Calcium and magnesium dependence of spontaneous and evoked afferent neural activity in the lateral-line organ of Xenopus laevis

1. Rates of spontaneous and mechanically evoked neural impulses from the Xenopus laevis lateral line were compared in the presence of calcium and/or magnesium or in the absence of either divalent cation, bath-applied to the synaptic side of the mechanoreceptors. 2. Spontaneous activity and total activity during mechanical stimulation decreased with increasing calcium or magnesium concentration. 3. Activity during stimulation minus spontaneous activity increased with increasing calcium concentration and decreased with increasing magnesium concentration. 4. At constant divalent cation concentration, increasing calcium/magnesium increased total stimulated activity minus spontaneous activity, while spontaneous activity remained essentially constant. 5. Membrane charge screening and mass action of divalent cations at voltage-dependent cation channels are hypothesized to account for the effects observed.     

Purinergic signaling in cochleovestibular hair cells and afferent neurons

Purinergic signaling in the mammalian cochleovestibular hair cells and afferent neurons is reviewed. The scope includes P2 and P1 receptors in the inner hair cells (IHCs) of the cochlea, the type I spiral ganglion neurons (SGNs) that convey auditory signals from IHCs, the vestibular hair cells (VHCs) in the vestibular end organs (macula in the otolith organs and crista in the semicircular canals), and the vestibular ganglion neurons (VGNs) that transmit postural and rotatory information from VHCs. Various subtypes of P2X ionotropic receptors are expressed in IHCs as well as P2Y metabotropic receptors that mobilize intracellular calcium. Their functional roles still remain speculative, but adenosine 5′-triphosphate (ATP) could regulate the spontaneous activity of the hair cells during development and the receptor potentials of mature hair cells during sound stimulation. In SGNs, P2Y metabotropic receptors activate a nonspecific cation conductance that is permeable to large cations as NMDG⁺ and TEA⁺. Remarkably, this depolarizing nonspecific conductance in SGNs can also be activated by other metabotropic processes evoked by acetylcholine and tachykinin. The molecular nature and the role of this depolarizing channel are unknown, but its electrophysiological properties suggest that it could lie within the transient receptor potential channel family and could regulate the firing properties of the afferent neurons. Studies on the vestibular partition (VHC and VGN) are sparse but have also shown the expression of P2X and P2Y receptors. There is still little evidence of functional P1 (adenosine) receptors in the afferent system of the inner ear.     

Effects of extracellular calcium on low frequency induced potentiation and habituation in the in vitro hippocampal slice preparation

The electrophysiological characteristics of frequency potentiation and habituation were investigated in two afferent systems of the in vitro hippocampal slice preparation. Low frequency stimulation (1 Hz) of the Schaffer collateral - commissural (Sch-comm) fibers results in a short-term potentiation of the amplitude and rate of rise of the EPSP and population spike responses recorded in the CA1 region. In contrast, 1-Hz stimulation of the perforant path (PP) evokes a short-term, habituation-like depression of the dentate granule cell EPSP and population spike. An inverse relationship was observed between stimulus intensity and the magnitude of frequency potentiation or habituation. Changes in afferent fiber volleys or general excitability of postsynaptic membranes did not contribute significantly to the development of either of these forms of short-term plasticity. Perfusion with a medium containing a high calcium - low magnesium concentration (4 mM Ca+2 and 1 mM Mg+2) produced a differential effect on CA1 and dentate evoked potentials. Following a 20-min exposure to this medium, the amplitude of CA1 potentials was increased while dentate responses were decreased. Frequency potentiation of CA1 responses and habituation of dentate responses were depressed or eliminated by the high calcium medium. The opposing influence of extracellular calcium on CA1 and dentate evoked potentials indicates a fundamental difference in the process of transmitter release in these systems, a characteristic that may contribute to the production of frequency potentiation and habituation.     

