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

Current Drug Development Overview: Targeting Voltage-Gated Calcium Channels for the Treatment of Pain

Voltage-gated calcium channels (VGCCs) are targeted to treat pain conditions. Since the discovery of their relation to pain processing control, they are investigated to find new strategies for better pain control. This review provides an overview of naturally based and synthetic VGCC blockers, highlighting new evidence on the development of drugs focusing on the VGCC subtypes as well as mixed targets with pre-clinical and clinical analgesic effects.

Voltage-dependent CaV3.2 and CaV2.2 channels in nociceptive pathways

Noxious stimuli like cold, heat, pH change, tissue damage, and inflammation depolarize a membrane of peripheral endings of specialized nociceptive neurons which eventually results in the generation of an action potential. The electrical signal is carried along a long axon of nociceptive neurons from peripheral organs to soma located in dorsal root ganglions and further to the dorsal horn of the spinal cord where it is transmitted through a chemical synapse and is carried through the spinal thalamic tract into the brain. Two subtypes of voltage-activated calcium play a major role in signal transmission: a low voltage-activated Ca_V3.2 channel and a high voltage-activated Ca_V2.2 channel. The Ca_V3.2 channel contributes mainly to the signal conductance along nociceptive neurons while the principal role of the Ca_V2.2 channel is in the synaptic transmission at the dorsal horn. Both channels contribute to the signal initiation at peripheral nerve endings. This review summarizes current knowledge about the expression and distribution of these channels in a nociceptive pathway, the regulation of their expression and gating during pain pathology, and their suitability as targets for pharmacological therapy.

Study of interaction of tramadol with amlodipine in mice

Objective:

To study a possible interaction between tramadol, an opioid analgesic and amlodipine, a dihydropyridine calcium channel blocker with proposed antinociceptive property.

Materials and Methods:

Albino mice of Haffkine strain were used for the study. The experiment was carried out using tail-flick method. Different doses of tramadol (50 mg/kg, 22.8 mg/kg and 10 mg/kg) were administered intraperitoneally to select the nonanalgesic dose. The animals were treated with different doses of amlodipine (2.5 mg/kg, 3.0 mg/kg, 3.5 mg/kg) to study its antinociceptive action. Combination of different doses of both the drugs were administered to study antinociceptive effect of the combination.

Results:

Tramadol, showed dose dependent antinociception which persisted for entire two hours of the study period. Antinociceptive action was seen with amlodipine at a dose of 3.5 mg/kg. Different doses of amlodipine (2.5 mg/kg, 3.0 mg/kg) in combination with the nonanalgesic dose of tramadol (10 mg/kg) produced a significant enhancement of antinociceptive effect of tramadol. Combination of 3.5 mg/kg dose of amlodipine with nonanalgesic dose of tramadol (10 mg/kg) further enhances antinociceptive activity.

Conclusion:

It is concluded that combination of amlodipine, a N - type calcium channel blocker, with tramadol produce significant enhancement of antinociceptive activity of tramadol.

What can naturally occurring mutations tell us about Ca(v)1.x channel function?

Voltage-gated Ca^2 + channels allow for Ca^2 +-dependent intracellular signaling by directly mediating Ca^2 + ion influx, by physical coupling to intracellular Ca^2 + release channels or functional coupling to other ion channels such as Ca^2 + activated potassium channels. L-type Ca^2 + channels that comprise the family of Ca_v1 channels are expressed in many electrically excitable tissues and are characterized by their unique sensitivity to dihydropyridines. In this issue, we summarize genetic defects in L-type Ca^2 + channels and analyze their role in human diseases (Ca^2 + channelopathies); e.g. mutations in Ca_v1.2 α1 cause Timothy and Brugada syndrome, mutations in Ca_v1.3 α1 are linked to sinoatrial node dysfunction and deafness while mutations in Ca_v1.4 α1 are associated with X-linked retinal disorders such as an incomplete form of congenital stationary night blindness. Herein, we also put the mutations underlying the channel's dysfunction into the structural context of the pore-forming α1 subunit. This analysis highlights the importance of combining functional data with structural analysis to gain a deeper understanding for the disease pathophysiology as well as for physiological channel function. This article is part of a Special Issue entitled: Calcium channels.

CRMP-2 peptide mediated decrease of high and low voltage-activated calcium channels, attenuation of nociceptor excitability, and anti-nociception in a model of AIDS therapy-induced painful peripheral neuropathy

Background

The ubiquity of protein-protein interactions in biological signaling offers ample opportunities for therapeutic intervention. We previously identified a peptide, designated CBD3, that suppressed inflammatory and neuropathic behavioral hypersensitivity in rodents by inhibiting the ability of collapsin response mediator protein 2 (CRMP-2) to bind to N-type voltage-activated calcium channels (CaV2.2) [Brittain et al. Nature Medicine 17:822–829 (2011)].

