Antagonism of the non-opioid component of ethanol-induced analgesia by the NMDA receptor antagonist MK-801

Neurobiological aspects of pain in the context of alcohol use disorder

Alcohol is an effective and widely utilized analgesic. However, the chronic use of alcohol can actually facilitate nociceptive sensitivity over time, a condition known as hyperalgesia. Excessive and uncontrollable alcohol drinking is also a hallmark feature of alcohol use disorder (AUD). Both AUD and chronic pain are typically accompanied by negative affective states that may underlie reinforcement mechanisms contributing to AUD maintenance or progression. Frequent utilization of alcohol to relieve pain in individuals suffering from AUD or other chronic pain conditions may thus represent a powerful negative reinforcement construct. This chapter will describe ties between alcohol-mediated pain relief and potential exacerbation of AUD. We describe neurobiological systems engaged in alcohol analgesia as well as systems recruited in the development and maintenance of AUD and hyperalgesia. Although few effective therapies exist for either chronic pain or AUD, the common interaction of these conditions will likely lead the way for promising new discoveries of more effective and even simultaneous treatment of AUD and co-morbid hyperalgesia. An abundance of neurobiological findings from multiple laboratories has implicated a potentiation of central amygdala (CeA) signaling in both pain and AUD, and these data also suggest that attenuation of stress-related systems (including corticotropin-releasing factor, vasopressin, and glucocorticoid receptor activity) would be particularly effective and comprehensive therapeutic strategies targeting the critical intersection of somatic and motivational mechanisms driving AUD, including alcohol-induced hyperalgesia.

Pharmacological mechanisms of alcohol analgesic-like properties in mouse models of acute and chronic pain

Alcohol use and chronic pain are highly comorbid. Acute alcohol use typically produces an analgesic effect. However, chronic use can worsen the progression of chronic pain. In rodent models, acute models of pain have primarily been used to investigate the relationship between alcohol and pain analgesia. Here, we use two models of chronic pain, chronic inflammatory and peripheral neuropathic pain, to investigate acute alcohol's antinociceptive and analgesic properties. We hypothesize that acute ethanol is acting through opioid receptors to create an analgesic-like effect in both reflexive and affective dimensions of pain. Using male and female C57BL/6J mice, oral ethanol administration (0-1.25 g/kg) showed a dose-dependent reversal of mechanical hypersensitivity in both Complete Freund's Adjuvant (CFA) and chronic constriction injury (CCI) models of chronic inflammatory and neuropathic pain. No sex differences were observed. Using the conditioned place preference (CPP) task to assess the subjective responses to ethanol's anti-nociceptive properties, CCI-injured animals showed a preference for the ethanol-paired side, suggesting a reduction in an aversive and pain-like state produced by nerve injury. These effects are likely mediated through the kappa and possibly the mu opioid systems, since ethanol-induced anti-nociception following CCI was fully reversed by pretreatment with the kappa selective antagonist, nor-BNI, or high doses of naltrexone. These data show that ethanol possesses analgesic-like properties in chronic inflammatory and neuropathic pain models in mice and provide new insight into ethanol as it relates to chronic pain.

Analgesic Effects of Alcohol: A Systematic Review and Meta-Analysis of Controlled Experimental Studies in Healthy Participants

Despite the long-standing belief in the analgesic properties of alcohol, experimental studies have produced mixed results. This meta-analysis aimed to clarify whether alcohol produces a decrease in experimentally-induced pain and to determine the magnitude of any such effect. PubMed, PsycINFO, and Embase databases were searched from inception until April 21, 2016 for controlled studies examining the effect of quantified dosages of alcohol on pain response to noxious stimulation. Eighteen studies involving 404 participants were identified providing alcohol versus no-alcohol comparisons for 13 tests of pain threshold (n = 212) and 9 tests of pain intensity ratings (n = 192). Random effects meta-analysis of standardized mean difference (SMD) provided robust support for analgesic effects of alcohol. A mean blood alcohol content (BAC) of approximately .08% (3-4 standard drinks) produced a small elevation of pain threshold (SMD [95% CI] = .35 [.17-.54], P = .002), and a moderate to large reduction in pain intensity ratings (SMD [95% CI] = .64 [.37-.91], P < .0001), or equivalently, a mean reduction of 1.25 points on a 0- to 10-point pain rating scale. Furthermore, increasing BAC resulted in increasing analgesia, with each .02% BAC increment producing an increase of SMD = .11 for pain threshold and SMD = .20 for reduced pain intensity. Some evidence of publication bias emerged, but statistical correction methods suggested minimal impact on effect size. Taken together, findings suggest that alcohol is an effective analgesic that delivers clinically-relevant reductions in ratings of pain intensity, which could explain alcohol misuse in those with persistent pain despite its potential consequences for long-term health. Further research is needed to corroborate these findings for clinical pain states.

