Drug-induced home cage conspecifics' behavior can potentiate behavioral sensitization in mice

Effects of social housing conditions on ethanol-induced behavioral sensitization in Swiss mice

RATIONALE

In previous animal model studies, it was shown that drug sensitization is dependent upon physical environmental conditions. However, the effects of social housing conditions on drug sensitization is much less known.

OBJECTIVE

The aim of the present study was to investigate the effects of social conditions, through the size of housing groups, on ethanol stimulant effects and ethanol-induced behavioral sensitization in mice.

MATERIALS AND METHODS

Male and female Swiss mice were housed in groups of different sizes (isolated mice, two mice per cage, four mice per cage and eight mice per cage) during a six-week period. A standard paradigm of ethanol-induced locomotor sensitization was then started with one daily injection of 2.5 g/kg ethanol for 8 consecutive days.

RESULTS

The results show that social housing conditions affect the acute stimulant effects of ethanol. The highest stimulant effects were observed in socially isolated mice and then gradually decreased as the size of the group increased. Although the rate of ethanol sensitization did not differ between groups, the ultimate sensitized levels of ethanol-induced stimulant effects were significantly reduced in mice housed in groups of eight.

CONCLUSIONS

These results are consistent with the idea that higher levels of acute and sensitized ethanol stimulant effects are observed in mice housed in stressful housing conditions, such as social isolation.

Long-term exposure to daily ethanol injections in DBA/2J and Swiss mice: Lessons for the interpretation of ethanol sensitization

Most mice ethanol sensitization studies focused on neurobiology at the expense of its behavioral characterization. Furthermore, relatively short ethanol exposures (10 to 20 injections) were used in these studies. The first aim of the present study is to better characterize the development and expression of ethanol sensitization after an extended exposure of 45 daily injections. In some previous studies, mice were classified as "respondent" and "resistant" to ethanol sensitization. The second aim of the present study is to test the long-term reliability of such categorizations and the consequences of their use on the interpretation of the ethanol sensitization results. Swiss and DBA/2J female mice received 45 consecutive daily ethanol administrations (respectively 2.5 and 2.0 g/kg) and their locomotor activity was daily recorded to test the development of ethanol sensitization. At the end of the procedure, a challenge test assessed the inter-group ethanol sensitization.The results of the present study show that ethanol sensitization continues to develop beyond 20 days to reach maximal levels after about 25 injections in DBA/2J mice and 40 injections in Swiss mice, although the core phase of the development of ethanol sensitization occurred in both strains during the first 20 days. Remarkably, ethanol sensitization after such a long daily ethanol treatment resulted in both an upward shift of the magnitude of ethanol stimulant effects and a prolongation of these effects in time (up to 30 minutes). Mice classified as "resistant to ethanol sensitization" according to previous studies developed very significant levels of ethanol sensitization when tested after 45 ethanol injections and are best described as showing a delayed development of ethanol sensitization. Furthermore, mice classified as respondent or resistant to ethanol sensitization also differ in their acute response to ethanol, such that it is difficult to ascertain whether these classifications are specifically related to the sensitization process.

Social Factors in Ethanol Sensitization

Behavioral sensitization is a neuroadaptive process characterized by an increase in a particular behavior after repeated exposure to drugs or other stimuli, such as stress. Sensitization can also be extended to neurochemical and neuroendocrine sensitization. Several factors can influence sensitization to the effects of ethanol. For instance, stress is an important component in addiction that can strengthen ethanol-induced behaviors. In animal models, stressful situations can be induced by alterations in social aspects of the animal's environment, such as maternal separation, social conflicts, and housing conditions. Social conflict models involve acute, chronic or intermittent interaction of an animal to a conspecific and can occur at any stage of life, including preweaning, adolescence or adulthood. These events can influence ethanol-induced behavioral sensitization in different ways, such as increases in locomotion, drug reward, and drug-taking behaviors. On the other hand, environmental enrichment can produce a protective phenotype against drug-related behaviors. In this chapter, we discuss findings regarding consequences of social stress and environmental enrichment on sensitization to ethanol.

