A radioreceptor assay for opiate drugs in human cerebrospinal fluid and plasma

We have developed a radioreceptor assay for opiates based on the ability of the plasma and CSF content of these drugs to compete for the binding of ³H-buprenorphine to opiate receptors in rat forebrain membranes. Since plasma proteins significantly inhibit total ³H-buprenorphine binding, and sodium ions reduce the affinity of opiate agonists for the receptor, it was necessary to extract opiates into an organic solvent (ether). The radioreceptor assay is particularly sensitive to buprenorphine and morphine, detecting these compounds at low picogram levels. The assay is simple to perform since 50 samples can be processed in a day, and is specific in that other drugs employed during anaesthesia such as benzodiazepines do not compete with ³H-buprenorphine for the opiate receptor. The extraction and binding techniques described should be applicable to other ³H-ligands which have high affinity for opiate receptors.