Tricyclic Antidepressant Agents. I. Comparison of the Inhibition of the Uptake of 3H‐Noradrenaline and 14C‐5‐Hydroxytryptamine in Slices and Crude Synaptosome Preparations of the Midbrain‐Hypothalamus Region of the Rat Brain

Abstract

The simultaneous uptake of 3-H-l-noradrenaline (NA) and 14-C-5-hydroxytryptamine (5-HT) in slices from the midbrain-hypothalamus region of the rat brain was compared with the corresponding uptake in crude synaptosome preparations of the same brain region. In both preparations the uptake of the two amines was selective at the concentration used (1 times 10- minus 7 M or lower). The KM values for the amines (NA: 2 times 10- minus 7 M in synaptosomes and 5 times 10- minus 7 M in slices; 5-HT: 8 times 10- minus 8 M in synaptosomes and 6 times 10- minus 7 M in slices) and the inhibitory concentrations (IC50) of the antidepressant agents were lower in the synaptosome experiments than in the slices experiments. Moreover the order of the inhibitory activities differed between the two preparations. In the slices experiments the NA uptake was inhibited most markedly by desipramine followed by imipramine greater than chlorimipramine = nortriptyline greater than or equal to amitriptyline greater than or equal to chlordesipramine whereas in the synaptosome experiments the order was desipramine greater than nortriptyline greater than or equal to chlordesipramine greater than or equal to imipramine greater than amitriptyline greater than or equal to chlorimipramine. For the 5-HT uptake in slices the order of activity was: chlorimipramine greater than imipramine greater than or equal to amitriptyline greater than or equal to chlordesipramine = desipramine greater than or equal to nortriptyline whereas in the synaptosome preparations the order was: chlorimipramine greater than imipramine greater than or equal to amitriptyline greater than or equal to chlordesipramine greater than nortriptyline = desipramine. The role of protein binding and diffusion barriers in the causation of the difference in the results obtained with the two preparations is discussed.