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Functional maturation of the pituitary-gonadal axis in the neonatal female rat
1Department of Physiology, University of Turku, Finland.
Insights
The neonatal rat pituitary feedback response to hCG stimulation begins around day 7-10. Elevated follicle-stimulating hormone (FSH) may accelerate ovarian response to luteinizing hormone (LH).
Area of Science:
- Reproductive Endocrinology
- Neonatal Physiology
- Hormonal Feedback Mechanisms
Background:
- The pituitary feedback response to ovarian stimulation is crucial for reproductive health.
- Understanding the developmental timing of this response in neonates is essential for identifying potential disruptions.
Purpose of the Study:
- To investigate the developmental onset of the pituitary feedback response to LH/hCG-stimulated ovarian activity in neonatal rats.
- To determine the influence of FSH on the appearance of ovarian response to LH/hCG.
Main Methods:
- Neonatal rats (1-10 days old) received hCG injections, with ovarian steroid (estradiol, progesterone, testosterone) and serum LH levels monitored.
- Combined recombinant human FSH and hCG treatments were administered to assess FSH's role.
Main Results:
- Ovarian testosterone and estradiol increased significantly after hCG treatment from Day 8.
- Progesterone levels rose significantly when hCG was given on Day 9.
- Serum LH decreased significantly after hCG injection starting from Day 7.
- Elevated FSH levels advanced LH suppression following hCG injection.
Conclusions:
- The negative feedback of ovarian activity on pituitary LH secretion is functional in neonatal rats from Day 10.
- Increased intraovarian testosterone levels indicate early androgenic dominance in this feedback.
- Elevated postnatal FSH levels can advance the onset of ovarian response to LH/hCG stimulation.
Abstract:
The development onset of the pituitary feedback response to LH/hCG-stimulated ovarian activity was studied in the neonatal rat. A single injection of hCG (600 IU/kg BW) was administered to groups of rats between 1 and 10 days of age, and the responses, i.e., ovarian estradiol (E2), progesterone (P), and testosterone (T) production as well as serum LH were monitored 3 days later. The first significant increase in ovarian T and E2 contents occurred in rats treated on Day 8 of life, and the first significant increase in P content occurred when hCG was administered at the age of 9 days. A significant decrease in serum LH, 3 days after hCG injection, was observed for the first time in animals treated on Day 7, but not in those aged 1-6 days. To study whether the appearance of the ovarian response to LH is dependent on FSH, rats received combined treatment with recombinant human (rec)FSH on Days 3-8 (0.3 IU s.c. twice daily) and a single injection of hCG (600 IU/kg BW s.c.) on Day 6. The elevated FSH levels from Day 3 onward advanced the suppression of the serum LH level after hCG injection, suggesting that enhanced action of FSH promotes the appearance of functional LH receptors neonatally. It was concluded that 1) the negative feedback of ovarian activity on pituitary LH secretion is functional in the neonatal rat from Day 10 of life; 2) increased intraovarian T levels reflect the androgenic dominance of this early feedback action; and 3) elevated postnatal levels of FSH may advance the onset of the ovarian response to LH/hCG stimulation.