Michael B. Hennessy - US grants

Evidence from young primates indicates that even a brief period of separation from the mother or other attachment figure can have a number of physiological consequences for the infant. These effects include increases in sympathetic activity, changes in neurotransmitter levels, and increased activity of a neuroendocrine system associated with stress--the pituitary-adrenal system. In the past, it has been possible to study such questions only in primates because other commonly studied species (e.g., laboratory rodents) were not thought to show true infant-mother attachment. With the expense of primate research, the increasing demand for primate subjects, and the dwindling populations of wild primates, an alternative animal model would be beneficial. It appears that guinea pig mothers and infants may display a "true" attachment. Moreover, both infant and mother guinea pigs exhibit activation of the pituitary-adrenal system during brief (30 minutes) separation. The studies in the current research will further explore this question. Several issues will be addressed within this research, including the ontogeny of the separation response, the effect of environmental variables in determining responsiveness, and the specificity of the mother's response to infant separation. These studies will contribute to our understanding of the consequences of mother-infant separation and the processes of attachment and elucidate the nature of the stress response in young animals.

Infants that are involuntarily separated from their mothers may suffer severe physiological and behavioral consequences. Most of what we know concerning the physiological effects has derived from studies using nonhuman primates. These effects include increases in sympathetic activity, changes in neurotransmitter levels, and increased activity of the pituitary-adrenal system, a neuroendocrine system associated with stress. For many years, it was only possible to study such questions in primates since common laboratory rodents have been thought to respond to separation in a fundamentally different way than to primates, and possibly humans. In his previous NSF support, Dr. Hennessy demonstrated that the responses of young guinea pigs to maternal separation are in many ways similar to those of primates. The guinea pig model provides an alternative to primate research which, in turn, offers many practical advantages. Dr. Hennessy would continue his research to further our understanding of how brief periods of maternal separations alter neuroendocrine and behavioral function. He will examine the role of corticotropin- releasing factor (CRF) since this hypothalamic neurohormone is of primary importance in the body's diverse physiological and behavioral responses in threatening situations. Dr. Hennessy will investigate the role of centrally and peripherally administered corticotropin-releasing factor and its antagonists in mediating stress-induced behavioral changes. These basic studies will contribute to our understanding of the consequences of mother-infant separation and processes of attachment and elucidate the nature of the stress response in young.

In infants of primate and other mammalian species, the presence of the mother, but not another familiar animal, can prevent or reduce hormonal stress responses to threatening conditions. This selective capacity of the mother to reduce stress responses seems to disappear after puberty, but other companions may attain this ability at about the same time. These normal developmental changes may influence social grouping patterns of the species and have implications for understanding how humans cope with stressful conditions. Many of the relations between stress hormones, social partners, and behavior, observed in primates, have also been observed in guinea pigs. Therefore, guinea pigs are the subjects of these initial experiments. Proposed studies will: (1) document in detail changes that occur from infancy to adulthood in the ability of various classes of social companions to reduce hormonal stress responses; (2) examine how these changes correlate with developmental changes in behavior and social grouping tendencies; and, (3) determine if these developmental changes are due to rising levels of gonadal hormones at the time of puberty.

Hormones have a huge impact on behavior and its development from infancy to adulthood. Puberty is the time of the most dramatic hormonal changes of all postnatal life. Yet with the exception of sexual behavior itself, surprisingly little is known about the effects of hormones on behavior during this important period. Proposed studies will provide basic information on how hormones of puberty influence the ability of social partners to moderate stress hormones and organize social behavior. Results will be of value to researchers examining how social support reduces various stress-related physiological responses at different portions of the life span. The proposed work will also further science education by involving undergraduate students planning careers in neuroscience.

In both our own and other species, important, but still poorly understood, relations exist between the body's physiological reaction to stress and the presence of significant social partners. On the one hand, the presence of social partners with which a strong bond has been formed can often reduce the physiological stress response, while on the other, reductions in stress-related hormone levels seem to promote the formation of social bonds. The current project will examine these effects and how they change over the life span as individuals form differing age-specific social relations and undergo normal developmental changes in neuroendocrine activity. These questions will be studied in guinea pigs because of their similarity to primates in the ways that social behavior affects stress hormone levels, and because these questions can be studied with greater experimental control, more economically, and in a reasonable period of time in this rodent species. This project will assess the impact of various social partners throughout the life span on stress hormone levels; examine how this impact varies with normal age-specific changes in social interactions and stress hormone responsiveness; and assess how altering hormone responsiveness affects the formation of adult social bonds. These studies will provide new information on the development of basic biobehavioral processes related to stress and social interactions, and they will provide a unique contribution in their longitudinal approach. The work will also integrate research and undergraduate education by involving promising undergraduates interested in careers in the life sciences in every aspect of the studies.

