Neurons communicate with each other, effector organs and sensory organs through the neurotransmitter – receptor pathway at synapses. Neurotransmitters can be divided into 4 major groups: 1. Amino acids (glumate, aspartate, serine, glycine and GABA); 2. Monoamines (norepinephrine, epinephrine, dopamine, histamine, and serotonin); 3. Peptides (opioid peptides, substance P, somatostatin); and 4. Others (acetylcholine, NO, nucleosides). Substance P is an undecapeptide in the tachykinin family responsible for transmitting pain from sensory neurons to the central nervous system. Short opioid peptides participate in pain pathways and modulate motivation, emotion, the response to stress and pain, and control food intake. Some opioid peptides have an analgesic effect. Opioid receptors are G protein coupled receptors, activated by endogenous opioid peptides like dynorphins, enkephalins, endorphins, endomorphins and nociceptin. Four subtypes of receptors (s, k, m and nociceptin) are responsible for different functions.