Stimulating Effect of Glycoprotein Hormone Free alpha-Subunit and Daily Gonadotropin Releasing Hormone Treatment on Prolactin Release from 50-Day Ovine Foetal Pituitary Explants

Abstract

The aim of our study was to determine whether free alpha of glycoprotein hormones (free alpha) plays a role in lactotroph function during early pituitary development in the sheep foetus. Detection and quantification of free alpha, luteinzing hormone beta-subunit (LHbeta) and prolactin immunolabelling were determined by immunocytochemistry at days 32, 37, 42, 50 and 63 of gestation. Free alpha- and LHbeta-containing cells were first detected in the ovine foetal pituitary gland on day 37 of gestation, while prolactin-containing cells were first identified on day 42. Analysis of serial sections suggested that free alpha immunoreactive cells were also LHbeta-positive, indicating that free alpha was mainly synthesized by gonadotrophs. In early foetal stages, free alpha occurred in the antero-medio ventral region of the pituitary gland, whereas prolactin-containing cells were more dorsally and more caudally localized. The free alpha-, LHbeta- and prolactin-immunostained area increased markedly between days 50 and 63 of gestation. To evaluate a possible functional relationship between gonadotrophs and lactotrophs, the effects of free alpha or gonadotropin releasing hormone (GnRH) on prolactin release were assayed. Chronic treatment of pituitary explants from male and female 42-day-old ovine foetuses for 8 days with 10-9 or 10-7 M ovine free alpha did not affect prolactin release. By contrast, free alpha administration on pituitary explants from male and female 50-day-old foetuses resulted in enhanced prolactin release. At this age, a daily (2 h per day) treatment with 10-8 M GnRH had similar stimulatory effect to free alpha whereas a 'first day' treatment (24 h on the first day) reduced prolactin release throughout the culture in males and had no effect in females. These results indicate that, despite early detection of free alpha at day 37 in the ovine foetal pituitary, its stimulatory effect on prolactin release occurs from day 50 of gestation, corresponding to the first period of lactotroph development in vivo. A daily treatment with GnRH mimics the effect of free alpha on prolactin release.

Paracrinicity: The Story of 30 Years of Cellular Pituitary Crosstalk

“…A trophic action of GnRH on PRL production has also been noticed in the sheep fetal pituitary ( 114 ). Consistent with the trophic action of gonadotrophs on lactotrophs is that, in the hypogonadal gonadectomised mouse, treatment with GnRH slightly increases PRL synthesis even in adult life ( 115 ).…”

Stimulation of Combinatorial Expression of Prolactin and Glycoprotein Hormone α-Subunit Genes by Gonadotropin-Releasing Hormone and Estradiol-17β in Single Rat Pituitary Cells during Aggregate Cell Culture

“…That such hyperplasia also seems to occur in our aggregate cell culture system and this is associated with coexpression of ␣GSU in the PRL cells is of potential interest for better understanding the pathogenesis of lactotroph hyperplasia and tumor formation. Indeed, ␣GSU may have an autocrine role in these PRL cells because it stimulates the development (25,37) and secretory activity of lactotrophs (38). Coexpression of PRL and ␣GSU, but not ␤GSUs, has been found in certain prolactinomas that also express the GnRH receptor in the same cells (39).…”

Encoding and Decoding Photoperiod in the Mammalian Pars Tuberalis

“…Free αGSU, known to stimulate PRL [71,72] and shown to be seasonally regulated in the PT, has been considered as a potential candidate but differences in expression observed between sheep and hamsters (upregulated or downregulated protein expression in LP, respectively) do not support its role as tuberalin (reviewed in [30]). However, its implication can still be considered, as its potential dimerization with a PT-specific β subunit has not yet been fully investigated.…”