Increased Synaptic Excitation and Abnormal Dendritic Structure of Prefrontal Cortex Layer V Pyramidal Neurons following Prolonged Binge-Like Consumption of Ethanol

Withdrawal from chronic alcohol impairs the serotonin-mediated modulation of GABAergic transmission in the infralimbic cortex in male rats

The infralimbic cortex (IL) is part of the medial prefrontal cortex (mPFC), exerting top-down control over structures that are critically involved in the development of alcohol use disorder (AUD). Activity of the IL is tightly controlled by γ-aminobutyric acid (GABA) transmission, which is susceptible to chronic alcohol exposure and withdrawal. This inhibitory control is regulated by various neuromodulators, including 5-hydroxytryptamine (5-HT; serotonin). We used chronic intermittent ethanol vapor inhalation exposure, a model of AUD, in male Sprague-Dawley rats to induce alcohol dependence (Dep) followed by protracted withdrawal (WD; 2 weeks) and performed ex vivo electrophysiology using whole-cell patch clamp to study GABAergic transmission in layer V of IL pyramidal neurons. We found that WD increased frequencies of spontaneous inhibitory postsynaptic currents (sIPSCs), whereas miniature IPSCs (mIPSCs; recorded in the presence of tetrodotoxin) were unaffected by either Dep or WD. The application of 5-HT (50 μM) increased sIPSC frequencies and amplitudes in naive and Dep rats but reduced sIPSC frequencies in WD rats. Additionally, 5-HT2A receptor antagonist M100907 and 5-HT2C receptor antagonist SB242084 reduced basal GABA release in all groups to a similar extent. The blockage of either 5-HT2A or 5-HT2C receptors in WD rats restored the impaired response to 5-HT, which then resembled responses in naive rats. Our findings expand our understanding of synaptic inhibition in the IL in AUD, indicating that antagonism of 5-HT2A and 5-HT2C receptors may restore GABAergic control over IL pyramidal neurons. SIGNIFICANCE STATEMENT: Impairment in the serotonergic modulation of GABAergic inhibition in the medial prefrontal cortex contributes to alcohol use disorder (AUD). We used a well-established rat model of AUD and ex vivo whole-cell patch-clamp electrophysiology to characterize the serotonin modulation of GABAergic transmission in layer V infralimbic (IL) pyramidal neurons in ethanol-naive, ethanol-dependent (Dep), and ethanol-withdrawn (WD) male rats. We found increased basal inhibition following WD from chronic alcohol and altered serotonin modulation. Exogenous serotonin enhanced GABAergic transmission in naive and Dep rats but reduced it in WD rats. 5-HT2A and 5-HT2C receptor blockage in WD rats restored the typical serotonin-mediated enhancement of GABAergic inhibition. Our findings expand our understanding of synaptic inhibition in the infralimbic neurons in AUD.

Pre- and postsynaptic signatures in the prelimbic cortex associated with "alcohol use disorder" in the rat

The transition to alcohol use disorder (AUD) involves persistent neuroadaptations in executive control functions primarily regulated by the medial prefrontal cortex. However, the neurophysiological correlates to behavioral manifestations of AUD are not fully defined. The association between cortical neuroadaptations and behavioral manifestations of addiction was studied using a multi-symptomatic operant model based on the DSM-5 diagnostic criteria for AUD. This model aimed to characterize an AUD-vulnerable and AUD-resistant subpopulation of outbred male Wistar rats and was combined with electrophysiological measurements in the prelimbic cortex (PL). Mirroring clinical observations, rats exhibited individual variability in their vulnerability to develop AUD-like behavior, including motivation to seek for alcohol (crit 1), increased effort to obtain the substance (crit 2), and continued drinking despite negative consequences (crit 3). Only a small subset of rats met all the aforementioned AUD criteria (3 crit, AUD-vulnerable), while a larger fraction was considered AUD-resilient (0 crit). The development of AUD-like behavior was characterized by disruptions in glutamatergic synaptic activity, involving decreased frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and heightened intrinsic excitability in layers 2/3 PL pyramidal neurons. These alterations were concomitant with a significant impairment in the ability of mGlu2/3 receptors to negatively regulate glutamate release in the PL but not in downstream regions like the basolateral amygdala or nucleus accumbens core. In conclusion alterations in PL synaptic activity were strongly associated with individual addiction scores, indicating their role as potential markers of the behavioral manifestations linked to AUD psychopathology.

