Input-output relation of a single synapse

The 70-year search for the voltage sensing mechanism of ion channels

This retrospective on the voltage‐sensing mechanisms and gating models of ion channels begins in 1952 with the charged gating particles postulated by Hodgkin and Huxley, viewed as charges moving across the membrane and controlling its permeability to Na⁺ and K⁺ ions. Hodgkin and Huxley postulated that their movement should generate small and fast capacitive currents, which were recorded 20 years later as gating currents. In the early 1980s, several voltage‐dependent channels were cloned and found to share a common architecture: four homologous domains or subunits, each displaying six transmembrane α‐helical segments, with the fourth segment (S4) displaying four to seven positive charges invariably separated by two non‐charged residues. This immediately suggested that this segment was serving as the voltage sensor of the channel (the molecular counterpart of the charged gating particle postulated by Hodgkin and Huxley) and led to the development of the sliding helix model. Twenty years later, the X‐ray crystallographic structures of many voltage‐dependent channels allowed investigation of their gating by molecular dynamics. Further understanding of how channels gate will benefit greatly from the acquisition of high‐resolution structures of each of their relevant functional or structural states. This will allow the application of molecular dynamics and other approaches. It will also be key to investigate the energetics of channel gating, permitting an understanding of the physical and molecular determinants of gating. The use of multiscale hierarchical approaches might finally prove to be a rewarding strategy to overcome the limits of the various single approaches to the study of channel gating.

Specific labeling of synaptic schwann cells reveals unique cellular and molecular features

Perisynaptic Schwann cells (PSCs) are specialized, non-myelinating, synaptic glia of the neuromuscular junction (NMJ), that participate in synapse development, function, maintenance, and repair. The study of PSCs has relied on an anatomy-based approach, as the identities of cell-specific PSC molecular markers have remained elusive. This limited approach has precluded our ability to isolate and genetically manipulate PSCs in a cell specific manner. We have identified neuron-glia antigen 2 (NG2) as a unique molecular marker of S100β+ PSCs in skeletal muscle. NG2 is expressed in Schwann cells already associated with the NMJ, indicating that it is a marker of differentiated PSCs. Using a newly generated transgenic mouse in which PSCs are specifically labeled, we show that PSCs have a unique molecular signature that includes genes known to play critical roles in PSCs and synapses. These findings will serve as a springboard for revealing drivers of PSC differentiation and function.

Reflectin as a Material for Neural Stem Cell Growth

Cephalopods possess remarkable camouflage capabilities, which are enabled by their complex skin structure and sophisticated nervous system. Such unique characteristics have in turn inspired the design of novel functional materials and devices. Within this context, recent studies have focused on investigating the self-assembly, optical, and electrical properties of reflectin, a protein that plays a key role in cephalopod structural coloration. Herein, we report the discovery that reflectin constitutes an effective material for the growth of human neural stem/progenitor cells. Our findings may hold relevance both for understanding cephalopod embryogenesis and for developing improved protein-based bioelectronic devices.

Crystallographic studies of pharmacological sites in pentameric ligand-gated ion channels

BACKGROUND

Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical transmission of nerve signals in the central and peripheral nervous system. On the functional side, these molecules respond to the binding of a neurotransmitter (glycine, GABA, acetylcholine or 5HT3) in the extracellular domain (ECD) by opening their ionotropic pore in the transmembrane domain (TMD). The response to the neurotransmitter binding can be modulated by several chemical compounds acting at topographically distinct sites, as documented by a large body of literature. Notably, these receptors are the target of several classes of world-wide prescribed drugs, including general anesthetics, smoking cessation aids, anxiolytics, anticonvulsants, muscle relaxants, hypnotics and anti-emetics. On the structural side recent progress has been made on the crystallization of pLGICs in its different allosteric states, especially pLGICs of bacterial origin. Therefore, structure-function relationships can now be discussed at the atomic level for pLGICs.

SCOPE OF REVIEW

This review focuses on the crystallographic structure of complexes of pLGICs with a number of ligands of pharmacological interest. First, we review structural data on two key functional aspects of these receptors: the agonist-induced activation and ion transport itself. The molecular understanding of both these functional aspects is important, as they are those that most pharmacological compounds target. Next, we describe modulation sites that have recently been documented by X-ray crystallography. Finally, we propose a simple geometric classification of all these pharmacological sites in pLGICs, based on icosahedrons.

