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.     

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 3 h 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.     

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.     

Brief daily postpartum separations from the litter alter dam response to psychostimulants and to stress

Neonatal handling induces several behavioral and neurochemical alterations in pups, including decreased responses to stress and reduced fear in new environments. However, there are few reports in the literature concerning the behavioral effects of this neonatal intervention on the dams during the postpartum period. Therefore, the aim of the current study was to determine if brief postpartum separation from pups has a persistent impact on the dam''s stress response and behavior. Litters were divided into two neonatal groups: 1) non-handled and 2) handled [10 min/day, from postnatal day (PND) 1 to 10]. Weaning occurred at PND 21 when behavioral tasks started to be applied to the dams, including sweet food ingestion (PND 21), forced swimming test (PND 28), and locomotor response to a psychostimulant (PND 28). On postpartum day 40, plasma was collected at baseline for leptin assays and after 1 h of restraint for corticosterone assay. Regarding sweet food consumption, behavior during the forced swimming test or plasma leptin levels did not differ between dams briefly separated and non-separated from their pups during the postpartum period. On the other hand, both increased locomotion in response to diethylpropion and increased corticosterone secretion in response to acute stress were detected in dams briefly separated from their pups during the first 10 postnatal days. Taken together, these findings suggest that brief, repeated separations from the pups during the neonatal period persistently impact the behavior and induce signs of dopaminergic sensitization in the dam.     

Factors influencing cortisol and behavioral responses to maternal separation in guinea pigs

Infant guinea pigs recently were found to respond to brief maternal separation with an increase in plasma cortisol levels. The present experiments were conducted to further characterize this response and compare it with the cortisol separation response previously observed in primates. In Experiment 1, separation of guinea pig pups from their mothers did not elevate the plasma cortisol levels of the pups at either 30 or 180 min when they remained alone in their home cages during the separation. Experiment 2 showed that cortisol levels of pups placed alone in a novel cage were greater at 30, 90, and 180 min than were those of pups placed in the cages together with their mothers. In contrast, the separated pups vocalized more than did pups tested with their mothers during the initial 30 min only. In Experiment 3, pups raised on inanimate surrogates responded less intensely to rearing-figure separation in terms of both cortisol and vocalizations than did mother-reared controls. Taken together, these results indicate both differences (response to home cage separation) and similarities (dissociation of cortisol and vocalization responses, effect of surrogate separation) in the separation responses of guinea pig and primate infants. The guinea pig model may provide a useful adjunct to primate studies for examining particular issues concerning physiological effects of brief separation from an attachment object.     

Hormonal and behavioral attachment responses in infant guinea pigs

Infant guinea pigs, 11/12 and 18/19 days of age, exhibited high rates of vocalizing and increases in plasma concentrations of cortisol when separated from their mothers for 30 min in a novel environment (unfamiliar cage in an unfamiliar room). Levels of vocalizations and plasma cortisol were significantly reduced when the pup was exposed to the novel environment with its own conscious or anesthetized mother, but not when it was exposed with an unfamiliar anesthetized lactating female. Pups spent equivalent amounts of time in physical contact with their own conscious mother, their own anesthetized mother and an unfamiliar anesthetized lactating female. Unexpectedly, when the unfamiliar cage was placed in an environmental chamber during separation, pups showed negligible increases in vocalizations and plasma cortisol levels, even when administered footshock just prior to exposure. These findings demonstrate that guinea pig pups exhibit adrenocorticoid as well as behavioral responses to maternal separation, provide further evidence for the existence of filial attachment in this species, and document an unexpected role of the test environment in determining responses during maternal separation. The guinea pig may afford a rodent model for examining the effects of attachment-object separation on the activity of the pituitary-adrenal system.     

Ontogeny of feeding controls in suckling and weanling rats.

To rationalize conflicting data in the literature, the latency of attachment of fed and fasted rat pups to the nipples of their anesthetized mother has been studied at various postnatal ages. At 10 and 14 days of age, the mode of fasting had a critical effect on the results: pups fasted in a 30 degrees C incubator had shorter attachment latencies than fed pups, whereas when pups were fasted in a 34 degrees C incubator or with a nonlactating foster mother at room temperature, their latencies were no different from those of fed pups. Thus maternal deprivation has an important influence on attachment latencies, unless other steps are taken to maintain the body temperature of fasted pups. When this variable is controlled, it is apparent that at 10 and 14 days of age, the feeding behavior of rat pups is not related to nutritional status. In contrast, from 17 days onward, latencies of fed pups were significantly longer than those of faster pups, regardless of the mode of fasting. This appearance of appetitive behavior during the 3rd postnatal wk was shown to have a temporal correlation with the beginning of weaning, as measured by the appearance of chow in the stomachs.     

