Chemical determinants involved in anandamide-induced inhibition of T-type calcium channels

Anandamide-Mediated Modulation of Nociceptive Transmission at the Spinal Cord Level

Three decades ago, the first endocannabinoid, anandamide (AEA), was identified, and its analgesic effect was recognized in humans and preclinical models. However, clinical trial failures pointed out the complexity of the AEA-induced analgesia. The first synapses in the superficial laminae of the spinal cord dorsal horn represent an important modulatory site in nociceptive transmission and subsequent pain perception. The glutamatergic synaptic transmission at these synapses is strongly modulated by two primary AEA-activated receptors, cannabinoid receptor 1 (CB1) and transient receptor potential vanilloid 1 (TRPV1), both highly expressed on the presynaptic side formed by the endings of primary nociceptive neurons. Activation of these receptors can have predominantly inhibitory (CB1) and excitatory (TRPV1) effects that are further modulated under pathological conditions. In addition, dual AEA-mediated signaling and action may occur in primary sensory neurons and dorsal horn synapses. AEA application causes balanced inhibition and excitation of primary afferent synaptic input on superficial dorsal horn neurons in normal conditions, whereas peripheral inflammation promotes AEA-mediated inhibition. This review focuses mainly on the modulation of synaptic transmission at the spinal cord level and signaling in primary nociceptive neurons by AEA via CB1 and TRPV1 receptors. Furthermore, the spinal analgesic effect in preclinical studies and clinical aspects of AEA-mediated analgesia are considered.

Conceptual DFT, QTAIM, and Molecular Docking Approaches to Characterize the T-Type Calcium Channel Blocker Anandamide

The anandamide is a relevant ligand due to its capacity of interacting with several proteins, including the T-type calcium channels, which play an important role in neuropathic pain and depression disorders. Hence, a detailed characterization of the chemical properties and conformational stability of anandamide may provide valuable information to understand its behavior in a biological context. Herein, conceptual DFT and QTAIM analyses were performed to theoretically characterize the chemical reactivity properties and the structural stability of conformations of anandamide, using the BP86/cc-pVTZ level of theory. Global reactivity description, based on conceptual DFT, indicates that the hardness increases and the electrophilicity index decreases for both, the hairpin and U-shape conformers relative to the extended conformers. Also, an increase in the chemical potential value and a decrease in the electronegativity and the electrophilicity index is observed in the ethanolamide open ring conformers in comparison with the corresponding closed ring structures. In addition, regarding the characterization of local reactivity descriptors, the maximum values of the Fukui and Parr functions indicate that the most probable location for a nucleophilic attack is either the hydroxyl oxygen located in the ethanolamide closed ring conformers or the carbonyl oxygen present in the open ring conformers. The most probable location for an electrophilic attack is in the alkyl double bond region in all anandamide conformers. According to the QTAIM results, the intramolecular hydrogen bond formation stabilizing the structure of anandamide has interaction energy values for the closed ring conformations of 12.33–12.46 kcal mol⁻¹, indicating a strong interaction. Lastly, molecular docking calculations determined that a region in the pore, denominate as pore-blocking, is a probable site for the interaction of anandamide with the human Ca_v3.2 isoform of the T-type calcium channel family. The pore-blocking site contains hydrophobic residues where the non-polar part in the final alkyl region of anandamide established mainly alkyl-alkyl interactions, while the polar part (the ethanolamide group) interacts with the polar residue S900. The information based on conceptual DFT presented may aid in the design of drugs with similar chemical characteristics as those identified in anandamide so as to bind anandamide-interacting proteins, including the T-type calcium channels.

Receptor-Dependent and Independent Regulation of Voltage-Gated Ca2+ Channels and Ca2+-Permeable Channels by Endocannabinoids in the Brain

The activity of specific populations of neurons in different brain areas makes decisions regarding proper synaptic transmission, the ability to make adaptations in response to different external signals, as well as the triggering of specific regulatory pathways to sustain neural function. The endocannabinoid system (ECS) appears to be a very important, highly expressed, and active system of control in the central nervous system (CNS). Functionally, it allows the cells to respond quickly to processes that occur during synaptic transmission, but can also induce long-term changes. The endocannabinoids (eCBs) belong to a large family of bioactive lipid mediators that includes amides, esters, and ethers of long-chain polyunsaturated fatty acids. They are produced “on demand” from the precursors located in the membranes, exhibit a short half-life, and play a key role as retrograde messengers. eCBs act mainly through two receptors, CB1R and CB2R, which belong to the G-protein coupled receptor superfamily (GPCRs), but can also exert their action via multiple non-receptor pathways. The action of eCBs depends on Ca²⁺, but eCBs can also regulate downstream Ca²⁺ signaling. In this short review, we focus on the regulation of neuronal calcium channels by the most effective members of eCBs-2-arachidonoylglycerol (2-AG), anandamide (AEA) and originating from AEA-N-arachidonoylglycine (NAGly), to better understand the contribution of ECS to brain function under physiological conditions.

Peripheral Mechanobiology of Touch-Studies on Vertebrate Cutaneous Sensory Corpuscles

The vertebrate skin contains sensory corpuscles that are receptors for different qualities of mechanosensitivity like light brush, touch, pressure, stretch or vibration. These specialized sensory organs are linked anatomically and functionally to mechanosensory neurons, which function as low-threshold mechanoreceptors connected to peripheral skin through Aβ nerve fibers. Furthermore, low-threshold mechanoreceptors associated with Aδ and C nerve fibers have been identified in hairy skin. The process of mechanotransduction requires the conversion of a mechanical stimulus into electrical signals (action potentials) through the activation of mechanosensible ion channels present both in the axon and the periaxonal cells of sensory corpuscles (i.e., Schwann-, endoneurial- and perineurial-related cells). Most of those putative ion channels belong to the degenerin/epithelial sodium channel (especially the family of acid-sensing ion channels), the transient receptor potential channel superfamilies, and the Piezo family. This review updates the current data about the occurrence and distribution of putative mechanosensitive ion channels in cutaneous mechanoreceptors including primary sensory neurons and sensory corpuscles.