Substance use and spine density: a systematic review and meta-analysis of preclinical studies

The elucidation of synaptic density changes provides valuable insights into the underlying brain mechanisms of substance use. In preclinical studies, synaptic density markers, like spine density, are altered by substances of abuse (e.g., alcohol, amphetamine, cannabis, cocaine, opioids, nicotine). These changes could be linked to phenomena including behavioral sensitization and drug self-administration in rodents. However, studies have produced heterogeneous results for spine density across substances and brain regions. Identifying patterns will inform translational studies given tools that now exist to measure in vivo synaptic density in humans. We performed a meta-analysis of preclinical studies to identify consistent findings across studies. PubMed, ScienceDirect, Scopus, and EBSCO were searched between September 2022 and September 2023, based on a protocol (PROSPERO: CRD42022354006). We screened 6083 publications and included 70 for meta-analysis. The meta-analysis revealed drug-specific patterns in spine density changes. Hippocampal spine density increased after amphetamine. Amphetamine, cocaine, and nicotine increased spine density in the nucleus accumbens. Alcohol and amphetamine increased, and cannabis reduced, spine density in the prefrontal cortex. There was no convergence of findings for morphine's effects. The effects of cocaine on the prefrontal cortex presented contrasting results compared to human studies, warranting further investigation. Publication bias was small for alcohol or morphine and substantial for the other substances. Heterogeneity was moderate-to-high across all substances. Nonetheless, these findings inform current translational efforts examining spine density in humans with substance use disorders.

Prefrontal cortex glutamatergic adaptations in a mouse model of alcohol use disorder

Alcohol use disorder (AUD) produces cognitive deficits, indicating a shift in prefrontal cortex (PFC) function. PFC glutamate neurotransmission is mostly mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type ionotropic receptors (AMPARs); however preclinical studies have mostly focused on other receptor subtypes. Here we examined the impact of early withdrawal from chronic ethanol on AMPAR function in the mouse medial PFC (mPFC). Dependent male C57BL/6J mice were generated using the chronic intermittent ethanol vapor-two bottle choice (CIE-2BC) paradigm. Non-dependent mice had access to water and ethanol bottles but did not receive ethanol vapor. Naïve mice had no ethanol exposure. We used patch-clamp electrophysiology to measure glutamate neurotransmission in layer 2/3 prelimbic mPFC pyramidal neurons. Since AMPAR function can be impacted by subunit composition or plasticity-related proteins, we probed their mPFC expression levels. Dependent mice had higher spontaneous excitatory postsynaptic current (sEPSC) amplitude and kinetics compared to the Naïve/Non-dependent mice. These effects were seen during intoxication and after 3-8 days withdrawal, and were action potential-independent, suggesting direct enhancement of AMPAR function. Surprisingly, 3 days withdrawal decreased expression of genes encoding AMPAR subunits (Gria1/2) and synaptic plasticity proteins (Dlg4 and Grip1) in Dependent mice. Further analysis within the Dependent group revealed a negative correlation between Gria1 mRNA levels and ethanol intake. Collectively, these data establish a role for mPFC AMPAR adaptations in the glutamatergic dysfunction associated with ethanol dependence. Future studies on the underlying AMPAR plasticity mechanisms that promote alcohol reinforcement, seeking, drinking and relapse behavior may help identify new targets for AUD treatment.

Targeting prefrontal cortex GABAergic microcircuits for the treatment of alcohol use disorder

Developing novel treatments for alcohol use disorders (AUDs) is of paramount importance for improving patient outcomes and alleviating the suffering related to the disease. A better understanding of the molecular and neurocircuit mechanisms through which alcohol alters brain function will be instrumental in the rational development of new efficacious treatments. Clinical studies have consistently associated the prefrontal cortex (PFC) function with symptoms of AUDs. Population-level analyses have linked the PFC structure and function with heavy drinking and/or AUD diagnosis. Thus, targeting specific PFC cell types and neural circuits holds promise for the development of new treatments. Here, we overview the tremendous diversity in the form and function of inhibitory neuron subtypes within PFC and describe their therapeutic potential. We then summarize AUD population genetics studies, clinical neurophysiology findings, and translational neuroscience discoveries. This study collectively suggests that changes in fast transmission through PFC inhibitory microcircuits are a central component of the neurobiological effects of ethanol and the core symptoms of AUDs. Finally, we submit that there is a significant and timely need to examine sex as a biological variable and human postmortem brain tissue to maximize the efforts in translating findings to new clinical treatments.

