Endocrine Abstracts

Introduction: The hypothalamic-pituitary-gonadal (HPG) axis is regulated by pulsatile GnRH secretion and its action on the anterior pituitary, leading to the synthesis and secretion of LH and FSH. KNDy neurons, expressing kisspeptin, neurokinin B and dynorphin A, are the GnRH/lH pulse generator and are key regulators of puberty and fertility. The HPG axis is active during prenatal and early postnatal life in humans and rodents, but then remains quiescent until puberty in both species. The response of gonadotropes to exogenous upstream GnRH signals such as kisspeptin and neurokinin during prepubertal development in mice is unknown.

Aim: To conduct a systematic assessment of LH responsiveness to exogenous kisspeptin and to the neurokinin-3 receptor agonist senktide at regular intervals postnatally and across pubertal maturation in mice.

Materials and Methods: Male and female J129Sv mice were treated with intraperitoneal injections of kisspeptin-10 (0.0023 nmol/g), senktide (0.023 nmol/g) or vehicle every 3 days from postnatal day (PND) 10 until completion of puberty. Serum LH was measured by an ultra-sensitive ELISA at baseline and 15 minutes after injection.

Results: Prepubertal mice responded in an age-dependent and sexually dimorphic manner to kisspeptin and senktide. LH levels increased following kisspeptin stimulation in both sexes, but responses were higher in females than in males from PND10 to PND16. The highest response was observed at PND10, with a pattern of decreasing response with advancing age. Senktide induced a robust increase in serum LH in females from PND10 to PND16, with a decrease in response with age, similar to that observed after kisspeptin treatment. In contrast, in males there was no increase in serum LH in response to senktide at any age. No difference in the onset of puberty or body weight was observed across treatment groups.

Conclusions: The assessment of serum LH levels following IP administration of kisspeptin and senktide in prepubertal male and female mice demonstrated distinct age and sexually dimorphic patterns of LH responses to GnRH upstream signals, with greater responses at early postnatal ages and in females than in males. These findings provide new evidence for sexual dimorphism of the HPG axis in mice early in postnatal life, prior to the onset of puberty, and raise the possibility of additional restraining factors on HPG axis activity in early life.