Beta-subunits are important modulators of the acute response to alcohol in human BK channels

BK ZERO isoform HEK293 stably transfected cell lines differing 3'UTRs to assess miR-9 regulation

Research has identified the large conductance voltage- and calcium-activated potassium channel (BK) as a key regulator of neuronal excitability genetically associated to behavioral alcohol tolerance. Sensitivity to ethanol at the molecular level is characterized by acute potentiation of channel activity. BK isoforms show variations in alcohol sensitivity and are differentially distributed on the plasma membrane surface in response to prolonged exposure. MicroRNA (MiRNA) targeting of alcohol-sensitive isoforms coupled with active internalization of BK channels in response to ethanol are believed to be key in establishing homeostatic adaptations that produce persistent changes within the plasma membrane of neurons. In fact, microRNA 9 (miR-9) upregulated expression is a key event in persistent alcohol tolerance mediating acute EtOH desensitization of BK channels. The exact nature of these interactions remains a current topic of discussion. To further study the effects of miR-9 on the expression and distribution of BK channel isoforms we designed an experimental model by transfecting human BK channel isoforms ZERO heterologous constructs in human embryonic kidney cells 293 (HEK293) cells respectively expressing 2.1 (miR-9 responsive), 2.2 (unresponsive) and control (no sequence) 3'untranslated region (3'UTR) miRNA recognition sites. We used imaging techniques to characterize the stably transfected monoclonal cell lines, and electrophysiology to validate channel activity. Finally, we used immunocytochemistry to validate isoform responsiveness to miR-9. Our findings suggest the cell lines were successfully transfected to express either the 2.1 or 2.2 version of ZERO. Patch clamp recordings confirm that these channels retain their functionality and immunohistochemistry shows differential responses to miR-9, making these cells viable for use in future alcohol dependence studies.

The molecular nature of the 17β-Estradiol binding site in the voltage- and Ca2+-activated K+ (BK) channel β1 subunit

The accessory β1 subunit modulates the Ca2+- and voltage-activated K+ (BK) channel gating properties mainly by increasing its apparent Ca2+ sensitivity. β1 plays an important role in the modulation of arterial tone and blood pressure by vascular smooth muscle cells (SMCs). 17β-estradiol (E2) increases the BK channel open probability (Po) in SMCs, through a β1 subunit-dependent modulatory effect. Here, using molecular modeling, bioinformatics, mutagenesis, and electrophysiology, we identify a cluster of hydrophobic residues in the second transmembrane domain of the β1 subunit, including the residues W163 and F166, as the binding site for E2. We further show that the increase in Po induced by E2 is associated with a stabilization of the voltage sensor in its active configuration and an increase in the coupling between the voltage sensor activation and pore opening. Since β1 is a key molecular player in vasoregulation, the findings reported here are of importance in the design of novel drugs able to modulate BK channels.

Drosophila: An Emergent Model for Delineating Interactions between the Circadian Clock and Drugs of Abuse

Endogenous circadian oscillators orchestrate rhythms at the cellular, physiological, and behavioral levels across species to coordinate activity, for example, sleep/wake cycles, metabolism, and learning and memory, with predictable environmental cycles. The 21st century has seen a dramatic rise in the incidence of circadian and sleep disorders with globalization, technological advances, and the use of personal electronics. The circadian clock modulates alcohol- and drug-induced behaviors with circadian misalignment contributing to increased substance use and abuse. Invertebrate models, such as Drosophila melanogaster, have proven invaluable for the identification of genetic and molecular mechanisms underlying highly conserved processes including the circadian clock, drug tolerance, and reward systems. In this review, we highlight the contributions of Drosophila as a model system for understanding the bidirectional interactions between the circadian system and the drugs of abuse, alcohol and cocaine, and illustrate the highly conserved nature of these interactions between Drosophila and mammalian systems. Research in Drosophila provides mechanistic insights into the corresponding behaviors in higher organisms and can be used as a guide for targeted inquiries in mammals.

Differential potassium channel gene regulation in BXD mice reveals novel targets for pharmacogenetic therapies to reduce heavy alcohol drinking

