Brain adaptations for a successful pregnancy

Pregnancy has long been the focus of endocrinologists and reproductive biologists. Possibly because of this, most physiology textbooks with chapters devoted to pregnancy, lactation and fetal/maternal physiology pay considerable attention to alterations in the maternal circulation, respiratory system and metabolism. Similarly, the importance of the hypothalamic–pituitary axis is acknowledged in the obligatory role of oxytocin in milk letdown and its participation in labour, and the additional involvement of the hypothalamic–pituitary–adrenal axis in some species in the initiation of labour. However, it is becoming evident that the maternal brain also undergoes significant changes during pregnancy that equip the mother physiologically and behaviourally for a successful pregnancy, delivery and mothering. This issue of The Journal of Physiology highlights this theme, which is at the intersection of reproductive biology and neuroscience. The invited contributions represent proceedings of a recent symposium supported by The Journal of Physiology and the International Union of Physiological Sciences at the 7th International Brain Research Organization World Congress of Neuroscience in Melbourne Australia. The symposium, entitled ‘Brain Adaptations for a Successful Pregnancy’ featured talks by leading scientists in the field who demonstrated how the dramatic hormonal changes that occur throughout pregnancy, birth and lactation can alter the brain to redirect its role from that of preservation of self only, to that devoted to support of a fetus and newborn. This requires a fascinating, neurally orchestrated series of physiological compromises and redirection of resources and energy. Two of the reviews in this symposium issue report on the attenuation of stress responses during pregnancy. Paula Brunton and John Russell (Brunton & Russell, 2008) use the immune stimulant, interleukin 1β, a well known activator of the hypothalamic–pituitary–adrenal (HPA) axis, and show how the glucocorticoid responses are reduced during pregnancy. This is due to an up-regulation of opioid inhibition of ascending fibres to the corticotrophin releasing hormone neurons in the hypothalamic paraventricular nucleus and is brought about by the neurosteroid allopregnanolone, a progesterone metabolite whose levels peak shortly before parturition. Inga Neumann and colleagues (Slattery & Neumann, 2008) note that hormonal responses to behavioural stressors are similarly attenuated during pregnancy and lactation and show how up-regulated actions of oxytocin and prolactin within the brain are involved in this attenuated stress response. They postulate that this may be important in preventing excessive levels of glucocorticoids that could be detrimental to healthy prenatal development and postnatal maternal care. David Grattan and colleagues (Augustine et al. 2008) discuss the adaptations required for a pregnant mother to meet the needs of a growing fetus and to prepare for the metabolic demands of lactation. This requires an increase in appetite and food intake brought about by alterations in the sensitivity of the hypothalamus to the hormone leptin, thus circumventing its known appetite-suppressing function. Finally, Sarah Spencer and colleagues (Spencer et al. 2008) report on their studies with the bacterial pyrogen lipopolysaccharide, and show how both the generation of prostaglandins within the brain, and the actions of these prostaglandins on thermogenesis are reduced specifically at term. Thus, fever, an important component of the host defence response, is attenuated, and the possible survival value of this adaptation is discussed. Altogether, these papers reveal that the brain displays remarkable plasticity during pregnancy, just as remarkable as those alterations seen in the maternal circulatory, endocrine and metabolic axes; it is time for the ‘maternal brain’ to be featured in our physiology textbooks. Finally, it is possible that the pregnancy-altered brain may provide a physiologically relevant, accessible preparation to understand other types of plasticity in the nervous system.

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