Psychosocial stress and chronic ethanol ingestion in male rats: Effects on elevated plus maze behavior and ultrasonic vocalizations

Behavioral analysis in laboratory rats: Challenges and usefulness of 50-kHz ultrasonic vocalizations

Many rodent species emit and detect vocalizations in the ultrasonic range. Rats use three classes of ultrasonic vocalizations depending on developmental stage, experience and the behavioral situation. Calls from one class emitted by juvenile and adult rats, the so-called 50-kHz calls, are typical for appetitive and social situations. This review provides a brief historical account on the introduction of 50-kHz calls in behavioral research followed by a survey of their scientific applications focusing on the last five years, where 50-kHz publications reached a climax. Then, specific methodological challenges will be addressed, like how to measure and report 50-kHz USV, the problem of assignment of acoustic signals to a specific sender in a social situation, and individual variability in call propensity. Finally, the intricacy of interpreting 50-kHz results will be discussed focusing on the most prevalent ones, namely as communicative signals and/or readouts of the sender's emotional status.

Paired Housing or a Socially-Paired Context Decreases Ethanol Conditioned Place Preference in Male Rats

Alcohol abuse dramatically affects individuals’ lives nationwide. The 2020 National Survey on Drug Use and Health (NSDUH) estimated that 10.2% of Americans suffer from alcohol use disorder. Although social support has been shown to aid in general addiction prevention and rehabilitation, the benefits of social support are not entirely understood. The present study sought to compare the benefits of social interaction on the conditioned ethanol approach behavior in rats through a conditioned place preference (CPP) paradigm in which a drug is paired with one of two distinct contexts. In experiment 1A, rats were single-housed and received conditioning trials in which ethanol was paired with the less preferred context. In experiment 1B, rats underwent procedures identical to experiment 1A, but were pair-housed throughout the paradigm. In experiment 1C, rats were single-housed, but concurrently conditioned to a socially-paired context and an ethanol-paired context. By comparing the time spent between the ethanol-paired environment and the saline-paired or socially-paired environment, we extrapolated the extent of ethanol approach behavior in the pair-housed, single-housed, and concurrently conditioned rats. Our results revealed that social interaction, both in pair-housed animals or concurrently socially-conditioned animals, diminished the ethanol approach behavior, which highlights the importance of social support in addiction prevention, treatment, and recovery programs.

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

Social Interaction With Relapsed Partner Facilitates Cocaine Relapse in Rats

Social factors strongly contribute to drug use and relapse, and epidemiological studies have found that members of peer groups influence each other to use drugs. However, previous animal models mostly failed to incorporate social factors and demonstrate the effects of social partners on drug addiction and relapse. In the present study, we investigated the transfer of relapse to cocaine seeking between drug-addicted partners in rats. Male Sprague-Dawley rats were pair-housed and subjected to training and extinction of cocaine self-administration and conditioned place preference (CPP). 24 h after extinction test, the targeted rats interacted with a cocaine-primed (relapsed) partner or stranger, or saline-injected (unrelapsed) partner for 30 min, after which the targeted rats were tested for drug seeking behavior. We found that social interaction with a relapsed partner increased drug seeking behavior in cocaine self-administration and CPP models in rats, while social interaction with an unrelapsed partner or relapsed stranger had no effect on cocaine seeking. Moreover, the effect of social interaction on cocaine seeking could last for at least 1 day. Our findings demonstrate a facilitation effect of relapsed social partners on drug relapse in rats and provide a novel animal model for social transfer of drug relapse.

Effects of pair housing on voluntary alcohol intake in male and female Wistar rats

A number of different voluntary alcohol intake paradigms are available for home cage drinking studies. Traditionally, these paradigms involve single housing in order for individual intake to be measured. This study aimed at investigating the effects of pair housing on voluntary alcohol intake. Male and female Wistar rats were housed in pairs or individually for studies of voluntary alcohol intake using the modified intermittent access paradigm with alcohol access during three consecutive days per week followed by four days of water only. Individual intake of 20% alcohol solution and water was measured during 12 sessions, i.e., 4 weeks. Pair-housed animals could interact freely with their cage mate for four consecutive days each week and were then separated by an inserted mesh divider for three consecutive days each week during alcohol intake sessions. Alcohol intake and preference were compared between pair-housed and individually housed rats. The results revealed higher alcohol intake in females than in males. Pair-housed males had a higher alcohol intake and preference during the first 3 weeks, but not during the fourth week, compared to individually housed males No effect of housing condition was observed in female rats. The alcohol intake was higher on the first day of access relative to the two consecutive days in pair-housed males and higher on the first two days relative to the third day in female rats. Social rank or female estrus cycle had no effect on alcohol intake or preference. Taken together, the use of a divider during alcohol intake sessions had no impact on alcohol intake in female rats and may not exert long-term influences in male rats. Future studies are needed in order to elucidate whether the use of a divider can constitute an experimental refinement as an alternative to individual housing in studies of voluntary alcohol intake using the limited access and/or intermittent access paradigms.

