Measurement of opioid peptides in biologic fluids by radioimmunoassay

Although this chapter is by no means exhaustive, it illustrates the broad biologic and clinical context of RIA analysis of opioid peptides in body fluids. The explosive growth in interest in the endorphin field which occurred during the last decade can explain some of the conceptual errors and methodological flaws which abound in the literature. For instance, one critical issue which is often overlooked is that of using the proper compartment (fluid). There is no evidence that opioid peptide secretion into CSF and plasma is coordinated and there is restricted passage of opioid peptides from plamsa into CSF. Far too many studies take the stand that opioid peptides as measured in plasma can be used as an index of CNS activity. CSF levels are likely to be a much better choice. Also relevant to functional indications is the selection of the proper peptide for measurement. The most frequent choice is β-endorphin, whereas the proenkephalin-derived peptides, which are more abundant, have received much less attention. This is partly on methodological grounds; for instance, commercial RIA kits are available for β-endorphin measurements, but not until recently for other opioid peptides