Comparative in vitro/in vivo autoradiography using the opiate ligand 3H-(-)-bremazocine

Autoradiograms of rat brain sections were compared obtained from animals receiving a tritiated drug through intravenous injection or from precut sections incubated in vitro. The benzomorphan analogue 3H-(-)-bremazocine was used as ligand and its distribution to all different opioid binding sites was followed. Although the general distribution of opioid binding sites visualized on 3H-LKB ultrofilms was independent of the methodological approach used, the maximal number of such sites (Bmax) was greater in brain sections incubated in vitro than after in vivo drug application. Since the number of binding sites is highly dependent on the particular incubation condition used, this finding has no further relevance.