Results and discussion

Here, we utilized SPOTScan analysis to identify an optimized variation of the CBD3 peptide (CBD3A6K) that bound with greater affinity to Ca²⁺ channels. Molecular dynamics simulations demonstrated that the CBD3A6K peptide was more stable and less prone to the unfolding observed with the parent CBD3 peptide. This mutant peptide, conjugated to the cell penetrating motif of the HIV transduction domain protein TAT, exhibited greater anti-nociception in a rodent model of AIDS therapy-induced peripheral neuropathy when compared to the parent TAT-CBD3 peptide. Remarkably, intraperitoneal administration of TAT-CBD3A6K produced none of the minor side effects (i.e. tail kinking, body contortion) observed with the parent peptide. Interestingly, excitability of dissociated small diameter sensory neurons isolated from rats was also reduced by TAT-CBD3A6K peptide suggesting that suppression of excitability may be due to inhibition of T- and R-type Ca²⁺ channels. TAT-CBD3A6K had no effect on depolarization-evoked calcitonin gene related peptide (CGRP) release compared to vehicle control.

Conclusions

Collectively, these results establish TAT-CBD3A6K as a peptide therapeutic with greater efficacy in an AIDS therapy-induced model of peripheral neuropathy than its parent peptide, TAT-CBD3. Structural modifications of the CBD3 scaffold peptide may result in peptides with selectivity against a particular subset of voltage-gated calcium channels resulting in a multipharmacology of action on the target.

Reductions in external divalent cations evoke novel voltage-gated currents in sensory neurons

It has long been recognized that divalent cations modulate cell excitability. Sensory nerve excitability is of critical importance to peripheral diseases associated with pain, sensory dysfunction and evoked reflexes. Thus we have studied the role these cations play on dissociated sensory nerve activity. Withdrawal of both Mg²⁺ and Ca²⁺ from external solutions activates over 90% of dissociated mouse sensory neurons. Imaging studies demonstrate a Na⁺ influx that then causes depolarization-mediated activation of voltage-gated Ca²⁺ channels (Ca_V), which allows Ca²⁺ influx upon divalent re-introduction. Inhibition of Ca_V (ω-conotoxin, nifedipine) or Na_V (tetrodotoxin, lidocaine) fails to reduce the Na⁺ influx. The Ca²⁺ influx is inhibited by Ca_V inhibitors but not by TRPM7 inhibition (spermine) or store-operated channel inhibition (SKF96365). Withdrawal of either Mg²⁺ or Ca²⁺ alone fails to evoke cation influxes in vagal sensory neurons. In electrophysiological studies of dissociated mouse vagal sensory neurons, withdrawal of both Mg²⁺ and Ca²⁺ from external solutions evokes a large slowly-inactivating voltage-gated current (I_DF) that cannot be accounted for by an increased negative surface potential. Withdrawal of Ca²⁺ alone fails to evoke I_DF. Evidence suggests I_DF is a non-selective cation current. The I_DF is not reduced by inhibition of Na_V (lidocaine, riluzole), Ca_V (cilnidipine, nifedipine), K_V (tetraethylammonium, 4-aminopyridine) or TRPM7 channels (spermine). In summary, sensory neurons express a novel voltage-gated cation channel that is inhibited by external Ca²⁺ (IC₅₀∼0.5 µM) or Mg²⁺ (IC₅₀∼3 µM). Activation of this putative channel evokes substantial cation fluxes in sensory neurons.

Hereditary pancreatitis amlodipine trial: a pilot study of a calcium-channel blocker in hereditary pancreatitis

OBJECTIVES

Hereditary pancreatitis (HP) is a form of recurrent acute pancreatitis (AP) mediated by mutations in cationic trypsinogen (PRSS1). Mutations cluster in the calcium-associated regulator regions of PRSS1. In rats, calcium-channel blockers (CCB) prevent hyperstimulation-associated AP. Because of the potential importance of hyperstimulation in triggering episodes of AP in HP, we designed a pilot study to evaluate the safety and potential benefit of CCB use in HP.

