Effects of acamprosate on attentional set-shifting and cellular function in the prefrontal cortex of chronic alcohol-exposed mice

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.

Chronic ethanol exposure produces long-lasting, subregion-specific physiological adaptations in RMTg-projecting mPFC neurons

Chronic ethanol exposure produces neuroadaptations in the medial prefrontal cortex (mPFC) which facilitate the maladaptive behaviors interfering with recovery from alcohol use disorder. Despite evidence that different cortico-subcortical projections play distinct roles in behavior, few studies have examined the physiological effects of chronic ethanol at the circuit level. The rostromedial tegmental nucleus (RMTg) is a GABAergic midbrain region involved in aversive signaling and is functionally altered by chronic ethanol exposure. Our recent work identified a dense input from the mPFC to the RMTg, yet the effects of chronic ethanol exposure on this circuitry is unknown. In the current study, we examined physiological changes after chronic ethanol exposure in prelimbic (PL) and infralimbic (IL) mPFC neurons projecting to the RMTg. Adult male Long-Evans rats were injected with fluorescent retrobeads into the RMTg and rendered dependent using a 14-day chronic intermittent ethanol (CIE) vapor exposure paradigm. Whole-cell patch-clamp electrophysiological recordings were performed in fluorescently-labeled (RMTg-projecting) and -unlabeled (projection-undefined) layer 5 pyramidal neurons 7-10 days following ethanol exposure. CIE significantly increased intrinsic excitability as well as excitatory and inhibitory synaptic drive in RMTg-projecting IL neurons. In contrast, no lasting changes in excitability were observed in RMTg-projecting PL neurons, although a CIE-induced reduction in excitability was observed in projection-undefined PL neurons. CIE also increased excitatory synaptic drive in RMTg-projecting PL neurons. These data uncover novel subregion- and circuit-specific neuroadaptations in the mPFC following chronic ethanol exposure and reveal that the IL mPFC-RMTg projection is uniquely vulnerable to long-lasting effects of chronic ethanol.

Animal Models of Excessive Alcohol Consumption in Rodents

The development of animal models that demonstrate excessive levels of alcohol consumption has played an important role in advancing our knowledge about neurobiological underpinnings and environmental circumstances that engender such maladaptive behavior. The use of these preclinical models has also provided valuable opportunities for discovering new and novel therapeutic targets that may be useful in the treatment of alcohol use disorder (AUD). While no single model can fully capture the complexities of AUD, the goal is to develop animal models that closely approximate characteristics of heavy alcohol drinking in humans to enhance their translational value and utility. A variety of experimental approaches have been employed to produce the desired phenotype of interest-robust and reliable excessive levels of alcohol drinking. Here we provide an updated review of five animal models that are commonly used. The models entail procedural manipulations of scheduled access to alcohol (time of day, duration, frequency), periods of time when access to alcohol is withheld, and history of alcohol exposure. Specially, the models involve (a) scheduled access to alcohol, (b) scheduled periods of alcohol deprivation, (c) scheduled intermittent access to alcohol, (d) scheduled-induced polydipsia, and (e) chronic alcohol (dependence) and withdrawal experience. Each of the animal models possesses unique experimental features that engender excessive levels of alcohol consumption. Both advantages and disadvantages of each model are described along with discussion of future work to be considered in developing more optimal models. Ultimately, the validity and utility of these models will lie in their ability to aid in the discovery of new and novel potential therapeutic targets as well as serve as a platform to evaluate treatment strategies that effectively reduce excessive levels of alcohol consumption associated with AUD.

The effects of acamprosate on prefrontal cortical function are mimicked by CaCl2 and they are influenced by the history of alcohol exposure

Alcohol use disorder is associated with functional changes in the medial prefrontal cortex (mPFC), which include altered glutamatergic transmission and deficits in executive functions that contribute to relapse. Acamprosate (calcium-bis N-acetylhomotaurinate) reduces alcohol craving and relapse, effects that are thought to be mediated by acamprosate's ability to ameliorate alcohol-induced dysregulation of glutamatergic signaling. Treatment with acamprosate and its active moiety calcium (CaCl2) both improve deficits in cognitive flexibility in postdependent mice following chronic intermittent ethanol (CIE) exposure. Here, we show that mice that self-administered alcohol under goal-directed conditions (i.e., operant responding on a fixed-ratio schedule) also display similar deficits in cognitive flexibility and altered glutamatergic signaling in the mPFC, both of which were improved with acamprosate or CaCl2. However, under conditions shown to bias behavior towards habitual responding (operant self-administration after CIE exposure, or on a variable interval schedule), alcohol-induced changes to glutamatergic transmission were unaffected by either acamprosate or CaCl2 treatment. Together, these findings suggest that the variable effects of acamprosate on synaptic signaling may reflect a shift in mPFC networks related to the loss of behavioral control in habitual alcohol-seeking.

