Differential innervation of individual melanotropes suggests a role for nonsynaptic inhibitory regulation of the developing and adult rat pituitary intermediate lobe

GABAB receptor subunit mRNAs are differentially regulated in pituitary melanotropes during development and detection of functioning receptors coincides with completion of innervation

This study examines the developmental expression of GABAB receptor subunits (GABAB(1a), GABAB(1b), GABAB(2)) in the pituitary intermediate lobe using in situ hybridization, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and Western blots. Receptor functionality was studied by baclofen-stimulated GTPgammaS binding. In the adult rat pituitary all three transcripts were detected in melanotropes, but not in glia, of the intermediate lobe. No transcripts of any subunit were detected in the neural lobe. Transcripts of GABAB(1a) and GABAB(1b), but not of GABAB(2), were detected in specific subpopulations of cells in the anterior lobe. All three transcripts were detected in melanotropes on gestational day 18 using in situ hybridization. Reverse transcriptase-polymerase chain reactions comparing postnatal day 2 and adult transcript levels in the neurointermediate lobe support in situ hybridization data that GABAB(1a) mRNA levels do not change, GABAB(1b) levels increase, and GABAB(2) levels decrease as the rat matures. Thus, GABAB receptor subunit transcripts are differentially regulated in melanotropes during development. In the adult rat both GABAB(1) and GABAB(2) proteins were detected in the neurointermediate lobe using Western blotting and in melanotropes by immunohistochemistry. Developmentally, GABAB(1) protein was not detected until postnatal day 7, but was clearly expressed by postnatal day 15 while GABAB(2) protein could not be detected until postnatal day 15. Functional receptors were found in the intermediate lobe at postnatal day 15 and in the adult. The demonstration of transcripts for GABAB(1a), GABAB(1b) and GABAB(2) subunits at gestational day 18 contrasted with the failure to detect any protein before postnatal day 7, suggesting that the regulation of GABAB subunit isoforms occurs differentially at both the transcriptional and translational level as development progresses. The disparity in the regulation of the receptor subunits may suggest that GABAB(1) could have other functions besides being part of the GABAB receptor heterodimer.

Axonal projections from the hypothalamus to the pituitary intermediate lobe in rats during ontogenesis: DiI tracing study

This study has determined ontogenetic schedule of axonal arrival from the hypothalamus in the pituitary intermediate lobe (IL) in rats using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a retrograde tracer. The brains with attached pituitaries were dissected in rats from the 20th embryonic day (E20) to the 20th postnatal day (P20). Anterior lobe was mechanically detached from the IL, material fixed in paraformaldehyde, and DiI crystals were applied on the IL laying on the posterior lobe (PL). The labeling of IL + PL resulted in staining of hypothalamic magnocellular neurons, which send their axons to the PL, and hypothalamic parvocellular neurons contributing to the innervation of the IL. Therefore, the magnocellular neurons were not taken into account when identifying the neurons projecting axons to the IL. Rare fluorescent neurons projecting their axons to the IL were detected as early as on E20 in the ventral part of the periventricular nucleus (Pe) and in the rostral part of the arcuate nucleus. Few DiI-labeled neurons were seen in Pe from P1 to P3. At P5, the fluorescent neurons were accumulated giving rise to the prominent cluster in the Pe, which was enlarged on later stages and occupied all the Pe. In addition to the Pe, fluorescent neurons first appeared in the retrochiasmatic region and around the ventromedial nucleus in young rats. Thus, the axons of hypothalamic neurons of the Pe and mediobasal hypothalamus first arrive in the IL in rats at the end of intrauterine development, although the principal innervation of the IL is the postnatal event.

