Differences in the characteristics of opioid receptor binding in the rat and marmoset

The density of mu and delta opioid binding sites in homogenates of rat and marmoset brain was determined using radiolabelled peptides with high receptor selectivity. In addition the affinity of mu and delta receptor binding of five drugs of the fentanyl series was measured. The ratio of mu: delta sites was similar within each species but the total receptor binding capacity in the rat was double that of the marmoset. The binding affinities for the delta site were dissimilar between the species and supports the hypothesis that the structure of this receptor shows phylogenetic differences. In contrast, mu site binding affinity was identical in rat and marmoset.