Alcohol (ethanol) dependence is a chronic relapsing brain disorder partially influenced by genetics and characterized by an inability to regulate harmful levels of drinking. Emerging evidence has linked genes that encode K_V7, K_IR, and K_Ca2 K⁺ channels with variation in alcohol-related behaviors in rodents and humans. This led us to experimentally test relations between K⁺ channel genes and escalation of drinking in a chronic-intermittent ethanol (CIE) exposure model of dependence in BXD recombinant inbred strains of mice. Transcript levels for K⁺ channel genes in the prefrontal cortex (PFC) and nucleus accumbens (NAc) covary with voluntary ethanol drinking in a non-dependent cohort. Transcripts that encode K_V7 channels covary negatively with drinking in non-dependent BXD strains. Using a pharmacological approach to validate the genetic findings, C57BL/6J mice were allowed intermittent access to ethanol to establish baseline consumption before they were treated with retigabine, an FDA-approved K_V7 channel positive modulator. Systemic administration significantly reduced drinking, and consistent with previous evidence, retigabine was more effective at reducing voluntary consumption in high-drinking than low-drinking subjects. We evaluated the specific K⁺ channel genes that were most sensitive to CIE exposure and identified a gene subset in the NAc and PFC that were dysregulated in the alcohol-dependent BXD cohort. CIE-induced modulation of nine genes in the NAc and six genes in the PFC covaried well with the changes in drinking induced by ethanol dependence. Here we identified novel candidate genes in the NAc and PFC that are regulated by ethanol dependence and correlate with voluntary drinking in non-dependent and dependent BXD mice. The findings that Kcnq expression correlates with drinking and that retigabine reduces consumption suggest that K_V7 channels could be pharmacogenetic targets to treat individuals with alcohol addiction.

Voltage-Sensitive Potassium Channels of the BK Type and Their Coding Genes Are Alcohol Targets in Neurons

Among all members of the voltage-gated, TM6 ion channel superfamily, the proteins that constitute calcium- and voltage-gated potassium channels of large conductance (BK) and their coding genes are unique for their involvement in ethanol-induced disruption of normal physiology and behavior. Moreover, in vitro studies document that BK activity is modified by ethanol with an EC₅₀~23 mM, which is near blood alcohol levels considered legal intoxication in most states of the USA (0.08 g/dL = 17.4 mM). Following a succinct introduction to our current understanding of BK structure and function in central neurons, with a focus on neural circuits that contribute to the neurobiology of alcohol use disorders (AUD), we review the modifications in organ physiology by alcohol exposure via BK and the different molecular elements that determine the ethanol response of BK in alcohol-naïve systems, including the role of an ethanol-recognizing site in the BK-forming slo1 protein, modulation of accessory BK subunits, and their coding genes. The participation of these and additional elements in determining the response of a system or an organism to protracted ethanol exposure is consequently analyzed, with insights obtained from invertebrate and vertebrate models. Particular emphasis is put on the role of BK and coding genes in different forms of tolerance to alcohol exposure. We finally discuss genetic results on BK obtained in invertebrate organisms and rodents in light of possible extrapolation to human AUD.

Ethanol Effect on BK Channels is Modulated by Magnesium

BACKGROUND

Alcoholics have been reported to have reduced levels of magnesium in both their extracellular and intracellular compartments. Calcium-dependent potassium channels (BK) are known to be one of ethanol (EtOH)'s better known molecular targets.

METHODS

Using outside-out patches from hippocampal neuronal cultures, we examined the consequences of altered intracellular Mg(2+) on the effects that EtOH has on BK channels.

RESULTS

We find that the effect of EtOH is bimodally influenced by the Mg(2+) concentration on the cytoplasmic side. More specifically, when internal Mg(2+) concentrations are ≤200 μM, EtOH decreases BK activity, whereas it increases activity when Mg(2+) is at 1 mM. Similar results are obtained when using patches from HEK cells expressing only the α-subunit of BK. When patches are made with the actin destabilizer cytochalasin D present on the cytoplasmic side, the potentiation caused by EtOH becomes independent of the Mg(2+) concentration. Furthermore, in the presence of the actin stabilizer phalloidin, EtOH causes inhibition even at Mg(2+) concentrations of 1 mM.

CONCLUSIONS

Internal Mg(2+) can modulate the EtOH effects on BK channels only when there is an intact, internal actin interaction with the channel, as is found at synapses. We propose that the EtOH-induced decrease in cytoplasmic Mg(2+) observed in frequent/chronic drinkers would decrease EtOH's actions on synaptic (e.g., actin-bound) BK channels, producing a form of molecular tolerance.

BK Channels in the Central Nervous System

Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels are widely distributed in the postnatal central nervous system (CNS). BK channels play a pleiotropic role in regulating the activity of brain and spinal cord neural circuits by providing a negative feedback mechanism for local increases in intracellular Ca(2+) concentrations. In neurons, they regulate the timing and duration of K(+) influx such that they can either increase or decrease firing depending on the cellular context, and they can suppress neurotransmitter release from presynaptic terminals. In addition, BK channels located in astrocytes and arterial myocytes modulate cerebral blood flow. Not surprisingly, both loss and gain of BK channel function have been associated with CNS disorders such as epilepsy, ataxia, mental retardation, and chronic pain. On the other hand, the neuroprotective role played by BK channels in a number of pathological situations could potentially be leveraged to correct neurological dysfunction.

