Single-site model of the neuronal 5-hydroxytryptamine uptake and imipramine-binding site

Recently, a high affinity [3H]imipramine-binding site of protein nature that appeared to be related to the 5-hydroxytryptamine (5-HT, serotonin) uptake mechanism was demonstrated. This binding site was only part of desipramine-displaceable [3H]imipramine binding, which contained a significant amount of additional binding not related to 5-HT uptake. The present study further investigates the [3H]imipramine-binding site of protein nature in the rat brain. Displacement by 5-HT and 6-methoxytetrahydro-beta-carboline (6-MeO-TH beta C) revealed monophasic displacement patterns with 60% displaceable binding. This binding fraction was abolished by protease treatment of the brain tissue prior to binding assay. Saturation studies of [3H]imipramine binding (1-30 nM) in rat cortex showed that the binding displaced by 30 microM 5-HT [Bmax 322 +/- 16 fmol/mg of protein, Kd 4.17 +/- 1.07 nM (means +/- SE)] was not different from the binding displaced by 1.0 microM norzimeldine (Bmax 349 +/- 15 fmol/mg of protein, Kd 4.47 +/- 1.07 nM) or 30 microM 6-MeO-TH beta C (Bmax 439 +/- 28 fmol/mg of protein, Kd 5.49 +/- 1.09 nM). When 100 microM desipramine was used in saturation studies, the binding was different from that displaced by 5-HT with Bmax 608 +/- 42 fmol/mg of protein and Kd 6.68 +/- 1.09 nM. Both displacement and saturation studies in which two displacing agents were combined indicated that most of the binding competed by 5-HT (30 microM) and norzimeldine (1.0 microM) is identical. Similarly, the binding displaced by 5-HT or norzimeldine is subsumed within 6-MeO-TH beta C (30 microM)-displaceable binding. Lesion studies with parachloroamphetamine, a selective toxin for 5-HT terminals, which resulted in a 83% reduction of [3H] 5-HT uptake ( [3H]noradrenaline uptake unaffected), abolished cortical [3H]imipramine binding displaced by 30 microM 5-HT or 1.0 microM norzimeldine. (greater than 80% reduction). However, with 100 microM desipramine as displacer, 40% of the binding remained in lesioned animals. The [3H]imipramine binding displaced by 30 microM 5-HT or 1.0 microM norzimeldine was sodium dependent, and an increase in NaCl concentration from 0 to 120 mM resulted in a 10-fold increase in affinity without effect on Bmax, whereas no change in binding was observed with increasing concentrations of LiCl.(ABSTRACT TRUNCATED AT 400 WORDS)