Presynaptic calcium currents and facilitation of serotonin release at synapses between cultured leech neurones

The role of presynaptic Ca²⁺ entry in facilitation of transmitter release has been analysed by voltage-clamp measurements at synapses formed in culture by Retzius and P neurones isolated from the central nervous system (CNS) of the leech. The transmitter released by Retzius cells is serotonin.

Synaptic transmission persisted in solutions containing raised concentrations of divalent cations, reduced concentrations of Na⁺, and tetraethylammonium (TEA⁺) and 4-AP (to block K⁺ currents). Ca²⁺ and Sr²⁺ were more effective in promoting transmitter release than Ba²⁺, as assessed by the postsynaptic potentials in P cells. The degree and time course of facilitation in Ca²⁺- and Sr²⁺-containing solutions were similar to those observed for synapses bathed in normal L-15 medium.

Transmitter release depended upon the amplitude and the duration of presynaptic depolarization and inward Ca²⁺ current. Peak Ca²⁺ currents and postsynaptic potentials occurred with depolarizing steps to +15 mV. Frequent or prolonged pulses depressed the postsynaptic potentials.

Pairs of depolarizing pulses that caused facilitation were accompanied by identical inward Ca²⁺ currents. These results indicate that the mechanism responsible for facilitated serotonin release must occur following Ca²⁺ entry and that residual Ca²⁺ plays a role.