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J. B. Williams and P. J. Sharp

Summary. The functional integrity of the components of the hypothalamo–pituitary–ovarian axis was examined in young and old laying hens. Ovarian function was tested by measuring the amount of progesterone released in response to an injection of LH, and pituitary function was investigated by measuring the increase in the plasma LH level after an injection of LH-RH. There were no differences between young and old birds in the response of the pituitary gland or the ovary to these stimuli. Hypothalamic function was investigated by studying the positive feedback action of a standard dose of progesterone on LH release; the positive feedback response was smaller (P < 0·005) in old hens.

It is suggested that the fall in the rate of lay in hens towards the end of their laying year is caused partly by a decrease in the response of the LH-positive feedback mechanism to progesterone.

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Seasonal changes in plasma LH concentrations were measured by radioimmunoassay every month for 1 year in nine Khaki Campbell drakes kept under natural lighting conditions in Kiel, Germany. The LH levels rose in all the birds in March and April and reached a mean ( ± S.E.) maximum of 2·4 ± 0·2 ng/ml early in May. Thereafter, LH levels remained high in three drakes, but decreased in the other six to between 0·9 and 0·4 ng/ml early in June and/or July and rose to about 2·5 ng/ml by the beginning of August.

Plasma LH levels started to fall in all nine drakes in September and by early October had reached winter values of about 1·2 to 1·5 ng/ml. A small transient increase in the level of circulating LH was observed in most drakes during November or December.

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Continuous variations in circulating levels of plasma luteinizing hormone (LH) were measured over a 24-hr period in laying hens, by radioimmunoassay.

A single LH peak, closely linked to the dark period, was associated with the occurrence of an ovulation. No other consistent changes in LH levels were found, apart from a small rise at the onset of darkness.

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I. C. Dunn and P. J. Sharp

Summary. Photoperiodic response curves for LH release were obtained for juvenile female domestic chickens at 8 weeks of age by measuring changes in plasma LH concentrations after increasing the daily photoperiod from 8 to 10·5, 12·75, 15·25, 17·75 or 20 h. The birds were bred either for meat production (broiler) or commercial egg-laying and were fed ad libitum or a restricted diet, similar to that used under commercial conditions. Ovarian and oviduct growth was stimulated by 2 weeks after transfer to 20 h light/day in the dwarf broiler strain, irrespective of the dietary treatment, but not in birds of the egg-laying strain. Baseline concentrations of plasma LH were higher in the egg-laying than in the dwarf broiler strain birds. A significant effect of dietary treatment was observed on the changes in concentration of plasma LH in the nonphotostimulated dwarf broiler, but not in the egg-laying bird. There was no significant interaction between dietary treatment and photoinduced LH release in birds of either strain. The shortest photoperiod needed to stimulate LH release (critical daylength) was < 10·5 h in the dwarf broilers and between 10·5 and 12·75 h in the egg-laying birds. The shortest photoperiod needed to stimulate the maximum release of LH (saturation daylength) was between 10·5 and 12·75 h in the dwarf broiler strain. The saturation daylength in birds of the egg-laying strain was longer, being between 12·75 and 15·25 h. It is concluded that there are differences in the photoperiodic response between chickens of different breeds and that they are not modified by restriction of food intake to 50–60% of that in control birds fed ad libitum.

Keywords: domestic chicken; LH; photoperiodism; restricted feeding

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J. Culbert, P. J. Sharp, and J. W. Wells

Spermatogenesis begins at about 16 weeks of age in Thornber 909 cockerels when subjected to a lighting schedule of 14 h light/day. Most birds produce semen by 24 weeks. The onset of spermatogenesis is associated with increases in the concentrations of plasma LH and testosterone (Sharp, 1975; Sharp, Culbert & Wells, 1977). In the present study these increases are related to changes in the concentration of circulating androst-4-ene-3,17-dione (androstenedione). This steroid is one of the androgens secreted by the mammalian testis (Eik-Nes & Hall, 1965) but little is known about its secretion in birds. Additionally, concentrations of LH, testosterone and androstenedione were measured for 4 months after the birds began to produce semen.

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P. J. Sharp, I. C. Dunn, and S. Cerolini

Summary. Egg-laying in hens exposed for more than 11 months to photostimulatory daylengths was intermittent and associated with a reduction in numbers of yellow-yolky ovarian follicles. Old laying hens (105 weeks) had lower concentrations of luteinizing hormone (LH) in the pituitary gland and plasma and reduced pituitary gland responsiveness to chicken LH-releasing-hormones (LHRH-I and II) in vivo when compared with young laying hens (28 weeks). Four weeks after transfer from 14 to 8 h light/day, egg production almost stopped in old, but not in young hens, although plasma LH concentrations decreased in all birds. After transfer from 14 to 20 h light/day, plasma LH increased in young, but not in old, hens, without a change in the rate of egg production. Reproductive function was enhanced in old hens returned to long days after induction of a moult and ovarian regression by reducing daylength and dietary restriction. Moulted hens had a greater rate of egg production, higher concentrations of plasma LH and a greater pituitary-gland responsiveness to LHRH-II in vivo than unmoulted control hens. After transfer from 14 to 8 h light/day, egg-laying decreased more rapidly in unmoulted than in moulted hens; transfer to 17 h light/day increased egg production in moulted, but not in unmoulted, birds. Induction of ovarian regression in old hens by dietary restriction alone also enhanced reproductive function after the dietary restriction was relaxed. Egg-laying was more persistent in hens brought into lay for a second year by transferring them from 3 to 11 h light/day than in hens transferred from 3 to 20 h light/day. Egg production was stimulated in hens maintained on 3 or 11 h light/day for 42 weeks, after transfer to 20 h light/day. Egg production ceased in hens maintained on 20 h light/day for 46 weeks, after transfer to 3 h light/day.