METHODS

Subjects 6 years or older had a PRSS1 mutation, recurrent AP, and pain. Total study duration was 16 weeks. Amlodipine was given during weeks 0 to 11. Dose (2.5, 5, or 10 mg) was based on weight (range, 0.08-0.17 mg x kg(-1) x d(-1)). Subjects filled a daily diary including pain (0-10 scale) and blood pressure reading. Clinical assessments occurred at weeks -4, 0, 1, 2, 6, 10, 11, and 12. Subjects filled a Medical Outcomes Study Short-Form Survey version 2 (SF-10 for children <14 years old) at weeks -4, 0, 6, and 10. Data were compared for weeks -4 to 0 and 6 to 10.

RESULTS

Nine subjects signed informed consent (4 males; 12-52 years old). Four were excluded during the screening phase. Drug was discontinued in one due to development of unilateral lower-extremity numbness. Four subjects (12-31 years old) completed the study. Mean blood pressure, laboratory tests, physical findings, and daily pain scores did not clinically significantly differ before and during drug therapy, but all reported reduced symptoms. Three reduced analgesic use. Three had improved scores on the Medical Outcomes Study Short-Form Survey version 2.

CONCLUSIONS

Amlodipine is generally safe in subjects with HP and does not increase pain or episodes of AP. Further research into the mechanism of CCB on pancreatitis would be important to provide a pathophysiologic basis to support further trials in HP.

Congenital stationary night blindness type 2 mutations S229P, G369D, L1068P, and W1440X alter channel gating or functional expression of Ca(v)1.4 L-type Ca2+ channels

Mutations in the CACNA1F gene (voltage-dependent L-type calcium channel alpha1F subunit) encoding retinal Ca(v)1.4 L-type Ca2+ channels cause X-linked recessive congenital stationary night blindness type 2 (CSNB2). Many of them are predicted to yield nonfunctional channels. Complete loss of Ca(v)1.4 function is therefore regarded as a pathogenetic mechanism for the impaired signaling from photoreceptors to second-order retinal neurons. We investigated the functional consequences of CSNB2 missense mutations S229P, G369D, and L1068P and the C-terminal truncation mutant W1440X. After expression in Xenopus laevis oocytes or human embryonic kidney tsA-201 cells, inward Ca2+ current (I(Ca)) and inward Ba2+ current (I(Ba)) could be recorded from mutations G369D and L1068P. G369D shifted the half-maximal voltage for channel activation (V(0.5,act)) significantly to more negative potentials (>11 mV), slowed inactivation, and removed Ca2+-dependent inactivation. The L1068P mutant yielded currents only in the presence of the channel activator BayK8644. Currents (I(Ba)) inactivated faster than wild type (WT) and recovered more slowly from inactivation (I(Ba) and I(Ca)). No channel activity could be measured for mutants S229P and W1440X after oocyte expression. No W1440X alpha1 protein was detected after expression in tsA-201 cells, whereas S229P (as well as G369D and L1068P) alpha1 subunits were expressed at levels indistinguishable from WT (n = 3). Our data provide unequivocal evidence that CSNB2 missense mutations can induce severe changes in Ca(v)1.4 function, which may decrease (L1068P and S229P) or even increase (G369D) channel activity. The lower activation range of G369D can explain the reduced dynamic range of photoreceptor signaling. Moreover, we demonstrate that loss of channel function of one (L1068P) CSNB2 mutation can be rescued by a Ca2+ channel activator.

Effects of activation of central nervous histamine receptors in cardiovascular regulation; studies in H(1) and H(2) receptor gene knockout mice

To elucidate the central roles of histamine receptors in cardiovascular regulatory system, systolic, mean, and diastolic blood pressures (BPs) and heart rate (HR) were examined in conscious H(1) receptor gene knockout (H(1)KO) mice, H(2) receptor gene knockout (H(2)KO) mice, H(1) and H(2) receptor gene double knockout (DKO) mice, and their respective control mice by the tail-cuff system. Histamine, histamine-trifluoromethyl-toluidine derivative (HTMT, an H(1) agonist), dimaprit (an H(2) agonist), and immepip (an H(3) agonist) were intrathecally administered to these KO mice and control mice. Basal BPs and HR were not different among these three KO mice and their control or wild-type mice. Intrathecal administration of histamine significantly increased BPs and decreased HR in control mice. The increases in BPs were produced by histamine in H(1)KO and H(2)KO mice and by HTMT and dimaprit in C57BL mice. The pressor responses by HTMT and dimaprit in C57BL mice were greater than those by histamine in H(1)KO and H(2)KO mice, although the same decreases in HR were induced by histamine in C57BL and H(1)KO mice and by dimaprit in C57BL mice. The selective stimulation of H(3) receptors by immepip produced a consistent decrease in BPs in control mice. These results obtained with the exogenous selective agonists of three histamine receptors suggest that the pressor responses to histamine are mediated through the stimulation of both H(1) and H(2) receptors, whereas the atropine-sensitive decrease in heart rate is mainly due to H(2) receptors which activate the vagal output to the heart.

