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 pregnan

Mother–infant bonding and the evolution of mammalian social relationships

A wide variety of maternal, social and sexual bonding strategies have been described across mammalian species, including humans. Many of the neural and hormonal mechanisms that underpin the formation and maintenance of these bonds demonstrate a considerable degree of evolutionary conservation across a representative range of these species. However, there is also a considerable degree of diversity in both the way these mechanisms are activated and in the behavioural responses that result. In the majority of small-brained mammals (including rodents), the formation of a maternal or partner preference bond requires individual recognition by olfactory cues, activation of neural mechanisms concerned with social reward by these cues and gender-specific hormonal priming for behavioural output. With the evolutionary increase of neocortex seen in monkeys and apes, there has been a corresponding increase in the complexity of social relationships and bonding strategies together with a significant redundancy in hormonal priming for motivated behaviour. Olfactory recognition and olfactory inputs to areas of the brain concerned with social reward are downregulated and recognition is based on integration of multimodal sensory cues requiring an expanded neocortex, particularly the association cortex. This emancipation from olfactory and hormonal determinants of bonding has been succeeded by the increased importance of social learning that is necessitated by living in a complex social world and, especially in humans, a world that is dominated by cultural inheritance

Changes in the sensory processing of olfactory signals induced by birth in sheep

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Importance of vaginocervical stimulation for the formation of maternal bonding in primiparous and multiparous parturient ewes

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