Question

what is mean by opioid receptors ?

Answer 1

These are the group of inhibitory G-protein coupled receptors that function by binding with opioid/opioid like ligands.
These receptors are activated when we are administered with opiate drugs like morphine. So once these receptors are activated we feel the releif from the pain in the presence of these drugs.

Answer 2

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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.