Steroid hormones: effect on brain development and function.

Hormones secreted by the adrenals, gonads and thyroid play an important role in mediating how the environment shapes the structure and function of the brain during early development, adult life and senescence. Many of these hormone effects occur at the level of gene transcription, via the actions of intracellular hormone receptors which are DNA-binding proteins. Other effects occur at the membrane level via receptors on the cell surface that produce rapid effects on bioelectrical activity and secondary messenger systems. Hormone effects on the brain are classified as organizational, occurring during development; cyclical, occurring during maturity; experiential, depending on the individual experiences; and disorganizational, leading to damage and destruction of neural tissue. Organizational effects, such as occur as a result of testosterone action during sexual differentiation, give rise to group differences; whereas experiential effects, in which hormone secretion is evoked on an individual basis according to personal life events, are responsible for individual differences even between identical twins having the same genetic constitution. Experiential effects, often involving stress and possibly thyroid hormones, may result in adaptation or may lead to disorganization and damage under extreme and deleterious conditions.