Enkephalins are considered the putative ligands for the δ receptors, β endorphins for the μ-receptors, and dynorphins for the κ receptors. They act on various receptors in the brain and spinal cord. The opioid peptides modulate nociceptive input in two ways: 1) block neurotransmitter release by inhibiting Ca 2+ influx into the presynaptic terminal, or 2) open potassium channels, which hyperpolarizes neurons and inhibits spike activity. These three opioid peptides are derived from a large protein precursor by three different genes: the proopiomelanocortin (POMC) gene, the proenkephalin gene and the prodynorphin gene. Moreover, three major classes of endogenous opioid peptides that interact with the above opiate receptors have been recognized in the CNS: β-endorphins, enkephalins and the dynorphins. The genes encoding each one of them have been cloned and found to be members of the G protein receptors.
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All three classes are widely distributed in the brain. Three classes of opioid receptors have been identified: μ-mu, δ-delta and κ-kappa. Opioids exert marked effects on mood and motivation and produce euphoria. Opioidergic neurotransmission is found throughout the brain and spinal cord and appears to influence many CNS functions, including nociception, cardiovascular functions, thermoregulation, respiration, neuroendocrine functions, neuroimmune functions, food intake, sexual activity, aggressive locomotor behavior as well as learning and memory. The brain has a neuronal circuit and endogenous substances to modulate pain. They are competitive antagonists of opiate receptors. Opiate antagonist is a drug that antagonizes the opioid effects, such as naloxone or maltroxone, etc. In general, these drugs modulate the incoming pain information in the spinal and central sites, as well as relieve pain temporarily, and are also known as opiate producing analgesia (OA). Several side effects resulting from opiate use include tolerance and drug dependence (addiction). There are currently no other effective pain therapeutic alternatives to opiates. The most effective clinically used drugs for producing temporary analgesia and relief from pain are the opioid family, which includes morphine, and heroin. It is characterized by the absence of all perception of sensory modalities, including loss of consciousness without loss of vital functions. The term central anesthesia refers to a drug that depresses the CNS.
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The term analgesic refers to a drug that relieves pain without loss of consciousness. Pain is modulated by two primary types of drugs that work on the brain: analgesics and anesthetics.
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Pain is interpreted and perceived in the brain. Most, if not all, ailments of the body cause pain.