Distribution ofsup3/supH- andsup14/supC-Labelled Potential Neurotransmitters on a Discontinuous Density Gradient after Uptake into Rat Cerebral Cortex Slices

摘要

The uptake and subcellular distribution of ten labelled substances (3H-γ-aminobutyrate, 3H-L-glutamate, 3H-L-aspartate, 3H-L-noradrenaline, 3H-dopamine, 3H-5-hydroxytryptamine, 14C-glycine, 14C-L-lysine, 14C-choline and 14C-histamine) was studied in rat cerebral cortex slices. After 10 min uptake at 25 °C, a nuclei-free homogenate of the slices was fractionated on a 10-step discontinuous sucrose density gradient, which gave three distinct bands of synaptosomes, at 1.0, 1.1 and 1.2 M sucrose. The quantitative distribution of label in the three synaptosomal bands is characteristic for each substance and allows the distinction of specific synaptosomal subpopulations responsible for the uptake of a certain number of them. In particular, the catecholamines (noradrenaline and dopamine) were accumulated mainly by the heaviest and the acidic amino acids (glutamate and aspartate) mainly by the lightest types of synaptosomes, the other six giving intermediate patterns. By fractionating a homogenate containing all cytoplasmic material, uptake into cellular compartments other than the synapses could be quantitatively determined. The importance of active uptake into glial cells as a synaptic inactivation mechanism for neurotransmitters is discussed, comparing the acidic amino acids (over 50 uptake into the cytoplasmic fractions) and the catecholamines (less than 25