Ontogeny of the photoperiodic control of prolactin and luteinizing hormone secretion in male and female bantams (Gallus domesticus)

Single-Cell RNA Sequencing Analysis of Chicken Anterior Pituitary: A Bird's-Eye View on Vertebrate Pituitary

It is well-established that anterior pituitary contains multiple endocrine cell populations, and each of them can secrete one/two hormone(s) to regulate vital physiological processes of vertebrates. However, the gene expression profiles of each pituitary cell population remains poorly characterized in most vertebrate groups. Here we analyzed the transcriptome of each cell population in adult chicken anterior pituitaries using single-cell RNA sequencing technology. The results showed that: (1) four out of five known endocrine cell clusters have been identified and designated as the lactotrophs, thyrotrophs, corticotrophs, and gonadotrophs, respectively. Somatotrophs were not analyzed in the current study. Each cell cluster can express at least one known endocrine hormone, and novel marker genes (e.g., CD24 and HSPB1 in lactotrophs, NPBWR2 and NDRG1 in corticotrophs; DIO2 and SOUL in thyrotrophs, C5H11ORF96 and HPGDS in gonadotrophs) are identified. Interestingly, gonadotrophs were shown to abundantly express five peptide hormones: FSH, LH, GRP, CART and RLN3; (2) four non-endocrine/secretory cell types, including endothelial cells (expressing IGFBP7 and CFD) and folliculo-stellate cells (FS-cells, expressing S100A6 and S100A10), were identified in chicken anterior pituitaries. Among them, FS-cells can express many growth factors, peptides (e.g., WNT5A, HBEGF, Activins, VEGFC, NPY, and BMP4), and progenitor/stem cell-associated genes (e.g., Notch signaling components, CDH1), implying that the FS-cell cluster may act as a paracrine/autocrine signaling center and enrich pituitary progenitor/stem cells; (3) sexually dimorphic expression of many genes were identified in most cell clusters, including gonadotrophs and lactotrophs. Taken together, our data provides a bird's-eye view on the diverse aspects of anterior pituitaries, including cell composition, heterogeneity, cell-to-cell communication, and gene expression profiles, which facilitates our comprehensive understanding of vertebrate pituitary biology.

Integrative analysis of transcriptomic data related to the liver of laying hens: from physiological basics to newly identified functions

Background

At sexual maturity, the liver of laying hens undergoes many metabolic changes to support vitellogenesis. In published transcriptomic approaches, hundreds of genes were reported to be overexpressed in laying hens and functional gene annotation using gene ontology tools have essentially revealed an enrichment in lipid and protein metabolisms. We reanalyzed some data from a previously published article comparing 38-week old versus 10-week old hens to give a more integrative view of the functions stimulated in the liver at sexual maturity and to move beyond current physiological knowledge. Functions were defined based on information available in Uniprot database and published literature.

Results

Of the 516 genes previously shown to be overexpressed in the liver of laying hens, 475 were intracellular (1.23–50.72 fold changes), while only 36 were predicted to be secreted (1.35–66.93 fold changes) and 5 had no related information on their cellular location. Besides lipogenesis and protein metabolism, we demonstrated that the liver of laying hens overexpresses several clock genes (which supports the circadian control of liver metabolic functions) and was likely to be involved in a liver/brain/liver circuit (neurotransmitter transport), in thyroid and steroid hormones metabolisms. Many genes were associated with anatomical structure development, organ homeostasis but also regulation of blood pressure. As expected, several secreted proteins are incorporated in yolky follicles but we also evidenced that some proteins are likely participating in fertilization (ZP1, MFGE8, LINC00954, OVOCH1) and in thyroid hormone maturation (CPQ). We also proposed that secreted proteins (PHOSPHO1, FGF23, BMP7 but also vitamin-binding proteins) may contribute to the development of peripheral organs including the formation of medullar bones to provide labile calcium for eggshell formation. Thirteen genes are uniquely found in chicken/bird but not in human species, which strengthens that some of these genes may be specifically related to avian reproduction.

Conclusions

This study gives additional hypotheses on some molecular actors and mechanisms that are involved in basic physiological function of the liver at sexual maturity of hen. It also revealed some additional functions that accompany reproductive capacities of laying hens, and that are usually underestimated when using classical gene ontology approaches.

