Genetics, Ethanol and the Fos Response: A Comparison of the C57BL/6J and DBA/2J Inbred Mouse Strains

Dynamic c-Fos changes in mouse brain during acute and protracted withdrawal from chronic intermittent ethanol exposure and relapse drinking

Alcohol dependence promotes neuroadaptations in numerous brain areas, leading to escalated drinking and enhanced relapse vulnerability. We previously developed a mouse model of ethanol dependence and relapse drinking in which repeated cycles of chronic intermittent ethanol (CIE) vapor exposure drive a significant escalation of voluntary ethanol drinking. In the current study, we used this model to evaluate changes in neuronal activity (as indexed by c‐Fos expression) throughout acute and protracted withdrawal from CIE (combined with or without a history of ethanol drinking). We analyzed c‐Fos protein expression in 29 brain regions in mice sacrificed 2, 10, 26, and 74 hours or 7 days after withdrawal from 5 cycles of CIE. Results revealed dynamic time‐ and brain region‐dependent changes in c‐Fos activity over the time course of withdrawal from CIE exposure, as compared with nondependent air‐exposed control mice, beginning with markedly low expression levels upon removal from the ethanol vapor chambers (2 hours), reflecting intoxication. c‐Fos expression was enhanced during acute CIE withdrawal (10 and 26 hours), followed by widespread reductions at the beginning of protracted withdrawal (74 hours) in several brain areas. Persistent reductions in c‐Fos expression were observed during prolonged withdrawal (7 days) in prelimbic cortex, nucleus accumbens shell, dorsomedial striatum, paraventricular nucleus of thalamus, and ventral subiculum. A history of ethanol drinking altered acute CIE withdrawal effects and caused widespread reductions in c‐Fos that persisted during extended abstinence even without CIE exposure. These data indicate that ethanol dependence and relapse drinking drive long‐lasting neuroadaptations in several brain regions.

Individual differences in voluntary ethanol consumption lead to differential activation of the central amygdala in rats: relationship to the anxiolytic and stimulant effects of low dose ethanol

BACKGROUND

Although alcohol use disorders and anxiety disorders are highly comorbid, the relationship between these 2 disorders is not fully understood. Previous work from our laboratory shows that anxiety-like behavior is highly variable in outbred Long-Evans rats and is related to the level of voluntary ethanol (EtOH) consumption, suggesting that basal anxiety state influences EtOH intake. To further examine the relationship between the acquisition of EtOH consumption and anxiety phenotype, Long-Evans rats were assessed for anxiety-like behavior and neuronal activation following voluntary EtOH consumption in a limited access drinking paradigm.

METHODS

Rats were allowed to self-administer EtOH (6% v/v) for 4 days using a limited access drinking in the dark paradigm and divided into high- and low-drinking groups based on a median split of average daily EtOH intake. Immediately following the fourth drinking session, animals were tested on the elevated plus maze and evaluated for anxiety-like behaviors. Fos immunoreactivity was assessed in the central and basolateral amygdala, as well as the bed nucleus of the stria terminalis.

RESULTS

High EtOH drinkers spent significantly more time on the open arms of the plus maze than low EtOH drinkers. High EtOH drinkers also had increased locomotor activity as compared to both low EtOH drinkers and water drinkers. Fos immunoreactivity was positively correlated with EtOH consumption in all brain regions examined, although Fos-positive cell counts were only significantly different between high and low EtOH drinkers in the central amygdala (CeA).

CONCLUSIONS

Our findings demonstrate that outbred rats will voluntarily consume behaviorally effective doses of EtOH in a short-term access model and EtOH consumption is positively correlated with increased neuronal activation in the CeA.

