Opioid receptors

Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs). Opioid receptors are distributed widely in the brain, and are found in the spinal cord and digestive tract.

By the mid-1960s, it had become apparent from pharmacologic studies that opiate drugs were likely to exert their actions at specific receptor sites, and that there were likely to be multiple such sites. Early studies had indicated that opiates appeared to accumulate in the brain. The receptors were first identified as specific molecules through the use of binding studies, in which opiates that had been labeled with radioisotopes were found to bind to brain membrane homogenates. The first such study was published in 1971, using ³H-levorphanol. In 1973, Candace Pert and Solomon H. Snyder published the first detailed binding study of what would turn out to be the μ opioid receptor, using ³H-naloxone. That study has been widely credited as the first definitive finding of an opioid receptor, although two other studies followed shortly after.

Purification of the receptor further verified its existence. The first attempt to purify the receptor involved the use of a novel opioid receptor antagonist called chlornaltrexamine that was demonstrated to bind to the opioid receptor. Caruso later purified the detergent-extracted component of rat brain membrane that eluted with the specifically bound ³H-chlornaltrexamine.

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