Malocclusion Generates Anxiety-Like Behavior Through a Putative Lateral Habenula-Mesencephalic Trigeminal Nucleus Pathway

Malocclusion is an important risk factor for temporomandibular disorder (TMD), a series of disorders characterized by dysfunction in the orofacial region involving the temporomandibular joint (TMJ) and jaw muscles. We recently showed that experimental unilateral anterior crossbite (UAC) produced masseter hyperactivity through a circuit involving the periodontal proprioception, trigeminal mesencephalic nucleus (Vme), and trigeminal motor nucleus (Vmo). Anxiety is a common complication in patients with TMD. The lateral habenula (LHb) is involved in emotional modulation and has direct projections to the Vme. Therefore, the present research examined whether UAC facilitates excitatory input from the LHb to the Vme and, subsequently, anxiety-like behaviors in rats. The LHb activation was evaluated by the electrophysiological recording, assessment of vesicular glutamate transporter-2 (VGLUT2) mRNA expression, and measurement of anxiety-like behaviors. The effects of LHb activity on Vme were evaluated by electrophysiological recording from Vme neurons and local changes in VGLUT2 protein density. UAC produced anxiety in modeled rats and increased neuronal activity in the LHb. VGLUT2 mRNA expression was also increased in the LHb. Further, VGLUT2-positive boutons were observed in close apposite upon parvalbumin (PV)-labeled Vme neurons. VGLUT2 protein expression was also increased in the Vme. Significantly, injection of VGLUT2-targeted shRNA into the LHb reduced the expression of VGLUT2 protein in the Vme, attenuated UAC-associated anxiety-like behaviors, and attenuated electrophysiological changes in the Vme neurons. In conclusion, we show that UAC activates the LHb neurons as well as the periodontal proprioceptive pathway to provide excitatory input to the Vme and produce anxiety in rats. These findings provide a rationale for suppressing activity of the LHb to attenuate both the physical and psychological effects of TMD.

Assessing Social Alcohol Drinking in Rodent Models: Are We There Yet?

Pathological and social consequences associated with excessive alcohol consumption and dependence constitute a serious concern for human well-being. The success of preventative and therapeutic strategies for alcohol use disorder depends on the development of appropriate animal models of alcohol consumption. Alcohol consumption in humans typically occurs in social settings. In contrast, the vast majority of animal models investigate alcohol drinking in socially isolated animals. A number of rodent model studies have attempted to assess drinking of individual animals within social setting. These studies identified varied directions of effects of social environment on alcohol consumption. However, the interpretation of these results is strongly hampered by technical issues associated with the existing approaches to assess drinking of individual animals within each group. This review analyzes the existing literature mostly focusing on the interpretation caveats of performed studies and highlights some of the new directions in these studies.

Acoustic Communication in Rats: Effects of Social Experiences on Ultrasonic Vocalizations as Socio-affective Signals

Ultrasonic vocalizations (USV) serve important communicative functions as socio-affective signals in rats. In aversive situations, such as inter-male aggression and predator exposure, 22-kHz USV are emitted. They likely function as appeasement signals during fighting and/or as alarm calls to warn conspecifics. In appetitive situations, 50-kHz USV are uttered, most notably during social interactions, such as rough-and-tumble play and mating. It is believed that they fulfill an affiliative function as social contact calls. Social experiences or their lack, such as social isolation, can have profound impact on the emission of 22- and 50-kHz USV by the sender in later life, albeit direction and strength of observed effects vary, with time point of occurrence and duration being critical determinants. Little, however, is known about how social experiences affect the behavioral responses evoked by 22- and 50-kHz USV in the recipient. By means of our 50-kHz USV radial maze playback paradigm, we recently showed that the behavioral response elicited in the recipient is affected by post-weaning social isolation. Rats exposed to four weeks of isolation during the rough-and-tumble play period did not display social approach behavior toward 50-kHz USV but some signs of social avoidance. We further found that physical environmental enrichment providing minimal opportunities for social interactions has similar detrimental effects. Together, this indicates that social experiences can affect socio-affective communication in rodents, both at the level of sender and recipient. Deficits seen following post-weaning social isolation or physical environmental enrichment might be useful to model aspects of neurodevelopmental disorders characterized by social and communication deficits, such as autism and schizophrenia.

