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

Lack of delta waves and sleep disturbances during non-rapid eye movement sleep in mice lacking alpha1G-subunit of T-type calcium channels

T-type calcium channels have been implicated as a pacemaker for brain rhythms during sleep but their contribution to behavioral states of sleep has been relatively uncertain. Here, we found that mice lacking alpha1(G) T-type Ca(2+) channels showed a loss of the thalamic delta (1-4 Hz) waves and a reduction of sleep spindles (7-14 Hz), whereas slow (<1 Hz) rhythms were relatively intact, when compared with the wild-type during urethane anesthesia and non-rapid eye movement (NREM) sleep. Analysis of sleep disturbances, as defined by the occurrence of brief awakening (BA) episodes during NREM sleep, revealed that mutant mice exhibited a higher incidence of BAs of >16 sec compared with the wild-type, whereas no difference was seen in BAs of <16 sec between the two genotypes. These results are consistent with the previous idea of the distinct nature of delta oscillations and sleep spindles from cortically generated slow waves. These results also suggest that the alpha1(G)-subunit of T-type calcium channels plays a critical role in the genesis of thalamocortical oscillations and contributes to the modulation of sleep states and the transition between NREM sleep and wake states.

Low threshold calcium currents in rat cerebellar Purkinje cell dendritic spines are mediated by T-type calcium channels

The functional role of low voltage activated (LVA) calcium channels in the cerebellar Purkinje cell dendritic tree is not completely understood. Since the localization of these channels will influence their possible roles in dendritic integration and induction of plasticity, we set out to characterize the LVA calcium current in Purkinje cell dendrites in acute cerebellar slices of young rats. Using a combination of electrophysiological recordings and two-photon laser scanning microscopy, we show that LVA calcium current recorded at the soma can be correlated with voltage-dependent calcium transients in Purkinje cell dendritic spines. Blocking sodium and potassium conductances allowed us to isolate and characterize a fast inactivating inward current activated positive to -55 mV. Activation and steady-state inactivation kinetics, voltage-dependent deactivation kinetics, and pharmacological experiments (using omega-agatoxin-IVA, mibefradil and nickel) show that this current is carried by T-type calcium channels. Furthermore, the LVA calcium transient observed in the dendritic spines of the Purkinje cell is well correlated with the current recorded at the soma, suggesting that T-type calcium channels are the main component of the LVA calcium input in spines. The fast rising phase of the calcium transient in spines and the absence of delay between the onset in the spine and the parent dendrite show that T-type calcium channels are present both in spines and dendrites of the Purkinje cell.

The 'window' T-type calcium current in brain dynamics of different behavioural states

All three forms of recombinant low voltage-activated T-type Ca(2)(+) channels (Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3) exhibit a small, though clearly evident, window T-type Ca(2)(+) current (I(Twindow)) which is also present in native channels from different neuronal types. In thalamocortical (TC) and nucleus reticularis thalami (NRT) neurones, and possibly in neocortical cells, an I(Twindow)-mediated bistability is the key cellular mechanism underlying the expression of the slow (< 1 Hz) sleep oscillation, one of the fundamental EEG rhythms of non-REM sleep. As the I(Twindow)-mediated bistability may also represent one of the cellular mechanisms underlying the expression of high frequency burst firing in awake conditions, I(Twindow) is of critical importance in neuronal population dynamics associated with different behavioural states.

Low voltage-activated calcium channels in vascular smooth muscle: T-type channels and AVP-stimulated calcium spiking

An important path of extracellular calcium influx in vascular smooth muscle (VSM) cells is through voltage-activated Ca2+ channels of the plasma membrane. Both high (HVA)- and low (LVA)-voltage-activated Ca2+ currents are present in VSM cells, yet little is known about the relevance of the LVA T-type channels. In this report, we provide molecular evidence for T-type Ca2+ channels in rat arterial VSM and characterize endogenous LVA Ca2+ currents in the aortic smooth muscle-derived cell line A7r5. AVP is a vasoconstrictor hormone that, at physiological concentrations, stimulates Ca2+ oscillations (spiking) in monolayer cultures of A7r5 cells. The present study investigated the role of T-type Ca2+ channels in this response with a combination of pharmacological and molecular approaches. We demonstrate that AVP-stimulated Ca2+ spiking can be abolished by mibefradil at low concentrations (<1 microM) that should not inhibit L-type currents. Infection of A7r5 cells with an adenovirus containing the Cav3.2 T-type channel resulted in robust LVA Ca2+ currents but did not alter the AVP-stimulated Ca2+ spiking response. Together these data suggest that T-type Ca2+ channels are necessary for the onset of AVP-stimulated calcium oscillations; however, LVA Ca2+ entry through these channels is not limiting for repetitive Ca2+ spiking observed in A7r5 cells.

In vivo intracellular responses of the medial geniculate neurones to acoustic stimuli in anaesthetized guinea pigs

In the present study, we investigated the auditory response features of the medial geniculate neurones, using in vivo intracellular recordings in anaesthetized guinea pigs. Of the 76 neurones examined, 9 showed 'off' or 'on-off' responses to an acoustic stimulus and thus were defined as 'off' or 'on-off' neurones. Among the remaining 67 neurones, 42 showed an excitatory postsynaptic potential (EPSP) to acoustic stimuli and 25 showed either a pure inhibitory postsynaptic potential (IPSP, 7 neurones), or an IPSP preceded by an EPSP (EPSP-IPSP type, 18 neurones). The EPSP responses exhibited a mean latency of 15.7 +/- 6.1 ms, which was significantly shorter than that of the IPSP responses (21.3 +/- 8.6 ms, P < 0.01). The IPSP responses also showed a significantly greater duration than the EPSP responses (208.5 +/- 128.2 ms versus 122.4 +/- 84.8 ms, P < 0.05), while there were no significant differences between the amplitudes of IPSP and EPSP (8.3 +/- 3.2 mV versus 8.7 +/- 5.3 mV). Of the 11 neurones that showed EPSP responses to acoustic stimuli and were histologically labelled, 7 were located in the lemniscal medial geniculate body (MGB) and 4 in the non-lemniscal MGB. Another 6 labelled neurones that showed IPSP responses to acoustic stimuli were located in the non-lemniscal MGB. With a membrane potential of above -72 mV, the neurones showed greater EPSP or IPSP to an acoustic stimulus when their membrane potential was depolarized. However, upon hyperpolarization to below -74 mV, the neurones shifted to low-threshold calcium spikes (LTS)/LTS bursts. In response to auditory stimuli of different durations, 'off' neurones that responded to the offset of the acoustic stimulus and were located in the non-lemniscal MGB showed different response latencies or deviations of latencies in addition to exhibiting different numbers of spikes, suggesting that the timing of the spikes could be another component utilized by thalamic neurones to encode information on the stimulus. Given that some non-lemniscal neurones are multisensory and project to the entire auditory cortex, the selective corticofugal inhibition in the non-lemniscal MGB would enable the ascending pathway to prepare the auditory cortex to receive subsequent auditory information, avoiding the interference of other sensory inputs.

