Growth hormone inhibits growth hormone secretion from the rainbow trout pituitary in vitro

Pituitary Hormones mRNA Abundance in the Mediterranean Sea Bass Dicentrarchus labrax: Seasonal Rhythms, Effects of Melatonin and Water Salinity

In fish, most hormonal productions of the pituitary gland display daily and/or seasonal rhythmic patterns under control by upstream regulators, including internal biological clocks. The pineal hormone melatonin, one main output of the clocks, acts at different levels of the neuroendocrine axis. Melatonin rhythmic production is synchronized mainly by photoperiod and temperature. Here we aimed at better understanding the role melatonin plays in regulating the pituitary hormonal productions in a species of scientific and economical interest, the euryhaline European sea bass Dicentrarchus labrax. We investigated the seasonal variations in mRNA abundance of pituitary hormones in two groups of fish raised one in sea water (SW fish), and one in brackish water (BW fish). The mRNA abundance of three melatonin receptors was also studied in the SW fish. Finally, we investigated the in vitro effects of melatonin or analogs on the mRNA abundance of pituitary hormones at two times of the year and after adaptation to different salinities. We found that (1) the reproductive hormones displayed similar mRNA seasonal profiles regardless of the fish origin, while (2) the other hormones exhibited different patterns in the SW vs. the BW fish. (3) The melatonin receptors mRNA abundance displayed seasonal variations in the SW fish. (4) Melatonin affected mRNA abundance of most of the pituitary hormones in vitro; (5) the responses to melatonin depended on its concentration, the month investigated and the salinity at which the fish were previously adapted. Our results suggest that the productions of the pituitary are a response to multiple factors from internal and external origin including melatonin. The variety of the responses described might reflect a high plasticity of the pituitary in a fish that faces multiple external conditions along its life characterized by marked daily and seasonal changes in photoperiod, temperature and salinity.

A Comparative Update on the Neuroendocrine Regulation of Growth Hormone in Vertebrates

Growth hormone (GH), mainly produced from the pituitary somatotrophs is a key endocrine regulator of somatic growth. GH, a pleiotropic hormone, is also involved in regulating vital processes, including nutrition, reproduction, physical activity, neuroprotection, immunity, and osmotic pressure in vertebrates. The dysregulation of the pituitary GH and hepatic insulin-like growth factors (IGFs) affects many cellular processes associated with growth promotion, including protein synthesis, cell proliferation and metabolism, leading to growth disorders. The metabolic and growth effects of GH have interesting applications in different fields, including the livestock industry and aquaculture. The latest discoveries on new regulators of pituitary GH synthesis and secretion deserve our attention. These novel regulators include the stimulators adropin, klotho, and the fibroblast growth factors, as well as the inhibitors, nucleobindin-encoded peptides (nesfatin-1 and nesfatin-1-like peptide) and irisin. This review aims for a comparative analysis of our current understanding of the endocrine regulation of GH from the pituitary of vertebrates. In addition, we will consider useful pharmacological molecules (i.e. stimulators and inhibitors of the GH signaling pathways) that are important in studying GH and somatotroph biology. The main goal of this review is to provide an overview and update on GH regulators in 2020. While an extensive review of each of the GH regulators and an in-depth analysis of specifics are beyond its scope, we have compiled information on the main endogenous and pharmacological regulators to facilitate an easy access. Overall, this review aims to serve as a resource on GH endocrinology for a beginner to intermediate level knowledge seeker on this topic.

Bipotential effects of estrogen on growth hormone synthesis and storage in vitro

Increased pulses of serum GH coincide with rising estrogens during the reproductive cycle, suggesting estrogen regulation. However, there is lack of agreement about estrogen's direct effects on the pituitary. Pituitaries from cycling female rats were dispersed and plated for 24 h in defined media containing vehicle or 0.001-250 nm 17beta-estradiol. Estrogen (0.01-10 nm) increased the percentages of GH antigen-bearing cells in the anterior pituitary significantly (1.3- to 1.6-fold) and 0.01-1 nm concentrations also stimulated significant increases in GH mRNA-bearing cells and in the integrated OD for GH mRNA. However, 100-250 nm either had no effect or, inhibitory effects on the area of label for GH mRNA. To test estrogen's effects on expression of GHRH receptors, cultures were stimulated with biotinylated analogs of GHRH and target cells detected by affinity cytochemistry. Estrogen increased GHRH target cells in populations from rats in all stages of the cycle tested. Basal expression of GHRH target cells declined at metestrus. Cultures treated with 0-1 nm estrogen were then dual labeled for bio-GHRH followed by immunolabeling for GH with the antirabbit IgG-ImmPRESS peroxidase polymer. Over 98% of GH cells bound GHRH and 90-96% of GHRH-bound cells contained GH in all treatment groups. Thus, low concentrations of estrogen may stimulate expression of more cells with GH proteins, biotinylated GHRH binding sites, and GH mRNA, whereas high concentrations have no effect, or may reduce GH mRNA. These bipotential effects may help explain the different findings reported in the literature.

Medical management of pituitary adenomas: structural and ultrastructural changes

The morphology of the various pituitary cell types is highly dynamic and allows recognition of many cellular functions. Most pituitary cells show morphologic changes that reflect stimulation or inhibition by hormones. Drugs have also been shown to alter the morphology of several pituitary tumor types, allowing a measure of therapeutic efficiency and a careful dissection of the mechanisms of action of various medical therapies. In some cases, these morphologic alterations can evoke diagnostic problems.

Dopaminergic innervation of the rainbow trout pituitary and stimulatory effect of dopamine on growth hormone secretion in vitro

To elucidate which factors regulate growth hormone (GH) secretion in rainbow trout, dopaminergic innervation of the rainbow trout pituitary along with the action of dopamine in vitro, were studied. Brains with attached pituitaries were double-labeled for putative dopaminergic neuronal fibers and somatotropes, using fluorescence immunohistochemistry. A direct dopaminergic innervation to the proximal pars distalis (PPD) with dopaminergic fibers terminating adjacent to somatotropes was demonstrated. Growth hormone secretion from whole pituitaries was measured in perifusate using a homologous GH-RIA. Dopamine (DA; 10(-7)-2x10(-6) g ml(-1)) increased basal GH secretion, with the GH secretion normalizing again after the DA exposure was halted. When pituitaries were pre-treated with somatostatin-14 (SRIF-14; 10(-12)-10(-9) g ml(-1)), before being exposed to different doses of DA, there was an inhibition of GH secretion which was not reversed after treatment of SRIF-14 was halted, unless DA was added. It is concluded that dopamine can function as a GH secretagogue in the rainbow trout pituitary gland.