Coupling of muscarinic receptor subtypes to ion channels: experiments on neuroblastoma hybrid cells

KCNQ potassium channels: physiology, pathophysiology, and pharmacology

KCNQ genes encode a growing family of six transmembrane domains, single pore-loop, K(+) channel alpha-subunits that have a wide range of physiological correlates. KCNQ1 (KvLTQ1) is co-assembled with the product of the KCNE1 (minimal K(+)-channel protein) gene in the heart to form a cardiac-delayed rectifier-like K(+) current. Mutations in this channel can cause one form of inherited long QT syndrome (LQT1), as well as being associated with a form of deafness. KCNQ1 can also co-assemble with KCNE3, and may be the molecular correlate of the cyclic AMP-regulated K(+) current present in colonic crypt cells. KCNQ2 and KCNQ3 heteromultimers are thought to underlie the M-current; mutations in these genes may cause an inherited form of juvenile epilepsy. The KCNQ4 gene is thought to encode the molecular correlate of the I(K,n) in outer hair cells of the cochlea and I(K,L) in Type I hair cells of the vestibular apparatus, mutations in which lead to a form of inherited deafness. The recently identified KCNQ5 gene is expressed in brain and skeletal muscle, and can co-assemble with KCNQ3, suggesting it may also play a role in the M-current heterogeneity. This review will set this family of K(+) channels amongst the other known families. It will highlight the genes, physiology, pharmacology, and pathophysiology of this recently discovered, but important, family of K(+) channels.

Muscarinic toxins from the green mamba

Muscarinic acetylcholine receptors are involved in many important physiological processes. Discovery of different subtypes of muscarinic receptors that are responsible for modulating specific physiological events was a key development in muscarinic receptor research. However, the lack of highly selective muscarinic agonists and antagonists has made the classification of a muscarinic receptor subtype responsible for the mediation or modulation of a particular response very difficult. Toxins have previously proved to be highly useful pharmacological tools, due to their high potency and selectivity. This review looks at a new class of muscarinic ligand isolated from the venom of the Eastern green mamba (Dendroaspis angusticeps). Just over a decade ago, it was found that two toxins from the green mamba venom appeared to distinguish between different muscarinic receptor subtypes. Since then, at least 10 more muscarinic toxins (MTs) have been isolated from mamba venom. In recent years, some of the MTs have been used as pharmacological tools; for example, to determine the muscarinic receptor subtype involved in inhibition of adenylyl cyclase in rat striatum. This review looks at the progress that has been made over the past 10 years in the area of MT research and examines whether or not these new peptides are a new way forward in the field of muscarinic receptor research.

Inhibition of M-type K+ current by linopirdine, a neurotransmitter-release enhancer, in NG108-15 neuronal cells and rat cerebral neurons in culture

The effect of linopirdine, a neurotransmitter-release enhancer, on the M-type K+-current, IK(M), was examined in NGPM1-27 cells, mouse neuroblastomaxrat glioma NG108-15 cells transformed to express m1-muscarinic acetylcholine (ACh) receptors, using the nystatin-perforated patch-recording mode under voltage-clamp conditions. The application of linopirdine induced the inward current associated with an inhibition of IK(M), which mimics an excitatory part of the ACh-induced responses in NGPM1-27 cells. The affinity of linopirdine for the inhibition of IK(M) was 24.7 microM in NGPM1-27 cells. In the presence of linopirdine, ACh failed to evoke a further inward current, but ACh still elicited an outward current, thus suggesting that the Ca2+-dependent K+ current is rather insensitive to linopirdine. Linopirdine also inhibited another voltage-gated potassium current (IK(V)) at the concentration of 72.3 microM. Finally, the inhibitory effect of linopirdine on IK(M) was confirmed in pyramidal neurons acutely dissociated from the rat cerebral cortex at 35.8 microM. The results suggest that linopirdine is thus considered to be an inhibitor of some type of K+ channels in both NGPM1-27 cells and the rat cerebral neurons.