Hyperexcitability and brain morphological differences in mice lacking the cystine/glutamate antiporter, system xc. J Neurosci Res 2021;99:3339-3353. [PMID: 34747522 DOI: 10.1002/jnr.24971]

System x_c ^- (Sx_c ^- ) is a heteromeric antiporter (L-cystine/L-glutamate exchanger) expressed predominately on astrocytes in the central nervous system. Its activity contributes importantly to the maintenance of the ambient extracellular glutamate levels, as well as, to cellular redox homeostasis. Since alterations in glutamate levels and redox modifications could cause structural changes, we analyzed gross regional morphology of thionin-stained brain sections and cellular and subcellular morphology of Golgi-Cox stained layer V pyramidal neurons in the primary motor cortex (PM1) of mice naturally null for SLC7A11 (SLC7A11^sut/sut )-the gene that encodes the substrate specific light chain (xCT) for Sx_c ^- . Intriguingly, in comparison to age- and sex-matched wild-type (SLC7A11^+/+ ) littermate controls, we found morphologic changes-including increased dendritic complexity and mushroom spine area in males and reduced corpus callosum and soma size in females-that have previously been described, in each case, as morphological correlates of excitability. Consistent with this, we found that both male and female SLC7A11^sut/sut mice had lower convulsive seizure thresholds and greater seizure severity than their sex-matched wild-type (SLC7A11^+/+ ) littermates after acute challenge with two pharmacologically distinct chemoconvulsants: the Glu receptor agonist, kainic acid (KA), or the GABA_A receptor antagonist, pentylenetetrazole (PTZ). These results suggest that the loss of Sx_c ^- signaling in males and females perturbs excitatory/inhibitory (E/I) balance in vivo, potentially through its regulation of cellular and subcellular morphology.

Loss of liver X receptor β in astrocytes leads to anxiety-like behaviors via regulating synaptic transmission in the medial prefrontal cortex in mice

Astrocytes are integral components of synaptic transmission, and their dysfunction leads to neuropsychiatric disorders such as anxiety and depression. Liver X receptor β (LXRβ) is expressed in astrocytes, and LXRβ global knockout mice shows impaired synaptic formation. In order to define the role of LXRβ in astrocytes, we used a conditional Cre-loxP system to specifically remove LXRβ from astrocytes. We found that this deletion caused anxiety-like but not depressive-like behaviors in adult male mice. This behavioral phenotype could be completely reproduced by selective deletion of LXRβ in astrocytes in the medial prefrontal cortex (mPFC). Pyramidal neurons in layer V of mPFC are involved in mood behaviors. We found that there was an increased spontaneous excitatory synaptic transmission in layer V pyramidal neurons of the mPFC of these mice. This was concurrent with increased dendritic complexity, despite normal appearance and number of dendritic spines. In addition, gene ontology analysis of RNA sequencing revealed that deletion of astrocytic LXRβ led to the enrichment of the process of synaptic transmission in mPFC. Finally, we also confirmed that renormalized excitatory synaptic transmission in layer V pyramidal neurons alleviated the anxiety in mice with astrocytic LXRβ deletion in mPFC. Together, our findings reveal that astrocytic LXRβ in mPFC is critical in the regulation of synaptic transmission, and this provides a potential new target for treatment of anxiety-like behavior.

Adolescent Ethanol Exposure Alters Cholinergic Function and Apical Dendritic Branching Within the Orbital Frontal Cortex

During adolescence, heavy binge-like ethanol consumption can lead to frontocortical structural and functional impairments. These impairments are likely driven by adolescence being a critical time point for maturation of brain regions associated with higher-order cognitive functioning. Rodent models of heavy binge-like ethanol exposure show consistent disruptions to the typical development of the prefrontal cortex (PFC). All deep cortical layers receive cholinergic projections that originate from the Nucleus basalis of Meynert (NbM) complex. These cholinergic projections are highly involved in learning, memory, and attention. Adolescent intermittent ethanol exposure (AIE) induces cholinergic dysfunction as a result of an epigenetic suppression of the genes that drive the cholinergic phenotype. The current study used a model of AIE to assess structural and functional changes to the frontal cortex and NbM following binge-like ethanol exposure in adolescence. Western blot analysis revealed long-term disruptions of the cholinergic circuit following AIE: choline acetyltransferase (ChAT) was suppressed in the NbM and vesicular acetylcholine transporter (VAChT) was suppressed in the orbitofrontal cortex (OFC). In vivo microdialysis for acetylcholine efflux during a spatial memory task determined changes in cholinergic modulation within the PFC following AIE. However, AIE spared performance on the spatial memory task and on an operant reversal task. In a second study, Golgi-Cox staining determined that AIE increased apical dendritic complexity in the OFC, with sex influencing whether the increase in branching occurred near or away from the soma. Spine density or maturity was not affected, likely compensating for a disruption in neurotransmitter function following AIE.

