Serotonin (5-hydroxytryptamine) is a monoamine neurotransmitter derived from tryptophan by addition of a hydroxyl group at the 5 position on the benzyl ring. The first step involves a tryptophan amino acid combining with molecular oxygen through the enzyme tryptophan hydroxylase. This results in the compound 5-hydroxytryptophan (5-HT). The carboxy group is then removed, leaving serotonin. Serotonin is later degraded to 5-hydroxyindoleacetic acid (5-HIAA) which is often disposed of in the cerbrospinal fluid (CSF).

Serotonin is synthesized in the body, but its precursor, tryptophan, is not. Tryptophan must be supplied in the diet in order for this neurotransmitter to be manufactured. Approximately 1% of dietary tryptophan is converted into serotonin. While serotonin synthesis is relatively simple, the metabolism and incorporation of tryptophan into serotonin generating cells is a complex process. There are multiple metabolic pathways for incorporating and storing tryptophan. Tryptophan reaches serotonin generating cells by binding to serum albumin, because tryptophan itself is largley nonpolar and has low solubility in the bloodstream. The binding of tryptophan to serum albumin is very pH dependent and physiological conditions such as acidosis or alkyosis can disrupt tryptophan transport.

Serotonin, being a neurotransmitter, exerts its effects by its interaction with a receptor. Stimulation of serotonin receptors leads to a number of different cell processes, depending on the receptor type. Some lead to the turnover of cellular lipids such as phosphoinositol phosphate and diaglyceride formation. Others trigger adenylate cyclase which then results in a cascade of cell signalling events.

Serotonin levels can be assayed directly in post-mortem autopsies or by looking for downstream degraded products in the CSF and urine. 5-HIAA in particular is often found in these two fluids. Because of the widespread use of serotonin in many different physiological processes from neurochemistry to digestion, understanding how serotonin levels correlate with physical and psychological pathologies is an area of intense study. Efforts to understand how serotonin levels may correlate with propensity for suicide and comitting violent crime are yielding interesting results.

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Many different serotonin receptors have been discovered; they have been divided into subtypes:

Serotonin's chemical structure:

                H
                 \
           H  H   N
            \  \ / \
 H   H       C--C   H
  \   \     / \  \
   O   C   C   H  H
    \ / \\/ \\
     C   C   C
    ||   |   /\
     C   C--N  H
    / \ //   \
   H   C      H
        \
         H

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