Characteristics of a human N-type calcium channel expressed in HEK293 cells

5-HT3 Receptors in Rat Dorsal Root Ganglion Neurons: Ca2+ Entry and Modulation of Neurotransmitter Release

Rat dorsal root ganglion (DRG) neurons express 5-hydroxytryptamine receptors (5-HT3Rs). To elucidate their physiological role in the modulation of sensory signaling, we aimed to quantify their functional expression in newborn and adult rat DRG neurons, as well as their ability to modulate the Ca²⁺-dependent neurotransmitter release, by means of electrophysiological techniques combined with fluorescence-based Ca²⁺ imaging. The selective 5-HT3R agonist mCPBG (10 μM) elicited whole-cell currents in 92.5% of adult DRG neurons with a significantly higher density current than in responding newborn cells (52.2%), suggesting an increasing serotoninergic modulation on primary afferent cells during development. Briefly, 5-HT3Rs expressed by adult DRG neurons are permeable to Ca²⁺ ions, with a measured fractional Ca²⁺ current (i.e., the percentage of total current carried by Ca²⁺ ions, Pf) of 1.0%, similar to the value measured for the human heteromeric 5-HT3_A/B receptor (P_f = 1.1%), but lower than that of the human homomeric 5-HT3_A receptor (P_f = 3.5%). mCPBG applied to co-cultures of newborn DRG and spinal neurons significantly increased the miniature excitatory postsynaptic currents (mEPSCs) frequency in a subset of recorded spinal neurons, even in the presence of Cd²⁺, a voltage-activated Ca²⁺ channel blocker. Considered together, our findings indicate that the Ca²⁺ influx through heteromeric 5-HT3Rs is sufficient to increase the spontaneous neurotransmitter release from DRG to spinal neurons.

Characterization of the triazine, T4, a representative from a novel series of CaV2 inhibitors with strong state-dependence, poor use-dependence, and distinctively fast kinetics

There is strong pharmacological, biological, and genetic evidence supporting the role of N-type calcium channels (CaV2.2) in nociception. There is also human validation data from ziconotide, the CaV2.2-selective peptidyl inhibitor used clinically to treat refractory pain. Unfortunately, ziconotide utility is limited by its narrow therapeutic window and required intrathecal route of administration. A major focus has been placed on identifying state-dependent CaV2.2 inhibitors to improve safety margins. Much less attention, however, has been given to characterizing the kinetics of CaV2.2 inhibitors as a means to further differentiate compounds and maximize therapeutic potential. Here we provide a detailed characterization of the CaV2.2 inhibitor T4 in terms of its state-dependence, use-dependence, kinetics, and mechanism of inhibition. Compound T4 displayed a >20-fold difference in potency when measured under inactivating conditions (IC50=1.1 μM) as compared to closed-state conditions (IC50=25 μM). At 3 μM, T4 produced a 15-fold hyperpolarizing shift in the inactivation curve for CaV2.2 while having no effect on channel activation. To assess the kinetic properties of T4 in a more physiological manner, its inhibition kinetics were assessed at 32°C using 2 mM Ca(2+) as the charge carrier. Surprisingly, the repriming rate for CaV2.2 channels at hyperpolarized potentials was similar in both the presence and absence of T4. This was in contrast to other compounds which markedly delayed repriming. Furthermore, T4 inhibited CaV2.2 channels more potently when channel inactivation was driven through a tonic sub-threshold depolarization rather than through a use-dependent protocol, despite similar levels of inactivation.

Omega-conotoxins as experimental tools and therapeutics in pain management

Neuropathic pain afflicts a large percentage of the global population. This form of chronic, intractable pain arises when the peripheral or central nervous systems are damaged, either directly by lesion or indirectly through disease. The comorbidity of neuropathic pain with other diseases, including diabetes, cancer, and AIDS, contributes to a complex pathogenesis and symptom profile. Because most patients present with neuropathic pain refractory to current first-line therapeutics, pharmaceuticals with greater efficacy in pain management are highly desired. In this review we discuss the growing application of ω-conotoxins, small peptides isolated from Conus species, in the management of neuropathic pain. These toxins are synthesized by predatory cone snails as a component of paralytic venoms. The potency and selectivity with which ω-conotoxins inhibit their molecular targets, voltage-gated Ca²⁺ channels, is advantageous in the treatment of neuropathic pain states, in which Ca²⁺ channel activity is characteristically aberrant. Although ω-conotoxins demonstrate analgesic efficacy in animal models of neuropathic pain and in human clinical trials, there remains a critical need to improve the convenience of peptide drug delivery methods, and reduce the number and severity of adverse effects associated with ω-conotoxin-based therapies.

Ca(v)2 channels mediate low and high voltage-activated calcium currents in Drosophila motoneurons

Different blends of membrane currents underlie distinct functions of neurons in the brain. A major step towards understanding neuronal function, therefore, is to identify the genes that encode different ionic currents. This study combined in situ patch clamp recordings of somatodendritic calcium currents in an identified adult Drosophila motoneuron with targeted genetic manipulation. Voltage clamp recordings revealed transient low voltage-activated (LVA) currents with activation between –60 mV and –70 mV as well as high voltage-activated (HVA) current with an activation voltage around –30 mV. LVA could be fully inactivated by prepulses to –50 mV and was partially amiloride sensitive. Recordings from newly generated mutant flies demonstrated that DmαG (Ca(v)3 homolog) encoded the amiloride-sensitive portion of the transient LVA calcium current. We further demonstrated that the Ca(v)2 homolog, Dmca1A, mediated the amiloride-insensitive component of LVA current. This novel role of Ca(v)2 channels was substantiated by patch clamp recordings from conditional mutants, RNAi knock-downs, and following Dmca1A overexpression. In addition, we show that Dmca1A underlies the HVA somatodendritic calcium currents in vivo. Therefore, the Drosophila Ca(v)2 homolog, Dmca1A, underlies HVA and LVA somatodendritic calcium currents in the same neuron. Interestingly, DmαG is required for regulating LVA and HVA derived from Dmca1A in vivo. In summary, each vertebrate gene family for voltage-gated calcium channels is represented by a single gene in Drosophila, namely Dmca1D (Ca(v)1), Dmca1A (Ca(v)2) and DmαG (Ca(v)3), but the commonly held view that LVA calcium currents are usually mediated by Ca(v)3 rather than Ca(v)2 channels may require reconsideration.

