Target-dependent regulation of acetylcholine secretion at developing motoneurons in Xenopus cell cultures

1. Myocyte-dependent regulation of acetylcholine (ACh) quantal secretion fr om developing motoneurons was studied in day-3 Xenopus nerve-muscle co-cult ures. Spontaneous synaptic currents (SSCs) were measured in manipulated syn apses by using whole-cell voltage-clamped myocytes. Changes in SSC amplitud e were assumed to reflect changes in the ACh content of secreted quantal pa ckets. Compared with natural synapses, motoneurons without any contact with a myocyte (naive neurons) released ACh in smaller quantal packets. 2. Bipolar cultured motoneurons, which were in contact with a myocyte with one axon branch (contact-end) but remained free at another axon branch (fre e-end), were further used to examine quantal ACh secretion. The ACh quantal size recorded at free-end terminals was similar to that of naive neurons a nd was smaller than that at the contact-end, indicating that myocyte contac t exerts differential regulation on quantal secretion in the same neuron. 3. Some of the neurons that formed a natural synapse with a myocyte continu ed to grow forward anc ACh quantal secretion from the free growth cone was examined. The ACh quantal size recorded at free growth cones was inversely proportional to the distance to the natural synapse, implying localized reg ulation of quantal secretion by the myocyte. 4. Chronic treatment of day-1 cultures with veratridine and d-tubocurarine, respectively, increased and decreased the neurotrophic action of myocytes when assayed on day 3. 5. Taken together, these findings suggest that the myocyte is an important postsynaptic target in the regulation of quantal secretion and that the tro phic action is spatially restricted to the neighbourhood of the neuromuscul ar junction.