Drugs of Abuse Differentially Alter the Neuronal Excitability of Prefrontal Layer V Pyramidal Cell Subtypes

The medial prefrontal cortex (mPFC) plays an important role in regulating executive functions including reward seeking, task flexibility, and compulsivity. Studies in humans have demonstrated that drugs of abuse, including heroin, cocaine, methamphetamine, and alcohol, alter prefrontal function resulting in the consequential loss of inhibitory control and increased compulsive behaviors, including drug seeking. Within the mPFC, layer V pyramidal cells, which are delineated into two major subtypes (type I and type II, which project to subcortical or commissurally to other cortical regions, respectively), serve as the major output cells which integrate information from other cortical and subcortical regions and mediate executive control. Preclinical studies examining changes in cellular physiology in the mPFC in response to drugs of abuse, especially in regard to layer V pyramidal subtypes, are relatively sparse. In the present study, we aimed to explore how heroin, cocaine, methamphetamine, ethanol, and 3,4-methylenedioxypyrovalerone (MDPV) alter the baseline cellular physiology and excitability properties of layer V pyramidal cell subtypes. Specifically, animals were exposed to experimenter delivered [intraperitoneal (i.p.)] heroin, cocaine, the cocaine-like synthetic cathinone MDPV, methamphetamine, ethanol, or saline as a control once daily for five consecutive days. On the fifth day, whole-cell physiology recordings were conducted from type I and type II layer V pyramidal cells in the mPFC. Changes in cellular excitability, including rheobase (i.e., the amount of injected current required to elicit action potentials), changes in input/output curves, as well as spiking characteristics induced by each substance, were assessed. We found that heroin, cocaine, methamphetamine, and MDPV decreased the excitability of type II cells, whereas ethanol increased the excitability of type I pyramidal cells. Together, these results suggest that heroin, cocaine, MDPV, and methamphetamine reduce mPFC commissural output by reducing type II excitability, while ethanol increases the excitability of type I cells targeting subcortical structures. Thus, separate classes of abused drugs differentially affect layer V pyramidal subtypes in the mPFC, which may ultimately give rise to compulsivity and inappropriate synaptic plasticity underlying substance use disorders.

Increased Synaptic Strength and mGlu2/3 Receptor Plasticity on Mouse Prefrontal Cortex Intratelencephalic Pyramidal Cells Following Intermittent Access to Ethanol

BACKGROUND

The medial prefrontal cortex (PFC) is crucial for regulating craving and alcohol seeking in alcohol use disorder (AUD) patients and alcohol seeking in animal models. Maladaptive changes in volitional ethanol (EtOH) intake have been associated with PFC function, yet synaptic adaptations within PFC have not been consistently detected in voluntary drinking rodent models. At least 80% of the neurons in PFC are glutamatergic pyramidal cells. Pyramidal cells provide the predominant cortical output to several brain regions relevant to AUD, including structures within the telencephalon (IT: e.g., basal ganglia, amygdala, other neocortical regions) and outside the telencephalon (ET: e.g., lateral hypothalamus, midbrain monoaminergic structures, thalamus).

METHODS

In addition to their anatomical distinctions, studies from several laboratories have revealed that prefrontal cortical IT and ET pyramidal cells may be differentiated by specific electrophysiological parameters. These distinguishable parameters make it possible to readily classify pyramidal cells into separable subtypes. Here, we employed and validated the hyperpolarization sag ratio as a diagnostic proxy for separating ET (type A) and IT (type B) neurons. We recorded from deep-layer prelimbic PFC pyramidal cells of mice 1 day after 4 to 5 weeks of intermittent access (IA) EtOH exposure.

RESULTS

Membrane properties were not altered by IA EtOH, but excitatory postsynaptic strength onto IT type B neurons was selectively enhanced in slices from IA EtOH mice. The increased excitatory drive was accompanied by enhanced mGlu_2/3 receptor plasticity on IT type B neurons, providing a potential translational approach to mitigate cognitive and motivational changes to PFC function related to binge drinking.

CONCLUSIONS

Together, these studies provide insight into the specific PFC neurocircuits altered by voluntary drinking. In addition, the findings provide an additional rationale for developing compounds that potentiate mGlu₂ and/or mGlu₃ receptor function as potential treatments for AUD.

Interaction of chronic intermittent ethanol and repeated stress on structural and functional plasticity in the mouse medial prefrontal cortex

Stress is a risk factor that plays a considerable role in the development and maintenance of alcohol (ethanol) abuse and relapse. Preclinical studies examining ethanol-stress interactions have demonstrated elevated ethanol drinking, cognitive deficits, and negative affective behaviors in mice. However, the neural adaptations in prefrontal cortical regions that drive these aberrant behaviors produced by ethanol-stress interactions are unknown. In this study, male C57BL/6J mice were exposed to chronic intermittent ethanol (CIE) and repeated forced swim stress (FSS). After two cycles of CIE x FSS, brain slices containing the prelimbic (PrL) and infralimbic (IfL) cortex were prepared for analysis of adaptations in dendritic spines and synaptic plasticity. In the PrL cortex, total spine density was increased in mice exposed to CIE. Immediately following induction of long-term potentiation (LTP), the fEPSP slope was increased in the PrL of CIE x FSS treated mice, indicative of a presynaptic adaptation on post-tetanic potentiation (PTP). In the IfL cortex, CIE exposure regardless of FSS experience resulted in an increase in spine density. FSS alone or when combined with CIE exposure increased PTP following LTP induction. Repeated FSS episodes increased IfL cortical paired-pulse facilitation, a second measure of presynaptic plasticity. In summary, CIE exposure resulted in structural adaptations while repeated stress exposure drove metaplastic changes in presynaptic function, demonstrating distinct morphological and functional changes in PrL and IfL cortical neurons. Thus, the structural and functional adaptations may be one mechanism underlying the development of excessive drinking and cognitive deficits associated with ethanol-stress interactions.