Sulfhydryl modification inhibits K+ (M) current with kinetics close to acetylcholine in rodent NG108-15 cells

The effects of sulfhydryl reagents on M-type voltage-dependent potassium currents (IK(M)) were examined in NG108-15 cells transformed to express ml muscarinic acetylcholine receptors (mAChRs), a NGPM1-27 clone. Focal application of glutathione at millimolar concentrations dissolved in acidic solutions caused a transient inward current in NGPM1-27 cells at holding potentials of -30mV, associated with an inhibition of IK(M). The glutathione-induced response was mimicked by cysteine. These effects were also reproduced by superfusion with micromolar concentrations of HgCl2, AgNO3, N-methylmaleimide and p-chloromercuribenzoic acid (pCMB), agents which target protein thiols. Glutathione, HgCl2, AgNO3 and pCMB inhibited the peak conductance of IK(M) without shifting the half activating voltage (V1/2), which was comparable to the acetylcholine (ACh)-induced response. The voltage dependence of time constants for IK(M) deactivation in sulfhydryl reagent-, ACh- and non-treated cells resembled, but differed from that in Ba(2+)-treated cells. These results reveal that there is an accessible cysteine moiety, but not a disulfide bond, either on the M channel protein itself or on a protein directly involved in agonist-M channel coupling.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Inositol trisphosphate/Ca2+ as messengers of bradykinin B2 and muscarinic acetylcholine m1-m4 receptors in neuroblastoma-derived hybrid cells

Neuroblastoma x glioma hybrid NG 108-15 and neuroblastoma x fibroblast hybrid NL308 cells possess endogenous bradykinin B2 receptors and m4 muscarinic acetylcholine receptors (mAChRs), which couple to phospholipase C and adenylate cyclase, respectively. Four genetic subtypes of mAChRs differed in their effects when stimulated in NG108-15 and NL308 cells overexpressing mAChRs. Broadly speaking, the principal effects fell into two categories: the odd-numbered receptors (m1 and m3) activated phospholipase C and increased inositol trisphosphate/Ca2+, as bradykinin did, whereas the even-numbered receptors (m2 and m4) inhibited adenylate cyclase via a pertussis toxin (PTx)-sensitive G-protein in NG108-15 cells. But all four types of NL308 cells overexpressing each m1, m2, m3 and m4 receptor activated phospholipase C, while keeping the PTx-sensitivity in m2/m4, but not in m1/m3 receptors. Coupling to ion channel effectors showed a comparable dichotomy in NG108-15 cells, while cross-activation occurred in NL308 cells.

Effects of thyrotropin-releasing hormone on rat motoneurons are mediated by G proteins

Numerous transmitter receptors are linked via GTP-binding proteins (G proteins) to membrane phosphoinositide metabolism by phospholipase C (PLC) and generation of second messengers such as activated protein kinase C (PKC), inositol trisphosphate (IP3) and/or elevations in intracellular calcium. In many cases, these same receptors also inhibit a resting ('leak') potassium current (IK(L)), thereby depolarizing neurons. It is unclear if activation of this PLC pathway mediates inhibition of IK(L) by neurotransmitter receptors. Therefore, we tested the contribution of this pathway to the TRH-induced inhibition of IK(L) in rat hypoglossal motoneurons (HMs) using conventional intracellular recording in brainstem slices. When HMs were recorded with electrodes containing 3 M KCl or 30 mM GTP (in KCl), TRH induced a depolarization that recovered quickly (within 8-10 min) and could be repeated with only modest tachyphylaxis (< 20%). However, with electrodes containing the non-hydrolyzable G protein activator, GTP gamma S (10 mM), the TRH-induced depolarization was long lasting (up to 1 h); with electrodes containing the G protein inhibitor, GDP beta S (20 mM) the tachyphylaxis with repeated TRH application was exaggerated (approximately 60%). Activation of PKC by phorbol dibutyrate (10 microM in perfusate) neither mimicked nor occluded the effects of TRH. There were no effects on membrane potential, input resistance (RN) or the response to TRH in HMs during long recordings with electrodes containing high concentrations of IP3 (60 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of inward current associated with M-potassium current inhibition in m1-muscarinic receptor-transformed NG108-15 cells by KST-5452, a novel cognition enhancer

The electrophysiological effects of KST-5452 [3-(m-phenoxybenzylidene)-quinuclidine], an M1 muscarinic acetylcholine receptor (muscarinic AChR) binding compound, were studied in NG108-15 neuroblastoma x glioma hybrid cells transfected with m1 muscarinic AChR cDNA. Application of KST-5452 to m1-transformed NGPM1-27 cells elicited a sustained inward current associated with decreased conductance and reduced M-current relaxations at a holding potential of -20 mV. The KST-5452-induced responses were blocked by pirenzepine, suggesting that KST-5452 acts as a potent excitant via M1 muscarinic AChRs in brain neurons.