T-type Ca2+ channels in absence epilepsy

Absence epilepsy accompanies the paroxysmal oscillations in the thalamocortical circuit referred as spike and wave discharges (SWDs). Low-threshold burst firing mediated by T-type Ca²⁺ channels highly expressed in both inhibitory thalamic reticular nuclei (TRN) and excitatory thalamocortical (TC) neurons has been correlated with the generation of SWDs. A generally accepted view has been that rhythmic burst firing mediated by T-type channels in both TRN and TC neurons are equally critical in the generation of thalamocortical oscillations during sleep rhythms and SWDs. This review examined recent studies on the T-type channels in absence epilepsy which leads to an idea that even though both TRN and TC nuclei are required for thalamocortical oscillations, the contributions of T-type channels to TRN and TC neurons are not equal in the genesis of sleep spindles and SWDs. Accumulating evidence revealed a crucial role of TC T-type channels in SWD generation. However, the role of TRN T-type channels in SWD generation remains controversial. Therefore, a deeper understanding of the functional consequences of modulating each T-type channel subtype could guide the development of therapeutic tools for absence seizures while minimizing side effects on physiological thalamocortical oscillations.     

Ca2+-dependent large conductance K+ currents in thalamocortical relay neurons of different rat strains

Mutations in genes coding for Ca²⁺ channels were found in patients with childhood absence epilepsy (CAE) indicating a contribution of Ca²⁺-dependent mechanisms to the generation of spike-wave discharges (SWD) in humans. Since the involvement of Ca²⁺ signals remains unclear, the aim of the present study was to elucidate the function of a Ca²⁺-dependent K⁺ channel (BK_Ca) under physiological conditions and in the pathophysiological state of CAE. The activation of BK_Ca channels is dependent on both voltage and intracellular Ca²⁺ concentrations. Moreover, these channels exhibit an outstandingly high level of regulatory heterogeneity that builds the basis for the influence of BK_Ca channels on different aspects of neuronal activity. Here, we analyse the contribution of BK_Ca channels to firing of thalamocortical relay neurons, and we test the hypothesis that BK_Ca channel activity affects the phenotype of a genetic rat model of CAE. We found that the activation of the β₂-adrenergic receptor/protein kinase A pathway resulted in BK_Ca channel inhibition. Furthermore, BK_Ca channels affect the number of action potentials fired in a burst and produced spike frequency adaptation during tonic activity. The latter result was confirmed by a computer modelling approach. We demonstrate that the β₂-adrenergic inhibition of BK_Ca channels prevents spike frequency adaptation and, thus, might significantly support the tonic firing mode of thalamocortical relay neurons. In addition, we show that BK_Ca channel functioning differs in epileptic WAG/Rij and thereby likely contributes to highly synchronised, epileptic network activity.     

Ca2+ signaling by T-type Ca2+ channels in neurons

Among the major families of voltage-gated Ca²⁺ channels, the low-voltage-activated channels formed by the Ca_v3 subunits, referred to as T-type Ca²⁺ channels, have recently gained increased interest in terms of the intracellular Ca²⁺ signals generated upon their activation. Here, we provide an overview of recent reports documenting that T-type Ca²⁺ channels act as an important Ca²⁺ source in a wide range of neuronal cell types. The work is focused on T-type Ca²⁺ channels in neurons, but refers to non-neuronal cells in cases where exemplary functions for Ca²⁺ entering through T-type Ca²⁺ channels have been described. Notably, Ca²⁺ influx through T-type Ca²⁺ channels is the predominant Ca²⁺ source in several neuronal cell types and carries out specific signaling roles. We also emphasize that Ca²⁺ signaling through T-type Ca²⁺ channels occurs often in select subcellular compartments, is mediated through strategically co-localized targets, and is exploited for unique physiological functions. Lucius Cueni and Marco Canepari contributed equally to this review.     

