Effect of ABT-639 on Cav3.2 channel activity and its analgesic actions in mouse models of inflammatory and neuropathic pain

Cav3.2 T-type calcium channels are important targets for pain relief in rodent models of inflammatory and neuropathic pain. Even though many T-type channel blockers have been tested in mice, only one molecule, ABT-639, has been tested in phase II clinical studies and did not produce analgesic effects over placebo. Here we examined the effects of ABT-639 on Cav3.2 channel activity in tsA-201 cells and dorsal root ganglion (DRG) neurons, in comparison with another established Cav3.2 inhibitor Z944. These experiments revealed that Z944 mediated ∼100-fold more potent inhibition of Cav3.2 currents than ABT-639, with the latter blocking channel activity by less than 15 percent when applied at a concentration of 30 μM. A slight increase in ABT-639 potency was observed at more depolarized holding potentials, suggesting that this compound may act preferentially on inactivated channels. We tested the effects of both compounds in the Complete Freund's Adjuvant (CFA) model of chronic inflammatory pain, and in partial sciatic nerve injury model of neuropathic pain in mice. In the neuropathic pain model, both Z944 and ABT-639 reversed mechanical hypersensitivity to similar degrees when delivered systemically, but remarkably, when delivered intrathecally, only Z944 was effective. In the CFA model, both compounds reversed thermal hyperalgesia upon systemic delivery, but only Z944 mediated pain relief upon intrathecal delivery, indicating that ABT-639 acts primarily at peripheral sites. ABT-639 lost its analgesic effects in CFA treated Cav3.2 null mice, indicating that these channels are essential for ABT-639-mediated pain relief despite its poor inhibition of Cav3.2 currents.

The role of HCN channels on the effects of T-type calcium channels and GABAA receptors in the absence epilepsy model of WAG/Rij rats

In this study we used ivabradine (IVA), a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker, to identify its effect on spike-wave discharges (SWDs); and aimed to determine the role of IVA on the effects of T-type calcium channel blocker NNC 55-0396, GABAA receptor agonist muscimol and antagonist bicuculline in male WAG/Rij rats. After tripolar electrodes for electrocorticogram (ECoG) recordings were placed on the WAG/Rij rats' skulls, 5, 10, and 20 mg/kg IVA were intraperitoneally administered for 7 consecutive days and ECoG recordings were obtained on days 0th, 3rd, 6th, and 7th for three hours before and after injections. While acute injection of 5, 10, and 20 mg/kg IVA did not affect the total number and the mean duration of SWDs, subacute administration (7 days) of IVA decreased the SWDs parameters 24 hours after the 7th injection. Interestingly, when IVA was administered again 24 hours after the 6th IVA injection, it increased the SWDs parameters. Western-blot analyses showed that HCN1 and HCN2 expressions decreased and HCN4 increased in the 5-month-old WAG/Rij rats compared to the 1-month-old WAG/Rij and 5-month-old native Wistar rats, while subacute IVA administration increased the levels of HCN1 and HCN2 channels, except HCN4. Subacute administration of IVA reduced the antiepileptic activity of NNC, while the proepileptic activity of muscimol and the antiepileptic activity of bicuculline were abolished. It might be suggested that subacute IVA administration reduces absence seizures by changing the HCN channel expressions in WAG/Rij rats, and this affects the T-type calcium channels and GABAA receptors.

KTt-45, a T-type calcium channel blocker, acts as an anticancer agent by inducing apoptosis on HeLa cervical cancer cell line

The abnormal expression in the T-type calcium channels is involved in various cancer types, thus inhibiting T-type calcium channels is one of approaches in cancer treatment. The fact that KTt-45 acted as a T-type calcium channel inhibitor as well as a pain-relief agent prompts us to address if KTt-45 plays any role against cancer cells. The results showed that KTt-45 caused cytotoxic effects towards HeLa cervical, Raji lymphoma, MCF-7 breast cancer, and A549 lung cancer cell lines with IC50 values less than 100 μM, in which highly selective toxicity was against HeLa cells (IC50 = 37.4 μM, SI > 3.2). Strikingly, the KTt-45 induced an accumulation of cytoplasmic vacuoles after 48 h treatment and mitochondrial-dependent apoptosis activation as evidenced by morphological features, chromatin condensation, nuclear fragmentation, and significant activation of caspase-9 as well as caspase-3. In conclusion, KTt-45 could inhibit cell growth and trigger mitochondrial-dependent apoptosis in HeLa cervical cancer cells. The results, taken together, strongly demonstrated that KTt-45 is a potential agent for further study on anticancer drug development which not only targets cancer cells but also helps to relieve neuropathic pain in cancer patients.

Direct Current Stimulation Modulates Synaptic Facilitation via Distinct Presynaptic Calcium Channels

Transcranial direct current stimulation (tDCS) is a subthreshold neurostimulation technique known for ameliorating neuropsychiatric conditions. The principal mechanism of tDCS is the differential polarization of subcellular neuronal compartments, particularly the axon terminals that are sensitive to external electrical fields. Yet, the underlying mechanism of tDCS is not fully clear. Here, we hypothesized that direct current stimulation (DCS)-induced modulation of presynaptic calcium channel conductance alters axon terminal dynamics with regard to synaptic vesicle release. To examine the involvement of calcium-channel subtypes in tDCS, we recorded spontaneous excitatory postsynaptic currents (sEPSCs) from cortical layer-V pyramidal neurons under DCS while selectively inhibiting distinct subtypes of voltage-dependent calcium channels. Blocking P/Q or N-type calcium channels occluded the effects of DCS on sEPSCs, demonstrating their critical role in the process of DCS-induced modulation of spontaneous vesicle release. However, inhibiting T-type calcium channels did not occlude DCS-induced modulation of sEPSCs, suggesting that despite being active in the subthreshold range, T-type calcium channels are not involved in the axonal effects of DCS. DCS modulates synaptic facilitation by regulating calcium channels in axon terminals, primarily via controlling P/Q and N-type calcium channels, while T-type calcium channels are not involved in this mechanism.

Electric field stimulation unmasks a subtle role for T-type calcium channels in regulating lymphatic contraction

We previously identified two isoforms of T-type, voltage-gated calcium (Cav3) channels (Cav3.1, Cav3.2) that are functionally expressed in murine lymphatic muscle cells; however, contractile tests of lymphatic vessels from single and double Cav3 knock-out (DKO) mice, exhibited nearly identical parameters of spontaneous twitch contractions as wild-type (WT) vessels, suggesting that Cav3 channels play no significant role. Here, we considered the possibility that the contribution of Cav3 channels might be too subtle to detect in standard contraction analyses. We compared the sensitivity of lymphatic vessels from WT and Cav3 DKO mice to the L-type calcium channel (Cav1.2) inhibitor nifedipine and found that the latter vessels were significantly more sensitive to inhibition, suggesting that the contribution of Cav3 channels might normally be masked by Cav1.2 channel activity. We hypothesized that shifting the resting membrane potential (Vm) of lymphatic muscle to a more negative voltage might enhance the contribution of Cav3 channels. Because even slight hyperpolarization is known to completely silence spontaneous contractions, we devised a method to evoke nerve-independent, twitch contractions from mouse lymphatic vessels using single, short pulses of electric field stimulation (EFS). TTX was present throughout to block the potential contributions of voltage-gated Na+ channels in perivascular nerves and lymphatic muscle. In WT vessels, EFS evoked single contractions that were comparable in amplitude and degree of entrainment to those occurring spontaneously. When Cav1.2 channels were blocked or deleted, only small residual EFS-evoked contractions (~ 5% of normal amplitude) were present. These residual, EFS-evoked contractions were enhanced (to 10-15%) by the KATP channel activator pinacidil (PIN) but were absent in Cav3 DKO vessels. Our results point to a subtle contribution of Cav3 channels to lymphatic contractions that can be unmasked in the absence of Cav1.2 channel activity and when the resting Vm is more hyperpolarized than normal.