Differential Influence of Pueraria lobata Root Extract and Its Main Isoflavones on Ghrelin Levels in Alcohol-Treated Rats

The study was carried out on alcohol-preferring male Wistar rats. The following drugs were repeatedly (28×) administered: acamprosate (500 mg/kg, p.o.), naltrexone (0.1 mg/kg, i.p), and Pueraria lobata (kudzu) root extract (KU) (500 mg/kg, p.o.) and its isoflavones: daidzin (40 mg/kg, p.o.) and puerarin (150 mg/kg, p.o.). Their effects on a voluntary alcohol intake were assessed. KU and alcohol were also given for 9 days in an experiment on alcohol tolerance development. Finally, total and active ghrelin levels in peripheral blood serum were measured by ELISA method. Acamprosate, naltrexone, daidzin, and puerarin, reducing the alcohol intake, caused an increase in both forms of ghrelin levels. On the contrary, though KU inhibited the alcohol intake and alcohol tolerance development, it reduced ghrelin levels in alcohol-preferring rats. The changes of ghrelin concentration could play a role as an indicator of the currently used drugs. The other effect on the KU-induced shift in ghrelin levels in the presence of alcohol requires further detailed study.

Neurobiological mechanisms of control in alcohol use disorder - moving towards mechanism-based non-invasive brain stimulation treatments

Alcohol use disorder (AUD) is characterized by excessive habitual drinking and loss of control over alcohol intake despite negative consequences. Both of these aspects foster uncontrolled drinking and high relapse rates in AUD patients. Yet, common interventions mostly focus on the phenomenological level, and prioritize the reduction of craving and withdrawal symptoms. Our review provides a mechanistic understanding of AUD and suggests alternative therapeutic approaches targeting the mechanisms underlying dysfunctional alcohol-related behaviours. Specifically, we explain how repeated drinking fosters the development of rigid drinking habits and is associated with diminished cognitive control. These behavioural and cognitive effects are then functionally related to the neurobiochemical effects of alcohol abuse. We further explain how alterations in fronto-striatal network activity may constitute the neurobiological correlates of these alcohol-related dysfunctions. Finally, we discuss limitations in current pharmacological AUD therapies and suggest non-invasive brain stimulation (like TMS and tDCS interventions) as a potential addition/alternative for modulating the activation of both cortical and subcortical areas to help re-establish the functional balance between controlled and automatic behaviour.

Altered Cortico-Subcortical Network After Adolescent Alcohol Exposure Mediates Behavioral Deficits in Flexible Decision-Making

Behavioral flexibility, the ability to modify behavior according to changing conditions, is essential to optimize decision-making. Deficits in behavioral flexibility that persist into adulthood are one consequence of adolescent alcohol exposure, and another is decreased functional connectivity in brain structures involved in decision-making; however, a link between these two outcomes has not been established. We assessed effects of adolescent alcohol and sex on both Pavlovian and instrumental behaviors and resting-state functional connectivity MRI in adult animals to determine associations between behavioral flexibility and resting-state functional connectivity. Alcohol exposure impaired attentional set reversals and decreased functional connectivity among cortical and subcortical regions-of-interest that underlie flexible behavior. Moreover, mediation analyses indicated that adolescent alcohol-induced reductions in functional connectivity within a subnetwork of affected brain regions statistically mediated errors committed during reversal learning. These results provide a novel link between persistent reductions in brain functional connectivity and deficits in behavioral flexibility resulting from adolescent alcohol exposure.

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.

Review of Orbitofrontal Cortex in Alcohol Dependence: A Disrupted Cognitive Map?

Alcoholism is a persistent worldwide problem associated with long-lasting impairments to decision making processes. Some aspects of dysfunction are thought to reflect alcohol-induced changes to relevant brain areas such as the orbitofrontal cortex (OFC). In this review, we will examine how chronic alcohol exposure alters OFC function to potentially contribute to maladaptive decision making, and explore experimental behavioral approaches that may be better suited to test whether alcohol dependence disrupts OFC's function. We argue that although past works suggest impairments in aspects of OFC function, more information may be gained by specifically targeting tasks to the broader function of OFC as put forth by the recent hypothesis of OFC as a "cognitive map" of task space. Overall, we suggest that such a focus could provide a better understanding of how OFC function changes in alcohol dependence, and could inform better assessment tools and treatment options for clinicians working with this population.

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.