MAJOR CONCLUSIONS

This review illustrates the wealth of structural insight gained by comparing all available structures of members of the pLGIC family to rationalize the pharmacology of structurally diverse drugs acting at topographically distinct sites. It will be highlighted how sites that had been described earlier using biochemical techniques can be rationalized using structural data. Surprisingly, the use of icosahedral symmetry allows to link together several modulation sites, in a way that was totally unanticipated.

GENERAL SIGNIFICANCE

Overall, understanding the interplay between the different modulation sites at the structural level should help the design of future drugs targeting pLGICs. This article is part of a Special Issue entitled structural biochemistry and biophysics of membrane proteins.

Presynaptic K(+) channels, vesicular Ca(2+)/H (+) antiport--synaptotagmin, and acetylcholinesterase, three mechanisms cutting short the cholinergic signal at neuromuscular and nerve-electroplaque junctions

In neuromuscular and nerve-electroplaque junctions, nerve impulses can be transmitted at high frequencies. This implies that transmission of individual impulses must be very brief. We describe three mechanisms which curtail the time course of individual impulses at these synapses: (1) opening of presynaptic K(+) channels (delayed rectifier) efficiently curtails the presynaptic action potential. Inhibition of K(+) channel by aminopyridines transforms the normally brief postsynaptic potential (2-3 ms) to a long-lasting "giant" potential (exceeding half a second); (2) a low-affinity Ca(2+)/H(+) antiport ensures rapid Ca(2+) sequestration into synaptic vesicles, curtailing the calcium signal and thereby the duration of transmitter release. Indeed vesicular Ca(2+)/H(+) antiport inhibition by bafilomycin or Sr(2+) prolongs the duration of the postsynaptic potential. We recently showed that synaptotagmin-1 is required for this antiport activity; thus the vesicular Ca(2+)/H(+) antiport might be synaptotagmin itself, or regulated by it; and (3) it is recalled that, in these junctions, acetylcholinesterase is highly concentrated in the synaptic cleft and that anticholinesterases lengthen the endplate time course. Therefore, at three different steps of synaptic transmission, an efficient mechanism curtails the local synaptic signal. When one of these three mechanisms is inhibited, the duration of individual impulses is prolonged, but the synapse loses its faculty to fire at high frequencies.

Synaptic plasticity in cephalopods; more than just learning and memory?

The outstanding behavioural capacity of cephalopods is underpinned by a highly sophisticated nervous system anatomy and neural mechanisms that often differ significantly from similarly complex systems in vertebrates and insects. Cephalopods exhibit considerable behavioural flexibility and adaptability, and it might be expected that this should be supported by evident cellular and synaptic plasticity. Here, we review what little is known of the cellular mechanisms that underlie plasticity in cephalopods, particularly from the point of view of synaptic function. We conclude that cephalopods utilise short-, medium-, and long-term plasticity mechanisms that are superficially similar to those so far described in vertebrate and insect synapses. These mechanisms, however, often differ significantly from those in other animals at the biophysical level and are deployed not just in the central nervous system, but also to a limited extent in the peripheral nervous system and neuromuscular junctions.

Structure and pharmacology of pentameric receptor channels: from bacteria to brain

Orthologs of the pentameric receptor channels that mediate fast synaptic transmission in the central and peripheral nervous systems have been found in several bacterial species and in a single archaea genus. Recent X-ray structures of bacterial and invertebrate pentameric receptors point to a striking conservation of the structural features within the whole family, even between distant prokaryotic and eukaryotic members. These structural data reveal general principles of molecular organization that allow allosteric membrane proteins to mediate chemoelectric transduction. Notably, several conformations have been solved, including open and closed channels with distinct global tertiary and quaternary structure. The data reveal features of the ion channel architecture and of diverse categories of binding sites, such as those that bind orthosteric ligands, including neurotransmitters, and those that bind allosteric modulators, such as general anesthetics, ivermectin, or lipids. In this review, we summarize the most recent data, discuss insights into the mechanism of action in these systems, and elaborate on newly opened avenues for drug design.

