Activity-dependent modulation of the presynaptic potassium current in the frog neuromuscular junction

The Frog Motor Nerve Terminal Has Very Brief Action Potentials and Three Electrical Regions Predicted to Differentially Control Transmitter Release

The action potential (AP) waveform controls the opening of voltage-gated calcium channels and contributes to the driving force for calcium ion flux that triggers neurotransmission at presynaptic nerve terminals. Although the frog neuromuscular junction (NMJ) has long been a model synapse for the study of neurotransmission, its presynaptic AP waveform has never been directly studied, and thus the AP waveform shape and propagation through this long presynaptic nerve terminal are unknown. Using a fast voltage-sensitive dye, we have imaged the AP waveform from the presynaptic terminal of male and female frog NMJs and shown that the AP is very brief in duration and actively propagated along the entire length of the terminal. Furthermore, based on measured AP waveforms at different regions along the length of the nerve terminal, we show that the terminal is divided into three distinct electrical regions: A beginning region immediately after the last node of Ranvier where the AP is broadest, a middle region with a relatively consistent AP duration, and an end region near the tip of nerve terminal branches where the AP is briefer. We hypothesize that these measured changes in the AP waveform along the length of the motor nerve terminal may explain the proximal-distal gradient in transmitter release previously reported at the frog NMJ.SIGNIFICANCE STATEMENT The AP waveform plays an essential role in determining the behavior of neurotransmission at the presynaptic terminal. Although the frog NMJ is a model synapse for the study of synaptic transmission, there are many unknowns centered around the shape and propagation of its presynaptic AP waveform. Here, we demonstrate that the presynaptic terminal of the frog NMJ has a very brief AP waveform and that the motor nerve terminal contains three distinct electrical regions. We propose that the changes in the AP waveform as it propagates along the terminal can explain the proximal-distal gradient in transmitter release seen in electrophysiological studies.

Synapse clusters are preferentially formed by synapses with large recycling pool sizes

Synapses are distributed heterogeneously in neural networks. The relationship between the spatial arrangement of synapses and an individual synapse's structural and functional features remains to be elucidated. Here, we examined the influence of the number of adjacent synapses on individual synaptic recycling pool sizes. When measuring the discharge of the styryl dye FM1-43 from electrically stimulated synapses in rat hippocampal tissue cultures, a strong positive correlation between the number of neighbouring synapses and recycling vesicle pool sizes was observed. Accordingly, vesicle-rich synapses were found to preferentially reside next to neighbours with large recycling pool sizes. Although these synapses with large recycling pool sizes were rare, they were densely arranged and thus exhibited a high amount of release per volume. To consolidate these findings, functional terminals were marked by live-cell antibody staining with anti-synaptotagmin-1-cypHer or overexpression of synaptopHluorin. Analysis of synapse distributions in these systems confirmed the results obtained with FM 1-43. Our findings support the idea that clustering of synapses with large recycling pool sizes is a distinct developmental feature of newly formed neural networks and may contribute to functional plasticity.

The presynaptic modulation of corticostriatal afferents by mu-opioids is mediated by K+ conductances

Population spikes associated with the paired pulse ratio protocol were used to measure the presynaptic inhibition of corticostriatal transmission caused by mu-opioid receptor activation. A 1 microM of [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO), a selective mu-opioid receptor agonist, enhanced paired pulse facilitation by 44+/-8%. This effect was completely blocked by 2 nM of the selective mu-receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH (CTOP). Antagonists of N- and P/Q-type Ca(2+) channels inhibited, whereas antagonists of potassium channels enhanced, synaptic transmission. A 1 microM of omega-conotoxin GVIA, a blocker of N-type Ca(2+) channels, had no effect on the action of DAMGO, but 400 nM omega-agatoxin TK, a blocker of P/Q-type Ca(2+)-channels, partially blocked the action of this opioid. However, 5 mM Cs(2+) and 400 microM Ba(2+), unselective antagonists of potassium conductances, completely prevented the action of DAMGO on corticostriatal transmission. These data suggest that presynaptic inhibition of corticostriatal afferents by mu-opioids is mediated by the modulation of K(+) conductances in corticostriatal afferents.

Repolarization of the presynaptic action potential and short-term synaptic plasticity in the chick ciliary ganglion

Stimulation-induced increases in synaptic efficacy have been described as being composed of multiple independent processes that arise from the activation of distinct mechanisms at the presynaptic terminal. In the chick ciliary ganglion, four components of short-term synaptic plasticity have been described: F1 and F2 components of facilitation, augmentation, and potentiation. In the present study, intracellular recording from the presynaptic calyciform nerve terminal of the chick ciliary ganglion revealed that the late repolarization and afterhypolarization (AHP) phases of the presynaptic action potential are affected by repetitive stimulation and that the time course of these effects parallel that of facilitation. The effects of these changes in the presynaptic action potential time course on calcium influx were tested by using the recorded action potential waveforms as voltage command stimuli during whole-cell patch-clamp recordings from acutely isolated chick ciliary ganglion neurons. The "facilitated" action potential waveform (slowed repolarization, decreased AHP amplitude) evoked calcium current with slightly but significantly greater total calcium influx. Taken together, these results are consistent with the hypothesis that activity-dependent changes in the presynaptic action potential are one of several mechanisms contributing to the facilitation phase of stimulation-induced increases in transmitter release in this preparation.