Neuronal Cav3 channelopathies: recent progress and perspectives

T-type, low-voltage activated, calcium channels, now designated Cav3 channels, are involved in a wide variety of physiological functions, especially in nervous systems. Their unique electrophysiological properties allow them to finely regulate neuronal excitability and to contribute to sensory processing, sleep, and hormone and neurotransmitter release. In the last two decades, genetic studies, including exploration of knock-out mouse models, have greatly contributed to elucidate the role of Cav3 channels in normal physiology, their regulation, and their implication in diseases. Mutations in genes encoding Cav3 channels (CACNA1G, CACNA1H, and CACNA1I) have been linked to a variety of neurodevelopmental, neurological, and psychiatric diseases designated here as neuronal Cav3 channelopathies. In this review, we describe and discuss the clinical findings and supporting in vitro and in vivo studies of the mutant channels, with a focus on de novo, gain-of-function missense mutations recently discovered in CACNA1G and CACNA1H. Overall, the studies of the Cav3 channelopathies help deciphering the pathogenic mechanisms of corresponding diseases and better delineate the properties and physiological roles Cav3 channels.

Analgesic transient receptor potential vanilloid-1-active compounds inhibit native and recombinant T-type calcium channels

BACKGROUND AND PURPOSE

T-type calcium (Ca_v 3) and transient receptor potential vanilloid-1 (TRPV1) channels play central roles in the control of excitability in the peripheral nervous system and are regarded as potential therapeutic pain targets. Modulators that either activate or inhibit TRPV1-mediated currents display analgesic properties in various pain models despite opposing effects on their connate target, TRPV1. We explored the effects of TRPV1-active compounds on Ca_v 3-mediated currents.

EXPERIMENTAL APPROACH

Whole-cell patch clamp recordings were used to examine the effects of TRPV1-active compounds on rat dorsal root ganglion low voltage-activated calcium currents and recombinant Ca_v 3 isoforms in expression systems.

KEY RESULTS

The classical TRPV1 agonist capsaicin as well as TRPV1 antagonists A-889425, BCTC, and capsazepine directly inhibited Ca_v 3 channels. These compounds altered the voltage-dependence of activation and inactivation of Ca_v 3 channels and delayed their recovery from inactivation, leading to a concomitant decrease in T-type current availability. The TRPV1 antagonist capsazepine potently inhibited Ca_v 3.1 and 3.2 channels (K_D  < 120 nM), as demonstrated by its slow off rate. In contrast, neither the TRPV1 agonists, Palvanil and resiniferatoxin, nor the TRPV1 antagonist AMG9810 modulated Ca_v 3-mediated currents.

CONCLUSIONS AND IMPLICATIONS

Analgesic TRPV1-active compounds inhibit Ca_v 3 currents in native and heterologous systems. Hence, their analgesic effects may not be exclusively attributed to their actions on TRPV1, which has important implications in the current understanding of nociceptive pathways. Importantly, our results highlight the need for attention in the experimental design used to address the analgesic properties of Ca_v 3 channel inhibitors.

A concise total synthesis of cochlearoid B

A T-type calcium channel inhibiting natural product cochlearoid B was synthesized in 7 linear steps.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Effects of the endogenous cannabinoid anandamide on voltage-dependent sodium and calcium channels in rat ventricular myocytes

BACKGROUND AND PURPOSE

The endocannabinoid anandamide (N-arachidonoyl ethanolamide; AEA) exerts negative inotropic and antiarrhythmic effects in ventricular myocytes.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp technique and radioligand-binding methods were used to analyse the effects of anandamide in rat ventricular myocytes.

KEY RESULTS

In the presence of 1-10 μM AEA, suppression of both Na(+) and L-type Ca(2+) channels was observed. Inhibition of Na(+) channels was voltage and Pertussis toxin (PTX) - independent. Radioligand-binding studies indicated that specific binding of [(3) H] batrachotoxin (BTX) to ventricular muscle membranes was also inhibited significantly by 10 μM metAEA, a non-metabolized AEA analogue, with a marked decrease in Bmax values but no change in Kd . Further studies on L-type Ca(2+) channels indicated that AEA potently inhibited these channels (IC50 0.1 μM) in a voltage- and PTX-independent manner. AEA inhibited maximal amplitudes without affecting the kinetics of Ba(2+) currents. MetAEA also inhibited Na(+) and L-type Ca(2+) currents. Radioligand studies indicated that specific binding of [(3) H]isradipine, was inhibited significantly by metAEA. (10 μM), changing Bmax but not Kd .

CONCLUSION AND IMPLICATIONS

Results indicate that AEA inhibited the function of voltage-dependent Na(+) and L-type Ca(2+) channels in rat ventricular myocytes, independent of CB1 and CB2 receptor activation.

Characterization of novel cannabinoid based T-type calcium channel blockers with analgesic effects

Low-voltage-activated (T-type) calcium channels are important regulators of the transmission of nociceptive information in the primary afferent pathway and finding ligands that modulate these channels is a key focus of the drug discovery field. Recently, we characterized a set of novel compounds with mixed cannabinoid receptor/T-type channel blocking activity and examined their analgesic effects in animal models of pain. Here, we have built on these previous findings and synthesized a new series of small organic compounds. We then screened them using whole-cell voltage clamp techniques to identify the most potent T-type calcium channel inhibitors. The two most potent blockers (compounds 9 and 10) were then characterized using radioligand binding assays to determine their affinity for CB1 and CB2 receptors. The structure–activity relationship and optimization studies have led to the discovery of a new T-type calcium channel blocker, compound 9. Compound 9 was efficacious in mediating analgesia in mouse models of acute inflammatory pain and in reducing tactile allodynia in the partial nerve ligation model. This compound was shown to be ineffective in Cav3.2 T-type calcium channel null mice at therapeutically relevant concentrations, and it caused no significant motor deficits in open field tests. Taken together, our data reveal a novel class of compounds whose physiological and therapeutic actions are mediated through block of Cav3.2 calcium channels.

