Rearing conditions alter social reactivity and D1 dopamine receptors in high- and low-aggressive mice

Fatty acid-binding protein 5 differentially impacts dopamine signaling independent of sex and environment

Epidermal/brain fatty acid-binding protein 5 (FABP5) plays an integral role in the intracellular trafficking of bioactive lipids/endocannabinoids and the subsequent initiation of cellular cascades affecting cannabinoid and dopamine (DA) systems. Social isolation (SI) and environmental enrichment (EE) during adolescence have been shown to impact DA signaling, and, specifically, DA transporter (DAT) and receptor levels of DA type 1 (D1) and 2 (D2); however, the relationship between FABP5, environment and DA signaling remains unclear. The present study quantified DAT and DA receptor levels in male/female FABP5-/- and FABP5+/+ mice raised in either SI or EE. Results showed that FABP5-/- mice had 6.09-8.81% greater D1 levels in striatal sub-regions of the caudal brain, independent of sex or environment. D1 levels were 8.03% greater only in the olfactory tubercle of enrichment-reared animals. In summary, these results supported that FABP5 plays an important function in regulating striatal DA signaling, and this may have important implications as a target with therapeutic potential for various psychiatric disorders.

Peer Social Environment Impacts Behavior and Dopamine D1 Receptor Density in Prairie Voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are socially monogamous rodents that form selective, long-lasting relationships with mates and with same-sex peers. It is unknown to what extent mechanisms supporting 'peer relationships' are similar to those involved in mate relationships. The formation of pair bonds is dependent on dopamine neurotransmission, whereas the formation of peer relationships is not, providing evidence of relationship type-specificity. The current study assessed endogenous structural changes in dopamine D1 receptor density in male and female voles across different social environments, including long-term same-sex partnerships, new same-sex partnerships, social isolation, and group housing. We also related dopamine D1 receptor density and social environment to behavior in social interaction and partner preference tests. Unlike prior findings in mate pairs, voles paired with new same-sex partners did not exhibit upregulated D1 binding in the nucleus accumbens (NAcc) relative to controls paired from weaning. This is consistent with differences in relationship type: D1 upregulation in pair bonds aids in maintaining exclusive relationships through selective aggression, and we found that formation of new peer relationships did not enhance aggression. Isolation led to increases in NAcc D1 binding, and even across socially housed voles, individuals with higher D1 binding exhibited increased social avoidance. These findings suggest that elevated D1 binding may be both a cause and a consequence of reduced prosociality. These results highlight the neural and behavioral consequences of different non-reproductive social environments and contribute to growing evidence that the mechanisms underlying reproductive and non-reproductive relationship formation are distinct. Elucidation of the latter is necessary to understand mechanisms underlying social behavior beyond a mating context.

Enrichment-induced differences in methamphetamine drug discrimination in male rats

Rats raised in an enriched environment show a decrease in sensitivity to the subjective effects of the psychostimulant d-amphetamine. The purpose of the present study was to determine if environmental enrichment during development alters the subjective effects of the more commonly abused drug methamphetamine. Male Sprague-Dawley rats were raised in either an enriched (EC) or an isolated condition (IC). EC and IC rats were trained on a two-lever operant procedure to discriminate 1.0 mg/kg (i.p.) methamphetamine from saline. Following acquisition of the discrimination a methamphetamine generalization curve (0.1-1.0 mg/kg) was determined. The antagonistic effects of dopamine D₁ receptor antagonist SCH23390 (0.0075-0.06 mg/kg) and the dopamine D₂ receptor antagonist eticlopride (0.01-0.3 mg/kg) were also tested. Finally, the ability of nicotine (0.05-0.5 mg/kg) to generalize and the ability of the nicotinic receptor antagonist mecamylamine (0.125-0.5 mg/kg) to antagonize the discriminative stimulus effects of methamphetamine were determined. EC rats were less sensitive to discriminative stimulus effects of methamphetamine compared to IC rats at a low 0.3 mg/kg dose and showed full antagonism of methamphetamine discrimination following SCH23390 compared to IC rats. There were no environmentally-induced differences in the effects of eticlopride. Nicotine only partially generalized to the effects of methamphetamine in both EC and IC rats. While mecamylamine failed to antagonize the effects of methamphetamine in either EC or IC rats. These results suggest that environmental enrichment decreases sensitivity to the discriminative effects of methamphetamine and the differences may be mediated through changes in the D₁ dopamine receptor.

