Opioid is the term used to designate a group of compounds that are opium-like in their properties. These drugs have effects on perception of pain, consciousness, motor control, mood, and autonomic function, and can induce physical dependence. Pharmacological studies suggested that there are at least 3 major classes of opioid receptors, designated delta, kappa, and mu. They differ in their affinity for various opioid ligands and in their cellular distribution. Studies of the receptors in the mouse and rat show that they are structurally related and are members of the family of 7 transmembrane-spanning G protein-coupled receptors. The kappa opiod receptor inhibits neurotransmitter release by reducing calcium ion currents and increasing potassium ion conductance. It is the receptor for beta-endorphin.
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