Fair M. Vassoler

Epigenetic inheritance of a cocaine-resistance phenotype

We delineated a heritable phenotype resulting from the self-administration of cocaine in rats. We observed delayed acquisition and reduced maintenance of cocaine self-administration in male, but not female, offspring of sires that self-administered cocaine. Brain-derived neurotrophic factor (Bdnf) mRNA and BDNF protein were increased in the medial prefrontal cortex (mPFC), and there was an increased association of acetylated histone H3 with Bdnf promoters in only the male offspring of cocaine-experienced sires. Administration of a BDNF receptor antagonist (the TrkB receptor antagonist ANA-12) reversed the diminished cocaine self-administration in male cocaine-sired rats. In addition, the association of acetylated histone H3 with Bdnf promoters was increased in the sperm of sires that self-administered cocaine. Collectively, these findings indicate that voluntary paternal ingestion of cocaine results in epigenetic reprogramming of the germline, having profound effects on mPFC gene expression and resistance to cocaine reinforcement in male offspring.

Cocaine-Induced Chromatin Remodeling Increases Brain-Derived Neurotrophic Factor Transcription in the Rat Medial Prefrontal Cortex, Which Alters the Reinforcing Efficacy of Cocaine

Cocaine self-administration alters patterns of gene expression in the brain that may underlie cocaine-induced neuronal plasticity. In the present study, male Sprague Dawley rats were allowed to self-administer cocaine (0.25 mg/infusion) 2 h/d for 14 d, followed by 7 d of forced abstinence. Compared with yoked saline control rats, cocaine self-administration resulted in increased brain-derived neurotrophic factor (BDNF) protein levels in the rat medial prefrontal cortex (mPFC). To examine the functional relevance of this finding, cocaine self-administration maintained under a progressive ratio schedule of reinforcement was assessed after short hairpin RNA-induced suppression of BDNF expression in the mPFC. Decreased BDNF expression in the mPFC increased the cocaine self-administration breakpoint. Next, the effect of cocaine self-administration on specific BDNF exons was assessed; results revealed selectively increased BDNF exon IV-containing transcripts in the mPFC. Moreover, there were significant cocaine-induced increases in acetylated histone H3 (AcH3) and phospho-cAMP response element binding protein (pCREB) association with BDNF promoter IV. In contrast, there was decreased methyl-CpG-binding protein 2 (MeCP2) association with BDNF promoter IV in the mPFC of rats that previously self-administered cocaine. Together, these results indicate that cocaine-induced increases in BDNF promoter IV transcript in the mPFC are driven by increased binding of AcH3 and pCREB as well as decreased MeCP2 binding at this BDNF promoter. Collectively, these results indicate that cocaine self-administration remodels chromatin in the mPFC, resulting in increased expression of BDNF, which appears to represent a compensatory neuroadaptation that reduces the reinforcing efficacy of cocaine.

Raphe GABAergic Neurons Mediate the Acquisition of Avoidance after Social Defeat

Serotonin (5-HT) modulates neural responses to socioaffective cues and can bias approach or avoidance behavioral decisions, yet the cellular mechanisms underlying its contribution to the regulation of social experiences remain poorly understood. We hypothesized that GABAergic neurons in the dorsal raphe nucleus (DRN) may participate in socioaffective regulation by controlling serotonergic tone during social interaction. We tested this hypothesis using whole-cell recording techniques in genetically identified DRN GABA and 5-HT neurons in mice exposed to social defeat, a model that induces long-lasting avoidance behaviors in a subset of mice responsive to serotonergic antidepressants. Our results revealed that social defeat engaged DRN GABA neurons and drove GABAergic sensitization that strengthened inhibition of 5-HT neurons in mice that were susceptible, but not resilient to social defeat. Furthermore, optogenetic silencing of DRN GABA neurons disinhibited neighboring 5-HT neurons and prevented the acquisition of social avoidance in mice exposed to a social threat, but did not affect a previously acquired avoidance phenotype. We provide the first characterization of GABA neurons in the DRN that monosynaptically inhibit 5-HT neurons and reveal their key role in neuroplastic processes underlying the development of social avoidance.

