A Potential Primate Model for Bone Loss Resulting from Medical Oophorectomy or Menopause*

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Abstract
This study examined the potential use of the GnRH agonist-treated female monkey as a model for bone loss after medical oophorectomy or the onset of menopause in women. Three female rhesus monkeys (13–16 yr of age) were treated continuously for 10 months with 25 μg/day GnRH agonist using osmostic minipumps. All three animals exhibited normal menstrual cycles before treatment. Within 5 weeks of the beginning of GnRH agonist treatment, serum progesterone and estradiol concentrations had fallen to low values and did not rise significantly during the remaining treatment period. The decline in ovarian steroidogenesis was correlated with a reduction in bone mineral density (BMD; bone mineral content/bone width) of the caudal vertebrae and humerus. The reduction of BMD of the caudal vertebrae occurred gradually. The downward trend was evident during the first 3 treatment months, but did not fall significantly below pretreatment levels until 9 months of GnRN agonist treatment. The overall decline in BMD for the caudal vertebrae was approximately 14% after 9 months of GnRH agonist treatment. The measured decline in BMD of the humerus was 11%. Serum osteocalcin levels rose more than 10-fold above pretreatment values between 4 and 7 months of GnRH agonist treatment before declining to levels that approached pretreatment concentrations between 8 and 10 months of treatment. Menstrual cycles were reinitiated within 4 weeks after the termination of treatment, as shown by luteal phase increases in serum progesterone concentrations. BMD of the humerus and caudal vertebrae remained subnormal 2 months posttreatment, but by 5 months had recovered to near-pretreatment values. These data suggest that ovarian hormone deprivation induced by GnRH agonist administration is associated with a decline in BMD in female monkeys, and that this animal model may be an excellent model for postmenopausal bone loss or bone reduction resulting from medical oophorectomy. The GnRH agonist-treated monkey also has the potential to be developed as a model for type I postmenopausal osteoporosis.

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