-Difluoromethylornithine Modifies Gonadotropin-Releasing Hormone Release and Follicle-Stimulating Hormone Secretion in the Immature Female Rat

Abstract

Polyamines play an essential role in tissue growth and differentiation, in body weight increment, in brain organization, and in the molecular mechanisms of hormonal action, intracellular signaling, and cell-to-cell communication. In a previous study, inhibition of their synthesis by alpha-difluoromethylornithine (DFMO), a specific and irreversible inhibitor of ornithine decarboxylase, during development in female rats, was followed by prolonged high follicle-stimulating hormone (FSH) serum level and a delayed puberty onset. Those changes were relatively independent of body mass and did not impair posterior fertility. The present work studies the mechanisms and site of action of polyamine participation in FSH secretion during development. DFMO was injected in female rats between Days 1 and 9 on alternate days. At 10 days of age, hypothalami from control and DFMO rats were perifused in vitro, and basal and potassium-induced gonadotropin-releasing hormone (GnRH) release were measured. The response to membrane depolarization was altered in DFMO hypothalami. Increased GnRH release in response to a low K+ concentration was evidenced. Adenohypophyses of the same treated prepubertal rats were perifused in vitro and the response to GnRH pulses was checked. In DFMO-treated rats, higher FSH release was observed, with no changes in LH or PRL secretion. Finally, pituitary GnRH receptor number in adenohypophyseal membranes from treated and control groups was quantified. A significant reduction in specific binding was evident in hypophyses from DFMO-treated rats when compared with binding in the control group. In summary, DFMO treatment in a critical developmental period in the female rat impacts the immature GnRH neuronal network and immature gonadotropes. A delay in maturation is evidenced by a higher sensitivity to secretagogs in both pituitary glands and hypothalamic explants. These events could explain the prolonged high FSH serum levels and delayed puberty onset seen in this experimental model.

Effects of Polyamines on the Release of Gonadotropin-Releasing Hormone and Gonadotropins in Developing Female Rats

“…The present results show a direct effect of some polyamines on hypothalamic and adenohypophyseal tissues. They are in agreement with previous results using DFMO in vivo: the lower concentration of PUT and SPD achieved, under DFMO treatment (Thyssen SM, Libertun C, unpublished data), was followed by an increase of FSH secretion and a delayed puberty onset (14,15). Therefore, from our in vitro and in vivo results, a relationship between polyamine levels and the output of the GnRH-gonadotropin system is demonstrated in early stages of development.…”

“…They would participate by inhibiting GnRH and FSH release. In agreement, when both polyamines are decreased by DFMO treatment in vivo, an increase in GnRH and FSH release is observed when the tissues are stimulated specifically by GnRH or unspecifically by KCl (14). Furthermore, DFMO acting directly on pituitary cells of 6-day-old females increased FSH release, precisely when polyamine concentrations are higher.…”

“…In addition, this was the only amine that was not inhibited by DFMO, and it was the only one that remained constant in both hypothalamus and anterior pituitary during postnatal development, as determined in a parallel work. These results indicate that spM was not responsible for the endocrine changes previously determined with the in vivo treatment with DFMO, where the female rats presented high FSH release and an associated puberty delay (14,15).…”

“…Polyamines, specifically putrescine, were also related to -y-aminobutyric acid (GAB A) synthesiSi a neurotransmitter critically involved in the regulation 0 gonadotropin secretion (13). In previous studies, we de f scribed that DFMO administered during the first 10 dayS 0 life in female rats, but not at later ages, was followed bY prolonged high follicle-stimulating hormone (FSH) seru J11 levels and a delayed puberty onset (14). Those changes we~e relatively independent of body mass and they did not impaIr posterior fertility.…”

“…DFMO treatment in a critical develoPmental period in the female rat impacted on the immature GnRH neuronal network and immature gonadotropes. A delay in maturation was evidenced by a higher sensitivity to secretagoges in both pituitary glands and hypothalamic explants (14,15). More recently, we identified and measured the four polyamines mentioned above in relevant neuroendocrine areas, i.e., hypothalamus and pituitary, of the developing rat, comparatively in males and females (Thyssen SM, Libertun C, unpublished data).…”