Post-weaning social isolation alters anxiety-related behavior and neurochemical gene expression in the brain of male prairie voles

Chronic variable stress in fathers alters paternal and social behavior but not pup development in the biparental California mouse (Peromyscus californicus)

Stress and chronically elevated glucocorticoid levels have been shown to disrupt parental behavior in mothers; however, almost no studies have investigated corresponding effects in fathers. The present experiment tested the hypothesis that chronic variable stress inhibits paternal behavior and consequently alters pup development in the monogamous, biparental California mouse (Peromyscus californicus). First-time fathers were assigned to one of three experimental groups: chronic variable stress (CVS, n=8), separation control (SC, n=7), or unmanipulated control (UC, n=8). The CVS paradigm (3 stressors per day for 7 days) successfully stressed mice, as evidenced by increased baseline plasma corticosterone concentrations, increased adrenal mass, decreased thymus mass, and a decrease in body mass over time. CVS altered paternal and social behavior of fathers, but major differences were observed only on day 6 of the 7-day paradigm. At that time point, CVS fathers spent less time with their pairmate and pups, and more time autogrooming, as compared to UC fathers; SC fathers spent more time behaving paternally and grooming the female mate than CVS and UC fathers. Thus, CVS blocked the separation-induced increase in social behaviors observed in the SC fathers. Nonetheless, chronic stress in fathers did not appear to alter survival or development of their offspring: pups from the three experimental conditions did not differ in body mass gain over time, in the day of eye opening, or in basal or post-stress corticosterone levels. These results demonstrate that chronic stress can transiently disrupt paternal and social behavior in P. californicus fathers, but does not alter pup development or survival under controlled, non-challenging laboratory conditions.

Disruption of social bonds induces behavioral and physiological dysregulation in male and female prairie voles

The social disruption of losing a partner may have particularly strong adverse effects on psychological and physiological functioning. More specifically, social stressors may play a mediating role in the association between mood disorders and cardiovascular dysfunction. This study investigated the hypothesis that the disruption of established social bonds between male and female prairie voles would produce depressive behaviors and cardiac dysregulation, coupled with endocrine and autonomic nervous system dysfunction. In Experiment 1, behaviors related to depression, cardiac function, and autonomic nervous system regulation were monitored in male prairie voles during social bonding with a female partner, social isolation from the bonded partner, and a behavioral stressor. Social isolation produced depressive behaviors, increased heart rate, heart rhythm dysregulation, and autonomic imbalance characterized by increased sympathetic and decreased parasympathetic drive to the heart. In Experiment 2, behaviors related to depression and endocrine function were measured following social bonding and social isolation in both male and female prairie voles. Social isolation produced similar levels of depressive behaviors in both sexes, as well as significant elevations of adrenocorticotropic hormone and corticosterone. These alterations in behavioral and physiological functioning provide insight into the mechanisms by which social stressors negatively influence emotional and cardiovascular health in humans.

Behavioral and physiological responses of female prairie voles (Microtus ochrogaster) to various stressful conditions

Stressful life events elicit hypothalamic-pituitary-adrenal (HPA) axis activation, which may alter psychological states or behavioral routines. Therefore, the current study focused on the HPA axis response to better understand such manifestations in female prairie voles (Microtus ochrogaster). In Experiment 1, females were stressed for 1 h via one of the four stressors: exposure to a novel environment, immobilization ("plastic mesh"), brief social defeat, or prolonged social defeat. Following a 30-min recovery, the females received a 5-min elevated plus maze (EPM) test and, subsequently, blood was collected to measure plasma corticosterone concentrations. Only immobilization stress induced an anxiety-like behavioral response in the EPM test and elevated plasma corticosterone levels compared to the control groups. Corticosterone concentrations were also significantly elevated following exposure to prolonged social defeat compared to the control conditions, but not after novel environment stress or short social defeat. In Experiment 2, females were exposed to immobilization stress over 1, 3, or 7 days in a daily (predictable; pIMO) or irregular (unpredictable; uIMO) schedule. The biobehavioral stress response in females exposed to pIMO for 3 or 7 days did not differ significantly from controls, suggesting these females habituated. By comparison, females exposed to uIMO over 3 or 7 days did not habituate behaviorally or physiologically, even producing augmented corticosterone levels. In both experiments, positive correlations were found between corticosterone levels and anxiety-like behaviors in the EPM test. Together, our data suggest that the stress response by female prairie voles is dependent on stress intensity, source, previous experience, and predictability. Furthermore, the HPA axis response, as evident by corticosterone levels, is associated with the impact that these factors have on behavioral routine.

