Putative histamine-gated chloride channel subunits of the insect visual system and thoracic ganglion

Histamine-gated chloride channels, members of the ligand-gated ion channel superfamily, are thought to be peculiar for arthropods. Their cognate ligand, histamine, is the transmitter of all arthropod photoreceptors and of thoracic mechanoreceptors. To identify putative histamine-gated chloride channel subunits we scanned the Drosophila genome for putative ligand-gated chloride channel subunits and found 12 candidate genes. We found four groups of transcripts based on their expression pattern. Only members of the last group show an expression pattern that is consistent with our knowledge about histamine-gated chloride channels in insects. In the brain these transcripts (Dm HA-Cl I and II) are exclusively present in interneurones postsynaptic to photoreceptors. Within the lamina (the first visual ganglion) only the L1-L3 neurones are labelled. The lack of non-photoreceptor dependent staining in the brain indicates that mechanosensory transmission differs between the head and the thorax/abdomen, and that the receptors responding to brain-intrinsic histaminergic cells use different signalling pathways. The putative histamine-gated chloride channels show the greatest homology mammalian glycine receptors. These ion-channels are the first specific molecular markers for postsynaptic cells in the insect visual system, thus representing ideal tools to study its physiology and development.