The pineal gland is located in the posterior portion of the roof of the third ventricle in the middle of the brain (Figure 6.4). It was originally thought to be the seat of the soul. This gland secretes melatonin, a hormone that is derived from serotonin (a modified amino acid, also known as 5-hydroxytryptamine). Shown to have connections with neurons from the eye, the pineal gland is believed to function as a tim-ing device that keeps internal events synchronized with the light-dark cycle of the environment. Mela-tonin levels are increased during the night and re-duced during the daylight. The fluctuation in the lev-els is believed to be responsible for sleep and many other actions, which are still obscure.
It is thought that melatonin participates in deter-mining the onset of puberty by reducing the secretion of GnRH from the hypothalamus. Melatonin may be involved in the daily changes in physiologic processes that follow a regular pattern.