Deletion of the Mitochondrial Matrix Protein CyclophilinD Prevents Parvalbumin Interneuron Dysfunctionand Cognitive Deficits in a Mouse Model of NMDA Hypofunction

Redox dysregulation and oxidative stress are final common pathways in the pathophysiology of a variety of psychiatric disorders, including schizophrenia. Oxidative stress causes dysfunction of GABAergic parvalbumin (PV)-positive interneurons (PVI), which are crucial for the coordination of neuronal synchrony during sensory and cognitive processing. Mitochondria are the main source of reactive oxygen species (ROS) in neurons and they control synaptic activity through their roles in energy production and intracellular calcium homeostasis. We have previously shown that in male mice transient blockade of NMDA receptors (NMDARs) during development [subcutaneous injections of 30 mg/kg ketamine (KET) on postnatal days 7, 9, and 11] results in long-lasting alterations in synaptic transmission and reduced PV expression in the adult prefrontal cortex (PFC), contributing to a behavioral phenotype that mimics multiple symptoms associated with schizophrenia. These changes correlate with oxidative stress and impaired mitochondrial function in both PVI and pyramidal cells. Here, we show that genetic deletion (Ppif-/-) of the mitochondrial matrix protein cyclophilin D (CypD) prevents perinatal KET-induced increases in ROS and the resulting deficits in PVI function, and changes in excitatory and inhibitory synaptic transmission in the PFC. Deletion of CypD also prevented KET-induced behavioral deficits in cognitive flexibility, social interaction, and novel object recognition (NOR). Taken together, these data highlight how mitochondrial activity may play an integral role in modulating PVI-mediated cognitive processes.SIGNIFICANCE STATEMENT Mitochondria are important modulators of oxidative stress and cell function, yet how mitochondrial dysfunction affects cell activity and synaptic transmission in psychiatric illnesses is not well understood. NMDA receptor (NMDAR) blockade with ketamine (KET) during development causes oxidative stress, dysfunction of parvalbumin (PV)-positive interneurons (PVI), and long-lasting physiological and behavioral changes. Here we show that mice deficient for the mitochondrial matrix protein cyclophilin D (CypD) show robust protection from PVI dysfunction following perinatal NMDAR blockade. Mitochondria serve as an essential node for a number of stress-induced signaling pathways and our experiments suggest that failure of mitochondrial redox regulation can contribute to PVI dysfunction.

Attentional set shifting in HAP3, LAP3, and cHAP mice is unaffected by either genetic differences in alcohol preference or an alcohol drinking history

Alcohol consumption may precede, or result from, behavioral inflexibility and contribute to individuals' difficulties ceasing drinking. Attentional set shifting tasks are an animal analog to a human behavioral flexibility task requiring recognition of a previous strategy as inappropriate, and the formation and maintenance of a novel strategy (Floresco, Block, & Tse, 2008). Abstinent individuals with alcohol use disorder, nonalcoholic individuals with a family history of alcoholism, and mice exposed to chronic-intermittent alcohol vapor show impaired behavioral flexibility (Gierski et al., 2013; Hu, Morris, Carrasco, & Kroener, 2015; Oscar-Berman et al., 2009). Behavioral flexibility deficits can be linked to frontal cortical regions connected to the striatum (Ragozzino, 2007), and alterations to the endocannabinoid system, implicated in drug seeking and consumption (Economidou et al., 2006; Serrano & Parsons, 2011), may affect these behaviors. Alcohol-preferring and nonpreferring rodents exhibit differences in CB1 receptor expression (CB1R; Hansson et al., 2007; Hungund & Basavarajappa, 2000), but whether dorsal striatal CB1Rs are important for other alcohol-related behaviors such as attentional set shifting tasks remains unclear. This study assesses whether selectively bred high (HAP) versus low alcohol-preferring mice differ in an operant attentional set shifting task or CB1R levels in the dorsal striatum and whether a history of voluntary alcohol consumption in crossed HAP mice exacerbates inflexibility. Contrary to our hypothesis, neither genetic differences in alcohol preference nor drinking affected set shifting. However, high alcohol-preferring mice-3 mice showed reduced levels of dorsal striatal CB1R compared with low alcohol-preferring-3 mice, suggesting that genetic differences in alcohol consumption may be mediated in part by striatal CB1R. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Selective impact of lateral orbitofrontal cortex inactivation on reinstatement of alcohol seeking in male Long-Evans rats

The orbitofrontal cortex (OFC) plays a fundamental role in motivated behavior and decision-making. In humans, OFC structure and function is significantly disrupted in drug using and dependent individuals, including those exhibiting chronic alcohol use and alcoholism. In animal models, the OFC has been shown to significantly influence the seeking of non-alcohol drugs of abuse. However direct investigations of the OFC during alcohol seeking and use have been more limited. In the studies reported here, we inactivated lateral (lOFC) or medial OFC (mOFC) subregions in rats during multiple stages of alcohol seeking. After one month of intermittent access to homecage 20% ethanol (EtOH), rats were trained to self-administer EtOH under an FR3 schedule and implanted with cannulae directed to lOFC or mOFC. We inactivated OFC subregions with baclofen/muscimol during EtOH self-administration, extinction, cue-induced reinstatement, and progressive ratio testing to broadly characterize the influence of these subregions on alcohol seeking. There were no significant effects of mOFC or lOFC inactivation during FR3 self-administration, extinction, or progressive ratio self-administration. However, lOFC, and not mOFC, inactivation significantly decreased cue-induced reinstatement of EtOH seeking. These findings contribute new information to the specific impact of OFC manipulation on operant alcohol seeking, support previous studies investigating the role of OFC in seeking and consumption of alcohol and other drugs of abuse, and indicate a specific role for lOFC vs. mOFC in reinstatement.

