Acoustic startle and fear-potentiated startle in alcohol-preferring (P) and -nonpreferring (NP) lines of rats

Neurosteroid [3α,5α]-3-hydroxy-pregnan-20-one enhances IL-10 production via endosomal TRIF-dependent TLR4 signaling pathway

Background

Previous studies demonstrated the inhibitory effect of allopregnanolone (3α,5α-THP) on the activation of inflammatory toll-like receptor 4 (TLR4) signals in RAW264.7 macrophages and the brains of selectively bred alcohol-preferring (P) rats. In the current study, we investigated the impact of 3α,5α-THP on the levels of IL-10 and activation of the TRIF-dependent endosomal TLR4 pathway.

Methods

The amygdala and nucleus accumbens (NAc) of P rats, which exhibit innately activated TLR4 pathways as well as RAW264.7 cells, were used. Enzyme-linked immunosorbent assays (ELISA) and immunoblotting assays were used to ascertain the effects of 3α,5α-THP on the TRIF-dependent endosomal TLR4 pathway and endosomes were isolated to examine translocation of TLR4 and TRIF. Additionally, we investigated the effects of 3α,5α-THP and 3α,5α-THDOC (0.1, 0.3, and 1.0 µM) on the levels of IL-10 in RAW264.7 macrophages. Finally, we examined whether inhibiting TRIF (using TRIF siRNA) in RAW264.7 cells altered the levels of IL-10.

Results

3α,5α-THP administration facilitated activation of the endosomal TRIF-dependent TLR4 pathway in males, but not female P rats. 3α,5α-THP increased IL-10 levels (+13.2 ± 6.5%) and BDNF levels (+21.1 ± 11.5%) in the male amygdala. These effects were associated with increases in pTRAM (+86.4 ± 28.4%), SP1 (+122.2 ± 74.9%), and PI(3)K-p110δ (+61.6 ± 21.6%), and a reduction of TIRAP (-13.7 ± 6.0%), indicating the activation of the endosomal TRIF-dependent TLR4 signaling pathway. Comparable effects were observed in NAc of these animals. Furthermore, 3α,5α-THP enhanced the accumulation of TLR4 (+43.9 ± 11.3%) and TRIF (+64.8 ± 32.8%) in endosomes, with no significant effect on TLR3 accumulation. Additionally, 3α,5α-THP facilitated the transition from early endosomes to late endosomes (increasing Rab7 levels: +35.8 ± 18.4%). In RAW264.7 cells, imiquimod (30 µg/mL) reduced IL-10 while 3α,5α-THP and 3α,5α-THDOC (0.1, 0.3, and 1.0 µM) restored IL-10 levels. To determine the role of the TRIF-dependent TLR4 signaling pathway in IL-10 production, the downregulation of TRIF (-62.9 ± 28.2%) in RAW264.7 cells led to a reduction in IL-10 levels (-42.3 ± 8.4%). TRIF (-62.9 ± 28.2%) in RAW264.7 cells led to a reduction in IL-10 levels (-42.3 ± 8.4%) and 3α,5α-THP (1.0 µM) no longer restored the reduced IL-10 levels.

Conclusion

The results demonstrate 3α,5α-THP enhancement of the endosomal TLR4-TRIF anti-inflammatory signals and elevations of IL-10 in male P rat brain that were not detected in female P rat brain. These effects hold significant implications for controlling inflammatory responses in both the brain and peripheral immune cells.

Evaluating habit formation across pairs of female and male selectively bred alcohol-preferring and non-preferring rats

Some individuals with alcohol use disorder (AUD) continue to drink because they have developed a habit where they do not consider the consequences of their actions. Genetically selected lines of alcohol-preferring and non-preferring rats allow for exploration of how specific endophenotypes, such as tendency to form habits, may be risk factors that interact with a genetic predisposition of AUD. While high alcohol drinking (HAD) and alcohol-preferring (P) rats were selectively bred to consume high amounts of freely available ethanol, they exhibit differences in alcohol-seeking behaviors as well as impulsive behaviors, and may represent different behavioral models of AUD. The goal of the current study was to compare the tendency to develop habitual behaviors across female and male HAD1, HAD2, and P rats and their respective alcohol non-preferring counterparts. Alcohol-naïve rats were trained on a variable interval schedule using a non-ethanol reinforcer and were then tested in two extinction sessions, one prior to a reinforcer devaluation (conditioned taste aversion) procedure and one after. There were no differences in total lever presses between P and alcohol non-preferring (NP) rats, but there were differences between HAD and low-alcohol drinking (LAD) rats. All six strains decreased lever pressing after reinforcer devaluation. However, P and NP females did not increase latency to first lever press after devaluation, suggesting some inclination toward habitual behavior that was not apparent in either the HAD or LAD lines. Selective breeding for alcohol preference does not seem to influence the tendency to form habits, whereas background strain and sex may have an influence on this behavior.

