Christopher R. Pryce

Long-term neurobehavioural impact of the postnatal environment in rats: manipulations, effects and mediating mechanisms

The major characteristics of the postnatal environment of the rat pup are its mother and littermates. The pup, which is poorly developed at birth, matures rapidly in this environment, and regulates the behaviour and physiology of the dam and littermates, as well as vice versa. The study of the impact of the rats postnatal environment on its long-term neurobehavioural development is of fundamental importance. In fact, it is one of the major examples--at the interface of the biological, social and medical sciences--of animal models for the study of the interaction between the environment and the genome in both the acute and chronic regulation of the phenotype. Specific experimental manipulations of the rat postnatal environment have been demonstrated to exert robust and marked effects on neurobiological, physiological and behavioural phenotypes in adulthood. In the present review we present some of the major findings, including some original data, and discuss what these existing data can tell us about the long-term neurobehavioural effects of the postnatal environment in rats, the external and internal mechanisms that mediate these effects, and the most appropriate directions for future basic and applied research in this area.

Long-term effects of early-life environmental manipulations in rodents and primates: Potential animal models in depression research

Depression is one of the most common human illnesses and is of immense clinical and economic significance. Knowledge of the neuro-psychology, -biology and -pharmacology of depression is limited, as is the efficacy of antidepressant treatment. In terms of depression aetiology, whilst the evidence for causal mechanisms is sparse, some genomic and environmental factors associated with increased vulnerability have been identified. With regards to the latter, the environments in which human infants and children develop are fundamental to how they develop, and parental loss, emotional and physical neglect, and abuse have been shown to be associated with: traits of depression, traits of predisposition to depression triggered by subsequent life events, and associated physiological abnormalities, across the life span. Studies of postnatal environmental manipulations in rodents and primates can potentially yield evidence that abnormal early-life experience leading to dysfunction of the neurobiology, physiology and behaviour of emotion is a general mammalian characteristic, and therefore, that this approach can be used to develop animal models for depression research, with aetiological, face, construct and predictive validity. The establishment of models with such validity, if at all achievable, will require a sophisticated combination of (1) appropriate postnatal manipulations that induce acute stress responses in the infant brain which in turn lead to long-term neurobiological consequences, and (2) appropriate behavioural and physiological assays to identify and quantify any depression-like phenotypes resulting from these long-term neurobiological phenotypes. Here, we review some of the evidence-positive and negative-that neglect-like environments in rat pups and monkey infants lead to long-term, depression-like behavioural traits of reduced motivation for reward and impaired coping with adversity, and to altered activity in relevant physiological homeostatic systems.

Effect of social isolation on stress-related behavioural and neuroendocrine state in the rat.

The present study investigated the effects of post-weaning social isolation (SI) on behavioural and neuroendocrine reactivity to stress of male and female rats. Innate aspects of fear and anxiety were assessed in the open field and elevated plus maze tests. Spontaneous startle reflex and conditioned fear response were further investigated. The neuroendocrine response of isolates was examined by measuring basal and stress release of ACTH and corticosterone and by evaluating the mRNA expression of mineralocorticoid (MR) and glucocorticoid (GR) receptors using in situ hybridization. Locomotor activity in the open field was not modified by chronic SI. In males, but not females, SI produced an anxiogenic profile in the elevated plus maze. Male isolates showed a trend towards increased startle reflex amplitude relative to socially-reared controls. Moreover, SI in males produced alterations of the HPA axis functioning as reflected by higher basal levels of ACTH, and enhanced release of ACTH and corticosterone following stress. In contrast, startle response or HPA axis functioning were not altered in female isolates. Social isolates from both genders showed reduced contextual fear-conditioning. Finally, the mRNA expression of MR and GR was not modified by SI. The results of the present study suggest that chronic SI increases emotional reactivity to stress and produces a hyperfunction of the HPA axis in adult rats, particularly in males.

