question: How are serotonin, dopamine, endorphins, and enkephalins related? Also what's the function of each on its own?
comment: Dopamine and serotonin are classical neurotransmitters. This means that they are one of a number of molecules that facilitate communication between nerve cells. Enkephalin is a neuropeptide. These are larger molecules that facilitate slower forms of communication between neurons because it takes time for them to be released and the effects they have typically make relatively persistent changes. Endorphins are also neuropeptides. Again these are involved in slower, forms of communication. To illustrate what I mean by slow and fast communication, compare an action (like raising your arm) to a mood (a feeling that occurs over a longer time period). Both are products of nerve cells talking to each other in different ways. I assume you're trying to get at something to do with pleasure/pain as: dopamine is typically thought of colloquially as the "reward" neurotransmitter, seratonin the "pleasure" neurotransmitter, endorphins are associated with inhibiting pain signals/producing pleasure (hence, endorphin = endogenous morphine), enkephalin is involved in pain signalling as well as it prevents/reduces pain messages to the brain.
passages: Section::::Comparative biology and evolution.:Unicellular organisms.
   Serotonin is one compound of the poison contained in stinging nettles ("Urtica dioica"), where it causes pain on injection in the same manner as its presence in insect venoms (see below). It is also naturally found in "Paramuricea clavata", or the Red Sea Fan.
  
  Serotonin and tryptophan have been found in chocolate with varying cocoa contents. The highest serotonin content (2.93 µg/g) was found in chocolate with 85% cocoa, and the highest tryptophan content (13.27–13.34 µg/g) was found in 70–85% cocoa. The intermediate in the synthesis from tryptophan to serotonin, 5-hydroxytryptophan, was not found.
  
  Section::::Comparative biology and evolution.:Invertebrates.
   Serotonin (5-HT): Overall, 5-HT synapses are more abundant and have a greater number of synaptic contacts in the NAcc shell than in the core. They are also larger and thicker, and contain more large dense core vesicles than their counterparts in the core.
  
  Section::::Function.
  
  Section::::Function.:Reward and reinforcement.
   Section::::History.
  
  Opioid neuropeptides were first discovered in 1974 by two independent groups of investigators:
  
  BULLET::::- John Hughes and Hans Kosterlitz of Scotland isolated – from the brain of a pig – what some called "enkephalins" (from the Greek , "").
   NKB, kisspeptin, and dynorphin together are found in the arcuate nucleus (ARC) known as the KNDy subpopulation. This subpopulation is targeted by a large number of steroid hormones and works to form a network that feeds back to GnRH pulse generator.
  
  Section::::Synthesis.
   The three types of endorphins that exist are made through the fragmentation of precursor proteins. The original protein is called proopiomelanocortin (POMC). This protein is fragmented into many different smaller proteins including beta-lipotropin (β-LPH). β-LPH, a pituitary hormone with little opiate activity, is then continually fragmented into different peptides giving rise to α-Endorphin, β-Endorphin, γ-Endorphin and many other peptides.
  
  Section::::Types.
  
  The class of endorphins includes three endogenous opioid peptides:
   The neurons of the raphe nuclei are the principal source of 5-HT release in the brain.
  
  There are nine raphe nuclei, designated B1-B9, which contain the majority of serotonin-containing neurons (some scientists chose to group the "nuclei raphes lineares" into one nucleus), all of which are located along the midline of the brainstem, and centered on the reticular formation.
   Section::::Perception of resource availability.:Outside the nervous system.:Bone metabolism.
   BULLET::::- Around the same time, in a calf brain, Rabi Simantov and Solomon H. Snyder of the United States found what Eric Simon (who independently discovered opioid receptors in vertebral brains) later termed "endorphin" by an abbreviation of "endogenous morphine", meaning "morphine produced naturally in the body". Studies have demonstrated that human and diverse animal tissues are capable of producing morphine, which is not a peptide. They have been continually researched for their pain relieving properties and role in the feeling of pleasure.
  
  Section::::Synthesis.
   There are three well-characterized families of opioid peptides produced by the body: enkephalins, B-endorphin, and dynorphins. The met-enkephalin peptide sequence is coded for by the enkephalin gene; the leu-enkephalin peptide sequence is coded for by both the enkephalin gene and the dynorphin gene. The proopiomelanocortin gene (POMC) also contains the met-enkephalin sequence on the N-terminus of beta-endorphin, but the endorphin peptide is not processed into enkephalin.
  
