We have previously demonstrated that [3H]dynorphin A selectively labels κ opioid receptors in guinea-pig whole brain. In these current studies, using protection from inactivation by β-chloronaltrexamine (β-CNA), we are able to demonstrate that although dynorphin A prefers κ receptors, it will label μ receptors when κ receptors are not available, or present in only a small number. Thus, differences in numbers of μ and κ receptors present in brain preparations are critical in determining the receptor binding profile of [3H]dynorphin A across species. Additionally, lthough all the prodynorphin derived peptides show κ preference, the ability of the other prodynorphin derived peptides to compete with [3H]dynorphin A for its receptor varies across species. Consequently, in a highly enriched κ preparation such as monkey cerebral cortex, [3H]dynorphin A appears to label κ receptors with substantial selectivity, and the other prodynorphin-derived peptides show less ability to compete with dynorphin A for its receptor. In contrast, in a κ-poor tissue such as rat brain, all of the prodynorphin-derived peptides, including dynorphin A-(1-8), show very similar potency. Thus, differences in μ and κ receptor numbers across brain regions and species lead to differences in the receptor binding profile of dynorphin A.
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