There are two main reasons why we investigated faeces as a potential source of AOS activation. First, it has long been known that soiled cage bedding is one of the most potent activators of AOS activity20,34. Bedding contains a mixture of mouse excretions, the most abundant of which are urine and faeces. Urine, currently the best-studied source of AOS ligands, contains a number of unique ligands including sulfated steroids and major urinary proteins9,10,12,15,17, but mammalian faecal chemosignals have not yet been systematically investigated. Second, molecular components of faeces vary across many biological states, including sex and species35,36,37,38, and information gleaned from faecal constituents might regulate animal behaviour.
(a) Overview of the AOS and the ex vivo preparation. (b) Example single-unit recording of a male mouse AOB neuron that responded to female mouse urine. Top is a raster plot, bottom is an average peristimulus time histogram (PSTH) from the same cell. (c) An AOB neuron exclusively responsive to female faecal extracts. (d) A neuron that selectively responded to urine. (e) Neuron that selectively responded to faeces. (f) A cell that responded equally to both urine and faeces. (g) Heat map of normalized change in firing rate (Norm. ΔR) following VNO stimulation with female urine or faeces. Thin black lines indicate divisions between clusters (89 cells from 56 animals). (h) Percentage of cells that responded exclusively to urine, exclusively to faeces extracts, to both urine and faeces extracts, or to other compounds in the stimulus battery (89 cells from 56 animals). (i) Venn diagram of response overlap for the pool of neurons that responded to urine and/or faeces (55 cells from 40 animals). (j) Histogram showing the discriminability index (dı) for the observed AOB neuron population (red) compared with the mean of 100,000 scrambled populations (grey; 55 cells from 40 animals).
Faecal extracts and urine at these dilutions were equally potent. The apparent selectivity of many MCs indicated that urine and faeces produce unique information in the AOS. We quantified the capacity for this information to be used to discriminate urine from faeces by calculating the discriminability index (dı) for all MC responses to urine and faeces based on the change in firing rate (ΔR) elicited by each (Fig. 1j). The dı values we observed across the MC population showed a strong bias towards high discriminability between urine andz faeces (Fig. 1j). 98.9% of 100,000 simulated MC populations, each with 55 urine and faeces responses chosen at random from our actual data, showed dı distributions that were statistically lower than the observed population. In sum, the strong but differential activation of MCs by faecal and urinary chemosignals indicated that urine and faeces provide unique information to the AOS.
Animal secretions (for example, tears and urine) contain sex-specific AOS cues4,7,34,48, and previous studies indicated that the bile acid pool also varies with sex35. Therefore, we investigated whether male and female faeces differentially activate the AOS. We delivered 300-fold diluted faecal extracts from BALB/cJ males and females to male ex vivo preparations, and recorded sex-specific activation of AOB MCs (Fig. 6). Many MCs responded selectively to faecal extracts from a specific sex (Fig. 6a) and others responded to faecal extracts of both sexes (Fig. 6b). We classified MC tuning curves to male and female urine and faeces using clustering algorithms, revealing variable tuning patterns (70 cells from 44 animals Fig. 6c). For the 29 cells that responded to male or female faeces, 34.5% of those cells responded selectively to female faeces, 20.7% responded selectively to male faeces and 44.8% responded nonselectively to both (Fig. 6d). We quantified the discriminability of male and female faeces by AOB MCs, finding that, as a population, sex discrimination based on faeces was lower than between female urine and faeces, but that the presence of many MCs with high dı values was significant (67.9% of 100,000 simulated MC populations showed statistically lower discriminability than observed; Fig. 6e). These results indicated that there are sex-specific differences in the concentrations and/or identities of faecal chemosignals.
Finally, we investigated MC tuning across all the polar sterols in our stimulus battery, including primary and secondary bile acids and sulfated glucocorticoids (Fig. 8). Cluster analysis revealed rich combinatorial tuning across sterols, but clearly showed that MCs tuned to bile acids were almost completely separable from MCs tuned to sulfated glucocorticoids (Fig. 8a,b). Overall, these results indicated that AOS bile acid tuning is not limited to molecules excreted by conspecifics, and that bile acids produce a complex combinatorial code in AOB MCs, similar to codes produced by sulfated steroids, major urinary proteins, formyl peptides, major histocompatibility proteins and exocrine-secreting gland peptides7,8,9,10,11,12,13,15,21,28,48,50.
