Noradrenaline-Induced Inhibition of Voltage-Sensitive Calcium Currents in NG108-15 Hybrid Cells

G protein regulation of neuronal calcium channels: back to the future

Neuronal voltage-gated calcium channels have evolved as one of the most important players for calcium entry into presynaptic endings responsible for the release of neurotransmitters. In turn, and to fine-tune synaptic activity and neuronal communication, numerous neurotransmitters exert a potent negative feedback over the calcium signal provided by G protein-coupled receptors. This regulation pathway of physiologic importance is also extensively exploited for therapeutic purposes, for instance in the treatment of neuropathic pain by morphine and other μ-opioid receptor agonists. However, despite more than three decades of intensive research, important questions remain unsolved regarding the molecular and cellular mechanisms of direct G protein inhibition of voltage-gated calcium channels. In this study, we revisit this particular regulation and explore new considerations.

Rabies virus infection prevents the modulation by alpha(2)-adrenoceptors, but not muscarinic receptors, of Ca(2+) channels in NG108-15 cells

In mouse neuroblastoma x rat glioma hybrid (NG108-15) cells, we examined whether rabies virus infection affects the voltage-dependent Ca(2+) current (I(Ca)) and agonist-induced I(Ca) inhibition. The viral infection had little effect on the current-voltage relationship for peak I(Ca) or on the late I(Ca) that remained at the end of a 200-ms step depolarization. Noradrenaline and carbachol, via alpha(2)-adrenoceptors and muscarinic receptors, respectively, reduced I(Ca) concentration dependently. The maximum effect of noradrenaline was attained at 10 microM with 19.4+/-1.8% inhibition of I(Ca), which was significantly decreased to 9.9+/-1.3% after viral infection. The decrease was not reversed with 100 microM noradrenaline, suggesting that it does not result from a decrease in agonist sensitivity of cells. The maximum effect of carbachol (300 microM; 27.7+/-2.9% inhibition) remained unchanged, despite carbachol sharing intracellular signaling pathways with noradrenaline. These results indicate that in NG108-15 cells, rabies virus infection does not alter the functional expression of voltage-dependent Ca(2+) channels, but it attenuates the alpha(2)-adrenoceptor-mediated I(Ca) inhibition, possibly through some change at the receptor level.

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.

Opioid inhibition of Ca2+ channel subtypes in bovine chromaffin cells: selectivity of action and voltage-dependence

Bovine chromaffin cells possess a mixture of high-voltage-activated Ca2+ channel subtypes: L-type, dihydropyridine-sensitive channels, and N-, P- and Q-types, omega-conotoxin MVIIC-sensitive channels. In these cells, we studied the reversible, naloxone-antagonized inhibition of Ba2+ currents by the opioid agonist met-enkephalin (IC50 = 272 nM). This inhibition could be resolved into a voltage-dependent and a voltage-independent component. The first was revealed by its slow Ba2+ current activation kinetics at 0 mV and by the current facilitation induced by short prepulses to +90 mV. The second was estimated as the residual inhibition persisting after the facilitation protocol. The two inhibitory components varied markedly from cell to cell and each contributed to about half of the total inhibition. Replacement of internal GTP by GDP-beta-S or cell pretreatment with pertussis toxin completely abolished the voltage-dependent inhibition by opioids, partially preserving the voltage-independent component. The opioid-induced inhibition was not selective for any Ca2+ channel subtype, being not prevented after the addition of specific Ca2+ channel antagonists. However, when separately analysing the contribution of each channel type to the voltage-dependent and voltage-independent modulation, a clear-cut distinction could be achieved. The voltage-independent inhibition was effective on all Ca2+ channel subtypes but predominantly on L-type Ca2+ channels. The voltage-dependent process was abolished by omega-conotoxin-MVIIC, but unaffected by nifedipine, and was thus sharply restricted to non-L-type channels (N-, P- and Q-types). Our data suggest a functionally distinct opioid receptor-mediated modulation of L- and non-L-type channels, i.e. of the two channel classes sharing major control of catecholamine secretion from bovine chromaffin cells.

