How does the retina transmit information about light-dark exposure to the pineal gland?
Light exposure to the retina is first relayed to the suprachiasmatic nucleus of the hypothalamus, an area of the brain well known to coordinate biological clock signals. Fibers from the hypothalamus descend to the spinal cord and ultimately project to the superior cervical ganglia, from which post-ganglionic neurons ascend back to the pineal gland. Thus, the pineal is similar to the adrenal medulla in the sense that it transduces signals from the sympathetic nervous system into a hormonal signal. Melatonin: Synthesis, Secretion and Receptors The precursor to melatonin is serotonin, a neurotransmitter that itself is derived from the amino acid tryptophan. Within the pineal gland, serotonin is acetylated and then methylated to yield melatonin. Synthesis and secretion of melatonin is dramatically affected by light exposure to the eyes. The fundamental pattern observed is that serum concentrations of melatonin are low during the daylight hours, and increase to a peak during the dark.
