Pitfalls in demonstrating an endogenous ligand of imipramine recognition sites

Further evidence for the existence of endogenous serotonin uptake inhibitors and their purification by calmodulin affinity chromatography

Experimental evidence suggests the occurrence of endogenous antidepressant-like compounds in the brain, blood plasma, and urine. After extensive purification by calmodulin-sepharose affinity chromatography, and further purification of the urine-derived sample by exclusion chromatography, at least three distinctive fractions have been identified. These fractions effectively inhibited serotonin uptake, imipramine, and/or paroxetine binding, and they apparently contained some compounds that were recognized by the anti-imipramine or anti-paroxetine antibodies. Their identification may have significant implications for studies of affective illness.

Further studies on the endogenous serotonin-uptake-inhibitor-like substances in the human cerebrospinal fluid

The properties of endogenous substances that inhibit 5HT uptake in human cerebrospinal fluid (CSF) were investigated. Human CSF was loaded onto a Sephadex G-25 column, and each fraction was tested for its ability to inhibit [3H]paroxetine binding in monkey brain preparations. We found four different inhibitory peaks with respective molecular weights (M.W.) of > 12400, 2000 and two of < 1350. The third and fourth peaks (F-3, F-4: < 1350 M.W.) of inhibitory activity were determined to consist of some monoamines (5HT, etc.) or their metabolites (5HIAA, etc.) and other unidentified compounds by using an HPLC-electrochemical detector. The second peak (F-2, M.W. about 2000) displaced [3H]paroxetine binding noncompetitively (decreased Bmax and did not change Kd) and inhibited [3H]5HT uptake noncompetitively (decreased Vmax and did not change Km), but had no effect on either [3H]norepinephrine uptake or [3H]dopamine uptake. These results suggest that the endogenous substances that selectively inhibit 5HT uptake are present in human CSF as low molecular weight compounds.

Anti-imipramine antibodies recognize endogenous serotonin uptake and imipramine binding inhibitors

Calf brain and human platelet extracts purified by Bio-Gel P2 column chromatography contained substances that inhibited serotonin uptake and 3H-imipramine binding. Some of these endogenous substances were also recognized by rabbit antibodies produced against imipramine. The data suggest the possible existence of endogenous serotonin uptake modulators, which may possess a partial molecular structure similar to that identified by the antibodies.

Platelet tritiated imipramine binding in psychiatric patients: relationship to symptoms and severity of depression

The clinical and research significance of reduced imipramine binding has remained unclear despite considerable investigation. This study used an assay of demonstrated reliability to investigate the clinical correlates of imipramine binding to platelets in 63 depressed and 33 nondepressed psychiatric patients and 40 healthy control subjects. Both patient groups had Bmax values significantly lower than those of the healthy controls. Unequivocal associations between binding parameters and individual symptoms or groups of symptoms were not established, but a negative correlation between Kd and the number of adverse life events experienced in the preceding 6 months was apparent. These findings provide no support for the view that reduced binding is a trait marker for susceptibility to depression and cast doubt on its specificity as a state marker for the syndrome of depression.

Non-specific inhibition of imipramine binding argues against an endogenous ligand

We have attempted to characterize the product(s) inhibiting [3H]imipramine binding from rat cortical membranes. Fractions inhibiting [3H]imipramine binding were analysed by gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography-mass spectrometry (HPLC-MS). Tissue extracts showed marked inhibitory activity (two different peaks); however, extracts containing no brain tissue (blanks) also exhibited inhibitory activity (one peak after HPLC pre-purification). The chemical composition of the compound that inhibited [3H]imipramine binding was identical in both cases (tissue and blank extracts) and consisted of zinc chelates, probably formed by the use of zinc sulfate during the extraction procedure. The second inhibitory peak in the tissue extracts was 5-hydroxytryptamine (serotonin, 5-HT). These results suggest that the inhibitory activity on [3H]imipramine binding attributed to a low-molecular weight putative endacoid is non-specific and due to ions introduced in the sample during purification.

Studies on the serotonin transporter in platelets

[3H]-Imipramine and [3H]-paroxetine label with high affinity a recognition site which is associated with the serotonergic transporter in blood platelets. The pharmacological profile of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of drugs to inhibit the uptake of serotonin. Dissociation kinetic experiments suggest that the substrate recognition site for serotonin may be different from the modulatory site which is labeled with [3H]-imipramine or [3H]-paroxetine. The existence of an endocoid acting on the imipramine receptor to modulate the serotonin transporter has been proposed by several laboratories. In clinical studies most laboratories have reported a decrease in Bmax of [3H]-imipramine binding in platelets from depressed untreated patients when compared with matched healthy volunteers. The Bmax of [3H]-imipramine binding in platelets appears to be a state-dependent biological marker in depression.

Circadian rhythm of the Bmax of [3H]-imipramine binding in rabbit platelets

[3H]-imipramine binding was measured in rabbit blood platelet membranes on a 24 h cycle. Animals were kept on a 14 h light (L) 10 h dark (D) schedule, and blood samples were collected at L + 2, L + 8, D + 2, D + 8 and L + 2 h on a following cycle. Significant differences were found for Bmax values of [3H]-imipramine binding, with highest values during the dark phase and lowest during the light phase. No significant differences were found in Kd values. These results suggest the existence of a circadian rhythm for the Bmax of [3H]-imipramine binding in blood platelets.