The pattern of lateral-line afferents in urodeles

Summary The organization of posterior and anterior afferents of the lateralline system was studied in several species of urodeles by means of transganglionic transport of horseradish peroxidase. The afferents of each lateral-line nerve form distinct fascicles in the medullary alar plate. Each of the two branches of the anterior lateral-line nerve is organized in two long and one short fascicles. The posterior lateral-line afferents form only two long fascicles. Each ordinary neuromast is supplied by only two afferents, which run in the two ventral medullary fiber bundles. It is suggested that afferents to hair cells displaying one type of polarity form together one bundle, but those contacting hair cells polarized in the opposite way form the second ventral bundle of one lateral-line branch. Thus, the lateral-line afferents may be organized in a directotopic fashion.The short dorsal fascicle formed only by the anterior lateral-line afferents receives fibers exclusively from small pit organs. Each pit organ is supplied by only one afferent. Anatomically, these pit organs resemble in many respects the electroreceptive ampullary organs of certain fish.Neurons labeled retrogradely via the anterior lateral-line nerve afferents have been attributed to the nervus trigeminus or facialis. In addition to the posterior lateral-line afferents, only few centrifugally projecting neurons were labeled. These neurons are discussed as efferents to the posterior lateral-line neuromasts.     

Spike-associated fast contraction of dendritic spines in cultured hippocampal neurons.

Dendritic spines have long been known to contain contractile elements and have recently been shown to express apparent spontaneous motility. Using high-resolution imaging of dendritic spines of green-fluorescent protein (GFP)-expressing, patch-clamped hippocampal neurons in dissociated culture, we find that bursts of action potentials, evoked by depolarizing current pulses, cause momentary contractions of dendritic spines. Blocking calcium currents with cobalt prevented these twitches. In additional experiments with neurons loaded via a micropipette with calcium-sensitive and insensitive dyes, spontaneous calcium transients were associated with a rapid contraction of the spine head. The spine twitch was prolonged by tetraethylammonium or bicuculline, which enhance calcium transients, and was blocked by the actin polymerization antagonist latrunculin-B. The spine twitch may be instrumental in modulating reactivity of the NMDA receptor to afferent stimulation, following back-propagating action potentials.     

Properties of synaptic transmission at the neuromuscular junction of the squid, Loligo opalescens

1. Spontaneous and evoked synaptic activity were recorded from the muscles of squid fin and mantle. These spontaneous synaptic potentials were large (up to 30 mV) and pleomorphic. Their amplitudes were not normally distributed, nor did they appear to be clustered in integral multiples of some "unit" event size. 2. Electrical stimulation of the nerve resulted in muscle twitches when the bath calcium concentration was a third normal or higher. The frequency of spontaneous synaptic events was unaffected by low calcium. 3. The large size of spontaneous events may mean that the synchronized release of only a few such "quanta" are sufficient to cause muscle action potentials and contraction. 4. The shapes of spontaneous events correlated poorly with their amplitudes, which is consistent with release from multiple synaptic sites with distinct properties.     

Efferent control of the electrical and mechanical properties of hair cells in the bullfrog's sacculus

Background

Hair cells in the auditory, vestibular, and lateral-line systems respond to mechanical stimulation and transmit information to afferent nerve fibers. The sensitivity of mechanoelectrical transduction is modulated by the efferent pathway, whose activity usually reduces the responsiveness of hair cells. The basis of this effect remains unknown.

Methodology and Principal Findings

We employed immunocytological, electrophysiological, and micromechanical approaches to characterize the anatomy of efferent innervation and the effect of efferent activity on the electrical and mechanical properties of hair cells in the bullfrog''s sacculus. We found that efferent fibers form extensive synaptic terminals on all macular and extramacular hair cells. Macular hair cells expressing the Ca²⁺-buffering protein calretinin contain half as many synaptic ribbons and are innervated by twice as many efferent terminals as calretinin-negative hair cells. Efferent activity elicits inhibitory postsynaptic potentials in hair cells and thus inhibits their electrical resonance. In hair cells that exhibit spiking activity, efferent stimulation suppresses the generation of action potentials. Finally, efferent activity triggers a displacement of the hair bundle''s resting position.