Adiponectin receptor 1-mediated stimulation of Cav3.2 channels in trigeminal ganglion neurons induces nociceptive behaviors in mice

BACKGROUND

Adipokines, including adiponectin, are implicated in nociceptive pain; however, the underlying cellular and molecular mechanisms remain unknown.

METHODS

Using electrophysiological recording, immunostaining, molecular biological approaches and animal behaviour tests, we elucidated a pivotal role of adiponectin in regulating membrane excitability and pain sensitivity by manipulating Cav3.2 channels in trigeminal ganglion (TG) neurons.

RESULTS

Adiponectin enhanced T-type Ca2+ channel currents (IT) in TG neurons through the activation of adiponectin receptor 1 (adipoR1) but independently of heterotrimeric G protein-mediated signaling. Coimmunoprecipitation revealed a physical association between AdipoR1 and casein kinase II alpha-subunits (CK2α) in the TG, and inhibiting CK2 activity by chemical inhibitor or siRNA targeting CK2α prevented the adiponectin-induced IT response. Adiponectin significantly activated protein kinase C (PKC), and this effect was abrogated by CK2α knockdown. Adiponectin increased the membrane abundance of PKC beta1 (PKCβ1). Blocking PKCβ1 pharmacologically or genetically abrogated the adiponectin-induced IT increase. In heterologous expression systems, activation of adipoR1 induced a selective enhancement of Cav3.2 channel currents, dependent on PKCβ1 signaling. Functionally, adiponectin increased TG neuronal excitability and induced mechanical pain hypersensitivity, both attenuated by T-type channel blockade. In a trigeminal neuralgia model induced by chronic constriction injury of infraorbital nerve, blockade of adipoR1 signaling suppressed mechanical allodynia, which was prevented by silencing Cav3.2.

CONCLUSION

Our study elucidates a novel signaling cascade wherein adiponectin stimulates TG Cav3.2 channels via adipoR1 coupled to a novel CK2α-dependent PKCβ1. This process induces neuronal hyperexcitability and pain hypersensitivity. Insight into adipoR-Cav3.2 signaling in sensory neurons provides attractive targets for pain treatment.

Structurally modified Cyclovirobuxine-D Buxus alkaloids as effective analgesic agents through Cav3.2 T-Type calcium channel inhibition

Cyclovirobuxine-D (CVB-D) is a Buxus alkaloid and a major active constituent in the Chinese medicinal herb Buxus microphylls. Traditionally, the natural alkaloid cyclovirobuxine-D has a long history of use as a traditional Chinese medicine for cardiovascular diseases as well as to treat a wide variety of medical conditions. As we found that CVB-D inhibited T-type calcium channels, we designed and synthesized a variety of fragments and analogues and evaluated them for the first time as new Cav3.2 inhibitors. Compounds 2-7 exhibited potency against Cav 3.2 channels, and two of them were more active than their parent molecules. As a result of the in vivo experiments, both compounds 3 and 4 showed significantly reduced writhes in the acetic acid-induced writhing test. Studies of molecular modeling have identified possible mechanism(s) of Cav3.2 binding. Moreover, the relationship between structure and activity was studied in a preliminary manner. Our results indicated that compounds 3 and 4 could play an important role in the discovery and development of novel analgesics.

Sex-specific hypnotic effects of the neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile are mediated by peripheral metabolism into an active hypnotic steroid

BACKGROUND

The novel synthetic neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH) blocks T-type calcium channels but does not directly modulate neuronal γ-aminobutyric acid type A (GABAA) currents like other anaesthetic neurosteroids. As 3β-OH has sex-specific hypnotic effects in adult rats, we studied the mechanism contributing to sex differences in its effects.

METHODS

We used a combination of behavioural loss of righting reflex, neuroendocrine, pharmacokinetic, in vitro patch-clamp electrophysiology, and in vivo electrophysiological approaches in wild-type mice and in genetic knockouts of the CaV3.1 T-type calcium channel isoform to study the mechanisms by which 3β-OH and its metabolite produces sex-specific hypnotic effects.

RESULTS

Adult male mice were less sensitive to the hypnotic effects of 3β-OH compared with female mice, and these differences appeared during development. Adult males had higher 3β-OH brain concentrations despite being less sensitive to its hypnotic effects. Females metabolised 3β-OH into the active GABAA receptor positive allosteric modulator (3α,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3α-OH) to a greater extent than males. The 3α-OH metabolite has T-channel blocking properties with sex-specific hypnotic and pharmacokinetic effects. Sex-dependent suppression of the cortical electroencephalogram is more pronounced with 3α-OH compared with 3β-OH.

CONCLUSIONS

The sex-specific differences in the hypnotic effect of 3β-OH in mice are attributable to differences in its peripheral metabolism into the more potent hypnotic metabolite 3α-OH.

Dietary Zinc Deficiency Induces Cav3.2-Dependent Nociceptive Hypersensitivity in Mice

Cav3.2 channels belong to the T-type calcium channel (T-channel) family, i.e., low voltage-activated calcium channels, and are abundantly expressed in the nociceptors, playing a principal role in the development of pathological pain. The channel activity of Cav3.2 is suppressed by zinc under physiological conditions. We thus tested whether dietary zinc deficiency would cause Cav3.2-dependent nociceptive hypersensitivity in mice. In the mice fed with zinc deficient diet for 2 weeks, plasma zinc levels declined by more than half, and mechanical allodynia developed. The dietary zinc deficiency-induced allodynia was restored by T-channel inhibitors or by Cav3.2 gene silencing. These data demonstrate that zinc deficiency induces Cav3.2-dependent nociceptive hypersensitivity in mice, thereby suggesting that pain experienced by patients with diseases accompanied by zinc deficiency (e.g., chronic kidney disease) might involve the increased Cav3.2 activity.

Reduction of neuronal hyperexcitability with modulation of T-type calcium channel or SK channel in essential tremor

Essential tremor is one of the most prevalent movement disorders. Propranolol and primidone are the first-line pharmacological therapies. They provide symptomatic control in less than 50% of patients. Topiramate, alprazolam, clonazepam, gabapentin, and botulinum toxin injections are the next line of treatments. These medications lead to modest improvements and are therefore commonly used as add-on agents. Surgical therapies, including deep brain stimulation (DBS) surgery and focused ultrasound beam targeted to the thalamus, are considered for treating tremor refractory to medications and lead to greater than 75% improvements in tremor symptoms. However, DBS is a costly and an invasive procedure; some patients report tolerance to benefits. Focused ultrasound therapy leading to brain lesions is associated with a possibility for permanent clinical deficits. Therefore, research efforts to develop the next generation of oral medications with greater benefits and lesser adverse effects are warranted. There is considerable evidence that the increased functions of calcium channels (P/Q-type and T-type channels) and reduced functions of calcium-activated potassium channels (SK channels) located in the neuronal membranes lead to tremor oscillations. Consequently, many new pharmacological studies have targeted these channels to leverage better clinical outcomes. The current review will discuss the pathophysiology, the specific importance of these channels, and the early clinical experience of using compounds targeting these channels to treat essential tremor.

