Olfactory sensitivity to amino acids in the blackspot sea bream (Pagellus bogaraveo): a comparison between olfactory receptor recording techniques in seawater

Commentary: Best practices for performing olfactory behavioral assays on aquatic animals: A guide for comparative physiologists

As more physiologists start to incorporate animal behavior into their experiments, especially in the olfactory behavior research field, some considerations are often overlooked, partly due to the inherited way that physiological experiments are traditionally designed and performed. Here we highlight some of these subtle but important considerations and make a case for why these might affect the results collected from behavioral assays. Our aim is to provide useful suggestions for increased standardization of methods so they can be more easily replicated among different experiments and laboratories. We have focused on areas that are less likely to be mentioned in the materials and methods section of a manuscript such as starvation, preliminary experiments, appropriate sample sizes and considerations when choosing an odorant for an assay. Additionally, we are strongly cautioning against the use of alarm cue to generate behavioral responses due to its highly unstable chemical properties/potency. Instead, we suggest using pure chemicals (made up of one known molecule) such as amino acids, bile acids, or polyamines that are commercially available and easier to make up in known concentrations. Lastly, we strongly suggest using environmentally relevant concentrations of these odorants. We believe these guidelines will help standardize these assays and improve replication of experiments within and between laboratories.

The evolution of olfactory sensitivity, preferences, and behavioral responses in Mexican cavefish is influenced by fish personality

Animals are adapted to their natural habitats and lifestyles. Their brains perceive the external world via their sensory systems, compute information together with that of internal states and autonomous activity, and generate appropriate behavioral outputs. However, how do these processes evolve across evolution? Here, focusing on the sense of olfaction, we have studied the evolution in olfactory sensitivity, preferences, and behavioral responses to six different food-related amino acid odors in the two eco-morphs of the fish Astyanax mexicanus. To this end, we have developed a high-throughput behavioral setup and pipeline of quantitative and qualitative behavior analysis, and we have tested 489 six-week-old Astyanax larvae. The blind, dark-adapted morphs of the species showed markedly distinct basal swimming patterns and behavioral responses to odors, higher olfactory sensitivity, and a strong preference for alanine, as compared to their river-dwelling eyed conspecifics. In addition, we discovered that fish have an individual 'swimming personality', and that this personality influences their capability to respond efficiently to odors and find the source. Importantly, the personality traits that favored significant responses to odors were different in surface fish and cavefish. Moreover, the responses displayed by second-generation cave × surface F2 hybrids suggested that olfactory-driven behavior and olfactory sensitivity is a quantitative genetic trait. Our findings show that olfactory processing has rapidly evolved in cavefish at several levels: detection threshold, odor preference, and foraging behavior strategy. Cavefish is therefore an outstanding model to understand the genetic, molecular, and neurophysiological basis of sensory specialization in response to environmental change.

Bile acids as putative social signals in Mozambique tilapia (Oreochromis mossambicus)

Chemical cues provide potential mates with information about reproductive status and resource-holding potential. In the Mozambique tilapia (Oreochromis mossambicus), males can distinguish female reproductive status through chemical cues, and accessibility of males to females depends on their position in the hierarchy, determined in part by chemical cues. Here, we hypothesized that tilapia faecal cues are attractive to conspecifics once released into the water. C18 solid-phase extracts of faeces from dominant males and pre-ovulatory females evoked stronger olfactory epithelium electrical responses (EOG) than, respectively, subordinate males and post-spawning females. Mass spectrometry of the reverse-phase C18 high-performance liquid chromatography fractions of these extracts with highest EOG, identified by amino acids and bile acids. Faeces from pre-ovulatory females contain significantly higher concentrations of cholic acid (CA) and taurocholic acid (TCH) than both post-spawning females and males. A pool of amino acids had no effect on aggression or attraction in males. However, males were attracted to the scent of pre-ovulatory female faeces, as well as CA and TCH, when applied separately. This attraction was accompanied by increased digging behaviour compared to the odour of post-spawning females. CA and TCH exert their action through separate receptor mechanisms. These findings are consistent with a role for faeces - and bile acids therein - in chemical communication in this species, acting as an attractant for males to reproductive females.

What do oysters smell? Electrophysiological evidence that the bivalve osphradium is a chemosensory organ in the oyster, Magallana gigas

The sensing of chemical cues is essential for several aspects of bivalve biology, such as the detection of food and pheromones. However, little is known about chemical communication systems in bivalves or the possible role of the osphradium as a chemosensory organ. To address this, we adapted an electrophysiological technique extensively used in vertebrates-the electro-olfactogram-to record from the osphradium in the Pacific oyster, Magallana gigas. This technique was validated using amino acids as stimulants. The osphradium proved to be sensitive to most proteinogenic L-amino acids tested, evoking tonic, negative, concentration-dependent 'electro-osphradiogram' (EOsG) voltage responses, with thresholds of detection in the range of 10^- 6 to 10^- 5 M. Conversely, it was insensitive to L-arginine and L-glutamic acid. The current study supports the hypothesis that the osphradium is, indeed, a chemosensory organ. The 'electro-osphradiogram' may prove to be a powerful tool in the isolation and characterization of pheromones and other important chemical cues in bivalve biology.

