The effects of long-term isolation on physiology and behavior in male guinea pigs

Does the early social environment prepare individuals for the future? A match-mismatch experiment in female wild cavies

Background

The social environment that mothers experience during pregnancy and lactation has a strong effect on the developing offspring. Whether offspring can be adaptively shaped to match an environment that is similar to the maternal one is still a major question in research. Our previous work in wild cavies showed that females whose mothers lived in a stable social environment with few social challenges during pregnancy and lactation (SE-daughters) developed different behavioral phenotypes than females whose mothers lived in an unstable social environment with frequent social challenges during pregnancy and lactation (UE-daughters). In the present study we investigated whether SE-daughters are better adapted to a stable social environment, similar to their maternal one, than are UE-daughters, for which the stable social environment represents a mismatch with their maternal one. For this purpose, we established pairs of one UE- and one SE-daughter and housed them together under stable social conditions for one week. Dominance ranks, behavioral profiles, glucocorticoid levels, cortisol responsiveness and body weight changes were compared between the groups. We hypothesized that SE-daughters fare better in a stable social setting compared to UE-daughters.

Results

After one week of cohabitation in the stable social condition, UE-daughters had higher glucocorticoid levels, tended to gain less body weight within the first three days and displayed higher frequencies of energy-demanding behaviors such as rearing and digging than SE-daughters. However, there was no difference in cortisol responsiveness as well as in dominance ranks between UE- and SE-daughters.

Conclusion

Higher glucocorticoid levels and less body weight gain imply that UE-daughters had higher energy demands than SE-daughters. This high energy demand of UE-daughters is further indicated by the increased display of rearing and digging behavior. Rearing implies increased vigilance, which is far too energy demanding in a stable social condition but may confer an advantage in an unstable social environment. Hence, SE-daughters seem to better match a stable social environment, similar to their maternal one, than do UE-daughters, who encountered a mismatch to their maternal environment. This data supports the environmental matching hypothesis, stating that individuals manage the best in environments that correspond to their maternal ones.

Critical period exists in the effects of isolation rearing on sensorimotor gating function but not locomotor activity in rat

Isolation-reared (IR) rats exhibit various cognitive and behavioral abnormalities in adulthood of which locomotor hyperactivity and impaired prepulse inhibition (PPI) of an acoustic startle reflex are the two cardinal characteristics. Using an amended social deprivation-resocialization paradigm, the present study examined the role of the developmental specificity of the effects of IR and its interaction with the non-competitive NMDA receptor antagonist, MK-801. Locomotor activity and PPI were measured in three groups of adult rats: social control, IR throughout life, and IR for the first two weeks after weaning followed by social housing. MK-801 was injected intraperitoneally (i.p.) 30 min prior to testing at doses of 0, 0.02, 0.1, and 0.2 mg/kg. The results revealed that locomotor activity increased only in rats in the IR throughout life group but not in the other two groups. The impairment of PPI was seen in rats in the IR for the first two weeks and the IR throughout life groups. Furthermore, the effect of MK-801 on PPI was bidirectional in rats of the IR for the first two weeks and IR throughout life groups but not in the social control group. These results suggest that the IR-induced behavioral effects are developed distinctively in terms of the critical period hypothesis, which strengthens the developmental hypothesis of schizophrenic-like dysfunctions.

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

Early preweaning methamphetamine and postweaning rearing conditions interfere with the development of peripheral stress parameters and neural growth factors in gerbils

Adrenal steroid hormones and neuronal growth factors are two interacting systemic factors that mediate the environment's influence on the brain's structure and function. In order to further elucidate their role and relationship in the effects of early stressful experience and isolated rearing (IR), this study measured blood corticosterone titres and relative adrenal weights and assessed nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) concentrations in brain regions of both hemispheres of young adult Mongolian gerbils injected on postnatal day 14 with a single high dose of methamphetamine (MA) or saline and raised after weaning either in an enriched or an impoverished environment. Irrespective of MA challenge, IR decreased corticosterone titres to about half, but increased relative adrenal weights. BDNF concentrations were decreased by IR in saline-injected animals in the left prefrontal and parietal cortices and right entorhinal and hippocampal cortices, and in the subcortical regions of both hemispheres. NGF concentrations were unaltered by IR in saline-injected animals, but increased in MA challenged animals in the entorhinal/hippocampal cortices and subcortical areas of both hemispheres. MA application induced shifts of the lateral asymmetry in NGF contents in prefrontal and entorhinal cortices. The results suggest that an early pharmacological traumatization can set a switch for further brain development, and that growth factor concentrations might possibly be influenced by peripheral stress hormones.