Social experience alters socially induced serotonergic fluctuations in the inferior colliculus

Past social experience and current social context shape the responses of animals to social signals. The serotonergic system is one potential mechanism by which both experiential and contextual factors could be conveyed to sensory systems, such as the auditory system, for multiple reasons. 1) Many features of the serotonergic system are sensitive to social experience. 2) Elevations in serotonergic activity are triggered by social partners, and variations in socially triggered serotonergic responses reflect behavioral differences among social encounters. 3) Serotonin is an auditory neuromodulator, altering how auditory neurons respond to sounds including conspecific vocalizations. In this study, we tested how social experience influences the socially triggered serotonergic response in the inferior colliculus, an auditory midbrain region with an important role in vocalization processing. We used carbon fiber voltammetry to measure serotonin during social interactions of male mice ( Mus musculus) from different social backgrounds: 4 weeks of grouped or individual housing. When paired with an unfamiliar male, both group-housed and individually housed males demonstrated elevations in serotonin; however, individually housed males exhibited socially triggered serotonergic responses with delayed time courses compared with the group-housed males. Furthermore, group-housed males displayed previously described correlations between the socially triggered serotonergic response and behaviors such as social investigation. In contrast, individually housed males did not show these serotonin-behavior relationships. These results suggest that social experience gained via social housing may shape the ability of the central serotonergic system to encode social context in sensory regions.

NEW & NOTEWORTHY We demonstrate that past social experience influences the fidelity with which the serotonergic system represents social context in an auditory region. Social experience altered the time course of socially triggered serotonergic responses and changed how the serotonergic system reflects behavioral variations among social encounters of the same context. These findings are significant to the study of communication, suggesting that centralized neuromodulatory systems potentially convey integrated information regarding past experience and current context to primary sensory regions.

Impact of social isolation and enriched environment during adolescence on voluntary ethanol intake and anxiety in C57BL/6J mice

This study was designed to determine the impact of an enriched environment in a previously established stress model of isolation during early development that induces high alcohol (ethanol) self-administration. The study was conducted with male and female C57BL/6J mice housed in isolation or in groups that were either provided or withheld enrichment during adolescence. The impact of these housing conditions was assessed during adulthood by measuring weight gain, quantifying voluntary ethanol intake, measuring plasma corticosterone levels, and assessing anxiety-like behavior. Results showed that, regardless of sex, mice that were single-housed during adolescence showed a significant increase in voluntary ethanol intake, which was not observed in isolated mice that were provided with nesting material during adolescence (compared to group-housed non-enriched control group). Basal corticosterone was not affected by housing, enrichment conditions, or sex. Corticosterone levels did not relate to levels of voluntary ethanol intake. However, corticosterone levels were higher after three weeks of ethanol intake. Surprisingly, mice that were group-housed during adolescence showed higher levels of anxiety-like behavior in the light/dark test. Overall, these results indicate that housing conditions during a critical developmental period can significantly modulate voluntary ethanol intake later in life.

The effects of rearing environment and chronic methylphenidate administration on behavior and dopamine receptors in adolescent rats

Rearing young rodents in socially isolated or environmentally enriched conditions has been shown to affect numerous components of the dopamine system as well as behavior. Methylphenidate (MPH), a commonly used dopaminergic agent, may affect animals differently based on rearing environment. Here we examined the interaction between environment and chronic MPH treatment at clinically relevant doses, administered via osmotic minipump. Young Sprague Dawley rats (PND 21) were assigned to environmentally enriched, pair-housed, or socially isolated rearing conditions, and treated with either 0, 2, 4, or 8 mg/kg/day MPH for 3 weeks. At the end of the treatment period, animals were tested for locomotor activity and anxiety-like behavior. The densities of D1-like and D2-like receptors were measured in the striatum using in vitro receptor autoradiography. Locomotor activity and anxiety-like behavior were increased in isolated animals compared to pair-housed and enriched animals. The density of D1-like receptors was greater in isolated animals, but there were no differences between groups in D2-like receptor density. Finally, there were no effects of MPH administration on any reported measure. This study provides evidence for an effect of early rearing environment on the dopamine system and behavior, and also suggests that MPH administration may not have long-term consequences.

