Alternative splicing in the cytoplasmic II-III loop of the N-type Ca channel alpha 1B subunit: functional differences are beta subunit-specific

Mechanism of gabapentinoid potentiation of opioid effects on cyclic AMP signaling in neuropathic pain

Over half of spinal cord injury (SCI) patients develop opioid-resistant chronic neuropathic pain. Safer alternatives to opioids for treatment of neuropathic pain are gabapentinoids (e.g., pregabalin and gabapentin). Clinically, gabapentinoids appear to amplify opioid effects, increasing analgesia and overdose-related adverse outcomes, but in vitro proof of this amplification and its mechanism are lacking. We previously showed that after SCI, sensitivity to opioids is reduced by fourfold to sixfold in rat sensory neurons. Here, we demonstrate that after injury, gabapentinoids restore normal sensitivity of opioid inhibition of cyclic AMP (cAMP) generation, while reducing nociceptor hyperexcitability by inhibiting voltage-gated calcium channels (VGCCs). Increasing intracellular Ca2+ or activation of L-type VGCCs (L-VGCCs) suffices to mimic SCI effects on opioid sensitivity, in a manner dependent on the activity of the Raf1 proto-oncogene, serine/threonine-protein kinase C-Raf, but independent of neuronal depolarization. Together, our results provide a mechanism for potentiation of opioid effects by gabapentinoids after injury, via reduction of calcium influx through L-VGCCs, and suggest that other inhibitors targeting these channels may similarly enhance opioid treatment of neuropathic pain.

CaV2.2 (N-type) voltage-gated calcium channels are activated by SUMOylation pathways

SUMOylation is an important post-translational modification process involving covalent attachment of SUMO (Small Ubiquitin-like MOdifier) protein to target proteins. Here, we investigated the potential for SUMO-1 protein to modulate the function of the CaV2.2 (N-type) voltage-gated calcium channel (VGCC), a protein vital for presynaptic neurotransmitter release. Co-expression of SUMO-1, but not the conjugation-deficient mutant SUMO-1ΔGG, increased heterologously-expressed CaV2.2 Ca2+ current density, an effect potentiated by the conjugating enzyme Ubc9. Expression of sentrin-specific protease (SENP)-1 or Ubc9 alone, had no effect on recombinant CaV2.2 channels. Co-expression of SUMO-1 and Ubc9 caused an increase in whole-cell maximal conductance (Gmax) and a hyperpolarizing shift in the midpoint of activation (V1/2). Mutation of all five CaV2.2 lysine residues to arginine within the five highest probability (>65 %) SUMOylation consensus motifs (SCMs) (construct CaV2.2-Δ5KR), produced a loss-of-function mutant. Mutagenesis of selected individual lysine residues identified K394, but not K951, as a key residue for SUMO-1-mediated increase in CaV2.2 Ca2+ current density. In synaptically-coupled superior cervical ganglion (SCG) neurons, SUMO-1 protein was distributed throughout the cell body, axons and dendrites and presumptive presynaptic terminals, whilst SUMO-1ΔGG protein was largely confined to the cell body, in particular, the nucleus. SUMO-1 expression caused increases in paired excitatory postsynaptic potential (EPSP) ratio at short (20-120 ms) inter-stimuli intervals in comparison to SUMO-1ΔGG, consistent with an increase in residual presynaptic Ca2+ current and an increase in release probability of synaptic vesicles. Together, these data provide evidence for CaV2.2 VGCCs as novel targets for SUMOylation pathways.

Tissue- and cell-specific expression of a splice variant in the II-III cytoplasmic loop of Cacna1b

Presynaptic Ca_V2.2 (N‐type) channels are fundamental for transmitter release across the nervous system. The gene encoding Ca_V2.2 channels, Cacna1b, contains alternatively spliced exons that result in functionally distinct splice variants (e18a, e24a, e31a, and 37a/37b). Alternative splicing of the cassette exon 18a generates two mRNA transcripts (+e18a‐Cacna1b and ∆e18a‐Cacna1b). In this study, using novel mouse genetic models and in situ hybridization (BaseScope™), we confirmed that +e18a‐Cacna1b splice variants are expressed in monoaminergic regions of the midbrain. We expanded these studies and identified +e18a‐Cacna1b mRNA in deep cerebellar cells and spinal cord motor neurons. Furthermore, we determined that +e18a‐Cacna1b is enriched in cholecystokinin‐expressing interneurons. Our results provide key information to understand cell‐specific functions of Ca_V2.2 channels.

The CaV2α1 EF-hand F helix tyrosine, a highly conserved locus for GPCR inhibition of CaV2 channels

The sensory neuron of Aplysia californica participates in several forms of presynaptic plasticity including homosynaptic depression, heterosynaptic depression, facilitation and the reversal of depression. The calcium channel triggering neurotransmitter release at most synapses is Ca_V2, consisting of the pore forming α1 subunit (Ca_V2α1), and auxiliary Ca_Vβ, and Ca_Vα2δ subunits. To determine the role of the Ca_V2 channel in presynaptic plasticity in Aplysia, we cloned Aplysia Ca_V2α1, Ca_Vβ, and Ca_Vα2δ and over-expressed the proteins in Aplysia sensory neurons (SN). We show expression of exogenous Ca_V2α1 in the neurites of cultured Aplysia SN. One proposed mechanism for heterosynaptic depression in Aplysia is through inhibition of Ca_V2. Here, we demonstrate that heterosynaptic depression of the Ca_V2 calcium current is inhibited when a channel with a Y-F mutation at the conserved Src phosphorylation site is expressed, showing the strong conservation of this mechanism over evolution. We also show that the Y-F mutation reduces heterosynaptic inhibition of neurotransmitter release, highlighting the physiological importance of this mechanism for the regulation of synaptic efficacy. These results also demonstrate our ability to replace endogenous Ca_V2 channels with recombinant channels allowing future examination of the structure function relationship of Ca_V2 in the regulation of transmitter release in this system.

