An historical view of the pineal gland and mental disorders.

Since Classical Antiquity numerous authors have linked the origin of some mental disorders to physical and functional changes in the pineal gland because of its attributed role in humans as the connection between the material and the spiritual world. The pineal organ was seen as a valve-like structure that regulated the flow of animal spirits through the ventricular system, a hypothesis that took on more vigour during the Middle Ages and the Renaissance. The framework for this theory was "the three cells of the brain", in which the pineal gland was even called the "appendix of thought". The pineal gland could also be associated with the boom, during this period, of certain legends about the "stone of folly". But the most relevant psychopathological role of this organ arrived with Descartes, who proposed that it was the seat of the human soul and controlled communications between the physical body and its surroundings, including emotions. After a period of decline during which it was considered as a mere vestigial remnant of evolution, the link between the pineal gland and psychiatric disorders was definitively highlighted in the 20th century, first with the use of glandular extracts in patients with mental deficiency, and finally with the discovery of melatonin in 1958. The physiological properties of melatonin reawakened interest in the relationship between the pineal gland and mental disorders, fundamentally the affective and sleep disorders, which culminated in the development of new pharmacological agents acting through melatonergic receptors (ramelteon and agomelatine).