LSD1-BDNF activity in lateral hypothalamus-medial forebrain bundle area is essential for reward seeking behavior

Gut Microbiota Regulates Epigenetic Remodelling in the Amygdala: A Role in Repeated Mild Traumatic Brain Injury (rMTBI)-Induced Anxiety

Gut microbiota serves in the development and maintenance of phenotype. However, the underlying mechanisms are still in its infancy. The current study shows epigenetic remodelling in the brain as a causal mechanism in the gut microbiota-brain axis. Like in trauma patients, gut dysbiosis and anxiety were comorbid in adult male Wistar rats subjected to repeated mild traumatic brain injuries (rMTBI). rMTBI caused epigenetic dysregulation of brain-derived neurotrophic factor (Bdnf) expression in the amygdala, owing to the formation of transcriptional co-repressor complex due to dynamic interaction between histone deacetylase and DNA methylation modification at the Bdnf gene promoter. The probiosis after faecal microbiota transplantation (FMT) from healthy naïve rats or by administration of single strain probiotic (SSP), Lactobacillus rhamnosus GG (LGG), recuperated rMTBI-induced anxiety. Concurrently, LGG infusion or naïve FMT also dislodged rMTBI-induced co-repressor complex resulting in the normalization of Bdnf expression and neuronal plasticity as measured by Golgi-Cox staining. Furthermore, sodium butyrate, a short-chain fatty acid, produced neurobehavioural effects similar to naïve FMT or LGG administration. Interestingly, the gut microbiota from rMTBI-exposed rats per se was able to provoke anxiety in naïve rats in parallel with BDNF deficits. Therefore, gut microbiota seems to be causally linked with the chromatin remodelling necessary for neuroadaptations via neuronal plasticity which drives experience-dependent behavioural manifestations.

CREB-Binding Protein Regulates Cocaine- and Amphetamine-Regulated Transcript Peptide Expression in the Lateral Hypothalamus: Implication in Reward and Reinforcement

Neuropeptide cocaine- and amphetamine-regulated transcript peptide (CARTp) is known to play an important role in reward processing. The rats conditioned to intra-cranial self-stimulation (ICSS) showed massive upregulation of CART protein and mRNA in the vicinity of the electrode implanted to deliver the electric current directly at the lateral hypothalamus (LH)-medial forebrain bundle (MFB) area. However, the underlying mechanisms leading to the upregulation of CART in ICSS animals remain elusive. We tested the putative role of CREB-binding protein (CBP), an epigenetic enzyme with intrinsic histone acetyltransferase (HAT) activity, in regulating CART expression during ICSS. An electrode was implanted in LH-MFB and the rats were conditioned to self-stimulation in an operant chamber. CBP siRNA was delivered ipsilaterally in the LH-MFB to knock-down CBP and the effects on lever press activity were monitored. While ICSS-conditioned rats showed distinct increase in CART, CBP and pCREB levels, enhanced CBP binding and histone acetylation (H3K9ac) were noticed on the CART promoter in chromatin immunoprecipitation assay. Direct infusion of CBP siRNA in the LH-MFB lowered lever press activity, CBP levels, histone acetylation at the CART promoter, and CART mRNA and peptide expression. Co-infusion of CARTp in LH-MFB rescued the waning effects of CBP siRNA on self-stimulation. We suggest that CBP-mediated histone acetylation may play a causal role in CART expression in LH, which in turn may drive the positive reinforcement of lever press activity.

AMPA and NMDA receptors in dentate gyrus mediate memory for sucrose in two port discrimination task

