Potassium-channel closure taken to TASK

A Direct Interaction between Cyclodextrins and TASK Channels Decreases the Leak Current in Cerebellar Granule Neurons

Simple Summary

Cyclodextrins are cyclic oligosaccharides used to deplete cholesterol from cellular membranes. The effects of methyl-β-cyclodextrin (MβCD) on cellular functions originate principally from reductions in cholesterol levels. In this study, using immunocytochemistry, heterologous expression of K2P channels, and cholesterol-depleting maneuvers, we provide evidence of expression in cultured rat cerebellar granule neurons (CGNs) of TWIK-1 (K2P1), TASK-1 (K2P3), TASK-3 (K2P9), and TRESK (K2P18) channels and their association with lipid rafts using the specific lipids raft markers. In addition, we show a direct blocking with MβCD of TASK-1 and TASK-3 channels as well as for the covalently concatenated heterodimer TASK-1/TASK-3.

Abstract

Two pore domain potassium channels (K2P) are strongly expressed in the nervous system (CNS), where they play a central role in excitability. These channels give rise to background K⁺ currents, also known as IK_SO (standing-outward potassium current). We detected the expression in primary cultured cerebellar granule neurons (CGNs) of TWIK-1 (K2P1), TASK-1 (K2P3), TASK-3 (K2P9), and TRESK (K2P18) channels by immunocytochemistry and their association with lipid rafts using the specific lipids raft markers flotillin-2 and caveolin-1. At the functional level, methyl-β-cyclodextrin (MβCD, 5 mM) reduced IK_SO currents by ~40% in CGN cells. To dissect out this effect, we heterologously expressed the human TWIK-1, TASK-1, TASK-3, and TRESK channels in HEK-293 cells. MβCD directly blocked TASK-1 and TASK-3 channels and the covalently concatenated heterodimer TASK-1/TASK-3 currents. Conversely, MβCD did not affect TWIK-1- and TRESK-mediated K+ currents. On the other hand, the cholesterol-depleting agent filipin III did not affect TASK-1/TASK-3 channels. Together, the results suggest that neuronal background K⁺ channels are associated to lipid raft environments whilst the functional activity is independent of the cholesterol membrane organization.

Differential expression of two-pore domain potassium channels in rat cerebellar granule neurons

Two pore domain potassium (K2P) channels are mostly present in the central nervous system (CNS) where they play important roles in modulating neuronal excitability. K2P channels give rise to background K(+) currents (IKSO) a key component in setting and maintaining the resting membrane potential in excitable cells. Here, we studied the expression and relative abundances of K2P channels in cerebellar granule neurons (CGNs), combining molecular biology, electrophysiology and immunologic techniques. The CGN IKSO was very sensitive to external pH, as previously reported. Quantitative determination of mRNA expression level demonstrated the existence of an accumulation pattern of transcripts in CGN that encode K2P9>K2P1>K2P3>K2P18>K2P2=K2P10>K2P4>K2P5 subunits. The presence of the major K2P subunits expressed was then confirmed by Western blot and immunofluorescence analysis, demonstrating robust expression of K2P1 (TWIK-1), K2P3 (TASK-1), K2P9 (TASK-3) and K2P18 (TRESK) channel protein. Based, on these results, it is concluded that K2P1, -3, -9 and -18 subunits represent the majority component of IKSO current in CGN.

Dopamine and gamma band synchrony in schizophrenia--insights from computational and empirical studies

Dopamine modulates cortical circuit activity in part through its actions on GABAergic interneurons, including increasing the excitability of fast-spiking interneurons. Though such effects have been demonstrated in single cells, there are no studies that examine how such mechanisms may lead to the effects of dopamine at a neural network level. With this motivation, we investigated the effects of dopamine on synchronization in a simulated neural network composed of excitatory and fast-spiking inhibitory Wang-Buzsaki neurons. The effects of dopamine were implemented through varying leak K+ conductance of the fast-spiking interneurons and the network synchronization within the gamma band (∼40 Hz) was analyzed. Parametrically varying the leak K+ conductance revealed an inverted-U shaped relationship, with low gamma band power at both low and high conductance levels and optimal synchronization at intermediate conductance levels. We also examined the effects of modulating excitability of the inhibitory neurons more generically using an idealized model with theta neurons, with similar findings. Moreover, such a relationship holds when the external input is both tonic and periodic. Our computational results mirror our empirical study of dopamine modulation in schizophrenia and healthy controls, which showed that amphetamine administration increased gamma power in patients but decreased it in controls. Together, our computational and empirical investigations indicate that dopamine can modulate cortical gamma band synchrony in an inverted-U fashion and that the physiologic effects of dopamine on single fast-spiking interneurons can give rise to such non-monotonic effects at the network level.

