Hippocampal-dependent learning and experience-dependent activation of the hippocampus are preferentially disrupted by ethanol

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

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

Mechanisms of alcohol influence on fear conditioning: a computational model

A connection between stress-related illnesses and alcohol use disorders is extensively documented. Fear conditioning is a standard procedure used to study stress learning and links it to the activation of amygdala circuitry. However, the connection between the changes in amygdala circuit and function induced by alcohol and fear conditioning is not well established. We introduce a computational model to test the mechanistic relationship between amygdala functional and circuit adaptations during fear conditioning and the impact of acute vs. repeated alcohol exposure. In accordance with experiments, both acute and prior repeated alcohol decreases speed and robustness of fear extinction in our simulations. The model predicts that, first, the delay in fear extinction in alcohol is mostly induced by greater activation of the basolateral amygdala (BLA) after fear acquisition due to alcohol-induced modulation of synaptic weights. Second, both acute and prior repeated alcohol shifts the amygdala network away from the robust extinction regime by inhibiting the activity in the central amygdala (CeA). Third, our model predicts that fear memories formed in acute or after chronic alcohol are more connected to the context. Thus, the model suggests how circuit changes induced by alcohol may affect fear behaviors and provides a framework for investigating the involvement of multiple neuromodulators in this neuroadaptive process.

Behavioral changes and dendritic remodeling of hippocampal neurons in adolescent alcohol-treated rats

Objective: Earlier, we and others have reported that alcohol exposure in adolescent rat impaired performance of a spatial memory task in the Morris water maze. The goal of the present study was to investigate the effects of acute adolescent alcohol treatment on the hippocampus-dependent (contextual fear conditioning) and hippocampus-independent (cued fear) memories. The study also looked at the structural changes in anterior CA1 hippocampal neurons in adolescent alcohol-treated rats. Methods: Adolescent female rats were administered with a single dose of alcohol (1.0, 1.5, or 2.0 g/kg) or vehicle either before training (pre-training) or after training (pre-testing). Experimental and control rats were trained in the fear conditioning paradigm, and 24 h later tested for both contextual fear conditioning as well as cued fear memory. Separate groups of rats were treated with either alcohol (2 g/kg) or vehicle and sacrificed 24 h later. Their brains were harvested and processed for rapid Golgi staining. Randomly selected CA1 pyramidal neurons were analyzed for dendritic branching and dendritic spine density. Results: Pre-training alcohol dose-dependently attenuated acquisition of hippocampus-dependent contextual fear conditioning but had no effect on the acquisition of amygdala-associated cued fear. When administered following training (pre-testing), alcohol did not alter either contextual conditioning or cued fear memory. Golgi stained CA1 pyramidal neurons in alcohol treated female rats had reduced basilar tree branching and less complex dendritic arborization. Conclusion: Alcohol specifically impaired hippocampal learning in adolescent rats but not amygdala-associated cued fear memory. Compared to vehicle-treated rats, CA1 hippocampal pyramidal neurons in alcohol-treated rats had less complex dendritic morphology. Together, these data suggest that adolescent alcohol exposure produces changes in the neuronal organization of the hippocampus, and these changes may be related to impairments in hippocampus-dependent memory formation.

Examining the engram encoding specificity hypothesis in mice

According to the encoding specificity hypothesis, memory is best recalled by retrieval cues that overlap with training cues. Human studies generally support this hypothesis. However, memories are thought to be stored in neuronal ensembles (engrams), and retrieval cues are thought to reactivate neurons in an engram to induce memory recall. Here, we visualized engrams in mice to test whether retrieval cues that overlap with training cues produce maximal memory recall via high engram reactivation (engram encoding specificity hypothesis). Using variations of cued threat conditioning (pairing conditioned stimulus [CS] with footshock), we manipulated encoding and retrieval conditions along multiple domains, including pharmacological state, external sensory cue, and internal optogenetic cue. Maximal engram reactivation and memory recall occurred when retrieval conditions closely matched training conditions. These findings provide a biological basis for the encoding specificity hypothesis and highlight the important interaction between stored information (engram) and cues available at memory retrieval (ecphory).

Using animal models to identify clinical risk factors in the older population due to alcohol use and misuse

The number of people over the age of 65 years old is increasing and understanding health risks associated with the aged population is important. Recent research has revealed that alcohol (ethanol) consumption levels in older demographics remains elevated and often occurs in a dangerous binge pattern. Given ethical constraints on investigating high level or binge pattern alcohol consumption in humans, animal models are often used to study the effects of ethanol. The current review highlights ongoing work revealing that aged rats are often more sensitive to the effects of acute ethanol compared to younger rats. Specifically, aged rats are more sensitive to the motor impairing, hypnotic, hypothermic, and often the cognitive effects of ethanol compared to younger rats. In addition, the development of ethanol tolerance following chronic exposure may have a different temporal pattern in aged rats compared to younger rats. However, the neurobiological mechanisms that cause the increased sensitivity to ethanol in aged animals have yet to be identified. Furthermore, the differential age effects of ethanol highlight clinical risk factors for alcohol misuse in the older human population. Future work is needed to determine underlying CNS mechanisms producing altered effects of ethanol in aged subjects and also the development of educational material concerning ethanol's effects across ages for health care providers working with the aged population.

Age modifies the effect of ethanol on behavior: Investigations in adolescent, adult and aged rats

The number of older people is increasing in most if not all countries in the world. In addition, the amount of alcohol consumption in the aged population is increasing and the consumption pattern is often in a binge fashion. However, little is known if the effects of alcohol, either acute or chronic exposure, vary in the older population compared to younger populations. The current mini-review will provide an overview of the effects of acute and chronic ethanol exposure at three different periods of development: adolescent, adult and aged on multiple different commonly studied behaviors. The overall conclusion is that biological age of the subject is a critical factor in understanding the effects of ethanol across the lifespan.

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

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

Male, but not female, Sprague Dawley rats display enhanced fear learning following acute ethanol withdrawal (hangover)

The present studies investigated the effects of withdrawal from a single binge-like dose of ethanol (hangover) on fear conditioning in male and female Sprague Dawley rats. In Experiment 1, males and females were given 0 or 3.5 g/kg ethanol intraperitoneally (i.p.) and then conditioned to contextual fear 24 h post injection. Withdrawal from acute ethanol enhanced expression of the conditioned freezing response in males, but not in females. Experiment 2 demonstrated that in males, withdrawal from acute ethanol administered 24 h prior to conditioning enhanced contextual fear conditioning, but not auditory-cued fear conditioning. In Experiment 3, male and female rats were given 3.5 g/kg ethanol, and blood ethanol concentrations (BECs) were assessed at various time points for determination of ethanol clearance. Female rats cleared ethanol at a higher rate than males, with 10 h required for females and 14 for males to eliminate ethanol from their systems. Because females cleared ethanol faster than males, in Experiment 4, females were conditioned 18 h after ethanol administration to keep the interval between ethanol clearance and fear conditioning similar to that of males. Withdrawal from acute ethanol given 18 h prior to conditioning did not affect both contextual and auditory-cued fear conditioning in females. In summary, these results highlight sex differences in the impact of withdrawal from acute ethanol (hangover) on fear learning; suggesting that males are more sensitive to hangover-associated enhancement of negative affect than females.

