Activity-dependent modulation of K+ currents at presynaptic terminals of mammalian central synapses.

The activity-dependent regulation of presynaptic K+ currents at the CA3-CA1 synapse in the rat hippocampus was investigated during a train of evoked afferent action potentials. The waveforms of presynaptic compound action potentials (cAPs) and presynaptic Ca2+ transients ([Ca2+]pre,t) were measured with fluorescent voltage-sensitive and Ca2+-sensitive indicators in rat brain slices. Under control conditions, presynaptic cAPs and the accompanying [Ca2+]pre,t displayed similar amplitudes for each stimulus, suggesting that there was no cumulative change of K+ and Ca2+ currents during the test train. However, when a subgroup of presynaptic K+ channels was blocked by a low concentration of 4-aminopyridine (4-AP, 40 μm), a significant facilitation of the [Ca2+]pre,t was observed. This phenomenon was not due to a direct action of 4-AP on presynaptic Ca2+ channels, but to cumulative suppression of the K+ conductance as indicated by the corresponding change in waveforms of the cAP and presynaptic fibre volley. The observed facilitation was not an artifact by virtue of increased fibre recruitment, nor was it related to the accumulation of extracellular K+; rather, it was dependent on Ca2+ influx and stimulation frequency. The time course of recovery from facilitation was closely related to the decay of the intracellular Ca2+ concentration. The facilitation was not blocked by a saturating concentration of 4-AP (8 mM) but was reduced during the application of the K+ channel blocker tetraethylammonium (TEA, 10 mM), implicating the involvement of TEA-sensitive K+ channels. Such activity-dependent suppression of presynaptic K+ conductance could lead to excessive transmitter release and might explain the hippocampal epileptiform activity that can be induced by application of 4-AP. Presynaptic Ca2+ influx through voltage-gated Ca2+ channels plays an important role in the release of neurotransmitters. In addition to the activity of presynaptic Ca2+ channels, the time course of presynaptic action potential repolarization, which is largely controlled by the activity of presynaptic K+ channels, determines the amount of Ca2+ influx. Modulation of neurotransmitter release by the waveform of action potentials has been shown in various preparations (Augustine, 1990; Wheeler et al. 1996; Sabatini & Regehr, 1997; Qian & Saggau, 1999). Since activation and inactivation of most presynaptic K+ channels is voltage dependent, any change in the membrane potential as the result of invading action potentials could potentially alter the activity of these channels and thereby modulate the presynaptic Ca2+ influx in response to a subsequent action potential. Furthermore, intracellular Ca2+ levels may also regulate K+ channels through Ca2+-dependent protein phosphorylation (Roeper et al. 1997). Thus, activity-dependent modulation of presynaptic K+ channels could provide neurons with a potential means for positive or negative feedback regulation of presynaptic Ca2+ influx. Activity-dependent modulation of K+ currents has been observed at both presynaptic and postsynaptic sites. At the soma of molluscan neurons, repetitive firing broadens the action potential by cumulatively reducing repolarizing K+ currents (Aldrich et al. 1979). In hippocampal pyramidal neurons, voltage-dependent inactivation of fast inactivating K+ currents was shown to determine the frequency of repetitive somatic action potential firing (Storm, 1988). The same mechanism is believed to modulate signal propagation in dendrites of hippocampal CA1 pyramidal neurons (Hoffman et al. 1997). In presynaptic terminals of rat pituitary, voltage-dependent inactivation of K+ channels has been proposed to explain the frequency-dependent broadening of presynaptic action potentials and facilitation of Ca2+ influx (Jackson et al. 1991). The inactivation of a fast inactivating K+ channel plays a significant role in determining the reliability of action potential propagation along Schaffer collateral axons in the hippocampus (Debanne et al. 1997). In the central nervous system, it is extremely difficult to directly assay presynaptic K+ channels due to the small structure of most presynaptic terminals and limitations of current electrophysiological techniques. Nevertheless, the duration of the presynaptic action potential, which is mediated to a great extent by voltage-gated K+ channels, determines the presynaptic Ca2+ influx. By measuring Ca2+ entry and the waveform of the presynaptic action potential with fluorescent Ca2+- and voltage-sensitive dyes, we were able to investigate the activity-dependent regulation of presynaptic K+ currents at the rat hippocampal CA3-CA1 synapse.