Trigeminal neuropathic pain is alleviated by inhibition of Cav3.3 T-type calcium channels in mice

In this brief report, we demonstrate that the Cav3.3 T-type voltage-gated calcium channel subtype is involved in our FRICT-ION model of chronic trigeminal neuropathic pain. We first showed that the Cacna1i gene encoding Cav3.3 is significantly upregulated in whole trigeminal ganglia of FRICT-ION mice compared to controls at week 10 post-injury. We confirmed protein upregulation of Cav3.3 compared to controls using Western blot analysis of whole trigeminal ganglia tissues. Finally, we demonstrated that intraperitoneal injection of a selective TAT-based Cav3.3 blocking peptide in FRICT-ION mice significantly reduces Cav3.3 protein expression at the peak anti-allodynic effect (4 hrs post-injection) of the attenuated neuropathic pain behavior. We also suggest that blockade of Cav3.3 may be more effective in attenuating trigeminal neuropathic pain in female than male FRICT-ION mice. Therefore, blocking or attenuating Cav3.3 function may be an effective strategy for the treatment of trigeminal neuropathic pain.

Effects of a patient-derived de novo coding alteration of CACNA1I in mice connect a schizophrenia risk gene with sleep spindle deficits

CACNA1I, a schizophrenia risk gene, encodes a subtype of voltage-gated T-type calcium channel CaV3.3. We previously reported that a patient-derived missense de novo mutation (R1346H) of CACNA1I impaired CaV3.3 channel function. Here, we generated CaV3.3-RH knock-in animals, along with mice lacking CaV3.3, to investigate the biological impact of R1346H (RH) variation. We found that RH mutation altered cellular excitability in the thalamic reticular nucleus (TRN), where CaV3.3 is abundantly expressed. Moreover, RH mutation produced marked deficits in sleep spindle occurrence and morphology throughout non-rapid eye movement (NREM) sleep, while CaV3.3 haploinsufficiency gave rise to largely normal spindles. Therefore, mice harboring the RH mutation provide a patient derived genetic model not only to dissect the spindle biology but also to evaluate the effects of pharmacological reagents in normalizing sleep spindle deficits. Importantly, our analyses highlighted the significance of characterizing individual spindles and strengthen the inferences we can make across species over sleep spindles. In conclusion, this study established a translational link between a genetic allele and spindle deficits during NREM observed in schizophrenia patients, representing a key step toward testing the hypothesis that normalizing spindles may be beneficial for schizophrenia patients.

[Rebound depolarization of substantia gelatinosa neurons and its modulatory mechanisms in rat spinal dorsal horn]

OBJECTIVE

To investigate the rebound depolarization of substantia gelatinosa (SG) neurons in rat spinal dorsal horn and explore its modulatory mechanisms to provide better insights into rebound depolarization-related diseases.

METHODS

Parasagittal slices of the spinal cord were prepared from 3- to 5-week-old Sprague-Dawley rats. The electrophysiologic characteristics and responses to hyperpolarization stimulation were recorded using whole-cell patch-clamp technique. The effects of hyperpolarization-activated cyclic nucleotide gated cation (HCN) channel blockers and T-type calcium channel blockers on rebound depolarization of the neurons were studied.

RESULTS

A total of 63 SG neurons were recorded. Among them, 23 neurons showed no rebound depolarization, 19 neurons showed rebound depolarization without spikes, and 21 neurons showed rebound depolarization with spikes. The action potential thresholds of the neurons without rebound depolarization were significantly higher than those of the neurons with rebound depolarization and spikes (-28.7∓1.6 mV vs -36.0∓2.0 mV, P<0.05). The two HCN channel blockers CsCl and ZD7288 significantly delayed the latency of rebound depolarization with spike from 45.9∓11.6 ms to 121.6∓51.3 ms (P<0.05) and from 36.2∓10.3 ms to 73.6∓13.6 ms (P<0.05), respectively. ZD7288 also significantly prolonged the latency of rebound depolarization without spike from 71.9∓35.1 ms to 267.0∓68.8 ms (P<0.05). The T-type calcium channel blockers NiCl2 and mibefradil strongly decreased the amplitude of rebound depolarization with spike from 19.9∓6.3 mV to 9.5∓4.5 mV (P<0.05) and from 26.1∓9.4 mV to 15.5∓5.0 mV (P<0.05), respectively. Mibefradil also significantly decreased the amplitude of rebound depolarization without spike from 14.3∓3.0 mV to 7.9∓2.0 mV (P<0.05).

CONCLUSION

Nearly two-thirds of the SG neurons have rebound depolarizations modulated by HCN channel and T-type calcium channel.

[Protection of T-type calcium channel blocker in spiral ganglion neurons of adult C57BL/6J mice]

OBJECTIVE

To explore the distribution and expression of three T-type calcium channel receptors (α1G; α1H; α1I) and understand their protective effects in spiral neurons of C57BL/6J mice.

METHODS

The distribution and expression of three T-type calcium channel receptors in spiral ganglion neurons were observed by in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR) in 6-8-week-old C57BL/6J mice. The mice of 24-26-week-old C57BL/6J were divided into 3 groups of zonisamide, benidipine and saline. And the expression changes of calcium-binding proteins calmodulin and calbindin were observed by immunohistochemistry.

RESULTS

Three subunits were expressed in spiral ganglion neurons. The decremented quantities were α1H (24.21 ± 0.10), α1I (14.88 ± 0.04) and α1G (10.42 ± 0.02). The expression level of calmodulin in spiral ganglion neurons was lower in the zonisamide-treated group than that in the saline-treated group (0.336 ± 0.041 vs 0.504 ± 0.020, P < 0.05). The expression level of calbindin in spiral ganglion neurons was lower in the zonisamide (0.482 ± 0.045) and benidipine-treated groups (0.511 ± 0.032) than that in the saline-treated group (0.611 ± 0.035, P < 0.05).

CONCLUSION

The expressions of calcium-binding proteins decrease after 4-week dosing of T-type calcium channel blockers in 24-26-week C57BL/6J mice. It implies a relief of calcium overload. T-type calcium channel blockers may protect the murine spiral ganglion neurons from degeneration.

Genome-wide association study implicates HLA-C*01:02 as a risk factor at the major histocompatibility complex locus in schizophrenia

BACKGROUND

We performed a genome-wide association study (GWAS) to identify common risk variants for schizophrenia.

METHODS

The discovery scan included 1606 patients and 1794 controls from Ireland, using 6,212,339 directly genotyped or imputed single nucleotide polymorphisms (SNPs). A subset of this sample (270 cases and 860 controls) was subsequently included in the Psychiatric GWAS Consortium-schizophrenia GWAS meta-analysis.

RESULTS

One hundred eight SNPs were taken forward for replication in an independent sample of 13,195 cases and 31,021 control subjects. The most significant associations in discovery, corrected for genomic inflation, were (rs204999, p combined = 1.34 × 10(-9) and in combined samples (rs2523722 p combined = 2.88 × 10(-16)) mapped to the major histocompatibility complex (MHC) region. We imputed classical human leukocyte antigen (HLA) alleles at the locus; the most significant finding was with HLA-C*01:02. This association was distinct from the top SNP signal. The HLA alleles DRB1*03:01 and B*08:01 were protective, replicating a previous study.

CONCLUSIONS

This study provides further support for involvement of MHC class I molecules in schizophrenia. We found evidence of association with previously reported risk alleles at the TCF4, VRK2, and ZNF804A loci.

[Calcium antagonists: current and future applications based on new evidence. Cardio-protective effect of calcium antagonists]

Calcium antagonists block calcium influx through the L-type calcium channel to produce dilatation of resistant arteries including coronary arteries. Due to arterial vasodilatation, calcium antagonists have beneficial effects for ischemic heart disease, left ventricular hypertrophy and heart failure. Some of calcium antagonists are known to have the inhibitory action for T-type or N-type calcium channel which might be beneficial for the inhibition of reactive tachycardia. Calcium antagonists may also retard the progression of atherosclerosis to prevent cardiovascular events.