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

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

Zebrafish exhibit associative learning for an aversive robotic stimulus

Zebrafish have quickly emerged as a species of choice in preclinical research, holding promise to advance the field of behavioral pharmacology through high-throughput experiments. Besides biological and heuristic considerations, zebrafish also constitute a fundamental tool that fosters the replacement of mammals with less sentient experimental subjects. Notwithstanding these features, experimental paradigms to investigate emotional and cognitive domains in zebrafish are still limited. Studies on emotional memories have provided sound methodologies to investigate fear conditioning in zebrafish, but these protocols may still benefit from a reconsideration of the independent variables adopted to elicit aversion. Here, we designed a fear-conditioning paradigm in which wild-type zebrafish were familiarized over six training sessions with an empty compartment and a fear-eliciting one. The fearful stimulus was represented by three zebrafish replicas exhibiting a fully synchronized and polarized motion as they were maneuvered along 3D trajectories by a robotic platform. When allowed to freely swim between the two compartments in the absence of the robotic stimulus (test session), zebrafish displayed a marked avoidance of the stimulus-paired one. To investigate whether fear conditioning was modulated by psychoactive compounds, two groups of zebrafish were administered ethanol (0.25% and 1.00%, ethanol/water, by volume) a few minutes before the test session. We observed that ethanol administration abolished the conditioned avoidance of the stimulus-paired compartment. Ultimately, this study confirms that robotic stimuli may be used in the design of fear-conditioning paradigms, which are sensitive to pharmacological manipulations.

Effect of Alcohol on Hippocampal-Dependent Plasticity and Behavior: Role of Glutamatergic Synaptic Transmission

Problematic alcohol drinking and alcohol dependence are an increasing health problem worldwide. Alcohol abuse is responsible for approximately 5% of the total deaths in the world, but addictive consumption of it has a substantial impact on neurological and memory disabilities throughout the population. One of the better-studied brain areas involved in cognitive functions is the hippocampus, which is also an essential brain region targeted by ethanol. Accumulated evidence in several rodent models has shown that ethanol treatment produces cognitive impairment in hippocampal-dependent tasks. These adverse effects may be related to the fact that ethanol impairs the cellular and synaptic plasticity mechanisms, including adverse changes in neuronal morphology, spine architecture, neuronal communication, and finally an increase in neuronal death. There is evidence that the damage that occurs in the different brain structures is varied according to the stage of development during which the subjects are exposed to ethanol, and even much earlier exposure to it would cause damage in the adult stage. Studies on the cellular and cognitive deficiencies produced by alcohol in the brain are needed in order to search for new strategies to reduce alcohol neuronal toxicity and to understand its consequences on memory and cognitive performance with emphasis on the crucial stages of development, including prenatal events to adulthood.

Acute alcohol and cognition: Remembering what it causes us to forget

Addiction has been conceptualized as a specific form of memory that appropriates typically adaptive neural mechanisms of learning to produce the progressive spiral of drug-seeking and drug-taking behavior, perpetuating the path to addiction through aberrant processes of drug-related learning and memory. From that perspective, to understand the development of alcohol use disorders, it is critical to identify how a single exposure to alcohol enters into or alters the processes of learning and memory, so that involvement of and changes in neuroplasticity processes responsible for learning and memory can be identified early. This review characterizes the effects produced by acute alcohol intoxication as a function of brain region and memory neurocircuitry. In general, exposure to ethanol doses that produce intoxicating effects causes consistent impairments in learning and memory processes mediated by specific brain circuitry, whereas lower doses either have no effect or produce a facilitation of memory under certain task conditions. Therefore, acute ethanol does not produce a global impairment of learning and memory, and can actually facilitate particular types of memory, perhaps particular types of memory that facilitate the development of excessive alcohol use. In addition, the effects on cognition are dependent on brain region, task demands, dose received, pharmacokinetics, and tolerance. Additionally, we explore the underlying alterations in neurophysiology produced by acute alcohol exposure that help to explain these changes in cognition and highlight future directions for research. Through understanding the impact that acute alcohol intoxication has on cognition, the preliminary changes potentially causing a problematic addiction memory can better be identified.

Conditioning the neuroimmune response to ethanol using taste and environmental cues in adolescent and adult rats

IMPACT STATEMENT

A combined odor and taste cue was paired with a binge-like ethanol exposure (4 g/kg intraperitoneal) using a single-trial learning paradigm. Re-exposure to the CS alone was sufficient to evoke a conditioned Interleukin (IL)-6 elevation in the amygdala in adolescents, an effect that was not observed in young adults. This demonstrates a particular sensitivity of adolescents to alcohol-associated cues and neuroimmune learning, whereas prior work indicated that adults require multiple pairings of ethanol to the CS in order to achieve a conditioned amygdala IL-6 response. While the role of immune conditioning has been studied in other drugs of abuse, these findings highlight a previously unknown aspect of alcohol-related learning. Given the emergent importance of the neuroimmune system in alcohol abuse, these findings may be important for understanding cue-induced reinstatement of alcohol intake among problem drinkers.

Rapid reacquisition of contextual fear following extinction in mice: effects of amount of extinction, acute ethanol withdrawal, and ethanol intoxication

RATIONALE

Many studies have found that ethanol intoxication and withdrawal impair initial acquisition or extinction of learned behaviors. Rapid reconditioning following extinction is a form of post-extinction re-emergence of conditioned behavior that has not been studied for its interaction with ethanol intoxication or withdrawal.

OBJECTIVES

The goals of this paper were to define the parameters that allow rapid post-extinction reacquisition of fear in mice and investigate the effect of acute ethanol withdrawal and intoxication on acquisition, extinction, and post-extinction reconditioning.

METHODS

We examined acquisition, extinction, and post-extinction reconditioning of contextual fear in male C57BL/6 mice. Acute ethanol withdrawal occurred 6 h following a 4 g/kg injection of 20% ethanol and acute ethanol intoxication occurred 5 min following a 1.5 g/kg injection of 20% ethanol.

RESULTS

A weak context-shock pairing caused rapid reacquisition of conditioned freezing following moderate, but not extensive extinction. Acute ethanol intoxication impaired initial conditioning and acute ethanol withdrawal impaired rapid reacquisition after extinction, but not reconditioning or extinction itself.

CONCLUSIONS

These findings show that rapid reconditioning occurs following moderate but not extensive extinction in C57BL/6J mice. Additionally, acute ethanol withdrawal and intoxication may differentially affect different phases of conditioning. Results are discussed in terms of current ideas about post-extinction behavior and ethanol's effects on memory.

