Changes in processes downstream of the hypothalamus are associated with seasonal follicle development in a songbird, the dark-eyed junco (Junco hyemalis)

https://doi.org/10.1016/j.ygcen.2018.10.012Get rights and content

Highlights

  • Study of mechanisms regulating seasonal breeding in females is understudied.

  • We studied wild female dark-eyed juncos in the time period preceding reproduction.

  • Prior to breeding, female juncos, both the pituitary and gonads are responsive to GnRH.

  • Expression of hormone receptors change as birds approach laying.

  • Multiple components of the HPG axis downstream of the hypothalamus are involved.

Abstract

Mechanisms related to seasonal reproductive timing in vertebrates have received far more study in males than in females, despite the fact that female timing decisions dictate when rearing of offspring will occur. Production and release of gonadotropin-releasing hormone (GnRH) by the hypothalamus stimulates the pituitary to secrete gonadotropins, initiating the beginning stages of gonadal recrudescence and production of the sex steroids, testosterone and estradiol, which are necessary to prime the liver for secretion of yolk precursors in breeding female birds. While stimulation by the hypothalamus can occur during the pre-breeding period, egg development itself is likely regulated downstream of the hypothalamus. We used GnRH challenges to examine variation in breeding-stage-specific patterns of pituitary and ovarian responsiveness in free-living female dark-eyed juncos (Junco hyemalis) and also examined the ovary and liver for variation in mRNA expression of candidate genes. Baseline LH levels increased during the transition from pre-breeding to egg-development, however no significant difference was observed in post-GnRH injection levels for LH or sex steroids (testosterone and estradiol). Interestingly, a stage by time-point interaction was observed, with post-GnRH LH levels increasing over baseline during the pre-breeding stage, but not during the egg-development stage. We observed a decrease in liver mRNA expression of estradiol receptor-alpha, and glucocorticoid and mineralocorticoid receptors and a decrease in glucocorticoid receptor expression levels in the ovary. A decline in FSH receptor expression across stages was also observed in the ovary. Combined, our data suggest seasonal variation in female’s sensitivity to signals of HPG activity and energetic or stress signals. These data provide additional insight into the physiological mechanisms regulating onset of clutch initiation.

Introduction

Most temperate-zone animals breed seasonally to ensure that offspring are born at an optimal time for rearing and survival (Baker, 1938, Murton and Westwood, 1977). Before young arrive, parents must undergo extensive physiological and behavioral preparation for breeding. Often, preparatory events must be initiated well in advance of seasonal increases in food availability (Gibb, 1950).

In most seasonally breeding species, environmental cues are perceived and transduced at the level of the hypothalamus, where gonadotropin-releasing hormone (GnRH) neurons trigger an endocrine cascade resulting in gonadal activation (Dawson, 2008, Jacobs and Wingfield, 2000). More specifically, GnRH triggers secretion of gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), from the anterior pituitary. LH stimulates the synthesis and release of steroids such as testosterone (T) and estradiol (E2) from the gonads (Adkins-Regan, 2008, Wingfield, 2012). These hormones are then transported to target tissues where they bind to receptors, thus shaping physiological, morphological, and behavioral traits. While the above sequence is fairly well established, particularly in males, it is less well known how changes in sensitivity of tissues downstream of the hypothalamus may influence the final transition leading to female clutch initiation in oviparous species (Caro, 2012). This investigation seeks to document whether changes in sensitivities at one or more levels downstream of the hypothalamus are observed as females transition from a non-breeding status in early spring into the onset of egg development.

Here, we asked whether female pituitary and gonadal hormonal responsiveness to stimulation with gonadotropin-releasing hormone (GnRH) changes from pre-breeding to egg-development. In addition, we also assessed whether expression of key genes in the ovary and liver are important for responding to endocrine signal changes during the transition into a reproductively mature state. The ovary is the site of follicular maturation and the liver is necessary for the production of yolk-precursors utilized in the developing follicle (Williams, 2012). Specifically, we asked whether expression for reproductive hormone receptors on the ovary (LHR, FSHR) and the liver (ERα) increase during the transition to breeding maturity, which would likely increase responsiveness to these hormones following stimulation with GnRH. Finally, we also assessed whether expression of the two main receptors for corticosterone (CORT), mineralocorticoid receptor (MR), which binds CORT with high affinity, and the low-affinity glucocorticoid receptor (GR), (De Kloet et al., 1998, Funder, J.W., 1997), display increases in expression during the transition to breeding. Reproduction is energetically expensive, and glucocorticoid hormones, which play an essential role in metabolism and energy regulation (Landys et al., 2006, Sapolsky et al., 2000) may play an integral role in the ability of the ovary and liver to be responsive to a seasonal increase in baseline circulating glucocorticoids leading up to reproduction (Romero, 2002). Alternatively, an increase in expression of these hormone receptors, particularly the low-affinity GR, may also increase the sensitivity of these tissues to stress-induced levels of CORT (Abraham et al., 2013, Bambino and Hsueh, 1981, Denari and Ceballos, 2006, Hsueh and Erickson, 1978, Kwok et al., 2007, Lattin et al., 2011), which may enable a delayed investment in and maturation of reproductive tissues when conditions are sub-optimal (Lattin et al., 2016, Wingfield et al., 1998).

