Calcium channel beta subunits differentially regulate the inhibition of N-type channels by individual Gbeta isoforms

Regulation of N-type calcium channels by nociceptin receptors and its possible role in neurological disorders

Activation of nociceptin opioid peptide receptors (NOP, a.k.a. opioid-like receptor-1, ORL-1) by the ligand nociceptin/orphanin FQ, leads to G protein-dependent regulation of Cav2.2 (N-type) voltage-gated calcium channels (VGCCs). This typically causes a reduction in calcium currents, triggering changes in presynaptic calcium levels and thus neurotransmission. Because of the widespread expression patterns of NOP and VGCCs across multiple brain regions, the dorsal horn of the spinal cord, and the dorsal root ganglia, this results in the alteration of numerous neurophysiological features. Here we review the regulation of N-type calcium channels by the NOP-nociceptin system in the context of neurological conditions such as anxiety, addiction, and pain.

Dopamine-mediated calcium channel regulation in synaptic suppression in L. stagnalis interneurons

D2 dopamine receptor-mediated suppression of synaptic transmission from interneurons plays a key role in neurobiological functions across species, ranging from respiration to memory formation. In this study, we investigated the mechanisms of D2 receptor-dependent suppression using soma-soma synapse between respiratory interneuron VD4 and LPeD1 in the mollusk Lymnaea stagnalis (L. stagnalis). We studied the effects of dopamine on voltage-dependent Ca²⁺ current and synaptic vesicle release from the VD4. We report that dopamine inhibits voltage-dependent Ca²⁺ current in the VD4 by both voltage-dependent and -independent mechanisms. Dopamine also suppresses synaptic vesicle release downstream of activity-dependent Ca²⁺ influx. Our study demonstrated that dopamine acts through D2 receptors to inhibit interneuron synaptic transmission through both voltage-dependent Ca²⁺ channel-dependent and -independent pathways. Taken together, these findings expand our understanding of dopamine function and fundamental mechanisms that shape the dynamics of neural circuit.

Presynaptic Calcium Channels

Presynaptic Ca²⁺ entry occurs through voltage-gated Ca²⁺ (Ca_V) channels which are activated by membrane depolarization. Depolarization accompanies neuronal firing and elevation of Ca²⁺ triggers neurotransmitter release from synaptic vesicles. For synchronization of efficient neurotransmitter release, synaptic vesicles are targeted by presynaptic Ca²⁺ channels forming a large signaling complex in the active zone. The presynaptic Ca_V2 channel gene family (comprising Ca_V2.1, Ca_V2.2, and Ca_V2.3 isoforms) encode the pore-forming α1 subunit. The cytoplasmic regions are responsible for channel modulation by interacting with regulatory proteins. This article overviews modulation of the activity of Ca_V2.1 and Ca_V2.2 channels in the control of synaptic strength and presynaptic plasticity.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α₁ subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein–protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein–protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity. J. Cell. Physiol. 999: 00–00, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc. J. Cell. Physiol. 230: 2019–2031, 2015. © 2015 Wiley Periodicals, Inc.

Phosphoinositides regulate ion channels

Phosphoinositides serve as signature motifs for different cellular membranes and often are required for the function of membrane proteins. Here, we summarize clear evidence supporting the concept that many ion channels are regulated by membrane phosphoinositides. We describe tools used to test their dependence on phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate, and consider mechanisms and biological meanings of phosphoinositide regulation of ion channels. This lipid regulation can underlie changes of channel activity and electrical excitability in response to receptors. Since different intracellular membranes have different lipid compositions, the activity of ion channels still in transit towards their final destination membrane may be suppressed until they reach an optimal lipid environment. This article is part of a Special Issue entitled Phosphoinositides.

Voltage-dependent regulation of CaV2.2 channels by Gq-coupled receptor is facilitated by membrane-localized β subunit

The pathway through which preferentially G_qPCRs inhibit Ca_V2.2 channels depends on which β subunits are present.

G protein–coupled receptors (GPCRs) signal through molecular messengers, such as Gβγ, Ca²⁺, and phosphatidylinositol 4,5-bisphosphate (PIP₂), to modulate N-type voltage-gated Ca²⁺ (Ca_V2.2) channels, playing a crucial role in regulating synaptic transmission. However, the cellular pathways through which G_qPCRs inhibit Ca_V2.2 channel current are not completely understood. Here, we report that the location of Ca_V β subunits is key to determining the voltage dependence of Ca_V2.2 channel modulation by G_qPCRs. Application of the muscarinic agonist oxotremorine-M to tsA-201 cells expressing M₁ receptors, together with Ca_V N-type α1B, α2δ1, and membrane-localized β2a subunits, shifted the current-voltage relationship for Ca_V2.2 activation 5 mV to the right and slowed current activation. Muscarinic suppression of Ca_V2.2 activity was relieved by strong depolarizing prepulses. Moreover, when the C terminus of β-adrenergic receptor kinase (which binds Gβγ) was coexpressed with N-type channels, inhibition of Ca_V2.2 current after M₁ receptor activation was markedly reduced and delayed, whereas the delay between PIP₂ hydrolysis and inhibition of Ca_V2.2 current was decreased. When the Gβγ-insensitive Ca_V2.2 α1C-1B chimera was expressed, voltage-dependent inhibition of calcium current was virtually abolished, suggesting that M₁ receptors act through Gβγ to inhibit Ca_V2.2 channels bearing membrane-localized Ca_V β2a subunits. Expression of cytosolic β subunits such as β2b and β3, as well as the palmitoylation-negative mutant β2a(C3,4S), reduced the voltage dependence of M₁ muscarinic inhibition of Ca_V2.2 channels, whereas it increased inhibition mediated by PIP₂ depletion. Together, our results indicate that, with membrane-localized Ca_V β subunits, Ca_V2.2 channels are subject to Gβγ-mediated voltage-dependent inhibition, whereas cytosol-localized β subunits confer more effective PIP₂-mediated voltage-independent regulation. Thus, the voltage dependence of G_qPCR regulation of calcium channels can be determined by the location of isotype-specific Ca_V β subunits.

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.