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

Adult male and female GFAP-TK transgenic rats experienced six weeks of chronic intermittent ethanol vapor inhalation (CIE). During the last week of CIE, a subset of male and female TK rats were fed with Valcyte to ablate neural progenitor cells (NPCs). Seventy-two hours after CIE cessation, all CIE and age-matched ethanol naïve controls experienced auditory trace fear conditioning (TFC). Twenty-four hours later all animals were tested for cue-mediated retrieval in the fear context. Adult male CIE rats showed a significant burst in NPCs paralleled by reduction in fear retrieval compared to naïve controls and Valcyte treated CIE rats. Adult female CIE rats did not show a burst in NPCs and showed similar fear retrieval compared to naïve controls and Valcyte treated CIE rats, indicating that CIE-mediated impairment in fear memory and its regulation by NPCs was sex dependent. Valcyte significantly reduced Ki-67 and NeuroD labeled cells in the dentate gyrus (DG) in both sexes, demonstrating a role for NPCs in reduced fear retrieval in males. Valcyte prevented adaptations in GluN2A receptor expression and synaptoporin density in the DG in males, indicating that NPCs contributed to alterations in plasticity-related proteins and mossy fiber projections that were associated with reduced fear retrieval. These data suggest that DG NPCs born during withdrawal and early abstinence from CIE are aberrant, and could play a role in weakening long-term memory consolidation dependent on the hippocampus.

Transgenic mice with increased astrocyte expression of CCL2 show altered behavioral effects of alcohol

Emerging research provides strong evidence that activation of CNS glial cells occurs in neurological diseases and brain injury and results in elevated production of neuroimmune factors. These factors can contribute to pathophysiological processes that lead to altered CNS function. Recently, studies have also shown that both acute and chronic alcohol consumption can produce activation of CNS glial cells and the production of neuroimmune factors, particularly the chemokine ligand 2 (CCL2). The consequences of alcohol-induced increases in CCL2 levels in the CNS have yet to be fully elucidated. Our studies focus on the hypothesis that increased levels of CCL2 in the CNS produce neuroadaptive changes that modify the actions of alcohol on the CNS. We utilized behavioral testing in transgenic mice that express elevated levels of CCL2 to test this hypothesis. The increased level of CCL2 in the transgenic mice involves increased astrocyte expression. Transgenic mice and their non-transgenic littermate controls were subjected to one of two alcohol exposure paradigms, a two-bottle choice alcohol drinking procedure that does not produce alcohol dependence or a chronic intermittent alcohol procedure that produces alcohol dependence. Several behavioral tests were carried out including the Barnes maze, Y-maze, cued and contextual conditioned fear test, light-dark transfer, and forced swim test. Comparisons between alcohol naïve, non-dependent, and alcohol-dependent CCL2 transgenic and non-transgenic mice show that elevated levels of CCL2 in the CNS interact with alcohol in tests for alcohol drinking, spatial learning, and associative learning.

New Implications for the Melanocortin System in Alcohol Drinking Behavior in Adolescents: The Glial Dysfunction Hypothesis

Alcohol dependence causes physical, social, and moral harms and currently represents an important public health concern. According to the World Health Organization (WHO), alcoholism is the third leading cause of death worldwide, after tobacco consumption and hypertension. Recent epidemiologic studies have shown a growing trend in alcohol abuse among adolescents, characterized by the consumption of large doses of alcohol over a short time period. Since brain development is an ongoing process during adolescence, short- and long-term brain damage associated with drinking behavior could lead to serious consequences for health and wellbeing. Accumulating evidence indicates that alcohol impairs the function of different components of the melanocortin system, a major player involved in the consolidation of addictive behaviors during adolescence and adulthood. Here, we hypothesize the possible implications of melanocortins and glial cells in the onset and progression of alcohol addiction. In particular, we propose that alcohol-induced decrease in α-MSH levels may trigger a cascade of glial inflammatory pathways that culminate in altered gliotransmission in the ventral tegmental area and nucleus accumbens (NAc). The latter might potentiate dopaminergic drive in the NAc, contributing to increase the vulnerability to alcohol dependence and addiction in the adolescence and adulthood.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

Behavioral sensitization to ethanol: Neural basis and factors that influence its acquisition and expression

Ethanol-induced behavioral sensitization (EBS) was first described in 1980, approximately 10 years after the phenomenon was described for psychostimulants. Ethanol acts on γ-aminobutyric acid (GABA) and glutamate receptors as an allosteric agonist and antagonist, respectively, but it also affects many other molecular targets. The multiplicity of factors involved in the behavioral and neurochemical effects of ethanol and the ensuing complexity may explain much of the apparent disparate results, found across different labs, regarding ethanol-induced behavioral sensitization. Although the mesocorticolimbic dopamine system plays an important role in EBS, we provide evidence of the involvement of other neurotransmitter systems, mainly the glutamatergic, GABAergic, and opioidergic systems. This review also analyses the neural underpinnings (e.g., induction of cellular transcription factors such as cyclic adenosine monophosphate response element binding protein and growth factors, such as the brain-derived neurotrophic factor) and other factors that influence the phenomenon, including age, sex, dose, and protocols of drug administration. One of the reasons that make EBS an attractive phenomenon is the assumption, firmly based on empirical evidence, that EBS and addiction-related processes have common molecular and neural basis. Therefore, EBS has been used as a model of addiction processes. We discuss the association between different measures of ethanol-induced reward and EBS. Parallels between the pharmacological basis of EBS and acute motor effects of ethanol are also discussed.

Brain plasticity and cognitive functions after ethanol consumption in C57BL/6J mice

Acute or chronic administrations of high doses of ethanol in mice are known to produce severe cognitive deficits linked to hippocampal damage. However, we recently reported that chronic and moderate ethanol intake in C57BL/6J mice induced chromatin remodeling within the Bdnf promoters, leading to both enhanced brain-derived neurotrophic factor (BDNF) expression and hippocampal neurogenesis under free-choice protocol. We performed here a series of cellular and behavioral studies to analyze the consequences of these modifications. We showed that a 3-week chronic free-choice ethanol consumption in C57BL/6J mice led to a decrease in DNA methylation of the Bdnf gene within the CA1 and CA3 subfields of the hippocampus, and upregulated hippocampal BDNF signaling pathways mediated by ERK, AKT and CREB. However, this activation did not affect long-term potentiation in the CA1. Conversely, ethanol intake impaired learning and memory capacities analyzed in the contextual fear conditioning test and the novel object recognition task. In addition, ethanol increased behavioral perseveration in the Barnes maze test but did not alter the mouse overall spatial capacities. These data suggested that in conditions of chronic and moderate ethanol intake, the chromatin remodeling leading to BDNF signaling upregulation is probably an adaptive process, engaged via epigenetic regulations, to counteract the cognitive deficits induced by ethanol.

