The pars tuberalis of the rhesus monkey secretes luteinizing hormone

Involvement of Luteinizing Hormone in Alzheimer Disease Development in Elderly Women

Alzheimer disease (AD) is a slow progressive neurodegenerative disease that affects more elderly women than elderly men. It impairs memory, typically progresses into multidomain cognitive decline that destroys the quality of life, and ultimately leads to death. About 5.3 million older Americans are now living with this disease, and this number is projected to rise to 14 million by 2050. Annual health-care costs in the United States alone are projected to increase to about US$1.1 trillion by 2050. The initial theory that decreasing estrogen levels leads to AD development in postmenopausal women has been proven inconclusive. For example, Women's Health Research Initiative Memory Study and the population-based nested case-control study have failed to demonstrate that estrogen/progesterone (hormone replacement therapy [HRT]) or estrogen replacement therapy could prevent the cognitive decline or reduce the risk of AD. This led to the realization that AD development could be due to a progressive increase in luteinizing hormone (LH) levels in postmenopausal women. Accordingly, a large number of studies have demonstrated that an increase in LH levels is positively correlated with neuropathological, behavioral, and cognitive changes in AD. In addition, LH has been shown to promote amyloidogenic pathway of precursor protein metabolism and deposition of amyloid β plaques in the hippocampus, a region involved in AD. Cognate receptors that mediate LH effects are abundantly expressed in the hippocampus. Reducing the LH levels by treatment with gonadotropin-releasing hormone agonists could provide therapeutic benefits. Despite these advances, many questions remain and require further research.

The Pars tuberalis of the monkey (Macaca fascicularis) hypophysis: cell types and hormone expression

The pars tuberalis (Pt) of most mammalian species contains specific cells which are structurally and functionally different from the pars distalis (Pd) cells. Pt-specific cells possess melatonin receptors and reveal morphological changes dependent on the duration of the photoperiod. Furthermore, in hamsters the transmission of photoperiodic stimuli to the endocrine system is influenced by melatonin, an effect which is likely to be mediated by Pt-specific cells. In monkeys, however, only little is known about this cell type. Therefore, we studied the ultrastructural differentiation of Pt-specific cells and describe the expression of different hormones and their mRNA by immunohistochemistry and in situ hybridization. Apparently the Pt consists of (1) cells similar to gonadotropic cells of the Pd, (2) folliculostellate cells and (3) a cell population which is morphologically and functionally clearly distinct from all other cell types found in the Pd. Morphologically they resemble the Pt-specific cells found in other species. Regarding the expression of secretory products there is evidence that they transcribe and translate the beta-TSH subunit. Although there is a strong signal for the mRNA of the common alpha-chain, protein staining is much weaker. POMC mRNA is expressed in the Pt while there is no evidence for PRL mRNA. The present results lead to the conclusion that the Pt of the monkey contains Pt-specific cells which express different hormonal subunits as was already shown for other species. In context with previous findings of melatonin receptors in the monkey Pt further investigations are necessary to establish the possible role of Pt-specific cells in the photoperiod-dependent generation of endocrine rhythms.

Targeting of gonadotropin-releasing hormone axons from preoptic area grafts to the median eminence

Implantation of normal GnRH neurons can reverse many of the reproductive deficiencies that characterize hypogonadal (hpg) mice. Since the GnRH axons follow a stereotyped trajectory to their target we investigated the possibility that host brain regions adjacent to the graft might provide signals that induced this directional growth. The role of the adenohypophysis in GnRH axonal outgrowth was studied in mice with co-grafts of fetal preoptic area (POA) and pituitary and in hypophysectomized hosts. When fetal pituitaries were grafted together with the POA, immunoreactive GnRH fibers did enter the glandular tissue but they also grew into the host median eminence. Surgical removal of the pituitary of hpg hosts prior to POA graft placement was also compatible with GnRH innervation of the host median eminence although in some individuals that innervation pattern was confined to the more caudal aspects. The results of these two experiments suggest that the anterior pituitary gland may be an attractive target for GnRH axons but that this tissue is not essential for directed GnRH axonal outgrowth to its target. To determine if the median eminence itself could direct the growth of GnRH axons, co-grafts of POA and a fetal medial basal hypothalamic (MBH) block, which was predominantly median eminence, were made. Immunocytochemistry showed that an intragraft mini-median eminence was formed with a highly organized and robust GnRH innervation. Ultrastructural analysis indicated that these axons terminated near fenestrated capillaries. However, even under these conditions some GnRH axons exited into the host median eminence. It now seems likely that a cellular component of the median eminence can provide a signal to attract GnRH axons. Whether this signal is produced by the specialized ependymal cells, by the endothelia, or by meningeal (pial) components must now be tested.