The attractive effects of amino acids and some classical substances on grass carp (Ctenopharyngodon idellus)

Grass carp (Ctenopharyngodon idellus) is one of the most essential fishing species in China. The bait for this fish is rapidly developing. However, the study on the attractants in the bait for this fish lacks. This study was designed to systematically investigate the effects of 16 kinds of test substances on the perspective of behaviour and physiology of grass carp by using different kinds of methods, including behavioral tests (maze test and biting-balls test) and electro-olfactogram (EOG). Our experiment's idea is mainly to imitate: in addition to vision, fish in nature also use smell to find food and finally swallow under the action of olfaction, taste, and other sensory systems. Firstly, the behavioral maze test was used to screen the attractive or suppressive effect of 16 test substances on grass carp, and the electronic olfactory recording method was used to further evaluate the olfactory response of grass carp to the eight stimuli selected from the maze test. Then, the best concentrations of these eight stimuli and their combination were investigated by the biting-balls test to compound a formula with the strongest appetite for grass carp. The results of behavioral maze test showed that dimethyl-β-propiothetin (DMPT), dimethylthetin (DMT), glycine, taurine, L-glutamic, L-alanine, L-proline, and L-arginine have different degrees of usefulness in attracting grass carp. The electro-olfactogram recoding showed that the EOG response of grass carp to the stimuli is a transient biphasic potential change and all of the eight stimuli could induce the EOG response of grass carp. The biting-balls test showed that glycine, L-glutamic, and L-arginine at 10^-2 mol/L had significant feeding stimulation and DMT at 10^-1 mol/L had significant feeding stimulation than the other groups. Finally, formula 9 composed of DMT, glycine, L-glutamic acid, and L-arginine has the greatest attraction for grass carp. The results of this study verified the attractive effect of some amino acids and other chemicals on grass carp fishing, and would provide support for the production of specific grass carp attractants.

Short- and Medium-Term Exposure to Ocean Acidification Reduces Olfactory Sensitivity in Gilthead Seabream

The effects of ocean acidification on fish are only partially understood. Studies on olfaction are mostly limited to behavioral alterations of coral reef fish; studies on temperate species and/or with economic importance are scarce. The current study evaluated the effects of short- and medium-term exposure to ocean acidification on the olfactory system of gilthead seabream (Sparus aurata), and attempted to explain observed differences in sensitivity by changes in the protonation state of amino acid odorants. Short-term exposure to elevated PCO₂ decreased olfactory sensitivity to some odorants, such as L-serine, L-leucine, L-arginine, L-glutamate, and conspecific intestinal fluid, but not to others, such as L-glutamine and conspecific bile fluid. Seabream were unable to compensate for high PCO₂ levels in the medium term; after 4 weeks exposure to high PCO₂, the olfactory sensitivity remained lower in elevated PCO₂ water. The decrease in olfactory sensitivity in high PCO₂ water could be partly attributed to changes in the protonation state of the odorants and/or their receptor(s); we illustrate how protonation due to reduced pH causes changes in the charge distribution of odorant molecules, an essential component for ligand-receptor interaction. However, there are other mechanisms involved. At a histological level, the olfactory epithelium contained higher densities of mucus cells in fish kept in high CO₂ water, and a shift in pH of the mucus they produced to more neutral. These differences suggest a physiological response of the olfactory epithelium to lower pH and/or high CO₂ levels, but an inability to fully counteract the effects of acidification on olfactory sensitivity. Therefore, the current study provides evidence for a direct, medium term, global effect of ocean acidification on olfactory sensitivity in fish, and possibly other marine organisms, and suggests a partial explanatory mechanism.

High olfactory sensitivity to conspecific intestinal fluid in the chameleon cichlid Australoheros facetus: could faeces signal dominance?

The present study shows that the olfactory potency of intestinal and bile fluids taken from dominant male chameleon cichlids Australoheros facetus is greater than those from subordinate males. Thus, dominant status may be communicated by odorants released in the intestinal fluid and bile acids may contribute towards this.

Lack of evidence for a role of olfaction on first maturation in farmed sea bass Dicentrarchus labrax

Chemical communication is widespread in the animal kingdom and olfaction constitutes a powerful channel for social and environmental cues. In fish, olfactory stimuli are known to influence physiological processes, including reproduction. Here we investigate the effects of olfaction on puberty in European sea bass Dicentrarchus labrax males. Intact sea bass coming to first maturity (puberty) are able to smell conspecific odours. However, induced anosmia during most of the spermatogenesis period had no effect on the sex ratio, gonad maturation state or gonado-somatic index at the time of reproduction. Furthermore anosmia decreased mRNA expression of brain KISS2 and pituitary LHb and FSHb, but not brain GnRH1 and GnRH3. Thus, although anosmia seems to modify gene expression of key reproduction related genetic factors, it seems to be insufficient to stop or delay growth or gonadal development and maturation.