L-Type Calcium Channels: The Low Down

L-type calcium channels couple membrane depolarization in neurons to numerous processes including gene expression, synaptic efficacy, and cell survival. To establish the contribution of L-type calcium channels to various signaling cascades, investigators have relied on their unique pharmacological sensitivity to dihydropyridines. The traditional view of dihydropyridine-sensitive L-type calcium channels is that they are high-voltage–activating and have slow activation kinetics. These properties limit the involvement of L-type calcium channels to neuronal functions triggered by strong and sustained depolarizations. This review highlights literature, both long-standing and recent, that points to significant functional diversity among L-type calcium channels expressed in neurons and other excitable cells. Past literature contains several reports of low-voltage–activated neuronal L-type calcium channels that parallel the unique properties of recently cloned CaV1.3 L-type channels. The fast kinetics and low activation thresholds of CaV1.3 channels stand in stark contrast to criteria currently used to describe L-type calcium channels. A more accurate view of neuronal L-type calcium channels encompasses a broad range of activation thresholds and recognizes their potential contribution to signaling cascades triggered by subthreshold depolarizations.

Design, Synthesis, and Preliminary Pharmacological Evaluation of 4-Aminopiperidine Derivatives as N-Type Calcium Channel Blockers Active on Pain and Neuropathic Pain

Several compounds with a 4-aminopiperidine scaffold decorated on both nitrogen atoms by alkyl or acyl moieties containing the structural motifs of verapamil and of flunarizine, as well as those that are more frequent in known N-type calcium channel antagonists, have been synthesized. Antinociceptive activity on the mouse hot-plate test was used to select molecules to be submitted to further studies. Active compounds were tested in vitro on a PC12 rat pheochromocytoma clonal cell line, to evaluate their action on N-type calcium channels, and on a rat model of neuropathic pain. Two compounds that show N-type calcium channel antagonism and are endowed with potent action on pain and neuropathic pain (3 and 18) have been selected for further studies.

Inhibitory effect of pranidipine on N-type voltage-dependent Ca2+ channels in mice

We investigated the N-type voltage-dependent calcium channel blocking action of pranidipine, a novel dihydropyridine (DHP) derivative. Pranidipine significantly suppressed KCl-induced intracellular calcium changes ([Ca(2+)](i)) in a dose-dependent fashion in dorsal root ganglion neurons. A patch-clamp investigation revealed a dose-dependent blocking effect on N-type currents. Intrathecal injection of pranidipine significantly shortened the licking time in the late phase of the formalin test, as occurs with cilnidipine and amlodipine, which act on L- and N-type channels. Conversely, nicardipine, which acts exclusively on L-type channels, had no antinociceptive effect. Our results indicate that pranidipine inhibits N-type calcium channels. Furthermore, it exerts an antinociceptive effect, which might be related to an attenuation of synaptic transmission by nociceptive neurons due to the blocking effect of pranidipine on N-type calcium channels in primary nociceptive afferent fibers.

Structures of the murine genes for the beta1- and beta4-subunits of the voltage-dependent calcium channel

The structures of the genomic DNA of the murine beta1- and beta4-subunits of the voltage-dependent calcium channel were mapped by comparing genomic and cDNA sequences. The gene structure analysis revealed that the murine beta1 gene consists of 15 translated exons spread over 19 kb of the genome, whereas the beta4 gene consists of 13 translated exons spread over 124 kb of the genome. Alternative spliced transcripts of the beta1 gene were also characterized. Although the total lengths differ, these subunits have exon-intron organization similar to the murine beta2- and beta3-, and human beta4-subunits, showing the highly conserved gene structure of this family. Furthermore, mRNA of these beta-subunits is strongly expressed in dorsal root ganglion neurons, which have many voltage-dependent calcium channels.

Spinal delivery of analgesics in experimental models of pain and analgesia

Systemic administration of analgesics can lead to serious adverse side effects compromising therapeutic benefit in some patients. Information coding pain transmits along an afferent neuronal network, the first synapses of which reside principally in the spinal cord. Delivery of compounds to spinal cord, the intended site of action for some analgesics, is potentially a more efficient and precise method for inhibiting the pain signal. Activation of specific proteins that reside in spinal neuronal membranes can result in hyperpolarization of secondary neurons, which can prevent transmission of the pain signal. This is one of the mechanisms by which opioids induce analgesia. The spinal cord is enriched in such molecular targets, the activation of which inhibit the transmission of the pain signal early in the afferent neuronal network. This review describes the pre-clinical models that enable new target discovery and development of novel analgesics for site-directed pain management.

