The development of sexually dimorphic sensitivity to growth hormone feedback of the Clonidine-induced growth hormone surge in the rat
M.S. (Master of Science)
Department of Biological Sciences
Somatotropin; Sexual dimorphism (Animals); Clonidine
These studies investigate the change in sensitivity to a2-adrenergic stimulation and growth hormone feedback during the development of the sexually dimorphic pattern of Growth Hormone (GH) release in the rat. Sensitivity to a2- adrenergic stimulation was tested with Clonidine (CLN, an a2-adrenergic agonist) which stimulates GH release in the adult male rat. Studies were carried out in a novel hypothalamic-pituitary co-perifusion system developed in this laboratory. This in vitro preparation allows coincubation of these tissues free from peripheral hormonal and extra-hypothalamic neural influences, allowing the assessment of specific treatments. Pre-pubertal rats of 10, 20, 25 and 30 days of age and 50 day old and adult rats were used. Initial studies assessed whether CLN had a stimulatory effect on GH release at these ages in each sex. Subsequent studies examined whether prior treatment with human GH (hGH) suppresses the CLN-induced GH surge as was previously demonstrated in the adult male rat. GH feedback may be involved in the sexually dimorphic secretory patterns. Data showed that both male and female rats are sensitive to a2-adrenergic stimulation by 10 days of age. In the male rat, there is an increase in GH release in response to CLN until 30 days of age, and sensitivity is maintained in adulthood. In the female, there is a GH surge in response to CLN until 30 days. At 50 days and in adulthood, this response is substantially diminished. In addition, a profound sexual dimorphism in the capacity of hGH to suppress the CLN-induced GH surge was demonstrated. By 25 days of age, hGH greatly attenuates the CLN-induced GH surge which continues in adulthood in male rats. In female rats, hGH did not depress the CLN-induced GH surge at any age. Thus, in the female rat, GH feedback does not develop, and the ability to respond to a2-adrenergic stimulation with a GH surge is greatly reduced. To evaluate the role of releasing factors in gender differences in sensitivity to a2-adrenergic stimulation and feedback, samples were assayed for GRF and SRIF. In media from tissue of both male and female prepubertal rats, CLN stimulated a surge in GRF release that was inhibited by hGH by 25 days of age. In females, by 50 days, this GRF response did not occur. SRIF levels provided no conclusive differences to explain the sex difference in feedback sensitivity. The findings in this study suggest that prior to puberty, females are sensitive to a2-adrenergic stimulation as males, and that this sensitivity is suppressed by peripubertal events. In addition, the sexually dimorphic GH secretory patterns may involve differences in sensitivity to GH feedback.
Maszak, Gary J., "The development of sexually dimorphic sensitivity to growth hormone feedback of the Clonidine-induced growth hormone surge in the rat" (1994). Graduate Research Theses & Dissertations. 5438.
Northern Illinois University
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Includes bibliographical references (pages -51)