SNX-482: A Novel Class E Calcium Channel Antagonist from Tarantula Venom

Spider Toxin SNX-482 Gating Modifier Spontaneously Partitions in the Membrane Guided by Electrostatic Interactions

Spider toxin SNX-482 is a cysteine-rich peptide that interferes with calcium channel activity by binding to voltage-sensing domains of the CaV2.3 subtype. Two mechanisms dominate the binding process of cysteine-rich peptides.: direct binding from the aqueous phase or through lateral diffusion from the membrane, the so-called reduction in dimensionality mechanism. In this work, via coarse-grained and atomistic molecular dynamics simulations, we have systematically studied the spontaneous partitioning of SNX-482 with membranes of different anionic compositions and explored via diffusional analysis both binding mechanisms. Our simulations revealed a conserved protein patch that inserts in the membrane, a preference for binding towards partially negatively charged membranes, and that electrostatics guides membrane binding by incrementing and aligning the molecular dipole. Finally, diffusivity calculations showed that the toxin diffusion along the membrane plane is an order of magnitude slower than the aqueous phase suggesting that the critical factor in determining the SNX-482-CaV2.3 binding mechanism is the affinity between the membrane and SNX-482.

Further Evidence that Inhibition of Neuronal Voltage-Gated Calcium Channels Contributes to the Hypnotic Effect of Neurosteroid Analogue, 3β-OH

We recently reported that a neurosteroid analogue with T-channel-blocking properties (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH), induced hypnosis in rat pups without triggering neuronal apoptosis. Furthermore, we found that the inhibition of the Ca_V3.1 isoform of T-channels contributes to the hypnotic properties of 3β-OH in adult mice. However, the specific mechanisms underlying the role of other subtypes of voltage-gated calcium channels in thalamocortical excitability and oscillations in vivo during 3β-OH-induced hypnosis are largely unknown. Here, we used patch-clamp recordings from acute brain slices, in vivo electroencephalogram (EEG) recordings, and mouse genetics with wild-type (WT) and Ca_V2.3 knock-out (KO) mice to further investigate the molecular mechanisms of neurosteroid-induced hypnosis. Our voltage-clamp recordings showed that 3β-OH inhibited recombinant Ca_V2.3 currents. In subsequent current-clamp recordings in thalamic slices ex vivo, we found that selective Ca_V2.3 channel blocker (SNX-482) inhibited stimulated tonic firing and increased the threshold for rebound burst firing in WT animals. Additionally, in thalamic slices we found that 3β-OH inhibited spike-firing more profoundly in WT than in mutant mice. Furthermore, 3β-OH reduced bursting frequencies in WT but not mutant animals. In ensuing in vivo experiments, we found that intra-peritoneal injections of 3β-OH were less effective in inducing LORR in the mutant mice than in the WT mice, with expected sex differences. Furthermore, the reduction in total α, β, and low γ EEG power was more profound in WT than in Ca_V2.3 KO females over time, while at 60 min after injections of 3β-OH, the increase in relative β power was higher in mutant females. In addition, 3β-OH depressed EEG power more strongly in the male WT than in the mutant mice and significantly increased the relative δ power oscillations in WT male mice in comparison to the mutant male animals. Our results demonstrate for the first time the importance of the Ca_V2.3 subtype of voltage-gated calcium channels in thalamocortical excitability and the oscillations that underlie neurosteroid-induced hypnosis.

