Elevation in presynaptic Ca²⁺ level accompanying initial nerve-muscle contact in tissue culture

Nerve-muscle cocultures were used to study the relationship between neuronal Ca²⁺ level and the earliest nerve-muscle interaction. Xenopus spinal cord neurons were loaded with Ca²⁺ indicators and monitored by digital video microscopy as a myoball was manipulated into contact with it. Transmitter release was measured from the myoball by whole-cell recording. We observed a 1.5- to 6-fold increase in Ca²⁺ level in the neurite upon contact with a myoball. Fifty percent of the pairs showing Ca²⁺ elevation were also positive for neurotransmission. This Ca²⁺ rise was suppressed by lifting away the myoball, by Ca²⁺-free solution, or by suramin, a polyanionic compound that interferes with cell surface receptors. This suppression was accompanied by a reduction in transmitter release. The specificity of the nerve-muscle contact-induced Ca²⁺ rise was shown by its absence upon neuron-neuron contact. Naturally formed nerve-muscle contacts also showed an elevation in presynaptic Ca²⁺ level. Thus, this elevation appears to be a physiological step in the early stage of synaptogenesis and is likely mediated by muscle-derived molecules.