Modulation of BK Channel Function by Auxiliary Beta and Gamma Subunits

The large-conductance, Ca(2+)- and voltage-activated K(+) (BK) channel is ubiquitously expressed in mammalian tissues and displays diverse biophysical or pharmacological characteristics. This diversity is in part conferred by channel modulation with different regulatory auxiliary subunits. To date, two distinct classes of BK channel auxiliary subunits have been identified: β subunits and γ subunits. Modulation of BK channels by the four auxiliary β (β1-β4) subunits has been well established and intensively investigated over the past two decades. The auxiliary γ subunits, however, were identified only very recently, which adds a new dimension to BK channel regulation and improves our understanding of the physiological functions of BK channels in various tissues and cell types. This chapter will review the current understanding of BK channel modulation by auxiliary β and γ subunits, especially the latest findings.

BK Channel β1 Subunit Contributes to Behavioral Adaptations Elicited by Chronic Intermittent Ethanol Exposure

BACKGROUND

Large conductance, calcium- and voltage-activated potassium (BK) channels regulate neuronal excitability and neurotransmission. They can be directly activated by ethanol (EtOH) and they may be implicated in EtOH dependence. In this study, we sought to determine the influence of the auxiliary β1 and β4 subunits on EtOH metabolism, acute sensitivity to EtOH intoxication, acute functional tolerance, chronic tolerance, and handling-induced convulsions during withdrawal.

METHODS

Motor coordination, righting reflex, and body temperature were evaluated in BK β1 and β4 knockout, heterozygous, and wild-type mice following acute EtOH administration. Chronic tolerance and physical dependence were induced by chronic intermittent inhalation of EtOH vapor.

RESULTS

Constitutive deficiency in BK β1 or β4 subunits did not alter the clearance rate of EtOH, acute sensitivity to EtOH-induced ataxia, sedation, and hypothermia, nor acute functional tolerance to ataxia. BK β1 deletion reduced chronic tolerance to sedation and abolished chronic tolerance to hypothermia, while BK β4 deletion did not affect these adaptations to chronic EtOH exposure. Finally, the absence of BK β1 accelerated the appearance, while the absence of BK β4 delayed the resolution, of the hyperexcitable state associated with EtOH withdrawal.

CONCLUSIONS

Altogether, the present findings reveal the critical role of BK β1 in behavioral adaptations to prolonged, repeated EtOH intoxication.

Molecular mechanism underlying β1 regulation in voltage- and calcium-activated potassium (BK) channels

Being activated by depolarizing voltages and increases in cytoplasmic Ca(2+), voltage- and calcium-activated potassium (BK) channels and their modulatory β-subunits are able to dampen or stop excitatory stimuli in a wide range of cellular types, including both neuronal and nonneuronal tissues. Minimal alterations in BK channel function may contribute to the pathophysiology of several diseases, including hypertension, asthma, cancer, epilepsy, and diabetes. Several gating processes, allosterically coupled to each other, control BK channel activity and are potential targets for regulation by auxiliary β-subunits that are expressed together with the α (BK)-subunit in almost every tissue type where they are found. By measuring gating currents in BK channels coexpressed with chimeras between β1 and β3 or β2 auxiliary subunits, we were able to identify that the cytoplasmic regions of β1 are responsible for the modulation of the voltage sensors. In addition, we narrowed down the structural determinants to the N terminus of β1, which contains two lysine residues (i.e., K3 and K4), which upon substitution virtually abolished the effects of β1 on charge movement. The mechanism by which K3 and K4 stabilize the voltage sensor is not electrostatic but specific, and the α (BK)-residues involved remain to be identified. This is the first report, to our knowledge, where the regulatory effects of the β1-subunit have been clearly assigned to a particular segment, with two pivotal amino acids being responsible for this modulation.

Ethanol modulation of mammalian BK channels in excitable tissues: molecular targets and their possible contribution to alcohol-induced altered behavior

In most tissues, the function of Ca²⁺- and voltage-gated K⁺ (BK) channels is modified in response to ethanol concentrations reached in human blood during alcohol intoxication. In general, modification of BK current from ethanol-naïve preparations in response to brief ethanol exposure results from changes in channel open probability without modification of unitary conductance or change in BK protein levels in the membrane. Protracted and/or repeated ethanol exposure, however, may evoke changes in BK expression. The final ethanol effect on BK open probability leading to either BK current potentiation or BK current reduction is determined by an orchestration of molecular factors, including levels of activating ligand (Ca²⁺_i), BK subunit composition and post-translational modifications, and the channel's lipid microenvironment. These factors seem to allosterically regulate a direct interaction between ethanol and a recognition pocket of discrete dimensions recently mapped to the channel-forming (slo1) subunit. Type of ethanol exposure also plays a role in the final BK response to the drug: in several central nervous system regions (e.g., striatum, primary sensory neurons, and supraoptic nucleus), acute exposure to ethanol reduces neuronal excitability by enhancing BK activity. In contrast, protracted or repetitive ethanol administration may alter BK subunit composition and membrane expression, rendering the BK complex insensitive to further ethanol exposure. In neurohypophyseal axon terminals, ethanol potentiation of BK channel activity leads to a reduction in neuropeptide release. In vascular smooth muscle, however, ethanol inhibition of BK current leads to cell contraction and vascular constriction.

