Slow Inward Research Articles

Noradrenergic current responses of neurons in rat oculomotor neural integrators.

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are involved in the control of horizontal and vertical gaze, respectively. A previous study showed that PHN neurons exhibit depolarized or hyperpolarized responses to noradrenaline (NA). However, the adrenoceptor types that participate in NA-induced responses and the effects of NA on INC neurons have not yet been investigated. Furthermore, the relationship between NA-induced responses and neuron types defined by neurotransmitter phenotypes has not been determined. In this study, we investigated NA-induced current responses in PHN and INC neurons and the relationships between these responses and neuron types using whole cell recordings in wild-type and transgenic rat brainstem slices. Local application of NA to the cell soma induced slow inward (SI) and slow outward (SO) currents that were mainly mediated by α1 and α2 adrenoceptors, respectively. These current responses were observed in both PHN and INC neurons, although the proportion of INC neurons that responded to NA was low. Analyses of the distributions of the current responses revealed that in the PHN, all fluorescently identified inhibitory neurons exhibited SI currents, whereas glutamatergic and cholinergic neurons exhibited both SI and SO currents. In the INC, glutamatergic and inhibitory neurons preferentially exhibited SI and SO currents, respectively. When the PHN and INC neurons were characterized by their firing pattern, we found that the proportions of the currents depended on their firing pattern. These results suggest that various modes of noradrenergic modulation in horizontal and vertical neural integrators are dependent on neuron type.NEW & NOTEWORTHY Noradrenergic modulation of oculomotor neural integrators involved in gaze control has not been elucidated. Here, we report that noradrenaline (NA)-induced slow inward (SI) and outward (SO) currents are mediated mainly by α1 and α2 adrenoceptors in neurons that participate in horizontal and vertical gaze control. The NA-induced current responses differed depending on the neurotransmitter phenotype and firing pattern. These results suggest various modes of noradrenergic modulation in horizontal and vertical integrator neurons.

Unravelling neuron-astrocyte communication in the dorsal raphe nucleus.

Introduction: Serotonin is a neuromodulator widely spread throughout the central nervous system and involved in a vast variety of behaviors, such as cognitive functions and emotional states. On this basis, serotonergic dysfunction has been related to several psychiatric disorders such as Major Depressive Disorder, schizophrenia, or autism. Despite serotonin’s widespread distribution, the location of serotonergic neurons is restricted to the midbrain raphe nuclei. In this context, astrocytes are known to participate in synaptic transmission modulating neural circuits; however, their role in the serotonergic system remains largely unknown. My study aims to elucidate the neuron-astrocyte signaling at serotonergic nuclei, looking at the dorsal raphe nucleus (DRN). Materials and methods: For the purpose of this project, we performed intracranial injections of viral constructs in adult mice to express the Ca2+ indicator GCaMP6 specifically in DRN astrocytes, Ca2+ imaging experiments to record astrocytic activity, electrophysiological recordings of neuronal activity using patch-clamp and immunohistochemistry techniques. Results and conclusions: Preliminary data show that astrocytes from DRN respond to serotonin with an increase in the amplitude and frequency of intracellular Ca2+ events, which is partly mediated by serotonin type 2 receptors. As a consequence of triggering Ca2+ signaling, we found that serotonin induces gliotransmission, shown by an increased frequency of Slow Inward Currents (SICs) and modulation of excitatory synaptic transmission in dorsal raphe neurons. In conclusion, these data suggest that astrocytes might play a role in synaptic transmission and neuronal excitability in the DRN and, therefore, in the serotonergic-mediated actions of DRN.

Purinergic modulation of neuronal activity in the rat prepositus hypoglossi nucleus.

In the nervous system, adenosine 5'-trisphosphate (ATP) functions as a neurotransmitter and binds to ionotropic P2X receptors and metabotropic P2Y receptors. Although ATP receptors are expressed in the prepositus hypoglossi nucleus (PHN), which is a brainstem structure involved in controlling horizontal gaze, it is unclear whether ATP indeed affects the activity of PHN neurons. In this study, we investigated the effects of ATP on spontaneous firing of PHN neurons using whole-cell recordings in rat brainstem slices. Bath application of ATP increased or decreased the spontaneous firing rate of the neurons in a dose-dependent manner, indicating that ATP indeed affects PHN neuronal activity. To clarify the mechanisms of the ATP effects, we investigated the current responses of PHN neurons to a local application of ATP. The ATP application induced a fast inward (FI) current, a slow inward (SI) current, and/or a slow outward (SO) current in the neurons. The agonists of P2X and P2Y receptors induced FI and SI currents, respectively. The SO currents were not induced by the ATP agonists but were induced by adenosine, which may be extracellularly converted from ATP by ectonucleotidases. An antagonist of adenosine P1 (A1 ) receptors abolished the adenosine-induced SO currents and bath application of adenosine decreased the spontaneous firing rate of all PHN neurons tested. These results indicate that PHN neurons express functional purinoceptors and show that the FI, SI, and SO currents were mediated via P2X, P2Y, and A1 receptors, respectively.