Development of ultrasonic vocalization responses in genetically heterogeneous National Institute of Health (N:NIH) rats. I. Influence of age,testing experience,and associated factors

The N:NIH strain of rats was developed by the National Institutes of Health to provide a maximally heterogeneous population as a base for selective breeding (Hansen & Spuhler, 1984). Using the N:NIH strain, this laboratory will selectively breed adult animals that exhibited extremes of high or low ultrasonic vocalization (USV) rates as infants. Because nothing was known about USV in N:NIH rats, we characterized the development of isolation-induced USV in the first generation of this strain born in our laboratory. In a longitudinal/cross-sectional study of pups tested at 3, 10, 15, and 18 days postnatally, N:NIH pups emitted their highest rates of USV at 3–4 days postnatally and calling remained high for 10 days before declining. USV rates were found to be a relatively environmentally stable behavioral trait in that repeated testing did not significantly affect the calling rates of either individuals or litters, and only at 3 days postnatal age did naturally occurring ambient temperature variations (6°C range) significantly affect USV responses. Individual differences in USV responses emerged by 10 days of age that were not simply correlations of body weight or rectal temperature, and pups at that age showed isolation calling rates that were highly predictive of their response levels 5 days later. © 1996 John Wiley & Sons, Inc.     

Early and later adoptions differently modify mother-pup interactions

Life events occurring during the perinatal period have strong long-term effects. In rats, prenatal stress, postnatal maternal separations, or adoptions at different periods are known to affect behavior and reactivity to stress in offspring. To determine the role of maternal factors on differential outcome adoptions, the authors investigated interactions between pups and the adopting mothers by assessing both pups' ultrasound emissions and maternal behavior. Early and late adoptions increased mother care at the moment of adoption and during mother-infant reunion after a separation procedure. However, although early adoption induced a decrease in pups' ultrasound emissions in response to a stressful separation, later adoptions enhanced it. Results suggest a sensitive period during which fostering may change pups' and dams' behavior.     

Behavioral and autonomic responses to peer separation in pigtail macaque monkey infants

Brief maternal separations of young nonhuman primates have been used extensively to study the behavior and physiology of attachment, loss, and bereavement. The physiological responses to the loss of alternative attachment figures, such as peers, is less well documented in nonhuman primates. This study examined both autonomic and behavioral responses of peer-reared pigtail macaque infants to separation. Eight infants were removed from their mothers at birth and reared in four peer pairs. At 6 months of age, each monkey was implanted with a multichannel biotelemetry device which transmitted heartrate, body temperature, EEG, EMG, and EOG. Blood was collected twice weekly for immunological assessment. Behavioral and physiological data, including sleep, were collected for 1 week of baseline, 2 weeks of separation, and 1 week of reunion. Behavioral and physiological results indicated agitation but not depression following separation from their peer attachment figures. We found reduced mitogenic responses to pokeweed consequent to peer separation, suggestive of altered B-cell function. REM variables were the only sleep measures affected by the separation, and were suggestive of agitation but not depression.     

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.     

Developmental changes in the suckling behavior of hamster pups: a comparison with rat pups

The suckling behavior of hamster pups was observed to undergo several transitions during the preweaning period. Pups 10 days of age and younger attached to the nipples of their anesthetized mothers and suckled. After 10 days of age, however, only animals which had been deprived of suckling (and food and water) reliably attached to their mothers' nipples. By 25 days of age, deprived pups no longer suckled. The pattern of response after nipple attachment also changed during development: pups younger than 5 days of age remained on 1 nipple after initial attachment, whereas older pups shifted from 1 nipple to another. The development of suckling in hamster pups resembles that in rat pups, although certain changes or transitions occur at different ages in the 2 species. These changes may provide important markers for neural and behavioral development in each species and for comparisons between the two.     

The effects of brief maternal separations on behavior and heart rate of two week old rat pups

Behavioral and cardiac rate responses to a novel test box were recorded from rat pups separated from their mothers for 18 hr and from littermates allowed to remain with their mothers. If body temperatures of separated pups were maintained at nest levels, they showed increased numbers of rises, squares crossed and head raises but did not differ from their mothered littermates in self-grooming or elimination. If no temperature source was supplied, separated pups lost 3°C and showed generally reduced levels of behavior. Heart rates of separated pups decreased markedly during separation, regardless of whether body temperatures were maintained. The cardiac rate response of mothered pups to the test box was a transient slowing in the first two min. Both separated groups showed acceleratory responses to the test box. Those whose temperatures had been maintained had more prolonged cardiac acceleration which reached levels close to the range of their mothered littermates.     

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 (α‐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 α‐MSH to pups injected with CRF. As expected, CRF markedly enhanced passive responding during a 60‐min period of separation. α‐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 α‐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. © 2009 Wiley Periodicals, Inc. Dev Psychobiol 51: 399–407, 2009.     

