Target size analysis of serotonin 5-HT1 and 5-HT2 receptors in bovine brain membranes

Freeze-dried crude synaptic membranes prepared from bovine cerebral cortex and striatum were exposed to high energy gamma ray from the source of 60Co. The size of serotonin 5-HT1 receptors labeled by [3H]serotonin and that of 5-HT2 receptors labeled by [3H]spiperone or [3H]ketanserin was determined by target size analyses. The values were 57,000 daltons, 145,000 daltons and 152,000 daltons for the cerebral cortex and 56,000 daltons, 141,000 daltons and 150,000 daltons for the striatum, respectively. The estimated sizes were deduced by reference to enzyme standards with known molecular masses and which were irradiated in parallel. Our results demonstrate that the molecular entities in situ for 5-HT1 receptors are distinct from those for 5-HT2 receptors, thus supporting data on the existence of two distinct populations of serotonin receptors, hitherto evidenced physiopharmacologically.