Central Mechanisms Mediating Thrombospondin-4-induced Pain States

Peripheral nerve injury induces increased expression of thrombospondin-4 (TSP4) in spinal cord and dorsal root ganglia that contributes to neuropathic pain states through unknown mechanisms. Here, we test the hypothesis that TSP4 activates its receptor, the voltage-gated calcium channel Cavα2δ1 subunit (Cavα2δ1), on sensory afferent terminals in dorsal spinal cord to promote excitatory synaptogenesis and central sensitization that contribute to neuropathic pain states. We show that there is a direct molecular interaction between TSP4 and Cavα2δ1 in the spinal cord in vivo and that TSP4/Cavα2δ1-dependent processes lead to increased behavioral sensitivities to stimuli. In dorsal spinal cord, TSP4/Cavα2δ1-dependent processes lead to increased frequency of miniature and amplitude of evoked excitatory post-synaptic currents in second-order neurons as well as increased VGlut2- and PSD95-positive puncta, indicative of increased excitatory synapses. Blockade of TSP4/Cavα2δ1-dependent processes with Cavα2δ1 ligand gabapentin or genetic Cavα2δ1 knockdown blocks TSP4 induced nociception and its pathological correlates. Conversely, TSP4 antibodies or genetic ablation blocks nociception and changes in synaptic transmission in mice overexpressing Cavα2δ1 Importantly, TSP4/Cavα2δ1-dependent processes also lead to similar behavioral and pathological changes in a neuropathic pain model of peripheral nerve injury. Thus, a TSP4/Cavα2δ1-dependent pathway activated by TSP4 or peripheral nerve injury promotes exaggerated presynaptic excitatory input and evoked sensory neuron hyperexcitability and excitatory synaptogenesis, which together lead to central sensitization and pain state development.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Expression of CaV 2.2 and splice variants of CaV 2.1 in oxytocin- and vasopressin-releasing supraoptic neurones

The magnocellular neurosecretory cells (MNCs) release vasopressin (VP) and oxytocin (OT) from their axon terminals into the circulation and from their somata and dendrites to exert paracrine effects on other MNCs. MNCs express several types of voltage-gated Ca(2+) channels, including Ca(V)2.1 and Ca(V)2.2. These two channels types are similar in structure and function in other cells, but although influx of Ca(2+) through Ca(V)2.2 triggers the release of both OT and VP into the circulation, Ca(V)2.1 is involved in stimulating the release of VP but not OT. Release of OT from MNC somata is also triggered by Ca(V)2.2 but not Ca(V)2.1. These observations could be explained by differences in the level of expression of Ca(V)2.1 in VP and OT MNCs or by differences in the way that the two channels interact with the exocytotic apparatus. We used immunohistochemistry to confirm earlier work suggesting that MNCs express variants of Ca(V)2.1 lacking portions of an internal loop that enables the channels to interact with synaptic proteins. We used an antibody that would recognise both the full-length Ca(V)2.1 and the deletion variants to show that OT MNCs express fewer Ca(V)2.1 channels than do VP MNCs in both somata and axon terminals. We used the reverse transcriptase-polymerase chain reaction and immunocytochemistry to test whether MNCs express similar deletion variants of Ca(V)2.2 and were unable to find any evidence to support this. Our data suggest that the different roles that Ca(V)2.1 and Ca(V)2.2 play in MNC secretion may be a result of the different levels of expression of Ca(V)2.1 in VP and OT MNCs, as well as the expression in MNCs of deletion variants of Ca(V)2.1 that do not interact with exocytotic proteins and therefore may be less likely to mediate exocytotic release.

C-terminal alternative splicing of CaV1.3 channels distinctively modulates their dihydropyridine sensitivity

The transcripts of L-type voltage-gated calcium channels (CaV) 1.3 undergo extensive alternative splicing. Alternative splicing, particularly in the C terminus, drastically modifies gating properties of the channel. However, little is known about whether alternative splicing could modulate the pharmacologic properties of CaV1.3 in a manner similar to the paralogous CaV1.2. Here we undertook the screening of different channel splice isoforms harboring splice variations in either the IS6 segment or the C terminus. Unexpectedly, while inclusion of exon 8a or 8, which code for IS6, did not alter dihydropyridine (DHP) sensitivity, distinct pharmacologic properties were observed for the various C-terminal splice isoforms. In the presence of external Ca(2+), fast inactivating splice variants including CaV1.342a and CaV1.343s with intact calmodulin-IQ domain interaction showed consistently low DHP sensitivity. Interestingly, attenuation of calcium-dependent inactivation with overexpression of calmodulin34 did not enhance the sensitivity of CaV1.342a, suggesting that the low DHP sensitivity may not be a result of fast channel inactivation. Alternatively, disruption of calmodulin-IQ domain binding in the CaV1.3Δ41 and full-length CaV1.342 channels was associated with heightened DHP sensitivity. In distinct contrast to the well-known modulation of DHP blockade of CaV1.2 channels, this study has therefore uncovered a novel mechanism for modulation of the pharmacologic properties of CaV1.3 channels through posttranscriptional modification of the C terminus.

Control of neuronal voltage-gated calcium ion channels from RNA to protein

Voltage-gated calcium ion (CaV) channels convert neuronal activity into rapid intracellular calcium signals to trigger a myriad of cellular responses. Their involvement in major neurological and psychiatric diseases, and importance as therapeutic targets, has propelled interest in subcellular-specific mechanisms that align CaV channel activity to specific tasks. Here, we highlight recent studies that delineate mechanisms controlling the expression of CaV channels at the level of RNA and protein. We discuss the roles of RNA editing and alternative pre-mRNA splicing in generating CaV channel isoforms with activities specific to the demands of individual cells; the roles of ubiquitination and accessory proteins in regulating CaV channel expression; and the specific binding partners that contribute to both pre- and postsynaptic CaV channel function.

