Abstract
Testis growth during early life is important for future male fertility and shows acceleration during the first months of life in humans. This acceleration coincides with the peak in gonadotropic hormones in the blood, while the role of hypothalamic factors remains vague. Using neonatal rats to assess this issue, we found that day 9 of life is likely critical for testis development in rats. Before this day, testicular growth was proportional to body weight gain, but after that the testes showed accelerated growth. Hypothalamic kisspeptin and its receptor mRNA levels begin to elevate 2 days later, at day 11. A significant increase in the mRNA levels for gonadotropin-releasing hormone (GnRH) receptors in the hypothalamus between days 5 and 7 was followed by a 3-fold decrease in GnRH mRNA levels in this brain region during the next 2 days. Starting from day 9, hypothalamic GnRH mRNA levels increased significantly and positively correlated with accelerated testicular growth. Triptorelin, an agonist of GnRH, at a dose that had no effect on testicular growth during “proportional” period, increased testis weights during the period of accelerated growth. The insensitivity of testicular growth to GnRH during “proportional” period was supported by inability of a 2.5-fold siRNA knockdown of GnRH expression in the hypothalamus of the 7-day-old animals to produce any effect on their testis weights. GnRH receptor blockade with cetrorelix was also without effect on testis weights during “proportional” period but the same doses of this GnRH antagonist significantly inhibited “accelerated” testicular growth. GnRH receptor mRNA levels in the pituitary as well as plasma LH concentrations were higher during “accelerated” period of testicular growth than during “proportional” period. In general, our data defined two distinct periods in rat testicular development that are primarily characterized by different responses to GnRH signaling.
Highlights
Testis growth during early life is important for future male fertility
We have shown that testicular growth undergoes acceleration in rats after day 9 of life
There were no differences in follicle-stimulating hormone (FSH)-b mRNA levels between the periods of proportional and accelerated testicular growth
Summary
Testis growth during early life is important for future male fertility. Testicular development depends on intrinsic and environmental factors in man [1,2,3] and rodents [4]. Secretions are key players in the hypothalamic mechanisms of the onset of puberty [12] All these factors are already expressed in both rat hypothalamus and pituitary at birth [13,14]. The data on mice deficient in kisspeptin or GnRH signaling provide evidence that hypothalamus begins to have a significant impact on rodent testis development somewhere within the first 2 weeks of life [16,21]. Functional significance of hypothalamic factors for testis growth during early postnatal period in genetically normal animals remains unclear To investigate these issues, we analyzed the growth of the testes and mRNA levels for GnRH, kisspeptin and their receptors in the preoptic area of the hypothalamus of rat pups during the first two weeks of life. The effects of siRNA inhibition of GnRH expression and treatment with GnRH agonist and antagonist on testicular growth in 1- and 2-week-old rats were assessed
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