Calcium channels in vertebrate neurons. Experiments on a neuroblastoma hybrid model

The long-term effect of toosendanin on current through nifedipine-sensitive Ca2+ channels in NG108-15 cells

Toosendanin is a triterpenoid derivative extracted from Melia toosendan Sieb et Zucc. Previous studies demonstrated that toosendanin could block neurotransmission and stimulate PC12 cell into differentiation and apoptosis. These actions of toosendanin were suggested to result from a continuous increase in Ca2+ influx, which led to intracellular Ca2+ overload. Here, we observed the long-term effect of toosendanin on Ca2+ channels in NG108-15 cells by whole-cell patch-clamp recording. Obtained data showed that a prolonged exposure to toosendanin induced a continuous increase in the Ca2+ influx in a concentration and time-dependent manner while a brief treatment induced an irreversible increase in Ca2+ influx in differentiated NG108-15 cells. The nifedipine-sensitive L-type currents were significantly increased after exposure to TSN while the nifedipine-resistant or omega-conotoxin MVIIC-sensitive currents were not affected.

Toosendanin, a triterpenoid derivative, increases Ca2+ current in NG108-15 cells via L-type channels

Toosendanin, a triterpenoid derivative extracted from Melia toosendan Sieb et Zucc, was demonstrated to be a selective presynaptic blocker and an effective antibotulismic agent in previous studies. Here, we observed its effects on Ca(2+) channels in NG108-15 cells by whole-cell patch-clamp recording. Obtained data showed that toosendanin concentration dependently increased the high-voltage-activated (HVA) Ca(2+) current with an EC(50) of 5.13 microM in differentiated NG108-15 cells. The enhancement effect was still observed when the cells were pretreated with 5 microM omega-conotoxin MVIIC. However, when the cells were preincubated with 5 microM nifedipine or 10 microM verapamil-containing solution, the effect was absent. In undifferentiated NG108-15 cells, which only express T-type Ca(2+) channels, toosendanin did not affect Ca(2+) currents. These results show that toosendanin increases Ca(2+) influx in NG108-15 cells via L-type Ca(2+) channels.

Recombinant GABA(B) receptors formed from GABA(B1) and GABA(B2) subunits selectively inhibit N-type Ca(2+) channels in NG108-15 cells

Efficient transfection of NG108-15 cells with GABA(B) receptor subunits was achieved using polyethylenimine. Baclofen modulated high voltage-activated Ca(2+) current in differentiated cells transfected with GABA(B1) and GABA(B2) receptor subunits or with the GABA(B2) subunit alone, but not with the GABA(B1) subunit alone. Characteristics of the current modulation were very similar for cells transfected with GABA(B1/2) and GABA(B2) subunits. Using antisense oligonucleotides against GABA(B1) subunits and also western immunoblotting, we are able to show that NG108-15 cells contain endogenous GABA(B1) subunits. Therefore, functional receptors can be formed by the combination of native GABA(B1) subunits with transfected GABA(B2) subunits, in agreement with the proposed heteromeric structure of GABA(B) receptors. Finally, we used selective channel blockers to identify the subtypes of Ca(2+) channels that are modulated by GABA(B) receptors. In fact, in differentiated NG108-15 cells, the recombinant GABA(B) receptors couple only to N-type Ca(2+) channels.

Modulation of calcium current by recombinant GABA(B) receptors

Two GABA(B) receptor subunits have been cloned: GABA(B1) and GABA(B2). In this study we investigate the coupling of recombinant GABA(B) receptors to calcium channels in differentiated NG108-15 cells, which exhibit many similarities to neurones but in which functional GABA(B) receptors are normally absent. Transfection of GABA(B1) and GABA(B2) subunit cDNAs enables baclofen-mediated inhibition of different calcium channel subtypes and a component of this modulation is voltage-dependent. When transfected individually, GABA(B2), but not GABA(B1), is able to enhance calcium current inhibition over background levels. Further, an antisense oligodeoxynucleotide to GABA(B1) reduces the average functional response in cells transfected with GABA(B2) alone. Assuming that the functional receptor is heteromeric, this suggests that GABA(B1), but not GABA(B2), is expressed endogenously in NG108-15 cells.