Fatherhood reduces the survival of adult-generated cells and affects various types of behavior in the prairie vole (Microtus ochrogaster )

Motherhood has profound effects on physiology, neuronal plasticity, and behavior. We conducted a series of experiments to test the hypothesis that fatherhood, similarly to motherhood, affects brain plasticity (such as cell proliferation and survival) and various behaviors in the highly social prairie vole (Microtus ochrogaster). In Experiment 1, adult males were housed with their same-sex cage mate (control), single-housed (isolation), or housed with a receptive female to mate and produce offspring (father) for 6 weeks. Fatherhood significantly reduced cell survival (assessed by bromodeoxyuridine labeling), but not cell proliferation (assessed by Ki67-labeling), in the amygdala, dentate gyrus of the hippocampus, and ventromedial hypothalamus, suggesting that fatherhood affects brain plasticity. In Experiment 2, neither acute (20 min) nor chronic (20 min daily for 10 consecutive days) pup exposure altered cell proliferation or survival in the brain, but chronic pup exposure increased circulating corticosterone levels. These data suggest that reduced cell survival in the brain of prairie vole fathers was unlikely to be due to the level of pup exposure and display of paternal behavior, and may not be mediated by circulating corticosterone. The effects of fatherhood on various behaviors (including anxiety-like, depression-like, and social behaviors) were examined in Experiment 3. The data indicated that fatherhood increased anxiety- and depression-like behaviors as well as altered aggression and social recognition memory in male prairie voles. These results warrant further investigation of a possible link between brain plasticity and behavioral changes observed due to fatherhood.

Developmental methamphetamine exposure results in short- and long-term alterations in hypothalamic-pituitary-adrenal-axis-associated proteins

Developmental exposure to methamphetamine (MA) causes long-term behavioral and cognitive deficits. One pathway through which MA might induce these deficits is by elevating glucocorticoid levels. Glucocorticoid overexposure during brain development can lead to long-term disruptions in the hypothalamic-pituitary-adrenal (HPA) axis. These disruptions affect the regulation of stress responses and may contribute to behavioral and cognitive deficits reported following developmental MA exposure. Furthermore, alterations in proteins associated with the HPA axis, including vasopressin, oxytocin, and glucocorticoid receptors (GR), are correlated with disruptions in mood and cognition. We therefore hypothesized that early MA exposure will result in short- and long-term alterations in the expression of HPA axis-associated proteins. Male mice were treated with MA (5 mg/kg daily) or saline from postnatal day (P) 11 to P20. At P20 and P90, mice were perfused and their brains processed for vasopressin, oxytocin, and GR immunoreactivity within HPA axis-associated regions. At P20, there was a significant decrease in the number of vasopressin-immunoreactive cells and the area occupied by vasopressin immunoreactivity in the paraventricular nucleus (PVN) of MA-treated mice, but no difference in oxytocin immunoreactivity in the PVN, or GR immunoreactivity in the hippocampus or PVN. In the central nucleus of the amygdala, the area occupied by GR immunoreactivity was decreased by MA. At P90, the number of vasopressin-immunoreactive cells was still decreased, but the area occupied by vasopressin immunoreactivity no longer differed from saline controls. No effects of MA were found on oxytocin or GR immunoreactivity at P90. Thus developmental MA exposure has short- and long-term effects on vasopressin immunoreactivity and short-term effects on GR immunoreactivity.

Central corticotropin releasing factor and social stress

Social interactions are a main source of stress in vertebrates. Social stressors, as well as other stressors, activate the hypothalamic–pituitary–adrenal (HPA) axis resulting in glucocorticoid release. One of the main components of the HPA axis is corticotropin releasing factor (CRF). The neuropeptide CRF is part of a peptide family including CRF, urocortin 1–3, urotensin 1–3, and sauvagine. The actions of the CRF family are mediated by at least two different receptors with different anatomical distribution and affinities for the peptides. The CRF peptides affect several behavioral and physiological responses to stress including aggression, feeding, and locomotor activity. This review will summarize recent research in vertebrates concerning how social stress interacts with components of the CRF system. Consideration will be taken to the different models used for social stress ranging from social isolation, dyadic interactions, to group dominance hierarchies. Further, the temporal effect of social stressor from acute, intermittent, to chronic will be considered. Finally, strains selected for specific behavior or physiology linked to social stress will also be discussed.