Although the phenomenon of memory formation and recall associated with the use of psychotropic drugs has been extensively studied, mechanisms underlying memories for natural reward have not been clarified. Herein, we test the hypothesis that glutamatergic receptors in the dentate gyrus play a role in memories associated with sucrose. We used pellet self-administration protocol to generate memories in two-port nose-poke discrimination task using male Wistar rats. During non-rewarded probe trial, the conditioned animals readily discriminated the active port versus inactive port and showed massive increase in mRNA expression of AMPA receptor subunit genes (gria2, gria3) as well as c-Fos protein in the DG. Access to sweet pellet further enhanced c-Fos expression in the DG. However, animals pre-treated with AMPA receptor antagonist CNQX (intra-DG), on exposure to operant chamber (no pellet), showed decreased discrimination as well as c-Fos expression. We suggest that AMPA receptors in DG mediate recall and consolidation of memories associated with sucrose consumption. CNQX pre-treated animals, if presented with sweet pellet on nose poke, exhibited high discrimination index coupled with increased c-Fos expression. In these CNQX treated rats, the DI was again decreased following administration of NMDA receptor antagonist AP5. We suggest that, although AMPA receptors are blocked, the access to sweet pellet may induce surge of glutamate in the DG, which in turn may reinstate memories via activation of erstwhile silent synapses in NMDA dependant manner.

Response of nitrergic system in the brain of rat conditioned to intracranial self-stimulation

The role of nitrergic system in modulating the action of psychostimulants on reward processing is well established. However, the relevant anatomical underpinnings and scope of the involved interactions with mesolimbic dopaminergic system have not been clarified. Using immunohistochemistry, we track the changes in neuronal nitric oxide synthase (nNOS) containing cell groups in the animals conditioned to intracranial self-stimulation (ICSS) via an electrode implanted in the lateral hypothalamus-medial forebrain bundle (LH-MFB) area. An increase in the nNOS immunoreactivity was noticed in the cells and fibers in the ventral tegmental area (VTA) and nucleus accumbens shell (AcbSh), the primary loci of the reward system. In addition, nNOS was up-regulated in the nucleus accumbens core (AcbC), vertical limb of diagonal band (VDB), locus coeruleus (LC), lateral hypothalamus (LH), superficial gray layer (SuG) of the superior colliculus, and periaqueductal gray (PAG). The brain tissue fragments drawn from these areas showed a change in nNOS mRNA expression, but in opposite direction. Intracerebroventricular (icv) administration of nNOS inhibitor, 7-nitroindazole (7-NI) showed decreased lever press activity in a dose-dependent manner in ICSS task. While an increase in the dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) efflux was noted in the microdialysates collected from the AcbSh of ICSS rats, pre-administration of 7-NI (icv route) attenuated the response. The study identifies nitrergic centers that probably mediate sensory, cognitive, and motor components of the goal-directed behavior.

Transgenerational Effects of Arsenic Exposure on Learning and Memory in Rats: Crosstalk between Arsenic Methylation, Hippocampal Metabolism, and Histone Modifications

Arsenic (As) is widely present in the natural environment, and exposure to it can lead to learning and memory impairment. However, the underlying epigenetic mechanisms are still largely unclear. This study aimed to reveal the role of histone modifications in environmental levels of arsenic (sodium arsenite) exposure-induced learning and memory dysfunction in male rats, and the inter/transgenerational effects of paternal arsenic exposure were also investigated. It was found that arsenic exposure impaired the learning and memory ability of F0 rats and down-regulated the expression of cognition-related genes Bdnf, c-Fos, mGlur1, Nmdar1, and Gria2 in the hippocampus. We also observed that inorganic arsenite was methylated to DMA and histone modification-related metabolites were altered, contributing to the dysregulation of H3K4me1/2/3, H3K9me1/2/3, and H3K4ac in rat hippocampus after exposure. Therefore, it is suggested that arsenic methylation and hippocampal metabolism changes attenuated H3K4me1/2/3 and H3K4ac while enhancing H3K9me1/2/3, which repressed the key gene expressions, leading to cognitive impairment in rats exposed to arsenic. In addition, paternal arsenic exposure induced transgenerational effects of learning and memory disorder in F2 male rats through the regulation of H3K4me2 and H3K9me1/2/3, which inhibited c-Fos, mGlur1, and Nmdar1 expression. These results provide novel insights into the molecular mechanism of arsenic-induced neurotoxicity and highlight the risk of neurological deficits in offspring with paternal exposure to arsenic.

Repeated ethanol exposure and withdrawal alters angiotensin-converting enzyme 2 expression in discrete brain regions: Implications for SARS-CoV-2 neuroinvasion

BACKGROUND

People with alcohol use disorder (AUD) may be at higher risk for COVID-19. Angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) are required for cellular entry by SARS-CoV-2, but information on their expression in specific brain regions after alcohol exposure is limited. We sought to clarify how chronic alcohol exposure affects ACE2 expression in monoaminergic brainstem circuits and other putative SARS-CoV-2 entry points.