Diversity and properties of calcium channel types in NG108-15 hybrid cells

We used an integral of the current-voltage relation as a new evaluation of Ca2+ current component composition in NG108-15 hybrid cells. We determined significant changes in the values and composition of Ca2+ currents during cell differentiation. Only low-voltage-activated Ca2+ currents could be observed in undifferentiated cells; after cell differentiation, high-voltage-activated currents appeared and the total Ca2+ current was increased about 30-fold. By pharmacological and biophysical separation, we determined four main types of Ca2+ channels in differentiated cells: approximately 50%, 20% and 17% of N, T and L types, respectively, and 12% of residual current, which is insensitive to classical blockers of low- and high-voltage-activated currents, with the exception of (omega-conotoxin GVIA. All current components displayed kinetics and pharmacological properties similar to neuronal ones. We also established a significant Ca2+ dependence of omega-conotoxin GVIA to inhibit N-type Ca2+ channels: 10 mM Ca2+ in bath solution reduced the toxin efficacy to block N channels three-fold. The residual component fitted the properties of Q-type Ca2+ channels: it was sensitive to (omega-conotoxin GVIA and very similar to the T-type channel with respect to its kinetics; however, the threshold of its activation was closer to the high-voltage-activated component (- 40 mV). Our results show the functional diversity of Ca2+ channels and demonstrate, for the first time, that presumably the Q type of an alpha1A family, which has biophysical and pharmacological properties distinct from the previously described T, L and N types in these cells, is co-expressed in NG108-15 cells.

Neurochemical and electrophysiological evidence for the existence of a functional gamma-hydroxybutyrate system in NCB-20 neurons

Clonal neurohybridoma NCB-20 cells express a valproate-insensitive succinic semialdehyde reductase activity that transforms succinic semialdehyde into gamma-hydroxybutyrate. This activity (1.14+/-0.16 nmol/min/mg protein) was similar to the lowest activity existing in adult rat brain. [3H]gamma-Hydroxybutyrate labels a homogeneous population of sites on NCB-20 cell membranes (Kd=250+/-44.4nM, Bmax=180+/-16.2fmol/mg protein) that apparently represents specific gamma-hydroxybutyrate binding sites characterized previously on brain cell membranes. Finally, an Na+-dependent uptake of [3H]gamma-hydroxybutyrate was expressed in NCB-20 cells with a Km of 35+21.1 microM and a Vmax of 80+/-14.2 pmol/min/mg protein. A three-day treatment with 1 mM dibutyryl-cyclic-AMP induced a three-fold increase in the cellular succinic semialdehyde reductase activity. In parallel, a K+-evoked release of [3H]gamma-hydroxybutyrate occurred. This release was Ca2+ dependent and was not present in undifferentiated cells. Cyclic-AMP treatment induced a decrease of [3H]gamma-hydroxybutyrate binding sites, which could be due to spontaneous gamma-hydroxybutyrate release. Patch-clamp experiments carried out on differentiated NCB-20 cells revealed the presence of Ca2+ conductances which were partially inhibited by 50 microM gamma-hydroxybutyrate. This gamma-hydroxybutyrate-induced effect was blocked by the gamma-hydroxybutyrate receptor antagonist NCS-382, but not by the GABA(B) antagonist CGP-55845. These results demonstrate the presence of an active gamma-hydroxybutyratergic system in NCB-20 cells which possesses the ability to release gamma-hydroxybutyrate. These cells express specific gamma-hydroxybutyrate receptors which modulate Ca2+ currents independently of GABA(B) receptors.

Ca2+ influx through voltage-gated Ca2+ channels regulates 5-HT3 receptor channel desensitization in rat glioma x mouse neuroblastoma hybrid NG108-15 cells

1. The kinetics of desensitization of the 5-HT3 receptor (5-HT3R)-gated ion channel were investigated using whole-cell and perforated-patch recording techniques in NG108-15 cells. 2. Rapid application of 5-HT (50 microM) elicited a 5-HT3R-mediated inward current response that desensitized completely in the continued presence of agonist. In the whole-cell recording configuration (holding potential of -70 mV) while buffering internal calcium (Cai2+) with 5 mM EGTA (0.5 mM added Ca2+; with an estimated free [Ca2+] of 30 nM), the rate of desensitization was initially rapid (with a half-time of approximately 230 ms), but dramatically slowed with time by 1120 +/- 160%. 3. This slowing in the rate of desensitization was reduced by stronger Ca2+ buffering (20 mM BAPTA, without added Ca2+), or by the bath application of cadmium (100 microM) to block voltage-gated Ca2+ channels. The rate of desensitization was also dependent on membrane potential. 4. In perforated-patch recordings, the rate of desensitization remained constant. However, a slowing in the desensitization rate could be induced by depolarizing cells immediately prior to the application of 5-HT. 5. The depolarization-induced slowing was blocked by incubating cells with BAPTA-AM (a membrane-permeant analogue of BAPTA) or by the bath application of cadmium. 6. These data suggest that Ca2+ influx through a cadmium-sensitive voltage-gated Ca2+ channel increases the cytoplasmic Ca2+ concentration ([Ca2+]i) and induces a dramatic slowing in the kinetics of desensitization of the 5-HT3R channel. These data provide evidence for cross-talk between voltage-gated Ca2+ channels and 5-HT3Rs in NG108-15 cells.

