Unmasking of a novel potassium current in Drosophila by a mutation and drugs

The adhesion G-protein-coupled receptor mayo/CG11318 controls midgut development in Drosophila

Adhesion G-protein-coupled receptors (aGPCRs) form a large family of cell surface molecules with versatile tasks in organ development. Many aGPCRs still await their functional and pharmacological deorphanization. Here, we characterized the orphan aGPCR CG11318/mayo of Drosophila melanogaster and found it expressed in specific regions of the gastrointestinal canal and anal plates, epithelial specializations that control ion homeostasis. Genetic removal of mayo results in tachycardia, which is caused by hyperkalemia of the larval hemolymph. The hyperkalemic effect can be mimicked by a raise in ambient potassium concentration, while normal potassium levels in mayoKO mutants can be restored by pharmacological inhibition of potassium channels. Intriguingly, hyperkalemia and tachycardia are caused non-cell autonomously through mayo-dependent control of enterocyte proliferation in the larval midgut, which is the primary function of this aGPCR. These findings characterize the ancestral aGPCR Mayo as a homeostatic regulator of gut development.

Identification and pharmacological characterization of the voltage-gated potassium channel Shab in diamondback moth, Plutella xylostella

BACKGROUND

Voltage-gated potassium channel Kv2 is the primarily delayed rectifier in insect nerves and muscles involved in several crucial biological processes, including action potential regulation, photoreceptor performance, and larval locomotor. It is a potential molecular target for developing a novel pesticide for mosquitos. However, there are few studies on the Kv2 channel in agricultural pests.

RESULTS

The only α-subunit gene of the Kv2 channel in Plutella xylostella (L.), PxShab, was cloned, and its expression profile was analyzed. The relative expression level of PxShab was highest in the pupal stage of both sexes and male adults but lowest in female adults. Meanwhile, PxShab had the highest expression in the head in both larvae and adults. Then, PxShab was stably expressed in the HEK-293 T cell line. Whole cell patch clamp recordings showed an outward current whose current-voltage relationship conformed to a typical delayed-rectifier potassium channel. 20 μM quinidine could effectively inhibit the potassium current, while the channel was insensitive to 4-AP even at 10 mM. Several potential compounds and botanical pesticides were assessed, and carvedilol (IC₅₀  = 0.53 μM) and veratrine (IC₅₀  = 2.22 μM) had a good inhibitory effect on the channel.

CONCLUSION

This study revealed the pharmacological properties of PxShab and screened out several high potency inhibitors, which laid the foundation for further functional research of PxShab and provides new insight into designing novel insecticides. © 2022 Society of Chemical Industry.

Fatty Acid and Related Potassium Kv2 Channel Blockers: Toxicity and Physiological Actions on Mosquitoes

Potassium channels constitute a very diverse group involved in neural signaling, neuronal activity, membrane potential maintenance, and action potential generation. Here, we tested the mammalian potassium channel blockers TRAM-34 and 5-hydroxydecanoate (5-HDC), as well as certain fatty acids (FA) that might fit in the lumen of the pore and block channel activity by obstructing K⁺ ion passage. Kv channel blockers could be leads for a novel pesticide type. Insecticidal activity was assessed by topical application to Anopheles gambiae adult mosquitoes, paralysis in a headless larval assay, at the cellular level with patch-clamp recordings of engineered HEK cells expressing AgKv2.1 channels, as well as central nervous system recordings from larval Drosophila melanogaster. With only one hydroxyl group difference, decanoic acid had a consistently greater effect than 5-HDC in blocking Kv channels, paralyzing larvae, and killing mosquitoes. The 11-dansylamino undecanoic acid (DAUDA) blockage of eukaryotic Kv channels is demonstrated for the first time, but it failed to kill adult mosquitoes. We synthesized alkyl esters from DAUDA and decanoic acid in an effort to improve cuticular penetration, but it had little impact upon adult toxicity. TRAM-34 and rolipram did not show activity on Kv channels nor potent insecticidal effect on adult mosquitoes. Furthermore, co-application of test compounds with permethrin did not increase mortality in adults. In conclusion, the compounds tested had modest insecticidal and synergistic activity.

Generation and characterization of new alleles of quiver (qvr) that encodes an extracellular modulator of the Shaker potassium channel

