Characterization and visualization of rat and guinea pig brain κ opioid receptors: Evidence for κ₁ and κ₂ opioid receptors

κ opioid receptors (κ receptors) have been characterized in homogenates of guinea pig and rat brain under in vitro binding conditions. κ receptors were labeled by using the tritiated prototypic κ opioid ethylketocyclazocine under conditions in which μ and δ opioid binding was suppressed. In the case of guinea pig brain membranes, a single population of high-affinity κ opioid receptor sites (κ sites; K(d) = 0.66 nM, B(max) = 80 fmol/mg of protein) was observed. In contrast, in the case of rat brain, two populations of κ sites were observed - high-affinity sites at low density (K(d) = 1.0 nM, B(max) = 16 fmol/mg of protein) and low-affinity sites at high density (K(d) = 13 nM, B(max) = 111 fmol/mg of protein). To test the hypothesis that the high- and low-affinity κ sites represent two distinct κ receptor subtypes, a series of opioids were tested for their abilities to compete for binding to the two sites. U-69,593 and Cambridge 20 selectively displaced the high-affinity κ site in both guinea pig and rat tissue, but were inactive at the rat-brain low-affinity site. Other κ opioid drugs, including U-50,488, ethylketocyclazocine, bremazocine, cyclazocine, and dynorphin (1-17), competed for binding to both sites, but with different rank orders of potency. Quantitative light microscopy in vitro autoradiography was used to visualize the neuroanatomical pattern of κ receptors in rat and guinea pig brain. The distribution patterns of the two κ receptor subtypes of rat brain were clearly different. The pattern of rat high-affinity κ sites paralleled that of guinea pig in the caudate-putamen, midbrain, central gray substance of cerebrum, and substantia nigra; interspecies differences were apparent throughout most of the rest of the brain. Collectively, these data provide direct evidence for the presence of two κ receptor subtypes; the U-69,593-sensitive, high-affinity κ₁ site predominates in guinea pig brain, and the U-69,593-insensitive, low-affinity κ₂ site predominates in rat brain.