Expression of granulocyte-macrophage colony stimulating factor (GM-CSF) in male germ cells: GM-CSF enhances sperm motility

The granulocyte-macrophage colony stimulating factor (GM-CSF) is a pleiotropic cytokine capable of stimulating proliferation, maturation and function of hematopoietic cells. Receptors for this cytokine are composed of two subunits, alpha and beta, and are expressed on myeloid progenitors and mature mononuclear phagocytes, monocytes, eosinophils and neutrophils, as well as in other nonhematopietic cells. We have recently demonstrated that bull spermatozoa express functional GM-CSF receptors that signal for increased glucose and Vitamin C uptake. In this study, we analyzed the expression of GM-CSF in bovine and human germ cells and its influence in bovine sperm motility. Reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization and immunoblotting analysis demonstrated that adult bovine and human testes expressed GM-CSF. In addition, immunolocalization studies confirmed the presence of GM-CSF in the germ cell line in bovine and human testes. Computer-assisted evaluation of patterns of sperm motility demonstrated that the addition of GM-CSF enhances several parameters of sperm motility in the presence of glucose or fructose substrates.

The mammalian pituitary intermediate lobe: an update on innervation and regulation

The pituitary intermediate lobe (IL) in mammals is an area of uniform endocrine cells which synthesize and release specific peptide products of the proopiomelanocortin gene. The lobe receives direct synaptic connections onto the endocrine cells from hypothalamic dopaminergic neurons. This review updates information on the dopaminergic as well as the gamma-aminobutyric acid inhibitory neuroregulation for the IL. It also provides a discussion of stimulatory molecules which are likely to affect peptide release, particularly the neurotransmitter serotonin, which may be present via uptake into the dopaminergic nerve terminals. Other stimulatory molecules discussed which are likely to significantly affect peptide secretion are norepinephrine, corticotropin-releasing factor, and several opiate peptides. A new direction of study involves the potential interaction of neurotrophic factors, which are present in all areas of the pituitary, and may be suggested to have a supportive role for the neural elements of the IL. The endocrine cells of the IL and their direct hypothalamic innervation are considered to be an easily accessed peripheral model for study of both neural-endocrine and neurotrophic-target cell interactions.

Glial somatostatin-14 expression in the rat pituitary intermediate lobe: a possible neurotrophic function during development?

Somatostatin-14 was first detected on gestational day 17 in radially-oriented, bipolar cells spanning the width of the intermediate lobe of the rat pituitary. Cells were prominent, and constituted approximately 50% of the lobe area. The presence of vimentin, the cellular shape, and the localization identified these cells as glia. At postnatal day 6, somatostatin-14 and vimentin staining appeared in stellate-shaped cells. This is in agreement with the change from bipolar to stellate shape these glia undergo after the onset of innervation ([13] Gary et al. Int. J. Devl. Neurosci. 13, 555-565, 1995). Glia were more abundant, relative to melanotropes, throughout embryonic and early postnatal development compared to adulthood. Reverse transcription-polymerase chain reaction data showed a high level of prosomatostatin mRNA in the intermediate lobe, compared to the anterior and neural lobes from postnatal day 2 animals, and a significant drop in intermediate lobe content in the adult. The correlation between the number of glia and high expression of somatostatin in neonatal relative to adult tissue, together with the close apposition of incoming axons to the abundant, radially oriented glia during innervation of the lobe, support a neurotrophic function of glia-derived somatostatin.

Heterogeneity in POMC expression among explanted melanotropes decreases with time in culture and bromocriptine treatment

The biosynthetic activity of rat intermediate lobe melanotropes in vivo is inhibited by stimulation of dopamine D2 receptors. Individual melanotropes are innervated differentially by dopaminergic axons and vary in their levels of pro-opiomelanocortin (POMC) mRNA. We tested the hypothesis that placement of the lobe in primary culture, which removes the inhibitory innervation, would increase POMC mRNA levels and abolish the heterogeneity in POMC expression. POMC mRNA levels increased successively in untreated melanotropes when tested on culture Days 10, 16, and 20; however, some heterogeneity in POMC expression persisted. If treated with a D2 receptor agonist (1 microM bromocriptine) from culture Day 1, POMC mRNA levels were decreased significantly throughout the testing period when compared to untreated cells with the same time in culture. Although some melanotropes still expressed high POMC levels, preparations appeared more homogeneous by Day 20. Melanotrope responses were reversible, since POMC mRNA levels were down-regulated by application and up-regulated by withdrawal of a D2 receptor agonist. A short agonist treatment resulted in subpopulations that responded differently to the agonist, possibly representing a mechanism for fine-tuning peptide hormone release.