Adaptation of body temperature and nociception to cold stress in preweanling rats

In order to investigate the response of rat pups to stress, 10-day-old rats (N = 10) were exposed to cold water (14 degrees C) for 5 min or to a control treatment (N = 10) for 5 consecutive days. On the sixth day when the rats were 15 days of age, all rat pups were immersed in cold water for 5 min. Body temperature and nociceptive threshold were recorded as functional indexes of the stress response to cold water. Results revealed that the rat pups that had been previously exposed to the cold stressor demonstrated a smaller decline in body temperature and a smaller increase in nociceptive threshold following cold-water immersion than rat pups which had no prior experience with the stressor. Thus, as measured by thermoregulatory and nociceptive function, 15-day-old rat pups are capable of adapting to repeated stressful experiences.     

Day-by-day maturation of the long-term expression of cocaine sensitization acquired before weaning in the rat

This study aimed to identify the ontogenetic period during which long-term expression of behavioral sensitization to cocaine begins to emerge. Rat pups aged 4, 8, 12, or 16 days received a pretreatment of 4 daily injections of 15 mg/kg sc cocaine paired with the test chamber for 45 min. Pups were then tested for sensitization in that context after abstinence intervals ranging from 2 to 10 days. On test days, pups were videotaped, and their behavior was scored later. Sensitization was detected after intervals of 2, 4, 5, or 9 days in pups aged 4-7, 8-11, 12-15, or 16-19 days during pretreatment, respectively. These results suggest that the mechanisms for long-term retention of sensitization mature incrementally in the rat, starting to emerge gradually after the 1st week of age, whereas those relevant to short-term retention and initiation of sensitization are present earlier.     

Influence of maternal proximity on behavioral and physiological responses to separation in infant rhesus monkeys (Macaca mulatta).

The effects of maternal proximity on the behavioral and physiological responses of infant rhesus macaques during 4 days of total or adjacent separations from the mother were studied. The 6 infants tested showed behavioral responses that differentiated the two separation conditions. Major differences were found in the quantity and quality of vocalizations, the occurrence of cage-biting and cage-shaking behavior, object exploration, and hunched and freezing postures. In particular, the structure of coo vocalizations clearly discriminated between the presence or the absence of the mother during separation. Cerebrospinal fluid (CSF) concentrations of dopamine and serotonin metabolites did not discriminate between the two separation conditions but showed a transient elevation at 24 hr after separation and were not different from baseline by 96 hr after separation. In contrast, both the plasma cortisol and the CSF norepinephrine metabolite responses tended to be greater and to persist for a longer period of time when infants were totally isolated. The results are discussed within the context of attachment and coping theories.     

Time-dependent effects of peer separation on lymphocyte proliferation responses in juvenile squirrel monkeys

This study evaluated the immunological, hormonal, and behavioral responses of juvenile squirrel monkeys to repeated social separations of varying length. Following a 3-hr separation, lymphocyte responses to stimulation with the mitogen concanavalin A (Con A) declined significantly, and these alterations were sustained after a 24-hr separation period. The responses to Con A and to a second mitogen, phytohemagglutinin (PHA), were suppressed after 2 days. At the end of a 7-day separation period, immune responses were not significantly different from basal values. Plasma cortisol levels were elevated above basal levels in all animals after the 3-hr, 24-hr, and 2-day separations, but were not elevated after the 7-day separation. While we observed no statistically significant changes in locomotor activity or calling behavior during any of the separations, the monkeys tended to be most active and to call most immediately following separation. Our findings concur with earlier reports indicating that social stressors can influence lymphocyte proliferation in nonhuman primates and that certain cell types might be differentially sensitive to stress, but also indicate that these influences are transient.     

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. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 395–403, 2013     

Cardiorespiratory regulation and activity patterns of rat pups studied with their mothers during the nursing cycle

Rat pups aged 10 to 19 days were implanted with chronic electrodes and recorded while freely moving in the litter situation, during nursing and throughout naturally occurring maternal absences. Heart rates were significantly higher in the mother's presence (p<.01) while respiratory rates and percent of time spent active were identical under the 2 conditions. Pups' inactive heart rates progressively increased during the 20 min of nursing, while no such time trends were evident in other measures. Inactive heart rates decreased significantly (p<.01) as a function of age; respiratory rates and activity during nursing showed similar trends. A histogram of number of periods of activity bursts of different durations while pups were nursing showed a preponderance of 1- to 5-sec bursts which were observed to be associated with active sucking and pawing movements. This pattern of short bursts of spontaneous activity was repeated almost identically when the mother was absent, although the behavior was qualitatively different. In a second experiment, increases in pups' body temperature of .2°C to 1.0°C were found to accompany active nursing. If such increases were imposed by an artificial heat source, heart rates of pups actually fell slightly. In a third study, heart rates were recorded from nursing mothers and found to be relatively unaffected by stimulation from the pups.