Spontaneous Na+ and Ca2+ spike firing of cerebellar Purkinje neurons at high pressure

 The effects of high pressure (up to 10.1 MPa) on the spontaneous firing of Purkinje neurons in guinea-pig cerebellar slices were studied using the macropatch clamp technique. Pressure did not significantly alter the single somatic Na⁺ spike parameters or the frequency of regular Na⁺ spike firing. When Na⁺ currents were blocked by 0.5–1 μM tetrodotoxin (TTX), a pressure of 10.1 MPa slightly reduced the dendritic Ca²⁺ spike amplitude to 90.2±3.1% of its control value, and slowed its kinetics. The effects of pressure on the single Ca²⁺ spike were even less prominent when K⁺ currents were blocked by 5 mM 4-aminopyridine (4-AP). Pressure prolonged the active period of Ca²⁺ spike firing to 152.2±10.4% of the control value. Within the active period pressure increased the inter-spike interval to 164.9±8.7% and suppressed the typical firing of doublets. The latter changes were reversed by a high extracellular potassium concentration ([K⁺]_o) and 1 μM 4-AP, whereas in the presence of 5 mM 4-AP the pattern was insensitive to pressure. A high [Ca²⁺]_o reduced the firing frequency and suppressed doublet firing in a manner reminiscent of the pressure effect, but these changes could not be reversed by 4-AP. A low [Ca²⁺]_o slightly increased the firing of doublets. These results show that the single somatic Na⁺ spike is insensitive and the dendritic Ca²⁺ spike is only mildly sensitive to pressure. However, alterations in Ca²⁺ spike firing pattern suggest that modulation of dendritic K⁺ currents induce depression of dendritic excitability at pressure. Received: 19 May 1998 / Received after revision: 15 July 1998 / Accepted: 3 September 1998     

Nonselective cation channels are essential for maintaining intracellular Ca2+ levels and spontaneous firing activity in the midbrain dopamine neurons

Intracellular Ca²⁺ and Ca²⁺-permeable ion channels are important in regulating the firing activity and pattern of midbrain dopamine neurons, but the role of Ca²⁺-permeable nonselective cation channels (NSCCs) on spontaneous firing activity is unclear. Therefore, we investigated how Ca²⁺-permeable NSCCs modulate spontaneous firing activity and cytosolic Ca²⁺ concentration ([Ca²⁺]_c) in acutely isolated midbrain dopamine neurons of the rat. Applications of voltage-dependent Ca²⁺ channels antagonists failed to abolish spontaneous firing activity completely, but they decreased firing rate and [Ca²⁺]_c. However, a blockade of NSCCs by 2-APB or SKF96365 more potently suppressed spontaneous firings with a depolarization of membrane potential and strong decreases in basal [Ca²⁺]_c levels. The depolarization of membrane potentials was attenuated by intracellular dialysis with 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA). NSCCs blockers inhibited oscillatory potentials and decreased basal [Ca²⁺]_c in the presence of tetrodotoxin. Apamin, a small-conductance Ca²⁺-activated K⁺ channel inhibitor, depolarized membrane potentials and enhanced firing rates. From these data, we conclude that NSCCs not only make up the tonic Ca²⁺ entry pathways to uphold basal [Ca²⁺]_c levels but also contribute to generation of spontaneous firings, thereby regulating spontaneous firing activities of the midbrain dopamine neurons.     

Modulation of a Ca2+-dependent K+-current by intracellular cAMP in rat thalamocortical relay neurons

