Enduring maternal influences in a precocial rodent

Pharmacological Prevention of Neonatal Opioid Withdrawal in a Pregnant Guinea Pig Model

Newborns exposed to prenatal opioids often experience intense postnatal withdrawal after cessation of the opioid, called neonatal opioid withdrawal syndrome (NOWS), with limited pre- and postnatal therapeutic options available. In a prior study in pregnant mice we demonstrated that the peripherally selective opioid antagonist, 6β-naltrexol (6BN), is a promising drug candidate for preventive prenatal treatment of NOWS, and a therapeutic mechanism was proposed based on preferential delivery of 6BN to fetal brain with relative exclusion from maternal brain. Here, we have developed methadone (MTD) treated pregnant guinea pigs as a physiologically more suitable model, enabling detection of robust spontaneous neonatal withdrawal. Prenatal MTD significantly aggravates two classic maternal separation stress behaviors in newborn guinea pigs: calling (vocalizing) and searching (locomotion) - natural attachment behaviors thought to be controlled by the endogenous opioid system. In addition, prenatal MTD significantly increases the levels of plasma cortisol in newborns, showing that cessation of MTD at birth engages the hypothalamic-pituitary-adrenal (HPA) axis. We find that co-administration of 6BN with MTD prevents these withdrawal symptoms in newborn pups with extreme potency (ID50 ∼0.02 mg/kg), at doses unlikely to induce maternal or fetal withdrawal or to interfere with opioid antinociception based on many prior studies in rodents and non-human primates. Furthermore, we demonstrate a similarly high potency of 6BN in preventing opioid withdrawal in adult guinea pigs (ID50 = 0.01 mg/kg). This high potency appears to run counter to our pharmacokinetic studies showing slow 6BN transit of both the placenta and maternal blood brain barrier in guinea pigs, and calls into question the preferential delivery mechanism. Rather, it suggests a novel receptor mechanism to account for the selectively high potency of 6BN to suppress opioid dependence at all developmental stages, even in adults, as compared to its well-established low potency as a classical opioid antagonist. In conclusion, 6BN is an attractive compound for development of a preventive therapy for NOWS.

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.

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.

Social Odors: Alarm Pheromones and Social Buffering

In this chapter, I describe 2 types of olfactory communication in rats, which appear to arouse anxiety and relief, respectively. In alarm pheromonal communication, rats release 4-methylpentanal and hexanal from their perianal region when they are stressed. These molecules activate the anxiety circuit, including the bed nucleus of the stria terminalis, when 4-methylpentanal and hexanal are simultaneously detected by the vomeronasal system and the main olfactory system, respectively. Consequently, recipient rats show a variety of anxiety responses, depending on the threatening stimuli. In appeasing olfactory communication, non-stressed rats release an appeasing olfactory signal, which is detected by the main olfactory system of other rats. When detected, this olfactory signal suppresses activation of the basolateral complex of the amygdala and, as a result, ameliorates stress responses elicited by an auditory conditioned stimulus during social buffering phenomenon. Because social buffering appears to be based on affinity and attachment to accompanying animals, the appeasing olfactory signal may arouse relief in rats. A definition of social buffering is also proposed as we still have no set definition for the term social buffering yet.

Isolation-induced vocalization in the infant rat depends on the nucleus accumbens

Mammalian infants vocalize when socially isolated. Vocalization guides the return of the caregiver and thereby maintains an environment critical to the infant's survival. Although the role of the periaqueductal gray area (PAG) in these vocalizations is established, other aspects of the relevant neural circuitry remain under-studied. Here we report that output from the nucleus accumbens (Acb) is necessary for isolation-induced vocalizations of infant rats aged postnatal days (PND) 11-13. Local inhibition via infusion of the GABA_A agonist muscimol (.8 μg/side) of the Acb, but not the dorsolateral striatum, blocked isolation-induced vocalizations, independent of whether the isolation was at room temperature, followed a brief reunion with the dam, or occurred in a cool (10 °C) environment. These findings highlight a possible anatomical area mediating the mammalian infant response to social separation and, more generally, to the development of social attachment.

Maternal stress and plural breeding with communal care affect development of the endocrine stress response in a wild rodent

Maternal stress can significantly affect offspring fitness. In laboratory rodents, chronically stressed mothers provide poor maternal care, resulting in pups with hyperactive stress responses. These hyperactive stress responses are characterized by high glucocorticoid levels in response to stressors plus poor negative feedback, which can ultimately lead to decreased fitness. In degus (Octodon degus) and other plural breeding rodents that exhibit communal care, however, maternal care from multiple females may buffer the negative impact on pups born to less parental mothers. We used wild, free-living degus to test this hypothesis. After parturition, we manipulated maternal stress by implanting cortisol pellets in 0%, 50-75%, or 100% of adult females within each social group. We then sampled pups for baseline and stress-induced cortisol, negative feedback efficacy, and adrenal sensitivity. From groups where all mothers were implanted with cortisol, pups had lower baseline cortisol levels and male pups additionally had weaker negative feedback compared to 0% or 50-75% implanted groups. Contrary to expectations, stress-induced cortisol did not differ between treatment groups. These data suggest that maternal stress impacts some aspects of the pup stress response, potentially through decreased maternal care, but that presence of unstressed mothers may mitigate some of these effects. Therefore, one benefit of plural breeding with communal care may be to buffer post-natal stress.

