Effects of sex steroids on growth hormone responses to clonidine and GHRH in reserpine pretreated rats

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Abstract
Administration of reserpine in a dose causing depletion of brain monoamines led to a complete suppression of the pulsatile secretory pattern of growth hormone (GH) in gonadectomized (GX) as well as in sham-operated male and female rats. In GX animals of both sexes treated with estradiol, but not in those treated with testosterone or dihydrotestosterone (DHT), the reserpine induced inhibition of GH release was partially antagonized. Administration of the alpha2-adrenoceptor agonist clonidine caused secretion of GH in reserpine pretreated, sham-operated rats. In GX male rats GH responses to clonidine were blunted, while in GX males treated with testosterone or estradiol, but not in those treated with DHT, the responses were restored. In female rats gonadectomy did not significantly affect the GH releasing effect of clonidine. However, administration of estradiol to GX females led to enhanced responses to the alpha2-agonist. Administration of the GH releasing hormone (GHRH) induced pronounced GH secretion in reserpine pretreated animals of both sexes; this effect was not significantly affected by gonadectomy. In GX males, however, GH responses to GHRH were enhanced by replacement with estradiol or testosterone, while in GX females, estradiol, but not testosterone, had the same effect.