Prostaglandin E2 (PGE2)-induced growth hormone (GH) release: effect of intrahypothalamic and intrapituitary implants

Ovariectomized rats were unilaterally implanted with a 23-gauge stainless steel cannula in different hypothalamic areas or in the pituitary gland and subsequently were treated with estrogen (sc, 10 micron g estradiol benzoate, Eb). Two days after the estrogen injection, an inner cannula containing PGE2 or PGF2alpha at its tip was inserted into the cannula. Other animals were implanted with an empty inner cannula. Plasma GH concentrations were measured by RIA in blood samples drawn from the jugular vein while the animals were lightly etherized before (-2) and at 20, 40, 60 and 120 min following the implantation. Plasma GH levels in control animals bearing an empty cannula in the body of the arcuate nucleus-median eminence region (BARH-ME) were significantly depressed by the ether stress. The implantation of PGF2alpha in this area was completely ineffective in preventing ether stress-induced decline in plasma GH. By contrast, PGE2 implanted in BARH-ME or the post-chiasmatic region of the hypothalamus (HARH-ME) elevated plasma GH 20 min following its implantation and partially prevented the subsequent decrease in GH levels induced by ether stress. PGE2 implants located in several other hypothalamic areas failed to induce GH release or to prevent the decline in GH levels induced by ether stress. However, PGE2 implanted in the pituitary gland elicited a marked increase in plasma GH at 20 min and completely prevented the subsequent ether stress-induced decline in GH levels. The results suggest that PGE2 can act at both hypothalamic (ARH-ME) and pituitary levels to stimulate GH release. At the hypothalamus, PGE2 may inhibit GH-inhibiting factor (GIF) release or induce release of GH releasing factor (GHRF).