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- The Bruce effect was first noted by HILDA M. BRUCE in 1959.
- The Bruce effect is also known as pregnancy block, it is shown by the rodents, it refers to the tendency of the pregnant female rodents to terminate their pregnancy, when they are exposed to the scent of another male, which is not the father of the embryo carried by the female.
- Still now the Bruce effect has not been confirmed in other non-rodent species.
- In 1959 Bruce kept pregnant female mice with males which were not the father of the carried embryo, it was seen that the female blocked their pregnancy, in the presence of an unfamiliar male. This effect blocking of the pregnancy by the female influenced by the scent of the male mice was termed as BRUCE effect, after the name of HILDA BRUCE.
- The male mouse urine contains MHC CLASS I peptides that binds to the receptor in the females mouse’s vomeronasal organ (it is an organ filled with mucous present in the nasal septum), it stimulates the vomeronasal system to act as a pump and the system draws in the substances actively. These chemical signals which are specific to each male are learned by the females. This chemosensory cue is coupled by the vasopressin, with an appropriate physiological response,
- It is seen that, if the vasopressin 1b gene is knocked out in females, then the presence of an unfamiliar male does not trigger pregnancy disruption.
- Exposure to the male urinal pheromones will activate a neuroendocrine pathway, which leads to the pregnancy failure. However, if these pheromones are pre-memorized by the female mice, then noradrenalin is secreted, which lowers down the receptivity of the olfactory bulb to these pheromones. Thus the pregnancy disruption is averted.
- The hormone oxytocin is also important in Bruce effect, if the males are treated by the oxytocin-antagonist hormone, then the females are unable to recognize the urinary pheromones of the their mate, and terminates pregnancy when exposed any male, known or unknown.
è The activation of vomeronasal neuron receptors by the male pheromones induces a complex neuroendocrine pathway. This works as follows.
i. The pheromonal information is carried by the nerves to the accessory olfactory lobe, and then to the corticomedial amygdala lobe and then to stria.
ii. These areas stimulate the hypothalamus.]
iii. The hypothalamus then secretes the dopamine,
iv. The dopamine prevents the secretion of prolactin from the anterior pituitary (prolactin is necessary for maintaining the corpus luteum, in the absence of the prolactin the corpus luteum undergoes luteolysis)
v. Now in the absence of the corpus luteum the progesterone is not released. Since the progesterone is necessary for maintaining the pregnancy.
vi. Thus in the absence of the progesterone the pregnancy fails.
è Role of oestrogen: oestradiol is a metabolic product of testosterone, the oestrogen (particularly the E2) is a crucial chemo-signal regulating the Bruce effect.
Small steroid molecules such as E2 are able to enter the bloodstream directly via the nasal ingestion, after ingestion the E2 travels to the uterus, which has high level of suitable receptors,
Normally E2 is essential for the preparation of the blastocyst and the uterus for implantation.
However the excessive E2 prevents implantation.
Factors governing the Bruce effect.
1. Timing of exposure: after mating the females experience twice daily surges of prolactin. The incidence of Bruce-phenomena depends on the timing of the exposure to the male pheromones, if the pheromone exposure coincides with the prolactin surges, then only there takes place the Bruce-effect.
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