Presynaptic membrane potential affects transmitter release in an identified neuron in Aplysia by modulating the Ca ²⁺ and K ⁺ currents

Abstract

We have examined the relationships between the modulation of transmitter release and of specific ionic currents by membrane potential in the cholinergic interneuron LIO of the abdominal ganglion of Aplysia californica. The presynaptic cell body was voltage-clamped under various pharmacological conditions and transmitter release from the terminals was assayed simultaneously by recording the synaptic potentials in the stsynaptic cell. When cell L1O was voltageclamped from a holding potential of -60 mV in the presence of tetrodotoxin, graded transmitter release was evoked by de- In some spike-generating neurons the presynaptic terminals controlling transmitter release are electrically sufficiently close to the cell body so that the release of transmitter from the terminals can be modified by injecting current into the cell body (1-4). Klein and Kandel (5) have recently found that changes in the Ca2+ current in the cell body of an Aplysia neuron parallel changes in transmitter release at the terminals. These two sets of observations suggested to us that we might be able to examine the relationships between transmitter release and specific ionic currents of the presynaptic membrane. Toward this end we have combined two separate techniques: (i) pharmacological separation and voltage-clamp analysis of the ionic currents in the cell body of the presynaptic neuron and (ii) assay of transmitter release obtained by means of intracellular recordings of the synaptic potential in the post-synaptic cells. We have found that this combined technique provides a powerful method for studying changes in specific ionic conductances associated with various presynaptic mechanisms for synaptic plasticity.For these studies we have used the identified cholinergic neuron L10 of the abdominal ganglion of Aplysia. We eliminated impulse conduction by blocking Na+ channels with tetrodotoxin (TTX) and found that graded depolarizing commands, applied to the membrane of the presynaptic cell body under voltage-clamp control, caused graded transmitter release from the terminals as determined by the graded changes in the synaptic potential in the postsynaptic cell. Although in many cases we probably lacked ideal voltage control of the terminals, this procedure nonetheless allowed sufficient control to study transmitter release while examining specific ionic currents in the soma of the presynaptic neuron. Voltage control can be further improved by axotomizing the presynaptic cell (6, 7) and by adding pharmacological agents that block each of the three known outward K+ currents (8) and thereby also lengthen the effective space constant of the neuron.By using these approaches, we have found that depolarizing the presynaptic membrane potential enhances transmitter release by two mechanisms: (i) by activation of a steady-state Ca2+ current and (ii) by decreasing the outward K+ currents activated by a step depolarization (9, 10). This decrease in the K+ currents increases the duration of the spike and, therefore, the transient Ca2+ current activat...

Transient Enhancement of Spike-Evoked Calcium Signaling by a Serotonergic Interneuron

“…Subthreshold depolarization can lead to Ca 2ϩ influx and an enhancement of neurotransmitter release (Shapiro et al 1980;Turecek and Trussell 2001). However, our data, and those of Katz and Frost (1995b) suggest that this is not the mechanism by which DSI enhances C2 distal neurite spike-evoked Ca 2ϩ signals or transmitter release.…”

Post-tetanic potentiation at an identified synapse in Aplysia is correlated with a Ca2+-activated K+ current in the presynaptic neuron: evidence for Ca2+ accumulation.

“…4 A and B), a treatment that blocks Ca2+-activated K+ current (18,23,27). IKCa, although at holding potential -40 mV a part of the resting leakage conductance is a Ca24-activated K+ current (13). (C) The slow time constants of decay of PTP (e) and post-tetanic Ca2"-activated K+ (0) currents are plotted as afunction of time after EGTA injection.…”

“…In addition, these analyses have been compromised because tetanic stimulation also gives rise to membrane potential changes in the presynaptic terminals that may have secondary effects on release, masking the actual time course of the facilitated release process. Recently, neurons have been described in Aplysia in which presynaptic release sites are accessible to intracellular electrodes and can be studied under voltage-clamp conditions, in which secondary changes in membrane potential can be controlled (13)(14)(15). One of these neurons, the identified cholinergic cell L10, makes connections to follower cells that undergo PTP.…”

“…MATERIALS AND METHODS Experiments were performed on over 100 desheathed abdominal ganglia removed from Aplysia californica weighing 50-125 g. Cell LLO was impaled with two independent electrodes of low resistance (1-5 MW) for current and voltage clamping (13,19). Follower cells were recorded from with a third electrode connected to a bridge circuit for intracellular recording and current passing.…”

Axon Physiology

“…A totally different scenario has been observed in the Aplysia (475,486,487) and the leech (382). In these studies on connected pairs of neurons, the authors reported that constant or transient depolarization of the membrane potential in the soma of the presynaptic neuron facilitates synaptic transmission evoked by single action potentials, in a graded manner (Fig.…”