Opioid Analgesics : the opioid receptors

Opioid analgesics affect the nervous system.

Shortly after the discovery and observance of endogenous opioid peptides, multiple classes of unique opioid receptors were found

There are four main opioid receptors, the mu receptor, the delta receptor, the kappa receptor, and the ORL-1 receptor.

The sigma receptors were once thought to be opioid receptors ,however, pharmacological testing indicated that the sigma receptors were activated by drugs completely unrelated to the opioids

The receptors are found on cell membranes of cells in the nervous system (neurons) and are found in unique distributions and have different effects.