Possible Role of CRF-Hcrt Interaction in the Infralimbic Cortex in the Emergence and Maintenance of Compulsive Alcohol-Seeking Behavior

Alcohol use disorder (AUD) is a chronic, relapsing disorder that is characterized by the compulsive use of alcohol despite numerous health, social, and economic consequences. Initially, the use of alcohol is driven by positive reinforcement. Over time, however, alcohol use can take on a compulsive quality that is driven by the desire to avoid the negative consequences of abstinence, including negative affect and heightened stress/anxiety. This transition from positive reinforcement- to negative reinforcement-driven consumption involves the corticotropin-releasing factor (CRF) system, although mounting evidence now suggests that the CRF system interacts with other neural systems to ultimately produce behaviors that are symptomatic of compulsive alcohol use, such as the hypocretin (Hcrt) system. Hypocretins are produced exclusively in the hypothalamus, but Hcrt neurons project widely throughout the brain and reach regions that perform regulatory functions for numerous behavioral and physiological responses-including the infralimbic cortex (IL) of the medial prefrontal cortex (mPFC). Although the entire mPFC undergoes neuroadaptive changes following prolonged alcohol exposure, the IL appears to undergo more robust changes compared with other mPFC substructures. Evidence to date suggests that the IL is likely involved in EtOH-seeking behavior, but ambiguities with respect to the specific role of the IL in this regard make it difficult to draw definitive conclusions. Furthermore, the manner in which CRF interacts with Hcrt in this region as it pertains to alcohol-seeking behavior is largely unknown, although immunohistochemical and electrophysiological experiments have shown that CRF and Hcrt directly interact in the mPFC, suggesting that the interaction between CRF and Hcrt in the IL may be critically important for the development and subsequent maintenance of compulsive alcohol seeking. This review aims to consolidate recent literature regarding the role of the IL in alcohol-seeking behavior and to discuss evidence that supports a functional interaction between Hcrt and CRF in the IL.

Glial gene networks associated with alcohol dependence

Chronic alcohol abuse alters the molecular structure and function of brain cells. Recent work suggests adaptations made by glial cells, such as astrocytes and microglia, regulate physiological and behavioral changes associated with addiction. Defining how alcohol dependence alters the transcriptome of different cell types is critical for developing the mechanistic hypotheses necessary for a nuanced understanding of cellular signaling in the alcohol-dependent brain. We performed RNA-sequencing on total homogenate and glial cell populations isolated from mouse prefrontal cortex (PFC) following chronic intermittent ethanol vapor exposure (CIE). Compared with total homogenate, we observed unique and robust gene expression changes in astrocytes and microglia in response to CIE. Gene co-expression network analysis revealed biological pathways and hub genes associated with CIE in astrocytes and microglia that may regulate alcohol-dependent phenotypes. Astrocyte identity and synaptic calcium signaling genes were enriched in alcohol-associated astrocyte networks, while TGF-β signaling and inflammatory response genes were disrupted by CIE treatment in microglia gene networks. Genes related to innate immune signaling, specifically interferon pathways, were consistently up-regulated across CIE-exposed astrocytes, microglia, and total homogenate PFC tissue. This study illuminates the cell-specific effects of chronic alcohol exposure and provides novel molecular targets for studying alcohol dependence.

Adolescent intermittent ethanol impairs behavioral flexibility in a rat foraging task in adulthood

Alcohol exposure is linked to behavioral flexibility deficits in humans, but it is unclear when the critical exposure occurred or if alcohol exposure alone is sufficient to produce behavior deficits. Increasing evidence shows that binge levels of alcohol during adolescence are particularly harmful to the brain, producing physiological and behavioral effects that can persist into adulthood. The present study determined whether adolescent intermittent ethanol (AIE) in rats impaired action selection in a discriminative stimulus task using a foraging response. Rats were exposed to ethanol during adolescence (5 g/kg/day, IG, 2-days-on/2-days-off, postnatal day 25-54). In adulthood, they learned to dig for food reward buried in one of two media, cued with one of two odors. AIE and control rats both learned to discriminate between olfactory cues, but AIE rats were impaired when reversing that learned association (first intra-dimensional reversal). However, AIE rats were faster to reinstate the original odor discrimination rule (second reversal), suggesting perseverative behavior. Next, the reward location was cued by digging media rather than odor. Both groups learned this extra-dimensional shift; however, control rats were slower to reach criterion. These findings are consistent with studies of people with substance abuse disorder, who learn new stimulus-response associations similarly to, or better than, control subjects, but perseverate when attempting to replace a well-learned association. These data suggest that adolescent binge-alcohol exposure contributes to behavioral flexibility deficits observed in adulthood.

