Orexin A effects on the olfactory bulb spontaneous activity and odor responsiveness in freely breathing rats

Function of orexin-1 receptor signaling in the olfactory tubercle in odor-guided attraction and aversion

While olfactory behaviors are influenced by neuromodulatory signals, the underlying mechanism remains unknown. The olfactory tubercle (OT), a component of the olfactory cortex and ventral striatum, consists of anteromedial (am) and lateral (l) domains regulating odor-guided attractive and aversive behaviors, respectively, in which the amOT highly expresses various receptors for feeding-regulated neuromodulators. Here we show functions of appetite-stimulating orexin-1 receptor (OxR1) signaling in the amOT. When odor-food reward associated mice underwent OxR1 antagonist injection in the amOT, their odor-attractive behavior was suppressed and odor-aversive behavior was conversely induced. Although odor-attractive mice showed activation of attraction-promoting dopamine receptor type 1-expressing D1 cells in the amOT, the antagonist injection increased activation of aversion-promoting D2 cells in the amOT and D1 cells in the lOT. The results highlight the amOT as the crucial structure integrating OxR1 signaling and odor information, thereby controlling metabolic status-dependent olfactory behavior through the cell type- and domain-specific activation.

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Signaling mechanisms underlying activity-dependent integration of adult-born neurons in the mouse olfactory bulb

Adult neurogenesis has fascinated the field of neuroscience for decades given the prospects of harnessing mechanisms that facilitate the rewiring and/or replacement of adult brain tissue. The subgranular zone of the hippocampus and the subventricular zone of the lateral ventricle are the two main areas in the brain that exhibit ongoing neurogenesis. Of these, adult-born neurons within the olfactory bulb have proven to be a powerful model for studying circuit plasticity, providing a broad and accessible avenue into neuron development, migration, and continued circuit integration within adult brain tissue. This review focuses on some of the recognized molecular and signaling mechanisms underlying activity-dependent adult-born neuron development. Notably, olfactory activity and behavioral states contribute to adult-born neuron plasticity through sensory and centrifugal inputs, in which calcium-dependent transcriptional programs, local translation, and neuropeptide signaling play important roles. This review also highlights areas of needed continued investigation to better understand the remarkable phenomenon of adult-born neuron integration.

Metabolic state modulates neural processing of odors in the human olfactory bulb

The olfactory and endocrine systems have recently been shown to reciprocally shape the homeostatic processes of energy intake. As demonstrated in animal models, the individual's metabolic state dynamically modulates how the olfactory bulb process odor stimuli using a range of endocrine signals. Here we aimed to determine whether the neural processing of odors in human olfactory bulb is modulated by metabolic state. Participants were exposed to food-associated odors, in separate sessions being hungry and sated, while neural responses from the olfactory bulb was obtained using electrobulbogram. We found significantly higher gamma power activity (51-100 Hz) in the OB's response to odors during the Hunger compared to Sated condition. Specifically, EBG gamma power were elevated while hungry already at 100 ms after odor onset, thereby suggesting intra-bulbar modulation according to metabolic state. These results demonstrate that, akin to other animal models, hunger state affects OB activity in humans. Moreover, we show that the EBG method has the potential to measure internal metabolic states and, as such, could be used to study specificities in olfactory processing of individuals suffering from pathologies such as obesity or anorexia.

An anatomically distinct subpopulation of orexin neurons project from the lateral hypothalamus to the olfactory bulb

Olfactory cues play a key role in natural behaviors such as finding food, finding mates, and avoiding predators. In principle, the ability of the olfactory system to carry out these perceptual functions would be facilitated by signaling related to an organism's physiological state. One candidate pathway includes a direct projection from the hypothalamus to the main olfactory bulb, the first stage of olfactory sensory processing. The pathway from the hypothalamus to the main olfactory bulb is thought to include neurons that express the neuropeptide orexin, although the proportion that is orexinergic remains unknown. A current model proposes that the orexin population is heterogeneous, yet it remains unknown whether the proportion that innervates the main olfactory bulb reflects a distinct subpopulation of the orexin population. Herein, we carried out combined retrograde tract tracing with immunohistochemistry for orexin-A in the mouse to define the proportion of hypothalamic input to the main olfactory bulb that is orexinergic and to determine what fraction of the orexin-A population innervates the bulb. The numbers and spatial positions of all retrogradely labeled neurons and all the orexin-A-expressing neurons were quantified in sequential sections through the hypothalamus. Retrogradely labeled neurons were found in the ipsilateral hypothalamus, of which 22% expressed orexin-A. The retrogradely labeled neurons that did and did not express orexin-A could be anatomically distinguished based on their spatial position and cell body area. Remarkably, only 7% of all the orexin-A neurons were retrogradely labeled, suggesting that only a small fraction of the orexin-A population directly innervate the main olfactory bulb. These neurons spatially overlapped with the orexin-A neurons that did not innervate the bulb, although the two cell populations were differentiated based on cell body area. Overall, these results support a model in which olfactory sensory processing is influenced by orexinergic feedback at the first synapse in the olfactory processing pathway.

Obesity wars: may the smell be with you

Classically, the regulation of energy balance has been based on central and peripheral mechanisms sensing energy, nutrients, metabolites, and hormonal cues. Several cellular mechanisms at central level, such as hypothalamic AMP-activated protein kinase (AMPK), integrate this information to elicit counterregulatory responses that control feeding, energy expenditure, and glucose homeostasis, among other processes. Recent data have added more complexity to the homeostatic regulation of metabolism by introducing, for example, the key role of "traditional" senses and sensorial information in this complicated network. In this regard, current evidence is showing that olfaction plays a key and bidirectional role in energy homeostasis. Although nutritional status dynamically and profoundly impacts olfactory sensitivity, the sense of smell is involved in food appreciation and selection, as well as in brown adipose tissue (BAT) thermogenesis and substrate utilization, with some newly described actors, such as olfactomedin 2 (OLFM2), likely playing a major role. Thus, olfactory inputs are contributing to the regulation of both sides of the energy balance equation, namely, feeding and energy expenditure (EE), as well as whole body metabolism. Here, we will review the current knowledge and advances about the role of olfaction in the regulation of energy homeostasis.