Persistent Na+ and K+-dominated leak currents contribute to respiratory rhythm generation in the pre-Bötzinger complex in vitro

A central problem in analyzing neural circuit function is establishing how intrinsic neuronal conductances contribute to the generation of network activity. We used real-time calcium activity imaging combined with whole-cell patch-clamp recording to analyze contributions of subthreshold conductances in the excitatory rhythm-generating network in the respiratory pre-Bötzinger complex (pre-BötC) of neonatal rat in vitro brainstem slice preparations. Voltage-clamp ramp recordings from imaged pre-BötC neurons revealed that persistent sodium (NaP) and K+-dominated leak currents primarily contribute to subthreshold I-V relations. We quantified NaP and leak conductance densities (g/C(m)) in intrinsic oscillatory bursters and intrinsically nonbursters, the two main electrophysiological phenotypes of inspiratory neurons within the pre-BötC. Densities of g(NaP) were significantly higher for intrinsic bursters, whereas leak conductance densities were not significantly different between intrinsic bursters and nonbursters. By pharmacologically manipulating g(NaP) and/or g(Leak) directly within the pre-BötC, we could modulate network oscillation frequency over a wide dynamic range and cause transitions between oscillatory and quiescent states. These results were consistent with models of the pre-BötC excitatory network consisting of heterogeneous mixtures of intrinsic bursters and nonintrinsic bursters incorporating g(NaP) and g(Leak) with parameter values found experimentally. We propose a paradigm whereby NaP and Leak represent a functional set of subthreshold conductances that endow the pre-BötC with rhythmogenic properties and represent targets for modulatory control of inspiratory rhythm generation.

Modifying the subunit composition of TASK channels alters the modulation of a leak conductance in cerebellar granule neurons

Two-pore domain potassium (K2P) channel expression is believed to underlie the developmental emergence of a potassium leak conductance [IK(SO)] in cerebellar granule neurons (CGNs), suggesting that K2P function is an important determinant of the input conductance and resting membrane potential. To investigate the role that different K2P channels may play in the regulation of CGN excitability, we generated a mouse lacking TASK-1, a K2P channel known to have high expression levels in CGNs. In situ hybridization and real-time PCR studies in wild-type and TASK-1 knock-outs (KOs) demonstrated that the expression of other K2P channels was unaltered in CGNs. TASK-1 knock-out mice were healthy and bred normally but exhibited compromised motor performance consistent with altered cerebellar function. Whole-cell recordings from adult cerebellar slice preparations revealed that the resting excitability of mature CGNs was no different in TASK-1 KO and littermate controls. However, the modulation of IK(SO) by extracellular Zn2+, ruthenium red, and H+ was altered. The IK(SO) recorded from TASK-1 knock-out CGNs was no longer sensitive to alkalization and was blocked by Zn2+ and ruthenium red. These results suggest that a TASK-1-containing channel population has been replaced by a homodimeric TASK-3 population in the TASK-1 knock-out. These data directly demonstrate that TASK-1 channels contribute to the properties of IK(SO) in adult CGNs. However, TASK channel subunit composition does not alter the resting excitability of CGNs but does influence sensitivity to endogenous modulators such as Zn2+ and H+.

Background potassium channel block and TRPV1 activation contribute to proton depolarization of sensory neurons from humans with neuropathic pain

Protons cause a sustained depolarization of human dorsal root ganglion (DRG) neurons [Baumann et al. (1996) Pain, 65, 31-38]. In the present study we sought to determine which ion channels are expressed in human DRG neurons that could mediate the sustained responses observed in the patch-clamp recordings. RT-PCR of material from the DRG tissue revealed the presence of mRNAs for a nonselective cation channel that is activated by protons (TRPV1) and background potassium channels that are blocked by protons (TASK-1, TASK-3 and Kir2.3). Highly acidic solution (pH 5.4) applied to cultured DRG neurons evoked prolonged currents that were associated with a net increase in membrane conductance. Consistent with the involvement of TRPV1, these proton-evoked currents were blocked by capsazepine and were only found in neurons that responded to capsaicin with an increase in membrane conductance. Less acidic extracellular solution (pH 6.0) evoked such currents only rarely, but was able to strongly enhance the currents evoked by capsaicin. Capsazepine (1 microm) blocked the currents evoked by capsaicin at pH 7.35, as well as the potentiated responses to capsaicin at pH 6.0. In neurons that were not excited by capsaicin, moderate extracellular acidification (pH 6.0) caused a sustained decrease in resting membrane conductance. The decrease in membrane conductance by protons was associated with inhibition of background potassium channels. This excitatory effect of protons was not blocked by capsazepine. We conclude that in most neurons the sustained depolarization in response to moderately acidic solutions is the result of blocked background potassium channels. In a subset of neurons, TRPV1 also contributes.

