Effects of ethanol on hippocampal neurogenesis depend on the conditioned appetitive response

Adolescent Intermittent Ethanol Drives Modest Neuroinflammation but Does Not Escalate Drinking in Male Rats

During adolescence, the brain is highly susceptible to alcohol-induced damage and subsequent neuroimmune responses, effects which may enhance development of an alcohol use disorder (AUD). Neuroimmune reactions are implicated in adolescent alcohol exposure escalating adulthood drinking. Therefore, we investigated whether intermittent alcohol exposure in male, adolescent rats (AIE) escalated adult drinking via two-bottle choice (2BC). We also examined the influence of housing environment across three groups: standard (group-housed with enrichment during 2BC), impoverished (group-housed without enrichment during 2BC), or isolation (single-housed without bedding or enrichment throughout). In the standard group immediately after AIE/saline and after 2BC, we also examined the expression of microglial marker, Iba1, reactive astrocyte marker, vimentin, and neuronal cell death dye, FluoroJade B (FJB). We did not observe an escalation of adulthood drinking following AIE, regardless of housing condition. Further, only a modest neuroimmune response occurred after AIE in the standard group: no significant microglial reactivity or neuronal cell death was apparent using this model, although some astrocyte reactivity was detected in adolescence following AIE that resolved by adulthood. These data suggest that the lack of neuroimmune response in adolescence in this model may underlie the lack of escalation of alcohol drinking, which could not be modified through isolation stress.

Expression of stable and reliable preference and aversion phenotypes following place conditioning with psychostimulants

RATIONALE AND OBJECTIVES

Drug-seeking behavior occurs more readily in some individuals than others. This phenomenon is considered in studies of drug self-administration in which high drug-seeking/taking individuals can be identified. In contrast, studies of conditioned place preference (CPP) often involve a random sample of drug-naïve rodents that includes phenotypes not considered relevant to addiction. The main objective of the current studies was to determine if a priori identification of different conditioning phenotypes could improve the validity and sensitivity of CPP expression as a preclinical test for vulnerability to addiction.

METHODS AND RESULTS

Analysis of cocaine place conditioning data from 443 Swiss-Webster mice revealed a trimodal distribution with peaks corresponding to means of k = 3 clusters. The cluster means occurred at high, low, or negative preference scores, the latter suggesting a phenotype acquiring conditioned place aversion (CPA). The same clusters were identified in mice conditioned with methamphetamine, MDPV, or amphetamine, and these clusters remained stable and reliable during three additional expression tests spaced at 24 h. A meta-analysis of effect sizes obtained from CPP literature revealed a positively skewed distribution affected by sample size, consistent with the existence of a CPA phenotype within the populations tested. A dopamine receptor antagonist, flupentixol, blocked cocaine CPP expression in a group containing all phenotypes, but sensitivity improved markedly when CPA phenotypes were excluded from the dataset.

CONCLUSIONS

These studies suggest that taking phenotype into consideration when designing place conditioning studies will improve their application as a preclinical tool in addiction biology and drug discovery.

Regulation of GABAA Receptor Subunit Expression in Substance Use Disorders

The modulation of neuronal cell firing is mediated by the release of the neurotransmitter GABA (γ-aminobuytric acid), which binds to two major families of receptors. The ionotropic GABAA receptors (GABA_ARs) are composed of five distinct subunits that vary in expression by brain region and cell type. The action of GABA on GABA_ARs is modulated by a variety of clinically and pharmacologically important drugs such as benzodiazepines and alcohol. Exposure to and abuse of these substances disrupts homeostasis and induces plasticity in GABAergic neurotransmission, often via the regulation of receptor expression. Here, we review the regulation of GABA_AR subunit expression in adaptive and pathological plasticity, with a focus on substance use. We examine the factors influencing the expression of GABA_AR subunit genes including the regulation of the 5′ and 3′ untranslated regions, variations in DNA methylation, immediate early genes and transcription factors that regulate subunit expression, translational and post-translational modifications, and other forms of receptor regulation beyond expression. Advancing our understanding of the factors regulating GABA_AR subunit expression during adaptive plasticity, as well as during substance use and withdrawal will provide insight into the role of GABAergic signaling in substance use disorders, and contribute to the development of novel targeted therapies.

Potential role for histone deacetylation in chronic diazepam-induced downregulation of α1-GABAA receptor subunit expression

Corroborating evidence indicate that the downregulation of GABAA receptor subunit expression may underlie tolerance to the anticonvulsant and anxiolytic actions of benzodiazepine (BZ) ligands that act as full allosteric modulators (FAMs) of GABA actions at a variety of GABAA receptor subtypes. We and others have shown that 10-14 days treatment with increasing doses of diazepam (a FAM) resulted in anticonvulsant tolerance and decreased the expression of the α1 GABAA receptor subunit mRNA and protein in frontal cortex. In addition, we have also shown that long-term treatment with imidazenil, a partial allosteric modulator of GABA action at selective GABAA receptor subtypes, fail to change the expression of the α1 subunit mRNA or induce tolerance to its anticonvulsant or anxiolytic action. However, little is known regarding the potential role of epigenetic mechanisms on long-term BZ-induced downregulation of GABAA receptor subunit. Therefore, we examined the role of histone acetylation and DNA methylation mechanisms on long-term diazepam-induced downregulation of the α1 subunit mRNA expression in rat frontal cortex. We found that 10 days treatment with increasing doses of diazepam but not imidazenil decreased the expression of the α1 GABAA receptor subunit mRNA and promoter acetylation in frontal cortex. In addition, we also found that 10 days treatment with diazepam but not imidazenil increased the expression of histone deacetylase (HDAC) 1 and 2 in frontal cortex. Thus, the increased expression of HDAC1 and HDAC2 (class 1 HDACs) and consequently increased histone deacetylation mechanism of this class 1 HDACs, may underlie long-term diazepam-induced decreased expression of the α1 GABAA receptor subunit mRNA in frontal cortex.

Distinct behavioral phenotypes in ethanol-induced place preference are associated with different extinction and reinstatement but not behavioral sensitization responses

Conditioned place preference (CPP) is a model to study the role of drug conditioning properties. In outbred strains, individual variability may affect some behavioral measures. However, there are few studies focusing on understanding how different phenotypes of ethanol conditioned behavior may influence its extinction, reinstatement, and behavioral adaptation measures. We used male Swiss Webster mice to study different phenotypes related to ethanol conditioning strength, reinstatement and behavioral sensitization. Mice went through a CPP procedure with ethanol (2.2 g/kg, i.p.). After that, one group of mice was submitted to repeated extinction sessions, while another group remained in their home cages without any drug treatment. Mice went through environmental and ethanol priming (1.0 g/kg, i.p.) reinstatement tests. Ethanol priming test reinstated the conditioned behavior only in the animals kept in the home-cage during the abstinence period. Besides, the ethanol conditioned behavior strength was positively correlated with the time required to be extinguished. In the second set of experiments, some mice went through a CPP protocol followed by behavioral sensitization (five i.p. administrations of ethanol 2.2 g/kg or saline per week, for 3 weeks) and another group of mice went through sensitization followed by CPP. No positive correlation was observed between ethanol CPP strength and the intensity of behavioral sensitization. Considering that different phenotypes observed in CPP strength predicted the variability in other CPP measures, we developed a statistics-based method to classify mice according to CPP strength to be used in the evaluation of ethanol conditioning properties.