Neuroendocrine Role of Receptors for Insulin and IGF-1 in the Pituitary Gland: Morphological Considerations
Previous investigations have demonstrated that interaction of peripheral insulin with neuroendocrine centers of the brain may play a key role in the regulation of body metabolism. Since the various levels of interaction are only incompletely known, the present study focused on the adenohypophysis of the rat, where the localization of the insulin receptor and the structurally and functionally closely related insulin-like growth factor-1 (IGF-1) receptor was investigated by light and electronmicroscopic immunocytochemistry. The two receptors were found on separate subpopulations of secretory cells of the pars distalis, the IGF-1 receptors preferentially occurred in the postero-lateral region, whereas insulin receptors were dispersed throughout the pars distalis. Almost 90% of cells containing insulin receptors were also shown to be immunoreactive for (β-endorphin. In contrast, IGF-1 receptors were almost exclusively located on follicle stimulating hormone secreting cells, suggesting a regulatory role of IGF-1 in the pituitary gonadotropin system. In addition, key proteins of receptor tyrosine kinase signal transduction, such as insulin receptor substrate-1 and phosphatidyl inositol-3 kinase, were demonstrated in the anterior lobe. Both markers were highly colocalized in two populations of morphologically distinct secretory cells, i.e. large, smooth-shaped cells and small-sized cells with irregular cytoplasmic protrusions. These data indicate the ability of insulin and IGF-1 receptors to transduce biological signals in the pituitary in vivo and further support the hypothesis that peripheral insulin acts as a regulatory hormone in the control of body homeostasis via various steps in the neuroendocrine axis, including the pituitary gland.
KeywordsInsulin Receptor Pituitary Gland Signal Transduction Protein Insulin Receptor Tyrosine Kinase Neuroendocrine Center
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