Mechanisms of dopamine activation of fast-spiking interneurons that exert inhibition in rat prefrontal cortex

Prefrontal cortical dopamine (DA) modulates pyramidal cell excitability directly and indirectly by way of its actions on local circuit GABAergic interneurons. DA modulation of interneuronal functions is implicated in the computational properties of prefrontal networks during cognitive processes and in schizophrenia. Morphologically and electrophysiologically distinct classes of putative GABAergic interneurons are found in layers II-V of rat prefrontal cortex. Our whole cell patch-clamp study shows that DA induced a direct, TTX-insensitive, reversible membrane depolarization, and increased the excitability of fast-spiking (FS) interneurons. The DA-induced membrane depolarization was reduced significantly by D1/D5 receptor antagonist SCH 23390, but not by the D2 receptor antagonist (-)sulpiride, D4 receptor antagonists U101958 or L-745870, alpha1-adrenoreceptor antagonist prazosin, or serotoninergic receptor antagonist mianserin. The D1/5 agonists SKF81297 or dihydrexidine, but not D2 agonist quinpirole, also induced a prolonged membrane depolarization. Voltage-clamp analyses of the voltage-dependence of DA-sensitive currents, and the effects of changing [K(+)](O) on reversal potentials of DA responses, revealed that DA suppressed a Cs(+)-sensitive inward rectifier K(+) current and a resting leak K(+) current. D1/D5, but not D2 agonists mimicked the suppressive effects of DA on the leak current, but the DA effects on the inward rectifier K(+) current were not mimicked by either agonist. In a subgroup of FS interneurons, the slowly inactivating membrane outward rectification evoked by depolarizing voltage steps was also attenuated by DA. Collectively, these data showed that DA depolarizes FS interneurons by suppressing a voltage-independent 'leak' K(+) current (via D1/D5 receptor mechanism) and an inwardly rectifying K(+) current (via unknown DA mechanisms). Additional suppression of a slowly inactivating K(+) current led to increase in repetitive firing in response to depolarizing inputs. This D1-induced increase in interneuron excitability enhances GABAergic transmission to PFC pyramidal neurons and could represent a mechanism via which DA suppresses persistent firing of pyramidal neurons in vivo.

Characterization of four types of background potassium channels in rat cerebellar granule neurons

Cerebellar granule neurons express a standing outward (background) K+ current (I(K,SO)) that regulates the resting membrane potential and cell excitability. As several tandem-pore (2P) K+ channel mRNAs are highly expressed in cerebellar granule cells, we studied whether, and which, 2P K+ channels contribute to I(K,SO). I(K,SO) was highly sensitive to changes in extracellular pH and was partially inhibited by acetylcholine, as reported previously. In cell-attached patches from cultured cerebellar granule neurons, four types of K+ channels were found to be active when membrane potential was held at -50 mV or +50 mV in symmetrical 140 mM KCl. Based on single-channel conductances, gating kinetics and modulation by pharmacological agents and pH, three K+ channels could be considered as functional correlates of TASK-1, TASK-3 and TREK-2, which are members of the 2P K+ channel family. The fourth K+ channel (Type 4) has not been described previously and its molecular correlate is not yet known. Based on the measurement of channel current densities, the Type 2 (TASK-3) and the Type 4 K+ channels were determined to be the major sources of I(K,SO) in cultured cerebellar granule neurons. The Type 1 (TASK-1) and Type 3 (TREK-2) activities were relatively low throughout cell growth in culture (1-10 days). Similar to TASK-1 and TASK-3, the Type 4 K+ channel was highly sensitive to changes in extracellular pH, showing a 78 % inhibition by changing the extracellular pH from 7.3 to 6.3. The results of this study show that three 2P K+ channels and an additional pH-sensing K+ channel (Type 4) comprise the I(K,SO) in cultured cerebellar granule neurons. Our results also show that the high sensitivity of I(K,SO) to extracellular pH comes from the high sensitivity of Type 2 (TASK-3) and Type 4 K+ channels.

