Hormone-dependent brain development.

Hormones may be defined as chemical messengers that are produced in specialized cells and exert biological effects on other cells of the same organism by acting either locally (as local hormones) or on distant target cells (as systemic hormones). Hence, neurotransmitters may be regarded as local hormones of the brain, and two different hormonal actions can be distinguished for neurotransmitters as well as for systemic hormones: (1) transient, i.e. reversible activational or inactivational effects and (2) persistent, i.e. more or less irreversible differentiational or organizational effects, if the hormones act during critical developmental periods, especially of the brain. Abnormal levels of systemic hormones and neurotransmitters produced by genetic defects or deficient environments and occurring during brain differentiation can act as 'teratogens'. They lead to malorganizations of the brain and permanent dysfunctions of fundamental processes of life, such as reproduction, metabolism, and/or information processing. Such malorganizations of the brain appear to be preventable by improving the external environment and/or by diagnosing and correcting abnormalities of systemic hormones and neurotransmitters during brain development. These principles of 'teratophysiology and teratopsychology' open new possibilities for preventive therapy.