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Opiates have been known to suppress pain for a long time. In the early 1970s, scientists had only hypothesized that the brain has a specific protein receptor for opiates but had never tested the idea. With the discovery of opiate receptors, researchers used a simple yet effective experiment to search for such proteins. The key tool is naloxone, an opiate antagonist, which binds to the opiate receptor without activating it. Naloxone can treat narcotic overdose in an ​emergency.

Read the description of the experiment below, and fill in blanks when needed.

Researchers put radioactive naloxone in a protein mixture containing opiate receptor candidates together with a test drug that is either an opiate or non-opiate. For example, the opiate drugs can be
Select Option morphineserotonin
, methadone, or heroin and these drugs compete and block naloxone’s binding to the opiate receptor (as natural binding mates of the opiate receptor). Then the proteins are trapped in a filter and measured for radioactivity. If naloxone binds to a certain protein in the mixture, the radioactivity of the resulting complex will be stable. Further evidence for the presence of the opiate protein receptor would be available if in the same protein mixture the
Select Option opiatenon-opiate
test drug blocks naloxone binding at a low concentration (e.g., ${10}^{-8}$M) or if the
Select Option opiatenon-opiate
test drug fails to block naloxone binding at a high concentration (e.g., ${10}^{-4}$M). Such results would prove the specificity of the receptor binding activity.
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