Anti-inflammatory influences on behavioral, but not cortisol, responses during maternal separation

Examination of the role of adrenergic receptor stimulation in the sensitization of neuroinflammatory-based depressive-like behavior in isolated Guinea pig pups

Early-life attachment disruption appears to sensitize neuroinflammatory signaling to increase later vulnerability for stress-related mental disorders, including depression. How stress initiates this process is unknown, but studies with adult rats and mice suggest sympathetic nervous system activation and/or cortisol elevations during the early stress are key. Guinea pig pups isolated from their mothers exhibit an initial active behavioral phase characterized by anxiety-like vocalizing. This is followed by inflammatory-dependent depressive-like behavior and fever that sensitize on repeated isolation. Using strategies that have been successful in adult studies, we assessed whether sympathetic nervous system activity and cortisol contributed to the sensitization process in guinea pig pups. In Experiment 1, the adrenergic agonist ephedrine (3 or 10 mg/kg), either alone or with cortisol (2.5 mg/kg), did not increase depressive-like behavior or fever during initial isolation the following day as might have been expected to if this stimulation was sufficient to account for the sensitization process. In Experiment 2, both depressive-like behavior and fever sensitized with repeated isolation, but beta-adrenergic receptor blockade with propranolol (10 or 20 mg/kg) did not affect either of these responses or their sensitization. The high dose of propranolol did, however, reduce vocalizing. These results suggest sympathetic nervous system activation is neither necessary nor sufficient to induce the presumptive neuroinflammatory signaling underlying sensitization of depressive-like behavioral or febrile responses in developing guinea pigs. Thus, processes mediating sensitization of neuroinflammatory-based depressive-like behavior following early-life attachment disruption in this model appear to differ from those previously found to underlie neuroinflammatory priming in adults.

Early Life Stress, Neuroinflammation, and Psychiatric Illness of Adulthood

Stress exposure during early stages of life elevates the risk of developing psychopathologies and psychiatric illness in later life. The brain and immune system are not completely developed by birth and therefore continue develop after birth; this post birth development is influenced by several psychosocial factors; hence, early life stress (ELS) exposure can alter brain structural development and function. A growing number of experimental animal and observational human studies have investigated the link between ELS exposure and increased risk of psychopathology through alternations in the immune system, by evaluating inflammation biomarkers. Recent studies, including brain imaging, have also shed light on the mechanisms by which both the innate and adaptive immune systems interact with neural circuits and neurotransmitters, which affect psychopathology. Herein, we discuss the link between the experience of stress in early life and lifelong alterations in the immune system, which subsequently lead to the development of various psychiatric illnesses.

Neuroinflammation, Early-Life Adversity, and Brain Development

The overarching objective is to review how early exposure to adversity interacts with inflammation to alter brain maturation. Both adversity and inflammation are significant risk factors for psychopathology. Literature relevant to the effects of adversity in children and adolescents on brain development is reviewed. These studies are supported by research in animals exposed to species-relevant stressors during development. While it is known that exposure to adversity at any age increases inflammation, the effects of inflammation are exacerbated at developmental stages when the immature brain is uniquely sensitive to experiences. Microglia play a vital role in this process, as they scavenge cellular debris and prune synapses to optimize performance. In essence, microglia modify the synapse to match environmental demands, which is necessary for someone with a history of adversity. Overall, by piecing together clinical and preclinical research areas, what emerges is a picture of how adversity uniquely sculpts the brain. Microglia interactions with the inhibitory neurotransmitter GABA (specifically, the subtype expressing parvalbumin) are discussed within contexts of development and adversity. A review of inflammation markers in individuals with a history of abuse is combined with preclinical studies to describe their effects on maturation. Inconsistencies within the literature are discussed, with a call for standardizing methodologies relating to the age of assessing adversity effects, measures to quantify stress and inflammation, and more brain-based measures of biochemistry. Preclinical studies pave the way for interventions using anti-inflammation-based agents (COX-2 inhibitors, CB2 agonists, meditation/yoga) by identifying where, when, and how the developmental trajectory goes awry.

Presence of mother prompts dissociation of sickness behavior, fever, and hypothalamic gene expression in lipopolysaccharide-injected guinea pig pups

During infection, sickness behaviors, such as a hunched stance with piloerection, can facilitate host resistance by supporting the generation and maintenance of fever. Fever, in turn, is mediated by hypothalamic neuroimmune signaling. Sickness behaviors, however, can also be influenced by social stimuli. In this study, guinea pig pups were injected with lipopolysaccharide to simulate a bacterial infection and then exposed to a novel, threatening environment while either with their mother or alone. We found that the presence of the mother suppressed sickness behavior, but enhanced fever, and had no measureable effect on gene expression of hypothalamic mediators of fever. This 3-way dissociation induced by the mother's presence is interpreted in terms of the differential adaptive consequences of behavioral and febrile responses for pups in this situation. The results contribute to a growing literature linking immunological and social processes.