Pharmacological consequences of the coexpression of BK channel α and auxiliary β subunits

Coded by a single gene (Slo1, KCM) and activated by depolarizing potentials and by a rise in intracellular Ca(2+) concentration, the large conductance voltage- and Ca(2+)-activated K(+) channel (BK) is unique among the superfamily of K(+) channels. BK channels are tetramers characterized by a pore-forming α subunit containing seven transmembrane segments (instead of the six found in voltage-dependent K(+) channels) and a large C terminus composed of two regulators of K(+) conductance domains (RCK domains), where the Ca(2+)-binding sites reside. BK channels can be associated with accessory β subunits and, although different BK modulatory mechanisms have been described, greater interest has recently been placed on the role that the β subunits may play in the modulation of BK channel gating due to its physiological importance. Four β subunits have currently been identified (i.e., β1, β2, β3, and β4) and despite the fact that they all share the same topology, it has been shown that every β subunit has a specific tissue distribution and that they modify channel kinetics as well as their pharmacological properties and the apparent Ca(2+) sensitivity of the α subunit in different ways. Additionally, different studies have shown that natural, endogenous, and synthetic compounds can modulate BK channels through β subunits. Considering the importance of these channels in different pathological conditions, such as hypertension and neurological disorders, this review focuses on the mechanisms by which these compounds modulate the biophysical properties of BK channels through the regulation of β subunits, as well as their potential therapeutic uses for diseases such as those mentioned above.

Large conductance voltage- and Ca2+-gated potassium (BK) channel β4 subunit influences sensitivity and tolerance to alcohol by altering its response to kinases

Tolerance is a well described component of alcohol abuse and addiction. The large conductance voltage- and Ca(2+)-gated potassium channel (BK) has been very useful for studying molecular tolerance. The influence of association with the β4 subunit can be observed at the level of individual channels, action potentials in brain slices, and finally, drinking behavior in the mouse. Previously, we showed that 50 mm alcohol increases both α and αβ4 BK channel open probability, but only α BK develops acute tolerance to this effect. Currently, we explore the possibility that the influence of the β4 subunit on tolerance may result from a striking effect of β4 on kinase modulation of the BK channel. We examine the influence of the β4 subunit on PKA, CaMKII, and phosphatase modulation of channel activity, and on molecular tolerance to alcohol. We record from human BK channels heterologously expressed in HEK 293 cells composed of its core subunit, α alone (Insertless), or co-expressed with the β4 BK auxiliary subunit, as well as, acutely dissociated nucleus accumbens neurons using the cell-attached patch clamp configuration. Our results indicate that BK channels are strongly modulated by activation of specific kinases (PKA and CaMKII) and phosphatases. The presence of the β4 subunit greatly influences this modulation, allowing a variety of outcomes for BK channel activity in response to acute alcohol.

Functional insights into modulation of BKCa channel activity to alter myometrial contractility

The large-conductance voltage- and Ca(2+)-activated K(+) channel (BKCa) is an important regulator of membrane excitability in a wide variety of cells and tissues. In myometrial smooth muscle, activation of BKCa plays essential roles in buffering contractility to maintain uterine quiescence during pregnancy and in the transition to a more contractile state at the onset of labor. Multiple mechanisms of modulation have been described to alter BKCa channel activity, expression, and cellular localization. In the myometrium, BKCa is regulated by alternative splicing, protein targeting to the plasma membrane, compartmentation in membrane microdomains, and posttranslational modifications. In addition, interaction with auxiliary proteins (i.e., β1- and β2-subunits), association with G-protein coupled receptor signaling pathways, such as those activated by adrenergic and oxytocin receptors, and hormonal regulation provide further mechanisms of variable modulation of BKCa channel function in myometrial smooth muscle. Here, we provide an overview of these mechanisms of BKCa channel modulation and provide a context for them in relation to myometrial function.

Nematodes feel a craving--using Caenorhabditis elegans as a model to study alcohol addiction

Alcohol is the most frequently-used addictive drug. However, the mechanism by which its consumption leads to addiction remains largely elusive. Given the conservation of behavioral reactions to alcohol, Caenorhabitis elegans (C. elegans) has been effectively used as a model system to investigate the relevant molecular targets and pathways mediating these responses. In this article, we review the roles of BK channels (also called SLO-1), the lipid microenvironment, receptors, the synaptic machinery, and neurotransmitters in both the acute and chronic effects of alcohol. We provide an overview of the genes and mechanisms involved in alcoholismrelated behaviors in C. elegans.

