Low molecular weight constituents of male mouse urine mediate the pregnancy block effect and convey information about the identity of the mating male.

Mouse urine contains a complex mixture of chemosignals including a variety of small volatile molecules that are bound to major urinary proteins. In addition to signalling maleness, male urine also conveys information about individuality, which allows recently mated female mice to distinguish the urinary chemosignals of the mating male from those of an unfamiliar male. The highly polymorphic nature of the major urinary proteins makes them a likely candidate for conveying individuality information in the context of the pregnancy block effect. This was investigated by comparing the pregnancy-blocking effectiveness of a high molecular weight urinary fraction, containing major urinary proteins, with that of a low molecular weight fraction containing volatile ligands. Not only was the high molecular weight fraction ineffective in blocking pregnancy, but it also appeared to be less important in signalling individuality than the low molecular fraction. The high molecular weight fraction was ineffective in inducing expression of the immediate early gene product egr-1 in the accessory olfactory bulb. In contrast, the low molecular weight fraction induced egr-1 expression in the mitral/tufted neurons in the anterior subregion of the accessory olfactory bulb, suggesting that they activate the V1R class of vomeronasal receptor neuron.