PERSPECTIVE

This meta-analysis provides robust evidence for the analgesic properties of alcohol, which could potentially contribute to alcohol misuse in pain patients. Strongest analgesia occurs for alcohol levels exceeding World Health Organization guidelines for low-risk drinking and suggests raising awareness of alternative, less harmful pain interventions to vulnerable patients may be beneficial.

Delta-opioid receptor antagonism leads to excessive ethanol consumption in mice with enhanced activity of the endogenous opioid system

The opioid system modulates the central reinforcing effects of ethanol and participates in the etiology of addiction. However, the pharmacotherapy of ethanol dependence targeted on the opioid system is little effective and varies due to individual patients' sensitivity. In the present study, we used two mouse lines with high (HA) and low (LA) activity of the endogenous opioid system to analyze the effect of opioid receptor blockade on ethanol drinking behavior. We found that LA and HA lines characterized by divergent magnitudes of swim stress-induced analgesia also differ in ethanol intake and preference. Downregulation of the opioid system in LA mice was associated with increased ethanol consumption. Treatment with a non-selective opioid receptor antagonist (naloxone) had no effect on ethanol intake in this line. Surprisingly, in HA mice, the blockage of opioid receptors led to excessive ethanol consumption. Moreover, naloxone selectively induced high levels of anxiety- and depressive-like behaviors in HA mice which was attenuated by ethanol. With the use of specific opioid receptor antagonists we showed that the naloxone-induced increase in ethanol drinking in HA mice is mediated mainly by δ and to a lower extent by μ opioid receptors. The effect of δ-opioid receptor antagonism was abolished in HA mice carrying a C320T transition in the δ-opioid receptor gene (EU446125.1), which impairs this receptor's function. Our results indicate that high activity of the opioid system plays a protective role against ethanol dependence. Therefore, its blockage with opioid receptor antagonists may lead to a profound increase in ethanol consumption.

The association between frequent alcohol drinking and opioid consumption after abdominal surgery: A retrospective analysis

Aims

It is perceived that patients with a history of frequent alcohol consumption require more opioids for postoperative pain control and experience less postoperative nausea and vomiting than patients without such a history. However, there is scarce evidence supporting this notion. The aim of this study was to assess association between frequent alcohol consumption and opioid requirement for postoperative pain control and occurrence of postoperative nausea and vomiting.

Methods

The medical records for 4143 patients using intravenous patient-control analgesia with opioids after abdominal surgery between January 2010 and September 2013 were obtained, and associations were sought between the cumulative opioid consumption (in intravenous morphine equivalence) per body weight (mg/kg) in the first 2 days after abdominal operation and several demographic and clinical variables by multiple regression analysis. The association between the occurrence of postoperative nausea and vomiting and several demographic and clinical variables was also sought by multiple logistic regression analysis.

Results

Frequent alcohol drinking, among other previously reported factors, was associated with increased opioid consumption for postoperative pain control (p < 0.001). The estimate effect of frequent alcohol drinking was 0.117 mg/kg. Frequent alcohol drinking was also associated with decreased risks of postoperative nausea (odds ratio = 0.59, p = 0.003) and vomiting (odds ratio = 0.49, p = 0.026).

Conclusions

Frequent alcohol drinking was associated with increased opioid consumption for postoperative pain control and decreased risks of postoperative nausea and vomiting after abdominal surgery.

Effect of Intravenous Ethanol on Capsaicin-Induced Hyperalgesia in Human Subjects

BACKGROUND

The objective of this study was to assess ethanol's (EtOH's) effects on capsaicin-induced hyperalgesia in healthy participants. Specifically, we investigated the change in area of capsaicin-induced hyperalgesia following 3 interventions: intravenous EtOH at 2 targeted breath alcohol concentrations (BrAC), or placebo.