Neurosteroid allopregnanolone (3α,5α-THP) inhibits inflammatory signals induced by activated MyD88-dependent toll-like receptors

We have shown that endogenous neurosteroids, including pregnenolone and 3α,5α-THP inhibit toll-like receptor 4 (TLR4) signal activation in mouse macrophages and the brain of alcohol-preferring (P) rat, which exhibits innate TLR4 signal activation. The current studies were designed to examine whether other activated TLR signals are similarly inhibited by 3α,5α-THP. We report that 3α,5α-THP inhibits selective agonist-mediated activation of TLR2 and TLR7, but not TLR3 signaling in the RAW246.7 macrophage cell line. The TLR4 and TLR7 signals are innately activated in the amygdala and NAc from P rat brains and inhibited by 3α,5α-THP. The TLR2 and TLR3 signals are not activated in P rat brain and they are not affected by 3α,5α-THP. Co-immunoprecipitation studies indicate that 3α,5α-THP inhibits the binding of MyD88 with TLR4 or TLR7 in P rat brain, but the levels of TLR4 co-precipitating with TRIF are not altered by 3α,5α-THP treatment. Collectively, the data indicate that 3α,5α-THP inhibits MyD88- but not TRIF-dependent TLR signal activation and the production of pro-inflammatory mediators through its ability to block TLR-MyD88 binding. These results have applicability to many conditions involving pro-inflammatory TLR activation of cytokines, chemokines, and interferons and support the use of 3α,5α-THP as a therapeutic for inflammatory disease.

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

Individual differences in voluntary alcohol consumption are associated with conditioned fear in the fear incubation model

Previous research in male Long Evans rats has shown a relationship between low voluntary alcohol consumption and high conditioned fear after a single training session. Here, we determined whether chronic intermittent access (CIA) to alcohol during adolescence/early adulthood or during adulthood would alter or be associated with auditory-cued conditioned fear levels using an extended training fear incubation procedure. This training procedure leads to low fear soon after training that grows over one month. Rats received 6 weeks of CIA to 20% alcohol or water from PND 26-66. Ten or eleven days later, the rats began behavioral testing that included 10 sessions of tone-shock pairings. Rats then received 4 weeks of CIA exposure during the 1-month fear incubation period and were tested for conditioned fear 6 days after the end of alcohol access. We found no evidence that voluntary alcohol consumption during adolescence/early adulthood or adulthood altered fear expression. However, we found that rats that consumed more alcohol during early adulthood (PND 54-66) had lower fear than low-consumption rats on day 1 of conditioned fear training and in the day 2 and 1-month tests. This extends associations we previously found between individual differences in alcohol consumption and conditioned fear to a different fear conditioning procedure. Combined with our previous data that show that the rate of instrumental extinction is associated with both alcohol consumption and conditioned fear, these data provide further support for the generality and reliability of a pair of phenotypes that encompass a wide variety of learning traits.

Punishment of alcohol-reinforced responding in alcohol preferring P rats reveals a bimodal population: Implications for models of compulsive drug seeking

Individual variations in animal behaviour can be used to describe relationships between different constructs, as well as the underlying neurobiological mechanisms responsible for such variation. In humans, variation in the expression of certain traits contributes to the onset of psychopathologies, such as drug addiction. Addiction is characterised by persistent drug use despite negative consequences, but it occurs in only a sub-population of drug users. Compulsive drug use is modelled in laboratory animals by punishing a drug-reinforced operant response. It has been reported that there is individual variability in the response to punishment, and in this report we aim to further define the conditions under which this variation can be observed. We have previously used footshock punishment to suppress alcohol seeking in an animal model of context-induced relapse to alcohol seeking after punishment-imposed abstinence. Here we present a re-examination of the training and punishment data from a large cohort of rats (n=499) collected over several years. We found evidence for a bimodal distribution in the response to punishment in alcohol preferring P rats. We only observed this population split when rats received constant shock intensity for three sessions, but not when increasing shock intensity was used. This observation provides evidence for the existence of two distinct groups of rats, defined by their response to punishment, in an otherwise homogeneous population. The implications of this observation are discussed in reference to prior observations using punishment of other addictive drugs (cocaine and methamphetamine), the potential causes of this phenomenon, and with broader implications for the cause of alcohol and drug addiction in humans.

Individual differences in conditioned fear are associated with levels of adolescent/early adult alcohol consumption and instrumental extinction

Previous research has shown a relationship between alcohol exposure and conditioned fear, but the nature of this relationship remains unclear. We determined whether chronic intermittent access to alcohol during adolescence and early adulthood would alter or be associated with the level of conditioned fear to an auditory cue in male Long Evans rats. Rats received 6 weeks of chronic intermittent access to 20% alcohol or water from PND 26-66 and began behavioral testing 10 days later. We found no evidence that voluntary alcohol consumption altered fear. However, we found that rats that consumed more alcohol had lower fear, as measured with conditioned suppression of lever-pressing and conditioned freezing to an auditory cue. We have previously shown that higher levels of alcohol consumption are correlated with faster instrumental extinction learning. Therefore, we determined whether instrumental extinction would be directly associated with conditioned fear in rats never given alcohol access. As predicted, we found that rats that exhibited faster instrumental extinction also exhibited lower conditioned fear, as measured with conditioned suppression of lever-pressing and conditioned freezing. Our results suggest that at least part of the relationship between alcohol consumption levels and fear learning differences may be due to pre-existing individual differences. In addition, our finding that conditioned fear and instrumental extinction abilities (both separately associated with alcohol consumption levels) are associated with each other suggests that alcohol consumption levels may be a marker that can distinguish two separate phenotypes that encompass a wide variety of learning traits.

