Effects of Mitochondrion on Calcium Transients at Intact Presynaptic Terminals Depend on Frequency of Nerve Firing

Abstract

The rate and the total amount of Ca2+ elevation in the presynaptic terminals of bullfrog sympathetic ganglia depend on the firing frequency of the terminals. Carbonyl cyanide m-chlorophenylhydrazone (CCCP), a mitochondrial uncoupler, was used for testing whether mitochondrial Ca2+ uptake is one of the mechanisms that underlie this frequency dependence. Fura-2 fluorimetry was used for measurement of intraterminal Ca2+. When stimulations of different durations (30 and 1.5 s) and frequencies (4 and 20 Hz) evoked Ca2+ transients with similar peak amplitudes (264 +/- 22 nM vs. 251 +/- 18 nM, means +/- SE), CCCP augmented the responses to the 4-Hz stimulation 8.9 times more strongly than it did the responses to the 20-Hz stimulation (249.7 +/- 81.5% vs. 25.3 +/- 10.2%). When stimulations delivered at the two frequencies had the same durations (1.5, 3, 6, 10, 20, and 30 s), CCCP enlarged the responses to the 4-Hz stimulations up to 4.2 times more than it did the responses to the 20-Hz stimulations. When the same number of stimuli (120) was delivered at the two frequencies, the effects of CCCP on the responses evoked by the 4-Hz train were again 6.8 times stronger than its effects on the responses to the 20-Hz stimulation. Therefore neither the peak amplitudes of the responses nor the durations of the stimulations dictated the extent to which the mitochondria modulated the peak [Ca2+]i. Instead, the extent of the modulation was governed by the frequency of stimulation. Specifically, the less frequent the Ca2+ influx, the stronger the mitochondrial modulation. Also, during nerve firing Ca2+ release from the ryanodine-sensitive store had a higher potential to influence the [Ca2+]i transients than did Ca2+ removal by the mitochondria for the first 6 s of the responses. On cessation of stimulation, CCCP reduced the initial rapid rate of Ca2+ decay. Thus uptake by the mitochondria was an important mechanism for Ca2+ removal after repetitive firing at the presynaptic terminals.

Mitochondrial Calcium Buffering Contributes to the Maintenance of Basal Calcium Levels in Mouse Taste Cells

“…However, antimycin A and oligomycin did not individually affect baseline calcium values in taste cells. This is not surprising because it has been shown in sympathetic ganglia that protonophores do not significantly affect ATP/ ADP ratios for Ն23 min (Peng 1998). This is in contrast to the rapid depletion of the ATP/ADP ratios caused by protonophores in cultured cerebellar granule cells (Budd and Nicholls 1996).…”

“…In addition to its role in ATP production, mitochondria make significant contributions to the regulation cytosolic calcium in many cells (Babcock and Hille 1998;Babcock et al 1997;Budd and Nicholls 1998;Duchen 2000a,b) and has important roles in the regulation of synaptic activity in neurons (Medler and Gleason 2002;Peng 1998;Sen et al 2007;Tang and Zucker 1997). Mitochondrial calcium buffering is important during continuous calcium-induced calcium release in submandibular acinar cells (Kopach et al 2008), directly modulates agonist-induced calcium signals (Johnson et al 2002;White and Reynolds 1995), and can functionally interact with calcium stores to regulate cytosolic calcium signals (Landolfi et al 1998;Malli et al 2005).…”

Interplay between Na+/Ca2+ Exchangers and Mitochondria in Ca2+ Clearance at the Calyx of Held

“…Investigating the quantitative role of different CCMs in nerve terminals will thus provide a basic understanding of the mechanisms underlying short-term plasticity. There is evidence for presynaptic roles of PMCA in retinal bipolar cells (Zenisek and Matthews, 2000), for Na/CaX in cultured hippocampal neurons (Reuter and Porzig, 1995) and in cerebellar granule cells (Regehr, 1997), and for mitochondria in sympathetic ganglion (Peng, 1998). Previous studies on presynaptic CCMs aimed to determine the contribution of specific mechanisms but have not provided a comprehensive picture of how different CCMs interact with each other as a function of the cytosolic [Ca 2ϩ ] attained after a stimulus.…”

Mitochondrial Ca2+Buffering Regulates Synaptic Transmission Between Retinal Amacrine Cells

“…FCCPand Ca 2ϩ -dependent exocytosis has also been previously demonstrated at excitatory synapses between hippocampal neurons (Scotti et al 1999) and at the frog neuromuscular junction (Alnaes and Rahamimoff 1975;Molgo and Pecot-Dechavassine 1988). Evidence for the importance of mitochondrial Ca 2ϩ buffering in regulation of evoked synaptic transmission has also been found at several peripheral synapses including the frog (Alnaes andRahamimoff 1975), lizard (David et al 1998) and crayfish (Tang and Zucker 1997) neuromuscular junctions as well as in bullfrog sympathetic ganglia (Peng 1998). Additionally, synaptic activity has been shown to stimulate the Ca 2ϩ -dependent activation of a mitochondrial conductance, possibly the Ca 2ϩ uniporter, at the squid giant synapse (Jonas et al 1999).…”

“…It has been demonstrated in several neuronal cell types that the glycolytic pathway can maintain the ATP/ADP ratio for tens of minutes (Kauppinen and Nicholls 1986;Peng 1998;Werth and Thayer 1994;White and Reynolds 1995). Nonetheless, it was necessary to establish that the FCCP-dependent Ca 2ϩ elevations we were detecting were not due to loss of ATP and inhibition of the Ca 2ϩ ATPases.…”