Mutational analysis ofCACNA1Gin idiopathic generalized epilepsy

Recent studies have strongly implicated low voltage-activated/T-type calcium channels (T-channels) in the etiology of epilepsy. Here, we report the results of a mutational analysis of the CACNA1G gene, encoding the T-channel Ca(V)3.1/(1G) subunit, using a cohort of 123 mostly Japanese and Hispanic patients with idiopathic generalized epilepsies (IGE) and 360 healthy control individuals. We found 13 variants, including five which involved amino acid substitutions. One variant, c.1709C>T (Ala570Val), is present in a sporadic case of juvenile myoclonic epilepsy (JME) with early childhood absence and astatic seizures, but was not found in control samples. Another variant, c.3265G>T (Ala1089Ser), was observed in three family members affected with JME, and also in one control individual. Two JME patients and three control individuals harbored a third variant, c.2968G>A (Asp980Asn). Although not statistically significant, slightly faster inactivation decay rates were observed in some mutant channels. Our collective findings flag CACNA1G as a potential susceptibility locus for IGE subsyndromes that warrants closer investigation.

Growth inhibition of human cancer cells in vitro by T-type calcium channel blockers

This paper describes the preliminary biological results that novel T-type calcium channel blockers inhibit the growth of human cancer cells by blocking calcium influx into the cell, based on unknown mechanism on the cell cycle responsible for cellular proliferation. Among the selected compounds from compound library, compound 9c (KYS05041) was identified to be nearly equipotent with Cisplatin against some human cancers in the micromolar range.

Randomized comparison of T-type versus L-type calcium-channel blockade on exercise duration in stable angina: results of the Posicor Reduction of Ischemia During Exercise (PRIDE) trial

BACKGROUND

Mibefradil is a T-type calcium-channel antagonist and arterial vasodilator with negative chronotropic effects. It is not known if T-type calcium-channel blockade is superior to L-type calcium-channel blockade in patients with stable angina pectoris.

METHODS

A multicenter, randomized, double-blind trial was conducted in patients with documented coronary disease and stable angina to compare a 360 mg dose of diltiazem CD with 100 mg dose of mibefradil. The primary end point was change in time to symptom-limited exercise termination from baseline to 8 weeks. Secondary efficacy parameters included time to onset of persistent ST-segment depression, time to awareness of angina, and change in exercise duration from baseline to 2 and 4 weeks of treatment.

RESULTS

A total of 121 patients were randomized to mibefradil and 113 to diltiazem CD. At 8 weeks, the increase in exercise duration was 24.5 seconds greater in the mibefradil group (P =.017; 95% CI 4.4-44.7 seconds). At 8 weeks, time to development of > or =1 mm ST-segment depression was greater by 45.3 seconds (P =.0025; 95% CI 16.2-74.5) with mibefradil, but time to development of angina was not significantly different.

CONCLUSION

T-type calcium-channel antagonism with mibefradil improved treadmill exercise parameters compared with diltiazem in patients with chronic stable angina. Further investigation and development of antagonists of T-type calcium channels with fewer adverse drug interactions is warranted and may be promising in the management of ischemic heart disease.

Age-dependent variation in contractility of adult cardiac myocytes

This study was designed to examine the influence of the age of adults on the contractile characteristics of the myocardium and to ascertain whether the age dependent variation is related to variation in sarcolemmal calcium channels. Cardiomyocytes were isolated from 2, 6 and 12-month-old, male Sprague-Dawley rats and the extent and velocity of contraction were recorded as a function of change in cell length. Age dependent increase in cell length and sarcomere length was significant (P<0.05). Extent of contraction increased with age and the velocities of contraction and relaxation normalized to total contraction decreased with age (P<0.05). Sensitivity to the L-type channel antagonist (verapamil, 1 microM) and the T-type channel antagonist (nickel chloride, 40 microM) was significant in 6 and 12-month-old animals. This differential response to calcium channel antagonists suggests that the age-dependent variation in contractility may be mediated by the variation in the distribution/function of sarcolemmal calcium channels.

Expression of T-type Ca(2+) channels in ventricular cells from hypertrophied rat hearts

In this study we examined the existence of T-type Ca(2+) current in ventricular myocytes isolated from rats with pressure-overload hypertrophy. The whole-cell clamp technique was used to record Ca(2+) currents in enzymatically dissociated ventricular cells. T- and L-type Ca(2+) currents were separated by applying voltage steps to different test potentials from a holding potential of -80 mV and -50 mV. T-type Ca(2+) current was defined as the difference between the currents from the two holding potentials. Ventricular myocytes from sham-operated rats showed only L-type Ca(2+) current (maximal density -13.9+/-1.3 pA/pF n=17), whereas ventricular myocytes isolated from rats with aortic stenosis showed both L- and T-type Ca(2+) currents. The average values of T- and L-type Ca(2+) current density were -4.8+/-0.4 pA/pF and -12.4+/-0.9 pA/pF (n=32), respectively. T-type Ca(2+) current was distinguished from L-type Ca(2+) current by its voltage dependence, its kinetics and by its strong blockade by nickel 50 microM. In conclusion, we have demonstrated that hypertrophied ventricular rat cells express T-type Ca(2+) channels and this finding strongly supports a role for this channel in regulating growth processes in cardiac tissue.

The effect of alpha2-delta and other accessory subunits on expression and properties of the calcium channel alpha1G

1. The effect has been examined of the accessory alpha2-delta and beta subunits on the properties of alpha1G currents expressed in monkey COS-7 cells and Xenopus oocytes. 2. In immunocytochemical experiments, the co-expression of alpha2-delta increased plasma membrane localization of expressed alpha1G and conversely, the heterologous expression of alpha1G increased immunostaining for endogenous alpha2-delta, suggesting an interaction between the two subunits. 3. Heterologous expression of alpha2-delta together with alpha1G in COS-7 cells increased the amplitude of expressed alpha1G currents by about 2-fold. This finding was confirmed in the Xenopus oocyte expression system. The truncated delta construct did not increase alpha1G current amplitude, or increase its plasma membrane expression. This indicates that it is the exofacial alpha2 domain that is involved in the enhancement by alpha2-delta. 4. Beta1b also produced an increase of functional expression of alpha1G, either in the absence or the presence of heterologously expressed alpha2-delta, whereas the other beta subunits had much smaller effects. 5. None of the accessory subunits had any marked influence on the voltage dependence or kinetics of the expressed alpha1G currents. These results therefore suggest that alpha2-delta and beta1b interact with alpha1G to increase trafficking of, or stabilize, functional alpha1G channels expressed at the plasma membrane.

Multiple structural domains contribute to voltage-dependent inactivation of rat brain alpha(1E) calcium channels

We have investigated the molecular determinants that mediate the differences in voltage-dependent inactivation properties between rapidly inactivating (R-type) alpha(1E) and noninactivating (L-type) alpha(1C) calcium channels. When coexpressed in human embryonic kidney cells with ancillary beta(1b) and alpha(2)-delta subunits, the wild type channels exhibit dramatically different inactivation properties; the half-inactivation potential of alpha(1E) is 45 mV more negative than that observed with alpha(1C), and during a 150-ms test depolarization, alpha(1E) undergoes 65% inactivation compared with only about 15% for alpha(1C). To define the structural determinants that govern these intrinsic differences, we have created a series of chimeric calcium channel alpha(1) subunits that combine the major structural domains of the two wild type channels, and we investigated their voltage-dependent inactivation properties. Each of the four transmembrane domains significantly affected the half-inactivation potential, with domains II and III being most critical. In particular, substitution of alpha(1C) sequence in domains II or III with that of alpha(1E) resulted in 25-mV negative shifts in half-inactivation potential. Similarly, the differences in inactivation rate were predominantly governed by transmembrane domains II and III and to some extent by domain IV. Thus, voltage-dependent inactivation of alpha(1E) channels is a complex process that involves multiple structural domains and possibly a global conformational change in the channel protein.

