Neurotrophins in the developing and adult primate adenohypophysis: a new pituitary hormone system?

Expression of neurotrophins and of the low-affinity neurotrophin receptor p75 was examined immunocytochemically in pituitary glands of twelve developing and adult macaques, ranging in age from fetal day 100 through age 5 years. Neurotrophins were identified by labeling with a rabbit polyclonal antiserum raised against purified mouse nerve growth factor, which recognizes brain-derived neurotropic factor and neurotrophin-3 as well. During pituitary morphogenesis, neurotrophins were present in epithelial cells distributed throughout all divisions of the anterior pituitary (pars distalis, pars intermedia, and pars tuberalis). Near term and in the adult, neurotrophin-immunoreactive cells were fewer in number and their distribution was limited to the pars distalis and pars tuberalis. A monoclonal antibody against the human neurotrophin receptor p75 heavily labeled mesenchymal boundary structures and blood vessels in the developing gland, and several populations of glial-like cells with a presumed paracrine function (folliculostellate cells in the pars distalis, and pituicytes and tanycytes in the neural lobe and infundibulum, respectively) as well as axons innervating the portal vasculature in postnatal specimens. These complementary patterns of neurotrophin and receptor expression suggest a possible inductive role for neurotrophins in pituitary morphogenesis and in the establishment of hypothalamic neural and hormonal control of pituitary function. In the adult anterior pituitary, examined using double-label immunocytochemistry for neurotrophins and conventional anterior-pituitary hormones, neurotrophins did not colocalize with human prolactin, human adrenocorticotropic hormone, recombinant human growth hormone, or the beta subunits of human luteinizing hormone, human follicle-stimulating hormone, or human thyrotropin. Neurotrophin-containing cells therefore appear to be a distinct population, suggesting novel paracrine or endocrine functions for this family of neuropeptides.