Expression of Tyrosine Hydroxylase in the Anterior Lobe of the Human Pituitary Gland: A Possible Local Autocrine/Paracrine Role in the Regulation of Pituitary Hormone Secretion

Abstract

The main source of dopamine (DA) regulating anterior pituitary function is derived from the hypothalamic tuberoinfundibular (TIDA) neurons. Recent studies performed on mice have indicated the presence of a local source of DA because tyrosine hydroxylase (TH), the rate‐limiting enzyme of DA biosynthesis, has been found within the AL. Here, we provide evidence for the presence of TH in human cadaveric pituitary glands. The presence of TH is a good indicator of an intrinsic or local production of DA. Serial axial sections of pituitary glands obtained from two (70 and 87 year old) male cadavers have been histologically prepared and stained with hematoxylin‐eosin (HE) and immunohistochemistry has been performed using specific antibodies for TH. TH immunoreactivity has been found in the AL of both pituitaries. Similar to the AL of the mouse, TH positive cells have been localized bilaterally, at the border of the intermediate lobe (IL) and the AL. Incidentally, a distinct area in the left side of the AL of one of the pituitary glands has been found avoiding HE staining, which indicates a proliferation of cells forming an adenoma called “incidentaloma”. Curiously, an increased number of TH immunoreactive cells have been found in the area of the incidentaloma compared to the normal AL tissue of the same gland. The increased number of TH positive cells within the incidentaloma and the asymptomatic nature of it may suggest a compensatory elevation of local production of DA for being able to inhibit hormone secretion of the adenoma in an autocrine/paracrine manner. Therefore, a new mechanism of DAergic control of pituitary hormone secretion locating within the AL of the human pituitary gland can be hypothesized.