Effects of peripherally administered corticotropin-releasing factor (CRF) and a CRF antagonist: does peripheral CRE activity mediate behavior of guinea pig pups during isolation?

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.

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.

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.

Adult males buffer the cortisol response of young guinea pigs: Changes with age, mediation by behavior, and comparison with prefrontal activity

In the guinea pig, the presence of the mother buffers hypothalamic-pituitary-adrenal (HPA) responses of her young during exposure to a novel environment, and can do so even if she is anesthetized. In contrast, under comparable conditions other conspecifics (siblings, other adult females) are less effective or ineffective in doing so. However, we recently observed that an unfamiliar adult male reduced plasma cortisol elevations and increased Fos in the prefrontal cortex of preweaning pups exposed to a novel enclosure for 120min. Here we found adult males buffered the adrenocortical response of preweaning pups at 60 as well as 120min and of periadolescent guinea pigs if exposure was of 120min. Further, because males vigorously engaged in social interactions with the young during exposure, we examined the effect of behavior by comparing the impact of conscious and unconscious (anesthetized) adult males. When tested with a conscious but not unconscious male, pups exhibited reduced plasma cortisol elevations. Pups, particularly females, had greater Fos induction in the prefrontal cortex when with a conscious versus unconscious adult male. Overall, we found that an unfamiliar adult male can buffer the cortisol response of guinea pigs both before and after weaning, though more-prolonged exposure appears necessary in the older animals. Further, unlike buffering by the biological mother, the effect of the male is mediated by behavioral interactions. Thus, the buffering of the infant guinea pig's cortisol response by the mother and an unfamiliar adult male involve different underlying mechanisms.

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.

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.

Developmental emergence of fear/threat learning: neurobiology, associations and timing

Pavlovian fear or threat conditioning, where a neutral stimulus takes on aversive properties through pairing with an aversive stimulus, has been an important tool for exploring the neurobiology of learning. In the past decades, this neurobehavioral approach has been expanded to include the developing infant. Indeed, protracted postnatal brain development permits the exploration of how incorporating the amygdala, prefrontal cortex and hippocampus into this learning system impacts the acquisition and expression of aversive conditioning. Here, we review the developmental trajectory of these key brain areas involved in aversive conditioning and relate it to pups' transition to independence through weaning. Overall, the data suggests that adult-like features of threat learning emerge as the relevant brain areas become incorporated into this learning. Specifically, the developmental emergence of the amygdala permits cue learning and the emergence of the hippocampus permits context learning. We also describe unique features of learning in early life that block threat learning and enhance interaction with the mother or exploration of the environment. Finally, we describe the development of a sense of time within this learning and its involvement in creating associations. Together these data suggest that the development of threat learning is a useful tool for dissecting adult-like functioning of brain circuits, as well as providing unique insights into ecologically relevant developmental changes.

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.

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.

Sociality and sickness: have cytokines evolved to serve social functions beyond times of pathogen exposure?

During pathogen exposure or some forms of stress, proinflammatory processes induce an array of motivated and behavioral adjustments termed "sickness behaviors". Although withdrawal from social interactions is a commonly observed sickness behavior, the relation between social behavior and sickness is much more complex. Sickness can suppress or stimulate social behavior. Sickness can serve as a social cue. Stressors that are social in nature can induce sickness behaviors, and sickness behavior can be readily suppressed by meaningful social stimuli. The nature, context, and timing of these effects together suggest that cytokine-induced behavior may play a role in mediating social interactions in various non-pathological conditions.

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.

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.

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.

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.

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

The present study examined the effect of anti-inflammatory agents on the behavioral and plasma cortisol responses of guinea pig pups during isolation in a novel environment. In Experiment 1, intracerebroventricular (i.c.v.) administration of interleukin-10 (IL-10), across a range of doses, significantly reduced passive behaviors of isolated pups. In Experiment 2, neither i.c.v. IL-10 nor alpha-melanocyte stimulating hormone affected the plasma cortisol response to the separation procedure. These results provide further evidence that the passive behaviors of isolated guinea pig pups are mediated by proinflammatory cytokines and indicate that cytokines do not affect behavior indirectly by means of an action on hypothalamic-pituitary-adrenal activity.

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.

Anti-inflammatory agents attenuate the passive responses of guinea pig pups: evidence for stress-induced sickness behavior during maternal separation

A previous study found that intracerebroventricular (ICV) infusion of 25 microg of alpha-MSH reduced the passive responses (crouched stance, eye-closing, piloerection) of guinea pig pups during a 3-h isolation in a novel environment. Because alpha-MSH has broad anti-inflammatory properties, the results suggested that proinflammatory factors play a role in mediating the behavior of isolated infants. The present study further investigated this possibility. In Experiment 1, injection of lipopolysacchride (LPS) increased the number of 60-s intervals in which pups expressed the same three responses during a 1-h test, and ICV infusion of alpha-MSH significantly reduced the effect of LPS on crouching and piloerection. In Experiment 2, the prostaglandin synthesis inhibitor indomethacin (10 mg/kg) reduced the number of 60-s intervals in which pups exhibited both crouching and the full suite of passive responses during a 3-h isolation in a novel environment. Together these results provide further support for the hypothesis that the passive behaviors exhibited during prolonged isolation are "stress-induced sickness behaviors" mediated by proinflammatory factors.