Conclusions and Significance

The hair cells of the bullfrog''s sacculus receive a rich efferent innervation with the heaviest projection to calretinin-containing cells. Stimulation of efferent axons desensitizes the hair cells and suppresses their spiking activity. Although efferent activation influences mechanoelectrical transduction, the mechanical effects on hair bundles are inconsistent.     

Cloning and expression of a calcium-activated chloride channel reveal a novel protein candidate

Purkinje neurons fire spontaneous action potentials at ~50 spikes/sec and generate more than 100 spikes/sec during cerebellum-mediated behaviors. Many voltage-gated channels, including Ca channels, can inactivate and/or facilitate with repeated stimulation, raising the question of how these channels respond to regular, rapid trains of depolarizations. To test whether Ca currents are modulated during firing, we recorded voltage-clamped Ca currents, predominantly carried by P-type Ca channels, from acutely dissociated mouse Purkinje neurons at 30-33{degree sign}C (1 mM Ca). With 0.5 mM intracellular EGTA, 1-second trains of either spontaneous action potential waveforms or brief depolarizing steps at 50 Hz evoked Ca tail currents that were stable, remaining within 5% of the first tail current throughout the train. Higher frequency trains (100 and 200 Hz) elicited a maximal inactivation of     

Afferent Neurons of the Zebrafish Lateral Line Are Strict Selectors of Hair-Cell Orientation

Hair cells in the inner ear display a characteristic polarization of their apical stereocilia across the plane of the sensory epithelium. This planar orientation allows coherent transduction of mechanical stimuli because the axis of morphological polarity of the stereocilia corresponds to the direction of excitability of the hair cells. Neuromasts of the lateral line in fishes and amphibians form two intermingled populations of hair cells oriented at 180° relative to each other, however, creating a stimulus-polarity ambiguity. Therefore, it is unknown how these animals resolve the vectorial component of a mechanical stimulus. Using genetic mosaics and live imaging in transgenic zebrafish to visualize hair cells and neurons at single-cell resolution, we show that lateral-line afferents can recognize the planar polarization of hair cells. Each neuron forms synapses with hair cells of identical orientation to divide the neuromast into functional planar-polarity compartments. We also show that afferent neurons are strict selectors of polarity that can re-establish synapses with identically oriented targets during hair-cell regeneration. Our results provide the anatomical bases for the physiological models of signal-polarity resolution by the lateral line.     

Pacemaker potentials are the physiologic basis of epileptiform activity in the buccal ganglia of Helix pomatia

Mechanisms of epileptic activity in nervous systems were studied using the identified neurons B1 through B4 in the buccal ganglia of the snail Helix pomatia as a model system. Activities were recorded with intracellular microelectrodes. Epileptiform activity was induced by bath application of an epileptogenic drug (pentylenetetrazol: 1 mM to 40 mM, or etomidate: 0.1 mM to 1.0 mM). Epileptiform potentials recorded from the somata of neurons consisted of paroxysmal depolarization shifts (PDSs). With increasing concentration of an epileptogenic drug, pacemaker potentials in neuron B3 developed into PDS. Simultaneously several types of chemical post-synaptic potentials were suppressed in amplitude. Since on the one hand epileptic seizures only appear when PDS are synchronized in many neurons and since on the other hand synaptic potentials were found to be suppressed during epileptic conditions, mechanisms underlying neuronal synchronization were studied. Evidence was found that, under epileptogenic conditions only, neurons were synchronized by an non-synaptic release of substances. Strong depolarizations accompanied by an increase in intracellular calcium concentration are known to induce an unspecific exocytosis. Thus, an unspecific exocytosis from the dendrites of PDS-generating neurons probably appears under epileptic conditions and synchronizes neighbouring neurons.     

