Posttetanic potentiation at the crayfish neuromuscular junction is dependent on both intracellular calcium and sodium ion accumulation

Abstract

The fluorescent indicator fura-2 was used to measure cytoplasmic calcium in presynaptic terminals in the crayfish Procambarus clarkii under conditions that raise intracellular sodium to examine whether sodium can elevate intracellular calcium concentration ([Ca2+]i) or prolong its efflux and thus influence the magnitude and duration of posttetanic potentiation (PTP). Sodium was elevated in presynaptic terminals at rest by either (1) injection of sodium into the excitatory axon, (2) application of veratridine to open sodium channels, or (3) addition of ouabain to block Na/K exchange, with [Ca2+]i increasing by either 430, 400, or 180 nM, respectively. Intracellular calcium concentration increased only when external calcium was present, indicating that calcium influx occurred through Na/Ca exchange. Additionally, ouabain enhanced excitatory junctional potentials (EJPs) eightfold. Elevation of sodium using a high-frequency stimulation in zero-calcium Ringer's did not elevate [Ca2+]i during the train or immediately afterward when calcium-containing Ringer's was re- introduced. This indicates that a physiological sodium load does not release calcium from internal stores or reverse Na/Ca exchange to levels where [Ca2+]i accumulation is detectable. We examined the ability of sodium to interfere with calcium efflux from presynaptic terminals by loading boutons with both sodium and calcium or calcium alone using high-potassium depolarization. Elevation of internal sodium slowed calcium efflux from the terminal (12.3 min) compared to calcium removal without a sodium load (4.0 min). When sodium loading was increased during a tetanus by application of ouabain, the time constants for decay of EJP potentiation, 17.3 min, and for [Ca2+]i, 35 min, were longer than control values, 4.4 min and 5.8 min, respectively. In addition, using lithium to inhibit the efflux of calcium by Na/Ca exchange following a PTP-inducing train also lengthened the decay of [Ca2+]i to 15.7 min. Intracellular sodium accumulation in presynaptic terminals slows the efflux of calcium through Na/Ca exchange, and may therefore augment and prolong PTP.

A quantitative measurement of the dependence of short-term synaptic enhancement on presynaptic residual calcium

“…However, since trains of action potentials are and Tank -Synaptic Enhancement and Resi dual Calcium used to elevate presynaptic [Caz+],, it is possible that ACaz+ acts in conjunction with some other product of presynaptic activity (e.g., voltage or Na+ ) to produce synaptic enhancement (Bittner and Baxter, 1991 (Mulkey and Zucker, 1992).…”

“…When [Ca2+], is raised using an ionophore, the amount of enhancement produced is comparable to that observed during augmentation after a short high-frequency train of action potentials. Table 1 (Charlton and Atwood, 1977;Zucker et al, 199 1;Mulkey and Zucker, 1992) and synaptic transmission is enhanced.…”

“…7A and experiments indicated by a in Table 1). Interpreting the effect of ouabain-induced A[Ca2+], on ejp enhancement is complicated by the fact that broadening of the action potential and depolarization of the presynaptic terminal occurs with ouabain incubation (Charlton and Atwood, 1977;Mulkey and Zucker 1992;Delaney and Tank, unpublished observations Figure 7A, that is, with 1 mM ouabain, except those indicated by *, which were obtained in the absence of ouabain after 60 set duration 65 Hz trains with approximately 1 mM fura-in presynaptic terminals to slow Ca2+ removal and block-evoked ejps. In different experiments [Ca2+], before addition of ouabain was between 140 and 190 nM.…”

Short-Term Presynaptic Plasticity

“…At other synapses the Na/Ca exchanger appears to be involved in prolonging the decay of calcium (Fig. 4C) (Mulkey and Zucker 1992;Regehr 1997;Zhong et al 2001). Following tetanic stimulation the Na/ Ca exchanger and the Ca-ATPase initially work One possibility is that calcium loads the mitochondria during tetanic stimulation, and then afterwards calcium leaks out of the mitochondria and thereby leads to a sustained elevation of calcium in the presynaptic bouton.…”

Ca2+Dynamics along Identified Synaptic Terminals inDrosophilaLarvae

“…In addition, it is not known what is responsible for extruding Ca 2ϩ from these terminals. At other synaptic terminals, there is evidence for Ca 2ϩ extrusion by the Na/Ca exchanger and/or the plasma membrane Ca 2ϩ ATPase (PMCA) (Mulkey and Zucker, 1992;Reuter and Porzig, 1995;Morgans et al, 1998) We examined the Ib terminals on muscle fibers 4 and 6 in Drosophila larvae and found an inverse relationship between bouton width and the increase in [Ca 2ϩ ] i for single APs but not for AP trains. The [Ca 2ϩ ] i decay after single APs and AP trains was directly correlated with bouton width.…”