Cav1.4alpha1 subunits can form slowly inactivating dihydropyridine-sensitive L-type Ca2+ channels lacking Ca2+-dependent inactivation

The neuronal L-type calcium channels (LTCCs) Cav1.2alpha1 and Cav1.3alpha1 are functionally distinct. Cav1.3alpha1 activates at lower voltages and inactivates more slowly than Cav1.2alpha1, making it suitable to support sustained L-type Ca2+ inward currents (ICa,L) and serve in pacemaker functions. We compared the biophysical and pharmacological properties of human retinal Cav1.4alpha1 using the whole-cell patch-clamp technique after heterologous expression in tsA-201 cells with other L-type alpha1 subunits. Cav1.4alpha1-mediated inward Ba2+ currents (IBa) required the coexpression of alpha2delta1 and beta3 or beta2a subunits and were detected in a lower proportion of transfected cells than Cav1.3alpha1. IBa activated at more negative voltages (5% activation threshold; -39mV; 15 mm Ba2+) than Cav1.2alpha1 and slightly more positive than Cav1.3alpha1. Voltage-dependent inactivation of IBa was slower than for Cav1.2alpha1 and Cav1.3alpha1( approximately 50% inactivation after 5 sec; alpha2delta1 + beta3 coexpression). Inactivation was not increased with Ca2+ as the charge carrier, indicating the absence of Ca2+-dependent inactivation. Cav1.4alpha1 exhibited voltage-dependent, G-protein-independent facilitation by strong depolarizing pulses. The dihydropyridine (DHP)-antagonist isradipine blocked Cav1.4alpha1 with approximately 15-fold lower sensitivity than Cav1.2alpha1 and in a voltage-dependent manner. Strong stimulation by the DHP BayK 8644 was found despite the substitution of an otherwise L-type channel-specific tyrosine residue in position 1414 (repeat IVS6) by a phenylalanine. Cav1.4alpha1 + alpha2delta1 + beta channel complexes can form LTCCs with intermediate DHP antagonist sensitivity lacking Ca2+-dependent inactivation. Their biophysical properties should enable them to contribute to sustained ICa,L at negative potentials, such as required for tonic neurotransmitter release in sensory cells and plateau potentials in spiking neurons.

Analgesic action of loperamide, an opioid agonist, and its blocking action on voltage-dependent Ca2+ channels

We investigated the relationship between the antinociceptive effect of the opiate agonist loperamide at the spinal level and its inhibitory effect on calcium influx. Intrathecal administration of loperamide showed a significant antinociceptive effect in the formalin test, which was not prevented by naloxone. On the other hand, no significant effects were observed by nicardipine, an L-type specific blocker, or by BAY K8644, an L-type specific agonist, suggesting no significant role of L-type calcium channels in nociceptive signal transduction. Loperamide suppressed the calcium influx in dorsal root ganglion neurons. As the antinociceptive effect of loperamide was not affected by naloxone or other calcium channel blocking toxins, and loperamide showed a direct inhibitory effect on calcium-influx, the analgesic effect of intrathecally injected loperamide might be due to its blockade of the voltage-dependent calcium channels at the terminals of the primary afferent fibers.

Pain perception in mice lacking the beta3 subunit of voltage-activated calcium channels

The importance of voltage-activated calcium channels in pain processing has been suggested by the spinal antinociceptive action of blockers of N- and P/Q-type calcium channels as well as by gene targeting of the alpha1B subunit (N-type). The accessory beta3 subunits of calcium channels are preferentially associated with the alpha1B subunit in neurones. Here we show that deletion of the beta3 subunit by gene targeting affects strongly the pain processing of mutant mice. We pinpoint this defect in the pain-related behavior and ascending pain pathways of the spinal cord in vivo and at the level of calcium channel currents and proteins in single dorsal root ganglion neurones in vitro. The pain induced by chemical inflammation is preferentially damped by deletion of beta3 subunits, whereas responses to acute thermal and mechanical harmful stimuli are reduced moderately or not at all, respectively. The defect results in a weak wind-up of spinal cord activity during intense afferent nerve stimulation. The molecular mechanism responsible for the phenotype was traced to low expression of N-type calcium channels (alpha1B) and functional alterations of calcium channel currents in neurones projecting to the spinal cord.