Ion channels in key marine invertebrates; their diversity and potential for applications in biotechnology

Of the intra-membrane proteins, the class that comprises voltage and ligand-gated ion channels represents the major substrate whereby signals pass between and within cells in all organisms. It has been presumed that vertebrate and particularly mammalian ion channels represent the apex of evolutionary complexity and diversity and much effort has been focused on understanding their function. However, the recent availability of cheap high throughput genome sequencing has massively broadened and deepened the quality of information across phylogeny and is radically changing this view. Here we review current knowledge on such channels in key marine invertebrates where physiological evidence is backed up by molecular sequences and expression/functional studies. As marine invertebrates represent a much greater range of phyla than terrestrial vertebrates and invertebrates together, we argue that these animals represent a highly divergent, though relatively underused source of channel novelty. As ion channels are exquisitely selective sensors for voltage and ligands, their potential and actual applications in biotechnology are manifold.

John Eccles' studies of spinal cord presynaptic inhibition

Presynaptic inhibition is one of many areas of neurophysiology in which Sir John Eccles did pioneering work. Frank and Fuortes first described presynaptic inhibition in 1957. Subsequently, Eccles and his colleagues characterized the process more fully and showed its relationship to primary afferent depolarization. Eccles' studies emphasized presynaptic inhibition of the group Ia monosynaptic reflex pathway but also included group Ib, II and cutaneous afferent pathways, and the dorsal column nuclei. Presynaptic inhibition of the group Ia afferent pathway was demonstrated by depression of monosynaptic excitatory postsynaptic potentials and inhibition of monosynaptic reflex discharges. Primary afferent depolarization was investigated by recordings of dorsal root potentials, dorsal root reflexes, cord dorsum and spinal cord field potentials, and tests of the excitability of primary afferent terminals. Primary afferent depolarization was proposed to result in presynaptic inhibition by reducing the amplitude of the action potential as it invades presynaptic terminals. This resulted in less calcium influx and, therefore, less transmitter release. Presynaptic inhibition and primary afferent depolarization could be blocked by antagonists of GABA(A) receptors, implying a role of interneurons that release gamma aminobutyric acid in the inhibitory circuit. The reason why afferent terminals were depolarized was later explained by a high intracellular concentration of Cl(-) ions in primary sensory neurons. Activation of GABA(A) receptors opens Cl(-) channels, and Cl(-) efflux results in depolarization. Another proposed mechanism of depolarization was an increase in extracellular concentration of K(+) following neural activity. Eccles' work on presynaptic inhibition has since been extended in a variety of ways.

Identifying hallmarks of consciousness in non-mammalian species

Most early studies of consciousness have focused on human subjects. This is understandable, given that humans are capable of reporting accurately the events they experience through language or by way of other kinds of voluntary response. As researchers turn their attention to other animals, "accurate report" methodologies become increasingly difficult to apply. Alternative strategies for amassing evidence for consciousness in non-human species include searching for evolutionary homologies in anatomical substrates and measurement of physiological correlates of conscious states. In addition, creative means must be developed for eliciting behaviors consistent with consciousness. In this paper, we explore whether necessary conditions for consciousness can be established for species as disparate as birds and cephalopods. We conclude that a strong case can be made for avian species and that the case for cephalopods remains open. Nonetheless, a consistent effort should yield new means for interpreting animal behavior.

Synaptic transfer of dynamic motion information between identified neurons in the visual system of the blowfly

Synaptic transmission is usually studied in vitro with electrical stimulation replacing the natural input of the system. In contrast, we analyzed in vivo transfer of visual motion information from graded-potential presynaptic to spiking postsynaptic neurons in the fly. Motion in the null direction leads to hyperpolarization of the presynaptic neuron but does not much influence the postsynaptic cell, because its firing rate is already low during rest, giving only little scope for further reductions. In contrast, preferred-direction motion leads to presynaptic depolarizations and increases the postsynaptic spike rate. Signal transfer to the postsynaptic cell is linear and reliable for presynaptic graded membrane potential fluctuations of up to approximately 10 Hz. This frequency range covers the dynamic range of velocities that is encoded with a high gain by visual motion-sensitive neurons. Hence, information about preferred-direction motion is transmitted largely undistorted ensuring a consistent dependency of neuronal signals on stimulus parameters, such as motion velocity. Postsynaptic spikes are often elicited by rapid presynaptic spike-like depolarizations which superimpose the graded membrane potential. Although the timing of most of these spike-like depolarizations is set by noise and not by the motion stimulus, it is preserved at the synapse with millisecond precision.