Endocannabinoid signalling in reward and addiction

Brain endocannabinoid (eCB) signalling influences the motivation for natural rewards (such as palatable food, sexual activity and social interaction) and modulates the rewarding effects of addictive drugs. Pathological forms of natural and drug-induced reward are associated with dysregulated eCB signalling that may derive from pre-existing genetic factors or from prolonged drug exposure. Impaired eCB signalling contributes to dysregulated synaptic plasticity, increased stress responsivity, negative emotional states and cravings that propel addiction. Understanding the contributions of eCB disruptions to behavioural and physiological traits provides insight into the eCB influence on addiction vulnerability.

Startle-response based tasks and laboratory measures of impulsivity in abstinent alcoholic patients

AIMS

It is well known that impulsivity is a risk factor for the development of Addictive Disorders, and more specifically Alcohol Use Disorders (AUD). Recently, the Startle-Response Based Tasks (SRBT) and its different forms of plasticity have been found to be impaired in the alcoholic population. This is the first study to explore the correlation between impulsivity laboratory tasks and the SRBT test, in order to determine whether impulsivity and startle response (SR) could be related and in turn, explain their association with Alcohol Dependence (AD).

METHODS

SUBJECTS

40 men, who met DSM-IV criteria for AD and had been abstinent for at least one month. Impulsivity was assessed using three laboratory tests: Continuous Performance Test (CPT), Stop-Signal Task (SST) and Differential Reinforcement for Low-Rate Responding (DRL6). Patients also underwent the SR test. They were compared to 40 matched controls.

RESULTS

Impulsivity laboratory measures tasks (SST and commissions of the CPT) correlated positively with the magnitude of SR (P < 0.05) and with habituation (P < 0.05). Scores on DRL6 correlated negatively with the magnitude of SR (P < 0.05). This was not found in the control group.

CONCLUSIONS

The fact that impulsivity laboratory measures and the SR are correlated in patients but not in controls, could imply the existence of a common link for these two measures in alcoholic patients. Our findings support the hypothesis of the existence of two different vulnerability pathways for the development of AUD: anxiety and disinhibitory behaviour.

Acoustic startle in alcohol-naïve male rats predicts subsequent voluntary alcohol intake and alcohol preference

AIMS

Acoustic startle response in rats is used to model sensorimotor reactivity. The aim of the study was to determine whether acoustic startle response in alcohol-naïve rats predicts subsequent increased voluntary alcohol drinking or alcohol preference.

METHODS

Startle responses to 90, 95 and 100 decibel (dB) white noise stimuli presented in counterbalanced semi-randomized order were tested in alcohol-naïve young adult male Wistar rats before voluntary alcohol intake was established with an intermittent alcohol access (IAA) model.

RESULTS

Startle amplitude in response to 95 or 100 dB stimuli was positively correlated with subsequent alcohol intake and alcohol preference following 3 months of IAA. Rats with high (median split) pre-IAA startle amplitude in response to 95 or 100 dB stimuli developed increased alcohol intake as well as increased alcohol preference following 3 months of IAA, relative to rats with low pre-IAA startle amplitude.

CONCLUSION

Startle response to moderate acoustic stimuli can be a predictive index of vulnerability to developing increased alcohol drinking.

Alcohol-preferring rats show decreased corticotropin-releasing hormone-2 receptor expression and differences in HPA activation compared to alcohol-nonpreferring rats

BACKGROUND

Corticotropin-releasing hormone (CRH) and urocortins (UCNs) bind to corticotropin-releasing hormone type 2 receptor (CRF2 receptor ), a Gs protein-coupled receptor that plays an important role in modulation of anxiety and stress responses. The Crhr2 gene maps to a quantitative trait locus (QTL) for alcohol preference on chromosome 4 previously identified in inbred alcohol-preferring (iP) and-nonpreferring (iNP) F2 rats.

METHODS

Real-time polymerase chain reaction was utilized to screen for differences in Crhr2 mRNA expression in the central nervous system (CNS) of male iP and iNP rats. DNA sequence analysis was then performed to screen for polymorphism in Crhr2 in order to identify genetic variation, and luciferase reporter assays were then applied to test their functional significance. Next, binding assays were used to determine whether this polymorphism affected CRF2 receptor binding affinity as well as CRF2 receptor density in the CNS. Finally, social interaction and corticosterone levels were measured in the P and NP rats before and after 30-minute restraint stress.

RESULTS

Crhr2 mRNA expression studies found lower levels of Crhr2 mRNA in iP rats compared to iNP rats. In addition, DNA sequencing identified polymorphisms in the promoter region, coding region, and 3'-untranslated region between the iP and iNP rats. A 7 bp insertion in the Crhr2 promoter of iP rats altered expression in vitro as measured by reporter assays, and we found that CRF2 receptor density was lower in the amygdala of iP as compared to iNP rats. Male P rats displayed decreased social interaction and significantly higher corticosterone levels directly following 30-minute restraint when compared to male NP rats.