METHODS

Brains were examined for ACE2 using immunofluorescence after 4 weeks of chronic intermittent ethanol (CIE) vapor inhalation. We also examined TMPRSS2, Cathepsin L, and ADAM17 by Western blot and RAS pathway mediators and pro-inflammatory markers via RT-qPCR.

RESULTS

ACE2 was increased in most brain regions following CIE including the olfactory bulb (OB), hypothalamus (HT), raphe magnus (RMG), raphe obscurus (ROB), locus coeruleus (LC), and periaqueductal gray (PAG). We also observed increased colocalization of ACE2 with monoaminergic neurons in brainstem nuclei. Moreover, soluble ACE2 (sACE2) was elevated in OB, HT, and LC. The increase in sACE2 in OB and HT was accompanied by upregulation of ADAM17, an ACE2 sheddase, while TMPRSS2 increased in HT and LC. Cathepsin L, an endosomal receptor involved in viral entry, was also increased in OB. Alcohol can increase Angiotensin II, which triggers a pro-inflammatory response that may upregulate ACE2 via activation of RAS pathway receptors AT1R/AT2R. ACE2 then metabolizes Angiotensin II to Angiotensin (1-7) and provokes an anti-inflammatory response via MAS1. Accordingly, we report that AT1R/AT2R mRNA decreased in OB and increased in the LC, while MAS1 mRNA increased in both OB and LC. Other mRNAs for pro-inflammatory markers were also dysregulated in OB, HT, raphe, and LC.

CONCLUSIONS

Our results suggest that alcohol triggers a compensatory upregulation of ACE2 in the brain due to disturbed RAS and may increase the risk or severity of SARS-CoV-2 infection.

Early-life exercise primes the murine neural epigenome to facilitate gene expression and hippocampal memory consolidation

Aerobic exercise is well known to promote neuroplasticity and hippocampal memory. In the developing brain, early-life exercise (ELE) can lead to persistent improvements in hippocampal function, yet molecular mechanisms underlying this phenomenon have not been fully explored. In this study, transgenic mice harboring the "NuTRAP" (Nuclear tagging and Translating Ribosome Affinity Purification) cassette in Emx1 expressing neurons ("Emx1-NuTRAP" mice) undergo ELE during adolescence. We then simultaneously isolate and sequence translating mRNA and nuclear chromatin from single hippocampal homogenates containing Emx1-expressing neurons. This approach allowed us to couple translatomic with epigenomic sequencing data to evaluate the influence of histone modifications H4K8ac and H3K27me3 on translating mRNA after ELE. A subset of ELE mice underwent a hippocampal learning task to determine the gene expression and epigenetic underpinnings of ELE's contribution to improved hippocampal memory performance. From this experiment, we discover gene expression - histone modification relationships that may play a critical role in facilitated memory after ELE. Our data reveal candidate gene-histone modification interactions and implicate gene regulatory pathways involved in ELE's impact on hippocampal memory.

ERK signaling is required for nicotine-induced conditional place preference by regulating neuroplasticity genes expression in male mice

Nicotine is an addictive compound that interacts with nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA), inducing a release of dopamine in the nucleus accumbens (NAc). When neurons undergo repeated exposure to nicotine, several adaptive changes in neuroplasticity occur. Activation of nAChRs involves numerous intracellular signaling cascades that likely contribute to neuroplasticity and ultimately the establishment of nicotine addiction. Nevertheless, the molecular mechanisms underlying this adaptation remain unclear. To explore the effects of nicotine on neuroplasticity, a stable nicotine-induced conditioned place preference (CPP) model was constructed by intravenous injection in mice. Using a PCR array, we observed significant changes in the expression of synaptic plasticity-related genes in the VTA (16 mRNAs) and NAc (40 mRNAs). When mice were pre-treated with PD98059, an extracellular signal-regulated kinase (ERK) inhibitor, more gene expression changes in the VTA (53 mRNAs) and NAc (60 mRNAs) were found. Moreover, PD98059 pre-treatment blocked the increased p-ERK/ERK and p-CREB/CREB ratios and decreased the expression of synaptic plasticity-related proteins such as SAP102, PSD95, synaptophysin, and BDNF, these changes might contribute to preventing the establishment of nicotine-induced CPP. Furthermore, neurons from the VTA and NAc of nicotine CPP mice had an increased dendritic spine density and complexity of dendritic morphology by Golgi staining. PD98059 also blocked this dynamic. These results demonstrate that repeated exposure to nicotine may remold the expression of neuroplasticity-related genes by activating the ERK signaling pathway in the VTA and NAc, and is related to the establishment of nicotine-induced CPP.