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.

Prenatal Stress Alters Hippocampal Neuroglia and Increases Anxiety in Childhood

Prenatal stress has been associated with detrimental outcomes of pregnancy, including altered brain development leading to behavioural pathologies. The neurosteroid allopregnanolone has been implicated in mediating some of these adverse outcomes following prenatal stress due to its potent inhibitory and anxiolytic effects on the brain. The aims of the current study were to characterise key markers for brain development as well as behavioural parameters, adrenocortical responses to handling and possible neurosteroid influences towards outcomes in guinea pig offspring in childhood. Pregnant guinea pig dams were exposed to strobe light for 2 h (9-11 a.m.) on gestational days 50, 55, 60, and 65 and were left to deliver spontaneously at term and care for their litter. Behavioural testing (open-field test, object exploration test) of the offspring was performed at postnatal day 18 (with salivary cortisol and DHEA measured), and brains were collected at post-mortem on day 21. Markers of brain development myelin basic protein (MBP) and glial fibrillary acidic protein (GFAP) were assessed via immunohistochemistry, and the neurosteroid allopregnanolone and its rate-limiting enzymes 5α-reductase types 1 and 2 (5αR1/2) were measured in neonatal brains by radioimmunoassay, reverse transcriptase polymerase chain reaction (RT-PCR), and Western blot, respectively. Brain-derived neurotrophic factor protein was measured as a marker of synaptic plasticity, and GABAA receptor subunit expression was also assessed using RT-PCR. Neonates born from mothers stressed during late pregnancy showed a reduction in both MBP (p < 0.01) and GFAP (p < 0.05) expression in the CA1 region of the hippocampus at 21 days of age. Pups of prenatally stressed pregnancies also showed higher levels of anxiety and neophobic behaviours at the equivalent of childhood (p < 0.05). There were no significant changes observed in allopregnanolone levels, 5αR1/2 expression, or GABAA receptor subunit expression in prenatally stressed neonates compared to controls. This study shows alterations in markers of myelination and reactive astrocytes in the hippocampus of offspring exposed to prenatal stress. These changes are also observed in offspring that show increased anxiety behaviours at the equivalent of childhood, which indicates ongoing structural and functional postnatal changes after prenatal stress exposure.

An ecologically relevant guinea pig model of fetal behavior

The laboratory guinea pig, Cavia porcellus, shares with humans many similarities during pregnancy and prenatal development, including precocial offspring and social dependence. These similarities suggest the guinea pig as a promising model of fetal behavioral development as well. Using innovative methods of behavioral acclimation, fetal offspring of female IAF hairless guinea pigs time mated to NIH multicolored Hartley males were observed longitudinally without restraint using noninvasive ultrasound at weekly intervals across the 10 week gestation. To ensure that the ultrasound procedure did not cause significant stress, salivary cortisol was collected both before and after each observation. Measures of fetal spontaneous movement and behavioral state were quantified from video recordings from week 3 through the last week before birth. Results from prenatal quantification of Interlimb Movement Synchrony and state organization reveal guinea pig fetal development to be strikingly similar to that previously reported for other rodents and preterm human infants. Salivary cortisol readings taken before and after sonography did not differ at any observation time point. These results suggest this model holds translational promise for studying the prenatal mechanisms of neurobehavioral development, including those that may result from adverse events. Because the guinea pig is a highly social mammal with a wide range of socially oriented vocalizations, this model may also have utility for studying the prenatal origins and trajectories of developmental disabilities with social-emotional components, such as autism.

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.

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.

Behavioural profiles are shaped by social experience: when, how and why

The comprehensive understanding of individual variation in behavioural profiles is a current and timely topic not only in behavioural ecology, but also in biopsychological and biomedical research. This study focuses on the shaping of behavioural profiles by the social environment in mammals. We review evidence that the shaping of behavioural profiles occurs from the prenatal phase through adolescence and beyond. We focus specifically on adolescence, a sensitive phase during which environmental stimuli have distinctive effects on the modulation of behavioural profiles. We discuss causation, in particular, how behavioural profiles are shaped by social stimuli through behavioural and neuroendocrine processes. We postulate a central role for maternal hormones during the prenatal phase, for maternal behaviour during lactation and for the interaction of testosterone and stress hormones during adolescence. We refer to evolutionary history and attempt to place developmental shaping into broader evolutionary historical trends. Finally, we address survival value. We argue that the shaping of behavioural profiles by environmental stimuli from the prenatal phase through adolescence represents an effective mechanism for repeated and rapid adaptation during the lifetime. Notably, the adolescent phase may provide a last chance for correction if the future environment deviates from that predicted in earlier phases.

