Pharmacological characterization of voltage-dependent calcium currents in rat hippocampal neurons

Modulation of voltage-gated Ca2+ current by 4-hydroxynonenal in dentate granule cells

Although recent studies have suggested that dentate granule cells play a key role in hippocampal functions, electrophysiological properties in these cells have not been sufficiently explored. In the present study, modification of voltage-gated Ca2+ channels by 4-hydroxynonenal (4HN), a major aldehydic product of membrane lipid peroxidation, in cultured dentate granule cells was examined using the whole-cell patch clamp technique. When whole-cell voltage clamp was applied, the cells exhibited a high-voltage-activated Ca2+ current, which was totally sensitive to 30 microM Cd2+ and partially sensitive to 2 microM nifedipine. 4HN enhanced the Ca2+ current in these cells. When L-type Ca2+ channels were blocked by application of nifedipine, the enhancement was completely canceled, whereas application of omega-conotoxin-GVIA or omega-agatoxin-IVA, blockers of N- and P/Q-type Ca2+ channels, respectively, had no effect. These results suggest that 4HN modulates L-type Ca2+ channels in the dentate granule cells, and thereby plays a role in the physiological and pathophysiological responses of these cells to oxidative stress.

Properties of a High-threshold Voltage-activated Calcium Current in Rat Cerebellar Granule Cells

Postmitotic cerebellar granule cells, maintained for 5 - 6 days in Dulbecco's modified essential medium supplemented with 25 mM KCl, have been studied in whole-cell recording conditions to characterize calcium currents. With 10 mM Ba2+ as the divalent charge carrier, and using a pipette solution highly buffered for Ca2+ (30 mM EGTA, 100 mM HEPES - Tris, pH 7.2), only a high-threshold voltage-activated barium current was recorded from a holding potential of -90 mV. The addition of 1 mM ATP to the pipette medium allowed stable recording for an average duration of 10 min, compatible with pharmacological studies of the barium current. Ninety-six per cent of the current was half-inactivated at low negative holding potential (-76 mV). A total block of current was obtained with 1 microM Cd2+. Sixty-three per cent of the mean current was abolished by 3 microM omega-conotoxin (omega-CgTx; Ki=10 nM for a 15 min application), but individual cells showed either full sensitivity to this toxin or incomplete sensitivity. Seventy-eight per cent of the mean current was also abolished by 10 microM nicardipine but with a higher Ki of 0.5 microM. After exposure to omega-CgTx, BAY K 8644 had no effect on the remaining current, though it was suppressed by nicardipine. No sensitivity to diltiazem, desmethoxyverapamil or flunarizine could be detected. Our major conclusion is that at least half of the channels have a mixed pharmacology, showing sensitivity to both omega-CgTx and dihydropyridine antagonists.

Calcium channel isoforms underlying synaptic transmission at embryonic Xenopus neuromuscular junctions

Studies on the amphibian neuromuscular junction have indicated that N-type calcium channels are the sole mediators of stimulus-evoked neurotransmitter release. We show, via both presynaptic and postsynaptic voltage-clamp measurements, that dihydropyridine (DHP)-sensitive calcium channels also contribute to stimulus-evoked release at developing Xenopus neuromuscular junctions. Whereas inhibition of postsynaptic responses by omega-conotoxin (omega-Ctx) GVIA has been taken previously as evidence that only N-type channels mediate transmitter release, we find that both N-type and DHP-sensitive calcium currents are sensitive to this toxin. The unusual sensitivity of DHP-sensitive calcium channels to omega-Ctx GVIA in presynaptic terminals raises the possibility that this channel type may have escaped detection in previous physiological studies on adult frog neuromuscular junctions. Alternatively, the additional channel isoforms may be present only during early development, when they may serve to strengthen collectively presynaptic release during critical periods of synaptogenesis.