Impact of Substance Use Disorder Pharmacotherapy on Executive Function: A Narrative Review

Substance use disorders are chronic, relapsing, and harmful conditions characterized by executive dysfunction. While there are currently no approved pharmacotherapy options for stimulant and cannabis use disorders, there are several evidence-based options available to help reduce symptoms during detoxification and aid long-term cessation for those with tobacco, alcohol and opioid use disorders. While these medication options have shown clinical efficacy, less is known regarding their potential to enhance executive function. This narrative review aims to provide a brief overview of research that has investigated whether commonly used pharmacotherapies for these substance use disorders (nicotine, bupropion, varenicline, disulfiram, acamprosate, nalmefene, naltrexone, methadone, buprenorphine, and lofexidine) effect three core executive function components (working memory, inhibitory control and cognitive flexibility). While pharmacotherapy-induced enhancement of executive function may improve cessation outcomes in dependent populations, there are limited and inconsistent findings regarding the effects of these medications on executive function. We discuss possible reasons for the mixed findings and suggest some future avenues of work that may enhance the understanding of addiction pharmacotherapy and cognitive training interventions and lead to improved patient outcomes.

The role of the orbitofrontal cortex in alcohol use, abuse, and dependence

One of the major functions of the orbitofrontal cortex (OFC) is to promote flexible motivated behavior. It is no surprise, therefore, that recent work has demonstrated a prominent impact of chronic drug use on the OFC and a potential role for OFC disruption in drug abuse and addiction. Among drugs of abuse, the use of alcohol is particularly salient with respect to OFC function. Although a number of studies in humans have implicated OFC dysregulation in alcohol use disorders, animal models investigating the association between OFC and alcohol use are only beginning to be developed, and there is still a great deal to be revealed. The goal of this review is to consider what is currently known regarding the role of the OFC in alcohol use and dependence. I will first provide a brief, general overview of current views of OFC function and its contributions to drug seeking and addiction. I will then discuss research to date related to the OFC and alcohol use, both in human clinical populations and in non-human models. Finally I will consider issues and strategies to guide future study that may identify this brain region as a key player in the transition from moderated to problematic alcohol use and dependence.

A novel multichoice touchscreen paradigm for assessing cognitive flexibility in mice

Cognitive flexibility refers to various processes which enable behaviors to be modified on the basis of a change in the contingencies between stimuli or responses and their associated outcomes. Reversal learning is a form of cognitive flexibility which measures the ability to adjust responding based on a switch in the stimulus–outcome contingencies of, typically two, perceptually distinct stimuli. Reversal tasks have provided valuable insight into the neural basis of cognitive flexibility, implicating brain regions including the lateral orbitofrontal cortex (lOFC) and dorsomedial prefrontal cortex (dmPFC). However, with two-stimulus reversal, it is difficult to determine whether response errors are due excessive perseveration, deficient learning, or other problems with updating. To address this limitation, we developed a mouse three-choice touchscreen-based visual reversal task, in which the contingencies of two stimuli were switched on reversal but a third, simultaneously presented, stimulus was never reinforced. We found that, in male C57BL/6J mice, responding at the previously rewarded stimulus predominated over the newly and never-reinforced stimuli during early reversal. Next, we showed that acute pharmacological inhibition of lOFC, but not dmPFC, impaired early reversal performance, relative to noninactivated controls. Interestingly, however, lOFC inactivation deficits were characterized by increased choice of the never-reinforced stimulus and a decrease in (perseverative-like) responding at the previously rewarded stimulus. These effects are inconsistent with the historical notion of lOFC mediating response inhibition and closer to recent views of the lOFC's role in response/outcome tracking. Overall, these findings provide initial support the utility of this novel paradigm for studying cognitive flexibility and its underlying neural substrates.

Neuropathic Pain Creates an Enduring Prefrontal Cortex Dysfunction Corrected by the Type II Diabetic Drug Metformin But Not by Gabapentin

Chronic pain patients suffer from pain-related cognitive deficits, even when taking commonly prescribed analgesics. These deficits are likely related to pain-related maladaptive plasticity in the frontal cortex. We sought to model cognitive deficits in mice with neuropathic pain to examine maladaptive morphological plasticity in the mPFC and to assess the effects of several therapeutics. We used an attentional set-shifting task in mice with spared nerve injury (SNI) who received either a single intrathecal injection of an analgesic dose of clonidine, 7 d of 100 mg/kg gabapentin, or 7 d of 200 mg/kg metformin. Male SNI mice were significantly more impaired in the set-shifting task than females. This deficit correlated with a loss of parvalbumin (PV) and reductions in axon initial segment (AIS) length in layers 5/6 of the infralimbic (IL) cortex. Acute pain relief with clonidine had no effect on set-shifting performance, whereas pain relief via 7 day treatment with gabapentin worsened the impairment in both SNI and sham mice. Gabapentin reversed the PV loss in the IL but had no effect on AIS length. Treatment with the AMPK-activator metformin completely reversed the pain-related cognitive impairment and restored AIS length in the IL but had little effect on PV expression. Our findings reveal that neuropathic pain-related cognitive impairments in male mice are correlated to bilateral morphological changes in PV interneurons and layer 5/6 IL pyramidal neuron AIS. Pain relief with metformin can reverse some of the functional and anatomical changes.SIGNIFICANCE STATEMENT Cognitive impairments are a comorbidity of neuropathic pain but are inadequately addressed by existing therapeutics. We used a neuropathic pain model in mice to demonstrate that male (but not female) mice show a robust pain-related deficit in attentional set-shifting, which is associated with structural plasticity in axon initial segments in the infralimbic cortex. These deficits were completely reversed by 7 day treatment with the antidiabetic drug metformin, suggesting that this drug can be repurposed for the treatment of neuropathic pain and its cognitive comorbidities. Our findings have implications for our understanding of how neuropathic pain causes structural plasticity in the brain, and they point to a marked sexual dimorphism in neuropathic pain mechanisms in mice.