The CRF system and social behavior: a review

The corticotropin-releasing factor (CRF) system plays a key role in a diversity of behaviors accompanying stress, anxiety and depression. There is also substantial research on relationships between social behaviors and the CRF system in a variety of taxa including fish, birds, rodents, and primates. Some of these relationships are due to the broad role of CRF and urocortins in stress and anxiety, but these peptides also modulate social behavior specifically. For example, the social interaction (SI) test is often used to measure anxiety-like behavior. Many components of the CRF system including CRF, urocortin1, and the R1 receptor have been implicated in SI, via general effects on anxiety as well as specific effects depending on the brain region. The CRF system is also highly responsive to chronic social stressors such as social defeat and isolation. Animals exposed to these stressors display a number of anxiety- and stress-related behaviors, accompanied by changes in specific components the CRF system. Although the primary focus of CRF research on social behavior has been on the deleterious effects of social stress, there are also insights on a role for CRF and urocortins in prosocial and affiliative behaviors. The CRF system has been implicated in parental care, maternal defense, sexual behavior, and pair bonding. Species differences in the ligands and CRF receptors have been observed in vole and bird species differing in social behavior. Exogenous administration of CRF facilitates partner preference formation in monogamous male prairie voles, and these effects are dependent on both the CRF R1 and R2 receptors. These findings are particularly interesting as studies have also implicated the CRF and urocortins in social memory. With the rapid progress of social neuroscience and in understanding the complex structure of the CRF system, the next challenge is in parsing the exact contribution of individual components of this system to specific social behaviors.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

Social isolation impairs adult neurogenesis in the limbic system and alters behaviors in female prairie voles

Disruptions in the social environment, such as social isolation, are distressing and can induce various behavioral and neural changes in the distressed animal. We conducted a series of experiments to test the hypothesis that long-term social isolation affects brain plasticity and alters behavior in the highly social prairie vole (Microtus ochrogaster). In Experiment 1, adult female prairie voles were injected with a cell division marker, 5-bromo-2'-deoxyuridine (BrdU), and then same-sex pair-housed (control) or single-housed (isolation) for 6 weeks. Social isolation reduced cell proliferation, survival, and neuronal differentiation and altered cell death in the dentate gyrus of the hippocampus and the amygdala. In addition, social isolation reduced cell proliferation in the medial preoptic area and cell survival in the ventromedial hypothalamus. These data suggest that long-term social isolation affects distinct stages of adult neurogenesis in specific limbic brain regions. In Experiment 2, isolated females displayed higher levels of anxiety-like behaviors in both the open field and elevated plus maze tests and higher levels of depression-like behavior in the forced swim test than controls. Further, isolated females showed a higher level of affiliative behavior than controls, but the two groups did not differ in social recognition memory. Together, our data suggest that social isolation not only impairs cell proliferation, survival, and neuronal differentiation in limbic brain areas, but also alters anxiety-like, depression-like, and affiliative behaviors in adult female prairie voles. These data warrant further investigation of a possible link between altered neurogenesis within the limbic system and behavioral changes.

Effects of social isolation on glucocorticoid regulation in social mammals

The regulation and function of the hypothalamic-pituitary-adrenocortical (HPA) axis and glucocorticoids have been well conserved across vertebrate species. Glucocorticoids influence a wide range of physiological functions that include glucose regulation, metabolism, inflammatory control, as well as cardiovascular, reproductive, and neuronal effects. Some of these are relatively quick-acting non-genomic effects, but most are slower-acting genomic effects. Thus, any stimulus that affects HPA function has the potential to exert wide-ranging short-term and long-term effects on much of vertebrate physiology. Here, we review the effects of social isolation on the functioning of the HPA axis in social species, and on glucocorticoid physiology in social mammals in particular. Evidence indicates that objective and perceived social isolation alter HPA regulation, although the nature and direction of the HPA response differs among species and across age. The inconsistencies in the direction and nature of HPA effects have implications for drawing cross-species conclusions about the effects of social isolation, and are particularly problematic for understanding HPA-related physiological processes in humans. The animal and human data are incommensurate because, for example, animal studies of objective isolation have typically not been modeled on, or for comparability with, the subjective experience of isolation in humans. An animal model of human isolation must be taken more seriously if we want to advance our understanding of the mechanisms for the effects of objective and perceived isolation in humans.