A critical period for social experience-dependent oligodendrocyte maturation and myelination

Early social isolation results in adult behavioral and cognitive dysfunction that correlates with white matter alterations. However, how social deprivation influences myelination and the significance of these myelin defects in the adult remained undefined. We show that mice isolated for 2 weeks immediately after weaning have alterations in prefrontal cortex function and myelination that do not recover with reintroduction into a social environment. These alterations, which occur only during this critical period, are phenocopied by loss of oligodendrocyte ErbB3 receptors, and social isolation leads to reduced expression of the ErbB3 ligand neuregulin-1. These findings indicate that social experience regulates prefrontal cortex myelination through neuregulin-1/ErbB3 signaling and that this is essential for normal cognitive function, thus providing a cellular and molecular context to understand the consequences of social isolation.

Chronic social isolation and chronic variable stress during early development induce later elevated ethanol intake in adult C57BL/6J mice

Experience with stress situations during early development can have long-lasting effects on stress- and anxiety-related behaviors. Importantly, this can also favor drug self-administration. These studies examined the effects of chronic social isolation and/or variable stress experiences during early development on subsequent voluntary ethanol intake in adult male and female C57BL/6J mice. The experiments were conducted to evaluate the effect of chronic isolation between weaning and adulthood (Experiment 1), chronic isolation during adulthood (Experiment 2), and chronic variable stress (CVS) alone or in combination with chronic social isolation between weaning and adulthood (Experiment 3) on subsequent voluntary ethanol intake. Mice were born in our facility and were separated into two housing conditions: isolate housed (one mouse/cage) or group housed (four mice/cage) according to sex. Separate groups were isolated for 40 days starting either at time of weaning postnatal day 21 (PD 21) (early isolation, Experiments 1 and 3) or at adulthood (PD 60: late isolation, Experiment 2). The effects of housing condition on subsequent ethanol intake were assessed starting at around PD 65 in Experiments 1 and 3 or PD 105 days in Experiment 2. In Experiment 3, starting at PD 32, isolate-housed and group-housed mice were either subjected to CVS or left undisturbed. CVS groups experienced random presentations of mild stressors for 14 days, including exposure to an unfamiliar open field, restraint, physical shaking, and forced swim, among others. All mice were tested for ethanol intake for 14 days using a two-bottle choice (ethanol 15% vol/vol vs. water) for a 2-h limited access procedure. Early social isolation resulted in greater ethanol intake compared with the corresponding group-housed mice (Experiment 1). In contrast, social isolation during adulthood (late isolation) did not increase subsequent ethanol intake compared with the corresponding group-housed mice (Experiment 2). For mice that did not experience CVS, early social isolation resulted in greater ethanol intake compared with group-housed mice (Experiment 3). CVS subsequently resulted in a significant increase in ethanol intake in group-housed mice, but CVS failed to further increase ethanol intake in mice that experienced chronic social isolation early in life (Experiment 3). Overall, female mice consumed more ethanol than males, whether isolated (early or late) or group housed. These results indicate that early but not late social isolation can subsequently influence ethanol consumption in C57BL/6J mice. Thus, the developmental timing of chronic social isolation appears to be an important factor in defining later effects on ethanol self-administration behavior. In addition, experience with CVS early in life results in elevated ethanol intake later in adulthood. Taken together, these results emphasize the important role of early stress experiences that modulate later voluntary ethanol intake during adulthood.

Nocturnal hyperactivity, increased social novelty preference and delayed extinction of fear responses in post-weaning socially isolated mice

When rodents are reared in isolation from young age onwards, they manifest a number of behavioural alterations in adulthood. Since some of these alterations resemble symptoms of psychiatric disorders, the post-weaning social isolation (ISO) manipulation is often applied to create rodent models of these disorders. In rats, ISO effects have been thoroughly characterised, but in mice they are less well documented. Therefore, we further evaluated behaviour of adult ISO mice with a test battery that focussed on abnormalities relevant to schizophrenia. We found that ISO mice were hyperactive during the dark phase. Also, ISO mice showed alterations in magnitude, habituation and prepulse-inhibition of the acoustic startle reflex, increased anxiety, increased social preference and changes in extinction of fear responses. We did not observe increased sensitivity to locomotor-activating effects of amphetamine. It is concluded that ISO of mice might serve as a useful model to test further hypotheses regarding pathogenesis occurring at specific developmental timeframes.