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca(2+) concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Regulation of Ca(V)2 calcium channels by G protein coupled receptors

Voltage gated calcium channels (Ca²⁺ channels) are key mediators of depolarization induced calcium influx into excitable cells, and thereby play pivotal roles in a wide array of physiological responses. This review focuses on the inhibition of Ca(V)2 (N- and P/Q-type) Ca²⁺-channels by G protein coupled receptors (GPCRs), which exerts important autocrine/paracrine control over synaptic transmission and neuroendocrine secretion. Voltage-dependent inhibition is the most widespread mechanism, and involves direct binding of the G protein βγ dimer (Gβγ) to the α1 subunit of Ca(V)2 channels. GPCRs can also recruit several other distinct mechanisms including phosphorylation, lipid signaling pathways, and channel trafficking that result in voltage-independent inhibition. Current knowledge of Gβγ-mediated inhibition is reviewed, including the molecular interactions involved, determinants of voltage-dependence, and crosstalk with other cell signaling pathways. A summary of recent developments in understanding the voltage-independent mechanisms prominent in sympathetic and sensory neurons is also included. This article is part of a Special Issue entitled: Calcium channels.

Structure and function of the β subunit of voltage-gated Ca²⁺ channels

The voltage-gated Ca²⁺ channel β subunit (Ca(v)β) is a cytosolic auxiliary subunit that plays an essential role in regulating the surface expression and gating properties of high-voltage activated (HVA) Ca²⁺ channels. It is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, Ras-related RGK GTPases, and other proteins. There are indications that Ca(v)β may carry out Ca²⁺ channel-independent functions. Ca(v)β knockouts are either non-viable or result in a severe pathophysiology, and mutations in Ca(v)β have been implicated in disease. In this article, we review the structure and various biological functions of Ca(v)β, as well as recent advances. This article is part of a Special Issue entitled: Calcium channels.

Ginsenoside Rb1 selectively inhibits the activity of L-type voltage-gated calcium channels in cultured rat hippocampal neurons

AIM

To investigate the effect of ginsenoside Rb1 on voltage-gated calcium currents in cultured rat hippocampal neurons and the modulatory mechanism.

METHODS

Cultured hippocampal neurons were prepared from Sprague Dawley rat embryos. Whole-cell configuration of the patch-clamp technique was used to record the voltage-gated calcium currents (VGCCs) from the hippocampal neurons,and the effect of Rb1 was examined.

RESULTS

Rb1 (2-100 μmol/L) inhibited VGCCs in a concentration-dependent manner, and the current was mostly recovered upon wash-out. The specific L-type Ca(2+) channel inhibitor nifedipine (10 μmol/L) occluded Rb1-induced inhibition on VGCCs. Neither the selective N-type Ca(2+) channel blocker ω-conotoxin-GVIA (1 μmol/L), nor the selective P/Q-type Ca(2+) channel blocker ω-agatoxin IVA (30 nmol/L) diminished Rb1-sensitive VGCCs. Rb1 induced a leftward shift of the steady-state inactivation curve of I(Ca) to a negative potential without affecting its activation kinetics or reversal potential in the I-V curve. The inhibitory effect of Rb1 was neither abolished by the adenylyl cyclase activator forskolin (10 μmol/L), nor by the PKA inhibitor H-89 (10 μmol/L).

CONCLUSION

Ginsenoside Rb1 selectively inhibits the activity of L-type voltage-gated calcium channels, without affecting the N-type or P/Q-type Ca(2+) channels in hippocampal neurons. cAMP-PKA signaling pathway is not involved in this effect.

Trafficking and stability of voltage-gated calcium channels

Voltage-gated calcium channels are important mediators of calcium influx into electrically excitable cells. The amount of calcium entering through this family of channel proteins is not only determined by the functional properties of channels embedded in the plasma membrane but also by the numbers of channels that are expressed at the cell surface. The trafficking of channels is controlled by numerous processes, including co-assembly with ancillary calcium channel subunits, ubiquitin ligases, and interactions with other membrane proteins such as G protein coupled receptors. Here we provide an overview about the current state of knowledge of calcium channel trafficking to the cell membrane, and of the mechanisms regulating the stability and internalization of this important ion channel family.

Rim1 modulates direct G-protein regulation of Ca(v)2.2 channels

Regulation of presynaptic voltage-gated calcium channels is critical for depolarization-evoked neurotransmitter release. Various studies attempted to determine the functional implication of Rim1, a component of the vesicle release machinery. Besides to couple voltage-gated Ca(2+) channels to the presynaptic vesicle release machinery, it was evidenced that Rim1 also prevents voltage-dependent inactivation of the channels through a direct interaction with the ancillary β-subunits, thus facilitating neurotransmitter release. However, facilitation of synaptic activity may also be caused by a reduction of the inhibitory pathway carried by G-protein-coupled receptors. Here, we explored the functional implication of Rim1 in G-protein regulation of Ca(v)2.2 channels. Activation of μ-opioid receptors expressed in HEK-293 cells along with Ca(v)2.2 channels produced a drastic current inhibition both in control and Rim1-expressing cells. In contrast, Rim1 considerably promoted the extent of current deinhibition following channel activation, favoring sustained Ca(2+) influx under prolonged activity. Our data suggest that Rim1-induced facilitation of neurotransmitter release may come as a consequence of a decrease in the inhibitory pathway carried by G-proteins that contributes, together with the slowing of channel inactivation, to maintain Ca(2+) influx under prolonged activity. The present study also furthers functional insights in the importance of proteins from the presynaptic vesicle complex in the regulation of voltage-gated Ca(2+) channels by G-proteins.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

G protein modulation of CaV2 voltage-gated calcium channels

Voltage-gated Ca(2+) channels translate the electrical inputs of excitable cells into biochemical outputs by controlling influx of the ubiquitous second messenger Ca(2+) . As such the channels play pivotal roles in many cellular functions including the triggering of neurotransmitter and hormone release by CaV2.1 (P/Q-type) and CaV2.2 (N-type) channels. It is well established that G protein coupled receptors (GPCRs) orchestrate precise regulation neurotransmitter and hormone release through inhibition of CaV2 channels. Although the GPCRs recruit a number of different pathways, perhaps the most prominent, and certainly most studied among these is the so-called voltage-dependent inhibition mediated by direct binding of Gβγ to the α1 subunit of CaV2 channels. This article will review the basics of Ca(2+) -channels and G protein signaling, and the functional impact of this now classical inhibitory mechanism on channel function. It will also provide an update on more recent developments in the field, both related to functional effects and crosstalk with other signaling pathways, and advances made toward understanding the molecular interactions that underlie binding of Gβγ to the channel and the voltage-dependence that is a signature characteristic of this mechanism.

