and ganglion of origin. The conservation of the molecular language that governs growth and communication between cells suggest that information gained in such studies may some day be applied to promote neural regeneration in mammals.