Prenatal maternal programming determines testosterone response during social challenge

The present study investigated in domestic guinea pigs whether the effects of prenatal social stress are pathological consequences of adverse social conditions; or whether mothers adjust their offspring to the environment they experience during pregnancy. As a prenatal stressor social instability was used: we studied male guinea pig offspring whose mothers lived in a stable social environment (SE-sons) or in an unstable social environment (UE-sons) during pregnancy. Eight experimental groups were established, consisting of one SE-son, one UE-son and five females, respectively. All experimental groups remained in a stable group composition for the whole investigation time. We hypothesized that if mothers prenatally adapt their offspring, in a stable social environment SE-sons will be dominant, display agonistic and courtship behavior more frequently, have higher body weights, be less stressed and have higher testosterone concentrations than UE-sons. Our results show no significant differences between SE- and UE-sons concerning behavior, body weight or plasma-cortisol concentrations. Hence no evidence exists that an unstable social environment during pregnancy has pathological consequences for the male offsprings' phenotype. However, SE-sons had significantly higher plasma testosterone concentrations than UE-sons in phases when females were receptive. A higher reactivity of the hypothalamic-pituitary-gonadal axis might enable SE-males to adjust testosterone levels to the present social situation: they are significantly elevated in decisive phases of female receptivity, but remain on a lower level before and after reproductive challenge. Thus, mothers who experienced social stability during pregnancy provide their sons prenatally with a promising reproductive strategy in competitive situations later in life.

Effect of early isolation on signal transfer in the entorhinal cortex–dentate–hippocampal system

Deprivation of socio-sensory interactions during early life impairs brain function in adulthood. In previous investigations we showed that early isolation severely affects neuron development in several structures of the hippocampal region, including the entorhinal cortex. In the present study we investigated the effects of early isolation on signal processing along the entorhinal cortex-dentate-CA3-CA1 system, a major memory circuit of the hippocampal region. Male and female guinea-pigs were assigned at 6-7 days of age to either a social or an isolated environment. At 90-100 days of age the animals were anesthetized and field potentials were recorded from the entorhinal cortex-dentate-CA3-CA1 circuit, driven by dorsal psalterium commissural volleys. Analysis of the input-output function in the different structures showed that in isolated males there was a small reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in layer II of the entorhinal cortex. No changes occurred in isolated females. In isolated males and females there was a reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in the dentate gyrus, CA3 and CA1, but this effect was larger in males. In isolated males, but not in females, the population spike/population excitatory postsynaptic potential ratio was reduced in all investigated structures, indicating that in males the size of the discharged neuron population was reduced more than due to the decreased input. Results show that isolation reduces the synaptic function in the whole entorhinal cortex-dentate gyrus-CA3-CA1 system. While the entorhinal cortex was moderately impaired, the dentate-hippocampal system was more severely affected. The impairment in the signal transfer along the entorhinal cortex-dentate gyrus-CA3-CA1 system was heavier in males, confirming the larger susceptibility of this sex to early experience. This work provides evidence that malfunctioning of a major hippocampal network may underlie the learning deficits induced by impoverished surroundings during early life.

Glucocorticoids and the development of agonistic behaviour during puberty in male golden hamsters

During puberty, the agonistic behaviour of male golden hamsters undergoes a transition from play fighting to adult aggression. Repeated exposure to social stress early in puberty accelerates this transition. The present study investigated the possible role of glucocorticoids on the maturation of agonistic behaviour. First, we compared serum cortisol levels following a 20-min restraint stress during early puberty, mid-puberty or adulthood. Across puberty, animals exhibited a two-fold increase in post-restraint cortisol levels. We also compared corticotrophin-releasing hormone (CRH) immunoreactive fibres projecting to the median eminence between animals in early puberty and adulthood. The CRH fibre density was two-fold greater in adults compared to juveniles. Furthermore, we investigated the effects of stress hormones on the maturation of agonistic behaviour. Male hamsters were injected daily with dexamethasone, a corticosteroid receptor type II agonist (0, 10 or 40 microg/100 g), early in puberty from postnatal day 31 (P-31) to P-36. When paired with a smaller and younger intruder on P-37, attack frequency did not differ between groups. However, dexamethasone-treated animals showed a dose-dependent decrease in the percentage of play-fighting attacks and an increase in the percentage of adult attacks. In summary, puberty can be described as a period of increasing hypothalamic-pituitary-adrenal activity in male golden hamsters. Moreover, increasing glucocorticoid levels influence the maturation of agonistic behaviour. These data shed new light on the neuroendocrine mechanisms that regulate the maturation of social behaviours during puberty.

