Differential effect of cholesterol on two types of 5-hydroxytryptamine binding sites

The specific binding of 3H]5-hydroxytryptamine (3H]5-HT, 3H]serotonin) to rat cerebral cortex is increased approximately 1.5 to 2.0 fold by cholesterol hydrogen succinate (CHS) and is solubilized into the supernatant fraction by 12 mM CHS. 3H]5-HT binding sites can be constituted by incubating the supernatant fraction obtained from CHS-treated cerebral cortex with cerebellum in which no significant 3H]5-HT binding is detectable. The constituted 3H]5-HT binding could be displaced by unlabeled 5-HT, d-lysergic acid diethylamide (d-LSD) and spiperone as could the binding to cortex membranes. Unlabeled 5-HT, d-LSD and spiperone each inhibited specific 3H]5-HT binding to constituted binding sites by 50% at about 1 X 10(-9) M. Specific 3H]spiperone binding was not detectable in the constituted membranes. Stearic acid which is reported to have similar effects on membrane fluidity as cholesterol also increased specific 3H]5-HT binding in cortical membranes. Stearic acid does not affect specific 3H]spiperone binding. These results suggest that 3H]5-HT and 3H]spiperone binding sites are affected differently by membrane fluidity.