Genomic mapping of social behavior traits in a F2 cross derived from mice selectively bred for high aggression

BACKGROUND

Rapid response to selection was previously observed in mice selected for high levels of inter-male aggression based on number of attacks displayed in a novel social interaction test after isolation housing. Attack levels in this high aggression line (NC900) increased significantly within just four generations of selective breeding, suggesting the presence of a locus with large effect. We conducted an experiment using a small (n ≈ 100) F2 cross between the ICR-derived, non-inbred NC900 strain and the low aggression inbred strain C57BL/6J, genotyped for 154 fully informative SNPs, to determine if a locus with large effect controls the high-aggression selection trait. A second goal was to use high density SNP genotyping (n = 549,000) in the parental strains to characterize residual patterns of heterozygosity within NC900, and evaluate regions that are identical by descent (IBD) between NC900 and C57BL/6J, to determine what impacts these may have on accuracy and resolution of quantitative trait locus (QTL) mapping in the F2 cross.

RESULTS

No evidence for a locus with major effect on aggressive behavior in mice was identified. However, several QTL with genomewide significance were mapped for aggression on chromosomes 7 and 19 and other social behavior traits on chromosomes 4, 7, 14, and 19. High density genotyping revealed that 28% of the genome is still segregating among the six NC900 females used to originate the F2 cross, and that segregating regions are present on every chromosome but are of widely different sizes. Regions of IBD between NC900 and C57BL/6J are found on every chromosome but are most prominent on chromosomes 10, 16 and X. No significant differences were found for amounts of heterozygosity or prevalence of IBD in QTL regions relative to global analysis.

CONCLUSIONS

While no major gene was identified to explain the rapid selection response in the NC900 line, transgressive variation (i.e. where the allele from the C57BL/6J increased attack levels) and a significant role for dominant gene action were hallmarks of the genetic architecture for aggressive behavior uncovered in this study. The high levels of heterozygosity and the distribution of minor allele frequency observed in the NC900 population suggest that maintenance of heterozygosity may have been under selection in this line.

Exploratory behavior in mice selectively bred for developmental differences in aggressive behavior

The development and expression of exploratory behavior was assessed in the Cairns lines of Institute for Cancer Research (ICR) mice that were selectively bred for differences in aggressive behavior, with a high-aggressive 900 line, low-aggressive 100 line, and control 500 line. Four paradigms were employed. Developmental changes were evident in the complex novel arena, with older males faster to contact a novel object, and ambulating more than young males. Within the control 500 line, older males showed longer latency to emerge from the home cage, and shorter latency to contact novel objects. In the 900 line, younger males showed this same pattern. R. B. Cairns proposed that line differences in aggressive behavior arise through alterations in developmental timing [Cairns et al. [1983] Life-span developmental psychology (Vol. 5). New York: Academic Press; Gariépy et al. [2001] Animal Behaviour 61: 933-947]. The early appearance of mature patterns of exploratory behavior in 900 line males supports this interpretation. The 900 line males also appear to be behaviorally inhibited in novel settings such as the light-dark box and the neohypophagia paradigm, compared to the 500 and 100 lines (Experiments 1, 2, and 4). Moreover, in the most complex apparatus, the novel arena, 900 line males were slowest to exit the home cage, and fastest to contact a novel object. The apparent contrast in these parameters of exploratory behavior is discussed in relation to T. C. Schneirla's [1965 Advances in the study of behavior (Vol. 1). New York: PN Academic] approach-withdrawal theory.

