Growth and synapse formation by identified leech neurones in culture: a review

Within hours after identified neurones have been isolated from the CNS of the leech, they begin to sprout and to form synapses. Electrical recordings made by loose-patch clamp show that the tip of the isolated neurone has distinct properties with a high density of sodium channels. Neurites grow out from this tip after about 30 min and continue to grow for the next few days. The extent of growth, the branching pattern and the distribution of calcium channels all depend critically upon the molecular composition of the substrate. The tip of the neurone also represents a preferred region for synapse formation. For example when the tips of two serotonin-containing neuromodulatory neurones, the Retzius cells, are placed in contact, chemical synapses develop within about 6 h. These chemical synapses are bidirectional and become stronger over the next 2 days. Electrical synapses between the two Retzius cells develop more slowly and appear only after about 20 h. When the tip of one Retzius cell is apposed to the soma of another, chemical transmission develops more slowly. When other regions of these same cells are placed in contact, electrical transmission can appear before chemical. Together these results show that specialized areas of neuronal membrane are involved in neurite extension and in the formation of specific synaptic connections.