Midbrain Dopamine Controls Anxiety-like Behavior by Engaging Unique Interpeduncular Nucleus Microcircuitry

BACKGROUND

Dopamine (DA) is hypothesized to modulate anxiety-like behavior, although the precise role of DA in anxiety behaviors and the complete anxiety network in the brain have yet to be elucidated. Recent data indicate that dopaminergic projections from the ventral tegmental area (VTA) innervate the interpeduncular nucleus (IPN), but how the IPN responds to DA and what role this circuit plays in anxiety-like behavior are unknown.

METHODS

We expressed a genetically encoded G protein-coupled receptor activation-based DA sensor in mouse midbrain to detect DA in IPN slices using fluorescence imaging combined with pharmacology. Next, we selectively inhibited or activated VTA→IPN DAergic inputs via optogenetics during anxiety-like behavior. We used a biophysical approach to characterize DA effects on neural IPN circuits. Site-directed pharmacology was used to test if DA receptors in the IPN can regulate anxiety-like behavior.

RESULTS

DA was detected in mouse IPN slices. Silencing/activating VTA→IPN DAergic inputs oppositely modulated anxiety-like behavior. Two neuronal populations in the ventral IPN (vIPN) responded to DA via D₁ receptors (D1Rs). vIPN neurons were controlled by a small population of D1R neurons in the caudal IPN that directly respond to VTA DAergic terminal stimulation and innervate the vIPN. IPN infusion of a D1R agonist and antagonist bidirectionally controlled anxiety-like behavior.

CONCLUSIONS

VTA DA engages D1R-expressing neurons in the caudal IPN that innervate vIPN, thereby amplifying the VTA DA signal to modulate anxiety-like behavior. These data identify a DAergic circuit that mediates anxiety-like behavior through unique IPN microcircuitry.

Contrasting effects of adolescent and early-adult ethanol exposure on prelimbic cortical pyramidal neurons

BACKGROUND

Adolescence and early-adulthood are vulnerable developmental periods during which binge drinking can have long-lasting effects on brain function. However, little is known about the effects of binge drinking on the pyramidal cells of the prelimbic cortex (PrL) during early and protracted withdrawal periods.

METHODS

In the present study, we performed whole-cell patch clamp recordings and dendritic spine staining to examine the intrinsic excitability, spontaneous excitatory post-synaptic currents (sEPSCs), and spine morphology of pyramidal cells in the PrL from rats exposed to chronic intermittent ethanol (CIE) during adolescence or early-adulthood.

RESULTS

Compared to chronic intermittent water (CIW)-treated controls, the excitability of PrL-L5 pyramidal neurons was significantly increased 21 days after adolescent CIE but decreased 21 days after early-adult CIE. No changes of excitability in PrL Layer (L) 5 were detected 2 days after either adolescent or early-adulthood CIE. Interestingly, decreases in sEPSC amplitude and increases in thin spines ratio were detected 2 days after adolescent CIE. Furthermore, decreased frequency and amplitude of sEPSCs, accompanied by a decrease in the density of total spines and non-thin spines were observed 21 days after adolescent CIE. In contrast, increased frequency and amplitude of sEPSCs, accompanied by increased densities of total spines and non-thin spines were found 21 days after early adult CIE.

CONCLUSION

CIE produced prolonged neuronal and synaptic alterations in PrL-L5, and the developmental stage, i.e., adolescence vs. early-adulthood when subjects receive CIE, is a key factor in determining the direction of these changes.

