The inhibitory effects of dantrolene on action potential-induced calcium transients in cultured rat dorsal root ganglion neurons

We investigated the actions of dantrolene Ca(2+)-induced on Ca(2+)-release (CICR) evoked by action potentials in cultured rat sensory neurons. The effect of dantrolene on action potential after-depolarization and voltage-activated calcium currents was studied in cultured neonatal rat dorsal root ganglion cells (DRG) using the whole-cell patch-clamp technique. Depolarizing current injection evoked action potentials and depolarizing after-potentials, which are activated as a result of CICR following a single action potential in some cells. The type of after-potentials was determined by inducing action potentials from the resting membrane potential. Extracellular application of dantrolene (10 microM) abolished after-depolarizations without affecting action potential properties. Furthermore, dantrolene significantly reduced repetitive action potentials after depolarizing current injection into these neurons, but had no significant effect on the steady-state current voltage relationship of calcium currents in these neurons. We conclude that dantrolene inhibits the induction of action potential after depolarizations by inhibiting CICR in cultured rat sensory neurons.     

Classification of vestibulo-spinal neurons of the cat Deiters' nucleus

Antidromic excitation of neurons of the lateral vestibular nucleus of Deiters in cats in response to stimulation of the vestibulo-spinal tract in the cervical segments of the spinal cord was studied by intracellular microelectrode recording. Individual components of the antidromic action potential and accompanying after-potentials were analyzed and fast and slow neurons distinguished. The vestibulo-spinal neurons were differentiated on the basis of after-potentials accompanying the antidromic action potential. The ratio between fast and slow neurons differed in individual groups. The parameters of the depolarization after-potentials were directly proportional to the duration of the refractory period of the neurons studied. An attempt was made to correlate differences in the responsiveness of neurons with an identical conduction velocity along their axons with the characteristics of the depolarization after-potential.     

Electrophysiological features of facial motoneurons in cats

Antidromic activation of facial motoneurons in cats during stimulation of different branches of the facial nerve was studied by intracellular recording. Time and amplitude characteristics of individual components of the antidromic action potentials were analyzed and fast and slow after-potentials distinguished. Correlation was found between the duration of the descending phase of the SD spike, duration of its after-hyperpolarization, and the spike conduction time along the axon. Data were obtained to show absence of a recurrent collateral pathway in motoneurons of the facial nucleus. The functional significance of the after-potentials is discussed.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 10, No. 3, pp. 261–270, May–June, 1978.     

Properties of action potentials in Drosera tentacles

Summary Action potentials of Drosera tentacles resemble those of vertebrate peripheral nerves in that they appear to be comprised of relatively uniform spikes, variable shoulders or negative after-potentials, and variable positive after-potentials. The peaking of the spike corresponds to a period of great refractoriness, while action potentials of low amplitude may be fired readily during the negative after-potential. The action potentials fired during the negative after-potential appear to be unlike those of peripheral nerves in that they are of abnormally brief duration. Also apparently different from the case in peripheral nerves is the dependence of the duration of an action potential on the interval separating it from the preceding action potential.Action potentials propagate from the neck of the stalk to its base at about 5 mm s^-1 at room temperature. Propagation may be reversed artificially, consistent with the possibility that the neuroid cells are electrically coupled.     

After-currents,after-potentials,excitability, and ventral root electrotonus in spinal motoneurons