METHODS

Eighteen participants participated in 3 test days in a randomized order. Each test day, participants received an intradermal capsaicin injection on the volar surface of the forearm, followed by either infusion of high concentration EtOH (targeted BrAC = 0.100 g/dl), low concentration EtOH (targeted BrAC = 0.040 g/dl), or placebo. The area of hyperalgesia was determined by von Frey technique at 2 time points, prior to EtOH infusion, and again when target BrAC was reached. The primary outcome was the percent change in the area of capsaicin-induced hyperalgesia. Additional outcome measures included the visual analogue scale of mood states (VAS), which was administered at each time point.

RESULTS

There was a marked 30% reduction in the area of capsaicin-induced hyperalgesia with infusion of a high concentration of EtOH (p < 0.05). Low concentration EtOH produced a 10% reduction in hyperalgesia area, although this finding did not reach significance. Further, participants reported significant feelings of euphoria and drowsiness at high concentrations of EtOH (p < 0.05), as measured by the VAS.

CONCLUSIONS

In a human model examining pain phenomena related to central sensitization, this study is the first to demonstrate that capsaicin-induced hyperalgesia is markedly attenuated by EtOH. The capsaicin experimental pain paradigm employed provides a novel approach to evaluate EtOH's effects on pain processing. The antihyperalgesic effects of EtOH observed have important clinical implications for the converging fields of substance abuse and pain medicine and may inform why patients with chronic pain often report alcohol use as a form of self-medication.

Ethanol-induced effects on sting extension response and punishment learning in the western honey bee (Apis mellifera)

Acute ethanol administration is associated with sedation and analgesia as well as behavioral disinhibition and memory loss but the mechanisms underlying these effects remain to be elucidated. During the past decade, insects have emerged as important model systems to understand the neural and genetic bases of alcohol effects. However, novel assays to assess ethanol's effects on complex behaviors in social or isolated contexts are necessary. Here we used the honey bee as an especially relevant model system since bees are typically exposed to ethanol in nature when collecting standing nectar crop of flowers, and there is recent evidence for independent biological significance of this exposure for social behavior. Bee's inhibitory control of the sting extension response (SER) and a conditioned-place aversion assay were used to study ethanol effects on analgesia, behavioral disinhibition, and associative learning. Our findings indicate that although ethanol, in a dose-dependent manner, increases SER thresholds (analgesic effects), it disrupts the ability of honey bees to inhibit SER and to associate aversive stimuli with their environment. These results suggest that ethanol's effects on analgesia, behavioral disinhibition and associative learning are common across vertebrates and invertebrates. These results add to the use of honey bees as an ethanol model to understand ethanol's effects on complex, socially relevant behaviors.

Association between the A107V substitution in the δ-opioid receptors and ethanol drinking in mice selected for high and low analgesia

Experimental evidence suggests that endogenous opioids play an important role in the development of ethanol addiction. In this study, we employed two mouse lines divergently bred for opioid-mediated stress-induced analgesia. In comparison with HA (high analgesia line) mice, LA (low analgesia line) mice, having lower opioid receptor system activity, manifest enhanced basal as well as stress-induced ethanol drinking. Here, we found that recently discovered C320T transition in exon 2 of the δ-opioid receptor gene (EU446125.1), which results in an A107V substitution (ACA23171.1), leads to higher ethanol preference in CT mice compared with CC homozygotes. This genetic association is particularly evident under chronic mild stress (CMS) conditions. The interaction between stress and ethanol intake was significantly stronger in HA than in LA mice. Ethanol almost completely attenuated the pro-depressive effect of CMS (assessed with the tail suspension test) in both the CC and CT genotypes in the HA line. In the LA mice, a lack of response to ethanol was observed in the CC genotype, whereas ethanol consumption strengthened depressive-like behaviours in CT individuals. Our results suggest that constitutively active A107V substitution in δ-opioid receptors may be involved in stress-enhanced vulnerability to ethanol abuse and in the risk of ethanol dependence.

The atypical cannabinoid O-1602 increases hind paw sensitisation in the chronic constriction injury model of neuropathic pain

O-1602 is an atypical cannabinoid that acts as an agonist at GPR55, a g protein-coupled receptor that previous studies have indicated may have a pronociceptive role in neuropathic pain. We administered O-1602 to both naive rats and rats that had undergone chronic constriction injury surgery. O-1602 did not cause any changes in hind paw responses to Von Frey hair testing in naive rats. However, O-1602 reversed the desensitising effects of ETOH, which was used as an active and opposing vehicle. Our results are consistent with the hypothesis that GPR55 has a pronociceptive role in neuropathic pain.

