(A) Serotonin # Serotonin (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesized in serotonergic neurons in CNS & enterochromaffin cells in the gastrointestinal tract of animals including humans.> Dynorphin is a class of peptides produced by many different populations of neurons, which has some opiate-like activity.> It is thus classed as an endogenous opioid peptide.> Dynorphin functions primarily as a kappa opioid receptor agonist, meaning that it acts mainly at kappa opioid receptors.> Substance Pis a neuropeptide: a short-chain polypeptide that functions as a neurotransmitter and as a neuromodulator. It belongs to the tachykinin neuropeptide family.> Enkephalin is a pentapeptide ending with either leucine ("leu") or methionine ("met"). Both are products of the proenkephalin gene. Met-enkephalin is Tyr-Gly-Gly-Phe-Met. Leu-enkephalin has Tyr-Gly-Gly-Phe-Leu. Enkephalins play many roles in regulating pain.