Kristin M. Kramer

STRESS IN FREE-RANGING MAMMALS: INTEGRATING PHYSIOLOGY, ECOLOGY, AND NATURAL HISTORY

Abstract We review developments in the study of stress in free-ranging mammals and summarize the physiological and behavioral components of the stress response. Both the sympathetic nervous system response and the regulation and reactivity of the hypothalamic–pituitary–adrenal (HPA) axis are discussed. In particular, we describe how the activity of the HPA axis at baseline levels follows circadian and circannual rhythms in ways that allow animals to respond to predictable environmental changes, focusing largely on the endpoint of this axis, the glucocorticoid hormones cortisol and corticosterone. Superimposed upon these rhythms are the elevated glucocorticoid levels characteristic of the stress response, which allow an animal to respond to unpredictable social, physical, or environmental challenges. Methods used to explore the stress response in free-ranging mammals are described. Both inter- and intraspecific variation in the stress response as they relate to the environment are discussed. Finally, how the regulation and reactivity of the HPA axis varies by life-history stage and sex in mammals is reviewed, focusing on reproduction and development.

Mechanisms underlying epigenetic effects of early social experience: the role of neuropeptides and steroids.

In mammals the neonatal period is a time of significant social interaction. This is true even in solitary species as females spend a significant amount of time nursing and caring for their offspring. In social species interactions may also include the father, older siblings and extended family members. This period is a time of significant development, including organization of the central nervous system, and therefore a time when the degree and type of social interaction influences the development and expression of social behavior in adulthood. The purpose of this review is to examine the possible mechanisms for the epigenetic effects of early social experience on the subsequent expression of social behavior. We propose that social interactions during the neonatal period organize the subsequent expression of behavior by altering sensitivity to neuropeptides and steroids. Both neuropeptides (e.g. oxytocin and arginine vasopressin) and steroids (e.g. estrogen) regulate or influence the expression of behaviors such as affiliation, aggression, sociosexual behavior, parental behavior, and responses to stress. Therefore, changes in sensitivity to these hormones via reorganization of receptors or changes in hormone production and secretion are potentially powerful mechanisms through which early social experience can mold subsequent social behaviors.

NEONATAL MANIPULATIONS OF OXYTOCIN ALTER EXPRESSION OF OXYTOCIN AND VASOPRESSIN IMMUNOREACTIVE CELLS IN THE PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS IN A GENDER-SPECIFIC MANNER

Early postnatal manipulations of oxytocin have long-term behavioral and physiological consequences; the present study examined the hypothesis that oxytocin or its absence influences the subsequent expression of either oxytocin or arginine vasopressin in the CNS. On postnatal day 1 female and male prairie voles (Microtus ochrogaster) received a single i.p. injection of oxytocin (3 microg), oxytocin antagonist (0.3 microg), or 50 microl of isotonic saline or were only handled. On postnatal days 1, 8 and 21, brains were fixed, sectioned and stained for oxytocin or vasopressin immunoreactivity and analyzed as a function of age, treatment and sex. Both oxytocin and vasopressin immunoreactivity were observed on day 1 in the supraoptic and paraventricular nuclei (PVN) of the hypothalamus. Numbers of oxytocin and vasopressin neurons increased with age in both nuclei. Females treated on postnatal day 1 with oxytocin or oxytocin antagonist displayed a significant increase in oxytocin immunoreactivity on day 21 in the PVN. In contrast, males treated with antagonist tended to have decreased vasopressin immunoreactivity in the same region. These results revealed that the effects of neonatal manipulation of oxytocin are age-dependent, site-specific and sexually dimorphic. The long-lasting effects of neonatal exposure to exogenous oxytocin and oxytocin antagonist indicate a role for oxytocin in the development of the CNS during the neonatal period, affecting the development of the oxytocinergic system in females and the vasopressinergic system in males. The developmental effects observed suggest one possible mechanism by which neonatal exposure to oxytocin or neonatal inhibition of endogenous oxytocin produces long-lasting behavioral and physiological alterations and could play a role in the development of male- and female-typical behavior.

