Analysis of two kinetically distinct components of transmitter release at a fast synapse of the mammalian central nervous system

6 Summary Summary Two kinetically distinct components of transmitter release have been observed in Ca 2+ uncaging experiments at a CNS synapse in rats, and both release components showed a steep dependence of release kinetics on [Ca 2+ ] i. These findings were based on simultaneous pre-and postsynaptic whole-cell voltage-clamp recordings from the calyx of Held synapse, combined with presynaptic Ca 2+ uncaging and [Ca 2+ ] i measurements. Presynaptic transmitter release rates were deconvolved from postsynaptic excitatory currents, which allowed for a correlation between obtained release kinetics and stimulus intensities in terms of presynaptic [Ca 2+ ] i. Furthermore, results could be confirmed by measurements of presynaptic changes in C m. In control experiments, possible artifacts that might have produced apparent biphasic release during Ca 2+ uncaging stimuli were examined. Ca 2+ uncaging led to a step-like and spatially homogeneous elevation of [Ca 2+ ] i , and moreover, it was shown that AMPA-receptor saturation, or desensitization were unlikely to have marked effects. Also the impact of rapid refilling of the readily-releasable pool of vesicles was investigated. It was found, that refilling could not account for release to become biphasic, especially regarding the steep [Ca 2+ ] i dependence of the slow release component. Therefore, intrinsic heterogeneity in release probabilities was proposed as the mechanism underlying biphasic release kinetics after Ca 2+ uncaging. But still, an additional positional heterogeneity within the fast release component was found to affect fast release during presynaptic depolarizations. Furthermore, the relative contribution of the fast and the slow release component to total release was found to depend on [Ca 2+ ] i. At higher [Ca 2+ ] i , more vesicles with fast kinetics were released as compared to milder Ca 2+ uncaging stimuli. Thus, submaximal release of the fast release component at low [Ca 2+ ] i was observed, for the first time at a CNS synapse, and this behavior cannot be explained by previous release models for the calyx of Held synapse. At higher [Ca 2+ ] i , the amount of fast release increased at the expense of the slow component, which indicates convertibility between fast and slowly releasing vesicles, and moreover, slowing of the release time constant of the fast component after modest pre-stimulation was observed experimentally. Based on these findings, a single pool of readily-releasable vesicles with intrinsic heterogeneity was suggested as the mechanism underlying the observed phenomena. Simulations …