Central oxytocin alters cortisol and behavioral responses of guinea pig pups during isolation in a novel environment

The neuropeptide oxytocin plays key roles in social bonding and stress reduction, and thus appears to be a likely mediator of maternal buffering of infant stress responses. In the guinea pig, the presence of the mother in a threatening environment buffers cortisol elevations as well as active (vocalizing) and passive (e.g. crouching) responses typical of isolation in this species; yet, effects of OT in guinea pig pups under any conditions have not been reported. Here, we examined the ability of intracerebroventricular (ICV) OT to moderate plasma cortisol levels and behavior in guinea pig pups isolated in a brightly lit, novel environment, and the ability of a highly selective OT antagonist (OTA) to reduce buffering by the mother. We found that ICV OT moderated cortisol levels and vocalizations, but increased time spent in the crouched stance, particularly in females. In addition, OT modulated other ongoing behaviors in a sex-dependent fashion. In females, OT reduced duration of walking and rearing, and increased time spent quiet, while in males OT increased duration of rearing. OTA, however, was without effect on cortisol levels or behavior. These findings, including sex differences in response, extend results from other species to the guinea pig. Further, while demonstrating that exogenous OT is sufficient to reduce biobehavioral stress responses typical of isolated guinea pig infants, the results suggest that endogenous OT is not necessary for maternal buffering of infant responses in this species.

Early Attachment Disruption, Inflammation, and Vulnerability for Depression in Rodent and Primate Models

Early experiments in nonhuman primates established the relation between disruption of filial attachment and depressive-like outcomes. Subsequent studies in rats and mice have been instrumental in linking depressive-like outcomes to disturbances in maternal behavior. Another aspect of attachment disruption, absence of the attachment object per se, may be studied more effectively in a different laboratory rodent-the guinea pig. Here, we discuss the rationale for using guinea pigs for this work. We then review guinea pig studies providing evidence for inflammatory mechanisms mediating both depressive-like behavior during separation as well as sensitization of stress responsiveness such as is thought to lead to increased vulnerability to depression at later ages. Finally, we discuss recent complementary work in adult monkeys that suggests cross-species generalizability of broad principles derived from the guinea pig experiments. Overall, the findings provide experimental support for human research implicating inflammatory mechanisms in the development of increased stress responsiveness and vulnerability to depression following attachment disruption and other forms of early-life stress. Specifically, the findings suggest inflammatory mechanisms may set in motion a cascade of underlying processes that mediate later increased stress responsiveness and, therefore, depression susceptibility.

Central neuroimmune activity and depressive-like behavior in response to repeated maternal separation and injection of LPS

Disruption of attachment relations in early life is linked to greater vulnerability to depressive illness at later ages. Evidence suggests this process involves stress-induced activation of central inflammatory factors, though the specific mediators and processes involved are not known. We used a guinea pig model in which effects of maternal separation appear more clearly due to absence of the attachment figure than is the case for other laboratory rodents. Separation in a novel environment on two consecutive days evoked a depressive-like behavioral response that sensitized during a final test 9 days later. At this time, prior separation blunted the response of prostaglandin synthesizing enzymes (COX-2 and mPGES) and chemokines (CXCL-1 and MCP-1) 120 min following injection with lipopolysaccharide and isolation in a novel cage. The blunted response was not associated with a greater plasma cortisol elevation. In addition, injection of saline just prior to isolation at the oldest age elicited small, but significant, elevations in several signaling molecules, particularly at 30 min. These results demonstrate lasting central inflammatory consequences of our separation procedure. However, contrary to expectations, sensitization of depressive-like behavior was not associated with an increase in expression of neuroimmune mediators to inflammatory challenge. Together with earlier findings, the results suggest a multi-step process in which inflammatory response to an initial separation affects downstream mediators to sensitize depressive-like behavior.

Neuroimmune Impacts of Early-Life Stress on Development and Psychopathology

Maltreatment and trauma in childhood, termed early-life stress (ELS), has long-term effects on the immune system. ELS impacts immune signaling at the time of exposure but also disrupts the developmental trajectory of certain immunological processes, both in the periphery and in the brain. One consequence of these early alterations is a heightened immune response to stressors later in life. However, chronic and sustained inflammatory response can also lead to excitotoxicity and prevent typical brain development. In this chapter, we discuss current progress toward understanding the contribution of neuroimmune signaling to ELS-attributable dysfunction or maladaptation with a focus on postnatal experiences. To do so we first present an operational definition of ELS. Then, we offer a brief overview of the immune system and neuroimmune development, followed by a section discussing the interaction between immunity, childhood trauma, and mental disorders in humans. We present evidence from animal models about immune alterations after ELS and discuss the ways in which ELS-induced immune changes ultimately affect brain and behavior, as well as the importance of individual differences and future directions in this field. Taken together, we submit that when encountered with ELS, some core brain circuits could develop differently via various mechanisms involving dysfunctional immune reprograming. However, given the remarkable plasticity of both the brain and the immune system, many of the deleterious effects of ELS may be mitigated with interventions that account for sex and target neuroimmune interactions over the lifespan.