Calcium chloride mimics the effects of acamprosate on cognitive deficits in chronic alcohol-exposed mice

RATIONALE

Acamprosate (calcium-bis N-acetylhomotaurinate) is the leading medication approved for the maintenance of abstinence, shown to reduce craving and relapse in animal models and human alcoholics. Acamprosate can improve executive functions that are impaired by chronic intermittent ethanol (CIE) exposure. Recent work has suggested that acamprosate's effects on relapse prevention are due to its calcium component, which raises the question whether its pro-cognitive effects are similarly mediated by calcium.

OBJECTIVES

This study examined the effects of acamprosate on alcohol-induced behavioral deficits and compared them with the effects of the sodium salt version of N-acetylhomotaurinate or calcium chloride, respectively.

METHODS

We exposed mice to alcohol via three cycles of CIE and measured changes in alcohol consumption in a limited-access paradigm. We then compared the effects of acamprosate and calcium chloride (applied subchronically for 3 days during withdrawal) in a battery of cognitive tasks that have been shown to be affected by chronic alcohol exposure.

RESULTS

CIE-treated animals showed deficits in attentional set-shifting and deficits in novel object recognition. Alcohol-treated animals showed no impairments in social novelty detection and interaction, or delayed spontaneous alternation. Both acamprosate and calcium chloride ameliorated alcohol-induced cognitive deficits to comparable extents. In contrast, the sodium salt version of N-acetylhomotaurinate did not reverse the cognitive deficits.

CONCLUSIONS

These results add evidence to the notion that acamprosate produces its anti-relapse effects through its calcium moiety. Our results also suggest that improved regulation of drug intake by acamprosate after withdrawal might at least in part be related to improved cognitive function.

Morphological and functional evidence of increased excitatory signaling in the prelimbic cortex during ethanol withdrawal

Excessive alcohol consumption in humans induces deficits in decision making and emotional processing, which indicates a dysfunction of the prefrontal cortex (PFC). The present study aimed to determine the impact of chronic intermittent ethanol (CIE) inhalation on mouse medial PFC pyramidal neurons. Data were collected 6-8 days into withdrawal from 7 weeks of CIE exposure, a time point when mice exhibit behavioral symptoms of withdrawal. We found that spine maturity in prelimbic (PL) layer 2/3 neurons was increased, while dendritic spines in PL layer 5 neurons or infralimbic (IL) neurons were not affected. Corroborating these morphological observations, CIE enhanced glutamatergic transmission in PL layer 2/3 pyramidal neurons, but not IL layer 2/3 neurons. Contrary to our predictions, these cellular alterations were associated with improved, rather than impaired, performance in reversal learning and strategy switching tasks in the Barnes maze at an earlier stage of chronic ethanol exposure (5-7 days withdrawal from 3 to 4 weeks of CIE), which could result from the anxiety-like behavior associated with ethanol withdrawal. Altogether, this study adds to a growing body of literature indicating that glutamatergic activity in the PFC is upregulated following chronic ethanol exposure, and identifies PL layer 2/3 pyramidal neurons as a sensitive target of synaptic remodeling. It also indicates that the Barnes maze is not suitable to detect deficits in cognitive flexibility in CIE-withdrawn mice.

Cognitive effects of labeled addictolytic medications

INTRODUCTION

Alcohol, tobacco, and illegal drug usage is pervasive throughout the world, and abuse of these substances is a major contributor to the global disease burden. Many pharmacotherapies have been developed over the last 50years to target addictive disorders. While the efficacy of these pharmacotherapies is largely recognized, their cognitive impact is less known. However, all substance abuse disorders are known to promote cognitive disorders like executive dysfunction and memory impairment. These impairments are critical for the maintenance of addictive behaviors and impede cognitive behavioral therapies that are regularly administered in association with pharmacotherapies. It is also unknown if addictolytic medications have an impact on preexisting cognitive disorders, and if this impact is modulated by the indication of prescription, i.e. abstinence, reduction or substitution, or by the specific action of the medication.

METHOD

We reviewed the cognitive effects of labeled medications for tobacco addiction (varenicline, bupropion, nicotine patch and nicotine gums), alcohol addiction (naltrexone, nalmefene, baclofen, disulfiram, sodium oxybate, acamprosate), and opioid addiction (methadone, buprenorphine) in human studies. Studies were selected following MOOSE guidelines for systematic reviews of observational studies, using the keywords [Cognition] and [Cognitive disorders] and [treatment] for each medication.