Individual housing induces altered immuno-endocrine responses to psychological stress in male mice

Social isolation and lack of social support have deleterious effects on health, thus being regarded as one of the most relevant causes of diseases in human and other mammalian species. However, only few are the studies aimed at evaluating the psychoneuroimmunological functions of individually housed subjects. The present study was designed to understand how the behavior and the physiology of male house mice might be affected by individual housing. We first analyzed whether individual housing of different duration (1-42 days) would result in immuno-endocrine dysfunction (experiment 1). Then we investigated whether housing conditions would affect the reaction to an acute mild psychological stress (experiments 2 and 3). There were three main findings: first, individually housing mice for increasing time periods did not induce any major immuno-endocrine effects compared to a stable sibling group housing. Therefore, prolonged isolation does not seem to dramatically impair mice immuno-endocrine functions. Second, when exposed to a mild acute stress, i.e. forced exposure to a novel environment, isolated mice showed higher basal corticosterone and lower type 1 (IL-2) and type 2 (IL-4) cytokines as well as splenocytes proliferation compared to group housed male mice. Finally, when faced with a free choice between a novel environment and their home cage, individually housed mice showed reduced neophobic responses resulting in increased exploration of the novel environment, thus suggesting a low anxiety profile. Altogether, our findings suggest that individual housing in itself does not change immunocompetence and corticosterone level, but does affect reactivity to a stressor. In fact, individually housed mice showed high behavioral arousal, as well as altered immuno-endocrine parameters, when challenged with mild psychological novelty-stress.

Presence of mother and unfamiliar female alters levels of testosterone, progesterone, cortisol, adrenocorticotropin, and behavior in maturing Guinea pigs

Although the guinea pig is characterized by precocial physical development and minimal active maternal care, studies suggest the presence of the mother can influence neuroendocrine and behavioral activity of offspring even well beyond weaning. Previous results may have been influenced by the procedure of housing weaned subjects with the mother to within 2 days of testing. The present study examined approximately 40-day-old guinea pigs housed apart from the mother for 0 (not rehoused), 2, or 10 days. Rehousing without the mother led to elevations in plasma testosterone (measured in males), progesterone (measured in females), cortisol, and adrenocorticotropin (ACTH) (both measured in males and females). Offspring housed without the mother for 10 days had the highest progesterone, cortisol, and ACTH levels. Testosterone elevations were observed in 2-day-, but not 10-day-, rehoused animals. Regardless of rehousing condition, 60 min isolation in a novel test cage elevated progesterone, cortisol, and ACTH, and reduced testosterone. These effects were all moderated if the subject was tested with the mother or another female. Sexual behavior toward the mother was observed frequently, but only in males housed apart from her prior to testing. Overall, males and females that had been housed apart from the mother interacted with her as they would an unfamiliar female. Our results corroborate previous findings, suggest the effect of housing apart from the mother on male testosterone is transitory, and indicate that continuous housing with the mother past weaning suppresses circulating progesterone in females and cortisol and ACTH in both sexes.

Effects of early environment on field CA3a pyramidal neuron morphology in the guinea-pig

There is evidence that early environmental conditions have profound effects on the morphology of the dentate granule cells. The aim of the present study was to obtain information about the effects of early environment on neuron morphology in the hippocampal field CA3, a structure closely linked to the dentate gyrus. The dendritic trees and the somata of field CA3a pyramidal neurons were quantified in Golgi-stained brains of guinea-pigs of both sexes raised in either a social or an isolated environment. Two pyramidal neuron types were found in CA3a, characterized by either a long or a short shaft. Environment affected the apical tree of the long-shaft neurons only in males and that of the short-shaft neurons in both sexes. In isolated males the long-shaft neurons had a decrease in the number of dendritic intersections (62-82%), branching points (76%) and length (71%) in the middle third of the apical tree. The short-shaft neurons had a decrease in the number of intersections at two distal levels only in both isolated males (26, 83%) and females (77, 82%). The shaft spine density was affected by environment in the long-shaft neurons of males only, with a density increase (110%) in isolated males. In both sexes the basal tree of only the long-shaft neurons was affected by environment. Isolated males had a decrease in the number of dendritic intersections (65-88%), primary dendrites (80%) and dendritic length (88%) and isolated females had a decrease in the number of intersections (51-89%), branching points (77%) and dendritic length (85%). The soma major axis of only the long-shaft neurons was affected by environment with a reduction in isolated males (90%) but an increase in isolated females (111%). These results demonstrate dendritic atrophy of CA3a pyramidal neurons following early isolation and a different reactivity to environment of the two CA3a pyramidal neuron types, their apical and basal trees and the two sexes. The dendritic atrophy of CA3a neurons caused by isolation is likely to be associated with an impairment in the physiology of the hippocampal formation and in the forms of memory in which the hippocampal formation plays a major role.