DESCRIPTION (provided by applicant): Maternal separation has long been associated with depressive illness. In some species of primates as well as guinea pigs, infants exhibit a 2-stage, active /passive response during maternal separation. The second stage of separation (termed "despair" in primates) has been a long-standing model for the study of depressive illness. Further, the "stress diathesis" model of depression proposes that the occurrence of early maternal separation and related stressors enhances vulnerability to later depression through a process that involves sensitization of central processes mediating stress responses. In guinea pigs, proinflammatory cytokines appear to be important mediators of the passive behavioral response, and sensitization of the response occurs when pups are separated on consecutive days. Studies proposed here will examine the role of proinflammatory factors in the sensitization process (Specific Aim 1) by attempting to block the behavioral sensitization as well as related changes in core temperature and central and peripheral proinflammatory markers with the anti-inflammatory cytokine, Interleukin-10. Under Specific Aim 2, studies will examine whether activation of a proinflammatory cascade or repeated maternal separation during the preweaning period will enhance passive responses, core temperature changes, central and peripheral proinflammatory markers, and plasma cortisol levels during a period of social isolation during the periadolescent period. The proposed work meets the objectives of the AREA mechanism by providing meaningful research experience to undergraduates who will participate in all phases of the experimental work. PUBLIC HEALTH RELEVANCE: The relevance of the work is that it would: (1) provide a potential means of incorporating a long-standing animal model of depression (the passive "despair" stage of maternal separation) with current notions of the role of proinflammatory factors in depressive illness;and (2) suggest a novel way in which proinflammatory factors might take part in the sensitization process central to the stress-diathesis model of depression.

DESCRIPTION (provided by applicant): A primary current focus in the study of depression is the means by which attachment disruption in childhood and other forms of early adversity increase vulnerability for developing major depressive disorder in later life. The mechanism for this effect remains obscure but is thought to involve sensitization of stress-related processes. A second recent emphasis in depression research has been the role of proinflammatory factors in mediating depressive symptoms. Research proposed here uses a guinea pig model to examine the possibility that proinflammatory activity induced by the stressor of maternal separation mediates a prolonged sensitization of depressive-like behavior. Guinea pigs exhibit a specific attachment process, display a depressive-like behavioral response to maternal separation that is mediated by proinflammatory activity, and show a sensitization of depressive-like behavior upon repeated separation that is accompanied by a sensitized core temperature response. Work proposed here will first determine if a prolonged (9 day) sensitization identified during the last funding period is mediated by a sensitization of proinflammatory activity (either enhanced proinflammatory response or increased sensitivity to a proinflammatory response (Aim 1). This will be done by attempting to block sensitization of depressive-like behavior and core temperature increase with anti-inflammatory agents (Interleukin-10 and indomethacin) and by determining whether prior separation increases the central cytokine and/or behavioral response to later immune challenge. Next, we will assess whether the sensitized behavioral response reflects sensitization of a broader underlying depressive-like state mediated by proinflammatory activity (Aim 2). This will be accomplished by determining if early separation enhances responsiveness in a validated behavioral test of depressive-like behavior (forced swim) that is independent of the initial inducing agent (separation), and whether the forced swim effect can be reversed by an anti-inflammatory compound.

Our experiences as children, particularly those with our attachment figures, have a disproportionately large impact on our later development. Among the most powerful of these early experiences are aversive circumstances or challenges. While most would consider early trauma to be a relatively uncommon experience in modern human society, evidence suggests otherwise. More than half of adults living across a range of North American and European countries report experiencing one or more significant early adverse experiences, often involving disruption of the attachment relationship. Recent studies on how these essentially social experiences become ?embedded? in our biology have strongly implicated an upregulation of neuroimmune signaling and inflammatory processes. While once thought to relate specifically to mood disorders, it is now clear that this enhanced neural inflammatory activity has broad effects on stress reactivity, leading to a relatively commonplace phenotype in adolescence marked by an ?inflammatory burden? that increases susceptibility to an array of stress-related mental disorders including, but not limited to, anxiety, schizophrenia, bipolar disorder, depression, and posttraumatic stress disorder. However, we still know extremely little about the specific developmental processes linking early stress to vulnerability in later life. Work with appropriate animal models is required for this task. The guinea pig is a particularly well-suited rodent for studying attachment disruption. Because guinea pigs are well-developed at birth, display strong evidence for an attachment process like that seen in primates, and receive little active maternal care, effects of isolation from the mother can be confidently attributed to attachment disruption as opposed to physical immaturity or changes in the mother's behavior upon return of the pup. Moreover, isolation in a threatening environment induces a neuroinflammatory-based behavioral reaction that sensitizes when animals are isolated again in infancy or adolescence. This effect thus provides an opportunity to directly assess how physiological changes during early attachment disruption are translated into specific neuroinflammatory mechanisms that sensitize behavioral stress reactivity in later life. Proposed studies will examine the role of sympathetic nervous system activation, either alone or together with stress levels of glucocorticoids, in initiating the sensitization process. On the basis of our preliminary results, we propose that the activity of these stress mediators will result in enhanced sensitivity to the prostaglandin PGE-2 via an EP-1 receptor-specific mechanism. We will examine behavioral sensitivity to PGE-2 and upregulation of EP-1 in cells activated by isolation in essential threat-related neurocircuitry of medial prefrontal cortex (mPFC), amygdala, and hypothalamus. The necessity of PGE-2 signaling and binding of EP-1-- particularly in mPFC--for behavioral sensitization to occur will be analyzed. Successful completion of these studies will provide critical insight into how neuroinflammatory activity resulting from attachment disruption can promote development of a phenotype prone to stress-related disorders in adolescence.