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P. J. SHARP

The basal hypothalamus of Coturnix quail contains gonadotrophin releasing factor (GRF) activity (Follett, 1970) but the precise location of the neurons producing the neurohormone(s) is unknown. Lesioning studies show that the destruction of either of two sites blocks photo-inducible testicular growth. One of these lies in the dorsal basal hypothalamus around the paraventricular organ and the other directly above the median eminence (Sharp & Follett, 1969). These two areas are morphologically distinct when considered either in terms of the distribution of monoamines (Sharp & Follett, 1968) or of tanycyte processes (Sharp, 1972).

To explore this problem further, pituitaries were taken from sexually mature quail and fragments of the cephalic lobes were implanted into the hypothalami of gonadectomized birds. By removing the negative feedback effect of gonadal steroids it was hoped to stimulate GRF activity. It was anticipated that, as in rats (Halász, Pupp & Uhlarik, 1962), implanted pituitary cells would

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P. J. SHARP

SUMMARY

Changes in plasma LH concentrations were followed in chickens of both sexes from hatch to sexual maturity using a radioimmunoassay. Mean levels of LH were lower in the females than in the males at all stages of development. These levels rose rapidly in both sexes during the first week after hatch to maxima of 6·5 ± 1·2 (s.e.m.) ng/ml (n = 6) in the males and 4·6 ± 0·6 ng/ml (n = 6) in the females. Thereafter levels of the hormone in the circulation stabilized in the males but fell over a period of 1 or 2 weeks in the females to 2·5–3 ng/ml.

Plasma LH levels started to rise steeply in both sexes when they were between 16 and 19 weeks old at the same time as there was an increase in the rate of comb growth. Afterwards in six of the males studied in detail the mean plasma LH level rose significantly (P < 0·01) over a period of 5–8 weeks from 8·1 ± 1·2 to 13·2 ± 1·9 ng/ml. In a parallel study on six females the rate of LH secretion increased for approximately 3 weeks and then decreased for about the same period forming a prepubertal LH peak. The first eggs were laid between 22 and 25 weeks of age when mean plasma LH levels had fallen to about 1·8 ng/ml. The mean plasma LH level in these hens when they were laying (1·8 ± 0·3 ng/ml) was significantly lower (P < 0·01) than when they were sexually immature (2·7 ± 0·3 ng/ml). The duration of the period of rapid comb growth in each bird was closely related in the males to the time during which prepubertal LH levels were rising rapidly, and in the females to the duration of the prepubertal LH peak.

Differences in mean plasma LH concentrations in individual birds of either sex before the onset of puberty appeared to be related to subsequent reproductive performance.

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P. J. SHARP and R. MASSA

In the laying hen, progesterone was shown to be converted in vitro in the pituitary gland and the hypothalamus to 5β-pregnane-3,20-dione (5β-DHP), 5β-pregnan-3α-ol-20-one (5β,3α-ol) and 5α-pregnane-3,20-dione (5α-DHP) and in the hyperstriatum dorsale to 5β-DHP and 5β,3α-ol. The conversion of progesterone to 5β-reduced metabolites was greater in the hyperstriatum dorsale than in the hypothalamus (P<0·001) and greater in the hypothalamus than in the pituitary gland (P <0·01). The conversion of progesterone to 5β-reduced metabolites was greater than its conversion to 5α-DHP in the pituitary gland (P <0·01) and the hypothalamus (P < 0·001).

The possibility was investigated that 5α-DHP and 5β-DHP may act as metabolic intermediaries in the mechanism by which progesterone exerts a positive feedback effect on LH release. Progesterone, 5α-DHP and 5β-DHP were injected into laying hens at doses of 0·05,0·25 and 1·25 mg/kg and the changes in the concentration of plasma LH were followed for 4 h thereafter. Secretion of LH was stimulated after treatment with progesterone or 5α-DHP but not 5β-DHP. Progesterone stimulated LH release more effectively than did 5α-DHP, since an increase in the concentration of plasma LH was observed after 0·25 mg progesterone/kg but not after the same dose of 5α-DHP. It was concluded that in the hen 5α-DHP is unlikely to play a role in the induction of the preovulatory release of LH.

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P. J. SHARP and G. BEUVING

SUMMARY

This study was undertaken in laying hens to investigate the possibility that a diurnal increase in the concentration of plasma corticosterone is directly responsible for timing the preovulatory surge of LH which results in the first egg of a sequence.

