The importance of central noradrenergic neurones in the formation of an olfactory memory in the prevention of pregnancy block.

The olfactory block to pregnancy in mice is caused by a primer pheromone acting via the accessory olfactory system which projects to the mediobasal hypothalamus via the corticomedial amygdala. Only pheromones from males that are different to those of the stud (strange male pheromones) block pregnancy-hence mating "imprints" recognition of stud male pheromones. The olfactory bulbs receive centrifugal noradrenergic projections from the brainstem via the medial olfactory striae, which terminate in both main and accessory bulbs. Removal of these projections by injection of 6-hydroxydopamine into the striae, 6 days prior to mating, effectively depletes olfactory bulb noradrenaline and results in a failure by the female to recognize the stud male. In this condition the stud male's pheromones now block his own pregnancy. However, removal of noradrenaline after "imprint" formation at mating does not prevent recognition of the study pheromone implying that noradrenaline is required for formation, but not recall of the memory of the stud male's odour. Noradrenaline turnover in the bulbs, after alpha-methylparatyrosine injection, increases after cervical stimulation. Significantly higher rates of turnover are found at 1, 2 and 3 h, but not at 6, 12, 24 or 48 h post-stimulation. This is in accordance with our finding that the minimum time of exposure to the stud male pheromone in order for the memory to be formed lies between 3 and 4.5 h. In those females allowed to remain with the male for 3 h post-coitus, only 30% remain pregnant on the re-introduction to the stud the following day, whereas in females exposed for 4.5 h, 86% remain pregnant.(ABSTRACT TRUNCATED AT 250 WORDS)