Selective inhibition of high voltage-activated L-type and Q-type Ca2+ currents by serotonin in rat melanotrophs

1. Whole-cell Ca2+ currents (ICa) from cultured rat melanotrophs were identified by their sensitivity to Ca2+ channel blockers, and their modulation by serotonin (5-HT) was studied. All cells displayed high voltage-activated (HVA; > -30 mV) Ca2+ currents. A low voltage-activated (LVA; > -60 mV) Ca2+ current was detected in 92% of the cells. 2. The whole-cell ICa was insensitive to omega-conotoxin GVIA (0.5-1 microM) indicating the absence of N-type Ca2+ channels. 3. At a holding potential (Vh) of -70 mV, the L-type channel blocker nifedipine reduced ICa in a dose-dependent manner with a half-maximal effective concentration (IC50) of 28 nM. The L-type current represented 39% of the total ICa. 4. omega-Agatoxin IVA (omega-Aga IVA) produced a biphasic dose-dependent inhibition of ICa, with IC50 values of 0.4 and 91 nM, indicating the presence of P-type and Q-type Ca2+ channels, which accounted respectively for 16 and 45% of the total ICa. The P-type current was also blocked by synthetic funnel-web spider toxin (sFTX 3.3; 1-10 microM) and was present only in a subpopulation (60-70%) of cells. 5. All cells possessed a Ca2+ current which was resistant to nifedipine (10 microM) and omega-Aga IVA (50 nM). This current was not affected by Ni2+ (40 microM) but was abolished by a low concentration of Cd2+ (10 microM) and by omega-conotoxin MVIIC (1 microM) indicating that it was a Q-type Ca2+ current. 6. 5-HT (10 microM) inhibited the whole-cell ICa in 70% of the cells tested (n = 120) by activating 5-HT1A and 5-HT2C receptors. 5-HT produced either a kinetic slowing of the activation phase (37% of the cells) or a scaling down (14% of the cells) of ICa. In the majority of cells (49%) both types of inhibition were found to coexist. 7. The effects of 5-HT were voltage dependent, rendered irreversible when GTP-gamma-S (30 microM) was present in the pipette solution and abolished by pretreatment of the cells with pertussis toxin (PTX; 150 ng ml-1, 18 h). 8. Low concentrations of omega-Aga IVA (20 nM), which blocked mainly P-type channels, did not reduce the effect of 5-HT on ICa. The scaling down effect of 5-HT on ICa was eliminated in the presence of nifedipine (10 microM) and the kinetic slowing effect of 5-HT persisted after blockade of L- and P-type channels but was abolished by omega-conotoxin MVIIC (1 microM). 9. We conclude that rat melanotrophs possess functional L-, P- and Q-type Ca2+ channels and that 5-HT inhibits selectively L-type and Q-type Ca2+ currents with different modalities. These effects are voltage dependent and mediated by a PTX-sensitive G-protein.

Osmo- and mechanosensitivity of the transient outward K+ current in a mammalian neuronal cell line

1. The transient outward current in NG108-15 cells was investigated with the whole-cell patch-clamp technique. The current was inhibited by external 4-aminopyridine or tetra-ethylammonium. The reversal potential shifted rightward with increased external K+ concentrations. 2. Current inactivation was markedly accelerated in hyperosmotic media (+30 mosmol l-1) and after nearby ejection of isosmotic solution with maximal acceleration occurring after 15-20 s and full recovery within 2-4 min, thus demonstrating an osmo- and mechanosensitivity of this current. Voltage-dependent Na+ and Ca2+ currents were unaffected. 3. Hyperosmotic solution shifted the voltage dependence of inactivation leftward. Inactivation was sensitive to reducing and oxidizing intracellular conditions. Reduction blocked the acceleration of current inactivation induced by hyperosmotic media, while oxidation did not hamper the response. 4. Action potentials had a decreased amplitude and a slower repolarization after hyperosmotic ejections. 5. It is concluded that the transient K+ current is osmo- and mechanosensitive, thus providing a mechanism for extracellular osmolarity to modulate neuronal excitability. The response appeared to be mediated through a changed sensitivity of the inactivating principle to the membrane electric field and was dependent on the redox state of the cell.