A multispecies approach for understanding neuroimmune mechanisms of stress

The relationship between stress challenges and adverse health outcomes, particularly for the development of affective disorders, is now well established. The highly conserved neuroimmune mechanisms through which responses to stressors are transcribed into effects on males and females have recently garnered much attention from researchers and clinicians alike. The use of animal models, from mice to guinea pigs to primates, has greatly increased our understanding of these mechanisms on the molecular, cellular, and behavioral levels, and research in humans has identified particular brain regions and connections of interest, as well as associations between stress-induced inflammation and psychiatric disorders. This review brings together findings from multiple species in order to better understand how the mechanisms of the neuroimmune response to stress contribute to stress-related psychopathologies, such as major depressive disorder, schizophrenia, and bipolar disorder.

Modulation of Binge-like Ethanol Consumption by IL-10 Signaling in the Basolateral Amygdala

Excessive ethanol consumption alters the neuroimmune system and particularly impacts the cytokine milieu of the CNS. Cytokine dysregulation has been shown to underlie addictive-like behaviors including alcohol abuse; however, many studies focus primarily on the proinflammatory cytokine profile during alcohol dependence. The current study furthers this research by determining the impact of excessive ethanol consumption on interleukin-10 (IL-10) and interleukin-4 (IL-4) activity in a model of non-dependent binge consumption called the "drinking in the dark" (DID) paradigm. Furthermore, the ability of IL-10 to modulate ethanol consumption was tested using site-directed pharmacology. Immunohistochemistry analyses determined that ethanol decreased IL-10 by 50 % in the basolateral amygdala (BLA) but had no effect on IL-4. Neither IL-10 nor IL-4, however, were altered in the central amygdala (CEA). Enzyme linked immunosorbent assays confirmed that IL-10 was decreased in the amygdala but not in the serum, suggesting changes of this cytokine with the DID paradigm are restricted to the central nervous system. Finally, bilateral infusions of IL-10 into the BLA, but not CeA, reduced binge-like drinking and corresponding blood ethanol concentrations without impacting either locomotor activity or anxiety-like behavioral correlates. Together, these data support the idea that alcohol abuse dysregulates specific anti-inflammatory cytokines; however, ameliorating alcohol-induced effects on cytokines, like IL-10, may prove to be an effective therapy in curbing excessive consumption.

A multispecies approach for understanding neuroimmune mechanisms of stress

The relationship between stress challenges and adverse health outcomes, particularly for the development of affective disorders, is now well established. The highly conserved neuroimmune mechanisms through which responses to stressors are transcribed into effects on males and females have recently garnered much attention from researchers and clinicians alike. The use of animal models, from mice to guinea pigs to primates, has greatly increased our understanding of these mechanisms on the molecular, cellular, and behavioral levels, and research in humans has identified particular brain regions and connections of interest, as well as associations between stress-induced inflammation and psychiatric disorders. This review brings together findings from multiple species in order to better understand how the mechanisms of the neuroimmune response to stress contribute to stress-related psychopathologies, such as major depressive disorder, schizophrenia, and bipolar disorder.

Depressive-like behavior, its sensitization, social buffering, and altered cytokine responses in rhesus macaques moved from outdoor social groups to indoor housing

Psychosocial stressors appear to promote the onset of depressive illness through activation and sensitization of inflammatory mechanisms. Here, adult male rhesus monkeys brought from large outdoor social groups to indoor housing for 8 days reliably exhibited a hunched, depressive-like posture. When rehoused indoors a second 8 days about 2 weeks later, monkeys housed alone, but not those with an affiliative partner, showed sensitization of the depressive-like hunched posture. Housing indoors also affected circulating pro-inflammatory cytokines: IL-1β showed increased responsiveness to immune challenge, and IL-1β and TNF-α showed reduced suppression by dexamethasone. Sensitivity of the anti-inflammatory cytokine IL-10 to immune challenge exhibited a relative increase from the first to the second round of indoor housing in animals housed in pairs, and a relative decrease in animals housed alone. Cytokine levels during indoor housing were positively correlated with duration of depressive-like behavior. Plasma cortisol levels increased but did not differentiate housing conditions or rounds. Results demonstrate a rapid induction and sensitization of depressive-like behavior to indoor individual housing, social buffering of sensitization, and associated inflammatory responses. This paradigm may provide a practical nonhuman primate model for examining inflammatory-mediated consequences of psychosocial stressors on depression and possible social buffering of these effects.

