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J Physiol Volume 585, Number 2, 361-381, December 1, 2007 DOI: 10.1113/jphysiol.2007.140988
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NEUROSCIENCE

Kinetics of both synchronous and asynchronous quantal release during trains of action potential-evoked EPSCs at the rat calyx of Held

 V. Scheuss1, H. Taschenberger1 and E. Neher1

1 Max Planck Institute for Biophysical Chemistry, Department of Membrane Biophysics, Göttingen, Germany

We studied the kinetics of transmitter release during trains of action potential (AP)-evoked excitatory postsynaptic currents (EPSCs) at the calyx of Held synapse of juvenile rats. Using a new quantitative method based on a combination of ensemble fluctuation analysis and deconvolution, we were able to analyse mean quantal size (q) and release rate ({xi}) continuously in a time-resolved manner. Estimates derived this way agreed well with values of q and quantal content (M) calculated for each EPSC within the train from ensemble means of peak amplitudes and their variances. Separate analysis of synchronous and asynchronous quantal release during long stimulus trains (200 ms, 100 Hz) revealed that the latter component was highly variable among different synapses but it was unequivocally identified in 18 out of 37 synapses analysed. Peak rates of asynchronous release ranged from 0.2 to 15.2 vesicles ms–1 (ves ms–1) with a mean of 2.3 ± 0.6 ves ms–1. On average, asynchronous release accounted for less than 14% of the total number of about 3670 ± 350 vesicles released during 200 ms trains. Following such trains, asynchronous release decayed with several time constants, the fastest one being in the order of 15 ms. The short duration of asynchronous release at the calyx of Held synapse may aid in generating brief postsynaptic depolarizations, avoiding temporal summation and preserving action potential timing during high frequency bursts.

(Received 20 July 2007; accepted after revision 29 September 2007; first published online 4 October 2007)
Corresponding author E. Neher: Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany. Email: eneher{at}gwdg.de

V. Scheuss and H. Taschenberger contributed equally to this work.






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