Ine and the dipeptide L-arginyl-glycine (ORNs #28-30, see matrix in Figure 2B). Olfactory receptor neuron #27 showed an additional weak sensitivity to glycyl-L-arginine, and ORN #25 was sensitive to all applied stimuli. In Figure 3B we give a closer look at the four ORNs showing a specific amino acid sensitivity to L-arginine. Interestingly, in these ORNs 
the mean maximum amplitude of responses to the dipeptide L-arginyl-glycine was much higher than that of all other peptide responses (group II as well as group I), but with 7566 still significantly lower thanresponses to the amino acid L-arginine alone. The reversesubstituted glycyl-L-arginine, however, showed only minor activity and the mean relative maximum amplitude was only 1161 . An analysis of the time course of the calcium transients triggered by amino acids, group I and group II peptides gave heterogeneous results. Figure 4A shows the time points of the mean maximum amplitude of the responses to each of the applied odorants. Calcium transients evoked by group I peptides generally had a delay of their mean maximum amplitude if compared to those of amino acids. The mean time point of the maximum amplitude of all group I peptide responses showed a significant shift from 9.160.3 s (amino acids) to 13.760.9 s (peptides of group I) after stimulation (Figure 4B and C). In contrast, the time points of the mean maximum amplitude of the responses of all group II peptides did not significantly differ from those of amino acids [7.361.3 s (amino acids) vs. 9.061.2 s (peptides of group II); Figure 4B and D]. Interestingly, in the four ORNs specifically sensitive to L-arginine (see also Figure 3), the delay of the mean maximum amplitude for the L-arginyl-glycine (7.561.5 s; Figure 4B) was almost identical to that of L-arginine (7.961.5 s; Figure 4B).DiscussionIt has long been known that fish as well as other aquatic vertebrates and invertebrates are able to smell amino acid odorants. This has been assessed in many studies that used a wide range of different neurophysiological techniques (extracellular recordings: [4,33,34], patch clamp: [3,8,9,35,36], calcium imaging: [2,5,6], voltage sensitive dyes: 
[37]). Behavioural studies have shown that amino acids are appetitive olfactory cues that Hexaconazole chemical information elicit an attractive response [38?0]. The main sources of amino acids in sea and freshwater are: (i) direct release and excretion by the biota, (ii) bacterial exoenzyme activity, (iii) living cell lysis, (iv) decomposition of dead and dying autotrophic and heterotrophic organisms, and (v) release from biofilms [41,42]. In natural aquatic environments the concentrations of dissolved free amino acids areOlfactory Responses to Amino Acids and PeptidesFigure 4. Group I and group II peptides elicit significantly different [Ca2+]i transients in individual olfactory receptor SMER 28 site neurons. (A) The mean time points of amino acid- and peptide-evoked calcium transient maxima varied for individual stimuli. Transients evoked by group I peptides show a tendency to reach their maximum amplitude later if compared to amino acid stimulations (green, group I peptides, 1 mM; number of responses averaged: AA mix, 67; L-arginine (Arg), 10; L-methionine (Met), 11; L-lysine (Lys), 6; L-arginyl-L-methionine (Arg-Met), 3; L-arginyl-Lmethionyl-L-arginine (Arg-Met-Arg), 4; L-methionyl-L-arginyl-L-methionine (Met-Arg-Met), 9; L-methionyl-L-arginine (Met-Arg), 9; L-arginyl-L-lysine (Arg-Lys), 4; L-arginyl-L-lysyl-L-arginine (Arg-Lys-Arg), 7;.Ine and the dipeptide L-arginyl-glycine (ORNs #28-30, see matrix in Figure 2B). Olfactory receptor neuron #27 showed an additional weak sensitivity to glycyl-L-arginine, and ORN #25 was sensitive to all applied stimuli. In Figure 3B we give a closer look at the four ORNs showing a specific amino acid sensitivity to L-arginine. Interestingly, in these ORNs the mean maximum amplitude of responses to the dipeptide L-arginyl-glycine was much higher than that of all other peptide responses (group II as well as group I), but with 7566 still significantly lower thanresponses to the amino acid L-arginine alone. The reversesubstituted glycyl-L-arginine, however, showed only minor activity and the mean relative maximum amplitude was only 1161 . An analysis of the time course of the calcium transients triggered by amino acids, group I and group II peptides gave heterogeneous results. Figure 4A shows the time points of the mean maximum amplitude of the responses to each of the applied odorants. Calcium transients evoked by group I peptides generally had a delay of their mean maximum amplitude if compared to those of amino acids. The mean time point of the maximum amplitude of all group I peptide responses showed a significant shift from 9.160.3 s (amino acids) to 13.760.9 s (peptides of group I) after stimulation (Figure 4B and C). In contrast, the time points of the mean maximum amplitude of the responses of all group II peptides did not significantly differ from those of amino acids [7.361.3 s (amino acids) vs. 9.061.2 s (peptides of group II); Figure 4B and D]. Interestingly, in the four ORNs specifically sensitive to L-arginine (see also Figure 3), the delay of the mean maximum amplitude for the L-arginyl-glycine (7.561.5 s; Figure 4B) was almost identical to that of L-arginine (7.961.5 s; Figure 4B).DiscussionIt has long been known that fish as well as other aquatic vertebrates and invertebrates are able to smell amino acid odorants. This has been assessed in many studies that used a wide range of different neurophysiological techniques (extracellular recordings: [4,33,34], patch clamp: [3,8,9,35,36], calcium imaging: [2,5,6], voltage sensitive dyes: [37]). Behavioural studies have shown that amino acids are appetitive olfactory cues that elicit an attractive response [38?0]. The main sources of amino acids in sea and freshwater are: (i) direct release and excretion by the biota, (ii) bacterial exoenzyme activity, (iii) living cell lysis, (iv) decomposition of dead and dying autotrophic and heterotrophic organisms, and (v) release from biofilms [41,42]. In natural aquatic environments the concentrations of dissolved free amino acids areOlfactory Responses to Amino Acids and PeptidesFigure 4. Group I and group II peptides elicit significantly different [Ca2+]i transients in individual olfactory receptor neurons. (A) The mean time points of amino acid- and peptide-evoked calcium transient maxima varied for individual stimuli. Transients evoked by group I peptides show a tendency to reach their maximum amplitude later if compared to amino acid stimulations (green, group I peptides, 1 mM; number of responses averaged: AA mix, 67; L-arginine (Arg), 10; L-methionine (Met), 11; L-lysine (Lys), 6; L-arginyl-L-methionine (Arg-Met), 3; L-arginyl-Lmethionyl-L-arginine (Arg-Met-Arg), 4; L-methionyl-L-arginyl-L-methionine (Met-Arg-Met), 9; L-methionyl-L-arginine (Met-Arg), 9; L-arginyl-L-lysine (Arg-Lys), 4; L-arginyl-L-lysyl-L-arginine (Arg-Lys-Arg), 7;.