Ricardo Miledi and the calcium hypothesis of neurotransmitter release

Ricardo Miledi has made significant contributions to our basic understanding of how synapses work. Here I discuss aspects of Miledi's research that helped to establish the requirement of presynaptic calcium for neurotransmitter release, from his earliest scientific studies to his classic experiments in the squid giant synapse.

Properties of synchronous and asynchronous release during pulse train depression in cultured hippocampal neurons

Neurotransmitter release displays at least two kinetically distinct components in response to a single action potential. The majority of release occurs synchronously with action-potential-triggered Ca(2+) influx; however, delayed release--also called asynchronous release--persists for tens of milliseconds following the peak Ca(2+) transient. In response to trains of action potentials, synchronous release eventually declines, whereas asynchronous release often progressively increases, an effect that is primarily attributed to the buildup of intracellular Ca(2+) during repetitive stimulation. The precise relationship between synchronous and asynchronous release remains unclear at central synapses. To gain better insight into the mechanisms that regulate neurotransmitter release, we systematically characterized the two components of release during repetitive stimulation at excitatory autaptic hippocampal synapses formed in culture. Manipulations that increase the Ca(2+) influx triggered by an action potential--elevation of extracellular Ca(2+) or bath application of tetraethylammonium (TEA)--accelerated the progressive decrease in synchronous release (peak excitatory postsynaptic current amplitude) and concomitantly increased asynchronous release. When intracellular Ca(2+) was buffered by extracellular application of EGTA-AM, initial depression of synchronous release was equal to or greater than control; however, it quickly reached a plateau without further depression. In contrast, asynchronous release was largely abolished in EGTA-AM. The total charge transfer following each pulse--accounting for both synchronous and asynchronous release--reached a steady-state level that was similar between control and EGTA-AM. A portion of the decreased synchronous release in control conditions therefore was matched by a higher level of asynchronous release. We also examined the relative changes in synchronous and asynchronous release during repetitive stimulation under conditions that highly favor asynchronous release by substituting extracellular Ca(2+) with Sr(2+). Initially, asynchronous release was twofold greater in Sr(2+). By the end of the train, the difference was approximately 50%; consequently, the total release per pulse during the plateau phase was slightly larger in Sr(2+) compared with Ca(2+). We thus conclude that while asynchronous release--like synchronous release--is limited by vesicle availability, it may be able to access a slightly larger subset of the readily releasable pool. Our results are consistent with the view that during repetitive stimulation, the elevation of asynchronous release depletes the vesicles immediately available for release, resulting in depression of synchronous release. This implies that both forms of release share a small pool of immediately releasable vesicles, which is being constantly depleted and refilled during repetitive stimulation.

Electrophysiological determination of the site involved in generating abnormal muscle responses in hemifacial spasm

In patients with hemifacial spasm (HFS), abnormal muscle responses due to abnormal cross-transmission are observed in facial muscles. However, the site in the facial nerve responsible for the cross-transmission remains a matter of controversy. We have developed a model in which by considering the electrophysiological parameters involved in producing the abnormal muscle response, we can determine the site of the abnormal cross-transmission within the facial nerve. This model was applied to HFS patients with three different etiologies: idiopathic, post-Bell's palsy, and post-XII-VII anastomosis. Our data show that: in idiopathic HFS, the cross-transmission may occur in the facial nerve at the level of the pontocerebellar angle; in post-Bell's palsy, it is inside the petrous bone; and in XII-VII anastomosis, it must be in the extracranial part of the facial nerve. The possible mechanisms for this cross talk are discussed in terms of ephaptic transmission or of a central hyperexcitability in the facial motor nucleus.

