Olfactory receptors and odour coding

The first step of olfactory detection involves interactions between odorant molecules and neuronal protein receptors. Odour coding results from the combinatory activation of a set of receptors and rests on their clonal expression and olfactory neurone connexion, which lead to formation of a specific sensory map in the cortex. This system, sufficient to discriminate myriads of odorants with a mere 350 different receptors, allows humans to smell molecules that are not natural (new cooking flavours, synthetic chemicals...). The extreme olfactory genome diversity explains the absence of odour semantics. Olfactory receptors are also involved in cellular chemotaxis.     

Homology of olfactory receptor neuron response characteristics inferred from hybrids between Asian and European corn borer moths (Lepidoptera: Crambidae)

First generation hybrid males from crosses between the Asian corn borer (ACB), Ostrinia furnacalis, and the “univoltine Z-strain” European corn borer (ECB), Ostrinia nubilalis, were examined with respect to behavioral and physiological responses to ACB and ECB pheromones. The hybrid males often flew to the pheromone of ECB Z-strain, but very rarely to the ACB pheromone. We mapped the tuning profiles of each ORN of the F₁ hybrids with respect to the relevant pheromone components and a common behavioral antagonist by employing differential cross-adaptation and varying doses of the ligands. In the trichoid sensilla of F₁ hybrid males, the three co-compartmentalized ORNs produced spikes that were very difficult to distinguish by size, unlike the parental populations. Comparing the responses to ACB and ECB components at different doses reveals overlapping profiles similar to males of both parental types, but more responsiveness to the ECB pheromone components. We were unable to detect any differences in the ORN tuning profiles when comparing males with different behavioral phenotypes. While the two ECB pheromone races have similar ORN tuning properties that are different from those in ACB, the spike-amplitude patterns of ECB E-strain and ACB have greater homology when compared to ECB Z-strain.     

Four hours of daily contact with sexually active males is sufficient to induce fertile ovulation in anestrous goats

The study was conducted on two consecutive years to determine whether ovulatory activity can be induced in anovulatory goats by exposing them to sexually active bucks for 4, 8, 12 or 16 h per day during 15 consecutive days. In experiment 1, females remained continuously in the experimental pens where they were in contact with males. One group remained isolated from males (controls) and four other groups were exposed to sexually active males for 4, 8, 12 or 16 h per day. In experiment 2, females were taken away to “resting” pens free of male odours between the periods of contact with bucks. They were allocated to 5 groups as in experiment 1. Ovulations were determined by progesterone plasma levels and transrectal ultrasonography. Pregnancy was determined by abdominal ultrasonography. In both experiments, more than 90% of females exposed to the bucks had at least one ovulation during the whole experiment whereas only 11 or 0% (experiments 1 and 2, respectively) did so in the control group (P < 0.001). Furthermore, the proportion of females ovulating did not differ among groups depending on duration of contact with bucks (P > 0.05). In both experiments, pregnancy rates were not affected by the daily duration of contact with males (P > 0.05). To conclude, 4 h of daily contact with sexually active males is sufficient to stimulate ovulatory activity in anovulatory goats and this effect is not due to the presence of olfactory cues from the males remaining in the pens.     

Rosette-type tegumental glands associated with aesthetasc sensilla in the olfactory organ of the Caribbean spiny lobster, Panulirus argus

The lateral antennular flagellum of decapod crustaceans bears unique olfactory sensilla, namely the aesthetascs, and other sensilla types. In this study, we identify a new major tissue in the lateral flagellum of the Caribbean spiny lobster, Panulirus argus, namely “aesthetasc tegumental glands” (ATGs), based on immunostaining with antibodies against CUB serine protease (Csp), in situ hybridization with csp-specific probes, labeling with the F-actin marker phalloidin, labeling with the nuclear marker Hoechst 33258, and staining with methylene blue. Each ATG has 12–20 secretory cells arranged in a rosette. Each secretory cell has a Csp-immunoreactive basal portion and an apical portion containing granular material (metachromatic staining indicative of acid mucopolysaccharides). At the center of each secretory rosette is a phalloidin-positive common locus that gives rise to a main drainage duct projecting toward the cuticle. Scanning electron and light microscopy show that thin ducts traverse the cuticle and connect to “peg pores” proximal to the bases of the aesthetascs, with 3.4 peg pores per aesthetasc. Since the number of common loci is correlated with the number of peg pores, we conclude that each pore represents the outlet of one ATG, and that the secretions are released from them. We conclude further that ATGs and aesthetascs are functionally linked. We hypothesize that ATG secretions have antifouling and/or friction-reducing properties, and that they are spread over the surface of the aesthetascs by antennular grooming. A review of the literature suggests that ATGs are common in decapod crustacean antennules, and that rosette glands and grooming might be functionally coupled in other body areas.This study was supported by NSF IBN 0077474 and NIH DC00312.     

Complex memories in honeybees: can there be more than two?

