Histone deacetylase inhibition differentially attenuates cue-induced reinstatement: An interaction of environment and acH3K9 expression in the dorsal striatum

Epidrugs in the Therapy of Central Nervous System Disorders: A Way to Drive on?

The polygenic nature of neurological and psychiatric syndromes and the significant impact of environmental factors on the underlying developmental, homeostatic, and neuroplastic mechanisms suggest that an efficient therapy for these disorders should be a complex one. Pharmacological interventions with drugs selectively influencing the epigenetic landscape (epidrugs) allow one to hit multiple targets, therefore, assumably addressing a wide spectrum of genetic and environmental mechanisms of central nervous system (CNS) disorders. The aim of this review is to understand what fundamental pathological mechanisms would be optimal to target with epidrugs in the treatment of neurological or psychiatric complications. To date, the use of histone deacetylases and DNA methyltransferase inhibitors (HDACis and DNMTis) in the clinic is focused on the treatment of neoplasms (mainly of a glial origin) and is based on the cytostatic and cytotoxic actions of these compounds. Preclinical data show that besides this activity, inhibitors of histone deacetylases, DNA methyltransferases, bromodomains, and ten-eleven translocation (TET) proteins impact the expression of neuroimmune inflammation mediators (cytokines and pro-apoptotic factors), neurotrophins (brain-derived neurotropic factor (BDNF) and nerve growth factor (NGF)), ion channels, ionotropic receptors, as well as pathoproteins (β-amyloid, tau protein, and α-synuclein). Based on this profile of activities, epidrugs may be favorable as a treatment for neurodegenerative diseases. For the treatment of neurodevelopmental disorders, drug addiction, as well as anxiety disorders, depression, schizophrenia, and epilepsy, contemporary epidrugs still require further development concerning a tuning of pharmacological effects, reduction in toxicity, and development of efficient treatment protocols. A promising strategy to further clarify the potential targets of epidrugs as therapeutic means to cure neurological and psychiatric syndromes is the profiling of the epigenetic mechanisms, which have evolved upon actions of complex physiological lifestyle factors, such as diet and physical exercise, and which are effective in the management of neurodegenerative diseases and dementia.

Bored at home?—A systematic review on the effect of environmental enrichment on the welfare of laboratory rats and mice

Boredom is an emotional state that occurs when an individual has nothing to do, is not interested in the surrounding, and feels dreary and in a monotony. While this condition is usually defined for humans, it may very well describe the lives of many laboratory animals housed in small, barren cages. To make the cages less monotonous, environmental enrichment is often proposed. Although housing in a stimulating environment is still used predominantly as a luxury good and for treatment in preclinical research, enrichment is increasingly recognized to improve animal welfare. To gain insight into how stimulating environments influence the welfare of laboratory rodents, we conducted a systematic review of studies that analyzed the effect of enriched environment on behavioral parameters of animal well–being. Remarkably, a considerable number of these parameters can be associated with symptoms of boredom. Our findings show that a stimulating living environment is essential for the development of natural behavior and animal welfare of laboratory rats and mice alike, regardless of age and sex. Conversely, confinement and under-stimulation has potentially detrimental effects on the mental and physical health of laboratory rodents. We show that boredom in experimental animals is measurable and does not have to be accepted as inevitable.

Epigenetic Effects of Addictive Drugs in the Nucleus Accumbens

Substance use induces long-lasting behavioral changes and drug craving. Increasing evidence suggests that epigenetic gene regulation contributes to the development and expression of these long-lasting behavioral alterations. Here we systematically review extensive evidence from rodent models of drug-induced changes in epigenetic regulation and epigenetic regulator proteins. We focus on histone acetylation and histone methylation in a brain region important for drug-related behaviors: the nucleus accumbens. We also discuss how experimentally altering these epigenetic regulators via systemically administered compounds or nucleus accumbens-specific manipulations demonstrate the importance of these proteins in the behavioral effects of drugs and suggest potential therapeutic value to treat people with substance use disorder. Finally, we discuss limitations and future directions for the field of epigenetic studies in the behavioral effects of addictive drugs and suggest how to use these insights to develop efficacious treatments.