Calcineurin involvement in the regulation of high-threshold Ca2+ channels in NG108-15 (rodent neuroblastoma x glioma hybrid) cells

1. We examined the relationship between calcineurin (protein phosphatase 2B (PP2B) and voltage-operated Ca2+ channels (VOCCs) in NG108-15 cells. PP2B expression in NG108-15 cells was altered by transfection with plasmid constructs containing a full length cDNA of human PP2B beta(3) in sense (CN-15) and antisense (CN-21) orientation. 2. Confocal immunocytochemical localization showed that in wild-type cells, PP2B immunoreactivity is uniformly distributed in undifferentiated cells and located at the inner surface of soma membrane and neurites in differentiated cells. 3. To test the Ca2+ dependence of the VOCC, we used high-frequency stimulation (HFS). The L- and N-type VOCCs decreased by 37 and 52%, respectively, whereas the T-type current was only marginally sensitive to this procedure. FK-506 (2 microM), a specific blocker of PP2B, reduced the inhibition of L- and N-type VOCCs induced by HFS by 30 and 33%, respectively. 4. In CN-15-transfected cells overexpressing PP2B, total high-voltage-activated (HVA) VOCCs were suppressed by about 60% at a test potential of +20 mV. Intracellular addition of EGTA or FK-506 into CN-15-transfected cells induced an up to 5-fold increase of HVA VOCCs. 5. These findings indicate that PP2B activity does not influence the expression of HVA Ca2+ channels, but modulates their function by Ca(2+)-dependent dephosphorylation. Thus HVA VOCCs, in a phosphorylated state under control conditions, are downregulated by PP2B upon stimulation, with the major effect on N-type VOCCs.

The effect of overexpression of auxiliary Ca2+ channel subunits on native Ca2+ channel currents in undifferentiated mammalian NG108-15 cells

1. High voltage activated (HVA) Ca2+ channels are composed of a pore-forming alpha 1 subunit and the accessory beta and alpha2-delta subunits. However, the subunit composition of low voltage activated (LVA), or T-type, Ca2+ channels has yet to be elucidated. We have examined whether native calcium channels in NG108-15 mouse neuroblastoma x rat glioma hybrid cells, which express predominantly LVA currents when undifferentiated, are modulated by overexpression of accessory calcium channel subunits. 2. Endogenous alpha 1A, B, C, C, and E, and low levels of beta and alpha 2-delta subunit protein were demonstrated in undifferentiated NG108-15 cells. 3. The alpha 2-delta, beta 2a or beta 1b accessory subunits were overexpressed by transfection of the cDNAs into these cells, and the effect examined on the endogenous Ca2+ channel currents. Heterologous expression, particularly of alpha 2-delta but also of beta 2a subunits clearly affected the profile of these currents. Both subunits induced a sustained component in the currents evoked by depolarizing voltages above -30 mV, and alpha 2-delta additionally caused a depolarization in the voltage dependence of current activation, suggesting that it also affected the native T-type currents. In contrast, beta 1b overexpression had no effect on the endogenous Ca2+ currents, despite immunocytochemical evidence for its expression in the transfected cells. 4 These results suggest that in NG108-15 cells, overexpression of the Ca2+ channel accessory subunits alpha 2-delta and beta 2a induce a sustained component of HVA current, and alpha 2-delta also influences the voltage dependence of activation of the LVA current. It is possible that native T-type alpha 1 subunits are not associated with beta subunits.

Endomorphins inhibit high-threshold Ca2+ channel currents in rodent NG108-15 cells overexpressing mu-opioid receptors

1. Extracellular application of the novel brain peptides endomorphin 1 (EM1) and endomorphin 2 (EM2) inhibited high-threshold Ca2+ channel currents in NGMO-251 cells, a daughter clone of NG108-15 mouse neuroblastoma x rat glioma hybrid cells, in which mu-opioid receptors are overexpressed. 2. In contrast, EM1 and EM2 did not induce this inhibition in the parental NG108-15 cells that predominantly express endogenous delta-receptors. 3. The IC50 for EM1 and EM2 was 7.7 and 23.1 nM, respectively. 4. EM-induced Ca2+ channel current inhibition was blocked by treatment or pretreatment of the cells with 100 microM N-methylmaleimide or 100 ng ml-1 pertussis toxin. 5. These results show that a decrease in conductance of Ca2+ channels results following interaction of EMs with cloned mu-receptors, which couple via Gi/Go-type G proteins, and that EMs fulfill one of the necessary synaptic conditions for them to be identified as neurotransmitters.