Modulation of T-type calcium channels by bioactive lipids

T-type calcium channels (T-channels/CaV3) have unique biophysical properties allowing a calcium influx at resting membrane potential of most cells. T-channels are ubiquitously expressed in many tissues and contribute to low-threshold spikes and burst firing in central neurons as well as to pacemaker activities in cardiac cells. They also emerged as potential targets to treat cancer and hypertension. Regulation of these channels appears complex, and several studies have indicated that CaV3.1, CaV3.2, and CaV3.3 currents are directly inhibited by multiple endogenous lipids independently of membrane receptors or intracellular pathways. These bioactive lipids include arachidonic acid and ω3 poly-unsaturated fatty acids; the endocannabinoid anandamide and other N-acylethanolamides; the lipoamino-acids and lipo-neurotransmitters; the P450 epoxygenase metabolite 5,6-epoxyeicosatrienoic acid; as well as similar molecules with 18-22 carbons in the alkyl chain. In this review, we summarize evidence for direct effects of these signaling molecules, the molecular mechanisms underlying the current inhibition, and the involved chemical features. The impact of this modulation in physiology and pathophysiology is discussed with a special emphasis on pain aspects and vasodilation. Overall, these data clearly indicate that T-current inhibition is an important mechanism by which bioactive lipids mediate their physiological functions.

Ligand determinants of fatty acid activation of the pronociceptive ion channel TRPA1

Background and purpose. Arachidonic acid (AA) and its derivatives are important modulators of cellular signalling. The transient receptor potential cation channel subfamily A, member 1 (TRPA1) is a cation channel with important functions in mediating cellular responses to noxious stimuli and inflammation. There is limited information about the interactions between AA itself and TRPA1, so we investigated the effects of AA and key ethanolamide and amino acid/neurotransmitter derivatives of AA on hTRPA1.

Experimental approach. HEK 293 cells expressing hTRPA1 were studied by measuring changes in intracellular calcium ([Ca]_i) with a fluorescent dye and by standard whole cell patch clamp recordings.

Key results. AA (30 μM) increased fluorescence in hTRPA1 expressing cells by 370% (notional EC₅₀ 13 μM). The covalent TRPA1 agonist cinnamaldehyde (300 μM) increased fluorescence by 430% (EC₅₀, 11 μM). Anandamide (230%) and N-arachidonoyl tyrosine (170%) substantially activated hTRPA1 at 30 μM, however, N-arachidonoyl conjugates of glycine and taurine were less effective while N-acyl conjugates of 5-HT did not affect hTRPA1. Changing the acyl chain length or the number and position of double bonds reduced fatty acid efficacy at hTRPA1. Mutant hTRPA1 (Cys621, Cys641 and Cys665 changed to Ser) could be activated by AA (100 μM, 40% of wild type) but not by cinnamaldehyde (300 μM).

Conclusions and implications. AA is a more potent activator of TRPA1 than its ethanolamide or amino acid/neurotransmitter derivatives and acts via a mechanism distinct from that of cinnamaldehyde, further underscoring the likelyhood of multiple pharmacologically exploitable sites on hTRPA1.

Ca(v)3.2 calcium channels: the key protagonist in the supraspinal effect of paracetamol

To exert its analgesic action, paracetamol requires complex metabolism to produce a brain-specific lipoamino acid compound, AM404, which targets central transient receptor potential vanilloid receptors (TRPV1). Lipoamino acids are also known to induce analgesia through T-type calcium-channel inhibition (Ca(v)3.2). In this study we show that the antinociceptive effect of paracetamol in mice is lost when supraspinal Ca(v)3.2 channels are inhibited. Therefore, we hypothesized a relationship between supraspinal Ca(v)3.2 and TRPV1, via AM404, which mediates the analgesic effect of paracetamol. AM404 is able to activate TRPV1 and weakly inhibits Ca(v)3.2. Interestingly, activation of TRPV1 induces a strong inhibition of Ca(v)3.2 current. Supporting this, intracerebroventricular administration of AM404 or capsaicin produces antinociception that is lost in Ca(v)3.2(-/-) mice. Our study, for the first time, (1) provides a molecular mechanism for the supraspinal antinociceptive effect of paracetamol; (2) identifies the relationship between TRPV1 and the Ca(v)3.2 channel; and (3) suggests supraspinal Ca(v)3.2 inhibition as a potential pharmacological strategy to alleviate pain.

Low-voltage-activated T-type Ca2+ channel inhibitors as new tools in the treatment of glioblastoma: the role of endostatin

Ca(2+) plays a key role in intracellular signaling and controls various cellular processes such as proliferation, differentiation, cell growth, death, and apoptosis. Aberrant changes in intracellular Ca(2+) levels can promote undesired cell proliferation and migration and are therefore associated with certain tumor types. Many research groups have suggested a potential role for voltage-gated Ca(2+) channels in the regulation of tumor growth and progression, particularly T-type channels due to their unique biophysical properties. T-type channels are expressed in normal tissues throughout the body and in different types of tumors such as breast carcinoma, retinoblastoma, neuroblastoma, and glioma. It has been demonstrated that increased functional expression of the α1 subunit of T-type channels plays a role in the abnormal proliferation of glioblastoma cells. As such, siRNA-mediated knockdown of the expression of the α1 subunit of T-type channels decreases the proliferation of these cells. Moreover, pharmacological blockade of T-type channels significantly decreases tumor growth. In this review, we focus on the use of T-type channel blockers for the potential treatment of cancers, particularly highly proliferative tumors such as glioblastoma. We conclude that T-type channel blockers such as endostatin can serve as a potential therapeutic tool for tumors whose proliferation depends on increased T-type channel expression.