Isolation housing elevates amphetamine seeking independent of nucleus accumbens glutamate receptor adaptations

Overdose death rates caused by psychostimulants have increased by 22.3% annually from 2008 to 2017. Cue-evoked drug craving progressively increases and contributes to perpetual relapse. Preclinical models have determined that glutamate receptor plasticity within the nucleus accumbens (NAc) drives amplified cue-evoked drug seeking after prolonged abstinence (>40 days). Isolated condition (IC) rearing increases cocaine and amphetamine (AMP) self-administration and cue-induced reinstatement. We tested the hypothesis that housing in the IC will augment AMP seeking after short and prolonged abstinence from AMP self-administration when compared with rats reared in the enrichment condition (EC). EC and IC male rats acquired stable AMP or SAL self-administration and were tested in a cue-induced AMP-seeking test after 1 and 40 days of abstinence. After the seeking test, the whole NAc was extracted and prepared for western blot analysis. Results indicate that IC rats had more active lever presses during a brief extinction interval and during the cue-induced seeking test. After 40 days of abstinence, IC rats had more active lever presses than EC rats during the cue-induced seeking test. Western blots indicated that the expression ratio between GluA1:mGlur5 was reduced only in IC-AMP-trained rats and the ratio between GluA1:mGlur1 was positively correlated with AMP seeking after prolonged abstinence in IC-AMP rats. These results indicate that IC housing engenders a vulnerable phenotype prone to persistent AMP seeking. The behavioural momentum of this vulnerable phenotype is further revealed when AMP-associated cues are presented following prolonged abstinence.

Epigenetic Mechanisms in Drug Relapse

A growing body of evidence from the past 15 years implicates epigenetic mechanisms in the behavioral effects of addictive drugs. The main focus of these studies has been epigenetic mechanisms of psychomotor sensitization and drug reinforcement, as assessed by the conditioned place preference and drug self-administration procedures. Some of these studies have documented long-lasting changes in the expression of epigenetic enzymes and molecules that persist for weeks after the last drug exposure. These observations have inspired more recent investigations on the epigenetic mechanisms of relapse to drug seeking after prolonged abstinence. Here, we review studies that have examined epigenetic mechanisms (e.g., histone modifications, chromatin remodeler-associated modifications, and DNA methylation) that contribute to relapse to cocaine, amphetamine, methamphetamine, morphine, heroin, nicotine, or alcohol seeking, as assessed in rodent models. We first provide a brief overview of studies that have examined persistent epigenetic changes in the brain after prolonged abstinence from noncontingent drug exposure or drug self-administration. Next, we review studies on the effect of either systemic or brain site-specific epigenetic manipulations on the reinstatement of drug-conditioned place preference after extinction of the learned preference, the reinstatement of drug seeking after operant drug self-administration and extinction of the drug-reinforced responding, and the incubation of drug craving (the time-dependent increase in drug seeking after cessation of drug self-administration). We conclude by discussing the implications of these studies for understanding mechanisms contributing to persistent relapse vulnerability after prolonged abstinence. We also discuss the implications of these results for translational research on the potential use of systemically administered epigenetic enzyme inhibitors for relapse prevention in human drug users.

Adolescent ethanol exposure and differential rearing environment affect taste reactivity to ethanol in rats

The identification and characterization of variables that influence "liking" and enhance vulnerability to repeated alcohol use are vital to understanding and treating alcohol use disorders. In the current study, we explore the influence of rearing environment and experimenter-administered adolescent ethanol on the hedonic value of ethanol, sucrose, and quinine. Male and female rats were reared for 30 days starting at postnatal day (PND) 21 in either an enriched, isolated, or standard condition and received 1.5 g/kg (intraperitoneally [i.p.]) 20% (w/v) ethanol or saline every other day for 12 days starting at PND 28. Thereafter, all rats had indwelling intraoral fistulae implanted and their taste reactivities to water, ethanol (5, 10, 20, 30, 40% v/v), sucrose (0.1, 0.25, 0.5 M), and quinine (0.1, 0.5 mM) were recorded and analyzed. Results indicated that enrichment elevated hedonic responding to sucrose compared to isolation, and induced a stronger negative relationship between hedonic responding and ethanol concentration compared to standard conditions. Enrichment also elevated aversive responding to ethanol and quinine compared to both isolated and standard condition rats. Adolescent ethanol injections marginally reduced aversive responding to quinine. These results replicate previous findings that environmental enrichment enhances both "liking" and aversion. In addition, the current findings suggest that, while adolescent ethanol injections may blunt aversive responses to quinine, they have no effect on aversive or hedonic responding to ethanol or sucrose. Together with existing literature, our results may suggest that experience with the taste of ethanol is necessary for alterations to ethanol "liking" and aversion.

