Differential In Vitro Secretion of Gonadotropin‐Releasing Hormone (GnRH) and [Hydroxyproline⁹]GnRH from the Rat Hypothalamus During Postnatal Development

Abstract

The differential secretion of gonadotropin-releasing hormone (GnRH) and [hydroxyproline9]GnRH (HypGnRH) has been recently reported from the adult rat hypothalamus. We report here in vitro cosecretion of HypGnRH and GnRH by the hypothalamus of 2-45 day-old-rats and provide evidence that they are differentially regulated throughout development. The secretion of both forms of GnRH was increased in a dependent manner during depolarization by high K+ solutions, and was stimulated by forskolin and 12-O-tetradecanoylphorbol-13-acetate (TPA), activators of adenylate cyclase and protein kinase C pathways, respectively. The proportion of HypGnRH in the release of GnRH-like peptides remained stable and high (33-40%) under basal and K+-induced conditions until days 13 and 21, respectively. By contrast, the proportion of HypGnRH in the total GnRH-like content of the developing hypothalamus continuously decreased (from 37% to 14%). Similarly, the proportion of HypGnRH: total GnRH-like material released remained stable in TPA- (30%) and forskolin- (50%) induced secretion until postnatal day 8. Evaluation of release over tissue store ratios revealed a 1.3-to 2.8-fold higher release of HypGnRH compared to GnRH according to the different secretions and postnatal periods examined. The preferential recruitment of HypGnRH was maintained under basal and K+ conditions during postnatal development, but it disappeared under TPA stimulation from day 13 onwards. After forskolin stimulation, the preferential mobilization of HypGnRH was markedly reduced from day 2 to day 13 but recovered its high perinatal level during puberty. Taken together, our results support the hypothesis that HypGnRH may play a specific role in development. In addition, a specific function of this peptide taking place during puberty through the activation of the adenylate cyclase pathway is suggested.

Molecular Polymorphism of Native Gonadotropin-Releasing Hormone (GnRH) Is Restricted to Mammalian GnRH and [Hydroxyproline⁹] GnRH in the Developing Rat Brain

“…Our data confirm that the hypothalamic GnRH-like content increased with advancing age in both sexes, reaching higher amounts in males than in females [38]. This increase was greater in the case of mGnRH than with mHypGnRH, and the relative proportion of mHypGnRH decreased in the developing hypothalamus [34]. The hydroxylated and non-hydroxylated forms of [Gly 11 ] precursors and their respective processed peptides (free-acid and native decapeptides) detected were used to examine the biosynthetic pathway of mGnRH and mHypGnRH by comparing [Gly 11 ] precursor to processed peptide molar ratios between hydroxylated and non-hydroxylated molecules.…”

“…But, in addition to a previous study [34], we established here that both decapeptides are present at an early developmental stage in association with some of their catabolites and precursors which have been already identified in the adult animals [15, 16] and in the GT1 cell line [35, 36]. The chromatography of large amounts of tissues and the use of two combined HPLC steps made it possible to detect these additional metabolic compounds which mostly coelute with mHypGnRH or mGnRH on a single RP-HPLC [37].…”

“…This processing must involve a rather specific mechanism, since precursor processing into the free-acid form is similar in both decapeptide families. It has been reported that the relative proportions of mGnRH and mHypGnRH present in the hypothalamic secretions do not parallel those in the tissue during development [34]. This results in high proportions of released mHypGnRH in comparison with the total GnRH-like peptides which are maintained during development by the preferential mobilization of the releasable pool of mHypGnRH.…”

Structure and Biological Activity of Gonadotropin-Releasing Hormone Isoforms Isolated from Rat and Hamster Brains

“…It is reported that [Hyp 9 ]GnRH is released from the hypothalamus by different mechanisms in adult [39]and developing rats [40]and can potentially act on the secretion of human chorionic gonadotropin secretion from perifused placental cells [41]. Nevertheless, its actions are not fully understood.…”

Physiology of the Adult Gonadotropin-Releasing Hormone Neuronal Network

“…178 The physiological significance of (Hyp 9 ) GnRH remains unknown. Gautron and colleagues have shown that as much as 10% of GnRH-like peptide in the hypothalamus exists as (Hyp 9 ) GnRH in adult rats.…”