Serotonin neurochemistry revisited: A new look at some old axioms.

A number of principles have long guided research into serotonin neurochemistry and some of them are presently reconsidered with a slightly different outlook. An increasing amount of experimental data does not always support these older axioms and as more empirical observations are made, some of these may require modification. For example, increases in 5-HT produced by injections of l-tryptophan do not necessarily indicate that tryptophan hydroxylase is less than saturated with its substrate in vivo; injections of l-tryptophan increase 5-HT and its turnover but do not appear to increase its release; 5-HT release can apparently occur from the cytoplasm and need not involve exocytosis; presynaptic autoreceptors do not appear to modulate 5-HT release; and, 5-HIAA most accurately reflects monoamine oxidase activity, not release of 5-HT or activity of 5-HT neurons.