Social interaction attenuates stress responses following chronic stress in maternally separated rats

Early life stress has been implicated as a risk factor for functional gastrointestinal (GI) disorders. Hypothalamic oxytocin (OXT) is well known to regulate social interactions and affiliative behaviors. We have shown that maternal separation (MS) induces GI dysmotility and impair hypothalamic OXT expression in response to chronic homotypic stress (CHS). We studied whether social interaction can improve GI dysmotility and OXT expression in MS rats. Male neonatal SD rats were exposed to MS for 180 min from postnatal day (PND)-2 to PND-14. After weaning, 3MS rats were housed together (pure MS). In another group, 1MS rat was housed with 2 control rats (mixed MS). Anxiety-like behaviors were evaluated in elevated plus maze (EPM). Solid gastric emptying (GE) and colonic transit (CT) were measured following CHS loading. Expression of corticotropin releasing factor (CRF) and OXT in the paraventricular nucleus (PVN) were evaluated by real time RT-PCR and immunohistochemistry. Pure MS rats demonstrated increased anxiety-like behaviors, which were significantly reduced in mixed MS rats. Delayed GE (31.5±2.8%, n=6) and accelerated CT [Geometric center (GC) =8.9±0.8, n=6] observed in pure MS rats were restored in mixed MS rats (GE=67.8±3.8%, GC=6.7±1.2, n=6, P<0.05) following CHS. OXT mRNA expression was upregulated, while CRF mRNA expression was downregulated in mixed MS rats, compared to pure MS rats. The number of OXT-immunoreactive cells was significantly increased following CHS at the PVN in mixed MS rats. Our study may contribute to the treatment strategies for GI motility disorders associated with early life stress.

Paternal responsiveness is associated with, but not mediated by reduced neophobia in male California mice (Peromyscus californicus)

Hormones associated with pregnancy and parturition have been implicated in facilitating the onset of maternal behavior via reductions in neophobia, anxiety, and stress responsiveness. To determine whether the onset of paternal behavior has similar associations in biparental male California mice (Peromyscus californicus), we compared paternal responsiveness, neophobia (novel-object test), and anxiety-like behavior (elevated plus maze, EPM) in isolated virgins (housed alone), paired virgins (housed with another male), expectant fathers (housed with pregnant pairmate), and new fathers (housed with pairmate and pups). Corticotropin-releasing hormone (CRH) and Fos immunoreactivity (IR) were quantified in brain tissues following exposure to a predator-odor stressor or under baseline conditions. New fathers showed lower anxiety-like behavior than expectant fathers and isolated virgins in EPM tests. In all housing conditions, stress elevated Fos-IR in the hypothalamic paraventricular nucleus (PVN). Social isolation reduced overall (baseline and stress-induced) Fos- and colocalized Fos/CRH-IR, and increased overall CRH-IR, in the PVN. In the central nucleus of the amygdala, social isolation increased stress-induced CRH-IR and decreased stress-induced activation of CRH neurons. Across all housing conditions, paternally behaving males displayed more anxiety-related behavior than nonpaternal males in the EPM, but showed no differences in CRH- or Fos-IR. Finally, the latency to engage in paternal behavior was positively correlated with the latency to approach a novel object. These results suggest that being a new father does not reduce anxiety, neophobia, or neural stress responsiveness. Low levels of neophobia, however, were associated with, but not necessary for paternal responsiveness.

Impaired adaptation of gastrointestinal motility following chronic stress in maternally separated rats