Provided that the ovary contained a mature follicle, i.m. injection of 0·5 or 2·0 but not 0·1 mg corticosterone/kg stimulated a preovulatory release of LH. The dose of 0·5 mg/kg was less effective than that of 2 mg/kg and induced release of LH in only four out of eight hens. However, it resulted in concentrations of plasma corticosterone which were outside the physiological range.

Variations in the concentrations of plasma corticosterone were measured in ten hens on two successive nights for 8·5 h starting at the onset of darkness. The birds were maintained on a lighting regimen of 14 h light/day. The hens were selected so that on the first night there was no preovulatory release of LH while on the second night there was the first preovulatory surge of LH of a sequence starting soon after the onset of darkness. No diurnal increase in the concentration of plasma corticosterone was observed during the first 6 h of darkness on either night nor was any increase seen before the preovulatory release of LH.

These observations suggest that corticosterone is not directly involved in the timing of the first preovulatory surge of LH of a sequence.

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R. MASSA and P. J. SHARP

The metabolism in vitro of [4-14C]testosterone to reduced derivatives was studied in the pituitary gland, hypothalamus and hyperstriatum dorsale of cockerels from hatch to sexual maturity. The most important metabolites were 5β-dihydrotestosterone (5β-DHT), 5β-androstane-3α,17β-diol (5β-3α-diol) and 5β-androstane-3β,17β-diol. Trace amounts of androstenedione and, in the hypothalamus only, of 5α-DHT were also detected. The amounts of 5β-reduced metabolites produced by all neuroendocrine tissues declined progressively during maturation with the steepest fall occurring during the first 2 weeks after hatch. At all ages studied, 5β-DHT was formed to the greatest extent by the hyperstriatum dorsale, to a lesser extent by the hypothalamus and in the smallest quantities by the pituitary gland. In the three tissues studied, 5β-3α-diol tended to be formed to the greatest extent by the pituitary gland.

No significant change was observed in the metabolism of testosterone to reduced derivatives in any of the neuroendocrine tissues after castration.

It was concluded that in the cockerel, unlike the rat, a change in 5α-reductase activity of the neuroendocrine tissues is unlikely to be involved in the initiation of puberty. The physiological significance of 5β-reductase activity in the neuroendocrine tissues remains to be established.

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J. B. WILLIAMS and P. J. SHARP

Peripheral blood samples were taken from laying hens at frequent intervals during various periods of the ovulatory cycle in order to detect small changes in the concentrations of progesterone and androgen which might be important in initiating the preovulatory release of LH.

Blood samples were taken from seven hens at 1 h intervals for 3 h when the ovary contained a mature (C1) follicle and on another occasion, when the largest ovarian follicle was immature. The concentrations of progesterone and androgens in the plasma were 30% higher when there was a mature C1 follicle present in the ovary than when there was not, but this increase was significant (P < 0·05) only for progesterone.

The concentrations of progesterone and androgens were also measured in blood samples taken at 30 min intervals during the 3 h before and after the initiation of the first preovulatory LH surge of a sequence. The hens were kept on a lighting schedule of 14 h light/day and the first LH surge of a sequence was initiated at the beginning of the dark period. Just after the onset of darkness there was a small increase in the concentration of LH in the plasma and a subsequent, larger preovulatory release of LH. The first increase in the level of LH was associated with a small rise in the concentrations of androgens and progesterone in the plasma while the preovulatory release of LH was accompanied by a much larger increase in the secretion of these steroids.

It is proposed that the increase in the level of LH in the plasma at the onset of darkness stimulates the maturing ovarian follicles to secrete progesterone and androgens and that the quantities of these steroids secreted (particularly of progesterone) depends on the maturity of the largest ovarian follicle. If the largest ovarian follicle is mature, then the increase in the level of LH in the plasma associated with the onset of darkness stimulates the secretion of a quantity of progesterone sufficient to cause the preovulatory surge of LH. A diurnal increase in the concentration of LH in the plasma could, therefore, be responsible for timing the preovulatory surges of LH so that they are only initiated at night.

