Patterns of expression of the immediate-early gene egr-1 in the accessory olfactory bulb of female mice exposed to pheromonal constituents of male urine

Male mice excrete large quantities of major urinary proteins that have been proposed to have an important pheromonal role either alone or by way of th eir bound ligands. We have found that these major urinary proteins are not only likely to mediate the pregnancy blocking effects of male urine, but th at they also convey the strain recognition signal of the male pheromone. Re cent molecular biological investigations have characterized two classes of pheromonal receptor in the vomeronasal organ that appear to project separat ely to anterior and posterior regions of the accessory olfactory bulb. Howe ver, it is not known whether these separate pathways handle fundamentally d ifferent types of pheromonal information. We have attempted to investigate this question using the expression of the immediate-early gene egr-1 as a m arker for activity of neurons in the accessory olfactory bulb of female mic e in response to putative pheromonal constituents. Exposure to 2,3 dihydro- exo-brevicomin and 2-sec-butyl-4,5-dihydro-thiazole, the main ligands bound to the major urinary proteins, elicited expression of egr-1 in clusters of presumed mitral neurons at the medial and lateral margins of the posterior accessory olfactory bulb. Whole male urine and a preparation of major urin ary proteins that had been stripped of their ligands induced egr-1 expressi on in mitral cells of the anterior half of the accessory olfactory bulb in addition to the posterior clusters. This would suggest that the anterior and posterior halves of the accessory olfactory bulb are processing different aspects of the male pheromone signa l with the anterior region, which responds preferentially to major urinary proteins, being principally concerned with the strain recognition component . (C) 1999 IBRO. Published by Elsevier Science Ltd.