Astrocyte-Dependent Slow Inward Currents (SICs) Participate in Neuromodulatory Mechanisms in the Pedunculopontine Nucleus (PPN).

Slow inward currents (SICs) are known as excitatory events of neurons caused by astrocytic glutamate release and consequential activation of neuronal extrasynaptic NMDA receptors. In the present article we investigate the role of these astrocyte-dependent excitatory events on a cholinergic nucleus of the reticular activating system (RAS), the pedunculopontine nucleus (PPN). It is well known about this and other elements of the RAS, that they do not only give rise to neuromodulatory innervation of several areas, but also targets neuromodulatory actions from other members of the RAS or factors providing the homeostatic drive for sleep. Using slice electrophysiology, optogenetics and morphological reconstruction, we revealed that SICs are present in a population of PPN neurons. The frequency of SICs recorded on PPN neurons was higher when the soma of the given neuron was close to an astrocytic soma. SICs do not appear simultaneously on neighboring neurons, thus it is unlikely that they synchronize neuronal activity in this structure. Occurrence of SICs is regulated by cannabinoid, muscarinic and serotonergic neuromodulatory mechanisms. In most cases, SICs occurred independently from tonic neuronal currents. SICs were affected by different neuromodulatory agents in a rather uniform way: if control SIC activity was low, the applied drugs increased it, but if SIC activity was increased in control, the same drugs lowered it. SICs of PPN neurons possibly represent a mechanism which elicits network-independent spikes on certain PPN neurons; forming an alternative, astrocyte-dependent pathway of neuromodulatory mechanisms.

Osmotic Edema Rapidly Increases Neuronal Excitability Through Activation of NMDA Receptor-Dependent Slow Inward Currents in Juvenile and Adult Hippocampus.

Cellular edema (cell swelling) is a principal component of numerous brain disorders including ischemia, cortical spreading depression, hyponatremia, and epilepsy. Cellular edema increases seizure-like activity in vitro and in vivo, largely through nonsynaptic mechanisms attributable to reduction of the extracellular space. However, the types of excitability changes occurring in individual neurons during the acute phase of cell volume increase remain unclear. Using whole-cell patch clamp techniques, we report that one of the first effects of osmotic edema on excitability of CA1 pyramidal cells is the generation of slow inward currents (SICs), which initiate after approximately 1 min. Frequency of SICs increased as osmolarity decreased in a dose-dependent manner. Imaging of real-time volume changes in astrocytes revealed that neuronal SICs occurred while astrocytes were still in the process of swelling. SICs evoked by cell swelling were mainly nonsynaptic in origin and NMDA receptor-dependent. To better understand the relationship between SICs and changes in neuronal excitability, recordings were performed in increasingly physiological conditions. In the absence of any added pharmacological reagents or imposed voltage clamp, osmotic edema induced excitatory postsynaptic potentials and burst firing over the same timecourse as SICs. Like SICs, action potentials were blocked by NMDAR antagonists. Effects were more pronounced in adult (8–20 weeks old) compared with juvenile (P15–P21) mice. Together, our results indicate that cell swelling triggered by reduced osmolarity rapidly increases neuronal excitability through activation of NMDA receptors. Our findings have important implications for understanding nonsynaptic mechanisms of epilepsy in relation to cell swelling and reduction of the extracellular space.