The amyloid precursor protein fragment His 657-Lys 676 inhibits noradrenaline- and enkephaline-induced suppression of voltage sensitive calcium currents in NG108-15 hybrid cells

We have investigated the effects of the C-terminal amyloid precursor protein fragment His 657-Lys 676 upon calcium currents in NG108-15 neuroblastoma x glioma hybrid cells. The amyloid precursor protein fragment His 657-Lys 676 (1-10 microM) did not affect calcium currents per se, but clearly blocked the calcium current suppression mediated by both adrenergic alpha 2B- and opioid delta receptors in a concentration-dependent manner. The reverse amyloid precursor protein fragment Lys 676-His 657 and the shorter amyloid precursor protein fragment Gly 659-Lys 676 did not affect calcium current suppression by adrenergic alpha 2B- and opioid delta receptors. The similar interaction of C-terminal amyloid precursor protein with adrenergic alpha 2B- and opioid delta receptors suggest that the effect occurs downstream of the receptor, possibly via the GTP binding protein Go.

Depression of high-threshold calcium currents by activation of human D2 (short) dopamine receptors expressed in differentiated NG108-15 cells

1. This study examined the regulation of calcium currents in differentiated NG108-15 cells that had been stably transfected with cDNA encoding the short isoform of the human D2 dopamine receptor. Whole cell calcium currents were recorded by nystatin-perforated patch clamp recording. 2. Transient low-threshold calcium currents elicited by depolarizations from -100 mV to -20 mV were reversibly depressed by NiCl2 (84 +/- 8% at 30 microM; n = 3) and by omega-agatoxin IVA (15 +/- 5%; 100 nM, n = 7). These currents were unaffected by hD2 receptor activation. 3. High-threshold calcium currents elicited by depolarizations from -80 mV to 0 mV were partly blocked by omega-conotoxin GVIA (67 +/- 6% at 100 nM, n = 4) and by the subsequent addition of the dihydropyridine, nisoldipine (94 +/- 3% at 1 microM). Consistent with the presence of at least two distinct types of high-threshold calcium channels, nisoldipine alone (38 +/- 15% at 1 microM, n = 6) did not preclude the inhibition caused by omega-conotoxin GVIA (69 +/- 13% at 100 nM, n = 4). The residual current was completely blocked by 100 microM CdCl2 (98.8 +/- 0.4%, n = 7). 4. In hD2-transfected cells, but not untransfected cells, high-threshold currents were depressed by quinpirole (30 +/- 4% at 100 nM; n = 15) with a pEC50 of 8.61 +/- 0.22 (n = 5), as well as by (-)-noradrenaline (28 +/- 5% at 1 microM, n = 9). Responses to both agonists were selectively antagonized by S-(-)sulpiride (100 nM) but not by the alpha-adrenoceptor antagonist, phentolamine (1O microM). The depression caused by (-)-noradrenaline was positively correlated with that of quinpirole for each cell(r2 = 0.91, slope = 0.99).5. hD2-receptor-mediated inhibition of high-threshold calcium currents was abolished by pretreatment of cells with omega-conotoxin GVIA (100 nM; n = 4). However, a component of the high-threshold current was reversibly depressed by omega-conotoxin GVIA (67% to 45% depression after 10 min wash). This current was also depressed by hD2 receptor activation (59 +/- 9% depression in 100 nM quinpirole, n = 3),and was completely blocked by nisoldipine (95 +/- 2% at 1 MicroM).6. These data demonstrate that activation of hD2(short) dopamine receptors can regulate both wconotoxinGVIA, and dihydropyridine-sensitive high-threshold calcium currents in neuroblastoma cells.Morever, the ability of human D2 dopamine receptors to regulate more than one type of calcium current supports the notion that these receptors have a diverse functional role in the central nervous system.

