Characterisation of sodium-dependent uptake of 5-hydroxytryptamine into guinea-pig brain

The uptake of 5-hydroxytryptamine (5-HT) was studied using isolated nerve endings (synaptosomes) prepared from the corpus striatum of the guinea pig. Analysis of temperature-dependent uptake revealed that two processes were involved, a sodium-dependent and sodium-independent mechanism. Kinetic studies indicated that the affinity of 5-HT (Ku) and the transport capacity (Vu) of the uptake process were dependent upon the external concentration of sodium. The transport capacity increased linearly with increasing concentrations of sodium up to 140 mmol/l, whereas the affinity for 5-HT was enhanced at concentrations of sodium up to 100 mmol/l, but reduced at 130 mmol/l sodium. The findings of this study suggest that uptake of 5-HT occurs as a result of a mobile carrier transport system which, depending upon the external concentration of sodium proceeds by a single or double displacement mechanism.