Social context induces two unique patterns of c-Fos expression in adolescent and adult rats

The study assessed possible age differences in brain activation patterns to low dose ethanol (.5 g/kg intraperitoneally) and the influence of social context on this activation. Early adolescent or young adult male Sprague-Dawley rats were placed either alone or with an unfamiliar partner of the same age and sex following saline or ethanol administration. c-Fos protein immunoreactivity was used to index neuronal activation in 15 regions of interest. Ethanol had little effect on c-Fos activation. In adolescents, social context activated an "autonomic" network including the basolateral and central amygdala, bed nucleus of the stria terminalis, lateral hypothalamus, and lateral septum. In contrast, when adult rats were alone, activation was evident in a "reward" network that included the substantia nigra, nucleus accumbens, anterior cingulate and orbitofrontal cortices, lateral parabrachial nucleus, and locus coeruleus.

Rostroventral caudate putamen involvement in ethanol withdrawal is influenced by a chromosome 4 locus

Physiological dependence and associated withdrawal episodes are thought to constitute a motivational force that sustains alcohol use and abuse and may contribute to relapse in dependent individuals. Although no animal model duplicates alcoholism, models for specific factors, like withdrawal, are useful for identifying potential genetic and neural determinants of liability in humans. Previously, we identified a quantitative trait locus (QTL) and gene (Mpdz, which encodes the multi-PDZ domain protein) on chromosome 4 with a large effect on alcohol withdrawal in mice. Using congenic mice that confirm this QTL and c-Fos expression as a high-resolution marker of neuronal activation, we report that congenic mice show significantly less neuronal activity associated with alcohol withdrawal in the rostroventral caudate putamen (rvCP), but not other parts of the striatum, compared with background strain mice. Moreover, bilateral rvCP lesions significantly increase alcohol withdrawal severity. Using retrograde (fluorogold) and anterograde (Texas Red conjugated dextran amine) tract tracing, we found that ∼25% of c-Fos immunoreactive rvCP neurons project to caudolateral substantia nigra pars reticulata (clSNr), which we previously found is crucially involved in withdrawal following acute and repeated alcohol exposure. Our results expand upon work suggesting that this QTL impacts alcohol withdrawal via basal ganglia circuitry associated with limbic function, and indicate that an rvCP-clSNr projection plays a critical role. Given the growing body of evidence that the syntenic region of human chromosome 9p and human MPDZ gene are associated with alcohol abuse, our results may facilitate research on alcohol dependence and associated withdrawal in clinical populations.

The regulation of neuronal gene expression by alcohol

In recent years there has been an explosion of interest in how genes regulate alcohol drinking and contribute to alcoholism. This work has been stimulated by the completion of the human and mouse genome projects and the resulting availability of gene microarrays. Most of this work has been performed in drinking animals, and has utilized the extensive genetic variation among different mouse strains. At the same time, a much smaller amount of effort has gone into the in vitro study of the mechanisms underlying the regulation of individual genes by alcohol. These studies at the cellular and sub-cellular level are beginning to reveal the ways in which alcohol can interact with the transcriptional, translational and post-translational events inside the cell. Detailed studies of the promoter regions within several individual alcohol-responsive genes (ARGs) have been performed and this work has uncovered intricate signaling pathways that may be generalized to larger groups of ARGs. In the last few years several distinct ARGs have been identified from 35,000 mouse genes, by both the "top-down" approach (ex vivo gene arrays) and the "bottom-up" methods (in vitro promoter analysis). These divergent methodologies have converged on a surprisingly small number of genes encoding ion channels, receptors, transcription factors and proteins involved in synaptic function and remodeling. In this review we will describe some of the most interesting cellular and microarray work in the field, and will outline specific examples of genes for which the mechanisms of regulation by alcohol are now somewhat understood.