In vivo expression of G-protein beta1gamma2 dimer in adult mouse skeletal muscle alters L-type calcium current and excitation-contraction coupling

A number of G-protein-coupled receptors are expressed in skeletal muscle but their roles in muscle physiology and downstream effector systems remain poorly investigated. Here we explored the functional importance of the G-protein betagamma (Gbetagamma) signalling pathway on voltage-controlled Ca(2+) homeostasis in single isolated adult skeletal muscle fibres. A GFP-tagged Gbeta(1)gamma(2) dimer was expressed in vivo in mice muscle fibres. The GFP fluorescence pattern was consistent with a Gbeta(1)gamma(2) dimer localization in the transverse-tubule membrane. Membrane current and indo-1 fluorescence measurements performed under voltage-clamp conditions reveal a drastic reduction of both L-type Ca(2+) current density and of peak amplitude of the voltage-activated Ca(2+) transient in Gbeta(1)gamma(2)-expressing fibres. These effects were not observed upon expression of Gbeta(2)gamma(2), Gbeta(3)gamma(2) or Gbeta(4)gamma(2). Our data suggest that the G-protein beta(1)gamma(2) dimer may play an important regulatory role in skeletal muscle excitation-contraction coupling.

Scanning mutagenesis of the I-II loop of the Cav2.2 calcium channel identifies residues Arginine 376 and Valine 416 as molecular determinants of voltage dependent G protein inhibition

Direct interaction with the beta subunit of the heterotrimeric G protein complex causes voltage-dependent inhibition of N-type calcium channels. To further characterize the molecular determinants of this interaction, we performed scanning mutagenesis of residues 372-387 and 410-428 of the N-type channel alpha1 subunit, in which individual residues were replaced by either alanine or cysteine. We coexpressed wild type Gbeta1gamma2 subunits with either wild type or point mutant N-type calcium channels, and voltage-dependent, G protein-mediated inhibition of the channels (VDI) was assessed using patch clamp recordings. The resulting data indicate that Arg376 and Val416 of the alpha1 subunit, residues which are surface-exposed in the presence of the calcium channel beta subunit, contribute significantly to the functional inhibition by Gbeta1. To further characterize the roles of Arg376 and Val416 in this interaction, we performed secondary mutagenesis of these residues, coexpressing the resulting mutants with wild type Gbeta1gamma2 subunits and with several isoforms of the auxiliary beta subunit of the N-type channel, again assessing VDI using patch clamp recordings. The results confirm the importance of Arg376 for G protein-mediated inhibition and show that a single amino acid substitution to phenylalanine drastically alters the abilities of auxiliary calcium channel subunits to regulate G protein inhibition of the channel.

Orientation of palmitoylated CaVbeta2a relative to CaV2.2 is critical for slow pathway modulation of N-type Ca2+ current by tachykinin receptor activation

The G(q)-coupled tachykinin receptor (neurokinin-1 receptor [NK-1R]) modulates N-type Ca(2+) channel (Ca(V)2.2 or N channel) activity at two distinct sites by a pathway involving a lipid metabolite, most likely arachidonic acid (AA). In another study published in this issue (Heneghan et al. 2009. J. Gen Physiol. doi:10.1085/jgp.200910203), we found that the form of modulation observed depends on which Ca(V)beta is coexpressed with Ca(V)2.2. When palmitoylated Ca(V)beta2a is coexpressed, activation of NK-1Rs by substance P (SP) enhances N current. In contrast, when Ca(V)beta3 is coexpressed, SP inhibits N current. However, exogenously applied palmitic acid minimizes this inhibition. These findings suggested that the palmitoyl groups of Ca(V)beta2a may occupy an inhibitory site on Ca(V)2.2 or prevent AA from interacting with that site, thereby minimizing inhibition. If so, changing the orientation of Ca(V)beta2a relative to Ca(V)2.2 may displace the palmitoyl groups and prevent them from antagonizing AA's actions, thereby allowing inhibition even in the presence of Ca(V)beta2a. In this study, we tested this hypothesis by deleting one (Bdel1) or two (Bdel2) amino acids proximal to the alpha interacting domain (AID) of Ca(V)2.2's I-II linker. Ca(V)betas bind tightly to the AID, whereas the rigid region proximal to the AID is thought to couple Ca(V)beta's movements to Ca(V)2.2 gating. Although Bdel1/beta2a currents exhibited more variable enhancement by SP, Bdel2/beta2a current enhancement was lost at all voltages. Instead, inhibition was observed that matched the profile of N-current inhibition from Ca(V)2.2 coexpressed with Ca(V)beta3. Moreover, adding back exogenous palmitic acid minimized inhibition of Bdel2/beta2a currents, suggesting that when palmitoylated Ca(V)beta2a is sufficiently displaced, endogenously released AA can bind to the inhibitory site. These findings support our previous hypothesis that Ca(V)beta2a's palmitoyl groups directly interact with an inhibitory site on Ca(V)2.2 to block N-current inhibition by SP.