Role of mTOR-regulated autophagy in spine pruning defects and memory impairments induced by binge-like ethanol treatment in adolescent mice

Adolescence is a brain maturation developmental period during which remodeling and changes in synaptic plasticity and neural connectivity take place in some brain regions. Different mechanism participates in adolescent brain maturation, including autophagy that plays a role in synaptic development and plasticity. Alcohol is a neurotoxic compound and its abuse in adolescence induces neuroinflammation, synaptic and myelin alterations, neural damage and behavioral impairments. Changes in synaptic plasticity and its regulation by mTOR have also been suggested to play a role in the behavioral dysfunction of binge ethanol drinking in adolescence. Therefore, by considering the critical role of mTOR in both autophagy and synaptic plasticity in the developing brain, the present study aims to evaluate whether binge ethanol treatment in adolescence would induce dysfunctions in synaptic plasticity and cognitive functions and if mTOR inhibition with rapamycin is capable of restoring both effects. Using C57BL/6 adolescent female and male mice (PND30) treated with ethanol (3 g/kg) on two consecutive days at 48‐hour intervals over 2 weeks, we show that binge ethanol treatment alters the density and morphology of dendritic spines, effects that are associated with learning and memory impairments and changes in the levels of both transcription factor CREB phosphorylation and miRNAs. Rapamycin administration (3 mg/kg) prior to ethanol administration restores ethanol‐induced changes in both plasticity and behavior dysfunctions in adolescent mice. These results support the critical role of mTOR/autophagy dysfunctions in the dendritic spines alterations and cognitive alterations induced by binge alcohol in adolescence.

Contrasting sex-dependent adaptations to synaptic physiology and membrane properties of prefrontal cortex interneuron subtypes in a mouse model of binge drinking

Alcohol use disorder (AUD) affects all sexes, however women who develop AUD may be particularly susceptible to cravings and other components of the disease. While many brain regions are involved in AUD etiology, proper prefrontal cortex (PFC) function is particularly important for top-down craving management and the moderation of drinking behaviors. Essential regulation of PFC output is provided by local inhibitory interneurons, yet how drinking affects interneuron physiology remains poorly understood, particularly in female individuals. To address this gap, we generated fluorescent reporter transgenic mice to label the two major classes of interneuron in deep layer prelimbic PFC, based on expression of parvalbumin (PV-IN) or somatostatin (SST-IN). We then interrogated PV-IN and SST-IN membrane and synaptic physiology in a rodent model of binge drinking. Beginning in late adolescence, mice received 3-4 weeks of intermittent access (IA) ethanol. We prepared acute brain slices one day after the last drinking session. PV-INs but not SST-INs from IA ethanol mice displayed increased excitability relative to controls, regardless of sex. On the contrary, synaptic adaptations to PV-INs differed based on sex. While drinking decreased excitatory synaptic strength onto PV-INs from female mice, PV-INs from IA ethanol male mice exhibited potentiated excitatory transmission relative to controls. In contrast, decreased synaptic strength onto SST-INs was observed following IA ethanol in all groups of mice. Together, these findings illustrate novel sex differences in drinking-related PFC pathophysiology. Discovering means to restore PV-IN and SST-IN dysfunction following extended drinking provides opportunities for developing new treatments for all AUD patients.

Heightened Exploratory Behavior Following Chronic Excessive Ethanol Drinking: Mediation by Neurotensin Receptor Type 2 in the Anterior Paraventricular Thalamus

BACKGROUND

Chronic, excessive alcohol drinkers, even without dependence, can exhibit changes in behavior and neurochemical systems. Identifying these changes and their relationship with one another could provide novel avenues for the prevention and treatment of alcohol use disorder. We recently demonstrated, in rats, that neurotensin (NTS) in the paraventricular thalamus (PVT) regulates excessive ethanol (EtOH) drinking. Here, we investigate the effects of chronic EtOH drinking on the PVT-NTS system and its contribution to EtOH-induced behavioral changes.

METHODS

We gave adult male Long-Evans rats 20% EtOH under the intermittent access 2-bottle-choice paradigm or maintained them on chow and water for up to 11 weeks. Prior to EtOH exposure and following several weeks of access, during acute abstinence, we tested these groups for multiple behaviors. In the 12th week, during acute abstinence, we examined gene expression and peptide levels of NTS and its receptors in the anterior and posterior subregions of the PVT. Finally, in chronic EtOH drinkers, during acute abstinence, we microinjected the NTS receptor type 2 (NTS2R) agonist, JMV-431, in the anterior PVT (aPVT) and examined subsequent EtOH intake and behavior.

RESULTS

Following chronic intermittent EtOH access, rats were classified by cluster analysis as high or low EtOH drinkers. High EtOH drinkers spent more time in the light chamber of a light-dark box and open arms of an elevated plus maze and entered fewer familiar holes in a hole-board apparatus. These differences were absent prior to EtOH exposure but were detectable as early as 4 weeks into drinking. Time in the light chamber following chronic drinking also predicted level of subsequent drinking. High EtOH drinkers also showed elevated protein levels of NTS2R in the aPVT, and pharmacological stimulation of aPVT NTS2R in low drinkers mimicked the increased time spent in the light chamber that was observed in high drinkers.

