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

R. J. D'Amato, B. L. Largent, A. M. Snowman, S. H. Snyder

Research output: Contribution to journalArticle

Abstract

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.

Original languageEnglish (US)
Pages (from-to)364-371
Number of pages8
JournalJournal of Pharmacology and Experimental Therapeutics
Volume242
Issue number1
StatePublished - Jan 1 1987

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology

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