Our earlier genetic screen uncovered a paraquat-sensitive leg-shaking mutant quiver¹ (qvr¹), whose gene product interacts with the Shaker (Sh) K⁺ channel. We also mapped the qvr locus to EY04063 and noticed altered day-night activity patterns in these mutants. Such circadian behavioral defects were independently reported by another group, who employed the qvr¹ allele we supplied them, and attributed the extreme restless phenotype of EY04063 to the qvr gene. However, their report adopted a new noncanonical gene name sleepless (sss) for qvr. In addition to qvr¹ and qvr^EY, our continuous effort since the early 2000s generated a number of novel recessive qvr alleles, including ethyl methanesulfonate (EMS)-induced mutations qvr² and qvr³, and P-element excision lines qvr^ip6 (imprecise jumpout), qvr^rv7, and qvr^rv9 (revertants) derived from qvr^EY. Distinct from the original intron-located qvr¹ allele that generates abnormal-sized mRNAs, qvr², and qvr³ had their lesion sites in exons 6 and 7, respectively, producing nearly normal-sized mRNA products. A set of RNA-editing sites are nearby the lesion sites of qvr³ and qvr^EY on exon 7. Except for the revertants, all qvr alleles display a clear ether-induced leg-shaking phenotype just like Sh, and weakened climbing abilities to varying degrees. Unlike Sh, all shaking qvr alleles (except for qvr^f01257) displayed a unique activity-dependent enhancement in excitatory junction potentials (EJPs) at larval neuromuscular junctions (NMJs) at very low stimulus frequencies, with qvr^EY displaying the largest EJP and more significant NMJ overgrowth than other alleles. Our detailed characterization of a collection of qvr alleles helps to establish links between novel molecular lesions and different behavioral and physiological consequences, revealing how modifications of the qvr gene lead to a wide spectrum of phenotypes, including neuromuscular hyperexcitability, defective motor ability and activity-rest cycles.

Nonreciprocal homeostatic compensation in Drosophila potassium channel mutants

Homeostatic control of intrinsic excitability is important for long-term regulation of neuronal activity. In conjunction with many other forms of plasticity, intrinsic homeostasis helps neurons maintain stable activity regimes in the face of external input variability and destabilizing genetic mutations. In this study, we report a mechanism by which Drosophila melanogaster larval motor neurons stabilize hyperactivity induced by the loss of the delayed rectifying K⁺ channel Shaker cognate B (Shab), by upregulating the Ca²⁺-dependent K⁺ channel encoded by the slowpoke (slo) gene. We also show that loss of SLO does not trigger a reciprocal compensatory upregulation of SHAB, implying that homeostatic signaling pathways utilize compensatory pathways unique to the channel that was mutated. SLO upregulation due to loss of SHAB involves nuclear Ca²⁺ signaling and dCREB, suggesting that the slo homeostatic response is transcriptionally mediated. Examination of the changes in gene expression induced by these mutations suggests that there is not a generic transcriptional response to increased excitability in motor neurons, but that homeostatic compensations are influenced by the identity of the lost conductance.NEW & NOTEWORTHY The idea that activity-dependent homeostatic plasticity is driven solely by firing has wide credence. In this report we show that homeostatic compensation after loss of an ion channel conductance is tailored to identity of the channel lost, not its properties.

Differential Effects of Toluene and Ethanol on Dopaminergic Neurons of the Ventral Tegmental Area

Drugs of abuse increase the activity of dopaminergic neurons of the ventral tegmental area (VTA), and output from the VTA is critical for both natural and drug-induced reward and reinforcement. Ethanol and the abused inhalant toluene both enhance VTA neuronal firing, but the mechanisms of this effect is not fully known. In this study, we used extracellular recordings to compare the actions of toluene and ethanol on DA VTA neurons. Both ethanol and toluene increased the firing rate of DA neurons, although toluene was ~100 times more potent than ethanol. The mixed ion channel blocker quinine (100 μM) blocked the increases in firing produced by ethanol and toluene, indicating some similarity in mechanisms of excitation. A mixture of antagonists of GABA and cholinergic receptors did not prevent toluene-induced or ethanol-induced excitation, and toluene-induced excitation was not altered by co-administration of ethanol, suggesting independent mechanisms of excitation for ethanol and toluene. Concurrent blockade of NMDA, AMPA, and metabotropic glutamate receptors enhanced the excitatory effect of toluene while having no significant effect on ethanol excitation. Nicotine increased firing of DA VTA neurons, and this was blocked by the nicotinic antagonist mecamylamine (1 μM). Mecamylamine did not alter ethanol or toluene excitation of firing but the muscarinic antagonist atropine (5 μM) or a combination of GABA antagonists (bicuculline and CGP35348, 10 μM each) reduced toluene-induced excitation without affecting ethanol excitation. The Ih current blocker ZD7288 abolished the excitatory effect of toluene but unlike the block of ethanol excitation, the effect of ZD7288 was not reversed by the GIRK channel blocker barium, but was reversed by GABA antagonists. These results demonstrate that the excitatory effects of ethanol and toluene have some similarity, such as block by quinine and ZD7288, but also indicate that there are important differences between these two drugs in their modulation by glutamatergic, cholinergic, and GABAergic receptors. These findings provide important information regarding the actions of abused inhalants on central reward pathways, and suggest that regulation of the activation of central dopamine pathways by ethanol and toluene partially overlap.