Alpha-melanocyte stimulating hormone reduces putative stress-induced sickness behaviors in isolated guinea pig pups

We have proposed that passive responses observed following maternal separation in guinea pig pups represent "stress-induced sickness behaviors" mediated by proinflammatory processes. In this study, the anti-inflammatory peptide, alpha-melanocyte stimulating hormone (alpha-MSH) administered intracerebroventricularly, but not intraperitoneally, reduced the passive responses of crouching, eye-closing, and extensive piloerection relative to levels following administration of vehicle. These findings support our hypothesis and are as would be expected if pro-inflammatory processes act centrally to promote the passive behaviors of separated guinea pig pups.

Passive responses of young guinea pigs during exposure to a novel environment: Influences of social partners and age

Guinea pig pups exhibit a two-stage, active/passive response during isolation in a novel environment that resembles the "protest" and "despair" stages observed in some species of primates. The present study examined social and developmental influences on the expression of the passive stage of responsiveness in young guinea pigs. Both preweaning and periadolescent guinea pigs displayed the passive stage during isolation in a novel environment. In preweaning pups and periadolescents that had been housed with the mother since birth, the presence of the mother in the novel environment prevented the passive stage from occurring. Levels of passive responses in the presence of an unfamiliar adult female were generally intermediate to those in the other two test conditions; however, periadolescents housed only with a peer between weaning and testing exhibited the passive stage of responsiveness not only when alone but also when with the mother or an unfamiliar female. The findings indicate that the passive responses can occur in periadolescent guinea pigs and that the test conditions under which they occur depend upon the periadolescents' social housing conditions prior to testing. Further, the results are consistent with the hypothesis that responses during the second stage constitute "stress-induced sickness behaviors."

Responses of guinea pig pups during isolation in a novel environment may represent stress-induced sickness behaviors

When guinea pig pups are isolated in a novel environment, they show an initial active phase of behavioral responsiveness characterized by vocalizations and locomotor activity. One earlier study found that after about an hour, pups began to exhibit a second, passive stage of responsiveness marked by a crouched stance, eye-closing, and extensive piloerection. The present experiments tested the hypothesis that the responses during the second, passive stage result from the isolation experience activating pathways underlying the acute phase response, i.e., that behaviors of the second stage represent "stress-induced sickness behaviors". We found the following: (1) the passive stage did not emerge if pups remained with the mother during exposure to a novel cage; (2) injection of lipopolysaccharide, which induces an acute phase response, also led pups to exhibit crouching, eye-closing, and piloerection; and, (3) isolation in the novel cage produced a rise in rectal temperature, but did not affect peripheral or central levels of interleukin-1beta (IL-1beta)-immunoreactivity. Overall, these results are consistent with the notion that stress-induced sickness behaviors can account for some of the behaviors of isolated guinea pig pups, though if this is the case, cytokines other than IL-1beta appear to be involved.

Centrally administered norepinephrine modifies the behavior induced by corticotropin-releasing factor in neonatal chicks

We previously reported that glucagon-like peptide-1 decreased corticotropin-releasing factor (CRF)-induced behaviors in neonatal chicks, and such an effect is hypothesized to act through norepinephrine (NE). Experiments were designed to explore the effect of the NE on CRF-induced behaviors. In experiment 1, the chicks were intracerebroventricularly (i.c.v.) administered saline, 0.1 microg of CRF, 50.0 microg of NE, or 0.1 microg of CRF with 50.0 microg of NE. Behavior was monitored for the 10 min immediately after i.c.v. injection, and plasma corticosterone was analyzed at the end of behavior tests. Compared with the control, chicks were excited by CRF as evidenced by increased spontaneous activity and distress vocalizations (DVs). NE decreased the spontaneous activity of chicks, and the differences diverged with time. DVs completely disappeared in the presence of NE, and sleep-like (sitting with eyes closed) behavior was observed in the same birds. NE-treated birds spent most of the time in a sleep-like posture irrespective of CRF treatment. CRF-treated chicks had increased plasma corticosterone, whereas NE injection caused a decrease in corticosterone. In experiment 2, the effect of NE was further studied using i.c.v. administration with either 0.1 microg of CRF alone or 0.1 microg of CRF plus 12.5, 25.0, or 50.0 microg of NE. NE dose dependently modified the CRF-induced locomotor activity in the open field, and DVs disappeared when chicks were given any dose of NE with CRF. With these findings taken together, it is suggested that central NE interacts the CRF-induced behaviors in neonatal chicks.