Diurnal and photoperiodic changes in thyrotrophin-stimulating hormone β expression and associated regulation of deiodinase enzymes (DIO2, DIO3) in the female juvenile chicken hypothalamus

Increased thyrotrophin-stimulating hormone β (TSHβ) expression in the pars tuberalis is assumed to be an early step in the neuroendocrine mechanism transducing photoperiodic information. The present study aimed to determine the relationship between long-photoperiod (LP) and diurnal TSHβ gene expression in the juvenile chicken by comparing LP-photostimulated birds with groups kept on a short photoperiod (SP) for 1 or 12 days. TSHβ expression increased by 3- and 23-fold after 1 and 12 days of LP-photostimulation both during the day and at night. Under both SP and LP conditions, TSHβ expression was between 3- and 14-fold higher at night than in the day, suggesting that TSHβ expression cycles in a diurnal pattern irrespective of photoperiod. The ratio of DIO2/3 was decreased on LPs, consequent to changes in DIO3 expression, although there was no evidence of any diurnal effect on DIO2 or DIO3 expression. Plasma prolactin concentrations revealed both an effect of LPs and time-of-day. Thus, TSHβ expression changes in a dynamic fashion both diurnally and in response to photoperiod.

Seasonal plasticity in the peptide neuronal systems: potential roles of gonadotrophin-releasing hormone, gonadotrophin-inhibiting hormone, neuropeptide Y and vasoactive intestinal peptide in the regulation of the reproductive axis in subtropical Indian weaver birds

Two experiments examined the expression of gonadotrophin-releasing and inhibiting hormones (GnRH-I, GnRH-II and GnIH), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in subtropical Indian weaver birds, which demonstrate relative photorefractoriness. Experiment 1 measured peptide expression levels in the form of immunoreactive (-IR) cells, percentage cell area and cell optical density in the preoptic area (GnRH-I), midbrain (GnRH-II), paraventricular nucleus (GnIH), mediobasal hypothalamus [dorsomedial hypothalamus (DMH), infundibular complex (INc), NPY and VIP] and lateral septal organ (VIP) during the progressive, breeding, regressive and nonbreeding phases of the annual reproductive cycle. GnRH-I was decreased in the nonbreeding and VIP was increased in INc in the breeding and regressive states. GnRH-II and NPY levels did not differ between the testicular phases. Double-labelled immunohistochemistry (IHC) revealed a close association between the GnRH/GnIH, GnRH/NPY, GnRH/VIP and GnIH/NPY peptide systems, implicating them interacting and playing roles in the reproductive regulation in weaver birds. Experiment 2 further measured these peptide levels in the middle of day and night in weaver birds that were maintained under short days (8 : 16 h light /dark cycle; photosensitive), exposed to ten long days (16 : 8 h light /dark cycle; photostimulated) or maintained for approximately 2 years on a 16 : 8 h light /dark cycle (photorefractory). Reproductively immature testes in these groups precluded the possible effect of an enhanced gonadal feedback on the hypothalamic peptide expression. There were group differences in the GnRH-I (not GnRH-II), GnIH, NPY and VIP immunoreactivity, albeit with variations in immunoreactivity measures in the present study. These results, which are consistent with those reported in birds with relative photorefractoriness, show the distribution and possibly a complex interaction of key neuropeptides in the regulation of the annual reproductive cycle in Indian weaver birds.

Red jungle fowl (Gallus gallus) as a model for studying the molecular mechanism of seasonal reproduction

Photoperiodism is an adaptation mechanism that enables animals to predict seasonal changes in the environment. Japanese quail is the best model organism for studying photoperiodism. Although the recent availability of chicken genome sequences has permitted the expansion from single gene to genome-wide transcriptional analysis in this organism, the photoperiodic response of the domestic chicken is less robust than that of the quail. Therefore, in the present study, we examined the photoperiodic response of the red jungle fowl (Gallus gallus), a predecessor of the domestic chicken, to test whether this animal could be developed as an ideal model for studying the molecular mechanisms of seasonal reproduction. When red jungle fowls were transferred from short-day- to long-day conditions, gonadal development and an increase in plasma LH concentration were observed. Furthermore, rapid induction of thyrotropin beta subunit, a master regulator of photoperiodism, was observed at 16 h after dawn on the first long day. In addition, the long-day condition induced the expression of type 2 deiodinase, the key output gene of photoperiodism. These results were consistent with the results obtained in quail and suggest that the red jungle fowl could be an ideal model animal for the genome-wide transcriptional analysis of photoperiodism.