Despite two decades of research since the discovery of VRs (refs 6, 7, 8, 9, 10, 11, 12, 15, 23, 24, 26, 27, 48), our understanding of the repertoire of ligands for this behaviourally relevant neural pathway remains incomplete. Technical improvements have made it possible to conduct AOS ligand screens using VSNs as bioassays6,9,10,15,20,40,50. However, VSNs are notoriously noisy17,28,30,51 and require extensive controls to avoid false positive results. AOB MCs, in contrast, have dramatically improved signal/noise ratios by virtue of synaptic pooling/averaging28. In this study, we utilized MC recordings from the ex vivo preparation (which maintains functional connectivity between the VNO and AOB) as the platform of a screen for AOS ligands.
Simple aqueous extraction procedures isolate AOS ligands from BALB/cJ female mouse faeces that, at 300-fold dilutions, produce equivalent AOB neuronal activity to 100-fold diluted BALB/cJ female mouse urine. This indicates that mouse faeces are rich in AOS ligands, and that the activity stimulated by these ligands is roughly equivalent to mouse urine, which is currently the best-known source of AOS ligands9,10,12,15,17. The concentration-related differences in stimulus potency are likely related to the relative dryness of raw faeces compared with mouse urine and the specific ratio of faeces:water used in extractions. That said, faeces are dry in the natural environment, so faecal ligands are likely to be highly concentrated before dissolution in nasal mucus. In vivo studies confirmed that dilute faecal extracts produced AOB activation similar to soiled bedding, further indicating that the faecal molecules in these aqueous extracts are biologically active. AOB MCs readily discriminate urine from faeces, indicating that faecal chemosignals produce unique information in the AOS.
MC tuning across bile acids and sulfated glucocorticoids shows that bile acids, similar to other classes of AOS ligands, activate the AOB with a complex combinatorial code28,39,40,50. The capacity of the AOS to distinguish between individual bile acids that differ across biological states (for example, sex, species, etc.) indicates that these ligands may drive state-specific behaviours. Future studies that investigate the behavioural impact of the combinations of bile acids found in natural samples will be necessary to identify these state-specific behaviours. It is worth noting that common themes of sex, species and other biological state-related differences exist across AOS ligand classes, and in nearly all cases, ligands that vary with biological state have been shown to influence mouse behaviours4,6,8,9,40,48,50,65. The apparent redundancy of sex, species and other biologically relevant information across AOS ligand classes raises the question: what is the biological benefit of processing all this redundant information? The answer is likely to require detailed investigation of the specific social contexts in which the information is encountered.
All life on earth depends on light. A variety of photoreceptors capture the light for a wide range of reactions. Photosynthetic organisms absorb the light necessary for energy transformation and charge separation facilitating photosynthesis. In addition to the bulk pigments there is a great diversity of photoreceptors present in minute concentrations that control development, metabolism and orientation of plants and microorganisms. Based on its spectral absorbance, the well-studied phytochrome system acts in the RL (red light) region as well as in the UV-A/BL (blue light) region where the above mentioned reactions are mediated by a variety of photoreceptors whose natures are largely unknown. Phyllogenetically the UV-A/BL photoreceptors seem to be more ancient pigments that eventually were replaced by the phytochrome system. However, there are many reports that suggest a coaction between the UV-A/BL receptors and the phytochrome system. In several cases the UV-A/BL activation is the prerequisite for the phytochrome reaction. Historically it was the German botanist Julius Sachs who first discovered in 1864 that phototropism in plants was due to BL reactions. It took over 70 years until Bunning (1937) and Galston and Baker (1949) rediscovered the BL response. Since then, an ever-increasing attention has been paid to this effect. In this contribution, the general aspect of UV-A/BL responses and especially the responsiveness of algae will be covered.