Corticotropin and Melanotropin
This chapter discusses the neuroendocrinology of the pars intermedia as well as the endocrinology of the corticotroph cells located in the pars anterior and pars distalis. Synthesis and secretion of pro-opiomelancortin (POMC)-derived peptides in the pars intermedia appears to be tonically inhibited by hypothalamic axons that release dopamine at their terminals. Corticotropin, P-lipotropin, and β-endorphin are the three principal bioactive products derived from POMC in pars anterior tissue. Adrenocortical hormones from the adrenal gland which feedback and inhibit corti- cotroph cells of the pars anterior do not regulate the POMC cells of the pars intermedia. In horses that possess tumors of the pars intermedia and a clinical diagnosis of Cushing’s disease, there is increased production of POMC combined with reduced processing of its cleavage products. Of course, POMC and its products are produced in hypothalamic neurons that project to many central nervous system areas, and effects on higher brain function may reflect neuropeptide effects of melanocyte-stimulating hormone-related peptides.