Environmental enrichment may protect against neural and behavioural damage caused by withdrawal from chronic alcohol intake

Exposure to stress and prolonged exposure to alcohol leads to neuronal damages in several brain regions, being the medial prefrontal cortex (mPFC) one of the most affected. These changes presumably reduce the ability of the organism to cope with these stimuli and may underlie a series of maladaptive behaviours among which include drug addiction and withdrawal. Drug-addicted individuals show a pattern of behavior similar to patients with lesions of the mPFC. This impairment in the decision-making could be one of the mechanisms responsible for the transition from the casual to compulsive drug use. The environmental enrichment (EE) has a protective effect on the neural and cognitive impairments induced by psychoactive drugs, including ethyl alcohol. The present study aims to determine the influence of withdrawal from intermittent long-term alcohol exposure on alcohol preference, emotional reactivity and neural aspects of early isolated or grouped reared rats kept under standard or complex environments and the influence of social isolation on these measures, as well. Our results point out new insights on this matter showing that the EE can attenuate the adverse effects of withdrawal and social isolation on rat's behavior. This effect is probably due to its protective action on the mPFC integrity, including the cingulate area 1 (Cg1), and the prelimbic (PrL) and infralimbic cortex (IL), what could account for the absence of changes in the emotional reactivity in EE alcohol withdrawal rats. We argue that morphological changes at these cortical levels can afford the emotional, cognitive and behavioural dysregulations verified following withdrawal from chronic alcohol intake.

Animal models of social contact and drug self-administration

Social learning theories of drug abuse propose that individuals imitate drug use behaviors modeled by social peers, and that these behaviors are selectively reinforced and/or punished depending on group norms. Historically, animal models of social influence have focused on distal factors (i.e., those factors outside the drug-taking context) in drug self-administration studies. Recently, several investigators have developed novel models, or significantly modified existing models, to examine the role of proximal factors (i.e., those factors that are immediately present at the time of drug taking) on measures of drug self-administration. Studies using these newer models have revealed several important conclusions regarding the effects of social learning on drug abuse: 1) the presence of a social partner influences drug self-administration, 2) the behavior of a social partner determines whether social contact will increase or decrease drug intake, and 3) social partners can model and imitate specific patterns of drug self-administration. These findings are congruent with those obtained in the human laboratory, providing support for the cross-species generality and validity of these preclinical models. This mini-review describes in detail some of the preclinical animal models used to study social contact and drug self-administration to guide future research on social learning and drug abuse.

Tests of unconditioned anxiety - pitfalls and disappointments

The plus-maze, the light-dark box and the open-field are the main current tests of unconditioned anxiety for mice and rats. Despite their disappointing achievements, they remain as popular as ever and seem to play an important role in an ever-growing demand for behavioral phenotyping and drug screening. Numerous reviews have repeatedly reported their lack of consistency and reliability but they failed to address the core question of whether these tests do provide unequivocal measures of fear-induced anxiety, that these measurements are not confused with measures of fear-induced avoidance or natural preference responses - i.e. discriminant validity. In the present report, I examined numerous issues that undermine the validity of the current tests, and I highlighted various flaws in the aspects of these tests and the methodologies pursued. This report concludes that the evidence in support of the validity of the plus-maze, the light/dark box and the open-field as anxiety tests is poor and methodologically questionable.

Alcohol disinhibition of behaviors in C. elegans

Alcohol has a wide variety of effects on physiology and behavior. One of the most well-recognized behavioral effects is disinhibition, where behaviors that are normally suppressed are displayed following intoxication. A large body of evidence has shown that alcohol-induced disinhibition in humans affects attention, verbal, sexual, and locomotor behaviors. Similar behavioral disinhibition is also seen in many animal models of ethanol response, from invertebrates to mammals and primates. Here we describe several examples of disinhibition in the nematode C. elegans. The nematode displays distinct behavioral states associated with locomotion (crawling on land and swimming in water) that are mediated by dopamine. On land, animals crawl and feed freely, but these behaviors are inhibited in water. We found that additional behaviors, including a variety of escape responses are also inhibited in water. Whereas alcohol non-specifically impaired locomotion, feeding, and escape responses in worms on land, alcohol specifically disinhibited these behaviors in worms immersed in water. Loss of dopamine signaling relieved disinhibition of feeding behavior, while loss of the D1-like dopamine receptor DOP-4 impaired the ethanol-induced disinhibition of crawling. The powerful genetics and simple nervous system of C. elegans may help uncover conserved molecular mechanisms that underlie alcohol-induced disinhibition of behaviors in higher animals.