Origin of the voltage dependence of G-protein regulation of P/Q-type Ca2+ channels

G-protein (Gbetagamma)-mediated voltage-dependent inhibition of N- and P/Q-type Ca(2+) channels contributes to presynaptic inhibition and short-term synaptic plasticity. The voltage dependence derives from the dissociation of Gbetagamma from the inhibited channels, but the underlying molecular and biophysical mechanisms remain largely unclear. In this study we investigated the role in this process of Ca(2+) channel beta subunit (Ca(v)beta) and a rigid alpha-helical structure between the alpha-interacting domain (AID), the primary Ca(v)beta docking site on the channel alpha(1) subunit, and the pore-lining IS6 segment. Gbetagamma inhibition of P/Q-type channels was reconstituted in giant inside-out membrane patches from Xenopus oocytes. Large populations of channels devoid of Ca(v)beta were produced by washing out a mutant Ca(v)beta with a reduced affinity for the AID. These beta-less channels were still inhibited by Gbetagamma, but without any voltage dependence, indicating that Ca(v)beta is indispensable for voltage-dependent Gbetagamma inhibition. A truncated Ca(v)beta containing only the AID-binding guanylate kinase (GK) domain could fully confer voltage dependence to Gbetagamma inhibition. Gbetagamma did not alter inactivation properties, and channels recovered from Gbetagamma inhibition exhibited the same activation property as un-inhibited channels, indicating that Gbetagamma does not dislodge Ca(v)beta from the inhibited channel. Furthermore, voltage-dependent Gbetagamma inhibition was abolished when the rigid alpha-helix between the AID and IS6 was disrupted by insertion of multiple glycines, which also eliminated Ca(v)beta regulation of channel gating, revealing a pivotal role of this rigid alpha-helix in both processes. These results suggest that depolarization-triggered movement of IS6, coupled to the subsequent conformational change of the Gbetagamma-binding pocket through a rigid alpha-helix induced partly by the Ca(v)beta GK domain, causes the dissociation of Gbetagamma and is fundamental to voltage-dependent Gbetagamma inhibition.

Orientation of the calcium channel beta relative to the alpha(1)2.2 subunit is critical for its regulation of channel activity

BACKGROUND

The Ca(v)beta subunits of high voltage-activated Ca(2+) channels control the trafficking and biophysical properties of the alpha(1) subunit. The Ca(v)beta-alpha(1) interaction site has been mapped by crystallographic studies. Nevertheless, how this interaction leads to channel regulation has not been determined. One hypothesis is that betas regulate channel gating by modulating movements of IS6. A key requirement for this direct-coupling model is that the linker connecting IS6 to the alpha-interaction domain (AID) be a rigid structure.

METHODOLOGY/PRINCIPAL FINDINGS

The present study tests this hypothesis by altering the flexibility and orientation of this region in alpha(1)2.2, then testing for Ca(v)beta regulation using whole cell patch clamp electrophysiology. Flexibility was induced by replacement of the middle six amino acids of the IS6-AID linker with glycine (PG6). This mutation abolished beta2a and beta3 subunits ability to shift the voltage dependence of activation and inactivation, and the ability of beta2a to produce non-inactivating currents. Orientation of Ca(v)beta with respect to alpha(1)2.2 was altered by deletion of 1, 2, or 3 amino acids from the IS6-AID linker (Bdel1, Bdel2, Bdel3, respectively). Again, the ability of Ca(v)beta subunits to regulate these biophysical properties were totally abolished in the Bdel1 and Bdel3 mutants. Functional regulation by Ca(v)beta subunits was rescued in the Bdel2 mutant, indicating that this part of the linker forms beta-sheet. The orientation of beta with respect to alpha was confirmed by the bimolecular fluorescence complementation assay.

CONCLUSIONS/SIGNIFICANCE

These results show that the orientation of the Ca(v)beta subunit relative to the alpha(1)2.2 subunit is critical, and suggests additional points of contact between these subunits are required for Ca(v)beta to regulate channel activity.

Presynaptic calcium channels: structure, regulators, and blockers

The central and peripheral nervous systems express multiple types of ligand and voltage-gated calcium channels (VGCCs), each with specific physiological roles and pharmacological and electrophysiological properties. The members of the Ca(v)2 calcium channel family are located predominantly at presynaptic nerve terminals, where they are responsible for controlling evoked neurotransmitter release. The activity of these channels is subject to modulation by a number of different means, including alternate splicing, ancillary subunit associations, peptide and small organic blockers, G-protein-coupled receptors (GPCRs), protein kinases, synaptic proteins, and calcium-binding proteins. These multiple and complex modes of calcium channel regulation allow neurons to maintain the specific, physiological window of cytoplasmic calcium concentrations which is required for optimal neurotransmission and proper synaptic function. Moreover, these varying means of channel regulation provide insight into potential therapeutic targets for the treatment of pathological conditions that arise from disturbances in calcium channel signaling. Indeed, considerable efforts are presently underway to identify and develop specific presynaptic calcium channel blockers that can be used as analgesics.

Direct G protein modulation of Cav2 calcium channels

The regulation of presynaptic, voltage-gated calcium channels by activation of heptahelical G protein-coupled receptors exerts a crucial influence on presynaptic calcium entry and hence on neurotransmitter release. Receptor activation subjects presynaptic N- and P/Q-type calcium channels to a rapid, membrane-delimited inhibition-mediated by direct, voltage-dependent interactions between G protein betagamma subunits and the channels-and to a slower, voltage-independent modulation involving soluble second messenger molecules. In turn, the direct inhibition of the channels is regulated as a function of many factors, including channel subtype, ancillary calcium channel subunits, and the types of G proteins and G protein regulatory factors involved. Twenty-five years after this mode of physiological regulation was first described, we review the investigations that have led to our current understanding of its molecular mechanisms.

Importance of voltage-dependent inactivation in N-type calcium channel regulation by G-proteins

Direct regulation of N-type calcium channels by G-proteins is essential to control neuronal excitability and neurotransmitter release. Binding of the G(betagamma) dimer directly onto the channel is characterized by a marked current inhibition ("ON" effect), whereas the pore opening- and time-dependent dissociation of this complex from the channel produce a characteristic set of biophysical modifications ("OFF" effects). Although G-protein dissociation is linked to channel opening, the contribution of channel inactivation to G-protein regulation has been poorly studied. Here, the role of channel inactivation was assessed by examining time-dependent G-protein de-inhibition of Ca(v)2.2 channels in the presence of various inactivation-altering beta subunit constructs. G-protein activation was produced via mu-opioid receptor activation using the DAMGO agonist. Whereas the "ON" effect of G-protein regulation is independent of the type of beta subunit, the "OFF" effects were critically affected by channel inactivation. Channel inactivation acts as a synergistic factor to channel activation for the speed of G-protein dissociation. However, fast inactivating channels also reduce the temporal window of opportunity for G-protein dissociation, resulting in a reduced extent of current recovery, whereas slow inactivating channels undergo a far more complete recovery from inhibition. Taken together, these results provide novel insights on the role of channel inactivation in N-type channel regulation by G-proteins and contribute to the understanding of the physiological consequence of channel inactivation in the modulation of synaptic activity by G-protein coupled receptors.

