The peripheral olfactory system of the domestic chicken: physiology and development

Amplification of olfactory signals by Anoctamin 9 is important for mammalian olfaction

Sensing smells of foods, prey, or predators determines animal survival. Olfactory sensory neurons in the olfactory epithelium (OE) detect odorants, where cAMP and Ca2+ play a significant role in transducing odorant inputs to electrical activity. Here we show Anoctamin 9, a cation channel activated by cAMP/PKA pathway, is expressed in the OE and amplifies olfactory signals. Ano9-deficient mice had reduced olfactory behavioral sensitivity, electro-olfactogram signals, and neural activity in the olfactory bulb. In line with the difference in olfaction between birds and other vertebrates, chick ANO9 failed to respond to odorants, whereas chick CNGA2, a major transduction channel, showed greater responses to cAMP. Thus, we concluded that the signal amplification by ANO9 is important for mammalian olfactory transduction.

Primary Cultures of Olfactory Neurons from the Avian Olfactory Epithelium

The culture of the olfactory epithelium has been a useful model for the study of neurogenesis since olfactory neurons regenerate continuously throughout the adult lifespan. Structurally and functionally mature olfactory neurons are generated in vitro from non-neuronal precursors in a process that resembles the in vivo counterparts. This chapter describes a technique for culture of olfactory neurons from the avian olfactory epithelium of embryonic chickens; this enables the controlled laboratory study of a critical sensory system that is unstudied in this major vertebrate class. The techniques described here are broadly applicable to other endothermic vertebrate species.

Embryonic exposure of chicken chicks (Gallus gallus domesticus) leads to heightened sensitivities towards the exposed scent

In chickens, food consumption can be altered by exposing the chicks to scents as embryos. We exposed eggs to an orange-scented food additive in the final days of incubation. Following hatching, we tested these exposed chicks’ ability to detect this scent at a variety of concentrations. We found that orange-exposed chicks responded to an orange-scented solution at lower concentrations than control chicks. This sensitization may allow chicks to be more effective at locating acceptable food items but requires further testing to determine its significance. Orange-exposed and control chicks were also tested with the scent of raspberry. Orange-exposed chicks responded to the raspberry presentation significantly more than the control chicks did, suggesting that the embryonic exposure to orange may have influenced how the chicks responded towards another fruity smell. This result suggests that chicks may be learning general characteristics of exposed scents while in the egg, though this needs further research.

Chronic odorant exposure upregulates acquisition of functional properties in cultured embryonic chick olfactory sensory neurons

Neuronal development and differentiation is modulated by activity-dependent mechanisms that stimulate endogenous neurogenesis and differentiation to promote adaptive survival of the organism. Studies on bird odor imprinting have shown how sensory stimuli or environmental influences can affect neonatal behavior, presumably by remodeling the developing nervous system. It is unclear whether these changes originate from the sensory neurons themselves or from the brain. Thus, we attempted to address this by using an in vitro system to separate the peripheral neurons from their central connections. Olfactory neurons from embryonic day 17 Gallus domesticus chicks were isolated, cultured, and exposed to 100 µM amyl acetate or phenethyl alcohol in 12-hr bouts, alternated with periods of no-odor exposure. On days 4 and 5 in vitro, cells were immunostained for olfactory marker protein, neuron-specific tubulin, and olfactory GTP-binding protein, and tested for odorant sensitivity using calcium imaging. While odorant exposure did not result in a significant increase in the overall number of neurons, it promoted neuron differentiation: a larger proportion of odorant-exposed cells expressed olfactory marker protein and the olfactory GTP-binding protein. When cell responsiveness was tested using calcium imaging, a greater proportion of odorant-exposed cells responded to stimulation with 100 µM amyl acetate or phenethyl alcohol. Thus, odorant exposure during development modulated the developmental trajectories of individual neurons, resulting in changes in protein expression associated with odorant signaling. This suggests that the neuronal changes in the periphery have an important contribution to the overall long-term functional changes associated with odor imprinting. © 2016 Wiley Periodicals, Inc.