CONCLUSIONS

This study identified Crhr2 as a candidate gene of interest underlying the chromosome 4 QTL for alcohol consumption that was previously identified in the P and NP model. Crhr2 promoter polymorphism is associated with reduced mRNA expression in certain brain regions, particularly the amygdala, and lowered the density of CRF2 receptor in the amygdala of iP compared to iNP rats. Together, these differences between the animals may contribute to the drinking disparity as well as the anxiety differences of the P and NP rats.

Effects of stress, acute alcohol treatment, or both on pre-pulse inhibition in high- and low-alcohol preferring mice

Pre-pulse inhibition of the acoustic startle reflex (PPI) is a measure of sensorimotor gating frequently used to assess information processing in both humans and rodents. Both alcohol and stress exposure can modulate PPI, making it possible to assess how stress and alcohol interact to influence information processing. Humans with an increased genetic risk for alcoholism are more reactive to stressful situations compared to those without a family history, and alcohol may have stress-dampening effects for those with high genetic risk. The purpose of the present study was to examine the effects of stress, acute alcohol exposure, or both on PPI in male and female mice selectively bred for high- (HAP2) and low- (LAP2) alcohol preference. Experiment 1 assessed the effects of various doses of acute alcohol on PPI. Experiments 2 and 3 assessed the effect of 10 days of restraint stress on subsequent PPI tested at 30 min (Experiment 2) or 24 h (Experiment 3) following the termination of stress exposure. Experiment 3 also examined the effects of acute alcohol treatment (0.75 g/kg) on PPI in mice previously exposed to stress or no stress. Results indicate that 0.75 and 1.0 g/kg doses of alcohol increased PPI in HAP2 but not LAP2 mice. When PPI was tested 30 min after stress exposure, stressed HAP2 mice showed a trend toward decreased PPI and stressed LAP2 mice showed a trend toward increased PPI. The combination of stress and alcohol treatment did not alter PPI in either line 24 h following the termination of stress exposure, suggesting that alcohol does not ameliorate the effect of stress on PPI. Stressed LAP2 mice had increased basal circulating corticosterone on the final stress exposure day compared to non-stressed LAP2 mice, and no difference was found between stressed and non-stressed HAP2 mice. The results suggest that high genetic risk for alcoholism may be related to increased sensitivity to alcohol and stress effects on PPI, and this sensitivity could signify an endophenotype for increased genetic risk to develop alcoholism.

Aberrant histone deacetylase2-mediated histone modifications and synaptic plasticity in the amygdala predisposes to anxiety and alcoholism

BACKGROUND

Epigenetic mechanisms have been implicated in psychiatric disorders, including alcohol dependence. However, the epigenetic basis and role of specific histone deacetylase (HDAC) isoforms in the genetic predisposition to anxiety and alcoholism is unknown.

METHODS

We measured amygdaloid HDAC activity, levels of HDAC isoforms, and histone H3 acetylation in selectively bred alcohol-preferring (P) and -nonpreferring (NP) rats. We employed HDAC2 small interfering RNA infusion into the central nucleus of amygdala (CeA) of P rats to determine the causal role of HDAC2 in anxiety-like and alcohol-drinking behaviors. Chromatin immunoprecipitation analysis was performed to examine the histone acetylation status of brain-derived neurotrophic factor (Bdnf) and activity-regulated cytoskeleton associated protein (Arc) genes. Golgi-Cox staining was performed to measure dendritic spine density.

RESULTS

We found that P rats innately display higher nuclear HDAC activity and HDAC2 but not HDAC 1, 3, 4, 5, and 6 protein levels and lower acetylation of H3-K9 but not H3-K14, in the CeA and medial nucleus of amygdala compared with NP rats. Acute ethanol exposure decreased amygdaloid HDAC activity and HDAC2 protein levels, increased global and gene (Bdnf and Arc)-specific histone acetylation, and attenuated anxiety-like behaviors in P rats but had no effects in NP rats. The HDAC2 knockdown in the CeA attenuated anxiety-like behaviors and voluntary alcohol but not sucrose consumption in P rats and increased histone acetylation of Bdnf and Arc with a resultant increase in protein levels that correlated with increased dendritic spine density.

CONCLUSIONS

These novel data demonstrate the role of HDAC2-mediated epigenetic mechanisms in anxiety and alcoholism.

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.

From QTL to candidate gene: a genetic approach to alcoholism research

A major focus of research in alcohol-related disorders is to identify the genes and pathways that modulate alcohol-seeking behavior. In light of this, animal models have been established to study various aspects of alcohol dependence. The selectively bred alcohol-preferring (P) and -nonpreferring (NP) lines were developed from Wistar rats to model high and low voluntary alcohol consumption, respectively. Using inbred P and NP strains, a strong QTL (LOD-9.2) for alcohol consumption was identified on rat chromosome 4. To search for candidate genes that underlie this chromosomal region, complementary molecular-based strategies were implemented to identify genetic targets that likely contribute to the linkage signal. In an attempt to validate these genetic targets, corroborative studies have been utilized including pharmacological studies, knock-out/transgenic models as well as human association studies. Thus far, three candidate genes, neuropeptide Y (Npy), alpha-synuclein (Snca), and corticotrophin-releasing factor receptor 2 (Crhr2), have been identified that may account for the linkage signal. With the recent advancements in bioinformatics and molecular biology, QTL analysis combined with molecular-based strategies provides a systematic approach to identify candidate genes that contribute to various aspects of addictive behavior.