Agonist-induced inhibition of inositol-trisphosphate-activated IK(Ca) in NG108-15 neuroblastoma hybrid cells

IK(Ca) activated by intracellular ionophoresis of inositol trisphosphate (IP3) or pressure-applied acetylcholine was inhibited by bradykinin and acetylcholine in NG108-15 cells transfected with m1 receptors. The inhibition of the IP3-evoked current was complete at 10 microM acetylcholine. This inhibition was not seen if the current was evoked by intracellular ionophoresis of calcium ions. Only receptors the activate the phosphoinositide system in these cells produced this inhibition, i.e. transfected muscarinic m1 and m3 and bradykinin receptors, but not muscarinic m2, m4 or adrenergic alpha 2 receptors. This inhibition was not sensitive to pertussis toxin or staurosporine. The concentrations of acetylcholine needed to inhibit the evoked current were identical to those needed to raise intracellular calcium but tenfold less than those needed for the agonist to activate IK(Ca). In a normal calcium-containing superfusate, recovery from inhibition required around 8 min (half-time 4 min) after removal of acetylcholine. When the experiment was performed in calcium-free medium no recovery was seen after 8 min washing in drug-free solution, but complete recovery was seen within 3 min (half-time 1.5 min) after adding calcium. Responses to repeated pressure applications of acetylcholine could be reversibly inhibited by acetylcholine and bradykinin. It seems, then, that there is no direct action of acetylcholine or bradykinin on the IK(Ca) channels themselves but that concentrations below those needed to activate IK(Ca) can empty and inhibit the IP3-sensitive calcium store. This may provide a mechanism for heterologous desensitization for phospholipase-C-linked receptor-mediated responses.

Voltage-dependent noradrenergic modulation of omega-conotoxin-sensitive Ca2+ channels in human neuroblastoma IMR32 cells

High-threshold (HVA) Ca2+ channels of human neuroblastoma IMR32 cells were effectively inhibited by noradrenaline. At potentials between -20 mV and +10 mV, micromolar concentrations of noradrenaline induced a 50%-70% depression of HVA Ba2+ currents and a prolongation of their activation kinetics. Both effects were relieved at more positive voltages or by applying strong conditioning pre-pulses (facilitation). Facilitation restored the rapid activation of HVA channels and recruited about 80% of the noradrenaline-inhibited channels at rest. Re-inhibition of Ca2+ channels after facilitation was slow (tau r 36-45 ms) and voltage-independent between -30 mV and -90 mV. The inhibitory action of noradrenaline was dose-dependent (IC50 = 84 nM), mediated by alpha 2-adrenergic receptors and selective for omega-conotoxin-sensitive Ca2+ channels, which represent the majority of HVA channels expressed by IMR32 cells. The action of noradrenaline was mimicked by intracellular applications of GTP[gamma S] and prevented by GDP[beta S] or by pre-incubation with pertussis toxin. The time course of noradrenaline inhibition measured during fast application (onset) and wash-out (offset) of the drug were independent of saturating agonist concentrations (10-50 microM) and developed with mean time constants of 0.56 s (tau on) and 3.6 s (tau off) respectively. The data could be simulated by a kinetic model in which a G protein is assumed to modify directly the voltage-dependent gating of Ca2+ channels. Noradrenaline-modified channels are mostly inhibited at rest and can be recruited in a steep voltage-dependent manner with increasing voltages.

Voltage- and time-dependent inhibition of neuronal calcium channels by a GTP-binding protein in a mammalian cell line