Relationship between ethanol-induced activity and anxiolysis in the open field, elevated plus maze, light-dark box, and ethanol intake in adolescent rats

It is yet unclear if ethanol-induced motor stimulation in the open field (OF) merely reflects psychomotor stimulating effects of the drug or if this stimulation is driven or modulated by ethanol's antianxiety properties. In the present study, adolescent rats were administered with different ethanol doses or remained untreated. They were sequentially assessed in the OF, elevated plus maze (EPM), and light-dark box (LDB) and then assessed for ethanol intake. The aims were to assess the relationship between measures of ethanol-induced activity and anxiolysis, analyze ethanol intake as a function of prior ethanol exposure, and associate behavioral responsiveness in these apparatus with ethanol intake during adolescence. The results suggested that the enhanced exploration of the OF observed after 2.5 and 3.25 g/kg ethanol reflected a motor-stimulating effect that appeared to be relatively independent of anxiolysis. The 1.25 g/kg dose induced motor stimulation in the OF and anti-anxiety effects in the EPM, but these effects were relatively independent. The 0.5 g/kg ethanol dose exerted significant anxiolytic effects in the EPM in the absence of stimulating effects in the OF. A multivariate regression analysis indicated that adolescents with a higher frequency of rearing behavior in the OF, higher percentage of open arm entries in the EPM, and lower propensity to enter the central area of the OF exhibited greater ethanol intake. These results indicate that the OF is a valid procedure for the measurement of ethanol-induced stimulation, and provide information toward characterizing subpopulations of adolescents at risk for initiating alcohol drinking.

The effects of social contact on drug use: behavioral mechanisms controlling drug intake

The social environment plays a critical role in determining the likelihood that an individual will use drugs or will develop a drug use disorder. Recent evidence obtained from preclinical studies reveals that proximal social factors (i.e., those factors that are immediately present at the time of drug exposure) exert a particularly strong influence on both drug-seeking and drug-taking behavior. These studies are advancing our understanding of the role of the social environment in drug use by showing that the rewarding and reinforcing effects of drugs depend on (a) whether other individuals are immediately present and (b) whether those individuals are also using drugs. Furthermore, the preclinical literature examining the role of social learning in behavior maintained by nondrug reinforcers reveals a number of behavioral mechanisms by which social contact may influence drug use, as well as potential ways the social environment may be modified to prevent or reduce drug use. Additional research is needed to determine potential age and sex differences in the effects of social contact on drug use, to determine the generality of the current findings across different pharmacological classes of drugs, and to determine the role of social contact on drug intake during different transitional stages of drug use disorders; however, enough evidence now exists to begin implementing social interventions in clinical and at-risk populations.

Central effects of ethanol interact with endogenous mu-opioid activity to control isolation-induced analgesia in maternally separated infant rats

Endogenous opioid activity plays an important role in ethanol consumption and reinforcement in infant rats. Opioid systems are also involved in mediation and regulation of stress responses. Social isolation is a stressful experience for preweanling rats and changes the effects of ethanol through opioid-dependent mechanisms. The present study assessed effects of intracisternal (i.c.) administration of a selective mu-opioid antagonist (CTOP) and i.p. administration of a nonspecific opioid antagonist (naloxone) on voluntary intake and behavior in socially isolated 12-day-old (P12) pups treated with 0.5 g/kg ethanol. Voluntary intake of 0.1% saccharin or water, locomotion, rearing activity, paw licking and grooming were assessed during short-term isolation from littermates (STSI; 8-min duration). Thermal nociceptive reactivity was measured before and after this intake test, with normalized differences between pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). Results indicated several effects of social isolation and ethanol mediated through the mu-opioid system. Effects of low dose ethanol (0.5 g/kg) and voluntary consumption of saccharin interacted with endogenous mu-opioid activity associated with STSI. Blockade of mu-opioid receptors on saccharin consumption and paw licking-grooming affected intoxicated animals. Low dose ethanol and ingestion of saccharin blunted effects of CTOP on rearing behavior and nociceptive reactivity. Central injections of CTOP stimulated paw licking and grooming dependent on ethanol dose and type of fluid ingested. Ethanol selectively increased saccharin intake during STSI in females, naloxone and CTOP blocked ethanol-mediated enhancement of saccharin intake. We suggest that enhancement of saccharin intake by ethanol during STSI is the product of synergism between isolation-induced mu-opioid activity that increases the pup's sensitivity to appetitive taste stimulation and the anxiolytic effects of 0.5 g/kg ethanol that decreases behaviors otherwise competing with independent ingestive activity.

Temporary inhibition of dorsal or ventral hippocampus by muscimol: distinct effects on measures of innate anxiety on the elevated plus maze, but similar disruption of contextual fear conditioning

Studies in rats, involving hippocampal lesions and hippocampal drug infusions, have implicated the hippocampus in the modulation of anxiety-related behaviors and conditioned fear. The ventral hippocampus is considered to be more important for anxiety- and fear-related behaviors than the dorsal hippocampus. In the present study, we compared the role of dorsal and ventral hippocampus in innate anxiety and classical fear conditioning in Wistar rats, examining the effects of temporary pharmacological inhibition by the GABA-A agonist muscimol (0.5 ug/0.5 ul/side) in the elevated plus maze and on fear conditioning to a tone and the conditioning context. In the elevated plus maze, dorsal and ventral hippocampal muscimol caused distinct behavioral changes. The effects of ventral hippocampal muscimol were consistent with suppression of locomotion, possibly accompanied by anxiolytic effects, whereas the pattern of changes caused by dorsal hippocampal muscimol was consistent with anxiogenic effects. In contrast, dorsal and ventral hippocampal muscimol caused similar effects in the fear conditioning experiments, disrupting contextual, but not tone, fear conditioning.

