Circadian rhythm of the preovulatory surge of luteinizing hormone and its relationships to rhythms of body temperature and locomotor activity in turkey hens

Estrogen upregulates the firing activity of hypothalamic gonadotropin-releasing hormone (GnRH1) neurons in the evening in female medaka

The reproductive function of vertebrates is regulated by the hypothalamic-pituitary-gonadal axis. In sexually mature females, gonadotropin-releasing hormone (GnRH) neurons in the preoptic area (POA) are assumed to be responsible for a cyclic large increase in GnRH release, the GnRH surge, triggering a luteinizing hormone (LH) surge, which leads to ovulation. Precise temporal regulation of the preovulatory GnRH/LH surge is important for successful reproduction because ovulation should occur after follicular development. The time course of the circulating level of estrogen is correlated with the ovulatory cycle throughout vertebrates. However, the neural mechanisms underlying estrogen-induced preovulatory GnRH surge after folliculogenesis still remain unclear, especially in non-mammals. Here, we used a versatile non-mammalian model medaka for the analysis of the involvement of estrogen in the regulation of POA-GnRH (GnRH1) neurons. Electrophysiological analysis using a whole brain-pituitary in vitro preparation, which maintains the hypophysiotropic function of GnRH1 neurons intact, revealed that 17β-estradiol (E₂ ) administration recovers the ovariectomy-induced lowered GnRH1 neuronal activity in the evening, indicating the importance of E₂ for upregulation of GnRH1 neuronal activity. The importance of E₂ was also confirmed by the fact that GnRH1 neuronal activity was low in short-day photoperiod-conditioned females (low E₂ model). However, E₂ failed to upregulate the firing activity of GnRH1 neurons in the morning, suggesting the involvement of additional time-of-day signal(s) for triggering GnRH/LH surges at an appropriate timing. We also provide morphological evidence for the localization of estrogen receptor subtypes in GnRH1 neurons. In conclusion, we propose a working hypothesis in which both estrogenic and time-of-day signals act in concert to timely upregulate the firing activity of GnRH1 neurons that trigger the GnRH surge at an appropriate timing in a female-specific manner. This neuroendocrinological mechanism is suggested to be responsible for the generation of ovulatory cycles in female teleosts in general.

Egg-laying increases body temperature to an annual maximum in a wild bird

Most birds, unlike reptiles, lay eggs successively to form a full clutch. During egg-laying, birds are highly secretive and prone to disturbance and predation. Using multisensor data loggers, we show that average daily body temperature during egg-laying is significantly increased (1 °C) in wild eider ducks (Somateria mollissima). Strikingly, this increase corresponds to the annual maximum body temperature (40.7 °C), representing a severe annual thermogenic challenge. This egg-laying-induced rise in body temperature may prove to be a common feature of wild birds and could be caused by habitat-related thermoregulatory adjustments and hormonal modulation of reproduction. We conclude our findings with new perspectives of the benefits of high body temperature associated with egg-laying of birds and the potential effect of heat stress that may occur with the future advent of heatwaves.

The effects of food supply on reproductive hormones and timing of reproduction in an income-breeding seabird

Current food supply is a major driver of timing of breeding in income-breeding animals, likely because increased net energy balance directly increases reproductive hormones and advances breeding. In capital breeders, increased net energy balance increases energy reserves, which eventually leads to improved reproductive readiness and earlier breeding. To test the hypothesis that phenology of income-breeding birds is independent of energy reserves, we conducted an experiment on food-supplemented ("fed") and control female black-legged kittiwakes (Rissa tridactyla). We temporarily increased energy costs (via weight handicap) in a 2 × 2 design (fed/unfed; handicapped/unhandicapped) during the pre-laying period and observed movement via GPS-accelerometry. We measured body mass, baseline hormones (corticosterone; luteinising hormone) before and after handicap manipulation, and conducted a gonadotropin-releasing hormone challenge. Females from all treatment groups foraged in similar areas, implying that individuals could adjust time spent foraging, but had low flexibility to adjust foraging distance. Consistent with the idea that income breeders do not accumulate reserves in response to increased food supply, fed birds remained within an energy ceiling by reducing time foraging instead of increasing energy reserves. Moreover, body mass remained constant until the onset of follicle development 20 days prior to laying regardless of feeding or handicap, implying that females were using a 'lean and fit' approach to body mass rather than accumulating lipid reserves for breeding. Increased food supply advanced endocrine and laying phenology and altered interactions between the hypothalamic-pituitary-adrenal axis and the hypothalamic-pituitary-gonadal axis, but higher energy costs (handicap) had little effect. Consistent with our hypothesis, increased food supply (but not net energy balance) advanced endocrine and laying phenology in income-breeding birds without any impact on energy reserves.