The effects of paradoxical sleep deprivation on amphetamine-induced behavioral sensitization in adult and adolescent mice

Drug-induced behavioral sensitization (BS), paradoxical sleep deprivation (PSD) and adolescence in rodents are associated with changes in the mesolimbic dopaminergic system. We compared the effects of PSD on amphetamine-induced BS in adult and adolescent mice. Adult (90 days old) and adolescent (45 days old) Swiss mice were subjected to PSD for 48h. Immediately after PSD, mice received saline or 2.0mg/kg amphetamine intraperitoneally (i.p.), and their locomotion was quantified in activity chambers. Seven days later, all the animals were challenged with 2.0mg/kg amphetamine i.p., and their locomotion was quantified again. Acute amphetamine enhanced locomotion in both adult and adolescent mice, but BS was observed only in adolescent mice. Immediately after its termination, PSD decreased locomotion of both saline- and amphetamine-treated adolescent mice. Seven days later, previous PSD potentiated both the acute stimulatory effect of amphetamine and its sensitization in adolescent mice. In adult animals, previous PSD revealed BS. Our data suggest that adolescent mice are more vulnerable to both the immediate and long-term effects of PSD on amphetamine-induced locomotion. Because drug-induced BS in rodents shares neuroplastic changes with drug craving in humans, our findings also suggest that both adolescence and PSD could facilitate craving-related mechanisms in amphetamine abuse.

Short-term sleep deprivation reinstates memory retrieval in mice: the role of corticosterone secretion

While the effects of sleep deprivation (SD) on the acquisition and consolidation phases of memory have been extensively characterized, its effects on memory retrieval remain overlooked. SD alone is a stressor, and stress-activated glucocorticoids promote bimodal effects on memory. Because we have recently demonstrated that 72h SD impairs memory retrieval in the plus-maze discriminative avoidance task (PM-DAT) in mice, this study investigated whether shorter SD periods would facilitate retrieval. In Experiment I, the temporal forgetting curve of the PM-DAT was determined and an interval between training/testing in which retrieval was no longer present was used in all subsequent experiments. In Experiments II and III, retrieval performance and corticosterone concentration, respectively, were quantified in mice that were sleep deprived for 12 or 24h before testing. In Experiments IV and V, the effects of the corticosterone synthesis inhibitor metyrapone were evaluated on 12h SD-induced retrieval reinstatement and corticosterone concentration enhancement, respectively. Experiment VI determined whether pre-test acute administration of exogenous corticosterone would mimic the facilitatory effects of 12h SD on retrieval. Thirty days after training, mice presented poor performance of the task; however, SD for 12h (but not for 24) before testing reinstated memory retrieval. This facilitatory effect was accompanied by increased corticosterone concentration, abolished by metyrapone, and mimicked by pre-test acute corticosterone administration. Collectively, short-term SD can facilitate memory retrieval by enhancing corticosterone secretion. This facilitatory effect is abolished by longer periods of SD.

Effects of aripiprazole, an atypical antipsychotic, on the motor alterations induced by acute ethanol administration in mice

Aripiprazole is an antipsychotic that acts as a partial agonist at dopamine receptors. As the effects of most drugs of abuse converge to enhance dopamine-mediated neurotransmission, the present study was designed to test the hypothesis that aripiprazole would inhibit the acute effects of ethanol, a widely abused substance. Male Swiss mice received acute injections and were evaluated for motor activity in three distinct tests. In the open field, ethanol (1.5, 2.5 and 3.5 g/kg) induced an increase in locomotion in a U-shaped dose-related fashion, whereas aripiprazole (0.1, 1 and 10 mg/kg) did not affect this parameter. All the doses of the antipsychotic were able to prevent the stimulant effects of 2.5 g/kg of ethanol. In the rotarod test, ethanol (2.5 and 3.5 g/kg) reduced the latency to fall from the apparatus, an effect also observed with the higher dose of aripiprazole. Contrary to what was observed in the open field, this antipsychotic did not interfere with the effects of ethanol in motor balance. Finally, we tested animals in the wire hang test, in which ethanol, but not aripiprazole, reduced latency to fall at all doses. In this test, aripiprazole did not change ethanol effects. The present data lead to the conclusion that aripiprazole prevents the stimulant effects of ethanol on locomotion, without interfering with the motor impairment induced by this drug.

Acute and chronic ethanol differentially modify the emotional significance of a novel environment: implications for addiction

Using open-field behaviour as an experimental paradigm, we demonstrated a complex interaction between the rewarding/stimulating effects and the anxiogenic/stressful effects of both novelty and acute or chronic amphetamine in mice. As a consequence of this interaction, acute amphetamine-induced hyperlocomotion was inhibited, whereas the expression of its sensitization was facilitated in a novel environment. In the present study, we aimed to investigate the interactions between exposure to a novel environment and the acute and chronic effects of ethanol (Eth), a drug of abuse known to produce anxiolytic-like behaviour in mice. Previously habituated and non-habituated male Swiss mice (3 months old) were tested in an open field after receiving an acute injection of Eth or following repeated treatment with Eth. Acute Eth administration increased locomotion with a greater magnitude in mice exposed to the apparatus for the first time, and this was thought to be related to the attenuation of the stressful effects of novelty produced by the anxiolytic-like effect of acute Eth, leading to a subsequent prevalence of its stimulant effects. However, locomotor sensitization produced by repeated Eth administration was expressed only in the previously explored environment. This result might be related to the well-known tolerance of Eth-induced anxiolytic-like behaviour following repeated treatment, which would restore the anxiogenic effect of novelty. Our data suggest that a complex and plastic interaction between the emotional and motivational properties of novelty and drugs of abuse can critically modify the behavioural expression of addiction-related mechanisms.