Voltage-activated calcium channels in thalamic neurons are considered important elements in the generation of thalamocortical burst firing during periods of electroencephalographic synchronization. A potent counterpart of calcium-mediated depolarization may reside in the activation of calcium-dependent potassium conductances. In the present study, thalamocortical relay cells that were acutely dissociated from the rat ventrobasal thalamic complex (VB) were studied using whole-cell patch-clamp techniques. The calcium-dependent potassium-current (I_K(Ca)) was evident as a slowly activating component of outward current sensitive to the calcium ions (Ca²⁺)-channel blocker methoxyverapamil (10 μM) and to substitution of external calcium by manganese. The I_K(Ca) was blocked by tetraethylammonium chloride (1 mM) and iberiotoxin (100 nM), but not apamin (1 μM). In addition, isolated VB neurons were immunopositive to anti-α_(913–926) antibody, a sequence-directed antibody to the α-subunit of “big” Ca²⁺-dependent K⁺-channel (BK_Ca) channels. Activators of the adenylyl cyclase cyclic adenosine monophosphate (cAMP) system, such as forskolin (20 μM), dibutyryl-cAMP (10 mM) and 3-isobutyl-1-methylxanthine (500 μM), selectively and reversibly suppressed I_K(Ca). These results suggest that a rise in intracellular cAMP level leads to a decrease in a calcium-dependent potassium conductance presumably mediated via BK_Ca type channels, thereby providing an additional mechanism by which neurotransmitter systems are able to control electrogenic activity in thalamocortical neurons and circuits during various states of electroencephalographic synchronization and de-synchronization.     

Ischemic injury decreases parvalbumin expression in a middle cerebral artery occlusion animal model and glutamate-exposed HT22 cells

Measles virus (MV) infection may lead to severe chronic CNS disease processes, including MV-induced encephalitis. Because the intracellular Ca²⁺ concentration ([Ca²⁺]_i) is a major determinant of the (patho-)physiological state in all cells we asked whether important Ca²⁺ conducting pathways are affected by MV infection in cultured cortical rat neurons. Patch-clamp measurements revealed a decrease in voltage-gated Ca²⁺ currents during MV-infection, while voltage-gated K⁺ currents and NMDA-evoked currents were unaffected. Calcium-imaging experiments using 50 mM extracellular KCl showed reduced [Ca²⁺]_i increases in MV-infected neurons, confirming a decreased activity of voltage-gated Ca²⁺ channels. In contrast, the group-I metabotropic glutamate receptor (mGluR) agonist DHPG evoked changes in [Ca²⁺]_i that were increased in MV-infected cells. Our results show that MV infection conversely regulates Ca²⁺ signals induced by group-I mGluRs and by voltage-gated Ca²⁺ channels, suggesting that these physiological impairments may contribute to an altered function of cortical neurons during MV-induced encephalitis.     

T-type channels-secretion coupling: evidence for a fast low-threshold exocytosis

T-type channels are transient low-voltage-activated (LVA) Ca²⁺ channels that control Ca²⁺ entry in excitable cells during small depolarizations around resting potential. Studies in the past 20 years focused on the biophysical, physiological, and molecular characterization of T-type channels in most tissues. This led to a well-defined picture of the functional role of LVA channels in controlling low-threshold spikes, oscillatory cell activity, muscle contraction, hormone release, cell growth and differentiation. So far, little attention has been devoted to the role of T-type channels in transmitter release, which mainly involves channel types belonging to the high-voltage-activated (HVA) Ca²⁺ channel family. However, evidence is accumulating in favor of a unique participation of T-type channels in fast transmitter release. Clear data are now reported in reciprocal synapses of the retina and olfactory bulb, synaptic contacts between primary afferent and second order nociceptive neurons, rhythmic inhibitory interneurons of invertebrates and clonal cell lines transfected with recombinant α₁ channel subunits. T-type channels also regulate the large dense-core vesicle release of neuroendocrine cells where Ca²⁺ dependence, rate of vesicle release, and size of readily releasable pool appear comparable to those associated to HVA channels. This suggests that when sufficiently expressed and properly located near the release zones, T-type channels can trigger fast low-threshold secretion. In this study, we will review the main findings that assign a specific task to T-type channels in fast exocytosis, discussing their possible involvement in the control of the Ca²⁺-dependent processes regulating exocytosis like vesicle depletion and vesicle recycling.     