Activation of nucleotide receptors inhibits high-threshold calcium currents in NG108-15 neuronal hybrid cells

A P2U (UTP-sensitive) nucleotide receptor has previously been cloned from NG108-15 neuroblastoma x glioma hybrid cells and it has been shown that activation of this receptor inhibits the M-type K+-current. We now report that UTP also inhibits Ca2+-currents in differentiated NG108-15 cells, but probably through a different nucleotide receptor. UTP (100 microM) inhibited the peak of the high-threshold current by 28.4 +/- 3.1% (n = 28) with no effect on the low-threshold current. Two components of high-threshold current were identified: one inhibited by 100 nM omega-conotoxin (CgTx) and one inhibited by 2 microM nifedipine and enhanced by 1 microM BAY K8644. UTP inhibited the former by 31.0 +/- 3.1%, with an IC50 of 2. 8 +/- 1.1 microM, and the latter 34.2 +/- 6.1% with an IC50 of 1.7 +/- 1.3 microM. Pertussis toxin pretreatment prevented inhibition of the CgTx-sensitive, nifedipine-resistant but not CgTx-resistant current. Inhibition was not prevented by intracellular BAPTA (20 mM) or cAMP (1mM). Effects of UTP on both currents were imitated by UDP, ATP, ADP, AP4A and ATPgammaS but weakly or not at all by 2-MeSATP, GTP, AMP-CPP or ITP. Since the receptors which inhibit Ca2+-currents are activated by ATP, it is suggested that they might mediate auto-inhibition of transmitter release by ATP if present on purinergic nerve terminals.

KEYWORDS

nucleotides, UTP, ATP, calcium currents, neuroblastoma cells

Modulation of neuronal calcium channels by arachidonic acid and related substances

Low-voltage-activated (l-v-a) and high-voltage-activated (h-v-a) Ca2+ currents (ICa) were recorded in whole-cell voltage clamped NG108-15 neuroblastoma x glioma hybrid cells. We studied the effects of arachidonic acid (AA), oleic acid, myristic acid and of the positively charged compounds tetradecyltrimethylammonium (C14TMA) and sphingosine. At pulse potentials > -20 mV, AA (25-100 microM) decreased l-v-a and h-v-a ICa equally. The decrease developed slowly and became continually stronger with increasing time of application. It was accompanied by a small negative shift and a slight flattening of the activation and inactivation curves of the l-v-a ICa. The shift of the activation curve manifested itself in a small increase of l-v-a ICa at pulse potentials < -30 mV. The effects were only partly reversible. The AA effect was not prevented by 50 microM 5, 8, 11, 14-eicosatetraynoic acid, an inhibitor of the AA metabolism, and not mimicked by 0.1-1 microM phorbol 12, 13-dibutyrate, an activator of protein kinase C. Probably, AA directly affects the channel protein or its lipid environment. Oleic and myristic acid acted similarly to AA but were much less effective. The positively charged compounds C14TMA and sphingosine had a different effect: They shifted the activation curve of l-v-a ICa in the positive direction and suppressed l-v-a more than h-v-a ICa; their effect reached a steady-state within 5-10 min and was readily reversible. C14TMA blocked l-v-a ICa with an IC50 of 4.2 microM while sphingosine was less potent.

Model experiments on squid axons and NG108-15 mouse neuroblastoma x rat glioma hybrid cells

