Steroid hormones and the brain: linking "nature" and "nurture".

Contrary to earlier belief, the genetic constitution of each cell of the body ("nature") is subject to modulation by environmental factors ("nurture") which act throughout the life of the organism to shape the individual characteristics. The nervous system adapts and changes with the environment that the organism experiences through genomic activity controlled by chemical messengers from other nerve cells and from endocrine secretions. The nervous system expresses receptors for a number of circulating hormones, and the location of these hormone receptors has revealed a great deal about the neuroanatomy of neuroendocrine and behavioral control processes. The brain controls the endocrine system through the hypothalamus and pituitary gland, and it responds to circulating hormones throughout each stage of life. These effects begin during early development (eg., sexual differentiation of the brain; effects of maternal or neonatal stress). They continue in adult life in response to cyclic events (eg., season of year; time of day, controlling reproduction and daily activity-sleep rhythms of behavior); and they also include the behavior of other animals which alters hormone output. Hormones also operate during the aging process and under conditions which induce neural damage such as hypoxia and stress. This overview summarizes involvement of steroid hormones of gonads and adrenals in many of these processes and also examines the features of the genomic activity which is modified by these hormones. This area of research is fruitful because it brings together molecular, anatomical, physiological and behavioral approaches in an attempt to understand the long-term plasticity of the nervous system.