Sex-specific expression of pheromones and other signals in gravid starfish

BACKGROUND

Many echinoderms form seasonal aggregations prior to spawning. In some fecund species, a spawning event can lead to population outbreaks with detrimental ecosystem impacts. For instance, outbreaks of crown-of-thorns starfish (COTS), a corallivore, can destroy coral reefs. Here, we examine the gene expression in gravid male and female COTS prior to spawning in the wild, to identify genome-encoded factors that may regulate aggregation and spawning. This study is informed by a previously identified exoproteome that attracts conspecifics. To capture the natural gene expression profiles, we isolated RNAs from gravid female and male COTS immediately after they were removed from the Great Barrier Reef.  RESULTS: Sexually dimorphic gene expression is present in all seven somatic tissues and organs that we surveyed and in the gonads. Approximately 40% of the exoproteome transcripts are differentially expressed between sexes. Males uniquely upregulate an additional 68 secreted factors in their testes. A suite of neuropeptides in sensory organs, coelomocytes and gonads is differentially expressed between sexes, including the relaxin-like gonad-stimulating peptide and gonadotropin-releasing hormones. Female sensory tentacles-chemosensory organs at the distal tips of the starfish arms-uniquely upregulate diverse receptors and signalling molecules, including chemosensory G-protein-coupled receptors and several neuropeptides, including kisspeptin, SALMFamide and orexin.

CONCLUSIONS

Analysis of 103 tissue/organ transcriptomes from 13 wild COTS has revealed genes that are consistently differentially expressed between gravid females and males and that all tissues surveyed are sexually dimorphic at the molecular level. This finding is consistent with female and male COTS using sex-specific pheromones to regulate reproductive aggregations and synchronised spawning events. These pheromones appear to be received primarily by the sensory tentacles, which express a range of receptors and signalling molecules in a sex-specific manner. Furthermore, coelomocytes and gonads differentially express signalling and regulatory factors that control gametogenesis and spawning in other echinoderms.

Two-Month administration of Methylphenidate improves olfactory sensitivity and suppresses appetite in individuals with obesity

INTRODUCTION

Olfaction contributes to feeding behaviour and is modulated by changes in dopamine levels. Methylphenidate (MPH) increases brain dopamine levels and has been shown to reduce appetite and promote weight loss in patients with attention deficit hyperactivity disorder. The objectives of this study were to test the effect of MPH on olfaction, appetite, energy intake and body weight on individuals with obesity.

METHODS

In a randomized, double-blind study, 12 participants (age 28.9±6.7 yrs) (BMI 36.1±4.5 kg/m2) were assigned to MPH (0.5mg/kg) (n=5) or Placebo (n=7) twice daily for 2 months. Appetite (Visual Analog Scale), odour threshold (Sniffin' Sticks®), energy intake (food menu), and body weight (DEXA scan) were measured at day 1 and day 60.

RESULTS

MPH intake significantly increased odour threshold scores (6.3±1.4 vs. 9.4±2.1 and 7.9±2.3 vs. 7.8±1.9, respectively; p=0.029) vs. Placebo. There was a significantly greater suppression of appetite sensations (desire to eat (p=0.001), hunger (p=0.008), and prospective food consumption (p=0.003)) and an increase in fullness (p=0.028) over time in the MPH vs. Placebo.

CONCLUSIONS

MPH suppressed appetite and improved olfactory sensitivity in individuals with obesity. These data provide novel findings on the favourable effects of MPH on appetite and weight regulation in individuals living with obesity.

Exploring the Role of Orexinergic Neurons in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease affecting about 2% of the population. A neuropeptide, orexin, is linked with sleep abnormalities in the parkinsonian patient. This study aimed to review the changes in the orexinergic system in parkinsonian subjects and the effects of orexin. A number of search techniques were used and presumed during the search, including cloud databank searches of PubMed and Medline using title words, keywords, and MeSH terms. PD is characterised by motor dysfunctions (postural instability, rigidity, tremor) and cognitive disorders, sleep-wake abnormalities grouped under non-motor disorders. The Orexinergic system found in the hypothalamus is linked with autonomic function, neuroprotection, learning and memory, and the sleep-wake cycle. Prepro-orexin, a precursor peptide (130 amino acids), gives rise to orexins (Orx-A and Orx-B). Serum orexin level measurement is vital for evaluating several neurological disorders (Alzheimer's disease, Huntington's disease, and PD). Orexinergic neurons are activated by hypoglycemia and ghrelin, while they are restrained by food consumption and leptin. Orexinergic system dysfunctioning was found to be linked with non-motor symptoms (sleep abnormalities) in PD. Orexinergic neuron's behaviour may be either inhibitory or excitatory depending on the environment in which they are present. As well, orexin antagonists are found to improve the abnormal sleep pattern. Since the orexinergic system plays a role in several psychological and neurological disorders, therefore, these disorders can be managed by targeting this system.

Sensitivity of olfactory sensory neurons to food cues is tuned to nutritional states by Neuropeptide Y signaling

Modulation of sensory perception by homeostatic feedback from physiological states is central to innate purposive behaviors. Olfaction is an important predictive modality for feeding-related behaviors and its modulation has been associated with hunger-satiety states. However, the mechanisms mapping internal states to chemosensory processing in order to modify behavior are poorly understood. In the zebrafish olfactory epithelium, a subset of olfactory sensory neurons (OSNs) and the terminal nerve projections express neuropeptide Y (NPY). Using a combination of neuronal activity and behavioral evaluation, we find that NPY signaling in the peripheral olfactory system of zebrafish is correlated with its nutritional state and is both necessary and sufficient for the olfactory perception of food-related odorants. NPY activity dynamically modulates the microvillar OSN activation thresholds and acts cooperatively with amino acid signaling resulting in a switch-like increase in OSN sensitivity in starved animals. We suggest that cooperative activation of phospholipase C by convergent signaling from NPY and amino acid receptors is central to this heightened sensitivity. This study provides ethologically relevant, physiological evidence for NPY signaling in the modulation of OSN sensitivity to food-associated amino acid cues. We demonstrate sensory gating directly at the level of OSNs and identify a novel mechanistic framework for tuning olfactory sensitivity to prevailing energy states.

