Differential discrimination of appropriate pathways by regenerating identified neurons in Helisoma.

Pathway selection by two different identified neurons (neuron 4 and neuron 5) in the pond snail Helisoma was studied by intracellular injection of the fluorescent dye, Lucifer Yellow CH. The axonal projections of these neurons in normal animals are remarkably constant. Axons of neuron 4 and 5 share a common nerve, the esophageal trunk (ET), which bifurcates to form the gastric nerve (GN) and the salivary nerve (SN). Neuron 5's axon traverses the GN and "avoids" when neuron 4 enters the GN it may make right angle turns, leave the gastric nerve cylinder, and extend directly to its target, the salivary glands. Anomalously located neuron 4 somata also may extend axons to their correct targets via unusual routes. Following ET crush both neurons 4 and 5 regenerate. Sprouts from neuron 5 encountering the bifurcation of the ET basically recapitulate the normal neuron 5 axonal projection. In contrast, neuron 4 sprouts indiscriminately into "correct" and "incorrect" branches of the ET. These experiments indicate that pathway selection by growing neurites can be differentially and specifically regulated for different neurons of the same nerve trunk. The fidelity of different regenerating neurons for selecting nerves which normally contain their axons differs. Though the mechanism of pathway selection cannot yet be identified, the regenerate axonal projections cannot be accounted for by direct guidance of sprouts by the surviving distal axon segments, as has been reported for some identified neurons in the leech.