On the significance of sialic acid in high affinity 5-hydroxytryptamine uptake by synaptosomes

Synaptosomes isolated from rat brain cortex incorporated [14C]5-hydroxytryptamine at 37 degrees C with high affinity. An apparent transport constant of Kt = 50nM was found. The high affinity uptake was decreased by treatment of synaptosomes with neuraminidase from Vibrio cholerae or Clostridium perfringens prior to incubation with [14C]5-hydroxytryptamine. The inhibition was related to the amount of sialic acid released, with a Ki value of 3.5 micrometer. A non-competitive type of inhibition was observed after treatment with neuraminidase. The inhibition caused by ouabain could not be enhanced by simultaneous treatment with neuraminidase. Neuraminidase did not lower the activity of (Na + K)-ATPase or Mg2-ATPase. These results suggest that sialic acid is involved in the 5-hydroxytryptamine uptake mechanism without functional linkage to the energy pump of the membrane, which maintains the sodium gradient necessary for 5-hydroxytryptamine transport.