Circadian rhythmicity of body temperature and metabolism

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

Integration of biological clocks and rhythms

Animals, plants, and microorganisms exhibit numerous biological rhythms that are generated by numerous biological clocks. This article summarizes experimental data pertinent to the often-ignored issue of integration of multiple rhythms. Five contexts of integration are discussed: (i) integration of circadian rhythms of multiple processes within an individual organism, (ii) integration of biological rhythms operating in different time scales (such as tidal, daily, and seasonal), (iii) integration of rhythms across multiple species, (iv) integration of rhythms of different members of a species, and (v) integration of rhythmicity and physiological homeostasis. Understanding of these multiple rhythmic interactions is an important first step in the eventual thorough understanding of how organisms arrange their vital functions temporally within and without their bodies.

Duration of the reproductive period decreases the frequency of preovulatory luteinizing hormone surges in heavy weight-sire line turkey hens

The frequency of preovulatory luteinizing hormone (LH) surges is an important determinant of ovulation and oviposition rates in turkeys. Egg production rate is relatively poor in heavy weight-sire line type turkey hens and declines with advancing duration of the reproductive period. The purpose of this study was to measure frequency and characteristics of preovulatory LH surges in turkey hens of a heavy weight-sire line type early, at peak of egg production (Early), and late, after egg production rate had declined (Late), in a reproductive period. The Early hens were photostimulated with a continuous photoperiod [24 h light (L):0 h dark (D)] at 40 weeks of age and sampled during peak egg production at about 47 weeks of age. The Late hens were photostimulated at 40 weeks of age with a long day photoperiod (14L:10D). After a 27-week egg production period, the Late hens were switched to the 24L:0D photoperiod and sampled at 74 weeks of age. Continuous lighting was used during blood sampling to allow the rhythm of preovulatory LH surges to free run. All hens were cannulated 3-5 days before starting sampling and hourly blood samples were collected for 200 h. All hens were necropsied and ovarian and oviductal morphologies were measured after serial bleeding. The Late hens had a longer interval between intra-clutch preovulatory LH surges than the Early hens, and a higher incidence of atretic ovarian follicles. The Early hens had higher baseline and surge amplitude LH concentrations but lower progesterone (P4) surge amplitude concentrations than the Late hens. The duration of preovulatory LH surges, incidence of "blind" preovulatory LH surges, baseline P4 concentrations, and overall estradiol-17beta (E2) concentrations were not different between Early and Late hens. In conclusion, a longer interval between preovulatory LH surges, lower LH baseline and surge amplitude concentrations, a higher incidence of atretic follicles, and higher P4 surge amplitude concentration were associated with the decline in egg production late in the reproductive period in a heavy weight-sire line of turkey hens.

Preovulatory surge patterns of luteinizing hormone, progesterone, and estradiol-17β in broiler breeder hens fed ad libitum or restricted fed

