Fred J. Karsch - US grants

The neuroendocrine regulation of ovarian cyclicity in a seasonally breeding species is the theme of this project. Studies will be performed to identify signals pertinent to photoperiodic, pineal, hypothalamic and steroidal regulation of pulsatile LH secretion and response to estradiol negative feedback in the female sheep. Particularl emphasis will be placed on the importance of patterning of these signals to generation of responses. The patterns of hypothalamic GnRH and pineal melatonin signals will be of special interest and will be studied using a portable back-pack infusion system. Stereotaxically placed knife-cuts in the hypothalamus and immunocytochemical localization of GnRH in neurons of the sheep brain will be used to examine the neural processing of photoperiodic and steroidal signals which regulate pulsatile LH secretion. Radioimmunoassays will be used to measure serum LH, FSH, GnRH, melatonin, estradiol and progesterone. It is expected that these findings will lead to new insight concerning ovarian cyclicity and seasonal breeding, an natural process of reversable fertility.

Virtually all animals undergo marked changes in body structure, function and behavior which enable them to adapt to, and survive, the hardships of a seasonally changing natural environment. The overall goal of the program is to understand the regulation of seasonal reproduction in an animal of world-wide importance, the domestic sheep. It has become clear through recent research that the process of seasonal reproduction in sheep is generated spontaneously by a self-sustained endogenous rhythm of neuroendocrine activity. Changes in day length provide time cues which permit appropriate phasing of this endogenous rhythm with cyclic changes in the environment. The specific goal of the research program is to define those portions of the annual cycle of day length which provide the temporal information leading to an appropriately timed breeding season, and to clarify the mechanisms by which this information is conveyed. The outcome of the research should lead to new insights into a fundamental problem of enormous biological significance and practical importance: the regulation of fertility. The results will also be more broadly applicable to a wide variety of physiological, metabolic and behavioral rhythms which form an integral part of the adaptive mechanisms in many animals. An understanding of these processes should have profound implications to the production of food and human nutrition as well as the reproductive biology of wild or endangered species that are seasonal breeders.

Virtually all animals undergo marked changes in body structure, function and behavior which enable them to adapt to, and survive, the hardships of a seasonally changing natural environment. The goal of Dr. Karsch's research is to understand the means by which the changing seasons regulate seasonal reproduction, a natural process of reversible fertility and infertility. He has selected domestic female sheep as the model system. In this species, the process of seasonal reproduction is generated spontaneously by a self-sustained endogenous rhythm of neuroendocrine activity. Changes in day length provide time cues which permit appropriate phasing of this endogenous rhythm with cyclic changes in the environment. Dr. Karsch will determine whether a short day photoperiodic cue is effective when given during anestrus, and whether a long day photoperiodic cue is ineffective when given during the breeding season. The specific photoperiodic messages will be restored by infusion of melatonin into pinelectomized sheep. Melatonin is a hormone secreted by the pineal gland only at night, and it enables the animal to tell how long the night is and thus the duration of the day. The outcome of this research should lead to new insights into a fundamental problem of enormous biological significance and practical importance: the regulation of fertility. The results will also be more broadly applicable to a wide variety of physiological, metabolic and behavioral rhythms which form an integral part of the adaptive mechanisms in many animals. An understanding of these processes should have profound implications to the reproductive biology of endangered species that are seasonal breeding. Since the studies will be done in domestic sheep, the findings will have a direct bearing on production in the food and fiber industries.

