Presynaptic release probability and readily releasable pool size are regulated by two independent mechanisms during posttetanic potentiation at the calyx of held synapse

At the immature calyx of Held, the fast decay phase of a Ca2+ transient induced by tetanic stimulation ( TS) was followed by a period of elevated [ Ca2+] i for tens of seconds, referred to as posttetanic residual calcium ( Cares). We investigated the source of Cares and its contribution to posttetanic potentiation ( PTP). After TS ( 100 Hz for 4 s), posttetanic Cares at the calyx of Held was largely abolished by tetraphenylphosphonium ( TPP (+)) or Ru360, which inhibit mitochondrial Na+- dependent Ca2+ efflux and Ca2+ uniporter, respectively. Whereas the control PTP lasted longer than Cares, inhibition of Cares by TPP+ resulted in preferential suppression of the early phase of PTP, the decay time course of which well matched with that of Cares. TS induced significant increases in release probability ( Pr) and the size of the readily releasable pool ( RRP), which were estimated from plots of cumulative EPSC amplitudes. TPP+ or Ru360 suppressed the posttetanic increase in Pr, whereas it had little effect on the increase in RRP size. Moreover, the posttetanic increase in Pr, but not in RRP size, showed a linear correlation with the amount of Cares. In contrast, myosin light chain kinase ( MLCK) inhibitors and blebbistatin reduced the posttetanic increase in RRP size with no effect on the increase in Pr. Application of TPP+ in the presence of MLCK inhibitor peptide caused further suppression of PTP. These findings suggest that Cares released from mitochondria and activation of MLCK are primarily responsible for the increase in Pr and that in the RRP size, respectively.