Electroregulated sodium channels of the somatic membrane of sympathetic neurons

Electrically-operated sodium channels in the somatic membrane of isolated neurons from the rat superior cervical ganglion have been studied by means of intracellular dialysis technique under voltage clamp conditions. It was shown that in this preparation sodium currents can be carried by two independent systems of sodium channels. The mathematical analysis of voltage-dependent TTX-sensitive fast sodium currents was performed by the Hodgkin-Huxley formalism; their kinetic properties were compared with those described in other objects. TTX-sensitive sodium channels in the somatic membrane of sympathetic neurons were found to be highly selective for Na+ ions. Kinetic and voltage-dependent characteristics of slow TTX-resistant sodium current were also described. This component of the sodium current was observed only in a few neurons (not more than 2%).     

A use-dependent sodium current modification induced by type I pyrethroid insecticides in honeybee antennal olfactory receptor neurons

We studied the mode of action of type I pyrethroids on the voltage-dependent sodium current from honeybee olfactory receptor neurons (ORNs), whose proper function in antenna is crucial for interindividual communication in this species. Under voltage-clamp, tetramethrin and permethrin induce a long lasting TTX-sensitive tail current upon repolarization, which is the hallmark of an abnormal prolongation of the open channel configuration. Permethrin and tetramethrin also slow down the sodium current fast inactivation. Tetramethrin and permethrin both bind to the closed state of the channel as suggested by the presence of an obvious tail current after the first single depolarization applied in the presence of either compounds. Moreover, at first sight, channel opening seems to promote tetramethrin and permethrin binding as evidenced by the progressive tail current summation along with trains of stimulations, tetramethrin being more potent at modifying channels than permethrin. However, a use-dependent increase in the sodium peak current along with stimulations suggests that the tail current accumulation could also be a consequence of progressively unmasked silent channels. Experiments with the sea anemone toxin ATX-II that suppresses sodium channels fast inactivation are consistent with the hypothesis that these silent channels are either in an inactivated state at rest, or that they normally inactivate before they open so that they do not participate to the control sodium current. In honeybee ORNs, three processes lead to a use-dependent pyrethroid-induced tail current accumulation: (i) a recruitment of silent channels that produces an increase in the peak sodium current, (ii) a slowing down of the sodium current inactivation produced by prolongation of channels opening and (iii) a typical deceleration in current deactivation. The use-dependent recruitment of silent sodium channels in honeybee ORNs makes pyrethroids more potent at modifying neuronal excitability.     

Voltage-gated influx ion channels expressed in Xenopus oocytes after the administration of brain mRNA]

Expression of "fast", TTX-sensitive sodium and high-threshold calcium channels in the membrane of Xenopus oocytes following mRNA injection from the rat brain has been detected using two microelectrode voltage clamp technique. Barium current through expressed calcium channels was blocked by 200 mumol/l Cd2+ and was insensitive to D-600 (20 mumol/l) and nitrendipine (50 mumol/l). Expressed barium current was inhibited within 20-40 min by omega-conotoxin, a peptide neurotoxin known to block high-threshold calcium channels of the neuronal membrane, in 1 mumol/l concentration. A steady-state inactivation curve for this current could be fitted by the Boltzmann relation with V1/2 = -50 mV and k = 14 mV. Voltage-dependent and pharmacological properties of calcium channels which appeared in the oocyte membrane following mRNA injection from the mammalian brain resembled most of all those of high-threshold inactivating (HTI- or N-type) calcium channels of neurons in spite they did not demonstrate prominent time-dependent inactivation. Evidences in favour of expressed calcium channels heterogeneity were not obtained.     

Characterization of large conductance Ca2+-activated K+ channels in cerebellar Purkinje neurons

