Formyl Peptide Receptors from Immune and Vomeronasal System Exhibit Distinct Agonist Properties

The formyl peptide receptor (Fpr) family is well known for its contribution to immune defense against pathogens in human and rodent leukocytes. Recently, several structurally related members of these receptors were discovered in sensory neurons of the mouse vomeronasal organ (VNO), key detectors of pheromones and related semiochemicals. Although the biological role of vomeronasal Fprs is not yet clear, the known contribution of other Fprs to host immune defense suggested that they could contribute to vomeronasal pathogen sensing. Precise knowledge about the agonist properties of mouse Fprs is required to determine their function. We expressed all seven mouse and three human Fprs using an in vitro system and tested their activation with 32 selected compounds by conducting high throughput calcium measurements. We found an intriguing functional conservation between human and mouse immune Fprs that is most likely a consequence of closely similar biological constraints. By contrast, our data suggest a neofunctionalization of the vomeronasal Fprs. We show that the vomeronasal receptor mFpr-rs1 can be activated robustly by W-peptide and structural derivatives but not by other typical ligands of immune Fprs. mFpr-rs1 exhibits a stereo-selective preference for peptides containing d-amino acids. The same peptide motifs are contained in pathogenic microorganisms. Thus, the ligand profile of mFpr-rs1 is consistent with a role in vomeronasal pathogen sensing.     

Generalization of predator recognition: Velvet geckos display anti-predator behaviours in response to chemicals from non-dangerous elapid snakes

Many prey species detect chemical cues from predators and modify their behaviours in ways that reduce their risk of predation. Theory predicts that prey should modify their anti-predator responses according to the degree of threat posed by the predator. That is, prey should show the strongest responses to chemicals of highly dangerous prey, but should ignore or respond weakly to chemicals from non-dangerous predators. However, if anti-predator behaviours are not costly, and predators are rarely encountered, prey may exhibit generalised antipredator behaviours to dangerous and non-dangerous predators. In Australia, most elapid snakes eat lizards, and are therefore potentially dangerous to lizard prey. Recently, we found that the nocturnal velvet gecko Oedura lesueurii responds to chemicals from dangerous and non-dangerous elapid snakes, suggesting that it displays gen-eralised anti-predator behaviours to chemicals from elapid snakes. To explore the generality of this result, we videotaped the be-haviour of velvet geckos in the presence of chemical cues from two small elapid snakes that rarely consume geckos: the nocturnal golden-crowned snake Cacophis squamulosus and the diurnal marsh snake Hemiaspis signata. We also videotaped geckos in tri-als involving unsceted cards (controls) and cologne-scented cards (pungency controls). In trials involving Cacophis and Hemi-aspis chemicals, 50% and 63% of geckos spent long time periods (> 3 min) freezing whilst pressed flat against the substrate, re-spectively. Over half the geckos tested exhibited anti-predator behaviours (tail waving, tail vibration, running) in response to Ca-cophis (67%) or Hemiaspis (63%) chemicals. These behaviours were not observed in control or pungency control trials. Our re-sults support the idea that the velvet gecko displays generalised anti-predator responses to chemical cues from elapid snakes. Generalised responses to predator chemicals may be common in prey species that co-occur with multiple, ecologically similar, dangerous predators.     

Evidence for cell proliferation in the sheep brain and its down-regulation by parturition and interactions with the young

Production of new neurons continues throughout life in the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus and is influenced by both endocrine and social factors. In sheep parturition is associated with the establishment of a selective bond with the young based on an olfactory learning. The possibility exists that endocrine changes at parturition together with interactions with the young modulate cell proliferation in the neurogenic zones. In the present study, we first investigated the existence of cell proliferation in sheep. Newly born cells labeled by the cell proliferation marker 5-bromo-2′-deoxyuridine (BrdU) were found in the SVZ, the main olfactory bulb (MOB) and the DG and completely co-localized with Ki-67, another mitotic marker. Forty to 50% of the BrdU-labeled cells contained GFAP suggestive of the presence of neural stem cells. Secondly, parturition with or without interactions with the lamb for 2 days, down-regulated the number of BrdU-labeled cells in the 3 proliferation sites in comparison to no pregnancy. An additional control provided evidence that this effect is specific to early postpartum period: estrus with interactions with males did not affect cell proliferation. Our results provide the first characterization of neural cell proliferation in the SVZ, the DG and unexpectedly in the MOB of adult sheep. We hypothesize that the down-regulation of cell proliferation observed in the early postpartum period could facilitate the olfactory perceptual and memory demands associated with maternal behavior by favouring the survival and integration of neurons born earlier.     