Spontaneous ovulations are induced by preovulatory surges of luteinizing hormone (LH) and progesterone (P4) during ovulatory cycles in birds, but estradiol-17beta (E2) levels are relatively constant. Egg production is enhanced in restricted fed (RF) in comparison with ad libitum fed (FF) broiler breeder hens, but changes in concentrations and peripheral patterns of LH, P4, and E2 during ovulatory cycles in broiler breeder hens are poorly documented. The hypothesis of this study was that high resolution patterns of peripheral LH, P4, and E2 during preovulatory surges would not be different between FF and RF broiler breeder hens. Seven FF and 6 RF broiler breeder hens were photostimulated with 16 L:8 D at 22 wk of age. At 28 wk of age, the hens were cannulated for serial blood sampling and switched to a 24L:0D photoperiod to allow preovulatory surges of LH and P4 to run freely. Three days after cannulation, hens were serially bled every 12 min for 36 h. The FF hens were heavier than the RF hens (5.60 +/- 0.35 vs. 3.60 +/- 0.28 kg, respectively; P < 0.01). During the 10 d before cannulation, total egg production of the FF and RF hens (8.3 +/- 1.4 and 6.8 +/- 1.3 eggs, respectively; P = 0.08) and normal egg production (5.6 +/- 1.8 and 6.5 +/- 1.8 eggs, respectively; P = 0.37) were not different. The FF hens, however, had more abnormal eggs than the RF hens (2.7 +/- 1.7 and 0.3 +/- 0.8 eggs, respectively; P < 0.01). None of the hormonal measurements was different between the FF and RF hens (P > 0.05). The concentrations of hormones for the FF and RF hens, respectively, were as follows: baseline LH (2.79 +/- 0.45 vs. 2.94 +/- 0.60 ng/mL) and P4 (1.68 +/- 0.56 vs. 1.41 +/- 0.43 ng/mL), overall mean LH (3.18 +/- 0.45 vs. 3.10 +/- 0.46 ng/mL) and P4 (2.32 +/- 0.55 vs. 2.09 +/- 0.91 ng/ mL), preovulatory surge amplitude of LH (5.43 +/- 1.27 vs. 3.88 +/- 1.24 ng/mL) and P4 (6.08 +/- 2.09 vs. 6.71 +/- 3.91 ng/ mL), preovulatory surge duration of LH (7.52 +/- 1.80 vs. 5.74 +/- 3.18 h) and P4 (7.52 +/- 1.42 vs. 8.20 +/- 1.24 h), and overall mean E2 (0.25 +/- 0.05 vs. 0.23 +/- 0.05 ng/mL). In conclusion, there were no differences in total egg production or normal egg production between FF and RF broiler breeder hens, but the FF hens laid more abnormal eggs. Also, there were no differences in the concentrations or peripheral patterns of LH, P4, and E2 during preovulatory surges between the FF and RF broiler breeder hens.

Development of a cannulation procedure for broiler breeder hens

A cannulation and serial bleeding procedure has been developed to monitor the peripheral patterns of hormones associated with reproduction for up to 10 d in broiler breeder hens. Hens were cannulated via the jugular vein and returned to individual cages. The unrestrained cannulated hens were connected to a tether and swivel system that permitted constant infusion for maintenance of the cannula prior to serial bleeding and unrestrained long-term serial bleeding for up to 10 d. In a short-term experiment hens were bled every 12 min for 36 h, and in a long-term experiment hens were bled hourly for 10 d. In these experiments, 1.5-mL blood samples were collected at each time point with sodium citrate as the anticoagulant. To avoid hemodilution, after removal of plasma the red blood cells were reconstituted with saline to the original volume and returned to the hen of origin. Collection of serial blood samples was successful from 94% of hens in the short-term experiment and 79% of hens in the long-term experiment. Egg production was not affected (P > 0.05) during the 6 wk following serial bleeding in the short-term experiment. For hens that continued laying, egg production for 10 d prior to cannulation was not different (P > 0.05) from egg production for the 10 d during serial bleeding in the long-term experiment. However, late in the reproductive cycle many hens (25%) stopped laying when serially bled. It is concluded that this cannulation procedure can be used to study short-term or long-term peripheral patterns of hormones associated with oviposition and ovulation in laying broiler breeder hens.

Interval between preovulatory surges of luteinizing hormone increases late in the reproductive period in turkey hens