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Scanning Mutagenesis Reveals a Role for Serine 189 of the Heterotrimeric G-Protein Beta 1 Subunit in the Inhibition of N-Type Calcium Channels

Direct interactions between the presynaptic N-type calcium channel and the β subunit of the heterotrimeric G-protein complex cause voltage-dependent inhibition of N-type channel activity, crucially influencing neurotransmitter release and contributing to analgesia caused by opioid drugs. Previous work using chimeras of the G-protein β subtypes Gβ1 and Gβ5 identified two 20–amino acid stretches of structurally contiguous residues on the Gβ1 subunit as critical for inhibition of the N-type channel. To identify key modulation determinants within these two structural regions, we performed scanning mutagenesis in which individual residues of the Gβ1 subunit were replaced by corresponding Gβ5 residues. Our results show that Gβ1 residue Ser189 is critical for N-type calcium channel modulation, whereas none of the other Gβ1 mutations caused statistically significant effects on the ability of Gβ1 to inhibit N-type channels. Structural modeling shows residue 189 is surface exposed, consistent with the idea that it may form a direct contact with the N-type calcium channel α1 subunit during binding interactions.

Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies encompass a class of epileptic seizure types that exhibit a polygenic and heritable etiology. Advances in molecular biology and genetics have implicated defects in certain types of voltage-gated calcium channels and their ancillary subunits as important players in this form of epilepsy. Both T-type and P/Q-type channels appear to mediate important contributions to seizure genesis, modulation of network activity, and genetic seizure susceptibility. Here, we provide a comprehensive overview of the roles of these channels and associated subunits in normal and pathological brain activity within the context of idiopathic generalized epilepsy.

A novel molecular inactivation determinant of voltage-gated CaV1.2 L-type Ca2+ channel

The inactivation of voltage-gated L-type Ca(2+) channels (Ca(V)1) regulates Ca(2+) entry and controls intracellular Ca(2+) levels that are essential for cellular activity. The molecular entities implicated in L-channel (Ca(V)1.2) inactivation are not fully identified. Here we show for the first time the functional impact of one of the two highly conserved clusters of six negatively charged glutamates and aspartate (802-807; poly ED motif) at the II-III loop of the alpha 1 subunits of rabbit of Ca(v)1.2, alpha(1)1.2 and alpha(1)1.2 DeltaN60-Delta1733) on voltage-dependent inactivation. Mutation of the poly ED motif to alanine or glutamine/asparagine greatly enhanced voltage-dependent inactivation, shifting the voltage dependence to negative potentials by >50 mV and conferring a neuronal like inactivation kinetics onto Ca(V)1.2. The large shift in the midpoint of inactivation of the steady-state inactivation kinetics was observed also in Ca(2+) or Ba(2+) and was not altered by the beta2A subunit. Missing from the fast inactivating neuronal P/Q (Ca(V)2.1)-, N (Ca(V)2.2)- or R (Ca(V)2.3)-type channels and modulating Ca(V)1.2 inactivation kinetics, the poly ED motif is likely to be a specific L-type Ca(2+) channels inactivating domain. Our results fit a model in which the poly ED either by itself or as part of a larger inactivating motif acts as Ca(V)1.2 specific built-in "stopper." In this model, Ca(V)1 accomplishes a large Ca(2+) influx during depolarization, possibly by the poly ED hindering occlusion at the pore. Furthermore, the selective designed poly ED perhaps clarifies major inactivation differences between L- and non-L-type calcium channels.

How do G proteins directly control neuronal Ca2+ channel function?

Ca2+ entry into neuronal cells is modulated by the activation of numerous G-protein-coupled receptors (GPCRs). Much effort has been invested in studying direct G-protein-mediated inhibition of voltage-dependent CaV2 Ca2+ channels. This inhibition occurs through a series of convergent modifications in the biophysical properties of the channels. An integrated view of the structural organization of the Gbetagamma-dimer binding-site pocket within the channel is emerging. In this review, we discuss how variable geometry of the Gbetagamma binding pocket can yield distinct sets of channel inhibition. In addition, we propose specific mechanisms for the regulation of the channel by G proteins that take into account the regulatory input of each Gbetagamma binding element.

Deletion of N-type calcium channels alters ethanol reward and reduces ethanol consumption in mice

N-type calcium channels are modulated by acute and chronic ethanol exposure in vitro at concentrations known to affect humans, but it is not known whether N-type channels are important for behavioral responses to ethanol in vivo. Here, we show that in mice lacking functional N-type calcium channels, voluntary ethanol consumption is reduced and place preference is developed only at a low dose of ethanol. The hypnotic effects of ethanol are also substantially diminished, whereas ethanol-induced ataxia is mildly increased. These results demonstrate that N-type calcium channels modulate acute responses to ethanol and are important mediators of ethanol reward and preference.

Topiramate Inhibits the Initiation of Plateau Potentials in CA1 Neurons by Depressing R-type Calcium Channels

PURPOSE

Cholinergic-dependent plateau potentials (PPs) are intrinsically generated conductances that can elicit ictal-type seizure activity. The aim of this study was to investigate the actions of topiramate (TPM) on the generation of PPs.

METHODS

We used whole-cell patch-clamp recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the effects of TPM on the PPs.

RESULTS

In current-clamp mode, action potentials evoked PPs after cholinergic receptor stimulation. Therapeutically relevant concentrations of TPM (50 microM) depressed the PPs evoked by action potentials. Surprisingly, in voltage-clamp mode, we discovered that the cyclic nucleotide-gated (CNG) current that underlies PP generation (denoted as I(tail)) was not depressed. However, significantly longer depolarizing voltage steps were required to elicit I(tail). This suggested that the calcium entry trigger for evoking PPs was depressed by TPM and not I(tail) itself. TPM had no effect on calcium spikes in control conditions; however, TPM did reduce calcium spikes after cholinergic-receptor stimulation. We recently found that R-type calcium spikes are enhanced by cholinergic-receptor stimulation. Therefore we isolated R-type calcium spikes with a cocktail containing tetrodotoxin, omega-conotoxin MVIIC, omega-conotoxin-GVIA, omega-agatoxin IVA, and nifedipine. R-type calcium spikes were significantly depressed by TPM. We also examined the effects of TPM on recombinant Ca(V)2.3 calcium channels expressed in tsA-201 cells. TPM depressed currents mediated by Ca(V)2.3 subunits by a hyperpolarizing shift in steady-state inactivation.

