Adolescent chronic intermittent toluene inhalation dynamically regulates the transcriptome and neuronal methylome within the rat medial prefrontal cortex.

Inhalants containing the volatile solvent toluene are misused to induce euphoria or intoxication. Inhalant abuse is most common during adolescence and can result in cognitive impairments during an important maturational period. Despite evidence suggesting that epigenetic modifications may underpin the cognitive effects of inhalants, no studies to date have thoroughly investigated toluene-induced regulation of the transcriptome or discrete epigenetic modifications within the brain. To address this, we investigated effects of adolescent chronic intermittent toluene (CIT) inhalation on gene expression and DNA methylation profiles within the rat medial prefrontal cortex (mPFC), which undergoes maturation throughout adolescence and has been implicated in toluene-induced cognitive deficits. Employing both RNA-seq and genome-wide Methyl CpG Binding Domain (MBD) Ultra-seq analysis, we demonstrate that adolescent CIT inhalation (10 000 ppm for 1 h/day, 3 days/week for 4 weeks) induces both transient and persistent changes to the transcriptome and DNA methylome within the rat mPFC for at least 2 weeks following toluene exposure. We demonstrate for the first time that adolescent CIT exposure results in dynamic regulation of the mPFC transcriptome likely relating to acute inflammatory responses and persistent deficits in synaptic plasticity. These adaptations may contribute to the cognitive deficits associated with chronic toluene exposure and provide novel molecular targets for preventing long-term neurophysiological abnormalities following chronic toluene inhalation.

Ketamine exerts its sustained antidepressant effects via cell-type-specific regulation of Kcnq2.

A single sub-anesthetic dose of ketamine produces a rapid and sustained antidepressant response, yet the molecular mechanisms responsible for this remain unclear. Here, we identified cell-type-specific transcriptional signatures associated with a sustained ketamine response in mice. Most interestingly, we identified the Kcnq2 gene as an important downstream regulator of ketamine action in glutamatergic neurons of the ventral hippocampus. We validated these findings through a series of complementary molecular, electrophysiological, cellular, pharmacological, behavioral, and functional experiments. We demonstrated that adjunctive treatment with retigabine, a KCNQ activator, augments ketamine's antidepressant-like effects in mice. Intriguingly, these effects are ketamine specific, as they do not modulate a response to classical antidepressants, such as escitalopram. These findings significantly advance our understanding of the mechanisms underlying the sustained antidepressant effects of ketamine, with important clinical implications.

Adenosine-to-Inosine RNA Editing Within Corticolimbic Brain Regions Is Regulated in Response to Chronic Social Defeat Stress in Mice.

Adenosine-to-inosine (A-to-I) RNA editing is a co-/posttranscriptional modification of double-stranded RNA, catalyzed by the adenosine deaminase acting on RNA (ADAR) family of enzymes, which results in recognition of inosine as guanosine by translational and splicing machinery causing potential recoding events in amino acid sequences. A-to-I editing is prominent within brain-specific transcripts, and dysregulation of editing at several well-studied loci (e.g., Gria2, Htr2c) has been implicated in acute and chronic stress in rodents as well as neurological (e.g., Alzheimer's) and psychopathological disorders such as schizophrenia and major depressive disorder. However, only a small fraction of recoding sites has been investigated within the brain following stress, and our understanding of the role of RNA editing in transcriptome regulation following environmental stimuli remains poorly understood. Thus, we aimed to investigate A-to-I editing at hundreds of loci following chronic social defeat stress (CSDS) in mice within corticolimbic regions responsive to chronic stress regulation. Adult male mice were subjected to CSDS or control conditions for 21 days and dynamic regulation of A-to-I editing was investigated 2 and 8 days following the final defeat within both the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA). Employing a targeted resequencing approach, which utilizes microfluidics-based multiplex polymerase chain reaction (PCR) coupled with next-generation sequencing, we analyzed A-to-I editing at ∼100 high-confidence editing sites within the mouse brain. CSDS resulted in acute regulation of transcripts encoding several ADAR enzymes, which normalized 8 days following the final defeat and was specific for susceptible mice. In contrast, sequencing analysis revealed modest and dynamic regulation of A-to-I editing within numerous transcripts in both the mPFC and BLA of resilient and susceptible mice at both 2 and 8 days following CSDS with minimal overlap between regions and time points. Editing within the Htr2c transcript and relative abundance of Htr2c messenger RNA (mRNA)variants were also observed within the BLA of susceptible mice 2 days following CSDS. These results indicate dynamic RNA editing within discrete brain regions following CSDS in mice, further implicating A-to-I editing as a stress-sensitive molecular mechanism within the brain of potential relevance to resiliency and susceptibility to CSDS.