CONCLUSIONS

Our findings suggest that chronic, excessive, but not lower level, EtOH drinking induces heightened or flexible exploratory behavior, which predicts future EtOH drinking and is partly mediated by elevated NTS2R signaling in the aPVT. These EtOH-induced alterations represent adaptations that could perpetuate excessive drinking and lead to the development of EtOH dependence.

Comorbid HIV infection and alcohol use disorders: Converging glutamatergic and dopaminergic mechanisms underlying neurocognitive dysfunction

Alcohol use disorders (AUDs) are highly comorbid with human immunodeficiency virus (HIV) infection, occurring at nearly twice the rate in HIV positive individuals as in the general population. Individuals with HIV who consume alcohol show worse long-term prognoses and may be at elevated risk for the development of HIV-associated neurocognitive disorders. The direction of this relationship is unclear, and likely multifactorial. Chronic alcohol exposure and HIV infection independently promote cognitive dysfunction and further may interact to exacerbate neurocognitive deficits through effects on common targets, including corticostriatal glutamate and dopamine neurotransmission. Additionally, drug and alcohol use is likely to reduce treatment adherence, potentially resulting in accelerated disease progression and subsequent neurocognitive impairment. The development of neurocognitive impairments may further reduce cognitive control over behavior, resulting in escalating alcohol use. This review will examine the complex relationship between HIV infection and alcohol use, highlighting impacts on dopamine and glutamate systems by which alcohol use and HIV act independently and in tandem to alter corticostriatal circuit structure and function to dysregulate cognitive function.

Medial prefrontal cortex in neurological diseases

The medial prefrontal cortex (mPFC) is a crucial cortical region that integrates information from numerous cortical and subcortical areas and converges updated information to output structures. It plays essential roles in the cognitive process, regulation of emotion, motivation, and sociability. Dysfunction of the mPFC has been found in various neurological and psychiatric disorders, such as depression, anxiety disorders, schizophrenia, autism spectrum disorders, Alzheimer's disease, Parkinson's disease, and addiction. In the present review, we summarize the preclinical and clinical studies to illustrate the role of the mPFC in these neurological diseases.

Prefrontal-accumbens opioid plasticity: Implications for relapse and dependence

In addiction, an individual's ability to inhibit drug seeking and drug taking is thought to reflect a pathological strengthening of drug-seeking behaviors or impairments in the capacity to control maladaptive behavior. These processes are not mutually exclusive and reflect drug-induced modifications within prefrontal cortical and nucleus accumbens circuits, however unlike psychostimulants such as cocaine, far less is known about the temporal, anatomical, and cellular dynamics of these changes. We discuss what is known regarding opioid-induced adaptations in intrinsic membrane physiology and pre-/postsynaptic neurotransmission in principle pyramidal and medium spiny neurons in the medial prefrontal cortex and nucleus accumbens from electrophysiological studies and explore how circuit specific adaptations may contribute to unique facets of opioid addiction.

M1-positive allosteric modulators lacking agonist activity provide the optimal profile for enhancing cognition

Highly selective positive allosteric modulators (PAMs) of the M₁ subtype of muscarinic acetylcholine receptor have emerged as an exciting new approach for improving cognitive function in patients suffering from Alzheimer's disease and schizophrenia. However, excessive activation of M₁ is known to induce seizure activity and have actions in the prefrontal cortex (PFC) that could impair cognitive function. We now report a series of pharmacological, electrophysiological, and behavioral studies in which we find that recently reported M₁ PAMs, PF-06764427 and MK-7622, have robust agonist activity in cell lines and agonist effects in the mouse PFC, and have the potential to overactivate the M₁ receptor and disrupt PFC function. In contrast, structurally distinct M₁ PAMs (VU0453595 and VU0550164) are devoid of agonist activity in cell lines and maintain activity dependence of M₁ activation in the PFC. Consistent with the previously reported effect of PF-06764427, the ago-PAM MK-7622 induces severe behavioral convulsions in mice. In contrast, VU0453595 does not induce behavioral convulsions at doses well above those required for maximal efficacy in enhancing cognitive function. Furthermore, in contrast to the robust efficacy of VU0453595, the ago-PAM MK-7622 failed to improve novel object recognition, a rodent assay of cognitive function. These findings suggest that in vivo cognition-enhancing efficacy of M₁ PAMs can be observed with PAMs lacking intrinsic agonist activity and that intrinsic agonist activity of M₁ PAMs may contribute to adverse effects and reduced efficacy in improving cognitive function.