RESULTS

971 articles were screened and 77 studies met the inclusion criteria and were reported in this review (for alcohol abuse, n=21, for tobacco n=22, for opioid n=34. However, very few comparative clinical trials have explored the chronic effects of addictolytic medications on cognition in addictive behaviors, and there are no clinical trials on the cognitive impact of nalmefene in patients suffering from alcohol use disorders.

DISCUSSION

Although some medications seem to enhance cognition in patients suffering from cognitive disorders, others could promote cognitive impairments, and our work highlights a lack of literature on this subject. In conclusion, more comparative clinical trials are needed to better understand the cognitive impact of addictolytic medications.

Chronic Alcohol, Intrinsic Excitability, and Potassium Channels: Neuroadaptations and Drinking Behavior

Neural mechanisms underlying alcohol use disorder remain elusive, and this lack of understanding has slowed the development of efficacious treatment strategies for reducing relapse rates and prolonging abstinence. While synaptic adaptations produced by chronic alcohol exposure have been extensively characterized in a variety of brain regions, changes in intrinsic excitability of critical projection neurons are understudied. Accumulating evidence suggests that prolonged alcohol drinking and alcohol dependence produce plasticity of intrinsic excitability as measured by changes in evoked action potential firing and after-hyperpolarization amplitude. In this chapter, we describe functional changes in cell firing of projection neurons after long-term alcohol exposure that occur across species and in multiple brain regions. Adaptations in calcium-activated (K_Ca2), voltage-dependent (K_V7), and G protein-coupled inwardly rectifying (K_ir3 or GIRK) potassium channels that regulate the evoked firing and after-hyperpolarization parallel functional changes in intrinsic excitability induced by chronic alcohol. Moreover, there are strong genetic links between alcohol-related behaviors and genes encoding K_Ca2, K_V7, and GIRK channels, and pharmacologically targeting these channels reduces alcohol consumption and alcohol-related behaviors. Together, these studies demonstrate that chronic alcohol drinking produces adaptations in K_Ca2, K_V7, and GIRK channels leading to impaired regulation of the after-hyperpolarization and aberrant cell firing. Correcting the deficit in the after-hyperpolarization with positive modulators of K_Ca2 and K_V7 channels and altering the GIRK channel binding pocket to block the access of alcohol represent a potentially highly effective pharmacological approach that can restore changes in intrinsic excitability and reduce alcohol consumption in affected individuals.

Glutamate plasticity woven through the progression to alcohol use disorder: a multi-circuit perspective

Glutamate signaling in the brain is one of the most studied targets in the alcohol research field. Here, we report the current understanding of how the excitatory neurotransmitter glutamate, its receptors, and its transporters are involved in low, episodic, and heavy alcohol use. Specific animal behavior protocols can be used to assess these different drinking levels, including two-bottle choice, operant self-administration, drinking in the dark, the alcohol deprivation effect, intermittent access to alcohol, and chronic intermittent ethanol vapor inhalation. Importantly, these methods are not limited to a specific category, since they can be interchanged to assess different states in the development from low to heavy drinking. We encourage a circuit-based perspective beyond the classic mesolimbic-centric view, as multiple structures are dynamically engaged during the transition from positive- to negative-related reinforcement to drive alcohol drinking. During this shift from lower-level alcohol drinking to heavy alcohol use, there appears to be a shift from metabotropic glutamate receptor-dependent behaviors to N-methyl-D-aspartate receptor-related processes. Despite high efficacy of the glutamate-related pharmaceutical acamprosate in animal models of drinking, it is ineffective as treatment in the clinic. Therefore, research needs to focus on other promising glutamatergic compounds to reduce heavy drinking or mediate withdrawal symptoms or both.

Orbitofrontal Neuroadaptations and Cross-Species Synaptic Biomarkers in Heavy-Drinking Macaques

