[3H]-SR 27897B: a selective probe for autoradiographic labelling of CCK-A receptors in the brain.

The binding and distribution of radiolabelled SR27897B, a potent CCK-A antagonist, was characterized using in vitro receptor autoradiography. Rapid imaging and quantitative analysis of [3H]SR27897B binding was obtained in a very short period of time (5 days) with a highly sensitive radioimager ensuring very short exposure times for isotopes such as tritium. Tritiated SR27897B binding sites are localized almost exclusively in the area postrema and the medical part of the nucleus tractus solitarius and in this nucleus the rostral-caudal distribution of CCK-A sites differed from that of sulphated CCK8 receptors. Receptor binding properties analyzed on 15 microns serial coronal sections showed on site receptor occupancy in these two regions with high affinity and selectivity characteristic of the CCK-A receptor. These results precisely locate the SR27897B binding sites and provide further support for the absence of heterogeneity of the CCK-A receptors in the rat brain.