[125I]dynorphin(1-8) produces a similar pattern of kappa-opioid receptor labelling to [3H]dynorphin(1-8) and [3H]etorphine in guinea pig brain: a quantitative autoradiographic study.

kappa-Opioid receptors were radiolabelled with the peptides [125I]dynorphin(1-8) and [3H]dynorphin(1-8) or with [3H]etorphine on guinea pig forebrain and cerebellar sections and visualized by quantitative autoradiography. All three radioligands yielded similar patterns of kappa-receptor localization. However, quantitative analysis showed that using saturating concentrations of the tritiated radioligands the apparent density of specific [3H]etorphine-labelled kappa-sites was 1.5-5.4 times greater than that achieved with [3H]dynorphin(-8). The apparent rank order of regional density of kappa-sites on a quantitative basis with all 3 radioligands was similar. A high density of kappa-receptors was found in the nucleus accumbens, striatum, globus pallidus, cerebral cortex (layers V-VI), hippocampal and cerebellar molecular layers, substantia nigra and substantia gelatinosa of the spinal cord. A lower density of these sites was associated with the thalamus, hypothalamus and the amygdaloid complex. Thus, in view of the advantages of using iodinated ligands in autoradiography this study has shown that [125I]dynorphin(1-8) is an acceptable ligand for labelling kappa-receptors in the brain.