Environmental enrichment and a selective metabotropic glutamate receptor2/3 (mGluR2/3) agonist suppress amphetamine self-administration: Characterizing baseline differences

A challenge for developing effective treatments for substance use disorders (SUDs) is understanding how environmental variables alter the efficacy of therapeutics. Environmental enrichment (EC) enhances brain development and protects against behaviors associated with drug abuse vulnerability when compared to rats reared in isolation (IC) or standard conditions (SC). EC rearing enhances the expression and function of metabotropic glutamate receptor_2/3 (mGlurR_2/3) and activating mGluR_2/3 reduces psychostimulant self-administration (SA). However, the ability for mGluR_2/3 activation to suppress amphetamine (AMP) SA in differentially reared rats is not determined. Therefore, we tested the hypothesis EC reduces AMP (SA) by augmenting mGluR_2/3 function. At postnatal day 21, male Sprague-Dawley rats were assigned to EC, IC, or SC environments for 30 days. Then, they acquired AMP SA and were moved to a progressive ratio (PR) schedule of reinforcement. EC, IC, and SC rats were pretreated with LY379268 (vehicle, 0.3 and 1 mg/kg), a selective mGluR_2/3 agonist, before PR behavioral sessions. Linear mixed effects analysis determined EC rats had reduced motivation for AMP SA when compared to IC or SC rats and that LY379268 dose-dependently suppressed AMP SA, but there was no evidence of an interaction. Cumming/Gardner-Altman estimation plots illustrate that the 0.3 mg/kg dose suppressed infusions in EC rats while the 1 mg/kg dose suppressed infusions in SC rats. LY379268 was incapable of suppressing the motivation for AMP SA in IC rats. Controlling for baseline differences in differentially reared rats remains a challenge. Normalizing to a baseline introduced error which is illustrated in the precision of the estimated effect size differences. The data indicate that environmental enrichment enhances the ability of a selective mGluR_2/3 agonist to suppress AMP SA and indicates the functional status of the mGluR_2/3 is formed during development. Therefore, environmental history must be considered when evaluating pharmacological therapeutics particularly those aimed at the mGluR_2/3.

Differential rearing alters taste reactivity to ethanol, sucrose, and quinine

RATIONALE

Early-life environment influences reinforcer and drug motivation in adulthood; however, the impact on specific components of motivation, including hedonic value ("liking"), remains unknown.

OBJECTIVES

The current study determined whether differential rearing alters liking and aversive responding to ethanol, sucrose, and quinine in an ethanol-naïve rat model.

METHODS

Male and female rats were reared for 30 days starting at postnatal day 21 in either an enriched (EC), isolated (IC), or standard condition (SC). Thereafter, all rats had indwelling intraoral fistulae implanted and their taste reactivity to water, ethanol (5, 10, 20, 30, 40% v/v), sucrose (0.1, 0.25, 0.5 M), and quinine (0.1, 0.5 mM) was recorded and analyzed.

RESULTS

EC rats had higher amounts of liking responses to ethanol, sucrose, and quinine and higher amounts of aversive responses to ethanol and quinine compared to IC rats. While EC and IC rats' responses were different from each other, they both tended to be similar to SCs, who fell in between the EC and IC groups.

CONCLUSIONS

These results suggest that environmental enrichment may enhance sensitivity to a variety of tastants, thereby enhancing liking, while isolation may dull sensitivity, thereby dulling liking. Altogether, the evidence suggests that isolated rats have a shift in the allostatic set-point which may, in part, drive increased responding for a variety of rewards including ethanol and sucrose. Enriched rats have enhanced liking of both sucrose and ethanol suggesting that enrichment may offer a unique phenotype with divergent preferences for incentive motivation.