Amount and time-course of release. The calcium hypothesis and the calcium-voltage hypothesis

Presynaptic Acetylcholine Receptors Modulate the Time Course of Action Potential-Evoked Acetylcholine Quanta Secretion at Neuromuscular Junctions

For effective transmission of excitation in neuromuscular junctions, the postsynaptic response amplitude must exceed a critical level of depolarization to trigger action potential spreading along the muscle-fiber membrane. At the presynaptic level, the end-plate potential amplitude depends not only on the acetylcholine quanta number released from the nerve terminals in response to the nerve impulse but also on a degree of synchronicity of quanta releases. The time course of stimulus-phasic synchronous quanta secretion is modulated by many extra- and intracellular factors. One of the pathways to regulate the neurosecretion kinetics of acetylcholine quanta is an activation of presynaptic autoreceptors. This review discusses the contribution of acetylcholine presynaptic receptors to the control of the kinetics of evoked acetylcholine release from nerve terminals at the neuromuscular junctions. The timing characteristics of neurotransmitter release is nowadays considered an essential factor determining the plasticity and efficacy of synaptic transmission.

Calcium entry related to active zones and differences in transmitter release at phasic and tonic synapses

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.

Contribution of Ca2+ inflow to quantal, phasic transmitter release from nerve terminals of frog muscle

Evoked quantal release from sections of frog endplates contained in an extracellular electrode has been investigated with Ca2+ inflow prevented by superfusing the extracellular space with a Ringer's solution containing Cd2+e or with an "intracellular", EGTA-buffered solution containing less than 0.1 microM Ca2+e. Pulse application and recording were by a perfused macro-patch-clamp electrode. The muscle outside the electrode (bath) was superfused with Ringer's solutions containing Cd2+b to block Ca2+ inflow and normal (1.8 mM) or elevated (10 mM) Ca2+b. The depolarization level of the terminal during current pulses that generated maximal Ca2+ inflow was used as unit relative depolarization. Starting from a threshold above 0.5 relative depolarization, the average release increased by a factor of about 1000 with increasing depolarization, reaching a plateau above 1.2 relative depolarization. The high level of plateau release extended to at least a relative depolarization of 4, i.e. to about +200 mV. When Ca2+ inflow was prevented in the section of the terminal within the electrode, release was depressed strongly for relative depolarizations around 1, i.e. at potentials at which Ca2+ inflow is high. However, for large depolarizations (> 1.5 relative units), the depression of release by block of Ca2+ inflow was weak or absent. The time course of release, measured in distributions of the delays of quanta after the depolarizing pulse, was unaffected by block of Ca2+ inflow. If the extra-electrode superfusion of Ca2+b of the muscle was elevated to 10 mM and Cd2+b was 0.1 mM or 0.5 mM, perfusion of the electrode with solutions below 0.1 microM Ca2+e raised the average release paradoxically. With 0.5 mM Cd2+b this paradoxical increase of release was, on average, 4-fold at 6 degrees C, and 19-fold at 16 degrees C. Quantal endplate currents recorded in less than 0.1 microM Ca2+e had slightly increased amplitudes, and decay time constants were prolonged by about 50%. The results are interpreted to support the Ca2+/voltage theory of release, which proposes that evoked, phasic release is controlled by both intracellular Ca2+ concentration and another membrane-depolarization-related factor. If the resting intracellular Ca2+ concentration is sufficiently high, large depolarizations can elicit release independent of the presence or absence of Ca2+ inflow.