Emotional valence and context of social influences on drug abuse-related behavior in animal models of social stress and prosocial interaction

RATIONALE

Social factors are important determinants of drug dependence and relapse.

OBJECTIVES

We reviewed pre-clinical literature examining the role of social experiences from early life through the development of drug dependence and relapse, emphasizing two aspects of these experiences: (1) whether the social interaction is appetitive or aversive and (2) whether the social interaction occurs within or outside of the drug-taking context.

METHODS

The models reviewed include neonatal care, isolation, social defeat, chronic subordination, and prosocial interactions. We review results from these models in regard to effects on self-administration and conditioned place preference established with alcohol, psychostimulants, and opiates.

RESULTS

We suggest that in general, when the interactions occur outside of the drug-taking context, prosocial interactions are protective against drug abuse-related behaviors, whereas social stressors facilitate these behaviors. By contrast, positive or negative social interactions occurring within the drug-taking context may interact with other risk factors to enhance or inhibit these behaviors.

CONCLUSIONS

Despite differences in the nature and complexity of human social behavior compared to other species, the evolving animal literature provides useful models for understanding social influences on drug abuse-related behavior that will allow for research on the behavioral and biological mechanisms involved. The models have contributed to understanding social influences on initiation and maintenance of drug use, but more research is needed to understand social influences on drug relapse.

The interaction of chronic restraint stress and voluntary alcohol intake: effects on spatial memory in male rats

Alcohol consumption and exposure to stressful life events activate similar neural pathways and thus result in several comparable physiological and behavioral effects. Alcoholics in treatment claim that life stressors are the leading cause of continued drinking or relapse. However, few studies have investigated the interactive effects of stress and alcohol on cognitive behavior. The effects of restraint stress, alcohol, and stress in combination with alcohol were examined on a spatial memory test, the object placement (OP) task. In addition, intake levels were measured to determine if stress altered general consumption of alcohol. Male Sprague-Dawley rats were assigned to one of four conditions: no alcohol/no stress control (CON), stress alone (STR), alcohol alone (ALC), and STR+alcohol (STR+ALC). Following each restraint stress bout, the STR+ALC and the ALC groups were given access to 8% alcohol for 1h using the two-bottle choice limited access paradigm. As predicted, the STR+ALC group significantly increased alcohol consumption, while the ALC group had consistent drinking over the 10-day treatment. On the OP task, STR and ALC groups performed at chance levels, whereas the CON and STR+ALC groups significantly discriminated between objects in the new and old locations. These data show that stress increases alcohol intake and the intake of alcohol is associated with reduction of the stress-induced impairment of spatial memory. The data have important implications for the development of alcohol abuse and its treatment.

On the relationships between ultrasonic calling and anxiety-related behavior in rats

In the present review, the phenomenon of ultrasonic vocalization in rats will be outlined, including the three classes of vocalizations, namely 40-kHz calls of pups, and 22- and 50-kHz calls of juvenile and adult rats, their general relevance to behavioral neuroscience, and their special relevance to research on anxiety, fear, and defense mechanisms. Here, the emphasis will be placed on 40- and 22-kHz calls, since they are typical for various situations with aversive properties. Among other topics, we will discuss whether such behavioral signals can index a certain affective state, and how these signals can be used in social neuroscience, especially with respect to communication. Furthermore, we will address the phenomenon of inter-individual variability in ultrasonic calling and what we currently know about the mechanisms, which may determine such variability. Finally, we will address the current knowledge on the neural and pharmacological mechanisms underlying 22-kHz ultrasonic vocalization, which show a substantial overlap with mechanisms known from other research on fear and anxiety, such as those involving the periaqueductal gray or the amygdala.