An alcohol-sensing site in the calcium- and voltage-gated, large conductance potassium (BK) channel

Ethanol alters BK (slo1) channel function leading to perturbation of physiology and behavior. Site(s) and mechanism(s) of ethanol-BK channel interaction are unknown. We demonstrate that ethanol docks onto a water-accessible site that is strategically positioned between the slo1 calcium-sensors and gate. Ethanol only accesses this site in presence of calcium, the BK channel's physiological agonist. Within the site, ethanol hydrogen-bonds with K361. Moreover, substitutions that hamper hydrogen bond formation or prevent ethanol from accessing K361 abolish alcohol action without altering basal channel function. Alcohol interacting site dimensions are approximately 10.7 × 8.6 × 7.1 Å, accommodating effective (ethanol-heptanol) but not ineffective (octanol, nonanol) channel activators. This study presents: (i) to our knowledge, the first identification and characterization of an n-alkanol recognition site in a member of the voltage-gated TM6 channel superfamily; (ii) structural insights on ethanol allosteric interactions with ligand-gated ion channels; and (iii) a first step for designing agents that antagonize BK channel-mediated alcohol actions without perturbing basal channel function.

BK channel β1 and β4 auxiliary subunits exert opposite influences on escalated ethanol drinking in dependent mice

Large conductance calcium-activated potassium (BK) channels play a key role in the control of neuronal activity. Ethanol is a potent activator of BK channel gating, but how this action may impact ethanol drinking still remains poorly understood. Auxiliary β subunits are known to modulate ethanol-induced potentiation of BK currents. In the present study, we investigated whether BK β1 and β4 subunits influence voluntary ethanol consumption using knockout (KO) mice. In a first experiment, mice were first subjected to continuous two-bottle choice (2BC) and were then switched to intermittent 2BC, which progressively increased ethanol intake as previously described in wildtype mice. BK β1 or β4 subunit deficiency did not affect ethanol self-administration under either schedule of access. In a second experiment, mice were first trained to drink ethanol in a limited-access 2BC paradigm. BK β1 or β4 deletion did not affect baseline consumption. Weeks of 2BC were then alternated with weeks of chronic intermittent ethanol (CIE) or air inhalation. As expected, a gradual escalation of ethanol drinking was observed in dependent wildtype mice, while intake remained stable in non-dependent wildtype mice. However, CIE exposure only produced a mild augmentation of ethanol consumption in BK β4 KO mice. Conversely, ethanol drinking increased after fewer CIE cycles in BK β1 KO mice than in wildtype mice. In conclusion, BK β1 or β4 did not influence voluntary ethanol drinking in non-dependent mice, regardless of the pattern of access to ethanol. However, deletion of BK β4 attenuated, while deletion of BK β1 accelerated, the escalation of ethanol drinking during withdrawal from CIE. Our data suggest that BK β1 and β4 subunits have an opposite influence on the negative reinforcing properties of ethanol withdrawal. Modulating the expression, distribution or interactions of BK channel auxiliary subunits may therefore represent a novel avenue for the treatment of alcoholism.

A BK (Slo1) channel journey from molecule to physiology

Calcium and voltage-activated potassium (BK) channels are key actors in cell physiology, both in neuronal and non-neuronal cells and tissues. Through negative feedback between intracellular Ca (2+) and membrane voltage, BK channels provide a damping mechanism for excitatory signals. Molecular modulation of these channels by alternative splicing, auxiliary subunits and post-translational modifications showed that these channels are subjected to many mechanisms that add diversity to the BK channel α subunit gene. This complexity of interactions modulates BK channel gating, modifying the energetic barrier of voltage sensor domain activation and channel opening. Regions for voltage as well as Ca (2+) sensitivity have been identified, and the crystal structure generated by the 2 RCK domains contained in the C-terminal of the channel has been described. The linkage of these channels to many intracellular metabolites and pathways, as well as their modulation by extracellular natural agents, has been found to be relevant in many physiological processes. This review includes the hallmarks of BK channel biophysics and its physiological impact on specific cells and tissues, highlighting its relationship with auxiliary subunit expression.

Large conductance, calcium- and voltage-gated potassium (BK) channels: regulation by cholesterol

Cholesterol (CLR) is an essential component of eukaryotic plasma membranes. CLR regulates the membrane physical state, microdomain formation and the activity of membrane-spanning proteins, including ion channels. Large conductance, voltage- and Ca²⁺-gated K⁺ (BK) channels link membrane potential to cell Ca²⁺ homeostasis. Thus, they control many physiological processes and participate in pathophysiological mechanisms leading to human disease. Because plasmalemma BK channels cluster in CLR-rich membrane microdomains, a major driving force for studying BK channel-CLR interactions is determining how membrane CLR controls the BK current phenotype, including its pharmacology, channel sorting, distribution, and role in cell physiology. Since both BK channels and CLR tissue levels play a pathophysiological role in human disease, identifying functional and structural aspects of the CLR-BK channel interaction may open new avenues for therapeutic intervention. Here, we review the studies documenting membrane CLR-BK channel interactions, dissecting out the many factors that determine the final BK current response to changes in membrane CLR content. We also summarize work in reductionist systems where recombinant BK protein is studied in artificial lipid bilayers, which documents a direct inhibition of BK channel activity by CLR and builds a strong case for a direct interaction between CLR and the BK channel-forming protein. Bilayer lipid-mediated mechanisms in CLR action are also discussed. Finally, we review studies of BK channel function during hypercholesterolemia, and underscore the many consequences that the CLR-BK channel interaction brings to cell physiology and human disease.

