Differential photoaffinity labelling of serotonin-S2 receptors and histamine-H1 receptors using 7-azidoketanserin.

7-Azidoketanserin, a potent photoaffinity probe for serotonin-S2 receptors was shown to irreversibly photoinactivate histamine-H1 receptors as well. The photolabelling of H1-receptors could be prevented by several selective histamine-H1 antagonists. In guinea pig cerebellum, a brain area that is highly enriched in H1-receptors, photolabelling could be directed exclusively to these receptors by adding a high concentration of pipamperone, which selectively blocks serotonin-S2 receptors. In rat pre-frontal cortex, a region that is enriched in serotonin-S2 receptors, pyrilamine was used to block H1-receptors, thereby directing the photolabelling exclusively to S2-receptors.