We investigated the role of large conductance, calcium-activated potassium channels (BK channels) in regulation of the excitability of cerebellar Purkinje neurons. Block of BK channels by iberiotoxin reduced the afterhyperpolarization of spontaneous action potentials in Purkinje neurons in acutely prepared cerebellar slices. To establish the conditions required for activation of BK channels in Purkinje neurons, the dependence of BK channel open probability on calcium concentration and membrane voltage were investigated in excised patches from soma of acutely prepared Purkinje cells. Single channel currents were studied under conditions designed to select for potassium currents and in which voltage-activated currents were largely inactivated. Micromolar calcium concentrations activated channels with a mean single channel conductance of 266 pS. BK channels were activated by both calcium and membrane depolarization, and showed no sign of inactivation. At a given calcium concentration, depolarization over a 60-mV range increased the mean open probability (P(O)) from < 0.1 to > 0.8. Increasing the calcium concentration shifted the voltage required for half maximal activation to more hyperpolarized potentials. The apparent affinity of the channels for calcium increased with depolarization. At -60 mV the apparent affinity was approximately 35 micro m decreasing to approximately 3 micro M at +40 mV. These results suggest that BK channels are unlikely to be activated at resting membrane potentials and calcium concentrations. We tested the hypothesis that Purkinje cell BK channels may be activated by calcium entry during individual action potentials. Significant BK channel activation could be detected when brief action potential-like depolarizations were applied to patches under conditions in which the sole source of calcium was flux across the plasma membrane via the endogenous voltage-gated calcium channels. It is proposed that BK channels regulate the excitability of Purkinje cells by contributing to afterhyperpolarizations and perhaps by shaping individual action potentials.     

Maitotoxin-induced membrane current in neuroblastoma cells

Maitotoxin (MTX) is a potent marine toxin isolated from the toxic dinoflagellate, Gambierdiscus toxicus. We have examined the possibility of MTX activating calcium channels using cultured neuroblastoma cells (N1E-115). MTX (10 ng/ml) produced a depolarization of the membrane, which was prevented by the removal of Ca2+ from the external medium. Under voltage clamp conditions, membrane currents were recorded with 50 mM Ba2+ as a charge carrier through calcium channels. After application of MTX (1 ng/ml), an inward current necessary to hold the membrane at -90 mV increased progressively. This was followed by a gradual decrease of the transient inward Ba2+ current through type I calcium channels recorded at -30 mV which was eventually abolished. A similar tendency was observed in the long-lasting inward Ba2+ current through type II calcium channels, which was recorded at +10 mV. The MTX action was antagonized by calcium channel blockers such as verapamil (100 microM) and La3+ (1 mM). A high concentration of verapamil (500 microM) blocked both types of calcium channels persistently. After washout of verapamil but while the calcium channels were still blocked, MTX (1 ng/ml) induced a steady-state current. The MTX-induced current showed an inward-rectifying property with a reversal potential of approximately -30 mV. The results suggest that the MTX-induced current does not flow through calcium channels. Thus, MTX may create a pore in the membrane with pharmacological properties similar to those of calcium channels.     

A new conotoxin isolated from Conus consors venom acting selectively on axons and motor nerve terminals through a Na+-dependent mechanism

A novel conotoxin was isolated and characterized from the venom of the fish-hunting marine snail Conus consors. The peptide was identified by screening chromatography fractions of the crude venom that produced a marked contraction and extension of the caudal and dorsal fins in fish, and noticeable spontaneous contractions of isolated frog neuromuscular preparations. The peptide, named CcTX, had 30 amino acids and the following scaffold: X11CCX7CX2CXCX3C. At the frog neuromuscular junction, CcTx at nanomolar concentrations selectively increased nerve terminal excitability so that a single nerve stimulation triggered trains of repetitive or spontaneous synaptic potentials and action potentials. In contrast, CcTx had no noticeable effect on muscle excitability even at concentrations 100 x higher than those that affected motor nerve terminals, as revealed by direct muscle stimulation. In addition, CcTx increased miniature endplate potential (MEPP) frequency in a Ca2+-free medium supplemented with ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid (EGTA). Blockade of voltage-dependent sodium channels with tetrodotoxin (TTX) either prevented or suppressed the increase of MEPP frequency induced by the toxin. CcTx also produced a TTX-sensitive depolarization of the nodal membrane in single myelinated axons giving rise, in some cases, to repetitive and/or spontaneous action potential discharges. In addition, CcTx increased the nodal volume of myelinated axons, as determined using confocal laser scanning microscopy. This increase was reversed by external hyperosmolar solutions and was prevented by pretreatment of axons with TTX. It is suggested that CcTx, by specifically activating neuronal voltage-gated sodium channels at the resting membrane potential, produced Na+ entry into nerve terminals and axons without directly affecting skeletal muscle fibres. CcTx belongs to a novel family of conotoxins that targets neuronal voltage-gated sodium channels.     

