Serotonin Group

A relationship similar to that between tyrosine and dopamine exists between the amino-acid tryptophan and the brain substance serotonin or 5-hydroxytryptamine (Figure 8.9). Tryptamine is known in the venom of scorpions, while serotonin is found widely in the venom of honeybees, centipedes, and at least two spiders. Serotonin is not found in the venom of ants or solitary wasps but social wasps have it in quantity, as much as 1 jLig per insect. It is present in the barbs of the larvae of the Tiger moth Arctia caja (Plate 11) and a saturnid butterfly larva. Their origins in insect venoms are presumably from phenylalanine but this has not been proven. The same substance is in the hairs of the common stinging nettle Urtica dioica.

It is interesting to compare the structure of some alkaloids (Chapter 9) with these brain chemicals, dopamine, noradrenaline and serotonin. Many of the alkaloids are made from phenylalanine and tryptophan. For example, the opiates alter the release and breakdown of at least four substances, noradrenaline, serotonin, dopamine and acetylcholine, all of which are believed to act as transmitters in different parts of the brain. A complex system of neurotransmitters also exists in insects, which use some of the same compounds.

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