Convergence of mechanosensory inputs onto neuromodulatory serotonergic neurons in the leech

By the frequency-dependent release of serotonin, Retzius neurons in the leech modulate diverse behavioral responses of the animal. However, little is known about how their firing pattern is produced. Here we have analyzed the effects of mechanical stimulation of the skin and intracellular stimulation of mechanosensory neurons on the electrical activity of Retzius neurons. We recorded the electrical activity of neurons in ganglia attached to their corresponding skin segment by segmental nerve roots, or in isolated ganglia. Mechanosensory stimulation of the skin induced excitatory synaptic potentials (EPSPs) and action potentials in both Retzius neurons in a ganglion. The frequency and duration of responses depended on the strength and duration of the skin stimulation. Retzius cells responded after T and P cells, but before N cells, and their sustained responses correlated with the activity of P cells. Trains of five impulses at 10 Hz in every individual T, P, or N cell in isolated ganglia produced EPSPs and action potentials in Retzius neurons. Responses to T cell stimulation appeared after the first impulse. In contrast, the responses to P or N cell stimulation appeared after two or more presynaptic impulses and facilitated afterward. The polysynaptic nature of all the synaptic inputs was shown by blocking them with a high calcium/magnesium external solution. The rise time distribution of EPSPs produced by the different mechanosensory neurons suggested that several interneurons participate in this pathway. Our results suggest that sensory stimulation provides a mechanism for regulating serotonin-mediated modulation in the leech.     

Secondary hair cells and afferent neurones of the squid statocyst receive both inhibitory and excitatory efferent inputs

Summary Intracellular recordings were obtained from the hair cells and afferent neurones of the angular acceleration receptor system of the statocyst of the squid,Alloteuthis subulata. Electrical stimulation of the efferent fibres in the crista nerve (minor) evoked responses in all of the secondary hair cells recorded from (n=211). 48% of the secondary air cells responded with a small depolarization, 15% with a hyperpolarization, and 37% with a depolarization followed by a hyperpolarization. The depolarizations and hyperpolarizations had mean stimulus to response delays of 6.7 ms and 24 ms, and reversal potentials of about –1 mV and –64 mV, respectively. Both types of potential increased in amplitude, up to a point, when the stimulus shock was increased and facilitation and/or summation effects could be obtained by applying multiple shocks. These data, together with the fact that both responses could be blocked by bath application of cobalt or cadmium, indicate that the secondary hair cells receive both inhibitory and excitatory efferent inputs and that these are probably mediated via chemical synapses. No efferent responses were seen in the primary hair cells but both depolarizing and hyperpolarizing efferent responses were obtained from the afferent neurones.     

Neural response directionality correlates of hair cell orientation in a teleost fish

The otolithic end organs in the ears of teleost fishes play important roles in hearing. Although previous studies have shown that afferent fibers innervating otolithic organs are directionally sensitive to acoustic stimulation, no study has demonstrated that directionality of the otolithic afferent neurons derives directly from morphological polarity of the hair cells that they innervate. In this study we investigated whether or not there exists such a structure and function relationship in one of the otolithic organs, the saccule, by using intracellular and extracellular tracing, histochemistry, and confocal imaging techniques. We observed a variety of morphologies of dendritic terminals of saccular ganglion neurons. Arbor innervation areas of these saccular neurons ranged from 893 microm2 to 21,393 microm2, and the number of dendritic endings fell into a range between 10 and 54. We found that the response directionality of saccular ganglion neurons correlates significantly with the morphological polarization of the hair cells in the regions that they innervate. Therefore, we provide direct evidence to support the hypothesis that fish are able to encode directional information about a sound source, particularly in elevation, using arrays of hair cells in the otolithic organs that are oriented specifically along the sound propagation axis.     