Differential activation of limbic circuitry associated with chronic ethanol withdrawal in DBA/2J and C57BL/6J mice

Although no animal model exactly duplicates clinically defined alcoholism, models for specific factors, such as the withdrawal syndrome, are useful for identifying potential neural determinants of liability in humans. The well-documented difference in withdrawal severity following chronic ethanol exposure, between the DBA/2J and C57BL/6J mouse strains, provides an excellent starting point for dissecting the neural circuitry affecting predisposition to physical dependence on ethanol. To induce physical dependence, we used a paradigm in which mice were continuously exposed to ethanol vapor for 72h. Ethanol-exposed and air-exposed (control) mice received daily injections of pyrazole hydrochloride, an alcohol dehydrogenase inhibitor, to stabilize blood ethanol levels. Ethanol-dependent and air-exposed mice were killed 7h after removal from the inhalation chambers. This time point corresponds to the time of peak ethanol withdrawal severity. The brains were processed to assess neural activation associated with ethanol withdrawal indexed by c-Fos immunostaining. Ethanol-withdrawn DBA/2J mice showed significantly (P<.05) greater neural activation than ethanol-withdrawn C57BL/6J mice in the dentate gyrus, hippocampus CA3, lateral septum, basolateral and central nuclei of the amygdala, and prelimbic cortex. Taken together with results using an acute model, our data suggest that progression from acute ethanol withdrawal to the more severe withdrawal associated with physical dependence following chronic ethanol exposure involves recruitment of neurons in the hippocampal formation, amygdala, and prelimbic cortex. To our knowledge, these are the first studies to use c-Fos to identify the brain regions and neurocircuitry that distinguish between chronic and acute ethanol withdrawal severity using informative animal models.

Neuroplasticity in brain reward circuitry following a history of ethanol dependence

Mitogen-activated and extracellular regulated kinase (MEK) and extracellular signal-regulated protein kinase (ERK) pathways may underlie ethanol-induced neuroplasticity. Here, we used the MEK inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (UO126) to probe the role of MEK/ERK signaling for the cellular response to an acute ethanol challenge in rats with or without a history of ethanol dependence. Ethanol (1.5 g/kg, i.p.) induced expression of the marker genes c-fos and egr-1 in brain regions associated with both rewarding and stressful ethanol actions. Under non-dependent conditions, ethanol-induced c-fos expression was generally not affected by MEK inhibition, with the exception of the medial amygdala (MeA). In contrast, following a history of dependence, a markedly suppressed c-fos response to acute ethanol was found in the medial pre-frontal/orbitofrontal cortex (OFC), nucleus accumbens shell (AcbSh) and paraventricular nucleus (PVN). The suppressed ethanol response in the OFC and AcbSh, key regions involved in ethanol preference and seeking, was restored by pre-treatment with UO126, demonstrating a recruitment of an ERK/MEK-mediated inhibitory regulation in the post-dependent state. Conversely, in brain areas involved in stress responses (MeA and PVN), an MEK/ERK-mediated cellular activation by acute ethanol was lost following a history of dependence. These data reveal region-specific neuroadaptations encompassing the MEK/ERK pathway in ethanol dependence. Recruitment of MEK/ERK-mediated suppression of the ethanol response in the OFC and AcbSh may reflect devaluation of ethanol as a reinforcer, whereas loss of an MEK/ERK-mediated response in the MeA and PVN may reflect tolerance to its aversive actions. These two neuroadaptations could act in concert to facilitate progression into ethanol dependence.

FOS expression induced by an ethanol-paired conditioned stimulus

To identify brain areas involved in ethanol-induced Pavlovian conditioning, brains of male DBA/2J mice were immunohistochemically analyzed for FOS expression after exposure to a conditioned stimulus (CS) previously paired with ethanol (2 g/kg) in two experiments. Mice were trained with a procedure that normally produces place preference (Before: ethanol before the CS) or one that normally produces place aversion (After: ethanol after the CS). Control groups received unpaired ethanol injections in the home cage (Delay) or saline only (Naïve). On the test day, mice were exposed to the 5-min CS 90 min before sacrifice. Before groups showed a conditioned increase in activity, whereas the After group showed a conditioned decrease in activity. FOS expression after a drug-free CS exposure was significantly higher in Before-group mice than in control mice in the bed nucleus of the stria terminalis (Experiment 1) and anterior ventral tegmental area (Experiments 1-2). Conditioned FOS responses were also seen in areas of the extended amygdala and hippocampus (Experiment 2). However, no conditioned FOS changes were seen in any brain area examined in After-group mice. Overall, these data suggest an important role for the mesolimbic dopamine pathway, extended amygdala and hippocampus in ethanol-induced conditioning.