Localization of luteinizing hormone beta-mRNA by in situ hybridization in the sheep pars tuberalis

The localization of luteinizing hormone beta (LH beta)-mRNA was studied by in situ hybridization in the pars tuberalis of sheep using a homologous sheep double-stranded 32P- or 35S-cDNA. The labelled cDNA probe detected one mRNA sequence in the pars tuberalis by Northern blot analysis; this sequence was similar to that detected in the pituitary. In situ, the labelling of LH beta-mRNA in the horizontal and sagittal tissue sections was found throughout the pars tuberalis. This labelling was prevented by adding an excess of cold probe or treating the sections by ribonuclease before in situ hybridization. Controls showed a labelling in the pars distalis, but not in the median eminence, hypothalamus, cerebral cortex and liver sections. Double labelling by in situ hybridization followed by immunohistochemistry using a specific LH beta-antiserum indicated that the labelling of LH beta-mRNA appeared more intense in LH-containing cells that were found only in the ventral part of the pars tuberalis. These results suggest that the entire pars tuberalis is able to produce the LH beta subunit, but that the level of translation greatly varies according to the location of the cells.

Luteinizing hormone in the bovine pars tuberalis: secretion in response to luteinizing hormone releasing hormone and intracellular isoforms

The objective of this study was to examine the physiological characteristics of gonadotropes in the bovine (b) pars tuberalis as assessed by their ability to release Luteinizing Hormone (LH) in response to LH-Releasing Hormone (LHRH) and the intracellular distribution of LH isoforms. At slaughter, the stalk median eminence and associated pars tuberalis as well as the anterior pituitary gland were collected from each of 7 castrate males. Each stalk median eminence and pituitary gland was mid-sagitally sectioned and weighed. One half of each tissue was immediately frozen and subsequently homogenized to determine the intracellular distribution of bLH isoforms. Tissue extracts were desalted by flow dialysis against water and chromatofocused on pH 10.5-7.0 gradients. The remaining half of the pituitary was sliced with a Staddie-Riggs slicer. The pituitary slices and the remaining half of the stalk median eminence were perifused (0.1 ml/min) for a total of 360 min with effluent samples (1.0 ml) collected every 10 min. At 130 min tissues were stimulated with 5 x 10(-8) M LHRH. Concentrations of LH in the effluent samples and the fractions collected from chromatofocusing were determined by radioimmunoassay. The release of LH in response to LHRH was 43.9% and 47.0% above basal secretion for the pars tuberalis and pituitary, respectively, suggesting similar degrees of responsiveness. Pars tuberalis and pituitary extracts resolved into nine LH isoforms during chromatofocusing and were coded with letters beginning with the most basic form. No differences (P greater than .05) were observed in distribution of LH isoforms between the pars tuberalis and the pituitary gland.(ABSTRACT TRUNCATED AT 250 WORDS)

The mammalian hypophysial pars tuberalis: a comparative immunocytochemical study

Despite its occurrence in most vertebrate species, the function of the hypophysial pars tuberalis (PT) remains obscure. Recent immunocytochemical studies have demonstrated the presence of hormone-containing cells in the few species studied. In the present study the secretory cell composition of the PT was characterized in a variety of mammals using immunocytochemistry. Species studies were the mouse, rat, guinea pig, rabbit, sheep, rhesus monkey, baboon, and human. Antisera were chosen on the basis of their ability to identify a distinct cell population in the pars distalis. A total of 21 antisera were used to identify GH, PRL, ACTH, beta-endorphin, LH, FSH, and TSH. Gonadotropes were identified in the PT of all eight species and were the predominant immunoreactive cell type in the human, baboon, rhesus monkey, sheep, guinea pig, rabbit, and mouse. Thyrotropes were detected in all species except the sheep. They were the predominant cell type in the rat but were less common than gonadotropes in other species. No other secretory cell types were found, with the exception of occasional somatotropes and mammotropes in some human specimens, and small clusters of opiocorticotropes in the guinea pig. Thus the general pattern in the mammalian PT is the presence of gonadotropes and thyrotropes and the absence of other pituitary cell types. In the human, baboon, and rat, all PT parenchymal cells can be identified immunocytochemically. However, in the rhesus monkey, sheep, guinea pig, rabbit, and mouse, the majority of PT cells do not react with any antisera, and thus their function is unknown. Follicles are common in the PT of most mammalian species, however, the luminal contents do not react with antisera to adenohypophysial hormones.

Luteinizing hormone and follicle-stimulating hormone production in the pars tuberalis of hypophysectomized rats

Luteinizing hormone and follicle-stimulating hormone were identified by immunocytochemistry in the same cells of the pituitary pars tuberalis, following hypophysectomy in the rat. A marked hyperplasia and hypertrophy of these cells occurred in response to hypophysectomy. These findings are discussed in light of the potential for hormone production by pars tuberalis gonadotropes following hypophysectomy performed as part of an experimental protocol, as well as that performed for palliation of human metstatic breast cancer.