Selective labeling of serotonin uptake sites in rat brain by [³H]citalopram contrasted to labeling of multiple sites by [³H]imipramine

Citalopram is a potent and selective inhibitor of neuronal serotonin uptake. In rat brain membranes [³H]citalopram demonstrates saturable and reversible binding with a K(D) of 0.8 nM and a maximal number of binding sites (B(max)) of 570 fmol/mg of protein. The drug specificity for [³H]citalopram binding and synaptosomal serotonin uptake are closely correlated. Inhibition of [³H]citalopram binding by both serotonin and imipramine is consistent with a competitive interaction in both equilibrium and kinetic analyses. The autoradiographic pattern of [³H]citalopram binding sites closely resembles the distribution of serotonin. By contrast, detailed equilibrium-saturation analysis of [³H]imipramine binding reveals two binding components, i.e. high affinity (K(D) = 9 nM, B(max) = 420 fmol/mg of protein) and low affinity (K(D) = 553 nM, B(max) = 8560 fmol/mg of protein) sites. Specific [³H]imipramine binding, defined as the binding inhibited by 100 μM desipramine, is displaced only partially by serotonin. Various studies reveal that the serotonin-sensitive portion of binding corresponds to the high affinity sites of [³H]imipramine binding whereas the serotonin-insensitive binding corresponds to the low affinity sites. Lesioning of serotonin neurons with p-chloroamphetamine causes a large decrease in [³H]citalopram and serotonin-sensitive [³H]imipramine binding with only a small effect on serotonin-insensitive [³H]imipramine binding. The dissociation rate of [³H]imipramine or [³H]citalopram is not altered by citalopram, imipramine or serotonin up to concentrations of 10 μM. The regional distribution of serotonin sensitive [³H]imipramine high affinity binding sites closely resembles that of [³H]citalopram binding. Thus, [³H]citalopram and [³H]imipramine appear to label common sites on the serotonin uptake complex. However, [³H]imipramine also binds to a lower affinity site which is insensitive to serotonin and does not appear to be located primarily on serotonin neurons.