Two Forms of Astrocyte Calcium Excitability Have Distinct Effects on NMDA Receptor-Mediated Slow Inward Currents in Pyramidal Neurons

Astrocytes display excitability in the form of intracellular calcium concentration ([Ca(2+)](i)) increases, but the signaling impact of these for neurons remains debated and controversial. A key unresolved issue is whether astrocyte [Ca(2+)](i) elevations impact neurons or not. Here we report that in the CA1 region of the hippocampus, agonists of native P2Y(1) and PAR-1 receptors, which are preferentially expressed in astrocytes, equally elevated [Ca(2+)](i) levels without affecting the passive membrane properties of pyramidal neurons. However, under conditions chosen to isolate NMDA receptor responses, we found that activation of PAR-1 receptors led to the appearance of NMDA receptor-mediated slow inward currents (SICs) in pyramidal neurons. In stark contrast, activation of P2Y(1) receptors was ineffective in this regard. The PAR-1 receptor-mediated increased SICs were abolished by several strategies that selectively impaired astrocyte [Ca(2+)](i) excitability and function. Our studies therefore indicate that evoked astrocyte [Ca(2+)](i) transients are not a binary signal for interactions with neurons, and that astrocytes result in neuronal NMDA receptor-mediated SICs only when appropriately excited. The data thus provide a basis to rationalize recent contradictory data on astrocyte-neuron interactions.

Methylmercury Induces a Spontaneous, Transient Slow Inward Chloride Current in Purkinje Cells of Rat Cerebellar Slices

Methylmercury (MeHg; 10-100 microM) induced a spontaneous, transient, slow inward current in Purkinje cells in rat cerebellar slices. Insensitivity of this current to tetrodotoxin suggests that its generation is not related to presynaptic firing. The present study was designed to attempt to identify the ionic origin of this current. Neither Gd(3+), a nonspecific cation channel blocker, nor tetrakis(2-pyridylmethyl)ethylethylenediamine, which chelates Zn(2+), could prevent this current. Following dialysis of cells with a low-[Cl(-)] pipette solution, the giant currents were inducible only when the cells were held at potentials more positive than 0 mV but not at potentials more negative than -60 mV. In addition, no giant currents were observed when cells were held at 0 mV under symmetrical [Cl(-)] conditions. Thus, this current seems to be mediated by Cl(-). However, it was insensitive to the glycine receptor antagonist strychnine. The anion channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or niflumic acid suppressed GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents. Niflumic acid also prevented appearance of this giant current; DIDS was only effective at more positive membrane potentials. Thus, this current seems to be carried by a voltage-dependent Cl(-) channel. Reducing extracellular Ca(2+) concentration and/or intracellular application of the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid seemed to be ineffective at preventing appearance of this current. Thus, these data do not seem to support the conclusion that this current is mediated by a Ca(2+)-activated Cl(-) channel. The role that this current plays in MeHg-induced neurotoxicity is unknown.

Theophylline Effects on Phasic Tension and Slow Inward Current of Isolated Frog Atrial Fibres: Influence on the Action of Isoprenaline

The action of theophylline on phasic tension and slow inward current (Isi) was investigated on isolated voltaged-clamped frog atrial fibres. The action of theophylline was both dose- and time-dependent; the most effective dose was 2.10(-3)M. Theophylline induced a rapid positive inotropic effect which cannot be attributed to a beta-adrenoceptor stimulation or to an adenosine antagonism; this effect was closely related to an increase in slow inward current (Isi), probably due to an inhibition of phosphodiesterase activity and a subsequent increase in cAMP content. The decline of the positive inotropic effect of theophylline was discussed in terms of additional effects of the drug. Theophylline, consistently with its phosphodiesterase inhibitory action, was shown to potentiate the effects of isoprenaline on phasic tension and Isi; however no potentiation was observed after reserpine-induced depletion of endogenous catecholamines. It might be thought that non-specific effects of reserpine on some steps of cardiac excitation-contraction coupling counteract the effects of theophylline.

Book Reviews

Books review in this article: Glial Cell Receptors edited by H. K. Kimelberg Pineal Research Reviews, Volume 6, edited by Russel J. Reiter Neural Darwinism: The Theory of Neuronal Group Selection by G. M. Edelman Receptor Biochemistry and Methodology, edited by J. C. Venter and L. C. Harrison Volume 8: Dopamine Receptors, edited by I. Creese and C. M. Fraser Volume 9: Structure and Physiology of the Slow Inward Calcium Channel, edited by J. C. Venter and D. Triggle The Alpha‐1 Adrenergic Receptors edited by R. R. Ruffolo

Effect of Sodium Pentobarbital on the Transmembrane Action Potential and the Slow Inward Current of Guinea Pig Ventricular Myocytes

Barbiturates, which are widely used in clinical anesthesia, exert a negative inotropic effect on the myocardium. To investigate the mechanism for the negative inotropism, we studied the effect of sodium pentobarbital (SP) on the action potential and the slow inward current recorded from guinea pig ventricular myocytes. We found that SP at 0.6, 1.0, and 1.6 microM decreased action potential duration at 50% repolarization by 17 +/- 6, 25 +/- 11, and 29 +/- 8%, respectively (p less than 0.05, n = 4). At the concentration range of 0.6-1.6 microM, resting potential and action potential amplitude were unaffected. Voltage-clamp studies in ventricular myocytes demonstrated that SP at 0.6, 1.0, and 1.6 microM reduced the peak slow inward current by 30 +/- 4, 36 +/- 14, and 67 +/- 5%, respectively (p less than 0.005, n = 4). In conclusion, SP, a frequently used anesthetic, decreases the slow inward current in guinea pig ventricular myocytes at clinically relevant concentrations.