Section snippets

Study species

The dark-eyed junco (Junco hyemalis) is a seasonally breeding songbird commonly found throughout North America (Nolan, 2002). Dark-eyed juncos are an excellent model organism because they have been studied extensively, are abundant, and are relatively easy to capture. Juncos are ground-nesters with clutches ranging between 3 and 5 eggs (mean and median = 4) per nest attempt (Nolan, 2002). Our field site was located in the Black Hills National Forest, South Dakota, USA, where juncos are observed

Breeding stage and LH responsiveness to GnRH

There was a significant main effect of time point (pre-injection or 15 min post injection) (F1,51 = 13.6, p < 0.001) and of breeding stage (pre-breeding or egg development) (F1,40 = 11.1, p < 0.010) on LH levels. There was also a significant interaction between time point and breeding stage (F1,51 = 4.3, p = 0.044) on LH levels. Post-hoc pairwise comparisons within each breeding stage revealed that following a GnRH challenge, females significantly elevated LH during the pre-breeding (t = −5.53,

Discussion

Despite knowledge that females play the critical role in determining onset of reproduction (Ball and Ketterson, 2008, Caro et al., 2009), research on the mechanisms regulating seasonal reproduction in songbirds has continued to focus more on males than on females (Caro, 2012, Dawson, 2015, Dawson, 2003). Here, we focused on female birds during the time period from an early reproductive stage prior to rapid yolking of follicles, to full reproductive status when follicles were yolking. While we

Acknowledgements

Thank you to Emily Bertucci for assistance with 2016 data collection. Also, thank you to the 2009 field assistants: Rachel Maranto, Matthew Boser, Connor Wood, and Elizabeth Schultz. Thank you to the Black Hills National Forest, and Jim and Connie Gorsuch for access to the 2009 field sites. Tony Williams kindly provided guidance on measuring VLDL levels. All procedures used in this study were conducted under the appropriate state and federal permits and approved by the NDSU and Bloomington

Competing Interests

The authors declare no competing or financial interests.

Funding

We gratefully acknowledge the following funding sources for support during this work: the National Science Foundation (NSF IOS-1257527, IOS-1738591 to TJG; NSF IOS-0820055 to EKD), South Dakota Ornithologists’ Union Nathaniel R. Whitney Jr. Memorial Research Grant, and the North Dakota State University Department of Biological Sciences.

References (80)

  • T.J. Greives et al.

    Early spring sex differences in luteinizing hormone response to gonadotropin releasing hormone in co-occurring resident and migrant dark-eyed juncos (Junco hyemalis)

    Gen. Comp. Endocrinol.

    (2016)
  • R.A. Hinde et al.

    The effect of male song on an estrogen-dependent behavior pattern in the female canary (Serinus canarius)

    Horm. Behav.

    (1976)
  • A.J.W. Hsueh et al.

    Glucocorticoid inhibition of FSH-induced estrogen production in cultured rat granulosa cells

    Steroids

    (1978)
  • G. Hu et al.

    Rapid mechanisms of glucocorticoid signaling in the Leydig cell

    Steroids

    (2008)
  • J.M. Jawor et al.

    Seasonal and individual variation in response to GnRH challenge in male dark-eyed juncos (Junco hyemalis)

    Gen. Comp. Endocrinol.

    (2006)
  • A.H.Y. Kwok et al.

    Cloning of chicken glucocorticoid receptor (GR) and characterization of its expression in pituitary and extrapituitary tissues

    Poult. Sci.

    (2007)
  • M.M. Landys et al.

    Actions of glucocorticoids at a seasonal baseline as compared to stress-related levels in the regulation of periodic life processes

    Gen. Comp. Endocrinol.