The urocortin 1 neurocircuit: Ethanol-sensitivity and potential involvement in alcohol consumption

One of the hallmarks of alcoholism is continued excessive consumption of alcohol-containing beverages despite the negative consequences of such behavior. The neurocircuitry regulating alcohol consumption is not well understood. Recent studies have shown that the neuropeptide urocortin 1 (Ucn1), a member of the corticotropin-releasing factor (CRF) family of peptides, could be an important player in the regulation of alcohol consumption. This evidence is accumulated along three directions of research: (1) Ucn 1-containing neurons are extremely sensitive to alcohol; (2) the Ucn1 neurocircuit may contribute to the genetic predisposition to high alcohol intake in mice and rats; (3) manipulation of the Ucn1 system alters alcohol consumption and sensitivity. This paper reviews the current knowledge of the Ucn1 neurocircuit and the evidence for its involvement in alcohol-related behaviors, and proposes a mechanism for its involvement in the regulation of alcohol consumption.

Effects of intracerebroventricular ethanol on ingestive behavior and induction of c-Fos immunoreactivity in selected brain regions

The early changes in the central nervous system (CNS) following drinking of ethanol (ETOH) are poorly understood. It is known that chronic intracerebroventricular (ICV) administration of ethanol to rats induces preference for imbibed alcohol solutions. These results suggest that ICV ethanol could alter taste preference. In the present study, we tested whether ETOH[ICV] could induce a conditioned taste preference (CTP) or aversion (CTA) and alter c-Fos immunoreactivity (c-Fos-IR) in brain regions associated with feeding, aversion, and/or reward. Acute ETOH[ICV], as tested in the ETOH-naïve rat, did not induce CTA nor affect the amount of water imbibed by treated rats. The effects of ETOH[ICV] on intake and preference were determined using a novel palatable (i.e. sweet) noncaloric 0.1% saccharin solution. A single dose of ETOH[ICV] in the ETOH-nai;ve animal induced a CTP for saccharin. ETOH[ICV] significantly increased c-Fos-IR in a number of brain sites associated with feeding and reward including the bed nucleus of the stria terminalis, lateral dorsal area (BSTLD); nucleus accumbens, shell area (AcbSh); hypothalamic paraventricular nucleus (PVN); and lateral septum, ventral area (LSV). Thus, ETOH induced a CTP, not CTA, via central mechanisms; it increased c-Fos-IR in specific sites associated with feeding and reward.

Ethanol induces gene expression via nuclear compartmentalization of receptor for activated C kinase 1

Scaffolding proteins such as receptor for activated C kinase (RACK) 1 are involved in the targeting of signaling proteins and play an important role in the regulation of signal transduction cascades. Recently, we found that in cultured cells and in vivo, acute ethanol exposure induces the nuclear compartmentalization of RACK1. To elucidate a physiological role for nuclear RACK1, the Tat protein transduction system was used to transduce RACK1 and RACK1-derived fragments into C6 glioma cells. We found that nuclear RACK1 is mediating the induction of the immediate early gene c-fos expression induced by ethanol. First, transduction of full-length RACK1 (Tat-RACK1) resulted in the induction of c-fos expression and enhancement of ethanol activities. Second, we determined that the C terminus of RACK1 (Tat-RACK1DeltaN) is mediating transcription. Third, we identified a dominant negative fragment of RACK1 that inhibited the nuclear compartmentalization of endogenous RACK1 and inhibited ethanol-induction of c-fos mRNA and protein expression. Last, acute exposure to ethanol or transduction of full-length Tat-RACK1 resulted in an increase in mRNA levels of an activator protein 1 site-containing gene, PAC1 (pituitary adenylate cyclase-activating polypeptide receptor type I), suggesting that nuclear RACK1 is involved in the regulation of the expression of genes that are altered upon acute ethanol treatment. These results may therefore have important implications for the study of alcohol addiction.