Three types of ionic current essentially determine the firing pattern of nerve cells: the persistent Na+ current, the M current and the low-voltage-activated Ca(2)+ current. The present article summarizes recent experiments concerned with the basic properties of these currents. Keynes and Meves (Proc R Soc Lond B (1993) 253, 61-68) studied the persistent or steady-state Na+ current on dialysed squid axons and measured the probability of channel opening both for the peak and the steady-state Na+ current (PF(peak) and PF(ss)) as a function of voltage. Whereas PF(peak) starts to rise at -50 mV and reaches a maximum at +40 to +50 mV, PF(ss) only begins to rise appreciably at around 0 mV and is still increasing at +100 mV. This differs from observations on vertebrate excitable tissues where the persistent Na+ current tums on in the threshold region and saturates at around 0 mV. Schmitt and Meves (Pflugers Arch (1993) 425, 134-139) recorded M current, a non-inactivating K+ current, from NGI08-15 neuroblastoma x glioma hybrid cells, voltage-clamped in the whole-cell mode, and studied the effects of phorbol 12,13-dibutyrate (PDB), an activator of protein kinase C (PKC), and arachidonic acid (AA). PDB and AA both decreased I(M), the effective concentrations being 0.1-1 mu M and 5-25 mu M, respectively; while the PDB effect was regularly observed, the M current depression by AA was highly variable from cell to cell. The PKC 19-31 peptide, an effective inhibitor of PKC, in a concentration of 1 muM almost totally prevented the effects of PDB and AA on M current, suggesting that both are mediated by PKC. Schmitt and Meves (Pflugers Arch (1994a) 426, Suppl R 59) measured low-voltage-activated (l-v-a) and high-voltage-activated (h-v-a) Ca2+ currents on NG108-15 cells and investigated the effect of AA and PDB on both types of current. At pulse potentials > -20 mV, AA (25-100 mu M) decreased 1-v-a and h-v-a I(Ca). The decrease was accompanied by a small negative shift and a slight flattening of the activation and inactivation curves of the l-v-a I(Ca). The AA effect was not prevented by 50 mu M eicosa-5,8,11,14-tetraynoic acid (ETYA), an inhibitor of AA metabolism, or PKC 19-31 peptide and not mimicked by 0.1-1 mu M PDB. Probably, AA acts directly on the channel protein or its lipid environment. The physiological relevance of these three sets of observations is briefly discussed.

Effects of three fragments of parathyroid hormone on calcium channel currents in neonatal rat ventricular cells

The effects of different fragments of bovine parathyroid hormone, bPTH-(1-34), bPTH-(1-84) and bPTH-(3-34), on two types of calcium channel currents in neonatal rat ventricular cells were compared in the present study. bPTH-(1-34) increased the amplitude of L channel currents, but not of the T channel currents. This effect of bPTH-(1-34) was sustained after a complete washout of the peptide from the bath. The intact PTH molecule, bPTH-(1-84), also increased L channel currents but not affecting T channel currents. While bPTH-(3-34) did not affect the amplitudes of either L or T channel currents by itself, pretreatment of cells with bPTH-(3-34) abolished the effects of both bPTH-(1-34) and bPTH-(1-84) on L channel currents. Moreover, the kinetics of L channel currents in the presence of bPTH-(1-34) or bPTH-(3-34) were different. bPTH-(1-34) increased the time constant of activation, but not of inactivation, of L channel currents from 1.8 to 2.5 ms (P < 0.05). In contrast, bPTH-(3-34) decreased the time constant of inactivation, but not of activation, of L channel currents from 159 to 117 ms (P < 0.05). These results indicate that different fragments of PTH exert different effects on the amplitudes or kinetics of cardiac calcium channel currents.

Cholinergic modulation of immunoglobulin secretion from avian plasma cells: the role of calcium

The existence of a functional connection between the nervous and immune systems has long been argued. To determine if such a link exists in the secretory immune system, we have examined the avian lacrimal gland (Harderian gland) which contains large numbers of plasma cells. We have shown that these plasma cells bind an antibody to muscarinic acetylcholine receptor and that carbachol, an acetylcholine agonist, increases the secretion rate of IgG by these cells above a constitutive baseline level. This neurotransmitter-dependent increase of immunoglobulin secretion requires an influx of Ca2+, whereas the constitutive baseline secretion is apparently less dependent on such a flux. Furthermore, the Ca2+ flux appears to be mediated by voltage-dependent calcium channels. These data support the hypothesis that plasma cells can respond to neurotransmitters and, in the case of acetylcholine, increase immunoglobulin secretion.

A point mutation of the alpha 2-adrenoceptor that blocks coupling to potassium but not calcium currents

The alpha 2A-adrenergic receptor (adrenoceptor) was stably expressed in AtT20 mouse pituitary tumor cells; adrenoceptor agonists inhibited adenylyl cyclase, inhibited voltage-dependent calcium currents, and increased inwardly rectifying potassium currents. An aspartic acid residue (Asp79) highly conserved among guanine nucleotide-binding protein (G protein)-coupled receptors was mutated to asparagine; in cells transfected with the mutant alpha 2-receptor, agonists inhibited adenylyl cyclase and calcium currents but did not increase potassium currents. Because distinct G proteins appear to couple adrenoceptors to potassium and calcium currents, the present findings suggest that the mutant alpha 2-adrenoceptor cannot achieve the conformation necessary to activate G proteins that mediate potassium channel activation.

