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F. J. CUNNINGHAM
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SUMMARY

A bioassay of gonadotrophin depending on the induction of ovulation in immature mice is described.

Dose-response data to various preparations of gonadotrophin are examined.

Results by this method in some instances contrasted more sharply with those of the ovarian augmentation assay for follicle-stimulating hormone than did the results of the assay for total gonadotrophin.

The addition of prolactin, adrenocorticotrophic hormone and stilboestrol did not interfere with the assay.

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B. F. Fitzgerald
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F. J. Cunningham
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Plasma concentrations of prolactin in anoestrous ewes were respectively lowered or raised by the separate infusion of dopamine or thyrotrophin releasing hormone (TRH). Combined treatment with dopamine and TRH lowered the concentration of prolactin in plasma but the values increased markedly after the treatment was stopped and reached a level equivalent to that found in ewes treated with TRH alone. The results are interpreted as evidence that both dopamine and TRH play a regulatory role in determining the secretion of prolactin in the ewe.

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R. J. ETCHES
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F. J. CUNNINGHAM
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SUMMARY

The existence of a circadian rhythm in the sensitivity of the hypothalamus of the laying hen to stimulation by progesterone was investigated by injecting 0·5 mg progesterone subcutaneously during the proposed period of maximum insensitivity. Following this treatment increases in plasma concentrations of both LH and progesterone were observed which were comparable to the spontaneous preovulatory rises in the plasma levels of the hormones.

The ability of either progesterone or luteinizing hormone releasing hormone (LH-RH) to induce premature ovulation varied according to the stage of follicular development. Neither hormone was more than 28% effective when injected within 6·5 h of the previous ovulation, whereas both hormones were 100% effective approximately 27 h after the terminal ovulation of a clutch sequence. Failure to ovulate in response to LH-RH given 6·5 h after ovulation was associated with a lack of progesterone secretion.

Both LH and progesterone were secreted when ovulation was induced by injections of either LH-RH or progesterone, and LH was secreted in response to progesterone given 6·5 h after ovulation. These results demonstrate that progesterone stimulates the secretion of LH and LH stimulates the secretion of progesterone. The precise physiological role of these two hormones, however, was not established.

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S. C. Stansfield
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F. J. Cunningham
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ABSTRACT

An in-vitro superfusion system was used to study the effects of the endogenous opioid peptides [Met]-enkephalin (and its long-lasting analogue [d-Ala2, Met]-enkephalinamide), [Leu]-enkephalin and β-endorphin and of the opiate antagonist naloxone, on the secretion of LHRH from the mediobasal hypothalamus of the cockerel. The effects of the compounds on both basal release of LHRH and on release stimulated by a depolarizing pulse of increased extra-cellular potassium ion (64 mmol/l) were investigated. None of the endogenous opioid peptides altered basal release of LHRH; however, both [Met]-enkephalin (10 μmol/l) and [d-Ala2,Met]-enkephalinamide (1 μmol/l) significantly (P<0·05) reduced the response to depolarization. Neither [Leu]-enkephalin nor β-endorphin (0·1–10 μmol/l) were effective. Naloxone (1 μmol/l) administered alone significantly (P<0·05) increased basal release of LHRH and abolished the inhibitory effects of [Met]-enkephalin and [d-Ala2,Met]-enkephalinamide on depolarization-induced release. These results suggest that the endogenous opioid peptides exert a tonic inhibitory influence on LHRH secretion by the mediobasal hypothalamus of the cockerel.

J. Endocr. (1987) 114, 103–110

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S. C. Stansfield
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F. J. Cunningham
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ABSTRACT

Using an in-vitro superfusion system, the relative importance of three distinct subtypes of the opiate receptor in the control of the secretion of LHRH from the mediobasal hypothalamus of the cockerel was investigated. Basal release of LHRH was increased by the antagonist naloxone, which shows some μ-receptor selectivity, in a manner which was reversed by the μ-receptor specific agonist [d-Ala2, N-Phe4-Gly-ol5]-enkephalin (DAGO) and the μ- and δ-specific agonist [d-Ala2,N-Phe4,Met(0)ol5]-enkephalin (FK 33–824). The †-specific agonist [d-Thr2,l-Leu5]-enkephalyl-Thr (DTLET) and the κ-specific agonist 1-methyl-2(3-thienylcarbonyl)-aminomethyl-5-(2-fluorophenyl)-H-2,3-dihydro-1,4-benzodiazepine (KC 6128; (+)-titfluadom) did not reverse the effect of naloxone. The δ-specific antagonist N,N-diallyl-Tyr-α-aminoisobutyricacid-Phe-Leu-OH (ICI 174,864) failed to influence basal release. Release of LHRH stimulated by increasing the potassium ion concentration of the superfusate to 48 mmol/l was reduced by DAGO in a manner which was reversed by naloxone, and by FK 33–824 in a manner which was reversed by both naloxone and ICI 174,864. The agonists DTLET and titfluadom did not affect stimulated release of LHRH. These results support the proposal that spontaneous release of LHRH is tonically inhibited by agonists acting through the μ-receptor whilst, in response to a stimulus, the δ-receptor, in addition to the δ-receptor, may be involved.