Learning by subtraction: Hippocampal activity and effects of ethanol during the acquisition and performance of response sequences

Learning is believed to be reflected in the activity of the hippocampus. However, neural correlates of learning have been difficult to characterize because hippocampal activity is integrated with ongoing behavior. To address this issue, male rats (n = 5) implanted with electrodes (n = 14) in the CA1 subfield responded during two tasks within a single test session. In one task, subjects acquired a new 3-response sequence (acquisition), whereas in the other task, subjects completed a well-rehearsed 3-response sequence (performance). Both tasks though could be completed using an identical response topography and used the same sensory stimuli and schedule of reinforcement. More important, comparing neural patterns during sequence acquisition to those during sequence performance allows for a subtractive approach whereby activity associated with learning could potentially be dissociated from the activity associated with ongoing behavior. At sites where CA1 activity was closely associated with behavior, the patterns of activity were differentially modulated by key position and the serial position of a response within the schedule of reinforcement. Temporal shifts between peak activity and responding on particular keys also occurred during sequence acquisition, but not during sequence performance. Ethanol disrupted CA1 activity while producing rate-decreasing effects in both tasks and error-increasing effects that were more selective for sequence acquisition than sequence performance. Ethanol also produced alterations in the magnitude of modulations and temporal pattern of CA1 activity, although these effects were not selective for sequence acquisition. Similar to ethanol, hippocampal micro-stimulation decreased response rate in both tasks and selectively increased the percentage of errors during sequence acquisition, and provided a more direct demonstration of hippocampal involvement during sequence acquisition. Together, these results strongly support the notion that ethanol disrupts sequence acquisition by disrupting hippocampal activity and that the hippocampus is necessary for the conditioned associations required for sequence acquisition.

Extinction learning is slower, weaker and less context specific after alcohol

Alcohol is frequently involved in psychological trauma and often used by individuals to reduce fear and anxiety. We examined the effects of alcohol on fear acquisition and extinction within a virtual environment. Healthy volunteers were administered alcohol (0.4 g/kg) or placebo and underwent acquisition and extinction from different viewpoints of a virtual courtyard, in which the conditioned stimulus, paired with a mild electric shock, was centrally located. Participants returned the following day to test fear recall from both viewpoints of the courtyard. Skin conductance responses were recorded as an index of conditioned fear. Successful fear acquisition under alcohol contrasted with impaired extinction learning evidenced by persistent conditioned responses (Experiment 1). Participants’ impairments in extinction under alcohol correlated with impairments in remembering object-locations in the courtyard seen from one viewpoint when tested from the other viewpoint. Alcohol-induced extinction impairments were overcome by increasing the number of extinction trials (Experiment 2). However, a test of fear recall the next day showed persistent fear in the alcohol group across both viewpoints. Thus, alcohol impaired extinction rather than acquisition of fear, suggesting that extinction is more dependent than acquisition on alcohol-sensitive representations of spatial context. Overall, extinction learning under alcohol was slower, weaker and less context-specific, resulting in persistent fear at test that generalized to the extinction viewpoint. The selective effect on extinction suggests an effect of alcohol on prefrontal involvement, while the reduced context-specificity implicates the hippocampus. These findings have important implications for the use of alcohol by individuals with clinical anxiety disorders.

Adolescent and adult rats differ in the amnesic effects of acute ethanol in two hippocampus-dependent tasks: Trace and contextual fear conditioning

Experience-produced deficits in trace conditioning and context conditioning have been useful tools for examining the role of the hippocampus in learning. It has also been suggested that learning in these tasks is especially vulnerable to neurotoxic effects of alcohol during key developmental periods such as adolescence. In five experiments we systematically examined the presence and source of age-dependent vulnerability to the memory-disrupting effects of acute ethanol in trace conditioning and contextual fear conditioning. In Experiment 1a pre-training ethanol disrupted trace conditioning more strongly in adolescent (postnatal day, PD30-35) than adult rats (PD65-75). In Experiment 1b when pre-training ethanol was accompanied by pre-test ethanol no deficit in trace conditioning was observed in adolescents, suggesting that state-dependent retrieval failure mediated ethanol's disruption of trace conditioning at this age. Experiment 2a and b examined the effect of ethanol pretreatment on context conditioning. Here, adult but not adolescent rats were impaired in conditioned freezing to context cues. Experiment 2c explored state-dependency of this effect. Pre-training ethanol continued to disrupt context conditioning in adults even when ethanol was also administered prior to test. Collectively these findings reveal clear age-dependent and task-dependent vulnerabilities in ethanol's disruptive effects on hippocampus-dependent memory. Adolescents were more disrupted by ethanol in trace conditioning than adults, and adults were more disrupted by ethanol in context conditioning than adolescents. We suggest that adolescents may be more susceptible to changes in internal state (state-dependent retrieval failure) than adults and that ethanol disrupted performance in trace and context conditioning through different mechanisms. Relevance of these findings to theories of hippocampus function is discussed.

Severe diffuse axon injury in chronic alcoholic rat medulla oblongata following a concussion blow

AIMS

We investigated the axonal morphological changes and expression of both tau protein and β-APP following concussion to the medulla oblongata, in a rat model of chronic alcoholism.

METHODS

Fifty-nine male Sprague-Dawley rats were randomly divided into EtOH, EtOH-TBI and control groups (water group, water-TBI group). To establish chronic alcoholic rats, rats were intragastrically given edible spirituous liquor twice daily. Rats also received a blow on the occipital tuberosity with an iron pendulum. Morphological changes and expression of tau and β-APP proteins in the medulla oblongata were examined.

RESULTS

(a) Nerve fibre thickening and twisting were observed in alcoholic rats, with nerve fibre changes becoming more significant following a concussion blow, which leads to some nerve fibres fracturing. (b) Transmission electron microscopy revealed that the nerve fibre myelin became loosened and displayed lamellar separation, which became more significant following concussion. (c) The integral optical density (IOD) sum value of β-APP of the EtOH-TBI group was lower than that in the EtOH group (P < 0.05); the Tau IOD sum value of the EtOH-TBI group was higher than that in the EtOH group (P < 0.05).

CONCLUSION

(a) Chronic alcoholism caused nerve fibre and neuronal morphology damage in the rat medulla oblongata, with structural damage becoming more significant following concussion. (b) Concussion changed the expression of β-APP and tau protein in chronic alcoholic rat medulla oblongata, suggesting that chronic alcoholism can lead to severe axonal injury following a concussion blow. (c) The effect of chronic alcoholism may be synergistic the concussion blow to promote animal injury and death.

Consequences of adolescent or adult ethanol exposure on tone and context fear retention: effects of an acute ethanol challenge during conditioning

BACKGROUND

An acute ethanol (EtOH) challenge prior to fear conditioning typically disrupts fear retention to contextual cues to a greater degree than fear retention to a discrete tone cue, and adolescent rats are less sensitive than adults to these EtOH-induced disruptions of context fear memory. Given that some research suggests that repeated EtOH exposure during adolescence may "lock-in" adolescent-typical EtOH sensitivity into adulthood, the purpose of this study was to determine whether adults exposed to EtOH as adolescents would be less sensitive to EtOH-induced disruptions of context fear.