The angiotensin AT2 receptor modulates T-type calcium current in non-differentiated NG108-15 cells

We report here that angiotensin II (AII) and the AT2 receptor-selective ligand, CGP 42112, modulate the T-type calcium current in non-differentiated NG108-15 cells, which express only AT2 receptors. Both peptides decrease the T-type calcium current at membrane potentials above -40 mV and shift the current-voltage curve at lower potentials with maximal effect between 5 and 10 min after application. These data describe a new cellular response to AII and suggest that the AT2 receptor mediates certain neurophysiological actions of this hormone.

Voltage-clamp study of calcium currents during differentiation in the NCB-20 neuronal cell line

1. Calcium currents (ICa) were studied in voltage-clamped NCB-20 cells. In undifferentiated cells, voltage steps from hyperpolarized potentials (-80/-100 mV) essentially revealed transient ICa showing characteristics classically described for "T-type" channels. In about 50% of the cells, there was a residual current at the end of the step; no ICa was elicited from a holding potential of -50 mV. 2. In contrast, 100% of the cells differentiated with dibutyryl cyclic AMP (cAMP) displayed a residual current in addition to the transient one, and depolarizing steps from a holding potential of -50 mV induced a sustained current. In these cells, Bay K 8644 elicited both a negative shift in voltage dependence and a moderate increase of the sustained component. 3. Although these changes in Ca2+ channel physiology result from chemically induced differentiation, they might not be directly related to the concomitant morphologic differentiation. 4. In undifferentiated NCB-20 cells, T-type Ca2+ currents can be elicited in relative isolation.

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.

Norepinephrine activates potassium conductance in neurons of the turtle cerebral cortex

Whole-cell voltage and current clamp recordings were obtained from cortical neurons of the pond turtle, Pseudemys scripta elegans. Norepinephrine (NE) induced an outward current in 50% of pyramidal neurons. This current had a reversal potential of -88.3 +/- 3.2 mV, consistent with a K+ conductance increase, and had a mean amplitude of 18.3 +/- 7.2 pA at -40 mV. The ionic dependence and pharmacological analyses are both consistent with alpha 2 adrenergic receptor stimulation. Inhibition of Na(+)-dependent action potentials with TTX did not diminish the NE-induced K+ conductance, indicating that NE acts directly on the postsynaptic neuron. In addition to effects on postsynaptic conductance, NE dramatically decreased the amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in 55% of pyramidal neurons. The decrease in spontaneous IPSCs was observed both in those neurons which exhibited an increase in K+ conductance in response to NE administration (81%) and in those which did not (33%). Thus, NE modulates neuronal excitability both directly by activating a postsynaptic K+ conductance and indirectly by decreasing spontaneous IPSCs.

Changes of quantal transmitter release caused by gadolinium ions at the frog neuromuscular junction

1. The actions of the trivalent cation, gadolinium (Gd3+), were studied on frog isolated neuromuscular preparations by conventional electrophysiological techniques. 2. Gd3+ (450 microM) applied to normal or formamide-treated cutaneous pectoris nerve-muscle preparations induced, after a short delay, a complete block of neuromuscular transmission. The reversibility of the effect was dependent on the time of exposure. 3. Gd3+ (5-450 microM) had no consistent effect on the resting membrane potential of the muscle fibres. 4. Gd3+ (5-40 microM) applied to preparations equilibrated in solutions containing high Mg2+ and low Ca2+ reduced the mean quantal content of endplate potentials (e.p.ps) in a dose-dependent manner. Under those conditions, 3,4-diaminopyridine (10 microM) consistently reversed the depression of evoked quantal release. 5. The calcium current entering motor nerve terminals, revealed after blocking presynaptic potassium currents with tetraethylammonium (10 mM) in the presence of elevated extracellular Ca2+ (8 mM), was markedly reduced by Gd3+ (0.2-0.5 mM). 6. Gd3+ (40-200 microM) increased the frequency of spontaneous miniature endplate potentials (m.e.p.ps) in junctions bathed either in normal Ringer solution or in a nominally Ca(2+)-free medium supplemented with 0.7 microM tetrodotoxin. This effect may be due to Gd3+ entry into the nerve endings since it is not reversed upon removal of extracellular Gd3+ with chelators (1 mM EGTA or EDTA). Gd3+ also enhanced the frequency of me.p.ps appearing after each nerve stimulus in junctions bathed in a medium containing high Mg2+ and low Ca2+. 7. Gd3+, in concentrations higher than 100 microM, decreased reversibly the amplitude of m.e.p.ps suggesting a postsynaptic action. 8. It is concluded that the block of nerve-impulse evoked quantal release caused by Gd3 + is related to its ability to block the calcium current entering the nerve endings, supporting the view that Gd3 + blocks N-type Ca2+ channels; while the enhancement of spontaneous quantal release is probably the result of Gd3 + entry into motor nerve endings. Besides its dual prejunctional effects on quantal release it is suggested that Gd3 + exerts a postsynaptic action on the endplate acetylcholine receptor-channel complex.

