Histamine: A Neurotransmitter Candidate for Drosophila Photoreceptors

: Recent experimental evidence suggests that histamine might be the synaptic transmitter used by invertebrate photoreceptors. In the present study, we have examined whether histamine is a transmitter candidate for Drosophila photoreceptors. Our findings are as follows; (a) Large amounts of histamine are synthesized by wild‐type heads, whereas heads from the eye‐deficient mutants, eyes absent and sine oculis, show reduced histamine synthesis, (b) Histidine de‐carboxylase activity is ⋍ 10‐fold higher in extracts of normal heads compared with that ia the mutants, (c) Histamine taken up by fly heads is metabolized into N‐acetylhistamine and imidazole‐4‐acetic acid, (d) Immunostaining of normal and sevenless heads with histamine‐specific antisera demonstrates that histamine is present in photoreceptors R1–6 and R8. (e) Histamine synthesized from exogenously supplied [³H]‐histidine can be released by depolarization with 50 mM K⁺, and the release is Ca²⁺ dependent. These observations strongly suggest that histamine is a major neurotransmitter used by Drosophila photoreceptors.