The relationship between duration of initial alcohol exposure and persistence of molecular tolerance is markedly nonlinear

The neuronal calcium- and voltage-activated BK potassium channel is modulated by ethanol, and plays a role in behavioral tolerance in vertebrates and invertebrates. We examine the influence of temporal parameters of alcohol exposure on the characteristics of BK molecular tolerance in the ventral striatum, an important component of brain reward circuitry. BK channels in striatal neurons of C57BL/6J mice exhibited molecular tolerance whose duration was a function of exposure time. After 6 h exposure to 20 mm (0.09 mg%) ethanol, alcohol sensitivity was suppressed beyond 24 h after withdrawal, while after a 1 or 3 h exposure, sensitivity had significantly recovered after 4 h. This temporally controlled transition to persistent molecular tolerance parallels changes in BK channel isoform profile. After withdrawal from 6 h, but not 3 h alcohol exposure, mRNA levels of the alcohol-insensitive STREX (stress axis-regulated exon) splice variant were increased. Moreover, the biophysical properties of BK channels during withdrawal from 6 h exposure were altered, and match the properties of STREX channels exogenously expressed in HEK 293 cells. Our results suggest a temporally triggered shift in BK isoform identity. Once activated, the transition does not require the continued presence of alcohol. We next determined whether the results obtained using cultured striatal neurons could be observed in acutely dissociated striatal neurons, after alcohol administration in the living mouse. The results were in remarkable agreement with the striatal culture data, showing persistent molecular tolerance after injections producing 6 h of intoxication, but not after injections producing only 3 h of intoxication.

BK channel and alcohol, a complicated affair

Alcohol is a fast acting molecule that alters behavior within a few minutes of absorption. Its rapid behavioral impact suggests early action on ion channels. Of all voltage-gated potassium ion channels, BK channels, a subcategory of potassium channels characterized by their large unitary conductance, and by their capacity of being activated synergistically by membrane potential and intracellular free calcium, are unique due to their high sensitivity to alcohol. In this review, we discuss BK channels structure and function, and how they help us understand the various ways BK channel mediates alcohol's effects on neuronal function and on behavior in the striatum.

BK Channels: mediators and models for alcohol tolerance

Enhanced acute tolerance predicts alcohol abuse. We describe work on the role of the calcium- and voltage-gated BK channel in alcohol tolerance, highlighting the lipid environment, BK protein isoform selection and auxiliary BK channel proteins. We show how ethanol, which had the reputation of a nonspecific membrane perturbant, is now being examined at realistic concentrations with cutting-edge techniques, providing novel molecular targets for therapeutic approaches to alcoholism. Addictive disorders impact our emotional, physical and financial status, and burden our healthcare system. Although alcohol is the focus of this review, it is highly probable, given the common neural and biochemical pathways used by drugs of abuse, that the findings described here will also apply to other drugs.

Sizing up ethanol-induced plasticity: the role of small and large conductance calcium-activated potassium channels

Small (SK) and large conductance (BK) Ca(2+)-activated K(+) channels contribute to action potential repolarization, shape dendritic Ca(2+)spikes and postsynaptic responses, modulate the release of hormones and neurotransmitters, and contribute to hippocampal-dependent synaptic plasticity. Over the last decade, SK and BK channels have emerged as important targets for the development of acute ethanol tolerance and for altering neuronal excitability following chronic ethanol consumption. In this mini-review, we discuss new evidence implicating SK and BK channels in ethanol tolerance and ethanol-associated homeostatic plasticity. Findings from recent reports demonstrate that chronic ethanol produces a reduction in the function of SK channels in VTA dopaminergic and CA1 pyramidal neurons. It is hypothesized that the reduction in SK channel function increases the propensity for burst firing in VTA neurons and increases the likelihood for aberrant hyperexcitability during ethanol withdrawal in hippocampus. There is also increasing evidence supporting the idea that ethanol sensitivity of native BK channel results from differences in BK subunit composition, the proteolipid microenvironment, and molecular determinants of the channel-forming subunit itself. Moreover, these molecular entities play a substantial role in controlling the temporal component of ethanol-associated neuroadaptations in BK channels. Taken together, these studies suggest that SK and BK channels contribute to ethanol tolerance and adaptive plasticity.