The spinal cord constitutes a volume conductor. Potential changes are recorded therefrom only as current flows. During the period of the after-potentials current flows in significant density only if the after-polarization differs at different points of the active neurons. Thus one does not record after-potentials in volume; one may record after-currents which are defined as the resultants of differences in after-potentials. Measurable excitability change during the period of the after-potentials, in the event no current flows, might be regarded as approximating the change of intrinsic polarization status at the region tested. In the presence of after-current flow excitability change would approximate the sum of intrinsic change and extrinsic change due to current flow. In giving rise by differences to current flow after-potentials come to act as agents, and events in one part of a neuron help to determine excitability in other parts. Since the intramedullary after-current flow is not the after-potential of the soma, it follows that ventral root electrotonus which results from axonal after-current flow cannot be considered the counterpart of somatic after-potential. Following conduction of an antidromic volley after-current flows between somata and axons. According to the signs of the recorded potential changes, after-current flow initially, and for approximately 45 msec., is in the direction from somata to axons. Thereafter, and for approximately another 75 msec., the direction of flow is reversed. During the period of after-current flow following antidromic conduction the excitability of neighboring motoneurons is altered in a manner that reproduces the phases of after-current flow. The initial phase, depression, was first described by Renshaw. The after-potentials of ventral root fibers have been studied. In a single action and in usual form, they consist of a negative after-potential of considerable magnitude and of some 35 msec. duration, and a positive after-potential detectable for approximately 120 msec. Variants and the influence of temperature change are described. The recovery cycle of ventral root axons in general compares with the after-potential cycle. Recovery of intramedullary motor axons differs from that of their extramedullary projections as ventral root fibers in a manner that is accountable to intramedullary flow of after-current. Since the intrinsic recovery process of the motoneuron somata cannot be measured in the presence of current flow it must be estimated by correcting the observed recovery for the influence of known current flows. When this is done the resultant in simplest form provides for intrinsic somatic recovery from refractoriness through a single phase of subnormality lasting some 60 msec. Conditions for the relatively undistorted recording of antidromic ventral root electrotonus are described. They include provisions that the proximal ventral root electrode must be within 12 mm. of the root-cord junction and that the distal electrode must be located in excess of 30 mm. from the distal severed end of the ventral root. Antidromic ventral root electrotonus is a counterpart of the current flows in the intramedullary stretch of the axons. Initially, during the phase of metadromal postivity of the intramedullary axons, electrotonus is negative. During the period of deflections Sp-An, that signify after-current flow into the axons, electrotonus is positive. Finally during the period of deflections Sn-Ap, that signify after-current flow outwards through the intramedullary axon membranes, electrotonus is negative. Electrotonic showing is not of sufficient magnitude to make the time course of ventral root electrotonus palpably different from that of the generating intramedullary currents.     

AHP's, HAP's and DAP's: How Potassium Currents Regulate the Excitability of Rat Supraoptic Neurones

We have constructed mathematical models of the electrical activity of two hypothalamic supraoptic neuro-secretory cell-types, and we support our models with new calcium imaging and in vitro electrophysiological data. These cells are neurones that project to the pituitary gland and secrete either of two hormones, oxytocin or vasopressin, into the blood from their axonal terminals. Oxytocin-secreting and vasopressin-secreting cells are closely related and physically they differ only subtly, however when physiologically stressed their discharge patterns are dramatically distinct. We first show how each potassium current contributes to the action-potentials and after-potentials observed in these cells, and we show how these after-potentials are correlated to intra-cellular calcium elevations. We then show how these currents regulate the excitability of these cells and consequently shape their discharge pattern.     

After-Potentials and Large Depolarizations of Single Nodes of Ranvier Treated with Veratridine

Potential differences between normal nodes of Ranvier (single fiber from the sciatic nerve of the frog, air-gap method) and a node exposed to 1 to 2.5 x 10^-6 gm veratridine per ml were measured. Negative after-potentials occurred immediately after application of the alkaloid when spike configuration and resting potential were virtually unchanged. The after-potentials decreased in magnitude and their time constant increased as the resting membrane was depolarized either by outward currents or by a train of impulses. Increase of (Na)_o markedly increased the amplitude of the after-potential. After prolonged application of veratridine or with higher concentrations, a large slow depolarization (rate of potential change about 7 mv per second) could be triggered by a train of impulses or even a single spike. This depolarization could promptly be terminated by withdrawing Na. It is concluded that, once the nodal membrane has become permeable to Na (as during a spike), veratridine prevents the normal return of P_Na to its resting value.     

