Commentary: alcohol depletion of magnesium: a new mechanism for tolerance to alcohol activation of BK channels?

ETHANOL (ETOH) EXPOSURE has consequences that reach into many areas of human physiology, with direct effects on many different molecules and additional indirect effects associated with withdrawal from EtOH and various molecular adaptations that develop subsequent to EtOH exposure. A recent paper by Marrero and colleagues (2015) has linked 2 otherwise disparate findings, where EtOH exposure depletes magnesium, and where EtOH activates the BK-type fast, calcium-activated potassium channel (BK), within a common mechanism. In particular, their results suggest that depletion of magnesium prevents EtOH from activating BK, thus perhaps representing a new form of tolerance. Magnesium has a number of diverse physiological roles, including as a cofactor for enzymes, for actin cytoskeleton organization, and as a regulator of Ca flux through calcium channels and other Ca-related signaling events. In addition, EtOH exposure has been shown to deplete magnesium, which has been observed in different tissues in preclinical models (although not tested in neurons) and in humans with alcohol use disorders (AUDs); in fact, magnesium repletion has been considered as a therapy for some aspects of AUDs (see references in Marrero et al., 2015). However, the specific mechanistic consequences of magnesium depletion for neuronal function have remained unclear. On the other hand, regulation of BKs by EtOH and other signaling molecules has been widely studied. EtOH has been reported to activate BK channels at more moderate levels of EtOH (20 to 25 mM in Marrero et al., 2015). BKs have rapid kinetics and can potently regulate action potential (AP) firing, especially by enhancing the repolarization during the downstroke of the AP. This is thought to decrease excitability by increasing the post-AP hyperpolarization and delaying the interval until the neuron can fire again (Gruss et al., 2001). Thus, the activation of BK by EtOH could reduce excitability and, in this way, may contribute to the intoxicating, sedating, or other acute effects of EtOH exposure. It is also possible that activating BK could have the opposite effect, allowing a neuron to repolarize faster and thus increase the ability to fire, similar to delayed rectifying K+ channels in fast-spiking interneurons (Rudy et al., 1999). Also, inhibition of BK might be thought to increase excitability (by blocking a K+ channel), but in some cases inhibiting BK can cause a slower repolarization and widening of the AP, increasing calcium influx and slowing firing due to recruitment of slower calcium-activated K+ channels. Adding to the complexity is that BKs are located on soma and dendrites, and also on presynaptic terminals where they regulate transmitter release, and BK channels in different subcellular locations could contain different BK subunits (Martin et al., 2004). In contrast, we note that other groups have found divergent results, for example, where EtOH actually inhibits BK function in central amygdala neurons (Li et al., 2014). This is discussed further below. Marrero and colleagues (2015) have added to our understanding of the regulation of BK by EtOH, and provided novel evidence that the ability of EtOH to excite BK is strongly regulated by magnesium levels and the actin cytoskeleton. These studies used electrophysiology to examine the function of individual BK channels in outside-out patches from hippocampal neurons, where a patch of membrane is pulled from the surface of a neuron using a glass micropipette, allowing direct examination of currents associated with opening and closing of BK channels in that small piece of plasma membrane. Also, the outside-out patch configuration retains the cytoskeleton associated with the plasma membrane, and allows perfusion of different molecules across the piece of membrane to determine how channel activity is regulated. Using this method, Marrero and colleagues (2015) found that the ability of EtOH to activate BK is strongly regulated by magnesium levels. EtOH activates BK when magnesium levels are nearer to normal (1 mM), while EtOH actually inhibits BK when magnesium levels are reduced (200 lM). While some previous studies have implicated magnesium as From the Department of Neurology (FWH), University of California at San Francisco, San Francisco, California. Received for publication July 30, 2015; accepted August 3, 2015. Reprint requests: Frederic Woodward Hopf, Department of Neurology, University of California at San Francisco, 675 Nelson Rising Lane, Room 490D, San Francisco, CA 94158; Tel. : 510-418-1966; Fax: 415502-7332; E-mail: woody@gallo.ucsf. edu Copyright© 2015 by the Research Society on Alcoholism.