The effects of a postnatal high-fat diet on puberty onset and estrous cyclicity in female rats – a kisspeptin study

Maria E.K. Lie

Student thesis: Master thesis

Abstract

The age of puberty onset in girls has advanced within the last few decades. This is considered to be due, at least in part, to the increasing prevalence of childhood obesity, as epidemiological studies have shown an association between early puberty onset in girls associated and obesity markers early in life or in peri-puberty. The increasing obesity epidemic has also been associated with an increased incidence of reproductive dysfunction in women and the increased demand for fertility treatment. These associations do not prove causality and other factors can contribute to the secular trends in pubertal timing and impaired reproductive function. However, due to the increasing obesity problem, it is becoming increasingly important to investigate this connection and the underlying mechanism of a nutritional regulation of puberty onset and reproductive impairment. In rodents, an association between a positive energy status during development and an advancement of puberty onset and an irregular estrous cycle, has also been reported, but like in humans little is known about the underlying mechanism. During the last decade, it has been revealed that the hypothalamic neuropeptide kisspeptin, encoded by the Kiss1 gene, is pivotal in the regulation of the hypothalamic-pituitary-gonadal (HPG) axis that controls maturation and reproduction and thereby drives neuroendocrine pubertal maturation. Furthermore, kisspeptin neurons have recently emerged as a putative conduit for a metabolic regulation of the HPG axis, with leptin being a positive modulator of Kiss1 expression. Little is known about the impact of an exposure to a high-fat diet on pubertal timing and reproductive function in rodents, and the mechanisms have only been studied scarcely. Therefore, it is relevant to investigate whether an increase in the fat energy percentage in the diet alone can advance pubertal timing and interfere with estrous cyclicity, and to what degree the kisspeptinergic system is affected in this. The first aim of this thesis was to evaluate the effects of a postnatal high-fat-diet, with 60% energy from fat, during three different periods (lactation, post weaning, or both periods) on puberty onset, revealed by vaginal opening (V.O.) in rats. To gain mechanistic insight concomitant changes in the kisspeptinergic system at peri-puberty were assessed by Kiss1 expression determined using qRT-PCR, and kisspeptin-immunoreactivity (kisspeptin-ir). No effects of a postnatal high-fat diet exposure, in any of the periods, on puberty onset or body weights were observed. In accordance, no changes in leptin, insulin, triglycerides, and estradiol were identified. As reported previously, a postnatal maturation of the kisspeptinergic system at peri-puberty and from peri-puberty to early adulthood was observed. However, no effects of the high-fat diet on Kiss1 expression and kisspeptin-ir in the two hypothalamic nuclei, the arcuate nucleus (ARC) and the anteroventral periventricular nucleus (AVPV), were detected. The second aim was to evaluate the effect of a high-fat diet exposure from weaning and for 40 days on estrous cyclicity in adult rats in relation to changes in the kisspeptinergic system. The high-fat diet induced an irregular estrous cyclicity by prolonging the estrous cycle in a subgroup of the rats, characterized by an unaffected body weight and increased leptin levels. Surprisingly, the high-fat fed rats, characterized by an increased body weight with a less pronounced increase in leptin as the irregular rats, had a regular estrous cycle. Kiss1 expression and kisspeptin-ir in ARC and AVPV were unchanged by the high-fat diet in both groups of rats. In conclusion, these data shows that a high-fat diet induces an irregular cycle in rats by extending the diestrous phase, maybe through leptin, without modulating hypothalamic Kiss1 expression or kisspeptin levels. The data also indicate that a postnatal high-fat diet has no effect on puberty onset or hypothalamic Kiss1 expression and kisspeptin levels in female rats. These findings suggest that an increase in fat energy percentage in the diet alone do not regulate puberty onset or affect the kisspeptinergic system. This is in line with several epidemiological studies unable to establish a correlation between obesity markers at puberty and an earlier puberty onset.

EducationsMolecular Biology, (Bachelor/Graduate Programme) Graduate
LanguageEnglish
Publication date30 May 2012
Supervisors[No Value] Rewitz, F. Kim & Jens D Mikkelsen

Keywords

  • HPG axis
  • kisspeptin
  • high-fat diet
  • puberty onset
  • the estrous cycle
  • LH surge
  • obesity
  • reproduction