Exposure to early life stress causes increased stress responsiveness and permanent changes in the central nervous system. We recently showed that delayed gastric emptying (GE) and accelerated colonic transit (CT) in response to acute restraint stress (ARS) were completely restored following chronic homotypic stress (CHS) in rats via upregulation of hypothalamic oxytocin (OXT) expression. However, it is unknown whether early life stress affects hypothalamic OXT circuits and gastrointestinal motor function. Neonatal rats were subjected to maternal separation (MS) for 180 min/day for 2 wk. Anxiety-like behaviors were evaluated by the elevated-plus-maze test. GE and CT were measured under nonstressed (NS), ARS, and CHS conditions. Expression of corticotropin-releasing factor (CRF) and OXT in the paraventricular nucleus (PVN) of the hypothalamus was evaluated by real time RT-PCR and immunohistochemistry. MS increased anxiety-like behaviors. ARS delayed GE and accelerated CT in control and MS rats. After CHS, delayed GE and accelerated CT were restored in control, but not MS, rats. CRF mRNA expression was significantly increased in response to ARS in control and MS rats. Increased CRF mRNA expression was still observed following CHS in MS, but not control, rats. In response to CHS, OXT mRNA expression was significantly increased in control, but not MS, rats. The number of OXT-immunoreactive cells was increased following CHS in the magnocellular part of the PVN in control, but not MS, rats. MS impairs the adaptation response of gastrointestinal motility following CHS. The mechanism of the impaired adaptation involves downregulation of OXT and upregulation of CRF in the hypothalamus in MS rats.

Developmental exposure to a serotonin agonist produces subsequent behavioral and neurochemical changes in the adult male prairie vole

Autistic spectrum disorders (ASDs) are classified as pervasive developmental disorders characterized by abnormalities in various cognitive and behavioral functions. Although exact underlying causes are still unknown, nearly 30% of autistic patients show elevated blood levels of serotonin (5-HT) and, therefore, various genetic and environmental factors that are known to elevate 5-HT levels may play a role in the development of ASDs. In the present study, we used the socially monogamous male prairie vole (Microtus ochrogaster) as an animal model to examine the effects of perinatal exposure to 5-methoxytryptamine (5-MT), a non-selective serotonin agonist, on subsequent behavioral and neurochemical changes in the brain. 5-MT treated males showed a decrease in affiliation and an increase in anxiety-related behavior, as well as a decrease in the density of 5-HT immunoreactive (ir) fibers in the amygdala and oxytocin-ir and vasopressin-ir cells in the paraventricular nucleus of the hypothalamus, compared to saline treated controls. These data indicate that exposure to 5-HT during early development can induce abnormalities in various neurochemical systems which, in turn, may underlie deficits in social and anxiety-related behaviors. In addition, these data will help to establish the prairie vole model to study the neurobiological underpinnings of complex neuropsychiatric disorders such as ASDs.

Social isolation affects partner-directed social behavior and cortisol during pair formation in marmosets, Callithrix geoffroyi

Pair-bonded relationships form during periods of close spatial proximity and high sociosexual contact. Like other monogamous species, marmosets form new social pairs after emigration or ejection from their natal group resulting in periods of social isolation. Thus, pair formation often occurs following a period of social instability and a concomitant elevation in stress physiology. Research is needed to assess the effects that prolonged social isolation has on the behavioral and cortisol response to the formation of a new social pair. We examined the sociosexual behavior and cortisol during the first 90-days of cohabitation in male and female Geoffroy's tufted-ear marmosets (Callithrix geoffroyi) paired either directly from their natal group (Natal-P) or after a prolonged period of social isolation (ISO-P). Social isolation prior to pairing seemed to influence cortisol levels, social contact, and grooming behavior; however, sexual behavior was not affected. Cortisol levels were transiently elevated in all paired marmosets compared to natal-housed marmosets. However, ISO-P marmosets had higher cortisol levels throughout the observed pairing period compared to Natal-P marmoset. This suggests that the social instability of pair formation may lead to a transient increase in hypothalamic-pituitary-adrenal (HPA) axis activity while isolation results in a prolonged HPA axis dysregulation. In addition, female social contact behavior was associated with higher cortisol levels at the onset of pairing; however, this was not observed in males. Thus, isolation-induced social contact with a new social partner may be enhanced by HPA axis activation, or a moderating factor.

Experimental models for anxiolytic drug discovery in the era of omes and omics

INTRODUCTION

Animal behavioral models have become an indispensable tool for studying anxiety disorders and testing anxiety-modulating drugs. However, significant methodological and conceptual challenges affect the translational validity and accurate behavioral dissection in such models. They are also often limited to individual behavioral domains and fail to target the disorder's real clinical picture (its spectrum or overlap with other disorders), which hinder screening and development of novel anxiolytic drugs.

AREAS COVERED

In this article, the authors discuss and emphasize the importance of high-throughput multi-domain neurophenotyping based on the latest developments in video-tracking and bioinformatics. Additionally, the authors also explain how bioinformatics can provide new insight into the neural substrates of brain disorders and its benefit for drug discovery.