Central nucleus of the amygdala and the effects of alcohol and alcohol-drinking behavior in rodents

This article will review key literature on the effects of alcohol on the amygdala and the involvement of the amygdala in regulating alcohol drinking in mice and rats. Special emphasis will be placed on the central nucleus of the amygdala (CeA) because this nucleus is a major component of the extended amygdala, which has been implicated in regulating alcohol-drinking behavior. Immunocytochemical and in situ hybridization studies indicate that acute high-dose ethanol administration increases c-fos expression in GABAergic neurons within the CeA of the rat, suggesting activation of these neurons by ethanol. A similar high-dose (4 g/kg ethanol) effect on c-fos expression in the CeA of C57 mice was also observed, whereas the DBA mice showed increased c-fos expression in the CeA in the dose range of 1.25-4.0 g/kg. Studies with DBA x C57 F2 intercross mice suggest that there may be a relationship between the neuronal activating effects of ethanol in the CeA and the locomotor stimulating effects of ethanol. Studies with rats examining the effects of acute ethanol or chronic alcohol drinking on local cerebral glucose utilization (LCGU) rates (as a measure of synaptic activity) indicated that (a) acute ethanol (0.25-2.0 g/kg) had little effect on LCGU rates in the CeA; (b) basal LCGU rates were reduced in the CeA as a result of chronic alcohol drinking; and (c) oral self-administration of ethanol increased LCGU values within the CeA. Microdialysis studies demonstrated that acute ethanol (2 g/kg) injection increased dopamine (DA) and serotonin (5-HT) release in the CeA. Microinjection studies indicate that GABA(A) receptors within the CeA are involved in oral ethanol self-administration. Overall, the findings from the various studies support a role for the CeA in mediating the stimulating actions of alcohol in mice and regulating alcohol-drinking behavior in mice and rats.

Interactive effects of nicotine and alcohol co-administration on expression of inducible transcription factors in mouse brain

Nicotine and alcohol are abused substances that are often used concurrently. Despite their combined usage, little is known about how they interact to produce changes in behavior and neural activity. Two experiments were conducted to identify interactions on both behavior and neural targets resulting from the co-administration of nicotine and alcohol. In Experiment 1, male C57BL/6J mice were administered saline, alcohol (2.4 g/kg, i.p.), nicotine (0.5 mg/kg, i.p.) or an alcohol/nicotine mixture and returned to their home cage. In Experiment 2, a higher dose of nicotine (1.0 mg/kg, i.p.) was included and animals were exposed to a novel environment. Several behavioral measures were analysed during novelty exposure. Immunohistochemical detection of inducible transcription factors (c-Fos and Egr1) was used in both experiments to identify changes in neural activation. Behavioral results suggested that the drugs were interacting in the production of behaviors. In particular, alcohol produced locomotor stimulation while it suppressed counts of rearing and leaning. When co-administered, nicotine appeared to counteract the alcohol-enhanced locomotor activity. Several brain regions were observed to have altered transcription factor expression in response to the different drug treatments, including amygdalar, hippocampal and cortical subregions. In a subset of these brain areas, nicotine and alcohol counteracted one another in the expression of transcription factors. These results identify several interactive target sites within the hippocampus, extended amygdala and cortical regions. The interactions appear to be a result of antagonizing actions of nicotine and alcohol. Finally, the results suggest that the combined use of nicotine and alcohol may offset the effects of the drug administered independently.

Alcohol and genetics: new animal models

In recent years, it has become increasingly evident that there is a genetic component to alcoholism. Attempts to isolate alcoholism genes have met with modest success, in part because alcoholism is a multigenic trait. Recently, experimental animal models and novel genetic manipulations have provided several clues as to the specific genes involved in alcoholism, and extensive research has identified many genes that might influence responses to alcohol. Although not all of these might be proven to influence drug sensitivity, research has provided evidence for the involvement of a few genes. Ultimately, findings from animal models that investigate the function of specific genes could aid the development of pharmacotherapies to treat alcohol dependence.