Pitfalls in demonstrating an endogenous ligand of imipramine recognition sites

The recognition sites for the 5-hydroxytryptamine (5-HT) uptake inhibitors imipramine and paroxetine may represent receptors for a presently unknown endogenous ligand, whose function would be to modulate 5-HT uptake. Attempts to isolate such a factor from rat brain tissue are described, following a published procedure. It is shown that chromatographic fractions found to inhibit the binding of [3H]imipramine and [3H]paroxetine to rat brain membranes consisted of material essentially unretained by the reverse-phase HPLC column, and they were of high osmolarity. Thus, the activity was probably unspecific in nature, and the presence in rat brain of the factor has not been unequivocally demonstrated.