Anxiolytic-like effects of oleamide in group-housed and socially isolated mice

Oleamide (cis-9,10-octadecenoamide) is an endogenous sleep-inducing lipid and prototypic member of a new class of biological signaling molecules identified in recent years. In the present study, the anxiolytic-like effect of oleamide was studied in several experimental models of anxiety in group-housed and socially isolated mice. As the results show, socially isolated mice exhibited an anxiogenic-like profile in the elevated plus-maze test, the light/dark test, and the hole-board test, which could be significantly reversed by oleamide (10 or 20 mg/kg, i.p.). Moreover, oleamide significantly reduced the anxiety levels in grouped-housed mice. In the isolation-induced aggressive test, oleamide markedly reduced the attacking duration and increased the attacking latency. It is concluded that oleamide has an anxiolytic-like effect in socially isolated or group-housed mice, which suggests that fatty acid amides might be involved in the regulation of anxiety-related behavior in mice.

Housing and rank status of male Long-Evans rats modify ethanol's effect on open-field behaviors

RATIONALE

Psychosocial stress is known to alter behavior of rodents. While psychosocial stress may alter the response to some drugs, the response to ethanol (EtOH) has not been evaluated.

OBJECTIVE

To examine open-field behaviors of triad- and singly housed rats treated acutely or voluntarily ingesting EtOH.

METHOD

Triad-housed rats were categorized as dominant, subdominant, or subordinate based on assessments of offensive and defensive behaviors. Open-field behaviors were monitored during a 10-min test in rats voluntarily ingesting a 6% solution of EtOH for 2 weeks (1), and after an i.p. injection of saline, 0.5 or 1.0 g kg(-1) of EtOH (2).

RESULTS

Daily intake of EtOH was highest in subdominant and lowest in dominant rats. Overall, open-field behaviors did not differ between water- and EtOH-consuming triad- or singly housed rats. The 0.5-g kg(-1) dose of EtOH enhanced locomotor activity only in triad-housed rats, center entries primarily in singly housed rats, and head-poke behavior in dominant and singly housed rats. Rearing behavior was not altered by the 0.5-g kg(-1) dose, but in singly housed rats, rearing behavior was depressed by the 1.0-g kg(-1) dose. This larger dose of EtOH had no effect on the other behaviors.

CONCLUSIONS

EtOH's effects on open-field behaviors show behavioral specificity and vary with the subject's housing and/or rank status. EtOH's acute anxiolytic-like effect was primarily evident in singly housed rats.

Effects of housing and nicotine on shuttle-box avoidance in male NMRI mice

The present study aimed to evaluate whether housing condition could interact with nicotine administration in influencing the acquisition of a two-way active avoidance task. Male NMRI mice were either group- or individually housed for 30 days and, after this period, evaluated both in the actimeter and, 24h later, in the elevated plus-maze. On the basis of the percentage of time spent in the open arms of the plus-maze, both group- and individually housed mice were sub-classified into three groups with high, moderate or low anxiety baseline levels. Effects of nicotine on the acquisition of the two-way active avoidance task was assessed in each of these groups of mice using an automated shuttle-box. Mice were treated with nicotine (0.35 mg/kg) or vehicle before each daily training session in the shuttle-box (30 trials) over 5 days. The results showed that motor activity counts were significantly higher in individually housed mice than in group-housed counterparts. The factor "anxiety" in the active avoidance test did not reach statistical significance but the factor "housing" did. The number of avoidances was higher in individually housed than in group-housed mice. Separate analyses performed for individually and group-housed animals indicated that group-housed nicotine-treated mice displayed fewer number of avoidances than saline-treated counterparts. It was concluded that housing condition may be a useful variable in evaluating the effects of nicotine on learning tasks. Indeed, the two-way active avoidance task seems to be sensitive to housing condition.

Characterization of dopamine D1 and D2 receptor function in socially housed cynomolgus monkeys self-administering cocaine

RATIONALE

Social rank has been shown to influence dopamine (DA) D(2) receptor function and vulnerability to cocaine self-administration in cynomolgus monkeys. The present studies were designed to extend these findings to maintenance of cocaine reinforcement and to DA D(1) receptors.

OBJECTIVE

Examine the effects of a high-efficacy D(1) agonist on an unconditioned behavior (eyeblinking) and a low-efficacy D(1) agonist on cocaine self-administration, as well as the effects of cocaine exposure on D(2) receptor function across social ranks, as determined by positron emission tomography (PET).