An integrated multiassay approach to the discovery of small-molecule N-type voltage-gated calcium channel antagonists

Abstract The N-type voltage-gated calcium channel (Cav2.2) has been intensively explored as a target for novel, small-molecule analgesic drugs because of its distribution in the pain pathway and its role in nociceptive processing. For example, Cav2.2 is localized at presynaptic terminals of pain fibers in the dorsal horn, and it serves as a downstream effector of μ-opioid receptors. Most importantly, antagonism of the channel by the highly specific and potent Cav2.2 blocker ω-conotoxin MVIIA (ziconotide) produces clinical efficacy in the treatment of severe, intractable pain. To identify novel small-molecule Cav2.2 inhibitors, we developed new tools and screening methods critical to enhance the efficiency and probability of success. First, we established and characterized a new cell line stably expressing the three subunits of the Cav2.2, including an α-subunit splice variant that is uniquely expressed by dorsal root ganglion neurons. Second, using this cell line, we validated and employed a fluorescence-based calcium flux assay. Third, we developed a new "medium-throughput" electrophysiology assay using QPatch-HT to provide faster turnaround on high-content electrophysiology data that are critical for studying ion channel targets. Lastly, we used a therapeutically relevant, ex vivo spinal cord calcitonin gene-related peptide-release assay to confirm activities in the other assays. Using this approach we have identified compounds exhibiting single-digit nM IC₅₀ values and with a positive correlation across assay methods. This integrated approach provides a more comprehensive evaluation of small-molecule N-type inhibitors that may lead to improved therapeutic pharmacology.

Control of chondrocyte regulatory volume decrease (RVD) by [Ca2+]i and cell shape

OBJECTIVES

Optimal matrix metabolism by articular chondrocytes is controlled by the 'set-point' volume which is determined mainly by membrane transporters. The signal transduction pathway(s) for the key membrane transporter which responds to cell swelling ('osmolyte channel') and mediates regulatory volume decrease (RVD) is poorly understood, so here the role of Ca2+ and the effects of 2D culture have been clarified.

METHODS

Changes to the volume and intracellular calcium levels ([Ca2+]i) of freshly isolated and 2D cultured bovine articular chondrocytes subjected to hypotonic challenge using a 43% reduction in medium osmolarity were studied by single-cell fluorescence microscopy. The effects of ethylene glycol tetraacetic acid (EGTA), REV5901 and Gd(3+) were studied and the role of Ca2+ influx determined by Mn2+ quench.

RESULTS

In freshly isolated cells, approximately 50% of chondrocytes exhibited 'robust RVD' (6[120]). RVD was inhibited by REV 5901 (4+/-2% responding) (3[23]) and 2 mM EGTA (18+/-5% responding) (4[166]) whereas Gd3+ had no effect (3[89]). The hypotonic challenge resulted in a Gd3+-insensitive rise in [Ca2+]i that did not correlate with RVD in all cells. Following 2D culture, chondrocytes also demonstrated Gd3+-insensitive RVD, but in contrast, the [Ca2+]i rise was blocked by this agent.

CONCLUSIONS

The data suggested that in freshly isolated and 2D cultured chondrocytes, the rise in [Ca2+]i occurring during hypotonic challenge could be related to RVD, but only in some cells. However, with 2D culture, the Ca2+ response switched to being Gd3+-sensitive, suggesting that as a result of changes to chondrocyte shape, stretch-activated cation channels although present, do not appear to play a role in volume regulation.

Protease treatment of cerebellar purkinje cells renders omega-agatoxin IVA-sensitive Ca2+ channels insensitive to inhibition by omega-conotoxin GVIA

The identification of currents carried by N- and P-type Ca(2+) channels in the nervous system relies on the use of omega-conotoxin (CTx) GVIA and omega-agatoxin (Aga) IVA. The peptide omega-Aga-IVA inhibits P-type currents at nanomolar concentrations and N-type currents at micromolar concentrations. omega-CTx-GVIA blocks N-type currents, but there have been no reports that it can also inhibit P-type currents. To assess the effects of omega-CTx-GVIA on P-type channels, we made patch-clamp recordings from the soma of Purkinje cells in cerebellar slices of mature [postnatal days (P) 40-50, P40-50] and immature (P13-20) rats, in which P-type channels carry most of the Ca(2+) channel current (>/=85%). These showed that micromolar concentrations of omega-CTx-GVIA inhibited the current in P40-50 cells (66%, 3 microM; 78%, 10 microM) and in P13-20 Purkinje cells (86%, 3 muM; 89%, 10 microM). The inhibition appeared to be reversible, in contrast to the known irreversible inhibition of N-type current. Exposure of slices from young animals to the enzyme commonly used to dissociate Purkinje cells, protease XXIII, abolished the inhibition by omega-CTx-GVIA but not by omega-Aga-IVA (84%, 30 nM). Our finding that micromolar concentrations of omega-CTx-GVIA inhibit P-type currents suggests that specific block of N-type current requires the use of submicromolar concentrations. The protease-induced removal of block by omega-CTx-GVIA but not by omega-Aga-IVA indicates a selective proteolytic action at site(s) on P-type channels with which omega-CTx-GVIA interacts. It also suggests that Ca(2+) channel pharmacology in neurons dissociated using protease may not predict that in neurons not exposed to the enzyme.

Ziconotide: a review of its pharmacology and use in the treatment of pain

Ziconotide is a powerful analgesic drug that has a unique mechanism of action involving potent and selective block of N-type calcium channels, which control neurotransmission at many synapses. The analgesic efficacy of ziconotide likely results from its ability to interrupt pain signaling at the level of the spinal cord. Ziconotide is a peptidic drug and has been approved for the treatment of severe chronic pain in patients only when administered by the intrathecal route. Importantly, prolonged administration of ziconotide does not lead to the development of addiction or tolerance. The current review discusses the various studies that have addressed the in vitro biochemical and electrophysiological actions of ziconotide as well as the numerous pre-clinical studies that were conducted to elucidate its antinociceptive mechanism of action in animals. In addition, this review considers the pivotal Phase 3 (and other) clinical trials that were conducted in support of ziconotide's approval for the treatment of severe chronic pain and tries to offer some insights regarding the future discovery and development of newer analgesic drugs that would act by a similar mechanism to ziconotide but which might offer improved safety, tolerability and ease of use.

Kinetics of internalization and degradation of N-type voltage-gated calcium channels: Role of the α2/δ subunit

The contribution of voltage-gated calcium channels to excitable cell function depends, critically, upon the mechanisms that control their expression at the cell surface. While co-assembly of the pore forming alpha(1) and auxiliary beta subunits enhances channel surface expression, the levels are still only 30-40% of those seen with the core alpha(1B)/beta(1b)/alpha(2)delta calcium channel complex. To rationalize this observation, it has been suggested that the alpha(2)/delta subunit might stabilize calcium channel expression at the cell surface. To test this notion, we have resolved the effect of the alpha(2)/delta subunit on the rates of binding, internalization and degradation of defined N-type calcium channel surface complexes expressed in HEK293 cells, through pulse-labeling with the selective, cell impermeable, radioligand [(125)I]-omega-CgTx. Through detailed kinetic and sensitivity analysis we show that alpha(1B)/beta(1b)/alpha(2)delta complexes are internalized slowly (k(int) 0.4/h), whereupon, most become degraded (k(deg) 0.02/h). In contrast, alpha(1B)/beta(1b) complexes are internalized more rapidly (k(int) 0.8/h), following which they are either quickly degraded (k(deg) 0.1/h) or are sequestered slowly (k(tra) 0.1/h) to a pool that is metabolically stable within the time-frame of our experiments (24h). In neither case did we find evidence for recycling via the cell surface. Thus, our data argue for a novel mechanism where complexes lacking an alpha(2)/delta subunit are cleared from the cell surface and are rapidly degraded or stored, possibly for further attempts at complexation as new alpha(2)/delta subunits become available. The slower rate of internalization of complexes containing the alpha(2)/delta subunit rationalizes the stabilizing effect this subunit has upon calcium channel surface expression and suggests a mechanism by which alpha(2)delta mutations may cause severe neurological deficits.