Modulation of Neural Microcircuits That Control Complex Dynamics in Olfactory Networks

Neuromodulation influences neuronal processing, conferring neuronal circuits the flexibility to integrate sensory inputs with behavioral states and the ability to adapt to a continuously changing environment. In this original research report, we broadly discuss the basis of neuromodulation that is known to regulate intrinsic firing activity, synaptic communication, and voltage-dependent channels in the olfactory bulb. Because the olfactory system is positioned to integrate sensory inputs with information regarding the internal chemical and behavioral state of an animal, how olfactory information is modulated provides flexibility in coding and behavioral output. Herein we discuss how neuronal microcircuits control complex dynamics of the olfactory networks by homing in on a special class of local interneurons as an example. While receptors for neuromodulation and metabolic peptides are widely expressed in the olfactory circuitry, centrifugal serotonergic and cholinergic inputs modulate glomerular activity and are involved in odor investigation and odor-dependent learning. Little is known about how metabolic peptides and neuromodulators control specific neuronal subpopulations. There is a microcircuit between mitral cells and interneurons that is comprised of deep-short-axon cells in the granule cell layer. These local interneurons express pre-pro-glucagon (PPG) and regulate mitral cell activity, but it is unknown what initiates this type of regulation. Our study investigates the means by which PPG neurons could be recruited by classical neuromodulators and hormonal peptides. We found that two gut hormones, leptin and cholecystokinin, differentially modulate PPG neurons. Cholecystokinin reduces or increases spike frequency, suggesting a heterogeneous signaling pathway in different PPG neurons, while leptin does not affect PPG neuronal firing. Acetylcholine modulates PPG neurons by increasing the spike frequency and eliciting bursts of action potentials, while serotonin does not affect PPG neuron excitability. The mechanisms behind this diverse modulation are not known, however, these results clearly indicate a complex interplay of metabolic signaling molecules and neuromodulators that may fine-tune neuronal microcircuits.

Zone-specific damage of the olfactory epithelium under protein restriction

Oxidative stress causes tissue damage, affecting age-related pathologies. Protein restriction (PR) provides a powerful intervention strategy for reducing oxidative stress, which may have a positive effect on individual organs. However, it is unknown whether PR intervention influences the olfactory system. Here, we investigated how 10 months of PR could affect the cell dynamics of the olfactory epithelium (OE) in mice. We found that PR reduced age-related loss of outer hair cells in the cochlea, providing preventive effects against age-related hearing loss. In contrast, PR resulted in reduced mature olfactory sensory neurons (OSNs), increased proliferative basal cells, and increased apoptotic OSNs in zone 1 (the only area containing neurons expressing NQO1 [quinone dehydrogenase 1]) of the OE in comparison with animals given a control diet. Substantial oxidative stress occurred in NQO1-positive cells and induced apoptotic OSNs in zone 1. These results indicate that in contrast to the positive effect on the auditory system, PR induces oxidative stress and structurally and functionally negative effects on OSNs in zone 1, which is probably involved in the bioactivation of NQO1.

Mechanism underlying starvation-dependent modulation of olfactory behavior in Drosophila larva

Starvation enhances olfactory sensitivity that encourage animals to search for food. The molecular mechanisms that enable sensory neurons to remain flexible and adapt to a particular internal state remain poorly understood. Here, we study the roles of GABA and insulin signaling in starvation-dependent modulation of olfactory sensory neuron (OSN) function in the Drosophila larva. We show that GABAB-receptor and insulin-receptor play important roles during OSN modulation. Using an OSN-specific gene expression analysis, we explore downstream targets of insulin signaling in OSNs. Our results suggest that insulin and GABA signaling pathways interact within OSNs and modulate OSN function by impacting olfactory information processing. We further show that manipulating these signaling pathways specifically in the OSNs impact larval feeding behavior and its body weight. These results challenge the prevailing model of OSN modulation and highlight opportunities to better understand OSN modulation mechanisms and their relationship to animal physiology.

Expression of feeding-related neuromodulatory signalling molecules in the mouse central olfactory system

Various neural systems cooperate in feeding behaviour, and olfaction plays crucial roles in detecting and evaluating food objects. While odour-mediated feeding behaviour is highly adaptive and influenced by metabolic state, hedonic cues and learning processes, the underlying mechanism is not well understood. Feeding behaviour is regulated by orexigenic and anorexigenic neuromodulatory molecules. However, knowledge of their roles especially in higher olfactory areas is limited. Given the potentiation of feeding behaviour in hunger state, we systemically examined the expression of feeding-related neuromodulatory molecules in food-restricted mice through quantitative PCR, in the olfactory bulb (OB), olfactory tubercle (OT), and remaining olfactory cortical area (OC). The OT was further divided into attraction-related anteromedial, aversion-related lateral and remaining central regions. Examination of 23 molecules including neuropeptides, opioids, cannabinoids, and their receptors as well as signalling molecules showed that they had different expression patterns, with many showing elevated expression in the OT, especially in the anteromedial and central OT. Further, in mice trained with odour-food association, the expression was significantly altered and the increase or decrease of a given molecule varied among areas. These results suggest that different olfactory areas are regulated separately by feeding-related molecules, which contributes to the adaptive regulation of feeding behaviour.

Orexin/hypocretin and dysregulated eating: Promotion of foraging behavior

At its discovery, orexin/hypocretin (OX) was hypothesized to promote food intake. Subsequently, with the identification of the participation of OX in numerous other phenomena, including arousal and drug seeking, this neuropeptide was proposed to be involved in highly motivated behaviors. The present review develops the hypothesis that the primary evolutionary function of OX is to promote foraging behavior, seeking for food under conditions of limited availability. Thus, it will first describe published literature on OX and homeostatic food intake, which shows that OX neurons are activated by conditions of food deprivation and in turn stimulate food intake. Next, it will present literature on excessive and binge-like food intake, which demonstrates that OX stimulates both intake and willingness to work for palatable food. Importantly, studies show that binge-like eating can be inhibited by OX antagonists at doses far lower than those required to suppress homeostatic intake (3 mg/kg vs. 30 mg/kg), suggesting that an OX-based pharmacotherapy, at the right dose, could specifically control dysregulated eating. Finally, the review will discuss the role of OX in foraging behavior, citing literature which shows that OX neurons, which are activated during the anticipation of food reward, can promote a number of phenomena involved in successful foraging, including food-anticipatory locomotor behavior, olfactory sensitivity, visual attention, spatial memory, and mastication. Thus, OX may promote homeostatic eating, as well as binge eating of palatable food, due to its ability to stimulate and coordinate the activities involved in foraging behavior.

Are people who have a better smell sense, more affected from satiation?