Innate differences in the expression of brain-derived neurotrophic factor in the regions within the extended amygdala between alcohol preferring and nonpreferring rats

BACKGROUND

Animal lines such as alcohol-preferring (P) and nonpreferring (NP) rats appear to be suitable animal models to investigate the biological basis of alcohol-drinking behaviors. The extended amygdala serves as a neuroanatomical substrate for alcohol-drinking behaviors. Brain-derived neurotrophic factor (BDNF) in the amygdala has been implicated in alcohol-drinking behaviors; however, its expression in the extended amygdala of P and NP rats is unknown. Therefore, we examined the basal expression of BDNF in the extended amygdala of alcohol naive P and NP rats.

METHODS

We determined the basal mRNA and protein levels of BDNF by in situ RT-PCR and immuno-histochemical procedure, respectively, in the amygdaloid [central nucleus of amygdala (CeA), medial nucleus of amygdala (MeA), and basolateral amygdala (BLA)], nucleus accumbal (NAc shell and core), and bed nucleus of stria terminalis (BNST) [lateral BNST (lBNST), medial BNST (mBNST), and ventral BNST (vBNST)] brain structures of P and NP rats. In addition, we examined the localization of BDNF in neurons using double-immunofluorescence labeling of BDNF with neuron-specific nuclear protein (NeuN) and also determined the number of NeuN-positive neurons in the amygdaloid structures of P and NP rats.

RESULTS

The mRNA and protein levels of BDNF were found to be significantly lower in both the CeA and MeA, but not in the BLA, of P compared with NP rats. We also found that BDNF was expressed in neurons in the amygdaloid structures of P and NP rats. In addition, we found that the number of NeuN-positive neurons was similar in the amygdaloid structures of P and NP rats. Interestingly, the mRNA and protein levels of BDNF were also significantly lower in the lBNST, mBNST, and vBNST of P compared with NP rats. On the other hand, mRNA and protein levels of BDNF were similar in the NAc shell and core structures of P and NP rats.

CONCLUSIONS

P and NP rats are selectively bred for higher and lower alcohol preference, respectively; therefore it is possible that lower BDNF levels in the amygdaloid and BNST structures may be associated with the excessive alcohol-drinking behaviors of P rats.

Genetic correlation between innate alcohol preference and fear-potentiated startle in selected mouse lines

BACKGROUND

There is a high rate of co-occurrence between anxiety and alcohol-use disorders in humans that may arise from the inheritance of common genes that increase the risk for both psychiatric disorders. The purpose of this study was to investigate whether a genetic relationship exists between innate alcohol preference and propensity to develop learned fear, using the fear-potentiated startle (FPS) paradigm, in 2 mouse lines selectively bred for high or low alcohol preference.

METHODS

Alcohol-naïve, male, and female mice from replicate pairs of lines selectively bred for high alcohol preference and low alcohol preference were randomly assigned to a fear-conditioned or control group. Mice in the fear-conditioned group received 20 pairings of a light stimulus and footshock; the control group received the same number of exposures to light and footshock, except that these stimuli were explicitly unpaired. During testing for FPS, acoustic stimuli were presented both in the presence and in the absence of the light stimulus.

RESULTS

In both replicate pairs of lines, mice selectively bred for high alcohol preference showed greater FPS than mice selectively bred for low alcohol preference. No sex differences in FPS were found in any line. Control groups did not show FPS.

CONCLUSION

These findings suggest that common genes mediate both innate alcohol preference and propensity to develop learned fear in these selected mouse lines.

Phenotypic characterization of genetically selected Sardinian alcohol-preferring (sP) and -non-preferring (sNP) rats

Sardinian alcohol-preferring (sP) and -non-preferring (sNP) rats are one of the pairs of rat lines selectively bred for high and low alcohol preference and consumption, respectively, under the homecage, continuous two-bottle choice regimen. sP rats meet most of the fundamental criteria for an animal model of alcoholism, in that they voluntarily consume sufficient amounts of alcohol to achieve significant blood alcohol levels and produce psychopharmacological effects, including anxiolysis and motor stimulation. sP rats are also willing to 'work' (such as lever-pressing) for alcohol. Chronic alcohol drinking in sP rats results in the development of tolerance to a given effect of alcohol (specifically, motor incoordination) and relapse-like drinking (the alcohol deprivation effect). Conversely, sNP rats avoid alcohol virtually completely; their avoidance for alcohol being resistant even to an environmental manipulation such as long-term exposure to alcohol plus sucrose. sP and sNP rats have been characterized for different phenotypes, possibly associated to their different alcohol preference and consumption. In comparison with sNP rats, alcohol-naive sP rats displayed (1) more anxiety-related behaviors; (2) higher initial sensitivity to the locomotor stimulating and sedative/hypnotic effects of alcohol; and (3) lower sensitivity to the aversive effects of alcohol. The present paper reviews the data collected to date on alcohol drinking behavior and other alcohol-related behaviors in sP and sNP rats. The behavioral profile of sP rats is also compared with that of other lines of selectively bred alcohol-preferring rats and the heterogeneity resulting from this comparison is discussed in terms of different animal models for the different forms of alcoholism.