The chicken type III GnRH receptor homologue is predominantly expressed in the pituitary, and exhibits similar ligand selectivity to the type I receptor

Two GnRH isoforms (cGnRH-I and GnRH-II) and two GnRH receptor subtypes (cGnRH-R-I and cGnRH-R-III) occur in chickens. Differential roles for these molecules in regulating gonadotrophin secretion or other functions are unclear. To investigate this we cloned cGnRH-R-III from a broiler chicken and compared its structure, expression and pharmacological properties with cGnRH-R-I. The broiler cGnRH-R-III cDNA was 100% identical to the sequence reported in the red jungle fowl and white leghorn breed. Pituitary cGnRH-R-III mRNA was approximately 1400-fold more abundant than cGnRH-R-I mRNA. Northern analysis indicated a single cGnRH-R-III transcript. A pronounced sex and age difference existed, with higher pituitary transcript levels in sexually mature females versus juvenile females. In contrast, higher expression levels occurred in juvenile males versus sexually mature males. Functional studies in COS-7 cells indicated that cGnRH-R-III has a higher binding affinity for GnRH-II than cGnRH-I (K(d): 0.57 vs 19.8 nM) with more potent stimulation of inositol phosphate production (ED(50): 0.8 vs 4.38 nM). Similar results were found for cGnRH-R-I, (K(d): 0.51 vs 10.8 nM) and (ED(50): 0.7 vs 2.8 nM). The initial rate of internalisation was faster for cGnRH-R-III than cGnRH-R-I (26 vs 15.8%/min). Effects of GnRH antagonists were compared at the two receptors. Antagonist #27 distinguished between cGnRH-R-I and cGnRH-R-III (IC(50): 2.3 vs 351 nM). These results suggest that cGnRH-R-III is probably the major mediator of pituitary gonadotroph function, that antagonist #27 may allow delineation of receptor subtype function in vitro and in vivo and that tissue-specific recruitment of cGnRH-R isoforms has occurred during evolution.

Differential regulation of gene expression and release of FSH and prolactin by long day and sulfamethazine in chicks

In several avian species long day exposure results in plasma elevation of gonadotropins and prolactin (PRL). We examined the early (12-72h) effects of photostimulation on mRNA transcripts and plasma levels of follicle stimulating hormone (FSH) and PRL in three-week old cockerels. In addition, the neuroendocrine influence of the compound, sulfamethazine (SMZ), known to enhance light-induced gonadal development in chicks, was studied when applied with or without long-day photostimulation. Both long day exposure and SMZ intake caused a rapid increase in FSHbeta mRNA transcripts at Zeitgeber time 48 (ZT48), while only SMZ stimulated secretion of the hormone into plasma during the course of the study. In contrast to SMZ treatment, photostimulation was more effective at stimulating PRL mRNA transcripts and secretion of PRL. Results demonstrate a differential role of long day exposure and SMZ intake on the regulation of FSH and PRL synthesis and secretion and suggest that some effects of SMZ on gonadal development may be mediated by the pituitary.

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

Although it is axiomatic that males and females differ in relation to many aspects of reproduction related to physiology, morphology and behaviour, relatively little is known about possible sex differences in the response to cues from the environment that control the timing of seasonal breeding. This review concerns the environmental regulation of seasonal reproduction in birds and how this process might differ between males and females. From an evolutionary perspective, the sexes can be expected to differ in the cues they use to time reproduction. Female reproductive fitness typically varies more as a function of fecundity selection, while male reproductive fitness varies more as a function sexual selection. Consequently, variation in the precision of the timing of egg laying is likely to have more serious fitness consequences for females than for males, while variation in the timing of recrudescence of the male testes and accompanying territory establishment and courtship are likely to have more serious fitness consequences for males. From the proximate perspective, sex differences in the control of reproduction could be regulated via the response to photoperiod or in the relative importance and action of supplementary factors (such as temperature, food supply, nesting sites and behavioural interactions) that adjust the timing of reproduction so that it is in step with local conditions. For example, there is clear evidence in several temperate zone avian species that females require both supplementary factors and long photoperiods in order for follicles to develop, while males can attain full gonadal size based on photoperiodic stimulation alone. The neuroendocrine basis of these sex differences is not well understood, though there are many candidate mechanisms in the brain as well as throughout the entire hypothalamo-pituitary-gonadal axis that might be important.