METHODS

Effects of the high-efficacy D(1) agonist SKF 81297 and cocaine (0.3-3.0 mg/kg) on spontaneous blinking were characterized in eight monkeys during 15-min observation periods. Next, the ability of the low-efficacy D(1) agonist SKF 38393 (0.1-17 mg/kg) to decrease cocaine self-administration (0.003-0.1 mg/kg per injection, IV) was assessed in 11 monkeys responding under a fixed-ratio 50 schedule. Finally, D(2) receptor levels in the caudate and putamen were assessed in nineteen monkeys using PET.

RESULTS

SKF 81297, but not cocaine, significantly increased blinking in all monkeys, with slightly greater potency in dominant monkeys. SKF 38393 dose-dependently decreased cocaine-maintained response rates with similar behavioral potency and efficacy across social rank. After an extensive cocaine self-administration history, D(2) receptor levels did not differ across social ranks.

CONCLUSIONS

These results suggest that D(1) receptor function is not substantially influenced by social rank in monkeys from well-established social groups. While an earlier study showed that dominant monkeys had higher D(2) receptor levels and were less sensitive to the reinforcing effects of cocaine during initial exposure, the present findings indicate that long-term cocaine use changed D(2) receptor levels such that D(2) receptor function and cocaine reinforcement were not different between social ranks. These findings suggest that cocaine exposure attenuated the impact of social housing on DA receptor function.

Spatial learning in male mice with different levels of aggressiveness: effects of housing conditions and nicotine administration

The main aim of the present investigation was to evaluate the possible modulation of spatial learning ability by housing conditions and level of aggressiveness in mice, also testing whether differences in locomotion and anxiety could influence this relationship. Additionally, we have examined effects of nicotine in the acquisition and retention of a spatial learning task in groups of mice differing in these variables. NMRI male mice were either group-housed or individually housed for 30 days and then classified into mice with short (SAL) and long (LAL) attack latency after a pre-screening agonistic encounter. Locomotor activity and baseline levels of anxiety of these groups were evaluated in the actimeter and elevated plus-maze. Results indicated that SAL and LAL individually housed mice displayed higher locomotion activity than LAL group-housed mice. In the plus-maze test, SAL and LAL individually housed mice showed more total and open arm entries than group-housed LAL mice, confirming the hyperactivity of individually housed mice and suggesting that isolation had no clear anxiolytic or anxiogenic actions. In the water-maze, we compared the performance of individually housed SAL, individually housed LAL mice, and group-housed LAL mice treated with nicotine (0.35 and 0.175 mg/kg) or vehicle. Nicotine did not improve acquisition in group-housed mice and even impaired it in individually housed mice. Retention of platform position was better in vehicle-treated individually housed mice in comparison with vehicle-treated group-housed mice. The present study demonstrates that housing conditions but not level of aggressiveness modify spontaneous locomotor activity and behaviors displayed on the elevated plus-maze test, and can also influence retention of a spatial learning task.

Sex difference in psychological behavior changes induced by long-term social isolation in mice

Social isolation can induce psychological behavior changes. It is interesting to know whether there is sex difference in responding to social isolation or not. The present study compared the behavior difference between male and female mice isolated for 1-4 months. The results showed that the isolated male mice had higher accounts of locomotor activity than the isolated female and group-housed ones. Both isolated male and female mice spent shorter time in the dark box than the group-housed mice in the light/dark test, and isolated male mice spent less time in the closed arms than isolated female and group-housed mice when isolated for 2, 3 and 4 months in the elevated plus-maze test. These results suggest that isolation induce an anxiolytic-like effect. The immobile time in the forced swimming test was shortened in male mice isolated for 1 and 2 months. Both isolated male and female mice showed shorter time in pentobarbital-induced loss of righting reflex and less body weight gain. These results demonstrated that there was a sex difference in psychological behavior changes in mice undergoing social isolation and the male mice were more easily affected by isolation.

A review of 25 years of the social interaction test

The social interaction test of anxiety was developed 25 years ago to provide an ethologically based test that was sensitive to both anxiolytic and anxiogenic effects. It is sensitive to a number of environmental and physiological factors that can affect anxiety. It has detected anxiogenic effects of peptides such as corticotropin-releasing factor (CRF) and adrenocorticotropic hormone (ACTH), and anxiolytic effects of neuropeptide Y and substance P receptor antagonists. It has successfully identified neuropharmacological sites of action of anxiogenic compounds and drug withdrawal. Effects of compounds acting on the gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT) systems have been extensively investigated after both systemic administration and microinjection into specific brain regions. The use of this test has, thus, played a crucial role in unravelling the neural basis of anxiety. It is hoped that in the next 25 years, the test will play a crucial role in determining the genetic basis of anxiety disorders.

