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Levels of β-endorphin immunoreactivity in cerebrospinal fluid were measured in 12 chronic pain patients undergoing the surgical implantation of an electrode into the periventricular gray matter. Cerebrospinal fluid fractions were collected following placement of a cannula into the third ventricle, following injection of metrizamide contrast medium into the ventricles, following implantation of the electrode, and following electrical stimulation. A second set of samples was collected on a non-surgical day before and after stimulation.Levels of β-endorphin immunoreactivity increased significantly from baseline levels to post-electrode implantation in one group of patients, but no significant change was seen following the onset of stimulation. Immunoreactivity increased significantly following metrizamide injection in a second group and was still elevated, in comparison to baseline, following electrode placement, but no increase was seen following the onset of stimulation. Levels of immunoreactive β-endorphin did not increase in either group after stimulation on a post-surgical day, despite consistent reports of pain relief. Addition of metrizamide or a related contrast medium, iothalamate meglumine (Conray) to the β-endorphin radioimmunoassay revealed that both compounds interfered with antigen-antibody binding and also quenched the gamma radiation emitted by iodinated peptide ligands. Due to these combined effects, the contrast media alone produced results similar to those of the β-endorphin standard. Moreover, similar observations were made when contrast media were incorporated into radioimmunoassays for met-enkephalin, dynorphin and cholecystokinin octapeptide.These findings indicate that increased levels of β-endorphin in cerebrospinal fluid are not directly associated with patient report of pain relief following periventricular gray stimulation. This study also suggests that the previously reported association between periventricular gray stimulation and elevated levels of β-endorphin immunoreactivity in ventricular cerebrospinal fluid are artifactual due to the interaction of contrast media with the radioimmunoassay.