Modulation of vertebrate neuronal calcium channels by transmitters

A large number of neurotransmitters have now been shown to reduce the amplitude and slow the activation kinetics of whole cell HVA ICa in a great diversity of neurons. These transmitters include L-glutamate (AMPA/kainate, metabotropic and NMDA receptors), GABA (via GABAB receptors, NA (via alpha 2 receptors), 5-HT, NA (via alpha 2 receptors), DA and several peptides. Both whole-cell and single-channel studies have demonstrated that the N-channel is the most common channel type to be blocked by transmitters, although an inhibition of the L-type channel has also occasionally been reported. The suppression of the N-type Ca current was commonly shown to be voltage-dependent, with a relief at large positive voltages. Strong evidence has been put forward showing that the transmitter action is mediated by a G-protein, with GDP-beta-S blocking transmitter action, and GTP-gamma-S directly inhibiting the Ca channel. Moreover, pertussis toxin blocked the transmitter action in most neurons, and following such block, injection of the G-protein Go restored transmitter action. A direct link between the G-protein and the Ca channel has been widely theorized to mediate the action of transmitters on certain neurons. There is also some evidence that certain transmitters in specific neurons mediate calcium channel inhibition through a 2nd messenger, perhaps protein kinase C. Transmitters have also been found, although uncommonly, to inhibit HVA L-type and LVA T-type channels. In addition, an enhancement of both HVA and LVA Ca currents by transmitters has been demonstrated, and substantial evidence exists for mediation of this action by cAMP.

Hearing fluctuation during migraine attacks. Activity of propranolol-HCl in rats

Transitory hearing impairment has been well documented in humans during basilar migraine attacks. Ischemia induced by activation of central nervous system adrenoreceptors by cAMP is one of the mechanisms that has been implicated, as well as calcium metabolism alterations. Propranolol-HCl has proven to be an effective treatment. Massive local adrenaline release is regarded as the primum movens mechanism triggering the liberation of other beta-receptor activators resulting in sterile edema and exudate. This exudate can be visualized (via ophthalmoscope) and is viewed as pathognomonic of a migraine attack. We investigated both the action of adrenaline and Propranolol-HCl on brainstem auditory potentials in 3 groups of young rats. The first group was perfused intra-arterially with adrenaline. The second group received the same treatment plus a prophylactic daily dose of Propranolol-HCl. The third group served as the control and received Propranolol-HCl, but was perfused with a Ringer solution. Alterations in blood flow and hearing sensitivity (BERA) following perfusion occurred in the first group only. Propranolol seems to exhibit a protective action during experimental attempts to induce migraine-like attacks in rats.

Inhibition of calcium currents by noradrenaline, somatostatin and opioids in guinea-pig submucosal neurones

1. Whole-cell recordings were made from submucosal neurones acutely dissociated from guinea-pigs. The actions of noradrenaline, somatostatin and [Met5]enkephalin on currents carried by calcium ions were studied. 2. On depolarization from a holding potential of -70 mV, an inward current activated at -40 mV, reached its peak amplitude at 10 mV and reversed to outward at 72 mV (with external calcium of 5 mM and internal caesium of 160 mM). 3. Cadmium, nickel and cobalt reversibly blocked the calcium current; concentrations causing 50% block were 2.5, 500 and 2000 microM respectively. The calcium current (holding at -70 or -30 mV) was reversibly blocked by omega-conotoxin (100 nM), and unaffected by Bay K 8644 (0.1-10 microM) and nifedipine (1 microM). Cadmium caused an outward shift in holding current at -30 mV, implying that there was a persistent inward calcium current at this potential. 4. Noradrenaline, somatostatin and [Met5]enkephalin decreased the calcium current. The maximal inhibition observed with any one agonist, or with a combination of two agonists, did not exceed 50%; concentrations giving half-maximal inhibition were 5.5 microM for noradrenaline, 4 nM for somatostatin and 1 microM for [Met5]enkephalin. The inhibition was independent of membrane potential. All three agonists also reduced the persistent calcium current at -30 mV. 5. Inhibition of the calcium current by noradrenaline occurred with a latency of not less than 175 ms; cadmium applied by the same method depressed the current within 5-45 ms. 6. Experiments with selective agonists and antagonists indicated that the receptor types involved in calcium current inhibition were alpha 2-adrenoceptors and delta-opioid receptors. Somatostatin acted at a distinct receptor. 7. Calcium currents were also inhibited by intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). Agonists were ineffective in cells pre-treated with pertussis toxin, but their action was restored when purified GTP-binding proteins (Go or Gi) were included in the intracellular recording solution. 8. It is concluded that noradrenaline, somatostatin and [Met5]enkephalin act at their respective receptors on guinea-pig submucosal neurones to inhibit a voltage-dependent calcium current. Activation of the same receptors also increases a potassium conductance in these cells: in both cases a pertussis-sensitive G protein is involved.