Acamprosate (calcium acetylhomotaurinate) enhances the N-methyl-D-aspartate component of excitatory neurotransmission in rat hippocampal CA1 neurons in vitro

The taurinate analog acamprosate (calcium acetylhomotaurinate) has received considerable attention in Europe for its ability to prevent relapse in abstained alcoholics. To determine the mechanism of acamprosate actions in the CNS, we superfused acamprosate onto rat hippocampal CA1 pyramidal neurons using an in vitro slice preparation. In current-and voltage-clamp recordings, acamprosate (100 to 100 microM) superfusion had little effect on resting membrane potential or input slope resistance. Acamprosate had no effect on Ca(2+)-dependent action potentials when tetrodotoxin was used to block Na+ spikes. In whole-cell voltage-clamp recordings, and in the presence of tetraethylammonium and Cs+ to block K+ channels, acamprosate had little effect on a Cd(2+)-sensitive inward current likely to be a high voltage-activated Ca2+ current. However, in both current- and voltage-clamp recordings, acamprosate significantly increased the N-methyl-D-aspartate (NMDA) component of excitatory postsynaptic potentials evoked by stimulation of Schaffer collaterals in the stratum radiatum, in the presence of the selective non-NMDA (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid kainate) glutamate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and the GABAA receptor antagonist bicuculline. Acamprosate had inconsistent or no effects on the stratum radiatum-evoked non-NMDA component of the excitatory postsynaptic potentials, in the presence of bicuculline and the NMDA antagonist DL-2-amino-5-phosphonovalerate. Acamprosate, on average, had little effect on the late inhibitory postsynaptic potentials thought to be mediated by GABAB receptors. In the presence of tetrodotoxin to block synaptic transmission, acamprosate dramatically increased inward current responses in most CA1 neurons to exogenous NMDA applied by pressure or superfusion, with reversal on washout of acamprosate. These data suggest that acamprosate may act postsynaptically to increase the NMDA component of excitatory transmission to hippocampal CA1 pyramidal neurons. Considering the known interaction of ethanol with NMDA receptors, this acamprosate modulation of NMDA receptor-mediated neurotransmission could provide a mechanism of action underlying the clinical efficacy of acamprosate.

Electrophysiological properties of neurones in cultures from postnatal rat dentate gyrus

Electrophysiological properties of neurofilament-positive neurones in dissociated cell cultures were prepared at postnatal days 4-5 from rat dentate gyrus and studied using the whole-cell patch-clamp technique. These cells expressed a fast-inactivating, 0.5 microM tetrodotoxin-sensitive Na+ current; a high-voltage-activated (HVA) Ca2+ current, which was 30 microM Cd(2+)- and partially 2 microM nicardipine-sensitive; and an inward rectifier current, which was sensitive to extracellularly applied 1 mM Cs+. The outward current pattern was composed of a delayed rectifier-like outward current sensitive to 20 mM tetraethylammonium (TEA) and a fast-inactivating, Ca(2+)-dependent outward current. This transient Ca(2+)-dependent K+ outward current was identified by a subtraction procedure. K+ currents recorded under conditions of blocked Ca2+ currents (after rundown of the HVA Ca2+ current or blocked by extracellularly applied Cd2+) were subtracted from control currents. By comparison with the current pattern of identified dentate granule cells, it is concluded that the investigated cell type originated from interneurones or projection neurones of the dentate hilus.

Calcium channel currents in undifferentiated human neuroblastoma (SH-SY5Y) cells: actions and possible interactions of dihydropyridines and omega-conotoxin

Ca2+ channel currents were recorded in undifferentiated human neuroblastoma (SH-SY5Y) cells with the whole-cell patch-clamp technique, using 10 mM Ba2+ as charge carrier. Currents were only evoked by depolarizations to -30 mV or more positive (holding potential -80 mV), inactivated partially during 200 ms depolarizing steps, and were abolished by 150 microM Cd2+. Currents could be enhanced by Bay K-8644 and partially inhibited by nifedipine, suggesting that they arose in part due to activation of L-type Ca2+ channels. Currents were also inhibited by the marine snail peptide omega-conotoxin GVIA (omega-CgTx). At a concentration of 10 nM inhibition by omega-CgTx was reversible, but at higher concentrations blockade was always irreversible. Although current inhibition by nifedipine was maximal at 1 microM, supramaximal concentrations reduced the inhibitory actions of omega-CgTx in a concentration-dependent manner. Ca2+ channel currents evoked from a holding potential of -50 mV showed no inactivation during 200 ms depolarizations but declined in amplitude with successive depolarizing steps (0.2 Hz). Current amplitudes could be restored by returning the holding potential to -80 mV. Currents evoked from -50 mV were inhibited by nifedipine and omega-CgTx to a similar degree as those evoked from -80 mV. Our results indicate that undifferentiated SH-SY5Y cells possess L- and N-type Ca2+ channels which can be distinguished pharmacologically but cannot be separated by using depolarized holding potentials. Furthermore, these data suggest that nifedipine has a novel action to inhibit blockade of N-type channels by omega-CgTx.