Cross-modulation and molecular interaction at the Cav3.3 protein between the endogenous lipids and the T-type calcium channel antagonist TTA-A2

T-type calcium channels (T/Ca(v)3-channels) are implicated in various physiologic and pathophysiologic processes such as epilepsy, sleep disorders, hypertension, and cancer. T-channels are the target of endogenous signaling lipids including the endocannabinoid anandamide, the ω3-fatty acids, and the lipoamino-acids. However, the precise molecular mechanism by which these molecules inhibit T-current is unknown. In this study, we provided a detailed electrophysiologic and pharmacologic analysis indicating that the effects of the major N-acyl derivatives on the Ca(v)3.3 current share many similarities with those of TTA-A2 [(R)-2-(4-cyclopropylphenyl)-N-(1-(5-(2,2,2-trifluoroethoxy)pyridin-2-yl)ethyl)acetamide], a synthetic T-channel inhibitor. Using radioactive binding assays with the TTA-A2 derivative [(3)H]TTA-A1 [(R)-2-(4-(tert-butyl)phenyl)-N-(1-(5-methoxypyridin-2-yl)ethyl)acetamide], we demonstrated that polyunsaturated lipids, which inhibit the Ca(v)3.3 current, as NAGly (N-arachidonoyl glycine), NASer (N-arachidonoyl-l-serine), anandamide, NADA (N-arachidonoyl dopamine), NATau (N-arachidonoyl taurine), and NA-5HT (N-arachidonoyl serotonin), all displaced [(3)H]TTA-A1 binding to membranes prepared from cells expressing Ca(v)3.3, with Ki in a micromolar or submicromolar range. In contrast, lipids with a saturated alkyl chain, as N-arachidoyl glycine and N-arachidoyl ethanolamine, which did not inhibit the Ca(v)3.3 current, had no effect on [(3)H]TTA-A1 binding. Accordingly, bio-active lipids occluded TTA-A2 effect on Ca(v)3.3 current. In addition, TTA-Q4 [(S)-4-(6-chloro-4-cyclopropyl-3-(2,2-difluoroethyl)-2-oxo-1,2,3,4-tetrahydroquinazolin-4-yl)benzonitrile], a positive allosteric modulator of [(3)H]TTA-A1 binding and TTA-A2 functional inhibition, acted in a synergistic manner to increase lipid-induced inhibition of the Ca(v)3.3 current. Overall, our results demonstrate a common molecular mechanism for the synthetic T-channel inhibitors and the endogenous lipids, and indicate that TTA-A2 and TTA-Q4 could be important pharmacologic tools to dissect the involvement of T-current in the physiologic effects of endogenous lipids.

Why do cannabinoid receptors have more than one endogenous ligand?

The endocannabinoid system was revealed following the understanding of the mechanism of action of marijuana's major psychotropic principle, Δ(9)-tetrahydrocannabinol, and includes two G-protein-coupled receptors (GPCRs; the cannabinoid CB1 and CB2 receptors), their endogenous ligands (the endocannabinoids, the best studied of which are anandamide and 2-arachidonoylglycerol (2-AG)), and the proteins that regulate the levels and activity of these receptors and ligands. However, other minor lipid metabolites different from, but chemically similar to, anandamide and 2-AG have also been suggested to act as endocannabinoids. Thus, unlike most other GPCRs, cannabinoid receptors appear to have more than one endogenous agonist, and it has been often wondered what could be the physiological meaning of this peculiarity. In 1999, it was proposed that anandamide might also activate other targets, and in particular the transient receptor potential of vanilloid type-1 (TRPV1) channels. Over the last decade, this interaction has been shown to occur both in peripheral tissues and brain, during both physiological and pathological conditions. TRPV1 channels can be activated also by another less abundant endocannabinoid, N-arachidonoyldopamine, but not by 2-AG, and have been proposed by some authors to act as ionotropic endocannabinoid receptors. This article will discuss the latest discoveries on this subject, and discuss, among others, how anandamide and 2-AG differential actions at TRPV1 and cannabinoid receptors contribute to making this signalling system a versatile tool available to organisms to fine-tune homeostasis.

Inhibition of human recombinant T-type calcium channels by N-arachidonoyl 5-HT

BACKGROUND AND PURPOSE

N-arachidonoyl 5-HT (NA-5HT) has anti-nociceptive effects reported to be mediated by inhibitory actions at the transient receptor potential vanilloid receptor 1 (TRPV1) and fatty acid amide hydrolase (FAAH). Anandamide and N-arachidonoyl dopamine (NA-DA), endocannabinoids that activate TRPV1 or are metabolized by FAAH, also inhibit T-type calcium channels (I(Ca) ). T-type I(Ca) are expressed by many excitable cells, including neurons involved in pain detection and processing. We sought to determine whether NA-5HT also modulates T-type I(Ca) .

EXPERIMENTAL APPROACH

Human recombinant T-type I(Ca) (Ca(V) 3 channels) expressed in HEK 293 cells were examined using standard whole-cell voltage-clamp electrophysiology techniques.

KEY RESULTS

NA-5HT completely inhibited Ca(V) 3 channels with a rank order of potency (pEC(50) ) of Ca(V) 3.1 (7.4) > Ca(V) 3.3 (6.8) ≥ Ca(V) 3.2 (6.6). The effects of NA-5HT were voltage-dependent, and it produced significant hyperpolarizing shifts in Ca(V) 3 steady-state inactivation relationships. NA-5HT selectively affected Ca(V) 3.3 channel kinetics.

CONCLUSIONS AND IMPLICATIONS

NA-5HT increases the steady-state inactivation of Ca(V) 3 channels, reducing the number of channels available to open during depolarization. These effects occur at NA-5HT concentrations at or below those at which NA-5HT affects TRPV1 receptors and FAAH. NA-5HT is one of the most potent inhibitors of T-type I(Ca) described to date, and it is likely to exert some of its biological effects, including anti-nociception, via inhibition of these channels.

N-Arachidonyl glycine does not activate G protein-coupled receptor 18 signaling via canonical pathways

Recent studies propose that N-arachidonyl glycine (NAGly), a carboxylic analogue of anandamide, is an endogenous ligand of the Gα(i/o) protein-coupled receptor 18 (GPR18). However, a high-throughput β-arrestin-based screen failed to detect activation of GPR18 by NAGly (Yin et al., 2009; JBC, 18:12328). To address this inconsistency, this study investigated GPR18 coupling in a native neuronal system with endogenous signaling pathways and effectors. GPR18 was heterologously expressed in rat sympathetic neurons, and the modulation of N-type (Ca(v)2.2) calcium channels was examined. Proper expression and trafficking of receptor were confirmed by the "rim-like" fluorescence of fluorescently tagged receptor and the positive staining of external hemagglutinin-tagged GPR18-expressing cells. Application of NAGly on GPR18-expressing neurons did not inhibit calcium currents but instead potentiated currents in a voltage-dependent manner, similar to what has previously been reported (Guo et al., 2008; J Neurophysiol, 100:1147). Other proposed agonists of GPR18, including anandamide and abnormal cannabidiol, also failed to induce inhibition of calcium currents. Mutants of GPR18, designed to constitutively activate receptors, did not tonically inhibit calcium currents, indicating a lack of GPR18 activation or coupling to endogenous G proteins. Other downstream effectors of Gα(i/o)-coupled receptors, G protein-coupled inwardly rectifying potassium channels and adenylate cyclase, were not modulated by GPR18 signaling. Furthermore, GPR18 did not couple to other G proteins tested: Gα(s), Gα(z), and Gα(15). These results suggest NAGly is not an agonist for GPR18 or that GPR18 signaling involves noncanonical pathways not examined in these studies.

