Presynaptic calcium currents in squid giant synapse

Abstract

A voltage clamp study has been performed in the presynaptic terminal of the squid stellate ganglion. After blockage of the voltage-dependent sodium and potassium conductances, an inward calcium current is demonstrated. Given a step-depolarization pulse, this voltage- and time-dependent conductance has an S-shaped onset. At the "break" of the voltage step, a rapid tail current is observed. From these results a kinetic model is generated which accounts for the experimental results and predicts for the time course and amplitude a possible calcium entry during presynaptic action potentials.

Voltage-dependent calcium channel in the squid axon.

“…and 0.93 ± 0.14 AA/cm2 (n = 4) in 8 mM Ca.. This dependence of peak Ca current on external Ca is similar to that recently reported at the presynaptic end of the squid giant synapsis (5). Fig.…”

“…In this preparation, depolarization-induced Ca entry, measured either with aequorin or 'Ca under voltage clamp conditions (2, 3), shows two definite components: an "earlier" one that is blocked by tetrodotoxin (TTX) and occurs via the Na channels, and a "late" one that is TTX insensitive, is blocked by Ca antagonists (4) (Mn2", Co2", D600), and shows a membrane potential dependency similar to that found at the presynaptic terminal (5) and in other excitable preparations (2). However, the fact that no direct demonstration of an inward Ca current has been possible in squid axons added to the recent finding that most of the Ca entry during prolonged K depolarization is Nai dependent (1,6) has led to the proposal (1) that the TTX-insensitive Ca entry, or so-called late Ca channel, is not a separate entity but represents the operation of the reversal mode of the Na/Ca exchange (1,7).…”

Presynaptic plasticity: The regulation of Ca2+-dependent transmitter release

“…Several time-consuming processes may allow Ca 2 + to accumulate for a short instant within the micro-environment, long enough to trigger exocytosis. First, Ca 2 + diffusion into the cytosol proceeds with limited velocity (diffusion co-efficient 10-s-10 -6 m'~/sec; Llinas, 1981a;Neher, 1986;Allbritton et al, 1993). Furthermore, the saturated Ca 2+ buffer components must diffuse away into the bulk cytosol while free components must enter the submembrane area.…”

Oral Administration of Pharmacologically Active Substances to Squid: A Methodological Description

“…Simultaneous electrophysiological recordings were made from the presynaptic and postsynaptic terminals of the giant synapse, following our standard procedure (Llinas et al, 1981). Injection of MPPϩ into the terminal was monitored electrically as well as visually (Fig.…”

“…Many aspects of the action potential, synaptic transmission, and fast axonal transport have been worked out in detail in the squid (Young, 1939;Bulloch, 1948;Bloedel et al, 1966;Katz and Miledi, 1967;Kusano et al, 1967;Llinas et al, 1981;Augustine et al, 1985;Serulle et al, 2007). The scale of the axon and the synaptic structures allows unparalleled access for experiments.…”