Pharmacological characterization of a non-inactivating outward current observed in mouse cerebellar Purkinje neurones

Whole-cell patch clamp recordings were used to investigate the properties of a non-inactivating outward current observed in mouse cerebellar Purkinje neurones at a holding potential of -20 mV. Increasing the external potassium (K(+)) concentration from 3 mM to 20 mM produced a rightward shift in the observed reversal potential of approximately 30 mV or approximately 40 mV for a K(+)-or a caesium (Cs(+))-based intracellular solution respectively, indicating the outward current was a K(+) current. The outward current was partially inhibited by the K(+) channel blocker, tetraethylammonium (TEA; IC(50)=0.15 mM). Subsequently, the background or TEA-insensitive current was measured in the presence of 1 mM TEA. The background current was reversibly inhibited by barium (Ba(2+); 300 microM, 50%) and potentiated by the application of arachidonic acid (AA; 1 mM, 62%). The volatile anaesthetic, halothane (1 mM), and the neuroprotectant, riluzole (500 microM), both reversibly inhibited the background current by 54% and 36% respectively. The background current was insensitive to changes in both intracellular and extracellular acidification. The GABA(B) and mu-opioid receptor agonists, baclofen and [D-Ala(2), N-MePhe(4)-Gly-ol(5)] enkephalin (DAMGO) both reversibly potentiated the outward current by 42% and 26% respectively. In contrast, the metabotropic glutamate receptor and acetylcholine receptor agonists, (S)-3,5-dihydroxyphenylglycine (DHPG) and muscarine both reversibly inhibited the outward current by 48% and 42% respectively. These data suggest that cerebellar Purkinje neurones possess a background current which shares several properties with recently cloned two-pore K(+) channels, particularly THIK-1.

Expression pattern in brain of TASK-1, TASK-3, and a tandem pore domain K(+) channel subunit, TASK-5, associated with the central auditory nervous system

TWIK-related acid-sensitive K(+) (TASK) channels contribute to setting the resting potential of mammalian neurons and have recently been defined as molecular targets for extracellular protons and volatile anesthetics. We have isolated a novel member of this subfamily, hTASK-5, from a human genomic library and mapped it to chromosomal region 20q12-20q13. hTASK-5 did not functionally express in Xenopus oocytes, whereas chimeric TASK-5/TASK-3 constructs containing the region between M1 and M3 of TASK-3 produced K(+) selective currents. To better correlate TASK subunits with native K(+) currents in neurons the precise cellular distribution of all TASK family members was elucidated in rat brain. A comprehensive in situ hybridization analysis revealed that both TASK-1 and TASK-3 transcripts are most strongly expressed in many neurons likely to be cholinergic, serotonergic, or noradrenergic. In contrast, TASK-5 expression is found in olfactory bulb mitral cells and Purkinje cells, but predominantly associated with the central auditory pathway. Thus, TASK-5 K(+) channels, possibly in conjunction with auxiliary proteins, may play a role in the transmission of temporal information in the auditory system.

Modulation of the two-pore domain acid-sensitive K+ channel TASK-2 (KCNK5) by changes in cell volume

The molecular identity of K(+) channels involved in Ehrlich cell volume regulation is unknown. A background K(+) conductance is activated by cell swelling and is also modulated by extracellular pH. These characteristics are most similar to those of newly emerging TASK (TWIK-related acid-sensitive K(+) channels)-type of two pore-domain K(+) channels. mTASK-2, but not TASK-1 or -3, is present in Ehrlich cells and mouse kidney tissue from where the full coding sequences were obtained. Heterologous expression of mTASK-2 cDNA in HEK-293 cells generated K(+) currents in the absence intracellular Ca(2+). Exposure to hypotonicity enhanced mTASK-2 currents and osmotic cell shrinkage led to inhibition. This occurred without altering voltage dependence and with only slight decrease in pK(a) in hypotonicity but no change in hypertonicity. Replacement with other cations yields a permselectivity sequence for mTASK-2 of K(+) > Rb(+) Cs(+) > NH(4)(+) > Na(+) congruent with Li(+), similar to that for the native conductance (I(K, vol)). Clofilium, a quaternary ammonium blocker of I(K, vol), blocked the mTASK-2-mediated K(+) current with an IC(50) of 25 microm. The presence of mTASK-2 in Ehrlich cells, its functional similarities with I(K, vol), and its modulation by changes in cell volume suggest that this two-pore domain K(+) channel participates in the regulatory volume decrease phenomenon.

