Melatonin synthesis in the human pineal gland: advantages, implications, and difficulties.

Rhythms in the mammalian pineal organ depend on afferent information that is derived from the endogenous clock residing in the hypothalamic suprachiasmatic nucleus (SCN). The best characterized function of the pineal gland is the nocturnally elevated synthesis of the hormone melatonin, which provides the body with the signal of the duration of the night period. The rate-limiting enzyme for melatonin synthesis is arylalkylamine N-acetyltransferase (AANAT). In contrast to the transcriptional regulation of the Aanat gene in rodents, a post-translational shaping of the melatonin pattern is indicated in the human pineal gland. Despite the fact that melatonin levels can be determined easily in various body fluids, the molecular elements involved in shaping the rhythmic hormone synthesis cannot be analyzed experimentally in the living organism. However, the use of post-mortem pineal material seems to constitute a valid approach to decipher the regulation of human melatonin synthesis.