Hypocretin/orexin depolarizes and decreases potassium conductance in locus coeruleus neurons

Recent studies demonstrated that noradrenergic locus coeruleus (LC) neurons are a particularly strong target of the novel neuropeptide, hypocretin (orexin). The present study sought to elucidate the action of hypocretin-B (HCRT) on LC neurons recorded intracellularly in rat brain slices. Bath (1.0 microM) or local puff application (50-100 microM in pipette) of HCRT depolarized LC neurons in rat brain slices and increased their spontaneous discharge rate. Depolarization evoked by HCRT was persistent in the presence of tetrodotoxin (TTX, 1 microM) and Co2+ (1 mM), indicating that HCRT directly activated LC neurons, and that its effect on the postsynaptic cell was not due to activation of TTX-sensitive sodium channels or Co2+-sensitive calcium channels. The apparent input resistance was significantly increased in the majority of LC neurons during the HCRT-evoked depolarization. Moreover, the HCRT-evoked depolarization was decreased in amplitude with hyperpolarization of membrane. The present results indicate that decreased potassium conductance is involved in the effect of HCRT on LC neurons.     

Voltage clamp analysis of sodium channels in normal and scorpion toxin-resistant neuroblastoma cells

Sodium currents mediated by voltage-sensitive sodium channels in normal and scorpion toxin-resistant neuroblastoma cells were measured using a giga-ohm seal recording method in the whole cell patch configuration. The voltage and time dependence of sodium currents were similar in normal and mutant cell lines. Half-maximal activation occurred for test depolarizations in the range of -7 to -11 mV. Half-maximal inactivation occurred for pre-pulses in the range of -62 to -69 mV. Scorpion toxin from Leiurus quinquestriatus (100 to 200 nM) increased the time constant for sodium channel inactivation 6- to 9-fold, increased the peak sodium current 2.0 +/- 0.5-fold, shifted the voltage dependence of sodium channel activation 7 to 11 mV to more negative potentials, and made the voltage dependence of inactivation less steep. These effects were observed for both normal and scorpion toxin-resistant neuroblastoma cells. However, the effect of Leiurus toxin on the rate of inactivation was half-maximal at 1.7 nM for the parental cell line N18, in contrast to 5.4 or 39 nM for the scorpion toxin-resistant clone LV30 and 24 or 51 nM for LV10. These results show that scorpion toxin resistance results from a specific change in channel properties that does not impair normal function but causes an increase in the apparent KD for Leiurus toxin action on sodium channels.     

Glycine triggers an intracellular calcium influx in oligodendrocyte progenitor cells which is mediated by the activation of both the ionotropic glycine receptor and Na+-dependent transporters

Using fluo-3 calcium imaging, we demonstrate that glycine induces an increase in intracellular calcium concentration ([Ca2+]i) in cortical oligodendrocyte progenitor (OP) cells. This effect results from a calcium entry through voltage-gated calcium channels (VGCC), as it is observed only in OP cells expressing such channels, and it is abolished either by removal of calcium from the extracellular medium or by application of an L-type VGCC blocker. Glycine-triggered Ca2+ influx in OP cells actually results from an initial depolarization that is the consequence of the activation of both the ionotropic glycine receptor (GlyR) and Na+-dependent transporters, most probably the glycine transporters 1 (GLYT1) and/or 2 (GLYT2) which are colocalized in these cells. Through this GlyR- and transporter-mediated effect on OP intrcellular calcium concentration [Ca2+]i, glycine released by neurons may, as well as other neurotransmitters, serve as a signal between neurons and OP during development.     

Glutamate receptor-mediated calcium entry in neurons derived from P19 embryonal carcinoma cells

We have examined the control of calcium elevation by glutamate in neurons derived from the mouse P19 embryonal carcinoma cell line. Following transient exposure to retinoic acid, P19 cells differentiate into neurons that express both NMDA and non-NMDA glutamate receptor subtypes. Fluorescence videomicroscopy using the indicator fura-2 revealed concentration-dependent elevation in cytosolic calcium levels with exposure to NMDA or kainate. Replacement of extracellular sodium with N-methylglucamine significantly reduced the action of kainate. Exposure to high K⁺ medium also elicited an elevation of cytosolic calcium in P19 cells, which was partially inhibited by the calcium channel antagonist nimodipine. These experiments suggest that the elevation in calcium produced by kainate involves the activation of voltage-gated calcium channels as a consequence of membrane depolarization, in contrast to direct calcium entry through NMDA receptor channels. Whole-cell recordings revealed that P19 NMDA receptors were highly permeable to calcium (P_Ca/P_Na = 5.6 ± 0.2). In most cells, channels gated by kainate displayed low permeability to calcium; the median permeability ratio, P_Ca/P_Na, was 0.053 (range 0.045 to 0.132). Activation of peak currents by NMDA, glycine, and kainate was half-maximal at 24 μM, 240 nM, and 81 μM, respectively. In addition, cadmium-sensitive currents through voltage-gated calcium channels were recorded in P19 cells bathed in barium/TEA chloride. Staining with antibodies directed against AMPA receptor subunits revealed widespread immunoreactivity for anti-GluR-B/C and anti-GluR-B/D. About half of the P19 cells were stained with antibodies selective for GluR-D but there was little or no immunoreactivity for the GluR-A subunit. © 1996 Wiley-Liss, Inc.     