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J. B. WILLIAMS and P. J. SHARP

Agricultural Research Council's Poultry Research Centre, King's Buildings, West Mains Road, Edinburgh, EH9 3JS

(Received 20 June 1977)

In the maturing hen the concentration of luteinizing hormone (LH) in the plasma rises to a prepubertal maximum and then declines over a 2–3 week period until the first egg is laid (Sharp, 1975). Sharp (1975) postulated that increased secretion of progesterone by the developing ovary may be responsible for this fall in the level of LH since the large, yolky follicles, which are thought to be steroidogenic (Furr, 1969), develop over a 2–3 week period before the onset of lay (Wilson & Sharp, 1975). The present experiment was designed to test this hypothesis by direct measurement of the amount of progesterone and LH in the blood of growing hens.

Variations in the level of LH in the peripheral plasma were determined by an homologous radioimmunoassay for avian LH (Follett, Scanes &

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SUSAN C. WILSON and P. J. SHARP

SUMMARY

Single intramuscular injections of 0·5 mg progesterone/kg resulted in increased LH secretion in laying hens but not in pullets with completely undeveloped sexual organs. Injections of the steroid were first able to stimulate LH release 8–10 weeks before the onset of lay when the comb, ovary and oviduct had started to grow and basal plasma LH concentrations were beginning to rise. At this time, injections of 10 μg synthetic LH-RH/kg resulted in an incremental change in plasma LH levels of around 26 ng/ml. A similar incremental change was observed after giving the same dose of LH-RH to pullets with no signs of sexual development.

Three to four weeks before the first eggs were laid, basal plasma LH levels started to fall, the pituitary became progressively more insensitive to synthetic LH-RH and injections of 0·5 mg progesterone/kg resulted in a reduced LH response. Ten μg LH-RH/kg caused incremental changes in blood levels of LH of less than 5 ng/ml.

The final stage of sexual maturation occurred during the week before the onset of lay and was characterized by a rapid growth of large yolky ovarian follicles and a further fall in the sensitivity of the pituitary to synthetic LH-RH. However, injections of 0·5 mg progesterone/kg resulted in a prolonged release of LH.

These observations are discussed in relation to the maturation of the positive feedback mechanism by which progesterone stimulates the secretion of LH.

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SUSAN C. WILSON and P. J. SHARP

SUMMARY

Changes in plasma LH concentrations after i.m. injections of 0·5 mg progesterone/kg at various stages of the ovulatory cycle were measured by radioimmunoassay. Four types of response were observed. (1) When the steroid was injected between 4 h after and 12 h before an ovulation, LH levels started to rise after 15–45 min and reached peak values within 90–120 min. The mean maximal incremental change in the level of LH was 1·58 ± 0·10 (s.e.m.) ng/ml (n = 37). (2) In contrast, when progesterone was injected 12–8 h before ovulation, i.e. immediately before a spontaneous pre-ovulatory LH surge, the resulting mean maximal incremental change in LH level, 0·79 ± 0·12 ng/ml (n = 9), was significantly smaller (P < 0·001). (3) If progesterone was injected 8–4 h before ovulation, i.e. when pre-ovulatory LH levels were rising, they immediately started to rise more rapidly and reached peak values within 45 min. The maximal incremental change in the level of LH under these circumstances, 2·34 ± 0·20 ng/ml (n = 12), was significantly greater (P < 0·001 in both cases) than the changes observed in the responses 1 and 2 described above. (4) Levels of LH generally showed no incremental change in response to injections of progesterone given 4–0 h before ovulation, i.e. when pre-ovulatory LH levels were falling.

It was concluded that the type of change in plasma LH levels induced by progesterone depended upon the stage of the ovulatory cycle at which the steroid was injected.

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H. M. FRASER and P. J. SHARP

MRC Unit of Reproductive Biology, 2 Forrest Road, Edinburgh, EH3 9ER and * Agricultural Research Council's Poultry Research Centre, King's Buildings, West Mains Road, Edinburgh, EH9 3JS

(Received 22 August 1977)

The decapeptide luteinizing hormone releasing hormone (LH-RH) stimulates the release of luteinizing hormone (LH) in birds as well as in mammals (Van Tienhoven & Schally, 1972) and a substance immunochemically similar to LH-RH is present in the chicken hypothalamus (Jeffcoate, Sharp, Fraser, Holland & Gunn, 1974). Avian LH-RH has still to be isolated and sequenced, however, and there is some doubt about whether the decapeptide is the naturally occurring LH-RH in the bird (Jackson, 1971).

In the hen, release of LH is induced by the positive feedback action of progesterone (Wilson & Sharp, 1975) which is presumably associated with the release of chicken LH-RH. To gain further information about the identity of chicken LH-RH and to investigate the site