Fear state induced by ethanol withdrawal may be due to the sensitization of the neural substrates of aversion in the dPAG

The neural substrate underlying the aversive effects induced by ethanol abstinence is still unclear. One candidate for such effects is the dorsal periaqueductal gray (dPAG), a core structure of the brain aversion system. The main aim of this study is to examine the role of the dPAG as a possible locus of the aversive effects following abrupt alcohol withdrawal. To this end, rats were subjected to an oral ethanol self-administration procedure, in which animals were offered 6-8% (v/v) ethanol solution for a period of 21 days followed by an abrupt discontinuation of the treatment on the two subsequent days. Control animals received control dietary fluid for similar periods of time. The effects of ethanol withdrawal were examined in the elevated plus-maze (EPM) (Exp. I), on the prepulse inhibition of startle to loud sounds (Exp. II) and on the freezing and escape responses induced by electrical stimulation of the dPAG (Exp. III). In Experiment III, rats were implanted with an electrode aimed at the dPAG and the number and duration of ultrasonic vocalizations (USVs) were also recorded in the rats that received dPAG stimulation at freezing and escape thresholds. Data obtained showed that ethanol withdrawal elicited significant "anxiety-like" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. Startle reflex and prepulse inhibition remained unchanged in withdrawn animals. In addition, discontinuation from the chronic ethanol regimen caused a reduction in the stimulation thresholds for freezing and escape and in the number and duration of USVs. Together, these effects have been interpreted in the frame of a high fear state elicited by activation of the dPAG. These findings are indicative that ethanol withdrawal sensitizes the substrates of fear at the level of this midbrain structure.

Acoustic Startle Reactivity During Acute Alcohol Withdrawal in Rats That Differ in Genetic Predisposition Toward Alcohol Drinking: Effect of Stimulus Characteristics

BACKGROUND

We have previously reported an association between greater alcohol withdrawal magnitude after a single alcohol exposure and a genetic predisposition toward low alcohol drinking in rats selectively bred for differences in alcohol intake when acoustic startle reactivity to a tone stimulus was used to index acute alcohol withdrawal. The purpose of this study was to examine whether the quality of the acoustic startle stimulus (noise versus tone) is important for detecting a genetic relationship between alcohol withdrawal magnitude and alcohol drinking behavior.

METHODS

Alcohol-naive male rats selectively bred for high alcohol intake [alcohol-preferring (P), high-alcohol-drinking (HAD)1, and HAD2] or low alcohol intake [alcohol-nonpreferring (NP), low-alcohol-drinking (LAD)1, and LAD2] received a single intragastric infusion of water or alcohol (4.0 g/20.3 ml/kg; 25% v/v), and acoustic startle test sessions were given at 14, 16, 18, 20, and 24 hr after infusion. Each test session consisted of a 5-min acclimation period followed by random presentation of various white noise stimuli (90, 100, 110, and 120 dB.)

RESULTS

Line differences in acoustic startle magnitude under control conditions were present in all three pairs of selectively bred lines; P rats showed a greater startle magnitude relative to NP rats, whereas both LAD lines showed a greater startle magnitude relative to both HAD lines. During alcohol withdrawal, the P, HAD1, and HAD2 lines showed enhanced startle magnitude compared with their water-treated controls. No change in startle magnitude during alcohol withdrawal was found in the NP, LAD1, or LAD2 lines.

CONCLUSIONS

In contrast to our prior findings, these results showed a genetic association between high alcohol drinking and a greater startle response magnitude to a noise stimulus during alcohol withdrawal. It seems that the genetic association between alcohol drinking and alcohol withdrawal, as assessed by the acoustic startle response, depends on the quality of the acoustic startle stimulus.

Deficits in amygdaloid cAMP-responsive element-binding protein signaling play a role in genetic predisposition to anxiety and alcoholism

We investigated the role of cAMP-responsive element-binding protein (CREB) in genetic predisposition to anxiety and alcohol-drinking behaviors using alcohol-preferring (P) and -nonpreferring (NP) rats. The levels of CREB, phosphorylated CREB, and neuropeptide Y (NPY) were innately lower in the central amygdala (CeA) and medial amygdala (MeA), but not in the basolateral amygdala (BLA), of P rats compared with NP rats. P rats displayed higher baseline anxiety-like behaviors and consumed higher amounts of alcohol compared with NP rats. Ethanol injection or voluntary intake reduced the higher anxiety levels in P rats. Ethanol also increased CREB function in the CeA and MeA, but not in the BLA, of P rats. Infusion of the PKA activator Sp-cAMP or NPY into the CeA decreased the alcohol intake and anxiety-like behaviors of P rats. PKA activator infusion also increased CREB function in the CeA of P rats. On the other hand, ethanol injection or voluntary intake did not produce any changes either in anxiety levels or on CREB function in the amygdaloid structures of NP rats. Interestingly, infusion of the PKA inhibitor Rp-cAMP into the CeA provoked anxiety-like behaviors and increased alcohol intake in NP rats. PKA inhibitor decreased CREB function in the CeA of NP rats. These novel results provide the first evidence to our knowledge that decreased CREB function in the CeA may be operative in maintaining the high anxiety and excessive alcohol-drinking behaviors of P rats.