Differential Sensitivity of N- and P/Q-Type Ca2+ Channel Currents to a μ Opioid in Isolectin B -Positive and -Negative Dorsal Root Ganglion Neurons

Opioids have a selective effect on nociception with little effect on other sensory modalities. However, the cellular mechanisms for this preferential effect are not fully known. Two broad classes of nociceptors can be distinguished based on their growth factor requirements and binding to isolectin B4(IB4). In this study, we determined the difference in the modulation of voltage-gated Ca2+ currents by [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO, a specific mu opioid agonist) between IB4-positive and -negative small dorsal root ganglion (DRG) neurons. Whole-cell voltage-clamp recordings were performed in acutely isolated DRG neurons in adult rats. Both 1-10 microM DAMGO and 1 to 10 microM morphine had a greater effect on high voltage-activated Ca2+ currents in IB4-negative than IB4-positive cells. However, DAMGO had no significant effect on T-type Ca2+ currents in both groups. The N-type Ca2+ current was the major subtype of Ca2+ currents inhibited by DAMGO in both IB4-positive and -negative neurons. Although DAMGO had no effect on L-type and R-type Ca2+ currents in both groups, it produced a larger inhibition on N-type and P/Q-type Ca2+ currents in IB4-negative than IB4-positive neurons. Furthermore, double labeling revealed that there was a significantly higher mu opioid receptor immunoreactivity in IB4-negative than IB4-positive cells. Thus, these data suggest that N-and P/Q-type Ca2+ currents are more sensitive to inhibition by the mu opioids in IB4-negative than IB4-positive DRG neurons. The differential sensitivity of voltage-gated Ca2+ channels to the mu opioids in subsets of DRG neurons may constitute an important analgesic mechanism of mu opioids.

Functional impact of alternative splicing of human T-type Cav3.3 calcium channels

Low-voltage-activated T-type (Cav3) Ca2+ channels produce low-threshold spikes that trigger burst firing in many neurons. The CACNA1I gene encodes the Cav3.3 isoform, which activates and inactivates much more slowly than the other Cav3 channels. These distinctive kinetic features, along with its brain-region-specific expression, suggest that Cav3.3 channels endow neurons with the ability to generate long-lasting bursts of firing. The human CACNA1I gene contains two regions of alternative splicing: variable inclusion of exon 9 and an alternative acceptor site within exon 33, which leads to deletion of 13 amino acids (Delta33). The goal of this study is to determine the functional consequences of these variations in the full-length channel. The cDNA encoding these regions were cloned using RT-PCR from human brain, and currents were recorded by whole cell patch clamp. Introduction of the Delta33 deletion slowed the rate of channel opening. Addition of exon 9 had little effect on kinetics, whereas its addition to Delta33 channels unexpectedly slowed both activation and inactivation kinetics. Modeling of neuronal firing showed that exon 9 or Delta33 alone reduced burst firing, whereas the combination enhanced firing. The major conclusions of this study are that the intracellular regions after repeats I and IV play a role in channel gating, that their effects are interdependent, suggesting a direct interaction, and that splice variation of Cav3.3 channels provides a mechanism for fine-tuning the latency and duration of low-threshold spikes.

Ethosuximide reverses paclitaxel- and vincristine-induced painful peripheral neuropathy

Paclitaxel (Taxol) is one of the most effective and frequently used chemotherapeutics for the treatment of solid tumours. However, paclitaxel produces peripheral neurotoxicity with patients reporting sensory abnormalities and neuropathic pain during and often persisting after paclitaxel therapy. The mechanisms underlying this dose-limiting side effect are currently unknown and there are no validated drugs for its prevention or control. Male Sprague-Dawley rats received four intraperitoneal (i.p.) injections on alternate days of 2 mg/kg paclitaxel. Behavioural assessment using von Frey filaments and acetone showed that such paclitaxel treatment induced a pronounced mechanical and cold allodynia/hyperalgesia. Thus these studies aim to test potential analgesics on established paclitaxel-induced pain. Paclitaxel-induced pain appears to be relatively resistant to opioid therapy i.p. 4 mg/kg morphine was ineffective and i.p. 8 mg/kg morphine only elicited up to a 50% reversal of mechanical allodynia/hyperalgesia. Interestingly, a maximally tolerated dose (i.p. 0.2 mg/kg) of the potent NMDA receptor antagonist MK-801 produced no significant reversal of the mechanical allodynia/hyperalgesia suggesting that NMDA receptors have little role in paclitaxel-induced pain. Ethosuximide (i.p. 450 mg/kg) an anti-epileptic and relatively selective T-type calcium channel blocker elicited a near complete reversal of mechanical allodynia/hyperalgesia. Repetitive dosing with ethosuximide (i.p. 100 or 300 mg/kg daily for 3 days) showed a dose-related consistent reversal of mechanical allodynia/hyperalgesia with no evidence of tolerance. Ethosuximide (i.p. 300 mg/kg) also reversed paclitaxel-induced cold allodynia and vincristine-induced mechanical allodynia/hyperalgesia. These data suggest that T-type calcium channels may play a role in chemotherapy-induced neuropathy and moreover identify ethosuximide as a new potential treatment for chemotherapy-induced pain.

Pre- and postsynaptic contributions of voltage-dependent Ca2+ channels to nociceptive transmission in rat spinal lamina I neurons

Activation of voltage-dependent Ca2+ channels (VDCCs) is critical for neurotransmitter release, neuronal excitability and postsynaptic Ca2+ signalling. Antagonists of VDCCs can be antinociceptive in different animal pain models. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the processing of pain-related information, but the role of VDCCs to the activity-dependent Ca2+ increase in lamina I neurons and to the synaptic transmission between nociceptive afferents and second order neurons in lamina I is not known. This has now been investigated in a lumbar spinal cord slice preparation from young Sprague-Dawley rats. Microfluorometric Ca2+ measurements with fura-2 have been used to analyse the Ca2+ increase in lamina I neurons after depolarization of the cells, resulting in a distinct and transient increase of the cytosolic Ca2+ concentration. This Ca2+ peak was reduced by the T-type channel blocker, Ni2+, by the L-type channel blockers, nifedipine and verapamil, and by the N-type channel blocker, omega-conotoxin GVIA. The P/Q-type channel antagonist, omega-agatoxin TK, had no effect on postsynaptic [Ca2+]i. The NMDA receptor channel blocker D-AP5 reduced the Ca2+ peak, whereas the AMPA receptor channel blocker CNQX had no effect. Postsynaptic currents, monosynaptically evoked by electrical stimulation of the attached dorsal roots with C-fibre and Adelta-fibre intensity, respectively, were reduced by N-type channel blocker omega-conotoxin GVIA and to a much lesser extent, by P/Q-type channel antagonist omega-agatoxin TK, and the L-type channel blockers verapamil, respectively. No difference was found between unidentified neurons and neurons projecting to the periaqueductal grey matter. This is the first quantitative description of the relative contribution of voltage-dependent Ca2+ channels to the synaptic transmission in lamina I of the spinal dorsal horn, which is essential in the processing of pain-related information in the central nervous system.

Localization of voltage-gated ion channels in mammalian brain

The intrinsic electrical properties of neurons are shaped in large part by the action of voltage-gated ion channels. Molecular cloning studies have revealed a large family of ion channel genes, many of which are expressed in mammalian brain. Much recent effort has focused on determining the contribution of the protein products of these genes to neuronal function. This requires knowledge of the abundance and distribution of the constituent subunits of the channels in specific mammalian central neurons. Here we review progress made in recent studies aimed at localizing specific ion channel subunits using in situ hybridization and immunohistochemistry. We then discuss the implications of these results in terms of neuronal physiology and neuronal mechanisms underlying the observed patterns of expression.

