Amplitude and Kinetics of Action Potential-Evoked Ca2+ Current and Its Efficacy in Triggering Transmitter Release at the Developing Calyx of Held Synapse

Y.-M. Yang
2006 Journal of Neuroscience  
Action potentials (APs) play a crucial role in evoking Ca 2ϩ currents (I Ca ) through voltage-gated calcium channels (VGCCs) and transmitter release. During development and neuromodulation, both depolarization and repolarization phases of APs change, but how such changes affect the characteristics of I Ca and its efficacy at central synapses is not clear. By paired voltage-clamp recordings of I Ca and excitatory postsynaptic currents (I EPSC ) with pseudo-APs and real APs, we examined these
more » ... es in the developing calyx of Held synapse of postnatal mice. We found that speeding the AP depolarization rate primarily reduces the number of activated VGCCs, whereas shortening the AP repolarization phase decreases the number of activated VGCCs and accelerates their kinetics. The I Ca -I ESPC relationships are well predicted by the integral but not the amplitude of I Ca , and exhibit development-and temperature-dependent shifts toward left, indicating an enhancement in downstream Ca 2ϩ coupling efficacy. Cross-correlation analyses of I Ca and I EPSC evoked by real APs and pseudo-APs demonstrated that AP shortening in the half-width from 0.4 ms at postnatal day 8 (P8)-P12 to 0.27 ms at P16 -P18 decreases I Ca integral by 36%, but increases I EPSC by 72% as a result of developmental upregulation in coupling efficacy. These counteracting actions maintain the release fraction evoked by an AP at ϳ10% of the maximal quantal output. We suggest that AP narrowing is a critical adaptation for the calyx of Held synapse to control the quantal output per AP and is likely important for the efficient use of the readily releasable pool of synaptic vesicles during high-frequency neurotransmission.
doi:10.1523/jneurosci.4889-05.2006 pmid:16723526 fatcat:cbyxsbkciraunfs5psookzahem