Role of corticotropin-releasing factor and corticosterone in behavioral sensitization to ethanol

The effect of MK-801 on stress-ethanol cross-sensitization is dissociable from its effects on nNOS activity

Locomotor behavioral sensitization represents an animal model for understanding neuroadaptive processes related to repeated drug exposure. Repeated stress can elicit a cross-sensitization to the stimulant response of ethanol, which involves neuronal nitric oxide synthase (nNOS). Activation of N-methyl d-aspartate (NMDA) glutamate receptors triggers nNOS and the synthesis of nitric oxide (NO). In this study, we investigated the effects of blocking NMDA receptors using the NMDA receptor antagonist MK-801 on the cross-sensitization between restraint stress and ethanol. We also evaluated the nNOS activity in the prefrontal cortex (PFC) and hippocampus. Mice were pretreated with saline or MK-801 30 min before an injection of saline or stress exposure for 14 days. On the following day, they were challenged with either saline or 1.8 g/kg ethanol. Swiss male mice pretreated with 0.25 mg/kg MK-801 exhibited a sensitized response to ethanol. Moreover, MK-801 potentiated the cross-sensitization between stress and ethanol. However, MK-801 prevented the enhanced nNOS activity in stress-exposed groups (challenged with saline or ethanol) in the PFC; the antagonist also prevented the ethanol-induced increase in nNOS activity and reduced this enzyme activity in mice exposed to stress in the hippocampus. These data indicate that systemic treatment with the NMDA antagonist potentiated, rather than blocked, ethanol-induced behavioral sensitization and that this effect is dissociable from the capacity of NMDA antagonists to reduce ethanol/stress-induced NOS stimulation in the PFC and hippocampus.

Extracting data from graphs: A case-study on animal research with implications for meta-analyses

Systematic reviews with meta-analyses are powerful tools that can answer research questions based on data from published studies. Ideally, all relevant data is directly available in the text or tables, but often it is only presented in graphs. In those cases, the data can be extracted from graphs, but this potentially introduces errors. Here, we investigate to what extent the extracted outcome and error values differ from the original data and if these differences could affect the results of a meta-analysis. Six extractors extracted 36 outcome values and corresponding errors from 22 articles. Differences between extractors were compared using overall concordance correlation coefficients (OCCC), differences between the original and extracted data were compared using concordance correlation coefficients (CCC). To test the possible influence on meta-analyses, random-effects meta-analyses on mean difference comparing original and extracted data were performed. The OCCCs and CCCs were high for both outcome values and errors, CCCs were >0.99 for the outcome and >0.92 for errors. The meta-analyses showed that the overall effect on outcome was very small (median: 0.025, interquartile range: 0.016-0.046). Therefore, data extraction from graphs is a good method to harvest data if it is not provided in the text or tables, and the original authors cannot provide the data.

Biological intersection of sex, age, and environment in the corticotropin releasing factor (CRF) system and alcohol

The neuropeptide corticotropin-releasing factor (CRF) is critical in neural circuit function and behavior, particularly in the context of stress, anxiety, and addiction. Despite a wealth of preclinical evidence for the efficacy of CRF receptor 1 antagonists in reducing behavioral pathology associated with alcohol exposure, several clinical trials have had disappointing outcomes, possibly due to an underappreciation of the role of biological variables. Although he National Institutes of Health (NIH) now mandate the inclusion of sex as a biological variable in all clinical and preclinical research, the current state of knowledge in this area is based almost entirely on evidence from male subjects. Additionally, the influence of biological variables other than sex has received even less attention in the context of neuropeptide signaling. Age (particularly adolescent development) and housing conditions have been shown to affect CRF signaling and voluntary alcohol intake, and the interaction between these biological variables is particularly relevant to the role of the CRF system in the vulnerability or resilience to the development of alcohol use disorder (AUD). Going forward, it will be important to include careful consideration of biological variables in experimental design, reporting, and interpretation. As new research uncovers conditions in which sex, age, and environment play major roles in physiological and/or pathological processes, our understanding of the complex interaction between relevant biological variables and critical signaling pathways like the CRF system in the cellular and behavioral consequences of alcohol exposure will continue to expand ultimately improving the ability of preclinical research to translate to the clinic. This article is part of the special issue on Neuropeptides.