Voltage-gated Ca<Superscript>2+</Superscript> channel mRNAs and T-type Ca<Superscript>2+</Superscript> currents in rat gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons play a pivotal role in the neuroendocrine regulation of reproduction. We have previously reported that rat GnRH neurons exhibit voltage-gated Ca²⁺ currents. In this study, oligo-cell RT-PCR was carried out to identify subtypes of the α₁ subunit of voltage-gated Ca²⁺ channels in adult rat GnRH neurons. GnRH neurons expressed mRNAs for all five types of voltage-gated Ca²⁺ channels. For T-type Ca²⁺ channels, α_1H was dominantly expressed in GnRH neurons. Electrophysiological analysis in acute slice preparations revealed that GnRH neurons from adult rats exhibited T-type Ca²⁺ currents with fast inactivation kinetics (~20 ms at −30 mV) and a time constant of recovery from inactivation of ~0.6 s. These results indicate that rat GnRH neurons express subtypes of the α₁ subunit for all five types of voltage-gated Ca²⁺ channel, and that α_1H was the dominant subtype in T-type Ca²⁺ channels.     

CaV3.2 is the major molecular substrate for redox regulation of T-type Ca2+ channels in the rat and mouse thalamus

Although T-type Ca²⁺ channels in the thalamus play a crucial role in determining neuronal excitability and are involved in sensory processing and pathophysiology of epilepsy, little is known about the molecular mechanisms involved in their regulation. Here, we report that reducing agents, including endogenous sulfur-containing amino acid l -cysteine, selectively enhance native T-type currents in reticular thalamic (nRT) neurons and recombinant Ca_V3.2 (α1H) currents, but not native and recombinant Ca_V3.1 (α1G)- and Ca_V3.3 (α1I)-based currents. Consistent with this data, T-type currents of nRT neurons from transgenic mice lacking Ca_V3.2 channel expression were not modulated by reducing agents. In contrast, oxidizing agents inhibited all native and recombinant T-type currents non-selectively. Thus, our findings directly demonstrate that Ca_V3.2 channels are the main molecular substrate for redox regulation of neuronal T-type channels. In addition, because thalamic T-type channels generate low-threshold Ca²⁺ spikes that directly correlate with burst firing in these neurons, differential redox regulation of these channels may have an important function in controlling cellular excitability in physiological and pathological conditions and fine-tuning of the flow of sensory information into the central nervous system.     

A newly identified Ca2+ dependent K+ channel in the smooth muscle membrane of single cells dispersed from the rabbit portal vein

We found a new type of Ca²⁺-dependent K⁺ channel in smooth muscle cell membranes of single cells of the rabbit portal vein. A slope conductance of the current was 180 pS when 142 mM K⁺ solution was exposed to both sides of the membrane (this channel was named the K_M channel, in comparison to the known K_L and K_S channels from the same membrane patch; Inoue et al. 1985). This K_M channel was less sensitive to the cytoplasmic Ca²⁺ concentration, [Ca²⁺]_i, but was sensitive to the extracellular Ca²⁺, [Ca²⁺]_o, e.g. in the outside-out membrane patch, lowering the [Ca²⁺]_o in the bath markedly reduced the open probability of this channel, and also in cell-attached configuration, lowering of the [Ca²⁺]_o using the internally perfused patch clamp electrode device reduced the opening of K_M channel. TEA⁺ (1–10 mM) reduced the amplitude of the elementary current through the K_M channel applied from each side of the membrane, but this agent inhibited the K_M channel to a greater extent when applied to the inner than to the outer surface of the membrane. Furthermore, this K_M channel had a weak voltage dependency, and the open probability of the channel remained much the same within a wide range of potential (from –60 mV to +60 mV). Whereas most Ca²⁺-dependent K⁺ channels are regulated mainly by [Ca²⁺]_i and possess a voltage dependency, these properties of the K_M channel differed from other Ca²⁺-dependent K⁺ channels. The elucidation of this K_M channel should facilitate explanations of the actions of external Ca²⁺ or TEA⁺ on the membrane potential, in the smooth muscles of the rabbit portal vein.     