Maternal separation increases later immobility during forced swim in guinea pig pups: evidence for sensitization of a depressive-like state

Early-life stress is thought to increase later vulnerability for developing depressive illness by sensitizing underlying stress-responsive systems. Guinea pig pups separated from their mother and isolated in a novel cage for 3 hr exhibit a sensitized depressive-like behavioral response when separated again the following day as well as weeks later. The behavioral response and its sensitization appear to be mediated by inflammatory factors. To determine if this sensitization is specific to the separation response or if it reflects a broader underlying depressive-like state, guinea pig pups that had either been separated for 3 hr or remained with their mothers were observed in the forced swim test the following 3 days. Earlier separation was found to increase the duration of immobility, a measure sensitive to antidepressant treatment. These results support the use of the guinea pig as a model for examining mechanisms of inflammatory-mediated sensitization of depression following stress in early life.

Owners' Perceptions of Their Animal's Behavioural Response to the Loss of an Animal Companion

Simple Summary

The loss of a companion animal is recognised as being associated with experiences of grief by the owner, but it is unclear how other animals in the household may be affected by such a loss. This paper investigates the behavioural responses of dogs and cats to the loss of an animal companion through owner-reported observations. There was consensus that behaviour changed as a result of loss including increased affectionate behaviour, territorial behaviour, and changes in food consumption and vocalisation.

Abstract

The loss of a companion animal is recognised as being associated with experiences of grief by the owner, but it is unclear how other animals in the household may be affected by such a loss. Our aim was to investigate companion animals’ behavioural responses to the loss of a companion through owner-report. A questionnaire was distributed via, and advertised within, publications produced by the Royal Society for the Prevention of Cruelty to Animals (RSPCA) across Australia and New Zealand, and through a selection of veterinary clinics within New Zealand. A total of 279 viable surveys were returned pertaining to 159 dogs and 152 cats. The two most common classes of behavioural changes reported for both dogs and cats were affectionate behaviours (74% of dogs and 78% of cats) and territorial behaviours (60% of dogs and 63% of cats). Both dogs and cats were reported to demand more attention from their owners and/or display affiliative behaviour, as well as spend time seeking out the deceased’s favourite spot. Dogs were reported to reduce the volume (35%) and speed (31%) of food consumption and increase the amount of time spent sleeping (34%). Cats were reported to increase the frequency (43%) and volume (32%) of vocalisations following the death of a companion. The median duration of reported behavioural changes in both species was less than 6 months. There was consensus that the behaviour of companion animals changed in response to the loss of an animal companion. These behavioural changes suggest the loss had an impact on the remaining animal.

Stability and change: Stress responses and the shaping of behavioral phenotypes over the life span

In mammals, maternal signals conveyed via influences on hypothalamic-pituitary-adrenal (HPA) activity may shape behavior of the young to be better adapted for prevailing environmental conditions. However, the mother's influence extends beyond classic stress response systems. In guinea pigs, several hours (h) of separation from the mother activates not only the HPA axis, but also the innate immune system, which effects immediate behavioral change, as well as modifies behavioral responsiveness in the future. Moreover, the presence of the mother potently suppresses the behavioral consequences of this innate immune activation. These findings raise the possibility that long-term adaptive behavioral change can be mediated by the mother's influence on immune-related activity of her pups. Furthermore, the impact of social partners on physiological stress responses and their behavioral outcomes are not limited to the infantile period. A particularly crucial period for social development in male guinea pigs is that surrounding the attainment of sexual maturation. At this time, social interactions with adults can dramatically affect circulating cortisol concentrations and social behavior in ways that appear to prepare the male to best cope in its likely future social environment. Despite such multiple social influences on the behavior of guinea pigs at different ages, inter-individual differences in the magnitude of the cortisol response remain surprisingly stable over most of the life span. Together, it appears that throughout the life span, physiological stress responses may be regulated by social stimuli. These influences are hypothesized to adjust behavior for predicted environmental conditions. In addition, stable individual differences might provide a means of facilitating adaptation to less predictable conditions.