Differential effects of intermittent and continuous exposure to novel environmental stimuli on the development of amphetamine-induced behavioral sensitization in mice: implications for addiction

BACKGROUND

Previous studies have demonstrated a preventive effect of continuous environmental enrichment during early development on the vulnerability of rodents to drug addiction-related behaviors. Recently, it was demonstrated that a continuous environmental enrichment could eliminate already established addiction-related behaviors in mice. The present study compared the effects of intermittent or continuous exposure to novel stimuli during repeated amphetamine (Amp) treatment on the development of behavioral sensitization (an animal model of addiction-related neuroadaptations) in adult mice.

METHODS

Three-month-old male Swiss mice were treated with 2.5mg/kg Amp every other day for 13 days in their home cages. Novel objects were presented in their home cages for 2h on non-drug treatment days (experiment 1) or for 24h/day during the 13 days of drug treatment (experiment 2). Seven days after the drug treatment had finished, the mice were challenged with 2.5mg/kg Amp, and their locomotor activity was quantified in a familiar open field for 10 min.

RESULTS

Intermittent exposure to the novel objects did not modify the acute Amp locomotor stimulatory effect but potentiated the development of Amp-induced locomotor sensitization. This enhanced sensitization was due to increased locomotion in the central squares of the apparatus, which suggests anxiolysis or increased impulsiveness. Conversely, continuous exposure to the novel objects potentiated the acute Amp locomotor stimulatory effect and blunted the development of Amp-induced locomotor sensitization.

CONCLUSIONS

We conclude that addiction-related behaviors can be differentially and critically modified depending on the schedule and period of the novelty exposure.

Effects of group exposure on single injection-induced behavioral sensitization to drugs of abuse in mice

BACKGROUND

Behavioral sensitization in rodents is hypothesized to reflect neuronal adaptations that are related to drug addiction in humans. We evaluated the effects of group exposure on the acute hyperlocomotion and behavioral sensitization induced by four drugs of abuse in C57BL/6 mice: methylenedioxymethamphetamine (MDMA), d-amphetamine, morphine and ethanol.

METHODS

In the priming session, animals received an ip injection of one of the drugs of abuse and were exposed to an open field either individually or in groups of four. Seven days later, we assessed behavioral sensitization in the challenge session. All animals received an ip injection of the same drug and were exposed to the open field in the same social conditions described for the priming session. Locomotion and social interaction were quantified during each session.

RESULTS

Acute MDMA, morphine and ethanol, but not d-amphetamine, increased social interaction. However, group exposure only potentiated MDMA-induced hyperlocomotion. After a challenge injection of each drug, there was no sensitization to the facilitating effect of MDMA, morphine or ethanol on social interaction, but locomotion sensitization developed to all drugs of abuse except ethanol. This sensitization was potentiated by group exposure in MDMA-treated animals, attenuated in morphine-treated animals and not modified in d-amphetamine-treated animals. Acute MDMA enhanced body contact and peaceful following, while acute morphine and ethanol increased social sniffing.

CONCLUSIONS

These results provide preclinical evidence showing that while different drugs of abuse affect different components of social interaction, the neuronal adaptations related to drug dependence can be critically and specifically influenced by group exposure.

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.

Chronic amphetamine transforms the emotional significance of a novel but not a familiar environment: implications for addiction