Distribution of the voltage-dependent calcium channel alpha1G subunit mRNA and protein throughout the mature rat brain

The molecular identity of a gene which encodes the pore-forming subunit (alpha1G) of a member of the family of low-voltage-activated, T-type, voltage-dependent calcium channels has been described recently. Although northern mRNA analyses have shown alpha1G to be expressed predominantly in the brain, the detailed cellular distribution of this protein in the central nervous system (CNS) has not yet been reported. The current study describes the preparation of a subunit specific alpha1G riboprobe and antiserum which have been used in parallel in situ mRNA hybridization and immunohistochemical studies to localize alpha1G in the mature rat brain. Both alpha1G mRNA and protein were widely distributed throughout the brain, but variations were observed in the relative level of expression in discrete nuclei. Immunoreactivity for alpha1G was typically localized in both the soma and dendrites of many neurons. Whilst alpha1G protein and mRNA expression were often observed in cells known to exhibit T-type current activity, some was also noted in regions, e.g. cerebellar granule cells, in which T-type activity has not been described. These observations may reflect differences between the subcellular distribution of channels that can be identified by immunohistochemical methods compared with electrophysiological techniques.

State-dependent inactivation of the alpha1G T-type calcium channel

We have examined the kinetics of whole-cell T-current in HEK 293 cells stably expressing the alpha1G channel, with symmetrical Na(+)(i) and Na(+)(o) and 2 mM Ca(2+)(o). After brief strong depolarization to activate the channels (2 ms at +60 mV; holding potential -100 mV), currents relaxed exponentially at all voltages. The time constant of the relaxation was exponentially voltage dependent from -120 to -70 mV (e-fold for 31 mV; tau = 2.5 ms at -100 mV), but tau = 12-17 ms from-40 to +60 mV. This suggests a mixture of voltage-dependent deactivation (dominating at very negative voltages) and nearly voltage-independent inactivation. Inactivation measured by test pulses following that protocol was consistent with open-state inactivation. During depolarizations lasting 100-300 ms, inactivation was strong but incomplete (approximately 98%). Inactivation was also produced by long, weak depolarizations (tau = 220 ms at -80 mV; V(1/2) = -82 mV), which could not be explained by voltage-independent inactivation exclusively from the open state. Recovery from inactivation was exponential and fast (tau = 85 ms at -100 mV), but weakly voltage dependent. Recovery was similar after 60-ms steps to -20 mV or 600-ms steps to -70 mV, suggesting rapid equilibration of open- and closed-state inactivation. There was little current at -100 mV during recovery from inactivation, consistent with Collapse

Key Words

• t-channel
• cumulative inactivation
• recovery from inactivation

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MESH Headings

• Algorithms
• Animals
• Calcium Channels/physiology
• Calcium Channels, T-Type
• Electric Stimulation
• Electrophysiology
• Ion Channel Gating/physiology
• Kinetics
• Membrane Potentials/physiology
• Models, Neurological
• Patch-Clamp Techniques
• Rats
• Sodium/metabolism
• Tumor Cells, Cultured

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Grants

• R01 NS024471 NINDS NIH HHS
• HL58728 NHLBI NIH HHS
• NS24471 NINDS NIH HHS

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Role of calcium electrogenesis in apical dendrites: generation of intrinsic oscillations by an axial current

Dendrites are covered with conductances whose function is still mysterious. Using intracellular recording and calcium imaging, we describe an electrogenic band of calcium channels in distal apical dendrites of layer 5 pyramidal neurons (Yuste et al., 1994). We now explore the functional consequences of this distal electrogenic area with multicompartmental numerical simulations. A calcium imaging and electrophysiological database from a single neuron, recorded under blocked sodium and potassium conductances, is replicated by simulations having increased dendritic calcium current. In these models a significant axial current flows from the apical dendrite into the somatic region, activating low-threshold calcium channels and generating oscillations similar to those seen in the electrophysiological data. We propose that the distal electrogenic area in apical dendrites serves to inject current into the soma and produce intrinsic oscillatory dynamics.

All thalamocortical neurones possess a T-type Ca2+ 'window' current that enables the expression of bistability-mediated activities

1. The existence of a non-negligible steady-state ('window') component of the low threshold, T-type Ca2+current (IT) and an appropriately large ratio of IT to ILeak conductance (i.e. gT/gLeak) have been shown to underlie a novel form of intrinsic bistability that is present in about 15 % of thalamocortical (TC) neurones. 2. In the present experiments, the dynamic clamp technique was used to introduce into mammalian TC neurones in vitro either an artificial, i.e. computer-generated, IT in order to enhance endogenous IT, or an artificial inward ILeak to decrease endogenous ILeak. Using this method, we were able to investigate directly whether the majority of TC neurones appear non-bistable because their intrinsic ionic membrane properties are essentially different (i.e. presence of a negligible IT 'window' component), or simply because they possess a gT or gLeak conductance that is insufficiently large or small, respectively. 3. The validity of the dynamic clamp arrangement and the accuracy of artificial IT were confirmed by (i) recreating the low threshold calcium potential (LTCP) with artificial IT following its block by Ni2+ (0.5-1 mM), and (ii) blocking endogenous LTCPs with an artificial outward IT. 4. Augmentation of endogenous IT by an artificial analog or introduction of an artificial inward ILeak transformed all non-bistable TC neurones to bistable cells that expressed the full array of bistability-mediated behaviours, i.e. input signal amplification, slow oscillatory activity and membrane potential bistability. 5. These results demonstrate the existence of a non-negligible IT 'window' component in all TC neurones and suggest that rather than being a novel group of neurones, bistable cells are merely representative of an interesting region of dynamical modes in the (gT, gLeak) parameter space that may be expressed under certain physiological or pathological conditions by all TC neurones and other types of excitable cells that possess an IT 'window' component with similar biophysical properties.

Troglitazone inhibits voltage-dependent calcium currents in guinea pig cardiac myocytes

BACKGROUND

It has been suggested that intracellular Ca2+ overload in cardiac myocytes leads to the development of diabetic cardiomyopathy. Troglitazone, an insulin-sensitizing agent, is a promising therapeutic agent for diabetes and has been shown to prevent diabetes-induced myocardial changes. To elucidate the underlying mechanism of troglitazone action on cardiac myocytes, the effects of troglitazone on voltage-dependent Ca2+ currents were examined and compared with classic Ca2+ antagonists (verapamil and nifedipine).

METHODS AND RESULTS

Whole-cell voltage-clamp techniques were applied in single guinea pig atrial myocytes. Under control conditions with CsCl internal solution, the voltage-dependent Ca2+ currents consisted of both T-type (ICa,T) and L-type (ICa,L) Ca2+ currents. Troglitazone effectively reduced the amplitude of ICa,L in a concentration-dependent manner. Troglitazone also suppressed ICa,T, but the effect of troglitazone on ICa,T was less potent than that on ICa,L. The current-voltage relationships for ICa,L and the reversal potential for ICa,L were not altered by troglitazone. The half-maximal inhibitory concentration of troglitazone on ICa,L measured at a holding potential of -40 mV was 6.3 micromol/L, and 30 micromol/L troglitazone almost completely inhibited ICa,L. Troglitazone 10 micromol/L did not affect the time courses for inactivation of ICa,L and inhibited ICa,L mainly in a use-independent fashion, without shifting the voltage-dependency of inactivation. This effect was different from those of verapamil and nifedipine. Troglitazone also reduced isoproterenol- or cAMP-enhanced ICa,L.