Compartmentalized beta subunit distribution determines characteristics and ethanol sensitivity of somatic, dendritic, and terminal large-conductance calcium-activated potassium channels in the rat central nervous system

Neurons are highly differentiated and polarized cells, whose various functions depend upon the compartmentalization of ion channels. The rat hypothalamic-neurohypophysial system (HNS), in which cell bodies and dendrites reside in the hypothalamus, physically separated from their nerve terminals in the neurohypophysis, provides a particularly powerful preparation in which to study the distribution and regional properties of ion channel proteins. Using electrophysiological and immunohistochemical techniques, we characterized the large-conductance calcium-activated potassium (BK) channel in each of the three primary compartments (soma, dendrite, and terminal) of HNS neurons. We found that dendritic BK channels, in common with somatic channels but in contrast to nerve terminal channels, are insensitive to iberiotoxin. Furthermore, analysis of dendritic BK channel gating kinetics indicates that they, like somatic channels, have fast activation kinetics, in contrast to the slow gating of terminal channels. Dendritic and somatic channels are also more sensitive to calcium and have a greater conductance than terminal channels. Finally, although terminal BK channels are highly potentiated by ethanol, somatic and dendritic channels are insensitive to the drug. The biophysical and pharmacological properties of somatic and dendritic versus nerve terminal channels are consistent with the characteristics of exogenously expressed alphabeta1 versus alphabeta4 channels, respectively. Therefore, one possible explanation for our findings is a selective distribution of auxiliary beta1 subunits to the somatic and dendritic compartments and beta4 to the terminal compartment. This hypothesis is supported immunohistochemically by the appearance of distinct punctate beta1 or beta4 channel clusters in the membrane of somatic and dendritic or nerve terminal compartments, respectively.

Acute alcohol action and desensitization of ligand-gated ion channels

Ethanol exerts its biological actions through multiple receptors, including ion channels. Ion channels that are sensitive to pharmacologically relevant ethanol concentrations constitute a heterogeneous set, including structurally unrelated proteins solely sharing the property that their gating is regulated by a ligand(s). Receptor desensitization is almost universal among these channels, and its modulation by ethanol may be a crucial aspect of alcohol pharmacology and effects in the body. We review the evidence documenting interactions between ethanol and ionotropic receptor desensitization, and the contribution of this interaction to overall ethanol action on channel function. In some cases, such as type 3 serotonin, nicotinic acetylcholine, GABA-A, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, ethanol actions on apparent desensitization play a significant role in acute drug action on receptor function. In a few cases, mutagenesis helped to identify different areas within a receptor protein that differentially sense n-alcohols, resulting in differential modulation of receptor desensitization. However, desensitization of a receptor is linked to a variety of biochemical processes that may alter protein conformation, such as the lipid microenvironment, post-translational channel modification, and channel subunit composition, the relative contribution of these processes to ethanol interactions with channel desensitization remains unclear. Understanding interactions between ethanol and ionotropic receptor desensitization may help to explain different ethanol actions 1) when ethanol is evaluated in vitro on cloned channel proteins, 2) under physiological or pathological conditions or in distinct cell domains with modified ligand concentration and/or receptor conformation. Finally, receptor desensitization is likely to participate in molecular and, possibly, behavioral tolerance to ethanol, which is thought to contribute to the risk of alcoholism.

Identification of a BK channel auxiliary protein controlling molecular and behavioral tolerance to alcohol

Tolerance, described as the loss of drug effectiveness over time, is an important component of addiction. The degree of acute behavioral tolerance to alcohol exhibited by a naïve subject can predict the likelihood of alcohol abuse. Thus, the determinants of acute tolerance are important to understand. Calcium- and voltage-gated (BK) potassium channels, consisting of pore forming alpha and modulatory beta subunits, are targets of ethanol (EtOH) action. Here, we examine the role, at the molecular, cellular, and behavioral levels, of the BK beta4 subunit in acute tolerance. Single channel recordings in HEK-293 cells show that, in the absence of beta4, EtOH potentiation of activity exhibits acute tolerance, which is blocked by coexpressing the beta4 subunit. BK channels in acutely isolated medium spiny neurons from WT mice (in which the beta4 subunit is well-represented) exhibit little tolerance. In contrast, neuronal BK channels from beta4 knockout (KO) mice do display acute tolerance. Brain slice recordings showed tolerance to EtOH's effects on spike patterning in KO but not in WT mice. In addition, beta4 KO mice develop rapid tolerance to EtOH's locomotor effects, whereas WT mice do not. Finally, in a restricted access ethanol self-administration assay, beta4 KO mice drink more than their WT counterparts. Taken together, these data indicate that the beta4 subunit controls ethanol tolerance at the molecular, cellular, and behavioral levels, and could determine individual differences in alcohol abuse and alcoholism, as well as represent a therapeutic target for alcoholism.

BK channel subunit composition modulates molecular tolerance to ethanol

BACKGROUND

The large conductance calcium-activated potassium channel (also called BK channel or Slo channels) is a well-studied target of alcohol action, and plays an important role in behavioral tolerance.