Social influences on endocrine activity in guinea pigs, with comparisons to findings in nonhuman primates

Guinea pigs exhibit a rich and varied social organization. Studies in recent years have demonstrated that social stimuli have widespread neuroendocrine effects in guinea pigs. Here, effects on the hypothalamic-pituitary-adrenal, adrenal medullary/sympathetic, and hypothalamic-pituitary-gonadal systems of both adult and developing guinea pigs are reviewed. These systems respond to various social variables, or factors that affect social variables, including: separation from attachment objects, housing conditions, changes in housing, the familiarity of the environment in which social interactions occur, foraging conditions, surrogate-rearing, agonistic interactions, and the establishment of dominance rank. Similarities and differences between these findings and those in nonhuman primates are discussed. It is argued that the guinea pig is well suited for the study of socioendocrine effects throughout the life span, and can provide a valuable complement to nonhuman primate research in this area.

Rats reared in social isolation show schizophrenia-like changes in auditory gating

Central sensory filtering processes can be demonstrated using a paired stimulus paradigm. Normal humans show a diminished vertex-recorded midlatency auditory-evoked potential to the second of paired clicks (0.5 s apart), a phenomenon termed auditory gating. Schizophrenics routinely fail to suppress the response to the second stimulus; thus, they do not gate. Previous animal studies of auditory gating have used psychotomimetic drug administration to induce a schizophrenia-like loss. However, a nonpharmacologic model of deficient gating would be advantageous. Isolation rearing of weanling rats produces impaired prepulse startle inhibition similar to that observed in schizophrenics. The present studied examined the effects of rearing status upon auditory gating. Male Sprague-Dawley rats raised in social isolation (ISO) were compared to socially raised rats (SOC). Across 10 baseline recording sessions, SOC rats showed substantial gating, while ISO rats failed to gate. Abnormal auditory gating is transiently normalized by nicotine, but not haloperidol, in schizophrenics. ISO rats given nicotine bitartrate showed gating in the normal range for 60 min. By contrast, haloperidol failed to normalize gating in ISO rats. Thus, isolation rearing of weanling rats appears to produce a stable schizophrenia-like gating deficiency that shows the same pattern of response to pharmacological interventions.

Social confrontation in male guinea pigs: behavior, experience, and complement activity

Because aggressive encounters are known to affect immune function in rodents, we hypothesized that individual behavior and social experience would contribute importantly to the impact of confrontation on the activity of the complement system (CA) in guinea pigs. CA was determined by lysis of Euglena gracilis cells (triggered by alternative pathway mechanisms). Males with different social experience were used: i) individually housed males (IH), ii) socially less-experienced males (LE), raised in large groups in the absence of adult animals; and iii) socially experienced males (EX) with additional fighting experience. An IH and LE male, respectively, was introduced into a group of EX residents (consisting of one male and two females). During a 26-day confrontation period the behavior of all animals was quantitatively recorded. IH and LE males showed a significant and persistent decrease in CA after confrontation (mean +/- SEM lysed cells/100 cells; IH: -16.5 +/- 4.0, LE: -16.5 +/- 3.5), whereas no significant changes from baseline were observed in EX males (-2.5 +/- 3.0). However, in social situations characterized by unstable dominance, EX males showed a lowered CA (-11.3 +/- 4.0) as well. Plasma cortisol concentrations determined in LE males were significantly elevated 4 h after confrontation but did not correlate with the long-term decrease in CA. The data indicate that the activity of the complement system can be influenced by psychosocial stressors, and suggest the importance of prior social experience for the guinea pig's ability to cope with social conflicts.