Modulation of low-voltage-activated T-type Ca²⁺ channels

Low-voltage-activated T-type Ca²⁺ channels contribute to a wide variety of physiological functions, most predominantly in the nervous, cardiovascular and endocrine systems. Studies have documented the roles of T-type channels in sleep, neuropathic pain, absence epilepsy, cell proliferation and cardiovascular function. Importantly, novel aspects of the modulation of T-type channels have been identified over the last few years, providing new insights into their physiological and pathophysiological roles. Although there is substantial literature regarding modulation of native T-type channels, the underlying molecular mechanisms have only recently begun to be addressed. This review focuses on recent evidence that the Ca(v)3 subunits of T-type channels, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are differentially modulated by a multitude of endogenous ligands including anandamide, monocyte chemoattractant protein-1, endostatin, and redox and oxidizing agents. The review also provides an overview of recent knowledge gained concerning downstream pathways involving G-protein-coupled receptors. This article is part of a Special Issue entitled: Calcium channels.

ZnT-1 enhances the activity and surface expression of T-type calcium channels through activation of Ras-ERK signaling

Zinc transporter-1 (ZnT-1) is a putative zinc transporter that confers cellular resistance from zinc toxicity. In addition, ZnT-1 has important regulatory functions, including inhibition of L-type calcium channels and activation of Raf-1 kinase. Here we studied the effects of ZnT-1 on the expression and function of T-type calcium channels. In Xenopus oocytes expressing voltage-gated calcium channel (CaV) 3.1 or CaV3.2, ZnT-1 enhanced the low-threshold calcium currents ( IcaT) to 182 ± 15 and 167.95 ± 9.27% of control, respectively ( P < 0.005 for both channels). As expected, ZnT-1 also enhanced ERK phosphorylation. Coexpression of ZnT-1 and nonactive Raf-1 blocked the ZnT-1-mediated ERK phosphorylation and abolished the ZnT-1-induced augmentation of IcaT. In mammalian cells (Chinese hamster ovary), coexpression of CaV3.1 and ZnT-1 increased the IcaT to 166.37 ± 6.37% compared with cells expressing CaV3.1 alone ( P < 0.01). Interestingly, surface expression measurements using biotinylation or total internal reflection fluorescence microscopy indicated marked ZnT-1-induced enhancement of CaV3.1 surface expression. The MEK inhibitor PD-98059 abolished the ZnT-1-induced augmentation of surface expression of CaV3.1. In cultured murine cardiomyocytes (HL-1 cells), transient exposure to zinc, leading to enhanced ZnT-1 expression, also enhanced the surface expression of endogenous CaV3.1 channels. Consistently, in these cells, endothelin-1, a potent activator of Ras-ERK signaling, enhanced the surface expression of CaV3.1 channels in a PD-98059-sensitive manner. Our findings indicate that ZnT-1 enhances the activity of CaV3.1 and CaV3.2 through activation of Ras-ERK signaling. The augmentation of CaV3.1 currents by Ras-ERK activation is associated with enhanced trafficking of the channel to the plasma membrane.

Mechanosensory neurons, cutaneous mechanoreceptors, and putative mechanoproteins

The mammalian skin has developed sensory structures (mechanoreceptors) that are responsible for different modalities of mechanosensitivity like touch, vibration, and pressure sensation. These specialized sensory organs are anatomically and functionally connected to a special subset of sensory neurons called mechanosensory neurons, which electrophysiologically correspond with Aβ fibers. Although mechanosensory neurons and cutaneous mechanoreceptors are rather well known, the biology of the sense of touch still remains poorly understood. Basically, the process of mechanosensitivity requires the conversion of a mechanical stimulus into an electrical signal through the activation of ion channels that gate in response to mechanical stimuli. These ion channels belong primarily to the family of the degenerin/epithelium sodium channels, especially the subfamily acid-sensing ion channels, and to the family of transient receptor potential channels. This review compiles the current knowledge on the occurrence of putative mechanoproteins in mechanosensory neurons and mechanoreceptors, as well as the involvement of these proteins on the biology of touch. Furthermore, we include a section about what the knock-out mice for mechanoproteins are teaching us. Finally, the possibilities for mechanotransduction in mechanoreceptors, and the common involvement of the ion channels, extracellular membrane, and cytoskeleton, are revisited.

Dehydroepiandrosterone (DHEA) inhibits voltage-gated T-type calcium channels

BACKGROUND AND PURPOSE

Dehydroepiandrosterone (DHEA) and its sulfated form, DHEAS, are the most abundant steroid hormones in the mammalian blood flow. DHEA may have beneficial effects in various pathophysiological conditions such as cardiovascular diseases or deterioration of the sense of well-being. However to date, the cellular mechanism underlying DHEA action remains elusive and may involve ion channel modulation. In this study, we have characterized the effect of DHEA on T-type voltage-activated calcium channels (T-channels), which are involved in several cardiovascular and neuronal diseases.

KEY RESULTS

Using the whole-cell patch-clamp technique, we demonstrate that DHEA inhibits the three recombinant T-channels (Ca(V)3.1, Ca(V)3.2 and Ca(V)3.3) expressed in NG108-15 cell line, as well as native T-channels in pulmonary artery smooth muscle cells. This effect of DHEA is both concentration (IC(50) between 2 and 7μM) and voltage-dependent and results in a significant shift of the steady-state inactivation curves toward hyperpolarized potentials. Consequently, DHEA reduces window T-current and inhibits membrane potential oscillations induced by Ca(V)3 channels. DHEA inhibition is not dependent on the activation of nuclear androgen or estrogen receptors and implicates a PTX-sensitive Gi protein pathway. Functionally, DHEA and the T-type inhibitor NNC 55-0396 inhibited KCl-induced contraction of pulmonary artery rings and their effect was not cumulative.