Expression and differential cell distribution of low-threshold Ca2+ channels in mammalian male germ cells and sperm

Numerous sperm functions including the acrosome reaction (AR) are associated with Ca(2+) influx through voltage-gated Ca(2+) (Ca(V)) channels. Although the electrophysiological characterization of Ca(2+) currents in mature sperm has proven difficult, functional studies have revealed the presence of low-threshold (Ca(V)3) channels in spermatogenic cells. However, the molecular identity of these proteins remains undefined. Here, we identified by reverse transcription polymerase chain reaction the expression of Ca(V)3.3 mRNA in mouse male germ cells, an isoform not previously described in these cells. Immunoconfocal microscopy revealed the presence of the three Ca(V)3 channel isoforms in mouse spermatogenic cells. In mature mouse sperm only Ca(V)3.1 and Ca(V)3.2 were detected in the head, suggesting its participation in the AR. Ca(V)3.1 and Ca(V)3.3 were found in the principal and the midpiece of the flagella. All Ca(V)3 channels are also present in human sperm, but only to a minor extent in the head. These findings were corroborated by immunogold transmission electron microscopy. Tail localization of Ca(V)3 channels suggested they may participate in motility, however, mibefradil and gossypol concentrations that inhibit Ca(V)3 channels did not significantly affect human sperm motility. Only higher mibefradil doses that can block high-threshold (HVA) Ca(V) channels caused small but significant motility alterations. Antibodies to HVA channels detected Ca(V)1.3 and Ca(V)2.3 in human sperm flagella.

Gating Effects of Mutations in the Cav3.2 T-type Calcium Channel Associated with Childhood Absence Epilepsy

Childhood absence epilepsy (CAE) is a type of generalized epilepsy observed in 2-10% of epileptic children. In a recent study by Chen et al. (Chen, Y., Lu, J., Pan, H., Zhang, Y., Wu, H., Xu, K., Liu, X., Jiang, Y., Bao, X., Yao, Z., Ding, K., Lo, W. H., Qiang, B., Chan, P., Shen, Y., and Wu, X. (2003) Ann. Neurol. 54, 239-243) 12 missense mutations were identified in the CACNA1H (Ca(v)3.2) gene in 14 of 118 patients with CAE but not in 230 control individuals. We have functionally characterized five of these mutations (F161L, E282K, C456S, V831M, and D1463N) using rat Ca(v)3.2 and whole-cell patch clamp recordings in transfected HEK293 cells. Two of the mutations, F161L and E282K, mediated an approximately 10-mV hyperpolarizing shift in the half-activation potential. Mutation V831M caused a approximately 50% slowing of inactivation relative to control and shifted half-inactivation potential approximately 10 mV toward more depolarized potentials. Mean time to peak was significantly increased by mutation V831M but was unchanged for all others. No resolvable changes in the parameters of the IV relation or current kinetics were observed with the remaining mutations. The findings suggest that several of the Ca(v)3.2 mutants allow for greater calcium influx during physiological activation and in the case of F161L and E282K can result in channel openings at more hyperpolarized (close to resting) potentials. This may underlie the propensity for seizures in patients with CAE.

Characterization and function of TWIK-related acid sensing K+ channels in a rat nociceptive cell

We examined the properties of a proton sensitive current in acutely dissociated, capsaicin insensitive nociceptive neurons from rat dorsal root ganglion (DRG). The current had features consistent with K(+) leak currents of the KCNK family (TASK-1, TASK-3; TWIK-related acid sensing K(+)). Acidity and alkalinity induced inward and outward shifts in the holding current accompanied by increased and decreased whole cell resistance consistent with a K(+) current. We used alkaline solutions to open the channel and examine its properties. Alkaline evoked currents (AECs; pH 10.0-10.75), reversed near the K(+) equilibrium potential (-74 mV), and were suppressed 85% in 0 mM K(+). AECs were insensitive to Cs(+) (1 mM) and anandamide (1 microM), but blocked by Ba(++) (1 mM), quinidine (100 microM) or Ruthenium Red (10 microM). This pharmacology was identical to that of rat TASK-3 and inconsistent with that of TASK-1 or TASK-2. The TASK-like AEC was not modulated by PKA (forskolin, kappa opioid agonists U69593 and GR8696, somatostatin) but was inhibited by PKC activator phorbol-12-myristate-13 acetate (PMA). When acidic solutions were used, we were able to isolate a Ba(++) and Ruthenium Red insensitive current that was inhibited by Zn(++). This Zn(++) sensitive component of the proton sensitive current was consistent with TASK-1. In current clamp studies, acidic pH produced sensitive changes in resting membrane potential but did not influence excitability (pH 7.2-6.8). In contrast, Zn(++) produced substantial changes in excitability at physiological pH. Alkaline solutions produced hyperpolarization followed by proportional burst discharges (pH 10.75-11.5) and increased excitability (at pH 7.4). In conclusion, multiple TASK currents were present in a DRG nociceptor and differentially contributed to distinct discharge mechanisms.

Voltage-gated calcium channels in adult rat inferior colliculus neurons

The inferior colliculus (IC) plays a key role in the processing of auditory information and is thought to be an important site for genesis of wild running seizures that evolve into tonic-clonic seizures. IC neurons are known to have Ca(2+) channels but neither their types nor their pharmacological properties have been as yet characterized. Here, we report on biophysical and pharmacological properties of Ca(2+) channel currents in acutely dissociated neurons of adult rat IC, using electrophysiological and molecular techniques. Ca(2+) channels were activated by depolarizing pulses from a holding potential of -90 mV in 10 mV increments using 5 mM barium (Ba(2+)) as the charge carrier. Both low (T-type, VA) and high (HVA) threshold Ca(2+) channel currents that could be blocked by 50 microM cadmium, were recorded. Pharmacological dissection of HVA currents showed that nifedipine (10 microM, L-type channel blocker), omega-conotoxin GVIA (1 microM, N-type channel blocker), and omega-agatoxin TK (30 nM, P-type channel blocker) partially suppressed the current by 21%, 29% and 22%, respectively. Since at higher concentration (200 nM) omega-agatoxin TK also blocks Q-type channels, the data suggest that Q-type Ca(2+) channels carry approximately 16% of HVA current. The fraction of current (approximately 12%) resistant to the above blockers, which was blocked by 30 microM nickel and inactivated with tau of 15-50 ms, was considered as R-type Ca(2+) channel current. Consistent with the pharmacological evidences, Western blot analysis using selective Ca(2+) channel antibodies showed that IC neurons express Ca(2+) channel alpha(1A), alpha(1B), alpha(1C), alpha(1D), and alpha(1E) subunits. We conclude that IC neurons express functionally all members of HVA Ca(2+) channels, but only a subset of these neurons appear to have developed functional LVA channels.

Abnormal Coronary Function in Mice Deficient in α1HT-type Ca2+Channels

Calcium ion (Ca2+) influx through voltage-gated Ca2+ channels is important for the regulation of vascular tone. Activation of L-type Ca2+ channels initiates muscle contraction; however, the role of T-type Ca2+ channels (T-channels) is not clear. We show that mice deficient in the alpha1H T-type Ca2+ channel (alpha(1)3.2-null) have constitutively constricted coronary arterioles and focal myocardial fibrosis. Coronary arteries isolated from alpha(1)3.2-null arteries showed normal contractile responses, but reduced relaxation in response to acetylcholine and nitroprusside. Furthermore, acute blockade of T-channels with Ni2+ prevented relaxation of wild-type coronary arteries. Thus, Ca2+ influx through alpha1H T-type Ca2+ channels is essential for normal relaxation of coronary arteries.

