opiate receptor [Gk, opion, poppy juice; L, recipere, to receive] , transmembrane proteins that bind to endogenous opioid neuropeptides and exogenous morphine and similar natural or synthetic compounds. The three major classes of these receptors are designated mu, kappa, and delta. Morphine preferentially stimulates mu receptors to produce analgesia, euphoria, respiratory depression, constipation, and pinpoint pupils. Some other drugs (e.g., butorphanol) can selectively block mu receptors while stimulating kappa receptors; this provides moderate to high pain relief with low abuse potential. Stimulation of delta receptors can also contribute to analgesia. The receptors are found in high concentrations in the dorsal horn of the spinal cord and in the brain regions involved with pain modulation or pain transmission (e.g., periaqueductal gray matter). Endogenous agonists at these receptors include endorphins, enkephalins, and dynorphins.