CONCLUSIONS

Altogether, the present data demonstrate that DHEA inhibits T-channels by a Gi protein dependent pathway. DHEA-induced alteration in T-channel activity could thus account for its therapeutic action and/or physiological effects.

T-type voltage-gated calcium channels as targets for the development of novel pain therapies

It is well recognized that voltage-gated calcium (Ca(2+)) channels modulate the function of peripheral and central pain pathways by influencing fast synaptic transmission and neuronal excitability. In the past, attention focused on the modulation of different subtypes of high-voltage-activated-type Ca(2+) channels; more recently, the function of low-voltage-activated or transient (T)-type Ca(2+) channels (T-channels) in nociception has been well documented. Currently, available pain therapies remain insufficient for certain forms of pain associated with chronic disorders (e.g. neuropathic pain) and often have serious side effects. Hence, the identification of selective and potent inhibitors and modulators of neuronal T-channels may help greatly in the development of safer, more effective pain therapies. Here, we summarize the available information implicating peripheral and central T-channels in nociception. We also discuss possible future developments aimed at selective modulation of function of these channels, which are highly expressed in nociceptors.

International Union of Basic and Clinical Pharmacology. LXXIX. Cannabinoid receptors and their ligands: beyond CB₁ and CB₂

There are at least two types of cannabinoid receptors (CB(1) and CB(2)). Ligands activating these G protein-coupled receptors (GPCRs) include the phytocannabinoid Δ(9)-tetrahydrocannabinol, numerous synthetic compounds, and endogenous compounds known as endocannabinoids. Cannabinoid receptor antagonists have also been developed. Some of these ligands activate or block one type of cannabinoid receptor more potently than the other type. This review summarizes current data indicating the extent to which cannabinoid receptor ligands undergo orthosteric or allosteric interactions with non-CB(1), non-CB(2) established GPCRs, deorphanized receptors such as GPR55, ligand-gated ion channels, transient receptor potential (TRP) channels, and other ion channels or peroxisome proliferator-activated nuclear receptors. From these data, it is clear that some ligands that interact similarly with CB(1) and/or CB(2) receptors are likely to display significantly different pharmacological profiles. The review also lists some criteria that any novel "CB(3)" cannabinoid receptor or channel should fulfil and concludes that these criteria are not currently met by any non-CB(1), non-CB(2) pharmacological receptor or channel. However, it does identify certain pharmacological targets that should be investigated further as potential CB(3) receptors or channels. These include TRP vanilloid 1, which possibly functions as an ionotropic cannabinoid receptor under physiological and/or pathological conditions, and some deorphanized GPCRs. Also discussed are 1) the ability of CB(1) receptors to form heteromeric complexes with certain other GPCRs, 2) phylogenetic relationships that exist between CB(1)/CB(2) receptors and other GPCRs, 3) evidence for the existence of several as-yet-uncharacterized non-CB(1), non-CB(2) cannabinoid receptors; and 4) current cannabinoid receptor nomenclature.

N-acyl amino acids and N-acyl neurotransmitter conjugates: neuromodulators and probes for new drug targets

The myriad functions of lipids as signalling molecules is one of the most interesting fields in contemporary pharmacology, with a host of compounds recognized as mediators of communication within and between cells. The N-acyl conjugates of amino acids and neurotransmitters (NAANs) have recently come to prominence because of their potential roles in the nervous system, vasculature and the immune system. NAAN are compounds such as glycine, GABA or dopamine conjugated with long chain fatty acids. More than 70 endogenous NAAN have been reported although their physiological role remains uncertain, with various NAAN interacting with a low affinity at G protein coupled receptors (GPCR) and ion channels. Regardless of their potential physiological function, NAAN are of great interest to pharmacologists because of their potential as flexible tools to probe new sites on GPCRs, transporters and ion channels. NAANs are amphipathic molecules, with a wide variety of potential fatty acid and headgroup moieties, a combination which provides a rich source of potential ligands engaging novel binding sites and mechanisms for modulation of membrane proteins such as GPCRs, ion channels and transporters. The unique actions of subsets of NAAN on voltage-gated calcium channels and glycine transporters indicate that the wide variety of NAAN may provide a readily exploitable resource for defining new pharmacological targets. Investigation of the physiological roles and pharmacological potential of these simple lipid conjugates is in its infancy, and we believe that there is much to be learnt from their careful study.

The Ca2+ channel beta subunit determines whether stimulation of Gq-coupled receptors enhances or inhibits N current

In superior cervical ganglion (SCG) neurons, stimulation of M₁ receptors (M₁Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M₁Rs. In addition, techniques that diminish AA's concentration during M₁R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M₁R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the β subunit (Ca_Vβ) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M₁Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca_V2.2 and coexpressed with Ca_Vβ1b, Ca_Vβ3, or Ca_Vβ4 but enhanced activity of N channels containing Ca_Vβ2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca_Vβ2a, Ca_Vβ3, and Ca_Vβ4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca_Vβ with Ca_V2.2. Further experiments with mutated, chimeric Ca_Vβ subunits and free palmitic acid revealed that palmitoylation of Ca_Vβ2a is essential for loss of inhibition. The data presented here fit a model in which Ca_Vβ2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca_Vβ2a toggles M₁R- or NK-1R–mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M₁Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca_Vβ2a.

