Electrical activity of the olfactory bulb of the pigeon

The avian olfactory system and hippocampus: Complementary roles in the olfactory and visual guidance of homing pigeon navigation

The homing pigeon is the foundational model species used to investigate the neural control of avian navigation. The olfactory system is critically involved in implementing the so-called olfactory map, used to locate position relative to home from unfamiliar locations. The hippocampal formation supports a complementary navigational system based on familiar visual landmarks. Insight into the neural control of pigeon navigation has been revolutionised by GPS-tracking technology, which has been crucial for both detailing the critical role of environmental odours for navigation over unfamiliar areas as well as offering unprecedented insight into the role of the hippocampal formation in visual landscape/landmark-based navigation, including a possible, unexpected role in visual-spatial perception.

The perfume of reproduction in birds: chemosignaling in avian social life

This article is part of a Special Issue "Chemosignals and Reproduction". Chemical cues were probably the first cues ever used to communicate and are still ubiquitous among living organisms. Birds have long been considered an exception: it was believed that birds were anosmic and relied on their acute visual and acoustic capabilities. Birds are however excellent smellers and use odors in various contexts including food searching, orientation, and also breeding. Successful reproduction in most vertebrates involves the exchange of complex social signals between partners. The first evidence for a role of olfaction in reproductive contexts in birds only dates back to the seventies, when ducks were shown to require a functional sense of smell to express normal sexual behaviors. Nowadays, even if the interest for olfaction in birds has largely increased, the role that bodily odors play in reproduction still remains largely understudied. The few available studies suggest that olfaction is involved in many reproductive stages. Odors have been shown to influence the choice and synchronization of partners, the choice of nest-building material or the care for the eggs and offspring. How this chemical information is translated at the physiological level mostly remains to be described, although available evidence suggests that, as in mammals, key reproductive brain areas like the medial preoptic nucleus are activated by relevant olfactory signals. Olfaction in birds receives increasing attention and novel findings are continuously published, but many exciting discoveries are still ahead of us, and could make birds one of the animal classes with the largest panel of developed senses ever described.

Telencephalic organization of the olfactory system in homing pigeons (Columba livia)

Pigeons use olfactory cues to navigate over unfamiliar areas, and any impairment of the olfactory system generates remarkable reduction of homing performance. Lesion and deprivation studies suggest a critical involvement of the right nostril and thus, the right olfactory bulb (OB) and the left piriform cortex (CPi) for initial orientation. This functional pattern suggests that OB and CPi are asymmetrically connected with a stronger projection from the right OB to the left CPi. However, the structural organization of the olfactory system is not unequivocally clarified yet. Thus, we re-analyzed the system by antero- and retrograde tract tracing with biotinylated dextran amine and choleratoxin subunit B, and we especially evaluated quantitative differences in the number of cells in the OB innervating the left and right CPi. Our anterograde tracing data verified a strong bilateral input to the CPi, and the prepiriform cortex (CPP), as well as small projections to the ipsilateral medial septum and the dorsolateral corticoid area and the nucleus taeniae of the amygdala in both hemispheres. Apart from the bilateral bulbar afferents, CPi in turn receives unequivocal input from the ipsilateral CPP, hyperpallium densocellulare, dorsal arcopallium, and from a cluster of cells located within the frontolateral nidopallium. Thus, an indirect connection between OB and CPi is only mediated by the CPP. For quantitative analysis of bulbar input to the CPi, we counted the number of ipsi- and contralaterally projecting neurons located in the OB after injections into the left or right CPi. Retrogradely labeled cells were found bilaterally in the OB with a higher number of ipsilaterally located cells. The bilaterality index did not differ after left- or right-sided CPi injections indicating that the functional lateralization of the olfactory system is not simply based on differences in the number of projecting axons of the major processing stream.

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.

