Stimulation of estrus in female mice by male urinary proteins

Stimulation of estrus in adult female mice was obtained with major urinary proteins (MUPs) with the natural volatile ligands bound. The MUP threshold concentration for this effect was about 1.8 mg/ml. MUPs without the ligands bound, as purified by organic extraction of hydrophobic compounds, stimulated estrus in mice only when dissolved in carrier urine of juvenile or castrated adult male mouse or ovariectomized female mouse. They did not stimulate estrus when dissolved in water. Mice that had the vomeronasal organ removed were insensitive to MUPs. It is concluded that MUPs are an integral part of the mouse male pheromones that stimulate hormonal activity in females and that the vomeronasal system is involved in the estrus-stimulating effect of the major urinary proteins.