Naproxen attenuates sensitization of depressive-like behavior and fever during maternal separation

Early life stress can increase susceptibility for later development of depressive illness though a process thought to involve inflammatory mediators. Isolated guinea pig pups exhibit a passive, depressive-like behavioral response and fever that appear mediated by proinflammatory activity, and which sensitize with repeated separations. Treatment with an anti-inflammatory can attenuate the behavioral response during the initial separation and separation the following day. Here we used the cyclooxygenase inhibitor naproxen to examine the role of prostaglandins in mediating the depressive-like behavior and core body temperature of young guinea pigs during an initial separation, separation the next day, and separation 10 days after the first. The passive, depressive-like behavior as well as fever sensitized with repeated separation. Three days of injection with 14 mg/kg of naproxen prior to the initial separation reduced depressive-like behavior during all three separations. A 28 mg/kg dose of naproxen, however, had minimal effect on behavior. Fever during the early separations was moderated by naproxen, but only at the higher dose. These results suggest a role of prostaglandins in the behavioral and febrile response to maternal separation, and particularly in the sensitization of depressive-like behavior following repeated separation.

Selective social buffering of behavioral and endocrine responses and Fos induction in the prelimbic cortex of infants exposed to a novel environment

In mammals, the presence of the mother can reduce or "buffer" stress responses of her young in threatening conditions. We compared the effect of the mother, a familiar littermate, and an unfamiliar adult male on three classes of response shown by guinea pig pups in a novel environment: short latency active behaviors, particularly vocalizing; slower developing passive behaviors that appear mediated by inflammatory mechanisms; and hypothalamic-pituitary-adrenal activity. We also examined Fos induction in the prelimbic cortex, a region hypothesized to mediate buffering effects. Only the mother significantly suppressed all classes of behavior. The greatest selectivity was observed for passive behavioral responses. Contrary to expectations, the adult male reduced plasma cortisol levels of pups as effectively as did the mother. The presence of the male also resulted in increased Fos induction in the prelimbic cortex and high levels of social interaction. Maternal buffering was not associated with prelimbic activity. These results confirm the ability of the mother to reduce active behavioral and HPA responses and suggest a specific maternal buffering effect on the later developing passive behavioral responses. The findings also demonstrate an unexpected ability of adult males to reduce HPA responses and raise the possibility that different social partners buffer HPA activity through different underlying processes.

Broken or maladaptive? Altered trajectories in neuroinflammation and behavior after early life adversity

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This paper reviews how early life adversity alters neuroimmune mechanisms.

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Neuroimmune sensitization from early life adversity impacts circuitry at discrete life stages.

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Neuroimmune and neurodevelopmental influences can impact behavior and vulnerability.

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Sexual dimorphism in immune and brain development yield distinct effects of early life adversity.

Exposure to adversity and stress early in development yields vulnerability to mental illnesses throughout the lifespan. Growing evidence suggests that this vulnerability has mechanistic origins involving aberrant development of both neurocircuitry and neuro-immune activity. Here we review the current understanding of when and how stress exposure initiates neuroinflammatory events that interact with brain development. We first review how early life adversity has been associated with various psychopathologies, and how neuroinflammation plays a role in these pathologies. We then summarize data and resultant hypotheses describing how early life adversity may particularly alter neuro-immune development with psychiatric consequences. Finally, we review how sex differences contribute to individualistic vulnerabilities across the lifespan. We submit the importance of understanding how stress during early development might cause outright neural or glial damage, as well as experience-dependent plasticity that may insufficiently prepare an individual for sex-specific or life-stage specific challenges.

Filial attachment and its disruption: insights from the guinea pig

Guinea pigs are precocial rodents that show evidence of a selective attachment to the mother who, in turn, exhibits little active maternal care. Effects of separation in guinea pigs are, therefore, more likely to reflect the disruption of attachment than the removal of, or alterations in, patterns of maternal care. Here, effects in guinea pigs of the presence or absence of the mother on psychobiological endpoints and of maternal separation on depressive-like behavior are reviewed. It is argued that results with guinea pigs often align more closely with those of nonhuman primates than those of laboratory rats and mice, and that the guinea pig offers a valuable translational model for studies of the consequences of attachment and its disruption.

Evidence for a neuroinflammatory mechanism in delayed effects of early life adversity in rats: relationship to cortical NMDA receptor expression