The Maternal Separation Paradigm and Adult Emotionality and Cognition in Male and Female Wistar Rats

A single 24-h maternal separation (MS) in the rat during the stress hyporesponsive period alters adult behavior and neuroendocrine stress response. The age of the animal at MS might be a crucial factor for effects in adulthood. We report here on adult behavioral effects of MS performed on postnatal day 4 (MS4), 9 (MS9), or 18 (MS18) in male and female Wistar rats. Unrelated subjects were used to avoid confounding litter effects. Subjects were tested on paradigms of unconditioned fear/anxiety, i.e., open field and elevated plus-maze, and on paradigms involving learning in an aversive situation, i.e., conditioned freezing, active avoidance, and water maze. In line with our predictions we obtained (a) sex differences that were consistent with enhanced fear/anxiety in males relative to females, (b) evidence that MS4 yielded deficits in active avoidance learning and conditioned freezing (trend level), whereas MS9 yielded enhanced active avoidance and water maze learning, (c) evidence (at trend level) that these effects of MS are greater in males than in females. There was no evidence for an effect of MS on paradigms of unconditioned fear/anxiety. We conclude that MS, irrespective of the age at separation, does not provide a robust environmental model of modified behavior in aversive situations.

Comparison of the effects of infant handling, isolation, and nonhandling on acoustic startle, prepulse inhibition, locomotion, and HPA activity in the adult rat.

This study examined whether early isolation (EI), early handling (EH), or early nonhandling (NH) in infant rats alters (a) prepulse inhibition (PPI) of the acoustic startle response (ASR) or its disruption by apomorphine, (b) motor activity or its stimulation by amphetamine, or (c) corticosterone activity (because of its modulation of dopamine activity), in adulthood and in comparison with a normal-husbandry postnatal control environment. EI did not affect PPI, reduced PPI disruption by apomorphine in males, and increased amphetamine-stimulated activity in males. NH increased the ASR, reduced activity in the open field, and increased corticosterone reactivity in males. In all paradigms, the effects of EH were similar to those of the control environment. This study provides an important contribution to the evidence on the relationship between postnatal experience and long-term neurobehavioral development in the rat and the relevance of this approach to animal models of neuropsychiatric disorder.

Comparison of the effects of early handling and early deprivation on conditioned stimulus, context, and spatial learning and memory in adult rats

Effects of manipulations of the rat pup-dam relationship on affective learning and memory in adulthood have received scant systematic investigation. The authors previously described how early handling (EH; 15 min isolation/day) and early deprivation (ED; 4 hr isolation/day) exert similar effects on spontaneous adult affect (open-field behavior, acoustic startle, endocrine stress response) relative to nonhandling (NH; C. R. Pryce, D. Bettschen, N. I. Bahr, & J. Feldon, 2001). The present study demonstrates that both EH and ED adults exhibit enhanced active avoidance relative to NH adults. Fear-conditioned context and conditioned stimulus (CS) freezing were unaffected in both EH and ED, but stress hormone responses to the CS were reduced in EH males and ED females relative to NH. In the water maze, ED adults exhibited enhanced spatial learning and memory relative to NH.

Effect of Sex on Fear Conditioning is Similar for Context and Discrete CS in Wistar, Lewis and Fischer Rat Strains

Enhanced fear in males relative to females, both innate and conditioned, is a well-described characteristic of behavior in the laboratory rat. In the case of aversive conditioning to foot shock in Long-Evans rats, it has been described that conditioning to general (nondiscrete) contextual cues is greater in male rats relative to female rats, whereas conditioning to a discrete, predictive stimulis (CS) is not. These findings have been combined with evidence for greater levels of hippocampal LTP in males in Sprague-Dawley rats to derive a model of hippocampal-LTP-mediated contextual and not CS, fear conditioning. The present study reports on an analysis of the effect of sex in contextual and discrete CS conditioning to foot shock, assessed via measurement of freezing behavior in a novel automated paradigm, in three rat strains: Wistar, Fischer, and Lewis. In Wistar rats, there was a consistent but nonsignificant tendency for males to demonstrate both more contextual and more CS conditioning than females; in Fischer rats, males demonstrated both more contextual and more CS conditioning than females; in Lewis rats, a markedly enhanced acquisition of freezing in males did not translate into a sex difference in either context or CS conditioning at expression. Therefore, within each strain the effect of sex was consistent between context and CS conditioning. These findings, taken together with the hippocampal LTP evidence, suggest that the latter mediates both contextual and discrete CS aversive conditioning, and contributes to sex differences in both these forms of conditioning, in those strains where these sex differences exist.

