Histamine acts as a neurotransmitter of photoreceptors in insects and other arthropods, where it directly activates a chloride channel and mediates rapid inhibitory responses. Homo- and heteromultimeric histamine-gated ion channels formed by HisCl-alpha2 or HisCl-alpha1 + alpha2 subunits from Drosophila melanogaster were characterized by two-electrode voltage-clamp measurements of functionally expressed ion channels in Xenopus laevis oocytes. The sensitivity of heteromultimeric histamine receptors with an EC(50) of 2.3 microM is lower than that of either homomultimeric receptor. They can be further distinguished from the homomultimeric channels by their reduced sensitivity to d-tubocurarine. Heteromultimeric channels generate a spontaneous current in the absence of any agonist. This spontaneous current can be blocked in the absence of an agonist by d-tubocurarine and the histamine antagonists cimetidine, thioperamide and pyrilamine. Homomultimeric HisCl-alpha2 channels are dually gated by histamine (IC(50)=9.4 microM) and GABA (IC(50)=1.0mM), both of which are full agonists. The action of both agonists can be blocked with comparable IC(50) values by the histamine antagonists cimetidine, thioperamide and pyrilamine but not by the GABA antagonist bicuculline. Picrotoxin blocked with an IC(50) of 403 microM. Our data show that histamine and GABA act on the same ion channel, which thus might function as a site of integration of the action of different neurotransmitters.