Potassium channels in Drosophila: historical breakthroughs, significance, and perspectives

Drosophila has enabled important breakthroughs in K(+) channel research, including identification and fi rst cloning of a voltage-activated K(+) channel, Shaker, a founding member of the K(V)1 family. Drosophila has also helped in discovering other K(+) channels, such as Shab, Shaw, Shal, Eag, Sei, Elk, and also Slo, a Ca(2+) - and voltage-dependent K(+) channel. These findings have contributed significantly to our understanding of ion channels and their role in physiology. Drosophila continues to play an important role in ion channel studies, benefiting from an unparalleled arsenal of genetic tools and availability of tens of thousands of genetically modified strains. These tools allow deletion, expression, or misexpression of almost any gene in question with temporal and spatial control. The combination of these tools and resources with the use of forward genetic approach in Drosophila further enhances its strength as a model system. There are many areas in which Drosophila can further help our understanding of ion channels and their function. These include signaling pathways involved in regulating and modulating ion channels, basic information on channels and currents where very little is currently known, and the role of ion channels in physiology and pathology.

Inhibition of ion channels and heart beat in Drosophila by selective COX-2 inhibitor SC-791

Recent findings suggest that modulation of ion channels might be implicated in some of the clinical effects of coxibs, selective inhibitors of cyclooxygenase-2 (COX-2). Celecoxib and its inactive analog 2,5-dimethyl-celecoxib, but not rofecoxib, can suppress or augment ionic currents and alter functioning of neurons and myocytes. To better understand these unexpected effects, we have recently investigated the mechanism of inhibition of human K_v2.1 channels by a highly selective COX-2 inhibitor SC-791. In this study we have further explored the SC-791 action on ion channels and heartbeat in Drosophila, which lacks cyclooxygenases and thus can serve as a convenient model to study COX-2-independent mechanisms of coxibs. Using intracellular recordings in combination with a pharmacological approach and utilizing available Drosophila mutants, we found that SC-791 inhibited voltage-activated K⁺ and L-type Ca²⁺ channels in larval body-wall muscles and reduced heart rate in a concentration-dependent manner. Unlike celecoxib and several other K⁺ channel blockers, SC-791 did not induce arrhythmia. Instead, application of SC-791 resulted in a dramatic slowing of contractions and, at higher concentrations, in progressively weaker contractions with gradual cessation of heartbeat. Isradipine, a selective blocker of L-type Ca²⁺ channels, showed a similar pattern of heart arrest, though no prolongation of contractions was observed. Ryanodine was the only channel modulating compound of those tested additionally that was capable of slowing contractions. Like SC-791, ryanodine reduced heart rate without arrhythmia. However, it could not stop heartbeat completely even at 500 µM, the highest concentration used. The magnitude of heart rate reduction, when SC-791 and ryanodine were applied together, was smaller than expected for independent mechanisms, raising the possibility that SC-791 might be interfering with excitation-contraction coupling in Drosophila heart.

Segmental differences in firing properties and potassium currents in Drosophila larval motoneurons

Potassium currents play key roles in regulating motoneuron activity, including functional specializations that are important for locomotion. The thoracic and abdominal segments in the Drosophila larval ganglion have repeated arrays of motoneurons that innervate body-wall muscles used for peristaltic movements during crawling. Although abdominal motoneurons and their muscle targets have been studied in detail, owing, in part, to their involvement in locomotion, little is known about the cellular properties of motoneurons in thoracic segments. The goal of this study was to compare firing properties among thoracic motoneurons and the potassium currents that influence them. Whole-cell, patch-clamp recordings performed from motoneurons in two thoracic and one abdominal segment revealed both transient and sustained voltage-activated K(+) currents, each with Ca(++)-sensitive and Ca(++)-insensitive [A-type, voltage-dependent transient K(+) current (I(Av))] components. Segmental differences in the expression of voltage-activated K(+) currents were observed. In addition, we demonstrate that Shal contributes to I(Av) currents in the motoneurons of the first thoracic segment.

Modulation of the frequency response of Shaker potassium channels by the quiver peptide suggesting a novel extracellular interaction mechanism

Recent studies have indicated that the Shaker potassium channel regulates sleep in Drosophila. The Drosophila quiver (qvr) gene encodes a novel potassium channel subunit that modulates the Shaker potassium channel. The Qvr peptide contains a signal sequence for extracellular localization and may regulate a unique feature of the Shaker I(A) current that confers special neuronal excitability patterns. Thus, studies of the Shaker channel properties in the qvr mutant background should provide an opportunity to uncover a new form of physiologic modulation of potassium channels. We have begun to investigate the impact of qvr protein on the Shaker channel properties and its implications in synaptic function in vivo. We studied synaptic transmission at the larval neuromuscular junction and characterized the transient potassium current I(A) in larval muscles. We identified two different functional states of I(A) in qvr larval muscles, as reflected by two distinct components, I(AF) and I(AS), differing in their kinetics of recovery from inactivation and sensitivity to a K(+) channel blocker. Correspondingly, qvr mutant larvae exhibit multiple synaptic discharges following individual nerve stimuli during repetitive activity.