EXPERT OPINION

The throughput and utility of animal models of anxiety and other brain disorders can be markedly increased by a number of ways: i) analyzing systems of several domains and their interplay in a wider spectrum of model species; ii) using a larger number of end points generated by video-tracking tools; iii) correlating behavioral data with genomic, proteomic and other physiologically relevant markers using online databases and iv) creating molecular network-based models of anxiety to identify new targets for drug design and discovery. Experimental models utilizing bioinformatics tools and online databases will not only improve our understanding of both gene-behavior interactions and complex trait interconnectivity but also highlight new targets for novel anxiolytic drugs.

Social influences on neurobiology and behavior: epigenetic effects during development

The quality of the social environment can have profound influences on the development and activity of neural systems with implications for numerous behavioral and physiological responses, including the expression of emotionality. Though social experiences occurring early in development may be particularly influential on the developing brain, there is continued plasticity within these neural circuits amongst juveniles and into early adulthood. In this review, we explore the evidence derived from studies in rodents which illustrates the social modulation during development of neural systems, with a particular emphasis on those systems in which a long-term effect is observed. One possible explanation for the persistence of dynamic changes in these systems in response to the environment is the involvement of epigenetic mechanisms, and here we discuss recent studies which support the role of these mechanisms in mediating the link between social experiences, gene expression, neurobiological changes, and behavioral variation. This literature raises critical questions about the interaction between neural systems, the concordance between neural and behavioral changes, sexual dimorphism in effects, the importance of considering individual differences in response to the social environment, and the potential of an epigenetic perspective in advancing our understanding of the pathways leading to variations in mental health.

Prairie voles as a novel model of socially facilitated excessive drinking

Social relationships strongly affect alcohol drinking in humans. Traditional laboratory rodents do not exhibit social affiliations with specific peers, and cannot adequately model how such relationships impact drinking. The prairie vole is a socially monogamous rodent used to study social bonds. The present study tested the prairie vole as a potential model for the effects of social affiliations on alcohol drinking. Same-sex adult sibling prairie voles were paired for five days, and then either separated into individual cages, or housed in pairs. Starting at the time of separation, the voles received unlimited access to alcohol in a two-bottle choice test versus water. Pair-housed siblings exhibited higher preference for alcohol, but not saccharin, than singly housed voles. There was a significant correlation between the amount of alcohol consumed by each member of a pair when they were housed together (r = 0.79), but not when housed apart (r = 0.20). Following automated analysis of circadian patterns of fluid consumption indicating peak fluid intake before and after the dark phase, a limited access two-hour two-bottle choice procedure was established. Drinking in this procedure resulted in physiologically relevant blood ethanol concentrations and increased Fos immunoreactivity in perioculomotor urocortin containing neurons (but not in nucleus accumbens or central nucleus of the amygdala). The high ethanol preference and sensitivity to social manipulation indicate that prairie voles can serve to model social influences on excessive drinking.

Loneliness matters: a theoretical and empirical review of consequences and mechanisms

As a social species, humans rely on a safe, secure social surround to survive and thrive. Perceptions of social isolation, or loneliness, increase vigilance for threat and heighten feelings of vulnerability while also raising the desire to reconnect. Implicit hypervigilance for social threat alters psychological processes that influence physiological functioning, diminish sleep quality, and increase morbidity and mortality. The purpose of this paper is to review the features and consequences of loneliness within a comprehensive theoretical framework that informs interventions to reduce loneliness. We review physical and mental health consequences of loneliness, mechanisms for its effects, and effectiveness of extant interventions. Features of a loneliness regulatory loop are employed to explain cognitive, behavioral, and physiological consequences of loneliness and to discuss interventions to reduce loneliness. Loneliness is not simply being alone. Interventions to reduce loneliness and its health consequences may need to take into account its attentional, confirmatory, and memorial biases as well as its social and behavioral effects.