Epigenetic mechanisms involved in the effects of stress exposure: focus on 5-hydroxymethylcytosine.

5-hydroxymethylcytosine (5hmC) is a recently re-discovered transient intermediate in the active demethylation pathway that also appears to play an independent role in modulating gene function. Epigenetic marks, particularly 5-methylcytosine, have been widely studied in relation to stress-related disorders given the long-lasting effect that stress has on these marks. 5hmC is a good candidate for involvement in the etiology of these disorders given its elevated concentration in mammalian neurons, its dynamic regulation during development of the central nervous system, and its high variability among individuals. Although we are unaware of any studies published to date examining 5 hmC profiles in human subjects who have developed a psychiatric disorder after a life stressor, there is emerging evidence from the animal literature that 5hmC profiles are altered in the context of fear-conditioning paradigms and stress exposure, suggesting a possible role for 5hmC in the biological underpinnings of stress-related disorders. In this review, the authors examine the available approaches for profiling 5hmC and describe their advantages and disadvantages as well as discuss the studies published thus far investigating 5hmC in the context of fear-related learning and stress exposure in animals. The authors also highlight the global versus locus-specific regulation of 5hmC in these studies. Finally, the limitations of the current studies and their implications are discussed.

Chronic intermittent toluene inhalation initiated during adolescence in rats does not alter voluntary consumption of ethanol in adulthood.

Voluntary inhalation of organic solvents, such as toluene, is particularly prevalent in adolescent populations and is considered to be a contributing factor to substance use and dependence later in life. While inhalants are often the initial "drug" experienced during this period, alcohol is another substance readily abused by adolescent populations. Although both substances are thought to have similar actions within the brain, our understanding of the implications of adolescent inhalant abuse upon subsequent exposure to alcohol remains to be investigated. Thus, this study aimed to assess locomotor responses to acute ethanol and voluntary ethanol consumption following a period of toluene inhalation throughout adolescence/early adulthood. Adolescent male Wistar rats (postnatal day [PN] 27) inhaled air or toluene (3000 ppm) for 1 h/day, 3 days/week for 4 (PN 27-52) or 8 weeks (PN 27-80) to mimic the patterns observed in human inhalant abusers. Following the exposure period, cross-sensitization to acute ethanol challenge (0.5 g/kg, intra-peritoneally [i.p.]), and voluntary consumption of 20% ethanol in a chronic intermittent 2-bottle choice paradigm, were assessed. Hepatic ethanol and acetaldehyde metabolism and liver histopathology were also investigated. Chronic intermittent toluene (CIT) exposure throughout adolescence for up to 8 weeks did not alter the behavioral response to acute ethanol or voluntary consumption of ethanol in adulthood, although an age-dependent effect on ethanol consumption was observed (p<0.05). Both liver function and pathology did not differ between treatment groups. Thus, in the paradigm employed, CIT exposure throughout adolescence and early adulthood did not predispose rats to subsequent locomotor sensitivity or voluntary consumption of ethanol in adulthood.

Specific impairments in instrumental learning following chronic intermittent toluene inhalation in adolescent rats.

RATIONALE: Inhalant abuse is prevalent in adolescent populations, with chronic use resulting in neurobiological and cognitive abnormalities in adulthood. However, the nature and persistence of cognitive dysfunction, particularly following adolescent inhalant abuse, remain equivocal. OBJECTIVE: The present study assessed specific cognitive processes beginning in late adolescence and adulthood following adolescent inhalation of toluene, a main component of many compounds readily abused. METHODS: Adolescent male Wistar rats (postnatal day (PN) 27) were exposed to chronic intermittent inhaled toluene (10,000 ppm) for 1 h/day, 3 days/week for 4 weeks (PN 27-52) to mimic the patterns observed in human adolescent inhalant abusers. Following toluene exposure, motor and cognitive function was assessed. RESULTS: Adolescent toluene exposure did not alter motor learning in the Rotarod task (PN 58) or acquisition, reversal, or retention of spatial learning in the Morris water maze (PN 55-64). In contrast, it delayed acquisition of instrumental responding for sucrose (5 % w/v) and impaired operant reversal learning and cue-induced reinstatement of sucrose seeking in adulthood (PN 57-100). CONCLUSION: This study demonstrates that exposure to toluene at an abuse concentration during adolescence results in specific impairments in aspects of instrumental learning, without altering motor function and spatial learning in late adolescence/early adulthood. Our data imply that persistent alterations in reward processing may occur following adolescent inhalant misuse.