Quinidine interaction with Shab K+ channels: pore block and irreversible collapse of the K+ conductance

Quinidine is a commonly used antiarrhythmic agent and a tool to study ion channels. Here it is reported that quinidine equilibrates within seconds across the Sf9 plasma membrane, blocking the open pore of Shab channels from the intracellular side of the membrane in a voltage-dependent manner with 1:1 stoichiometry. On binding to the channels, quinidine interacts with pore K(+) ions in a mutually destabilizing manner. As a result, when the channels are blocked by quinidine with the cell bathed in an external medium lacking K(+), the Shab conductance G(K) collapses irreversibly, despite the presence of a physiological [K(+)] in the intracellular solution. The quinidine-promoted collapse of Shab G(K) resembles the collapse of Shaker G(K) observed with 0 K(+) solutions on both sides of the membrane: thus the extent of G(K) drop depends on the number of activating pulses applied in the presence of quinidine, but is independent of the pulse duration. Taken together the observations indicate that, as in Shaker, the quinidine-promoted collapse of Shab G(K) occurs during deactivation of the channels, at the end of each activating pulse, with a probability of 0.1 per pulse at 80 mV. It appears that when Shab channels are open, the pore conformation able to conduct is stable in the absence of K(+), but on deactivation this conformation collapses irreversibly.

Mechanisms of Kv2.1 channel inhibition by celecoxib--modification of gating and channel block

BACKGROUND AND PURPOSE

Selective cyclooxygenase-2 (COX-2) inhibitors such as rofecoxib (Vioxx) and celecoxib (Celebrex) were developed as NSAIDs with reduced gastric side effects. Celecoxib has now been shown to affect cellular physiology via an unexpected, COX-independent, pathway - by inhibiting K(v)2.1 and other ion channels. In this study, we investigated the mechanism of the action of celecoxib on K(v)2.1 channels.

EXPERIMENTAL APPROACH

The mode of action of celecoxib on rat K(v)2.1 channels was studied by whole-cell patch-clamping to record currents from channels expressed in HEK-293 cells.

KEY RESULTS

Celecoxib reduced current through K(v)2.1 channels when applied from the extracellular side. At low concentrations (<or=3 microM), celecoxib accelerated kinetics of activation, deactivation and inactivation. Recovery of rat K(v)2.1 channels from inactivation could be characterized by two components, with celecoxib selectively accelerating the slow component of recovery at <or=10 microM. At >3 microM, celecoxib led to closed-channel block with relative slowing of activation. At 30 microM, it additionally induced open-channel block that manifested in use-dependent inhibition and slower recovery from inactivation.

CONCLUSIONS AND IMPLICATIONS

Celecoxib reduced current through K(v)2.1 channels by modifying gating and inducing closed- and open-channel block, with the three effects manifesting at different concentrations. These data will help to elucidate the mechanisms of action of this widely prescribed drug on ion channels and those underlying its neurological, cardiovascular and other effects.

Inhibition of delayed rectifier potassium channels and induction of arrhythmia: a novel effect of celecoxib and the mechanism underlying it

Selective inhibitors of cyclooxygenase-2 (COX-2), such as rofecoxib (Vioxx), celecoxib (Celebrex), and valdecoxib (Bextra), have been developed for treating arthritis and other musculoskeletal complaints. Selective inhibition of COX-2 over COX-1 results in preferential decrease in prostacyclin production over thromboxane A2 production, thus leading to less gastric effects than those seen with nonselective COX inhibitors such as acetylsalicylic acid (aspirin). Here we show a novel effect of celecoxib via a mechanism that is independent of COX-2 inhibition. The drug inhibited the delayed rectifier (Kv2) potassium channels from Drosophila, rats, and humans and led to pronounced arrhythmia in Drosophila heart and arrhythmic beating of rat heart cells in culture. These effects occurred despite the genomic absence of cyclooxygenases in Drosophila and the failure of acetylsalicylic acid, a potent inhibitor of both COX-1 and COX-2, to inhibit rat Kv2.1 channels. A genetically null mutant of Drosophila Shab (Kv2) channels reproduced the cardiac effect of celecoxib, and the drug was unable to further enhance the effect of the mutation. These observations reveal an unanticipated effect of celecoxib on Drosophila hearts and on heart cells from rats, implicating the inhibition of Kv2 channels as the mechanism underlying this effect.

Effects of mutant Drosophila K+ channel subunits on habituation of the olfactory jump response

The olfactory-jump response assay was used to analyze habituation in Drosophila mutants of potassium (K(+)) channel subunits. As with physiological assays of the giant fiber-mediated escape reflex, mutations at loci that encode K(+) channel subunits have distinct effects on habituating the olfactory-jump response. The data for slowpoke and ether à go-go indicate similar effects on habituation of the olfactory-jump response and the giant fiber-mediated escape. Habituation in the olfactory jump assay in Hyperkinetic and Shaker mutants was drastically different from the degree of defect in the giant fiber-mediated escape reflex, indicating differential control mechanisms underlying the two forms of non-associative conditioning.