  Section::::Effects on Stress.
   Serotonin pathway
  
  A serotonin pathway identifies aggregate projections from neurons which synthesize and communicate the monoamine neurotransmitter serotonin. These pathways are relevant to many psychiatric and neurological disorders.
  
  Section::::Function.
   Axons from the neurons of the raphe nuclei form a neurotransmitter system reaching almost every part of the central nervous system. Axons of neurons in the lower raphe nuclei terminate in the cerebellum and spinal cord, while the axons of the higher nuclei spread out in the entire brain.
  
  Section::::Perception of resource availability.:Nervous system.:Ultrastructure and function.
   There are two classes of receptors for neurotrophins: p75 and the "Trk" family of Tyrosine kinases receptors.
  
  Section::::Types.
  
  Section::::Types.:Nerve growth factor.
   Endorphins (contracted from "endogenous morphine") are endogenous opioid neuropeptides and peptide hormones in humans and other animals. They are produced by the central nervous system and the pituitary gland. The term "endorphins" implies a pharmacological activity (analogous to the activity of the corticosteroid category of biochemicals) as opposed to a specific chemical formulation. It consists of two parts: "endo-" and "-orphin"; these are short forms of the words "endogenous" and "morphine", intended to mean "a morphine-like substance originating from within the body". The class of endorphins includes three compounds—α-endorphin (alpha endorphins), β-endorphin (beta endorphins), and γ-endorphin (gamma endorphins)—which preferentially bind to μ-opioid receptors. The principal function of endorphins is to inhibit the communication of pain signals; they may also produce a feeling of euphoria very similar to that produced by other opioids.
   Section::::Functions.:Cellular effects.:Storage, release, and reuptake.
   Section::::Role in synaptic transmission.:GABAergic signaling.
   Section::::Function.
   BULLET::::- Others: acetylcholine (ACh), anandamide, etc.
  
  In addition, over 50 neuroactive peptides have been found, and new ones are discovered regularly. Many of these are "co-released" along with a small-molecule transmitter. Nevertheless, in some cases a peptide is the primary transmitter at a synapse. β-endorphin is a relatively well-known example of a peptide neurotransmitter because it engages in highly specific interactions with opioid receptors in the central nervous system.
   BULLET::::- Madsen, K, Erritzøe, DF, Mortensen, EL, Gade, A, Madsen, J, Baaré, WFC, Knudsen, GM & Hasselbalch, SG 2011, "Cognitive function is related to fronto-striatal serotonin transporter levels—a brain PET study in young healthy subjects" Psychopharmacology, vol 213, nr. 2-3, s. 573-81.
   Section::::Anatomy.
  
  The mesolimbic pathway is a collection of dopaminergic (i.e., dopamine-releasing) neurons that project from the ventral tegmental area (VTA) to the ventral striatum, which includes the nucleus accumbens (NAcc) and olfactory tubercle. It is one of the component pathways of the medial forebrain bundle, which is a set of neural pathways that mediate brain stimulation reward.
   Section::::Disease linkage.:Depression.
   To examine the role of monoamine transporters in models of depression DAT, NET, and SERT knockout (KO) mice and wild-type littermates were studied in the forced swim test (FST), the tail suspension test, and for sucrose consumption. The effects of DAT KO in animal models of depression are larger than those produced by NET or SERT KO, and unlikely to be simply the result of the confounding effects of locomotor hyperactivity; thus, these data support reevaluation of the role that DAT expression could play in depression and the potential antidepressant effects of DAT blockade.
   Section::::Biological function.
  
  Section::::Biological function.:Synthesis.
   Section::::Emotional processing and neural circuits.:Reward.
   Function of β-endorphin has been known to be associated with hunger, thrill, pain, maternal care, sexual behavior, and reward cognition. In the broadest sense, β-endorphin is primarily utilized in the body to reduce stress and maintain homeostasis. In behavioral research, studies have shown that β-endorphin is released via volume transmission into the ventricular system in response to a variety of stimuli, and novel stimuli in particular.
  
  Section::::Formation and structure.
  