5-Hydroxytryptamine in the Sub-cellular Particles of Rabbit Brain

IN the course of experiments on the sub-cellular distribution of substance P in the rabbit brain1, we have confirmed Whittaker's finding2that the rat fundus stimulating action of his “synaptic vesicle fraction” was higher than that of the mitochondrial or supernatant fraction. As pointed out by him, however, the total amount of brain 5-hydroxytryptamine (5HT) thus estimated was about several times as much as that reported by previous workers3. This should be attributable to his omitting extracting 5HT by acetone. Using the acetone extraction3and Vane's rat fundus preparation4, therefore, we attempted to compare the sub-cellular distribution of brain 5HT with and without acetone treatment. Methods of preparation of the rabbit brain homogenates and of sub-cellular fraction were as described previously1. As illustrated in Fig. 1, the amount of 5HT estimated is lowest in the vesicle fraction, while that in the mitochondrial fraction is relatively high. It is apparent at a glance that the 5HT distribution obtained by omitting acetone treatment as in Whittaker's experiment is quite similar to that of substance P1,9. In fact, it was found that the rat's fundus was considerably sensitive to substance P, and the 5HT equivalence of acetone-insoluble aliquot of each fraction measured with the fundus preparation ran parallel to their substance P content (Fig. 1).