INTRODUCTION

Drug addiction is a chronic and relapsing disease that often begins during adolescence.

Epidemiological evidence documents an association between marijuana use during adolescence and subsequent abuse of drugs such as heroin and cocaine (1, 2). While many factors including societal pressures, family, culture, and drug availability can contribute to this apparent `gateway’ association, little is known about the neurobiological basis underlying such potential vulnerability.

Of the neural substrates that have been investigated, the enkephalinergic opioid system is  consistently altered by developmental marijuana exposure (3–5), perhaps reflecting neuroanatomical interactions between cannabinoid receptor type 1 and the enkephalinergic system (6, 7).

Debates exist, however, regarding the relationship between proenkephalin (Penk) dysregulation and opiate susceptibility. We previously reported that adult rats exposed to Δ9-tetrahydrocannabinol (THC; primary psychoactive component of marijuana) during adolescence exhibit increased heroin self administration (SA) as well as increased expression of Penk, the gene encoding the opioid neuropeptide enkephalin, in the nucleus accumbens shell (NAcsh), a mesolimbic structure critically involved in reward-related behaviors (3).

Although these data suggest that increased NAcsh Penk expression and heroin SA behavior are independent consequences of adolescent THC exposure, they do not address a possible causal relationship between THCinduced  Penk upregulation in NAcsh and enhanced behavioral susceptibility to opiates.

Moreover, insights regarding the neurobiological mechanisms by which adolescent THC exposure maintains upregulation of Penk into adulthood remain unknown.

Here, we take advantage of viral-mediated gene transfer strategies to show that adulthood addiction-like behaviors induced by adolescent THC exposure are dependent on discrete regulation of NAcsh Penk gene expression. A number of recent studies have demonstrated an important role for histone methylation in the regulation of drug-induced behaviors and transcriptional plasticity, particularly alteration of repressive histone H3 lysine 9 (H3K9) methylation at NAc gene promotors (8, 9).

We report here that one mechanism by which adolescent THC exposure may mediate Penk upregulation in adult NAcsh is through reduction of H3K9 di- and trimethylation, a functional consequence of which may be decreased transcriptional repression of Penk.

ABSTRACT

Background

Marijuana use by teenagers often predates the use of harder drugs, but the neurobiological underpinnings of such vulnerability are unknown. Animal studies suggest enhanced heroin self-administration (SA) and dysregulation of the endogenous opioid system in the nucleus accumbens shell (NAcsh) of adults following adolescent Δ9-tetrahydrocannabinol (THC) exposure. However, a causal link between Penk expression and vulnerability to heroin has yet to be established.

Methods

To investigate the functional significance of NAcsh  Penk tone, selective viral mediated knockdown and overexpression of Penk was performed, followed by analysis of subsequent heroin SA behavior. To determine whether adolescent THC exposure was associated with chromatin alteration, we analyzed levels of histone H3 methylation in the NAcsh via ChIP atfive sites flanking the Penk gene transcription start site.

Results

Here, we show that regulation of the proenkephalin (Penk) opioid neuropeptide gene in NAcsh directly regulates heroin SA behavior. Selective viral-mediated knockdown of Penk in striatopallidal neurons attenuates heroin SA in adolescent THC-exposed rats, whereas Penk overexpression potentiates heroin SA in THC-naïve rats. Furthermore, we report that adolescent THC exposure mediates Penk upregulation through reduction of histone H3 lysine 9 (H3K9) methylation in the NAcsh, thereby disrupting the normal developmental pattern of H3K9 methylation.

Conclusions

These data establish a direct association between THC-induced NAcsh Penk upregulation and heroin SA and indicate that epigenetic dysregulation of Penk underlies the long term effects of THC.

Source:  Biol Psychiatry. 2012 November 15; 72(10): 803–810. doi:10.1016/j.biopsych.2012.04.026.