Involvement of transient receptor potential-like channels in responses to mGluR-I activation in midbrain dopamine neurons

We investigated the involvement of store-operated channels (SOCs) and transient receptor potential (TRP) channels in the response to activation of the group I metabotropic glutamate receptor subtype 1 (mGluR1) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG, puff application) in dopamine neurons in rat brain slices. The mGluR1-induced conductance reversed polarity close to 0 mV and at more positive potentials when extracellular potassium concentrations were increased, indicating the involvement of a cationic channel. DHPG currents but not intracellular calcium responses were reduced by low extracellular sodium concentrations but were not affected by sodium channel blockers, tetrodotoxin and saxitoxin or by inhibition of the h-current with cesium. Abolition of calcium responses with intracellular BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; 10 mm) did not affect current responses, indicating they were not calcium activated. Extracellular application of non-selective SOCs and TRP channel blockers 2-aminoethoxydiphenylborane (2-APB), SKF96365, ruthenium red and flufenamic acid (but not gadolinium) reduced DHPG current and calcium responses. Intracellular application of ruthenium red and 2-APB did not affect DHPG currents, indicating that IP3 and ryanodine receptors did not mediate their actions. Single-cell PCR revealed the presence of TRPC1 and 5 mRNA in most dopamine neurons and subtypes 3, 4 and 6 in some. Store depletion evoked calcium entry indicative of SOCs, providing the first functional observation of such channels in native central neurons. Store depletion with either cyclopiazonic acid or ryanodine abolished calcium but not current responses to DHPG. The electrophysiological and pharmacological properties of the mGluR1-induced inward current are consistent with the involvement of TRP channels whereas calcium responses are dependent on the function of SOCs in voltage clamp recordings.     

Hyperosmolar D-mannitol reverses the increased membrane excitability and the nodal swelling caused by Caribbean ciguatoxin-1 in single frog myelinated axons

The effects of hyperosmolar D-mannitol were studied on single frog myelinated nerve fibres previously poisoned with Caribbean ciguatoxin-1 (C-CTX-1), a new toxin isolated from the pelagic fish Caranx latus inhabiting the Caribbean region. In current-clamped myelinated axons, C-CTX-1 (50-120 nM) caused spontaneous and repetitive action potential discharges after a short delay. In addition, the toxin produced a marked swelling of nodes of Ranvier of myelinated axons that reached a steady state within about 90 min, as revealed by using confocal laser scanning microscopy. The increased excitability and the nodal swelling caused by C-CTX-1 were prevented or reversed by an external hyperosmotic solution containing 100 mM D-mannitol. Moreover, the C-CTX-1-induced nodal swelling was completely prevented by the blockade of voltage-sensitive sodium channels by tetrodotoxin (TTX). It is suggested that C-CTX-1, by increasing nerve membrane excitability, enhances Na(+) entry into nodes of Ranvier through TTX-sensitive sodium channels, which directly or indirectly disturb the osmotic equilibrium between intra- and extra-axonal media resulting in an influx of water that was responsible for the long-lasting nodal swelling. The fact, that hyperosmolar D-mannitol either reversed or prevented the neurocellular actions of C-CTX-1, is of particular interest since it provides the rational basis for its use to treat the neurological symptoms of ciguatera fish poisoning in the Caribbean area.     

Activation and inactivation of batrachotoxin-modified sodium channels of nerve fiber membranes in the frog

Currents through batrachotoxin (BTX)-modified sodium channels in frog myelinated nerve were measured under voltage-clamp conditions. Nonlinearity of "instantaneous" current-voltage relations was taken into account when determining steady-state parameters of channel activation. BTX induces the shift of voltage dependence of channel activation towards more negative potentials by 67 mV, without changes in its steepness. Current kinetics and effect of preceding depolarization on current size suggest that BTX-modified channels are capable for partial inactivation. High level of steady-state conductance of BTX-modified channels can be explained by suggestion that open state of the channel is energetically more profitable than inactivated one. It is concluded that effect of BTX on inactivation is different in principle from that of pronase and protein reagents.     

