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Keywords:

  • Histamine;
  • Neurotransmitter;
  • Photoreceptor;
  • Histidine decarboxylase;
  • Invertebrate;
  • Drosophila.

Abstract

: 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.