Drug-induced epigenetic changes produce drug tolerance.

Charles F Stevens,Yan Wang,Harish R Krishnan,Nigel S Atkinson,Alfredo Ghezzi,Jerry C P Yin

doi:10.1371/journal.pbio.0050265

Abstract

Tolerance to drugs that affect neural activity is mediated, in part, by adaptive mechanisms that attempt to restore normal neural excitability. Changes in the expression of ion channel genes are thought to play an important role in these neural adaptations. The slo gene encodes the pore-forming subunit of BK-type Ca2+-activated K+ channels, which regulate many aspects of neural activity. Given that induction of slo gene expression plays an important role in the acquisition of tolerance to sedating drugs, we investigated the molecular mechanism of gene induction. Using chromatin immunoprecipitation followed by real-time PCR, we show that a single brief sedation with the anesthetic benzyl alcohol generates a spatiotemporal pattern of histone H4 acetylation across the slo promoter region. Inducing histone acetylation with a histone deacetylase inhibitor yields a similar pattern of changes in histone acetylation, up-regulates slo expression, and phenocopies tolerance in a slo-dependent manner. The cAMP response element binding protein (CREB) is an important transcription factor mediating experience-based neuroadaptations. The slo promoter region contains putative binding sites for the CREB transcription factor. Chromatin immunoprecipitation assays show that benzyl alcohol sedation enhances CREB binding within the slo promoter region. Furthermore, activation of a CREB dominant-negative transgene blocks benzyl alcohol–induced changes in histone acetylation within the slo promoter region, slo induction, and behavioral tolerance caused by benzyl alcohol sedation. These findings provide unique evidence that links molecular epigenetic histone modifications and transcriptional induction of an ion channel gene with a single behavioral event.

Highlights

Drug tolerance can be defined as reduced responsiveness to an effect of a drug caused by prior exposure to the drug [1]
It has been shown that sedation by a variety of methods induces slo gene expression in the nervous system, that slo mutations block the acquisition of behavioral tolerance, and that transgenic induction of slo phenocopies the tolerant phenotype
We use the fruit fly Drosophila melanogaster as a model system to determine how the nervous system becomes tolerant to the sedative effects of organic solvents

Summary

Introduction

Drug tolerance can be defined as reduced responsiveness to an effect of a drug caused by prior exposure to the drug [1]. Most organic solvents are potent central nervous system depressants that produce sedation if inhaled or consumed in sufficient quantities. These properties have led to the use of such solvents both as anesthetics and as drugs of abuse. Rapid tolerance is defined as reduced drug responsiveness caused by a single prior exposure to the drug. In flies, this manifests itself as a reduction in the duration of sedation. The transcriptional regulation of the slo gene appears to be of general importance for the production of tolerance to sedation by various organic solvents [2,3,4]. We continue to use BA as a model organic solvent anesthetic in this endeavor because it is well tolerated by flies and can be administered

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Conclusion