CONCLUSIONS

We have found that TPM reduces ictal-like activity in CA1 hippocampal neurons through a novel inhibitory action of R-type calcium channels.

Syntaxin 1A regulation of weakly inactivating N-type Ca2+ channels

N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they interact with proteins of the release apparatus, including syntaxin 1A and SNAP-25. In previous studies on N- or P/Q-type Ca2+ channels, syntaxin 1A co-expression reduced current amplitudes, increased voltage-dependent inactivation and/or enhanced G-protein inhibition. However, these studies were conducted in Ca2+ channels that exhibited significant voltage-dependent inactivation. We previously reported that N-type current in bovine chromaffin cells exhibits very little voltage-dependent inactivation and we identified the Ca2+ channel subunits involved. This study was undertaken to determine the effect of syntaxin 1A on this weakly inactivating Ca2+ channel. Co-expression of syntaxin 1A with the weakly inactivating bovine N-type Ca2+ channels in Xenopus oocytes did not appear to alter inactivation but dramatically reduced current amplitudes, without changing cell surface expression. To further understand the mechanisms of syntaxin 1A regulation of this weakly inactivating channel, we examined mutants of the alpha1B subunit, beta2a subunit and syntaxin 1A. We determined that the synprint site of alpha1B and the C-terminal third of syntaxin 1A were necessary for the reduced current amplitude. In addition we show that enhanced G-protein-dependent modulation of the Ca2+ current by syntaxin 1A cannot explain the large suppression of Ca2+ current observed. Of most significance, syntaxin 1A increased voltage-dependent inactivation in channels containing mutant beta2a subunits that cannot be palmitoylated. Our data suggest that changes in inactivation can not explain the reduction in current amplitude produced by co-expressing syntaxin and a weakly inactivating Ca2+ channel.

Ca2+ Channels As Integrators of G Protein-Mediated Signaling in Neurons

The observations from Dunlap and Fischbach that transmitter-mediated shortening of the duration of action potentials could be caused by a decrease in calcium conductance led to numerous studies of the mechanisms of modulation of voltage-dependent calcium channels. Calcium channels are well known targets for inhibition by receptor-G protein pathways, and multiple forms of inhibition have been described. Inhibition of Ca(2+) channels can be mediated by G protein betagamma-subunits or by kinases, such as protein kinase C and tyrosine kinases. In the last few years, it has been shown that integration of G protein signaling can take place at the level of the calcium channel by regulation of the interaction of the channel pore-forming subunit with different cellular proteins.

Auxiliary subunit regulation of high-voltage activated calcium channels expressed in mammalian cells

The effects of auxiliary calcium channel subunits on the expression and functional properties of high-voltage activated (HVA) calcium channels have been studied extensively in the Xenopus oocyte expression system, but are less completely characterized in a mammalian cellular environment. Here, we provide the first systematic analysis of the effects of calcium channel beta and alpha(2)-delta subunits on expression levels and biophysical properties of three different types (Ca(v)1.2, Ca(v)2.1 and Ca(v)2.3) of HVA calcium channels expressed in tsA-201 cells. Our data show that Ca(v)1.2 and Ca(v)2.3 channels yield significant barium current in the absence of any auxiliary subunits. Although calcium channel beta subunits were in principle capable of increasing whole cell conductance, this effect was dependent on the type of calcium channel alpha(1) subunit, and beta(3) subunits altogether failed to enhance current amplitude irrespective of channel subtype. Moreover, the alpha(2)-delta subunit alone is capable of increasing current amplitude of each channel type examined, and at least for members of the Ca(v)2 channel family, appears to act synergistically with beta subunits. In general agreement with previous studies, channel activation and inactivation gating was regulated both by beta and by alpha(2)-delta subunits. However, whereas pronounced regulation of inactivation characteristics was seen with the majority of the auxiliary subunits, effects on voltage dependence of activation were only small (< 5 mV). Overall, through a systematic approach, we have elucidated a previously underestimated role of the alpha(2)-delta(1) subunit with regard to current enhancement and kinetics. Moreover, the effects of each auxiliary subunit on whole cell conductance and channel gating appear to be specifically tailored to subsets of calcium channel subtypes.

A Single Gβ Subunit Locus Controls Cross-talk between Protein Kinase C and G Protein Regulation of N-type Calcium Channels

The modulation of N-type calcium channels is a key factor in the control of neurotransmitter release. Whereas N-type channels are inhibited by Gbetagamma subunits in a G protein beta-isoform-dependent manner, channel activity is typically stimulated by activation of protein kinase C (PKC). In addition, there is cross-talk among these pathways, such that PKC-dependent phosphorylation of the Gbetagamma target site on the N-type channel antagonizes subsequent G protein inhibition, albeit only for Gbeta(1)-mediated responses. The molecular mechanisms that control this G protein beta subunit subtype-specific regulation have not been described. Here, we show that G protein inhibition of N-type calcium channels is critically dependent on two separate but adjacent approximately 20-amino acid regions of the Gbeta subunit, plus a highly conserved Asn-Tyr-Val motif. These regions are distinct from those implicated previously in Gbetagamma signaling to other effectors such as G protein-coupled inward rectifier potassium channels, phospholipase beta(2), and adenylyl cyclase, thus raising the possibility that the specificity for G protein signaling to calcium channels might rely on unique G protein structural determinants. In addition, we identify a highly specific locus on the Gbeta(1) subunit that serves as a molecular detector of PKC-dependent phosphorylation of the G protein target site on the N-type channel alpha(1) subunit, thus providing for a molecular basis for G protein-PKC cross-talk. Overall, our results significantly advance our understanding of the molecular details underlying the integration of G protein and PKC signaling pathways at the level of the N-type calcium channel alpha(1) subunit.