1. Inhibitory modualtion of the three Ca2+ channel current components by neurotransmitters was studied using the whole-cell patch-clamp method in a mammalian cell line, NG108-15. 2. In cells differentiated with dibutyryl cyclic AMP, both the low-voltage-activated current (ILVA) and omega CgTX-sensitive high-voltage-activated current (I omega CgTX) could be inhibited by [D-phen2, D-phen5]enkephalin, acetylcholine and noradrenaline. In contrast, differentiation with prostaglandin E1 and theophylline eliminated the agonist-induced inhibition of ILVA, but enhanced that of I omega CgTX. The DHP-sensitive high-voltage-activated current was unaffected by the transmitters in most of the cells. 3. The inhibition was prevented by pre-treatment of cells with pertussis toxin, suggesting involvement of a G-protein. Long treatment of the cells with phorbol ester did not prevent the inhibition. 4. The inhibition was always partial: the maximal inhibition was 40% for ILVA and 70% for I omega CgTX. 5. The inhibition of ILVA and I omega CgTX was relieved during depolarization. Half-maximal relief of inhibition of I omega CgTX was attained at 0 mV, irrespective of agonist concentration. 6. The kinetics of removal and re-establishment of inhibition were voltage dependent. Both processes were single exponentials and had identical time constants at a given membrane potential. Time constants were 124 ms at -40 mV, 160 ms at 0 mV and 8 ms at 60 mV, at any agonist concentration. 7. Time courses of tail currents were unaltered by the inhibition. 8. The inhibition of the omega CgTX-sensitive Ca2+ channel can be described as a shift in gating modes; with an additional voltage-dependent gating state activated by the agonists. The voltage-dependent properties of this modulation allow inhibition of Ca2+ channel to be overcome by high-frequency trains of action potentials.

Neuromodulation

The ability of the nervous system to respond to the environment and to learn depends upon the tuning of neuronal electrical activity, loosely called neuromodulation. The substrates for electrical activity and, therefore, neuromodulation are ion channels which may be either synaptic or extrasynaptic. Neuromodulation is dynamic and most frequently involves neurotransmitters and hormones acting via G-protein-coupled pathways.

Cannabinoid receptor agonists inhibit Ca current in NG108-15 neuroblastoma cells via a pertussis toxin-sensitive mechanism

Cannabinoid receptor ligands irreversibly inhibited peak voltage-activated Ca currents (44%) in NG108-15 cells; this inhibition was Pertussis toxin-sensitive. Inhibition was largely due to a reduction in the omega-conotoxin sensitive portion of high-voltage activated (HVA) current, although there was also a significant decrease in low-voltage activated current (56%) and in the nifedipine-sensitive portion of HVA current (41%).

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.

Pharmacology of the putative M4 muscarinic receptor mediating Ca-current inhibition in neuroblastoma x glioma hybrid (NG 108-15) cells

1. We have assessed the potency of a range of agonists and antagonists on the muscarinic receptor responsible for inhibiting the Ca-current (ICa) in NG 108-15 hybrid cells. 2. Acetylcholine (ACh), oxotremorine-M and carbachol were potent 'full' agonists (EC50 values were 0.11 microM, 0.14 microM and 2 microM, respectively). Maximum inhibition of peak high-threshold ICa by these agonists was 39.5%. (+/-)-Muscarine, methylfurmethide and arecaidine propargyl ester (APE) were 'partial' agonists, with EC50 values of 0.54 microM, 0.84 microM and 0.1 microM, respectively. 3. Atropine, pirenzepine and himbacine were potent antagonists of muscarinic inhibition of ICa, with apparent pKB values of 9.8, 7.74 and 8.83, respectively. Methoctramine was relatively weak (pKB = 7.63). Atropine and pirenzepine depressed maximum responses to agonists, probably because these antagonists have relatively slow dissociation rates. 4. The characteristic pharmacological profile found for the M4 receptors in these functional experiments (himbacine high affinity, pirenzepine moderate to high affinity, methoctramine low affinity) corresponds well with data from earlier binding experiments (Lazareno et al., 1990). Since mRNA hybridising to probes for the m4 receptor genotype can be detected in these cells, it is suggested that these pharmacological characteristics identify the equivalent expressed receptor subtype M4.

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.