CONCLUSIONS

These results demonstrate that troglitazone inhibits voltage-dependent Ca2+ currents (T-type and L-type) and then antagonizes the effects of isoproterenol in cardiac myocytes, thus possibly playing a role in preventing diabetes-induced intracellular Ca2+ overload and subsequent myocardial changes.

Different effects of L-type and T-type calcium channel blockers on the hypnotic potency of triazolam and zolpidem in rats

We examined the effects of an L-type Ca2+ channel blocker, nilvadipine (0.5 and 2.0 mg/kg), and that of a T-type Ca2+ channel blocker, flunarizine (10.0 and 40.0 mg/kg), on the hypnotic potency of both a benzodiazepine (BZ)-hypnotic, triazolam (1.0 mg/kg), and a non-BZ hypnotic, zolpidem (20.0 mg/kg), in rats. The polysomnogram was recorded for 6 h after administration of the vehicle solution alone, or after one of the Ca2+ channel blockers, with or without one of the hypnotics. Both Ca2+ channel blockers prolonged the increased total time of non-rapid eye movement (non-REM) sleep induced by either hypnotic. In the case of triazolam, however, the non-REM sleep-enhancing effect induced by nilvadipine was greater than that induced by flunarizine. These findings indicate that the hypnotic action of triazolam is potentiated more strongly by an L-type Ca2+ channel blocker than by a T-type Ca2+ channel blocker.

Morphological transformation induced by activation of the mitogen-activated protein kinase pathway requires suppression of the T-type Ca2+ channel

Transformation of fibroblasts by various oncogenes, including ras, mos, and src accompanies with characteristic morphological changes from flat to round (or spindle) shapes. Such morphological change is believed to play an important role in establishing malignant characteristics of cancer cells. Activation of the mitogen-activated protein kinase (MAPK) pathway is a converging downstream event of transforming activities of many oncogene products commonly found in human cancers. Intracellular calcium is known to regulate cellular morphology. In fibroblasts, Ca2+ influx is primarily controlled by two types of Ca2+ channels (T- and L-types). Here, we report that the T-type current was specifically inhibited in cells expressing oncogenically activated Ras as well as gain-of-function mutant MEK (MAPK/extracellular signal-regulated kinase (ERK) kinase, a direct activator of MAPK), whereas treatment of ras-transformed cells with a MEK-specific inhibitor restored T-type Ca2+ channel activity. Using a T-type Ca2+ channel antagonist, we further found that suppression of the T-type Ca2+ channel by the activated MAPK pathway is a prerequisite event for the induction and/or maintenance of transformation-associated morphological changes.

Comparative effects of mibefradil and other calcium antagonists on resistance arteries of different end organs

The biphasic contractile responses of rat isolated mesenteric, renal, coronary and basilar small arteries to potassium-induced depolarization were investigated. The tonic phase is assumed to be exclusively the result of L-type calcium channel (LCC) activation, whereas in the generation of the phasic phase T-type calcium channels (TCC) may be involved. In order to evaluate whether TCC blockade has any influence on depolarization-induced contractions the effects of the LCC antagonists nifedipine, diltiazem and verapamil were compared with those of the combined L- and TCC antagonist mibefradil. Small arteries (size 393.6 +/- 4.8 microns, n = 104) were dissected from the respective organs of male Wistar rats (300-350 g) and studied in an isometric wire myograph. The effects of increasing concentrations of the calcium antagonists on repetitive potassium-induced contractions were quantified by means of cumulative concentration-response curves. A comparison was made with mesenteric vessels of SHR and WKY for nifedipine and mibefradil. Nifedipine was the most potent compound in blocking both the phasic phase (reduction 66-77%) and the tonic phase (IC50 = 1.1-5.4 nM). The effect of mibefradil on the phasic response was comparable to that of verapamil and diltiazem. With respect to the tonic response mibefradil was comparable to verapamil (IC50 = 19.6-178.9 nM). These findings indicate that the TCC blockade does not contribute to the vasodilator effect of mibefradil under the conditions investigated.

The effects of verapamil and diltiazem on N-, P- and Q-type calcium channels mediating dopamine release in rat striatum

1. The putative inhibitory effects of verapamil and diltiazem on neuronal non-L-type Ca2+ channels were studied by investigating their effects on either K+- or veratridine-evoked [3H]-dopamine ([3H]-DA) release in rat striatal slices. Involvement of N-, P- and Q-type channels was identified by sensitivity of [3H]-DA release to omega-conotoxin GVIA (omega-CTx-GVIA), omega-agatoxin IVA (omega-Aga-IVA) and omega-conotoxin MVIIC (omega-CTx-MVIIC), respectively. 2. KCl (50 mM)-evoked [3H]-DA release was abolished in the absence of Ca2+, and was insensitive to dihydropyridines (up to 30 microM). It was significantly blocked by omega-CTx-GVIA (1 microM), omega-Aga-IVA (30 nM) and was confirmed to be abolished by omega-CTx-MVIIC (3 microM), indicating involvement of N-, P- and Q-type channel subtypes. 3. Verapamil and diltiazem inhibited K+-evoked [3H]-DA release in a concentration-dependent manner. The inhibitory effects of verapamil or diltiazem (each 30 microM) were fully additive to the effect of omega-CTx-GVIA (1 microM), whereas co-application with omega-Aga-IVA (30 nM) produced similar effects to those of omega-Aga-IVA alone. 4. As shown previously, veratridine-evoked [3H]-DA release in Ca2+ containing medium exclusively involves Q-type Ca2+ channels. Here, diltiazem (30 microM) did not inhibit veratridine-evoked [3H]-DA release, whereas verapamil (30 microM) partially inhibited it, indicating possible involvement of Q-type channels in verapamil-induced inhibition. However, verapamil (30 microM) inhibited this release even in the absence of extracellular Ca2+, suggesting that Na+ rather than Q-type Ca2+ channels are involved. 5. Taken together, our results suggest that verapamil can block P- and at higher concentrations possibly N- and Q-type Ca2+ channels linked to [3H]-DA release, whereas diltiazem appears to block P-type Ca2+ channels only.

A role for T-type Ca2+ channels in the synergistic control of aldosterone production by ANG II and K+

Independently, plasma K+ and ANG II stimulate aldosterone secretion from adrenal glomerulosa (AG) cells, but together they synergistically control production. We studied mechanisms to mediate this synergy using bovine AG cells studied under physiological conditions (in 1.25 mM Ca2+ at 37 degrees C). Increasing K+ from 2 to 5 mM caused a potentiation of ANG II-induced aldosterone secretion and a substantial membrane depolarization ( approximately 21 mV). ANG II inhibited a K+-selective conductance in both 2 and 5 mM K+ but caused only a slight depolarization because, under both conditions, membrane potential was close to the reversal potential of the ANG II-induced current. ANG II activated calcium/calmodulin-dependent protein kinase II (CaMKII) equivalently in 2 and 5 mM K+. However, CaMKII activation caused a hyperpolarizing shift in the activation of T-type Ca2+ channels, such that substantially more current was elicited at membrane potentials established by 5 mM K+. We propose that synergy in aldosterone secretion results from K+-induced depolarization and ANG II-induced modulation of T-type channel activation, such that together they promote enhanced steady-state Ca2+ flux.

