The different receptor classes are distinguished by their different responses to drugs. Agonists produce a response, while antagonists block response to natural epinephrine. In the pineal gland, serotonin is further modified by acetylation and O-methylation to give melatonin . Serotonin is a neurotransmitter, particularly of neurons in the hypothalamus and brainstem, innervating the spinal cord, and stimulant of smooth muscle contraction and vasoconstrictor. It is also store and released by mast cells.
Lysergic acid is an antagonist of serotonin receptors.
Prozac acts by enhancing the availability of serotonin. Melatonin is secreted by the pineal gland, which converts neural signals into endocrine output. Eventually, melatonin downregulates the signals that lead to its synthesis, so that the levels rise and fall in a cyclic fashion approximating the 24-hour daily rhythm. The cycle is suppressed by continous illumination, but is maintained in continuous darkness. Chocolate is not just a treat . . .