Noradrenergic inhibition and voltage-dependent facilitation of omega-conotoxin-sensitive Ca channels in insulin-secreting RINm5F cells

We found that, besides dihydropyridine-sensitive Ca channels, insulin-secreting RINm5F cells also contain a minority (15-25%) of omega-conotoxin (omega-CgTx)-sensitive channels that show a high-affinity binding to [125I] omega-CgTx (Kd 51 pM). Noradrenaline (NA, 10 microM) slows down Ca-channel activation in these cells and produces a sizeable reduction of Ca currents that is relieved by strong pre-conditioning depolarizations (facilitation). The action of NA is mimicked by intracellular application of GTP-gamma-S and is prevented by pertussis toxin (PTX) or by cell pre-incubation with omega-CgTx. This suggests specific noradrenergic inhibition of omega-CgTx-sensitive Ca channels that is modulated by membrane potentials and PTX-sensitive G-protein activation.

Mechanisms underlying presynaptic inhibition through alpha 2-adrenoceptors in guinea-pig submucosal neurones

1. Intracellular recordings were made from submucosal neurones of the guinea-pig ileum. The actions of noradrenaline, somatostatin and [Met5]enkephalin on nicotinic synaptic potentials (EPSPs) were studied. 2. In one series of experiments, agonists were applied by superfusion; noradrenaline (0.1-20 microM) decreased EPSP amplitude by 95-100% in all neurones. Similar application of somatostatin (1-100 nM) inhibited EPSPs in about half the neurones by a maximum of 40%. [Met5]enkephalin (0.1-10 microM) did not alter EPSPs. Idazoxan and yohimbine competitively antagonized the action of noradrenaline with dissociation equilibrium constants of 20 and 30 nM respectively. 3. In another series of experiments, noradrenaline and somatostatin were applied locally from a pipette so that they reached presynaptic terminals but not the cell bodies or axons of the presynaptic cell: noradrenaline inhibited EPSPs by 90% in all neurones but somatostatin had no effect. When applied locally to the cell bodies giving rise to the presynaptic fibres, both agonists inhibited EPSPs in half the neurones by 40%. 4. When noradrenaline was applied locally to presynaptic terminals, the latency to onset of noradrenaline to inhibit EPSPs was 45-160 ms; cadmium applied similarly depressed EPSPs in 5-50 ms. 5. Pertussis toxin pre-treatment only partially blocked presynaptic inhibition caused by noradrenaline but abolished the reduction of EPSP amplitude by somatostatin. 6. It is concluded that noradrenaline and somatostatin reduce the amplitude of the fast EPSP because they hyperpolarize cell bodies and prevent action potential initiation. Noradrenaline, but not somatostatin, has an additional action to inhibit acetylcholine release by acting at nerve terminal receptors. 7. The presynaptic inhibitory action of noradrenaline results from activation of alpha 2-adrenoceptors at nerve terminals but the mechanism(s) by which these presynaptic receptors act cannot be explained adequately by either activation of a potassium conductance and/or inhibition of a calcium conductance.

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.

Functional characterization of neuronal pre and postsynaptic alpha 2-adrenoceptor subtypes in guinea-pig submucosal plexus

1. The alpha 2-adrenoceptors on cell bodies of submucosal neurones, on presynaptic cholinergic nerve terminals innervating submucosal neurones, and on presynaptic sympathetic fibres innervating submucosal arterioles were characterized in functional studies by use of subtype selective ligands. 2. Both membrane hyperpolarization and presynaptic inhibition of nicotinic excitatory synaptic potentials (e.p.s.ps) produced by UK 14304 were similarly antagonized by idazoxan, yohimbine. SKF 104078, WB 4101 and ARC-239. Antagonism was competitive and dissociation equilibrium constants were the same for both effects. 3. Vasoconstriction of submucosal arterioles in response to stimulation of the sympathetic nerves (20 Hz for 2 s) was inhibited by UK 14304 and clonidine: concentrations producing half-maximum responses were 6 nm and 10 nM respectively. Idazoxan, yohimbine, WB 4101 and SKF 104078 antagonized this action, with dissociation constants similar to those for antagonism of the postsynaptic membrane hyperpolarization and presynaptic inhibition of nicotinic e.p.s.ps. 4. Oxymetazoline was a partial agonist when membrane hyperpolarization or presynaptic inhibition of nicotinic e.p.s.ps were measured but a full agonist when presynaptic inhibition of sympathetically-mediated arteriolar vasoconstriction was measured. As an agonist, oxymetazoline produced half maximum responses at 80-120 nM; the dissociation constant for oxymetazoline as an antagonist was 130 nM. 5. Neither prazosin nor chlorpromazine (up to 30 microM) altered any of the three responses to alpha 2-adrenoceptor agonists. 6. It is concluded that alpha 2-adrenoceptors present on submucosal neuronal cell bodies, on presynaptic cholinergic nerve terminals and on presynaptic sympathetic nerve terminals are the alpha 2A subtype. However, functional characterization of this subtype differs from that provided by ligand binding studies.