Preservation of motor neuron Ca2+ channel sensitivity to insulin-like growth factor-1 in brain motor cortex from senescent rat

Despite the multiple effects on mammals during development, the effectiveness of the insulin-like growth factor-1 (IGF-1) to sustain cell function and structure in the brain of senescent mammals is almost completely unknown. To address this issue, we investigated whether the effects of IGF-1 on specific targets are preserved at later stages of life. Voltage-gated Ca2+ channels (VGCC) are well-characterized targets of IGF-1. VGCC regulate membrane excitability and gene transcription along with other functions that have been found to be impaired in the brain of senescent rodents. As the voluntary control of movement has been reported to be altered in the elderly, we investigated the expression, function and responsiveness of high (HVA)- and low-voltage-activated (LVA) Ca2+ channels to IGF-1, using the whole-cell configuration of the patch-clamp and RT-PCR in the specific region of the rat motor cortex that controls hindlimb muscle movement. We detected the expression of alpha 1A, alpha 1B and alpha 1E genes encoding the HVA Ca2+ channels P/Q, N and R, respectively, but not alpha 1C, alpha 1D, alpha 1S encoding the L-type Ca2+ channel in this region of the brain cortex. IGF-1 enhanced Ca2+ channel currents through P/Q- and N-type channels but not significantly through the R-type or LVA channels. IGF-1 enhanced the amplitude but did not modify the voltage dependence of Ca2+ channel currents in young (2- to 4-week-old), young adult (7-month-old) and senescent (28- to 29-month-old) rats. These results support the concept that despite the reported decrease in circulating (liver) and local (central nervous system) production of IGF-1 with ageing, key neuronal targets such as the VGCC remain responsive to the growth factor throughout life.

A mechanism for the direct regulation of T-type calcium channels by Ca2+/calmodulin-dependent kinase II

Low-voltage-activated (LVA) Ca2+ channels are widely distributed throughout the CNS and are important determinants of neuronal excitability, initiating dendritic and somatic Ca2+ spikes that trigger and shape the pattern of action potential firing. Here, we define a molecular mechanism underlying the dynamic regulation of alpha1H channels (Cav3.2), by Ca2+/CaM-dependent protein kinase II (CaMKII). We show that channel regulation is selective for the LVA alpha1H Ca2+ channel subtype, depends on determinants in the alpha1H II-III intracellular loop, and requires the phosphorylation of a serine residue absent from unregulated alpha1G (Cav3.1) channels. These studies identify the alpha1H channel as a new substrate for CaMKII and provide the first molecular mechanism for the direct regulation of T-type Ca2+ channels by a protein kinase. Our data suggest a novel mechanism for modulating the integrative properties of neurons.

Regulation of granule cell excitability by a low-threshold calcium spike in turtle olfactory bulb

Granule cells excitability in the turtle olfactory bulb was analyzed using whole cell recordings in current- and voltage-clamp mode. Low-threshold spikes (LTSs) were evoked at potentials that are subthreshold for Na spikes in normal medium. The LTSs were evoked from rest, but hyperpolarization of the cell usually increased their amplitude so that they more easily boosted Na spike initiation. The LTS persisted in the presence of TTX but was antagonized by blockers of T-type calcium channels. The voltage dependence, kinetics, and inactivation properties of the LTS were characteristic of a low-threshold calcium spike. The threshold of the LTS was slightly above the resting potential but well below the Na spike threshold, and the LTS was often evoked in isolation in normal medium. Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) had only minimal effects on the LTS but revealed the presence of a high-threshold Ca2+ spike (HTS), which was antagonized by Cd2+. The LTS displayed paired-pulse attenuation, with a timescale for recovery from inactivation of about 2 s at resting membrane potential. The LTS strongly boosted Na spike initiation; with repetitive stimulation, the long recovery of the LTS governed Na spike initiation. Thus the olfactory granule cells possess an LTS, with intrinsic kinetics that contribute to sub- and suprathreshold responses on a timescale of seconds. This adds a new mechanism to the early processing of olfactory input.

Thalamic control of visceral nociception mediated by T-type Ca2+ channels

Sensations from viscera, like fullness, easily become painful if the stimulus persists. Mice lacking alpha1G T-type Ca2+ channels show hyperalgesia to visceral pain. Thalamic infusion of a T-type blocker induced similar hyperalgesia in wild-type mice. In response to visceral pain, the ventroposterolateral thalamic neurons evokeda surge of single spikes, which then slowly decayed as T type-dependent burst spikes gradually increased. In alpha1G-deficient neurons, the single-spike response persisted without burst spikes. These results indicate that T-type Ca2+ channels underlie an antinociceptive mechanism operating in the thalamus andsupport the idea that burst firing plays a critical role in sensory gating in the thalamus.

Ionic mechanisms of burst firing in dissociated Purkinje neurons

Cerebellar Purkinje neurons have intrinsic membrane properties that favor burst firing, seen not only during complex spikes elicited by climbing fiber input but also with direct electrical stimulation of cell bodies. We examined the ionic conductances that underlie all-or-none burst firing elicited in acutely dissociated mouse Purkinje neurons by short depolarizing current injections. Blocking voltage-dependent calcium entry by cadmium or replacement of external calcium by magnesium enhanced burst firing, but it was blocked by cobalt replacement of calcium, probably reflecting block of sodium channels. In voltage-clamp experiments, we used the burst waveform of each cell as a voltage command and used ionic substitutions and pharmacological manipulations to isolate tetrodotoxin (TTX)-sensitive sodium current, P-type and T-type calcium current, hyperpolarization-activated cation current (Ih), voltage-activated potassium current, large-conductance calcium-activated potassium current, and small-conductance calcium-activated potassium (SK) current. Measured near the middle of the first interspike interval, TTX-sensitive sodium current carried the largest inward current, and T-type calcium current was also substantial. Current through P-type channels was large immediately after a spike but decayed rapidly. These inward currents were opposed by substantial components of voltage-dependent and calcium-dependent potassium current. Termination of the burst is caused partly by decay of sodium current, together with a progressive buildup of SK current after the first interspike interval. Although burst firing depends on the net balance between multiple large currents flowing after a spike, it is surprisingly robust, probably reflecting complex interactions between the exact voltage waveform and voltage and calcium dependence of the various currents.

Ethanol influences on native T-type calcium current in thalamic sleep circuitry

Ethanol is known to disrupt normal sleep rhythms; however, the cellular basis for this influence is unknown. This study uses an in vitro slice preparation coupled with electrophysiological recordings to probe neuronal responses to acute ethanol exposure. Recordings were conducted in ferret and rat thalamic slices, since thalamic circuitry is an integral component of sleep/wake cycles and sleep spindles. A key mediator of spindle wave activity is the low-threshold calcium current (T-type current). The T-type current underlies burst responses in the lateral geniculate and thalamic reticular nuclei that are important in spindle propagation. Whole cell patch recordings in thalamic brain slices revealed that ethanol has a differential, dose-dependent effect on the native T-type current in thalamic relay cells. Low concentrations of ethanol (2.5, 5, and 10 mM) enhance T-type current (n = 35), whereas higher concentrations of ethanol (20 and 50 mM) decrease T-type current (n = 27). To address whether this dose-dependent effect was due to variation between cells, in a subset we verified the differential effect within the same cell (n = 7). In an effort to examine whether the biphasic effects on the current were due to the order of ethanol exposures, we varied the order of high and low ethanol concentrations within the same cell. The ability of ethanol to perturb calcium currents in thalamic relay cells may provide a mechanistic framework for the well documented disruptions in sleep/wake behavior in subjects with ethanol exposure.

