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S. R. Milligan

Summary. When female voles were allowed contact with the stud male for only 1 h at the time of mating, 55% exhibited pregnancy failure when exposed to a strange male 48 h later. When females were made pseudopregnant by hormone treatment and vaginal stimulation (i.e. no stud male involved), 87% exhibited luteal failure when exposed to a strange male. It is suggested that the characteristics of the stud male are rapidly imprinted upon the female at the time of mating and that this imprinting is important in preventing the female showing a blocking response to this male upon any subsequent exposure.

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S. R. Milligan

Summary. The tissue accumulation of 125I-labelled human serum albumin 30 min after intravenous injection was used as an index of uterine vascular permeability. In ovariectomized mice, all sham and experimental instillation procedures produced a 6–10-fold increase in vascular permeability. Some effects were also manifest in the contralateral, control horn. In ovariectomized rats, instillation of saline and arachis oil increased vascular permeability 3–7-fold. After 3 or more days of progesterone treatment following oestradiol priming, fluorocarbon and arachis oil instillation produced marked vascular responses, but these were not restricted to the transient period in which the uterus would respond with decidualization. An IUD prevented the response to arachis oil instillation. These results indicate that the uterus is very sensitive to any manipulation and are consistent with decidualization representing a specialization of a normal uterine inflammatory response.

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S. R. MILLIGAN

Summary.

Although M. agrestis normally ovulates following coitus, virgin females in the present study ovulated when separated from a male by a single barrier of wire mesh. A smaller number ovulated when tactile contact with the male was eliminated. Changing the male behind the barrier appeared particularly effective in inducing ovulation. Control females, in the absence of males, did not ovulate. Corpora lutea resulting from the ovulations normally started to degenerate within about 3 days of their formation. Short-term changes in the vaginal smear and in sexual behaviour accompanied the ovarian changes. Direct histological evidence indicated that ovulations could be repeated once; indirect evidence (based on vaginal smears) suggested that short ovulatory cycles could occur. The significance of these results in relation to the ovulatory mechanism of the vole is discussed.

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S. R. Milligan

Summary.

Pregnancy blocking in M. agrestis was associated with a rapid degeneration of CL, growth of follicles, a loss of embryos and return of the uterus to its non-pregnant state, and a return to cornified vaginal smears. These results are discussed in relation to the proposal that the immediate cause of pregnancy block is a failure of prolactin secretion resulting in a failure of CL function.

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S. R. Milligan

Summary.

Pregnancy failed in a high proportion of newly mated Microtus agrestis females when they were exposed to a strange male between 48 and 72 hr after mating with a stud male. This effect of the strange male was testosterone dependent. Direct contact with the strange male was normally necessary, and even a single barrier of wire mesh between the female and strange male prevented the male from exerting his pregnancy-blocking effect. The results suggest that the stimuli mediating pregnancy blocking in the vole may differ from those operative in the mouse.

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S. R. MILLIGAN

Summary.

A study of the relationship between the mating behaviour of the vole and the induction of ovulation and CL function is described. A single intromission or an injection of LH-RF constitute stimuli which induce ovulation, but normally give rise to CL that degenerate soon after formation. More prolonged mating, or mechanical stimulation of the vagina and cervix given after a separate ovulatory stimulus, result in the maintenance of the CL. Mechanical genital stimulation is effective in inducing CL maintenance when given up to 48 hr after an LH-RF injection. Similarities, therefore, are apparent between the vole, an induced ovulator, and spontaneous ovulators such as the rat, mouse and hamster.

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S. R. Milligan

Summary. Female voles were subjected to various regimens of subcutaneous injections of oestradiol-17β, oestradiol benzoate and/or progesterone. Ovulation occurred in only a few of the mature females and in none of the immature animals. There was no indication of any increased LH levels in blood samples taken every 2 h for 50 h after 150 μg oestradiol-17β.

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S. R. MILLIGAN

Department of Agricultural Science, University of Oxford, Parks Road, Oxford

(Received 29th November 1974)

Although members of the species Microtus agrestis are induced ovulators (Austin, 1957; Breed, 1967), ovulation may be induced by stimuli other than those associated with mating, e.g. when females are separated from adult males by wire mesh (Milligan, 1974). Tactile contact between the sexes is not necessary for this ovulatory response as ovulation still occurred when two barriers of mesh, 2·5 cm apart, separated the male and female (Milligan, 1974). The present experiments continued the investigation of the nature of the effective stimuli for ovulation which are provided by non-coital contact with a male.

Laboratory bred voles from the `mixed-type' colony described by Milligan (1974) were used. Males were fertile adults from the breeding colony; all females had shown cornified vaginal smears on 3 consecutive days before the experiments. In Exp. 1, virgin females were treated

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S. R. Milligan

Summary. Female voles mated with a male and experiencing a single ejaculatory series exhibited a rapid and marked elevation of plasma LH levels. The profile of the LH surge had a broad peak 20–40 min after mating. Pentobarbitone sodium administered immediately after mating did not affect the magnitude of the surge. The LH response to exogenous LH-RH was not affected by the state of receptivity of the female or by gonadectomy. There was no evidence of any priming effect of 25 ng LH-RH on the pituitary response to a second injection of LH-RH 20 or 60 min later. These results are consistent with the primary cause of the LH surge in the reflexly ovulating vole being a rapid secretion of LH-RH.

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S. R. Milligan and L. Martin

Summary. The hydrostatic pressures generated during controlled flushing of the mouse uterus increased at implantation and under conditions of uterine closure. These pressures may be responsible for inducing tissue damage during flushing. The possibility that samples collected by flushing might be contaminated with interstitial fluid or plasma was studied using intravenously administered51 Cr-labelled EDTA and 125I-labelled human serum albumin as markers. The presence of both tracers was detected in all flushings and was greatest in flushings from uteri with luminal closure and early implantation sites. These observations raise serious doubts about the validity of the flushing technique for analysing uterine luminal constituents in mice.