Selection for stress-induced analgesia affects the mouse hippocampal transcriptome

Stress responsiveness, including pain sensitivity and stress-induced analgesia (SIA), depends on genotype and, partially, is mediated by hippocampus. The present study examined differences in constitutive gene expression in hippocampus in lines of mice bred for high (HA) and low (LA) swim SIA. Between the lines, we found 1.5-fold or greater differences in expression of 205 genes in the hippocampus in nonstressed animals. The identity of these genes indicates that selective breeding for swim SIA affected many aspects of hippocampal neurons physiology, including metabolism, structural changes, and cellular signaling. Genes involved in calcium signaling pathway, including Slc8a1, Slc8a2, Prkcc, and Ptk2b, were upregulated in LA mice. In HA mice, robust upregulation of genes coding some transcription factors (Klf5) or receptors for neurotensin (Ntsr2) and GABA (Gabard) suggests the genetic basis for a novel mechanism of the non-opioid type of SIA in HA animals. Additional groups of differentially expressed genes represented functional networks involved in carbohydrate metabolism, gene expression regulation, and molecular transport. Our data indicate that selection for a single and very specific stress response trait, swim SIA, alters hippocampal gene expression. The results suggest that individual stress responsiveness may be associated with characteristics of the constitutive hippocampal transcriptome.

Analgesic properties of oleoylethanolamide (OEA) in visceral and inflammatory pain

Oleoylethanolamide (OEA) is a natural fatty acid amide that mainly modulates feeding and energy homeostasis by binding to peroxisome proliferator-activated receptor-alpha (PPAR-alpha) [Rodríguez de Fonseca F, Navarro M, Gómez R, Escuredo L, Navas F, Fu J, et al. An anorexic lipid mediator regulated by feeding. Nature 2001;414:209-12; Fu J, Gaetani S, Oveisi F, Lo Verme J, Serrano A, Rodríguez de Fonseca F, et al. Oleoylethanolamide regulates feeding and body weight through activation of the nuclear receptor PPAR-alpha. Nature 2003;425:90-3]. Additionally, it has been proposed that OEA could act via other receptors, including the vanilloid receptor (TRPV1) [Wang X, Miyares RL, Ahern GP. Oleoylethanolamide excites vagal sensory neurones, induces visceral pain and reduces short-term food intake in mice via capsaicin receptor TRPV1. J Physiol 2005;564:541-7.] or the GPR119 receptor [Overton HA, Babbs AJ, Doel SM, Fyfe MC, Gardner LS, Griffin G, et al. Deorphanization of a G protein-coupled receptor for oleoylethanolamide and its use in the discovery of small-molecule hypophagic agents. Cell Metab 2006;3:167-175], suggesting that OEA might subserve other physiological roles, including pain perception. We have evaluated the effect of OEA in two types of nociceptive responses evoked by visceral and inflammatory pain in rodents. Our results suggest that OEA has analgesic properties reducing the nociceptive responses produced by administration of acetic acid and formalin in two experimental animal models. Additional research was performed to investigate the mechanisms underlying this analgesic effect. To this end, we evaluated the actions of OEA in mice null for the PPAR-alpha receptor gene and compared its actions with those of PPAR-alpha receptor wild-type animal. We also compared the effect of MK-801 in order to evaluate the role of NMDA receptor in this analgesia. Our data showed that OEA reduced visceral and inflammatory responses through a PPAR-alpha-activation independent mechanism. Co-administration of subanalgesic doses of MK-801 and OEA produced an analgesic effect, suggesting the participation of glutamatergic transmission in the antinociceptive effect of OEA. This study represents a novel approach to the examination of the effectiveness of OEA in nociceptive responses and provides a framework for understanding its biological functions and endogenous targets in visceral and inflammatory pain.

Internal and External Factors Affecting the Development of Neuropathic Pain in Rodents. Is It All About Pain?