In turkey hens, the egg production rate is relatively high early during a reproductive period, but declines as the period progresses. Among lines with different egg production potential, the interval between preovulatory surges of LH is the primary determinant of the egg production rate. The main objective of this study was to determine whether the decline in egg production rate late during an egg production period is also associated with a difference in the interval between LH preovulatory surges. A group of photosensitive turkey hens (Early) were photostimulated with continuous light (24L:0D) at 40 wk of age to induce egg laying, and serial blood samples were collected after about 3 wk of egg production. A second group of hens (Late) were housed in floor pens and photostimulated with 14L:10D at 40 wk of age for a normal 36-wk reproduction period and were then switched to 24L:0D lighting for 2 wk before collection of serial blood samples. Continuous light photostimulation was used for at least 2 wk before and during serial blood sampling to avoid potential masking effects of diurnal lighting on the interval between LH surges. The Early (n = 12) and Late (n = 16) hens were cannulated 3 days before being serially bled hourly for 10 days. The mean interval between preovulatory surges of LH was shorter in the Early hens than in the Late hens (26.1 +/- 2.5 h and 34.7 +/- 3.9 h, respectively). The intra-hen LH surge interval coefficient of variation was lower in the Early hens than in the Late hens (7.2% and 18.6%, respectively). The inter-hen LH surge interval coefficient of variation was similar in the Early and Late hens (9.5% and 11.2%, respectively). The incidence of blind surges of LH (those not retrospectively associated with ovipositions) was not different between Early and Late laying hens (8.4% +/- 15.2% and 7.3% +/- 14.6%, respectively). In conclusion, in turkey hens, longer intervals and greater intra-hen variation between LH surges were associated with a poorer rate of egg production late in the reproductive period relative to early in the reproductive period.

Concentration change patterns of luteinizing hormone and progesterone and distribution of hierarchical follicles in normal and arrested laying turkey hens

Young photosensitive turkey hens of a line selected for increased egg production (Egg line) were photostimulated with constant light [24 h light:0 h darkness] at 30 wk of age. Egg laying became arrested in 6 of 12 the hens after only 2 to 3 wk of laying. Ovarian morphology and changes in concentrations of plasma hormones [luteinizing hormone (LH), progesterone (P4), and estradiol-17beta (E2)] over 10 d of serial bleeding were compared between the arrested laying and normal laying hens. The number of ovarian follicles heavier than 1.0 g was much greater in arrested laying hens, and some of the arrested laying hens presented a polycystic ovarian follicle condition. The oviducts of the arrested laying hens were fully developed and were similar in weight to those of normal laying hens. In arrested laying hens the plasma concentration of LH was relatively low (1.72 +/- 0.30 ng mL(-1)) and without preovulatory surges. In normal laying hens the baseline concentration of LH was 2.60 +/- 0.71 ng mL(-1), and the interval between LH surges was 26.8 h. In the arrested laying hens, the plasma concentration of P4 was relatively high (4.66 +/- 1.28 ng mL(-1)) and without preovulatory surges. In normal laying hens the baseline concentration of P4 between surges was 1.76 +/- 0.24 ng mL(-1). Plasma E2 concentrations were not different between normal laying and arrested laying hens. In conclusion, ovulations and ovipositions ceased in the arrested laying hens, but the entrance of follicles into the follicular hierarchy and hierarchical growth continued, leading to an accumulation of numerous mature follicles in the ovary. In addition, some of the accumulated mature follicles might have resumed growing, leading to the formation of cystic ovarian follicles.

Frequency of Luteinizing Hormone Surges and Egg Production Rate in Turkey Hens1

Whether the interval between preovulatory surges of LH was different between lines of turkey hens with either poor (RBC3 line, peak at 55%) or excellent rate of egg production (Egg line, peak at 85%) was examined. Laying hens were cannulated and bled hourly for 10 days at peak of production. A constant light photoschedule was used to avoid diurnal masking of innate circadian rhythms. The mean interval between LH surges in the RBC3 line was longer than in the Egg line and had a higher coefficient of variation. A few longer LH surge intervals (72 h) were found in some RBC3 line hens (2 of 7 hens), but none were found in Egg line hens (0 of 11 hens). All progesterone (P4) surges were coupled with LH surges, but not all LH-P4 surges were coupled with ovipositions (blind LH-P(4) surges). The percentage of blind LH-P4 surges was not different between lines. The baseline concentration of LH was higher in Egg line than RBC3 line hens, but LH surge amplitude, and surge duration were not different. The baseline and surge amplitude concentrations of P4 were not different between lines, nor was the concentration of estradiol-17beta. The longer interval between LH surges was the major factor tested that was associated with the poorer egg production rate in RBC3 line hens in comparison to Egg line hens. A higher incidence of blind LH surges further contributed to lower egg production in RBC3 line turkey hens.