The T-type calcium channel as a new therapeutic target for Parkinson’s disease

Parkinson’s disease (PD) is one of the most prevalent movement disorder caused by degeneration of the dopaminergic neurons in substantia nigra pars compacta. Deep brain stimulation (DBS) at the subthalamic nucleus (STN) has been a new and effective treatment of PD. It is interesting how a neurological disorder caused by the deficiency of a specific chemical substance (i.e., dopamine) from one site could be so successfully treated by a pure physical maneuver (i.e., DBS) at another site. STN neurons could discharge in the single-spike or the burst modes. A significant increase in STN burst discharges has been unequivocally observed in dopamine-deprived conditions such as PD, and was recently shown to have a direct causal relation with parkinsonian symptoms. The occurrence of burst discharges in STN requires enough available T-type Ca²⁺ currents, which could bring the relatively negative membrane potential to the threshold of firing Na⁺ spikes. DBS, by injection of negative currents into the extracellular space, most likely would depolarize the STN neuron and then inactivate the T-type Ca²⁺ channel. Burst discharges are thus decreased and parkinsonian locomotor deficits ameliorated. Conversely, injection of positive currents into STN itself could induce parkinsonian locomotor deficits in animals without dopaminergic lesions. Local application of T-type Ca²⁺ channel blockers into STN would also dramatically decrease the burst discharges and improve parkinsonian locomotor symptoms. Notably, zonisamide, which could inhibit T-type Ca²⁺ currents in STN, has been shown to benefit PD patients in a clinical trial. From the pathophysiological perspectives, PD can be viewed as a prototypical disorder of “brain arrhythmias”. Modulation of relevant ion channels by physical or chemical maneuvers may be important therapeutic considerations for PD and other diseases related to deranged neural rhythms.     

Calcium-dependent atrial slow action potentials generated with phosphatidic acid or phospholipaseD

Phosphatidic acid (PA) formed following phosphatidylinositol hydrolysis has been proposed as a necessary step in receptor-mediated Ca²⁺ flux. This study demonstrates that PA generates Ca²⁺-dependent slow action potentials (APs) in rat atrium partially depolarized with 22 mM K⁺. The slow response was not due to release of endogenous catecholamines or prostaglandin formation since propranolol and indomethacin failed to attenuate the PA-induced slow AP in normal and reserpinzed rats. PA-induced slow APs demonstrated Ca²⁺-dependence such that increasing [Ca²⁺]_o from 0.5 to 5.0 mM caused the amplitude of the slow AP to rise linearly with the logarithm of [Ca²⁺]_o. Phospholipase D (PLD) but not phospholipase C, was able to induce a slow AP, possibly through PA formation. Adenosine attenuated the PA and PLD-induced slow response and aminophylline reversed these effects. The observation that PA and PLD generate Ca²⁺-dependent slow APs in depolarized rat atrium supports a role for PA mediating Ca²⁺ influx.Supported by Grants HL 10384 and HL 19242     

Small (SKCa) Ca2+-activated K+ channels in cultured rat hippocampal pyramidal neurones

 Small (SK_Ca) Ca²⁺-activated K⁺ channels were identified in membrane patches excised from cultured CA1-CA3 pyramidal neurones of the neonatal rat hippocampus. When recorded in low-K⁺ extracellular solution ([K⁺]_o=2.5 mM), SK_Ca channels had a low conductance (@3 pS at 0 mV), were activated by ≥175 nM Ca²⁺ (P _o=0.54 at 500 nM Ca²⁺) and there were two open-time components (2.1 and @70 ms) to their activity. These properties of single SK_Ca channels are similar to those of slow after-hyperpolarization channels (sAHP) previously inferred from fluctuation analysis of the sAHP current. It is concluded that the SK_Ca channel reported here may be the channel that generates the sAHP in hippocampal pyramidal neurones. Received: 9 July 1998 / Received after revision: 5 October 1998 / Accepted: 7 October 1998     

