Neural basis of olfactory memory in the context of pregnancy block

In mice, only strange male pheromones block pregnancy; pheromones of the familiar male with which the female has mated have the capacity to block pregnancy but are ineffective with the consort female. Hence, some form of recognition/memory to the stud male is formed at mating. By infusing lignocaine locally into the accessory olfactory bulb and second order olfactory synapses in the medial nucleus of the amygdala, this study localizes changes that occur in the accessory olfactory bulb at mating to be subsequently important in preventing the stud male's pheromones from blocking pregnancy. Further attention is focused on the dendrodendritic synapses between mitral and granule cells in the accessory olfactory bulb. Blockade of the GABA receptors (granule to mitral cell synapse) in the accessory bulb without mating, but in the presence of male pheromones, prevents any male from blocking pregnancy. Conversely inhibition of protein kinase C, a second messenger system activated by excitatory amino acids (mitral to granule cell synapse), in the accessory bulb during a 4-h period after mating permits all male pheromones including the stud's to activate pregnancy block. While blockade of protein kinase C activity during the critical exposure time for memory formation prevents memory formation, infusions of a protein synthesis inhibitor (anisomycin) are without effect. However, protein synthesis inhibition in the accessory olfactory bulb in the late phase of the critical exposure time (3-6 h after mating) does prevent memory formation. These studies show that changes in synaptic plasticity in the accessory olfactory bulb following mating are critical to recognition of the stud male's pheromones, hence preventing these from subsequently blocking pregnancy.