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  • Title: Discrimination of three types of opioid binding sites in rat brain in vivo.
    Author: Rosenbaum JS, Holford NH, Richards ML, Aman RA, Sadée W.
    Journal: Mol Pharmacol; 1984 Mar; 25(2):242-8. PubMed ID: 6321946.
    Abstract:
    Opiate receptor sites in the rat brain were defined in vivo by measuring the binding of etorphine, sufentanil, diprenorphine, and naloxone in saturation and cross-competition experiments. The binding data were analyzed simultaneously, using a computerized curve-fitting technique with an extended least-squares nonlinear regression program. Three types of binding sites could be distinguished: site 1 (18 pmoles/g of brain), site 2 (15 pmoles/g of brain), and site 3 (20 pmoles/g of brain). Site 1 is bound selectively by sufentanil (the ratio of the apparent equilibrium dissociation constants K2/K1 approximately equal to 1200), etorphine (K2/K1 approximately equal to 20), and naloxone (K2/K1 approximately equal to 15), and it resembles the mu binding site previously demonstrated in vitro. Diprenorphine binds to both site 1 and site 2 with high affinity and a slight (approximately 3.7-fold) selectivity for site 1 over site 2. The latter site may represent a mixture of the delta and kappa binding sites. The third site displays relatively high affinity for naloxone, but it is clearly different from sites 1 and 2, as it exhibits a lack of affinity for sufentanil, etorphine, and diprenorphine. This binding site population does not resemble any of the known opiate binding sites. Recent in vitro binding studies revealed that site 3 (now termed lambda site) is highly labile in vitro and was, therefore, not previously detected. These results suggest significant differences between in vitro and in vivo opioid receptor binding characteristics.
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