Ethanol withdrawal and proclivity are inversely related in rats selectively bred for differential saccharin intake

Withdrawal severity and voluntary alcohol consumption are inversely related in rats and mice. The present study demonstrated this empirical relation and extended it in two ways. First, the rats were selectively bred for low (LoS) and high (HiS) saccharin intake, a phenotype that correlates positively with ethanol intake and inversely with emotional reactivity. Withdrawal has not yet been studied in these rats. Second, proclivity to consume ethanol was measured as conditioned preference for an ethanol-paired flavor. After 2 weeks of forced exposure to ethanol and a period of abstinence, LoS rats showed elevated acoustic startle; HiS rats did not (Exp. 1). When ethanol- and no-ethanol solutions were available freely during conditioning, both LoS and HiS rats preferred a flavor paired with 4% ethanol, but only HiS rats preferred a flavor paired with 10% ethanol (Exp. 2A); when exposure to the two solutions was controlled, all groups except LoS males preferred flavors paired with 4% or 10% ethanol (Exp. 2B). Thus, as predicted, withdrawal was more severe in the line with less ethanol proclivity (LoS). These results implicate basic associative and affective processes in individual differences in patterns of alcohol use.

Different expression of brain-derived neurotrophic factor in the nucleus accumbens of alcohol-preferring (P) and -nonpreferring (NP) rats

Recent studies suggest that the gene that encodes brain-derived neurotrophic factor (BDNF) might be linked with vulnerability to alcohol abuse. We have now compared BDNF protein levels in several brain regions between alcohol-naive alcohol-preferring (P) and -nonpreferring (NP) rats using the enzyme-linked immunosorbent assay (ELISA) procedure. The results showed that BDNF levels in the nucleus accumbens of the P rats were significantly lower than those of the NP rats, suggesting that this innate difference may contribute to the disparate alcohol drinking behavior of the P and NP rats.

High-alcohol-drinking rats exhibit persistent freezing responses to discrete cues following Pavlovian fear conditioning

We previously reported that high-alcohol-drinking (HAD) rats exhibited selective deficits in active avoidance learning and that those deficits were partially reversed by moderate doses of ethanol under certain training conditions [Pharmacol. Biochem. Behav. 75 (2003) 89]. In that study, we hypothesized that HAD deficits resulted from exaggerated fear in the conditioning context and that the anxiolytic properties of ethanol, along with prior exposure to the conditioning apparatus, were responsible for the facilitated avoidance learning that was observed in HAD rats following moderate doses of ethanol. The current study was designed to test whether HAD rats exhibit behaviors consistent with increased fear in aversive learning contexts. We used a standard Pavlovian fear conditioning paradigm to assess behavioral freezing in HAD (HAD-1 and HAD-2) and low-alcohol-drinking (LAD; LAD-1 and LAD-2) rats. No significant differences were observed between HAD-1 and HAD-2 or between LAD-1 and LAD-2 rats, indicating that the replicate lines performed similarly in this study. Both HAD and LAD rats exhibited robust fear conditioning during training. Although no differences were observed between HAD and LAD rats during fear training, HAD rats failed to extinguish freezing behavior in response to the discrete tone conditional stimulus during subsequent fear retention tests. Thus, HAD rats demonstrated prolonged cue-elicited fear that was resistant to extinction.

Amphetamine-modified acoustic startle responding and prepulse inhibition in adult and adolescent alcohol-preferring and -nonpreferring rats

Selective breeding has been used to develop the alcohol-preferring (P) and -nonpreferring (NP) rats, with the P rat having lower CNS levels of dopamine (DA) and reduced DA innervation in the nucleus accumbens compared with the NP rat. The acoustic startle response (ASR) and prepulse inhibition (PPI) of the ASR are experimental behaviors altered by DA agonists. We examined whether functional differences in amphetamine (AMPH)-modified ASR and PPI exist between P and NP rats. AMPH [0.0 (saline), 1.0, 2.0, or 4.0 mg/kg] was injected 15 min prior to placement into a startle apparatus. After a 5-min habituation period, rats were given approximately twelve 95-, 105-, or 115-dB white-noise burst (ASR) and PPI trials. As adults, P rats were sensitive to AMPH potentiation of the ASR to a greater extent than NP rats. During adolescence, P and NP rats had similar levels of AMPH-potentiated ASR. As adults, NP rats displayed potentiated, rather than disrupted, PPI at the 1.0-mg/kg dose, whereas P rats displayed the expected disrupted PPI at the 4.0-mg/kg dose. As adolescents, NP rats did not display significant differences in PPI after AMPH, whereas P rats displayed dose-dependent disruption of PPI, which was significant at the 4.0-mg/kg dose. The limited effect of AMPH on increasing the ASR and the presence of AMPH-potentiated PPI at the lowest dose in the adult NP rat suggests reduced functioning of the interactions between DA circuits and the neurocircuitry mediating the ASR and PPI, compared with P rats. However, the neurocircuitry mediating PPI does not appear to be fully developed in the adolescent NP rat. The present findings also indicate that lower levels of DA content and immunoreactive fibers in the P rat may not reflect reduced DA neuronal activity, because the P rat displayed AMPH-potentiated ASR, and, at the highest dose, AMPH disruption of PPI during both adulthood and adolescence.

