The timing of phasic transmitter release is Ca ²⁺ -dependent and lacks a direct influence of presynaptic membrane potential

Abstract

T ransmitter release occurs when an action potential (AP) invades the presynaptic nerve terminal and opens voltagegated Ca 2ϩ channels, allowing a brief influx of Ca 2ϩ ions into the presynaptic terminal (1-5). It is thought that the brief increase in release probability underlying phasic transmitter release (6) is caused by the transient increase in local [Ca 2ϩ ] i (intracellular free Ca 2ϩ concentration) at the sites of vesicle fusion, resulting from this AP-induced Ca 2ϩ influx. The local [Ca 2ϩ ] i signal for vesicle fusion arises from Ca 2ϩ microdomains (7,8) formed by individual Ca 2ϩ channels (9), or, alternatively, from the overlap of Ca 2ϩ microdomains created by several neighboring Ca 2ϩ channels (4, 10). We previously inferred the amplitude and the time course of the local [Ca 2ϩ ] i signal seen by readily releasable vesicles, on the basis of the intracellular Ca 2ϩ sensitivity of transmitter release determined by Ca 2ϩ uncaging (ref. 11; see also refs. 12-14). This back-calculation, however, assumes that the local [Ca 2ϩ ] i at the sites of vesicle fusion is the only determinant of transmitter release probability on a short timescale.It is controversial whether factors other than the time course of local [Ca 2ϩ ] i influence the timing and the amount of phasic release. At the neuromuscular junction, it has been shown that a strong reduction of release probability, imposed by reducing the amount of Ca 2ϩ influx, does not change the time course of phasic release during a presynaptic AP (15)(16)(17). It has been proposed that this invariance of the time course of transmitter release against changes in Ca 2ϩ influx implies that additional factors, other than the rapid rise and fall of the [Ca 2ϩ

Rapid Vesicular Release, Quantal Variability, and Spillover Contribute to the Precision and Reliability of Transmission at a Glomerular Synapse

“…Such rapid calcium transients could be experienced by the calcium sensor if it is within tens of nanometers of the entry site ["nanodomains" (Neher, 1998)], as reported recently (Fedchyshyn and Wang, 2005), or if the intracellular [Ca 2ϩ ] is shaped by buffers (Meinrenken et al, 2002). However, we cannot rule out the possibility of some calciumdependent speeding of release (Felmy et al, 2003;Bollmann and Sakmann, 2005) if it were masked by a calcium-dependent broadening of the AP (Schneggenburger et al, 1999). Because the release time course is rapid compared with the miniature EPSC, estimates of P R from MPFA at the peak of the quantal response will correspond to the final release probability.…”

Physiological Temperatures Reduce the Rate of Vesicle Pool Depletion and Short-Term Depression via an Acceleration of Vesicle Recruitment

“…These q Ca values also did not change from RT to PT (in part because of faster I Ca inactivation at PT) (Forsythe et al, 1998). Because membrane potential does not directly affect release from the calyx of Held (Felmy et al, 2003), steady-state exocytosis may be compared with EPSC charge between the 20th and 50th stimuli of a high-frequency train. Steady-state exocytosis was substantially increased at PT, but to a lesser extent than steady-state EPSC charge in P8 -P10 animals.…”

Depolarization Initiates Phasic Acetylcholine Release by Relief of a Tonic Block Imposed by Presynaptic M2 Muscarinic Receptors

“…MICE. M 2 R knockout mice (M 2 -KO) (Gomeza et al 1999), 1.5-3 mo of age, were used. They had a mixed genetic background (129J1 X CF-1; 50%/50%).…”

“…The G 2 dimeric form of AChE was purified from Torpedo californica electric organ by affinity chromatography, after solubilization with phosphatidylinositol-specific phospholipase C (Futerman et al 1985). Its specific activity was about 3,000 units per mg protein, one unit corresponding to hydrolysis of 1 mol min Ϫ1 of acetylthiocholine, assayed according to Ellman et al (1961) (see also Slutsky et al 2001).…”