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 Ca(2+) transient induced by tetanic stimulation (TS) was followed by a period of elevated [Ca(2+)](i) for tens of seconds, referred to as posttetanic residual calcium (Ca(res)). We investigated the source of Ca(res) and its contribution to posttetanic potentiation (PTP). After TS (100 Hz for 4 s), posttetanic Ca(res) at the calyx of Held was largely abolished by tetraphenylphosphonium (TPP(+)) or Ru360, which inhibit mitochondrial Na(+)-dependent Ca(2+) efflux and Ca(2+) uniporter, respectively. Whereas the control PTP lasted longer than Ca(res), inhibition of Ca(res) by TPP(+) resulted in preferential suppression of the early phase of PTP, the decay time course of which well matched with that of Ca(res). TS induced significant increases in release probability (P(r)) 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 P(r), whereas it had little effect on the increase in RRP size. Moreover, the posttetanic increase in P(r), but not in RRP size, showed a linear correlation with the amount of Ca(res). In contrast, myosin light chain kinase (MLCK) inhibitors and blebbistatin reduced the posttetanic increase in RRP size with no effect on the increase in P(r). Application of TPP(+) in the presence of MLCK inhibitor peptide caused further suppression of PTP. These findings suggest that Ca(res) released from mitochondria and activation of MLCK are primarily responsible for the increase in P(r) and that in the RRP size, respectively.
Lee
Jae Sung
JS
National Research Laboratory for Cell Physiology, Department of Physiology, Seoul National University College of Medicine and Neuroscience Research Institute, Seoul National University Medical Research Center, Seoul 110-799, Korea.
Kim
Myoung-Hwan
MH
Ho
Won-Kyung
WK
Lee
Suk-Ho
SH
D000818:Animals; D001933:Brain Stem / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004558:Electric Stimulation; D004789:Enzyme Activation / Q000502:physiology; D019706:Excitatory Postsynaptic Potentials; D066298:In Vitro Techniques; D008928:Mitochondria / Q000378:metabolism; D009219:Myosin-Light-Chain Kinase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009861:Onium Compounds / Q000494:pharmacology; D009943:Organophosphorus Compounds / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011336:Probability; D011493:Protein Kinase C / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017975:Ruthenium Compounds / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism
D002121:Calcium Channel Blockers; D009861:Onium Compounds; D009943:Organophosphorus Compounds; D047428:Protein Kinase Inhibitors; C112020:Ru 360; D017975:Ruthenium Compounds; D012964:Sodium; D011493:Protein Kinase C; D009219:Myosin-Light-Chain Kinase; D002118:Calcium; C013289:tetraphenylphosphonium