Changes in light intensity can influence age at sexual maturity in domestic pullets

1. Shaver White and ISA Brown pullets were reared to 140 d in groups of 8 in cages on a 10-h photoperiod of incandescent light and maintained at an illuminance of 3 or 25 lux, or transferred from 3 to 25 lux or from 25 to 3 lux at 63 or 112 d of age. 2. There was no significant difference in sexual maturity, measured as eggs per 100 bird.d at 139 and 140 d, for ISA Brown maintained on 3 or 25 lux, but Shaver White pullets exposed to constant 3 lux matured significantly later than those maintained on 25 lux. 3. In Shaver Whites, sexual maturity was significantly delayed by an increase from 3 to 25 lux at 63 and 112 d, and advanced by a decrease from 25 to 3 lux at 112 d. Sexual maturity of ISA Browns was not significantly affected by a change in illuminance at 63 or 112 d, though responses were in the same direction as for Shaver Whites. 4. In both breeds, total feed consumed to 112 d was higher for birds on 3 lux than 25 lux, but lower between 112 d and 140 d when birds on 25 lux underwent rapid sexual development. In both breeds, body weight at 63 d was higher for birds exposed to 3 lux than 25 lux, but body weight gain thereafter was similar for the two light intensities. 5. In both breeds, plasma luteinising hormone (LH) concentration at 63 and 112 d was lower in birds maintained on 3 lux than 25 lux. At 63 and 112 d, transfers from 25 to 3 lux depressed, whereas transfers from 3 to 25 lux at 63 d, but not at 112 d, increased plasma LH. 6. Advances or delays in sexual maturity induced by changes in illuminance were not correlated with differences in feed intake, body weight gain, or with changes in plasma LH. 7. One possible explanation for the inverse relationship between the direction of change in illuminance at 63 and 112 d in pullets exposed to a 10-h photoperiod and the age at which they became sexually mature is that changes in light intensity and/or spectral composition affect the entrainment of the circadian rhythm of photoinducibility, to effect a phase shift in the photoinducible phase and/or the responsiveness of phototransduction pathways.

A neuroendocrine model for prolactin as the key mediator of seasonal breeding in birds under long- and short-day photoperiods

Seasonal breeding is associated with sequential increases in plasma luteinizing hormone (LH) and prolactin in the short-day breeding emu, and in long-day breeding birds that terminate breeding by the development of reproductive photorefractoriness. A model of the avian neuroendocrine photoperiodic reproductive response is proposed, incorporating a role for prolactin, to account for neuroendocrine mechanisms controlling both long- and short-day breeding. The breeding season terminates after circulating concentrations of prolactin increase above a critical threshold to depress gonadotropin releasing hormone (GnRH) neuronal and gonadotrope (LH) activity. Subsequently, photorefractoriness develops for prolactin secretion and for LH secretion, independently of high plasma prolactin. The breeding season in the emu is advanced compared with long-day breeders, because after photorefractiness for both LH and prolactin secretion is dissipated, plasma concentrations of both hormones increase to maximum values while days are still short.

Effect of delayed step-up lighting on plasma luteinizing hormone and reproductive function in broiler breeders

The aim of the study was to establish, in contemporary broiler breeders, whether delayed photostimulation at 313 d results in a reproductive response similar to that after photostimulation at 134 d (standard practice). The standard lighting program was compared with a novel program in which daily hours of light were reduced to 3 h during rearing and kept at 3 h until photostimulation at 264 d (8 h) or at 313 d (16 h). This experiment was done with hens fed ad libitum or feed-restricted hens. In photostimulated and nonphotostimulated hens, feed restriction delayed the onset of egg production and enhanced the subsequent rate of laying. Standard photostimulation advanced the onset of lay and increased the subsequent rate of lay in hens fed ad libitum and feed-restricted hens. Delayed photostimulation of hens did not impair the photoinduced increase in the concentration of plasma luteinizing hormone (LH) or egg production. Delayed photostimulation in cockerels failed to stimulate LH secretion. Unexpectedly, for feed-restricted hens, transfer from 3 to 8 h light/d at 264 d resulted in an increased in plasma LH and increased egg production. A similar increase in plasma LH was observed for cockerels subjected to the same lighting treatment. We concluded that, in broiler breeder hens, the reproductive response to photostimulation is not impaired if photostimulation is delayed for up to 313 d. Cockerels may not respond well to delayed photostimulation.

Photo-induction of hypothalamic gonadotrophin releasing hormone-I mRNA in the domestic chicken: a role for oestrogen?

Photoinduced changes in GnRH neuronal function were investigated in prepubertal and in midpubertal cockerels and somatically mature hens. Photostimulation of short day mid-pubertal cockerels and somatically mature out-of-lay hens for 7 days significantly increased (P<0.05) total hypothalamic gonadotrophin releasing hormone-I (GnRH-I) mRNA. The increase in GnRH-I mRNA was associated with increased (P<0.05) plasma LH in the hens but not mid-pubertal cockerels. Photostimulation of short day prepubertal cockerels for 7 days also stimulated LH release (P<0.05) but in contrast did not increase total hypothalamic GnRH-I mRNA. Plasma LH and hypothalamic GnRH-I mRNA were depressed in (P<0.001) short day prepubertal cockerels chronically treated with oestradiol benzoate (0.5 mg/kg, on alternate days). However, photostimulation of oestrogenized prepubertal cockerels for 7 days stimulated LH release (P<0.001) and increased hypothalamic GnRH-I mRNA (P<0.001). It is concluded that photostimulatory inputs to GnRH neurones have the potential to increase GnRH-I mRNA transcription or stability and to increase GnRH-I release. The extent to which increased levels of GnRH-I mRNA or increased GnRH release from GnRH neurones are observed after photostimulation may depend on the interaction between the drive on GnRH-I neurone function, which increases at the onset of puberty, and the inhibitory action of oestrogen produced locally in the hypothalamus.