Selective reduction by isolation rearing of 5-HT1A receptor-mediated dopamine release in vivo in the frontal cortex of mice

Serotonin (5-HT)1A receptors modulate in vivo release of brain monoaminergic neurotransmitters which may be involved in isolation-induced aggressive behavior. The present study examined the effect of isolation rearing on the 5-HT1A receptor-mediated modulation of dopamine (DA), 5-HT and noradrenaline (NA) release in the frontal cortex of mice. The selective 5-HT1A receptor agonist (S)-5-[-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242) increased the release of DA and NA and decreased the release of 5-HT in the frontal cortex of mice. The effect of MKC-242 on DA release was significantly less in isolation-reared mice than in group-reared mice, while effects of the drug on NA and 5-HT release did not differ between both groups. The effect of the other 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin on cortical DA release was also less in isolation-reared mice than in group-reared mice, and that of the drug on cortical 5-HT release did not differ between both groups. In contrast to MKC-242-induced DA release, amphetamine-induced increase in cortical DA release in vivo was greater in isolation-reared mice. The present findings suggest that isolation rearing enhances the activity of cortical dopaminergic neurons and reduces selectively the 5-HT1A receptor-mediated release of DA in the cortex.

Handling, genetic and housing effects on the mouse stress system, dopamine function, and behavior

This research was designed to examine how early stimulation (i.e., handling), subsequent housing conditions and genetic factors interact to produce adult differences in stress regulation. High-aggressive (NC900) and low-aggressive (NC100) mice were handled for 3 weeks potspartum and were subsequently isolated or grouped until observed as adults in an open field or a dyadic test. In NC100, handling abolished the temporal variations seen in open-field activity among the nonhandled subjects and reduced corticosterone (CORT) activation. In NC900, these two measures were unaffected by handling. Only among handled NC100 did subsequent group rearing further reduce CORT activation. By contrast, handling caused an up-regulation of D1 dopamine receptors in both lines, and, in NC100, this effect was increased by group rearing. In a dyadic encounter with another male mouse, subjects of both lines showed handling effects. NC100 froze less rapidly and NC900 attacked more rapidly. This multifactorial design showed that the systemic effects of handling are modulated by genetic background, and that measures of these effects are affected by experience beyond infancy. Our findings also showed that the effects of handling vary when assessed across different physiological systems and across social and nonsocial testing conditions.

Attenuating effects of the isolated rearing condition on increased brain serotonin and dopamine turnover elicited by novelty stress

Isolation and acute environmental change are risk factors in human depression. In the present study, we investigated the differences in the brain monoamine activity of rats between two rearing conditions, isolated and group. Moreover, we examined the responses to novelty stress. Male F344 rats aged 11 weeks were divided into the above two groups. Four weeks later they were further divided into non-stress and stress groups. The latter received 20 min exposure to novelty stress. Isolation significantly changed brain monoamine levels, with the levels of dopamine (DA) in the nucleus accumbens and midbrain, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the midbrain, and 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus increasing. Serotonin (5-HT) levels also increased in all brain areas except the raphe nuclei. HVA levels in the raphe nuclei decreased. Novelty stress significantly altered brain monoamine levels. DA, DOPAC, and HVA levels in the prefrontal cortex decreased, as did those of 5-HT in the prefrontal cortex and hippocampus. DA levels in the nucleus accumbens increased. Isolation attenuated the enhanced brain monoamine turnover elicited by novelty stress. The enhanced DA turnover ratio in the prefrontal cortex of the group-reared group was attenuated in the isolated-reared group, and the unchanged DA turnover ratio in the nucleus accumbens of the group-reared group declined in the isolated-reared group. The enhanced 5-HT turnover ratio in the prefrontal cortex, nucleus accumbens, and hippocampus of the group-reared group was attenuated in the isolated-reared group. Isolation may exacerbate adaptation to stress, and be related to the etiology of human depression.