Mutations in cytochrome c oxidase subunit VIa cause neurodegeneration and motor dysfunction in Drosophila

Mitochondrial dysfunction is involved in many neurodegenerative disorders in humans. Here we report mutations in a gene (designated levy) that codes for subunit VIa of cytochrome c oxidase (COX). The mutations were identified by the phenotype of temperature-induced paralysis and showed the additional phenotypes of decreased COX activity, age-dependent bang-induced paralysis, progressive neurodegeneration, and reduced life span. Germ-line transformation using the levy(+) gene rescued the mutant flies from all phenotypes including neurodegeneration. The data from levy mutants reveal a COX-mediated pathway in Drosophila, disruption of which leads to mitochondrial encephalomyopathic effects including neurodegeneration, motor dysfunction, and premature death. The data present the first case of a mutation in a nuclear-encoded structural subunit of COX that causes mitochondrial encephalomyopathy rather than lethality, whereas several previous attempts to identify such mutations have not been successful. The levy mutants provide a genetic model to understand the mechanisms underlying COX-mediated mitochondrial encephalomyopathies and to explore possible therapeutic interventions.

Genetic modifications of seizure susceptibility and expression by altered excitability in Drosophila Na(+) and K(+) channel mutants

A seizure-paralysis repertoire characteristic of Drosophila "bang-sensitive" mutants can be evoked electroconvulsively in tethered flies, in which behavioral episodes are associated with synchronized spike discharges in different body parts. Flight muscle DLMs (dorsal longitudinal muscles) display a stereotypic sequence of initial and delayed bouts of discharges (ID and DD), interposed with giant fiber (GF) pathway failure and followed by a refractory period. We examined how seizure susceptibility and discharge patterns are modified in various K(+) and Na(+) channel mutants. Decreased numbers of Na(+) channels in nap(ts) flies drastically reduced susceptibility to seizure induction, eliminated ID, and depressed DD spike generation. Mutations of different K(+) channels led to differential modifications of the various components in the repertoire. Altered transient K(+) currents in Sh(133) and Hk mutants promoted ID induction. However, only Sh(133) but not Hk mutations increased DD seizure and GF pathway failure durations. Surprisingly, modifications in sustained K(+) currents in eag and Shab mutants increased thresholds for DD induction and GF pathway failure. Nevertheless, both eag and Shab, like Sh(133), increased DD spike generation and recovery time from GF pathway failure. Interactions between channel mutations with the bang-sensitive mutation bss demonstrated the role of membrane excitability in stress-induced seizure-paralysis behavior. Seizure induction and discharges were suppressed by nap(ts) in bss nap double mutants, whereas Sh heightened seizure susceptibility in bss Sh(133) and bss Sh(M) double mutants. Our results suggest that individual seizure repertoire components reflect different neural network activities that could be differentially altered by mutations of specific ion channel subunits.

Differential contributions of Shaker and Shab K+ currents to neuronal firing patterns in Drosophila

Different K(+) currents participate in generating neuronal firing patterns. The Drosophila embryonic "giant" neuron culture system has facilitated current- and voltage-clamp recordings to correlate distinct excitability patterns with the underlying K(+) currents and to delineate the mutational effects of identified K(+) channels. Mutations of Sh and Shab K(+) channels removed part of inactivating I(A) and sustained I(K), respectively, and the remaining I(A) and I(K) revealed the properties of their counterparts, e.g., Shal and Shaw channels. Neuronal subsets displaying the delayed, tonic, adaptive, and damping spike patterns were characterized by different profiles of K(+) current voltage dependence and kinetics and by differential mutational effects. Shab channels regulated membrane repolarization and repetitive firing over hundreds of milliseconds, and Shab neurons showed a gradual decline in repolarization during current injection and their spike activities became limited to high-frequency, damping firing. In contrast, Sh channels acted on events within tens of milliseconds, and Sh mutations broadened spikes and reduced firing rates without eliminating any categories of firing patterns. However, removing both Sh and Shal I(A) by 4-aminopyridine converted the delayed to damping firing pattern, demonstrating their actions in regulating spike initiation. Specific blockade of Shab I(K) by quinidine mimicked the Shab phenotypes and converted tonic firing to a damping pattern. These conversions suggest a hierarchy of complexity in K(+) current interactions underlying different firing patterns. Different lineage-defined neuronal subsets, identifiable by employing the GAL4-UAS system, displayed different profiles of spike properties and K(+) current compositions, providing opportunities for mutational analysis in functionally specialized neurons.