Yolk hormones influence in ovo chemosensory learning, growth, and feeding behavior in domestic chicks

In this study, we assessed whether prenatal exposure to elevated yolk steroid hormones can influence in ovo chemosensory learning and the behavior of domestic chicks. We simulated a maternal environmental challenge by experimentally enhancing yolk progesterone, testosterone, and estradiol concentrations in hen eggs prior to incubation. The embryos from these hormones-treated eggs (HO) as well as sham embryos (O) that had received the vehicle-only were exposed to the odor of fish oil (menhaden) between embryonic Days 11 and 20. An additional group of control embryos (C) was not exposed to the odor. All chicks were tested following hatching for their feeding preferences between foods that were or were not odorized with the menhaden odor. In the 3-min choice tests, the behavior of O chicks differed significantly according to the type of food whereas C and HO chicks showed no preference between odorized and non-odorized food. Our result suggests weaker response in HO chicks. In addition, HO chicks showed impaired growth and reduced intake of an unfamiliar food on the 24-h time scale compared to controls. Our data suggest that embryonic exposure to increased yolk hormone levels can alter growth, chemosensory learning, and the development of feeding behaviors.

GnRH neuronal migration and olfactory bulb neurite outgrowth are dependent on FGF receptor 1 signaling, specifically via the PI3K p110α isoform in chick embryo

Fibroblast growth factor (FGF) signaling is essential for both olfactory bulb (OB) morphogenesis and the specification, migration, and maturation of the GnRH-secreting neurons. Disruption of FGF signaling contributes to Kallmann syndrome characterized by both anosmia and sexual immaturity. However, several unanswered questions remain as to which specific FGF receptor (FGFR)-1 signaling pathways are necessary for OB and GnRH neuronal development. Here, using pharmacological phosphatidylinositol 3-kinase (PI3K) isoform-specific inhibitors, we demonstrate a central role for the PI3K p110α isoform as a downstream effector of FGFR1 signaling for both GnRH neuronal migration and OB development. We show that signaling via the PI3K p110α isoform is required for GnRH neuronal migration in explant cultures of embryonic day (E) 4 chick olfactory placodes. We also show that in ovo administration of LY294002, a global PI3K inhibitor as well as an inhibitor to the PI3K p110α isoform into the olfactory placode of E3 chick embryo impairs GnRH neuronal migration toward the forebrain. In contrast, in ovo PI3K inhibitor treatment produced no obvious defects on primary olfactory sensory neuron axonal targeting and bundle formation. We also demonstrate that anosmin-1 and FGF2 induced neuronal migration of immortalized human embryonic GnRH neuroblast cells (FNC-B4-hTERT) is mediated by modulating FGFR1 signaling via the PI3K p110α isoform, specifically through phosphorylation of the PI3K downstream effectors, Akt and glycogen synthase kinase-3β. Finally, we show that neurite outgrowth and elongation of OB neurons in E10 chick OB explants are also dependent on the PI3K p110α isoform downstream of FGFR1. This study provides mechanistic insight into the etiology of Kallmann syndrome.

MHC genotype predicts mate choice in the ring-necked pheasant Phasianus colchicus

Females of several vertebrate species selectively mate with males on the basis of the major histocompatibility complex (MHC) genes. As androgen-mediated maternal effects have long-lasting consequences for the adult phenotype, both mating and reproductive success may depend on the combined effect of MHC genotype and exposure to androgens during early ontogeny. We studied how MHC-based mate choice in ring-necked pheasants (Phasianus colchicus) was influenced by an experimental in ovo testosterone (T) increase. There was no conclusive evidence of in ovo T treatment differentially affecting mate choice in relation to MHC genotype. However, females avoided mating with males with a wholly different MHC genotype compared with males sharing at least one MHC allele. Females also tended to avoid mating with MHC-identical males, though not significantly so. These findings suggest that female pheasants preferred males with intermediate MHC dissimilarity. Male MHC heterozygosity or diversity did not predict the expression of ornaments or male dominance rank. Thus, MHC-based mating preferences in the ring-necked pheasant do not seem to be mediated by ornaments' expression and may have evolved mainly to reduce the costs of high heterozygosity at MHC loci for the progeny, such as increased risk of autoimmune diseases or disruption of coadapted gene pools.