Drug-Induced Glucocorticoids and Memory for Substance Use

The biological stress response of the body forms one of the foundations of adaptive behavior, including promoting (and impairing) different forms of memory. This response transcends stressful experiences and underlies reactions to challenges and even reinforcers such as addictive substances. Nevertheless, drug-induced stress responses are rarely incorporated into models of addiction. We propose here that drug-induced stress responses (particularly glucocorticoids) play a crucial role in addictive behavior by modulating the formation of memories for substance-use experiences. We review the contributions of amygdala-, striatum-, and hippocampus-based memory systems to addiction, and reveal common effects of addictive drugs and acute stress on these different memories. We suggest that the contributions of drug-induced stress responses to memory may provide insights into the mechanisms driving addictive behavior.

Convergent Neuronal Plasticity and Metaplasticity Mechanisms of Stress, Nicotine, and Alcohol

Stress and tobacco smoking are risk factors for alcoholism, but the underlying neural mechanisms are not well understood. Although stress, nicotine, and alcohol have broad, individual effects in the brain, some of their actions converge onto the same mechanisms and circuits. Stress and nicotine augment alcohol-related behaviors, in part via modulation of alcohol-evoked neuronal plasticity and metaplasticity mechanisms. Stress modulates alcohol-evoked plasticity via the release of signaling molecules that influence synaptic transmission. Nicotine also activates some of the same signaling molecules, cells, and circuits, producing a convergence of both stress and nicotine onto common plasticity mechanisms that influence alcohol self-administration. We describe several forms of alcohol-induced plasticity, including classic Hebbian plasticity at glutamatergic synapses, and we highlight less appreciated forms, such as non-Hebbian and GABAergic synaptic plasticity. Risk factors such as stress and nicotine initiate lasting neural changes that modify subsequent alcohol-induced synaptic plasticity and increase the vulnerability to alcohol addiction.

Region-specific expression of precursor and mature brain-derived neurotrophic factors after chronic alcohol exposure

BACKGROUND

Alcohol abuse is a serious health problem worldwide that causes a variety of physical and mental disorders. Research has shown that the brain-derived neurotrophic factor (BDNF) plays an important role in alcohol addiction. The BDNF precursor (proBDNF) exhibits different actions than BDNF through separate receptors and pathways in the central nervous system. However, the effects of proBDNF and BDNF in alcohol addiction are not fully known.

OBJECTIVES

The objective was to identify the expression patterns and effects of proBDNF and BDNF after chronic alcohol exposure.

METHODS

A total of 40 male adult mice were studied. A mouse psychomotor sensitization (PS) model was established to explore the effects of BDNF and proBDNF treatment following chronic alcohol exposure. Reverse transcription PCR (RT-PCR) was performed to measure mRNA levels for BDNF, TrkB, P75NTR, and sortilin in the prefrontal cortex, hippocampus, and dorsal striatum of Kunming mice after chronic alcohol exposure.

RESULTS

In Kunming mice, chronic alcohol exposure up-regulated BDNF and TrkB mRNA levels in the prefrontal cortex, but decreased sortilin and P75 mRNA levels in the dorsal striatum. No changes in mRNA levels were found in other measured brain regions in the alcohol and control groups.

CONCLUSION

Chronic alcohol exposure induced the region-specific expression of BDNF and proBDNF and their respective receptors in the brain. These results suggest that BDNF and proBDNF signaling pathways may play major roles in alcohol preference and addiction.

Differential effects of context on psychomotor sensitization to ethanol and cocaine

Repeated drug injections lead to sensitization of their stimulant effects in mice, a phenomenon sometimes referred to as drug psychomotor sensitization. Previous studies showed that sensitization to cocaine is context dependent as its expression is reduced in an environment that was not paired with cocaine administration. In contrast, the effects of the test context on ethanol sensitization remain unclear. In the present study, female OF1 mice were repeatedly injected with 1.5 g/kg ethanol to test for both the effects of context novelty/familiarity and association on ethanol sensitization. A first group of mice was extensively pre-exposed to the test context before ethanol sensitization and ethanol injections were paired with the test context (familiar and paired group). A second group was not pre-exposed to the test context, but ethanol injections were paired with the test context (nonfamiliar and paired group). Finally, a third group of mice was not pre-exposed to the test context and ethanol was repeatedly injected in the home cage (unpaired group). Control groups were similarly exposed to the test context, but were injected with saline. In a second experiment, cocaine was used as a positive control. The same behavioral procedure was used, except that mice were injected with 10 mg/kg cocaine instead of ethanol. The results show a differential involvement of the test context in the sensitization to ethanol and cocaine. Cocaine sensitization is strongly context dependent and is not expressed in the unpaired group. In contrast, the expression of ethanol sensitization is independent of the context in which it was administered, but is strongly affected by the relative novelty/familiarity of the environment. Extensive pre-exposure to the test context prevented the expression of ethanol sensitization. One possible explanation is that expression of ethanol sensitization requires an arousing environment.

