Studies of nerve-muscle interactions in Xenopus cell culture: analysis of early synaptic currents

We have studied the spontaneous and nerve-evoked synaptic currents during the initial period of nerve-muscle contact in Xenopus cell cultures. The precise timing of the contact was achieved by physically manipulating embryonic muscle cells into contact with co-cultured spinal neurons. Previous studies have shown that physical contact of the muscle membrane induces pulsatile release of acetylcholine (ACh) from the growth cone of these neurons, resulting in spontaneous synaptic currents (SSCs) in the muscle cell within seconds following the contact. In the present work, we first showed that these SSCs at the manipulated nerve-muscle contacts are similar to those observed at naturally occurring synapses. We then examined the possible cellular mechanisms responsible for the marked variation in SSC amplitude and showed that it most likely results from differences in either the amount of ACh contained in each release event or the extent of close membrane apposition near the release sites. During the first 20 min following the nerve-muscle contact, there was an increase in the frequency and mean amplitude of the SSCs. During a similar period, the evoked synaptic currents (ESCs), which were induced by suprathreshold electrical stimulation of the neuronal soma, also showed an increase in the mean amplitude and a reduction in the delay of onset following the stimulus. These postcontact changes in the efficacy of synaptic transmission may be related to an increase in the total area of close membrane apposition between the nerve and muscle cells. This was suggested by the finding that neurite-muscle adhesion increases over a similar postcontact period. The transition from low- to high-efficacy transmission during the early phase of contact may reflect the process of selective adhesion between the cells, and thus signify the formation of specific synapse. Analysis of the fluctuation in the ESC amplitude at the early nerve-muscle contact suggests that evoked release of ACh occurs as multiples of a quantal unit. However, this unit is apparently related to only a small subpopulation of SSCs of relatively high amplitudes.(ABSTRACT TRUNCATED AT 400 WORDS)