As the time of delivery nears, powerful hormones swing into action. Although the most obvious players are oxytocin, which stimulates uterine contractions and milk letdown, and prolactin, which instigates milk production, other hormones trigger changes inside the brain, too. Neuroanatomists at the Victor Segalen Bordeaux 2 University in France have observed a dramatic structural remodeling of the hypothalamus, a brain region that acts as a major regulator of the hormones associated with basic emotional behaviors such as fighting and sex. Neurons in a part of the hypothalamus known as the medial preoptic area, or mPOA, grow bigger and become more active. Indeed, lesions of the mPOA can eliminate maternal behavior.
Meanwhile the hypothalamus ramps up the feelings of pleasure a mother receives. Robert S. Bridges of the Tufts Cummings School of Veterinary Medicine and his colleagues found different concentrations of opioid receptors in female rats depending on whether the rodent was a virgin, pregnant or lactating. But the phenomenon fades with experience. Females that go through several pregnancies show a decline in sensitivity to their own opioids, much like addicts who require ever greater doses of a drug to get high.
The drug analogy, by the way, is not spurious. Animals may in fact be engaging in maternal behavior simply because it feels good. Many human mothers report a very pleasurable feeling as they breastfeed their infants. After pups attach to a female rat's nipple, the mom receives a “hit” of reinforcing opiate. But the rat's body imposes a natural limit. As the pups continue to suckle, the mother's core body temperature rises. Soon enough the mother begins to feel uncomfortable and moves away. Later, desiring another burst of opiates, the rat comes back to the nest, the pups reattach and the cycle begins again.
As an added benefit, maternal hormones may well make the brain more resilient. In 2010 neurobiologist Teresa Morales Guzmán of the National Autonomous University of Mexico showed that the brain of a lactating female is more resistant to the effects of a neurotoxin. The hormones of pregnancy appear to construct a neural shield that protects a mother from damage that otherwise might compromise a rat's ability to care for its young.