Interactions between membrane conductances underlying thalamocortical slow-wave oscillations

Neurons of the central nervous system display a broad spectrum of intrinsic electrophysiological properties that are absent in the traditional "integrate-and-fire" model. A network of neurons with these properties interacting through synaptic receptors with many time scales can produce complex patterns of activity that cannot be intuitively predicted. Computational methods, tightly linked to experimental data, provide insights into the dynamics of neural networks. We review this approach for the case of bursting neurons of the thalamus, with a focus on thalamic and thalamocortical slow-wave oscillations. At the single-cell level, intrinsic bursting or oscillations can be explained by interactions between calcium- and voltage-dependent channels. At the network level, the genesis of oscillations, their initiation, propagation, termination, and large-scale synchrony can be explained by interactions between neurons with a variety of intrinsic cellular properties through different types of synaptic receptors. These interactions can be altered by neuromodulators, which can dramatically shift the large-scale behavior of the network, and can also be disrupted in many ways, resulting in pathological patterns of activity, such as seizures. We suggest a coherent framework that accounts for a large body of experimental data at the ion-channel, single-cell, and network levels. This framework suggests physiological roles for the highly synchronized oscillations of slow-wave sleep.

Mechanisms of lateral inhibition in the olfactory bulb: efficiency and modulation of spike-evoked calcium influx into granule cells

Granule cells are axonless local interneurons that mediate lateral inhibitory interactions between the principal neurons of the olfactory bulb via dendrodendritic reciprocal synapses. This unusual arrangement may give rise to functional properties different from conventional lateral inhibition. Although granule cells spike, little is known about the role of the action potential with respect to their synaptic output. To investigate the signals that underlie dendritic release in these cells, two-photon microscopy in rat brain slices was used to image calcium transients in granule cell dendrites and spines. Action potentials evoked calcium transients throughout the dendrites, with amplitudes increasing with distance from soma and attaining a plateau level within the external plexiform layer, the zone of granule cell synaptic output. Transient amplitudes were, on average, equal in size in spines and adjacent dendrites. Surprisingly, both spine and dendritic amplitudes were strongly dependent on membrane potential, decreasing with depolarization and increasing with hyperpolarization from rest. Both the current-voltage relationship and the time course of inactivation were consistent with the known properties of T-type calcium channels, and the voltage dependence was blocked by application of the T-type calcium channel antagonists Ni2+ and mibefradil. In addition, mibefradil reduced action potential-mediated synaptic transmission from granule to mitral cells. The implication of a transiently inactivating calcium channel in synaptic release from granule cells suggests novel mechanisms for the regulation of lateral inhibition in the olfactory bulb.

Divalent metals differentially block cloned T-type calcium channels

We tested divalent metals including Cu2+, Pb2+, and Zn2+ to determine their pharmacological profiles for blockade of cloned T-type Ca2+ channels (alpha1G, alpha1 H, and alpha1I). Effects of the metals were also evaluated for native low and high voltage-activated Ca2+ channels in rat sympathetic pelvic neurons. Cu2+ and Zn2+ blocked three T-type channel isoforms in a concentration-dependent manner with a higher affinity for alpha1H currents (IC50 = 0.9 microM and 2.3 microM). In pelvic neurons, only Zn2+ showed strong selectivity for T-type Ca2+ currents over high voltage-activated Ca2+ currents. Conversely, Pb2+ block on Ca2+ channels did not show distinctive selectivity. Taken together, these results suggest that besides Ni2+, Cu2+ and Zn2+ can be used as selective blockers of alpha1 H at low concentrations.

Labyrinthectomy changes T-type calcium channels in vestibular neurones of the guinea pig

In the vestibular nuclei of the awake guinea pig, all neurones are spontaneously active. After unilateral labyrinthectomy, this activity virtually disappears on the ipsilateral side, but is completely restored one week later. In a recent study, we observed that the restoration of spontaneous activity was correlated with an increase in pacemaker activity. In the current study, we found that the ratio of medial vestibular nucleus (MVN) neurones endowed with one of the currents known to play a role in pacemaker activity (i.e. low-threshold calcium current; LTCC) increased from 29% in control guinea pigs to 65% in animals labyrinthectomised on the ipsilateral side one week earlier. Yet this change was not correlated with a modification of the ratio of neurones expressing any of the three related protein-channels (alpha1G, alpha1H and alpha1I).

Modulation of Ca(v)3.2 T-type Ca2+ channels by protein kinase C

Although T-type Ca2+ channels have been implicated in numerous physiological functions, their regulations by protein kinases have been obscured by conflicting reports. We investigated the effects of protein kinase C (PKC) on Ca(v)3.2 T-type channels reconstituted in Xenopus oocytes. Phorbol-12-myristate-13-acetate (PMA) strongly enhanced the amplitude of Ca(v)3.2 channel currents (approximately 3-fold). The augmentation effects were not mimicked by 4alpha-PMA, an inactive stereoisomer of PMA, and abolished by preincubation with PKC inhibitors. Our findings suggest that PMA upregulates Ca(v)3.2 channel activity via activation of oocyte PKC.

A T-type calcium channel required for normal function of a mammalian mechanoreceptor

The dorsal root ganglia (DRG) contain a variety of mechanoreceptors, but no molecular markers uniquely identify specific mechanoreceptor subtypes. We have used DNA microarrays and subtracted cDNA libraries to isolate genes that are specifically expressed by one type of mouse mechanoreceptor. The T-type calcium channel Ca(v)3.2 was exclusively expressed in the DRG by D-hair receptors, a very sensitive mechanoreceptor. Pharmacological blockade of T-type calcium channels indicated that this channel may be essential for normal D-hair receptor excitability including mechanosensitivity. This is the first evidence that a calcium channel is required for normal function of a vertebrate mechanoreceptor.

Enhancement of low-voltage-activated calcium currents by group II metabotropic glutamate receptors in rat retinal ganglion cells

Current through voltage-gated calcium channels of rat retinal ganglion cells was recorded using the whole-cell patch-clamp technique. All cells displayed high-voltage-activated currents, and 75% of these also displayed low-voltage-activated (LVA) currents. Currents could be separated on the basis of their voltage/time dependence and sensitivity to nickel ions. The group II metabotropic glutamate receptor (mGluR) agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC; 100 microM) increased LVA current by 40% as did the nonselective mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (tACPD; 100 microM). Neither the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (100 microM) nor 5-hydroxytryptamine (100 microM) enhanced LVA current. In the presence of (S)-alpha-methyl-4-carboxyphenylglycine (100 microM), a group I/II mGluR antagonist, the tACPD-induced enhancement of LVA current was blocked. The voltage dependence of the activation or inactivation kinetics was unchanged in the presence of tACPD. Inclusion in the pipette solution of GDP-beta-S (1 mM) blocked the enhancement of the LVA current by APDC, whereas GTP-gamma-S (0.5 mM) prevented recovery of the enhancement. The tACPD-mediated enhancement of the LVA current was still present in cells pretreated with pertussis or cholera toxins (500 ng x ml(-1)). Genistein (10 microM) prevented the enhancement of the LVA current. These results suggest that LVA current can be enhanced by activation of mGluR2, by a mechanism that is G-protein dependent and may involve a protein tyrosine kinase step.

Coincidence detection in pyramidal neurons is tuned by their dendritic branching pattern

Neurons display a variety of complex dendritic morphologies even within the same class. We examined the relationship between dendritic arborization and the coupling between somatic and dendritic action potential (AP) initiation sites in layer 5 (L5) neocortical pyramidal neurons. Coupling was defined as the relative reduction in threshold for initiation of a dendritic calcium AP due to a coincident back-propagating AP. Simulations based on reconstructions of biocytin-filled cells showed that addition of oblique branches of the main apical dendrite in close proximity to the soma (d < 140 microm) increases the coupling between the apical and axosomatic AP initiation zones, whereas incorporation of distal branches decreases coupling. Experimental studies on L5 pyramids in acute brain slices revealed a highly significant (n = 28, r = 0.63, P < 0.0005) correlation: increasing the fraction of proximal oblique dendrites (d < 140 microm), e.g., from 30 to 60% resulted on average in an increase of the coupling from approximately 35% to almost 60%. We conclude that variation in dendritic arborization may be a key determinant of variability in coupling (49 +/- 17%; range 19-83%; n = 37) and is likely to outweigh the contribution made by variations in active membrane properties. Thus coincidence detection of inputs arriving from different cortical layers is strongly regulated by differences in dendritic arborization.