Effets des anesthésiques intraveineux sur les neurones du système nerveux central : mécanismes d'action cellulaires et moléculaires

The mechanisms of action of intravenous anaesthetics are not yet completely elucidated. Until recently, most of the studies had focused on the interactions between anaesthetics and lipid bilayers. It has been proposed that loss of consciousness is produced by disorganization of the lipid phase of nerve membranes, which impairs the action potential propagation. However, new data obtained with sophisticated neuropharmacological tools such as the patch clamp technique have recently contributed to challenge this hypothesis. Indeed, several lines of evidence suggest that intravenous anaesthetics are thought to induce loss of consciousness by blocking the excitatory synaptic transmission. This can be achieved presynaptically, by inhibiting glutamate release from nerve endings via alterations in the gating properties of voltage-dependent calcium channels. Blockade of excitatory synaptic transmission can also occur at the postsynaptic level by antagonizing the glutamate receptors of the N-methyl D-aspartate subtype. Some anaesthetic agents including ketamine also block the nicotinic receptors, however the relevance of this finding with respect to clinical anaesthesia requires further investigation. Preliminary data also suggest that propofol and etomidate elicit uncoupling of gap junctions between astrocytes, which represent a major nonneuronal cell population in the central nervous system. This phenomenon might indirectly contribute to the hypnotic action of these compounds. Whether loss of consciousness involves preferential target structures within the brain remains to be delineated. A better understanding of the mechanisms of action of general anaesthetics might contribute to generate new agents with more pharmacological selectivity and less undesirable side-effects.

Reduction of the voltage-dependent calcium current in Aplysia neurons by pentobarbital

Effects of pentobarbital on the calcium current of Aplysia neurons were investigated under current- and voltage-clamp conditions using the conventional two-microelectrode technique. Pentobarbital attenuated the progressive broadening of repeated action potentials of somata, suggesting a reduction in the calcium current. When calcium ion was replaced with barium ion in the perfusing solution, in which neither sodium nor potassium ions carried transmembrane currents, the barium current (IBa) which flowed through the calcium channel of the cell membrane was generated by depolarizing pulses of several hundred milliseconds applied every 1 min from a holding potential of -50 mV. The IBa was not affected by tetrodotoxin (30 microM). The current was decreased by pentobarbital (0.1-5 mM) in a dose-dependent manner. The inhibition was much greater at a lower pH of the perfusate, indicating that the uncharged form of the agent was responsible. The voltage-dependent inactivation of the IBa proceeded with two time constants [190 +/- 21 and 2020 +/- 146 msec (N = 4) at -10 mV], both of which were shortened by adding 1 mM pentobarbital [to 120 +/- 18 and 540 +/- 51 msec (N = 4), respectively]. The IBa recovered from the inactivation with two time constants [60 +/- 7 and 871 +/- 76 msec (N = 3) at -50 mV]. The anesthetic (1 mM) prolonged both of them, to 124 +/- 20 and 1480 +/- 172 msec (N = 3), respectively, resulting in a use-dependent depression of the current at 2-Hz stimulation. Pentobarbital reduced the IBa to a greater extent when the holding potential was more positive (-30 instead of -50 mV), indicating a higher affinity of the drug to the inactivated state of the channel. These findings suggest that the attenuation of the progressive broadening of successive spikes by pentobarbital is due to a decrease in the voltage- and time-dependent calcium current, ending in depression of transmitter release from the nerve terminal.

Effects of acetylcholine and cholinolytics on the synaptic transmission in the ampullae of Lorenzini of the skate

1. The effects of ACh, D-tubocuraine and atropine on the electrical activity of the ampullae of Lorenzini of the skate were studied. 2. ACh (10(-5)M) caused an increase in frequency of afferent discharges in all receptors under study. 3. When excitability was abolished by high Mg2+ solution (50 mM) perfusion with 50 mM Mg2+ plus ACh did not induce firing in the afferent fibres. 4. Cholinergic antagonists (D-tub and atr) did not block the synaptic transmission. 5. The results suggest that ACh is unlikely to be the synaptic transmitter in the ampullae of Lorenzini.

Adenosine receptor agonists inhibit K+-evoked Ca2+ uptake by rat brain cortical synaptosomes

The uptake of Ca2+ by a K+-depolarized rat brain cerebral cortical crude synaptosomal preparation (P2 fraction) was investigated. The characteristics of the Ca2+ uptake system are similar to those observed by other investigators. The preparation is also a suitable model with which to study the effects of adenosine on Ca2+ uptake and neurotransmitter release, as it is generally accepted that K+-evoked Ca2+ uptake is intimately related to depolarization-induced release of neurotransmitters. We have demonstrated that an extracellular receptor is involved in mediating the adenosine-evoked inhibition of K+-evoked Ca2+ uptake. The pharmacological properties of the receptor suggest that it may be similar in some respects to the A2-receptor associated with adenylate cyclase. The adenosine uptake inhibitor, dipyridamole, potentiated the action of adenosine, suggesting that re-uptake is important in controlling the extracellular adenosine concentration and thus in the regulation of the adenosine receptor. The adenosine receptor antagonist theophylline inhibited the effects of adenosine. Calmodulin inhibited K+-evoked uptake of Ca2+ by the synaptosomal fraction.