Relationship between prolactin receptor mRNA in the anterior pituitary gland and hypothalamus and reproductive state in male and female bantams (Gallus domesticus)

The aim of this study was to test the hypothesis that prolactin may up- and down-regulate prolactin receptor gene expression in the anterior pituitary gland and hypothalamus respectively. Experiments were carried out in bantams (Gallus domesticus). Comparisons were made of concentrations of PRLR mRNA in the anterior pituitary gland and basal and preoptic hypothalamus in adult males and females held on long days (low vs high plasma prolactin); in 3-week-old juvenile male and females on short days (high vs low plasma prolactin); in 8-week-old juvenile male and females on short days (both low plasma prolactin); in adult laying, incubating, and out-of-lay (high, very high, and low plasma prolactin, respectively); in adult cockerels exposed to long or short days (high vs low prolactin); and in adult hens exposed to long or short days (high vs low prolactin). There was a sex difference in anterior pituitary and basal hypothalamic PRLR mRNA, with lower values in both tissues in females than in males. Compared with laying and out-of-lay hens, anterior pituitary and basal hypothalamic PRLR mRNA concentrations in incubating hens were increased and decreased, respectively. In adult birds of either sex held on long or short days, there was no difference in pituitary PRLR mRNA, while basal hypothalamic PRLR mRNA was lower on short days. PRLR mRNA in the preoptic hypothalamus was not affected by sex, reproductive state, or photoperiod. It is concluded that there is no consistent relationship between plasma prolactin, in the physiological range, and the concentration of PRLR mRNA in the anterior pituitary gland, basal hypothalamus, and preoptic hypothalamus.

Effect of photostimulation on concentrations of plasma prolactin in castrated bantams (Gallus domesticus)

The annual breeding cycle of 'unimproved' breeds of domestic chicken, including the bantam, at temperate latitudes, is terminated by decreasing daylength in autumn and is initiated in late winter, while daylengths are still short. Observations on photoperiodic birds that terminate seasonal breeding by the development of long day photorefractoriness suggest that the photoinduced pattern of prolactin secretion is associated with the pattern of gonadal growth and regression. It was predicted that, if there is a causal relationship between photoinduced changes in prolactin secretion and gonadal function in birds then, in the bantam, the pattern of prolactin secretion observed after photostimulation would not be the same as in birds terminating breeding by the development of long day photorefractoriness. Experiments were carried out on surgically castrated bantams to avoid confounding the effects of photostimulation and the stimulatory actions of testicular hormones on prolactin secretion. Transfer of photosensitive castrated bantams from 8 to 14, 16, 18 or 20 h light/day initially stimulated prolactin release and, subsequently, after 20-30 days, concentrations of plasma prolactin progressively decreased. After 148 days of photostimulation, concentrations of plasma prolactin approached but were still higher than short day controls. Transfer of photosensitive castrated bantam cockerels from 8 to 12 h light/day stimulated a slower increase in plasma prolactin that subsequently remained higher than in other photostimulated groups. A further 4 h increase in photoperiod in the birds exposed for 148 days to 12 or 16 h light/day resulted, respectively, in a transitory increase and no increase in prolactin secretion. Recovery of photosensitivity for prolactin release was observed in the birds transferred to 18 or 20 h light/day for 148 days after treatment with 8 h light/day for 35 days. Attempts to obtain an independent hormonal correlate of the prolactin responses to photostimulation by measurement of plasma luteinizing hormone (LH) were unsuccessful. The concentration of plasma LH in castrated bantams did not change in response to a change in photoperiod. These observations show that the photoinduced pattern of prolactin release in the bantam, a species which terminates seasonal breeding in response to decreasing daylength, is the same as that in birds which terminate seasonal breeding by the development of long day photorefractoriness. It is concluded that the photoinduced pattern of prolactin secretion in birds can be dissociated from the neuroendocrine mechanisms controlling the termination of seasonal breeding.