Lactation, Stress, and Neural Plasticity

Bearing and nursing offspring profoundly reorganize brain structure in mammals (including women). During the postpartum period, how nursing specifically (as opposed to offspring exposure or hormonal fluctuations of pregnancy and birth) can change the brain and behavior is incompletely understood. In women, breastfeeding is associated with a lower risk of breast and reproductive cancers, diabetes, Alzheimer’s disease, and postpartum depression. Our goal is to understand how the hormones that support milk production and let down (prolactin and oxytocin, respectively) could confer resistance to disease, with particular emphasis on postpartum depression.

What happens to the body during lactation?
Approximately 20% of women do not initiate breastfeeding…

A very pregnant rat.

and only 50% of women are breastfeeding at 6 months postpartum (the recommended minimum; CDC, American Academy of Pediatrics), although breastfeeding rates are rising (CDC, 2016 Breastfeeding Report Card). Approximately 70% of women are not breastfeeding one year postpartum, which is considered the optimal length of time to breastfeed. Numerous studies indicate that women who do not breastfeed are at greater risk for postpartum depression, and several prospective studies indicate that asymptomatic women who do not breastfeed have a higher risk for subsequently developing depression compared with women who breastfeed. These studies suggest that not breastfeeding or early discontinuation of breastfeeding might increase the risk of or exacerbate postpartum depression in some women. Rodent models are essential for understanding the proximate endocrine mechanisms that confer resistance and vulnerability to psychiatric illness following birth. In my lab, we are identifying how the absence of nursing in postpartum rats regulates stress-related behaviors and neural plasticity relevant to depression.

The hippocampus is reorganized during pregnancy and postpartum.

A cluster of cells expressing Ki-67.

Maternal experience transiently suppresses hippocampal neurogenesis and dendritic arborization but increases synaptic density during the postpartum period. Additionally, in depressed individuals, hippocampal volume reduces over time and is significantly correlated with the length of time depressed. Thus, hippocampal changes during the postpartum could contribute to postpartum depression. Our work suggests that the absence of nursing disinhibits hippocampal neurogenesis and cell proliferation (left) in postpartum rats. This work will be imperative for understanding how hippocampal reorganization during the postpartum period might contribute to postpartum depression, particularly for women who do not breastfeed.