Electrophysiological study on the high K+ sensitivity of rat glomerulosa cells

 Elevation of extracellular potassium concentration by as little as some tenth of mM activates rat adrenal glomerulosa cells. In the present study some factors responsible for this high K⁺ sensitivity were examined. Using whole-cell voltage-clamp technique we found that both T-type and L-type voltage-dependent Ca²⁺ channels have very low threshold potential (–71 and –58 mV, resp.). By means of patch-clamp technique combined with single-cell fluorimetry we also provided evidence that the activation of I_gl, a K⁺-activated inward rectifying current is associated with Ca²⁺ influx. Both the low activation threshold of voltage-dependent Ca²⁺ channels and the function of I_gl contribute to the exceptional K⁺ sensitivity of the glomerulosa cells. Received: 30 September 1997 / Accepted: 4 November 1997     

Calcium-dependent inhibition of T-type calcium channels by TRPV1 activation in rat sensory neurons

We studied the inhibitory effects of transient receptor potential vanilloid-1 (TRPV1) activation by capsaicin on low-voltage-activated (LVA, T-type) Ca²⁺ channel and high-voltage-activated (HVA; L, N, P/Q, R) currents in rat DRG sensory neurons, as a potential mechanism underlying capsaicin-induced analgesia. T-type and HVA currents were elicited in whole-cell clamped DRG neurons using ramp commands applied before and after 30-s exposures to 1 μM capsaicin. T-type currents were estimated at the first peak of the I–V characteristics and HVA at the second peak, occurring at more positive potentials. Small and medium-sized DRG neurons responded to capsaicin producing transient inward currents of variable amplitudes, mainly carried by Ca²⁺. In those cells responding to capsaicin with a large Ca²⁺ influx (59% of the total), a marked inhibition of both T-type and HVA Ca²⁺ currents was observed. The percentage of T-type and HVA channel inhibition was prevented by replacing Ca²⁺ with Ba²⁺ during capsaicin application or applying high doses of intracellular BAPTA (20 mM), suggesting that TRPV1-mediated inhibition of T-type and HVA channels is Ca²⁺-dependent and likely confined to membrane nano-microdomains. Our data are consistent with the idea that TRPV1-induced analgesia may derive from indirect inhibition of both T-type and HVA channels which, in turn, would reduce the threshold of nociceptive signals generation (T-type channel inhibition) and nociceptive synaptic transmission (HVA-channels inhibition).     

Impaired hippocampal Ca homeostasis and concomitant K channel dysfunction in a mouse model of rett syndrome during anoxia

Methyl-CpG-binding protein 2 (MeCP2) deficiency causes Rett syndrome (RTT), a neurodevelopmental disorder characterized by severe cognitive impairment, synaptic dysfunction, and hyperexcitability. Previously we reported that the hippocampus of MeCP2-deficient mice (Mecp2^−/y), a mouse model for RTT, is more susceptible to hypoxia. To identify the underlying mechanisms we now focused on the anoxic responses of wildtype (WT) and Mecp2^−/y CA1 neurons in acute hippocampal slices. Intracellular recordings revealed that Mecp2^−/y neurons show only reduced or no hyperpolarizations early during cyanide-induced anoxia, suggesting potassium channel (K⁺ channel) dysfunction. Blocking adenosine-5′-triphosphate-sensitive K⁺ channels (K_ATP-) and big-conductance Ca²⁺-activated K⁺ channels (BK-channels) did not affect the early anoxic hyperpolarization in either genotype. However, blocking Ca²⁺ release from the endoplasmic reticulum almost abolished the anoxic hyperpolarizations in Mecp2^−/y neurons. Single-channel recordings confirmed that neither K_ATP- nor BK-channels are the sole mediators of the early anoxic hyperpolarization. Instead, anoxia Ca²⁺-dependently activated various small/intermediate-conductance K⁺ channels in WT neurons, which was less evident in Mecp2^−/y neurons. Yet, pharmacologically increasing the Ca²⁺ sensitivity of small/intermediate-conductance K_Ca channels fully restored the anoxic hyperpolarization in Mecp2^−/y neurons. Furthermore, Ca²⁺ imaging unveiled lower intracellular Ca²⁺ levels in resting Mecp2^−/y neurons and reduced anoxic Ca²⁺ transients with diminished Ca²⁺ release from intracellular stores. In conclusion, the enhanced hypoxia susceptibility of Mecp2^−/y hippocampus is primarily associated with disturbed Ca²⁺ homeostasis and diminished Ca²⁺ rises during anoxia. This secondarily attenuates the activation of K_Ca channels and thereby increases the hypoxia susceptibility of Mecp2^−/y neuronal networks. Since cytosolic Ca²⁺ levels also determine neuronal excitability and synaptic plasticity, Ca²⁺ homeostasis may constitute a promising target for pharmacotherapy in RTT.     

