Delayed pubertal development by hypothalamic suppression causes an increase in periosteal modeling but a reduction in bone strength in growing female rats

Abstract

The timing of the pubertal growth is a critical event in skeletal development. A delay in the onset of puberty has been correlated with increased stress fracture incidence in young women and as a result, suboptimal skeletal development may affect long-term bone strength. Gonadotropin releasing hormone antagonist (GnRH-a) injections were used to delay the onset of puberty in growing female rats. Twenty-three-day-old female rats were injected with a GnRH-antagonist at 2 dosage levels (n =15/group). The low dose group (1.25 mg/kg/dose) received daily injections for 27 days (sacrifice 49 days). The high dose received (5.0mg/kg/dose) only 5 days per week over a 26 day period (sacrifice 48 days). Calcein injections measured bone formation activity on the periosteal and endocortical surfaces. Standard histomorphometric and biomechanical analyses were performed on the femora and ash content was measured on the tibiae of all animals. Serum estradiol and insulin-like growth factor (IGF)-1 levels were assayed. Significant delays in pubertal development occurred in the two GnRH-a groups as evidenced by delayed vaginal openings, decreased uterine and ovarian weights and suppressed estradiol levels compared to control. Femoral lengths were significantly shorter in the experimental groups and serum IGF-1 levels were higher than control. Bone strength and stiffness were significantly lower in the GnRH-a groups. Cortical bone area was decreased and total area was not different between groups. There was a significant decrease in % Ct.Ar/T.Ar. The decreased bone strength may have resulted from a decrease in the amount and distribution of bone, however, stress and Young's modulus were also decreased. There was a different response between endocortical formation indices and periosteal formation indices to the GnRH-a protocol. Endocortical bone formation rates decreased and there was an increase in periosteal labeled surface. A dose response between bone strength and GnRH-a dosage was found. The data suggest that hypothalamic Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access

Lack of sexual dimorphism in femora of the eusocial and hypogonadic naked mole-rat: A novel animal model for the study of delayed puberty on the skeletal system

“…In mice and rats, a delay in pubertal onset has also been shown to reduce trabecular number, increase trabecular separation, and reduce femoral length and strength [5, 53, 59-61]. Subordinate NMRs circumvent the observed detrimental effects of hypogonadism on the skeletal system that are evident in humans, rats and mice.…”

“…Ovariectomy models often manifest with high GnRH, since its inhibition by sex steroids and inhibin is reduced, whereas GnRH-antagonists affect not only the entire hypothalamic-pituitary-gonadal axis, but also have a tendency to increase body weight, serum IGF-1 levels, and complete suppression of the onset of puberty is dose dependent [5, 62, 63]. NMRs on the other hand, are naturally suppressed from entering puberty and do not show differences in other hypothalamic pituitary hormones or serum IGF-1 levels, a clear advantage to other models.…”

“…Chronic estrogen deficiency during growth and maturity has negative effects on the skeleton of both sexes. Young female athletes commonly show a delay in the onset of puberty, delayed menarche, and increased fracture risk despite the bone-strengthening effects generally associated with high levels of activity [4, 5]. Even in female athletes that have completed puberty, exercise induced amenorrhea, associated with low body fat, is a common occurrence, and, like delayed menarche, leads to suboptimal skeletal development, and a concomitant decrease in bone strength, bone mass, and an increase in the incidence of bone fragility [4, 5].…”

“…Suboptimal skeletal development during puberty affects long-term bone strength, leading to an increase in fracture risk later in life [5, 14-16]. The effects of hypogonadism on both human and rodent (mice and rat) skeletal tissue have been extensively characterized in females and to a lesser degree in males.…”

A Delay in Pubertal Onset Affects the Covariation of Body Weight, Estradiol, and Bone Size

“…We analyzed data from a study that used two doses of gonadotropin releasing hormone antagonist (GnRH-a) to delay pubertal onset [29]. Forty-five female Sprague-Dawley rats (23 days of age; Charles Rivers Laboratories, Wilmington, MA, USA) were randomly assigned to a control group (C; n = 15) and two experimental groups ( n = 15/group) that received injections of GnRH-a.…”