Active Uptake of [³h]5‐ht by Synaptic Vesicles From Rat Brain

The question of whether synaptic vesicles accumulate [ 3 H]5‐HT by an active process was investigated in a mixed population of vesiclcs from whole rat brain. The temperature dependence and the effect of metabolic inhibitors were studied in synaptosomal suspensions and vesicular fractions. Arrhenius plots for synaptosomes differed from those for vesicles as did the temperature coefficients for these two fractions. For synaptosomes the Q 10 was 7 and for vesicles 1.6. However, if ATP was added to the incubation, the temperature dependence of vesicular amine accumulation became manifest; the Arrhenius plot resembled that of synaptosomes and the Q 10 was greater than 20 indicating strong temperature dependence. In the presence of ATP, vesicular uptake was stimulated approx 8‐fold. Ouabain, dinitrophenol and NEM inhibited synaptosomal uptake but failed to affect [ 3 H]5‐HT accumulation by vesicles in the absence of ATP. When ATP was added, vesicular uptake was also blocked by NEM but was unaffected by either ouabain or DNP. Total observed uptake consisted of two components, one ATP‐dependent and one nonsaturable and ATP‐independent. The active process had a K m = 1.25 × 10 −7 M and could be completely blocked by either 10 −3 M or 10 −7 M‐reserpine. Active vesicular [ 3 H]5‐HT uptake was magnesium dependent and was inhibited by sodium and potassium. Cation effects on uptake were specific and could not be accounted for by either changes in osmotic pressure or ionic strength. It was concluded that synaptic vesicles from whole rat brain accumulate [ 3 H]5‐HT by an active process.