Ca(2+)‐dependent and ‐independent components of transmitter release at the frog neuromuscular junction.

Abstract

SUMMARY1. When a Ca2+ chelator, bis (0-aminophenoxy)ethane-NNN'N-tetraacetic acid (BAPTA), was loaded into the presynaptic nerve terminal of the frog neuromuscular junction (NMJ), facilitation, measured as an increase in endplate potential (EPP) amplitudes during a train of ten stimulations at 100 Hz, was greatly decreased within 20 min of BAPTA-AM (the acetoxymethyl ester of BAPTA) perfusion, and remained at a constant low level thereafter, suggesting that [Ca2+]i at the presynaptic nerve terminal was buffered by BAPTA.2. Detailed examination of the two components of facilitation of EPP amplitude in the BAPTA-loaded NMJs showed that the fast component was lost almost completely, while the slow component was unaffected by loaded BAPTA. Augmentation and potentiation were also unaffected by BAPTA.3. Under external Ca2+-free conditions (with 1 mM-EGTA), both augmentation and potentiation of miniature endplate potential (MEPP) frequency were clearly observed after tetanic stimulation in the normal NMJ, and were also unaffected by loaded BAPTA. 4. The above findings strongly support the residual Ca2+ hypothesis for the fast component of facilitation, and suggest that the three slower processes (the slow component of facilitation, augmentation and potentiation) occur independently of [Ca2+]i. This Ca2+ independence was supported by the fact that facilitation and potentiation have multiplicative effects on the amount of release.5. The quantal content of the first EPP in the train remained unchanged throughout the time course of BAPTA loading for most NMJs. This suggests that [Ca2+]

The effect of ions and second messengers on long‐term potentiation of chemical transmission in avian ciliary ganglia

“…As the frequency of tetanic stimulation increases, the inward calcium signals at the terminal overlap and summate, indicating that the calcium level here is dependent on the frequency of stimulation up to the level of saturation. This high calcium level in the terminal accounts for PTP which is a product of residual calcium (Erulkar & Rahamimoff, 1978; see however Tanabe & Kijima, 1992). Also, it is proposed (Kuba & Kumamoto, 1990) that this enlarged calcium influx persistently affects the calcium-dependent second messenger systems in the presynaptic terminal during ganglionic LTP.…”

“…and central synapses including the ciliary ganglion (Martin & Pilar, 1964c), the hippocampus (Racine & Milgram, 1983) and the neuromuscular junction (Magleby & Zengel, 1975;Delaney et al, 1989). The level of PTP has been shown to be directly related to the extracellular calcium concentration (Erulkar & Rahamimoff, 1978; but see Tanabe & Kijima, 1992). It has been suggested that PTP may be the result of the activation of calcium channels during a tetanic stimulation causing a calcium ion acccumulation as a consequence of the saturation of calcium pumps or their run-down due to limited energy resources in the presence of high calcium loads (Zucker, 1989).…”

Ca2+ dynamics at the frog motor nerve terminal

“…Endplate potentials (EPPs) were extracellularly recorded at the endplate region with a surface electrode under temperature control at 22±0.5°C as described before [35]. This was inevitably done in separate experiments from that of fluorescence measurements, because the latter was made on the preparations held up-side down on the stage of an inverted microscope.…”

Reduction by intracellular calcium chelation of acetylcholine secretion without occluding the effects of adenosine at frog motor nerve endings

“…frequency and quantal content being relatively unaffected by intracellular BAPTA. However, in the experiments in their more recent paper, the data of Kijima & Tanabe (1992) show that quantal content is decreased in the presence of 100 tiM BAPTA-AM (see e.g. Figure 6b and c in their paper).…”

“…frequency. In more recent experiments, Tanabe & Kijima (1992) have shown only a decrease in the fast phase of facilitation, with m.e.p.p. frequency and quantal content being relatively unaffected by intracellular BAPTA.…”