Mechanisms of conotoxin inhibition of N-type (Ca(v)2.2) calcium channels

N-type (Ca(v)2.2) voltage-gated calcium channels (VGCC) transduce electrical activity into other cellular functions, regulate calcium homeostasis and play a major role in processing pain information. Although the distribution and function of these channels vary widely among different classes of neurons, they are predominantly expressed in nerve terminals, where they control neurotransmitter release. To date, genetic and pharmacological studies have identified that high-threshold, N-type VGCCs are important for pain sensation in disease models. This suggests that N-type VGCC inhibitors or modulators could be developed into useful drugs to treat neuropathic pain. This review discusses the role of N-type (Ca(v)2.2) VGCCs in nociception and pain transmission through primary sensory dorsal root ganglion (DRG) neurons (nociceptors). It also outlines the potent and selective inhibition of N-type VGCCs by conotoxins, small disulfide-rich peptides isolated from the venom of marine cone snails. Of these conotoxins, ω-conotoxins are selective N-type VGCC antagonists that preferentially block nociception in inflammatory pain models, and allodynia and/or hyperalgesia in neuropathic pain models. Another conotoxin family, α-conotoxins, were initially proposed as competitive antagonists of muscle and neuronal nicotinic acetylcholine receptors (nAChR). Surprisingly, however, α-conotoxins Vc1.1 and RgIA, also potently inhibit N-type VGCC currents in the sensory DRG neurons of rodents and α9 nAChR knockout mice, via intracellular signaling mediated by G protein-coupled GABAB receptors. Understanding how conotoxins inhibit VGCCs is critical for developing these peptides into analgesics and may result in better pain management. This article is part of a Special Issue entitled: Calcium channels.

MKP3 eliminates depolarization-dependent neurotransmitter release through downregulation of L-type calcium channel Cav1.2 expression

Release of neurotransmitters is a fundamental and regulated process that is essential for normal brain functioning. Regulation of this process is potentially important for any neuronal process, and disruption of the release process may contribute to the pathophysiology associated with psychiatric diseases. In this work it is shown that expression of the negative regulator of mitogen-activated protein kinase (MAPK) signaling the MAPK phosphatase MKP3/DUSP6 eliminates depolarization-dependent release of dopamine in rat PC12 cells. Pharmacologic interventions with latrotroxin (LTX) or A23187, which make the cells permeable to calcium, reestablish the dopamine release. Calcium imaging also reveals that calcium influx is impaired in MKP3-expressing cells. Because acute pharmacologic inhibition of MAPKs has no effect on dopamine release in naïve PC12 cells, the MKP3-mediated elimination of neurotransmitter release must be caused by a long-term process, such as changes in gene expression. In support of this the expression of the L-type calcium channel cav1.2 alpha subunit (Cacna1c) is decreased in MKP3-expressing PC12 cells. With the reintroduction of cav1.2 expression, neurotransmitter release is restored in the MKP3-expressing PC12 cells. Thus, MKP3 expression reduces neurotransmitter release by decreasing the expression of cav1.2. Because MKP3 is increased when neuronal activity is elevated, this process could play a role in regulating neurotransmitter homeostasis.

Alternative splicing: functional diversity among voltage-gated calcium channels and behavioral consequences

Neuronal voltage-gated calcium channels generate rapid, transient intracellular calcium signals in response to membrane depolarization. Neuronal Ca(V) channels regulate a range of cellular functions and are implicated in a variety of neurological and psychiatric diseases including epilepsy, Parkinson's disease, chronic pain, schizophrenia, and bipolar disorder. Each mammalian Cacna1 gene has the potential to generate tens to thousands of Ca(V) channels by alternative pre-mRNA splicing, a process that adds fine granulation to the pool of Ca(V) channel structures and functions. The precise composition of Ca(V) channel splice isoform mRNAs expressed in each cell are controlled by cell-specific splicing factors. The activity of splicing factors are in turn regulated by molecules that encode various cellular features, including cell-type, activity, metabolic states, developmental state, and other factors. The cellular and behavioral consequences of individual sites of Ca(V) splice isoforms are being elucidated, as are the cell-specific splicing factors that control splice isoform selection. Altered patterns of alternative splicing of Ca(V) pre-mRNAs can alter behavior in subtle but measurable ways, with the potential to influence drug efficacy and disease severity. This article is part of a Special Issue entitled: Calcium channels.

Opioid inhibition of N-type Ca2+ channels and spinal analgesia couple to alternative splicing

Alternative pre-mRNA splicing predominates in the nervous systems of complex organisms including humans dramatically expanding the potential size of the proteome. Cell-specific alternative pre-mRNA splicing is thought to optimize protein function for specialized cellular tasks, but direct evidence for this is limited. Transmission of noxious thermal stimuli relies on the activity of N-type Ca_V2.2 calcium channels in nociceptors. Using an exon replacement strategy in mice, we show that mutually exclusive splicing in the Ca_V2.2 gene modulates N-type channel function in nociceptors leading to a change in morphine analgesia. Exon 37a enhances μ-opioid receptor mediated inhibition of N-type calcium channels by promoting activity-independent inhibition. In the absence of e37a spinal morphine analgesia is weakened in vivo without influencing the basal response to noxious thermal stimuli. Our data suggest that highly specialized, discrete cellular responsiveness in vivo can be attributed to alternative splicing events regulated at the level of individual neurons.