D(3) and D(2) dopamine receptor agonists differentially modulate isolation-induced social-emotional reactivity in mice

Following isolation housing, mice typically exhibit heightened emotional reactivity to mild social stimulation. Aggression, social avoidance and a variety of defensive behaviors that differ in terms of motor activation (e.g. freezing, escape) can be observed depending on strain. Previous studies suggested that D(2)-like dopamine (DA) receptors play an important, albeit strain specific, role in the mediation of particular forms of defensive behavior. D(3) receptors are subtypes of D(2)-like receptors that are highly expressed in limbic areas of the brain and, therefore, they have been hypothesized to mediate emotional behavior. This study examined the effects of the putative D(3) receptor agonists 7-OH-DPAT and PD128907 on social-emotional behavior in isolated C57BL/6J and A/J mice. These effects were compared with those of the selective D(2) receptor agonist PNU91356A. All three DA agonists increased non-locomotor forms of defensive behavior (e.g. freezing, upright defensive posture). These effects were observed at low doses in C57BL/6J and at higher doses in A/J mice. Only the D(3) receptor agonists were effective in increasing locomotor forms of defensive behavior (i.e. escape, jump) at higher doses. These effects were more pronounced in C57BL/6J mice than A/J mice. The increases in stationary and locomotor defensive behavior were accompanied by marked reduction in social investigation in both the strains. Aggressive behavior was also abolished in the aggressive C57BL/6J strain. These results support previous findings and suggest that DA agonists potentiate defensive behavior and/or social fearfulness. They also suggest that D(3) and D(2) DA receptors differentially modulate the expression of social-emotional reactivity and indicate the importance of strain in examining the effects of DA ligands on emotional behavior.

D1 dopamine receptors and the reversal of isolation-induced behaviors in mice

In a previous study, it was demonstrated that the high rates of social reactivity exhibited by isolated male mice in a dyadic encounter were mediated, at least in part, by an increased sensitivity of the D1 dopamine receptors. The present research was guided by the hypothesis that the behavioral effects of isolation are reversible, and that changes in dopaminergic function support this reversibility. To this end, mice selectively bred for high and low levels of aggression were reared in isolation from weaning (21 days) to puberty (45 days), at which point they were either assigned to groups or left in isolation until day 69. By comparison to the continuous isolation condition, mice that eventually formed groups exhibited significantly less reactivity in a dyadic test conducted on day 69, showed a reduced response to dihydrexidine (DHX), and a decreased density of D1 dopamine receptors. This experiment provided evidence for the plasticity of the neurobiological system supporting reactive responses, and confirmed the view that its functional organization is open to experientially-induced changes.

Effects of the putative dopamine D3 receptor antagonist PNU 99194A on motor behavior and emotional reactivity in C57BL/6J mice

Due to the regional expression of D3 dopamine receptors in limbic areas of the brain, there has been considerable interest in the potential role of this receptor subtype in mediating emotional behavior. Previous studies in habituated rats have shown that the putative dopamine D3 receptor antagonist 5,6-dimethoxy-2-(di-n-propylamino)indan (PNU 99194A) increased locomotor behavior. The present study examined the effects PNU 99194A on motor and emotional behaviors in C57BL/6J mice. Motor behavior was assessed in both habituated and nonhabituated mice. Emotional behavior was assessed using the elevated plus-maze and a social context involving an isolated C57BL/6J mouse and a nonaggressive conspecific. In mice habituated to the activity chamber prior to drug administration, PNU 99194A increased locomotion and rearing at lower doses (5, 10 mg/kg) whereas higher doses (20, 30 mg/kg) reduced these behaviors early in the test session. Thigmotaxis was increased independently of the effects on motor behavior. In mice exposed to the activity chamber for the first time, PNU 99194A produced a weak motor activation at lower doses and an initial decrease in motor behavior at higher doses that was followed by an increase in locomotion later in the test session. PNU 99194A had no systematic effects on activity in the elevated plus-maze, but dose-dependently increased flight reactivity in the social reactivity paradigm. These and previous findings raise questions about the role of dopamine D3 receptors in mediating motor behavior and emotional reactivity as well as the pharmacology of this putative dopamine D3 receptor antagonist.