Overexpressed Ca(v)beta3 inhibits N-type (Cav2.2) calcium channel currents through a hyperpolarizing shift of ultra-slow and closed-state inactivation

It has been shown that β auxiliary subunits increase current amplitude in voltage-dependent calcium channels. In this study, however, we found a novel inhibitory effect of β3 subunit on macroscopic Ba²⁺ currents through recombinant N- and R-type calcium channels expressed in Xenopus oocytes. Overexpressed β3 (12.5 ng/cell cRNA) significantly suppressed N- and R-type, but not L-type, calcium channel currents at “physiological” holding potentials (HPs) of −60 and −80 mV. At a HP of −80 mV, coinjection of various concentrations (0–12.5 ng) of the β3 with Ca_v2.2α₁ and α₂δ enhanced the maximum conductance of expressed channels at lower β3 concentrations but at higher concentrations (>2.5 ng/cell) caused a marked inhibition. The β3-induced current suppression was reversed at a HP of −120 mV, suggesting that the inhibition was voltage dependent. A high concentration of Ba²⁺ (40 mM) as a charge carrier also largely diminished the effect of β3 at −80 mV. Therefore, experimental conditions (HP, divalent cation concentration, and β3 subunit concentration) approaching normal physiological conditions were critical to elucidate the full extent of this novel β3 effect. Steady-state inactivation curves revealed that N-type channels exhibited “closed-state” inactivation without β3, and that β3 caused an ∼40-mV negative shift of the inactivation, producing a second component with an inactivation midpoint of approximately −85 mV. The inactivation of N-type channels in the presence of a high concentration (12.5 ng/cell) of β3 developed slowly and the time-dependent inactivation curve was best fit by the sum of two exponential functions with time constants of 14 s and 8.8 min at −80 mV. Similar “ultra-slow” inactivation was observed for N-type channels without β3. Thus, β3 can have a profound negative regulatory effect on N-type (and also R-type) calcium channels by causing a hyperpolarizing shift of the inactivation without affecting “ultra-slow” and “closed-state” inactivation properties.

Functional roles of cytoplasmic loops and pore lining transmembrane helices in the voltage-dependent inactivation of HVA calcium channels

Voltage-dependent inactivation of calcium channels is a key mechanism for regulating intracellular calcium levels and neuronal excitability. In sodium and potassium channels, the molecular determinants that govern fast inactivation involve pore block by a cytoplasmic gating particle. As we discuss here, there is an increasing body of evidence that is consistent with a qualitatively similar inactivation mechanism in high-voltage-activated calcium channels. Work from a number of laboratories has implicated both cytoplasmic regions and the pore-lining S6 transmembrane helices in the inactivation process. Together with our recent findings, this leads us to propose a model in which the intracellular domain I-II linker region acts as a 'hinged lid' that physically occludes the pore by docking to the cytoplasmic ends of the S6 segments. We further propose that the ancillary calcium channel Beta subunits differentially modulate inactivation kinetics by binding to and thereby regulating the mobility of the putative inactivation gate. Indeed, additional evidence suggests that the carboxy terminus, amino terminus and domain III-IV linker regions of the channel modulate inactivation rates through interactions with the I-II linker per se, or indirectly via the ancillary Beta subunits. Taken together, the fast voltage-dependent inactivation of calcium channels appears reminiscent of that of sodium channels, but appears to show a more complex regulation through intramolecular interactions between the putative inactivation gate and other cytoplasmic regions.

A minimal model for G protein-mediated synaptic facilitation and depression

G protein-coupled receptors are ubiquitous in neurons, as well as other cell types. Activation of receptors by hormones or neurotransmitters splits the G protein heterotrimer into Galpha and Gbetagamma subunits. It is now clear that Gbetagamma directly inhibits Ca2+ channels, putting them into a reluctant state. The effects of Gbetagamma depend on the specific beta and gamma subunits present, as well as the beta subunit isoform of the N-type Ca2+ channel. We describe a minimal mathematical model for the effects of G protein action on the dynamics of synaptic transmission. The model is calibrated by data obtained by transfecting G protein and Ca2+ channel subunits into tsA-201 cells. We demonstrate with numerical simulations that G protein action can provide a mechanism for either short-term synaptic facilitation or depression, depending on the manner in which G protein-coupled receptors are activated. The G protein action performs high-pass filtering of the presynaptic signal, with a filter cutoff that depends on the combination of G protein and Ca2+ channel subunits present. At stimulus frequencies above the cutoff, trains of single spikes are transmitted, while only doublets are transmitted at frequencies below the cutoff. Finally, we demonstrate that relief of G protein inhibition can contribute to paired-pulse facilitation.

N-type Ca2+ channel

Ca(2+) entry through voltage-dependent Ca(2+) channels (VDCCs) regulates various aspects of physiological function, including neurotransmitter release, regulation of cell membrane excitability, and control of gene expression. VDCCs are classified into several sub-types (L-, N-, P/Q-, R-, and T-types) based on electrophysiological and pharmacological properties. Each type of channels except the T-type is composed of at least four subunits, designated alpha(1), alpha(2), beta, and delta. During the past decade, a number of genes encoding these subunits have been cloned, and cDNA expression studies using heterologous expression systems have revealed the intricate nature of subunit interaction and many biophysical aspects of channel function. In recent years, an entirely new strategy has been introduced in attempts to clarify the physiological role of each of the VDCCs, and this has proven to be very useful in defining previously unknown in vivo functions of VDCCs. In this article, we briefly review the recent advances in our understanding of VDCCs with special emphasis on the N-type channel, which is mainly expressed in neural tissues and is the essential component of neurotransmitter release. We will mainly discuss the subunit composition, channel regulation by G proteins and exocytotic proteins, and the mouse phenotypes in which N-type channel subunits have been deleted by gene targeting technology.