Reduction of the Slow Inward Current of Isolated Rat Ventricular Cells by Thiamylal and Halothane

The barbiturates and halothane exert a negative inotropic effect on the myocardium. A reduction in the slow inward current, carried mainly by calcium ions, is an important factor for the underlying mechanism because the calcium current during the action potential provides the calcium ions for accompanying contraction, supplies Ca ions to the sarcoplasmic reticulum for subsequent contractions, and induces Ca release from the store site. It has been suggested that reduction in the slow inward current caused by anesthetics is indicated by depression of the slow action potential of the partially depolarized myocardium. In order to assess directly the effect of anesthetics on the slow inward current, we carried out voltage clamp experiments with single isolated rat ventricular cells obtained by an enzymatic dissociation method. Thiamylal (10(-4) mol . l-1) and halothane (1%) decreased the slow inward current to 60 +/- 5% (mean +/- s.d., n = 8) and to 65 +/- 10% (mean +/- s.d., n = 8) of the control value, respectively, without changing the configuration of the current-voltage curve. The results provide further evidence for anesthetic reduction of the slow inward current of the myocardium, and suggest that the negative inotropic effect is at least partly due to the reduction in that current.

Tl+-ions: comparison on the effects on the slow inward current and contractility of ventricular tissues.

To conclude our investigation of thallium effects on cardiac tissues, we studied the slow inward current of sheep cardiac Purkinje fibres exposed to 10-7 to 10-5 ᴍ Tl+ for extended periods of up to 80 min. Our previous results had suggested a possible involvement of the slow inward current during thallium intoxication: a) the modification of contractility staircases observed during thallium exposure, b) action potential recordings of ventricular muscle, c) changes in spontaneous beating in sino-atrial preparations. The thallium levels chosen were between those yielding strong positive inotropic transients and those producing a marked long­term decay of contraction force. The slow inward current was measured using a conventional two-microelectrode-technique and the standard voltage clamp protocol for this current system. The experimental work was restricted to the determination of d∞, the kinetics of activation of the slow inward current and of īsi, the current voltage relation of the current system. This was necessary since the effects of thallium were known to be short-lived and therefore frequent repeat runs of the voltage clamp program had to be performed in order to obtain the time courses of possible transient changes. The results showed that the slow inward current was first increased and then declined at the low concentration of 10-7 ᴍ Tl+. At 10-5 m Tl+ the initial increase was smaller, whereas the decay of the slow inward current proceeded to lower values. Comparison with contractility measure­ments at the same concentrations of thallium showed a distinct parallelism between changes of the slow inward current and myocardial contractility. Despite this apparent relationship, we do not conclude that the contractile events are primarily a result of changes of the slow inward current, since thallium does not seem to specifically alter the parameters of the slow inward current at the membrane level.

Voltage Clamp Studies on the Slow Inward Current during the Excitation ofNitellopsis obtusa

The ionic currents across the plasmalemma of Nitellopsis obtusa were measured in voltage clamp experiments. Depolarization of the cell by 30-100 mV from the level of the resting potential resulted in (1) a rapid inward current, (2) a subsequent slow inward current, and (3) a stationary outward current. The first current component changed sign at —20 to —30 mV. The second component decreased to a minimum at this clamped level. With increasing depolarizing steps some slow transient current component reappeared without changing sign. This transient inward current occurred also when the potential was clamped either at large depolarizing (+80 mV) or at large hyperpolarizing (—300 mV) potentials. In cases when the slow inward current component was evident cessation of protoplasmic streaming was observed. The ATPase inhibitor dicyclo hexylcarbodiimide (DCCD) at a concentration of 2 x 10~5 M in the external medium inhibited the slow transient inward current without affecting the first rapid current component. It is suggested that the irreversible slow transient current component reflects the onset of some active ion-transport system in the plasmalemma during cell excitation.