Mice (Mus musculus) lacking a vomeronasal organ can discriminate MHC-determined odortypes

Major histocompatibility complex (MHC) genes in mammals (H-2 in mice) play a major role in regulating immune function. They also bestow individuality in the form of a chemical signature or odortype. At present, the respective contributions of the olfactory epithelium and the vomeronasal organ (VNO) in the recognition of individual odortypes are not well defined. We examined a possible role for the VNO in the recognition of MHC odortypes in mice by first removing the organ (VNX) and then training the mice to distinguish the odors of two congenic strains of mice that differed only in their MHC type. C57BL/6J mice (bb at H-2) and C57BL/6J-H-2(k) (kk at H-2) provided urine for sensory testing. Eight VNX and six sham-operated mice were trained to make the discrimination. Neither the number of training trials-to-criterion nor the rate of learning differed significantly for VNX and sham-operated mice. We conclude that the VNO is not necessary for learning to discriminate between MHC odortypes.     

Patterns of neural activation associated with exposure to odors from a familiar winner in male golden hamsters

The neural mechanisms underlying recognition of familiar individuals and responses appropriate to them are not well known. Previous studies with male golden hamsters have shown that, after a series of brief aggressive encounters, a loser selectively avoids his own, familiar winner but does not avoid other males. Using this paradigm, we investigated activity in 20 areas of the brain using immunohistochemistry for c-Fos and Egr-1 during exposure to a familiar winner compared to control groups not exposed to another male. Behavioral data showed that 1 day after fights males that lost avoided the familiar winner, suggesting that they recognized this individual. The c-Fos and Egr-1 immunohistochemistry showed that the losers exposed to familiar winners had a greater density of stained cells in the basolateral amygdala, the CA1 region of anterior dorsal hippocampus and the dorsal subiculum than control groups had in these areas. These results suggest that these brain areas may be involved in the memory for other males, the learned fear of familiar winners, or related processes.     

A candidate olfactory receptor subtype highly conserved across different insect orders

Candidate olfactory receptors of the moth Heliothis virescens were found to be extremely diverse from receptors of the fruitfly Drosophila melanogaster and the mosquito Anopheles gambiae, but there is one exception. The moth receptor type HR2 shares a rather high degree of sequence identity with one olfactory receptor type both from Drosophila (Dor83b) and from Anopheles (AgamGPRor7); moreover, in contrast to all other receptors, this unique receptor type is expressed in numerous antennal neurons. Here we describe the identification of HR2 homologues in two further lepidopteran species, the moths Antheraea pernyi and Bombyx mori, which share 86-88% of their amino acids. In addition, based on RT-PCR experiments HR2 homologues were discovered in antennal cDNA of the honey bee (Apis mellifera; Hymenoptera), the blowfly (Calliphora erythrocephala; Diptera) and the mealworm (Tenebrio molitor; Coleoptera). Comparison of all HR2-related receptors revealed a high degree of sequence conservation across insect orders. In situ hybridization of antennal sections from the bee and the blowfly support the notion that HR2-related receptors are generally expressed in a very large number of antennal cells. This, together with the high degree of conservation suggests that this unique receptor subtype may fulfill a special function in chemosensory neurons of insects.     

