Evaluation of the binding of serotonin by isolated CNS acidic lipids

Binding of serotonin by rat lipids was examined in an organic solvent-aqueous partition system. Only phospholipids and sulfatide were found to have appreciable activity: this technique was unsuitable for gangliosides due to their poor extractibility. Binding by phospholipid was abolished and that by sulfatide was greatly inhibited by increasing ionic strength in the aqueous phase. At an ionic strength of 0.3 M the apparent affinity of sulfatide for serotonin was about 3×10³ M. Both tryptamine and 5-methoxytryptamine were much more effective than serotonin in inhibiting the binding of radioactive serotonin, suggesting that the observed binding is simply a charge neutralization with little specificity. Binding of serotonin by mixed brain gangliosides was examined in an equilibrium dialysis system. Without adequate precautions, the chemical lability of serotonin was found to produce spurious data when binding was assessed by the distribution of radiolabel. Binding of serotonin by ganglioside was also greatly inhibited by increasing ionic strength: at 0.3 M an apparent affinity of about 10³ M was found. While dopamine did not inhibit the binding of radioactive serotonin, tryptamine, 5-methoxytryptamine, and serotonin were equally effective inhibitors.