Depolarization of primary afferents during fictitious scratching in thalamic cats

The dorsal cord and dorsal root potentials were recorded in immobilized thalamic cats during fictitious scratching evoked by mechanical stimulation of the ear. Depolarization of primary afferents was shown to be simulated by the central scratching generator. Antidromic spike discharges appeared at the peak of the primary afferent depolarization waves in certain afferent fibers. Similar discharges arise in the resting state in response to stimulation of limb mechanoreceptors. It is suggested that during real scratching primary afferent depolarization and antidromic spikes evoked by it may effectively modulate the level of the afferent flow to spinal neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 173–176, March–April, 1978.     

Activity of in situ middle cervical ganglion neurons in dogs, using extracellular recording techniques

Neuronal activity in the in situ middle cervical ganglion of dogs was investigated using extracellular recording techniques. The recorded action potentials were frequently active during specific phases of the cardiac cycle, particularly during systole, and this activity persisted following acute decentralization of the ganglion. The activity of these action potentials was modified when systemic arterial pressure was altered by isoproterenol, noradrenaline, adrenaline, or partial occlusion of the aorta, whether in the intact or acutely decentralized preparation. These neurons were active between systolic pressures of 70 and 180 mmHg (1 mmHg = 133.322 Pa). Action potentials were frequently modified by mechanical distortion of the superior vena cava, ventricular epicardium, or adventitia of the aorta, whether the preparation was acutely decentralized or not. Seventy percent of these action potentials were unaffected by stimulation (1 ms, 4 V, 0.5 Hz) of a cardiopulmonary nerve and 27% were suppressed by such stimulation. Five of the neurons were activated by such stimulation. It is presumed that the latter neurons had axons in a cardiopulmonary nerve and most likely were efferent sympathetic postganglionic neurons. Sixty-three percent of these spontaneously active phase-locked units were modified by stimulation of a ramus or an ansa. It is postulated that some of the neurons in the middle cervical ganglia can be modified by afferent axons arising from receptors in thoracic organs, in particular from the great vessels and heart, whether in an intact or acutely decentralized preparation. The majority of these neurons are presumed not to be afferent neurons or efferent postganglionic neurons, as they are not activated directly by electrical stimulation of axons in cardiopulmonary nerves. Rather they are presumed to be interneurons. These results lend support to the thesis that considerable integration of neuronal activity related to thoracic cardiovascular dynamics occurs within the middle cervical ganglia of dogs.     

Increased responsiveness of rat mast cells to compound 48/80 due to removal of extracellular magnesium. Effects of ouabain and EGTA.

A decreased secretory response of mast cells to compound 48/80 (12% of control value) after preincubation of the cells with magnesium but without calcium was partially restored by removal of magnesium. EGTA (10 microM) blocked the restoration and decreased the restored secretory activity again, while this was further increased by ouabain (1 mM). Furthermore, ouabain completely restored the decreased secretion (50% of control value) due to preincubation without the divalent cations. This may indicate that magnesium influences a pool of cellular calcium that is involved in the stimulus-secretion coupling and is available to EGTA, and ouabain did not counteract the inhibitory mechanism of magnesium.     

EGTA induced fast potentials in rat small intestine.

Rat small intestine exhibits spontaneous slow-waves and spikes in normal solution. When treated with 0.3 – 0.7 mM EGTA, in calcium free solution, normal rhythmicity disappears and fast rhythmic potentials of duration intermediate to slow-waves and spikes appear. These induced fast potentials are absent in sodium free solution and are eliminated by verapamil, a calcium channel blocker. Application of EGTA to cat, guinea pig, mouse, and toad intestine did not yield fast potentials. The fast potentials appear to result from sodium entering through channels usually used by calcium. The fast potentials may be a phenomena exclusive to rat small intestine.     