METHODS

Using patch clamp electrophysiology, we examined human BK channels expressed in HEK293 cells to test whether tolerance to ethanol occurs in excised patches and whether it is influenced by subunit composition. Three combinations were examined: hSlo, hSlo + beta(1), and hSlo + beta(4).

RESULTS

The 2 components of BK alcohol adaptation (Component 1: rapid tolerance to acute potentiation, and Component 2: a more slowly developing decrease in current density) were observed, and varied according to subunit combination. Using a 2-exposure protocol, Component 1 tolerance was evident in 2 of the 3 combinations, because it was more pronounced for hSlo and hSlo + beta(4).

CONCLUSIONS

Thus, rapid tolerance in human BK occurs in cell-free membrane patches, independent of cytosolic second messengers, nucleotides or changes in free calcium. Alcohol pretreatment for 24 hours altered subsequent short-term plasticity of hSlo + beta(4) channels, suggesting a relationship between classes of tolerance. Finally, Component 2 reduction in current density showed a striking dependency on channel composition. Twenty-four hour exposure to 25 mM ethanol resulted in a down-regulation of BK current in hSlo and hSlo + beta(4) channels, but not in hSlo + beta(1) channels. The fact that hSlo + beta(1) channels show less sensitivity to acute challenge, in conjunction with less Component 1 and Component 2 tolerance, suggests subunit composition is an important factor for these elements of alcohol response.

Ethanol modulates BKCa channels by acting as an adjuvant of calcium

Ethanol modulation of calcium- and voltage-gated potassium (slo1) channels alters neuronal excitability, cerebrovascular tone, brain function, and behavior, yet the mechanism of this modulation remains unknown. Using patch-clamp electrophysiology on recombinant BK(Ca) channels cloned from mouse brain and expressed in Xenopus laevis oocytes, we demonstrate that ethanol, even at concentrations maximally effective to modulate BK(Ca) channel function (100 mM), fails to gate the channel in absence of activating calcium. Moreover, ethanol does not modify intrinsic, voltage- or physiological magnesium-driven gating. The alcohol works as an adjuvant of calcium by selectively facilitating calcium-driven gating. This facilitation, however, renders differential ethanol effects on channel activity: potentiation at low (<10 microM) and inhibition at high (>10 microM) calcium, this dual pattern remaining largely unmodified by coexpression of brain slo1 channels with the neuronally abundant BK(Ca) channel beta(4) subunit. Calcium recognition by either of the slo1 high-affinity sensors (calcium bowl and RCK1 Asp362/Asp367) is required for ethanol to amplify channel activation by calcium. The Asp362/Asp367 site, however, is necessary and sufficient to sustain ethanol inhibition. This inhibition also results from ethanol facilitation of calcium action; in this case, ethanol favors channel dwelling in a calcium-driven, low-activity mode. The agonist-adjuvant mechanism that we advance from the calcium-ethanol interaction on slo1 might be applicable to data of ethanol action on a wide variety of ligand-gated channels.

Physiology and pharmacology of alcohol: the imidazobenzodiazepine alcohol antagonist site on subtypes of GABAA receptors as an opportunity for drug development?

Alcohol (ethanol, EtOH) has pleiotropic actions and induces a number of acute and long-term effects due to direct actions on alcohol targets, and effects of alcohol metabolites and metabolism. Many detrimental health consequences are due to EtOH metabolism and metabolites, in particular acetaldehyde, whose high reactivity leads to nonspecific chemical modifications of proteins and nucleic acids. Like acetaldehyde, alcohol has been widely considered a nonspecific drug, despite rather persuasive evidence implicating inhibitory GABA(A) receptors (GABA(A)Rs) in acute alcohol actions, for example, a GABA(A)R ligand, the imidazobenzodiazepine Ro15-4513 antagonizes many low-to-moderate dose alcohol actions in mammals. It was therefore rather surprising that abundant types of synaptic GABA(A)Rs are generally not responsive to relevant low concentrations of EtOH. In contrast, delta-subunit-containing GABA(A)Rs and extrasynaptic tonic GABA currents mediated by these receptors are sensitive to alcohol concentrations that are reached in blood and tissues during low-to-moderate alcohol consumption. We recently showed that low-dose alcohol enhancement on highly alcohol-sensitive GABA(A)R subtypes is antagonized by Ro15-4513 in an apparently competitive manner, providing a molecular explanation for behavioural Ro15-4513 alcohol antagonism. The identification of a Ro15-4513/EtOH binding site on unique GABA(A)R subtypes opens the possibility to characterize this alcohol site(s) and screen for compounds that modulate the function of EtOH/Ro15-4513-sensitive GABA(A)Rs. The utility of such drugs might range from novel alcohol antagonists that might be useful in the emergency room, to drugs for the treatment of alcoholism, as well as alcohol-mimetic drugs to harness acute positive effects of alcohol.