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.

Serotonin receptor activation reduces calcium current in an acutely dissociated adult central neuron

The release of serotonin (5-HT) from the terminals of serotonergic (raphe) neurons is under inhibitory feed-back control. 5-HT, acting on raphe cell body autoreceptors, also mediates inhibitory postsynaptic potentials as a result of release from collaterals from neighboring raphe neurons. This may involve a ligand (5-HT)-gated increase in the membrane potassium conductance, leading to a decrease in action potential frequency, which could indirectly reduce calcium influx into nerve terminals. In this report we demonstrate that 5-HT can also directly reduce calcium influx at potentials including and bracketing the peak of calcium current activation. Using acutely isolated, patch-clamped dorsal raphe neurons, we found that low concentrations of 5-HT and the 5-HT1A-selective agonist 8-OH-DPAT reversibly decrease whole-cell calcium current. This effect is antagonized by the putative 5-HT1A-selective antagonist NAN 190. Hence, the inhibition of calcium current may serve a physiological role in these cells and elsewhere in the brain.

A transient outward current in NG108-15 neuroblastoma x glioma hybrid cells

Outward currents were recorded from voltage-clamped NG108-15 mouse neuroblastoma X rat glioma hybrid cells, differentiated with prostaglandin E1. Depolarising voltage steps from -70 mV, evoked a transient outward current from a threshold of -30 mV. The outward current showed complete inactivation at potentials positive to -10 mV. Inactivation was removed by hyperpolarisation with half-inactivation at -53 mV. The time course of the inactivation could be best fitted by two exponentials with mean time constants of 280 ms and 1.6 s at +80 mV. Tail current measurements showed a shift in the reversal potential with changes in external K+ concentration, consistent with K+ as the current-carrying ion. The outward current amplitude was reversibly reduced by 4-aminopyridine, and the time course of inactivation modified. In the presence of other K+ channel blockers (tetraethylammonium, barium and tetrahydroaminoacridine) the amplitude of the outward current was also reversibly reduced, but with a negligible effect on its time course. The current was unaffected by dendrotoxin, d-tubocurarine, apamin, Cd2+ and Ni2+, and by replacing external Ca2+ with Co2+ or Mg2+. In current clamp, action potential duration was greatly increased by 4-aminopyridine. The findings show that the NG108-15 cell line displays a transient outward current that resembles IK(A) but with a higher than usual threshold and relatively slow inactivation, and that this current is likely to be important for action potential repolarisation.

Cromakalim, a vasodilator, differentially inhibits Ca2+ currents in NG108-15 neuroblastoma x glioma hybrid cells

Extracellular perfusion with the antihypertensive agent cromakalim produced an inhibition of 22-66% in the low-threshold transient Ca2+ (T-like) current in NG108-15 hybrid cells. Cromakalim suppressed the high-threshold and long-lasting Ba2+ current (L-like Ca2+ current) by 29-73%, but had almost no effect on the high-threshold and inactivating Ba2+ current (N-like Ca2+ current). IC50 for T-like and L-like currents was the same at about 100 microM. The inhibitory effect developed relatively fast and was reversible. These results indicate that cromakalim can selectively inhibit the activity of inward Ca2+ currents.