Postnatal maternal separation in rats causes a reduction of GABAergic parvalbumin-containing interneurons in the prefrontal cortex that first occurs in adolescence. This parvalbumin loss can be prevented by pre-adolescent treatment with a non-steroidal anti-inflammatory drug that also protects against excitotoxicity. Therefore, the neuropsychiatric disorders associated with early life adversity and interneuron dysfunction may involve neuroinflammatory processes and/or aberrant glutamatergic activity. Here, we aimed to determine whether delayed parvalbumin loss after maternal separation was due to inflammatory activity, and whether central administration of the anti-inflammatory cytokine interleukin (IL)-10 could protect against such loss. We also investigated the effects of maternal separation and IL-10 treatment on cortical NMDA receptor expression. Male rat pups were isolated for 4h/day between postnatal days 2-20. IL-10 was administered intracerebroventricularly through an indwelling cannula between P30 and 38. Adolescent prefrontal cortices were analyzed using Western blotting and immunohistochemistry for parvalbumin and NMDA NR2A subunit expression. We demonstrate that central IL-10 administration during pre-adolescence protects maternally separated animals from parvalbumin loss in adolescence. Linear regression analyses revealed that increased circulating levels of the pro-inflammatory cytokines IL-1β and IL-6 predicted lowered parvalbumin levels in maternally separated adolescents. Maternal separation also increases cortical expression of the NR2A NMDA receptor subunit in adolescence, which is prevented by IL-10 treatment. These data suggest that inflammatory damage to parvalbumin interneurons may occur via aberrant glutamatergic activity in the prefrontal cortex. Our findings provide a novel interactive mechanism between inflammation and neural dysfunction that helps explain deleterious effects of early life adversity on prefrontal cortex interneurons.

Early maternal separation leads to down-regulation of cytokine gene expression

Exposure to stressors may lead to subsequent alterations in the immune response. The precise mechanisms underlying such vulnerability are poorly understood, but may be hypothesized to include changes in cytokine systems. Maternal separation was used as a model of exposure to early life stressors. Subsequent cytokine gene expression was studied using a cytokine gene expression array. Maternal separation resulted in significant down-regulation of the expression of 6 cytokine genes; chemokine ligand 7, chemokine receptor 4, interleukin 10, interleukin-1beta, interleukin 5 receptor alpha and integrin alpha M. Specific cytokines may be involved in mediating the effects of early adversity on subsequent immunosuppression. Further work is needed to delineate fully the relationship between early adversity, immune alterations, and behavioural changes.

Maternal inhibition of infant behavioral response following isolation in novel surroundings and inflammatory challenge

During isolation in a novel environment, guinea pig pups gradually begin to display passive behavior that appears to be mediated by proinflammatory activity, that is, "sickness behavior.". Administration of substances that increase proinflammatory activity [corticotropin-releasing factor (CRF), lipopolysaccharide (LPS)] prior to isolation induces passive behavior from the beginning of the isolation episode. Here, we show that reunion with the mother in the novel environment rapidly and potently suppresses the passive behavior of isolated pups (Experiment 1); inhibits the passive behavior of pups administered CRF (10 µg, subcutaneous; Experiment 2); and inhibits the passive behavior of male, though not female, pups administered LPS (250 µg/kg, intraperitoneal; Experiment 3). Together these findings suggest that the presence of the mother either recruits other processes that moderate the impact of proinflammatory processes on brain mechanisms mediating the passive response or initiates compensatory mechanisms that counter the effect of proinflammatory activity. Further, the results suggest that for physically ill animals of social species, the adaptive advantage that accrues from maintaining normal social interactions may sometimes outweigh the advantage gained by engaging in sickness behavior.

Sensitization of depressive-like behavior during repeated maternal separation is associated with more-rapid increase in core body temperature and reduced plasma cortisol levels

Infant guinea pigs exhibit a 2-stage response to maternal separation: an initial active stage, characterized by vocalizing, and a second passive stage marked by depressive-like behavior (hunched posture, prolonged eye-closure, extensive piloerection) that appears to be mediated by proinflammatory activity. Recently we found that pups showed an enhanced (i.e., sensitized) depressive-like behavioral response during repeated separation. Further, core body temperature was higher during the beginning of a second separation compared to the first, suggesting a more-rapid stress-induced febrile response to separation the second day, though the possibility that temperature was already elevated prior to the second separation could not be ruled out. Therefore, the present study examined temperature prior to, and during, 2 daily separations. We also examined the temperature response to a third separation conducted 3 days after the second, and assessed the effect of repeated separation on plasma cortisol levels. Core temperature did not differ just prior to the separations, but showed a more-rapid increase and then decline during both a second and third separation than during a first. Temperature responses were not associated with changes in motor activity. Depressive-like behavior was greater during the second and third separations. Pups separated a first time showed a larger plasma cortisol response at the conclusion of separation than did animals of the same age separated a third time. In all, the results indicate that the sensitization of depressive-like behavior during repeated separations over several days is accompanied by a more-rapid febrile response that may be related to a reduction of glucocorticoid suppression.