Deep Brain Stimulation of the Nucleus Accumbens Shell Attenuates Cocaine Reinstatement through Local and Antidromic Activation

Accumbal deep brain stimulation (DBS) is a promising therapeutic modality for the treatment of addiction. Here, we demonstrate that DBS in the nucleus accumbens shell, but not the core, attenuates cocaine priming-induced reinstatement of drug seeking, an animal model of relapse, in male Sprague Dawley rats. Next, we compared DBS of the shell with pharmacological inactivation. Results indicated that inactivation using reagents that influenced (lidocaine) or spared (GABA receptor agonists) fibers of passage blocked cocaine reinstatement when administered into the core but not the shell. It seems unlikely, therefore, that intrashell DBS influences cocaine reinstatement by inactivating this nucleus or the fibers coursing through it. To examine potential circuit-wide changes, c-Fos immunohistochemistry was used to examine neuronal activation following DBS of the nucleus accumbens shell. Intrashell DBS increased c-Fos induction at the site of stimulation as well as in the infralimbic cortex, but had no effect on the dorsal striatum, prelimbic cortex, or ventral pallidum. Recent evidence indicates that accumbens DBS antidromically stimulates axon terminals, which ultimately activates GABAergic interneurons in cortical areas that send afferents to the shell. To test this hypothesis, GABA receptor agonists (baclofen/muscimol) were microinjected into the anterior cingulate, and prelimbic or infralimbic cortices before cocaine reinstatement. Pharmacological inactivation of all three medial prefrontal cortical subregions attenuated the reinstatement of cocaine seeking. These results are consistent with DBS of the accumbens shell attenuating cocaine reinstatement via local activation and/or activation of GABAergic interneurons in the medial prefrontal cortex via antidromic stimulation of cortico-accumbal afferents.

Ventral Tegmental Afferents in Stress-Induced Reinstatement: The Role of cAMP Response Element-Binding Protein

The transcription factor cAMP response element-binding protein (CREB) is required for stress- but not drug-induced reinstatement of cocaine conditioned place preference. To reveal the neural circuitry associated with this CREB dependence, we injected a retrograde tracer into the ventral tegmental area (VTA) and identified afferents that were activated after stress or cocaine exposure in both naive and cocaine-conditioned mice. Neuronal activation, as assessed by Fos expression, was greatly reduced in the dorsal and ventral bed nucleus of the stria terminalis (BNST), lateral septum, and nucleus accumbens shell in mice lacking CREB (CREBαΔ mice) after a 6 min swim stress but not after cocaine exposure (20 mg/kg). Additionally, activation of VTA afferent neurons in the ventral BNST and the infralimbic cortex in CREBαΔ mice was blunted in response to stress. This pattern of neuronal activation persisted in mice that were conditioned to a cocaine place preference procedure before stress exposure. Furthermore, lidocaine inactivation (0.4 μl, 4%) studies demonstrated the necessity of BNST activation for swim-stress-induced reinstatement of cocaine-conditioned reward. Together, the present studies demonstrate that CREB is required for the activation of a unique circuit that converges on the dopamine reward pathway to elicit reinstatement of drug reward and points to the BNST as a key intersection between stress and reward circuits.

Mechanisms of transgenerational inheritance of addictive-like behaviors

Genetic factors are implicated in the heritability of drug abuse. However, even with advances in current technology no specific genes have been identified that are critical for the transmission of drug-induced phenotypes to subsequent generations. It is now evident that epigenetic factors contribute to disease heritability and represent a link between genes and the environment. Recently, epigenetic mechanisms have been shown to underlie drug-induced structural, synaptic, and behavioral plasticity by coordinating the expression of gene networks within the brain. Therefore, the epigenome provides a direct mechanism for drugs of abuse to influence the genetic events involved in the development of addiction as well as its heritability to subsequent generations. In this review we discuss the mechanisms underlying intergenerational epigenetic transmission, highlight studies that demonstrate this phenomenon with particular attention to the field of addiction, and identify gaps for future studies.

Deep brain stimulation for the treatment of addiction: basic and clinical studies and potential mechanisms of action

RationaleDeep brain stimulation (DBS) has achieved substantial success as a treatment for movement disorders such as Parkinson’s disease. The therapeutic efficacy and relative lack of serious side effects resulted in the expansion of DBS into the treatment of many other diseases, including obsessive–compulsive disorder, Tourette’s, and depression, among others. More recently, a limited number of basic and clinical studies indicated that DBS may also be useful in the treatment of various addictions.ObjectivesHere, we briefly summarize the history of DBS and review the basic and clinical studies focused on DBS and addiction. We also examine the potential mechanisms that may underlie the effects of DBS.Results and conclusionsThe available data indicate that DBS is a promising therapeutic modality for the treatment of addiction. Thus far, the nucleus accumbens and subthalamic nucleus are the most promising sites for DBS, reversing aspects of addiction. The mechanisms underlying DBS are complex and likely vary from region to region. Emerging evidence indicates that DBS of the nucleus accumbens produces its effects, at least in part, by antidromic activation of cortico-accumbal afferents that stimulate inhibitory medial prefrontal cortex interneurons via recurrent collaterals.