T-type calcium channel inhibition underlies the analgesic effects of the endogenous lipoamino acids

Lipoamino acids are anandamide-related endogenous molecules that induce analgesia via unresolved mechanisms. Here, we provide evidence that the T-type/Cav3 calcium channels are important pharmacological targets underlying their physiological effects. Various lipoamino acids, including N-arachidonoyl glycine (NAGly), reversibly inhibited Cav3.1, Cav3.2, and Cav3.3 currents, with potent effects on Cav3.2 [EC(50) approximately 200 nm for N-arachidonoyl 3-OH-gamma-aminobutyric acid (NAGABA-OH)]. This inhibition involved a large shift in the Cav3.2 steady-state inactivation and persisted during fatty acid amide hydrolase (FAAH) inhibition as well as in cell-free outside-out patch. In contrast, lipoamino acids had weak effects on high-voltage-activated (HVA) Cav1.2 and Cav2.2 calcium currents, on Nav1.7 and Nav1.8 sodium currents, and on anandamide-sensitive TRPV1 and TASK1 currents. Accordingly, lipoamino acids strongly inhibited native Cav3.2 currents in sensory neurons with small effects on sodium and HVA calcium currents. In addition, we demonstrate here that lipoamino acids NAGly and NAGABA-OH produced a strong thermal analgesia and that these effects (but not those of morphine) were abolished in Cav3.2 knock-out mice. Collectively, our data revealed lipoamino acids as a family of endogenous T-type channel inhibitors, suggesting that these ligands can modulate multiple cell functions via this newly evidenced regulation.

Dual modulation of CNS voltage-gated calcium channels by cannabinoids: Focus on CB1 receptor-independent effects

The neuromodulatory effects of cannabinoids in the central nervous system have mainly been associated with G-protein coupled cannabinoid receptor (CB1R) mediated inhibition of voltage-gated calcium channels (VGCCs). Numerous studies show, however, that cannabinoids can also modulate VGCCs independent of CB1R activation. Nevertheless, despite the fact that endocannabinoids have a nearly equal efficacy for direct and CB1R-mediated effects on VGCC, the role of the direct cannabinoid-VGCC interaction has been largely underestimated. In this review, we summarize recent studies on the modulation of different types of VGCCs by cannabinoids, highlight the evidence for and implications of the CB1R-independent modulation, and put forward the concept, that direct interaction of cannabinoids and VGCCs is as important in regulation of VGCCs function as the CB1R-mediated effects.

Regulation of voltage-gated Ca2+ channels by lipids

Great skepticism has surrounded the question of whether modulation of voltage-gated Ca(2+) channels (VGCCs) by the polyunsaturated free fatty acid arachidonic acid (AA) has any physiological basis. Here we synthesize findings from studies of both native and recombinant channels where micromolar concentrations of AA consistently inhibit both native and recombinant activity by stabilizing VGCCs in one or more closed states. Structural requirements for these inhibitory actions include a chain length of at least 18 carbons and multiple double bonds located near the fatty acid's carboxy terminus. Acting at a second site, AA increases the rate of VGCC activation kinetics, and in Ca(V)2.2 channels, increases current amplitude. We present evidence that phosphatidylinositol 4,5-bisphosphate (PIP(2)), a palmitoylated accessory subunit (beta(2a)) of VGCCs and AA appear to have overlapping sites of action giving rise to complex channel behavior. Their actions converge in a physiologically relevant manner during muscarinic modulation of VGCCs. We speculate that M(1) muscarinic receptors may stimulate multiple lipases to break down the PIP(2) associated with VGCCs and leave PIP(2)'s freed fatty acid tails bound to the channels to confer modulation. This unexpectedly simple scheme gives rise to unanticipated predictions and redirects thinking about lipid regulation of VGCCs.

Inhibition of human recombinant T-type calcium channels by the endocannabinoid N-arachidonoyl dopamine

BACKGROUND AND PURPOSE

N-arachidonoyl dopamine (NADA) has complex effects on nociception mediated via cannabinoid CB(1) receptors and the transient receptor potential vanilloid receptor 1 (TRPV1). Anandamide, the prototypic CB(1)/TRPV1 agonist, also inhibits T-type voltage-gated calcium channel currents (I(Ca)). These channels are expressed by many excitable cells, including neurons involved in pain detection and processing. We sought to determine whether NADA and the prototypic arachidonoyl amino acid, N-arachidonoyl glycine (NAGly) modulate T-type I(Ca)

EXPERIMENTAL APPROACH

Human recombinant T-type I(Ca) (Ca(V)3 channels) expressed in HEK 293 cells and native mouse T-type I(Ca) were examined using standard whole-cell voltage clamp electrophysiology techniques.

KEY RESULTS

N-arachidonoyl dopamine completely inhibited Ca(V)3 channels with a rank order of potency (pEC(50)) of Ca(V)3.3 (6.45) > or = Ca(V)3.1 (6.29) > Ca(V)3.2 (5.95). NAGly (10 micromol.L(-1)) inhibited Ca(V)3 I(Ca) by approximately 50% or less. The effects of NADA and NAGly were voltage- but not use-dependent, and both compounds produced significant hyperpolarizing shifts in Ca(V)3 channel steady-state inactivation relationships. By contrast with anandamide, NADA and NAGly had modest effects on Ca(V)3 channel kinetics. Both NAGly and NADA inhibited native T-type I(Ca) in mouse sensory neurons.

CONCLUSIONS AND IMPLICATIONS

N-arachidonoyl dopamine and NAGly increase the steady-state inactivation of Ca(V)3 channels, reducing the number of channels available to open during depolarization. These effects occur at NADA concentrations at or below to those affecting CB(1) and TRPV1 receptors. Together with anandamide, the arachidonoyl neurotransmitter amides, NADA and NAGly, represent a new family of endogenous T-type I(Ca) modulators.

Polyunsaturated fatty acid modulation of voltage-gated ion channels

Arachidonic acid (AA) was found to inhibit the function of whole-cell voltage-gated (VG) calcium currents nearly 16 years ago. There are now numerous examples demonstrating that AA and other polyunsaturated fatty acids (PUFAs) modulate the function of VG ion channels, primarily in neurons and muscle cells. We will review and extract some common features about the modulation by PUFAs of VG calcium, sodium, and potassium channels and discuss the impact of this modulation on the excitability of neurons and cardiac myocytes. We will describe the fatty acid nature of the membrane, how fatty acids become available to function as modulators of VG channels, and the physiologic importance of this type of modulation. We will review the evidence for molecular mechanisms and assess our current understanding of the structural basis for modulation. With guidance from research on the structure of fatty acid binding proteins, the role of lipids in gating mechanosensitive (MS) channels, and the impact of membrane lipid composition on membrane-embedded proteins, we will highlight some avenues for future investigations.