Proinflammatory activity and the sensitization of depressive-like behavior during maternal separation

When guinea pig pups are isolated for a few hours in a novel environment, they exhibit a distinctive passive behavioral response that appears to be mediated by proinflammatory activity. Recently, we observed that pups separated on two consecutive days show an enhanced (sensitized) passive response on the second day. In Experiment 1, pups receiving intracerebroventricular infusion of 50 ng of the anti-inflammatory cytokine interleukin-10 prior to a first separation failed to show a sensitized behavioral response to separation the next day. In Experiment 2, pups separated on Days 1 and 2, or just 2, showed an increase in passive responding during separation on Day 5. Pups injected with the bacterial antigen lipopolysacchride (LPS; 75 μg/kg body weight, intraperitoneal) prior to separation on Day 1 showed an increase in passive behavior several days later not shown by pups injected with saline prior to Day 1 separation. However, injection of LPS without separation on the first day did not enhance responding during an initial separation on the second day. These results suggest that immune activation is necessary, but not sufficient, to account for the sensitization of passive behavior of isolated guinea pig pups the following day, that boosting proinflammatory activity during an initial separation may promote sensitization several days later, and that the sensitized response persists for at least several days.

Persistent sensitization of depressive-like behavior and thermogenic response during maternal separation in pre- and post-weaning guinea pigs

Early attachment disruption is thought to promote later onset of depressive illness through a process involving sensitization. Maternal separation in guinea pig pups produces depressive-like behavior and core body temperature fluctuations that appear to be mediated by proinflammatory activity. In pups near the age of weaning (~20 days of age), these responses are increased during repeated separations occurring over several days. Here, enhanced depressive-like behavior and core body temperature responses were observed during repeated separations in guinea pigs from ~10 to 30 days of age. The sensitization lasted for more than a week, with the greatest temperature response occurring during the final separation. These results demonstrate persisting sensitization of behavioral and thermogenic responses to maternal separation over the age range in which these responses are known to occur. The findings are consistent with the hypothesis that proinflammatory activity contributes to the sensitization response and provide further suggestion that the impact of early attachment disruption on susceptibility to depression may involve proinflammatory processes.

Behavioral effects of peripheral corticotropin-releasing factor during maternal separation may be mediated by proinflammatory activity

When guinea pig pups are separated from their mothers in a novel environment, an initial period of active behavior (vocalizing, locomotor activity) wanes after an hour or so and is replaced by a second, passive stage characterized by a crouched stance, closed eyes, and extensive piloerection. If pups are given a peripheral injection of 7-14μg of corticotropin-releasing factor (CRF) prior to testing, the passive behaviors occur immediately upon separation. We found that intracerebroventricular infusion of 1-10μg of CRF did not increase passive behavior relative to vehicle infusion, but that peripheral injection of the anti-inflammatory cytokine, interleukin-10, attenuated the passive behavior induced by peripheral CRF injection. These results together with previous findings suggest that peripheral CRF administration affects behavior of separated guinea pig pups through a mechanism that involves peripheral proinflammatory activity. The possible role of endogenous peripheral CRF in the behavioral response of untreated pups during maternal separation is considered.

The role of neuroinflammation in the release of aversive odor cues from footshock-stressed rats: Implications for the neural mechanism of alarm pheromone

Stressed animals have been known to release aversive chemosignals toward which conspecifics show avoidance-like responses. The present studies assessed whether inflammatory cytokine responses provoked by footshock stress modulate odor signals released from male rats. Male rats were exposed to 30min of intermittent footshock (60 shocks, 1.0mA, 100ms each, variable ITI of 30s) or remained in their home cages as non-stressed controls. Real time RT-PCR analysis of brain tissues indicated that footshock increased the pro-inflammatory cytokine, IL-1β and hnCRH as well as c-fos mRNA expressions in the paraventricular nucleus, and the bed nucleus of the stria terminalis, and increased plasma corticosterone levels. Soiled bedding collected from rats exposed to 30-min, but not 5-min, of footshock elicited a differential response, as expressed by decreased sniffing and increased avoidance in male test subjects. Soiled bedding from rats given corticosterone injection (s.c. 1.25 or 3.75mg/ml) 3h before bedding collection evoked no avoidance response in odor-recipients. Furthermore, ICV infusion of the anti-inflammatory cytokine IL-10 (20 or 200ng) into the stimulus animals 30-min before a 30-min footshock session, had no effect on plasma corticosterone levels in the stimulus animals, but attenuated the release of aversive odor as indicated by dose-dependently diminished avoidance in odor-recipient rats. These results demonstrated that stressed rats release odorant cues that cause other rats to move away from the source of the signal. Such stress-induced chemosignals may be mediated by inflammatory cytokine responses in the brain.