It is important to know the factors that will influence animal models of neuropathic pain. A good reproducibility and predictability in different strains of animals for a given test increases the clinical relevance and possible targeting. An obligatory requirement for enabling comparisons of results of different origin is a meticulous definition of the specific sensitivities of a model for neuropathic pain and a description of the test conditions. Factors influencing neuropathic pain behavior can be subdivided in external and internal factors. The most important external factors are; timing of the measurement of pain after induction of neuropathy, circadian rhythms, seasonal influences, air humidity, influence of order of testing, diet, social variables, housing and manipulation, cage density, sexual activity, external stress factors, and influences of the experimenter. The internal factors are related to the type of animal, its genetic background, gender, age, and the presence of homeostatic adaptation mechanisms to specific situations or stress. In practice, the behavioral presentations to pain depend on the combination of genetic and environmental factors such as accepted social behavior. It also depends on the use of genetic manipulation of the animals such as in transgenic animals. These make the interpretation of data even more difficult. Differences of pain behavior between in- and outbred animals will be better understood by using modern analysis techniques. Substrains of animals with a high likelihood for developing neuropathic pain make the unraveling of specific pathophysiological mechanisms possible. Concerning the effect of stress on pain, it is important to differentiate between external and internal stress such as social coping behavior. The individual dealing with this stress is species sensitive, and depends on the genotype and the social learning. In the future, histo-immunological and genetic analysis will highlight similarities of the different pathophysiological mechanisms of pain between different species and human subjects. The final objective for the study of pain is to describe the genetics of the eliciting pain mechanisms in humans and to look for correlations with the knowledge from basic research. Therefore, it is necessary to know the genetic evolution of the different mechanisms in chronic pain. In order to be able to control the clinical predictability of a putative treatment the evolutionary pharmacogenomic structure of specific transmitters and receptors must be clarified.

Reduction of stress-induced analgesia following ethanol exposure in mice

In the present study, we examined the effects of ethanol treatment on the subsequent expression of opioid and nonopioid forms of swim stress-induced analgesia (SSIA). In Experiment 1, mice were injected with ethanol (2.5 g/kg, i.p.) or an equal volume of saline once a day for two days. Animals received no treatment on day 3. On day 4, the animals were tested for opioid (3-min swim in water maintained at 32 degrees C) or nonopioid (3-min swim in water maintained at 20 degrees C) SSIA in the hotplate test (52 degrees C). Mice pretreated with ethanol injections showed a decrease in nonopioid SSIA, but not in opioid SSIA. In Experiment 2, mice were given an ethanol solution (10%) or tap water to drink for 15 days. On day 16, all animals were given tap water to drink. On day 17, the animals were tested for opioid or nonopioid SSIA. Neither form of SSIA was modified in mice that drank the ethanol solution. These results show that ethanol pretreatment can modify nonopioid endogenous analgesic responses in mice. Further, the route of administration influences the effects of ethanol pretreatment on SSIA.

Ginsenoside Rf, a trace component of ginseng root, produces antinociception in mice

Ginseng root, a traditional oriental medicine, contains more than a dozen biologically active saponins called ginsenosides, including one present in only trace amounts called ginsenoside-Rf (Rf). Previously, we showed that Rf inhibits Ca2+ channels in mammalian sensory neurons through a mechanism requiring G-proteins, whereas a variety of other ginsenosides were relatively ineffective. Since inhibition of Ca2+ channels in sensory neurons contributes to antinociception by opioids, we tested for analgesic actions of Rf. We find dose-dependent antinociception by systemic administration of Rf in mice using two separate assays of tonic pain: in the acetic acid abdominal constriction test, the ED50 was 56+/-9 mg/kg, a concentration similar to those reported for aspirin and acetaminophen in the same assay; in the tonic phase of the biphasic formalin test, the ED50 was 129+/-32 mg/kg. Rf failed to affect nociception measured in three assays of acute pain: the acute phase of the formalin test, and the thermal (49 degrees C) tail-flick and increasing-temperature (3 degrees C/min) hot-plate tests. The simplest explanation is that Rf inhibits tonic pain without affecting acute pain, but other possibilities exist. Seeking a cellular explanation for the effect, we tested whether Rf suppresses Ca2+ channels on identified nociceptors. Inhibition was seen on large, but not small, nociceptors. This is inconsistent with a selective effect on tonic pain, so it seems unlikely that Ca2+ channel inhibition on primary sensory neurons can fully explain the behavioral antinociception we have demonstrated for Rf.