Regulation of N-type voltage-gated calcium channels (Cav2.2) and transmitter release by collapsin response mediator protein-2 (CRMP-2) in sensory neurons

Collapsin response mediator proteins (CRMPs) mediate signal transduction of neurite outgrowth and axonal guidance during neuronal development. Voltage-gated Ca(2+) channels and interacting proteins are essential in neuronal signaling and synaptic transmission during this period. We recently identified the presynaptic N-type voltage-gated Ca(2+) channel (Cav2.2) as a CRMP-2-interacting partner. Here, we investigated the effects of a functional association of CRMP-2 with Cav2.2 in sensory neurons. Cav2.2 colocalized with CRMP-2 at immature synapses and growth cones, in mature synapses and in cell bodies of dorsal root ganglion (DRG) neurons. Co-immunoprecipitation experiments showed that CRMP-2 associates with Cav2.2 from DRG lysates. Overexpression of CRMP-2 fused to enhanced green fluorescent protein (EGFP) in DRG neurons, via nucleofection, resulted in a significant increase in Cav2.2 current density compared with cells expressing EGFP. CRMP-2 manipulation changed the surface levels of Cav2.2. Because CRMP-2 is localized to synaptophysin-positive puncta in dense DRG cultures, we tested whether this CRMP-2-mediated alteration of Ca(2+) currents culminated in changes in synaptic transmission. Following a brief high-K(+)-induced stimulation, these puncta became loaded with FM4-64 dye. In EGFP and neurons expressing CRMP-2-EGFP, similar densities of FM-loaded puncta were observed. Finally, CRMP-2 overexpression in DRG increased release of the immunoreactive neurotransmitter calcitonin gene-related peptide (iCGRP) by approximately 70%, whereas siRNA targeting CRMP-2 significantly reduced release of iCGRP by approximately 54% compared with control cultures. These findings support a novel role for CRMP-2 in the regulation of N-type Ca(2+) channels and in transmitter release.

L- and N-current but not M-current inhibition by M1 muscarinic receptors requires DAG lipase activity

Stimulation of postsynaptic M(1) muscarinic receptors (M(1)Rs) increases firing rates of both sympathetic and central neurons that underlie increases in vasomotor tone, heart rate, and cognitive memory functioning. At the cellular level, M(1)R stimulation modulates currents through various voltage-gated ion channels, including KCNQ K+ channels (M-current) and both L- and N-type Ca2+ channels (L- and N-current) by a pertussis toxin-insensitive, slow signaling pathway. Depletion of phosphatidylinositol-4,5-bisphosphate (PIP2) during M(1)R stimulation suffices to inhibit M-current. We found previously that following PIP2 hydrolysis by phospholipase C, activation of phospholipase A2 and liberation of a lipid metabolite, most likely arachidonic acid (AA) are necessary for L- and N-current modulation. Here we examined the involvement of a third lipase, diacylglycerol lipase (DAGL), in the slow pathway. We documented the presence of DAGL in superior cervical ganglion neurons, and then tested the highly selective DAGL inhibitor, RHC-80267, for its capacity to antagonize M(1)R-mediated modulation of whole-cell Ca2+ currents. RHC-80267 significantly reduced L- and N-current inhibition by the muscarinic agonist oxotremorine-M (Oxo-M) but did not affect their inhibition by exogenous AA. Moreover, voltage-dependent inhibition of N-current by Oxo-M remained in the presence of RHC-80267, indicating selective action on the slow pathway. RHC also blocked inhibition of recombinant N-current. In contrast, RHC-80267 had no effect on native M-current inhibition. These data are consistent with a role for DAGL in mediating L- and N-current inhibition. These results extend our previous findings that the signaling pathway mediating L- and N-current inhibition diverges from the pathway initiating M-current inhibition.

Novel CaV2.1 clone replicates many properties of Purkinje cell CaV2.1 current

The P-type calcium current is mediated by a voltage-sensing CaV2.1 alpha subunit in combination with modulatory auxiliary subunits. In Purkinje neurones, this current has distinctively slow inactivation kinetics that may depend on alternative splicing of the alpha subunit and/or association with different CaVbeta subunits. To better understand the molecular components of P-type calcium current, we cloned a CaV2.1 cDNA from total mouse brain. The full-length CaV2.1 isoform that we isolated (GenBank AY714490) contains sequences recently shown to be present in Purkinje neurones. In agreement with previously published work, the alternatively spliced amino acid V421, implicated in slow inactivation, was not encoded in AY714490 and was absent from reverse transcription-polymerase chain reaction products generated from single Purkinje cells. Next, we studied the expression of the four known mouse auxiliary CaVbeta2 isoforms in Purkinje neurones. Confirmation of the presence of CaVbeta2a in Purkinje cells, previously shown by others to slow CaV2.1 kinetics, led us to characterize its influence on current dynamics. We studied currents generated by the clone AY714490 coexpressed in tsA201 cells with four different CaVbeta subunits. In addition to the well-documented slowing of open-state inactivation kinetics, coexpression with the CaVbeta2a subunit also protected CaV2.1 channels from closed-state inactivation and prevented the channel from inactivating during physiological trains of action potential-like stimuli. This strong resistance to inactivation parallels the property of Purkinje neurone P-type currents and is suggestive of a role for CaVbeta2a in modulating the inactivation properties of P-type calcium currents in Purkinje neurones.