Introduction

The olfactory system is affected by the nutritional balance and chemical state of the body, serving as an internal sensor. All bodily functions are affected by energy loss, including olfaction; hunger can alter odour perception.

Objective

In this study, we investigated the effect of fasting on olfactory perception in humans, and also assessed perceptual changes during satiation.

Methods

The “Sniffin’ Sticks” olfactory test was applied after 16 h of fasting, and again at least 1 h after Ramadan supper during periods of satiation. All participants were informed about the study procedure and provided informed consent. The study protocol was approved by the local Ethics Committee of Gaziosmanpaşa Taksim Education and Research Hospital (09/07/2014 no: 60). The study was conducted in accordance with the basic principles of the Declaration of Helsinki.

Results

This prospective study included 48 subjects (20 males, 28 females) with a mean age of 33.6 ± 9.7 (range 20–72) years; their mean height was 169.1 ± 7.6 (range 150.0–185.0) cm, mean weight was 71.2 ± 17.6 (range 50.0–85.0) kg, and average BMI was 24.8 ± 5.3 (range 19.5–55.9). Scores were higher on all items pertaining to olfactory identification, thresholds and discrimination during fasting vs. satiation (p < 0.05). Identification (I) results: Identification scores were significantly higher during the fasting (median = 14.0) vs. satiation period (median = 13.0). Threshold (T) results: Threshold scores were significantly higher during the fasting (median = 7.3) vs. satiation period (median = 6.2). Discrimination (D) results: Discrimination scores were significantly higher during the fasting (median = 14.0) vs. satiation period (median = 13.0). The total TDI scores were 35.2 (fasting) vs. 32.6 (satiation). When we compared fasting threshold value of >9 and ≤9, the gap between the fasting and satiety thresholds was significantly greater in >9 (p < 0.05).

Conclusion

Olfactory function improved during fasting and declined during satiation. The olfactory system is more sensitive, and more reactive to odours, under starvation conditions, and is characterised by reduced activity during satiation. This situation was more pronounced in patients with a better sense of smell. Olfaction-related neurotransmitters should be the target of further study.

Nutrient Sensing: Another Chemosensitivity of the Olfactory System

Olfaction is a major sensory modality involved in real time perception of the chemical composition of the external environment. Olfaction favors anticipation and rapid adaptation of behavioral responses necessary for animal survival. Furthermore, recent studies have demonstrated that there is a direct action of metabolic peptides on the olfactory network. Orexigenic peptides such as ghrelin and orexin increase olfactory sensitivity, which in turn, is decreased by anorexigenic hormones such as insulin and leptin. In addition to peptides, nutrients can play a key role on neuronal activity. Very little is known about nutrient sensing in olfactory areas. Nutrients, such as carbohydrates, amino acids, and lipids, could play a key role in modulating olfactory sensitivity to adjust feeding behavior according to metabolic need. Here we summarize recent findings on nutrient-sensing neurons in olfactory areas and delineate the limits of our knowledge on this topic. The present review opens new lines of investigations on the relationship between olfaction and food intake, which could contribute to determining the etiology of metabolic disorders.

The incretin hormone glucagon-like peptide 1 increases mitral cell excitability by decreasing conductance of a voltage-dependent potassium channel

Key points

The gut hormone called glucagon‐like peptide 1 (GLP‐1) is a strong moderator of energy homeostasis and communication between the peripheral organs and the brain.

GLP‐1 signalling occurs in the brain; using a newly developed genetic reporter line of mice, we have discovered GLP‐synthesizing cells in the olfactory bulb.

GLP‐1 increases the firing frequency of neurons (mitral cells) that encode olfactory information by decreasing activity of voltage‐dependent K channels (Kv1.3).

Modifying GLP‐1 levels, either therapeutically or following the ingestion of food, could alter the excitability of neurons in the olfactory bulb in a nutrition or energy state‐dependent manner to influence olfactory detection or metabolic sensing.

The results of the present study uncover a new function for an olfactory bulb neuron (deep short axon cells, Cajal cells) that could be capable of modifying mitral cell activity through the release of GLP‐1. This might be of relevance for the action of GLP‐1 mimetics now widely used in the treatment of diabetes.

Abstract

The olfactory system is intricately linked with the endocrine system where it may serve as a detector of the internal metabolic state or energy homeostasis in addition to its classical function as a sensor of external olfactory information. The recent development of transgenic mGLU‐yellow fluorescent protein mice that express a genetic reporter under the control of the preproglucagon reporter suggested the presence of the gut hormone, glucagon‐like peptide (GLP‐1), in deep short axon cells (Cajal cells) of the olfactory bulb and its neuromodulatory effect on mitral cell (MC) first‐order neurons. A MC target for the peptide was determined using GLP‐1 receptor binding assays, immunocytochemistry for the receptor and injection of fluorescence‐labelled GLP‐1 analogue exendin‐4. Using patch clamp recording of olfactory bulb slices in the whole‐cell configuration, we report that GLP‐1 and its stable analogue exendin‐4 increase the action potential firing frequency of MCs by decreasing the interburst interval rather than modifying the action potential shape, train length or interspike interval. GLP‐1 decreases Kv1.3 channel contribution to outward currents in voltage clamp recordings as determined by pharmacological blockade of Kv1.3 or utilizing mice with Kv1.3 gene‐targeted deletion as a negative control. Because fluctuations in GLP‐1 concentrations monitored by the olfactory bulb can modify the firing frequency of MCs, olfactory coding could change depending upon nutritional or physiological state. As a regulator of neuronal activity, GLP‐1 or its analogue may comprise a new metabolic factor with a potential therapeutic target in the olfactory bulb (i.e. via intranasal delivery) for controlling an imbalance in energy homeostasis.

The gut hormone called glucagon‐like peptide 1 (GLP‐1) is a strong moderator of energy homeostasis and communication between the peripheral organs and the brain.

GLP‐1 signalling occurs in the brain; using a newly developed genetic reporter line of mice, we have discovered GLP‐synthesizing cells in the olfactory bulb.

GLP‐1 increases the firing frequency of neurons (mitral cells) that encode olfactory information by decreasing activity of voltage‐dependent K channels (Kv1.3).

Modifying GLP‐1 levels, either therapeutically or following the ingestion of food, could alter the excitability of neurons in the olfactory bulb in a nutrition or energy state‐dependent manner to influence olfactory detection or metabolic sensing.