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

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

A-, T-, and H-type currents shape intrinsic firing of developing rat abducens motoneurons

During postnatal development, profound changes take place in the excitability of nerve cells, including modification in the distribution and properties of receptor-operated channels and changes in the density and nature of voltage-gated channels. We studied here the firing properties of abducens motoneurons (aMns) in transverse brainstem slices from postnatal day (P) 1-13 rats. Recordings were made from aMNs in the whole-cell configuration of the patch-clamp technique. Two main types of aMn could be distinguished according to their firing profile during prolonged depolarizations. Both types were identified as aMns by their fluorescence following retrograde labelling with the lipophilic carbocyanine DiI in the rectus lateralis muscle. The first type (BaMns) exhibited a burst of action potentials (APs) followed by an adaptation of discharge and were encountered in approximately 70 % of aMns. Their discharge profile resembled that of adult aMns and was encountered in all aMns after P9. BaMns exhibited a hyperpolarization-induced rebound potential that was blocked by low concentrations of Ni2+ or by Ca2+-free external solution. This current had the properties of the T-type current. Action potentials of BaMns showed a complex afterhyperpolarization (AHP). An inward rectification was evidenced following hyperpolarization and was blocked by external application of caesium or ZD7288, indicating the presence of the hyperpolarization-activated cationic current (IH). Blocking the IH current almost doubled the input resistance of BaMns. The second class of aMns (DaMns) displayed a delayed excitation that was mediated by A-type K+ currents and was observed only between P4 and P9. DaMns exhibited immature characteristics: an action potential with a simple AHP, a linear current-voltage relation and a large input resistance. The number of aMns remained unchanged when both types were present (P5-P6) and later in development when only BaMns were encountered (P19), suggesting that DaMns mature into BaMns during postnatal development. We conclude that aMns display profound reorganization in their intrinsic excitability during postnatal development.

Regulation of alpha1G T-type calcium channel gene (CACNA1G) expression during neuronal differentiation

Down-regulation of T-type Ca channel current and mRNA occurs following differentiation of Y79 retinoblastoma cells. To understand how the decrease in expression is linked to cell differentiation, we examined transcriptional regulation of the Cav3.1 Ca channel gene, CACNA1G. We identified two putative promoters (A and B) in 1.3 kb of cloned genomic DNA. Reverse transcriptase-polymerase chain reaction and 5' rapid amplification of cDNA ends-polymerase chain reaction analyses demonstrated that two transcripts with different 5' untranslated regions are generated by different transcription start sites, with promoter A favoured in undifferentiated cells and promoter B favoured in differentiated cells. Functional analyses of the promoter sequence revealed that both promoters are active. Enhancer and repressor sequences were identified upstream of promoter A and B, respectively. These results suggest that the down-regulation of alpha1G mRNA in differentiated Y79 cells is mediated primarily by decreased activity of promoter A, which could occur in conjunction with repression of the activity of promoter B. The decrease in T-type Ca channel expression in Y79 cells may be an essential signal affecting phenotypic maturation and expression of other ion channel subtypes in the differentiated cells.

T-type calcium channel gene alpha (1G) is not associated with childhood absence epilepsy in the Chinese Han population

We investigated whether the T-type calcium channel gene alpha (1G) is associated with childhood absence epilepsy (CAE), a form of idiopathic generalized epilepsy. We carried out direct sequencing of exons 1-37 and the exon-intron boundaries of the alpha (1G) gene in 48 Han Chinese patients with CAE and 48 normal controls. We found no mutation in the exons of alpha (1G). However, we did identify six single nucleotide polymorphisms (SNPs). Using two of these as markers, we carried out a case-control study in 192 patients with CAE and 192 normal controls. The allele and genotype distributions of all the SNPs studied were not significantly different between cases and control groups, thus the alpha (1G) gene is not an important susceptibility gene for CAE, at least in the Chinese population.

Maintenance of sympathetic tone by a nickel chloride-sensitive mechanism in the rostral ventrolateral medulla of the adult rat

In urethane-anaesthetised artificially ventilated Sprague-Dawley rats, bilateral microinjection of the divalent cation nickel chloride (Ni(2+); 50 mM, 50 nl) into the rostral ventrolateral medulla elicited a dramatic inhibition of splanchnic sympathetic nerve activity (-44+/-6%) and a marked depressor response (-35+/-7 mmHg). Selective blockade of high-voltage activated Ca(2+) channels with omega-agatoxin IVA (P/Q-type), omega-conotoxin GVIA (N-type) and nifedipine (L-type) did not decrease arterial pressure or splanchnic sympathetic nerve activity when injected separately into the rostral ventrolateral medulla, or combined with kynurenate. Injection of caesium chloride or ZD 7288, a blocker of the hyperpolarization-activated cation current, into the rostral ventrolateral medulla had no effect on arterial pressure or splanchnic sympathetic nerve activity. Bilateral microinjection of nickel chloride into the caudal ventrolateral medulla/pre-Bötzinger complex elicited small increases in splanchnic sympathetic nerve activity (+17+/-13%) and arterial pressure (+12+/-4 mmHg). These were substantially smaller than those evoked by blockade of glutamatergic receptors or high-voltage activated Ca(2+) channels in this area. Injection of kynurenate or high-voltage activated Ca(2+) channel blocker, but not Ni(2+), in this area evoked respiratory termination. The results indicate the existence of a distinct mechanism maintaining the tonic activity of rostral ventrolateral medulla presympathetic neurons that is different from that maintaining the tonic activity in the caudal ventrolateral medulla/pre-Bötzinger region. We conclude that ion channels that are sensitive to Ni(2+), but are insensitive to high-voltage activated (L, P/Q, N) Ca(2+) channel blockers, and are located postsynaptically on the presympathetic rostral ventrolateral medulla neurons are responsible for the tonic activity of the presympathetic neurons in rostral ventrolateral medulla. These channels could well be the low-voltage-activated (or T-type) Ca(2+) channels although other conductances cannot be conclusively excluded.

Afterhyperpolarization regulates firing rate in neurons of the suprachiasmatic nucleus

Cluster I neurons of the suprachiasmatic nucleus (SCN), which are thought to be pacemakers supporting circadian activity, fire spontaneous action potentials that are followed by a monophasic afterhyperpolarization (AHP). Using a brain slice preparation, we have found that the AHP has a shorter duration in cells firing at higher frequency, consistent with circadian modulation of the AHP. The AHP is supported by at least three subtypes of K(Ca) channels, including apamin-sensitive channels, iberiotoxin-sensitive channels, and channels that are insensitive to both of these antagonists. The latter K(Ca) channel subtype is involved in rate-dependent regulation of the AHP. Voltage-clamped, whole-cell Ca(2+) channel currents recorded from SCN neurons were dissected pharmacologically, revealing all of the major high-voltage activated subtypes: L-, N-, P/Q-, and R-type Ca(2+) channel currents. Application of Ca(2+) channel antagonists to spontaneously firing neurons indicated that predominantly L- and R-type currents trigger the AHP. Our findings suggest that apamin- and iberiotoxin-insensitive K(Ca) channels are subject to diurnal modulation by the circadian clock and that this modulation either directly or indirectly leads to the expression of a circadian rhythm in spiking frequency.