The actions of pentobarbitone, procaine and tetrodotoxin on synaptic transmission in the olfactory cortex of the guinea-pig

1 It has been suggested that the depression of excitatory synaptic potentials produced by general anaesthetics can be attributed to a partial blockade of impulse conduction in the terminal branches of axons. This hypothesis has been tested by comparing the actions of pentobarbitone, procaine and tetrodotoxin (TTX) on synaptic transmission in the guinea-pig olfactory cortex. 2 Pentobarbitone (0.1-0.3mM) depressed the evoked synaptic potentials without any significant depression of impulse conduction in the afferent fibres of the lateral olfactory tract (1.o.t). It had no effect on the electrical excitability of either the l.o.t axons or the postsynaptic neurones. 3 Tetrodotoxin (TTX; 1-5x10(-8 M) slowed conduction of impulses in the l.o.t. and decreased the amplitude of the l.o.t compound action potential in proportion to the concentration applied. All concentrations of TTX elevated the electrical threshold of the l.o.t. axons and there was evidence to suggest that the threshold of the postsynaptic neurones was also elevated. The synaptic potentials were depressed in direct proportion to the depression of the l.o.t. compound action potential. 4 Procaine (0.1-0.5 mM) exhibited a pattern of activity intermediate between pentobarbitone and TTX. The most marked effect, seen at all concentrations tested, was a slowing of impulse conduction and a decrease in the electrical excitability of the l.o.t. axons. 5 It is concluded that general anaesthetics (exemplified by pentobarbitone) depress synaptic transmission by interfering with the processes involved in chemical transmission and not by blocking impulse conduction in the terminal branches of afferent nerves.

Pathways and polarities of synaptic interactions in the inner retina of the mudpuppy: II. Insight revealed by an analysis of latency and threshold

Intracellular recording experiments in the mudpuppy (Necturus maculosus) have demonstrated that depolarizing bipolars show a one-half to one log unit higher threshold to light stimuli and a longer latency when compared to hyperpolarizing bipolars. This threshold difference cannot be ascribed to differences in rod and cone connections. Experimentally it was possible to use these differences to evaluate postbipolar cell connections; such experiments support the idea that some neurons are connected to one or the other bipolar cell type, while on-off cells receive input from both.

Serial synapses in Aplysia

Serial synapses occur between small profiles in the neuropil of Aplysia abdominal ganglion. Material was fixed in phosphate buffered OsO4, embedded in epon, and sections were stained with uranyl acetate and lead citrate. A class of synapses had the following characteristics: (1) synaptic vesicles clustered against the presynaptic membrane, (2) a widened extracellular space of about 20 nm containing electron-dense material, (3) straightening of the pre- and postsynaptic membranes, and (4) no postsynaptic membrane specialization. Some density between the presynaptic membrane and the adjacent synaptic vesicles was occasionally observed. Synapses occurred between small profiles in the neuropil (typical profile diameters were 1--3 micrometer). In this sample of approximately 100 synapses, four serial synapses were identified. The serial synaptic profiles were all small. In addition to the finding of serial synapses, 40% of the postsynaptic profiles contained vesicles similar to the synaptic vesicles seen in presynaptic profiles. Serial synapses may be the anatomical substrate of presynaptic inhibition and facilitation and of dishabituation.

Suppression of transmitter release at the neuromuscular junction

Tetrodotoxin and tetraethylammonium were used on frog and rat endplates, to inhibit voltage-activated sodium and potassium currents. Under these conditions, large endplate potentials can be evoked by applying super-threshold depolarizing current pulses (of 50-100 ms duration) to the motor nerve endings. When the current intensity is increased several­ fold above threshold, the response is suppressed during the current flow, and delayed until the pulse is terminated. This phenomenon is similar to and complements earlier observations on the squid giant synapse.