Chapter 6: Serotonin
• Serotonin = 5hydroxytryptamine (5HT)
– Trytophan (precursor)
• Converted by tryptophan hydroxylase
– To 5hydroxytryptophan (5HTP)
• Converted by aromatic amino acid decarboxylase (AADC)
– To 5hydroxytryptamine (5HT)
• The first step is the rate limiting step.
– Takes the longest
• Like we talked about with dopamine
– Thus, tryptophan hydroxylase is the rate limiting enzyme
• Only serotonergic neurons contain this enzyme.
• Thus, labeling for tryptophan hydroxylase is one way to identify
• Also notice that the enzyme in the second step is the same for catecholamines and
– Aromatic amino acid decarboxylase (AADC)
Turkey Dinner effect…
• Although many people believe that increasing tryptophan in the diet can increase
serotonin levels, it is more complicated than that.
Turkey dinner effect – not true
• The ratio of tryptophan to other large amino acids determines the rate that
tryptophan enters the brain
Competition for crossing the blood brain barrier.
• Consequently it is high carbohydrate and low protein diets that increases brain
Insulin release is stimulated by high carb diet.
Insulin causes most amino acids to be removed from the blood stream.
But not tryptophan
Less competition for tryptophan, means that more crosses the BBB
• Tryptophan depletion can cause the return of depressive symptoms in patients that
have recovered from depression.
– Indication of serotonin’s role in mood regulation.
What enzyme? MAO breaks down 5HT
COMT won’t break down serotonin
Some drugs can cause release, independent of action potentials
• Amphetamine like drugs – Paracholoroamphetamine