Direct Gonad-Pituitary Feedback: An Analysis of Effects of Intracranial Estrogenic Depots on Gonadotrophin Secretion

Publisher Website
Google Scholar
Abstract
In spayed rats, intrapituitary ovarian grafts or estrogenic depots caused castration cells to regress in localized regions. The volume of tissue affected was proportional to the estrogen concentration of the depot. Intrahypothalamic pellets tended to reduce the size, but not the population density, of the castration cells, but only at higher doses. These slight effects were not localized, however. These findings suggest that direct but localized effects of intrapituitary hormone depots may fail to influence enough pituitary tissue to permit the change to be detected at the periphery. The possibility that a hormone can act directly casts doubt upon the interpretation that the effects of intrahypothalamic hormone depots reveal an indirect feedback pathway—via neural “receptors.” Intrahypothalamic administration may merely be the most efficient method of distributing a compound to the entire adenohypophysis. The pertinence of this “implantation paradox” is discussed in relation to the large body of work aimed at unraveling the interplay between target organ hormone feedback and hypothalamic hypophysiotrophic mechanisms.