Formation of a Schiff base between the ethylamine residue of serotonin and an appropriate carbonyl residue at the receptor site may be among the forces holding serotonin onto the receptor. Reduction of this imine may provide a means of permanently labeling receptors as a preliminary to their isolation.

The Pictet–Spengler condensation, involving interaction of properly activated aromatic compounds having an ethylamine side chain (e.g. dopamine) and aldehydes (e.g. acetaldehyde, aldehyde derivatives of biogenic amines, etc.), leads to the formation of tetrahydroisoquinoline derivatives like salsolinol or tetrahydropapaveroline. It may he arrested in the presence of compounds like ascorbate, cysteine, and glutathione. Cysteine is by far the most effective in preventing this condensation. The mechanism of inhibition is different with each agent: ascorbate and glutathione act either as reducing or complexing agents, while cysteine, which is the most effective of compounds studied so far, is a displacing agent and tends to form thiazolidine derivatives with various substituted aldehydes.

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