The effects of cations upon the action potentials recorded from neurohaemal tissue of the stick insect

Summary The ionic requirement for the action potentials recorded from the neurohaemal tissue on the lateral branch of the median nerve inCarausius morosus has been studied using extracellular electrodes. Sodium-free, magnesium-free, or calcium-free salines produce irreversible block of the action potentials following prolonged exposure to the nerves. Reducing the sodium concentration to 4 mM has little effect on the amplitude of the action potentials, whilst increasing the sodium concentration to 100 mM reduces the amplitude by 50%. Neither tetrodotoxin nor procaine has any effect on these action potentials.Reducing the magnesium concentration to 1 mM increases the amplitude of the action potentials, whilst increasing the concentration of magnesium reduces the amplitude.The amplitude of the action potentials is linearly related to the log of the external calcium concentration, and the action potentials are blocked by both cobalt ions and lanthanum ions.It is concluded that calcium is the major charge carrier of the inward current in these neurosecretory axons which is the first report of calcium dependent action potentials in a nerve axon. Furthermore, small amounts of sodium and magnesium are necessary to maintain electrical activity. Magnesium is a competitive inhibitor of the calcium currents.We are grateful to the Science Research Council for financial support, and to Mrs. J. Birch for the printing of the electron micrographs.     

Properties of action potentials carried by divalent cations in identified leech neurons

Properties of divalent cation potentials carried by either Sr2+ or Ca2+ ions in Na+-free, TEA-Ringer solution were characterized in identified neurons of two species of leeches (Macrobdella and Haementeria). In Macrobdella, the overshoot of the potentials varied logarithmically with [Sr2+]0 (28.5 mV per 10-fold change). The overshoot, Vmax, and duration of the potentials increased with increasing divalent cation concentration and saturated at about 20 to 30 mM [Sr2+]0. The Vmax, amplitude, and duration of the potentials were reversibly blocked by Co2+ and Mn2+. The block by Mn2+ could be well-fitted by a reverse Langmuir-curve with an apparent KI of 100 micromolar. The local anesthetic procaine also reversibly inhibited the Vmax and duration of the potentials. The inhibition was greater at alkaline pH suggesting that procaine blocks the calcium channel from inside the membrane. The identified leech neurons examined in Macrobdella varied considerably in their ability to sustain somatic divalent cation potentials. Stimulation of T cells and most motoneurons produced no or only weak potentials, whereas stimulation of Retzius, N, Nut, and AP cells evoked overshooting potentials of several seconds' duration. Stimulation of the ALG cell of Haementeria in normal Ringer solution evoked a slowly-rising, purely Ca2+-dependent potential of approximately 100 ms duration. This response was TTX-resistant, unaffected by complete removal of Na+ from the Ringer solution, and abolished by 1 mM Mn2+. The overshoot varied logarithmically with a slope of 28 mV/decade change in [Ca2+]0.     

Response properties of chemosensory peg sensilla on the pectines of scorpions

By behavioral and anatomical criteria, the pectinal sensory appendages of scorpions appear to be chemoreceptive organs specialized for detection of substances on substrates. These comb-like, midventral appendages contain tens of thousands of minute (<5 μm), truncated setae, called pegs, arranged in dense, two-dimensional arrays on the ventral surface. In this study we used extracellular recording techniques to examine spontaneous and stimulated activity of sensory neurons within individual pegs. Chronic recordings lasting several days showed long-term fluctuations in spontaneous activity of sensory units in single peg sensilla, with peak activity coinciding with the animal's normal period of foraging. Several units were identified by the stereotypical waveforms of action potentials they elicit. Near-range olfactory stimulation of peg sensilla by volatile alcohols, aldehydes, ketones, esters, and carboxylic acids produced dose-dependent patterns of neural response. Contact stimulation with these chemicals, or water, or mechanical deflection of the peg tip also evoked activity in identifiable units. The peg sensilla appear to be broadly sensitive to odorants and tastants, suggesting they function similarly to the antennae of mandibulate arthropods. Accepted: 12 April 1997