Effects of prenatal stress on hypothalamic-pituitary-adrenal (HPA) axis function over two generations of guinea pigs (Cavia aperea f. porcellus)

Prenatal stress can alter hypothalamic-pituitary-adrenal axis function with potential consequences for later life. The aim of our study was to examine in guinea pigs (Cavia aperea f. porcellus) the effects of stress experienced during F0 pregnancy on glucocorticoid levels in plasma and feces, as well as challenge performance, in F1 offspring (n=44) and fecal glucocorticoid levels in F2 offspring (n=67). F1 animals were either born to F0 dams that had been stressed with strobe light during early to mid pregnancy, resulting in a short term increase but long-term down-regulation of maternal glucocorticoid levels, or to undisturbed F0 dams. The same stressor was used as a challenge for F1 offspring at age 26 days and around 100 days. Basal plasma cortisol concentrations during early F1 development, as well as overall glucocorticoid levels at challenge tests, were lower in F1 animals that were prenatally stressed than in control animals. Fecal cortisol metabolites were initially at lower levels in prenatally stressed F1 animals, relative to control animals, but shifted to higher levels around day 68, with an additional sex difference. Effects were also seen in the F2 generation, as male but not female offspring of prenatally stressed F1 animals had significantly higher levels of cortisol metabolites in feces after weaning. We conclude that stress exposure of F0 dams resulted in lower basal glucocorticoid levels in F1 offspring during the pre-pubertal phase and during stress exposure, but higher glucocorticoid levels in post-adolescent F1 animals. Also in males of F2 generation effects of stress exposure of F0 dams were detected.

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.

An investigation of the effects of maternal separation and novelty on central mechanisms mediating pituitary-adrenal activity in infant guinea pigs (Cavia porcellus)

In mammalian species in which the young exhibit a strong filial attachment (e.g., monkeys, guinea pigs), numerous studies have shown that even brief separation from the attachment figure potently elevates circulating concentrations of glucocorticoids and adrenocorticotropic hormone (ACTH). However, effects of separation on central regulation of this stress response are not known. Therefore, we investigated central mechanisms mediating pituitary-adrenal activation during maternal separation and novelty exposure in guinea pig (Cavia porcellus) pups. Corticotropin-releasing factor (CRF) mRNA expression in the hypothalamic paraventricular nucleus (PVN), and plasma cortisol and ACTH levels, were elevated only during separation in a novel environment. C-Fos activity was elevated in the medial amygdala (MeA) and reduced in the bed nucleus of the stria terminalis (BNST) during novelty exposure, regardless of separation. On the other hand, c-Fos activity was elevated in the PVN during separation, regardless of novelty exposure. These results demonstrate independent and combined effects of separation and novelty in regions of the guinea pig CNS that regulate pituitary-adrenal activity. Moreover, they suggest that a pathway from MeA to BNST to PVN mediates responses to novelty in the guinea pig pup, as in the adult rat, though inputs from other cell populations appear required to fully account for the HPA activity observed here.

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.

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.

Effects of neonatal novelty exposure on sexual behavior, fear, and stress-response in adult rats

Environmental stimuli in early life may result in permanent behavioral and physiological changes. Present study evaluated the effects of exposing pups to a novel environment on behaviors (open-field test and sexual behavior) and prolactin stress-responses in adult male rats. Half of a litter was daily removed outside (OUT) from the nest and stimulated by handling for 3 min, while the other half remained inside (IN) the nest and was also handled for the same period during the first 10 days postpartum. Maternal behavior after all the pups were returned to the nest was not different between IN and OUT littermates. In adulthood, OUT males showed increased general and central locomotion activity in the open-field test, reduced sexual behavior, and attenuated prolactin secretion in response to restraint stress compared with the IN littermates. The repeated exposition of rat pups to a novel environment is a causal factor for the long-lasting behavioral and endocrine changes. The premature exposition of the pup to unfamiliar environments decreases fear and stress-response, and also reduces sexual behavior. We suggest that the absence of the odor of the mother may be crucial to explain the effects detected in adulthood.

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."

Isolation induced changes in Guinea Pig Cavia porcellus pup distress whistles

Guinea Pig Cavia porcellus pups emit high-pitched distress whistles when separated from their mother. In order to assess the influence of the duration of a brief isolation period on whistle acoustic structure, we recorded the distress whistles of six 8-day old pups separated for 15 min from their group in a novel environment and compared the mean values of the first and last 30 whistle notes. Acoustic analysis revealed, throughout the session, a significant decrease in whistle duration, an increase in mean frequency and a tendency for a decrease in number of harmonics in the first part of the note. Results demonstrate that, throughout a brief isolation period, the vocal response of Guinea Pig pups to isolation undergoes structural changes possibly related to time-dependent changes in motivational state.