The results of the present study uncover a new function for an olfactory bulb neuron (deep short axon cells, Cajal cells) that could be capable of modifying mitral cell activity through the release of GLP‐1. This might be of relevance for the action of GLP‐1 mimetics now widely used in the treatment of diabetes.

New insights on food intake control by olfactory processes: the emerging role of the endocannabinoid system

The internal state of the organism is an important modulator of perception and behavior. The link between hunger, olfaction and feeding behavior is one of the clearest examples of these connections. At the neurobiological level, olfactory circuits are the targets of several signals (i.e. hormones and nutrients) involved in energy balance. This indicates that olfactory areas are potential sensors of the internal state of the organism. Thus, the aim of this manuscript is to review the literature showing the interplay between metabolic signals in olfactory circuits and its impact on food intake.

Cellular and molecular cues of glucose sensing in the rat olfactory bulb

In the brain, glucose homeostasis of extracellular fluid is crucial to the point that systems specifically dedicated to glucose sensing are found in areas involved in energy regulation and feeding behavior. Olfaction is a major sensory modality regulating food consumption. Nutritional status in turn modulates olfactory detection. Recently it has been proposed that some olfactory bulb (OB) neurons respond to glucose similarly to hypothalamic neurons. However, the precise molecular cues governing glucose sensing in the OB are largely unknown. To decrypt these molecular mechanisms, we first used immunostaining to demonstrate a strong expression of two neuronal markers of glucose-sensitivity, insulin-dependent glucose transporter type 4 (GLUT4), and sodium glucose co-transporter type 1 (SGLT1) in specific OB layers. We showed that expression and mapping of GLUT4 but not SGLT1 were feeding state-dependent. In order to investigate the impact of metabolic status on the delivery of blood-borne glucose to the OB, we measured extracellular fluid glucose concentration using glucose biosensors simultaneously in the OB and cortex of anesthetized rats. We showed that glucose concentration in the OB is higher than in the cortex, that metabolic steady-state glucose concentration is independent of feeding state in the two brain areas, and that acute changes in glycemic conditions affect bulbar glucose concentration alone. These data provide new evidence of a direct relationship between the OB and peripheral metabolism, and emphasize the importance of glucose for the OB network, providing strong arguments toward establishing the OB as a glucose-sensing organ.

The orexinergic system influences conditioned odor aversion learning in the rat: a theory on the processes and hypothesis on the circuit involved

A large variety of behaviors that are essential for animal survival depend on the perception and processing of surrounding smells present in the natural environment. In particular, food-search behavior, which is conditioned by hunger, is directly driven by the perception of odors associated with food, and feeding status modulates olfactory sensitivity. The orexinergic hypothalamic peptide orexin A (OXA), one of the central and peripheral hormones that triggers food intake, has been shown to increase olfactory sensitivity in various experimental conditions including the conditioned odor aversion learning paradigm (COA). COA is an associative task that corresponds to the association between an olfactory conditioned stimulus (CS) and a delayed gastric malaise. Previous studies have shown that this association is formed only if the delay separating the CS presentation from the malaise is short, suggesting that the memory trace of the odor is relatively unstable. To test the selectivity of the OXA system in olfactory sensitivity, a recent study compared the effects of fasting and of central infusion of OXA during the acquisition of COA. Results showed that the increased olfactory sensitivity induced by fasting and by OXA infusion was accompanied by enhanced COA learning performances. In reference to the duration of action of OXA, the present work details the results obtained during the successive COA extinction tests and suggests a hypothesis concerning the role of the OXA component of fasting on the memory processes underlying CS-malaise association during COA. Moreover, referring to previous data in the literature we suggest a functional circuit model where fasting modulates olfactory memory processes through direct and/or indirect activation of particular OXA brain targets including the olfactory bulb, the locus coeruleus (LC) and the amygdala.

A unified survival theory of the functioning of the hypocretinergic system

This article advances the theory that the hypocretinergic (orexinergic) system initiates, coordinates, and maintains survival behaviors and survival-related processes (i.e., the Unified Survival Theory of the Functioning of the Hypocretinergic System or "Unified Hypocretinergic Survival Theory"). A priori presumptive support for the Unified Hypocretinergic Survival Theory emanates from the fact that neurons that contain hypocretin are located in the key executive central nervous system (CNS) site, the lateral hypothalamus, that for decades has been well-documented to govern core survival behaviors such as fight, flight, and food consumption. In addition, the hypocretinergic system exhibits the requisite morphological and electrophysiological capabilities to control survival behaviors and related processes. Complementary behavioral data demonstrate that all facets of "survival" are coordinated by the hypocretinergic system and that hypocretinergic directives are not promulgated except during survival behaviors. Importantly, it has been shown that survival behaviors are selectively impacted when the hypocretinergic system is impaired or rendered nonfunctional, whereas other behaviors are relatively unaffected. The Unified Hypocretinergic Survival Theory resolves the disparate, perplexing, and often paradoxical-appearing results of previous studies; it also provides a foundation for future hypothesis-driven basic science and clinical explorations of the hypocretinergic system.

Hypothalamus-olfactory system crosstalk: orexin a immunostaining in mice

It is well known that olfaction influences food intake, and conversely, that an individual’s nutritional status modulates olfactory sensitivity. However, what is still poorly understood is the neuronal correlate of this relationship, as well as the connections between the olfactory bulb and the hypothalamus. The goal of this report is to analyze the relationship between the olfactory bulb and hypothalamus, focusing on orexin A immunostaining, a hypothalamic neuropeptide that is thought to play a role in states of sleep/wakefulness. Interestingly, orexin A has also been described as a food intake stimulator. Such an effect may be due in part to the stimulation of the olfactory bulbar pathway. In rats, orexin positive cells are concentrated strictly in the lateral hypothalamus, while their projections invade nearly the entire brain including the olfactory system. Therefore, orexin appears to be a good candidate to play a pivotal role in connecting olfactory and hypothalamic pathways. So far, orexin has been described in rats, however, there is still a lack of information concerning its expression in the brains of adult and developing mice. In this context, we revisited the orexin A pattern in adult and developing mice using immunohistological methods and confocal microscopy. Besides minor differences, orexin A immunostaining in mice shares many features with those observed in rats. In the olfactory bulb, even though there are few orexin projections, they reach all the different layers of the olfactory bulb. In contrast to the presence of orexin projections in the main olfactory bulb, almost none have been found in the accessory olfactory bulb. The developmental expression of orexin A supports the hypothesis that orexin expression only appears post-natally.

