Effects of gonadal steroids on neuronal morphology in the adult mammalian brain: A life history perspective

This chapter reviews effects of gonadal hormones on the brain and behavior during four epochs in mammalian life: puberty, the estrous cycle, pregnancy and lactation, and during seasonal changes in behavior. We focus particularly on how the natural fluctuations in levels of estradiol or testosterone during these life stages in rodents are associated with changes in the infrastructure of nuclei in the spinal cord and forebrain, including the amygdala, hippocampus, and hypothalamus. A central theme is the relationships between hormone-related morphological changes and changes in behavior. Gonadal hormones generally have trophic effects on synapses, dendrites, and axons, and these are often associated with the emergence of specific behaviors. Whether and how morphological changes in the brain are causally related to changes in behavior, and understanding how experience interacts with hormones to influence morphological plasticity in the brain are discussed. Additionally, we highlight evidence for new mechanisms of hormone action, including through classical hormone receptor proteins located at extranuclear sites and novel G-protein-coupled hormone receptors. Understanding how these mechanisms interact with hormone-induced changes in gene expression will aid in dissecting the roles of hormones and experience in regulating morphological plasticity of neurons throughout the life span.