Calcium currents of neuroblastoma x glioma hybrid cells after cultivation with dibutyryl cyclic AMP and nickel

The long-term modulation of calcium (Ca2+) currents (ICa) was studied in 108CC15 neuroblastoma x glioma hybrid (NxG) cells grown under various culture conditions. The following results were obtained: 1. Addition of 1 mM dibutyryl cyclic adenosine monophosphate (db-cAMP) or 0.1 microM forskolin to the culture medium increased a transient component of ICa two-fold within 3 days, from 21.0 +/- 1.6 pA/pF (n = 22) to a maximum of 40.0 +/- 2.6 pA/pF (n = 28). Under these conditions, cells also expressed a slowly inactivating ICa component (maximum after 3 days, 20.5 +/- 1.6 pA/pF, n = 28). 2. The fast inactivating ICa as well as the db-cAMP-induced slowly inactivating ICa were completely down-regulated during incubation of NxG cells with the inorganic Ca2+ channel blocker, nickel (Ni2+, 100 microM). The suppressing effect was reversed within 3 days of incubation in db-cAMP-containing medium lacking Ni2+. 3. Binding studies on membrane preparations of control and Ni2(+)-pretreated NxG cells revealed a marked difference in the maximal (+)3H-PN200-110 binding. The difference was seen in undifferentiated as well as in db-cAMP-incubated cells. 4. The protein synthesis blocker, cycloheximide, suppressed both the db-cAMP-induced increase and the reappearance of ICa following Ni2+ pretreatment. It is suggested that chronic application of db-cAMP or Ni2+ to NxG cells increases and decreases the number of Ca2+ channel proteins, respectively.

Selective coupling of different muscarinic acetylcholine receptors to neuronal calcium currents in DNA-transfected cells

Acetylcholine (ACh) can inhibit calcium currents (ICa) in nerve cells by activating muscarinic ACh receptors (mAChR). There are several different genetic subtypes of mAChR. It is not known which subtype(s) are responsible for ICa inhibition. To resolve this issue, we measured ICa inhibition by ACh with patch-clamp recording, by using Ba2+ as charge carrier, in clones of NG108-15 neuroblastoma x glioma hybrid cells transfected with DNA for mAChRI, II, III and IV. Control (non-transfected) cells showed a mean maximum inhibition of peak ICa of 12.8 +/- 1.8% (n = 36) at 1 mM ACh. No consistent increase in inhibition was detected in vector-transfected cells, or in cells transformed to express mAChRI or mAChRIII. In contrast, inhibition was significantly increased in clones transformed to express mAChRII or mAChRIV. Inhibition was not correlated with the number of muscarinic receptors as determined by 3H-quinuclidinyl benzilate binding. Inhibition in both control and transfected cells was prevented by pretreatment with pertussis toxin (PTx). Inhibition persisted in the presence of extracellular or intracellular dibutyryl cyclic AMP, and hence is not because of inhibition of adenylate cyclase. We conclude that the inhibition of neuronal ICa is mediated preferentially by mAChRII and mAChRIV, via a PTx-sensitive GTP-binding protein.

Antibodies to the GTP binding protein, Go, antagonize noradrenaline-induced calcium current inhibition in NG108-15 hybrid cells

The voltage-dependent calcium current in chemically differentiated NG108-15 cells is depressed by noradrenaline acting on alpha-adrenoreceptors. The response is absent in cells pretreated with pertussis toxin, implicating the involvement of a G-protein. To identify this G-protein, we have studied the response to noradrenaline in cells preinjected with antibodies specific for two G-proteins, Gi and Go. Cells injected with the Gi antibody responded normally to noradrenaline. In contrast, the response to noradrenaline in cells injected with the Go antibody was markedly attenuated. We conclude that Go is employed in coupling alpha-adrenoreceptors to the calcium channels in NG108-15 cells.