Sub-cellular Ca2+ dynamics affected by voltage- and Ca2+-gated K+ channels: Regulation of the soma-growth cone disparity and the quiescent state in Drosophila neurons

Using Drosophila mutants and pharmacological blockers, we provide the first evidence that distinct types of K(+) channels differentially influence sub-cellular Ca(2+) regulation and growth cone morphology during neuronal development. Fura-2-based imaging revealed in cultured embryonic neurons that the loss of either voltage-gated, inactivating Shaker channels or Ca(2+)-gated Slowpoke BK channels led to robust spontaneous Ca(2+) transients that preferentially occurred within the growth cone. In contrast, loss of voltage-gated, non-inactivating Shab channels did not show such a disparity and sometimes produced soma-specific Ca(2+) transients. The fast spontaneous transients in both the soma and growth cone were suppressed by the Na(+) channel blocker tetrodotoxin, indicating that these Ca(2+) fluctuations stemmed from increases in membrane excitability. Similar differences in regional Ca(2+) regulation were observed upon membrane depolarization by high K(+)-containing saline. In particular, Shaker and slowpoke mutations enhanced the size and dynamics of the depolarization-induced Ca(2+) increase in the growth cone. In contrast, Shab mutations greatly prolonged the Ca(2+) increase in the soma. Differential effects of these excitability mutations on neuronal development were indicated by their distinct alterations in growth cone morphology. Loss of Shaker currents increased the size of lamellipodia and the number of filopodia, structures associated with the actin cytoskeleton. Interestingly, loss of Slowpoke currents strongly influenced tubulin regulation, enhancing the number of microtubule loop structures per growth cone. Together, our findings support the idea that individual K(+) channel subunits differentially regulate spontaneous sub-cellular Ca(2+) fluctuations in growing neurons that may influence activity-dependent growth cone formation.

Divalent cation modulation of a-type potassium channels in acutely dissociated central neurons from wide-type and mutant Drosophila

Drosophila mutants provide an ideal model to study channel-type specificity of ion channel regulation in situ. In this study, the effects of divalent cations on voltage-gated K+ currents were investigated in acutely dissociated central neurons of Drosophila third instar larvae using the whole-cell patch-clamp recording. Our data showed that micromolar Cd2+ enhanced the peak inactivating current (I(A)) without affecting the delayed component (I(K)). The same results were obtained in Ca(2+)-free external solution, and from slo1 mutation, which eliminates transient Ca(2+)-activated K+ current. Micromolar Cd2+ and Zn2+, and millimolar Ca2+ and Mg2+ all shifted the steady-state inactivation curve of I(A) without affecting the voltage-dependence of I(A) activation, whereas millimolar Cd2+ markedly affected both the activation and steady-state inactivation curves for I(A). Divalent cations affected I(A) with different potency; the sequence was: Zn2+ > Cd2+ > Ca2+ > Mg2+. The modulation of I(A) by Cd2+ was partially inhibited in Sh(M), a null Shaker (one of I(A)-encoding genes) mutation. Taken together, the channel-type specificity, the asymmetric effects on I(A) activation and inactivation kinetics, and the diverse potency of divalent cations all strongly support the idea that physiological divalent cations modulate A-type K+ channels through specific binding to extracellular sites of the channels.

Dopamine modulation of two delayed rectifier potassium currents in a small neural network

Delayed rectifier potassium currents [I(K(V))] generate sustained, noninactivating outward currents with characteristic fast rates of activation and deactivation and play important roles in shaping spike frequency. The pyloric motor network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, is made up of one interneuron and 13 motor neurons of five different classes. Dopamine (DA) increases the firing frequencies of the anterior burster (AB), pyloric (PY), lateral pyloric (LP), and inferior cardiac (IC) neurons and decreases the firing frequencies of the pyloric dilator (PD) and ventricular dilator (VD) neurons. In all six types of pyloric neurons, I(K(V)) is small with respect to other K(+) currents. It is made up of at least two TEA-sensitive components that show differential sensitivity to 4-aminopyridine and quinidine, and have differing thresholds of activation. One saturable component is activated at potentials above -25 mV, whereas the second component appears at more depolarized voltages and does not saturate at voltage steps up to +45 mV. The magnitude of the components varies among cell types but also shows considerable variation within a single type. A subset of PY neurons shows a marked enhancement in spike frequency with DA; DA evokes a pronounced reversible increase in I(K(V)) conductance of < or = 30% in the PY neurons studied, and on average significantly increases both components of I(K(V)). The AB neuron also shows a reversible 20% increase in the steady state I(K(V)). DA had no effect on I(K(V)) in PD, LP, VD, and IC neurons. The physiological roles of these currents and their modulation by DA are discussed.