Different effects of omega-conotoxin GVIA at excitatory and inhibitory synapses in rat CA1 hippocampal neurons

The nature of the coupling mechanism of presynaptic calcium channels involved in the release of neurotransmitters in the mammalian central nervous system is unknown. Using intracellular recordings from CA1 neurons in the rat hippocampal slice preparation, we show that the N-type calcium channels antagonist omega-conotoxin GVIA (omega-CgTx) blocks partially the excitatory (EPSP) and totally the inhibitory (IPSP) synaptic transmission in CA1 hippocampal pyramidal neurons. In addition, the inhibitory effect of omega-CgTx on IPSPs is strongly depressed by intrahippocampal injection of PTX, while the effect on EPSP is not. The results suggest that the nature or the regulation of calcium channels might be different, depending on the location of these channels on excitatory or inhibitory terminals.

Enhancement of calcium currents in rat hippocampal CA1 neurons induced by kindling epileptogenesis

Kindling of the Schaffer collaterals in the dorsal hippocampus of the rat induced an epileptogenic focus in area CA1. Pyramidal neurons were acutely isolated from this area in fully kindled rats one day after the last class five generalized seizure. Calcium currents were measured in these cells under the whole-cell patch voltage-clamp condition after blockade of sodium and potassium currents. Voltage-dependent calcium currents were activated by depolarizing voltage steps from different prepulse potentials. Calcium currents activated at 0 mV consisted of a sustained component and two voltage-dependent inactivating components. Current inactivation was fitted with two exponentials (time-constants of 13 and 72 ms) and a constant. When cells from kindled rats were compared with those from controls, the amplitudes of the slow-inactivating and the sustained component were significantly enhanced by 36% and 39%, respectively; the fast inactivating current showed only a small enhancement. Inactivation kinetics, time-to-peak and voltage dependency of activation and steady-state inactivation were unchanged. Shape and size of the analysed cells from kindled rats were not different from those in controls. We concluded that an increased specific calcium conductance of as yet unknown origin underlies the larger current. The magnitude of the observed changes is such that it will considerably increase calcium influx and consequently raise intracellular calcium concentration during tetanic stimulation and subsequent periods of paroxysmal activity. This increase will modulate calcium-dependent factors that regulate neuronal excitability and may lead to the enhanced excitability found in kindled tissue.

Kinetic and pharmacological properties of high- and low-threshold calcium channels in primary cultures of rat hippocampal neurons

The kinetic, permeability and pharmacological properties of Ca currents were investigated in primary cultures of rat hippocampal neurons. The low-voltage-activated (LVA) Ca current turned on positive to -60 mV and fully inactivated in a voltage-dependent way. This current was depressed by nickel (Ni, 40 microM) and amiloride (500 microM) and was insensitive to omega-conotoxin (omega-CgTx) (4 microM) and to the Ca agonist Bay K 8644 (5 microM). The high-voltage-activated (HVA) Ca current turned on positive to -40 mV and inactivated slowly and incompletely. This current was much less sensitive than the LVA current to Ni and amiloride but more sensitive to cadmium. omega-CgTx blocked only partially this current (about 50%) in an irreversible way. Bay K 8644 had a clear agonistic action almost exclusively on the omega-CgTx-resistant HVA current component. The present results suggest that the HVA channels, quite homogeneous for their kinetic properties and sensitivity to holding potentials, can be pharmacologically separated in two classes: (i) omega-CgTx-sensitive and Bay-K-8644-insensitive (omega-S/BK-I) and (ii) omega-CgTx-insensitive and Bay-K-8644-sensitive (omega-I/BK-S), the latter displaying a stronger Ca-dependent inactivation.

Neurotransmitters inhibit the omega-conotoxin-sensitive component of Ca current in neuroblastoma x glioma hybrid (NG 108-15) cells, not the nifedipine-sensitive component

Voltage-dependent calcium currents (ICa) in NG 108-15 cells consisted of three pharmacologically distinct components: a transient low-voltage-activated (LVA) current, sensitive to Ni2+; a high-voltage-activated (HVA) current sensitive to the dihydropyridine antagonist, nifedipine and a HVA current sensitive to omega-conotoxin GVIA (CgTx). The voltage sensitivities and decay kinetics of the two HVA currents were indistinguishable. The neurotransmitters acetylcholine (ACh) and noradrenaline inhibited ICa. This inhibition was not occluded by Ni2+ or nifedipine, but was abolished by CgTx. It is therefore concluded that the neurotransmitter-sensitive component of ICa is restricted to that component of HVA current inhibitable by omega-conotoxin.

