Hippocampal neural progenitor cells play a distinct role in fear memory retrieval in male and female CIE rats

Abstinence and Fear Experienced during This Period Produce Distinct Cortical and Hippocampal Adaptations in Alcohol-Dependent Rats

Previous studies demonstrate that ethanol dependence induced by repeating cycles of chronic intermittent ethanol vapor exposure (CIE) followed by protracted abstinence produces significant gray matter damage via myelin dysfunction in the rodent medial prefrontal cortex (mPFC) and alterations in neuronal excitability in the mPFC and the dentate gyrus (DG) of the hippocampus. Specifically, abstinence-induced neuroadaptations have been associated with persistent elevated relapse to drinking. The current study evaluated the effects of forced abstinence for 1 day (d), 7 d, 21 d, and 42 d following seven weeks of CIE on synaptic plasticity proteins in the mPFC and DG. Immunoblotting revealed reduced expression of CaMKII in the mPFC and enhanced expression of GABAA and CaMKII in the DG at the 21 d time point, and the expression of the ratio of GluN2A/2B subunits did not change at any of the time points studied. Furthermore, cognitive performance via Pavlovian trace fear conditioning (TFC) was evaluated in 3 d abstinent rats, as this time point is associated with negative affect. In addition, the expression of the ratio of GluN2A/2B subunits and a 3D structural analysis of neurons in the mPFC and DG were evaluated in 3 d abstinent rats. Behavioral analysis revealed faster acquisition of fear responses and reduced retrieval of fear memories in CIE rats compared to controls. TFC produced hyperplasticity of pyramidal neurons in the mPFC under control conditions and this effect was not evident or blunted in abstinent rats. Neurons in the DG were unaltered. TFC enhanced the GluN2A/2B ratio in the mPFC and reduced the ratio in the DG and was not altered by abstinence. These findings indicate that forced abstinence from CIE produces distinct and divergent alterations in plasticity proteins in the mPFC and DG. Fear learning-induced changes in structural plasticity and proteins contributing to it were more profound in the mPFC during forced abstinence.

Progenitor Cells Play a Role in Reinstatement of Ethanol Seeking in Adult Male and Female Ethanol Dependent Rats

Female and male glial fibrillary acidic protein-thymidine kinase (GFAP-TK) transgenic rats were made ethanol dependent via a six-week chronic intermittent ethanol vapor (CIE) and ethanol drinking (ED) procedure. During the last week of CIE, a subset of male and female TK rats was fed valcyte to ablate dividing progenitor cells and continued the diet until the end of this study. Following week six, all CIE rats experienced two weeks of forced abstinence from CIE-ED, after which they experienced relapse to drinking, extinction, and reinstatement of ethanol seeking sessions. CIE increased ED in female and male rats, with females having higher ethanol consumption during CIE and relapse sessions compared with males. In both sexes, valcyte reduced the levels of Ki-67-labeled progenitor cells in the subgranular zone of the dentate gyrus and did not alter the levels in the medial prefrontal cortex (mPFC). Valcyte increased ED during relapse, increased lever responses during extinction and, interestingly, enhanced latency to extinguish ethanol-seeking behaviors in males. Valcyte reduced the reinstatement of ethanol-seeking behaviors triggered by ethanol cues in females and males. Reduced seeking by valcyte was associated with the normalization of cytokines and chemokines in plasma isolated from trunk blood, indicating a role for progenitor cells in peripheral inflammatory responses. Reduced seeking by valcyte was associated with increases in tight junction protein claudin-5 and oligodendrogenesis in the dentate gyrus and reduction in microglial activity in the dentate gyrus and mPFC in females and males, demonstrating a role for progenitor cells in the dentate gyrus in dependence-induced endothelial and microglial dysfunction. These data suggest that progenitor cells born during withdrawal and abstinence from CIE in the dentate gyrus are aberrant and could play a role in strengthening ethanol memories triggered by ethanol cues via central and peripheral immune responses.

Single cell molecular alterations reveal target cells and pathways of conditioned fear memory

OBJECTIVES

Recent evidence indicates that hippocampus is important for conditioned fear memory (CFM). Though few studies consider the roles of various cell types' contribution to such a process, as well as the accompanying transcriptome changes during this process. The purpose of this study was to explore the transcriptional regulatory genes and the targeted cells that are altered by CFM reconsolidation.