Mechanism of anesthesia revealed by shunting actions of isoflurane on thalamocortical neurons

By using thalamic brain slices from juvenile rats and the whole cell recording technique, we determined the effects of aqueous applications of the anesthetic isoflurane (IFL) on tonic and burst firing activities of ventrobasal relay neurons. At concentrations equivalent to those used for in vivo anesthesia, IFL induced a hyperpolarization and increased membrane conductance in a reversible and concentration-dependent manner (ionic mechanism detailed in companion paper). The increased conductance short-circuited the effectiveness of depolarizing pulses and was the main cause for inhibition of tonic firing of action potentials. Despite the IFL-induced hyperpolarization, which theoretically should have promoted bursting, the shunt blocked the low-threshold Ca2+ spike (LTS) and associated burst firing of action potentials as well as the high-threshold Ca2+ spike (HTS). Increasing the amplitude of either the depolarizing test pulse or hyperpolarizing prepulse or increasing the duration of the hyperpolarizing prepulse partially reversed the blockade of the LTS burst. In voltage-clamp experiments on the T-type Ca2+ current, which produces the LTS, IFL decreased the spatial distribution of imposed voltages and hence impaired the activation of spatially distant T channels. Although IFL may have increased a dendritic leak conductance or decreased dendritic Ca2+ currents, the somatic shunt appeared to block initiation of the LTS and HTS as well as their electrotonic propogation to the axon hillock. In summary, IFL hyperpolarized thalamocortical neurons and shunted voltage-dependent Na+ and Ca2+ currents. Considering the importance of the thalamus in relaying different sensory modalities (i.e., somatosensation, audition, and vision) and motor information as well as the corticothalamocortical loops in mediating consciousness, the shunted firing activities of thalamocortical neurons would be instrumental for the production of anesthesia in vivo.     

Stimulation-induced calcium signalling and ion transport in rat pancreatic acini

In the present study we have characterized receptor-mediated Ca²⁺ signalling patterns as well as Ca²⁺-mediated ion transport mechanisms in collagenase isolated rat pancreatic acini. Measurements of the initial Ca²⁺ response to maximal carbachol stimulation revealed a rapid increase in [Ca²⁺]_i, which, in general, occurred synchronously throughout the cells. Less frequently, not all cells in the acinus responded to carbachol, but did respond to subsequent stimulation with bombesin, indicating that not all cells possess receptors for all the applied agonists. In view of the heterogeneity in the agonist-evoked Ca²⁺ responses, ionomycin was used to assess the role of Ca²⁺ in activating K⁺, Na⁺ and Cl^- transport mechanisms. Ionomycin induced a rise in [Ca²⁺]_i, thereby increasing Cl^- permeability as well as stimulating K⁺ efflux, probably through non-specific cation channels. However, the resting K⁺ efflux was insensitive to blockers of non-specific cation channels, indicating the existence of a selective resting K⁺ conductance. Ionomycin also stimulated influx of Na⁺, which in part was mediated by non-specific cation channels. The changes in ion fluxes measured in the present study revealed that when [Ca²⁺]_i is raised in rat pancreatic acini, they gain Na⁺ and Cl^- and lose K⁺, with non-specific cation channels being essential for this process.