METHODS

Male Sprague-Dawley rats were given 4 g/kg intragastric EtOH (25% v/v) or water every 48 hours for a total of 11 exposures during adolescence (postnatal day [P] 28 to 48) or adulthood (P70-90). After a 22-day non-EtOH period, animals were acutely challenged with 1 g/kg intraperitoneal EtOH or saline 10 minutes prior to tone or context (noncued) fear conditioning. Tone and context fear retention was subsequently examined.

RESULTS

Regardless of age or exposure history, typical deficits in context fear retention were evident after EtOH challenge during conditioning. Similarly, tone fear retention was disrupted in all animals that were trained in the presence of EtOH, which was somewhat surprising given the relative resistance of tone fear retention to an acute EtOH challenge.

CONCLUSIONS

These results do not support the notion of a "lock-in" of adolescent-typical EtOH sensitivity as there was no influence of exposure age on sensitivity to the disruptive effects of an acute EtOH challenge. Thus, it appears that not all adolescent-like EtOH sensitivities persist into adulthood after prior EtOH exposure during adolescence.

Substance abuse, memory, and post-traumatic stress disorder

A large body of literature demonstrates the effects of abused substances on memory. These effects differ depending on the drug, the pattern of delivery (acute or chronic), and the drug state at the time of learning or assessment. Substance use disorders involving these drugs are often comorbid with anxiety disorders, such as post-traumatic stress disorder (PTSD). When the cognitive effects of these drugs are considered in the context of the treatment of these disorders, it becomes clear that these drugs may play a deleterious role in the development, maintenance, and treatment of PTSD. In this review, we examine the literature evaluating the cognitive effects of three commonly abused drugs: nicotine, cocaine, and alcohol. These three drugs operate through both common and distinct neurobiological mechanisms and alter learning and memory in multiple ways. We consider how the cognitive and affective effects of these drugs interact with the acquisition, consolidation, and extinction of learned fear, and we discuss the potential impediments that substance abuse creates for the treatment of PTSD.

Consequences of ethanol exposure on cued and contextual fear conditioning and extinction differ depending on timing of exposure during adolescence or adulthood

Some evidence suggests that adolescents are more sensitive than adults to ethanol-induced cognitive deficits and that these effects may be long-lasting. The purpose of Exp 1 was to determine if early-mid adolescent [postnatal day (P) 28-48] intermittent ethanol exposure would affect later learning and memory in a Pavlovian fear conditioning paradigm differently than comparable exposures in adulthood (P70-90). In Exp 2 animals were exposed to ethanol during mid-late adolescence (P35-55) to assess whether age of initiation within the adolescent period would influence learning and memory differentially. Male Sprague-Dawley rats were given 4 g/kg i.g. ethanol (25%) or water every 48 h for a total of 11 exposures. After a 22 day non-ethanol period, animals were fear conditioned to a context (relatively hippocampal-dependent task) or tone (amygdala-dependent task), followed by retention tests and extinction (mPFC-dependent) of this conditioning. Despite similar acquisition, a deficit in context fear retention was evident in animals exposed to ethanol in early adolescence, an effect not observed after a comparable ethanol exposure in mid-late adolescence or adulthood. In contrast, animals that were exposed to ethanol in mid-late adolescence or adulthood showed enhanced resistance to context extinction. Together these findings suggest that repeated ethanol imparts long-lasting consequences on learning and memory, with outcomes that differ depending on age of exposure. These results may reflect differential influence of ethanol on the brain as it changes throughout ontogeny and may have implications for alcohol use not only throughout the developmental period of adolescence, but also in adulthood.

Age differences in fear retention and extinction in male Sprague-Dawley rats: effects of ethanol challenge during conditioning

Pavlovian fear conditioning is an ideal model to investigate how learning and memory are influenced by alcohol use during adolescence because the neural mechanisms involved have been studied extensively. In Exp 1, adolescent and adult male Sprague-Dawley rats were non-injected or injected with saline, 1 or 1.5 g/kg ethanol intraperitoneally 10 min prior to tone or context conditioning. Twenty-four hours later, animals were tested for tone or context retention and extinction, with examination of extinction retention conducted 24h thereafter. In Exp 2, a context extinction session was inserted between the tone conditioning and the tone fear retention/extinction days to reduce pre-CS baseline freezing levels at test. Basal levels of acquisition, fear retention, extinction, and extinction retention after tone conditioning were similar between adolescent and adult rats. In contrast adolescents showed faster context extinction than adults, while again not differing from adults during context acquisition, retention or extinction retention. In terms of ethanol effects, adolescents were less sensitive to ethanol-induced context retention deficits than adults. No age differences emerged in terms of tone fear retention, with ethanol disrupting tone fear retention at both ages in Exp 1, but at neither age in Exp 2, a difference seemingly due to group differences in pre-CS freezing during tone testing in Exp 1, but not Exp 2. These results suggest that age differences in the acute effects of ethanol on cognitive function are task-specific, and provide further evidence for age differences cognitive functioning in a task thought to be hippocampally related.

Synaptic plasticity in the hippocampus shows resistance to acute ethanol exposure in transgenic mice with astrocyte-targeted enhanced CCL2 expression

It has been shown that ethanol exposure can activate astrocytes and microglia resulting in the production of neuroimmune factors, including the chemokine CCL2. The role of these neuroimmune factors in the effects of ethanol on the central nervous system has yet to be elucidated. To address this question, we investigated the effects of ethanol on synaptic transmission and plasticity in the hippocampus from mice that express elevated levels of CCL2 in the brain and their non-transgenic littermate controls. The brains of the transgenic mice simulate one aspect of the alcoholic brain, chronically increased levels of CCL2. We used extracellular field potential recordings in acutely isolated hippocampal slices to identify neuroadaptive changes produced by elevated levels of CCL2 and how these neuroadaptive changes affect the actions of acute ethanol. Results showed that synaptic transmission and the effects of ethanol on synaptic transmission were similar in the CCL2-transgenic and non-transgenic hippocampus. However, long-term potentiation (LTP), a cellular mechanism thought to underlie learning and memory, in the CCL2-transgenic hippocampus was resistant to the ethanol-induced depression of LTP observed in the non-transgenic hippocampus. Consistent with these results, ethanol pretreatment significantly impaired cued and contextual fear conditioning in non-transgenic mice, but had no effect in CCL2-transgenic mice. These data show that chronically elevated levels of CCL2 in the hippocampus produce neuroadaptive changes that block the depressing effects of ethanol on hippocampal synaptic plasticity and support the hypothesis that CCL2 may provide a neuroprotective effect against the devastating actions of ethanol on hippocampal function.

Effects of alcohol on the acquisition and expression of fear-potentiated startle in mouse lines selectively bred for high and low alcohol preference

RATIONALE

Anxiety disorders and alcohol use disorders frequently co-occur in humans perhaps because alcohol relieves anxiety. Studies in humans and rats indicate that alcohol may have greater anxiolytic effects in organisms with increased genetic propensity for high alcohol consumption.