Temporally Dependent Changes in Cocaine-Induced Synaptic Plasticity in the Nucleus Accumbens Shell are Reversed by D1-Like Dopamine Receptor Stimulation

Dopaminergic and glutamatergic inputs to the nucleus accumbens shell have a central role in reward processing. Non-contingent cocaine administration generates a number of long-term AMPA receptor-dependent changes in synaptic efficacy. However, the synaptic consequences of cocaine self-administration and the potential role of dopamine in these processes remain unclear. Here, we examined the influence of D1 dopamine receptor (D1DR) activation on excitatory synaptic plasticity in the accumbens shell of adult rats following cocaine self-administration. Our results indicated that during the first 2 days following cocaine exposure both pre- and post-synaptic mechanisms contribute to a net decrease in AMPA receptor-mediated signaling. This is reflected by decreased frequency of miniature EPSCs (mEPSCs) attributable to enhanced cannabinoid receptor activity, decreased mEPSC amplitude, and increased paired-pulse ratio of evoked EPSCs. In contrast, the only changes observed in the shell 3–4 weeks following cocaine self-administration were increased mEPSCs amplitudes and AMPA/NMDA ratios. We further found that although these cocaine-induced neuroadaptations during early and late abstinence have different synaptic expression mechanisms, they were normalized by stimulation of D1DRs. Thus, pre-exposure to the D1DR agonist, SKF38393, during the initial period of abstinence increased excitatory synaptic strength, but reduced excitatory signaling after weeks of abstinence. Taken together, these results indicate that the direction of changes in excitatory transmission induced by cocaine self-administration switches over the first few weeks of abstinence. Moreover, D1DRs gate the stability of these cocaine-induced changes at glutamatergic synapses in the accumbens shell by utilizing multiple temporally distinct mechanisms, which has implications for the treatment of cocaine craving and addiction.

Female adolescent exposure to cannabinoids causes transgenerational effects on morphine sensitization in female offspring in the absence of in utero exposure

Female adolescent marijuana use is increasing, yet the effect on future offspring is unknown. Here, adolescent female Sprague Dawley rats (postnatal-day 30; PN30) were given subcutaneous (s.c.) injections with the cannabinoid agonist WIN-55,212 (WIN) or its vehicle (VEH) for three consecutive days using a twice-daily, increasing dosage regimen (1 mg/kg day 1; 2 mg/kg day 2; 4 mg/kg day 3). As adults (PN60), females were mated with drug-naïve males. Their adult female offspring (VEH-F1 or WIN-F1) were tested for behavioral sensitization by administering morphine (0 or 7.5 mg/kg s.c.) every other day for a total of five administrations. Following five days of abstinence, all animals received a morphine challenge (7.5 mg/kg s.c.) and locomotor activity was monitored. At completion of behavioral testing, mu opioid receptor (OPRM1), FosB, cFos, and dopamine receptor mRNA expression was measured in the nucleus accumbens as well as OPRM1 and corticotropin-releasing hormone mRNA in the paraventricular nucleus. In addition, plasma corticosterone levels were examined. On the day of challenge, morphine-pretreated WIN-F1 animals demonstrated a significantly enhanced response to morphine compared to morphine-pretreated VEH-F1 animals. Also following the morphine challenge, significantly higher levels of OPRM1 in the nucleus accumbens were observed in WIN-F1 animals. Together, these findings demonstrate transgenerational effects of adolescent exposure to cannabinoids in the absence of any in utero exposure.

Enhanced anxiety in the male offspring of sires that self-administered cocaine.

We previously showed that paternal cocaine exposure reduced the reinforcing efficacy of cocaine in male offspring. Here, we sought to determine whether paternal cocaine experience could also influence anxiety levels in offspring. Male rats were allowed to self‐administer cocaine (controls received saline passively) for 60 days and then were bred with naïve females. Measures of anxiety and cocaine‐induced anxiogenic effects were assessed in the adult offspring. Cocaine‐sired male offspring exhibited increased anxiety‐like behaviors, as measured using the novelty‐induced hypophagia and defensive burying tasks, relative to saline‐sired males. In contrast, sire cocaine experience had no effect on anxiety‐like behaviors in female offspring. When challenged with an anxiogenic (but not anorectic) dose of cocaine (2.5 mg/kg, i.p.), anxiety‐like behavior was enhanced in all animals to an equal degree regardless of sire drug experience. Since anxiety and depression are often co‐morbid, we also assessed measures of depressive‐like behavior. Sire cocaine experience had no effect on depression‐like behaviors, as measured by the forced swim task, among male offspring. In a separate group of naïve littermates, select neuronal correlates of anxiety were measured. Male offspring of cocaine‐experienced sires showed increased mRNA and protein expression of corticotropin‐releasing factor receptor 2 in the hippocampus. Together, these results indicate that cocaine‐experienced sires produce male progeny that have increased baseline anxiety, which is unaltered by subsequent cocaine exposure.