Effects of voltage-gated calcium channel subunit genes on calcium influx in cultured C. elegans mechanosensory neurons

Voltage-gated calcium channels (VGCCs) serve as a critical link between electrical signaling and diverse cellular processes in neurons. We have exploited recent advances in genetically encoded calcium sensors and in culture techniques to investigate how the VGCC alpha1 subunit EGL-19 and alpha2/delta subunit UNC-36 affect the functional properties of C. elegans mechanosensory neurons. Using the protein-based optical indicator cameleon, we recorded calcium transients from cultured mechanosensory neurons in response to transient depolarization. We observed that in these cultured cells, calcium transients induced by extracellular potassium were significantly reduced by a reduction-of-function mutation in egl-19 and significantly reduced by L-type calcium channel inhibitors; thus, a main source of touch neuron calcium transients appeared to be influx of extracellular calcium through L-type channels. Transients did not depend directly on intracellular calcium stores, although a store-independent 2-APB and gadolinium-sensitive calcium flux was detected. The transients were also significantly reduced by mutations in unc-36, which encodes the main neuronal alpha2/delta subunit in C. elegans. Interestingly, while egl-19 mutations resulted in similar reductions in calcium influx at all stimulus strengths, unc-36 mutations preferentially affected responses to smaller depolarizations. These experiments suggest a central role for EGL-19 and UNC-36 in excitability and functional activity of the mechanosensory neurons.

Pharmacological characterization of recombinant N-type calcium channel (Cav2.2) mediated calcium mobilization using FLIPR

The N-type voltage-gated calcium channel (Ca(v)2.2) functions in neurons to regulate neurotransmitter release. It comprises a clinically relevant target for chronic pain. We have validated a calcium mobilization approach to assessing Ca(v)2.2 pharmacology in two stable Ca(v)2.2 cell lines: alpha1(B), alpha2delta, beta(3)-HEK-293 and alpha1(B), beta(3)-HEK-293. Ca(v)2.2 channels were opened by addition of KCl and Ca(2+) mobilization was measured by Fluo-4 fluorescence on a fluorescence imaging plate reader (FLIPR(96)). Ca(v)2.2 expression and biophysics were confirmed by patch-clamp electrophysiology (EP). Both cell lines responded to KCl with adequate signal-to-background. Signals from both cell lines were inhibited by omega-conotoxin (ctx)-MVIIa and omega-conotoxin (ctx)-GVIa with IC(50) values of 1.8 and 1nM, respectively, for the three-subunit stable, and 0.9 and 0.6nM, respectively, for the two-subunit stable. Other known Ca(v)2.2 blockers were characterized including cadmium, flunarizine, fluspirilene, and mibefradil. IC(50) values correlated with literature EP-derived values. Novel Ca(v)2.2 pharmacology was identified in classes of compounds with other primary pharmacological activities, including Na(+) channel inhibitors and antidepressants. Novel Na(+) channel compounds with high potency at Ca(v)2.2 were identified in the phenoxyphenyl pyridine, phenoxyphenyl pyrazole, and other classes. The highest potency at Ca(v)2.2 tricyclic antidepressant identified was desipramine.

Effects of isoflurane and xenon on Ba2+-currents mediated by N-type calcium channels

BACKGROUND

Isoflurane and xenon are inhalation general anaesthetics with differing clinical profiles and contrasting synaptic actions. Both agents have been shown to depress excitatory synaptic responses. Whether this is via pre-synaptic or post-synaptic mechanisms has not been determined clearly. N-type calcium channels are a putative pre-synaptic target for these agents. We tested whether N-type calcium channels were sensitive to isoflurane and xenon and whether there was any stereoselectivity in the effect of isoflurane.

METHODS

We used patch-clamp electrophysiology on isolated HEK293 cells stably expressing N-type calcium channels to investigate the effects of isoflurane and xenon on barium currents mediated by N-type calcium channels.

RESULTS

Racemic isoflurane caused a concentration-dependent reduction (11-35%) in the peak current through the N-type channels in the concentration range 0.15-1.22 mM. In the clinically relevant concentration range the inhibition was small. At an isoflurane concentration of 0.31 mM (equivalent to 1 MAC), the peak N-type current was inhibited by 14 (1)%. The optical isomers of isoflurane were found to be equally potent at inhibiting currents through N-type channels. The inert gas anaesthetic xenon was found to have no measureable effect on N-type channels at a concentration of 3.4 mM (approximately 1 MAC).

CONCLUSIONS

These results suggest that N-type calcium channels are not the targets mediating general anaesthesia with these two inhalation agents.

Adenine nucleotides inhibit recombinant N-type calcium channels via G protein-coupled mechanisms in HEK 293 cells; involvement of the P2Y13 receptor-type

1. N-type Ca(2+) channel modulation by an endogenous P2Y receptor was investigated by the whole-cell patch-clamp method in HEK 293 cells transfected with the functional rabbit N-type calcium channel. 2. The current responses (I(Ca(N))) to depolarizing voltage steps were depressed by ATP in a concentration-dependent manner. Inclusion of either guanosine 5'-O-(3-thiodiphosphate) or pertussis toxin into the pipette solution as well as a strongly depolarizing prepulse abolished the inhibitory action of ATP. 3. In order to identify the P2Y receptor subtype responsible for this effect, several preferential agonists and antagonists were studied. Whereas the concentration-response curves of ADP and adenosine 5'-O-(2-thiodiphosphate) indicated a higher potency of these agonists than that of ATP, alpha,beta-methylene ATP, UTP and UDP were considerably less active. The effect of ATP was abolished by the P2Y receptor antagonists suramin and N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, 2'deoxy-N(6)-methyladenosine-3',5'-diphosphate or 2-methylthio AMP. 4. Using reverse transcription and polymerase chain reaction, mRNA for the P2Y(1), P2Y(4), P2Y(6), P2Y(11) and P2Y(13) receptor subtypes, but not the P2Y(2), and P2Y(12) subtypes, was detected in HEK 293 cells. 5. Immunocytochemistry confirmed the presence of P2Y(1), and to a minor extent that of P2Y(4), but not of P2Y(2) receptors. 6. Hence, it is tempting to speculate that P2Y(13) receptors may inhibit N-type Ca(2+) channels via the betagamma subunits of the activated G(i) protein.