OBJECTIVES AND METHODS

The purpose of this study was to investigate the effects of moderate doses of alcohol (0.5, 1.0, 1.5 g/kg) on the acquisition and expression of anxiety-related behavior using a fear-potentiated startle (FPS) procedure. Experiments were conducted in two replicate pairs of mouse lines selectively bred for high- (HAP1 and HAP2) and low- (LAP1 and LAP2) alcohol preference; these lines have previously shown a genetic correlation between alcohol preference and FPS (HAP > LAP; Barrenha and Chester, Alcohol Clin Exp Res 31:1081-1088, 2007). In a control experiment, the effect of diazepam (4.0 mg/kg) on the expression of FPS was tested in HAP2 and LAP2 mice.

RESULTS

The 1.5 g/kg alcohol dose moderately decreased the expression of FPS in both HAP lines but not LAP lines. Alcohol had no effect on the acquisition of FPS in any line. Diazepam reduced FPS to a similar extent in both HAP2 and LAP2 mice.

CONCLUSIONS

HAP mice may be more sensitive to the anxiolytic effects of alcohol than LAP mice when alcohol is given prior to the expression of FPS. These data collected in two pairs of HAP/LAP mouse lines suggest that the anxiolytic response to alcohol in HAP mice may be genetically correlated with their propensity toward high alcohol preference and robust FPS.

Behavioral pharmacogenetic analysis on the role of the α4 GABA(A) receptor subunit in the ethanol-mediated impairment of hippocampus-dependent contextual learning

BACKGROUND

A major effect of low-dose ethanol is impairment of hippocampus-dependent cognitive function. α4/δ -containing GABA(A) Rs are highly expressed within the dentate gyrus region of the hippocampus where they mediate a tonic inhibitory current that is sensitive to the enhancement by low ethanol concentrations. These receptors are also powerful modulators of learning and memory, suggesting that they could play an important role in ethanol's cognitive impairing effects. The goal of this study was to develop a high-throughput cognitive ethanol assay, amenable to use in genetically modified mice that could be used to test this hypothesis.

METHODS

We developed a procedure where preexposure to a conditioning chamber is used to rescue the "immediate shock deficit." Using this task, ethanol can be specifically targeted at the hippocampus-dependent process of contextual learning without interfering with pain sensitivity or behavioral performance.

RESULTS

Validation of this task in C57BL/6 mice indicated that 1.0 g/kg ethanol and 10 mg/kg allopregnanolone disrupt contextual learning. Ro15-4513 reversed the effects of ethanol but not allopregnanolone, whereas it produced an impairment when given alone. The high-throughput nature of this task allowed for its application in a large cohort of α4 GABA(A) R KO mice. Loss of the α4 GABA(A) R subunit produced an enhanced sensitivity to the cognitive impairing effects of ethanol. This is consistent with the enhanced ethanol sensitivity of synaptic GABA(A) Rs that has been previously observed in the dentate gyrus in these mice, but inconsistent with the reduced ethanol sensitivity of extrasynaptic GABA(A) Rs observed in the same cells.

CONCLUSIONS

Overall, these findings are consistent with our hypothesis that ethanol acts directly at GABA(A) receptors to impair hippocampus-dependent cognitive function. Furthermore, validation of this high-throughput assay will allow for future studies to use anatomically and temporally restricted genetic manipulations to probe more deeply into the neural mechanisms of ethanol action on learning and memory circuits.

Effect of acute ethanol and acute allopregnanolone on spatial memory in adolescent and adult rats

The effects of ethanol differ in adolescent and adult rats on a number of measures. The evidence of the effects of ethanol on spatial memory in adolescents and adults is equivocal. Whether adolescents are more or less sensitive to ethanol-induced impairment of spatial memory acquisition remains unclear; with regard to the effects of acute ethanol on spatial memory retrieval there is almost no research looking into any age difference. Thus, we examined the effects of acute ethanol on spatial memory in the Morris Watermaze in adolescents and adults. Allopregnanolone (ALLO) is a modulator of the GABA(A) receptor and has similar behavioral effects as ethanol. We sought to also determine the effects of allopreganolone on spatial memory in adolescent and adults. Male adolescent (post natal [PN]28-30) and adult (PN70-72) rats were trained in the Morris Watermaze for 6 days and acute doses of ethanol (saline, 1.5 and 2.0 g/kg) or ALLO (vehicle, 9 and 18 mg/kg) were administered on Day 7. A probe trial followed on Day 8. As expected, there were dose effects; higher doses of both ethanol and ALLO impaired spatial memory. However, in both the ethanol and ALLO conditions adolescents and adults had similar spatial memory impairments. The current results suggest that ethanol and ALLO both impair hippocampal-dependent spatial memory regardless of age in that once learning has occurred, ethanol or ALLO does not differentially impair the retrieval of spatial memory in adolescents and adults. Given the mixed results on the effect of ethanol on cognition in adolescent rats, additional research is needed to ascertain the factors critical for the reported differential results.

Varenicline, a partial agonist at neuronal nicotinic acetylcholine receptors, reduces nicotine-induced increases in 20% ethanol operant self-administration in Sprague-Dawley rats

Alcohol and nicotine use disorders are often treated as separate diseases, despite evidence that approximately 80-90% of alcohol dependent individuals are also heavy smokers. Both nicotine and ethanol have been shown to interact with neuronal nicotinic acetylcholine receptors (nAChRs), suggesting these receptors are a common biological target for the effects of nicotine and ethanol in the brain. There are few studies that have examined the effects of co-administered nicotine and ethanol on the activity of nAChRs in rodents. In the present study, we show that Sprague-Dawley rats, a strain often used for nicotine studies but not as often for voluntary ethanol intake studies, will consume 20% ethanol using both the intermittent-access two-bottle-choice and operant self-administration models without the need for sucrose fading. We show that nicotine (0.2 mg/kg and 0.8 mg/kg, s.c.) significantly increases operant 20% ethanol self-administration and varenicline (2 mg/kg, s.c), a partial agonist at nAChRs, significantly decreases operant ethanol self-administration and nicotine-induced increases in ethanol self-administration. This suggests that nAChRs play an important role in increasing ethanol self-administration and that varenicline may be an efficacious treatment for alcohol and nicotine co-dependencies.

Alcohol inhibition of the NMDA receptor function, long-term potentiation, and fear learning requires striatal-enriched protein tyrosine phosphatase

Alcohol's deleterious effects on memory are well known. Acute alcohol-induced memory loss is thought to occur via inhibition of NMDA receptor (NMDAR)-dependent long-term potentiation in the hippocampus. We reported previously that ethanol inhibition of NMDAR function and long-term potentiation is correlated with a reduction in the phosphorylation of Tyr(1472) on the NR2B subunit and ethanol's inhibition of the NMDAR field excitatory postsynaptic potential was attenuated by a broad spectrum tyrosine phosphatase inhibitor. These data suggested that ethanol's inhibitory effect may involve protein tyrosine phosphatases. Here we demonstrate that the loss of striatal-enriched protein tyrosine phosphatase (STEP) renders NMDAR function, phosphorylation, and long-term potentiation, as well as fear conditioning, less sensitive to ethanol inhibition. Moreover, the ethanol inhibition was "rescued" when the active STEP protein was reintroduced into the cells. Taken together, our data suggest that STEP contributes to ethanol inhibition of NMDAR function via dephosphorylation of tyrosine sites on NR2B receptors and lend support to the hypothesis that STEP may be required for ethanol's amnesic effects.