The neuroprotective impact of the leak potassium channel TASK1 on stroke development in mice

Oxygen depletion (O(2)) and a decrease in pH are initial pathophysiological events in stroke development, but secondary mechanisms of ischemic cell death are incompletely understood. By patch-clamp recordings of brain slice preparations we show that TASK1 and TASK3 channels are inhibited by pH-reduction (42+/-2%) and O(2) deprivation (36+/-5%) leading to membrane depolarization, increased input resistance and a switch in action potential generation under ischemic conditions. In vivo TASK blockade by anandamide significantly increased infarct volumes at 24 h in mice undergoing 30 min of transient middle cerebral artery occlusion (tMCAO). Moreover, blockade of TASK channels accelerated stroke development. Supporting these findings TASK1(-/-) mice developed significantly larger infarct volumes after tMCAO accompanied by worse outcome in functional neurological tests compared to wild type mice. In conclusion, our data provide evidence for an important role of functional TASK channels in limiting tissue damage during cerebral ischemia.

Methanandamide activation of a novel current in mouse trigeminal ganglion sensory neurons in vitro

Anandamide is an endogenous agonist for cannabinoid receptors and produces analgesia by acting at these receptors in several sites in the brain and peripheral nervous system. Anandamide is also an agonist at the TRPV1 receptor, a protein that serves as an important integrator of noxious stimuli in sensory neurons. Although anandamide actions at CB1 and TRPV1 receptors can explain many of its effects on sensory neurons, some apparently CB1- and TRPV1-independent effects of anandamide have been reported. To explore possible mechanisms underlying these effects we examined the actions of the stable anandamide analog methanandamide on the membrane properties of trigeminal ganglion neurons from mice with TRPV1 deleted. We found that methanandamide and anandamide activate a novel current in a subpopulation of small trigeminal ganglion neurons. Methanandamide activated the current (EC(50) 2 microM) more potently than it activates TRPV1 under the same conditions. The methanandamide-activated current reverses at 0 mV and does not inactivate at positive potentials but declines rapidly at negative membrane potentials. Activation of the current is not mediated via cannabinoid receptors and does not appear to involve G proteins. The phytocannabinoid Delta(9)-tetrahydrocannabinol, the endocannabinoid-related molecules N-arachidonoyl dopamine and N-arachidonoyl glycine and the non-specific TRPV channel activator 2-aminoethoxydiphenyl borate do not mimic the effects of methanandamide. The molecular identity of the current remains to be established, but we have identified a potential new effector for endocannabinoids in sensory neurons, and activation of this current may underlie some of the previously reported CB1 and TRPV1-independent effects of these compounds.

The endogenous cannabinoid system stimulates glucose uptake in human fat cells via phosphatidylinositol 3-kinase and calcium-dependent mechanisms

BACKGROUND

The endogenous cannabinoid system participates in the regulation of energy balance, and its dysregulation may be implicated in the pathogenesis of obesity. Adipose tissue endocannabinoids may produce metabolic and endocrine effects, but very few data are available in human adipose tissue and in primary human fat cells.

EXPERIMENTAL DESIGN

We measured expression of type 1 and type 2 cannabinoid receptors (CNR), enzymes of cannabinoids synthesis and degradation in human omental, sc abdominal, and gluteal adipose tissue from lean and obese subjects. Furthermore, we assessed the effect of CNR1 stimulation on glucose uptake and intracellular transduction mechanisms in primary human adipocytes. Then we assessed the reciprocal regulation between CNR1 and peroxisome proliferator-activated receptor-gamma (PPARgamma). Finally, we tested whether leptin and adiponectin are regulated by CNR1 in human adipocytes.

RESULTS

We found that most genes of the endocannabinoid system are down-regulated in gluteal fat and up-regulated in visceral and sc abdominal adipose tissue of obese patients. Treatment of adipocytes with rosiglitazone markedly down-regulated CNR1 expression, whereas Win 55,212 up-regulated PPARgamma. Win 55,212 increased (+50%) glucose uptake, the translocation of glucose transporter 4, and intracellular calcium in fat cells. All these effects were inhibited by SR141716 and wortmannin and by removing extracellular calcium. Win 55,212 and SR141716 had no effect on expression of adiponectin and leptin.

CONCLUSIONS

These results indicate a role for the local endocannabinoids in the regulation of glucose metabolism in human adipocytes and suggest a role in channeling excess energy fuels to adipose tissue in obese humans.

Temperature-dependent modulation of CaV3 T-type calcium channels by protein kinases C and A in mammalian cells

Modulation of low voltage-activated Ca(V)3 T-type calcium channels remains poorly characterized compared with high voltage-activated Ca(V)1 and Ca(V)2 calcium channels. Notably, it is yet unresolved whether Ca(V)3 channels are modulated by protein kinases in mammalian cells. In this study, we demonstrate that protein kinase A (PKA) and PKC (but not PKG) activation induces a potent increase in Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3 currents in various mammalian cell lines. Notably, we show that protein kinase effects occur at physiological temperature ( approximately 30-37 degrees C) but not at room temperature ( approximately 22-27 degrees C). This temperature dependence could involve kinase translocation, which is impaired at room temperature. A similar temperature dependence was observed for PKC-mediated increase in high voltage-activated Ca(V)2.3 currents. We also report that neither Ca(V)3 surface expression nor T-current macroscopic properties are modified upon kinase activation. In addition, we provide evidence for the direct phosphorylation of Ca(V)3.2 channels by PKA in in vitro assays. Overall, our results clearly establish the role of PKA and PKC in the modulation of Ca(V)3 T-channels and further highlight the key role of the physiological temperature in the effects described.

Towards the discovery of novel T-type calcium channel blockers

Despite their presence in many tissues and their potential implication in various disease states, low-voltage activated T-type calcium channels (T-channels) have only recently become targets of interest. Unfortunately, the lack of selective T-channel blockers has hampered further characterisation of these channels. The recent availability of cloned T-channels, the Ca(V)3 proteins, facilitates identification of novel T-channel blockers. Also, studies performed in knockout animals have fostered novel interest. Selective inhibition of T-channels may have clinical importance in cardiovascular diseases, some forms of epilepsy, sleep disorders, pain and possibly cancer. This review focuses on novel research approaches to discover potent and selective T-channel modulators. These molecules may be potential drugs for treating human diseases, as well as important tools to decipher the physiological role of these channels.