Spinally delivered N-, P/Q- and L-type Ca2+-channel blockers potentiate morphine analgesia in mice

We studied the antinociceptive effects induced at the spinal level by N-, P/Q- and L-type voltage-dependent Ca2+-channel (VDCC) blockers given alone or in combination with morphine, the test responses being the algesic ones induced by acute thermal and mechanical stimuli. When given alone, intrathecal omega-agatoxin IVA (P/Q-type blocker) produced a potent dose-dependent inhibition in the tail-flick and tail-pressure over the dose range 0.33-33 pmol/mouse. Omega-conotoxin GVIA (N-type blocker) also produced dose-dependent inhibitions, but its antinociception against thermal stimuli was weaker than against mechanical stimuli. Calciseptine (L-type blocker) slightly reduced both nociceptive responses, but only at 33 pmol. At their subthreshold doses, intrathecal omega-agatoxin IVA, omega-conotoxin GVIA and calciseptine each significantly enhanced morphine analgesia in the tail-flick and tail-pressure tests, the rank order of potencies being N-> or =P/Q->L-type. These results indicate that combining a low-dose VDCC blocker, especially the N- or P/Q-type, with morphine may be a very useful way of minimizing the dose of morphine and may reduce side effects.

Omega-conotoxin CVID inhibits a pharmacologically distinct voltage-sensitive calcium channel associated with transmitter release from preganglionic nerve terminals

Neurotransmitter release from preganglionic parasympathetic neurons is resistant to inhibition by selective antagonists of L-, N-, P/Q-, R-, and T-type calcium channels. In this study, the effects of different omega-conotoxins from genus Conus were investigated on current flow-through cloned voltage-sensitive calcium channels expressed in Xenopus oocytes and nerve-evoked transmitter release from the intact preganglionic cholinergic nerves innervating the rat submandibular ganglia. Our results indicate that omega-conotoxin CVID from Conus catus inhibits a pharmacologically distinct voltage-sensitive calcium channel involved in neurotransmitter release, whereas omega-conotoxin MVIIA had no effect. omega-Conotoxin CVID and MVIIA inhibited depolarization-activated Ba(2+) currents recorded from oocytes expressing N-type but not L- or R-type calcium channels. High affinity inhibition of the CVID-sensitive calcium channel was enhanced when position 10 of the omega-conotoxin was occupied by the smaller residue lysine as found in CVID instead of an arginine as found in MVIIA. Given that relatively small differences in the sequence of the N-type calcium channel alpha(1B) subunit can influence omega-conotoxin access (Feng, Z. P., Hamid, J., Doering, C., Bosey, G. M., Snutch, T. P., and Zamponi, G. W. (2001) J. Biol. Chem. 276, 15728-15735), it is likely that the calcium channel in preganglionic nerve terminals targeted by CVID is a N-type (Ca(v)2.2) calcium channel variant.

Involvement of T-type calcium channels in excitatory junction potentials in rat resistance mesenteric arteries

1. We investigated the role of voltage-operated calcium channels in sympathetic transmission and depolarization-induced contractions in the rat mesenteric artery. In particular, we investigated the role of the T-type voltage-operated calcium channels (T-channels) in mediating excitatory junction potentials (EJPs). 2. EJPs were evoked by electrical field stimulation (trains of five stimuli at 0.9 Hz) in small mesenteric arteries. The average resting membrane potential was -59.8+/-0.5 mV (n=65). Trains of stimuli evoked individual EJPs with the peak EJP of 6+/-0.2 mV (n=34) occurring with the second stimulus. Trains of EJPs were inhibited 90% by tetrodotoxin (0.1 micro M) or by omega-conotoxin GVIA (GVIA, 10 nM) indicating their neural origin. 3. The EJPs were not inhibited by the L-type calcium channel blocker nicardipine at 0.1 micro M, a concentration sufficient to abolish the contraction to potassium depolarization. However, mibefradil (3 micro M), considered a relatively selective T-channel antagonist, inhibited the EJPs by about 50%. This concentration of mibefradil did not inhibit GVIA-sensitive electrically-evoked twitches of the rat vas deferens. Thus the action of mibefradil in reducing EJPs is unlikely to be due to either inhibition of L- or N-type channels but is probably due to inhibition of T-channels. 4. The finding that Ni(2+) (300 micro M), an inhibitor of T-type calcium channels, also reduced EJP amplitude by about 80% but did not block electrically-evoked twitches in the rat vas deferens, further supports an important role of T-channels in mediating small depolarizations associated with the EJPs evoked by sympathetic nerve stimulation.

Antinociceptive action of amlodipine blocking N-type Ca2+ channels at the primary afferent neurons in mice

We investigated the antinociceptive action of amlodipine, a dihydropyridine derivative, which acts on both L- and N-type voltage-dependent Ca2+ channels (VDCCs), in mice. Intrathecal injection of amlodipine (300 nmol/kg) significantly shortened the licking time in the late phase of a formalin test, while no effect was found with another dihydropyridine derivative, nicardipine (300 nmol/kg). Cilnidipine and omega-conotoxin GVIA also showed marked analgesic effects under the same experimental conditions. Transcripts of alpha1A, alpha1B, alpha1E, alpha1F, alpha1H, beta3, and beta4 subunits were detected by polymerase-chain reaction (PCR) in the dorsal root ganglion, suggesting the existence of a variety of voltage-dependent Ca2+ channels. Electrophysiological experiments showed that amlodipine and cilnidipine inhibit N-type currents in the dorsal root ganglion cells. These results suggest that amlodipine, cilnidipine, and omega-conotoxin GVIA exert their antinociceptive actions by blocking N-type Ca2+ channels in the primary nociceptive afferent fibers. Blocking of the Ca2+ channels results in attenuation of synaptic transmission of nociceptive neurons. Furthermore, it is suggested that some N-type Ca2+ channel blockers might have therapeutic potential as analgesics when applied directly into the subarachnoidal space.

High-voltage-activated calcium channel messenger RNA expression in the 140-3 neuroblastoma-glioma cell line

Expression of calcium channel alpha1 subunits in oocytes or cell lines has proven to be a powerful method in the analysis of structure-function relations, but these experimental systems are of limited value in the examination of neuron-specific functions such as transmitter release. Cell lines derived from neurons are often capable of these functions, but their intrinsic calcium channel alpha1 subunits are complicating factors in experimental design. We have examined the biophysical and molecular properties of calcium channels in a little studied neuroblastoma-glioma hybrid cell line, 140-3, a close relative of the NG108-15 cell line, to test whether this cell line might serve a role as an expression system for neural mechanisms. This cell was selected as it contains an intact transmitter release mechanism yet secretes little in response to depolarization. Patch-clamp recording revealed only a prominent low-threshold, rapidly inactivating calcium current with a single-channel conductance of approximately 7 pS that was identified as T type. A search for calcium channel alpha1 subunit messenger RNA in the 140-3 cells with three different tests only revealed alpha1C, whereas alpha1A-alpha1C were present in the parent NG108-15 line. We made a particular effort to search for alpha1E, since this subunit has been associated with a low-voltage-activated current. Our findings suggest that, since the principal nerve terminal-associated calcium channels (alpha1A, alpha1B, alpha1E) are absent in the 140-3 cell, this cell line may prove a particularly useful model for the analysis of the role of high-voltage-activated calcium channels in complex functions of neuronal cells.