Neuronal nicotinic acetylcholine receptors as pharmacotherapeutic targets for the treatment of alcohol use disorders

Alcohol use disorders (AUDs) are complex, and developing effective treatments will require the combination of novel medications and cognitive behavioral therapy approaches. Epidemiological studies have shown there is a high correlation between alcohol consumption and tobacco use, and the prevalence of smoking in alcoholics is as high as 80% compared to about 30% for the general population. Both preclinical and clinical data provide evidence that nicotine administration increases alcohol intake and non-specific nicotinic receptor antagonists reduce alcohol-mediated behaviors. As nicotine interacts specifically with the neuronal nicotinic acetylcholine receptor (nAChR) system, this suggests that nAChRs play an important role in the behavioral effects of alcohol. In this review, we discuss the importance of nAChRs for the treatment of AUDs and argue that the use of FDA approved nAChR ligands, such as varenicline and mecamylamine, approved as smoking cessation aids may prove to be valuable treatments for AUDs. We also address the importance of combining effective medications with behavioral therapy for the treatment of alcohol dependent individuals.

Alcohol Pathophysiology

There is no specialized alcohol addiction area in the brain; rather, alcohol acts on a wide range of excitatory and inhibitory nervous networks to modulate neurotransmitters actions by binding with and altering the function of specific proteins. With no hemato-encephalic barrier for alcohol, its actions are strongly related to the amount of intake. Heavy alcohol intake is associated with both structural and functional changes in the central nervous system with long-term neuronal adaptive changes contributing to the phenomena of tolerance and withdrawal. The effects of alcohol on the function of neuronal networks are heterogeneous. Because ethanol affects neural activity in some brain sites but is without effect in others, its actions are analyzed in terms of integrated connectivities in the functional circuitry of neuronal networks, which are of particular interest because of the cognitive interactions discussed in the manuscripts contributing to this review. Recent molecular data are reviewed as a support for the other contributions dealing with cognitive disturbances related to alcohol acute and addicted consumption.

Acute ethanol exposure elevates muscarinic tone in the septohippocampal system

The septohippocampal system has been implicated in the cognitive deficits associated with ethanol consumption, but the cellular basis of ethanol action awaits full elucidation. In the medial septum/diagonal band of Broca (MS/DB), a muscarinic tone, reflective of firing activity of resident cholinergic neurons, regulates that of their noncholinergic, putatively GABAergic, counterparts. Here we tested the hypothesis that ethanol alters this muscarinic tone. The spontaneous firing activity of cholinergic and noncholinergic MS/DB neurons were monitored in acute MS/DB slices from C57Bl/6 mice. Exposing the entire slice to ethanol increased firing in both cholinergic and noncholinergic neurons. However, applying ethanol focally to individual MS/DB neurons increased firing only in cholinergic neurons. The differential outcome suggested different mechanisms of ethanol action on cholinergic and noncholinergic neurons. Indeed, with bath-perfused ethanol, the muscarinic antagonist methyl scopolamine prevented the increase in firing in noncholinergic, but not cholinergic, MS/DB neurons. Thus, the effect on noncholinergic neuronal firing was secondary to ethanol's direct action of acutely increasing muscarinic tone. We propose that the acute ethanol-induced elevation of muscarinic tone in the MS/DB contributes to the altered net flow of neuronal activity in the septohippocampal system that underlies compromised cognitive function.

Alcohol selectively reduces anxiety but not fear: startle response during unpredictable versus predictable threat

Recent theory and empirical research have suggested that fear and anxiety are distinct processes with separable neurobiological substrates. Furthermore, a laboratory procedure has been developed to manipulate fear versus anxiety independently via administration of predictable or unpredictable electric shock, respectively. Benzodiazepines appear to selectively reduce anxiety but not fear in this procedure. The primary aim of this experiment was to determine if alcohol produced a similar selective reduction in anxiety. Intoxicated (target blood alcohol concentration of .08%) and nonintoxicated participants viewed a series of colored squares separated by variable intertrial intervals (ITIs) in 3 conditions. In the predictable shock condition, shocks were administered contingently during every square. In the unpredictable shock condition, shocks were administered noncontingently during both squares and ITIs. In the no-shock condition, no shocks were administered at any time. Alcohol significantly reduced startle potentiation during cues signaling unpredictable but not predictable shock, consistent with the thesis that alcohol selectively reduces anxiety but not fear. In addition, alcohol's effect on startle potentiation during unpredictable shock was mediated by vigilance. This anxiolytic effect may clarify the nature of alcohol's reinforcing effects in social and problem drinkers.

Extinction learning of stimulus reward contingencies: The acute effects of alcohol

BACKGROUND

Recent theories suggest that extinction is, at least partly, new learning suppressing original associations between a conditioned stimulus and a conditioned response without severing those associations. During extinction alcohol via its effects on inhibitory control may reduce the ability to suppress the original associations between a conditioned stimulus and a conditioned response leading to an impairment of extinction learning. Thus, the present study is set out to examine the effects of alcohol on extinction learning to enhance current knowledge on mechanisms of extinction and conditions that might hamper extinction, which is an important aspect for the treatment of alcohol-dependent patients.

METHODS

Light to moderate social drinkers (N=32) acquired an instrumental reward seeking response. Extinction training of the reward seeking response was performed after administration of a dose of 0.8 g/kg alcohol resulting in a peak blood alcohol concentration ranging from 112 to 184 mg/dL. In addition, we assessed subjective alcohol effects and administered a Stop-Signal task which measures the ability to inhibit a pre-potent motor response.

RESULTS

Alcohol influenced subjective ratings of light-headedness and increased the Stop-Signal reaction time indicating disinhibiting effects. However, our results did not show any impairment of learning of extinction after the administration of alcohol. Behavioural as well as attentional responses indicated extinction of conditioned responses for both experimental groups.

CONCLUSIONS

These findings suggest that alcohol at a dose that impairs performance in a task of inhibitory control does not impair learning of extinction.

Repeated ethanol exposure affects the acquisition of spatial memory in adolescent female rats

Ethanol has been reported to disrupt spatial learning and memory in adolescent male rats. The present study was undertaken to determine the effects of ethanol on the acquisition of spatial memory in adolescent female rats. Adolescent female rats were subjected to repeated ethanol or saline treatments, and spatial learning was tested in the Morris water maze. For comparison, adult female rats were subjected to similar ethanol treatment and behavioral assessments as for adolescent rats. Ethanol-treated adolescent rats took longer and swam greater distances to find the hidden platform than saline controls. In the probe trial, ethanol-treated adolescent rats showed a trend towards reduced time spent in the target quadrant, and made significantly fewer target location crossings than saline-treated controls. Adult saline-treated control rats did not learn the spatial memory task as well as the adolescent saline-treated rats. Although ethanol in adult rats increased both latency and swim distance to find the platform, in the probe trial there was no difference between ethanol-treated adult rats and age-matched saline controls. Ethanol did not alter swim speed or performance in the cued visual task at either age. Together, these data suggest that ethanol specifically impairs the acquisition of spatial memory in adolescent female rats. Since adult females did not learn the task, ethanol-induced alterations in water maze performance may not reflect true learning and memory dysfunction.