Amphetamine modifies ethanol intake of psychosocially stressed male rats

Studies of socially housed rodents have provided significant information regarding the consequences of exposure to stressors. Psychosocial stressors are known to alter the ingestion of ethanol and the activity of the dopaminergic neuronal system. Since both stressors and ethanol are known to affect the function of dopaminergic neurons, we employed amphetamine to assess the role of this neural system on the ingestion of ethanol by psychosocially stressed male rats. Male rats housed two per cage were designated as dominant or subdominant rats based on evaluations of agonistic behavior and body weight changes. The dyad-housed rats and a group of single-housed rats were sequentially assessed for ethanol intake after injections of saline or amphetamine (0.3, 0.9 or 2.7 mg/kg i.p.) both prior to dyad housing and subsequently again during dyad-housing. Prior to dyad housing ethanol intake of future subdominant rats was higher than that of future dominant rats. Dyad-housing significantly increased ethanol intake of dominant rats. Pre-dyad the highest dose of amphetamine potently depressed ethanol ingestion. Sensitivity to amphetamine's depressant effect on ethanol intake was higher at the dyad test in all subjects, most prominently in single-housed rats. In contrast to the single-housed rats, the dyad-housed rats displayed saccharin anhedonia. It can be concluded that dopaminergic system modulates, at least partially, the psychosocial stress-induced changes in ethanol intake. Furthermore, the level of ethanol ingestion at the pre-dyad test was predictive of future hierarchical status.

Current animal models of anxiety, anxiety disorders, and anxiolytic drugs

PURPOSE OF REVIEW

The perception that 'classical' anxiety tests are deficient was formulated in the mid-1990s. Recent clinical trials also demonstrate that the predictive power of such tests is low, which emphasizes the need for developing models of higher translational value. Several novel models are proposed each year. Here, we investigate their impact on anxiolytic-related studies performed in 2010 and 2011.

RECENT FINDINGS

Here, we depict as 'classical' all the tests that were developed at the same time as or earlier than the elevated plus-maze test. No test equaled its success in the subsequent decades; therefore, we consider it the endpoint of the period when the methodological bases of current laboratory research were laid down. Fourteen classical tests were employed in the investigated period, which were used in more than 80% of studies. Concurrently, 36 'nonclassical' tests were used and six novel tests were also proposed. These accounted for fewer than 20% of studies. 'Classical' tests were often performed under unconventional conditions that putatively increased their translational value. Taken together, half of the studies involved at least one innovative step. Yet, the new procedures were infrequently used. Out of the 36 'nonclassical' tests, only 11 were used more than once, while the amendments to 'classical tests' were almost entirely laboratory specific.

SUMMARY

Our analysis shows that there is a large interest in performing anxiety research innovatively. However, efforts are highly divergent and result in large numbers of poorly validated and infrequently used novel approaches. Thus, models with increased translational value still need to be developed.

Effects of stress on alcohol drinking: a review of animal studies

RATIONALE

While stress is often proposed to play a significant role in influencing alcohol consumption, the relationship between stress and alcohol is complex and poorly understood. Over several decades, stress effects on alcohol drinking have been studied using a variety of animal models and experimental procedures, yet this large body of literature has generally produced equivocal results.

OBJECTIVES

This paper reviews results from animal studies in which alcohol consumption is evaluated under conditions of acute/sub-chronic stress exposure or models of chronic stress exposure. Evidence also is presented indicating that chronic intermittent alcohol exposure serves as a stressor that consequently influences drinking.

RESULTS

The effects of various acute/sub-chronic stress procedures on alcohol consumption have generally been mixed, but most study outcomes suggest either no effect or decreased alcohol consumption. In contrast, most studies indicate that chronic stress, especially when administered early in development, results in elevated drinking later in adulthood. Chronic alcohol exposure constitutes a potent stressor itself, and models of chronic intermittent alcohol exposure reliably produce escalation of voluntary alcohol consumption.

CONCLUSIONS

A complex and dynamic interplay among a wide array of genetic, biological, and environmental factors govern stress responses, regulation of alcohol drinking, and the circumstances in which stress modulates alcohol consumption. Suggestions for future directions and new approaches are presented that may aid in developing more sensitive and valid animal models that not only better mimic the clinical situation, but also provide greater understanding of mechanisms that underlie the complexity of stress effects on alcohol drinking.

Differential sensitivity to amphetamine's effect on open field behavior of psychosocially stressed male rats

RATIONALE

Studies of socially housed rodents have provided significant information regarding the mechanisms of stress and of stress-related disorders.

OBJECTIVE

Since psychosocial stress is known to alter the functional activity of dopaminergic system, we employed amphetamine (AMP) to evaluate the involvement dopamine in mediating the behavioral consequences of psychosocial stress.

METHODS

Male rats housed two per cage were designated as dominant (DOM) or subdominant (Sdom) based on initial evaluations of agonistic behaviors and body weight changes. Diad-housed rats and a group of single-housed (SiH) rats were tested in an open field after injections of saline or amphetamine (0.9 or 2.7 mg/kg IP) prior to and again while diad-housing.