Olfaction under metabolic influences

Recently published work and emerging research efforts have suggested that the olfactory system is intimately linked with the endocrine systems that regulate or modify energy balance. Although much attention has been focused on the parallels between taste transduction and neuroendocrine controls of digestion due to the novel discovery of taste receptors and molecular components shared by the tongue and gut, the equivalent body of knowledge that has accumulated for the olfactory system, has largely been overlooked. During regular cycles of food intake or disorders of endocrine function, olfaction is modulated in response to changing levels of various molecules, such as ghrelin, orexins, neuropeptide Y, insulin, leptin, and cholecystokinin. In view of the worldwide health concern regarding the rising incidence of diabetes, obesity, and related metabolic disorders, we present a comprehensive review that addresses the current knowledge of hormonal modulation of olfactory perception and how disruption of hormonal signaling in the olfactory system can affect energy homeostasis.

Olfactory ability and object memory in three mouse models of varying body weight, metabolic hormones, and adiposity

Physiological and nutritional state can modify sensory ability and perception through hormone signaling. Obesity and related metabolic disorders present a chronic imbalance in hormonal signaling that could impact sensory systems. In the olfactory system, external chemical cues are transduced into electrical signals to encode information. It is becoming evident that this system can also detect internal chemical cues in the form of molecules of energy homeostasis and endocrine hormones, whereby neurons of the olfactory system are modulated to change animal behavior towards olfactory cues. We hypothesized that chronic imbalance in hormonal signaling and energy homeostasis due to obesity would thereby disrupt olfactory behaviors in mice. To test this idea, we utilized three mouse models of varying body weight, metabolic hormones, and visceral adiposity - 1) C57BL6/J mice maintained on a condensed-milk based, moderately high-fat diet (MHF) of 32% fat for 6 months as the diet-induced obesity model, 2) an obesity-resistant, lean line of mice due to a gene-targeted deletion of a voltage-dependent potassium channel (Kv 1.3-null), and 3) a genetic model of obesity as a result of a gene-targeted deletion of the melanocortin 4 receptor (MC4R-null). Diet-induced obese (DIO) mice failed to find a fatty-scented hidden peanut butter cracker, based solely on olfactory cues, any faster than an unscented hidden marble, initially suggesting general anosmia. However, when these DIO mice were challenged to find a sweet-scented hidden chocolate candy, they had no difficulty. Furthermore, DIO mice were able to discriminate between fatty acids that differ by a single double bond and are components of the MHF diet (linoleic and oleic acid) in a habituation-dishabituation paradigm. Obesity-resistant, Kv1.3-null mice exhibited no change in scented object retrieval when placed on the MHF-diet, nor did they perform differently than wild-type mice in parallel habituation-dishabituation paradigms of fatty food-related odor components. Genetically obese, MC4R-null mice successfully found hidden scented objects, but did so more slowly than lean, wild-type mice, in an object-dependent fashion. In habituation-dishabituation trials of general odorants, MC4R-null mice failed to discriminate a novel odor, but were able to distinguish two fatty acids. Object memory recognition tests for short- and long-term memory retention demonstrated that maintenance on the MHF diet did not modify the ability to perform these tasks independent of whether mice became obese or were resistant to weight gain (Kv1.3-null), however, the genetically predisposed obese mice (MC4R-null) failed the long-term object memory recognition performed at 24h. These results demonstrate that even though both the DIO mice and genetically predisposed obese mice are obese, they vary in the degree to which they exhibit behavioral deficits in odor detection, odor discrimination, and long-term memory.

Neuropeptide Y enhances olfactory mucosa responses to odorant in hungry rats

Neuropeptide Y (NPY) plays an important role in regulating appetite and hunger in vertebrates. In the hypothalamus, NPY stimulates food intake under the control of the nutritional status. Previous studies have shown the presence of NPY and receptors in rodent olfactory system, and suggested a neuroproliferative role. Interestingly, NPY was also shown to directly modulate olfactory responses evoked by a food-related odorant in hungry axolotls. We have recently demonstrated that another nutritional cue, insulin, modulates the odorant responses of the rat olfactory mucosa (OM). Therefore, the aim of the present study was to investigate the potential effect of NPY on rat OM responses to odorants, in relation to the animal's nutritional state. We measured the potential NPY modulation of OM responses to odorant, using electro-olfactogram (EOG) recordings, in fed and fasted adult rats. NPY application significantly and transiently increased EOG amplitudes in fasted but not in fed rats. The effects of specific NPY-receptor agonists were similarly quantified, showing that NPY operated mainly through Y1 receptors. These receptors appeared as heterogeneously expressed by olfactory neurons in the OM, and western blot analysis showed that they were overexpressed in fasted rats. These data provide the first evidence that NPY modulates the initial events of odorant detection in the rat OM. Because this modulation depends on the nutritional status of the animal, and is ascribed to NPY, the most potent orexigenic peptide in the central nervous system, it evidences a strong supplementary physiological link between olfaction and nutritional processes.

Orexins: tissue localization, functions, and its relation to insulin secretion and diabetes mellitus

Orexins play a role in many biological functions include sleep, feeding, and energy balance. They also regulate circadian rhythms and the way that we feel pain. Orexins have been identified in a variety of tissues including the cerebrospinal fluid, blood, hypothalamus, spinal cord, sensory ganglion, enteric nervous system, pituitary, adrenal, salivary and lacrimal glands, testis, vestibular gland, and skin. Orexins play a role in a variety of biological functions including arousal, sleeping, food and fluid intake, pain, memory, perception of odor, and sexual activity. Orexins have also been implicated in the regulation of glucose metabolism. The expression of orexin is induced by hypoglycemia, low food, pregnancy, and hemodialysis. In contrast, factors that inhibit the expression of orexins include obstructive sleep apnea, aging, depression, obesity, traumatic brain injury, and inflammatory molecules such as liposaccharide. In conclusion, orexins are widely distributed and involved in a large variety of biological activities.

The Orexin-1 Receptor Antagonist SB-334867 Reduces Alcohol Relapse Drinking, but not Alcohol-Seeking, in Alcohol-Preferring (P) Rats

PRINCIPLE

The orexin system has been hypothesized to regulate drug-seeking and drug self-administration behaviors, including ethanol (EtOH) seeking and consumption. However, studies on the effects of orexin receptor antagonists have not been conducted on robust alcohol-relapse behavior.

