The binding of 5-hydroxytryptamine to butanol extracts of rat brain stem

Butanol extracts from myelin fragments: identification of 5-hydroxytryptamine binding components

To investigate the nature of 5-HT binding components originated from myelin butanol extracts (i.e. proteolipids), the recombinate experiments with acidic lipids were planned. Binding assay of 14C . 5-Ht to recombinates was carried out by Sephadex lh20 column chromatography. Among several recombinate fractions, recombinate of sulphatides, phosphatidylserine and phosphatidylinositol showed the same elution profile and binding capacity for 14C . 5-HT with those of the myelin extracts. The displacement studies with other neurotransmitters revealed that the recombinate of these three acidic lipids completely regenerated the specificity of the original myelin proteolipids. Namely, tryptamine, DA and ACh inhibited the 5-HT binding but NA had no effect. A double reciprocal plot of 5-HT binding represented a multiple binding mode, not a straight line. This observation suggested that, at least, three binding components (or sites) are implicated in the binding of 14C . 5-HT to this recombinate system having 3 Kds of 2.2 x 10(-7), 4.1 x 10(-7) and 4.2 x 10(-6) m. All these observations infer that the 5-HT binding components of myelin proteolipids are mainly three acidic lipids in nature.

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 X 10(3) 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 10(3) M was found. While dopamine did not inhibit the binding of radioactive serotonin, tryptamine, 5-methoxytryptamine, and serotonin were equally effective inhibitors.

Butanol extracts from myelin fragments-II. Some properties of 5-hydroxytryptamine binding

The myelin fragments of rat brain stem were treated with butanol-water mixtures, and the butanol extracts (total extract=TE) were incubated with 5 x10(-7) M of C14.5-HT. After incubation, protein, lipid phosphorus and radioactivity were analyzed by Sephadex LH20 column chromatography. Two peaks of components eluted in chloroform-methanol 4:1 (peak I and II) showed the binding capacity for C14.5-HT. The displacement studies with unlabeled 5-HT (5 X 10(-4) M) suggested that peak II was the saturable binding component to 5-HT. On the other hand, butanol extracts from the synaptic membranes of rat brain stem did not show binding for C14.5-HT. Various compounds were studied to determine their inhibitory effects on the saturable binding of 5-HT to TE. The results indicated that acetylcholine, dopamine and tryptamine inhibited the 5-HT binding but LSD, reserpine, colchicine, vinblastine, 5-hydroxytryptophan (5-HTP) and 5-hydroxy-3-indole acetic acid (5-HIAA) had no effect.

5-Hydroxytryptamine binding to butanol extracts from myelin fragments

The myelin fraction of rat brain stem was treated with butanol-water mixtures, and the extracted proteolipids were separated by Sephadex LH20 column chromatography. 2 peaks of proteolipids eluted in chloroform-methanol 4/1 showed the binding capacity for C14 - 5-HT. This finding suggests the necessity of the more careful investigations for the probability of proteolipids as receptor proteins in the central nervous system.