Reduced tolerance to certain pharmacological effects of ethanol after chronic administration in perinatally undernourished rats

We have previously reported that recovered adult rats undernourished at perinatal age failed to develop tolerance to the anticonflict effect of ethanol after chronic ethanol administration (1 g/kg/day during 30 days) (4). To further study the extent of this finding, we examined the effect of a similar chronic ethanol treatment on the hypothermic and anticonvulsant effects of ethanol in perinatally deprived rats. Hypoalgesic activity was assessed in ethanol treated rats during 15 days. After chronic ethanol treatment, a similar development of tolerance to the hypothermic effect of ethanol was observed in control and deprived rats. However, tolerance to the anticonvulsant and hypoalgesic effect of ethanol was significantly reduced in deprived as compared with control animals. Thus, early undernutrition differentially affects the development of tolerance elicited by chronic ethanol administration.

The interaction of dopamine, cocaine, and cocaethylene with ethanol on central nervous system depression in mice

The interactions between dopamine, cocaine, cocaethylene, and ethanol were studied in Swiss-Webster mice. The loss of the righting reflex (LORR) was used as a measure of CNS depression. Animals were injected intraperitoneally (IP) with ethanol (4.0 g/kg). which caused a LORR. Immediately upon regaining of the righting reflex, mice were injected intracerebroventricularly (ICV) with saline, dopamine (0.1, 0.5, or 1.0 mumol/kg), cocaine (1, 15, or 25 mumol/kg), or cocaethylene (1, 15, or 25 mumol/kg). In the presence of systemic ethanol, all three compounds produced CNS depression in a dose-dependent manner. The dopamine D2-receptor antagonist sulpiride and the D1-receptor antagonist fluphenazine were given acutely ICV with dopamine in the presence of systemic ethanol to examine whether these antagonists could block the return to the LORR produced by dopamine. Sulpiride, however, actually enhanced the interaction between ethanol and dopamine in a dose-dependent manner as measured by the LORR; fluphenazine neither blocked nor enhanced the effect of dopamine in the presence of systemic ethanol. In addition, these antagonists had no effect on cocaine- and cocaethylene-induced CNS depression in the presence of systemic ethanol. The results of this study showed that the neurotransmitter dopamine and both cocaine and cocaethylene can promote further CNS depression in the presence of systemic ethanol, and that dopamine was significantly more potent than cocaine and cocaethylene as measured by the return to the LORR.

The genetics of pain and pain inhibition

The present review summarizes the current state of knowledge about the genetics of pain-related phenomena and illustrates the scope and power of genetic approaches to the study of pain. We focus on work performed in our laboratories in Jastrzebiec, Poland; Portland, OR; and Los Angeles, which we feel demonstrates the continuing usefulness of classical genetic approaches, especially when used in combination with newly available molecular genetic techniques.

Defective habituation to nociceptive stimulation in alcohol-avoiding ANA rats

Brain opioidergic mechanisms participate in the regulation of motivational and ingestive behaviours. Since alcohol is believed to activate endogenous opioid systems and to produce opioid-mediated antinociception, the present experiments were performed to find out if alcohol-induced antinociception differs between the alcohol-preferring AA and alcohol-avoiding ANA rat lines. Alcohol doses relevant to the voluntary alcohol intake by the AA rats (0.5-1.0 g/kg, intraperitoneally) failed to alter tail-flick (TF) latency in a 55 degrees C water bath by either rat line. However, repeated measurement of TF latency, even without any alcohol treatment, prolonged tail-flick latencies in AA but not in ANA rats. Prolongation of TF latency was also seen in non-selected Wistar rats, indicating that the ANA rats respond abnormally in this test. The antinociceptive effects of swimming-induced stress (3 min at 15 degrees C) and those of cumulative morphine administration (0.5-16.0 mg/kg, subcutaneously) were similar in both rat lines. Using higher, motor-impairing alcohol doses with repeated baseline TF determinations, it was observed that a dose of 1.5 g/kg induced slight antinociception only in the AA rats, while 2.0 g/kg produced similar effects in both rat lines. It is thus concluded that the alcohol-preferring AA rats do not show any enhanced alcohol-induced antinociception at relevant alcohol doses. However, the alcohol-avoiding ANA rats appear to have a defective ability to habituate to repeated sensory stimuli, which could contribute to their alcohol avoidance by preventing the development of tolerance to aversive effects of alcohol.