State-dependent access to the batrachotoxin receptor on the sodium channel

Batrachotoxin causes sustained opening of voltage-gated sodium channels. Toxin binds irreversibly to wild type channels; however, it dissociates rapidly from channels with mutation F1710C in transmembrane segment IVS6. This dissociation requires channel activation, suggesting that the activation gate guards the toxin-binding site. Here we show that activity-dependent toxin dissociation was not affected by external sodium, arguing against a binding site within the pore, and demonstrate that dissociation occurred only during the first few milliseconds after membrane depolarization, as if the toxin leaves its binding site during closed states that precede the final open state in the activation pathway. Toxin interaction with preopen states may facilitate subsequent channel opening, thus accounting for the batrachotoxin-induced negative shift in channel activation.     

Clindamycin-induced alteration of ganglionic function. I. Direct effects on ganglion cell properties

The influence of the lincosamide antibiotic, clindamycin, on the properties of bullfrog sympathetic ganglion B cells has been determined in vitro using conventional voltage recording methods or single microelectrode voltage-clamp recording techniques. Individual neurons were depolarized with both bath application or local perfusion of clindamycin. The amplitude of the depolarization was not altered by pretreatment with 50 microM (+)-tubocurarine, 10-microM atropine, or 1.5 microM tetrodotoxin (TTX), indicating that the clindamycin-induced depolarization does not result from either the activation of (1) nicotinic receptors, (2) muscarinic receptors, or (3) voltage-gated sodium channels. Clindamycin partially inhibited IM, an action which accounts for part of the clindamycin-induced depolarization. The duration of the hyperpolarizing afterpotential (HAP) following the action potential was decreased in the presence of clindamycin. Clindamycin decreased the amplitude and maximum rate of rise (MRR) of TTX-insensitive action potentials. As calcium influx is thought to contribute to the depolarizing phase of the TTX-insensitive spikes, we suggest that the decrease in HAP duration by clindamycin results from a decrease in the somal calcium current. Further, it is suggested that a decrease in IM and HAP duration may be responsible for the increased excitability exhibited during exposure to clindamycin.     

Role of 5-HT1A and 5-HT3 receptors in serotonergic activation of sensory neurons in relation to itch and pain behavior in the rat

Serotonin (5-hydroxytryptamine, 5-HT) released by platelets, mast cells, and immunocytes is a potent inflammatory mediator which modulates pain and itch sensing in the peripheral nervous system. The serotonergic receptors expressed by primary afferent neurons involved in these sensory functions are not fully identified and appear to be to a large extent species dependent. Moreover, the mechanisms through which 5-HT receptor activation is coupled to changes in neuronal excitability have not been completely revealed. Using a combination of in vitro (calcium and voltage imaging and patch-clamp) and in vivo behavioral methods, we used both male and female Wistar rats to provide evidence for the involvement of two 5-HT receptor subtypes, 5-HT1A and 5-HT3, in mediating the sustained and transient effects, respectively, of 5-HT on rat primary afferent neurons involved in pain and itch processing. In addition, our results are consistent with a model in which sustained serotonergic responses triggered via the 5-HT1A receptor are due to closure of background potassium channels, followed by membrane depolarization and action potentials, during which the activation of voltage-gated calcium channels leads to calcium entry. Our results may provide a better understanding of mammalian serotonergic itch signaling.     

European Neuroscience Association Pre‐ and post‐synaptic muscarinic receptors in thin slices of rat adrenal gland

The effects of activation of muscarinic receptors on chromaffin cells and splanchnic nerve terminals were studied in a rat adrenal slice preparation. In chromaffin cells, muscarine induced a transient hyperpolarization followed by a depolarization associated with cell spiking. The hyperpolarization was blocked by charybdotoxin (1 μm ) and tetraethylammonium chloride (TEA, 1 mm ), but was not affected by 200 μm Cd²⁺ or removal of external Ca²⁺, consistent with activation of BK channels. This would follow internal Ca²⁺ mobilization, as shown by Ca²⁺ imaging with fura-2 on isolated chromaffin cells in culture. Under voltage-clamp, outward BK currents were insensitive to MT3 toxin, a specific muscarinic m4 receptor antagonist. In contrast, muscarine-induced depolarization was due to a m4 receptor-mediated inward current blocked by MT3 toxin. This current was permeable to cations and was associated with Ca²⁺ entry and subsequently, Ca²⁺-induced Ca²⁺ release. Finally, both muscarine (25 μm ) and oxotremorine (10 μm ) decreased the amplitude and frequency of KCl-evoked excitatory postsynaptic currents, without affecting quantal size, consistent with a presynaptic inhibitory effect. Taken together, our data suggest that activation of m4 and probably m3 muscarinic receptors results in a strong, long-lasting excitation of chromaffin cells, as well as an uncoupling of synaptic inputs onto these cells.     

