The ontogeny of neuronal and glial elements within the intermediate lobe of the rat pituitary: Significance for the regulation of melanotrope proopiomelanocortin synthesis and peptide release

Melanotropes of the rat pituitary intermediate lobe are regulated by axons from neurons lying in the mediobasal hypothalamus. Melanotropes produce proopiomelano-cortin, which is processed to {dollar}alpha{dollar}-melanocyte stimulating hormone and {dollar}beta{dollar}-endorphin. This study was designed to temporally define developmental establishment of pre- and post-synaptic components of neuronal regulation of melanotropes. Onset of dopaminergic innervation was determined immunohistochemically, occurring at late postnatal day three or early postnatal day four. HPLC analysis of neurointermediate lobe dopamine content confirmed the immunohistochemical data. Onset of innervation decreased melanotrope proliferation as measured by ({dollar}sp3{dollar}H) thymidine incorporation. Dopamine D{dollar}sb2{dollar} receptor isoforms were demonstrated immunohistochemically with the 444 amino acid D{dollar}sb2{dollar} receptor isoform observed at postnatal day one, while the 415 amino acid isoform was not detected until postnatal day four. Ligand binding analysis with ({dollar}sp3{dollar}H) spiperone demonstrated that these early postnatal receptors were functional.; A second aim of these studies was to determine normal adult distribution of glial cells and to characterize their ontogeny within the intermediate lobe. Immunohistochemical localization of glial fibrillary acidic protein demonstrated a rostro-caudal gradient of glial cell distribution. Further analysis identified these cells as a sub-population of S-100 immunopositive glial cells, and glial fibrillary acidic protein expression could be induced by lactation and adrenalectomy, or suppressed by salt-loading. Immunohistochemical analyses of the developing intermediate lobe using antisera against vimentin, S-100, and glial fibrillary acidic protein yielded intriguing results. Immature glia initially express vimentin and are radially oriented within the lobe, similar to glia described during cortical histogenesis. At postnatal day six, the developing glia begin expressing S-100, and a subpopulation of these cells begin expressing glial fibrillary acidic protein by postnatal day 25.