Chemical communication in tilapia: a comparison of Oreochromis mossambicus with O. niloticus

In allopatric speciation species differentiation generally results from different selective pressures in different environments, and identifying the traits responsible helps to understand the isolation mechanism(s) involved. Male Mozambique tilapia (Oreochromis mossambicus) use urine to signal dominance; furthermore, 5β-pregnane-3α,17,20β-triol-3α-glucuronide (and its α-epimer, 5β-pregnane-3α,17,20α-triol-3α-glucuronide), in their urine is a potent pheromone, the concentration of which is correlated with social status. The Nile tilapia (Oreochromisniloticus) is a close relative; species divergence probably resulted from geographical separation around 6 million years ago. This raises the question of whether the two species use similar urinary chemical cues during reproduction. The olfactory potency of urine, and crude extracts, from either species was assessed by the electro-olfactogram and the presence of the steroid glucuronides in urine from the Nile tilapia by liquid-chromatography/mass-spectrometry. Both species showed similar olfactory sensitivity to urine and respective extracts from either species, and similar sensitivity to the steroid glucuronides. 5β-Pregnan-3α,17α,20β-triol-3α-glucuronide was present at high concentrations (approaching 0.5mM) in urine from Nile tilapia, with 5β-pregnan-3α,17α,20α-triol-3α-glucuronide present at lower concentrations, similar to the Mozambique tilapia. Both species also had similar olfactory sensitivity to estradiol-3-glucuronide, a putative urinary cue from females. Together, these results support the idea that reproductive chemical cues have not been subjected to differing selective pressure. Whether these chemical cues have the same physiological and behavioural roles in O. niloticus as O. mossambicus remains to be investigated.

Effects of diet supplementation with white tea and methionine on lipid metabolism of gilthead sea bream juveniles (Sparus aurata)

A growth trial was performed with gilthead sea bream juveniles (Sparus aurata) to evaluate the effect of diet supplementation with white tea and methionine on fish performance and lipid metabolism. For that purpose, four diets were formulated: a fish meal-based diet (Control) and diets identical to the control diet but supplemented with 2.9 % white tea (Tea), 0.3 % methionine (Met) or 2.9 % white tea plus 0.3 % methionine (Tea + Met). Growth performance and feed efficiency parameters, whole-body and liver composition, plasma metabolites concentration and liver glucose 6-phosphate dehydrogenase (G6PDH), malic enzyme (ME) and fatty acid synthetase (FAS) activities were determined. Feed intake was higher in fish fed methionine-supplemented diets, whereas this parameter and growth was decreased in fish fed white tea supplementation. Feed efficiency and protein efficiency ratio were not affected by diet composition. Plasma HDL cholesterol and total lipids concentration were higher in fish fed white tea-supplemented diets. Whole-body lipid, plasma glucose, liver glycogen concentration and liver G6PDH, ME and FAS activities were lower in fish fed white tea-supplemented diets. Results of the present study indicate that methionine seems to act as a feed attractant in diets for sea bream juveniles. Additionally, white tea is an important modulator of lipid metabolism in sea bream juveniles.

Amino acid- vs. peptide-odorants: responses of individual olfactory receptor neurons in an aquatic species

Amino acids are widely used waterborne olfactory stimuli proposed to serve as cues in the search for food. In natural waters the main source of amino acids is the decomposition of proteins. But this process also produces a variety of small peptides as intermediate cleavage products. In the present study we tested whether amino acids actually are the natural and adequate stimuli for the olfactory receptors they bind to. Alternatively, these olfactory receptors could be peptide receptors which also bind amino acids though at lower affinity. Employing calcium imaging in acute slices of the main olfactory epithelium of the fully aquatic larvae of Xenopus laevis we show that amino acids, and not peptides, are more effective waterborne odorants.

Olfactory detectability of L-amino acids in the European honeybee (Apis mellifera)

The honeybee is one of several insect model systems for the study of olfaction, yet our knowledge regarding the spectrum of odorants detectable by Apis mellifera is limited. One class of odorants that has never been tested so far are the amino acids, which are important constituents of floral nectar. Using the proboscis extension response paradigm, we assessed whether the odor of amino acids is detectable for honeybees and determined olfactory detection thresholds for those amino acids that were detectable. We found that honeybees are able to detect the odor of 5 of the 20 proteinogenic amino acids when presented at a concentration of 50 or 100 mM. Median olfactory detection thresholds for these 5 amino acids were 12.5 mM with L-tyrosine and L-cysteine, 50 mM with L-tryptophan and L-asparagine, and 100 mM with L-proline. All detection thresholds were much higher than reported concentrations of amino acids in floral nectars. We conclude that in the foraging and feeding context, honeybees are likely to detect amino acids through taste rather than olfaction. Across-species comparisons of the detectability of and sensitivity to amino acids suggest that the number of functional genes coding for olfactory receptors may affect both a species' sensitivity for odorants and the breadth of its spectrum of detectable odorants.