Synaptic plasticity in spinal lamina I projection neurons that mediate hyperalgesia

Inflammation, trauma, or nerve injury may cause enduring hyperalgesia, an enhanced sensitivity to painful stimuli. Neurons in lamina I of the spinal dorsal horn that express the neurokinin 1 receptor for substance P mediate this abnormal pain sensitivity by an unknown cellular mechanism. We report that in these, but not in other nociceptive lamina I cells, neurokinin 1 receptor-activated signal transduction pathways and activation of low-threshold (T-type) voltage-gated calcium channels synergistically facilitate activity- and calcium-dependent long-term potentiation at synapses from nociceptive nerve fibers. Thereby, memory traces of painful events are retained.

Axotomy differentially regulates voltage-gated calcium currents in mice sensory neurones

Medium sized dorsal root ganglion neurones are involved in tactile sensation and responsible for allodynia following nerve injury. We examined the effects of sciatic nerve injury on the expression of low and high voltage-gated calcium currents in medium sized neurones isolated from lumbar dorsal root ganglia of adult mice. Based on the relative expression of these calcium channel types, three populations of medium sized neurones were identified in controls. Type I, II and III populations were characterised respectively by small, predominant and no low voltage-gated current compared to the high voltage-gated current. Five days after nerve injury, calcium current expression was differentially affected by axotomy in these three subsets of medium neurones. Altogether, these results suggest that calcium channels are heterogeneously distributed among the medium sized neurones. This heterogeneity should provide specificity not only to sensory functions but also to sensory responses following nerve injury.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Actions of U-92032, a T-type Ca2+ channel antagonist, support a functional linkage between I(T) and slow intrathalamic rhythms

Thalamic relay neurons express high levels of T-type Ca(2+) channels, which support the generation of robust burst discharges. This intrinsically mediated form of phasic spike firing is thought to be critical in the generation of slow (3-4 Hz) synchronous oscillatory activity of absence epilepsy. Recordings made from brain slices or whole animals have shown that slow synchronous absence-like activity can be abolished when Ca(2+)-dependent burst firing in relay neurons is interrupted by the pharmacological or genetic inactivation of T-channels. Because succinimide drugs act as incomplete and nonspecific antagonists, we tested whether the novel T-channel antagonist U-92032 could provide stronger support for a role of T-channels in slow oscillatory activity. Ca(2+)-dependent rebound (LTS) bursts were recorded using whole cell current clamp in relay cells of the ventral basal complex (VB) from thalamic slices of adult rats. We used LTS kinetics to measure the availability of T-channels in VB cells after TTX. U-92032 (1 and 10 microM) reduced the maximum rate of depolarization of the isolated LTS by 51% and 90%, respectively, compared with the 35% reduction due to 2 mM methylphenylsuccinimide (MPS), the active metabolite of the antiabsence drug methsuximide. U-92032 (1 and 10 microM) also suppressed evoked, slow oscillations in thalamic slices with a time course similar for observed intracellular effects. Unlike MPS, we observed no substantial effects of short-term U-92032 applications (< or =2 h) on the generation of action potentials in VB cells. Our findings show U-92032 is a more potent, effective, and specific T-channel antagonist than previously studied succinimide antiabsence drugs and that it dramatically reduces epileptiform synchronous activity. This suggests that U-92032 or other specific T-channel antagonists may provide effective drug treatments for absence epilepsy.

Novel neuronal and astrocytic mechanisms in thalamocortical loop dynamics

In this review, we summarize three sets of findings that have recently been observed in thalamic astrocytes and neurons, and discuss their significance for thalamocortical loop dynamics. (i) A physiologically relevant 'window' component of the low-voltage-activated, T-type Ca(2+) current (I(Twindow)) plays an essential part in the slow (less than 1 Hz) sleep oscillation in adult thalamocortical (TC) neurons, indicating that the expression of this fundamental sleep rhythm in these neurons is not a simple reflection of cortical network activity. It is also likely that I(Twindow) underlies one of the cellular mechanisms enabling TC neurons to produce burst firing in response to novel sensory stimuli. (ii) Both electrophysiological and dye-injection experiments support the existence of gap junction-mediated coupling among young and adult TC neurons. This finding indicates that electrical coupling-mediated synchronization might be implicated in the high and low frequency oscillatory activities expressed by this type of thalamic neuron. (iii) Spontaneous intracellular Ca(2+) ([Ca(2+)](i)) waves propagating among thalamic astrocytes are able to elicit large and long-lasting N-methyl-D-aspartate-mediated currents in TC neurons. The peculiar developmental profile within the first two postnatal weeks of these astrocytic [Ca(2+)](i) transients and the selective activation of these glutamate receptors point to a role for this astrocyte-to-neuron signalling mechanism in the topographic wiring of the thalamocortical loop. As some of these novel cellular and intracellular properties are not restricted to thalamic astrocytes and neurons, their significance may well apply to (patho)physiological functions of glial and neuronal elements in other brain areas.

The making of a complex spike: ionic composition and plasticity

Climbing fiber (CF) activation evokes a large all-or-nothing electrical response in Purkinje cells (PCs), the complex spike. It has been suggested that the role of CFs (and thus complex spikes) is that of a "teacher" in simple learning paradigms such as associative eyeblink conditioning. An alternative hypothesis describes the olivocerebellar system as part of a timing device and denies a role of the CF input in learning. To date, neither of these hypotheses nor others can definitively be verified or discounted. Similarly, the complex spike evades a clear understanding when it comes to the cellular events underlying complex spike generation. What is known, however, is that complex spikes are associated with large dendritic calcium signals that are required for the induction of long-term depression (LTD) at the parallel fiber (PF)-PC synapse. PF-LTD is a form of long-term synaptic plasticity that has been suggested to underlie certain forms of cerebellar motor learning. In contrast to the PF input, the CF input has been considered invariant. Our recent discovery of LTD at the CF input shows that complex spikes are less static than previously assumed. In addition to depression of CF-evoked excitatory postsynaptic currents, long-lasting, selective reduction of slow complex spike components could be observed after brief CF tetanization. To understand the functional implications of CF-LTD, it is crucial to know the types of currents constituting the specific complex spike components. Here we review the "anatomy" of the complex spike as well as our observations of activity-dependent complex spike waveform modifications. In addition, we discuss which properties CF-LTD might add to the circuitry of the cerebellar cortex.

Involvement of T-type calcium channels in excitatory junction potentials in rat resistance mesenteric arteries

1. We investigated the role of voltage-operated calcium channels in sympathetic transmission and depolarization-induced contractions in the rat mesenteric artery. In particular, we investigated the role of the T-type voltage-operated calcium channels (T-channels) in mediating excitatory junction potentials (EJPs). 2. EJPs were evoked by electrical field stimulation (trains of five stimuli at 0.9 Hz) in small mesenteric arteries. The average resting membrane potential was -59.8+/-0.5 mV (n=65). Trains of stimuli evoked individual EJPs with the peak EJP of 6+/-0.2 mV (n=34) occurring with the second stimulus. Trains of EJPs were inhibited 90% by tetrodotoxin (0.1 micro M) or by omega-conotoxin GVIA (GVIA, 10 nM) indicating their neural origin. 3. The EJPs were not inhibited by the L-type calcium channel blocker nicardipine at 0.1 micro M, a concentration sufficient to abolish the contraction to potassium depolarization. However, mibefradil (3 micro M), considered a relatively selective T-channel antagonist, inhibited the EJPs by about 50%. This concentration of mibefradil did not inhibit GVIA-sensitive electrically-evoked twitches of the rat vas deferens. Thus the action of mibefradil in reducing EJPs is unlikely to be due to either inhibition of L- or N-type channels but is probably due to inhibition of T-channels. 4. The finding that Ni(2+) (300 micro M), an inhibitor of T-type calcium channels, also reduced EJP amplitude by about 80% but did not block electrically-evoked twitches in the rat vas deferens, further supports an important role of T-channels in mediating small depolarizations associated with the EJPs evoked by sympathetic nerve stimulation.