Ionic basis of the caesium-induced depolarisation in rat supraoptic nucleus neurones

1. The effects of external Cs(+) on magnocellular neurosecretory cells were studied during intracellular recordings from 93 supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Bath application of 3-5 mM Cs(+) provoked reversible membrane depolarisation and increased firing rate in all of the neurones tested. Voltage-current analysis revealed an increase in membrane resistance between -120 and -55 mV. The increase in resistance was greater below -85 mV than at more positive potentials. 3. Voltage-clamp analysis showed that external Cs(+) blocked the hyperpolarisation-activated inward current, I(H). Under current clamp, application of ZD 7288, a selective blocker of I(H), caused an increase in membrane resistance at voltages < or = -65 mV. Voltage-current analysis further revealed that blockade of I(H) caused hyperpolarisation when the initial voltage was < -60 mV but had no effect at more positive values. 4. Current- and voltage-clamp analysis of the effects of Cs(+) in the presence of ZD 7288, or ZD 7288 and tetraethyl ammonium (TEA), revealed an increase in membrane resistance throughout the range of voltages tested (-120 to -45 mV). The current blocked by Cs(+) in the absence of I(H) was essentially voltage independent and reversed at -100 mV. The reversal potential shifted by +22.7 mV when external [K(+)] was increased from 3 to 9 mM. We conclude that, in addition to blocking I(H), external Cs(+) blocks a leakage K(+) current that contributes significantly to the resting potential of rat magnocellular neurosecretory cells.

Identification of a novel tetramerization domain in large conductance K(ca) channels

More than 50 genes are known to encode K(+) channel monomers and can coassemble to form hetero-tetrameric K(+) channels. However, only a subset of possible monomer combinations come together to form functional ion channels. The assembly and tetramerization of appropriate channel monomers is mediated by association domains (ADs). To identify such domains in human large-conductance Ca(2+)-activated K(+) channels (hSlo1), we screened hSlo1 domains for self-association using yeast two-hybrid assays. Putative ADs were subjected to functional assays in Xenopus oocytes and further characterized by coprecipitation, native gel electrophoresis, and sucrose density gradient centrifugation assays. This led to the identification of a single intracellular association domain localized near the channel pore and required for channel function. We conclude that this novel tetramerization domain, referred to as BK-T1, promotes the assembly of hSlo1 monomers into functional K(Ca) channels.

Group I metabotropic glutamate receptors elicit epileptiform discharges in the hippocampus through PLCbeta1 signaling

Activation of metabotropic glutamate receptors (mGluRs) produces multiple effects in cortical neurons, resulting in the emergence of network activities including epileptiform discharges. The cellular mechanisms underlying such network responses are largely unknown. We examined the properties of group I mGluR-mediated cellular responses in CA3 neurons and attempted to determine their role in the generation of the network activities. Group I mGluR stimulation causes depolarization of hippocampal neurons. This depolarization is primarily mediated by two sets of conductance change: the opening of a voltage-dependent cationic conductance (mediating I(mGluR(V))) and the closing of a voltage-independent (background) K(+) conductance. I(mGluR(V)) was no longer elicited by group I mGluR agonists in the presence of U73122, a phospholipase C (PLC) blocker. Also, the current could not be activated in hippocampal CA3 neurons from PLCbeta1 knock-out mice. In contrast, suppression of PLC signaling did not affect the group I mGluR-mediated suppression of background K(+) conductance. Thus, the suppression of the background K(+) conductance occurred upstream to PLC activation, whereas the generation of I(mGluR(V)) occurred downstream to PLC activation. Group I mGluR agonists normally elicited rhythmic single cell and population burst responses in the CA3 neurons. In the absence of an I(mGluR(V)) response, CA3 neurons in slices prepared from PLCbeta1-/- mutant mice could no longer generate these responses. The results suggest that I(mGluR(V)) expression in CA3 hippocampal neuron is PLCbeta1-dependent and that I(mGluR(V)) plays a necessary role in the generation of rhythmic single cell bursts and synchronized epileptiform discharges in the CA3 region of the hippocampus.