Structural elements in domain IV that influence biophysical and pharmacological properties of human alpha1A-containing high-voltage-activated calcium channels

We have cloned two splice variants of the human homolog of the alpha1A subunit of voltage-gated Ca2+ channels. The sequences of human alpha1A-1 and alpha1A-2 code for proteins of 2510 and 2662 amino acids, respectively. Human alpha1A-2alpha2bdeltabeta1b Ca2+ channels expressed in HEK293 cells activate rapidly (tau+10mV = 2.2 ms), deactivate rapidly (tau-90mV = 148 micros), inactivate slowly (tau+10mV = 690 ms), and have peak currents at a potential of +10 mV with 15 mM Ba2+ as charge carrier. In HEK293 cells transient expression of Ca2+ channels containing alpha1A/B(f), an alpha1A subunit containing a 112 amino acid segment of alpha1B-1 sequence in the IVS3-IVSS1 region, resulted in Ba2+ currents that were 30-fold larger compared to wild-type (wt) alpha1A-2-containing Ca2+ channels, and had inactivation kinetics similar to those of alpha1B-1-containing Ca2+ channels. Cells transiently transfected with alpha1A/B(f)alpha2bdeltabeta1b expressed higher levels of the alpha1, alpha2bdelta, and beta1b subunit polypeptides as detected by immunoblot analysis. By mutation analysis we identified two locations in domain IV within the extracellular loops S3-S4 (N1655P1656) and S5-SS1 (E1740) that influence the biophysical properties of alpha1A. alpha1AE1740R resulted in a threefold increase in current magnitude, a -10 mV shift in steady-state inactivation, and an altered Ba2+ current inactivation, but did not affect ion selectivity. The deletion mutant alpha1ADeltaNP shifted steady-state inactivation by -20 mV and increased the fast component of current inactivation twofold. The potency and rate of block by omega-Aga IVA was increased with alpha1ADeltaNP. These results demonstrate that the IVS3-S4 and IVS5-SS1 linkers play an essential role in determining multiple biophysical and pharmacological properties of alpha1A-containing Ca2+ channels.

Mechanism of rise and decay of thapsigargin-evoked calcium signals in MDCK cells

We studied the effect of thapsigargin on intracellular calcium levels ([Ca2+]i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. Thapsigargin elevated [Ca2+]i dose dependently with an EC50 of approximately 0.15 microM. The Ca2+ signal consisted of a slow rise, a gradual decay and a plateau. Depletion of the endoplasmic reticulum Ca2+ store with thapsigargin for 7 min abolished the [Ca2+]i increases evoked by bradykinin. Removal of extracellular Ca2+ reduced the thapsigargin response by approximately 50%. The Ca2+ signal was initiated by Ca2+ release from the internal store followed by capacitative Ca2+ entry (CCE). The thapsigargin-evoked CCE was abolished by La3 and Gd3+, and was partly inhibited by SKF 96365 and econazole. After depletion of the internal Ca2+ store for 30 min with another inhibitor of the internal Ca2+ pump, cyclopiazonic acid, thapsigargin failed to increase [Ca2+]i, thus suggesting that the thapsigargin-evoked Ca2+ influx was solely due to CCE. We investigated the mechanism of decay of the thapsigargin response. Pretreatment with La3+ (or Gd3+) or alkalization of extracellular medium to pH 8 significantly potentiated the Ca2+ signal; whereas pretreatment with carbonylcyanide m-chlorophynylhydrozone (CCCP) or removal of extracellular Na+ had no effect. Collectively, our results imply that thapsigargin increased [Ca2+]i in MDCK cells by depleting the internal Ca2+ store followed by CCE, with both pathways contributing equally. The decay of the thapsigargin response might be significantly governed by efflux via the plasmalemmal Ca2+ pump.

Mechanism of rise and decay of 2,5-di-tert-butylhydroquinone-induced Ca2+ signals in Madin Darby canine kidney cells

We examined the effect of 2,5-di-tert-butylhydroquinone (BHQ) on intracellular Ca2+ concentrations ([Ca2+]i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. BHQ increased [Ca2+]i in a dose-dependent manner with an EC50 of 40 microM. The Ca2+ signal showed a slow onset, a gradual decay and a sustained plateau in normal Ca2+ medium. Depletion of the endoplasmic reticulum Ca2+ store by incubation with 0.1 mM BHQ for 6 min abolished the [Ca2+]i increase evoked by bradykinin or ATP, suggesting that BHQ depleted the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store. Removal of extracellular Ca2+ reduced the BHQ response by 50%. The Ca2+ signal was initiated by Ca2+ release from the internal store, followed by capacitative Ca2+ entry which was abolished by 100 microM La3+ or 50 microM Gd3+ and was partly inhibited by 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF 96365). After depletion of the endoplasmic reticulum Ca2+ store, by incubation with another inhibitor of the endoplasmic reticulum Ca2+ pump, thapsigargin for 30 min, BHQ did not elevate [Ca2+]i, suggesting that the BHQ-induced Ca2+ influx was largely due to capacitative Ca2+ entry, and that BHQ released Ca2+ from the thapsigargin-sensitive endoplasmic reticulum store. We investigated the mechanism of decay of the BHQ response. Pretreatemt with La3+ (or Gd3+) or alkalization of the extracellular medium to pH 8 significantly potentiated the Ca2+ signal, whereas pretreatment with carbonylcyanide m-chlorophenylhydrazone (CCCP) or oligomycin, or removal of extracellular Na+, had no effect. Collectively, our results suggest that BHQ increased [Ca2+]i in MDCK cells by depleting the endoplasmic reticulum Ca2+ store followed by capacitative Ca2+ entry, with both pathways contributing equally. The decay of the BHQ response is effected by Ca2+ efflux via the plasma membrane Ca2+ pump, but not by efflux via Na+/Ca2+ exchange or sequestration by the mitochondria.

