Anterior pituitary cascading physiological responses. Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by the hypothalamus. Neuroendocrine cells in the hypothalamus project axons to the median eminence, at the base of the brain. At this site, these cells can release substances into small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophyseal portal vessels). Aside from hypothalamic control of the anterior pituitary, other systems in the body have been shown to regulate the anterior pituitary’s function. GABA can either stimulate or inhibit the secretion of luteinizing hormone (LH) and growth hormone (GH) and can stimulate
Pituitary pars intermedia dysfunction by corticotropes in the pars distalis of the equine pituitary. In a normal horse, this accounts for the majority of ACTH production. ACTH produced by the pars distalis is subject to negative feedback in a normal horse, so high cortisol levels reduce ACTH production by the pituitary, subsequently reducing cortisol levels. In a horse with PPID, ACTH levels are high as a result of pars intermedia production, but it is not subject to negative feedback regulation. Despite the high levels of ACTH, cortisol levels vary, and are sometimes lower than normal. Additionally, hyperplasia of the adrenal cortex is infrequent. The