Cold transduction by inhibition of a background potassium conductance in rat primary sensory neurones

Transduction in cutaneous cold receptors is poorly understood at present. We have studied this question using dorsal root ganglion (DRG) neurones in primary culture as a model of the otherwise inaccessible receptor terminal. Whole-cell recordings during cooling from 32 to 20 degrees C revealed a large depolarization (>8mV) in 22 of 88 DRG neurones (25%), sometimes accompanied by action potentials. In cold-sensitive neurones cooling inhibited a time-independent background K+ current (Icold) which was resistant to tetraethylammonium and 4-aminopyridine. Ouabain elicited a substantially smaller depolarization than cooling, and no action potentials. We conclude that excitation by cooling in this model is primarily due to inhibition of Icold and that the previously suggested role of the Na+/K+ adenosine triphosphatase is secondary. We suggest that Icold may underlie cold transduction in cutaneous thermoreceptors.

The endocannabinoid anandamide is a direct and selective blocker of the background K(+) channel TASK-1

TASK-1 encodes an acid- and anaesthetic-sensitive background K(+) current, which sets the resting membrane potential of both cerebellar granule neurons and somatic motoneurons. We demonstrate that TASK-1, unlike the other two pore (2P) domain K(+) channels, is directly blocked by submicromolar concentrations of the endocannabinoid anandamide, independently of the CB1 and CB2 receptors. In cerebellar granule neurons, anandamide also blocks the TASK-1 standing-outward K(+) current, IKso, and induces depolarization. Anandamide-induced neurobehavioural effects are only partly reversed by antagonists of the cannabinoid receptors, suggesting the involvement of alternative pathways. TASK-1 constitutes a novel sensitive molecular target for this endocannabinoid.

Genomic organization and chromosomal localization of the murine 2 P domain potassium channel gene Kcnk8: conservation of gene structure in 2 P domain potassium channels

A 2 P domain potassium channel expressed in eye, lung, and stomach, Kcnk8, has recently been identified. To initiate further biochemical and genetic studies of this channel, we assembled the murine Kcnk8 cDNA sequence, characterized the genomic structure of the Kcnk8 gene, determined its chromosomal localization, and analyzed its activity in a Xenopus laevis oocyte expression system. The composite cDNA has an open reading frame of 1029 bp and encodes a protein of 343 amino acids with a predicted molecular mass of 36 kDa. Structure analyses predict 2 P domains and four potential transmembrane helices with a potential single EF-hand motif and four potential SH3-binding motifs in the COOH-terminus. Cloning of the Kcnk8 chromosomal gene revealed that it is composed of three exons distributed over 4 kb of genomic DNA. Genome database searching revealed that one of the intron/exon boundaries identified in Kcnk8 is present in other mammalian 2 P domain potassium channels genes and many C. elegans 2P domain potassium channel genes, revealing evolutionary conservation of gene structure. Using fluorescence in situ hybridization, the murine Kcnk8 gene was mapped to chromosome 19, 2B, the locus of the murine dancer phenotype, and syntenic to 11q11-11q13, the location of the human homologue. No significant currents were generated in a Xenopus laevis oocyte expression system using the composite Kcnk8 cDNA sequence, suggesting, like many potassium channels, additional channel subunits, modulator substances, or cellular chaperones are required for channel function.

The role of Ca2+ stores in the muscarinic inhibition of the K+ current IK(SO) in neonatal rat cerebellar granule cells

Cerebellar granule neurons (CGNs) possess a standing outward potassium current (IK(SO)) which shares many similarities with current through the two-pore domain potassium channel TASK-1 and which is inhibited following activation of muscarinic acetylcholine receptors. The action of muscarine on IK(SO) was unaffected by the M2 receptor antagonist methoctramine (100 nM) but was blocked by the M3 antagonist zamifenacin, which, at a concentration of 100 nM, shifted the muscarine concentration-response curve to the right by around 50-fold. Surprisingly, M3 receptor activation rarely produced a detectable increase in [Ca2+]i unless preceded by depolarization of the cells with 25 mM K+. Experiments with thapsigargin and ionomycin suggested that the endoplasmic reticulum Ca2+ stores in CGNs were depleted at rest. In contrast, cerebellar glial cells in the same fields of cells possessed substantial endoplasmic reticulum Ca2+ stores at rest. Pretreatment of the cells with BAPTA AM, thapsigargin or the phospholipase C (PLC) inhibitor U-73122 all blocked the muscarine-induced Ca2+ signal but had little or no effect on muscarinic inhibition of IK(SO). Raising [Ca2+]i directly with ionomycin caused a small but significant inhibition of IK(SO). It is concluded that muscarine acts on M3 muscarinic acetylcholine receptors both to inhibit IK(SO) and to mobilize Ca2+ from intracellular stores in CGNs. While the mobilization of Ca2+ occurs through activation of PLC, this does not seem to be the primary mechanism underlying muscarinic inhibition of IK(SO).