Ca2+ influx through both L- and N-type Ca2+ channels increases c-fos expression by electrical stimulation of sympathetic neurons

During direct membrane depolarization, Ca2+ influx primarily through L-type Ca2+ (L-) channels initiates activity-dependent gene transcription. This is surprising given that in most neurons a minority of the total Ca2+ current arises from L-channel activity. However, many studies have stimulated Ca2+ influx with unphysiological stimuli such as chronic membrane depolarization using high K+ medium. Few studies have tested whether other Ca2+ channels stimulate gene transcription in adult neurons as a consequence of direct electrical stimulation. Therefore, we evaluated the role of L- and N-type Ca2+ (N-) channel activity in regulating mRNA levels of c-fos, an activity-dependent transcription factor, in adult rat superior cervical ganglion (SCG) neurons as the majority of Ca2+ channels are N-type, while only a minority are L-type. Changes in c-fos mRNA levels were measured using semi-quantitative and single-cell RT-PCR. Phosphorylation of CREB (pCREB) and changes in c-Fos levels were visualized in dissociated cells by immunocytochemistry. Increases in pCREB, c-fos mRNA and c-Fos protein with either K+ or electrical depolarization required Ca2+ influx. These results support previous findings that elevated c-fos levels result from pCREB stimulating c-fos transcription. Elevation of pCREB, c-fos and c-Fos with K+ depolarization depended on L-channel activity. By contrast, antagonizing either channel at 10-Hz stimulation minimized these increases despite unequal numbers of the two channel types. Transition to exclusive L-channel involvement occurred with increasing frequency of stimulation (from 10 to 20 to 50 Hz). Our results demonstrate that N- and L-channel participation in regulating c-fos expression is encoded in the pattern of electrical stimulation.

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.

Dehydration increases L-type Ca(2+) current in rat supraoptic neurons

The magnocellular neurosecretory cells of the hypothalamus (MNCs) regulate water balance by releasing vasopressin (VP) and oxytocin (OT) as a function of plasma osmolality. Release is determined largely by the rate and pattern of MNC firing, but sustained increases in osmolality also produce structural adaptations, such as cellular hypertrophy, that may be necessary for maintaining high levels of neuropeptide release. Since increases in Ca(2+) current could enhance exocytotic secretion, influence MNC firing patterns, and activate gene transcription and translation, we tested whether Ca(2+) currents in MNCs acutely isolated from the supraoptic nucleus (SON) of the hypothalamus are altered by 16-24 h of water deprivation. A comparison of whole-cell patch-clamp recordings demonstrated that dehydration causes a significant increase in the amplitude of current sensitive to the L-type Ca(2+) channel blocker nifedipine (from -56 +/- 6 to -99 +/- 10 pA; P < 0.001) with no apparent change in other components of Ca(2+) current. Post-recording immunocytochemical identification showed that this increase in current occurred in both OT- and VP-releasing MNCs. Radioligand binding studies of tissue from the SON showed there is also an increase in the density of binding sites for an L-type Ca(2+) channel ligand (from 51.5 +/- 4.8 to 68.1 +/- 4.1 fmol (mg protein)(-1); P < 0.05), suggesting that there was an increase in the number of L-type channels on the plasma membrane of the MNCs or some other cell type in the SON. There were no changes in the measured number of binding sites for an N-type Ca(2+) channel ligand. Dehydration was not associated with changes in the levels of mRNA coding for Ca(2+) channel alpha(1) subunits. These data are consistent with the hypothesis that a selective increase of L-type Ca(2+) current may contribute to the adaptation that occurs in the MNCs during dehydration.

The role of voltage-gated calcium channels in pain and nociception

The voltage-gated calcium channels (VGCCs) are a large and functionally diverse group of ion channels found throughout the central nervous system (CNS) and the periphery. Neuronal functions include the control of neurotransmitter release and neuronal excitability in important pain pathways. In the current review we will give an overview of the data that has been generated in support of these channels performing a pivotal role in the pain pathway.

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.

Cell-specific alternative splicing increases calcium channel current density in the pain pathway

N-type calcium channels are critical for pain transduction. Inhibitors of these channels are powerful analgesics, but clinical use of current N-type blockers remains limited by undesirable actions in other regions of the nervous system. We now demonstrate that a unique splice isoform of the N-type channel is restricted exclusively to dorsal root ganglia. By a combination of functional and molecular analyses at the single-cell level, we show that the DRG-specific exon, e37a, is preferentially present in Ca(V)2.2 mRNAs expressed in neurons that contain nociceptive markers, VR1 and Na(V)1.8. Cell-specific inclusion of exon 37a correlates closely with significantly larger N-type currents in nociceptive neurons. This unique splice isoform of the N-type channel could represent a novel target for pain management.

Use-dependent blockade of Cav2.2 voltage-gated calcium channels for neuropathic pain

The translocation of extracellular calcium (Ca(2+)) via voltage-gated Ca(2+) channels (VGCCs) in neurons is involved in triggering multiple physiological cell functions but also the abnormal, pathophysiological responses that develop as a consequence of injury. In conditions of neuropathic pain, VGCCs are involved in supplying the signal Ca(2+) important for the sustained neuronal firing and neurotransmitter release characteristic of these syndromes. Preclinical data have identified N-type VGCCs (Ca(v)2.2) as key participants in contributing to these Ca(2+) signaling events and clinical data with the peptide blocker Prialt have now validated Ca(v)2.2 as a bona fide target for future drug discovery efforts to identify new and novel therapeutics for neuropathic pain. Imperative for the success of such an endeavor will be the ability to identify compounds selective for Ca(v)2.2, versus other VGCCs, but also compounds which demonstrate effective blockade during the pathophysiological states of neuropathic pain without compromising channel activity associated with sustaining normal housekeeping cellular functions. An approach to obtain this research target profile is to identify compounds, which are more potent in blocking Ca(v)2.2 during higher frequencies of firing as compared to the slower more physiologically-relevant frequencies. This may be achieved by identifying compounds with enhanced potency for the inactivated state of Ca(v)2.2. This commentary explores the rationale and options for engineering a use-dependent blocker of Ca(v)2.2. It is anticipated that this use-dependent profile of channel blockade will result in new chemical entities with an improved therapeutic ratio for neuropathic pain.