An Update on CRF Mechanisms Underlying Alcohol Use Disorders and Dependence

Alcohol is the most commonly used and abused substance worldwide. The emergence of alcohol use disorders, and alcohol dependence in particular, is accompanied by functional changes in brain reward and stress systems, which contribute to escalated alcohol drinking and seeking. Corticotropin-releasing factor (CRF) systems have been critically implied in the transition toward problematic alcohol drinking and alcohol dependence. This review will discuss how dysregulation of CRF function contributes to the vulnerability for escalated alcohol drinking and other consequences of alcohol consumption, based on preclinical evidence. CRF signaling, mostly via CRF1 receptors, seems to be particularly important in conditions of excessive alcohol taking and seeking, including during early and protracted withdrawal, relapse, as well as during withdrawal-induced anxiety and escalated aggression promoted by alcohol. Modulation of CRF1 function seems to exert a less prominent role over low to moderate alcohol intake, or to species-typical behaviors. While CRF mechanisms in the hypothalamic-pituitary-adrenal axis have some contribution to the neurobiology of alcohol abuse and dependence, a pivotal role for extra-hypothalamic CRF pathways, particularly in the extended amygdala, is well characterized. More recent studies further suggest a direct modulation of brain reward function by CRF signaling in the ventral tegmental area, nucleus accumbens, and the prefrontal cortex, among other structures. This review will further discuss a putative role for other components of the CRF system that contribute for the overall balance of CRF function in reward and stress pathways, including CRF2 receptors, CRF-binding protein, and urocortins, a family of CRF-related peptides.

Impaired Ethanol-Induced Sensitization and Decreased Cannabinoid Receptor-1 in a Model of Posttraumatic Stress Disorder

BACKGROUND AND PURPOSE

Impaired striatal neuroplasticity may underlie increased alcoholism documented in those with posttraumatic stress disorder (PTSD). Cannabinoid receptor-1 (CB1) is sensitive to the effects of ethanol (EtOH) and traumatic stress, and is a critical regulator of striatal plasticity. To investigate CB1 involvement in the PTSD-alcohol interaction, this study measured the effects of traumatic stress using a model of PTSD, mouse single-prolonged stress (mSPS), on EtOH-induced locomotor sensitization and striatal CB1 levels.

METHODS

Mice were exposed to mSPS, which includes: 2-h restraint, 10-min group forced swim, 15-min exposure to rat bedding odor, and diethyl ether exposure until unconsciousness or control conditions. Seven days following mSPS exposure, the locomotor sensitizing effects of EtOH were assessed. CB1, post-synaptic density-95 (PSD95), and dopamine-2 receptor (D2) protein levels were then quantified in the dorsal striatum using standard immunoblotting techniques.

RESULTS

Mice exposed to mSPS-EtOH demonstrated impaired EtOH-induced locomotor sensitization compared to Control-EtOH mice, which was accompanied by reduced striatal CB1 levels. EtOH increased striatal PSD95 in control and mSPS-exposed mice. Additionally, mSPS-Saline exposure increased striatal PSD95 and decreased D2 protein expression, with mSPS-EtOH exposure alleviating these changes.

CONCLUSIONS

These data indicate that the mSPS model of PTSD blunts the behavioral sensitizing effects of EtOH, a response that suggests impaired striatal neuroplasticity. Additionally, this study demonstrates that mice exposed to mSPS and repeated EtOH exposure decreases CB1 in the striatum, providing a mechanism of interest for understanding the effects of EtOH following severe, multimodal stress exposure.

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.

Preclinical evidence implicating corticotropin-releasing factor signaling in ethanol consumption and neuroadaptation

The results of many studies support the influence of the corticotropin-releasing factor (CRF) system on ethanol (EtOH) consumption and EtOH-induced neuroadaptations that are critical in the addiction process. This review summarizes the preclinical data in this area after first providing an overview of the components of the CRF system. This complex system involves hypothalamic and extra-hypothalamic mechanisms that play a role in the central and peripheral consequences of stressors, including EtOH and other drugs of abuse. In addition, several endogenous ligands and targets make up this system and show differences in their involvement in EtOH drinking and in the effects of chronic or repeated EtOH treatment. In general, genetic and pharmacological approaches paint a consistent picture of the importance of CRF signaling via type 1 CRF receptors (CRF(1)) in EtOH-induced neuroadaptations that result in higher levels of intake, encourage alcohol seeking during abstinence and alter EtOH sensitivity. Furthermore, genetic findings in rodents, non-human primates and humans have provided some evidence of associations of genetic polymorphisms in CRF-related genes with EtOH drinking, although additional data are needed. These results suggest that CRF(1) antagonists have potential as pharmacotherapeutics for alcohol use disorders. However, given the broad and important role of these receptors in adaptation to environmental and other challenges, full antagonist effects may be too profound and consideration should be given to treatments with modulatory effects.

