Characterization of solubilized serotonin (S2) receptors in rat brain

[3H]Spiperone binding sites were solubilized from rat frontal cortex and striatum by means of the mild detergent, lysolecithin. In the frontal cortex, the binding sites were extracted from a microsomal membrane fraction which was found to be enriched in serotonin (S2) receptors when labeled either with [3H]spiperone or with [3H]lysergic acid diethylamide. Although the extraction yield was relatively low, the [3H]spiperone binding sites solubilized from the frontal cortex retained the high-affinity characteristics of serotonin (S2) receptors in the original membrane: low KD (1.4 nM); binding saturable, reversible, and stereospecific; and displaying a high affinity toward the most potent serotonin antagonists (pirenperone, pipamperone, ketanserin, methysergide, and mianserin). There was a very good correlation between the drug potencies in both soluble and membrane preparations. The molecular dispersion of the soluble extract was assessed by several criteria, including a low sedimentation coefficient in sucrose gradient. No specific binding sites could be extracted from the cerebellum. In contrast, few binding sites endowed with the characteristics of serotonin (S2) receptors were detected in lysolecithin extracts from rat striatum. However, the S2 sites became more apparent when a tetralin derivative was added to the incubation medium in order to prevent [3H]spiperone binding on solubilized dopamine receptors. In contrast, when microsomal membranes from rat striatum were treated with digitonin, the solubilized [3H]spiperone binding sites were only of a dopaminergic nature.