Transfer of β subunit regulation from high to low voltage-gated Ca2+ channels

High voltage-activated Ca(2+) channel expression and gating is controlled by their beta subunits. Although the sites of interaction are known at the atomic level, how beta modulates gating remains to be determined. Using a chimeric approach, beta subunit regulation was conferred to a low voltage-activated channel. Regulation was dependent on a rigid linker connecting the alpha(1) interaction domain to IS6. Chimeric channels also revealed a role for IS6 in channel gating. Taken together, these results support a direct coupling model where beta subunits alter movements in IS6 that occur as the channel transits between closed, open, and inactivated states.

Chronic ethanol exposure induces an N-type calcium channel splice variant with altered channel kinetics

Chronic ethanol exposure increases the density of N-type calcium channels in brain. We report that ethanol increases levels of mRNA for a splice variant of the N channel specific subunit alpha1 2.2 that lacks exon 31a. Whole cell recordings demonstrated an increase in N-type current with a faster activation rate and a shift in activation to more negative potentials after chronic alcohol exposure, consistent with increased abundance of channels containing this variant. These results identify a novel mechanism whereby chronic ethanol exposure can increase neuronal excitability by altering levels of channel splice variants.

Alternative splicing in the voltage-sensing region of N-Type CaV2.2 channels modulates channel kinetics

The CaV2.2 gene encodes the functional core of the N-type calcium channel. This gene has the potential to generate thousands of CaV2.2 splice isoforms with different properties. However, the functional significance of most sites of alternative splicing is not established. The IVS3-IVS4 region contains an alternative splice site that is conserved evolutionarily among CaValpha1 genes from Drosophila to human. In CaV2.2, inclusion of exon 31a in the IVS3-IVS4 region is restricted to the peripheral nervous system, and its inclusion slows the speed of channel activation. To investigate the effects of exon 31a in more detail, we generated four tsA201 cell lines stably expressing CaV2.2 splice isoforms. Coexpression of auxiliary CaVbeta and CaValpha2delta subunits was required to reconstitute currents with the kinetics of N-type channels from neurons. Channels including exon 31a activated and deactivated more slowly at all voltages. Current densities were high enough in the stable cell lines co-expressing CaValpha2delta to resolve gating currents. The steady-state voltage dependence of charge movement was not consistently different between splice isoforms, but on gating currents from the exon 31a-containing CaV2.2 isoform decayed with a slower time course, corresponding to slower movement of the charge sensor. Exon 31a-containing CaV2.2 is restricted to peripheral ganglia; and the slower gating kinetics of CaV2.2 splice isoforms containing exon 31a correlated reasonably well with the properties of native N-type currents in sympathetic neurons. Our results suggest that alternative splicing in the S3-S4 linker influences the kinetics but not the voltage dependence of N-type channel gating.

N- and P/Q-type Ca2+ channels regulate synaptic efficacy between spinal dorsolateral funiculus terminals and motoneurons

Ca2+ influx through voltage-gated Ca2+ channels mediates synaptic transmission at numerous central synapses. However, electrophysiological and pharmacological evidence linking Ca+ channel activity with neurotransmitter release in the vertebrate mature spinal cord is scarce. In the current report, we investigated in a slice preparation from the adult turtle spinal cord, the effects of various Ca+ channel antagonists on neurotransmission at terminals from the dorsolateral funiculus synapsing motoneurons. Bath application of tetrodotoxin or NiCl2 prevented the monosynaptic excitatory postsynaptic potentials (EPSPs), and this effect was mimicked by exposure to a zero-Ca2+ solution. Application of polypeptide toxins that block N- and P/Q-type channels (omega-CTx-GVIA and omega-Aga-IVA) reduced the EPSP amplitude in a dose-dependent manner. By analyzing the input resistance and the EPSP time course, and using a paired pulse protocol we determined that both toxins act at presynaptic level to modulate neurotransmitter release. RT-PCR studies showed the expression of N- and P/Q-type channel mRNAs in the turtle spinal cord. Together, these results indicate that N- and P/Q-type Ca2+ channels may play a central role in the regulation of neurotransmitter release in the adult turtle spinal cord.

Cumulative inactivation of N-type CaV2.2 calcium channels modified by alternative splicing

The Ca(V)2 family of voltage-gated calcium channels, present in presynaptic nerve terminals, regulates exocytosis and synaptic transmission. Cumulative inactivation of these channels occurs during trains of action potentials, and this may control short-term dynamics at the synapse. Inactivation during brief, repetitive stimulation is primarily attributed to closed-state inactivation, and several factors modulate the susceptibility of voltage-gated calcium channels to this form of inactivation. We show that alternative splicing of an exon in a cytoplasmic region of the Ca(V)2.2 channel modulates its sensitivity to inactivation during trains of action potential waveforms. The presence of this exon, exon 18a, protects the Ca(V)2.2 channel from entry into closed-state inactivation specifically during short (10 ms to 3 s) and small depolarizations of the membrane potential (-60 mV to -50 mV). The reduced sensitivity to closed-state inactivation within this dynamic range likely underlies the differential responsiveness of Ca(V)2.2 splice isoforms to trains of action potential waveforms. Regulated alternative splicing of Ca(V)2.2 represents a possible mechanism for modulating short-term dynamics of synaptic efficacy in different regions of the nervous system.

The impact of splice isoforms on voltage-gated calcium channel alpha1 subunits

Semi-conserved exon boundaries in members of the CACNA1 gene family result in recurring pre-mRNA splicing patterns. The resulting variations in the encoded pore-forming subunit of the voltage-gated calcium channel affect functionally significant regions, such as the vicinity of the voltage-sensing S4 segments or the intracellular loops that are important for protein interaction. In addition to generating functional diversity, RNA splicing regulates the quantitative expression of other splice isoforms of the same gene by producing transcripts with premature stop codons which encode two-domain or three-domain channels. An overview of some of the known splice isoforms of the alpha(1) calcium channel subunits and their significance is given.