Determinants of excitability at transition zones in Kv1.1-deficient myelinated nerves

This study examines the role of K channel segregation and fiber geometry at transition zones of mammalian nerve terminals in the peripheral nervous system. Mutant mice that are deficient in Kv1.1, a fast Shaker K channel normally localized beneath the myelin sheath, display three types of cooling-induced abnormal hyperexcitability localized to regions before the transition zones of myelinated nerves. The first type is stimulus-evoked nerve backfiring that is absent at birth, peaks at postnatal day 17 (P17), and subsides in adults. The second type is spontaneous activity that has a more delayed onset, peaks at P30, and also disappears in older mice (>P60). TEA greatly amplifies this spontaneous activity with an effective dosage of approximately 0.7 mM, and can induce its reappearance in older mutant mice (>P100). These first two types of hyperexcitability occur only in homozygous mutants that are completely devoid of Kv1.1. The third type occurs in heterozygotes and represents a synergism between a TEA-sensitive channel and Kv1.1. Heterozygotes exposed to TEA display no overt phenotype until a single stimulation is given, which is then followed by an indefinite phase of repetitive discharge. Computer modeling suggests that the excitability of the transition zone near the nerve terminal has at least two major determinants: the preterminal internodal shortening and axonal slow K channels. We suggest that variations in fiber geometry create sites of inherent instability that is normally stabilized by a synergism between myelin-concealed Kv1.1 and a slow, TEA-sensitive K channel.

Determinants of excitability at transition zones in Kv1.1-deficient myelinated nerves

This study examines the role of K channel segregation and fiber geometry at transition zones of mammalian nerve terminals in the peripheral nervous system. Mutant mice that are deficient in Kv1.1, a fast Shaker K channel normally localized beneath the myelin sheath, display three types of cooling-induced abnormal hyperexcitability localized to regions before the transition zones of myelinated nerves. The first type is stimulus-evoked nerve backfiring that is absent at birth, peaks at postnatal day 17 (P17), and subsides in adults. The second type is spontaneous activity that has a more delayed onset, peaks at P30, and also disappears in older mice (>P60). TEA greatly amplifies this spontaneous activity with an effective dosage of approximately 0.7 mM, and can induce its reappearance in older mutant mice (>P100). These first two types of hyperexcitability occur only in homozygous mutants that are completely devoid of Kv1.1. The third type occurs in heterozygotes and represents a synergism between a TEA-sensitive channel and Kv1.1. Heterozygotes exposed to TEA display no overt phenotype until a single stimulation is given, which is then followed by an indefinite phase of repetitive discharge. Computer modeling suggests that the excitability of the transition zone near the nerve terminal has at least two major determinants: the preterminal internodal shortening and axonal slow K channels. We suggest that variations in fiber geometry create sites of inherent instability that is normally stabilized by a synergism between myelin-concealed Kv1.1 and a slow, TEA-sensitive K channel.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Temperature-sensitive neuromuscular transmission in Kv1.1 null mice: role of potassium channels under the myelin sheath in young nerves

In mammalian myelinated nerves, the internodal axon that is normally concealed by the myelin sheath expresses a rich repertoire of K channel subtypes thought to be important in modulating action potential propagation. The function of myelin-covered K channels at transition zones, however, has remained unexplored. Here we show that deleting the voltage-sensitive potassium channel Kv1.1 from mice confers a marked temperature-sensitivity to neuromuscular transmission in postnatal day 14 (P14)-P21 mice. Using immunofluorescence and electrophysiology, we examined contributions of four regions of the peripheral nervous system to the mutant phenotype: the nerve trunk, the myelinated segment preceding the terminal, the presynaptic terminal membrane itself, and the muscle. We conclude that the temperature-sensitive neuromuscular transmission is accounted for solely by a deficiency in Kv1.1 normally concealed in the myelinated segments just preceding the terminal. This paper demonstrates that under certain situations of physiological stress, the functional role of myelin-covered K channels is dramatically enhanced as the transition zone at the neuromuscular junction is approached.

Temperature-sensitive neuromuscular transmission in Kv1.1 null mice: role of potassium channels under the myelin sheath in young nerves

In mammalian myelinated nerves, the internodal axon that is normally concealed by the myelin sheath expresses a rich repertoire of K channel subtypes thought to be important in modulating action potential propagation. The function of myelin-covered K channels at transition zones, however, has remained unexplored. Here we show that deleting the voltage-sensitive potassium channel Kv1.1 from mice confers a marked temperature-sensitivity to neuromuscular transmission in postnatal day 14 (P14)-P21 mice. Using immunofluorescence and electrophysiology, we examined contributions of four regions of the peripheral nervous system to the mutant phenotype: the nerve trunk, the myelinated segment preceding the terminal, the presynaptic terminal membrane itself, and the muscle. We conclude that the temperature-sensitive neuromuscular transmission is accounted for solely by a deficiency in Kv1.1 normally concealed in the myelinated segments just preceding the terminal. This paper demonstrates that under certain situations of physiological stress, the functional role of myelin-covered K channels is dramatically enhanced as the transition zone at the neuromuscular junction is approached.

3,4-Diaminopyridine-induced impairment in frog motor nerve terminal response to high frequency stimulation

The refractory period of the presynaptic Na+ current (INa) of the frog neuromuscular junction before and after the block of the presynaptic delayed rectifier K+ conductance by 3,4-diaminopyridine (3,4-DAP) was studied by the perineurial recording technique. Application of 3,4-DAP 0.45 mM greatly prolonged the refractory period of the last nodes of Ranvier of frog motor axons. Suppression of the repetitive activity caused by 3,4-DAP by 3-aminobenzoic acid ethyl ester (tricaine) 0.46 mM (a local anesthetic) decreased the refractory period back towards normal values. These results indicate that 3,4-DAP impairs conduction of high frequency nerve impulses along the last nodes of Ranvier due to its block of presynaptic K+ conductance. The spontaneous activation of the most excitable, last nerve segments seemed to be the main factor causing such impairment. This phenomenon could explain in part the adverse motor effects shown by some patients treated with high doses of 3,4-DAP.