Role of multiple calcium and calcium-dependent conductances in regulation of hippocampal dentate granule cell excitability

We have constructed a detailed model of a hippocampal dentate granule (DG) cell that includes nine different channel types. Channel densities and distributions were chosen to reproduce reported physiological responses observed in normal solution and when blockers were applied. The model was used to explore the contribution of each channel type to spiking behavior with particular emphasis on the mechanisms underlying postspike events. T-type calcium current in more distal dendrites contributed prominently to the appearance of the depolarizing after-potential, and its effect was controlled by activation of BK-type calcium-dependent potassium channels. Coactivation and interaction of N-, and/or L-type calcium and AHP currents present in somatic and proximal dendritic regions contributed to the adaptive properties of the model DG cell in response to long-lasting current injection. The model was used to predict changes in channel densities that could lead to epileptogenic burst discharges and to predict the effect of altered buffering capacity on firing behavior. We conclude that the clustered spatial distributions of calcium related channels, the presence of slow delayed rectifier potassium currents in dendrites, and calcium buffering properties, together, might explain the resistance of DG cells to the development of epileptogenic burst discharges.

Molecular characterization of two members of the T-type calcium channel family

In this chapter we review our recent studies on the cloning of two novel cDNAs (alpha 1G and alpha 1H), and present electrophysiological evidence that they encode low voltage-activated, T-type calcium channels (CavT.1 and CavT.2, respectively). The nucleotide sequences of these T channels are very different from high voltage-activated Ca2+ channels, which explains why they were not cloned earlier using homology-based strategies. We used a bioinformatic approach, cloning the first fragment in silico. We then used this fragment to screen human heart and rat brain lambda gt10 libraries, leading to the cloning of two full-length cDNAs derived from distinct genes (CACNA1G and CACNA1H). The deduced amino acid sequences of the T channels (alpha 1G and alpha 1H) are also very different from previously cloned Ca2+ and Na+ channels; however, there are regions of structural similarity. For example, the T channels also contain four repeats, and within each repeat there are six putative membrane-spanning regions and a pore loop. Expression of these cloned channels in either Xenopus oocytes or HEK-293 cells leads to the formation of typical T-type currents. As observed for native T currents, these channels activate at potentials near the resting membrane potential, inactivate rapidly, deactivate slowly, and have a tiny single-channel conductance. The currents generated by alpha 1G and alpha 1H are nearly identical in terms of their voltage dependence and kinetics. We present preliminary evidence that nickel may serve as a valuable tool in discriminating between these subtypes.

L- and T-type voltage-gated Ca2+ currents in adrenal medulla endothelial cells

We investigated voltage-dependent Ca2+ channels of bovine adrenal medulla endothelial cells with the whole cell version of the patch-clamp technique. Depolarization elicited an inward current that was carried by Ca2+ and was composed of a transient (T) current, present in all the cells tested, and a sustained (L) current, present in 65% of them. We separated these currents and measured their individual kinetic and gating properties. The activation threshold for T current was approximately -50 mV, and its maximum amplitude was -49.8 +/- 4.8 pA (means +/- SE, n = 19) at 0 mV. The time constant was 10.2 +/- 1.5 ms (n = 4) for activation and 18.4 +/- 2.8 ms (n = 4) for inactivation. The L current activated at -40 mV, and it reached a plateau at -20.1 +/- 2.3 pA (n = 6). Its activation time course was a single exponential with an activation time contant of 26.8 +/- 2.3 ms (n = 4). Current-voltage curves, kinetics, gating, response to BAY K 8644, nifedipine, amiloride, and different selectivity for Ba2+ and Ca2+ indicated that the underlying channels for the observed currents are only of the T- and L-types that resemble those of the endocrine secretory cells.

A T-type calcium channel from mouse brain

A member of the low-voltage-activated calcium channel family was identified in mouse brain by taking advantage of amino acid sequences that have been evolutionary conserved. The identified sequence is similar to that of the recently cloned rat alpha1G T-type calcium channel, but there are differences in two insertions in the intracellular connecting loops. Northern blot analysis indicates that its expression is strong in the brain. In situ hybridization revealed that, in mouse brain, the alpha1G mRNA is found in the cerebellum, hippocampus, thalamus and olfactory bulb. In contrast to L-type calcium channel currents, IBa and ICa through the alpha1G channel expressed in HEK293 cells did not differ in terms of current density, voltage dependence of current activation, inactivation and deactivation, and speed of recovery from voltage-dependent inactivation. The kinetics of ICa inactivation were significantly slower than those of IBa. The expressed alpha1G channel has a relatively high sensitivity to mibefradil, but is only slightly affected by Ni2+.

Differential distribution of three members of a gene family encoding low voltage-activated (T-type) calcium channels

Low voltage-activated (T-type) calcium currents are observed in many central and peripheral neurons and display distinct physiological and functional properties. Using in situ hybridization, we have localized central and peripheral nervous system expression of three transcripts (alpha1G, alpha1H, and alpha1I) of the T-type calcium channel family (CaVT). Each mRNA demonstrated a unique distribution, and expression of the three genes was largely complementary. We found high levels of expression of these transcripts in regions associated with prominent T-type currents, including inferior olivary and thalamic relay neurons (which expressed alpha1G), sensory ganglia, pituitary, and dentate gyrus granule neurons (alpha1H), and thalamic reticular neurons (alpha1I and alpha1H). Other regions of high expression included the Purkinje cell layer of the cerebellum, the bed nucleus of the stria terminalis, the claustrum (alpha1G), the olfactory tubercles (alpha1H and alpha1I), and the subthalamic nucleus (alpha1I and alpha1G). Some neurons expressed high levels of all three genes, including hippocampal pyramidal neurons and olfactory granule cells. Many brain regions showed a predominance of labeling for alpha1G, including the amygdala, cerebral cortex, rostral hypothalamus, brainstem, and spinal cord. Exceptions included the basal ganglia, which showed more prominent labeling for alpha1H and alpha1I, and the olfactory bulb, the hippocampus, and the caudal hypothalamus, which showed more even levels of all three transcripts. Our results are consistent with the hypothesis that differential gene expression underlies pharmacological and physiological heterogeneity observed in neuronal T-type calcium currents, and they provide a molecular basis for the study of T-type channels in particular neurons.

Cloning and expression of a novel member of the low voltage-activated T-type calcium channel family

Low voltage-activated Ca2+ channels play important roles in pacing neuronal firing and producing network oscillations, such as those that occur during sleep and epilepsy. Here we describe the cloning and expression of the third member of the T-type family, alpha1I or CavT.3, from rat brain. Northern analysis indicated that it is predominantly expressed in brain. Expression of the cloned channel in either Xenopus oocytes or stably transfected human embryonic kidney-293 cells revealed novel gating properties. We compared these electrophysiological properties to those of the cloned T-type channels alpha1G and alpha1H and to the high voltage-activated channels formed by alpha1Ebeta3. The alpha1I channels opened after small depolarizations of the membrane similar to alpha1G and alpha1H but at more depolarized potentials. The kinetics of activation and inactivation were dramatically slower, which allows the channel to act as a Ca2+ injector. In oocytes, the kinetics were even slower, suggesting that components of the expression system modulate its gating properties. Steady-state inactivation occurred at higher potentials than any of the other T channels, endowing the channel with a substantial window current. The alpha1I channel could still be classified as T-type by virtue of its criss-crossing kinetics, its slow deactivation (tail current), and its small (11 pS) conductance in 110 mM Ba2+ solutions. Based on its brain distribution and novel gating properties, we suggest that alpha1I plays important roles in determining the electroresponsiveness of neurons, and hence, may be a novel drug target.