Pyrethroid insecticides: Actions of deltamethrin and related compounds on insect axonal sodium channels

The actions of deltamethrin and eight other pyrethroids were tested on isolated giant axons of the cockroach Periplaneta americana, using microelectrode and oil-gap, single-fibre electrophysiological recording techniques. Deltamethrin at micromolar concentrations induced a slow progressive depolarization of the axon membrane accompanied by a gradual reduction in action potential amplitude. The deltamethrin-induced depolarization was enhanced by an increase in stimulation frequency and was reduced in the presence of the sodium channel blocking agent saxitoxin (1 × 10^?7 M).Other synthetic pyrethroids (biopermethrin and its 1S enantiomer, biotetramethrin, s-bioallethrin, bioresmethrin and its 1S enantiomer, cismethrin and kadethrin) were also studied. In contrast to the findings with deltamethrin all other compounds, apart from the 1S isomers which were inactive, induced prolonged negative (depolarizing) after-potentials. Deltamethrin appears to affect a small fraction of sodium channels which are held in a modified open-state, whereas the pyrethroids which generate large negative after-potentials appear to induce a brief alteration of the open-state sodium channels with a larger number of channels affected. Differences between the actions of pyrethroids on insect axonal sodium channels and whole insects are discussed.     

Electrical phenomena in nerve; squid giant axon

The action of a number of agents, which may be classified as "stabilizers" and "unstabilizers" on the electrical oscillations and after-potentials in the squid giant axon has been examined. The effects on the spike, "positive overshoot," and "potassium potential" were also observed, but where possible concentrations were employed which left these phenomena unaltered. Veratrine augmented the oscillations and the negative after-potential, particularly with repetitive stimulation. Yohimbine caused a small long lasting positive after-potential and depressed the oscillations, effects also enhanced with repetitive activity. Cocaine and procaine suppressed the oscillations and the negative after-potential but DDT was completely inert. An elevation in the medium calcium depressed the oscillations and the naturally occurring negative after-potential; negative after-potentials induced with veratrine were increased by calcium. A decrease in the potassium augmented the oscillations and the negative after-potential. A hypothesis is presented in which these effects are interpreted in terms of potassium concentration at the fiber surface as regulated by a labile permeability and metabolism. This is discussed in relation to the available evidence for these factors. It is a pleasure to acknowledge the author's indebtedness to Dr. D. E. S. Brown, Director, and to his staff at the Bermuda Biological Station for Research for the cooperation and special facilities provided during the initiation of this work. Dr. T. Baylor of Princeton University very kindly provided the camera and film used in Bermuda.     

The influence of high hydrostatic pressure on cocaine and veratrine action in a vertebrate nerve

The application of high hydrostatic pressure to toad sciatic nerve causes a gain in sodium and a loss of potassium which are not affected by cocaine. However, cocaine action is enhanced by high pressure when counteracting veratrine depolarization and when blocking the action potential. Various effects of elevated pressure on the after-potentials are presented and the role of ions in these processes is discussed.     

The nervous system ofNectonema munidae andGordius aquaticus,with implications for the ground pattern of the Nematomorpha

 The nervous system of Nectonema munida is shown to be composed of a brain, a ventral nerve cord with an anterior and a posterior enlargement, a dorsal nerve cord and a plexus-like basiepidermal nervous system. The ultrastructure of these parts is given. Additionally, the ventral nerve cord of Gordius aquaticus is ultrastructurally described. The results are compared with the literature to work out the ground pattern of the Nematomorpha according to the nervous system. This contains a circumpharyngeal brain with a main subpharyngeal portion and a weak suprapharyngeal portion, a ventral and dorsal intraepidermal nerve cord and a peripheral nervous system. The ground pattern of the nervous system of Nematomorpha is then compared to that of other Nemathelminthes. The form of the brain and the distribution of perikarya are derived characters of the Nematomorpha. The existence of an unpaired ventral and an unpaired dorsal nerve cord and the position of these two cords in epidermal cords are synapomorphies of the Nematomorpha and the Nematoda. Accepted: 7 July 1996     