TET1-induced DNA demethylation in dentate gyrus is important for reward conditioning and reinforcement

Neuroadaptations in neurocircuitry of reward memories govern the persistent and compulsive behaviors. The study of the role of hippocampus in processing of reward memory and its retrieval is critical to our understanding of addiction and relapse. The aim of this study is to probe the epigenetic mechanisms underlying reward memory in the frame of dentate gyrus (DG). To that end, the rats conditioned to the food baited arm of a Y-maze and subjected to memory probe trial. The hippocampus of conditioned rats displayed higher mRNA levels of Ten-eleven translocase 1 (Tet1) and brain-derived neurotrophic factor (Bdnf) after memory probe trial. The DNA hydroxymethylation and TET1 occupancy at the Bdnf promoters showed concomitant increase. Stereotactic administration of Tet1 siRNA in the DG before and after conditioning inhibited reward memory formation and recall, respectively. Administration of Tet1 siRNA impaired the reward memory recall that was reinstated following administration of exogenous BDNF peptide or after wash-off period of 8 days. Infusion of a MEK/ERK inhibitor, U0126 in the DG inhibited reward memory retrieval. The TET1-induced DNA demethylation at the Bdnf promoters raised BDNF levels in the hippocampus, thereby setting the stage for reward memory retrieval. The study underscores the causative role of TET1 in the DG for reward memory formation and recall.

Repeated ethanol exposure and withdrawal alters ACE2 expression in discrete brain regions: Implications for SARS-CoV-2 infection

Emerging evidence suggests that people with alcohol use disorders are at higher risk for SARS-CoV-2. SARS-CoV-2 engages angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) receptors for cellular entry. While ACE2 and TMPRSS2 genes are upregulated in the cortex of alcohol-dependent individuals, information on expression in specific brain regions and neural populations implicated in SARS-CoV-2 neuroinvasion, particularly monoaminergic neurons, is limited. We sought to clarify how chronic alcohol exposure affects ACE2 and TMPRSS2 expression in monoaminergic brainstem circuits and other putative SARS-CoV-2 entry points. C57BL/6J mice were exposed to chronic intermittent ethanol (CIE) vapor for 4 weeks and brains were examined using immunofluorescence. We observed increased ACE2 levels in the olfactory bulb and hypothalamus following CIE, which are known to mediate SARS-CoV-2 neuroinvasion. Total ACE2 immunoreactivity was also elevated in the raphe magnus (RMG), raphe obscurus (ROB), and locus coeruleus (LC), while in the dorsal raphe nucleus (DRN), ROB, and LC we observed increased colocalization of ACE2 with monoaminergic neurons. ACE2 also increased in the periaqueductal gray (PAG) and decreased in the amygdala. Whereas ACE2 was detected in most brain regions, TMPRSS2 was only detected in the olfactory bulb and DRN but was not significantly altered after CIE. Our results suggest that previous alcohol exposure may increase the risk of SARS-CoV-2 neuroinvasion and render brain circuits involved in cardiovascular and respiratory function as well as emotional processing more vulnerable to infection, making adverse outcomes more likely. Additional studies are needed to define a direct link between alcohol use and COVID-19 infection.