Repeated parental deprivation in the infant common marmoset (callithrix jacchus, primates) and analysis of its effects on early development

BACKGROUND We describe a successful demonstration that repeated early deprivation of parental care (ED), as used to study effects of early life stress in rats, can be performed in a primate and that it constitutes an early life stressor. METHODS Seven breeding pairs of marmoset monkeys each provided control twins (CON) and twins that were subjected to ED for 30-120 min/day on postnatal days (PND) 2-28. Urine samples were obtained to monitor the acute effect of ED on cortisol and catecholamine levels. Behavior samples were obtained in the home cage to monitor the effects of ED on infant and infant-parent behavior. RESULTS Early deprivation of parental care caused acute increases in cortisol, epinephrine, and norepinephrine. At PND 28, basal cortisol was reduced in ED compared with CON infants, and ED infants were smaller than CON infants. Early deprivation infants tended to spend more time in the suckling position than did CON. Early deprivation infants demonstrated more distress vocalization than CON infants, even though the parental care they received in the home cage was similar. Early deprivation infants tended to play less socially than did CON. CONCLUSIONS To our knowledge, this is the first demonstration that a repeated early life stressor of the type developed in rats can also be applied to a primate species.

Comparison of maternal separation and early handling in terms of their neurobehavioral effects in aged rats

In the rat, relative to pup nonhandling (NH), early handling (EH) leads to old-adult offspring with a hyporesponsive HPA axis, superior spatial cognition, and greater hippocampal (HIPP) neuronal density. The present study compared the effects of EH and repeated maternal separation (MS), in the form of 6-hr separation on each of 4 days beginning at day 12, on spatial cognition, corticosterone (CORT) levels, and HIPP characteristics, in aged rats. Male Wistar rat pups were exposed to EH, MS, NH or our normal in-house husbandry (CON) and tested at 18-20 months. Relative to NH and CON, EH demonstrated superior spatial cognition, reduced CORT stress response, reduced CA-field volume and no change in HIPP neuronal number. MS demonstrated a trend to superior spatial cognition, an unaffected CORT stress response, reduced CA-field volume and no change in HIPP neuronal number. These findings are important in terms of the life-span mechanisms via which postnatal manipulations induce neurobehavioral effects, and the mechanisms via which CORT and HIPP structure relate to HIPP function.

Early deprivation and behavioral and physiological responses to social separation/novelty in the marmoset.

Long-term effects of adverse early environment on neurobehavioral development have been reported for rodents and primates. The present study used daily early deprivation (ED), a paradigm developed for rats, for the first time in a nonhuman primate, the common marmoset, and investigated its effects on the behavioral and physiological responses to social separation/novelty (SSN) challenge tests in juveniles. On postnatal days (PNDs) 2-28, infants (n=5 twin pairs) were removed from the parents and placed alone in an isolation chamber for 30-120 min (9 h/week). Parents and control subjects (n=5 twin pairs) were briefly restrained (CON). At Weeks 18-20, behavioral responses of ED and CON juveniles to six 60-min SSN tests in an isolated cage, comprising 45 min alone and 15 min reunion with the father, were measured. Baseline and post-test urine samples were collected for measurement of cortisol. ED subjects exhibited significantly lower basal SSN urinary cortisol than CON, whilst SSN response cortisol values were similar in ED and CON. When alone, ED subjects were significantly less mobile and emitted significantly less contact calls than CON. Following reunion, ED subjects were significantly less in contact with or being carried by the father than CON and demonstrated significantly more tail piloerection. Although they require validation by additional parameters (e.g. cardiovascular), these data strongly suggest that early-life stress alters endocrine and behavioral responsiveness to psychosocial challenge in this primate and in a direction that could model important changes in disorders of human affective state.