Ethanol excitation of dopaminergic ventral tegmental area neurons is blocked by quinidine

The dopaminergic (DA) neurons in the ventral tegmental area (VTA) are important for the reinforcing effects of ethanol. We have shown that ethanol directly excites DA VTA neurons and reduces the afterhyperpolarization (AHP) that follows spontaneous action potentials in these neurons. These data suggested that ethanol may be increasing the firing rate of DA VTA neurons by modulating currents that contribute to the AHP, either by reducing a K+ current or by increasing the inward current Ih. In the present study, different blockers of K+ channels and Ih were tested to determine whether any could prevent the ethanol excitation of DA VTA neurons. Extracellular single-unit recordings and whole-cell patch-clamp recordings were made from DA VTA neurons in brain slices from Fischer-344 rats and ethanol (40-120 mM) and channel blockers were applied in the bath. Ethanol excitation was not reduced by blockade of Ih with cesium (5 mM) or ZD7288 (30 microM), or by block of G-protein-coupled inwardly rectifying K+ channels with barium (500 microM). Tetraethylammonium (TEA) ion (2-10 mM), which blocks the large conductance calcium-dependent potassium K+ current and some types of delayed rectifier currents, had no effect on the ethanol-induced excitation. Interestingly, ethanol excitation of DA VTA neurons was blocked by quinidine (20-80 microM), a drug that blocks many types of delayed rectifier K+ channels, including some insensitive to TEA. This effect of quinidine was concentration-dependent and reversible. These results suggest that ethanol excites DA VTA neurons by reducing a quinidine-sensitive K+ current.

Olfactory receptor axons influence the development of glial potassium currents in the antennal lobe of the moth Manduca sexta

In the olfactory (antennal) lobe of the moth Manduca sexta, olfactory receptor axons strongly influence the distribution and morphology of glial cells. In the present study, we asked whether the development of the electrophysiological properties of the glial cells is influenced by the receptor axons. Whole-cell currents were measured in antennal lobe glial cells in acute brain slices prepared from animals at different stages of metamorphic development (stages 3, 6, and 12). Outward currents were induced by depolarizing voltage steps from a holding potential of -70 mV. At all developmental stages investigated, the outward currents were partly blocked by bath application of the potassium channel blocker 4-aminopyridine (4AP, 10 mM) or by including tetraethylammonium (TEA, 30 mM) in the pipette solution. The relative contribution of the 4AP-sensitive current to the outward current increased from 18% at stages 3 and 6 to 42% at stage 12, while the TEA-sensitive current increased from 18% at stage 3 to 81% at stage 6, and then declined again to 40% at stage 12. In contrast, in the absence of receptor axons, these changes in the contribution of the TEA- and 4AP-sensitive currents to the total outward current did not occur; rather, the current profile remained in the most immature state (stage 3). The results suggest that olfactory receptor axons are essential for development of the mature pattern of glial potassium currents.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Distinct roles of CaMKII and PKA in regulation of firing patterns and K(+) currents in Drosophila neurons

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the cAMP-dependent protein kinase A (PKA) cascades have been implicated in neural mechanisms underlying learning and memory as supported by mutational analyses of the two enzymes in Drosophila. While there is mounting evidence for their roles in synaptic plasticity, less attention has been directed toward their regulation of neuronal membrane excitability and spike information coding. Here we report genetic and pharmacological analyses of the roles of PKA and CaMKII in the firing patterns and underlying K(+) currents in cultured Drosophila central neurons. Genetic perturbation of the catalytic subunit of PKA (DC0) did not alter the action potential duration but disrupted the frequency coding of spike-train responses to constant current injection in a subpopulation of neurons. In contrast, selective inhibition of CaMKII by the expression of an inhibitory peptide in ala transformants prolonged the spike duration but did not affect the spike frequency coding. Enhanced membrane excitability, indicated by spontaneous bursts of spikes, was observed in CaMKII-inhibited but not in PKA-diminished neurons. In wild-type neurons, the spike train firing patterns were highly reproducible under consistent stimulus conditions. However, disruption of either of these kinase pathways led to variable firing patterns in response to identical current stimuli delivered at a low frequency. Such variability in spike duration and frequency coding may impose problems for precision in signal processing in these protein kinase learning mutants. Pharmacological analyses of mutations that affect specific K(+) channel subunits demonstrated distinct effects of PKA and CaMKII in modulation of the kinetics and amplitude of different K(+) currents. The results suggest that PKA modulates Shaker A-type currents, whereas CaMKII modulates Shal-A type currents plus delayed rectifier Shab currents. Thus differential regulation of K(+) channels may influence the signal handling capability of neurons. This study provides support for the notion that, in addition to synaptic mechanisms, modulations in spike activity patterns may represent an important mechanism for learning and memory that should be explored more fully.