The postspike positivity of unmedullated fibers of dorsal root origin

The positivity following the spike in the action potential of unmedullated nerve fibers of dorsal root origin (d.r.C) has been shown to be homologous with the first positive potential (P₁) of other varieties of nerve fibers. Thus it is only through the large size of the positivity that this group of nerve fibers is set apart from other groups. New findings accentuate and make more explicit the difference of d.r.C fiber behavior from that of the sympathetic unmedullated fibers. Support of the conclusion is derived from re-examination of the A fibers as well as from observations on the d.r.C fibers. Increase in size of the P₁'s in a tetanus of the d.r.C fibers can occur if the frequency is high enough; and it does not occur in an A fiber tetanus if the frequency is low enough. Frequency is also critical in the obtainment of increasing P₁'s in a tetanus of sympathetic C fibers. Decrease in the size of the P₁'s in the course of a tetanus is attributable to development of the negative after-potential (N a-p). In rested d.r.C fibers the N a-p is latent. But it appears during a tetanus, develops in size, and after the tetanus leads to a long lasting and clearly defined second positive potential. Absence of a supernormal period during the N a-p of the d.r.C fibers is accounted for. An analysis is made of the apparent increase in size of the spike elevations during a tetanus, for the two subgroups of the C fibers. The difference between the after-potentials of A fibers and of sympathetic C fibers is correlated with the shapes of the curves of cathodal electrotonus of these fibers.     

A wide-acting peptidergic interneurone in the snail Helix pomatia : Graded recruitment and local firing of terminal processes

Simultaneous pre- and postsynaptic intracellular recordings were used to study the mechanism of presynaptic terminal recruitment in a multifunction interneurone in the snail Helix pomatia. The interneurone was presynaptic to at least 20 neurones. The synaptic efficiency was correlated with the presence of presynaptic depolarizing after-potentials (DAPs) electrotonically produced by the delayed firing of remote terminal processes. These processes have large swellings filled with neurosecretory vesicles. The terminals were recruited in a graded manner when the interneurone was fired with a prolonged current. The terminal recruitment was enhanced by stimulating various efferent nerves, which presumably activated presynaptic receptors for dopamine. A few animals (three out of 300) had two electrically coupled interneurones. Simultaneous recordings from both cells showed that the terminals could be fired independently of the soma-axon activity. The graded and local firing of the presynaptic terminals was attributed to the electrical load that the large boutons exert on electrotonically spreading presynaptic impulses.     

Phosphorylation and dephosphorylation of distinct isoforms of the heavy neurofilament protein NF-H

Summary 1. Previous immunohistochemical studies led to the suggestion that distinctly phosphorylated neurofilament isoforms exist in different types of neurons. We have recently examined this hypothesis by direct biochemical experiments, which revealed that the heavy neurofilament protein NF-H of bovine ventral root cholinergic neurons is more acidic and markedly more phosphorylated than that of bovine dorsal root neurons.2. In the present study we employed this system to study the degree to which distinctly phosphorylated NF-H isoforms differ in the extents to which they can be phosphorylated and dephosphorylatedin vitro. This was performed utilizing alkaline phosphatase and protein kinase PK40^ERK, which is specific to serines of Lys-Ser-Pro (KSP) repeats. The results obtained reveal that:3. The more extensively phosphorylated ventral root NF-H is dephosphorylated more rapidly than dorsal root NF-H.4. Ventral root NF-H and dorsal root NF-H in their native form are both poor substrates of PK40^ERK.5. Following dephosphorylation, ventral root and dorsal root NF-H are phosphorylated extensively and differentially by this kinase. Under these conditions, PK40^ERK catalyzes the incorporation of, respectively, 4.2±1.3 and 2.8±0.6 mol of phosphate per molecule of ventral root NF-H and dorsal root NF-H. The ratio of phosphates incorporated into ventral root NF-H to those incorporated into dorsal root NF-H is 1.46±0.17.6. These findings support the hypothesis that different classes of neurons contain distinctly phosphorylated neurofilaments and show that ventral root and dorsal root neurons are a useful model system for studying the distinct characteristics of neurofilament phosphorylation in different types of neurons.     

Synaptic mechanisms of spike generation in bursts by neurons of the carp tectum and olfactory bulb

Synaptic mechanisms of burst activity generation in certain neurons of the tectum opticum and mechanisms of generation of stimulation-induced group discharges by certain secondary neurons of the olfactory bulb were analyzed in carp (Cyprinus carpio L.). Spikes of the spontaneous discharge in neurons of the tectum were accompanied by depolarizing after-potentials, which caused the burst discharges of these cells. Evidence is given in support of the synaptic nature of the after-potential; it is suggested that it is generated by a recurrent collateral mechanism. Synaptic bombardment causing the appearance of a group discharge in olfactory bulb neurons and groups of spikes in their spontaneous activity was found to be intermittent in character. These features of unit activity in the olfactory bulb are shown to be connected with the presence of excitatory synaptic interaction between several neurons, probably dendro-dendritic in nature.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiay, Vol. 14, No. 5, pp. 483–490, September–October, 1982.     