J. Endocr. (1987) 114,111–117

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B. P. Fitzgerald
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F. J. Cunningham
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Metoclopramide and bromocriptine, drugs which are respectively antagonistic and agonistic to dopamine receptor activity, were used to investigate the control of prolactin secretion in the ewe during pregnancy. Normal concentrations of prolactin were found during pregnancy, i.e. values were low and stable during the first 100 days but thereafter increased progressively to reach maximum values at day 140. Treatment with bromocriptine suppressed levels to undetectable values (< 1·4 μg/l plasma) between days 40 and 100 of pregnancy. Later in gestation, however, bromocriptine administered between days 120 and 140 was less effective, indicating that the pituitary gland was less responsive to the same degree of inhibition. Treatment with metoclopramide alone provoked a progressively greater release of prolactin as pregnancy advanced whereas in bromocriptine-treated animals an associated release of prolactin in response to metoclopramide was seen only between days 120 and 140 of pregnancy.

These results are interpreted as evidence that the raised concentrations of prolactin characteristic of late pregnancy in the ewe are due to a reduced responsiveness of the pituitary gland to inhibitory stimuli, or to a decreased secretion of endogenous dopamine into the hypophysial portal blood system or to both.

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SUSAN C. WILSON
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F. J. CUNNINGHAM
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The role of oestradiol in the regulation of LH release in the hen was studied by use of the anti-oestrogen, tamoxifen (ICI 46,474).

Intramuscular injection of laying hens with 2 or 4 mg tamoxifen on 2 successive days delayed or prevented the occurrence of the preovulatory release of LH and ovulation expected on day 3. Ovulation could be restored by i.v. injection of 20 pg LH releasing hormone (LH-RH). Tamoxifen at a dose of 1 mg affected neither the timing of the preovulatory release of LH nor ovulation. Treatment with 2 or 4 mg tamoxifen on 2 successive days reduced the effectiveness of an i.m. injection of progesterone to stimulate a release of LH. Injection of 1, 2 or 4 mg tamoxifen on 2 successive days significantly raised basal levels of LH in the blood at 24 h after the last injection. This was associated with an increase in the capacity of the pituitary gland to respond to an injection of synthetic LH-RH by a release of LH.

These studies suggest that oestradiol has at least two roles in the regulation of LH release in the hen. First, it maintains a low basal level of LH in the blood by reducing the responsiveness of the pituitary gland to LH-RH. Secondly, oestradiol has a facilitative role in the mechanism by which progesterone stimulates the preovulatory release of LH.

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B. E. SENIOR
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F. J. CUNNINGHAM
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The rôles of oestrogen and progesterone in the control of the release of luteinizing hormone (LH) required for ovulation in the laying hen have been extensively discussed (Van Tienhoven, 1961; Furr, Bonney, England & Cunningham, 1973), but firm conclusions have not been reached. In laying hens progesterone and LH levels in plasma rise simultaneously 4–7 h before ovulation (Cunningham & Furr, 1972; Furr et al. 1973) and administration of progesterone advances the time of ovulation (Fraps & Dury, 1943). Oestrone and oestradiol also rise at this time (Peterson & Common, 1972; Senior, 1973a), but oestrogens have only been shown to inhibit ovulation (Fraps, 1954, 1955). The purpose of the present work was to examine the temporal relationship between the levels of oestradiol and LH in plasma throughout the 12–14 h period before ovulation.

Thirteen White Leghorn laying hens were caged singly in a light-proof room and exposed to 14

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ANNE STOCKELL HARTREE
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F. J. CUNNINGHAM
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SUMMARY

Separation and partial purification of chicken pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH) has been obtained by methods which were effective in the purification of the corresponding human and horse hormones. Increases in chicken LH activity were observed after chromatography on DEAE-cellulose and on Amberlite IRC-50 suggesting removal of an LH inhibitor. The biological potencies of chicken FSH and LH preparations when assayed in mammals were very much lower than those of the corresponding mammalian fractions on a weight basis. A weak immunological cross-reaction between chicken and human pituitary LH was used to estimate LH in chicken pituitary fractions and the results were compared with bioassays of the same fractions.

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SUSAN C. WILSON
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F. J. CUNNINGHAM
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Plasma concentrations of corticosterone and LH were measured in hens in relation to the ovulation of an egg either early or late in a sequence and also in hens injected with gonadal steroids. In hens about to ovulate an egg early in the sequence, the plasma concentration of LH rose at the onset of darkness while levels of corticosterone remained low. As levels of LH fell during the 4 h preceding ovulation, those of corticosterone increased steeply to reach a maximum at about the time of ovulation, which is shortly after dawn. In contrast, in hens about to ovulate an egg late in a sequence, the concentration of LH in plasma fell and that of corticosterone tended to increase during the hours of darkness. During the 7 h preceding ovulation the temporal relationship of LH to corticosterone was similar to that observed before an early ovulation such that levels of corticosterone increased rapidly on the descending slope of the preovulatory LH surge and reached maximum values at the time of oviposition and ovulation.

Injections of oestradiol benzoate and testosterone propionate did not alter plasma concentrations of either corticosterone or LH. On the other hand, the injection of progesterone was associated with a rapid fall in levels of corticosterone. It is suggested that progesterone may suppress the pituitary–adrenal system and thus modulate the pattern of corticosterone secretion during the ovulatory cycle.

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