The ability to arrange with conspecifics depends on social experiences around puberty

The study examined the effect of age on the ability to arrange with unfamiliar conspecifics in differentially reared male guinea pigs. The experiments were conducted with 50-, 90-, 150-, and 210-240-day-old-males, which either had been reared in large colonies (CRM) or each with a single female (FRM). In each age class, seven pairs of unfamiliar CRM and seven pairs of unfamiliar FRM were placed in chronic confrontations for 6-10 days in the presence of an unfamiliar female in 2 m2 enclosures. Body weights were determined directly before and 52 h, 124 h, and 220 h after the onset of the experiments. CRM showed no substantial losses in body weights irrespective of whether the experiments were conducted before, around, or after sexual maturity (which is achieved at 2-3 months of age). In contrast pairs of FRM arranged without problems only at 50 days of age. Around puberty, social stress distinctly increased, and 2 and 4-5 months later the confrontations even had to be ceased after 6 days, because losers extremely decreased in body weights. It is concluded that in male guinea pigs the time around sexual maturity is crucial for the development of social skills necessary to arrange with unfamiliar conspecifics in a nonstressful way.

Taste preferences, body weight gain, food and fluid intake in singly or group-housed rats

Two behavioral experiments were performed to determine if the housing conditions modify taste preferences, body weight gain, food and fluid intake, and alimentary diurnal pattern in adult male rats. In Experiment 1, a two-bottle 24-h preference test (salt, sweet, sour, bitter solutions versus deionized water) was performed in singly, dually or multiply housed rats. In Experiment 2, the same sapid solutions as Experiment 1 and water were contemporaneously offered to singly, dually, or multiply housed rats. Crowded rats drank more water, sweet solution, and total fluid, but less salt solution than singly or dually housed rats during dark and whole-day periods. All rats preferred sour solution, but not bitter solution, to water. In both experiments, crowded rats gained less body weight and ate less food than dually or isolated rats. These results suggest that the housing conditions influence taste preferences, food and fluid intake, body weight gain, but not alimentary diurnal pattern in rats. An important implication of these results is that in experiments in which appetite and taste are dependent variables, all rats should be housed under the same social and environmental conditions.

Free choice ethanol intake of laboratory rats under different social conditions

To study the effects of different kinds of social deprivation on voluntary ethanol (ETOH) intake male Wistar rats were housed by (a) individual caging, (b) "contact" caging (partial social deprivation), and (c) group caging (four individuals per cage). In the latter condition the individuals were separated once a week from each other for 24 h. The rats simultaneously received water 5%, 10% and 20% ETOH for a period of 14 weeks. Additional control animals received water. Isolated individuals drank significantly more alcohol than group-housed or contact-caged rats. After a few days they preferred the 20% solution. Circadian measures revealed a discontinuous intake of high doses (greater than 0.5 g/kg/h) during short time periods. Contact-caged rats consumed much less ETOH, but both the preference for 20% ETOH and the circadian course of intake were similar to those occurring after isolation. ETOH intake of group-housed individuals was low. These individuals preferred the 5% solution and continuously consumed small ETOH doses. During the period of short-term isolation they drank even more ETOH than long-term isolated individuals. In contrast to the latter, the enhancement of intake decreased after some weeks. It is suggested that the differences between the housing groups not only reflect different degrees of isolation stress, but may also be explained by a contribution of different reinforcing or aversive psychotropic effects of ETOH. Reduction of isolation stress is probably most important in the situation of short term separation, whereas dose-dependent reinforcement via social stimulation or sedation may affect the drug taking behavior under the other social conditions.

Social stress in guinea pigs

Confrontation studies between eight pairs of two 7-8 months old male guinea pigs, each raised with one female from their 30th day of age, were conducted in an enclosure of 2 m2 in the presence of an unfamiliar female. The opponents were chronically kept together for up to 8 days. When they were not separated after 52 hr, one or (in 1 case) both males fell into a comatose state and died 5-8 days after the onset of the confrontation. Four hr after the onset of the confrontations both opponents showed significantly increased plasma glucocorticoid (CS) titers. At this time as well as 24 hr earlier, prospective winners (PW) and prospective losers (PL) did not yet differ in any physiological parameter measured. Forty-eight hr later, PW were characterized by a low body weight loss, low CS, high testosterone (T) and elevated plasma-catecholamine (CA) values. In contrast, PL showed a high body weight loss, very high CS titers, low T-titers and more elevated CA values than PW. Despite these highly significant differences in physiological parameters, PL and PW did not yet differ in the frequency of any agonistic behavioral element recorded. Seventy-two hr after the onset of the chronic cohabitation, losers began to behave more and more passive, they ceased feeding and drinking and were less and less involved in social interactions. The physiological differences between winners and losers intensified.(ABSTRACT TRUNCATED AT 250 WORDS)