Dynamics of volume-averaged intracellular Ca in a rat CNS nerve terminal during single and repetitive voltage-clamp depolarizations.

Key Points: The intracellular concentration of free calcium ions ([Ca ] ) in a nerve terminal controls both transmitter release and synaptic plasticity. The rapid triggering of transmitter release depends on the local micro- or nanodomain of highly elevated [Ca ] in the vicinity of open voltage-gated Ca channels, whereas short-term synaptic plasticity is often controlled by global changes in residual [Ca ] , averaged over the whole nerve terminal volume. Here we describe dynamic changes of such global [Ca ] in the calyx of Held - a giant mammalian glutamatergic nerve terminal, which is particularly suited for biophysical studies. We provide quantitative data on Ca inflow, Ca buffering and Ca clearance. These data allow us to predict changes in [Ca ] in the nerve terminal in response to a wide range of stimulus protocols at high temporal resolution and provide a basis for the modelling of short-term plasticity of glutamatergic synapses.

Abstract: Many aspects of short-term synaptic plasticity (STP) are controlled by relatively slow changes in the presynaptic intracellular concentration of free calcium ions ([Ca ] ) that occur in the time range of a few milliseconds to several seconds. In nerve terminals, [Ca ] equilibrates diffusionally during such slow changes, such that the globally measured, residual [Ca ] that persists after the collapse of local domains is often the appropriate parameter governing STP. Here, we study activity-dependent dynamic changes in global [Ca ] at the rat calyx of Held nerve terminal in acute brainstem slices using patch-clamp and microfluorimetry. We use low concentrations of a low-affinity Ca indicator dye (100 μm Fura-6F) in order not to overwhelm endogenous Ca buffers. We first study voltage-clamped terminals, dialysed with pipette solutions containing minimal amounts of Ca buffers, to determine Ca binding properties of endogenous fixed buffers as well as the mechanisms of Ca clearance. Subsequently, we use pipette solutions including 500 μm EGTA to determine the Ca binding kinetics of this chelator. We provide a formalism and parameters that allow us to predict [Ca ] changes in calyx nerve terminals in response to a wide range of stimulus protocols. Unexpectedly, the Ca affinity of EGTA under the conditions of our measurements was substantially lower (K  = 543 ± 51 nm) than measured in vitro, mainly as a consequence of a higher than previously assumed dissociation rate constant (2.38 ± 0.20 s ), which we need to postulate in order to model the measured presynaptic [Ca ] transients.