METHODS

A fear conditioning experiment was established on adult male C57 mice, after day 3 tone-cued CFM reconsolidation test, hippocampus cells were dissociated. Using single cell RNA sequencing (scRNA-seq) technique, alterations of transcriptional genes expression were detected and cell cluster analysis were performed and compared with those in sham group.

RESULTS

Seven non-neuronal and eight neuronal cell clusters (including four known neurons and four newly identified neuronal subtypes) has been explored. Among them, CA subtype 1 has characteristic gene markers of Ttr and Ptgds, which is speculated to be the outcome of acute stress and promotes the production of CFM. The results of KEGG pathway enrichment indicate the differences in the expression of certain molecular protein functional subunits in long-term potentiation (LTP) pathway between two types of neurons (DG and CA1) and astrocytes, thus providing a new transcriptional perspective for the role of hippocampus in the CFM reconsolidation. More importantly, the correlation between the reconsolidation of CFM and neurodegenerative diseases-linked genes is substantiated by the results from cell-cell interactions and KEGG pathway enrichment. Further analysis shows that the reconsolidation of CFM inhibits the risk-factor genes App and ApoE in Alzheimer's Disease (AD) and activates the protective gene Lrp1.

CONCLUSIONS

This study reports the transcriptional genes expression changes of hippocampal cells driven by CFM, which confirm the involvement of LTP pathway and suggest the possibility of CFM-like behavior in preventing AD. However, the current research is limited to normal C57 mice, and further studies on AD model mice are needed to prove this preliminary conclusion.

Building Compassion and Human Bridges through Research Collaborations

Our ENLACE binational summer research program was established with the aim of encouraging the participation of high school and college students in research in the sciences and engineering, while promoting cross-border friendships between the United States and Mexico. The program unites students around science and engineering questions and concurrently engages them in a rich curriculum that promotes understanding of broader societal issues of equity, inclusion, tolerance, and social justice. Because we built our program around hope and cooperation, it is our aspiration and promise that walls and borders-all kinds of walls and borders-can be eradicated through kindness, compassion, and respect for others. ENLACE is not just a program we organize every summer, it is also a program that defines who we are as people and the kind of contribution we want to make in the world.

Isoxazole-9 reduces enhanced fear responses and retrieval in ethanol-dependent male rats

Plasticity in the dentate gyrus (DG) is strongly influenced by ethanol, and ethanol experience alters long-term memory consolidation dependent on the DG. However, it is unclear if DG plasticity plays a role in dysregulation of long-term memory consolidation during abstinence from chronic ethanol experience. Outbred male Wistar rats experienced 7 weeks of chronic intermittent ethanol vapor exposure (CIE). Seventy-two hours after CIE cessation, CIE and age-matched ethanol-naïve Air controls experienced auditory trace fear conditioning (TFC). Rats were tested for cue-mediated retrieval in the fear context either twenty-four hours (24 hr), ten days (10 days), or twenty-one days (21 days) later. CIE rats showed enhanced freezing behavior during TFC acquisition compared to Air rats. Air rats showed significant fear retrieval, and this behavior did not differ at the three time points. In CIE rats, fear retrieval increased over time during abstinence, indicating an incubation in fear responses. Enhanced retrieval at 21 days was associated with reduced structural and functional plasticity of ventral granule cell neurons (GCNs) and reduced expression of synaptic proteins important for neuronal plasticity. Systemic treatment with the drug Isoxazole-9 (Isx-9; small molecule that stimulates DG plasticity) during the last week and a half of CIE blocked altered acquisition and retrieval of fear memories in CIE rats during abstinence. Concurrently, Isx-9 modulated the structural and functional plasticity of ventral GCNs and the expression of synaptic proteins in the ventral DG. These findings identify that abstinence-induced disruption of fear memory consolidation occurs via altered plasticity within the ventral DG, and that Isx-9 prevented these effects.