Neuroplastic Changes in the Superior Colliculus and Hippocampus in Self-rewarding Paradigm: Importance of Visual Cues

Coincident excitation via different sensory modalities encoding objects of positive salience is known to facilitate learning and memory. With a view to dissect the contribution of visual cues in inducing adaptive neural changes, we monitored the lever press activity of a rat conditioned to self-administer sweet food pellets in the presence/absence of light cues. Application of light cues facilitated learning and consolidation of long-term memory. The superior colliculus (SC) of rats trained on light cue showed increased neuronal activity, dendritic branching, and brain-derived neurotrophic factor (BDNF) protein and mRNA expression. Concomitantly, the hippocampus showed augmented neurogenesis as well as BDNF protein and mRNA expression. While intra-SC administration of U0126 (inhibitor of ERK 1/2 and long-term memory) impaired memory formation, lidocaine (local anaesthetic) hindered memory recall. The light cue-dependent sweet food pellet self-administration was coupled with increased efflux of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens shell (AcbSh). In conditioned rats, pharmacological inhibition of glutamatergic signalling in dentate gyrus (DG) reduced lever press activity, as well as DA and DOPAC secretion in the AcbSh. We suggest that the neuroplastic changes in the SC and hippocampus might represent memory engrams sculpted by visual cues encoding reward information.

Repeated mild traumatic brain injuries perturb the mitochondrial biogenesis via DNA methylation in the hippocampus of rat

Mitochondrial biogenesis in the brain is impaired in various neurological disorders including traumatic brain injury (TBI). The long-lasting effects of TBI may be, in part, attributed to epigenetic mechanisms such as DNA methylation. However, the role of DNA methylation on regulatory elements of nuclear and mitochondrial genome in mitochondrial biogenesis is not known. We examined the epigenetic regulation of mitochondrial transcription factor A (TFAM), and further probed its implications in mitochondrial dysfunction in the hippocampus of rats subjected to repeated mild TBI (rMTBI) using weight drop injury paradigm. rMTBI-induced hypermethylation at TFAM promoter resulted in deficits in its protein levels in mitochondria after immediate (48 h) and protracted (30 d) time points. Further, rMTBI also caused hypomethylation of mitochondrial DNA (mtDNA) promoters (HSP1 and HSP2), which further culminated into low binding of TFAM. rMTBI-induced changes weakened mitochondrial biogenesis in terms of reduced mtDNA-encoded rRNA, mRNA, and protein levels leading to shortages of ATP. To verify the potential role of mtDNA methylation in rMTBI-induced persistent mitochondrial dysfunction, rMTBI-induced rats were treated with methionine, a methyl donor. Methionine treatment restored the methylation levels on HSP1 and HSP2 resulting in efficient binding of TFAM and normalized the rRNA, mRNA, and protein levels. These findings suggest the crucial role of DNA methylation at nuclear and mitochondrial promoter regions in mitochondrial gene expression and ATP activity in the hippocampus after rMTBI.

Neuropeptide CART modulates dopamine turnover in the nucleus accumbens: Insights into the anatomy of rewarding circuits

Neuropeptide cocaine- and amphetamine-regulated transcript (CART) is known to influence the activity of the canonical mesolimbic dopaminergic pathway and modulate reward seeking behaviour. CART neurons of the lateral hypothalamus (LH) send afferents to the ventral tegmental area (VTA) and paraventricular thalamic nucleus (PVT) and these nuclei, in turn, send secondary projections to nucleus accumbens. We try to dissect the precise sites of CART's action in these circuits in promoting reward. Rats were implanted with bipolar electrode targeted at the lateral hypothalamus-medial forebrain bundle (LH-MFB) and trained to press the lever through intracranial self-stimulation (ICSS) protocol. CART (55-102) administered directly into posterior VTA (pVTA) or PVT of the conditioned rats significantly increased the number of lever presses, indicating reward-promoting activity of the peptide. Concomitant increase in dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) efflux was noted in the microdialysate collected from the nucleus accumbens shell (AcbSh). On the other hand, immunoneutralization of endogenous CART with CART antibodies injected directly in the pVTA or PVT reduced the lever press activity as well as DA and DOPAC efflux in the AcbSh. Injection of CART (1-39) in pVTA or PVT was ineffective. We suggest that CART cells in the LH-MFB area send afferents to (a) pVTA and influence dopaminergic neurons projecting to AcbSh and (b) PVT, from where the secondary neurons may feed into the AcbSh. Excitation of the CARTergic pathway to the pVTA as well as the PVT seems to promote DA release in the AcbSh and contribute to the generation of reward.