Effects of stress on parental care are sexually dimorphic in prairie voles

The effects of stress on parental care are poorly understood, especially in biparental species where males also display care. Data from previous studies in prairie voles, as well as parallels with pair-bonding behavior, suggest the hypothesis that a stressful experience might facilitate parental care in males but not in females. In the present study, male and female prairie voles were exposed to either a 3-min swim stressor or no stressor; 45 min later each animal was tested in a parental care paradigm. Following the parental care test, blood samples were collected and assayed for corticosterone (CORT). After the stressor males, but not females, showed significant changes in parental behavior including significantly more time in kyphosis (arched-back huddling), and a tendency to spend more time licking and grooming pups. In males, CORT levels measured following the parental care test were inversely related to licking and grooming but positively correlated with retrievals. These findings support earlier studies suggesting that the neuroendocrine substrates of parental behavior, as well as the effects of stressors, are sexually dimorphic in this species.

Developmental effects of oxytocin on neural activation and neuropeptide release in response to social stimuli.

Previous studies have revealed that the neuropeptide hormone oxytocin (OT) has developmental effects on subsequent social behavior and on mechanisms underlying social behavior such as OT neurons and estrogen receptor alpha. This suggests that OT might also have developmental effects on neural responses to social stimuli. This was tested in socially monogamous prairie voles (Microtus ochrogaster) by manipulating OT on the first day of life and then assessing the response to a heterosexual pairing in adulthood. The response to cohabitation was assessed by quantifying neural activation in regions of the brain associated with sociosexual behavior and anxiety using c-Fos immunoreactivity. Additionally, immunocytochemistry was used to label OT and vasopressin neurons and plasma was assayed for both neuropeptides. Treatment effects were evident in females, but not in males. Blockade of OT receptors with an OT antagonist on the first day of life resulted in neural activation of the central amygdala in response to a pairing with a novel male in adulthood. The central amygdala does not normally express c-Fos after a heterosexual pairing in reproductively naïve prairie voles. Treatment effects also were observed in vasopressin immunoreactivity in the SON with OT-treated females showing a decrease.

The organizational effects of oxytocin on the central expression of estrogen receptor α and oxytocin in adulthood

BackgroundPrevious studies have demonstrated that neonatal manipulation of oxytocin (OT) has effects on the expression of estrogen receptor α (ERα) and the central production of oxytocin observed in juveniles (at weaning, 21 days of age). The goal of this study was to determine whether the effects of neonatal manipulation of OT last into adulthood, and if the effects differ from those observed during the early postnatal period. On the first day of life, prairie voles (Microtus ochrogaster) received one of three doses of OT (High, 3 μg; Med, 0.3 μg; Low, 0.03 μg), an OT antagonist, or isotonic saline. Another group was handled, but not injected. Then as adults, brains were collected, sectioned, and stained for ERα or OT using immunocytochemistry.ResultsIn females, treatment with OT increased the expression of ERα immunoreactivity in the ventral lateral septum (0.03 μg) and the ventromedial nucleus of the hypothalamus and central amygdala (0.3 μg). In males, OT antagonist increased ERα expression in the bed nucleus of the stria terminalis. There was no apparent effect of OT on the number of cells producing OT in the paraventricular nucleus of the hypothalamus.ConclusionThe current results suggest that neonatal manipulation of OT has long-term organizational effects on the expression of ERα in both males and females. The lack of effect on OT neurons in the paraventricular nucleus suggests that some developmental effects of OT previously observed in weanlings do not persist into adulthood. Developmental effects of OT on ERα patterns were sexually dimorphic, dose-dependent, and site-specific.