The neuronal beta 4 subunit increases the unitary conductance of L-type voltage-gated calcium channels in PC12 cells

beta subunits of voltage-gated calcium channels influence channel behavior in numerous ways, including enhancing the targeting of alpha1 subunits to the plasma membrane and shifting the voltage dependence of activation and inactivation. Of the four beta subunits that have been identified, beta 4 is of particular interest because mutation of its alpha1 subunit interaction domain produces severe neurological defects. Its differential distribution in the hippocampus prompted us to examine whether this subunit was responsible for the heterogeneity of hippocampal L-type calcium channels. To study the functional effects of the beta 4 subunit on native L-type calcium channels, we transfected beta 4 cDNA subcloned out of embryonic hippocampal neurons into PC12 cells, a cell line that contains the beta 1, beta 2, and beta 3 subunits but not the beta 4 subunit. Cell-attached single-channel recordings of L-type channel activity from untransfected and transfected PC12 cells compared with recordings obtained from hippocampal neurons revealed an effect of the beta 4 subunit on single-channel conductance. L-type channels in untransfected PC12 cells had a significantly smaller conductance (19.8 picosiemens (pS)) than L-type channels in hippocampal neurons (22 pS). After transfection of beta 4, however, L-type single-channel conductance was indistinguishable between the two cell types. Our data suggest that calcium channel beta 4 subunits affect the conductance of L-type calcium channels and that native hippocampal L-type channels contain the beta 4 subunit.

Comparative effects of methylmercury and Hg(2+) on human neuronal N- and R-type high-voltage activated calcium channels transiently expressed in human embryonic kidney 293 cells

Expression cDNA clones of alpha1B-1 or alpha1E-3 subunits coding for human neuronal N-(Cav2.2) or R-subtype (Cav2.3) Ca2+ channels, respectively, was combined with alpha2-bdelta and beta3-a Ca2+ channel subunits, and transfected into human embryonic kidney cells for transient expression to determine whether specific types of neuronal voltage-sensitive Ca2+ channels are affected differentially by methylmercury (MeHg) and Hg2+. For both Ca2+ channel subtypes, MeHg (0.125-5.0 microM) or Hg2+ (0.1-5 microM) caused a time- and concentration-dependent reduction of current. MeHg caused an initial, rapid component and a subsequent more gradual component of inhibition. The rapid component of block was completed between 100 and 150 s after beginning treatment. At 0.125 to 1.25 microM, MeHg caused a more gradual decline in current. Apparent IC50 values were 1.3 and 1.1 microM for MeHg, and 2.2 and 0.7 microM for Hg2+ on N- and R-types, respectively. For N-type current, effects of Hg2+ were initially greater on the peak current than on the sustained current remaining at the end of a test pulse; subsequently, Hg2+ blocked both components of current. For R-type current, Hg2+ affected peak and sustained current approximately equally. Kinetics of inactivation also seemed to be affected by Hg2+ in cells expressing N-type but not R-type current. Washing with MeHg-free solution could not reverse effects of MeHg on either type of current. The effect of Hg2+ on N- but not R-type current was partially reversed by Hg2+-free wash solution. Therefore, different types of Ca2+ channels have differential susceptibility to neurotoxic mercurials even when expressed in the same cell type.

Immunological characterization of T-type voltage-dependent calcium channel CaV3.1 (alpha 1G) and CaV3.3 (alpha 1I) isoforms reveal differences in their localization, expression, and neural development

Low voltage-activated calcium channels (LVAs; "T-type") modulate normal neuronal electrophysiological properties such as neuronal pacemaker activity and rebound burst firing, and may be important anti-epileptic targets. Proteomic analyses of available alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 sequences suggest numerous potential isoforms, with specific alpha 1G/Ca(V)3.1 or alpha 1I/Ca(V)3.3 domains postulated to be conserved among isoforms of each T-type channel subtype. This information was used to generate affinity-purified anti-peptide antibodies against sequences unique to alpha 1G/Ca(V)3.1 or alpha 1I/Ca(V)3.3, and these antibodies were used to compare and contrast alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 protein expression by western blotting and immunohistochemistry. Each antibody reacted with appropriately sized recombinant protein in HEK-293 cells. Regional and developmental differences in alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 protein expression were observed when the antibodies were used to probe regional brain dissections prepared from perinatal mice and adult rodents and humans. Mouse forebrain alpha 1G/Ca(V)3.1 (approximately 240 kDa) was smaller than cerebellar (approximately 260 kDa) alpha 1G/Ca(V)3.1, and expression of both proteins increased during perinatal development. In contrast, mouse midbrain and diencephalic tissues evidenced an alpha 1I/Ca(V)3.3 immunoreactive doublet (approximately 230 kDa and approximately 190 kDa), whereas other brain regions only expressed the small alpha 1I/Ca(V)3.3 isoform. A unique large alpha 1I/Ca(V)3.3 isoform (approximately 260 kDa) was expressed at birth and eventually decreased, concomitant with the appearance and gradual increase of the small alpha 1I/Ca(V)3.3 isoform. Immunohistochemistry supported the conclusion that LVAs are expressed in a regional manner, as cerebellum strongly expressed alpha 1G/Ca(V)3.1, and olfactory bulb and midbrain contained robust alpha 1I/Ca(V)3.3 immunoreactivity. Finally, strong alpha 1I/Ca(V)3.3, but not alpha 1G/Ca(V)3.1, immunoreactivity was observed in brain and spinal cord by embryonic day 14 in situ. Taken together, these data provide an anatomical and biochemical basis for interpreting LVA heterogeneity and offer evidence of developmental regulation of LVA isoform expression.