Sex-dependent Cav2.3 channel contribution to the secondary hyperalgesia in a mice model of central sensitization

Central sensitization (CS) is characteristic of difficult to treat painful conditions, such as fibromyalgia and neuropathies and have sexual dimorphism involved. The calcium influx in nociceptive neurons is a key trigger for CS and the role of Cav2.1 and Cav2.2 voltage gated calcium channels (VGCC) in this role were evidenced with the use of ω-agatoxin IVA and ω-agatoxin MVIIA blockers, respectively. However, the participation of the α1 subunit of the voltage-gated channel Cav2.3, which conducts R-type currents, in CS is unknown. Furthermore, the role of sexual differences in painful conditions is still poorly understood. Thus, we investigated the role of Cav2.3 in capsaicin-induced secondary hyperalgesia in mice, which serve as a CS model predictive of the efficacy of novel analgesic drugs. Capsaicin injection in C57BL/6 mice caused secondary hyperalgesia from one to five hours after injection, and the effects were similar in male and female mice. In female but not male mice, intrathecal treatment with the Cav2.3 inhibitor SNX-482 partially and briefly reversed secondary hyperalgesia at a dose (300 pmol/site) that did not cause adverse effects. Moreover, Cav2.3 expression in the dorsal root ganglia (DRG) and spinal cord was reduced by intrathecal treatment with an antisense oligonucleotide (ASO) targeting Cav2.3 in female and male mice. However, ASO treatment was able to provide a robust and durable prevention of secondary hyperalgesia caused by capsaicin in female mice, but not in male mice. Thus, our results demonstrate that Cav2.3 inhibition, especially in female mice, has a relevant impact on a model of CS. Our results provide a proof of concept for Cav2.3 as a molecular target. In addition, the result associated to the role of differences in painful conditions linked to sex opens a range of possibilities to be explored and needs more attention. Thus, the relevance of testing Cav2.3 inhibition or knockdown in clinically relevant pain models is needed.

Alterations in Oscillatory Behavior of Central Medial Thalamic Neurons Demonstrate a Key Role of CaV3.1 Isoform of T-Channels During Isoflurane-Induced Anesthesia

Although the central medial nucleus (CeM) of the thalamus is an essential part of the arousal system for sleep and anesthesia initiation, the precise mechanisms that regulate its activity are not well studied. We examined the role of CaV3.1 isoform of T-type calcium channels (T-channels) in the excitability and rhythmic activity of CeM neurons during isoflurane (ISO)-induced anesthesia by using mouse genetics and selective pharmacology. Patch-clamp recordings taken from acute brain slices revealed that CaV3.1 channels in CeM are inhibited by prototypical volatile anesthetic ISO (250 and 500 μM) and selective T-channels blocker 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2). Both TTA-P2 and ISO attenuated tonic and burst firing modes, and hyperpolarized CeM neurons from wild type (WT) mice. These effects were greatly diminished or abolished in CaV3.1 null mice. Our ensuing in vivo local field potential (LFP) recordings from CeM indicated that the ability of TTA-P2 and anesthetic concentrations of ISO to promote δ oscillation was substantially weakened in CaV3.1 null mice. Furthermore, escalating ISO concentrations induced stronger burst-suppression LFP pattern in mutant than in WT mice. Our results demonstrate for the first time the importance of CaV3.1 channels in thalamocortical oscillations from the non-specific thalamic nuclei that underlie clinically important effects of ISO.

Age-dependent contribution of P/Q- and R-type Ca2+ channels to neuromuscular transmission in lethargic mice

