Variation in pituitary expression of mRNAs encoding the putative inhibin co-receptor (betaglycan) and type-I and type-II activin receptors during the chicken ovulatory cycle

Abstract

Secretion of LH and FSH from the anterior pituitary is regulated primarily by hypothalamic GnRH and ovarian steroid hormones. More recent evidence indicates regulatory roles for certain members of the transforming growth factor (TGF ) superfamily including inhibin and activin. The aim of this study was to identify expression of mRNAs encoding key receptors and ligands of the inhibin/activin system in the hen pituitary gland and to monitor their expression throughout the 24-25-h ovulatory cycle. Hens maintained on long days (16 h light/ 8 h dark) were killed 20, 12, 6 and 2 h before predicted ovulation of a midsequence egg (n=8 per group). Anterior pituitary glands were removed, RNA extracted and cDNA synthesized. Plasma concentrations of LH, FSH, progesterone and inhibin A were measured. Realtime quantitative PCR was used to quantify pituitary expression of mRNAs encoding betaglycan, activin receptor (ActR) subtypes (type I, IIA), GnRH receptor (GnRH-R), LH subunit, FSH subunit and GAPDH. Levels of mRNA for inhibin/activin A and B subunits, inhibin subunit, follistatin and ActRIIB mRNA in pituitary were undetectable by quantitative PCR (<2 amol/reaction). Significant changes in expression (P<0·05) of ActRIIA and betaglycan mRNA were found, both peaking 6 h before ovulation just prior to the preovulatory LH surge and reaching a nadir 2 h before ovulation, just after the LH surge. There were no significant changes in expression of ActRI mRNA throughout the cycle although values were correlated with mRNA levels for both ActRIIA (r=0·77; P<0·001) and betaglycan (r=0·45; P<0·01). Expression of GnRH-R mRNA was lowest 20 h before ovulation and highest (P<0·05) 6 h before ovulation; values were weakly correlated with betaglycan (r=0·33; P=0·06) and ActRIIA (r=0·34; P=0·06) mRNA levels. Expression of mRNAs encoding LH and FSH subunit were both lowest (P<0·05) after the LH surge, 2 h before ovulation. These results are consistent with an endocrine, but not a local intrapituitary, role of inhibin-related proteins in modulating gonadotroph function during the ovulatory cycle of the hen, potentially through interaction with betaglycan and ActRIIA. In contrast to mammals, intrapituitary expression of inhibin/activin subunits and follistatin appears to be extremely low or absent in the domestic fowl.

GnRH and GnRH receptors: distribution, function and evolution

“…D . F E R N A L D GnRH-Rb1 and GnRHb2 are less widely expressed in reptiles (Ikemoto et al, 2004) and birds (Sun et al, 2001;Lovell et al, 2005), while in mammals, GnRH-Ra2 and Rb1 exhibit wider distribution in the whole brain (Millar, 2003;Millar et al, 2004). Given that the GnRH-Rs do not show high fidelity, both GnRH-Rs are likely to be involved in the neural networks controlling reproduction.…”

“…For example, the distribution of GnRH‐Ra1 and Rb2 is widespread throughout the brain in teleosts (Illing et al , 1999; Madigou et al , 2000; Okubo et al , 2000 b , 2001, 2003; Bogerd et al , 2002; Peter et al , 2003; Gonzalez‐Martinez et al , 2004; Moncaut et al , 2005; Parhar et al , 2005; Soga et al , 2005; Chen & Fernald, 2006) and in amphibians (Troskie et al , 2000; Wang et al , 2001). However, GnRH‐Rb1 and GnRHb2 are less widely expressed in reptiles (Ikemoto et al , 2004) and birds (Sun et al , 2001; Lovell et al , 2005), while in mammals, GnRH‐Ra2 and Rb1 exhibit wider distribution in the whole brain (Millar, 2003; Millar et al , 2004). Given that the GnRH‐Rs do not show high fidelity, both GnRH‐Rs are likely to be involved in the neural networks controlling reproduction.…”

“…For example, GnRH1 projecting into the pituitary acts on GnRH‐Ra1 and GnRH‐Rb2 in an African cichlid A. burtoni (Chen & Fernald, 2006), while both GnRH1 and GnRH2 project to the pituitary and act through only GnRH‐Rb2 in the bullfrog Rana catesbeiana (Wang et al , 2001). It is a general rule that only one GnRH receptor subtype in each species regulates gonadotropin release in the pituitary, such as GnRH‐Rb2 in European sea bass Dicentrarchus labrax (L.) and African cichlid A. burtoni (Au et al , 2006; Moles et al , 2007), GnRH‐Ra1 in goldfish C. auratus and Nile tilapia O. niloticus (Habibi, 1991; Parhar et al , 2005), GnRH‐Rb1 in birds (Lovell et al , 2005) and GnRH‐Ra2 in mammals (Karges et al , 2003; Ulloa‐Aguirre et al , 2004). These studies suggest that GnRH receptors may be selected by their ability to detect synchronous release of GnRH in the pituitary.…”

Identification of a Novel Pituitary-Specific Chicken Gonadotropin-Releasing Hormone Receptor and Its Splice Variants1

“…Further studies will be needed to confirm this hypothesis. In addition to controlling sexual maturation, GnRH can also affect daily ovulatory cycles, and a transient increase in GNRHR1 mRNA was observed with a maximum reached 6 h before ovulation [32]. We did not measure levels of chicken GNRHR2 expression during an ovulatory cycle, but it is not excluded that daily fluctuations in gene expression may occur.…”

Differential expression of mRNAs encoding the putative inhibin co-receptor (betaglycan) and activin type-I and type-II receptors in preovulatory and prehierarchical follicles of the laying hen ovary

“…Various factors have been shown to modulate secretion of inhibin A and progesterone from cultured chicken F1-F3 granulosa cells including FSH, LH, TGF , insulin-like growth factor-I and activin A (Onagbesan & Peddie 1995, Lovell et al 2002a. Changes in plasma inhibin A mirror the F1 granulosa inhibin A content, peaking around the time of the LH surge (Lovell et al 2000) and may be involved in endocrine regulation of pituitary gonadotroph function, through interaction with betaglycan and ActRIIA as described by Lovell et al (2005). The fact that the transition from F2 to F1 coincides with a significant increase in granulosa betaglycan and ActRIIA expression, with no alteration in ActRI, suggests that the F1 granulosa itself may be a key target for inhibin action in an autoregulatory feedback manner.…”