On the function of histamine in the central nervous system of arthropods

Histamine is localized in the CNS of many invertebrate taxa by various methods. It is described to act as a neurotransmitter, neuromodulator, and neurohormone. Because of its distribution throughout these taxa it is tempting to forward the following hypothesis: Histamine acts as a neuroactive substance in all invertebrates. Its function as a transmitter of photoreceptors is restricted to arthropods. A special system of six large histamine-immunoreactive neurons exists exclusively in arachnids.