OBJECTIVES

This study assessed the effects of the orexin-1 receptor antagonist, SB-334867, on alcohol-seeking behavior and responding for alcohol under relapse conditions.

METHODS

Adult alcohol-preferring (P) rats self-trained in 2-lever operant chambers to administer 15% EtOH (vol/vol) on a fixed-ratio-5 and water on a fixed-ratio-1 schedule of reinforcement. After 10 weeks, rats underwent extinction training for 7 sessions. Animals were then maintained in their home cages for 2 weeks before being tested for Pavlovian Spontaneous Recovery (PSR; a measure of alcohol seeking) for 4 sessions. Rats were then allowed a week in their home cages before being returned to the operant chamber with access to EtOH and water (relapse). Thirty minutes before the PSR and relapse test sessions, rats received 0, 10, or 20 mg/kg SB-334867.

RESULTS

Responses on the EtOH lever during the 1st PSR test session were ~70 presses/session (3-fold higher than baseline); SB-334867 did not alter responses on the EtOH lever. Under relapse conditions, P rats increased responding on the EtOH lever from 250 (at baseline) to 350 responses/session; both doses of SD-334867 prevented this increase.

CONCLUSIONS

The results of this study suggest that activation of orexin-1 receptors is not involved in intrinsically initiated EtOH seeking, but may regulate the consummatory behavior of EtOH consumption.

The endocannabinoid 2-arachidonoyl-glycerol controls odor sensitivity in larvae of Xenopus laevis

Cannabinoids modulate the activity of many neuronal cells, among them sensory neurons in the olfactory epithelium. Here we show that the endocannabinoid 2-arachidonoyl-glycerol (2-AG) is synthesized in both olfactory receptor neurons and glia-like sustentacular cells in larval Xenopus laevis. Its production in the latter depends on the hunger state of the animal. The essential effect of 2-AG in olfactory receptor neurons is the control of odorant detection thresholds via cannabinoid CB(1) receptor activation. Hunger renders olfactory neurons more sensitive. Endocannabinoid modulation in the nose may therefore substantially influence food-seeking behavior.

The endocannabinoid 2-arachidonoyl-glycerol controls odor sensitivity in larvae of Xenopus laevis

Cannabinoids modulate the activity of many neuronal cells, among them sensory neurons in the olfactory epithelium. Here we show that the endocannabinoid 2-arachidonoyl-glycerol (2-AG) is synthesized in both olfactory receptor neurons and glia-like sustentacular cells in larval Xenopus laevis. Its production in the latter depends on the hunger state of the animal. The essential effect of 2-AG in olfactory receptor neurons is the control of odorant detection thresholds via cannabinoid CB(1) receptor activation. Hunger renders olfactory neurons more sensitive. Endocannabinoid modulation in the nose may therefore substantially influence food-seeking behavior.

The role of the terminal nerve and GnRH in olfactory system neuromodulation

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

Nutritional status modulates behavioural and olfactory bulb Fos responses to isoamyl acetate or food odour in rats: roles of orexins and leptin

Food odours are major determinants for food choice, and their detection depends on nutritional status. The effects of different odour stimuli on both behavioural responses (locomotor activity and sniffing) and Fos induction in olfactory bulbs (OB) were studied in satiated or 48-h fasted rats. We focused on two odour stimuli: isoamyl acetate (ISO), as a neutral stimulus either unknown or familiar, and food pellet odour, that were presented to quiet rats during the light phase of the day. We found significant effects of nutritional status and odour stimulus on both behavioural and OB responses. The locomotor activity induced by odour stimuli was always more marked in fasted than in satiated rats, and food odour induced increased sniffing activity only in fasted rats. Fos expression was quantified in periglomerular, mitral and granular OB cell layers. As a new odour, ISO induced a significant increase in Fos expression in all OB layers, similar in fasted and satiated rats. Significant OB responses to familiar odours were only observed in fasted rats. Among the numerous peptides shown to vary after 48 h of fasting, we focused on orexins (for which immunoreactive fibres are present in the OB) and leptin, as a peripheral hormone linked to adiposity, and tested their effects of food odour. The administration of orexin A in satiated animals partially mimicked fasting, since food odour increased OB Fos responses, but did not induce sniffing. The treatment of fasted animals with either an orexin receptors antagonist (ACT-078573) or leptin significantly decreased both locomotor activity, time spent sniffing food odour and OB Fos induction in all cell layers, thus mimicking a satiated status. We conclude that orexins and leptin are some of the factors that can modify behavioural and OB Fos responses to a familiar food odour.

Central and peripheral regulation of food intake and physical activity: pathways and genes

A changing environment and lifestyle on the background of evolutionary engraved and perinatally imprinted physiological response patterns is the foremost explanation for the current obesity epidemic. However, it is not clear what the mechanisms are by which the modern environment overrides the physiological controls of appetite and homeostatic body-weight regulation. Food intake and energy expenditure are controlled by complex, redundant, and distributed neural systems involving thousands of genes and reflecting the fundamental biological importance of adequate nutrient supply and energy balance. There has been much progress in identifying the important role of hypothalamus and caudal brainstem in the various hormonal and neural mechanisms by which the brain informs itself about availability of ingested and stored nutrients and, in turn, generates behavioral, autonomic, and endocrine output. Some of the genes involved in this "homeostatic" regulator are crucial for energy balance as manifested in the well-known monogenic obesity models. However, it can be clearly demonstrated that much larger portions of the nervous system of animals and humans, including the cortex, basal ganglia, and the limbic system, are concerned with the procurement of food as a basic and evolutionarily conserved survival mechanism to defend the lower limits of adiposity. By forming representations and reward expectancies through processes of learning and memory, these systems evolved to engage powerful emotions for guaranteed supply with, and ingestion of, beneficial foods from a sparse and often hostile environment. They are now simply overwhelmed with an abundance of food and food cues no longer contested by predators and interrupted by famines. The anatomy, chemistry, and functions of these elaborate neural systems and their interactions with the "homeostatic" regulator in the hypothalamus are poorly understood, and many of the genes involved are either unknown or not well characterized. This is regrettable because these systems are directly and primarily involved in the interactions of the modern environment and lifestyle with the human body. They are no less "physiological" than metabolic-regulatory mechanisms that have attracted most of the research during the past 15 years.