The role of TGF beta signaling in the formation of the dorsal nervous system is conserved between Drosophila and chordates

Transforming growth factor beta signaling mediated by Decapentaplegic and Screw is known to be involved in defining the border of the ventral neurogenic region in the fruitfly. A second phase of Decapentaplegic signaling occurs in a broad dorsal ectodermal region. Here, we show that the dorsolateral peripheral nervous system forms within the region where this second phase of signaling occurs. Decapentaplegic activity is required for development of many of the dorsal and lateral peripheral nervous system neurons. Double mutant analysis of the Decapentaplegic signaling mediator Schnurri and the inhibitor Brinker indicates that formation of these neurons requires Decapentaplegic signaling, and their absence in the mutant is mediated by a counteracting repression by Brinker. Interestingly, the ventral peripheral neurons that form outside the Decapentaplegic signaling domain depend on Brinker to develop. The role of Decapentaplegic signaling on dorsal and lateral peripheral neurons is strikingly similar to the known role of Transforming growth factor beta signaling in specifying dorsal cell fates of the lateral (later dorsal) nervous system in chordates (Halocythia, zebrafish, Xenopus, chicken and mouse). It points to an evolutionarily conserved mechanism specifying dorsal cell fates in the nervous system of both protostomes and deuterostomes.     

Planar and vertical induction of anteroposterior pattern during the development of the amphibian central nervous system

In amphibians and other vertebrates, neural development is induced in the ectoderm by signals coming from the dorsal mesoderm during gastrulation. Classical embryological results indicated that these signals follow a “vertical” path, from the involuted dorsal mesoderm to the overlying ectoderm. Recent work with the frog Xenopus laevis, however, has revealed the existence of “planar” neural-inducing signals, which pass within the continuous sheet or plane of tissue formed by the dorsal mesoderm and presumptive neurectoderm. Much of this work has made use of Keller explants, in which dorsal mesoderm and ectoderm are cultured in a planar configuration with contact along only a single edge, and vertical contact is prevented. Planar signals can induce the full anteroposterior (A-P) extent of neural pattern, as evidenced in Keller explants by the expression of genes that mark specific positions along the A-P axis. In this review, classical and modern molecular work on vertical and planar inductionwill be discussed. This will be followed by a discussion of various models for vertical induction and planar induction. It has been proposed that the A-P pattern in the nervous system is derived from a parallel pattern of inducers in the dorsal mesoderm which is “imprinted” vertically onto the overlying ectoderm. Since it is now known that planar signals can also induce A-P neural pattern, this kind of model must be reassessed. The study of planar induction of A-P pattern in Xenopus embryos provides a simple, manipulable, two-dimensional system in which to investigate pattern formation. © 1993 John Wiley & Sons, Inc.     

Ultrastructure and immunocytochemistry of the nervous system of the larvae ofLingula anatina andGlottidia sp. (Brachiopoda)

Summary Planktotrophic brachiopod larvae ofGlottidia sp. have been investigated for the occurrence of glyoxylic acid induced fluorescence in catecholamines (CA), and serotonin-like (5-HT) and neuropeptide FMRFamidelike (FMRFamide) immunoreactivity (ir). The location of CA, 5-HT-ir and FMRFamide-ir cells and processes were compared with the location of neurons and nerve processes found by transmission electron microscopy. The apical ganglion contains 5-HT-ir and FMRFamideir cells and processes and CA processes. From the dorsal part of the apical ganglion extend dorsal 5-HT-ir and FMRFamide-ir processes; from the nine pairs of tentacles stage (9. pt) they project to the ventral ganglion. These dorsal lophophore processes follow themusculus lophophoralis and them. brachialis. The 5-HT-ir and some of the FMRFamide-ir processes project along the muscles to the tentacles. From the ventral part of the apical ganglion extend CA, 5-HT-ir and FMRFamide-ir processes which follow the ciliary band of the lophophore and project to the tentacles. An intense band of CA processes was also observed in the lophophore, but the dorsal/ventral location could not be ascertained. The ventral ganglion contains 5-HT-ir and FMRFamide-ir cells which project either caudally on the metasome or rostrally as part of the dorsal lophophore processes. The neuropil of the ventral ganglion contains CA, 5-HT-ir and FMRFamide-ir processes. The nervous system of the planktotrophic brachiopod larvae seems to consist of a ventral lophophore system innervating the ciliary bands and a dorsal lophophore system including the ventral ganglion innervating the body musculature. The latter system develops later in ontogeny and is regarded as a specialization due to the presence of shells and associated musculature. The former system is regarded as homologous with the nervous system of actinotroch larvae (Phoronida) and planktotrophic larvae of the echinoderms.     

