Calcium responses in the dendrites of sensory cells do not only reflect signal transduction cascades, but also events linked to sensory adaptation (Leinders-Zufall et al., 1998). As these are also linked to second messenger cascades, we specifically addressed whether Gi or Go are involved in sensory adaptation by choosing appropriate odorant pulse protocols. Single-pulse stimulation was used to test the early response phase (phasic response, Fig. 6C). Double pulses were used to probe for adaptation or sensitization of ORNs: responses to the second stimulus are always lower than those to the first stimulus. If Gi or Go were involved in adaptation or sensitization, we would expect a modified response to the second odor pulse in the respective mutants. The response to the second pulse was quantified by subtracting the single-pulse response from the double-pulse response (Fig. 6C). The tonic response component was measured using a 10-s long odor pulse protocol, and was isolated by subtracting the response of a 1-s short odor pulse prior to quantification (Fig. 6C). We found that dOr22a-cells responded both to the first pulse and to the second pulse, and that they responded to 10-s long pulses for the entire length of the 10 s (Fig. 6C). Interestingly, responses to the second pulse (adapted response) and the late component of the response to the 10-s pulse (tonic response) had an inverse odorant-concentration response: for best ligands high concentrations led to weaker responses, for intermediate ligands the response was concentration independent (two-way anova, F2,35 = 3, P = 0.6 for EtHE, F2,42 = 0.19, P = 0.8 for EtBE, n = 5–7 flies), and for weak ligands the response increased with increasing concentration (two-way anova, F2,35 = 4.2, P = 0.02 for HepL, F2,24 = 4.8, P = 0.02 for MeBM, n = 5–7 flies), indicating stronger adaptation to better ligands (Fig. 6E and F). Overall, however, with increasing phasic response, the adapted and the tonic responses increased only slightly, as seen by the significant but shallow regression slope in Fig. S4. Reducing the effective concentration of Go or of Gi in olfactory receptor neurons reduced but did not abolish calcium responses for all aspects of the odor response: the phasic response (Fig. 6D, two-way anova, F2,43 = 45, P < 2.7e-11 for EtHE, F2,44 = 45, P < 1.9e-11 for EtBE, F2,41 = 24.7, P < 8.9e-08 for HepL, F2,27 = 8.5, P = 0.001 for MeBM, n = 5–7 flies), the adapted response (Fig. 6E, two-way anova, F2,35 = 11.7, P< 1.2e-4 for EtHE, F2,42 = 23.6, P < 1.3e-07 for EtBE, F2,35 = 13.4, P < 4.6e-05 for HepL, F2,24 = 4.4, P = 0.02 for MeBM, n = 5–7 flies) and the tonic response (Fig. 6F, two-way anova, F2,37 = 9.3, P < 5.0e-4 for EtHE, F2,37 = 8.4, P < 9.3e-04 for EtBE, F2,33 = 1.4, P = 0.2 for HepL, F2,23 = 1.1, P = 0.34 for MeBM, n = 5–7 flies; note that the tonic response of weak ligands was not a statistically significant response), arguing in favor of a role of these G proteins that is directly related to the receptor protein itself and its signal transduction mechanism, rather than to an associated second messenger cascade.