Cognitive impairments, uncontrolled drinking, and neuropathological cortical changes characterize alcohol use disorder. Dysfunction of the orbitofrontal cortex (OFC), a critical cortical subregion that controls learning, decision-making, and prediction of reward outcomes, contributes to executive cognitive function deficits in alcoholic individuals. Electrophysiological and quantitative synaptomics techniques were used to test the hypothesis that heavy drinking produces neuroadaptations in the macaque OFC. Integrative bioinformatics and reverse genetic approaches were used to identify and validate synaptic proteins with novel links to heavy drinking in BXD mice. In drinking monkeys, evoked firing of OFC pyramidal neurons was reduced, whereas the amplitude and frequency of postsynaptic currents were enhanced compared with controls. Bath application of alcohol reduced evoked firing in neurons from control monkeys, but not drinking monkeys. Profiling of the OFC synaptome identified alcohol-sensitive proteins that control glutamate release (e.g., SV2A, synaptogyrin-1) and postsynaptic signaling (e.g., GluA1, PRRT2) with no changes in synaptic GABAergic proteins. Western blot analysis confirmed the increase in GluA1 expression in drinking monkeys. An exploratory analysis of the OFC synaptome found cross-species genetic links to alcohol intake in discrete proteins (e.g., C2CD2L, DIRAS2) that discriminated between low- and heavy-drinking monkeys. Validation studies revealed that BXD mouse strains with the D allele at the C2cd2l interval drank less alcohol than B allele strains. Thus, by profiling of the OFC synaptome, we identified changes in proteins controlling glutamate release and postsynaptic signaling and discovered several proteins related to heavy drinking that have potential as novel targets for treating alcohol use disorder.SIGNIFICANCE STATEMENT Clinical research identified cognitive deficits in alcoholic individuals as a risk factor for relapse, and alcoholic individuals display deficits on cognitive tasks that are dependent upon the orbitofrontal cortex (OFC). To identify neurobiological mechanisms that underpin OFC dysfunction, this study used electrophysiology and integrative synaptomics in a translational nonhuman primate model of heavy alcohol consumption. We found adaptations in synaptic proteins that control glutamatergic signaling in chronically drinking monkeys. Our functional genomic exploratory analyses identified proteins with genetic links to alcohol and cocaine intake across mice, monkeys, and humans. Future work is necessary to determine whether targeting these novel targets reduces excessive and harmful levels of alcohol drinking.

Differential potassium channel gene regulation in BXD mice reveals novel targets for pharmacogenetic therapies to reduce heavy alcohol drinking

Alcohol (ethanol) dependence is a chronic relapsing brain disorder partially influenced by genetics and characterized by an inability to regulate harmful levels of drinking. Emerging evidence has linked genes that encode K_V7, K_IR, and K_Ca2 K⁺ channels with variation in alcohol-related behaviors in rodents and humans. This led us to experimentally test relations between K⁺ channel genes and escalation of drinking in a chronic-intermittent ethanol (CIE) exposure model of dependence in BXD recombinant inbred strains of mice. Transcript levels for K⁺ channel genes in the prefrontal cortex (PFC) and nucleus accumbens (NAc) covary with voluntary ethanol drinking in a non-dependent cohort. Transcripts that encode K_V7 channels covary negatively with drinking in non-dependent BXD strains. Using a pharmacological approach to validate the genetic findings, C57BL/6J mice were allowed intermittent access to ethanol to establish baseline consumption before they were treated with retigabine, an FDA-approved K_V7 channel positive modulator. Systemic administration significantly reduced drinking, and consistent with previous evidence, retigabine was more effective at reducing voluntary consumption in high-drinking than low-drinking subjects. We evaluated the specific K⁺ channel genes that were most sensitive to CIE exposure and identified a gene subset in the NAc and PFC that were dysregulated in the alcohol-dependent BXD cohort. CIE-induced modulation of nine genes in the NAc and six genes in the PFC covaried well with the changes in drinking induced by ethanol dependence. Here we identified novel candidate genes in the NAc and PFC that are regulated by ethanol dependence and correlate with voluntary drinking in non-dependent and dependent BXD mice. The findings that Kcnq expression correlates with drinking and that retigabine reduces consumption suggest that K_V7 channels could be pharmacogenetic targets to treat individuals with alcohol addiction.

Deficits in the extinction of ethanol-seeking behavior following chronic intermittent ethanol exposure are attenuated with positive allosteric modulation of mGlu5

Alcoholism is a chronic relapsing disorder characterized by periods of heavy alcohol consumption and unsuccessful attempts at abstinence. Relapse is one of the most problematic aspects in the treatment of alcoholism and is triggered by ethanol-associated cues. Extinction-based cue exposure therapies have proven ineffective in the treatment of alcoholism. However, positive allosteric modulation of mGlu5 with CDPPB enhances the extinction learning of alcohol-seeking behavior. The current study investigated the impact of chronic alcohol exposure on the extinction of ethanol-seeking behavior. Adult Wistar rats were trained to self-administer alcohol with a light/tone stimulus serving as the alcohol cue. After training, one group of rats was exposed to chronic intermittent ethanol (CIE) daily for a period of 2 weeks to induce ethanol dependence. Control rats were exposed to air for the same period of time. Both groups were then retrained to self-administer ethanol and subsequently tested for changes in extinction learning. CIE exposed rats consumed more ethanol compared to their pre-CIE levels and to control rats. During extinction training, CIE rats responded significantly more on the previously active lever and required more sessions to reach extinction criteria compared to control rats. Treatment with CDPPB facilitated extinction in control rats and attenuated the increased resistance to extinction in CIE-exposed rats. These results demonstrate that chronic ethanol exposure not only alters ethanol intake, but also the extinction of ethanol-seeking behaviors. The ability to attenuate deficits through modulation of mGlu5 provides a potential target for pharmacological manipulation that could ultimately reduce relapse in alcoholics.