Insulin‐induced hypoglycaemia suppresses pulsatile luteinising hormone secretion and arcuate Kiss1 cell activation in female mice

Abstract

Stress suppresses pulsatile luteinising hormone (LH) secretion in a variety of species,although the mechanism underlying this inhibition of reproductive function remains unclear. Metabolic stress, particularly hypoglycaemia, is a clinically-relevant stress type that is modelled with bolus insulin injection (insulin-induced hypoglycaemia).The present study utilised ovariectomised C57BL/6 mice to test the hypothesis that acute hypoglycaemia suppresses pulsatile LH secretion via central mechanisms.Pulsatile LH secretion was measured in 90-minute sampling periods immediately prior to and following i.p. injection of saline or insulin. The secretion of LH was not altered over time in fed animals or acutely fasted (5 hours) animals following an i.p. saline injection. By contrast, insulin elicited a robust suppression of pulsatile LH secretion in fasted animals, preventing LH pulses in five of six mice. To identify the neuroendocrine site of impairment, a kisspeptin challenge was performed in saline or insulin pre-treated animals in a cross-over design. LH secretion in response to exogenous kisspeptin was not different between animals pre-treated with saline or insulin, indicating normal gonadotrophin-releasing hormone cell and pituitary responses during acute hypoglycaemia. Based on this finding, the effect of insulin-induced hypoglycaemia on arcuate kisspeptin (Kiss1) cell function was determined using c-Fos as a marker of neuronal activation. Insulin caused a significant suppression in the percentage of Kiss1 cells in the arcuate nucleus that contained c-Fos compared to saline-injected controls. Taken together, these data support the hypothesis that insulin-induced hypoglycaemia suppresses pulsatile LH secretion in the female mouse via predominantly central mechanisms, which culminates in the suppression of the arcuate Kiss1 population. K E Y W O R D S arcuate nucleus, GnRH, hypoglycaemia, LH pulses, metabolic stress 1 | INTRODUC TI ON Acute stress threatens homeostasis and therefore initiates specific physiological responses to enable survival. 1 One common result of the body's response to acute stress is a disruption of reproductive function mediated, in part, by the suppression of gonadotrophin secretion. Luteinising hormone (LH) is secreted in an episodic or pulsatile pattern in males and in females throughout most of the ovarian How to cite this article: McCosh RB, Kreisman MJ, Tian K, Ho BS, Thackray VG, Breen KM. Insulin-induced hypoglycaemia suppresses pulsatile luteinising hormone secretion and arcuate Kiss1 cell activation in female mice.

Impact of chronic variable stress on neuroendocrine hypothalamus and pituitary in male and female C57BL/6J mice

“…Indeed, prior studies have shown that a range of different acute stressors can change ARC Kiss1 mRNA levels as well cFos expression in these cells. 62,63,71 Interestingly, we find here that CVS does not impact upon Kiss1 mRNA expression in either hypothalamic population in mice and, similarly, 18 days of chronic stress has been reported to have no effect on hypothalmic Kiss1 mRNA in rats. 24 Thus, it remains unclear how CVS may be impacting upon pulsatile LH secretion, as is the case for the LH surge mechanism.…”

“…Because the ARC kisspeptin neurones represent the GnRH pulse generator, 70 it is likely that different stressors impact upon these cells. Indeed, prior studies have shown that a range of different acute stressors can change ARC Kiss1 mRNA levels as well cFos expression in these cells 62,63,71 . Interestingly, we find here that CVS does not impact upon Kiss1 mRNA expression in either hypothalamic population in mice and, similarly, 18 days of chronic stress has been reported to have no effect on hypothalmic Kiss1 mRNA in rats 24 .…”

“…56 There is considerable evidence that psychological, immune and metabolic stressors suppress reproductive hormone secretion and function. [58][59][60][61][62][63] We find here that 14 days of CVS had no substantial impact on circulating LH levels or the mRNA expression of Gnrh1 and Kiss1 in the hypothalamus, or on Lhb and Fshb in the pituitary.…”

“…25,26 In rats, 4 weeks of CVS induces a small extension to oestrous cycle length, 64 as found here, whereas exposure to 8 weeks of CVS had no effect on a range of reproductive hormones in females. 65 Previous studies in mice have shown that acute episodes of stress potently suppress pulsatile LH secretion [61][62][63] and, presumably, this will be occurring on a daily basis in CVS mice.…”

GLUT1-mediated glycolysis supports GnRH-induced secretion of luteinizing hormone from female gonadotropes

“…Sex steroids from the ovaries regulate this axis through both positive and negative feedback mechanisms. Several studies implicate the gonadotrope as a point for nutrient regulation of the HPG-axis in addition to evidence that kisspeptin and GnRH neurons in the central nervous system regulate LH pulsatility in response to glucose availability 5 – 8 . In normal women and in women with polycystic ovary syndrome, BMI negatively correlates with circulating LH 9 .…”