Moderate doses of ethanol partially reverse avoidance learning deficits in high-alcohol-drinking rats

We previously reported that ethanol-naive high-alcohol-drinking (HAD1 and HAD2) rats exhibited selective deficits in active avoidance learning, as compared to low-alcohol-drinking (LAD1 and LAD2) rats, in a signaled bar-pressing task [Alcohol. Clin. Exp. Res. 24 (2000) 1778]. In the current study, we used appetitive and aversive learning tasks to assess whether administration of ethanol influences approach and avoidance learning in HAD and LAD rats. Rats were administered 0.0, 0.5, 1.0, or 1.5 g ethanol/kg body weight during appetitive and aversive conditioning sessions. We found that ethanol impaired acquisition of the appetitive conditioned response in a dose-dependent manner in both HAD and LAD rats, with 1.5 g/kg ethanol producing the greatest deficits. Notably, moderate doses of ethanol (0.5 and 1.0 g/kg) partially reversed avoidance learning deficits in HAD rats, but only when appetitive conditioning preceded aversive conditioning. The highest dose (1.5 g/kg EtOH) abolished avoidance responding altogether in HAD rats. Avoidance responding in LAD rats was not affected by any dose of ethanol. These results are consistent with previous studies suggesting that alcohol preference may be associated with increased fear or anxiety, but the conditions under which ethanol produces a reduction of fear and anxiety in HAD rats appear to be relatively complex.

Further evidence of an inverse genetic relationship between innate differences in alcohol preference and alcohol withdrawal magnitude in multiple selectively bred rat lines

BACKGROUND

We have previously shown that a genetic association exists between low alcohol drinking and high alcohol withdrawal magnitude after acute alcohol exposure in alcohol-naïve rats. However, the behavioral rating scale used in this prior study was not optimal for assessing the magnitude of mild alcohol withdrawal. The present study examined whether a genetic relationship is again found between alcohol preference and alcohol withdrawal magnitude when a sensitive measure is used to index mild alcohol withdrawal in rats.

METHODS

Alcohol-naïve, male rats selectively bred for alcohol preference (P, HAD1, HAD2) or nonpreference (NP, LAD1, LAD2) received a single intragastric infusion of alcohol (4.0 g/20.3 ml/kg body weight; 25% v/v) or water followed by acoustic startle testing.

RESULTS

Startle probability and magnitude was greater in water-treated P than in water-treated NP rats. During alcohol withdrawal, startle probability and magnitude was suppressed in P rats and elevated in NP rats relative to water-treated controls. Startle probability and magnitude was greater in water-treated LAD1 rats than in water-treated HAD1 rats. During alcohol withdrawal, startle probability and magnitude was suppressed in HAD1 and elevated in LAD1 rats relative to water-treated controls at 20 hr after acute alcohol exposure. Startle probability and magnitude did not differ between water-treated HAD2 and water-treated LAD2 rats. During alcohol withdrawal, there was a trend toward decreased startle probability and magnitude in HAD2 rats compared with water-treated controls.

CONCLUSIONS

The acoustic startle response to a tone stimulus is a sensitive measure of mild alcohol withdrawal in rats. Rats selectively bred for low alcohol intake showed greater alcohol withdrawal magnitude than did rats selectively bred for high alcohol intake. These results provide further evidence that an inverse genetic association exists between alcohol withdrawal magnitude and propensity toward alcohol drinking in rats.

Effects of ethanol on startle responding in alcohol-preferring and -non-preferring rats

The objectives of the present study were to determine (a) if differences exist between the selectively bred alcohol-preferring (P) and -non-preferring (NP) lines of rats in the acoustic startle response (ASR) and prepulse inhibition (PPI), and (b) the effects of ethanol on these measures. Alcohol-naïve adult female P and NP rats received a single i.p. injection of saline or ethanol (0.25, 0.5, 1. 0, or 1.5 g/kg) and were placed in the startle apparatus 10 min later. After a 5-min acclimation period, rats received five alternating trials of a startle stimulus alone (SSA) (115-dB white noise) or a PPI trial (90-dB white noise preceding a 115-dB white noise). Analysis of the ASR revealed that P rats exhibited higher startle amplitudes than did NP rats with saline injections. The 0. 5-g/kg ethanol dose reduced the startle amplitude in P, but not NP, rats. The 1.0- and 1.5-g/kg ethanol doses nearly abolished the ASR in the NP line, whereas only the highest ethanol dose had this effect in the P line. Vehicle-treated P and NP rats exhibited comparable PPI levels, but only P rats showed a significant disruption (30%) at the 0.50-g/kg ethanol dose. Neither P nor NP rats were affected by ethanol treatment at the 0.25-g/kg dose. Overall, the results suggest that: (a) the difference in baseline ASR may indicate line differences in the neurocircuitry mediating this response, possibly reflecting higher innate levels of emotional reactivity in the P line; (b) the P line may be more sensitive than the NP line to the effects of ethanol in reducing emotional reactivity; and (c) low-dose ethanol may have a greater disruptive effect on sensorimotor gating mechanisms in the P than NP rat.