Stress-induced sickness behaviors: an alternative hypothesis for responses during maternal separation

During maternal separation, some primate and nonprimate species show a biphasic (active/passive) response. The second stage is characterized by reduced activity, a hunched body posture, and other behaviors. Traditionally, the second stage has been referred to as "despair" and is considered an animal model for human depression. Recent research in psychoneuroimmunology suggests an alternative hypothesis--that behaviors occurring during the second passive phase represent stress-induced "sickness behaviors." This perspective more readily accounts for findings in widely divergent species, does not require assumptions regarding the ability to express complex emotional states, is empirically testable, and aligns the separation model with recent hypotheses regarding the nature and ontogeny of depressive illness.

Effects of a novel potential antidepressant on the behavior and cortisol levels of isolated guinea pig pups

A novel, potential antidepressant, E-6006 citrate (E-6039), dose-dependently reduced the vocalizations emitted by isolated guinea pig pups. The (+)-E-6006, but not the (-)-E-6006, enantiomer also reduced vocalizing. There were no reliable effects of E-6039 on locomotor activity, crouching, or other behavioral measures, but both E-6039 and the (+)-E-6006 enantiomer elevated plasma cortisol levels during isolation. The contrasting effects of E-6039 on vocalizations and plasma cortisol are discussed in terms of E-6039's putative ability to inhibit release of substance P. The reduction in the vocalizations of isolated guinea pig pups corroborates positive results with this drug in other antidepressant screens utilizing mice and rats, and provides further support for the potential of E-6039 as an antidepressant compound.

Stress during pregnancy alters the offspring hypothalamic, pituitary, adrenal, and testicular response to isolation on the day of weaning

Subjecting pregnant female rats to situations that activate the hypothalamic-pituitary-adrenal (HPA) axis can have long-term effects on the development of the offspring. Restraint under bright lights is a common method of stressing pregnant females that results in consistent behavioral changes in the offspring. We investigated the effects of gestationally administered restraint, bright lights, and heat on the HPA axis response of 21-day-old offspring following exposure to isolation in a novel environment or under resting conditions. Corticotropin-releasing factor titers in the hypothalamus were unaffected following isolation. Nonetheless, adrenocorticotropin hormone (ACTH) was found to be lower in the gestationally stressed offspring prior to or following the isolation period. Corticosterone was attenuated in gestationally stressed offspring following the postnatal stressor and there was also a tendency for the gestationally stressed females to have lower concentrations of aldosterone. Plasmatic testosterone levels were higher in the gestationally stressed males following the period of isolation. The present data suggest that the HPA axis of the offspring is differentially affected by the gestational stress procedure, that is, it is attenuated at the level of the pituitary and adrenal, but not at the level of the hypothalamus. These data have implications for behavioral differences observed in gestationally stressed animals.

Is there a future for neuropeptide receptor ligands in the treatment of anxiety disorders?

This review provides an overview of preclinical and clinical evidence of a role for the neuroactive peptides cholecystokinin (CCK), corticotropin-releasing factor (CRF), neuropeptide Y (NPY), tachykinins (i.e., substance P, neurokinin [NK] A and B), and natriuretic peptides in anxiety and/or stress-related disorders. Results obtained with CCK receptor antagonists in animal studies have been highly variable, and clinical trials with several of these compounds in anxiety disorders have been unsuccessful so far. However, future investigations using CCK receptor antagonists with better pharmacokinetic characteristics and animal models other than those validated with the classical anxiolytics benzodiazepines may permit a more precise evaluation of the potential of these compounds as anti-anxiety agents. Results obtained with peptide CRF receptor antagonists in animal models of anxiety convincingly demonstrated that the blockade of central CRF receptors may yield anxiolytic-like activity. However, the discovery of nonpeptide and more lipophilic CRF receptor antagonists is essential for the development of these agents as anxiolytics. Similarly, there is clear preclinical evidence that the central infusion of NPY and NPY fragments selective for the Y1 receptor display anxiolytic-like effects in a variety of tests. However, synthetic nonpeptide NPY receptor agonists are still lacking, thereby hampering the development of NPY anxiolytics. Unlike selective NK1 receptor antagonists, which have variable effects in anxiety models, peripheral administration of selective NK2 receptor antagonists and central infusion of natriuretic peptides produce clear anxiolytic-like activity. Taken as a whole, these findings suggest that compounds targeting specific neuropeptide receptors may become an alternative to benzodiazepines for the treatment of anxiety disorders.