Identification of carbonic anhydrase activity in bullfrog olfactory receptor neurons: histochemical localization and role in CO2 chemoreception

The objectives of this study were to determine: (1) the frequency and distribution of carbonic anhydrase (CA) activity in the bullfrog nasal cavities, and (2)␣whether inhibition of nasal CA affects the olfactory receptor response to CO₂ or other odorants. It was found, using Hansson's staining technique, that some olfactory receptor neurons exhibited CA activity and that these CA-positive receptors were distributed throughout the nasal cavity with peak densities in the dorsal and ventral sensory epithelial regions. To test for the role of CA in olfactory transduction, electro-olfactograms (EOGs) were recorded from the surface of the ventral sensory epithelium in response to 2-s pulses of 5% CO₂ and amyl acetate before and after topical CA inhibition with acetazolamide (10⁻³mol · l⁻¹). In 52 bullfrogs, 1222 sites on the ventral epithelium were tested resulting in 23 locations that exhibited a response to 5% CO₂. Inhibition of CA caused an immediate 65% reduction in the EOG response to CO₂ while the response to amyl acetate was not affected. These results, along with the histochemical localization of CA in some olfactory receptor neurons, indicate that CA plays a role in the detection of CO₂ in frog olfactory neurons and that only a small population of olfactory receptor neurons are CO₂ sensitive. Accepted: 31 July 1997     

Temporal coding of pheromone pulses and trains in Manduca sexta

Summary We investigated the ability of pheromone-sensitive olfactory receptors of male Manduca sexta to respond to 20-ms pulses of bombykal, the major component of the conspecific pheromonal blend. Isolated pulses of bombykal elicited a burst of activity which decreased exponentially with a time constant of 160–250 ms. Trains of pulses delivered at increasing frequencies (0.5–10 Hz) elicited temporally modulated responses at up to 3 Hz. Concentration of the stimulus (1, 10, 100 ng per odor source) had a marginal effect on the temporal resolution of the receptors. Within a train, the responses to individual pulses remained constant, except for 10-Hz trains (short-term adaptation). A dose-dependent decline of responsiveness was observed during experiments (long-term adaptation). Although individual neurons may not respond faithfully to each pulse of a train, the population of receptors sampled in this study appears to be capable of encoding the onset of odor pulses at frequencies of up to at least 3 Hz.Abbreviations BAL bombykal or (E,Z)-10,12-hexadecadienal - C15 (E,Z)-11,13-pentadecadienal - HAL (E)-2-hexenal - EAG electroantennogram - P1, P2, P3 single stimulus pulses - PSTH peri-stimulus histogram - SC synchronization coefficient - 0.5, 1, 2, 3, 10 Hz stimulus trains     

The putative pheromone androstadienone activates cortical fields in the human brain related to social cognition

Using 15O-butanol positron emission tomography (PET), we measured regional cerebral blood flow changes in five healthy young women during exposure to androstadienone, a putative human pheromone, as well as pleasant (gamma-methyl-ionone), unpleasant (methyl-thio-butanoate), and neutral (dipropylene glycol; vehicle compound) odours. Compared with the odorous substances, androstadienone activated a widely distributed neuronal network. Two large cortical fields exhibited consistent activation in each contrast: the anterior part of the inferior lateral prefrontal cortex (PFC) and the posterior part of the superior temporal cortex (STP). Intriguingly, these areas were deactivated by gamma-methyl-ionone and methyl-thio-butanoate. These brain regions can be identified as cortical fields underlying other than olfactory functions, including various aspects of social cognition and attention.     

A new aldehyde oxidase selectively expressed in chemosensory organs of insects

Signal termination is a crucial step in the dynamic of the olfactory process. It involves different classes of odorant-degrading enzymes. Whereas aldehyde oxidase enzymatic activities have been demonstrated in insect antennae by previous biochemical studies, the corresponding enzymes have never been characterized at the molecular level. In the cabbage armyworm Mamestra brassicae, we isolated for the first time an aldehyde oxidase partial cDNA specifically expressed in chemosensory organs, with the strongest expression in antennae of both sexes. In these organs, expression was restricted to the olfactory sensilla. Our results suggest that the corresponding enzyme could degrade aldehyde odorant compounds, such as pheromones or plant's volatiles.