Sh and eag K(+) channel subunit interaction in frog oocytes depends on level and time of expression

Subcellular clustering of ion channels critically affects neuronal function. Coexpression of Eag and Sh channel subunits in Xenopus oocytes leads to accelerated decay of the Sh-like transient K(+) current (Chen, M.-L., T. Hoshi, and C.-F. Wu. 1996. Neuron. 17:535-542). We report that such interaction depends critically on functional expression level (controlled by RNA injection quantities and indicated by current amplitudes) and developmental time after RNA injection. The accelerated decay became apparent 3 days after coinjection and increased thereafter. This was observed in different ionic conditions and at different voltage steps. However, decay was not accelerated at low expression levels, either within 1-2 days after injection or with reduced amounts of RNA. With sequential RNA injection, preformation of either Eag or Sh channels prevented interactions with the other subunit. The carboxyl terminus of Eag was found to be involved in accelerating, and in retarding recovery from, N-type inactivation. The interaction was reduced upon patch excision in macropatch measurements, suggesting involvement of cytosolic factors. We have reproduced the absence of interaction between Eag and Sh reported previously within 2 days after RNA injection and with low levels of current expression (Tang, C.-Y., C. T. Schulteis, R. M. Jiménez, and D. M. Papazian. 1998. Biophys. J. 75:1263-1270). Our findings demonstrate that heterologous expression of channels in Xenopus oocytes is a dynamic process influenced by cell physiology and development. These factors must be considered in interpreting the functional properties of heterologously expressed channels.

A novel leg-shaking Drosophila mutant defective in a voltage-gated K(+)current and hypersensitive to reactive oxygen species

1,1'-Dimethyl-4,4'-bipyridinium dichloride (methyl viologen; paraquat), an herbicide that causes depletion of NADPH and generates excessive reactive oxygen species (ROS) in vivo, has been used to screen for ROS-sensitive Drosophila mutants. One mutant so isolated, named quiver(1) (qvr(1)), has a leg-shaking phenotype. Mutants of the Shaker (Sh), Hyperkinetic (Hk), and ether a go-go (eag) genes, which encode different K(+) channel subunits that regulate the A-type K(+) current (I(A)) in different ways, exhibit leg shaking under ether anesthesia and have heightened metabolic rates and shortened life spans. We found that Sh, Hk, and eag mutant flies were all hypersensitive to paraquat. Double-mutant combinations among the three channel mutations and qvr(1) had drastically enhanced sensitivity to paraquat. Synaptic transmission at the larval neuromuscular junction was increased in the qvr(1) mutant to the level of Sh mutants. Similar to eag Sh double mutants, double mutants of eag and qvr(1) showed striking enhancement in synaptic transmission and a wings-down phenotype, the hallmarks of extreme hyperexcitability. Voltage-clamp experiments demonstrated that the qvr(1) mutation specifically disrupted the Sh-dependent I(A) current without altering the other currents [I(K), Ca(2+)-activated fast (I(CF)) and slow (I(CS)) currents, and I(Ca)] in larval muscles. Several deficiency strains of the qvr locus failed to complement qvr(1) and confirmed that ether-induced leg shaking, reduced I(A) current, and paraquat hypersensitivity map to the same locus. Our results suggest that the qvr gene may encode a novel K(+) channel-related polypeptide and indicate a strong link between a voltage-activated K(+) current and vulnerability to ROS.

Mutations affecting the delayed rectifier potassium current in Drosophila

Mutations and pharmacological agents have been used to resolve and analyze several K(+) currents in Drosophila. Mutations that affect channels carrying the voltage-activated I(A) and the Ca(2+) -activated I(CF) have helped greatly in analyzing the structure, function and regulation of these channels. We now report mutations that selectively affect the delayed rectifier current, I(K). Flies mutagenized with ethylmethanesulfonate were screened for temperature-induced paralysis. Paralytic mutants identified in the screen were examined for K(+) currents in the larval body-wall muscles. The z66 mutant larvae showed a significant reduction in I(K). The mutation did not affect other K(+) currents (I(A), I(CF), or I(CS) ) or the Ca(2+) channel current in the muscles. Another mutation, z4, which showed reduced I(K), failed to complement z66. Genetic analysis localized the gene disrupted by z66 and z4 to the left arm of chromosome 3, in the 63A1-63B6 region on polytene chromosomes. The z66 and the z4 mutations, which lie in the Shab K(+) channel gene, provide an opportunity to undertake analysis of the functioning of these channels and to study the role of these channels in membrane excitability.

Molecular separation of two behavioral phenotypes by a mutation affecting the promoters of a Ca-activated K channel

The Drosophila slowpoke gene encodes a BK-type calcium-activated potassium channel. Null mutations in slowpoke perturb the signaling properties of neurons and muscles and cause behavioral defects. The animals fly very poorly compared with wild-type strains and, after exposure to a bright but cool light or a heat pulse, exhibit a "sticky-feet" phenotype. Expression of slowpoke arises from five transcriptional promoters that express the gene in neural, muscle, and epithelial tissues. A chromosomal deletion (ash2(18)) has been identified that removes the neuronal promoters but not the muscle-tracheal cell promoter. This deletion complements the flight defect of slowpoke null mutants but not the sticky-feet phenotype. Electrophysiological assays confirm that the ash2(18) chromosome restores normal electrical properties to the flight muscle. This suggests that the flight defect arises from a lack of slowpoke expression in muscle, whereas the sticky-feet phenotype arises from a lack of expression in nervous tissue.