Histone acetylation in the nucleus accumbens shell modulates ethanol-induced locomotor activity in DBA/2J mice

BACKGROUND

A growing body of literature suggests that epigenetic mechanisms, including histone acetylation, may play key roles in drug abuse and the development of addiction. Experiments in this study were designed to investigate the role of histone acetylation in ethanol (EtOH)-induced locomotor sensitization.

METHODS

Immunohistochemical, Western blotting, and site-directed pharmacological techniques were used to explore the roles of histone acetylation at histone H3 (acH3K9) in both the expression of and acquisition of EtOH-induced locomotor sensitization. A commonly used sensitization protocol, in which animals were exposed to repeated injections of a low dose of EtOH while in their home cage, was used to examine this behavioral phenomenon. Additionally, site-directed administration of the histone deacetylase inhibitor (HDACi) Trichostatin A (TSA), in the absence of repeated EtOH injections, was used to examine the role of hyperacetylation in the nucleus accumbens (NAC) shell in EtOH-induced locomotor sensitization.

RESULTS

Sensitized mice displayed elevated acH3K9 immunoreactivity (IR) localized to the shell of the NAC. This augmentation in acH3K9 IR was confirmed, in a separate experiment, using Western blot analyses. Next, repeated intra-accumbal infusions of TSA, in the absence of repeated EtOH injections, were sufficient to induce an augmented locomotor response to a later injection of a low dose (2.0 g/kg, intraperitoneally) of EtOH, indicative of cross-sensitization to this locomotor stimulation between TSA and EtOH. Finally, a local infusion of TSA into the shell of the accumbens was also associated with a significant increase in acH3K9 IR within this region.

CONCLUSIONS

Together, the present observations suggest that histone acetylation, particularly within the shell of the NAC, is important for the development and expression of EtOH-induced locomotor sensitization.

Involvements of stress hormones in the restraint-induced conditioned place preference

The conditioned place preference (CPP) paradigm is widely used when examining the reinforcing effects of drugs. Some previous studies have shown that an acute stressor, such as restraint could also induce CPP. Although the modulating effects of stress hormones on various forms of learning are well known, the finding that a stressor has a potentially direct role in the reinforcement mechanism is novel. This study focused on the function of stress hormones in restraint-induced CPP in Wistar rats administered agonist or antagonist of 2 critical stress hormones prior to conditioning. Results showed that peripheral applications of corticosterone (CORT, 1, 3, 5, and 10 mg/kg, subcutaneously) failed to induce CPP. Furthermore, a glucocorticoid (GC) antagonist (mifepristone, 10, 40, or 100 mg/kg, sc) failed to block the restraint-induced CPP. Intracerebroventricular injection of a selective corticotropin-releasing factor receptor 1 (CRFR1) antagonist antalarmin (1 μg/5 μl), on the contrary, completely blocked the restraint-induced CPP. We concluded that CRFR1 plays an essential role in the neural mechanism of restraint-induced CPP. Negative feedback of CORT from peripheral sources may not be involved in this phenomenon.

Locomotor sensitization to EtOH: contribution of β-Endorphin

Alcohol use disorders, like all drug addictions, involve a constellation of adaptive changes throughout the brain. Neural activity underlying changes in the rewarding properties of alcohol reflect changes in dopamine transmission in mesolimbic and nigrostriatal pathways and these effects are modulated by endogenous opioids such as β-Endorphin. In order to study the role of β-Endorphin in the development of locomotor sensitization to repeated EtOH exposure, we tested transgenic mice that vary in their capacity to synthesize this peptide as a result of constitutive modification of the Pomc gene. Our results indicate that mice deficient in β-Endorphin show attenuated locomotor activation following an acute injection of EtOH (2.0 g/kg) and, in contrast to wildtype mice, fail to demonstrate locomotor sensitization after 12 days of repeated EtOH injections. These data support the idea that β-Endorphin modulates the locomotor effects of EtOH and contributes to the neuroadaptive changes associated with chronic use.