Adrenaline enhances odorant contrast by modulating signal encoding in olfactory receptor cells

Olfactory perception is influenced by hormones. Here we report that adrenaline can directly affect the signal encoding of olfactory receptor cells. Application of adrenaline suppressed action potentials near threshold and increased their frequency in response to strong stimuli, resulting in a narrower dynamic range. Under voltage-clamp conditions, adrenaline enhanced sodium current and reduced T-type calcium current. Because sodium current is the major component of spike generation and T-type calcium current lowers the threshold in olfactory receptor cells, the effects of adrenaline on these currents are consistent with the results obtained under current-clamp conditions. Both effects involved a common cytoplasmic pathway, cAMP-dependent phosphorylation. We suggest that adrenaline may enhance contrast in olfactory perception by this mechanism.

Structure and functional characterization of a novel human low-voltage activated calcium channel

We have isolated and characterized overlapping cDNAs encoding a novel, voltage-gated Ca2+ channel alpha1 subunit, alpha1H, from a human medullary thyroid carcinoma cell line. The alpha1H subunit is structurally similar to previously described alpha1 subunits. Northern blot analysis indicates that alpha1H mRNA is expressed throughout the brain, primarily in the amygdala, caudate nucleus, and putamen, as well as in several nonneuronal tissues, with relatively high levels in the liver, kidney, and heart. Ba2+ currents recorded from human embryonic kidney 293 cells transiently expressing alpha1H activated at relatively hyperpolarized potentials (-50 mV), rapidly inactivated (tau = 17 ms), and slowly deactivated. Similar results were observed in Xenopus oocytes expressing alpha1H. Single-channel measurements in human embryonic kidney 293 cells revealed a single-channel conductance of approximately 9 pS. These channels are blocked by Ni2+ (IC50 = 6.6 microM) and the T-type channel antagonists mibefradil (approximately 50% block at 1 microM) and amiloride (IC50 = 167 microM). Thus, alpha1H-containing channels exhibit biophysical and pharmacological properties characteristic of low voltage-activated, or T-type, Ca2+ channels.

Calcium channel subtypes differ at two types of cholinergic synapse in lumbar sympathetic neurones of guinea-pigs

1. The involvement of different presynaptic Ca2+ channels in transmission at 'weak' (subthreshold) and 'strong' (suprathreshold) synapses was investigated in guinea-pig paravertebral ganglia isolated in vitro. Selective Ca2+ channel antagonists were used to block excitatory synaptic currents evoked by stimulating single preganglionic axons. 2. The N-type Ca2+ channel blocker, omega-conotoxin GVIA (100 nM), reduced peak synaptic conductance by similar amounts at weak synapses (by 39 +/- 6 %) and strong synapses (34 +/- 6 %). 3. The P-type Ca2+ channel blocker, omega-agatoxin IVA (40 nM), significantly reduced transmitter release at weak synapses (by 42 +/- 6 %) but had only a small effect at strong synapses (reduced by 6 +/- 2 %). 4. Blockers of Q-, L- or T-type Ca2+ channels had no significant effects on peak synaptic conductance at either type of synapse. 5. We conclude that the two functionally distinct types of preganglionic terminal in sympathetic ganglia which synapse on the same neurone differ in their expression of particular types of voltage-dependent Ca2+ channels. Both types utilize N-type channels and channels resistant to blockade by specific antagonists, but Ca2+ entry through P-type channels makes a substantial contribution to acetylcholine release only at weak synapses.

Effects of mibefradil, a blocker of T-type Ca2+ channels, in single myocytes and intact muscle of guinea-pig heart

We investigated the effects of a relatively selective blocker of the T-type Ca2+ channels, mibefradil (MBF), in the isovolumic left ventricles of the isolated, perfused hearts of guinea-pigs and single myocytes isolated from the ventricles of this species. In the myocytes superfused with 0 Na+ solution containing 200 microM lidocaine and pulsed from -90 mV to -40 mV to +5 mV, MBF proved to be about 3 times more potent inhibitor of the T-type than of the L-type Ca2+ current. The effect on the L-type current was strongly voltage and use dependent. In the ventricles and in the myocytes contraction was reduced by 50% by about 1 microM MBF, the concentration 12 times higher than this increasing the coronary flow by 50%. In myocytes the decrease in unloaded shortening paralleled inhibition of the T-type rather, than of the L-type Ca2+ current. Inhibition of electrically stimulated contraction of the myocytes was three times stronger than inhibition of the caffeine contractures regarded as an index of sarcoplasmic reticulum (SR) Ca2+ content. These findings are consistent with the hypothesis that the T-type Ca2+ channels may contribute to release of Ca2+ from the SR. It is concluded that MBF has a definite negative inotropic effect in the ventricular myocardium of guinea-pig heart at the concentrations found in the blood of the patients submitted to the clinical trials.

Inhibition of T-type voltage-gated calcium channels by a new scorpion toxin

The biophysical properties of T-type voltage-gated calcium channels are well suited to pacemaking and to supporting calcium flux near the resting membrane potential in both excitable and non-excitable cells. We have identified a new scorpion toxin (kurtoxin) that binds to the alpha 1G T-type calcium channel with high affinity and inhibits the channel by modifying voltage-dependent gating. This toxin distinguishes between alpha 1G T-type calcium channels and other types of voltage-gated calcium channels, including alpha 1A, alpha 1B, alpha 1C and alpha 1E. Like the other alpha-scorpion toxins to which it is related, kurtoxin also interacts with voltage-gated sodium channels and slows their inactivation. Kurtoxin will facilitate characterization of the subunit composition of T-type calcium channels and help determine their involvement in electrical and biochemical signaling.

Duality of the voltage-dependent calcium influx in adrenal glomerulosa cells

Both T- and L-type calcium channels are expressed in bovine adrenal glomerulosa cells and both channels are sensitive to moderate depolarizations of the cell membrane induced by angiotensin II (AngII) or physiological concentrations of extracellular K+. These channels present distinct pharmacology, L-type channels being more sensitive to dihydropyridines, whereas T channels are inhibited by lower concentrations of mibefradil, a new type of calcium antagonist currently used for treating hypertension. The activity of these channels is also differently modulated by AngII, which inhibits T channels through activation of protein kinase C and L channels through a Pertussis toxin-sensitive G protein. Finally, whereas the activity of L-type channels is directly reflected on the levels of the cytosolic calcium concentration ([Ca2+]c), T-type channels are more closely related to the control of steroidogenesis, possibly through a kind of "calcium pipeline" linking the plasma membrane to the mitochondria. In conclusion, two types of calcium channels, with distinct functions and differential modulation by AngII, are activated by agonists of aldosterone biosynthesis in adrenal glomerulosa cells. Most importantly, these channels have distinct sensitivities to currently used antihypertensive therapeutic drugs.

Molecular characterization of a novel family of low voltage-activated, T-type, calcium channels

Low voltage-activated, T-type, calcium channels are thought to be involved in pacemaker activity, low threshold Ca2+ spikes, neuronal oscillations and resonance, and rebound burst firing. Mutations in T-type channel genes may be a contributing factor to neurological and cardiovascular disorders, such as epilepsy, arrhythmia, and hypertension. Due to the lack of selective blockers, little is known about their structure or molecular biology. This review discusses our recent findings on the cloning, chromosomal localization, and functional expression, of two novel channels, alpha1G and alpha1H. The biophysical properties of these cloned channels (distinctive voltage dependence, kinetics, and single channel conductance) demonstrates that these channels are members of the T-type Ca2+ channel family.