Alternative splicing of Cav2 genes and their functional significance

Alternative splicing is one of the most pharmacologically and physiologically significant mechanisms for the functional diversity of the mammalian genomes. Here I review recent results on the diversity of the Ca(v)2 subclass of voltage-dependent Ca(2+) channel gene in neurons. Although the entire picture of alternative splicing is not yet understood, emerging evidences suggest the Ca(v)2 isoforms permit optimization of Ca(2+) signaling in different regions of the brain with specific pharmacological ligands.

Characteristics of block by Pb2+ of function of human neuronal L-, N-, and R-type Ca2+ channels transiently expressed in human embryonic kidney 293 cells

Lead (Pb(2+)) is a well-known inhibitor of voltage-dependent Ca(2+) channels in their native environments in several types of cells. However, its effects on discrete Ca(2+) channel phenotypes in isolation have not been well studied. We compared how specific subtypes of human neuronal high-voltage-activated Ca(2+) channels were affected by acute exposure to Pb(2+). Expression cDNA clones of human alpha(1C), alpha(1B), or alpha(1E) subunit genes encoding neuronal L-, N-, and R-subtypes of Ca(2+) channels, respectively, along with a constant alpha(2)delta and beta(3) subunits were transfected into human embryonic kidney 293 cells. Currents through the respective transiently expressed channels were measured using whole-cell recording techniques with Ba(2+) (20 mM) as charge carrier. Extracellular bath applications of Pb(2+) significantly reduced current amplitude through all three types of Ca(2+) channels in a concentration-dependent manner. The order of potency was: alpha(1E) (IC(50) = 0.10 microM), followed by alpha(1C) (IC(50) = 0.38 microM) and alpha(1B) (IC(50) = 1.31 microM). Pb(2+)-induced perturbation of function of alpha(1C) and alpha(1B) containing Ca(2+) channels was more easily reversed than for alpha(1E)-containing Ca(2+) channels after washing with Pb(2+) free solution. The current-voltage relationships were not altered after 3-min exposure to Pb(2+) for any of the three types. However, the steady-state inactivation relationships were shifted to more negative potentials for channels containing alpha(1B) and alpha(1E) subunits, but not for those containing alpha(1C) subunits. Pb(2+) accelerated the inactivation time of current in all three subtypes of Ca(2+) channels in a concentration- and voltage-dependent manner. Therefore, different subtypes of Ca(2+) channels exhibit differential susceptibility to Pb(2+) even when expressed in the same cell type. Current expressed by alpha(1E)-containing channels is more sensitive to Pb(2+) than that expressed by alpha(1C)- or alpha(1B)-containing channels. Several Ca(2+) channel phenotypes are quite sensitive to the inhibitory action of Pb(2+). Furthermore, it seems that Pb(2+) is more likely to combine with Ca(2+) channels in the closed state.

Alternative splicing of the beta 4 subunit has alpha1 subunit subtype-specific effects on Ca2+ channel gating

Ca2+ channel beta subunits are important molecular determinants of the kinetics and voltage dependence of Ca2+ channel gating. Through direct interactions with channel-forming alpha1 subunits, beta subunits enhance expression levels, accelerate activation, and have variable effects on inactivation. Four distinct beta subunit genes each encode five homologous sequence domains (D1-5), three of which (D1, D3, and D5) undergo alternative splicing. We have isolated from human spinal cord a novel alternatively spliced beta4 subunit containing a short form of domain D1 (beta4a) that is highly homologous to N termini of Xenopus and rat beta3 subunits. The purpose of this study was to compare the gating properties of various alpha1 subunit complexes containing beta4a with those of complexes containing a beta4 subunit with a longer form of domain D1, beta4b. Expression in Xenopus oocytes revealed that, relative to alpha1A and alpha1B complexes containing beta4b, the voltage dependence of activation and inactivation of complexes containing beta4a were shifted to more depolarized potentials. Moreover, alpha1A and alpha1B complexes containing beta4a inactivated at a faster rate. Interestingly, beta4 subunit alternative splicing did not influence the gating properties of alpha1C and alpha1E subunits. Experiments with beta4 deletion mutants revealed that both the N and C termini of the beta4 subunit play critical roles in setting voltage-dependent gating parameters and that their effects are alpha1 subunit specific. Our data are best explained by a model in which distinct modes of activation and inactivation result from beta-subunit splice variant-specific interactions with an alpha1 subunit gating structure.

Identification and characterization of novel human Ca(v)2.2 (alpha 1B) calcium channel variants lacking the synaptic protein interaction site

The physical interaction between the presynaptic vesicle release complex and the large cytoplasmic region linking domains II and III of N-type (Ca(v)2.2) calcium channel alpha(1)B subunits is considered to be of fundamental importance for efficient neurotransmission. By PCR analysis of human brain cDNA libraries and IMR32 cell mRNA, we have isolated novel N-type channel variants, termed Ca(v)2.2-Delta1 and Delta2, which lack large parts of the domain II-III linker region, including the synaptic protein interaction site. They appear to be widely expressed across the human CNS as indicated by RNase protection assays. When expressed in tsA-201 cells, both novel variants formed barium-permeable channels with voltage dependences and kinetics for activation that were similar to those observed with the full-length channel. All three channel types exhibited the hallmarks of prepulse facilitation, which interestingly occurred independently of G-protein betagamma subunits. By contrast, the voltage dependence of steady-state inactivation seen with both Delta1 and Delta2 channels was shifted toward more depolarized potentials, and recovery from inactivation of Delta1 and Delta2 channels occurred more rapidly than that of the full-length channel. Moreover, the Delta1 channel was dramatically less sensitive to both omega-conotoxin MVIIA and GVIA than either the Delta2 variant or the full-length construct. Finally, the domain II-III linker region of neither variant was able to effectively bind syntaxin in vitro. These results suggest that the structure of the II-III linker region is an important determinant of N-type channel function and pharmacology. The lack of syntaxin binding hints at a unique physiological function of these channels.