Olfactory dysfunction in patients with narcolepsy with cataplexy is restored by intranasal Orexin A (Hypocretin-1)

Until recently, olfactory dysfunction was an unknown feature of narcolepsy. Orexin A, also called hypocretin-1, is abnormally decreased or undetectable in the cerebrospinal fluid of narcoleptic patients with cataplexies. As hypothalamic orexin-containing neurons project throughout the entire olfactory pathway, from the olfactory mucosa to the olfactory cortex, disturbed orexinergic transmission may crucially be involved in impaired olfactory performance of narcolepsy patients. In our study we analysed the olfactory performance (threshold, discrimination, identification and sum score of these measurements, the TDI score) of narcoleptic patients with cataplexies (n = 10) and of age-, gender-, BMI- and smoker/non-smoker-matched healthy controls (n = 10). We then in a double-blind, randomized, placebo-controlled cross-over design applied orexin A intranasally to seven of the patients and measured 2-phenyl-ethyl alcohol (PEA) single-staircase odour detection thresholds. Compared to the controls, patients showed significantly lower scores for olfactory threshold (patients: median 8.0, range 4.0-10.5; controls: median 9.4, range 7.5-13.3; P < 0.05), discrimination (patients: median 12.5, range 10-15; controls: median 15.0, range 12-16; P < 0.005), identification (patients: median 13.0, range 10-16; controls: median 14.0, range 13-16; P < 0.05) and TDI score (patients: median 33.4, range 30-36; controls: median 38.4, range 35-43; P < 0.0001). In all patients, the PEA olfactory threshold score increased after administration of orexin A (median 11.5, range 6.5-13.25) compared to placebo (median 7.75, range 6.25-11.25; P < 0.05). Our results support the hypothesis that mild olfactory dysfunction is an intrinsic symptom of narcolepsy with cataplexies. The observation that intranasal orexin A restores olfactory function is in favour of this hypothesis. Furthermore, our data support that the pathophysiological mechanism underlying olfactory dysfunction in narcolepsy is the lack of CNS orexin.

Distribution of orexins-containing fibers and contents of orexins in the rat olfactory bulb

Orexin-A and -B (identical to hypocretin-1 and -2) are hypothalamic neuropeptides that regulate appetite and arousal. Orexins-producing neurons project their axons to various brain regions, including the olfactory bulb. In the present study, to understand the relationship between orexins and olfaction, we investigated the distribution of the orexin-A- and -B-immunoreactive (ir) fibers in the rat olfactory bulb and the contents of orexin-A and -B in the rat olfactory bulb after food deprivation for 48 h by using immunohistochemistry and radioimmunoassay, respectively. Both orexin-A- and -B-ir fibers are similarly wide spread from the glomerular layer of the olfactory bulb where the terminals of the peripheral olfactory nerves make synapses with the mitral cells or the tufted cells, to the piriform cortex. Dense orexin-A- and -B-ir fibers were observed mainly in the granular cell layer and anterior olfactory nucleus. The contents of orexin-A and -B (pg/10 mg wet weight tissue) in fed rats (mean+/-S.E.M., n=6) were 2.72+/-0.24 and 6.31+/-0.63, respectively. Fasting for 48 h significantly reduced the contents of orexin-B, but not orexin-A. Orexins in the rat olfactory bulb may be involved in not only olfactory system but also energy balance.

Eating for pleasure or calories

A changing environment and lifestyle on the background of evolutionary engraved or perinatally imprinted physiological response patterns is the foremost explanation for the current obesity epidemic. However, it is not clear what the mechanisms are by which the modern environment overrides the physiological controls of appetite and homeostatic body weight regulation. Major advances have been made regarding crosstalk between metabolic signals and the cognitive/emotional brain that primarily deals with the environment. On one hand, metabolic signals such as leptin and ghrelin have previously unexpected direct effects on learning and memory, as well as liking and wanting. On the other hand, brain areas involved in reward, cognition, and executive control can override metabolic regulation by talking to the hypothalamus.

Fasting increases and satiation decreases olfactory detection for a neutral odor in rats

Olfaction plays a fundamental role in feeding behavior, but changes in olfactory acuity according to feeding states have never been precisely demonstrated in animals. The present study assesses the olfactory detection performance of fasted or satiated rats placed under a strictly controlled food-intake regimen. We did this using a conditioned odor aversion (COA) protocol which induced in rats an almost total aversion to an ISO-odorized drink at 10(-5) (1 microl in 100 ml of water). The rats (either fasted or satiated) were then presented with different concentrations of ISO-odorized water to compare their ability to detect and so avoid the ISO drink. In both states, the rats consumed significantly larger volumes of ISO at 10(-10), 10(-9) and 10(-8) than at 10(-5), suggesting lower detection at these three concentrations, although the fasted rats consumed significantly less ISO drink than did the satiated ones, showing better ISO detection at these concentrations. These experiments provide original data demonstrating the expected fact that olfactory sensitivity increases in fasted animals. Since these results were obtained using a neutral odor, we suggest that olfactory acuity increases during fasting, enabling animals to more easily detect both food and environmental odors such as those of predators. This would have an obvious eco-ethological role by increasing the relevance of olfactory inputs when seeking food.

Cannabinoid action in the olfactory epithelium

The perception of odors is influenced by a variety of neuromodulators, and there is growing evidence that modulation already takes place in the olfactory epithelium. Here we report on cannabinergic actions in the olfactory epithelium of Xenopus laevis tadpoles. First we show that CB1 receptor-specific antagonists AM251, AM281, and LY320135 modulate odor-evoked calcium changes in olfactory receptor neurons. Second, we localize CB1-like immunoreactivity on dendrites of olfactory receptor neurons. Finally, we describe the cannabinergic influence on odor-induced spike-associated currents in individual olfactory receptor neurons. Here we demonstrate that the cannabinergic system has a profound impact on peripheral odor processing and discuss its possible function.

Olfactory neuronal dynamics in behaving animals

More than 50 years have passed since the first recording of neuronal responses to an odor stimulus from the primary olfactory brain area, the main olfactory bulb. During this time very little progress has been achieved in understanding neuronal dynamics in the olfactory bulb in awake behaving animals, which is very different from that in anesthetized preparations. In this paper we formulate a new framework containing the main reasons for studying olfactory neuronal dynamics in awake animals and review advances in the field within this new framework.