Abstinence from ethanol dependence produces concomitant cortical gray matter abnormalities, microstructural deficits and cognitive dysfunction

Previous studies demonstrate that ethanol dependence induced by repeating cycles of chronic intermittent ethanol vapor exposure (CIE) followed by protracted abstinence (CIE-PA) produces significant alterations in oligodendrogenesis in the rodent medial prefrontal cortex (mPFC). Specifically, CIE-PA produced an unprecedented increase in premyelinating oligodendroglial progenitor cells and myelin, which have been associated with persistent elevated drinking behaviors during abstinence. The current study used neuroimaging and electron microscopy to evaluate the integrity of enhanced myelin and microstructural deficits underlying enhanced myelination in the mPFC in male rats experiencing forced abstinence for 1 day (D), 7D, 21D and 42D following seven weeks of CIE. In vivo diffusion tensor imaging (DTI) detected altered microstructural integrity in the mPFC and corpus callosum (CC). Altered integrity was characterized as reduced fractional anisotropy (FA) in the CC, and enhanced mean diffusivity (MD) in the mPFC in 7D abstinent rats. Increased MD occurred concomitantly with increases in myelin associated proteins, flayed myelin and enhanced mitochondrial stress in the mPFC in 7D abstinent rats, suggesting that the increases in myelination during abstinence was aberrant. Evaluation of cognitive performance via Pavlovian conditioning in 7D abstinent rats revealed reduced retrieval and recall of fear memories dependent on the mPFC. These findings indicate that forced abstinence from moderate to severe alcohol use disorder produces gray matter damage via myelin dysfunction in the mPFC and that these microstructural changes were associated with deficits in PFC dependent behaviors.

Recovery of Hippocampal-Dependent Learning Despite Blunting Reactive Adult Neurogenesis After Alcohol Dependence

Background:

The excessive alcohol drinking that occurs in alcohol use disorder (AUD) causes neurodegeneration in regions such as the hippocampus, though recovery may occur after a period of abstinence. Mechanisms of recovery are not clear, though reactive neurogenesis has been observed in the hippocampal dentate gyrus following alcohol dependence and correlates to recovery of granule cell number.

Objective:

We investigated the role of neurons born during reactive neurogenesis in the recovery of hippocampal learning behavior after 4-day binge alcohol exposure, a model of an AUD. We hypothesized that reducing reactive neurogenesis would impair functional recovery.

Methods:

Adult male rats were subjected to 4-day binge alcohol exposure and two approaches were tested to blunt reactive adult neurogenesis, acute doses of alcohol or the chemotherapy drug, temozolomide (TMZ).

Results:

Acute 5 g/kg doses of EtOH gavaged T6 and T7 days post binge did not inhibit significantly the number of Bromodeoxyuridine-positive (BrdU+) proliferating cells in EtOH animals receiving 5 g/kg EtOH versus controls. A single cycle of TMZ inhibited reactive proliferation (BrdU+ cells) and neurogenesis (NeuroD+ cells) to that of controls. However, despite this blunting of reactive neurogenesis to basal levels, EtOH-TMZ rats were not impaired in their recovery of acquisition of the Morris water maze (MWM), learning similarly to all other groups 35 days after 4-day binge exposure.

Conclusions:

These studies show that TMZ is effective in decreasing reactive proliferation/neurogenesis following 4-day binge EtOH exposure, and baseline levels of adult neurogenesis are sufficient to allow recovery of hippocampal function.

Acute Ethanol Exposure Enhances Synaptic Plasticity in the Dorsal Striatum in Adult Male and Female Rats

Background:

Acute (ex vivo) and chronic (in vivo) alcohol exposure induces neuroplastic changes in the dorsal striatum, a critical region implicated in instrumental learning.

Objective:

Sex differences are evident in alcohol reward and reinforcement, with female rats consuming higher amount of alcohol in operant paradigms compared to male rats. However, sex differences in the neuroplastic changes produced by acute alcohol in the dorsal striatum have been unexplored.

Methods:

Using electrophysiological recordings from dorsal striatal slices obtained from adult male and female rats, we investigated the effects of ex vivo ethanol exposure on synaptic transmission and synaptic plasticity. Ethanol (44 mM) enhanced basal synaptic transmission in both sexes. Ethanol also enhanced long-term potentiation in both sexes. Other measures of synaptic plasticity including paired-pulse ratio were unaltered by ethanol in both sexes.

Results:

The results suggest that alterations in synaptic plasticity induced by acute ethanol, at a concentration associated with intoxication, could play an important role in alcohol-induced experience-dependent modification of corticostriatal circuits underlying the learning of goal-directed instrumental actions and formation of habits mediating alcohol seeking and taking.

Conclusions:

Taken together, understanding the mechanism(s) underlying alcohol induced changes in corticostriatal function may lead to the development of more effective therapeutic agents to reduce habitual drinking and seeking associated with alcohol use disorders.