RESULTS

Compared to future DOM rats, saline-injected future Sdom rats entered the open field center less frequently, spent less time in rearing behavior and groomed less. At the pre-diad test AMP treatment elevated locomotor activity of all rats, while stimulation of center entries was more marked in future DOM rats. At the diad test, AMP's locomotor stimulant effect was evident in all experimental groups with DOM rats showing higher effects compared to Sdom and SiH rats. Amphetamine's stimulation of center entries in DOM rats was similar to the pre-diad test, but it was diminished in Sdom rats, while stimulation of rearing behavior was most evident in diad-housed rats.

CONCLUSION

The dopaminergic system modulates the psychosocial stress-induced differences in explorative and emotional behaviors. Furthermore, behavioral traits like frequency of grooming behavior and of center entries were predictive of future hierarchical status.

Behavioral traits predicting alcohol drinking in outbred rats: an investigation of anxiety, novelty seeking, and cognitive flexibility

BACKGROUND

Most adults in Western society consume alcohol regularly without negative consequences. For a small subpopulation, however, drinking can quickly progress to excessive and chronic intake. Given the dangers associated with alcohol abuse, it is critical to identify traits that may place an individual at risk for developing these behaviors. To that end, we used a rat model to determine whether anxiety-related behaviors, novelty seeking, or cognitive flexibility predict excessive alcohol drinking under both limited and continuous access conditions.

METHODS

Adult male rats were assessed in a series of behavioral tasks (elevated plus maze [EPM], locomotor activity, and discrimination/reversal learning in a Y-maze) followed by 6 weeks of daily, 1-hour access to alcohol in a free-choice, 2-bottle paradigm (10% alcohol vs. tap water). Next, subjects were given the opportunity to consume alcohol for 72 hours in drinking chambers that permit separate measures of each drinking bout. Half of the animals experienced a 2-week deprivation period between the limited and continuous access sessions.

RESULTS

Time spent on the open arms of the EPM, but not novelty seeking or discrimination/reversal learning, predicted alcohol consumption during limited, 1-h/d access sessions to alcohol. Anxiety-related behavior also predicted the escalation of intake when animals were given 72 hours of continuous access to alcohol. Bout size, but not frequency, was responsible for the increased consumption by high-anxiety subjects during this period. Finally, intake during limited access sessions predicted intake during continuous access, but only in subjects with low intake during limited access.

CONCLUSIONS

These findings confirm that preexisting anxiety-related behavior predicts alcohol intake under several schedules of alcohol access. Moreover, when access is unlimited, the high-anxiety-related group exhibited an increase in bout size, but not frequency, of drinking. In addition, we show that modest intake when alcohol is restricted may or may not progress to excessive intake when the drug is freely available.

Bidirectional interactions between acute psychosocial stress and acute intravenous alcohol in healthy men

BACKGROUND

The biological mechanisms by which acute stress increases alcohol consumption are unclear. One potential mechanism is that stress acts by altering the pharmacological and subjective effects of alcohol. Acute stress produces a cascade of physiological and psychological effects, each with a distinctive time course. In this study, we investigated whether different phases of response to an acute stress alter the subjective effects of intravenous alcohol, by administering the drug at 2 different times after the stress.

METHODS

Healthy men (n = 25) participated in 2 sessions: 1 with the Trier Social Stress Test and the other with a nonstressful control task, each followed by infusions of intravenous alcohol (targeting 40 mg% in 5 minutes) and placebo. One group of participants received alcohol within 1 minute of completing the tasks (Alc0, n = 11), followed by placebo 30 minutes later. In the other group (Alc30, n = 14), the order of alcohol and placebo infusions was reversed. Subjective effects (i.e., anxiety, stimulation, want more) and physiological measures (heart rate, blood pressure, salivary cortisol) were measured before and at repeated intervals after the tasks and infusions.

RESULTS

Stress did not change the subjective effects of alcohol in either group. However, when individual differences in alcohol responses were considered, stress differentially altered the stimulant-like and sedative effects of alcohol. Among individuals who exhibited predominantly stimulant responses to alcohol in the nonstressful condition, stress decreased the stimulant-like effects of alcohol and "wanting more." By contrast, among participants who did not report stimulation after alcohol in the control session, stress decreased the sedative effects and increased "want more." In addition, alcohol administered immediately after the Trier Social Stress Test dampened cortisol responses yet prolonged negative subjective responses to the stress.

CONCLUSIONS

These findings demonstrate that there are bidirectional relationships between alcohol and stress. Alcohol influences responses to stress, and stress changes reactions to alcohol, depending on an individual's pattern of response to alcohol. This study highlights the fact that stress-alcohol interactions vary among individual drinkers, suggesting that the effects of stress on motivation to drink alcohol may also differ between individuals.