The developmental stage of chicken embryos modulates the impact of in ovo olfactory stimulation on food preferences

Like mammals, bird embryos are capable of chemosensory learning, but the ontogeny of their feeding preferences has not been examined. We tested if the timing of stimulation in chicken embryos modulates the impact of in ovo olfactory stimulation on later food preferences. We exposed chicken embryos to an olfactory stimulus for a 4-day period in the middle or toward the end of the incubation period. The chicks were tested for their preference between foods with and without the olfactory stimulus in 3-min choice tests and on a 24-h time scale. Regardless of the type of food (familiar or novel) or the duration of the test, the control chicks not exposed to the olfactory stimulus consistently showed significant preferences for non-odorized foods. Chicks that were exposed in ovo to the olfactory stimulus did not show a preference for odorized or non-odorized foods. Only those chicks that were exposed to the olfactory stimulus toward the end of the incubation period differed from the controls and incorporated a higher proportion of odorized food into their diets on a 24-h time scale. This result indicates that olfactory stimulation at the end of embryonic development has a stronger impact on later feeding preferences. Our findings contribute to the growing pool of recent data appreciating the impact of olfactory signals on behavior regulation in avian species.

Phylogenic outline of the olfactory system in vertebrates

Phylogenic outline of the vertebrate olfactory system is summarized in the present review. In the fish and the birds, the olfactory system consists only of the olfactory epithelium (OE) and the olfactory bulb (B). In the amphibians, reptiles and mammals, the olfactory system is subdivided into the main olfactory and the vomeronasal olfactory systems, and the former consists of the OE and the main olfactory bulb (MOB), while the latter the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB). The subdivision of the olfactory system into the main and the vomeronasal olfactory systems may partly be induced by the difference between paraphyletic groups and monophyletic groups in the phylogeny of vertebrates.

Pheromones in birds: myth or reality?

Birds are anosmic or at best microsmatic… This misbelief persisted until very recently and has strongly influenced the outcome of communication studies in birds, with olfaction remaining neglected as compared to acoustic and visual channels. However, there is now clear empirical evidence showing that olfaction is perfectly functional in birds and birds use olfactory information in a variety of ethological contexts. Although the existence of pheromones has never been formally demonstrated in this vertebrate class, different groups of birds, such as petrels, auklets and ducks have been shown to produce specific scents that could play a significant role in within-species social interactions. Behavioral experiments have indeed demonstrated that these odors influence the behavior of conspecifics. Additionally, in quail, deprivation of olfactory inputs decreases neuronal activation induced by sexual interactions with a female. It seems therefore well established that birds enjoy a functional sense of smell and a fast growing body of experimental evidence suggests that they use this channel of olfactory communication to control their social life. The unequivocal identification of an avian pheromone is, however, still ahead of us but there are now many exciting opportunities to unravel the behavioral and physiological particularities of chemical communication in birds.

The underestimated role of olfaction in avian reproduction?

Until the second half of the 20th century, it was broadly accepted that most birds are microsmatic if not anosmic and unable to detect and use olfactory information. Exceptions were eventually conceded for species like procellariiforms, vultures or kiwis that detect their food at least in part based on olfactory signals. During the past 20-30 years, many publications have appeared indicating that this view is definitely erroneous. We briefly review here anatomical, electrophysiological and behavioral data demonstrating that birds in general possess a functional olfactory system and are able to use olfactory information in a variety of ethological contexts, including reproduction. Recent work also indicates that brain activation induced by sexual interactions with a female is significantly affected by olfactory deprivation in Japanese quail. Brain activation was measured via immunocytochemical detection of the protein product of the immediate early gene c-fos. Changes observed concerned two brain areas that play a key role in the control of male sexual behavior, the medial preoptic nucleus and the bed nucleus of the stria terminalis therefore suggesting a potential role of olfaction in the control of reproduction. The widespread idea that birds are anosmic or microsmatic is thus not supported by the available experimental data and presumably originates in our anthropomorphic view that leads us to think that birds do not smell because they have a rigid beak and nostrils and do not obviously sniff. Experimental analysis of this phenomenon is thus warranted and should lead to a significant change in our understanding of avian biology.