Voluntary exercise reduces the neurotoxic effects of 6-hydroxydopamine in maternally separated rats

Maternal separation has been associated with development of anxiety-like behaviour and learning impairments in adult rats. This has been linked to changes in brain morphology observed after exposure to high levels of circulating glucocorticoids during the stress-hyporesponsive period (P4-P14). In the present study, adult rats that had been subjected to maternal separation (180 min/day for 14 days) during the stress-hyporesponsive period, received unilateral infusions of a small dose of 6-hydroxydopamine (6-OHDA, 5 microg/4 microl saline) into the medial forebrain bundle. The results showed that voluntary exercise had a neuroprotective effect in both non-stressed and maternally separated rats in that there was a decrease in forelimb akinesia (step test) and limb use asymmetry (cylinder test). Maternal separation increased forelimb akinesia and forelimb use asymmetry and reduced the beneficial effect of exercise on forelimb akinesia. It also reduced exploratory behaviour, consistent with anxiety-like behaviour normally associated with maternal separation. Exercise appeared to reduce dopamine neuron destruction in the lesioned substantia nigra when expressed as a percentage of the non-lesioned hemisphere. However, this appeared to be due to a compensatory decrease in completely stained tyrosine hydroxylase-positive neurons in the contralateral, non-lesioned substantia nigra. In agreement with reports that maternal separation increases the 6-OHDA-induced loss of dopamine terminals in the striatum, there was a small increase in dopamine neuron destruction when expressed as a percentage of the non-lesioned hemisphere but there was no difference in dopamine cell number, suggesting that exposure to maternal separation did not exacerbate dopamine cell loss.

Biological contribution to social influences on alcohol drinking: evidence from animal models

Social factors have a tremendous influence on instances of heavy drinking and in turn impact public health. However, it is extremely difficult to assess whether this influence is only a cultural phenomenon or has biological underpinnings. Research in non-human primates demonstrates that the way individuals are brought up during early development affects their future predisposition for heavy drinking, and research in rats demonstrates that social isolation, crowding or low social ranking can lead to increased alcohol intake, while social defeat can decrease drinking. Neurotransmitter mechanisms contributing to these effects (i.e., serotonin, GABA, dopamine) have begun to be elucidated. However, these studies do not exclude the possibility that social effects on drinking occur through generalized stress responses to negative social environments. Alcohol intake can also be elevated in positive social situations, for example, in rats following an interaction with an intoxicated peer. Recent studies have also begun to adapt a new rodent species, the prairie vole, to study the role of social environment in alcohol drinking. Prairie voles demonstrate a high degree of social affiliation between individuals, and many of the neurochemical mechanisms involved in regulation of these social behaviors (for example, dopamine, central vasopressin and the corticotropin releasing factor system) are also known to be involved in regulation of alcohol intake. Naltrexone, an opioid receptor antagonist approved as a pharmacotherapy for alcoholic patients, has recently been shown to decrease both partner preference and alcohol preference in voles. These findings strongly suggest that mechanisms by which social factors influence drinking have biological roots, and can be studied using rapidly developing new animal models.

The impact of early life family structure on adult social attachment, alloparental behavior, and the neuropeptide systems regulating affiliative behaviors in the monogamous prairie vole (microtus ochrogaster)

Early social attachments lie at the heart of emotional and social development in many mammals, including humans. In nature, monogamous prairie voles (Microtus ochrogaster) experience considerable natural variation in early social attachment opportunities due to differences in family structure [e.g., single-mothers (SM), solitary breeding pairs, and communal groups]. We exploited some of this natural variation in family structure to examine the influence of early social environment on the development of adult social behavior. First, we characterized the parental care received by pups reared biparentally (BP) or by SM in the laboratory. Second, we examined whether BP- and SM-reared offspring differed in adult nurturing, bonding, and emotional behaviors. Finally, we investigated the effects of rearing condition on neuropeptide systems that regulate adult social behavior [oxytocin (OT), vasopressin, and corticotropin-releasing factor, (CRF)]. Observations revealed that SM-reared pups were exposed more frequently (P < 0.01), licked and groomed less (P < 0.01), and matured more slowly (P < 0.01) than BP-reared pups. In adulthood, there were striking socio-behavioral differences: SM-reared females showed low spontaneous, pup-directed alloparental behavior (P < 0.01) and both males and females from the SM-reared condition showed delayed partner preference formation. While rearing did not impact neuropeptide receptor densities in the ventral forebrain as we predicted, SM-reared animals, particularly females, had increased OT content (P < 0.01) and greater dorsal raphe CRF2 densities (P < 0.05) and both measures correlated with licking and grooming experienced during the first 10 days of life. These results suggest that naturalistic variation in social rearing conditions can introduce diversity into adult nurturing and attachment behaviors.