Spontaneous release of transmitter from growth cones of embryonic neurones.

A nerve process grows by inserting new membrane material at its advancing tip, the growth cone. In embryonic cell culture and in embryos of Xenopus laevis, many growth cones establish functional synaptic transmission within minutes after contact with muscle cells. The rapidity of synapse formation suggests that the growth cone may have already acquired the appropriate neurotransmitter and the machinery for transmitter release before encountering the target cell. Here, we have used a patch of outside-out embryonic muscle membrane formed with gigaohm seal at the tip of a micropipette as an extracellular probe for the presence of channel-activating substances near the growth cones of the isolated Xenopus embryonic neurones in culture. We report that single-channel activity resembling that of muscle acetylcholine receptor channels was induced when the probe was positioned near the growth cones of 50% of the neurones, suggesting the spontaneous release of acetylcholine (ACh) from these growth cones. The release of material from growth cones may occur as a consequence of the incorporation of new membrane during neurite extension; it may also have a role in the interaction between the growth cone and its immediate environment.