These observations are consistent with the view that poor persistence of laying in hens <2 years old and exposed continuously to long days is caused, in part, by a reduction in hypothalamic–gonadotroph function. This reduction in neuroendocrine function may be due, in part, to the development of relative photorefractoriness.

Keywords: ovary; ageing; chicken; LH; egg-laying; seasonal breeding; photorefractoriness

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J. B. March, P. J. Sharp, P. W. Wilson, and H. M. Sang

The hypothesis that the onset of incubation behaviour (broodiness) in the domestic hen is induced by an increase in prolactin secretion was investigated by actively immunizing bantam hens against recombinant-derived chicken prolactin. A second objective was to establish whether active immunization against prolactin affects photoinduced onset of egg laying and the rate of egg production. The immunogen was a fusion protein (βgals–prolactin, 23 kDa) produced in Escherichia coli, comprising chicken prolactin (without the nine amino-terminal amino acids) fused to 18 amino acids of E. coli β-galactosidase. A control immunogen was produced in the same strain of E. coli harbouring the same plasmid vector used to produce βgals–prolactin minus the prolactin gene sequence. Hens were immunized i.m. with 1 mg of protein containing 0.8–0.9 mg of fusion protein in Freund's incomplete adjuvant at 4–8 week intervals beginning before or after egg laying, which was induced by increasing the daily photoperiod. The βgals–prolactin immunogen, but not the control immunogen, stimulated the production of antibodies to chicken prolactin. In Expts 1, 2 and 3, hens were placed in floor pens with nest boxes after photostimulation to induce broodiness. In these experiments, immunization with βgals–prolactin reduced the incidence or delayed the development of broodiness. This effect was more pronounced if immunization was initiated before, rather than after, the onset of egg laying. In Expts 1 and 2 hens were immunized with βgals–prolactin before photostimulation. The presence of antibodies to prolactin in their blood did not affect photoinduced onset of egg laying. In Expt 4, hens were maintained in individual cages to suppress the development of broodiness to measure the rate of egg laying. In these conditions, hens immunized with βgals–prolactin laid eggs at the same rate as did control hens. It is concluded that in the domestic hen increased prolactin secretion plays an important role in the initiation of incubation behaviour, and that active immunization against prolactin does not affect photoinduced onset or subsequent rate of egg laying.

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P L Rangel, A Rodríguez, S Rojas, P J Sharp, and C G Gutierrez

The chicken ovary is organized into a hierarchy of yellow yolky follicles that ovulate on successive days. Active or passive immunization of laying hens against testosterone blocks ovulation without affecting follicle development. Testosterone may play a role in pre-ovulatory follicle maturation by stimulating granulosa progesterone production. We assessed whether this stimulus is dose-related and depends on the maturity of the donor follicle, and if it does so by stimulating granulosa cell STAR, P450 cholesterol side-chain cleavage (P450scc), and LH receptor (LHCGR) mRNAs expression. Progesterone production by granulosa cells from F1, F3, and F4 follicles, cultured for 3 h without testosterone was greater in cells collected 11–14 h than 1–4 h after ovulation. These differences in progesterone production were less pronounced after granulosa cells had been cultured for 24 h. Culture of granulosa cells for 3 or 24 h with testosterone (1–100 ng/ml) stimulated progesterone production in cells collected from F4, F3, or F1 follicles 1–4, or 11–14 h after ovulation. Testosterone (0–4000 ng/ml) alone or in combination with LH (0–100 ng/ml) increased progesterone production by F1 granulosa cells, collected 1–4 and 11–14 h after ovulation and cultured for 3 h. Finally, testosterone (10 or 100 ng/ml) increased STAR, P450scc, and LHCGR mRNAs, when added to 3 h cultures of F1 granulosa cells. In conclusion, testosterone stimulates granulosa cell progesterone production in hen pre-ovulatory hierarchical follicles irrespective of maturational state, acting alone or additively with LH. We propose that testosterone promotes granulosa cell maturation to facilitate the pre-ovulatory release of LH.

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P L Rangel, P J Sharp, and C G Gutierrez

The preovulatory release of luteinizing hormone (LH) in the domestic hen occurs after the initiation of a preovulatory surge of testosterone. The objective of this study was to determine whether this testosterone surge has functional significance in the endocrine control of ovulation. Groups of laying hens (n=10–22) were treated with the androgen receptor antagonist, flutamide, at 8 h intervals for 24 h at doses of 0, 31.25, 62.5, 125 and 250 mg. All doses reduced egg laying (P < 0.001), with the highest dose being the most effective. In a second study, laying hens (n=9) were treated with 250 mg flutamide at 8 h intervals for 24 h with a control group being given placebo (n=10). Blood samples were taken for hormone measurements at 2 h intervals for 18 h starting 4 h before the onset of darkness. The percentage of hens laying per day did not differ between groups before treatment (control, 88% vs flutamide, 86%). Ovulation was blocked in all hens treated with flutamide within 2 days while the control hens continued to lay at the pretreatment rate (80%). Preovulatory surges of plasma testosterone, progesterone, oestradiol and LH were observed in control hens but with the exception of testosterone, flutamide treatment blocked the progesterone, oestradiol and LH surges. LH concentrations declined progressively with time in the flutamide-treated hens. It is concluded that inhibition of testosterone action blocks egg laying and the preovulatory surges of progesterone, luteinizing hormone and oestradiol demonstrating a key role for the preovulatory release of testosterone in the endocrine control of ovulation in the domestic hen.