Avian chemoreception: an electrophysiological approach

The first detailed physiological evidence for olfactory and trigeminal chemoreception in an avian species is provided by a series of investigations in the chicken (Gallus domesticus). Initial work indicated that the activity of avian olfactory bulb neurones closely resembles that of other vertebrates, exhibiting variable spontaneous temporal firing patterns with mean firing rates between those reported for mammals and reptiles. Application of odors directly to the olfactory epithelium showed that like mammals, avian olfactory bulb neurones respond in the form of inhibition and excitation with accompanying changes in temporal firing pattern. When exposed to a range of concentrations of a single odor, all responsive neurones exhibited an ability to discriminate small step-changes in concentration producing clear stimulus response relationships. Avian trigeminal chemoreception was also investigated by examining the responses of single mucosal receptors in the nasal cavity and palate. Slowly and rapidly adapting nasal mechanoreceptors were identified, some of which exhibited chemical sensitivity when exposed to ammonia gas, acetic acid vapor or carbon dioxide. These results demonstrate that polymodal nociceptors are present in avian nasal mucosa and represent the first attempt in any species to quantify the responses of single trigeminal receptors to a range of concentrations of noxious airborne chemicals. Collectively, the findings demonstrate how an electrophysiological approach can improve our understanding of the underlying sensory physiology relating to avian perception of the chemical environment.

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.

Stimulus-response functions of single avian olfactory bulb neurones

This study investigated olfactory processing in a functional context by examining the responses of single avian olfactory bulb neurones to two biologically important gases over relevant concentration ranges. Recordings of extracellular spike activity were made from 80 single units in the left olfactory bulb of 11 anaesthetised, freely breathing adult hens (Gallus domesticus). The units were spontaneously active, exhibiting widely variable firing rates (0.07-47.28 spikes/s) and variable temporal firing patterns. Single units were tested for their response to an ascending concentration series of either ammonia (2.5-100 ppm) or hydrogen sulphide (1-50 ppm), delivered directly to the olfactory epithelium. Stimulation with a calibrated gas delivery system resulted in modification of spontaneous activity causing either inhibition (47% of units) or excitation (53%) of firing. For ammonia, 20 of the 35 units tested exhibited a response, while for hydrogen sulphide, 25 of the 45 units tested were responsive. Approximate response thresholds for ammonia (median threshold 3.75 ppm (range 2.5-60 ppm, n=20)) and hydrogen sulphide (median threshold 1 ppm (range 1-10 ppm, n=25)) were determined with most units exhibiting thresholds near the lower end of these ranges. Stimulus response curves were constructed for 23 units; 16 (the most complete) were subjected to a linear regression analysis to determine whether they were best fitted by a linear, log or power function. No single function provided the best fit for all the curves (seven were linear, eight were log, one was power). These findings show that avian units respond to changes in stimulus concentration in a manner generally consistent with reported responses in mammalian olfactory bulb neurones. However, this study illustrates a level of fine-tuning to small step changes in concentration (<5 ppm) not previously demonstrated in vertebrate single olfactory bulb neurones.

Chemosensory input and lateralization of brain function in the domestic chick

One-day old domestic chicks (Gallus gallus domesticus) show concentration-dependent behavioural responses to olfactory cues. In the present study we investigated the lateralized olfactory responses of 1-day-old chicks to the odours of eugenol and iso-amyl acetate. In experiment 1 different concentrations of each odour were presented in repeated trials to chicks housed individually. The odours were presented together with a small coloured bead at which the chick pecked. When tested with the highest concentration of eugenol (100% v/v), the chicks demonstrated more head shaking when their left nostril was occluded (RN; right nostril in use) than when their right nostril was occluded (LN; left nostril in use). No such lateralization occurred in response to iso-amyl acetate. This result was confirmed in a second experiment in which the chicks were tested with unscented stimuli, 100% eugenol and 100% isoamyl acetate. In experiment 3 we found that occluding both the chicks' nostrils abolished the head shaking response to eugenol and to iso-amyl acetate. Thus, the chicks' head shaking responses to the odorants eugenol and iso-amyl acetate are mediated primarily by inputs from within the nasal cavity, and not by oral or occular inputs. The present results are consistent with the hypothesis that there is lateralization to olfactory cues and that it is dependent on the involvement of receptors inside the nasal cavity. We suggest that differences in lateralized olfactory responses to different odours are affected by the relative involvement of intranasal olfactory and trigeminal chemoreceptors.