Selective action of myasthenic syndrome antibodies on calcium channels in a rodent neuroblastoma x glioma cell line

1. The effect of Lambert-Eaton myasthenic syndrome (LEMS) immunoglobulin G (IgG) on Ca2+ channels in undifferentiated mouse neuroblastoma x rat glioma hybrid cells (NG 108 15) was studied using the whole-cell patch clamp technique. 2. Sustained inward Ca2+ channel currents were evoked by depolarizing pulses from holding potentials of -80 and -40 mV, and were blocked by 5 microM-nitrendipine (L-type currents). Transient inward Ca2+ channel currents were activated from a holding potential of -80 mV by small depolarizing steps (T-type currents). Noradrenaline (10 microM) was without effect on transient currents. 3. LEMS IgG selectively reduced sustained (L-type) Ca2+ channel current amplitudes evoked from either holding potential used. In the presence of nitrendipine (5 microM), there was no significant effect of LEMS IgG on the remaining transient (T-type) Ca2+ channel current amplitudes. 4. Studies of the potential for maximal inward current indicated that voltage sensitivities of both L- and T-type Ca2+ channel current amplitudes were unaffected by LEMS IgG, whether recorded in the presence or absence of nitrendipine. LEMS IgG had no significant effect on the time-to-peak or decay of Ca2+ channel currents. 5. It is concluded that LEMS IgG acts selectively to cause functional loss of L-type, but not T-type, Ca2+ channels in NG 108 15 cells. Any effect of LEMS IgG on N-type channels (not present in these undifferentiated cells) was not studied here. LEMS IgG also acts at motor nerve terminal Ca2+ channels leading to muscle weakness. Thus antigenic similarities must exist between L-type channels in NG 108 15 cells and Ca2+ channels at motor nerve terminals.

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.

Neurotransmitter inhibition of neuronal calcium currents by changes in channel voltage dependence

The voltage-dependent calcium current of many neurons is depressed by transmitters such as noradrenaline, GABA, and kappa-opiate agonists. This modulation probably constitutes a major mechanism of presynaptic inhibition. Although recent work has implicated GTP-binding proteins in the mechanism of current inhibition, it is still unknown how the activation of those proteins alters the operation of the channels. In their initial description of the phenomenon, Dunlap and Fischbach proposed that noradrenaline acts by somehow reducing the number of functions calcium channels in the cell. By contrast with this hypothesis, I have found that inhibition of Ca2+ current is primarily due to a transmitter-induced change in the voltage-dependence with which channels are opened. Transmitters profoundly alter the voltage-dependence of channel activation, but there is little or no change in the number of functional channels activated by very large depolarizations. There is also little effect on the voltage-dependence of inactivation.

Noradrenaline- and Enkephalin-Induced Inhibition of Voltage-Sensitive Calcium Currents in NG108-15 Hybrid Cells

Voltage-sensitive calcium currents were recorded from chemically differentiated neuroblastoma x glioma hybrid (NG108-15) cells using the whole-cell clamp technique. Both noradrenaline and [D-Ala2, D-Leu5] enkephalin (DADLE) reversibly depressed the amplitude of the calcium current by up to 30%. The response to noradrenaline occluded that to DADLE suggesting that both agonists depress the same fraction of current. The response to DADLE but not that to noradrenaline desensitized rapidly. Cells responded normally to noradrenaline when desensitized to the opioid. Responses to either agonist were absent in cells pre-incubated with pertussis toxin. In addition the response to noradrenaline became irreversible in cells dialysed internally with a non-hydrolysable analogue of GTP. The response to noradrenaline was not affected by treatment of the cells with either membrane-permeable analogues of cAMP or a combination of forskolin and isobutylmethylxanthine. It is concluded that both noradrenaline and DADLE depress the same fraction of voltage-dependent calcium current in NG108-15 cells; that the responses are mediated by a pertussis-sensitive GTP-binding protein but are not secondary to a reduction in the intracellular concentration of cAMP; and that desensitization of the opioid response occurs at a site linked intimately to the opioid receptor rather than at a common site in the transduction pathway between receptor activation and reduction in the calcium channel current.