Main Cause of Bruce Effect in Mice

The main cause of the Bruce effect in mice is:

Correct Answer: Blockage of prolactin release

The Bruce effect occurs when a strange (unfamiliar) male mouse is introduced into a group of recently bred (pregnant) female mice. The pheromones of the strange male cause blockage of prolactin release from the anterior pituitary, which results in termination of early pregnancy.

Mechanism of the Bruce Effect

1. Strange male mouse pheromones detected by VNO of recently bred female
2. VNO → limbic system → hypothalamus
3. Hypothalamus increases dopamine release (dopamine is a prolactin-inhibiting factor)
4. Dopamine → inhibits prolactin secretion from anterior pituitary → prolactin release is blocked
5. Without prolactin, the corpus luteum (CL) regresses → progesterone drops
6. Loss of progesterone support → pregnancy terminated (embryo reabsorbed)

Why Prolactin Blockage Terminates Pregnancy

In mice (unlike large animals), prolactin is required to maintain the corpus luteum (CL) in early pregnancy. The CL produces progesterone (the pregnancy hormone). Without prolactin → CL degenerates → progesterone drops → pregnancy is lost.

Comparison of Pheromone Effects in Mice

• Whitten effect: Male pheromone → GnRH release → estrus synchronization in non-pregnant females
• Bruce effect: Strange male pheromone → prolactin blockage → pregnancy termination in recently bred females