Initial synaptic transmission at the growth cone in Xenopus nerve-muscle cultures.

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Abstract
The excellent visibility of cultured cells allows the early events during formation of the neuromuscular junction to be suitably studied. In various culture systems, synaptic transmission occurs early after nerve-muscle contact. Early synaptic potentials are small in amplitude and slow in time course reflecting a low acetylline [ACh] receptor density at the site of nerve contact. ACh receptors accumulate later at the contact region. Initial synaptic transmission was examined at the growth cone-muscle contact in Xenopus nerve-muscle cultures. The approaching growth cone was observed under a phase-contrast microscope while the membrane potential of its target muscle cell was continuously monitored by using an intracellular microelectrode. The innervating neuron was stimulated extracellularly at the cell body. No synaptic potential was evoked when the growth cone was contacting the muscle only at the tip of filopodia. As soon as the main portion of the growth cone contacted the muscle membrane, nerve-evoked synaptic potentials were detected after stimulation of the nerve. This immediate appearance of synaptic potentials raises the possibility that ACh could be released at the growth cone even prior to contact with muscle cells. As the area of contact enlarged during the observation period the amplitude of endplate potentials also increased. Spontaneous synaptic potentials (miniature endplate potentials) were rarely observed in these early growth cone-muscle contacts. Although there were several inherent difficulties, quantal analysis of the end-plate potentials was attempted by using binomial statistics. Nerve-evoked transmitter release at the growth cone occurs in a quantal fashion.