Spontaneous and odour evoked activity in single avian olfactory bulb neurones

Extracellular single unit recordings were made from various depths in the left olfactory bulb of ten anaesthetised, freely breathing adult hens (Gallus domesticus) using glass insulated tungsten microelectrodes. The 66 spontaneously active neurons recorded had widely variable firing rates (mean 4.9 spikes/s, range 0.1-32.4 spikes/s) and variable temporal firing patterns (regular, bursting and random discharge). Interspike interval histograms were constructed for each unit and tested for goodness of fit to theoretical distributions. The activity of 23 units fitted gamma distributions, six units fitted lognormal distributions, three units fitted Weibull distributions, one unit fitted an exponential distribution and 11 units had bimodal distributions. Responses of some units to odour stimuli (Clove oil, Geraniol, Limonene and Ammonia all at 10% vapour saturation) delivered directly to the olfactory epithelium were examined. Odour stimulation modified spontaneous activity in 29 of the 44 units tested, causing either inhibition (56%) or excitation (44%) of firing. Of the 35 units stimulated with more than one odour, 15 responded to multiple stimuli, showing excitation or inhibition only or both inhibition and excitation, depending on the odour applied. Ammonia most commonly elicited a response (70% of applications) and all the odours were capable of inducing both inhibition and excitation of spontaneous firing. Examination of transverse sections of adult hen olfactory bulb in the region corresponding to the recording sites allowed unit recording depth to be related to cellular layers. There was some evidence of different neurone properties in each bulb layer. The spontaneous activity and odour responses observed resemble those seen in other vertebrate groups and the firing rates observed are between those previously reported for mammals and reptiles. This study provides detailed physiological evidence for a well-developed functional olfactory system in an avian species.

Responses to odorants by the domestic chick

We have developed a method to measure responses to graded concentrations of volatile chemicals by domestic chicks (Gallus gallus domesticus). Concentration-response curves were obtained, from which 50% effective concentration (EC50) values were determined. One-day-old chicks were presented with a 4-mm diameter colored bead, at which they pecked readily, affixed to a tube containing log dilutions of odorant. The chick was exposed to the odorant when it pecked at the bead. Various methods of presentation were tried. The method preferred minimized the number of chicks required by allowing repeated testing. Habituation to visual cues was prevented by changing the color of the bead on each presentation. The number of pecks directed at the bead and the amount of head shaking that occurred while the chick was exposed to the stimulus during the 10-s trial were scored from video recordings. Chicks demonstrated increased amounts of head shaking with increasing concentrations of isoamyl acetate or allyl sulfide; low concentrations stimulated pecking and higher concentrations suppressed it. Coincident EC50 values for pecking and head shaking (approximately 1% for isoamyl acetate and allyl sulfide) indicate that they may be controlled by the same mechanism, albeit inversely. There was no relationship between the amount of pecking and the concentration of eugenol, but the EC50 value for head shaking was at the 30% concentration. The results demonstrate that one-day-old chicks show graded responses to graded concentrations of odors and that they demonstrate differential sensitivity to different odorants. Possible involvement of the trigeminal system in these responses is considered.

Olfaction in birds: differential embryonic expression of nine putative odorant receptor genes in the avian olfactory system

We have isolated nine putative odorant receptor genes from the chick, named COR1 to COR9, that belong to the large multigene family of olfactory G protein-coupled receptors found in the fish, rat, mouse, dog, and human. By combining genomic DNA blot analysis, low stringency library screenings, and several PCR analyses, we were able to detect approximately 20 COR genes in the chick genome highly related to COR1-9. By in situ hybridization of newborn and adult, COR expression was detected only in the olfactory epithelium, and exhibited a random spatial distribution. During development, COR expression was observed as early as embryonic stage E5. Different levels of gene expression were observed for the COR1-9 genes: at E5, COR1-6 expression was high compared to the expression of COR7, COR8, and COR9. Surprisingly, at E5, a row of COR1-6 positive cells probably associated with the olfactory nerve extended outside the olfactory placode, reaching the anterior pole of the developing forebrain. These results suggest that, in addition to their role as putative odorant receptors, some COR may play a role in the development of the avian olfactory system.