Oxidative damage and HSP70 expression in masseter muscle induced by psychological stress in rats

Psychological stressors are generally associated with masseter muscle dysfunction and disorders in emotional response. In addition, oxidative states and HSP70 expression, which are involved in the physical and pathological changes of the masseter muscle, could be altered in the stressed tissues and organs. However, the link between psychological stress and the redox homeostasis or the expression of HSP70 in masseter muscles in rats has not been examined. Therefore, we used a communication box paradigm to induce psychological stress in rats. The successful establishment of the animal model was evidenced by an increase in plasma corticosterone and adrenocorticotropic hormone (ACTH). Meanwhile, the stressed rats showed a decrease in the number of entries on open arms, percentage of time spent in open arms, and distance moved in the elevated plus-maze test. The stressed rats also displayed a decrease in the time spent in the center zone, active velocity, and the distance moved in the open-field test. These results demonstrate affective-like behavioral changes in the stressed rats. Moreover, compared with the control rats, a decrease in SOD, GSH-Px and catalase activities and an increase in MDA content were observed in the masseter muscles in stressed rats after 3 weeks and 5 weeks, and the HSP70 expression was elevated in muscles in the rats exposed to stress for 5 weeks. These results indicate that psychological stress induces oxidative damage and up-regulates the expression of HSP70 in masseter muscles in rats, which are associated with behavior resembling anxiety.

Trait anxiety and ethanol: anxiolysis in high-anxiety mice and no relation to intake behavior in an addiction model

Anxiety has been proposed to play a role in the development of alcohol addiction, but the exact mechanisms by which this occurs remain unclear. The present study aimed to verify the relationship between basal anxiety levels, the anxiolytic-like effect of ethanol, and ethanol intake in mice exposed to an addiction model. In one experiment Swiss mice were characterized as high-anxiety (HA), medium-anxiety (MA), or non-anxiety (NA) in the elevated plus maze and then received saline or ethanol 2 g/kg acutely and chronically and were again exposed to the same test. NA mice decreased while MA mice maintained anxiety indices over the test days, regardless of treatment. HA ethanol-treated mice showed an anxiolytic-like effect, both acutely and chronically, while the saline-treated ones maintained their basal anxiety levels. In another experiment HA and MA mice were exposed to an addiction model based on a 3-bottle free-choice paradigm (ethanol 5% and 10%, and water) consisting of four phases: acquisition (10 weeks), withdrawal (W, 2 weeks), reexposure (2 weeks), and quinine-adulteration (2 weeks). HA and MA control mice had access only to water. Mice were characterized as addicted, heavy-drinker and light-drinker [Fachin-Scheit DJ, Ribeiro AF, Pigatto G, Goeldner FO, Boerngen-Lacerda R. Development of a mouse model of ethanol addiction: naltrexone efficacy in reducing consumption but not craving. J Neural Transm 2006;113:1305-21.]. No difference was observed between HA and MA mice in their preference for and intake of ethanol. No correlation was observed between ethanol intake, during any phase, and anxiety indices measured in the basal tests and during the W phase. The differences in anxiety indices between HA and MA groups persisted in the test performed during ethanol withdrawal, suggesting a "trait" anxiety profile. The data suggest that despite the fact that high anxiety trait levels are important for the anxiolytic-like effects of ethanol, they are not a determining factor for high ethanol intake, at least not under these experimental conditions.

Do rats really express neophobia towards novel objects? Experimental evidence from exposure to novelty and to an object recognition task in an open space and an enclosed space

Three set of experiments were performed in an enclosed space (open-field) and in an open space (elevated platform). The surface of the open-field and the elevated platform were divided in nine equal squares. Rats were exposed (without previous habituation) in a unique session (experiment 1) or three consecutive sessions (experiment 2) either to an open-field (enclosed space) or to an elevated platform (open space) with and without an object on the centre of the field. In experiment 3, rats were exposed (without previous habituation) either to an enclosed or an open space on five consecutive sessions, one session a day. They were tested in an object recognition test in sessions 1, 3 and 5. In sessions 2 and 4, no objects were present. In experiment 1, we recorded the latency, frequency and duration of entries into different areas of the field. In experiment 3, we recorded the latency, frequency and duration of contacts with objects in addition to entries into different areas of the field. The first experiment demonstrates that rats exposed for the first time to an enclosed or an open space do not express neophobia toward novel objects in the absence of walls that surround an open-field. They crossed frequently into and spent more time in areas occupied with an object than in unoccupied areas. After two sessions of habituation to an empty open space or an empty enclosed space, the latency of first approach to a novel object is reduced while the frequency and duration of approaches are increased. The third experiment on object recognition confirmed that rats do not avoid novel objects; they made frequent visit and spent more time in the corner of the field occupied with an object than in empty corners. Recording of crossings provided detailed information about the patterns of exploratory behavior of rats but failed to reveal discrimination between novel and familiar objects which was evident in both open and enclosed space with recording of contacts with objects on the fifth exposure.