Both drug-induced locomotor sensitization and reactivity to novelty in rodents have been related to drug-craving mechanisms in humans. We investigated whether the exposure to a completely novel environment would modulate the expression of locomotor sensitization induced by repeated administration of amphetamine (Amp) in mice. In addition to locomotion, different open-field behavioural parameters were used to evaluate the possible involvement of anxiogenic-like effects induced by Amp, novelty or a combination of the two. In order to avoid misinterpretations due to different locomotor baseline conditions, we used an open-field illumination condition in which previous exposure to the apparatus did not modify locomotion (although it reliably increased grooming behaviour). Acute Amp administration increased locomotion in mice previously habituated to the open field (Hab) but not in mice exposed to the apparatus for the first time (Nov). This absence of Amp-induced locomotor activation in Nov mice may be related to higher anxiety-like levels, because these animals displayed longer freezing duration. However, only Nov mice developed locomotor sensitization. Because Amp challenge in Amp pre-treated Nov mice did not induce an increase in freezing behaviour, the locomotor sensitization in Nov mice might be related to the tolerance of Amp-induced anxiogenic-like behaviour in novel environments. Repeated Amp administration increased motivation to explore the environment in Nov mice in that these animals presented a within-session locomotion-habituation deficit. Our data suggest that a complex and plastic interaction between the anxiogenic and motivational properties of both novelty and Amp can critically modify the behavioural expression of craving-related mechanisms.

Environmental novelty and illumination modify ethanol-induced open-field behavioral effects in mice

Both spontaneous and drug-induced animal behaviors can be modified by exposure to novel stimuli or different levels of environmental illumination. However, research into how these factors specifically impact ethanol (ETH)-induced behavioral effects is currently lacking. We aimed to investigate the effects of these two factors, considered separately or in conjunction, on ETH-induced acute hyperlocomotor effect and its sensitization in adult male Swiss mice. Mice were placed in a novel or familiar open-field under normal light (200 lx) or low light (9 lx) immediately after receiving an ip injection of either 1.8 g/kg ETH or saline (SAL). After 7 days, all animals received an ip challenge injection of 1.8 g/kg ETH, and were placed in the open-field under the same light conditions described above. Novelty increased central locomotion and decreased grooming, while low light increased grooming. Acute ETH administration increased both total and peripheral locomotion and these effects were potentiated by low light. Both low light and novelty were able to facilitate ETH-induced locomotor sensitization, which was detected by the central locomotion parameter. However, there was no synergism between the effects of these two modulating factors on ETH-induced behavioral sensitization. We conclude that both the acute behavioral effects of ETH and behavioral sensitization induced by previous administration of this drug can be critically modified by environmental factors. In addition, our study stresses the importance of using different behavioral parameters to evaluate the interaction between environmental factors and ETH effects.

Chronic social stress in adolescence influenced both amphetamine conditioned place preference and locomotor sensitization

We previously reported that chronic social stress (SS) in adolescence, but not in adulthood, increased the locomotor-activating effects of nicotine in females, and not males, when tested in adulthood. However, SS rats had decreased locomotor response to nicotine when tested in adolescence. Here, we investigated age-related changes in the effects of SS on both conditioned place preference (CPP) and locomotor sensitization to amphetamine. In the CPP experiment, SS females tested in adolescence had increased preference for the 1.0 mg/kg dose of amphetamine, whereas SS rats of both sexes showed a decrease in CPP for the 0.5 mg/kg dose when tested as adults. Irrespective of time of testing, SS males and females had enhanced locomotor sensitization compared to controls. Thus, adolescent SS produced both immediate and enduring effects on behavioral responses to amphetamine, likely by altering the development of the mesocorticolimbic dopamine system, which holds implications for vulnerability to addiction.

Sleep rebound attenuates context-dependent behavioural sensitization induced by amphetamine

We have recently demonstrated that paradoxical sleep deprivation (PSD) potentiates the induction of amphetamine (AMPH)-induced behavioural sensitization by increasing its conditioned component. In the present study, the effects of sleep rebound (induced by 24 h recovery period from PSD) were studied on AMPH-induced behavioural sensitization. Sleep rebound attenuated the acute locomotor-stimulating effect of AMPH. AMPH-induced behavioural sensitization was context-specific and was also attenuated by sleep rebound. These results strengthen the notion that sleep conditions can influence AMPH-induced behavioural sensitization.

Sex does matter: comments on the prevalence of male-only investigations of drug effects on rodent behaviour

Despite abundant evidence of sex differences in the effects of drugs on nonsexual behaviour in rats and mice, most researchers continue to investigate male animals exclusively. This was evident from a survey of all relevant research reports published during the period February 2005-September 2006 (inclusive) in recent issues of five representative behavioural pharmacological journals. Reasons for excluding female animals from most studies are discussed along with attempts to justify the use of either male or female animals only, and the value of including both sexes (especially when a drug effect is poorly understood). Although there are other factors that can influence the effects of drugs, such as strain, age and social density, the sex of experimental animals is the easiest to control and thus is well suited to inclusion in pharmacological investigations. It is accordingly suggested that, as has been recommended many times in the past, animals' sex should play a more important part in future research than is still currently the case.