Maternal separation produces, and a second separation enhances, core temperature and passive behavioral responses in guinea pig pups

During separation in a novel cage, guinea pig pups exhibit passive behavior that appears due to increased proinflammatory activity. To determine if separation also produces a febrile response, the present study used telemetry to provide continuous core temperature measurement of pups exposed to a novel cage for 3h while either alone or with their mother on two consecutive days. Separation from the mother increased core temperature, with the clearest effects occurring early during separation the second day. The increased temperature was not associated with an increase in locomotor activity. Further, passive behavior during isolation exhibited pronounced sensitization from the first to second day of separation. These results show that separation produces an increase in core temperature in our testing situation, and suggest that this increase represents true fever. The findings also provide further support for the hypothesis that maternal separation induces aspects of an acute phase response in guinea pig pups. The potential role of proinflammatory activity in promoting change across days in temperature and behavior is discussed.

Sickness-related odor communication signals as determinants of social behavior in rat: a role for inflammatory processes

Infected animals are avoided by conspecifics, suggesting that the inflammatory cascade may play a significant role in odor communication. Injection of male rats with the bacterial mimetic, lipopolysaccharide (LPS, 100 microg/kg, i.p.), decreased investigation through a wire-mesh partition between healthy male partners. This avoidance response was observed in adult males in response to soiled bedding collected from sick rats, regardless of whether LPS was injected peripherally (100 microg/kg, i.p.) or centrally (0.25 or 2.5 microg, icv). The release of sickness-related odor cues was dose-dependently blocked by icv infusion of the anti-inflammatory cytokine, interleukin-10 (IL-10; 20 or 200 ng), and reproduced by icv infusion of pro-inflammatory cytokine, IL-1beta (5 or 50 ng). Subcutaneous pretreatment with either estradiol benzoate (20 microg/kg) or testosterone propionate (50 or 500 microg/kg) to adult males that were administered LPS inhibited release of aversive odor cues, but these hormones alone did not influence odor properties. Importantly, the avoidance response to sickness-related odor was not associated with changes in plasma corticosterone, testosterone, or IL-6 levels of odor donors. However, plasma IL-1beta concentrations of sick animals was in fact predictive of aversive responses in conspecifics, suggesting that the inflammatory cascade, but not plasma steroid hormones, is likely to mediate aversive properties in odor that functions to signal illness state to conspecifics.

Alpha-melanocyte-stimulating hormone attenuates behavioral effects of corticotropin-releasing factor in isolated guinea pig pups

During a 3-hr period of social isolation in a novel environment, guinea pig pups exhibit an initial active phase of behavioral responsiveness, characterized primarily by vocalizing, which is then followed by a stage of passive responsiveness in which pups display a distinctive crouch, eye-closing, and extensive piloerection. Prior treatment of pups with alpha-melanocyte-stimulating hormone (alpha-MSH) reduces each of the passive behaviors. The onset of passive responding during separation can be accelerated with peripheral injection of corticotropin-releasing factor (CRF). To examine whether CRF produces its effects through a mechanism similar to that of prolonged separation, we examined the effect of administering alpha-MSH to pups injected with CRF. As expected, CRF markedly enhanced passive responding during a 60-min period of separation. alpha-MSH delivered by either intracerebroventricular infusion or intraperitoneal injection significantly reduced each of the passive behavioral responses without significantly affecting active behavior. These findings, together with previous results indicating that it is the anti-inflammatory property of alpha-MSH that is responsible for its behavioral effects during prolonged separation, suggest that peripheral CRF speeds the induction of passive responding through a mechanism involving enhanced proinflammatory activity.

Separation, Sickness, and Depression: A New Perspective on an Old Animal Model

Studies of prolonged separation from the attachment figure that were conducted with infant monkeys during the middle of the 20th century identified a passive behavioral response, termed "despair," that appeared to model human depressive illness. Studies in guinea pigs, which exhibit filial attachment that resembles attachment in monkeys, have described a similar passive response to briefer periods of maternal separation. Recent evidence indicates that elements of the immune system mediate the passive behavioral response of guinea pigs. These findings accord well with current ideas that immune responses contribute to depressive illness, suggest new hypotheses about how maternal separation might promote depression, and give us a rodent model in which such hypotheses might be tested.

Early attachment-figure separation and increased risk for later depression: potential mediation by proinflammatory processes

Early maternal separation and other disruptions of attachment relations are known to increase risk for the later onset of depressive illness in vulnerable individuals. It is suggested here that sensitization involving proinflammatory processes may contribute to this effect. This argument is based on: (1) current notions of the role of proinflammatory cytokines in depressive illness; (2) evidence that proinflammatory cytokines mediate depressive-like behavior during separation in a rodent model of infant attachment; and (3) comparisons of the effects of early proinflammatory activation versus maternal separation on later proinflammatory activity and biobehavioral processes related to depression. The possible interaction of proinflammatory processes and corticotropin-releasing factor in the sensitization process is discussed.