β-Subunits of voltage-gated calcium channels (VGCCs) regulate assembly and membrane localization of the pore-forming α1-subunit and strongly influence channel function. β4-Subunits normally coassociate with α1A-subunits which comprise P/Q-type (Cav2.1) VGCCs. These control acetylcholine (ACh) release at adult mammalian neuromuscular junctions (NMJs). The naturally occurring lethargic (lh) mutation of the β4-subunit in mice causes loss of the α1-binding site, possibly affecting P/Q-type channel expression or function, and thereby ACh release. End-plate potentials and miniature end-plate potentials were recorded at hemidiaphragm NMJs of 5-7-week and 3-5-month-old lh and wild-type (wt) mice. Sensitivity to antagonists of P/Q- [ω-agatoxin IVA (ω-Aga-IVA)], L- (nimodipine), N- (ω-conotoxin GVIA), and R-type [C192H274N52O60S7 (SNX-482)] VGCCs was compared in juvenile and adult lh and wt mice. Quantal content (m) of adult, but not juvenile, lh mice was reduced compared to wt. ω-Aga-IVA (~60%) and SNX-482 (~ 45%) significantly reduced m in adult lh mice. Only Aga-IVA affected wt adults. In juvenile lh mice, ω-Aga-IVA and SNX-482 decreased m by >75% and ~20%, respectively. Neither ω-conotoxin GVIA nor nimodipine affected ACh release in any group. Immunolabeling revealed α1E and α1A, β1, and β3 staining at adult lh, but not wt NMJs. Therefore, in lh mice, when the β-subunit that normally coassociates with α1A to form P/Q channels is missing, P/Q-type channels partner with other β-subunits. However, overall participation of P/Q-type channels is reduced and compensated for by R-type channels. R-type VGCC participation is age-dependent, but is less effective than P/Q-type at sustaining NMJ function.

Modulation/physiology of calcium channel sub-types in neurosecretory terminals

The hypothalamic-neurohypophysial system (HNS) controls diuresis and parturition through the release of arginine-vasopressin (AVP) and oxytocin (OT). These neuropeptides are chiefly synthesized in hypothalamic magnocellular somata in the supraoptic and paraventricular nuclei and are released into the blood stream from terminals in the neurohypophysis. These HNS neurons develop specific electrical activity (bursts) in response to various physiological stimuli. The release of AVP and OT at the level of neurohypophysis is directly linked not only to their different burst patterns, but is also regulated by the activity of a number of voltage-dependent channels present in the HNS nerve terminals and by feedback modulators. We found that there is a different complement of voltage-gated Ca(2+) channels (VGCC) in the two types of HNS terminals: L, N, and Q in vasopressinergic terminals vs. L, N, and R in oxytocinergic terminals. These channels, however, do not have sufficiently distinct properties to explain the differences in release efficacy of the specific burst patterns. However, feedback by both opioids and ATP specifically modulate different types of VGCC and hence the amount of AVP and/or OT being released. Opioid receptors have been identified in both AVP and OT terminals. In OT terminals, μ-receptor agonists inhibit all VGCC (particularly R-type), whereas, they induce a limited block of L-, and P/Q-type channels, coupled to an unusual potentiation of the N-type Ca(2+) current in the AVP terminals. In contrast, the N-type Ca(2+) current can be inhibited by adenosine via A(1) receptors leading to the decreased release of both AVP and OT. Furthermore, ATP evokes an inactivating Ca(2+)/Na(+)-current in HNS terminals able to potentiate AVP release through the activation of P2X2, P2X3, P2X4 and P2X7 receptors. In OT terminals, however, only the latter receptor type is probably present. We conclude by proposing a model that can explain how purinergic and/or opioid feedback modulation during bursts can mediate differences in the control of neurohypophysial AVP vs. OT release.

Cav2.3 channels are critical for oscillatory burst discharges in the reticular thalamus and absence epilepsy

Neurons of the reticular thalamus (RT) display oscillatory burst discharges that are believed to be critical for thalamocortical network oscillations related to absence epilepsy. Ca²+-dependent mechanisms underlie such oscillatory discharges. However, involvement of high-voltage activated (HVA) Ca²+ channels in this process has been discounted. We examined this issue closely using mice deficient for the HVA Ca(v)2.3 channels. In brain slices of Ca(v)2.3⁻/⁻, a hyperpolarizing current injection initiated a low-threshold burst of spikes in RT neurons; however, subsequent oscillatory burst discharges were severely suppressed, with a significantly reduced slow afterhyperpolarization (AHP). Consequently, the lack of Ca(v)2.3 resulted in a marked decrease in the sensitivity of the animal to γ-butyrolactone-induced absence epilepsy. Local blockade of Ca(v)2.3 channels in the RT mimicked the results of Ca(v)2.3⁻/⁻ mice. These results provide strong evidence that Ca(v)2.3 channels are critical for oscillatory burst discharges in RT neurons and for the expression of absence epilepsy.

