Autoradiography of [³H]U-69593 binding sites in rat brain: evidence for κ opioid receptor subtypes

In vitro quantiative autoradiography was used to determine the distribution of [³H]U-69593 binding sites in rat brain. The highest density of binding was found in areas of the tel- and diencephalon while certain areas of the mes- and metencephalon were moderately labeled. The distribution of [³H]U-69593 binding sites corresponds closely (but not precisely) to the distribution of sites labeled by [³H]EKC and [³H]bremazocine at room temperature but differs substantially from the distribution of sites labeled by [³H]EKC at 4°C (as described by others). These anatomical findings support previous biochemical evidence indicating that [³H]U-69593 selectively labels a κ opioid receptor subtype with characteristics that differ from the κ receptors labeled by [³H]EKC at 4°C.