    (2006)
  • C.R. Lattin et al.

    Does corticosterone regulate the onset of breeding in free-living birds?: the CORT-flexibility hypothesis and six potential mechanisms for priming corticosteroid function

    Horm. Behav.

    (2016)
  • K.J. Livak et al.

    Analysis of relative gene expression data using real-time quantitative PCR and the 2- ΔΔCT method

    Methods

    (2001)
  • M.A. Mitchell et al.

    Plasma zinc as an index of vitellogenin production and reproductive status in the domestic fowl

    Comp. Biochem. Physiol. A Physiol.

    (1991)
  • K.A. Rosvall et al.

    Sources of variation in HPG axis reactivity and individually consistent elevation of sex steroids in a female songbird

    Gen. Comp. Endocrinol.

    (2013)
  • T.D. Williams et al.

    Individual variation in plasma estradiol-17β and androgen levels during egg formation in the European starling Sturnus vulgaris: implications for regulation of yolk steroids

    Gen. Comp. Endocrinol.

    (2004)
  • J.C. Wingfield et al.

    Effects of temperature on photoperiodically induced reproductive development, circulating plasma luteinizing hormone and thyroid hormones, body mass, fat deposition and molt in mountain white-crowned sparrows, Zonotrichia leucophrys oriantha

    Gen. Comp. Endocrinol.

    (2003)
  • J.C. Wingfield et al.

    Interrelationship of day length and temperature on the control of gonadal development, body mass, and fat score in white-crowned sparrows, Zonotrichia leucophrys gambelii

    Gen. Comp. Endocrinol.

    (1996)
  • J.C. Wingfield et al.

    Effects of day length and temperature on gonadal development, body mass, and fat depots in white-crowned sparrows, Zonotrichia leucophrys pugetensis

    Gen. Comp. Endocrinol.

    (1997)
  • C. Zhang et al.

    Expression of messenger ribonucleic acids of luteinizing hormone and follicle-stimulating hormone receptors in granulosa and theca layers of chicken preovulatory follicles

    Gen. Comp. Endocrinol.

    (1997)
  • W.M. Zinzow-Kramer et al.

    Evaluation of reference genes for quantitative real-time PCR in the brain, pituitary, and gonads of songbirds

    Horm. Behav.

    (2014)
  • S.B. Abraham et al.

    Cortisol, obesity, and the metabolic syndrome: a cross-sectional study of obese subjects and review of the literature

    Obesity

    (2013)
  • E. Adkins-Regan

    Do hormonal control systems produce evolutionary inertia?

    Philos. Trans. R. Soc. Lond. B Biol. Sci.

    (2008)
  • J.M. Bahr et al.

    Regulation of the follicular hierarchy and ovulation

    J. Exp. Zool. Part Ecol. Genet. Physiol.

    (1984)
  • J.R. Baker

    The evolution of breeding seasons

    Evolution

    (1938)
  • G.F. Ball

    The neural integration of environmental information by seasonally breeding birds

    Am. Zool.

    (1993)
  • G.F. Ball et al.

    Sex differences in the response to environmental cues regulating seasonal reproduction in birds

    Philos. Trans. R. Soc. B Biol. Sci.

    (2008)
  • T.H. Bambino et al.

    Direct inhibitory effect of glucocorticoids upon testicular luteinizing hormone receptor and steroidogenesis in vivo and in vitro

    Endocrinology

    (1981)
  • J. Bêty et al.

    Body condition, migration, and timing of reproduction in snow geese: a test of the condition-dependent model of optimal clutch size

    Am. Nat.

    (2003)
  • K.E. Cain et al.

    Competitive females are successful females; phenotype, mechanism, and selection in a common songbird

    Behav. Ecol. Sociobiol.

    (2012)
  • S.P. Caro et al.

    Local adaptation of timing of reproduction: females are in the driver’s seat

    Funct. Ecol.

    (2009)
  • W.O. Challenger et al.

    Follicular development and plasma yolk precursor dynamics through the laying cycle in the European starling (Sturnus vulgaris)

    Physiol. Biochem. Zool.

    (2001)
  • A. Dawson

    Control of the annual cycle in birds: endocrine constraints and plasticity in response to ecological variability

    Philos. Trans. R. Soc. Lond. B Biol. Sci.

    (2008)
  • A. Dawson

    Photoperiodic control of the annual cycle in birds and comparison with mammals

    Ardea

    (2003)
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