Growth hormone-releasing hormone and morphine attenuate growth hormone secretagogue-induced activation of the arcuate nucleus in the male rat

Growth hormone secretagogues (GHS) administered systemically selectively in duce growth hormone (GH) release from the pituitary and the expression of F os protein in arcuate nucleus neurons. Both the control of GH release and t he expression of the GHS receptor in the arcuate nucleus are thought to be regulated, at least in part, by the negative feedback actions of GH. In thi s study, we utilized the immunocytochemical detection of Fos protein to exa mine the effects of morphine- and GH-releasing hormone (GHRH)-induced GH re lease on the activation of arcuate nucleus neurons following GHS administra tion. Given alone, two structurally different GHS induced significant amoun ts of Fos-LI in the arcuate nucleus of male rats, suggesting activation of cells in this region. Prior administration of morphine or GHRH significantl y reduced the number of Fos-positive cells in the arcuate nucleus of rats i njected with either GHS, although when given together, morphine and GHRH di d not produce a greater reduction in Fos expression than when given alone, In no case was there a complete reduction in Fos expression, indicating tha t some arcuate nucleus neurons are not subject to the feedback effects of e ndogenous GH. These results provide evidence that, in the male rat, GH can feedback to the hypothalamus, altering the responsiveness of neurons involv ed in the central response to GHS.