Autoradiographic distribution in rat brain of a non-conventional binding site for the opiate [3H]diprenorphine

[3H]Diprenorphine, in the presence of saturating concentrations of mu, delta and kappa opiate receptor blocking agents, was found to label an additional binding site in rat brain, for which various opiates showed moderate affinity. Using light microscopic autoradiography, a unique, highly discrete distribution of binding sites was found, with a high density only in the globus pallidus, substantia nigra, interpeduncular nucleus, parabrachial nuclei and locus coeruleus. The identity of this binding site remains unclear.