Autoradiographic study of neurogenesis in the duck olfactory bulb

Neurogenesis was studied in the duck olfactory bulb by injection of tritiated thymidine into the eggs at 53 h and at the 3rd, 5th, 8th, 10th, 12th, 14th and 18th days of incubation. The large neurons appear before the small ones: the mitral cells arise between embryonic day 3 (E3) and E5, the tufted cells between E5 and E8 and the granular cells between E12 and E14. The periglomerular cells could be formed after E18. The order of appearance of this 4 main neurons of the duck olfactory bulb is the same as that in the mouse. All the neurons, except maybe for the periglomerular cells, are principally formed before the hatching of the duckling and the olfactory sense seems to have acquired most of its principal functional aptitudes at this moment.

Quantitative study of synapse formation in the duck olfactory bulb

Synaptic emergence and development in the duck olfactory bulb was quantitatively studied by electron microscopy from the 14th day of incubation (E 14) to the adult stage. Overall synaptic density in this bulb grew considerably during the last weeks of embryonic life and the first postnatal week. The pattern of synaptic density development was similar in the four main architectonic layers of the bulb. However, lower density values were observed in the mitral and inner granule cell layers. In the glomerular layer (GL), axodendritic synapse density was always higher than dendrodentritic synapse density. In the external plexiform layer, most synapses were dendrodendritic and were established between the gemmules of the granule cells (GC) and the dendrites of the mitral cells (MC) or tufted cells (TC). Synapses established by MC and TC on GC gemmules, or by GC on MC and TC dendrites had densities very similar to each other at all the stages studied. Reciprocal synapses already appeared at E 14; their density grew until a week after birth (P7) and thereafter remained stable. In the internal granular layer, the density of asymmetrical synapses was always higher than that of symmetrical synapses. Excitatory synapses formed earlier on MC and TC than inhibitory synapses. The ratio of inhibitory-to-excitatory synapses rose rapidly after birth, reaching 2.5 in the adult duck. The density of excitatory synapses received by granule cells was as high in the external plexiform layer as in the inner granule layer, at all stages of GC development. However, the ratio of received-to-formed synapses fell in these cells from 8.42 at E 14 to 2 after birth. These results are discussed as a function of the evolution of the different synaptic balances during olfactory bulb development. Synaptic development in the duck olfactory bulb at birth is relatively close to the adult state. It appears sufficiently advanced to enable the olfactory system to function in a way compatible with the relatively independent behavior displayed by the duckling.

Frequency analysis of olfactory system EEG in cat, rabbit, and rat

EEG activity in the 35--85 c/sec range has been reported from the rhinencephalon of numerous species of homeotherms, and from numerous parts of the forebrain in carnivores and primates, including man. Unimodal distributions of frequencies within this range for each of 3 species (cat, rat and rabbit) are reported here with proportionality between means and variances. Theoretical analysis has shown that the basis for this oscillatory activity lies in the feedback synaptic interactions of assemblies of excitatory and inhibitory neurons, and that there is a neural basis for frequency convergence and high amplitude near 40 c/sec. Given the synaptic mechanisms, the unimodal spectral distributions, and the widespread occurrence of EEG activity in this range, it clearly represents a unique and identifiable form of brain activity.

Forebrain projections of the pigeon olfactory bulb

The olfactory system of the pigeon (Columba livia) was examined. Our electrophysiological and experimental neuroanatomical (Fink-Heimer technique) data showed that axons from the olfactory bulb terminated in both sides of the forebrain. The cortex prepiriformis (olfactory cortex), the hyperstriatum ventrale and the lobus parolfactorius comprised the uncrossed terminal field. The crossed field included the paleostriatum primitivum and the caudal portion of the lobus parolfactorius, areas which were reached through the anterior commissure. In this report the relationships between areas that receive olfactory information and the possible roles that olfaction plays in the birds' behavior are discussed.

Sensitivity to odours in the embryo of the domestic fowl

Sensitivity to odours in the embryo of the domestic fowl was investigated on the day before hatching. Embryos were tested with four odorants: dichloroethane, cineole, amyl acetate and formic acid. Three odorants (dichloroethane, formic acid and cineole) produced an increase in the heart rate and a rise in the rates of beak-clapping and the first two increased the amount of head-shaking. Odorants had little effect on other types of activity. The response to amyl acetate varied between experiments. Blocking the nostrils with wax abolished the response. Some implications of these results are discussed briefly.