Two types of high-threshold calcium currents inhibited by omega-conotoxin in nerve terminals of rat neurohypophysis

1. The neurohypophysis comprises the nerve terminals of hypothalamic neurosecretory cells, which contain arginine vasopressin (AVP) and oxytocin. The secretory terminals of rat neurohypophyses were acutely dissociated. The macroscopic calcium currents (ICa) of these isolated peptidergic terminals were studied using 'whole-cell' patch-clamp recording techniques. 2. There are two types ('Nt' (where the subscript 't' denotes terminal) and 'L') of high-threshold voltage-activated ICa in the terminals, which can be distinguished by holding at different potentials i.e. -90 and -50 mV. Replacement of Ca2+ in the bathing solution by Ba2+ increased the amplitude of ICa, primarily due to an increase in the L-type component. Both inward currents were eliminated by adding 50 microM-Cd2+ or when in a Ca(2+)-free bathing solution. 3. omega-Conotoxin GVIA (omega-CgTx) has been widely used as a Ca2+ channel blocker. However, whether this toxin can discriminate between different types of Ca2+ channels is still a subject of controversy. We applied omega-CgTx over a wide range of concentrations (0.01-2 microM) to examine its effects on both Nt- and L-type ICa in these terminals. At a concentration of 30 nM, omega-CgTx selectively reduced, by 48%, the amplitude of Nt-type ICa. In contrast, a higher concentration (300 nM) of omega-CgTx was necessary to inhibit the L-type ICa. 4. omega-CgTx inhibited both Nt- and L-type ICa in a dose-dependent manner, and the half-maximum inhibition (IC50) of the ICa by the toxin was 50 and 513 nM, respectively, which was approximately a tenfold difference. The reduction in both types of currents did not result from any shift in their current-voltage or steady-state inactivation relationships. 5. In contrast, omega-CgTx, at a concentration of 300 nM, had no effect on the tetrodotoxin-sensitive sodium current (INa) of the isolated peptidergic nerve terminals. Furthermore, omega-CgTx did not reduce the long-lasting, non-inactivating ICa in the isolated non-neuronal secretory cells of the pars intermedia (PI) (intermediate lobe of the pituitary). 6. Our studies suggest that omega-CgTx might exert specific blocking effects on both Nt- and L-type Ca2+ channels, but that in the isolated peptidergic nerve terminals, the Nt-type component is more susceptible to this toxin.

Characterization of a Voltage-dependent Calcium Current in the Human Neuroblastoma Cell Line SH-SY5Y During Differentiation

In the presence of retinoic acid, cultured human neuroblastoma SH-SY5Y cells grow processes indicative of neuronal differentiation. A voltage-gated Ca current is already present in undifferentiated cells. A gradual increase of the Ca current density occurs during cell differentiation. According to kinetic and pharmacological properties, Ca currents in differentiated cells are indistinguishable from those elicitable in undifferentiated cells and resemble features of the high-voltage activated currents present in mammalian neuronal cells. omega-conotoxin strongly depresses high-voltage activated currents, both in undifferentiated and in differentiated SH-SY5Y cells. Interestingly, the Ca agonist Bay K 8644 is effective, albeit with great variability from cell to cell, only in differentiated cells and only when barium is the current carrier through the Ca channels. A diversity of high-voltage activated Ca channels of distinct pharmacology has been recently observed in other kinds of neurons. This requires a redefinition of the role that voltage-dependent Ca channel subtypes can play in mammalian neurons.

Ca2+ channels in rat central and peripheral neurons: high-threshold current resistant to dihydropyridine blockers and omega-conotoxin

Block of Ca2+ channel current by dihydropyridines and by omega-conotoxin (omega-CgTx) was studied in a variety of freshly dissociated rat neurons. In most neurons, including those from dorsal root ganglia, sympathetic ganglia, spinal cord, cerebral cortex, and hippocampus, nitrendipine and omega-CgTx each blocked a fraction of the high-threshold current, but a substantial fraction of current remained even when the two blockers were applied together at saturating concentrations. An extreme case was cerebellar Purkinje neurons, in which very little current was blocked by either nitrendipine or omega-CgTx. These results demonstrate the existence in mammalian neurons of high-threshold channels that are resistant to both omega-CgTx and dihydropyridine blockers. Such channels might underlie instances of synaptic transmission and other processes that depend on Ca2+ entry but are not sensitive to these blockers.