Foraging honeybees are likely to learn visual and chemical cues associated with many different food sources. Here, we explore how many such sources can be memorized and recalled. Marked bees were trained to visit two (or three) sugar feeders, each placed at a different outdoor location and carrying a different scent. We then tested the ability of the bees to recall these locations and fly to them, when the training scents were blown into the hive, and the scents and food at the feeders were removed. When trained on two feeder locations, each associated with a different scent, the bees could correctly recall the location associated with each scent. However, this ability broke down when the number of scents and feeder locations was increased to three. Performance was partially restored when each of the three training feeders was endowed with an additional cue, namely, a distinct colour. Our results suggest that bees can recall a maximum of two locations when each is associated with a different scent. However, this number can be increased if the scent cues are augmented by visual cues. These findings have implications for the ways in which associations are established and laid down in honeybee memory.     

Effects of androgens on behavioral and vomeronasal responses to chemosensory cues in male terrestrial salamanders (Plethodon shermani)

Chemosensory stimuli and sex steroid hormones are both required for the full expression of social behaviors in many species. The terrestrial salamander, Plethodon shermani, is an emerging nonmammalian system for investigating the nature and evolution of pheromonal communication, yet little is known regarding the role of sex steroid hormones. We hypothesized that increased circulating androgen levels in male P. shermani enhance chemoreception through morphological, behavioral, and physiological mechanisms. Experimental elevation of plasma androgens increased development of cirri, morphological structures thought to enhance the transfer of chemosensory cues from the substrate to the vomeronasal organ (VNO). Elevated plasma androgens also increased expression of a chemo-investigatory behavior (nose tapping) and increased preference for some female-derived chemosensory cues. Male-produced courtship pheromones activated a large number of cells in the VNO as measured by the method of agmatine uptake. However, androgen levels did not affect the total number of vomeronasal cells activated by male-produced courtship pheromones. Future studies will determine whether androgens potentially modulate responsiveness of the VNO to female-derived (as opposed to male-derived) chemosensory cues.     

Retronasal perception of odors

Odors often produce different sensations when presented in front of the nose or intraorally, when eaten. It is a long-standing question whether these differences in sensations are due, for example, to the additional mechanical sensations elicited by the food in the mouth or additional odor release during mastication. To study this phenomenon in detail, a stimulation technique has been developed that allows controlled ortho- or retronasal presentation of odorous stimuli. Results from psychophysical, electrophysiological, and imaging studies suggest that there are clear differences in the perception of ortho- and retronasal stimuli. This 'duality of the sense of smell' is also observed in a clinical context where some patients exhibit good retronasal olfactory function with little or no orthonasal function left, and vice versa. The differences between ortho- and retronasal perception of odors are thought to be, at least partly, due to absorption of odors to the olfactory epithelium, which appears to differ in relation to the direction of the airflow across the olfactory epithelium.     

1H, 15N, and 13C Chemical shift assignments of the navel orange worm pheromone-binding protein-1 (Atra-PBP1)

A pheromone-binding protein from navel orange worm, Amyelois transitella (Atra-PBP1) binds to non-polar pheromone molecules and facilitates the transport and delivery of pheromone to the membrane-bound pheromone receptors. We report complete NMR chemical shift assignments of Atra-PBP1 obtained at pH 4.5 and 25°C (BMRB No. 15601).     

Molecular mechanisms of olfactory reception IV. Some biochemical characteristics of the camphor receptor from rat olfactory epithelium

Some parameters of the receptor element from the rat olfactory epithelium are evaluated; it is characterized by a high affinity for camphor (K_D = 1.5 · 10^?9 M). Triton X-100 has no marked effect on the binding of [³H]camphor. Neither RNAase nor phospholipase C affected [³H]camphor-binding activity. Pronase and trypsin abolished [³H]camphor binding activity by 65 and 40%, respectively. Sulfhydryl reagents decrease the binding of [³H]camphor by a factor of 5–8. The isoelectric point of the receptor solubilized with Triton X-100 is 4.8, as determined by isoelectric focusing. The molecular weight of the receptor as determined by gel electrophoresis is about 120 000. It is proposed that the camphor receptor is a membrane protein containing sulfhydryl groups and playing a key role in olfactory reception.     

Bacterial olfaction

Sensing their environment is a crucial ability of all life forms. In higher eukaryotes the sensing of airborne volatile compounds, or olfaction, is well developed. In plants, slime moulds and yeast there is also compelling evidence that these organisms can smell their environment and respond accordingly. Here we show that bacteria are also capable of olfaction. Bacillus licheniformis was able to sense airborne volatile metabolites produced by neighbouring bacterial cultures and cells could respond to this chemical information in a coordinated way. When Bacillus licheniformis was grown in a microtitre plate adjacent to a bacterial culture of the same or a different species, growing in complex medium, biofilm formation and pigment production were elicited by volatile molecules. A weaker response occurred in increasingly distant wells. The emitted volatile molecule was identified as ammonia. These data demonstrate that B. licheniformis has evolved the ability collect information about its environment from the surrounding air and physiologically respond to it in a manner similar to olfaction. This is the first time that a behavioural response triggered by odorant molecules received through the gas phase is described in bacteria.