Effects of ApC, a sea anemone toxin, on sodium currents of mammalian neurons

We have characterized the actions of ApC, a sea anemone polypeptide toxin isolated from Anthopleura elegantissima, on neuronal sodium currents (I(Na)) using current and voltage-clamp techniques. Neurons of the dorsal root ganglia of Wistar rats (P5-9) in primary culture were used for this study. These cells express tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) I(Na). In current-clamp experiments, application of ApC increased the average duration of the action potential. Under voltage-clamp conditions, the main effect of ApC was a concentration-dependent increase in the TTX-S I(Na) inactivation time course. No significant effects were observed on the activation time course or on the current peak-amplitude. ApC also produced a hyperpolarizing shift in the voltage at which 50% of the channels are inactivated and caused a significant decrease in the voltage dependence of Na+ channel inactivation. No effects were observed on TTX-R I(Na). Our results suggest that ApC slows the conformational changes required for fast inactivation of the mammalian Na+ channels in a form similar to other site-3 toxins, although with a greater potency than ATX-II, a highly homologous anemone toxin.     

Prolongation and enhancement of gamma-aminobutyric acid receptor mediated excitation by chronic treatment with estradiol in developing rat hippocampal neurons

GABA(A) receptor activation during brain development is a critical source of excitation. This is due to the positive equilibrium potential for chloride relative to resting membrane potential, resulting in membrane depolarization sufficient to open voltage sensitive calcium channels. The gonadal steroid estradiol has pronounced trophic effects on the developing hippocampus, promoting cell survival and synaptogenesis. In the current study, we investigated the effect of estradiol on GABA(A) receptor-mediated calcium transients in cultured neonatal hippocampal neurons, from Sprague-Dawley rats, using the calcium sensitive dye, Fura-2-AM. Treatment of hippocampal neurons with physiological levels of estradiol significantly increased the peak amplitude of calcium transients, increased the number of cells responding to the GABA(A) agonist muscimol with membrane depolarization, and delayed the rate of clearance of free intracellular calcium. These effects were significantly attenuated by pretreatment with the oestrogen receptor antagonist ICI-182,780. This suggests that estradiol, via its action on the oestrogen receptor, prolongs the developmental duration of depolarizing GABA. Estradiol likely maintains GABA-mediated excitation by promoting increased protein levels of the active/phosphorylated form of the chloride cotransporter Na+K+2CL- and L-type voltage sensitive calcium channels containing the alpha1C subunit. We propose that a component of the trophic effects of estradiol on hippocampal development results from enhanced calcium influx subsequent to GABA(A) receptor activation.     

The role of altered sodium currents in action potential abnormalities of cultured dorsal root ganglion neurons from trisomy 21 (down syndrome) human fetuses

Trisomy 21 (Down syndrome) results in abnormalities in electrical membrane properties of cultured human fetal dorsal root ganglion (DRG) neurons. Action potentials have faster rates of depolarization and repolarization, with decreased spike duration, compared to diploid neurons. In order to analyze the faster depolarization rate observed in trisomic neurons, we examined sodium currents of cultured human fetal DRG neurons from trisomy 21 and control subjects, using the whole-cell patch-clamp technique. The neurons were replated in culture to reduce dendritic spines. Two components of the sodium current were identified: (1) a fast, tetrodotoxin (TTX)-sensitive current; and (2) a slow, TTX-resistant component. The inactivation curves of both current types in trisomic neurons showed a shift of approximately 10 mV towards more depolarized potentials compared to control neurons. Thus, whereas essetially all of the fast sodium channels were inactivated at normal resting potentials in control neurons, approximately 10% of these channels were available for activation in trisomy 21 cells. Furthermore, the fast current showed accelerated activation kinetics in trisomic neurons. The slow sodium current of trisomic neurons showed slower deactivation kinetics than control cells. No differences were observed between trisomic and control neurons in the maximal conductance or current densities of either fast or slow current components. These data indicate that the greater rate of depolarization in trisomy 21 neurons at resting potentials is primarily due to activation of residual fast sodium channels that also have a faster time course of activation.