Electrophysiological characterisation of the human N-type Ca2+ channel III: pH-dependent inhibition by a synthetic macrocyclic polyamine

The effects of a novel synthetic macrocyclic polyamine (LY310315) were investigated on recombinant human N-type Ca2+ channels stabley expressed in HEK293 cells. LY310315 proved to be a potent and reversible N-type Ca2+ channel antagonist. Inhibition by this compound was dose-dependent with an IC50 of approximately 0.4 microM at pH 7.35. LY310315 blocked very rapidly at all concentrations tested. Upon washout, recovery of the Ca2+ current developed with a time constant of approximately 30 s. Use-dependence in the development of block indicated that voltage-dependent transitions in the channel protein were required to permit significant inhibition. Application of > 100 times the IC50 dose of LY310315 to the interior of the cell produced no detectable Ca2+ current inhibition. LY310315 had no effects on the kinetics of channel activation or deactivation but did slightly slow the rate of macroscopic inactivation observed during a 300 ms test depolarisation. In the presence of LY310315 the activation curve was significantly shallower. This resulted in a shift in the activation midpoint voltage to a more depolarised levels. LY310315-induced inhibition of human N-type channels was strongly dependent on the extracellular pH, with increased potency seen upon extracellular acidification. Although most effective against N-type Ca2+ channels, LY310315 was also found to inhibit both P-type and L-type Ca2+ channels. LY310315 proved to be a weak blocker of Na+ currents, but produced approximately 50% of the K+ currents of AtT20 cells at a concentration of 0.5 microM.

Using gadolinium to identify stretch-activated channels: technical considerations

Gadolinium (Gd3+) blocks cation-selective stretch-activated ion channels (SACs) and thereby inhibits a variety of physiological and pathophysiological processes. Gd3+ sensitivity has become a simple and widely used method for detecting the involvement of SACs, and, conversely, Gd3+ insensitivity has been used to infer that processes are not dependent on SACs. The limitations of this approach are not adequately appreciated, however. Avid binding of Gd3+ to anions commonly present in physiological salt solutions and culture media, including phosphate- and bicarbonate-buffered solutions and EGTA in intracellular solutions, often is not taken into account. Failure to detect an effect of Gd3+ in such solutions may reflect the vanishingly low concentrations of free Gd3+ rather than the lack of a role for SACs. Moreover, certain SACs are insensitive to Gd3+, and Gd3+ also blocks other ion channels. Gd3+ remains a useful tool for studying SACs, but appropriate care must be taken in experimental design and interpretation to avoid both false negative and false positive conclusions.

Human autoantibodies specific for the alpha1A calcium channel subunit reduce both P-type and Q-type calcium currents in cerebellar neurons

The pharmacological properties of voltage-dependent calcium channel (VDCC) subtypes appear mainly to be determined by the alpha1 pore-forming subunit but, whether P-and Q-type VDCCs are encoded by the same alpha1 gene presently is unresolved. To investigate this, we used IgG antibodies to presynaptic VDCCs at motor nerve terminals that underlie muscle weakness in the autoimmune Lambert-Eaton myasthenic syndrome (LEMS). We first studied their action on changes in intracellular free Ca2+ concentration [Ca2+]i in human embryonic kidney (HEK293) cell lines expressing different combinations of human recombinant VDCC subunits. Incubation for 18 h with LEMS IgG (2 mg/ml) caused a significant dose-dependent reduction in the K+-stimulated [Ca2+]i increase in the alpha1A cell line but not in the alpha1B, alpha1C, alpha1D, and alpha1E cell lines, establishing the alpha1A subunit as the target for these autoantibodies. Exploiting this specificity, we incubated cultured rat cerebellar neurones with LEMS IgG and observed a reduction in P-type current in Purkinje cells and both P- and Q-type currents in granule cells. These data are consistent with the hypothesis that the alpha1A gene encodes for the pore-forming subunit of both P-type and Q-type VDCCs.

Structural diversity of the voltage-dependent Ca2+ channel alpha1E-subunit

Voltage-operated Ca2+ channels are heteromultimeric proteins. Their structural diversity is caused by several genes encoding homologous subunits and by alternative splicing of single transcripts. Isoforms of alpha1 subunits, which contain the ion conducting pore, have been deduced from each of the six cDNA sequences cloned so far from different species. The isoforms predicted for the alpha1E subunit are structurally related to the primary sequence of the amino terminus, the centre of the subunit (II-III loop), and the carboxy terminus. Mouse and human alpha1E transcripts have been analysed by reverse transcription-polymerase chain reaction and by sequencing of amplified fragments. For the II-III loop three different alpha1E cDNA fragments are amplified from mouse and human brain, showing that isoforms originally predicted from sequence alignment of different species are expressed in a single one. Both predicted alpha1E cDNA fragments of the carboxy terminus are identified in vivo. Two different alpha1E constructs, referring to the major structural difference in the carboxy terminus, were stably transfected in HEK293 cells. The biophysical properties of these cells were compared in order to evaluate the importance in vitro of the carboxy terminal insertion found in vivo. The wild-type alpha1E subunit showed properties, typical for a high-voltage activated Ca2+ channel. The deletion of 43 amino acid residues at the carboxy terminus does not cause significant differences in the current density and the basic biophysical properties. However, a functional difference is suggested, as in embryonic stem cells, differentiated in vitro to neuronal cells, the pattern of transcripts indicative for different alpha1E isoforms changes during development. In human cerebellum the longer alpha1E isoform is expressed predominantly. Although, it has not been possible to assign functional differences to the two alpha1E constructs tested in vitro, the expression pattern of the structurally related isoforms may have functional importance in vivo.

Effects of Ca2+ and Na+ channel inhibitors in vitro and in global cerebral ischaemia in vivo

In the present study we have examined the effects of the small organic molecules: NNC 09-0026 ((-)-trans-1-butyl-4-(4-dimethylaminophenyl)-3-[(4-trifluoromethyl-ph eno xy) methyl] piperidine dihydrochloride); SB 201823-A (4-[2-(3,4-dichlorophenoxy)ethyl]-1-pentyl piperidine hydrochloride); NS 649 (2-amino-1-(2,5-dimethoxyphenyl)-5-trifluoromethyl benzimidazole); CNS 1237 (N-acenaphthyl-N'-4-methoxynaphth-1-yl guanidine) and riluzole on human omega-conotoxin sensitive N-type voltage-dependent Ca2+ channel currents (ICa) expressed in HEK293 cells, on Na+ channel currents (INa) in acutely isolated cerebellar Purkinje neurones in vitro and in the gerbil model of global cerebral ischaemia in vivo. Estimated IC50 values for steady-state inhibition of ICa were as follows; NNC 09-0026, 1.1 microM; CNS 1237, 4.2 microM; SB 201823-A, 11.2 microM; NS 649, 45.7 microM and riluzole, 233 microM. Estimated IC50 values for steady-state inhibition of Na+ channel currents were as follows: NNC 09-0026, 9.8 microM; CNS 1237, 2.5 microM; SB 201823-A, 4.6 microM; NS 649, 36.7 microM and riluzole, 9.4 microM. In the gerbil model of global cerebral ischaemia the number of viable cells (mean +/- S.E.M.) per 1 mm of the CA1 was 215 +/- 7 (sham operated), 10 +/- 2 (ischaemic control), 44 +/- 15 (NNC 09-0026 30 mg/kg i.p.), 49 +/- 19 (CNS 1237 30 mg/kg i.p.), 11 +/- 2 (SB 201823-A 10 mg/kg i.p.), 17 +/- 4 (NS 649 50 mg/kg i.p.) and 48 +/- 18 (riluzole 10 mg/kg i.p.). Thus NNC 09-0026, CNS 1237 and riluzole provided significant neuroprotection when administered prior to occlusion while SB 201823-A and NS 649 failed to protect. These results indicate that the Ca2+ channel antagonists studied not only inhibited human N-type voltage-dependent Ca2+ channels but were also effective blockers of rat Na+ channels. Both NNC 09-0026 and CNS 1237 showed good activity at both Ca2+ and Na+ channels and this may contribute to the observed neuroprotection.

