Integrated Channel Plasticity Contributes to Alcohol Tolerance in Neurohypophysial Terminals THOMAS K. KNOTT, ALEJANDRO M. DOPICO, GOVINDAN DAYANITHI, JOS ´ E LEMOS, and STEVEN N. TREISTMAN Departments of Neurobiology (T.K.K., S.N.T.) and Physiology (J.L.), University of Massachusetts Medical School, Worcester, Massachusetts; Department of Pharmacology, University of Tennessee at Memphis, School of Medicine, Memphis, Tennessee (A.M.D.); and Institut National de la Sante ´ et de la Recherche Me ´ dicale-U432, University of Montpellier II, Montpellier, France (G.D.) Received January 8, 2002; accepted April 11, 2002 This article is available online at http://molpharm.aspetjournals.org ABSTRACT Short-term ethanol challenge results in the reduction of peptide hormone release from the rat neurohypophysis. However, rats that have been maintained on an ethanol-containing diet for 3 to 4 weeks exhibit tolerance to this effect. Mechanistic under- pinnings of this tolerance were probed by examining four ion channel conductances critical for neurohormone release. The voltage-gated L-type calcium channel and the functionally linked calcium-activated BK channel represent a functional dyad. Although these channels show opposite drug responses in the naive terminal (i.e., the L-type Ca 2+ channel is inhibited whereas the BK channel is potentiated), the effect of long-term alcohol exposure is to decrease sensitivity to the short-term administration of drug in both instances. In addition to the shift in sensitivity, current density increased for the L-type Ca 2+ current and decreased for the BK current, consistent with a compensatory change. Sensitivity to alcohol was also altered for two other channel types studied. Inhibition of the voltage- gated transient Ca 2+ current was lessened after long-term treatment. I A, which is not sensitive to the drug at clinically relevant concentrations in terminals from the naive rat, acquires sensitivity after long-term exposure, representing a potentially novel type of tolerance. However, neither the transient Ca 2+ current nor I A shows a change in current density, demonstrating the selectivity of this aspect of tolerance. Overall, these results demonstrate that channel plasticity can explain at least a por- tion of the behavioral tolerance resulting from changes in sen- sitivity of peptide hormone release. Furthermore, they suggest that an understanding of tolerance requires the examination of dynamically coupled channel populations. Tolerance represents a critical element of drug action, as well as an example of neuronal plasticity. Various forms of ethanol tolerance have been described, characterized by their time frame (Kalant, 1998). The molecular underpinnings of tolerance are not yet understood. Typically, studies have used either preparations amenable to exploration at the mo- lecular level, for which the role of the molecules studied are not understood in terms of physiological or behavioral events, or a physiological or behavioral function is examined, for which the underlying molecular components are unclear. The rat hypothalamic-neurohypophysial system provides an ideal model to study the short-term and long-term actions of eth- anol. Short-term ethanol challenge blocks the release of ar- ginine vasopressin and oxytocin (OT) from both the intact neurohypophysis and from isolated neurohypophysial termi- nals (Wang et al., 1991a,b; Knott et al., 2000). The diuretic effect of short-term alcohol exposure exhibits tolerance after prolonged ethanol exposure (Schrier et al., 1979; Crabbe et al., 1981; Pohorecky, 1985). Excitable cell function requires the dynamic interplay of a variety of ion channels and intracellular signaling pathways. The voltage-gated L-type calcium channel and the BK chan- nel, which is activated by both Ca 2+ and voltage, represent an interactive dyad, in which Ca 2+ entry through the volt- age-gated calcium channel activates the Ca 2+ -activated BK channel. In terminals, the activation of voltage-gated Ca 2+ channels provides the rise of intracellular Ca 2+ that triggers hormone release, and activation of Ca 2+ -activated potassium channels completes a feedback loop in which membrane repolarization terminates release. The biophysical basis of alcohol action on both of these channels has been described in the neurohy- pophysial terminal, as well as with cloned channels in ex- pression systems and planar bilayers (Wang et al., 1994; Dopico et al., 1996, 1998, 1999a; Chu et al., 1998). Ethanol inhibits the L-type channel and potentiates the BK channel (Wang et al., 1991a,b, 1994; Dopico et al., 1996). These ac- tions produce the reduction of peptide hormone release that follows ethanol ingestion. In both channels, ethanol modu- lates the gating properties of the channel, leaving parame- ABBREVIATIONS: OT, oxytocin; BK, calcium-activated potassium channel; VGCC, voltage-gated calcium channels; I Ca , transient calcium current; I A , potassium A-current; ELISA, enzyme-linked immunosorbent assay; HEDTA, N-hydroxy-EDTA; TEA, tetraethylammonium; 4-AP, 4-aminopyr- idine; ANOVA, analysis of variance; HP, holding potential. 0026-895X/02/6201-135–142$7.00 MOLECULAR PHARMACOLOGY Vol. 62, No. 1 Copyright © 2002 The American Society for Pharmacology and Experimental Therapeutics 1592/993577 Mol Pharmacol 62:135–142, 2002 Printed in U.S.A. 135 at ASPET Journals on July 20, 2018 molpharm.aspetjournals.org Downloaded from