CA2-POTENTIALS AND DISTAL FUNCTIONAL RELEASE SITES( IMAGING OF CNS AXONS IN CULTURE INDICATES RELIABLE COUPLING BETWEEN SINGLE ACTION)

A combination of Ca2+ imaging and current-clamp recording in cultured cortical neurons was used to evaluate the reliability of coupling betw een the action potential and rises in Ca2+ at distal release sites as a possible source of variability in CNS synaptic transmission. Local d omains of enhanced Ca2+ influx were observed at varicosities on axon c ollaterals. Functional assay of vesicle turnover using FM1-43 and para llel electron microscopy confirmed that these varicosities were releas e sites. Single action potentials reliably (>95% of the time) resulted in a presynaptic Ca2+ transient at all presumed release sites includi ng those on distal collaterals. Variability in the amplitude of presyn aptic Ca2+ transients at individual boutons was estimated to be on ave rage less than 20%. We conclude that the coupling of somatic action po tentials to distal release sites is generally a reliable process, alth ough nonlinearity in the relationship between Ca2+ influx and neurotra nsmitter release may amplify the effects of relatively small fluctuati ons in Ca2+ influx.