Preparation and purification of antibodies specific to human neuronal voltage-dependent calcium channel subunits

Neuronal voltage-dependent calcium channels (VDCCs) each comprising of alpha 1, alpha 2 delta, and beta subunits, are one mechanism by which excitable cells regulate the flux of calcium ions across the cell membrane following depolarisation Studies have shown the expression of several alpha 1 and beta subtypes within neuronal tissue. The comparative distribution of these in normal human brain is largely unknown. The aim of this work is to prepare antibodies directed specifically to selected subunits of human neuronal VDCCs for use in biochemical and mapping studies of calcium channel subtypes in the brain. Previous studies have defined DNA sequences specific for each subunit Comparison of these sequences allows the selection of unique amino acid sequences for use as immunogens which are prepared as glutathione-S-transferase (GST) fusion proteins in E. coli. Polyclonal antibodies raised against these fusion proteins are purified by Protein A chromatography, followed by immunoaffinity chromatography and extensive adsorptions using the appropriate fusion protein-GST Sepharose 4B columns. The resultant antibodies are analysed for specificity against the fusion proteins by ELISA, and by immunofluorescence and Western immunoblot analysis of recombinant HEK293 cells stably transfected with cDNAs encoding alpha 1, alpha 2 delta and beta subunits.

Electrophysiological properties of the human N-type Ca2+ channel: I. Channel gating in Ca2+, Ba2+ and Sr2+ containing solutions

We have characterized the properties of the human N-type Ca2+ channel produced by the stable co-expression of the alpha(1B-1), alpha(2b)delta and beta(1b) subunits. The channel displayed the expected pharmacology with respect to the toxins omega-CTx-GVIA and omega-CTx-MVIIC, which depressed currents in a voltage-independent fashion. We characterized a variety of biophysical properties of the channel under conditions in which either Ca2+, Ba2+ or Sr2+ was the sole extracellular divalent ion. In all three ions, current-voltage relationships revealed that the channel was clearly high-voltage activated. Current activation was significantly slower in Ca2+ than either Sr2+ or Ba2+. Construction of conductance-voltage relationships from tail current measurements indicated that the channel was more high-voltage activated in Ca2+ than in either Sr2+ or Ba2+. The rank order of current amplitude at +4 mV was Ba2+ > Sr2+ > or = Ca2+. Elevation of the extracellular concentration of Ba2+ increased maximal current amplitude and shifted the current-voltage relationship to the right. In all three ions channel inactivation was complex consisting of three distinct exponentials. Recovery from inactivation was slow taking several seconds to reach completion. Steady-state inactivation curves revealed that channel inactivation became detectable at holding potentials of between -101 and -91 mV depending on the permeating species. The rank order of mid-points of steady state inactivation was (most negative) Sr2+ > Ca2+ > Ba2+ (most positive). Deactivation of the N-type Ca2+ channel was voltage-dependent and very fast in all three ions. The deactivation rate in Ba2+ was significantly slower than that in both Ca2+ and Sr2+, however the voltage-dependence of deactivation rate was indistinguishable in all three ions.

Stretch-induced stimulation of lower airway nitric oxide formation in the guinea-pig: inhibition by gadolinium chloride

The effect of stretch on lower airway nitric oxide formation was studied in normoxic tracheostomized anaesthetized guinea-pigs. Increase of level of positive end-expiratory pressure caused increased lower airway nitric oxide formation, as measured by its presence in exhaled tracheal air. The L-type calcium channel blocker, verapamil, did not decrease lower airway nitric oxide formation. Neither the local anaesthetic xylocaine nor the ganglion blocker trimetaphan affected exhaled nitric oxide, excluding local and centrally-mediated neuronal reflexes. Intravenous administration of gadolinium chloride (GdCl3, 50 mg/kg) induced a rapid and pronounced decrease (75%) in the basal level of exhaled nitric oxide. GdCl3 completely abolished lower airway nitric oxide formation induced by ventilation with positive end-expiratory pressure (7 cm H2O). GdCl3 induced hypoxaemia, but there was no indication for the development of lung oedema. The results indicate that positive end-expiratory pressure stimulates lower airway nitric oxide formation in the guinea-pig. GdCl3 inhibits lower airway nitric oxide formation in the guinea-pig in vivo, perhaps by interference with stretch-induced cellular calcium-influx.

Receptor-mediated modulation of recombinant neuronal class E calcium channels

The modulation of a cloned neuronal calcium channel was studied in a human embryonic kidney cell line (HEK293). The HEK293 cells were stably transfected with the alpha1Ed cDNA, containing the pore forming subunit of a neuronal class E calcium channel. Inward currents of 25 +/- 1.9 pA/pF (n = 79) were measured with the cloned alpha1Ed-subunit. The application of the peptide hormone somatostatin, carbachol, ATP or adenosine reduced the amplitude of Ca2+ and Ba2+ inward currents and exhibited a slowing of inactivation. This inhibitory effect by somatostatin was significantly impaired after pre-incubating the transfected cell line with pertussis toxin (PTX). Internal perfusion of the cells with the G-protein-inactivating agent GDP-beta-S or with the permanently activating agent GTP-gamma-S also attenuated the somatostatin effect. The inhibition indicates that modulation of the alpha1Ed-mediated Ca2+ current involves pertussis toxin-sensitive G-proteins. The block of Ca2+ and Ba2+ inward currents by somatostatin is also found in cells expressing a truncated alpha1Ed-subunit which lacks a 129-bp fragment in the C-terminus. This fragment corresponds to the major structural difference between two native human alpha1E splice variants. As somatostatin inhibits inward currents through both, the cloned alpha1Ed- and the truncated alpha1Ed-DEL-subunit, the hormone-mediated modulation is independent from the presence of the 129-bp insertion in the C-terminus.