Infertility, overpopulation and reproductive disorders are major problems facing Society. This proposal focuses on integration of the three great communication systems in the body - the endocrine, the central nervous and the immune systems - and how this impacts the ovulatory cycle of females. The importance of the endocrine and nervous systems in orchestrating the events of the cycle, and in coordinating reproductive processes with the external environment, has long been recognized. Only recently, however, has the role of the immune system come to the forefront. The immune system elaborates a cascade of neural and humoral signals when challenged by inflammatory stimuli and invasion of the body by foreign organisms. Through mechanisms not fully understood, these immune signals powerfully inhibit reproduction. The broad objective of this research is to determine how an immune challenge impacts the neuroendocrine axis to inhibit the ovarian cycle. The general approach is to integrate current knowledge of the physiologic basis for the estrous cycle of sheep with observed changes in neuroendocrine activity following a standardized immune challenge (endotoxin). In this manner, processes critical to reproductive suppression will be pinpointed. Specific Aim 1 elucidates the influence of an immune challenge on the estrous cycle. This will be achieved by assessing the effects endotoxin on progression of the hormonal events of the follicular and luteal phases of the cycle, and by assessing the influence of ovarian steroids on neuroendocrine responses to an immune challenge. Specific Aim 2 addresses mechanisms whereby an immune challenge inhibits pulsatile secretion of GnRH and LH. This includes investigations directed at both the hypothalamic level (secretion of GnRH into pituitary portal blood) and the pituitary level (responsiveness to GnRH). Specific Aim 3 focuses on the impact of an immune challenge on surge secretion of GnRH and LH. This includes how an immune challenge impacts activation of estradiol receptive neurons in the hypothalamus, the transduction of the surge inducing signal to the GnRH neuronal network, and the actual surge processes of GnRH and LH release. The research incorporates a variety of approaches including monitoring secretion of hormones from the hypothalamus, the pituitary and the ovary, and immunocytochemical analysis of hypothalamic systems known to be crucial to reproduction. The health-related relevance of this work is that it will provide an in depth integrated analysis of how immune challenges, infectious and inflammatory disease, physical injury and trauma, and a variety of noxious stimuli lead to reproductive failure.

In mammals a key hormone involved in reproduction is follicle-stimulating hormone, FSH. This hormone is released by the pituitary gland, which is connected to part of the brain called the hypothalamus. The mechanisms that regulate the timed release of this hormone are largely unknown. Recently it has been shown that the blood circulating to the pituitary from the brain can include an unknown compound that stimulates FSH release (FSH-releasing factor, or FSH-RF) in a pulsatile fashion. This action seems to be independent from the release regulated by the known contribution of the compound activin, or of another releasing hormone called GnRH. This project utilizes biochemical, pharmacological and anatomical techniques, with a novel model system to separate the neural supply from the blood supply to the pituitary, to localize the timing and source of this novel releasing factor as a step towards identifying it.
Results will be very important for neuroendocrinology by clarifying release mechanisms for a major hormone involved in puberty as well as reproduction, and will have a likely impact also in veterinary and agricultural fields.

The broad question addressed by this research is: how does stress inhibit reproduction? One way stress acts to inhibit reproduction is by reducing secretion of gonadotropic hormones from the anterior lobe of the pituitary gland. Of importance, cortisol, which is a hormone secreted by the adrenal cortex in increased amounts during stress, suppresses gonadotropin secretion in non-stressed sheep when delivered in amounts that mimic plasma cortisol concentrations during stress. The first component of this research project uses a physiological approach to test the hypothesis that increased plasma cortisol is essential for suppression of gonadotropin secretion in sheep experiencing real-life psychosocial stressors (e.g., isolation, confinement, predator cues). It then addresses underlying mechanism by testing if stress-induced suppression of gonadotropin secretion is mediated via the type II glucocorticoid receptor, and if stress acts to reduce responsiveness of the pituitary gland to gonadotropin-releasing hormone. Cellular and molecular approaches and a pituitary-cell culture system will then be employed to distinguish actions of cortisol at the plasma membrane from intracellular actions, and to determine if cortisol-induced suppression of gonadotropin secretion is linked to reduced receptor for gonadotropin-releasing hormone on pituitary cells. This research will provide important novel insight into both integrative and cellular mechanisms whereby stress inhibits reproduction in sheep by demonstrating that cortisol, acting via the type II glucocorticoid receptor on cells of the anterior pituitary gland, is essential for stress-induced suppression of gonadotropin secretion. This, in turn, has global relevance as it could lead to enhanced food and fiber production in regions where sheep are agriculturally important. It also impacts preservation of endangered species maintained in captivity or in conditions of deteriorating habitat, and it bears broadly on fertility regulation. This research will also benefit professional development of new investigators by providing research opportunities for undergraduate, graduate and postdoctoral trainees.