Termination of the afferent fiber in the spinal cord of the turtle Testudo horsfieldi and three-dimensional reconstruction of the sensory-motoneuron connection

Horseradish peroxidase (HRP) tracing methods and subsequent computer reconstruction were used to study the structural organization of sensory-motoneuron connections in turtles. HRP was applied through suction electrodes to thin dorsal and ventral root filaments of superfused isolated lumbar spinal cord of the turtle Testudo horsfieldi. Single motoneurons were labeled ionophoretically with intracellular glass microelectrodes. Labeled elements were examined under a light microscope. The Eutectic neuron tracing system and its associated program were used for three-dimensional reconstructions and morphometry. The distribution of afferent fibers of the dorsal root and their terminations were presented in a new scheme in which new zones, in addition to those that were already well known, were shown, including the following: in the Lissauer zone, motor nuclei, and ventrolateral funiculus, as well as in the contralateral medial gray matter (laminae IV–V). Unlike in frogs, the motoneuron dendritic field in turtles was restricted to an ellipsoid space with a short axis in the rostrocaudal direction (300–500 µm). The afferent fibers of the dorsal root connected to motoneurons produced very short branches in a restricted rostrocaudal direction (50–70 μm). One fiber collateral of the dorsal root had about 80 synapse-like enlargements (approximately tenfold fewer than in frogs). Putative sensory-motoneuron contacts were found on the I–VII-order dendritic segments of the dorsal and ventro-medial dendritic trees. It was shown that, in turtles, only one first-order collateral of the dorsal root fiber participated in the sensory-motoneuron connection with a small number (about 4) of putative contacts, which is also one order less than in frogs. It is likely that the simplification of the synapse structure in turtles is compensated by a higher efficiency of the signal transmission comparable to that in mammals.     

SALMFamide2 and serotonin immunoreactivity in the nervous system of some acoels (Xenacoelomorpha)

Acoel worms are simple, often microscopic animals with direct development, a multiciliated epidermis, a statocyst, and a digestive parenchyma instead of a gut epithelium. Morphological characters of acoels have been notoriously difficult to interpret due to their relative scarcity. The nervous system is one of the most accessible and widely used comparative features in acoels, which have a so‐called commissural brain without capsule and several major longitudinal neurite bundles. Here, we use the selective binding properties of a neuropeptide antibody raised in echinoderms (SALMFamide2, or S2), and a commercial antibody against serotonin (5‐HT) to provide additional characters of the acoel nervous system. We have prepared whole‐mount immunofluorescent stainings of three acoel species: Symsagittifera psammophila (Convolutidae), Aphanostoma pisae, and the model acoel Isodiametra pulchra (both Isodiametridae). The commissural brain of all three acoels is delimited anteriorly by the ventral anterior commissure, and posteriorly by the dorsal posterior commissure. The dorsal anterior commissure is situated between the ventral anterior commissure and the dorsal posterior commissure, while the statocyst lies between dorsal anterior and dorsal posterior commissure. S2 and serotonin do not co‐localise, and they follow similar patterns to each other within an animal. In particular, S2, but not 5‐HT, stains a prominent commissure posterior to the main (dorsal) posterior commissure. We have for the first time observed a closed posterior loop of the main neurite bundles in S. psammophila for both the amidergic and the serotonergic nervous system. In I. pulchra, the lateral neurite bundles also form a posterior loop in our serotonergic nervous system stainings.