Alternative splicing of the rat Cav 3.3 T-type calcium channel gene produces variants with distinct functional properties1

Molecular diversity in T-type Ca(2+) channels is produced by expression of three genes, and alternative splicing of those genes. Prompted by differences noted between rat and human Ca(v)3.3 sequences, we searched for splice variants. We cloned six variants, which are produced by splicing at exon 33 and exon 34. Expression of the variants differed between brain regions. The electrophysiological properties of the variants displayed similar voltage-dependent gating, but differed in their kinetic properties. The functional impact of splicing was inter-related, suggesting an interaction. We conclude that alternative splicing of the Ca(v)3.3 gene produces channels with distinct properties.

T-Type calcium channel alpha1G and alpha1H subunits in human retinoblastoma cells and their loss after differentiation

Human retinoblastoma cells are multipotent retinal precursor cells capable of differentiating into photoreceptors, neurons, and glia. The current-voltage relation of the undifferentiated cells is dominated by a transient inward current that disappears shortly after differentiation. In 20 mM Ba(2+)-containing bath solutions, the current has an activation midpoint near -25 mV and appears to be fully inactivated at -20 mV. Sr(2+) and Ca(2+) are preferred charge carriers relative to Ba(2+), and the current vanishes in the absence of these divalent cations. Cd(2+) blocks the current with an IC(50) of 160 microM, and Ni(2+) blocks in a biphasic manner with IC(50)s of 22 and 352 microM. The current is unaffected when sodium is replaced with other monovalent cations, and it is insensitive to nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC. RT-PCR revealed the presence of alpha 1G and alpha 1H mRNA in undifferentiated cells, but following differentiation, a striking reduction of both alpha 1G and alpha 1H mRNA was found, and this was paralleled by the loss of T-type Ca channel currents. alpha 1I subunit mRNA levels were low in undifferentiated and differentiated cells. These results suggest that T-type Ca channels could play a role in undifferentiated retinoblastoma cell physiology since alpha 1G and alpha 1H Ca channel subunit expression is reduced in cells that have differentiated and exited the cell cycle.

Cloning and expression of the human T-type channel Ca(v)3.3: insights into prepulse facilitation

The full-length human Ca(v)3.3 (alpha(1I)) T-type channel was cloned, and found to be longer than previously reported. Comparison of the cDNA sequence to the human genomic sequence indicates the presence of an additional 4-kb exon that adds 214 amino acids to the carboxyl terminus and encodes the 3' untranslated region. The electrophysiological properties of the full-length channel were studied after transient transfection into 293 human embryonic kidney cells using 5 mM Ca(2+) as charge carrier. From a holding potential of -100 mV, step depolarizations elicited inward currents with an apparent threshold of -70 mV, a peak of -30 mV, and reversed at +40 mV. The kinetics of channel activation, inactivation, deactivation, and recovery from inactivation were very similar to those reported previously for rat Ca(v)3.3. Similar voltage-dependent gating and kinetics were found for truncated versions of human Ca(v)3.3, which lack either 118 or 288 of the 490 amino acids that compose the carboxyl terminus. A major difference between these constructs was that the full-length isoform generated twofold more current. These results suggest that sequences in the distal portion of Ca(v)3.3 play a role in channel expression. Studies on the voltage-dependence of activation revealed that a fraction of channels did not gate as low voltage-activated channels, requiring stronger depolarizations to open. A strong depolarizing prepulse (+100 mV, 200 ms) increased the fraction of channels that gated at low voltages. In contrast, human Ca(v)3.3 isoforms with shorter carboxyl termini were less affected by a prepulse. Therefore, Ca(v)3.3 is similar to high voltage-activated Ca(2+) channels in that depolarizing prepulses can regulate their activity, and their carboxy termini play a role in modulating channel activity.

Differential discrimination of fast and slow synaptic waveforms by two low-voltage-activated calcium channels

Electrophysiological analysis of human embryonic kidney 293 cells stably expressing recombinant channels was used to compare how the biophysical properties of the low-voltage-activated Ca(2+) channels encoded by the alpha(1G) (Ca(V)3.1) or alpha(1I) (Ca(V)3.3) subunits shape their responses to excitatory synaptic potentials. In medium containing 2 mM extracellular Ca(2+) standard current-voltage relationships demonstrated both channel types to be clearly low-voltage activated with significant slowly activating current responses being observed at -66 mV. At all test potentials examined, activation of Ca(V)3.3 was substantially slower than that of Ca(V)3.1. To probe how these different T-type channels might respond to excitatory postsynaptic potentials (EPSPs), mock EPSPs with different kinetic profiles were created from the sum of exponentials. These waveforms were then used as command templates in voltage-clamp experiments. Ca(V)3.1-mediated channels responded effectively to both rapidly decaying mock EPSPs and slowly decaying EPSPs. In contrast, Ca(V)3.3-mediated channels were poorly gated by rapidly decaying EPSPs but were effectively activated by the more prolonged synaptic response. When activated with mock EPSPs Ca(V)3.3-mediated currents were more resistant to steady-state depolarisation of the pre-stimulus holding potential. Ca(V)3.3 currents were also more resistant to repetitive application of prolonged EPSPs, which caused substantial inactivation of Ca(V)3.1-mediated currents. The addition of a single mock action potential to the peak of a rapidly decaying EPSP voltage-clamp template greatly enhanced the currents produced by either Ca(V)3.1 or Ca(V)3.3-expressing cells. This facilitatory effect was considerably greater for Ca(V)3.3-mediated channels. From these data we suggest that the slow activation kinetics of Ca(V)3.3-mediated T-type channels enable them to respond selectively to either slow or suprathreshold synaptic potentials.

Developmental expression patterns of alpha1H T-type Ca2+ channels during spermatogenesis and organogenesis in mice

The objectives of the present study were to investigate the expression patterns of T-type Ca2+ channel mRNA during spermatogenesis and organogenesis in mice. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to identify the subtypes of calcium channels present in the round spermatids isolated from mouse testes by flow cytometry. Transcripts of L-type (alpha1D), non-L-type (alpha1E) and T-type Ca2+ channels were detected in round spermatids. Analysis of PCR products of T-type Ca2+ channels indicated that only alpha1H subunits were detected in round spermatids. The appearance and differential distribution of alpha1H T-type Ca2+ channel mRNA during mouse spermatogenesis and postimplantation embryogenesis (embryonic (E) days E9, E12, E15) were investigated by in situ hybridization with digoxigenin-labeled RNA probes coupled with alkaline phosphatase detection. In testes from adult and immature mice (postnatal 2 and 3 weeks), alpha1H T-type Ca2+ channel mRNA was expressed in all developing germ cells and sertoli cells. On E9 and E12, tissues of the central nervous system, such as the telencephalon, expressed alpha1H T-type Ca2+ channel mRNA. On E15, signals were detected throughout all organs of the embryo. These findings indicate that the expression of alpha1H T-type Ca2+ channels is spatio-temporally regulated during spermatogenesis and organogenesis.