405 The Effects of Disease-Induced Inflammation on Reproductive Neuroendocrinology: Evidence from Sheep

Abstract

Abstract Reproduction is dictated by the cohesive functioning of the organs of the hypothalamic-pituitary-gonadal (HPG) axis. As the name suggests, reproduction starts within the hypothalamus through the production and pulsatile secretion of gonadotropin releasing hormone (GnRH) followed by subsequent secretion of luteinizing hormone (LH) from the anterior pituitary and then steroidogenesis and gametogenesis in the gonads. Additionally, the HPG axis is tightly regulated by internal and external cues and, over the last twenty years, kisspeptin has been identified as a key modulator for reproduction through its stimulatory actions on GnRH neurons to secrete GnRH and subsequently LH. Growing evidence indicates that these kisspeptin expressing neurons are negatively affected by stressors (e.g., inflammation, social stress, metabolic stress) resulting in impaired reproduction. Inflammation caused by lipopolysaccharide (LPS, endotoxin), a component of the cell membrane of Gram-negative bacteria (e.g., Escherichia coli), is of particular interest due to its relationship with common livestock ailments, such as mastitis and metritis, and its suppression of GnRH and LH secretions. However, there is limited information about the influence disease-induced inflammation has on kisspeptin expressing neurons. In rats, acute endotoxin inflammation will cause about a 50% decrease in the number of kisspeptin immunopositive cells in the arcuate nucleus along with a suppression in LH secretion. In ewes, an acute dose will decrease the proportion of activated kisspeptin neurons in the ARC and POA. However, the mechanisms by which inflammation affects reproduction are not completely understood, especially in domestic livestock. Through a series of experiments, we aim to elucidate these mechanisms and effects of endotoxinemia in both an acute and a chronic model. To accomplish these goals, we administered endotoxin acutely to ewes. Overall, the suppression of LH secretion at the level of hypothalamus is possible, however, more research is required to further elucidate the mechanism. To evaluate a chronic model of systemic inflammation, we evaluated routes of endotoxin administration by randomly assigning twenty two castrated male sheep into one of five groups - control (CON), single acute IV dose (SAD; 400 ng/kg BW LPS) administered on Day 1, daily acute IV dose (DAD; 400 ng/kg BW LPS), daily increasing IV dose (DID; starting with 400 ng/kg BW LPS and increasing 20% each day), or steady subcutaneous dose (SSD; 20 µg/kg BW LPS over 24 hours), all administered during a one-week study period. Chronic inflammation, induced by a daily rising dose of IV administered LPS, suppresses LH secretion through effects on kisspeptin protein expression in the middle arcuate nucleus of the hypothalamus of sheep. Taken together, systemic inflammation associated with infection and disease likely impairs reproduction in part through suppression of kisspeptin-expressing neurons.