BK Channels: mediators and models for alcohol tolerance

Abstract

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.

Figure 1

Alcohol response and adaptation strategies discussed. This review focuses on the BK channel, a calcium- and voltage-activated potassium channel that plays a dominant role in shaping neuronal activity, and which is strongly affected by alcohol. Shown here are four mechanisms influencing or underlying the response and adaptation of the BK protein to alcohol: (i) Membrane lipids affect BK function, alcohol response and acute tolerance. Lipid composition might differ between individuals and populations, or might be altered in response to alcohol exposure. Membrane lipid heterogeneity is illustrated by different colors. (ii) Epigenetic mechanisms, influencing the post-transcriptional expression of BK are an important mediator of molecular tolerance. MiR-9 is an example of a microRNA mediator that will be discussed in this review. (iii) Post-translational mechanisms, such as phosphorylation–dephosphorylation of the channel protein also influence tolerance. (iv) BK channel subunit composition might differ between populations and individuals, or might be modified in response to alcohol exposure, altering the response to alcohol. Why is there such a variety of routes? One possibility is that these different mechanisms produce different types of tolerance, modulating, for example the duration of tolerance, and that they are invoked differentially according to parameters such as the timing or concentration characteristics of drug exposure.

Figure 2

Modulation of BK channel function and distribution in neuronal terminals by chronic ethanol. After 24 h of exposure, BK channels are (i) not potentiated by acute challenge; (ii) less clustered in the membrane; (iii) less dense within remaining clusters; and (iv) more internalized (adapted from Ref. [9]). The light gray boxes indicate the membrane patch exposure to alcohol.

Figure 3

BK channel α subunit expression levels and sensitivity to alcohol. (a) The ratio of mRNAs coding for three of the most abundant BK channel splice variants [Insertless (IL), ALCOREX and STREX] in neurons is illustrated by the size of the channel icon (i). Thus, IL is by far the most abundant variant in naive cells. After alcohol exposure, the ratio IL/ALCOREX/STREX is profoundly altered (ii). IL mRNA representation decreases dramatically, whereas STREX, and to a lesser extent, ALCOREX, increases. This effect is initiated by an alcohol-mediated increase in microRNA (mir-9) and subsequent destruction of a subset of BK transcripts. Green boxes below each splice variant illustrate their respective sensitivity to acute alcohol, as well as the presence or absence of tolerance. Although IL BK channels are potentiated by shorter alcohol exposure, they lose their sensitivity after longer treatment. By contrast, STREX BKs (which represent a greater proportion of the post-exposure BK population) retain their lack of sensitivity after longer exposure to the drug. The sensitivity of ALCOREX sensitivity after chronic alcohol treatment is undetermined. Thus, the overall BK message is reduced and the representation of splice variant species is reordered, resulting in a more alcohol-resistant BK population. (b) Regulation of a BK channel transcripts in supraoptic nucleus neurons by alcohol. Only a subset of transcripts contain the miR-9 Recognition Element (MRE; pink boxes) in their untranslated regions (UTRs). After exposure to alcohol, miR-9 (light gray bands) is upregulated and binds to the MRE region, leading to a downregulation of splice variants [faded bans, (iii)] containing this MRE. The result is an altered isoform landscape in which alcohol-resistant BK isoforms predominate. X4 (green box), STREX (yellow box) and P27 represent three abundant inserts found in this brain region. A similar phenomenon was observed in the striatum [25].

Figure 4

Role of the BK channel β4 peptide in tolerance at the molecular, cellular and behavioral levels. Somatic BK channels (green bars) are widely expressed in somata of wild type striatal medium spiny neurons (a). At the single channel level, somatic BK channel open probability is rapidly [within 2 min, (b); bottom graphs show the timeline of alcohol effects] enhanced by acute alcohol. This effect is transitory when only the BK core α subunit is expressed [(b), α BK). By contrast, in the presence of the β4 auxiliary subunit, alcohol potentiation is sustained [(b), αβ4 BK). In the same neurons, alcohol drastically decreased neuronal excitability by potentiating the activity of BK channels activity, leading to a shortened action potential width [(a), white line models the alcohol effects on action potentials). Enhanced excitability shows tolerance when β4 subunits are absent [(c), β4 KO) but not when they are present [(c), WT). At the behavioral level, the alcohol intake of mice not expressing the β4 subunit is markedly higher than that of WT mice (d). These studies provide direct evidence for the role of BK, and in particular the BK β subunit, on acute alcohol tolerance and alcohol drinking behavior. Illustration is adapted from Ref. [3].