Peptide neurotoxins that affect voltage-gated calcium channels: a close-up on ω-agatoxins

Peptide neurotoxins found in animal venoms have gained great interest in the field of neurotransmission. As they are high affinity ligands for calcium, potassium and sodium channels, they have become useful tools for studying channel structure and activity. Peptide neurotoxins represent the clinical potential of ion-channel modulators across several therapeutic fields, especially in developing new strategies for treatment of ion channel-related diseases. The aim of this review is to overview the latest updates in the domain of peptide neurotoxins that affect voltage-gated calcium channels, with a special focus on ω-agatoxins.

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Ca2+ clearance mechanisms in neurohypophysial terminals of the rat

The importance of intracellular calcium ([Ca2+]i) in the release of vasopressin (AVP) and oxytocin from the central nervous system neurohypopyhysial nerve terminals has been well-documented. To date, there is no clear understanding of Ca2+ clearance mechanisms and their interplay with transmembrane Ca2+ entry, intracellular [Ca2+]i transients, cytoplasmic Ca2+ stores and hence the release of AVP at the level of a single nerve terminal. Here, we studied the mechanism of Ca2+ clearance in freshly isolated nerve terminals of the rat neurohypophysis using Fura-2 Ca2+ imaging and measured the release of AVP by radioimmuno assay. An increase in the K+ concentration in the perfusion solution from 5 to 50 mM caused a rapid increase in [Ca2+]i and AVP release. Returning K+ concentration to 5 mM led to rapid restoration of both responses to basal level. The K+-evoked [Ca2+]i and AVP increase was concentration-dependent, reliable, and remained of constant amplitude and time course upon successive applications. Extracellular Ca2+ removal completely abolished the K+-evoked responses. The recovery phase was not affected upon replacement of NaCl with sucrose or drugs known to act on intracellular Ca2+ stores such as thapsigargin, cyclopiazonic acid, caffeine or a combination of caffeine and ryanodine did not affect either resting or K+-evoked [Ca2+]i or AVP release. By contrast, the plasma membrane Ca2+ pump inhibitor, La3+, markedly slowed down the recovery phase. The mitochondrial respiration uncoupler, carbonyl cyanide 3-chlorophenylhydrazone (CCCP), slightly but significantly increased the basal [Ca2+]i, and also slowed down the recovery phase of both [Ca2+]i and release responses. In conclusion, we show in nerve terminals that (i) Ca2+ extrusion through the Ca2+ pump in the plasma membrane plays a major role in the Ca2+ clearance mechanisms of (ii) Ca2+ uptake by mitochondria also contributes to the Ca2+ clearance and (iii) neither Na+/Ca2+ exchangers nor Ca2+ stores are involved in the Ca2+ clearance or in the maintenance of basal [Ca2+]i or release of AVP.

Pharmacological modulators of voltage-gated calcium channels and their therapeutical application

Calcium channels (CCs) play an important role in the transduction of action potential to the cytosol. An influx of Ca(2+) is essential for muscle contraction, neurotransmitter, and hormonal release. Level of cytosolic Ca(2+) controls activities of many enzymes and regulatory proteins. Voltage-gated calcium channels (VGCCs) serve as sensors for membrane depolarization. Blood pressure reduction is due to relaxation of actomyosine filaments in vascular smooth muscles. Calcium channel blockers (CCBs) are traditionally used for treatment of cardiovascular diseases. Neurotransmitter release from presynaptic neurons is triggered by Ca(2+) influx. Blockers of neuronal CCs may be applied for pain treatment. Overload of neurons by Ca(2+) is toxic. CCBs may be applied for prevention of some neurodegenerative disorders.