Early exposure to oxytocin affects the age of vaginal opening and first estrus in female rats

Neonatal exposure to exogenous oxytocin (OT) can have long-term effects on the subsequent expression of adult behavior and physiology. Here, we test the prediction that early postnatal exposure to OT can affect the timing of sexual maturation in females, as indicated by the age of vaginal opening and the onset of first estrus. To test this hypothesis, female Sprague-Dawley rats received one of four treatments beginning on the day of birth and continuing for the next 6 days. Three groups received an intraperitoneal injection of one of the following: OT (1 mug/g), an OT antagonist (OTA, 0.1 mug/g) or isotonic saline (vehicle control). The fourth group was handled but not injected. Females were then examined to determine the day of vaginal opening and first estrus. The potential effects of OT on body weight were also measured, with females being weighed on postnatal days 1-7, 70, 91 and 136. Treatment with OT significantly delayed the age of vaginal opening and the onset of first estrus. There was no effect on weight. Results indicate that early exposure to OT can affect the timing and development of female sexual maturation.

Photoperiod alters central distribution of estrogen receptor α in brain regions that regulate aggression

Testosterone or its metabolite, estrogen, regulates aggression in males of many mammalian species. Because plasma testosterone levels are typically positively correlated with both aggression and reproduction, aggression is expected to be higher when males are in reproductive condition. However, in some photoperiodic species such as Siberian hamsters (Phodopus sungorus), males are significantly more aggressive in short day lengths when the testes are regressed and circulating testosterone concentrations are reduced. These results led to the formation of the hypothesis that aggression is modulated independently of circulating steroids in Siberian hamsters. Thus, recent studies have been designed to characterize the role of other neuroendocrine factors in modulating aggression. However, aggression may be mediated by testosterone or estrogen despite basal concentrations of these steroids by increasing sensitivity to steroids in specific brain regions. Consistent with this hypothesis, we found that males housed under short days have increased expression of estrogen receptor alpha in the bed nucleus of the stria terminalis, medial amygdala, and central amygdala. Neural activation in response to an aggressive encounter was also examined across photoperiod.

Parental regulation of central patterns of estrogen receptor α

Reduced levels of estrogen receptor alpha (ERalpha) in the medial amygdala (MeA) and bed nucleus of stria terminalis (BST) have been hypothesized to play a significant role in the expression of male behaviors associated with monogamy. Therefore, the regulation of ERalpha could be a critical factor in determining male behavior and the evolution of monogamy. Central expression of ERalpha immunoreactivity was compared in hybrid offspring from crosses between two phenotypically distinct populations of prairie voles (Microtus ochrogaster). Illinois voles (IL) are socially monogamous and display low levels of ERalpha, while Kansas voles (KN) display some characteristics associated with polygyny and have higher levels of ERalpha. In offspring from hybrid crosses, the pattern of ERalpha expression was dependent upon parentage; the two types of hybrid crosses did not produce the same ERalpha pattern in the offspring. In the BST and MeA, hybrid males expressed ERalpha patterns consistent with those of males from their mothers population, while hybrid females had ERalpha patterns typical of females belonging to their fathers population. The parental-specific patterns of ERalpha expression are suggestive of genomic imprinting, therefore, the vole ERalpha (Esr1) gene was cloned and sequenced, and examined for allele-specific expression. Results from this study indicate that while maternal factors may play a major role the expression of ERalpha in their male offspring, genomic imprinting is unlikely to be involved, suggesting another mechanism is responsible.

Estrogen receptor α and vasopressin in the paraventricular nucleus of the hypothalamus in Peromyscus

The purpose of this study was to determine the presence of estrogen receptor alpha (ERalpha) and the relationship between neurons that express ERalpha and produce vasopressin (AVP) in the paraventricular nucleus of the hypothalamus (PVN) in new world mice of the genus Peromyscus. Brains were collected from male and female Peromyscus californicus, Peromyscus leucopus, Peromyscus maniculatus, and Peromyscus polionotus, and double labeled for the expression of ERalpha and AVP immunoreactivity (IR). The number of cells expressing ERalpha-IR and AVP-IR was determined in the medial and posterior region of the PVN. The results indicate that Peromyscus is the first taxonomic group reported to have ERalpha widely distributed in the PVN, occurring in both medial and posterior regions of the PVN. While estrogen can regulate the production of AVP, AVP and ERalpha were rarely colocalized. There was, however, a significant inverse relationship between the number of cells that expressed ERalpha-IR and the number expressing AVP-IR. There were no sex differences in the expression of ERalpha-IR or AVP-IR.