Pinealocytes in the pineal gland secrete melatonin which induces sleepiness and inhibits gonadotropin and growth hormone secretion. The synthesis of melatonin is derived from which of the following?
(D) Melatonin is synthesized from tryptophan by way of serotonin. Choline (choice A) is a precursor in the synthesis of the neurotransmitter acetylcholine. Glucose (choice B) is the precursor for the synthesis of the neurotransmitter glutamate (choice C) which, in turn is the precursor for the synthesis of other neurotransmitters such as glycine and gamma aminobutyric acid (GABA). Tyrosine (choice E) is the precursor for the synthesis of the catecholamines (dopamine, norepinephrine, and epinephrine).