Stress during pregnancy alters rat offspring morphology and ultrasonic vocalizations

Stress during pregnancy, or prenatal stress, is known to alter offspring behavior, morphology and physiology. We found that a heat, light and restraint stressor applied during the third trimester of pregnancy: 1) decreased the weight gain of adult female rats during pregnancy; 2) reduced the weight of pups, as well as the anogenital distance of male offspring, at birth; and 3) increased the number of ultrasonic vocalizations emitted by pups during isolation in a novel environment on Postnatal Day 14. These results closely approximate those we previously observed after peripheral administration of corticotropin-releasing factor to pregnant females during the third trimester. Together, the studies strongly suggest a role for corticotropin-releasing factor and/or other hormones of the hypothalamic-pituitary-adrenal system in mediating some of the effects of gestational stress.

Evidence that endogenous corticotropin-releasing factor suppresses behavioral responses of guinea pig pups to brief isolation in novel surroundings

Guinea pig pups were injected subcutaneously with a corticotropin-releasing factor antagonist (CRF12-41) or saline vehicle and then placed into a novel cage for 30 or 60 min. Isolated 20- to 26-day-old pups vocalized more and exhibited more locomotor activity when given 15 to 150 micrograms of CRF12-41 than when given saline. The presence of the mother in the test cage prevented the antagonist from affecting behavior. The influence of the antagonist during isolation was not limited to guinea pigs near weaning age: CRF12-41 increased levels of vocalizing in isolated, 4- to 6-day-old pups, though no changes were seen in locomotor activity. Results support the hypothesis that endogenous corticotropin-releasing factor, perhaps acting at a peripheral binding site, suppresses the active behavioral response characteristic of pups during the early phase of isolation in novel surroundings.

Acute modulation of active carrier-mediated brain-to-blood transport of corticotropin-releasing hormone

The unidirectional brain-to-blood transport system for corticotropin-releasing hormone (CRH) across the blood-brain barrier could be instrumental in the homeostasis of central CRH. To characterize this system, the intracerebroventricular injection of 125I-CRH was used in mice. CRH was rapidly transported out of the brain with a half-time disappearance (t1/2) of 15 min, much faster than albumin (t1/2 = 50 min). Kinetic analysis revealed a saturable component with a low maximum velocity (apaproximately 0.020 nmol x min(-1) x brain(-1)) and low capacity (Michaelis constant approximately 1.4 nmol/brain). Transport was inhibited by verapamil, ouabain, and colchicine but not by cyclosporin. Transport was increased by corticosterone and inhibited by tumor necrosis factor-alpha and beta-endorphin. These results suggest that the specific unidirectional brain-to-blood transport system for CRH is dependent on energy and calcium channels, involves microtubules, is independent of the P-glycoprotein transporter, and is acutely modulated by adrenal steroids, cytokines, and endogenous opiates. This suggests its participation in the control of the stress response.

Hypothalamic-pituitary-adrenal responses to brief social separation

Involuntary separation from close social companions is widely held to lead to pathophysiological outcomes. Presumably, the relationship with, or category of, the separated individual determines the nature of the physiological response. Here, experiments examining the consequences of brief involuntary separation on the activity of the stress-responsive hypothalamic-pituitary-adrenal (HPA) system are reviewed. Only those studies designed specifically to assess the effect of the absence of the social partner are considered. Evidence for HPA activation in response to social separation has been obtained in a number of species; yet, many studies find no effect of separation of affiliative partners on HPA activity. The occurrence of an HPA response to separation does not appear to be related to the phylogenetic position or cognitive capacity of the species studied, nor is it a universal response to mother-infant separation. Rather, it is suggested that the pattern of results can be largely understood in the context of attachment. Separation of partners exhibiting signs of emotional attachment leads to an immediate and persistent HPA response, whereas separation of partners that are affiliative, but do not exhibit attachment, has little or no effect on HPA activity.

Peripheral administration of a corticotropin-releasing factor antagonist increases the vocalizing and locomotor activity of isolated guinea pig pups

Guinea pig pups vocalized more and were more active during a 60-min period of isolation in a novel environment when injected SC with 50 micrograms of the corticotropin-releasing factor (CRF) antagonist, [D-Phe]CRF12-41, than when injected with saline vehicle only. More pups exhibited eye closing and extensive piloerection when injected with saline than when injected with the antagonist. Plasma levels of cortisol following testing were not affected by the injections. These results support the hypothesis that endogenous CRF contributes to the normal decline in vocalizing and locomotor activity that guinea pig pups show with continued isolation. The results also suggest that endogenous CRF plays a role in the eye closing and piloerection of isolated pups. The finding that the dose of the antagonist used altered behavior, yet was not sufficient to lower plasma cortisol levels, supports earlier evidence that the observed effects of CRF are not due to the actions of ACTH or glucocorticoids.