Calcium Entry Related to Active Zones and Differences in Transmitter Release at Phasic and Tonic Synapses

Abstract

Synaptic functional differentiation of crayfish phasic and tonic motor neurons is large. For one impulse, quantal release of neurotransmitter is typically 100-1000 times higher for phasic synapses. We tested the hypothesis that differences in synaptic strength are determined by differences in synaptic calcium entry. Calcium signals were measured with the injected calcium indicator dyes Calcium Green-1 and fura-2. Estimated Ca(2+) entry increased almost linearly with frequency for both axons and was two to three times larger in phasic terminals. Tonic terminal Ca(2+) at 10 Hz exceeded phasic terminal Ca(2+) at 1 Hz, yet transmitter release was much higher for phasic terminals at these frequencies. Freeze-fracture images of synapses revealed on average similar numbers of prominent presynaptic active zone particles (putative ion channels) for both neurons and a two- to fourfold phasic/tonic ratio of active zones per terminal volume. This can account for the larger calcium signals seen in phasic terminals. Thus, differences in synaptic strength are less closely linked to differences in synaptic channel properties and calcium entry than to differences in calcium sensitivity of transmitter release.

Synaptic Vesicles: Test for a Role in Presynaptic Calcium Regulation

“…The maximum volume-averaged [Ca 2ϩ ] i attained in the MN terminals during high-frequency stimulation at 22°C was ϳ0.4 M. This value is in general agreement with [Ca 2ϩ ] i reported in other MN terminals during stimulation (ϳ1 M; crayfish, Tank et al, 1995, Msghina et al, 1999frog, Suzuki et al, 2000;mouse, David and Barrett, 2000) but is well below estimates for microdomain [Ca 2ϩ ] of 10 -30 M (Meinrenken et al, 2003). However, both values contrast sharply with the optimal Ca 2ϩ concentration (500 M) for inducing H ϩ release from SVs isolated from sheep brain (Gonçalves et al, 1998).…”

The Neuromuscular Junctions of the Slow and the Fast Excitatory Axon in the Closer of the CrabEriphia spinifronsAre Endowed with Different Ca2+Channel Types and Allow Neuron-Specific Modulation of Transmitter Release by Two Neuropeptides

“…Terminals of slow and fast neurons innervating crustacean muscles differ in morphological and physiological parameters such as number of release sites, quantal content, and facilitation or depression of transmitter release (Hoyle and Wiersma, 1958;Bittner, 1968;Rathmayer and Hammelsbeck, 1985;Atwood and Wojtowicz, 1986;King et al, 1996;Bradacs et al, 1997;Nguyen et al, 1997;Lnenicka et al, 1998;Msghina et al, 1998Msghina et al, , 1999. While studying peptidergic modulation of release by the FMRFamide-like DF 2 (DRNFLRFamide) and proctolin, we noted that DF 2 affected the slow and fast axons differentially.…”

“…Channels in the immediate vicinity of release sites have a higher effectiveness than channels more distant, such as R-and probably also N-type channels (Wu et al, 1999;Qian and Noebels, 2001). In addition, at least in crayfish slow and fast neuromuscular terminals, the Ca 2ϩ sensitivity of the release seems to differ (Msghina et al, 1999).…”

Synaptic Vesicle Recycling Adapts to Chronic Changes in Activity

“…In both systems, the synaptic release dynamics could be converted from tonic to phasic (or vice versa) in parallel with chronic changes in activity Atwood, 1985, 1989;Reid et al, 2003). However, the differences in presynaptic machinery between tonic or phasic release was not linked to synaptic vesicle dynamics but rather to vesicle pool sizes, Ca 2ϩ dependence of release, and the presynaptic glutamate synthesis (Shupliakov et al, 1995;Msghina et al, 1999;Quigley et al, 1999;Reid et al, 1999). In contrast, in the systems we analyzed here, the regulatory mechanisms targeted vesicle recycling kinetics.…”