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H. M. BRUCE

Summary.

The incidence of the pregnancy-block caused by alien males was not augmented by increasing the numbers of males to which the female was exposed, but it was reduced by the presence of other females in proportion to their numbers.

When females were deliberately crowded during pregnancy an increased susceptibility to disease was noted, but there was no evidence of the hormonal disturbance otherwise associated with such stress.

A mild degree of stress was indicated even when the housing conditions were adequate, among mixed populations formed after mating, but not among pregnant females housed together in the absence of males.

Pregnancy failure was more than three times as frequent under inadequate physical conditions of housing during Days 1 to 3 after mating, than under standard control conditions.

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H. M. BRUCE

Summary.

Repeated suppression of luteotrophic activity by olfactory stimulation from males other than the stud male, was without effect on the subsequent fertility of female mice at least up to 5 months, the longest period recorded.

Females in which such suppression had occurred, however, showed a tendency towards spontaneous pseudopregnancies following the block, in place of the normal 4-day oestrous cycle. This suggests that such olfactory stimulation may affect the release of luteotrophin rather than its production.

The reaction of the recently-mated female to each period of exposure to alien males was apparently not affected by her reaction to the same situation on previous occasions.

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H. M. BRUCE

Summary.

(1) The time relations of the olfactory block to pregnancy in the mouse have been examined. (2) The female is most vulnerable within 48 hr of coitus (80% pregnancies blocked) and the reaction rapidly disappears after the start of implantation. (3) Exposure of at least 48 hr duration is necessary to elicit the maximum number of blocked pregnancies but, during the period of maximum vulnerability, the reaction occurs in some females with exposure lasting only 12 hr. (4) Over 90% of females in which pregnancy is blocked return to oestrus within 4 days of the beginning of exposure, with a peak (56%) on the third day.

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H. M. BRUCE

Summary.

Experiments are described that illustrate the close control over mammary-gland activity exercised by the stimulus of suckling in pregnant and non-pregnant rats.

By selection of suitable foster litters, chosen to reduce to a minimum the change in the demand for milk at each exchange, lactation was maintained for 10 to 12 months in three females. Sufficient milk was produced regularly to support a uniform rate of growth of the young, giving a total of 2 to 4 kg milk per female during the course of a single lactation.

Oestrus recurred regularly during prolonged lactation at intervals of about 18 days. When mating was permitted, pregnancy was normal, parturition taking place as expected in the non-lactating animal. Lactation was not affected by the presence of the male or by the initiation of pregnancy, provided the young were left with the mother. Towards the end of pregnancy, however, lactation always failed despite continued suckling. It was thus not possible to maintain a continuous lactation from the birth of one litter to the birth of the next. This was true also of lactations concurrent with post-partum pregnancies.

At various intervals up to 3 months since the previous lactation, mammary-gland activity sufficient to support normal litter growth was induced in parous females after some weeks by suckling alone. Virgin females could not be brought into lactation by this means.

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H. M. BRUCE

Summary.

Pregnancy and pseudopregnancy both failed in a high proportion of mice when the recently mated female was housed with or near strange males, particularly males of a different strain, or when she was housed with a castrated male. In these circumstances, the female returned to oestrus 4 to 5 days after the original mating and implantation did not take place.

The use of genetically marked test males of a different strain from the stud males showed that superfoetation did not occur.

By contrast, pregnancy was not blocked when the female was returned to her own stud male after being separated from him for 24 hr, or when she was kept in the presence of other females. The evidence suggests that the presence of other females may even help towards stabilizing a pregnancy.

The histology of the ovaries from females having blocked pregnancies showed a varying degree of pituitary-gonadotrophin stimulation.

The fate of the blastocyst from the first mating remains as yet undetermined.

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H. M. BRUCE

Summary.

Evidence is presented that olfactory stimulation and nutritional stress are separate and distinct pregnancy-blocking agents, and that the latter is not a complicating factor in the reaction set up by the former.

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H. M. BRUCE

The recently-mated female mouse can be removed from her stud partner within a few hours of coitus and returned to him 24 hr later without disturbance to the pregnancy. [Outbred albino females: Bruce (1959); inbred CBA females: Godowicz (1967).] It is only when she is exposed to another male at this time that pregnancy may be blocked. This presupposes at least partial recognition of the stud male as an individual, amounting to about 25% for outbred albino Ρ males and about 30% for males of an outbred Dutch strain (Parkes & Bruce, 1961). In wild house mice pregnancy rate was reduced to 16% among recently-mated females exposed to strange males from 56% among control females similarly disturbed but without the intervention of a strange male (Chipman & Fox, 1966). This implies an even greater variation among wild house mice. Experiments have been undertaken to investigate the possibility that such individual differences are still retained between males belonging to inbred strains.

Vaginal smears were examined daily from young virgin females paired with fertile males. When the vaginal plug was found (Day 0), the female was separated from the stud male and housed alone. After 24 hr she was introduced into the box containing the test male, where she remained for 3 days. At the

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H. M. Massa and N. W. Bruce

The effects of noradrenaline on the rates of secretion of ovarian progesterone and 20α-hydroxypregn-4-en-3-one (20α-OHP), blood flow and oxygen consumption were examined in rats on day 16 of pregnancy. A modified venous outflow technique was used to infuse noradrenaline directly into the ovary, without recirculation, and to monitor subsequent changes in the ovary. Noradrenaline was infused for periods of 10 min at a low and a high concentration, which achieved effective blood concentrations of about 6.25 and 25 ng ml−1, respectively. Each period of noradrenaline infusion was interspersed by a 10 min period of infusion of its ascorbic acid carrier. Two series of infusions of low and high concentrations of noradrenaline were carried out on each rat. Neither the infusion of the ascorbic acid carrier nor of the low concentration of noradrenaline had any effect on ovarian progestin secretion. The high concentration of noradrenaline reduced blood flow by 30% but had no apparent effect on progestin secretion or oxygen consumption. Collectively, these findings question the generally accepted view that noradrenaline has a physiological role in the regulation of progesterone secretion. Further, putative luteotrophins need to be examined in the intact ovary as well as under in vitro and indirect in vivo conditions to determine their physiological role.

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A. S. PARKES and H. M. BRUCE

Summary.

Newly mated female mice were placed singly for periods of up to 3 days in boxes recently vacated by five males of a different strain (alien males). When the soiled boxes were renewed twice daily, the females showed pregnancy-block to the same extent as when placed in the proximity of alien males for similar periods. When the soiled boxes were renewed once daily, the incidence of pregnancy-block was much reduced. An equal amount of disturbance, caused by similarly transferring the females to clean cages, did not affect the incidence of pregnancy.

These results support previous work in indicating that the smell of the alien male is the operative factor in pregnancy-block, and further suggest that the odorous substances involved are highly evanescent.

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H. M. BRUCE, R. B. LAND and D. S. FALCONER

Summary.

Pregnancy-block after exposure of recently inseminated females to strange males was studied in mice of the Q strain. In one series of experiments the incidence of blocked pregnancy was very low. This strain displayed a normal increase in the frequency of oestrus when the females were exposed to males, hence the low pregnancy-block rate could not be attributed to anosmia. In other experiments, Q females displayed a high incidence of pregnancy-block after a period of adaptation. The initial low rate of pregnancy-block was evidently due to disturbance resulting from unaccustomed daily handling during the test.