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D. B. CRIGHTON

Summary.

In a preliminary experiment carried out in Essex sows, three treatments were compared with a control group of lactating sows. The treatments were: (1) intermittent separation of the sow from the young, (2) injection of pregnant mare serum gonadotrophin (pmsg), and (3) a combination of treatments 1 and 2.

None of the control sows exhibited oestrus during lactation. Of the oestrus-induction methods used, only the combination of separation and pmsg injection induced oestrus consistently.

In an extension of the preliminary experiment, the combined treatment was tested in Essex, Large White and Large White × Landrace sows. Of forty sows treated overall, thirty-three exhibited oestrus during lactation and twenty-four carried pregnancies to term as a result of mating at the induced oestrus. The mean interval between parturitions was reduced by approximately 25 days in sows becoming pregnant during lactation. There appeared to be no detrimental effects of 2 or 3 weeks of concurrent lactation on pregnancies established during lactation.

The results are discussed in relation to the possible modes of action of separation and pmsg injection in the lactating sow.

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D. B. CRIGHTON

The ovarian ascorbic acid depletion (oaad) assay of Parlow (1958) is an established method for the measurement of luteinizing hormone (lh). Tests using purified preparations of other anterior pituitary hormones have shown the bio-assay to be specific for lh (McCann & Taleisnik, 1960; Schmidt-Elmendorff & Loraine, 1962) but vasopressin possesses appreciable activity (McCann & Taleisnik, 1960). Pelletier (1964) has reported that hypophysectomized ewe plasma caused a significant response when assayed by this method. Evidence has been produced suggesting that the plasma acted by releasing lh from the pituitary glands of the assay rats (Pelletier, 1965).

Recently, determinations of the pituitary content of lh in the sow at various reproductive stages and after ovariectomy have been carried out in this laboratory. A preliminary report of this work has

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R. B. LAND, D. B. CRIGHTON and G. E. LAMMING

Summary.

The pituitaries of Merino × Blackface ewes were found to contain more lh at mid-cycle (Day 10) than Finn × Blackface ewes. The content in purebred Blackface ewes was intermediate. No differences were observed in the relative activities of fsh or lh in the pituitaries of the three breed types around the time of oestrus, or of fsh on Day 10 of the cycle. It is postulated that more lh may be released during mid-cycle by the more highly fertile breed type.

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MAXINE GRIFFITHS, K. J. COOPER and D. B. CRIGHTON

It is established that stimuli associated with the presence of a sexually mature male can influence the oestrous cycle of the underfed rat (Cooper & Haynes, 1967). This effect is still present when physical contact between the sexes is prevented, the lengthened and irregular cycles characteristic of undernutrition becoming significantly shorter (McNeilly, Cooper & Crighton, 1970; Purvis, Cooper & Haynes, 1971). A similar response to the proximity of the male has been demonstrated in the mouse, the lengthened and irregular oestrous cycles usually observed when mice are grouped not being found in the presence of a male (Whitten, 1957). Castration of the male prevented the acceleration of oestrous behaviour, but the stimulus value of the male for the female returned following androgen therapy (Bronson & Whitten, 1968). The object of the work

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A. S. McNEILLY, K. J. COOPER and D. B. CRIGHTON

Dietary restriction in the rat has been shown to affect the oestrous cycle, increasing the length of the di-oestrous period and producing a reduction in the cellular content of the vaginal smear (Mulinos & Pomerantz, 1940). Cooper & Haynes (1967) demonstrated that the introduction of a male to the cage of a female rat subjected to dietary restriction resulted in a shortening of the cycle regardless of the stage at which the male was introduced. In those animals which failed to mate at the first oestrus after introduction of the male (detected by the vaginal smear technique), the subsequent cycle again appeared to be of shorter duration than those before the introduction of the male, indicating that the female continued to respond to the presence of the male. The present
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W. HARESIGN, J. P. FOSTER, N. B. HAYNES, D. B. CRIGHTON and G. E. LAMMING

Summary.

Plasma progesterone determinations were carried out on blood samples collected daily from Clun Forest ewes during the normal oestrous cycle and also after administration of LH-releasing hormone (LH-RH) during seasonal anoestrus.

Levels of plasma progesterone at oestrus ranged from 0·1 to 0·5 ng/ml and luteal phase levels from 3 to 6 ng/ml. Levels found during seasonal anoestrus were within the range of those observed at oestrus. Following treatment with LH-RH, there was an increase in the plasma LH level in all cases and ovulation occurred in twenty-three out of twenty-seven treated ewes. In the animals which ovulated, the plasma progesterone concentration either remained basal (eighteen animals) or rose to a lower level (<2 ng/ml) than that found during the luteal phase of the cycle.

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D. B. CRIGHTON, J. P. FOSTER, W. HARESIGN and SUSAN A. SCOTT

Department of Physiology and Environmental Studies, University of Nottingham School of Agriculture, Sutton Bonington, Loughborough, Leicestershire

(Received 20th November 1974)

It has been shown that administration of a single intravenous injection of 150 or 300 μg synthetic LH-releasing hormone (LH-RH) to seasonally anoestrous Clun Forest ewes induced LH release in all animals and ovulation in the majority (Foster & Crighton, 1973, 1974). The induced LH peak was, however, much smaller than that observed at oestrus (Foster & Crighton, 1974). Luteal function, as assessed by peripheral plasma progesterone levels, was absent in the majority of treated animals (Crighton, Foster, Haresign, Haynes & Lamming, 1973; Haresign, Foster, Crighton, Haynes & Lamming, 1975), possibly because of an inadequate release of LH before ovulation. As an alternative means of administration to simulate more accurately the natural preovulatory LH peak, a single dose of 150 μg LH-RH was administered subcutaneously or intramuscularly instead of intravenously,