Mechanism of augmented premature responses in canine ventricular muscle.

HomeCirculation ResearchVol. 44, No. 5Mechanism of augmented premature responses in canine ventricular muscle. Free AccessAbstractPDF/EPUBAboutView PDFSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessAbstractPDF/EPUBMechanism of augmented premature responses in canine ventricular muscle. H Iinuma and K Kato H IinumaH Iinuma Search for more papers by this author and K KatoK Kato Search for more papers by this author Originally published1 May 1979https://doi.org/10.1161/01.RES.44.5.624Circulation Research. 1979;44:624–629 Previous Back to top Next FiguresReferencesRelatedDetailsCited By FRANZ M (2003) The Electrical Restitution Curve Revisited:, Journal of Cardiovascular Electrophysiology, 10.1046/j.1540.8167.90303.x, 14:s10, (S140-S147), Online publication date: 1-Oct-2003. 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The M cell., Circulation Research, 68:6, (1729-1741), Online publication date: 1-Jun-1991.Antzelevitch C, Sicouri S, Litovsky S, Lukas A, Krishnan S, Di Diego J, Gintant G and Liu D (1991) Heterogeneity within the ventricular wall. Electrophysiology and pharmacology of epicardial, endocardial, and M cells., Circulation Research, 69:6, (1427-1449), Online publication date: 1-Dec-1991. Litovsky S and Antzelevitch C (1989) Rate dependence of action potential duration and refractoriness in canine ventricular endocardium differs from that of epicardium: Role of the transient outward current, Journal of the American College of Cardiology, 10.1016/0735-1097(89)90490-7, 14:4, (1053-1066), Online publication date: 1-Oct-1989. ENDRESEN K and AMLIE J (1989) Electrical Restitution and Conduction Intervals of Ventricular Premature Beats in Man: Influence of Heart Rate, Pacing and Clinical Electrophysiology, 10.1111/j.1540-8159.1989.tb05048.x, 12:8, (1347-1354), Online publication date: 1-Aug-1989. Tseng G (1988) Calcium current restitution in mammalian ventricular myocytes is modulated by intracellular calcium., Circulation Research, 63:2, (468-482), Online publication date: 1-Aug-1988.Hiraoka M and Kawano S (1987) Mechanism of increased amplitude and duration of the plateau with sudden shortening of diastolic intervals in rabbit ventricular cells., Circulation Research, 60:1, (14-26), Online publication date: 1-Jan-1987. (1997) Activity-dependent changes in the electrical behaviour of sheep cardiac Purkinje fibres, Proceedings of the Royal Society of London. Series B. Biological Sciences, 10.1098/rspb.1985.0073, 225:1241, (457-479), Online publication date: 22-Oct-1985. Franz M, Schaefer J, Schöttler M, Seed W and Noble M (1983) Electrical and mechanical restitution of the human heart at different rates of stimulation., Circulation Research, 53:6, (815-822), Online publication date: 1-Dec-1983.Hashimoto H, Suzuki K, Miyake S and Nakashima M (1983) Effects of calcium antagonists on the electrical alternans of the ST segment and on associated mechanical alternans during acute coronary occlusion in dogs., Circulation, 68:3, (667-672), Online publication date: 1-Sep-1983. Iinuma H, Dreifus L, Bianchi J and Michelson E (1983) Supernormal response and conduction in depressed isolated canine ventricular muscle exposed to high potassium concentration, Journal of the American College of Cardiology, 10.1016/S0735-1097(83)80197-1, 1:3, (830-839), Online publication date: 1-Mar-1983. Blaustein A, Gaasch W, Adam D and Levine H (1981) Myocardial relaxation. V. Postextrasystolic contraction-relaxation in the intact dog heart., Circulation, 64:2, (345-351), Online publication date: 1-Aug-1981. Boyet M and Jewell B (1981) Analysis of the effects of changes in rate and rhythm upon electrical activity in the heart, Progress in Biophysics and Molecular Biology, 10.1016/0079-6107(81)90003-1, 36, (1-52), . Coraboeuf E (1980) Voltage Clamp Studies of the Slow Inward Current The Slow Inward Current and Cardiac Arrhythmias, 10.1007/978-94-009-8890-3_3, (25-95), . May 1, 1979Vol 44, Issue 5 Advertisement Article InformationMetrics Copyright © 1979 by American Heart Associationhttps://doi.org/10.1161/01.RES.44.5.624 Originally publishedMay 1, 1979 PDF download Advertisement