Effect of tricyclic antidepressants on the uptake of noradrenaline and 5-hydroxytryptamine by rat brain slices incubated in buffer or human plasma

Summary A method is described for measuring the inhibition of transmitter uptake into noradrenaline (NA) and 5-hydroxytryptamine (5-HT) neurons incubated in plasma from patients receiving tricyclic antidepressants. The potency was determined of the tricyclic antidepressants nortriptyline and chlorimipramine in inhibiting NA and 5-HT uptake by rat brain slices incubated in buffer or human plasma. As expected, nortriptyline produced greater inhibition of NA than of 5-HT uptake, and chlorimipramine had more effect on 5-HT uptake. These drugs caused 10 to 100 times more inhibition of monoamine uptake from buffer than from plasma, probably because they were bound to plasma proteins. Plasma from patients taking nortriptyline inhibited NA uptake by brain slices 35—55% of the value found in each subject in a pretreatment sample. During long term therapy the concentration of a drug in plasma should be in equilibrium with its concentration at central receptor sites. Thus, it seems likely that the present results reflect the inhibition of uptake by the central monoaminergic neurons of patients taking tricyclic antidepressants. The method also permits evaluation of inter-individual differences in the effects of various antidepressants on NA and 5-HT nerve terminals. It can also be used to evaluate the relative effects of various antidepressants on these two monoaminergic systems.