Glycerotoxin from Glycera convoluta stimulates neurosecretion by up-regulating N-type Ca2+ channel activity

We report here the purification of glycerotoxin from the venom of Glycera convoluta, a novel 320 kDa protein capable of reversibly stimulating spontaneous and evoked neurotransmitter release at the frog neuromuscular junction. However, glycerotoxin is ineffective at the murine neuromuscular junction, which displays a different subtype of voltage- dependent Ca(2+) channels. By sequential and selective inhibition of various types of Ca(2+) channels, we found that glycerotoxin was acting via Ca(v)2.2 (N-type). In neuroendocrine cells, it elicits a robust, albeit transient, influx of Ca(2+) sensitive to the Ca(v)2.2 blockers omega-conotoxin GVIA and MVIIA. Moreover, glycerotoxin triggers a Ca(2+) transient in human embryonic kidney (HEK) cells over-expressing Ca(v)2.2 but not Ca(v)2.1 (P/Q-type). Whole-cell patch-clamp analysis of Ca(v)2.2 expressing HEK cells revealed an up-regulation of Ca(2+) currents due to a leftward shift of the activation peak upon glycerotoxin addition. A direct interaction between Ca(v)2.2 and this neurotoxin was revealed by co-immunoprecipitation experiments. Therefore, glycerotoxin is a unique addition to the arsenal of tools available to unravel the mechanism controlling Ca(2+)-regulated exocytosis via the specific activation of Ca(v)2.2.

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.

The presence of Ca2+ channel beta subunit is required for mitogen-activated protein kinase (MAPK)-dependent modulation of alpha1B Ca2+ channels in COS-7 cells

In rat sensory neurones, voltage-dependent calcium channels (VDCCs), including the N-type, are tonically up-regulated via Ras/mitogen-activated protein kinase (MAPK) signalling. To determine whether VDCC beta subunit is involved in this process, the role of the four neuronal betas (beta1b, beta2a, beta3, beta4) in MAPK-dependent modulation of alpha1B (Ca(v)2.2, N-type) Ca(2+) channels has been examined in COS-7 cells. MAPK is exclusively activated by MAPK kinase (MEK), and here, acute application of a MEK-specific inhibitor UO126, significantly inhibited peak alpha1B Ca2+ channel current (I(max)) within a period of 5-10 min, regardless of which beta subunit was co-expressed (25-50 %, P < 0.01). With beta2a however, the percentage inhibition of I(max) was less than that observed with any other beta (ANOVA: F(3,34) = 6.48, P < 0.01). UO126 also caused a hyperpolarising shift (6 +/- 1 mV, P < 0.001) in the voltage dependence of beta2a current activation, such that inhibition occurred only at depolarised potentials (> +5 mV) whereas at more negative potentials the current amplitude was enhanced. A marked change in beta2a current kinetics, perceived either as decreased activation or increased inactivation, was also associated with UO126 application. A similar effect of UO126 on beta4 current kinetics was also observed. The beta2a-specific effects of UO126 on current inhibition and voltage dependence of activation were abolished when alpha1B was co-expressed with de-palmitoylated beta2a(C3,4S), in which amino terminal cysteines 3 and 4 had been mutated to serines. In the absence of beta subunit, UO126 had no effect on alpha1B Ca2+ channel current. Together, these data suggest an absolute requirement for beta in MAPK-dependent modulation of these channels. Since beta subunits vary both in their temporal expression and localisation within neurones, beta subunit-dependent modulation of N-type Ca2+ channels via MAPK could provide an important new mechanism by which to fine-tune neurotransmitter release.

Role for G protein Gbetagamma isoform specificity in synaptic signal processing: a computational study

Computational modeling is used to investigate the functional impact of G protein-mediated presynaptic autoinhibition on synaptic filtering properties. It is demonstrated that this form of autoinhibition, which is relieved by depolarization, acts as a high-pass filter. This contrasts with vesicle depletion, which acts as a low-pass filter. Model parameters are adjusted to reproduce kinetic slowing data from different Gbetagamma dimeric isoforms, which produce different degrees of slowing. With these sets of parameter values, we demonstrate that the range of frequencies filtered out by the autoinhibition varies greatly depending on the Gbetagamma isoform activated by the autoreceptors. It is shown that G protein autoinhibition can enhance the spatial contrast between a spatially distributed high-frequency signal and surrounding low-frequency noise, providing an alternate mechanism to lateral inhibition. It is also shown that autoinhibition can increase the fidelity of coincidence detection by increasing the signal-to-noise ratio in the postsynaptic cell. The filter cut, the input frequency below which signals are filtered, depends on several biophysical parameters in addition to those related to Gbetagamma binding and unbinding. By varying one such parameter, the rate at which transmitter unbinds from autoreceptors, we show that the filter cut can be adjusted up or down for several of the Gbetagamma isoforms. This allows for great synapse-to-synapse variability in the distinction between signal and noise.

Prostaglandin E(2) inhibits calcium current in two sub-populations of acutely isolated mouse trigeminal sensory neurons

Prostaglandins are important mediators of pain and inflammation. We have examined the effects of prostanoids on voltage-activated calcium currents (I(Ca)) in acutely isolated mouse trigeminal sensory neurons, using standard whole cell voltage clamp techniques. Trigeminal neurons were divided into two populations based on the presence (Type 2) or absence (Type 1) of low voltage-activated T-type I(Ca). The absence of T-type I(Ca) is highly correlated with sensitivity to mu-opioid agonists and the VR1 agonist capsaicin. In both populations of cells, high voltage-activated I(Ca) was inhibited by PGE(2) with an EC(50) of about 35 nM, to a maximum of 30 %. T-type I(Ca) was not inhibited by PGE(2). Pertussis toxin pre-treatment abolished the effects of PGE(2) in Type 2 cells, but not in Type 1 cells, whereas treatment with cholera toxin prevented the effects of PGE(2) in Type 1 cells, but not in Type 2 cells. Inhibition of I(Ca) by PGE(2) was associated with slowing of current activation and could be relieved with a large positive pre-pulse, consistent with inhibition of I(Ca) by G protein betagamma subunits. Reverse transcription-polymerase chain reaction of mRNA from trigeminal ganglia indicated that all four EP prostanoid receptors were present. However, in both Type 1 and Type 2 cells the effects of PGE(2) were only mimicked by the selective EP(3) receptor agonist ONO-AE-248, and not by selective agonists for EP(1) (ONO-DI-004), EP(2) (ONO-AE1-259) and EP(4) (ONO-AE1-329) receptors. These data indicate that two populations of neurons in trigeminal ganglia differing in their calcium channel expression, sensitivity to mu-opioids and capsaicin also have divergent mechanisms of PGE(2)-mediated inhibition of calcium channels, with Gi/Go type G proteins involved in one population, and Gs type G proteins in the other.