Phrenic motor neuron loss in aged rats

Sarcopenia is the age-related reduction of muscle mass and specific force. In previous studies, we found that sarcopenia of the diaphragm muscle (DIAm) is evident by 24 mo of age in both rats and mice and is associated with selective atrophy of type IIx and IIb muscle fibers and a decrease in maximum specific force. These fiber type-specific effects of sarcopenia resemble those induced by DIAm denervation, leading us to hypothesize that sarcopenia is due to an age-related loss of phrenic motor neurons (PhMNs). To address this hypothesis, we determined the number of PhMNs in young (6 mo old) and old (24 mo old) Fischer 344 rats. Moreover, we determined age-related changes in the size of PhMNs, since larger PhMNs innervate type IIx and IIb DIAm fibers. The PhMN pool was retrogradely labeled and imaged with confocal microscopy to assess the number of PhMNs and the morphometry of PhMN soma and proximal dendrites. In older animals, there were 22% fewer PhMNs, a 19% decrease in somal surface area, and a 21% decrease in dendritic surface area compared with young Fischer 344 rats. The age-associated loss of PhMNs involved predominantly larger PhMNs. These results are consistent with an age-related denervation of larger, more fatigable DIAm motor units, which are required primarily for high-force airway clearance behaviors. NEW & NOTEWORTHY Diaphragm muscle sarcopenia in rodent models is well described in the literature; however, the relationship between sarcopenia and frank phrenic motor neuron (MN) loss is unexplored in these models. We quantify a 22% loss of phrenic MNs in old (24 mo) compared with young (6 mo) Fischer 344 rats. We also report reductions in phrenic MN somal and proximal dendritic morphology that relate to decreased MN heterogeneity in old compared with young Fischer 344 rats.

Emerging role for the medial prefrontal cortex in alcohol-seeking behaviors

The medial prefrontal cortex (mPFC) plays an important role in high-order executive processes and sends highly organized projections to sub-cortical regions controlling mood, motivation and impulsivity. Recent preclinical and clinical studies have demonstrated alcohol-induced effects on the activity and composition of the PFC which are implicated in associative learning processes and may disrupt executive control over impulsivity, leading to an inability to self-limit alcohol intake. Animal studies have begun to dissect the role of the mPFC circuitry in alcohol-seeking behavior and withdrawal, and have identified a key role for projections to sub-cortical sites including the extended amygdala and the nucleus accumbens (NAc). Importantly, these studies have highlighted that alcohol can have contrasting effects on the mPFC compared to other addictive substances and also produce differential effects on the structure and activity of the mPFC following short-term versus long-term consumption. Because of these differences, how the mPFC influences the initial aspects of alcohol-seeking behavior and how we can better understand the long-term effects of alcohol use on the activity and connectivity of the mPFC need to be considered. Given the lack of preclinical data from long-term drinking models, an increased focus should be directed towards identifying how long-term alcohol use changes the mPFC, in order to provide new insights into the mechanisms underlying the transition to dependence.

Investigating Methodological Differences in the Assessment of Dendritic Morphology of Basolateral Amygdala Principal Neurons-A Comparison of Golgi-Cox and Neurobiotin Electroporation Techniques

Quantitative assessments of neuronal subtypes in numerous brain regions show large variations in dendritic arbor size. A critical experimental factor is the method used to visualize neurons. We chose to investigate quantitative differences in basolateral amygdala (BLA) principal neuron morphology using two of the most common visualization methods: Golgi–Cox staining and neurobiotin (NB) filling. We show in 8-week-old Wistar rats that NB-filling reveals significantly larger dendritic arbors and different spine densities, compared to Golgi–Cox-stained BLA neurons. Our results demonstrate important differences and provide methodological insights into quantitative disparities of BLA principal neuron morphology reported in the literature.

Social and environmental enrichment has different effects on ethanol and sucrose consumption in mice

BACKGROUND

Factors leading to the harmful consumption of substances, like alcohol and sucrose, involve a complex interaction of genes and the environment. While we cannot control the genes we inherit, we can modify our environment. Understanding the role that social and environmental experiences play in alcohol and sucrose consumption is critical for developing preventative interventions and treatments for alcohol use disorders and obesity.

METHODS

We used the drinking in the dark two-bottle choice (2BC) model of ethanol and sucrose consumption to compare male C57BL/6 mice housed in the IntelliCage (an automated device capable of simultaneously measuring behaviors of up to 16 mice living in an enriched social environment) with mice housed in standard isolated and social environments.

RESULTS

Consistent with previous publications on ethanol-naïve and -experienced mice, social and environmental enrichment reduced ethanol preference. Isolated mice had the highest ethanol preference and IntelliCage mice the least, regardless of prior ethanol experience. In mice with no prior sucrose experience, the addition of social and environmental enrichment increased sucrose preference. However, moving isolated mice to enriched conditions did not affect sucrose preference in sucrose-experienced mice.

CONCLUSIONS

The impact of social and environmental enrichment on ethanol consumption differs from sucrose consumption suggesting that interventions and treatments developed for alcohol use disorders may not be suitable for sucrose consumption disorders.