A, Orco (left), Orco mut (middle) and Or22a (right) proteins visualized in adult antennal sections with specific antibodies (red). The proteins show expression in cell bodies (arrowhead) and dendrites (arrow). Or22a-expressing cells are housed in few sensilla opposite to arista (a). Scale bar 50 mm. B, Normalized ab3A neuron spike frequency (fnorm) upon Etb stimulation wild type flies (Orco, n = 12), for Orco null mutants (no Orco, n = 15), and mutants rescued with Orco mut (“Orco mut flies”; n = 14; P = 0.016 vs. Control, Mann-Whitney U test). C, fnorm as in B upon Etb stimulation in Orco mut flies (n = 17) before (Control) and after forskolin injection.