INSULIN-RECEPTORS IN THE PITUITARY-GLAND - MORPHOLOGICAL EVIDENCE FORINFLUENCE ON OPIOID PEPTIDE-SYNTHESIZING CELLS

Previous investigations have demonstrated that peripheral insulin has a significant influence on brain function and that the interaction of insulin with neuropeptides in neuroendocrine regions may be pivotal fo r the regulation of body metabolism and energy balance. Since the vari ous levels of interactions are only incompletely known, the focus of t he present study has been the adenohypophysis of the rat, in which the presence and localization of insulin receptors and the structurally a nd functionally closely related insulin-like growth factor-1 (IGF-1) r eceptor has been investigated by light- and electron-microsopic immuno cytochemistry. The two receptors are found on separate subpopulations of secretory cells of the pars distalis with a preponderance of IGF-1 receptors in a postero-lateral portion of large endocrine cells, insul in receptors being more widely dispersed throughout the pars distalis in a population of smaller, irregularly shaped cells. Insulin receptor s, but not IGF-1 receptors, are also located in a subpopulation of sec retory cells in the intermediate lobe. Phosphotyrosine, a marker for s ubstrates of receptor tyrosine kinases, has been detected in numerous cells throughout the anterior and intermediate lobe, including the cel l populations containing insulin or IGF-1 receptors, indicating their ability to transduce biological signals in the pituitary in vivo. Almo st 90% of cells containing insulin receptors are also immunoreactive f or beta-endorphin. In contrast, IGF-1 receptors are almost exclusively located on cells secreting follicle-stimulating hormone, suggesting a regulatory role of IGF-1 in the pituitary gonadotropin system. The re lationship between beta-endorphin and insulin receptors provides furth er evidence for the hypothesis that peripheral insulin acts as a regul atory hormone in the control of body energy homeostasis via various st eps of the neuroendocrine axis, including opioid peptides in the hypot halamus and pituitary known to play an important role in the regulatio n of feeding behaviour.