Role of L- and N-type Ca2+ channels in muscarinic receptor-mediated facilitation of ACh and noradrenaline release in the rat urinary bladder

1. 3H-Noradrenaline (NA) and 14C-acetylcholine (ACh) released by electrical field stimulation were measured simultaneously in strips from the body of rat urinary bladder. 2. omega-Conotoxin GVIA (omega-CgTX; 20-100 nM) suppressed the non-facilitated transmitter release evoked by intermittent stimulation (IS), whereas nifedipine (1 microM) did not affect release. 3. Continuous electrical stimulation (CS) facilitated NA and ACh release via an atropine-sensitive mechanism. omega-CgTX reduced the facilitated release of NA (44% depression) but did not affect ACh release. Nifedipine depressed ACh release (43%) but not NA release. Combined administration of nifedipine and omega-CgTX (20 nM) produced a greater suppression of NA and ACh release (86 and 91%, respectively). 4. Maximal muscarinic facilitation of NA (5-fold) and ACh (17-fold) release occurred following administration of eserine, an anticholinesterase agent. Release of both NA and ACh was depressed by nifedipine (70 and 83%, respectively) but not by omega-CgTX. Combined application of omega-CgTX and nifedipine elicited a further depression of NA (95%) but not ACh release. 5. When NA and ACh release was facilitated with phorbol dibutyrate (0.5 microM), nifedipine inhibited ACh (67%) but not NA release, whereas omega-CgTX inhibited NA (73%) but not ACh release. Combined administration of both Ca2+ channel blockers did not elicit greater inhibition. 6. Bay K 8644, the L-type Ca2+ channel activator, increased ACh release in a dose-dependent manner (up to 5-fold) but did not significantly change NA release. 7. Both omega-CgTX (20-100 nM) and nifedipine (100 nM-1 microM) significantly decreased (50-80%) the neurally evoked contractions of the bladder strips. 8. It is concluded that L-type Ca2+ channels play a major role in muscarinic facilitation of NA and ACh release in the urinary bladder but are not essential for non-facilitated release. Other types of Ca2+ channels, including N-type, are involved to varying degrees in non-facilitated and facilitated release under different experimental conditions.

Inhibition of neuronal high voltage-activated calcium channels by insect peptides: a comparison with the actions of omega-conotoxin GVIA

The whole cell variant of the patch clamp technique was used to investigate the actions of two novel insect peptides on high voltage-activated Ca2+ currents in cultured dorsal root ganglion (DRG) neurones. The insect peptides (PMP-D2 and PMP-C) were isolated originally from insect brains and fat bodies, and have been found to have similar three-dimensional structures to the N-type Ca2+ channel inhibitor omega-conotoxin GVIA (omega-CgTx GVIA). High voltage-activated Ca2+ currents were activated from a holding potential of -90 mV by depolarizing step commands to 0 mV. Extracellular application of synthetic PMP-D2 or PMP-C (1 microM) attenuated high voltage-activated Ca2+ currents. The effects of PMP-C were strongly dependent on the frequency of current activation, but inhibition was apparent and reached a steady state after 20 steps when currents were evoked for 30 msec at 0.1 Hz. The actions of the two insect peptides overlapped both with each other and with omega-CgTx GVIA, suggesting that N-type Ca2+ current was predominantly sensitive to these peptides. Low voltage-activated T-type current and 1,4-dihydropyridine sensitive L-type Ca2+ currents were insensitive to 1 microM PMP-D2 and PMP-C, which indicates a degree of selectivity. The presence of a fucose group on PMP-C abolished the ability of this peptide to attenuate high voltage-activated Ca2+ currents, which may reflect a mechanism by which peptide function could be regulated in insects. The electrophysiological data are supported by studies on 45Ca2+ influx into rat cerebrocortical synaptosomes. Both PMP-D2 (10 microM), PMP-C (10 microM) and omega-CgTx GVIA (1 microM) attenuated a proportion of 45Ca2+ influx into the synaptosomes, but additive effects of these peptides were not observed. We conclude that these naturally occurring peptides obtained from invertebrate preparations have inhibitory effects on N-type Ca2+ channels. Although the peptides have related three-dimensional structures, they have distinct amino acid sequences and appear to have different mechanisms of action to produce inhibition of mammalian neuronal high voltage-activated Ca2+ channels.

Selective G-protein regulation of neuronal calcium channels

We examined the properties and regulation of Ca channels resulting from the expression of human alpha1B and alpha1E subunits stably expressed in KEK293 cells. The ancillary subunits beta1B and alpha2/delta were also stably expressed in these cell lines. Ca currents in alpha1B-expressing cells had the properties of N-type currents. Ca currents in cells expressing alpha1E exhibited a novel profile that was similar to the properties of the "R type" Ca current. Introduction of GTP-gamma-S into alpha1B cells greatly enhanced the extent of prepulse facilitation of the Ca current, whereas it had only a very small effect in alpha1E-expressing cells. Activation of somatostatin receptors endogenous to HEK293 cells or kappa opioid receptors, expressed in the cells after transfection, inhibited Ca currents in alpha1B-expressing cells. This inhibition was blocked by pertussis toxin and was partially relieved by a depolarizing prepulse. In contrast, no inhibitory effects were noted in cells expressing alpha1E channels under the same circumstances. HEK293 cells normally contained G-proteins from all of the four major families. Inhibition of Ca currents by kappa agonists in alpha1B-expressing cells was enhanced slightly by the cotransfection of several G-protein alpha subunits. kappa agonists, however, had no effect in alpha1E-containing cells, even after overexpression of different G-protein alpha-subunits. In summary, these results demonstrate that there is a large difference in the susceptibility of alpha1B- and alpha1E-based Ca channels to regulation by G-proteins. This is so despite the fact that the two types of Ca channels show substantial similarities in their primary sequences.

The effects of ifenprodil and eliprodil on voltage-dependent Ca2+ channels and in gerbil global cerebral ischaemia

Ifenprodil and eliprodil are both non-competitive NMDA receptor antagonists which have been shown to inhibit neuronal Ca2+ channel currents. We have examined the effects of these agents on two defined subtypes of voltage-dependent Ca2+ channels and in the gerbil model of global cerebral ischaemia. Recombinantly expressed human alpha 1B-1 alpha 2b beta 1-3 Ca2+ subunits in HEK293 cells, which results in an omega-conotoxin-sensitive neuronal N-type voltage-dependent Ca2+ channel and omega-Aga IVA sensitive Ca2+ channels (P-type) in acutely isolated cerebellar Purkinje neurones were reversibly inhibited by ifenprodil and eliprodil. Human N-type Ca2+ channel currents were inhibited by ifenprodil and eliprodil with IC50 values of 50 microM and 10 microM respectively whereas P-type Ca2+ channel currents were inhibited reversibly by ifenprodil and eliprodil with approximate IC50 values of 60 microM and 9 microM respectively. Maximum current block observed for both channel subtypes was approximately 80% for both ifenprodil and eliprodil. For neuroprotection studies, animals were subjected to 5 min bilateral carotid artery occlusion with or without administration of either ifenprodil or eliprodil (5, 10 or 20 mg/kg i.p.) immediately after surgery followed by two further doses (2.5, 5 or 10 mg/kg, respectively) at 3 and 6 h post-occlusion. Both compounds provided significant protective effects against ischaemia-induced neurodegeneration in the CA1 region of the hippocampus. These results indicate that both ifenprodil and eliprodil protect against ischaemia-induced neurodegeneration when administered post-occlusion and that they also block N and P-type voltage-dependent Ca2+ channels.