Modulation of ethanol state-dependent learning by dorsal hippocampal NMDA receptors in mice

The possible role of N-methyl-D-aspartate (NMDA) receptors of dorsal hippocampus on ethanol state-dependent learning was studied in adult male mice (Pasteur Institute, Iran). As a model of memory, a single-trial step-down passive avoidance task was used. All animals were bilaterally implanted with cannulae into the CA1 regions of dorsal hippocampi. Results show that intraperitoneal (i.p.) administration of ethanol (0.5 and 1 g/kg) 30 min before training impaired memory performance in animals when tested 24h later. Pretest administration of the same doses of ethanol-induced state-dependent retrieval of the memory acquired under pretraining ethanol (1 g/kg, i.p.) influence. Pretest intra-CA1 microinjection of NMDA (0.001, 0.01, and 0.1 microg/mouse) by itself had no effect on memory retrieval and ethanol-induced amnesia. However, pretest intra-CA1 administration of the same doses of NMDA with an ineffective dose of ethanol (0.25 g/kg, i.p.) significantly restored the retrieval and potentiated ethanol state-dependent learning. On the other hand, pretest administration of a competitive NMDA receptor antagonist D-AP5 (D-(-)-2-Amino-5-phosphonopentanoic acid) (0.01, 0.1, and 1 microg/mouse, intra-CA1) or a noncompetitive NMDA receptor antagonist MK-801 maleate [(5S, 10R)-(+)-5-Methyl-10, 11-dihydro-5H-dibenzo [a, d] cyclohepten-5, 10-imine maleate] (0.25, 0.5, and 1 g/mouse, intra-CA1) 5 min before the administration of ethanol (1 g/kg, i.p.) significantly inhibited ethanol state-dependent learning. Intra-CA1 pretest administration of D-AP5 (0.01, 0.1, and 1 microg/mouse) or MK-801 maleate [5S, 10R)-(+)-5-Methyl-10, 11-dihydro-5H-dibenzo [a, d] cyclohepten-5, 10-imine maleate] (0.25, 0.5, and 1 microg/mouse) alone did not affect memory retention. It may be concluded that dorsal hippocampal NMDA receptors are involved in mediating ethanol state-dependent learning.

Alcohol-induced retrograde memory impairment in rats: prevention by caffeine

RATIONALE

Ethanol and caffeine are two of the most widely consumed drugs in the world, often used in the same setting. Animal models may help to understand the conditions under which incidental memories formed just before ethanol intoxication might be lost or become difficult to retrieve.

OBJECTIVES

Ethanol-induced retrograde amnesia was investigated using a new odor-recognition test.

MATERIALS AND METHODS

Rats thoroughly explored a wood bead taken from the cage of another rat, and habituated to this novel odor (N1) over three trials. Immediately following habituation, rats received saline, 25 mg/kg pentylenetetrazol (a seizure-producing agent known to cause retrograde amnesia) to validate the test, 1.0 g/kg ethanol, or 3.0 g/kg ethanol. The next day, they were presented again with N1 and also a bead from a new rat's cage (N2).

RESULTS

Rats receiving saline or the lower dose of ethanol showed overnight memory for N1, indicated by preferential exploration of N2 over N1. Rats receiving pentylenetetrazol or the higher dose of ethanol appeared not to remember N1, in that they showed equal exploration of N1 and N2. Caffeine (5 mg/kg), delivered either 1 h after the higher dose of ethanol or 20 min prior to habituation to N1, negated ethanol-induced impairment of memory for N1. A combination of a phosphodiesterase-5 inhibitor and an adenosine A(2A) antagonist, mimicking two major mechanisms of action of caffeine, likewise prevented the memory impairment, though either drug alone had no such effect. Binge alcohol can induce retrograde, caffeine-reversible disruption of social odor memory storage or recall.

Post-training ethanol disrupts trace conditioned fear in rats: effects of timing of ethanol, dose and trace interval duration

Ethanol has complex effects on memory performance, although hippocampus-dependent memory may be especially vulnerable to disruption by acute ethanol intoxication occurring during or shortly after a training episode. In the present experiments, the effects of post-training ethanol on delay and trace fear conditioning were examined in adolescent rats. In Experiment 1, 30-day-old Sprague-Dawley rats were given delay or trace conditioning trials in which a 10s flashing light CS was paired with a 0.5 mA shock US. For trace groups, the trace interval was 10 s. On days 31-33, animals were administered ethanol once daily (0.0 or 2.5 g/kg via intragastric intubation), and on day 34 animals were tested for CS-elicited freezing. Results showed that post-training ethanol affected the expression of trace, but had no effect on delay conditioned fear. Experiment 2 revealed that this effect was dose-dependent; doses lower than 2.5 g/kg were without effect. Experiment 3 evaluated whether proximity of ethanol to the time of training or testing was critical. Results show that ethanol administration beginning 24h after training was more detrimental to trace conditioned freezing than administration that was delayed by 48 h. Finally, in Experiment 4 animals were trained with one of three different trace intervals: 1, 3 or 10s. Results indicate that post-training administration of 2.5 g/kg ethanol disrupted trace conditioned fear in subjects trained with a 10s, but not with a 1 or 3s, trace interval. Collectively the results suggest that ethanol administration impairs post-acquisition memory processing of hippocampus-dependent trace fear conditioning.

Ethanol enhances reactivated fear memories

Although ethanol has been shown to impair acquisition of memory, its effect on consolidated memories is not clear. Recent reports revealed that memory retrieval converted consolidated memory into a labile state and initiated the reconsolidation process. In the present study, we have demonstrated the effect of ethanol on reactivated fear memory. We used contextual fear conditioning where rats were conditioned with mild footshock, re-exposed to the training context for 2 min, immediately injected with ethanol or saline, and finally tested 48 h after re-exposure. Ethanol-treated groups demonstrated longer freezing and the effect lasted for 2 weeks. Reactivation is necessary for this effect. Injection of ethanol itself did not induce a fearful response. Reactivated and ethanol-treated rats exhibited longer freezing than non-reactivated controls, suggesting that ethanol does not inhibit the memory decline but facilitates the fear memory. Two minute re-exposures induced no or little extinction. The effect of ethanol was specific for 2-min reactivation, which induces reconsolidation. Moreover, we found that picrotoxin inhibited the memory enhancement that was produced by ethanol administered just after the reactivation. These studies demonstrate that ethanol enhances reactivated contextual fear memories via activation of GABA(A) receptors.